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Cite this: Dalton Trans., 2014, 43, 4520
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Computational insights into carbon–carbon homocoupling reactions mediated by organolanthanide(III) complexes† Christos E. Kefalidis,* Lionel Perrin‡ and Laurent Maron* Homocoupling of terminal alkynes into trienediyl complexes by alkyl samarocenes is known experimentally. By means of computational techniques, we investigated the mechanism of this reaction in detail. The overall reaction sequence is: σ-bond metathesis, dimerisation of metallocenes, and homocoupling of two acetylides into trienediyl. We show that the rate-determining step corresponds to the homocoupling of two anionic acetylides. This coupling takes place at a bis-samarocene dimer complex in
Received 17th October 2013, Accepted 17th December 2013
which the bridging mode of the two acetylide moieties is critical for the reaction to proceed. The limited energy barrier for the homocoupling of the carbanions originates from a synergistic effect of the two
DOI: 10.1039/c3dt52937a
samarium centres within the dimer. Variation of the steric demand of both substrates and lanthanocenes
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allowed rationalising all the experimental data available for these systems.
Introduction The reactivity of lanthanide(III) complexes proceeds frequently at a fixed oxidation state – and among others – via concerted σbond metathesis, insertion, or α-/β-elimination.1–3 It has been shown both experimentally and computationally that C–C bond coupling cannot be achieved by σ-bond metathesis at a single lanthanide(III) centre since it requires the presence of a carbon atom at the β-position of the 4-center transition state.4 This limitation has been circumvented by the use of Ln(II) reagents, and mainly illustrated by Sm(II) complexes. The versatility of the coupling reaction induced by a Sm(II) Single Electron Transfer (SET) has been strongly illustrated by the chemistry of SmI2.5–9 This chemistry has been expanded by the use of Cp*2Sm(THF)n (Cp* = η5-C5Me5, n = 0, 2).10–14 However, C–C homocoupling reactions were achieved using lanthanocene(III) complexes with terminal alkynes as substrates. Of utmost importance, this dimerisation of terminal alkynes appeared in 1990 from the group of Evans.15 In that study the phenylacetylene was used as the substrate, reacting with different mono- (in II or III oxidation state) or bimetallic (in III oxidation state) samarocene complexes to afford a new class of trienediyl complexes of samarium of the type
Université de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3dt52937a ‡ Present address: Université Lyon 1, CNRS, UMR 5246, ICBMS, 43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France.
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{(Cp*2SmIII)2(µ-η2:η2-Ph–CvCvCvC–Ph)}.15 It should be noted that the latter complex was also previously prepared from the oxidation of two Cp*2Sm(THF)2 molecules with the diyne Ph– CuC–CuC–Ph.16 This was the first example in the organolanthanide(III) chemistry of forming a carbon–carbon bond by coupling two anionic ligands. X-ray characterisation of this bimetallic complex, along with the absence of characteristic bands of triple bonds in the infra-red region,16 allowed identification of the same dimeric product obtained by different synthetic routes.15 This reactivity was extended to include other terminal alkynes HCuCR (where R = CH2CH2Ph, CH2CH2CHMe2, and CHMe2), producing the same type of dinuclear complex.17 However, starting from hindered acetylides like [Cp*2Sm(CuCtBu)(THF)], the homocoupling reaction could not be observed. In contrast the uncoupled dimers [Cp*2Sm(CuCtBu)]2 were characterised as products.17 Similarly, the use of different ancillary ligands than Cp*, for example CpMe (η5-C5H4Me) or CptBu (η5-C5H4tBu), also led to the formation of the corresponding uncoupled binuclear complexes.18,19 In 1993, Teuben’s group reported a related study using early lanthanide alkyls of the type Cp*2LnCH(SiMe3)2. These precursors, upon the reaction with terminal alkynes HCuCR, gave bis-µ-acetylides [Cp*2Ln(CuCR)]2 (where Ln = Ce, R = Me or tBu; Ln = La, R = Me). These complexes were shown to be unstable in solution. They easily afford the corresponding trienediyl bimetallic species of the type {[Cp*2Ln]2(µ-η2:η2-RC4R)} upon a homo C–C coupling step. These reactions were characterised by NMR techniques and their molecular structures were determined by X-ray diffraction.20 Independently, Marks et al.21 reported the same year a related study
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Dresden–Koln type, using large core potentials (RECP),25,26 augmented by an f polarisation function (exp = 1.000). The Si atoms were represented by the same type of RECP27 and augmented by a d polarisation function (exp = 0.284).28 For the remaining atoms the all-electron double zeta basis set 6-31G(d,p) was used.29 Enthalpy energies were obtained at T = 298.15 K based on the harmonic approximation. Intrinsic Reaction Paths (IRPs) were traced from the various transition structures to verify the reactant to product linkage.30 Natural population analysis (NPA) was performed using Weinhold’s methodology.31
Results and discussion Scheme 1 Proposed mechanisms for the formation of trienediyls from the acetylide monomers.
using the same alkyl precursor of lanthanum, but with different terminal alkynes HCuCR (R = tBu and Ph). The outcome of the reaction was again the formation of the same type of trienediyl bimetallic complex. These authors proposed based on kinetic studies that the dimer [Cp*2La(µ2-CuCR)]2 is the direct kinetic precursor of the trienediyl complex. Consequently, they speculate about two different mechanisms as possible routes for this process: a concerted one and an insertive one as shown in Scheme 1a.21 Alternatively, Evans and coworkers proposed that a heterolytic cleavage of Sm-CCPh is likely to occur to form an anionic phenylacetylide fragment and a cationic samarocene one. By forming the dimer, the coupling reaction will occur then followed by a rearrangement of the samarocene fragments in order to give the final trienediyl product (Scheme 1b).17 Having in mind the previous mechanistic scenarios, there is a need to complete and give a clearer picture of this type of reactivity, which remains elusive up to now. The underlying mechanistic issue is the interplay between two lanthanocenes III complexes that mediate the homocoupling of two carbanions. In this report, we address this mechanistic aspect for this important class of reactions in lanthanide chemistry. We will also highlight and explain the importance of stereo-electronic effects in the metallocenes and acetylides that govern the reaction in order to rationalise the experimental observations. This will be done by considering, explicitly, experimental and model systems in our mechanistic density functional theory investigations.
As mentioned in the Introduction, among the different synthetic routes that have been followed to obtain diphenylbutatrienediyl derivatives, the first one was via the classical lanthanide reaction of Sm(III) alkyl complexes with phenylacetylene (following equation).11,15
We will consider this reaction as a baseline for our mechanism exploration. The above transformation of two monomers of phenylacetylene to give the corresponding trienediyl complex has been proposed to be a two-step reaction. The first corresponds to a σ-bond metathesis (σ-BM) between the terminal acetylenic C–H bond and the Sm–C bond of (Cp*)2Ln–CH(SiMe3)2. It affords the corresponding phenylalkynide with the simultaneous formation of bis(trimethylsilyl)methane CH2(SiMe3)2, as is shown in Fig. 1. The second refers to the C–C homocoupling step.
Computational section The molecular electronic structure calculations required for the depiction of equilibrium and transition state molecular structures were performed at the density functional theory (DFT) level using the B3PW9122,23 hybrid functional as implemented in the Gaussian program code.24 The three lanthanides (La, Ce, and Sm) were described by the corresponding scalar relativistic pseudopotentials of the Stuttgart–
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Fig. 1 Enthalpic reaction profile for the σ-bond metathesis of the Sm alkyl complex (in kcal mol−1). Figures in parentheses refer to the Cp ligand, while numbers in blue correspond to natural charges.
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σ-Bond metathesis As shown by the alternation of the natural charges of the four atoms involved in a kite-shaped geometry, the transition state associated with the metallation of the terminal alkyne is indeed a classical σ-BM one (Fig. 1). This is the most important criteria for characterizing such a transition state as σ-BM.4,32,33 In particular, the energy barrier that is needed to pass from the separated reactants to the alkynide intermediate is of only 7.1 kcal mol−1. From a thermodynamic point of view, the reaction is exothermic, with the resulting phenylalkynide intermediate that is more stable with respect to the entrance channel by 29.1 kcal mol−1. Based on these energetics, this reaction proceeds almost spontaneously. It is noteworthy that by moving from the Cp* ligand to the less bulky Cp ligand, the difference in the activation barrier is about 1 kcal mol−1 and the exothermicity is in the same range. The latter shows that the substituents on cyclopentadienyl ligands on samarocene do not play any role on both the thermodynamics and kinetics of the reaction. The combination of a low activation barrier and high exothermicity is also found to be true for the La or Ce analogue complexes. These analogues are characterised by a similar chemical environment (both ligands and substrates), and their corresponding enthalpy profiles are similar, as shown in Fig. 2 (ΔH# = 5.0 and 5.3 kcal mol−1, and ΔrH = −25.7 and 27.1 kcal mol−1, respectively for La and Ce). It should be noted that for all combinations of alkyne with cerium or lanthanum precursors experimental results are available,20,21 except in the case of cerium phenylacetylene reactivity which was considered in the present study for the sake of comparison. In order to confirm the adequacy between experimental results and computed energetics, we investigated well-established reactions from the literature with a variety of substrates using the [Ln]CH(TMS)2 precursor alkyl complexes of La and Ce.20,21 In particular, we replaced the phenyl substituent with a methyl one and kept intact the metallocene fragment. Though this modification strongly modifies both steric and electronic properties of the alkynides, we found that activation
Fig. 2
barriers do not dramatically change, as well as the thermodynamics of the reaction. The same thermodynamic and kinetic trends are observed for the bulky tBu substituent. In general, the formation of the phenylalkynide complex in all cases is more exothermic than the tbutyl- and methyl-acetylide analogues. The latter difference in thermodynamics is due to the ability of the phenyl group to accept more density from the more electron rich Sm than the La and the Ce one, resulting in the stabilization of the corresponding monomer. In all cases, the activation barriers are close to 5 kcal mol−1, with the tBu being systematically higher by almost 2 kcal mol−1 compared to the other substituents (Fig. 2). It is worth noting that the computed geometry of the (Cp*)2CeCH(SiMe3)2 is in very good agreement with the molecular structure which is determined by single-crystal X-ray analysis.34 Alkynides homocoupling The second and definite step of the proposed mechanism corresponds to the homo C–C coupling of the two anionic moieties to afford the final C–C coupled product. This commences by the formation of the dimer as a result of the weak intermolecular interaction between the π-system of the alkynylide ligand of one monomer and the Sm centre of the second monomer. This doubly bridged interaction only requires 2.4 kcal mol−1 to be established. This low endothermicity is likely attributable to the steric demand that is apparent in the dimeric structure between the phenylacetylide groups and the methyl groups of the cyclopentadienyl ligands. This is further clarified by the change in some important distances and angles as the dimer forms. In particular, the Sm–Cacetylide bond distance is increased substantially by almost 0.15 Å, being 2.583 and 2.588 Å in the dimer, and 2.436 Å in the monomer. To a lesser extent, the bond distance of the CuC of the alkynides is increased by only 0.007 Å, being almost of the same value for the C–Cphenyl bond as well. Moreover, the ∠SmCC moves from linearity by almost 20°, while the ∠CCCphenyl decreases by almost 7°. It should be noted at this point that the geometries obtained computationally for the trienediyl bimetallic complexes are in excellent agreement with
Enthalpic reaction profiles for the σ-bond metathesis of a series of La and Ce alkyl complexes (in kcal mol−1).
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the X-ray one.15 In conclusion, the energy needed for all the distortions within the phenylacetylide fragment is overcompensated by the energy gained from the electrostatic attractions between the two monomers. As previously mentioned, the final trienediyl dinuclear complex results from the C–C homo coupling of the two terminal anionic charged phenylacetylide moieties. Interestingly, similar transition states were postulated to proceed in the respective bis-µ-alkynyl titanocene complexes35 and were confirmed computationally by Dobado et al.36 In the present study, the energy barrier for this step is 10.7 kcal mol−1 relatively to the precursor dimer. This reaction formally couples two terminal alkynyls and is the rate-determining step of the entire mechanism. The surprisingly low energy barrier for the homocoupling of two negatively charged moieties, as phenylacetylide is, needs further explanation. This can be done by inspecting the geometrical changes on passing from the precursor dimer to the transition state, in conjunction with the changes in electron density of the atoms involved in the coupling. For the C–C coupling to proceed, the electronic density of the negatively charged α-carbons of the alkynides has to be reduced. This is achieved by the electrophilic assistance from the β-carbon of each triple bond that accumulates negative charge due to its interaction with the second samarium centre. The natural population analysis shows the aforementioned statement. The charge distribution is depicted in Fig. 3. More precisely, the α-carbons of the alkynyls on the dimer acquire a negative charge of −0.45 |e| each, whereas each of the β-carbons has a negative charge of −0.15 |e|. At the transition state the corresponding charges are almost equal, being −0.32 |e| for all but one which is −0.35 |e| (Fig. 3). The NPA analysis is in line with the changes in some important geometrical features of the active site of the reactivity. In particular, there is a substantial decrease of the distance between the β-carbons of the alkynyls with respect to the vicinal Sm centers, being 2.724 and 2.561 Å at the transition state. The corresponding distances are respectively 3.184 and 3.268 Å in the dimer (denoted as Sm1–C*α and Sm2–Cα respectively in Table 1). The noticeable
Fig. 3 Enthalpic reaction profile, in kcal mol−1, for the C–C coupling step starting from the Cp*2Sm(CCPh) monomer.
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shortening of the C–C distance is directly related to the newly formed bond. In contrast, there is no significant change in the Sm–Calkynide distance. All the above theoretical observations account for the low barrier for this process compared to that reported in the literature.37 Finally, C–C coupling is exothermic by 24.5 kcal mol−1 and the overall exothermicity is 51.1 kcal mol−1 with respect to starting materials.
Acetylide substituent effects It has been experimentally highlighted that the coupling of alkynides into trienediyl depends on the steric properties of both the acetylide and metallocene substituents. We will focus now on the ligand effects (R groups) on the C–C coupling step as it is indeed the rate-determining step from samarocene alkyl complexes and terminal alkynes to trienediyl. In order to examine the effect introduced by the R substituents, we have chosen methylacetylide as a model reactive ligand, as well as the experimentally used tbutylacetylide. The choice of these two substrates is also made, apart from the fact that they introduce different steric as well as electronic effects compared to the phenyl substituent, based on the availability in the literature of analogous systems, for which experimental results are accessible for direct comparisons.17,20,21 From a thermodynamic point of view, whereas the dimerisation of Cp*2Sm(CuCMe) is almost athermic, this process requires 7.7 kcal mol−1 to proceed in the case of the more hindered Cp*2Sm(CuCtBu) (Fig. 4). The same trend is observed concerning the activation barrier, with an increase of 6.2 kcal mol−1 with respect to the precursor dimer. When R = Me, the activation barrier is found to be almost 3 kcal mol−1 higher than for phenylacetylide, but two times higher when R is the tbutylacetylide relative to the same phenylacetylide. However, from the computed kinetic and thermodynamic parameters, the coupling could be expected to occur for all substrates considered, though the coupling of tBuCuCH should be slower. This is not experimentally the case for the tbutyl substituent.15 The explanation for this controversy is given by the nature of the “real” precursor that is Cp*2Sm(CuCtBu)(THF), in which a THF molecule binds the Sm center. Such a THF molecule, which usually inhibits the reactivity in lanthanide chemistry, requires here +13.6 kcal mol−1 to decoordinate from the Sm center. Having this in mind and adding another 7.7 kcal mol−1 that are needed for the formation of the dimer, as well as the 20 kcal mol−1 to overcome the activation barrier, leads to an overall barrier of 41.3 kcal mol−1. This barrier makes the process kinetically not feasible. In addition, with respect to the THF adduct, the whole process is endothermic by 7.2 kcal mol−1, which is large enough to prevent this reaction from occurring. Experimentally, when the same reaction conditions were applied for the case of phenylacetylene, the reaction occurred only after heating the system at 120° for 3 days affording the trienediyl complex. The corresponding sum of energy that is required in this case is much lower than for tbutyl, being 26.4 kcal mol−1, which explains the experimentally observed low reactivity.15
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Table 1
CuCR R = Me Reactant TS Product R = tBu Reactant TS Product R = Ph Reactant TS Product
Dalton Transactions Selected interatomic distances (Å) and angles (°) for the C–C coupling step of different substituent alkynides using samarium complexes
Sm1–C1
Sm1–C′1
Sm1–C′2
Sm2–C1
Sm2–C′1
Sm2–C2
C1–C′1
Sm1–Sm2
Sm1–C1–C2
C′1–C1–C2
2.590 2.580 3.168
2.961 3.001 2.784
3.090 2.603 2.497
3.019 2.927 2.776
2.586 2.538 3.155
3.012 2.714 2.494
3.152 2.023 1.329
4.619 5.155 5.803
166.0 153.1 141.2
−125.9 −125.6 −157.6
2.713 2.597 3.407
2.695 2.992 2.836
3.635 2.698 2.522
2.720 3.000 2.750
2.695 2.584 3.098
3.554 2.685 2.539
3.318 1.991 1.324
4.275 5.235 5.893
133.2 158.1 147.1
−174.9 −121.7 −159.1
2.583 2.554 3.223
3.002 3.004 2.805
3.184 2.724 2.507
3.041 3.004 2.806
2.588 2.561 3.230
3.268 2.689 2.503
3.202 2.017 1.315
4.623 5.200 5.900
159.9 157.3 140.6
−138.0 −120.9 −159.3
Fig. 4 Enthalpic reaction profile, in kcal mol−1, for the C–C coupling step starting from Cp*2Sm(CCR) monomers (where R = Me or tBu).
In terms of the binding mode, a close inspection of some critical geometrical features reveals important insights into the observed reactivity (Table 1). In particular, the most relevant distances in the dimers are very similar for the methyl and phenyl substituents, with a few exceptions, like the Sm2⋯C2 that is longer by almost 0.25 Å for phenylacetylide with respect to the methylacetylide case. This
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is mainly due to the larger repulsions that are developed between the phenyl group and the Cp* ligands, with respect to the corresponding one in the case of the methyl substituent. It should be noted that in both cases the dimer can be characterised as two discrete monomeric fragments; this is not the case for tbutylacetylides which correspond to a bimetallic system. In the latter case, the terminal carbon atoms of the bulky alkynides adopt an almost ideal 4-center symmetric bridging coordination mode (d(Sm1–C1) = 2.713 Å, and d(Sm2–C1) = 2.720 Å). Also, the CuC bonds of both tbutylacetylides are almost perpendicular to the intermetallic axis (∠C′1–C1–C2 = −174.9°) as a consequence of the bulkiness of the tbutyl substituents that induces steric effects with the methyl substituents of the cyclopentadienyl ligands. This bonding situation also results in a shorter intermetallic distance (4.275 Å) compared to the others (∼4.620 Å) whose importance will be highlighted later. In contrast with the substantial changes found in this set of three dinuclear complexes, no major variation could be pointed out in the geometries of the corresponding transition states. However, in the case of the tbutyl substituent, passing from the bimetallic complex to the transition state leads to significant variations, in contrast with the other two cases. More particularly, the Sm1–C′2 and Sm2–C2 distances are dramatically shortened by almost 1.0 Å. The corresponding difference in the other two cases under study is 0.3–0.6 Å. As
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we have shown previously, it is mandatory that the β-carbons of the alkynides have to develop a strong interaction with neighbouring metals in the transition state; however, in the methyl and the phenyl case the β-carbons are much more close to the metals, so they do not need to vary their geometry dramatically, which is not the case in the tbutyl one, with the latter being translated in the extra energy required. In the meantime, the approach of the two bulky groups in the transition state with respect to the methyl groups of the Cp* ligands also affects the energy of the activation barrier. These two observations can serve as an explanation for the higher calculated activation barrier of this process for the tbutyl case compared to the other two cases. The same trend is followed when the same series of substrates are used CuC–R (R = Me, tBu, Ph), but with different lanthanides as cerium and lanthanum (Fig. 5). Apart from the combination of Ln = Ce and R = Ph, all other complexes were experimentally studied and the final trienediyl complexes were characterized by X-rays. In general, the computational protocol used herein is giving geometries very close to the experimental one, allowing further DFT analysis. Firstly, in terms of kinetics, there is an increase in the corresponding activation barriers in both cases compared to the samarocene one in a range of 1.3 to 4.0 kcal mol−1. The largest difference was found for the methyl substituents and the smallest for the tbutyl substituents meaning that steric effects play the most important role in the CC coupling step. In addition, starting from the same alkynide, differences in calculated activation barriers between the La and Ce cases are not significant (Fig. 5). This indicates that the metal plays an almost negligible role in the reactivity, as expected and previously suggested experimentally17,20,21 and theoretically.4,33,38,39 From the thermodynamic point of view, it should be noted that the formation of the corresponding uncoupled bimolecular intermediates from the approach of two monomeric fragments corresponds to a relatively high exothermic process, being in contrast with what was observed computationally for the samarocene analogues, apart from the methyl case which is only slightly exothermic. This is in line with the
Fig. 5
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experimental observations that demonstrate the existence in solution of {Cp*2Ln (CCR)}2 dimeric species, where Ln = La and R = Me, tBu, Ph, and Ln = Ce and R = Me, tBu, while the opposite is true for the uncoupled {Cp*2Sm(CCPh)}2 type of precursors.17,20,21 Moreover, the thermodynamics of this step refers to an exothermic reaction, nevertheless not as exothermic as in the case of samarocene analogues. Table 2 summarises the most important bond distances as well as critical angles of the C–C coupling step of lanthanum and cerium complexes. The first observation is the increase of the Ln1–C1 and Ln2–C′1 sigma bonds compared to the samarium one by more than 0.1 Å, as expected based on the differences in ionic radii between trivalent lanthanum (1.160 Å), trivalent cerium (1.143 Å), and trivalent samarium (1.079 Å).40 However, this increase is observed when the comparison is made between the phenyl and methyl substituents of the alkynides. In the case of the tbutyl bulky groups, the difference is not significant since the steric effects induced by these two substituents have governed the geometry around metals. It is also worth noting that independent from the metal centre, Ce or La, the most critical geometrical features, is almost identical for the tbutyl and phenyl cases and differentiate from the less bulky methyl. Another interesting feature is that the dimerisation of the alkynide monomers does not lead to bis-µ dimers but to bimetallic intermediates in which bridging acetylides are σ-bonded to one Sm and σ-bonded to the other. The latter conclusion is extracted by careful inspection of the corresponding geometries. In particular, in all the reactants, the alkynide terminal carbon atom is separated by an almost equal distance of the two lanthanide centres (Table 2). Likewise, for the cases of Ce and La having tBu and Ph as alkynide substituents, the triple bond is almost perpendicular to the intermetallic axis. The latter is clear by the ∠Ln–C1–C2 being about 136° and the angle of C′1–C1–C2 which is about 170°, as well as the long distances of La1–C′2 and La2–C′2. Moreover, the NPA analysis for all the species that are involved in this process is in close resemblance with what is shown previously in the samarium case (see ESI Fig. S1†).
Enthalpic reaction profiles in kcal mol−1 for the C–C coupling step of {Cp*2Ln(CCR)}2 intermediates (where Ln = La, Ce, and R = Me, tBu, Ph).
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Table 2 Selected interatomic distances (Å) and angles (°) for the C–C coupling step of different substituent alkynides using lanthanum and cerium complexes
CuCR R = Me Reactant TS Product R = tBu Reactant TS Product R = Ph Reactant TS Product
La1–C1 Ce1–C1
La1–C′1 Ce1–C′1
La1–C′2 Ce1–C′2
La2–C1 Ce2–C1
La2–C′1 Ce2–C′1
La2–C2 Ce2–C2
C1–C′1 C1–C′1
La1–La2 Ce1–Ce2
La1–C1–C2 Ce1–C1–C2
C′1–C1–C2 C′1–C1–C2
2.735 2.705 2.641 2.626 3.079 3.091
2.811 2.813 3.074 3.057 2.837 2.823
3.355 3.318 2.690 2.672 2.591 2.571
2.854 2.878 3.000 2.982 2.834 2.820
2.721 2.699 2.617 2.600 3.077 3.087
3.271 3.175 2.767 2.753 2.587 2.567
3.325 3.292 2.025 2.025 1.341 1.339
4.456 4.465 5.306 5.271 5.764 5.763
153.7 158.6 152.1 152.3 140.1 140.4
−147.4 −141.2 −126.0 −125.9 −154.0 −153.7
2.770 2.755 2.657 2.644 3.084 3.089
2.783 2.766 3.061 3.047 2.821 2.805
3.561 3.554 2.740 2.724 2.654 2.633
2.808 2.790 3.057 3.043 2.819 2.803
2.761 2.745 2.649 2.635 3.093 3.100
3.594 3.589 2.759 2.747 2.647 2.626
3.397 3.380 1.985 1.986 1.330 1.328
4.403 4.374 5.371 5.341 5.763 5.754
135.8 135.1 155.5 155.7 136.1 136.3
−171.7 −172.5 −123.4 −123.3 −157.8 −158.5
2.761 2.742 2.635 2.616 3.120 3.132
2.785 2.772 3.074 3.061 2.850 2.838
3.591 3.572 2.768 2.759 2.608 2.585
2.815 2.804 3.077 3.062 2.849 2.834
2.747 2.730 2.642 2.623 3.118 3.127
3.553 3.526 2.762 2.748 2.608 2.586
3.333 3.313 1.974 1.991 1.327 1.325
4.443 4.420 5.376 5.335 5.823 5.822
137.2 138.3 155.3 155.7 139.4 139.7
−167.9 −167.1 −122.1 −121.8 −154.5 −155.2
Cyclopentadienyl substituent effects We examined the steric effects of the cyclopentadienyl ligands on the C–C coupling step, by replacing all their methyl substituents but one by hydrogen atoms leading to CpMe, or totally giving unsubstituted Cp. In both cases we limited our study to tbutylacteylide as a substrate, since there are experimental data reported on {CpMe2Sm(CCtBu)}2 by Evans’ group (Fig. 6).18 Experimentally, as soon as the tbutylacetylide complex is formed, it yields the bimetallic species with the acetylides being in the µ-fashion. Hence, varying the substituents of the cyclopentadienyl ligands allows stoppage of the reaction at the bimetallic complex and consequently prevents the C–C coupling. The computed thermodynamics of the reaction shows that it is highly exothermic (41.1 kcal mol−1) and gives a direct explanation to the experimental observation. It should be noted that the formation of the hypothetical product of C–C coupling corresponds to an endothermic process, which is 8.9 kcal mol−1 less stable than the bimetallic species, as is shown in Fig. 6. This, along with the high energy barrier of 36.8 kcal mol−1, excludes any possible formation of the 1,4-di-tert-butylbutatrienyl derivative. However the exact origin of this high activation
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Fig. 6 Enthalpic reaction profile, in kcal mol−1, for the C–C coupling step starting from CpMe2Sm(CCtBu) monomers. Figures in parentheses refer to the Cp ligands.
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Table 3 Selected interatomic distances (Å) and angles (°) for the C–C coupling step of tbutylalkynides using samarium complexes bearing different cyclopentadienyl ligands
CpR C5H5 Reactant TS Product C5H5Me Reactant TS Product C5Me5 Reactant TS Product
Sm1–C1
Sm1–C′1
Sm1–C′2
Sm2–C1
Sm2–C′1
Sm2–C2
C1–C′1
Sm1–Sm2
Sm1–C1–C2
C′1–C1–C2
2.574 2.483 2.753
2.576 2.878 2.669
3.471 2.592 2.520
2.578 2.859 2.672
2.573 2.477 2.833
3.470 2.637 2.497
3.421 1.981 1.351
3.851 4.983 5.293
136.0 155.3 137.4
−175.5 −125.3 −150.5
2.578 2.494 2.812
2.600 2.878 2.674
3.290 2.617 2.514
2.601 2.872 2.675
2.579 2.489 2.825
3.283 2.629 2.511
3.435 1.981 1.345
3.875 5.000 5.327
150.1 156.6 139.1
−160.3 −124.2 −150.7
2.713 2.597 3.407
2.695 2.992 2.836
3.635 2.698 2.522
2.720 3.000 2.750
2.695 2.584 3.098
3.554 2.685 2.539
3.318 1.991 1.324
4.275 5.235 5.893
133.2 158.1 147.1
−174.9 −121.7 −159.1
barrier is unclear. In order to gain insights into the nature of this discrepancy we have considered the Cp ligand as a borderline case. From the geometrical point of view, replacement of all the methyl groups of Cp* by H atoms leads to an expected significant decrease of the intermetallic distance, both for the reactants and the products, as is shown in Table 3. Additionally, in [Cp2Sm(CCtBu)]2, the bulky alkynides bridge almost symmetrically the two lanthanide centres, while a slight distortion from symmetry was identified in the case of the CpMe. In the latter case, the tbutylalkynide groups are bent due to the steric congestion induced by the methyl substituents of the cyclic ligands. This is highlighted by the ∠Ln–C1– C2, being 150.1°, as well as the angle of C′1–C1–C2 which is −160°. Nevertheless, there is no conclusive observation from the geometrical viewpoint which clarifies the difference in kinetics between the Cp or CpMe with the Cp* supporting ligands. For that purpose, we computed isodesmic reactions in which Cp and Cp*, or CpMe and Cp* are exchanged as anions. This formal exchange has been computed for the corresponding reactant, transition state and product of the C–C coupling reaction, as depicted in Scheme 2. For the reactant, the exchange of CpMe with Cp* is favouring the less bulky CpMe by 72.9 kcal mol−1. The same trend is computed for the transition state: the stabilisation energy is 56.1 kcal mol−1 being lower by 16.8 kcal mol−1 with respect to the reactants one. This justifies the observed difference in activation barriers initially computed. For the products, the isodesmic balance is 49.9 kcal mol−1 in favour of CpMe; this stabilisation is even lower and reflects the endothermicity of the C–C coupling step of tbutyl-alkynides with CpMe as ligands (+8.9 kcal mol−1). This trend is also evident for the pair of Cp and Cp* ligands;
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however, the transition state is stabilised by only 1.9 kcal mol−1 in comparison with the CpMe case. These trends suggest that a stronger Sm–acetylide bond is developed in Cp- and CpMe-based than in Cp*-based complexes especially for the reactants, and to a lesser extent for the corresponding transition states. This is in line with the shortening of the Sm–C1 bond distances along the Cp*, CpMe and Cp series (Table 3). This most likely results from a better orbital overlap between the alkynides and the samarium centres in less encumbered complexes. Consequently, the steric bulk induced by the Cp* ligand destabilizes the reactant in favour of the kinetics of the homocoupling reaction. The analysis of the NPA charges for the three ancillary ligands (Scheme 3) reveals that there is a charge transfer from the Cp-type ligand to the π* of the alkynide ligands, in a kind of “push–pull” effect. This confers a radical character to the transition state that favours the formation of the C–C bond, as already demonstrated in this study. Interestingly, it is found that the less substituted the Cp ring the less charge transfer to the π* is observed. This is in line with the greater donation ability of the methyl substituted Cp ligand.
Conclusions This computational study on the homocoupling of acetylides mediated by two lanthanide(III) centres clearly highlights the interplay of stereoelectronic effects of both metallocenes and substituents in order to process the coupling of two carbanions. The set of ligands and substrates considered herein leads to some optimal geometrical parameters for the reaction
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Scheme 2 Enthalpy energy differences for some isodesmic reactions of CpMe with Cp* anions, in kcal mol−1. Figures in parentheses correspond to the isodesmic reaction of Cp neat with Cp* anions.
Scheme 3
Selected natural charges for the reactants and transition states bearing Cp, or CpMe or Cp* as ancillary ligands.
to proceed. Among them, the ideal intermetallic distance threshold for C–C coupling to proceed is close to 4.2 Å. In addition, we have shown that each lanthanide plays two complementary roles. For each acetylide motif, one lanthanide σbinds the acetylide terminal carbon atom whereas the second lanthanide brings electrophilic assistance via π-coordination of the triple bond. At one lanthanide centre, for symmetry reasons, the weakening of the Ln–acetylide σ-bond is compensated by the π-bonding to the second acetylide triple bond in which density is accumulated. This synergistic effect is mandatory for the reaction to proceed with a realistic energy barrier. The mechanism described here is closer to the early picture given by Marks21 but shares common features with the polarization of the triple bond as proposed by Evans.17
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In terms of ancillary ligand effects, we have shown that steric constraints raise the energy of reactants, and hence facilitate the reaction. Hence, the right combination of the sterics, induced by the ancillary cyclopentadienyl ligands, and the substrate along with the metal is essential for an efficient route of forming the desired trienediyls.
Acknowledgements CINES and CalMip are acknowledged for generous grant of computing time. LM is also grateful to the Humboldt Foundation.
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F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. M. Dolg, H. Stoll, A. Savin and H. Preuss, Theor. Chim. Acta, 1989, 75, 173–194. M. Dolg, H. Stoll and H. Preuss, Theor. Chim. Acta, 1993, 85, 441–450. A. Bergner, M. Dolg, W. Kuchle, H. Stoll and H. Preuss, Mol. Phys., 1993, 80, 1431–1441. A. Hollwarth, M. Bohme, S. Dapprich, A. W. Ehlers, A. Gobbi, V. Jonas, K. F. Kohler, R. Stegmann, A. Veldkamp and G. Frenking, Chem. Phys. Lett., 1993, 208, 237– 240. (a) R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 1971, 54, 724–728; (b) W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 56, 2257–2261; (c) P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213–222. (a) C. Gonzalez and H. B. Schlegel, J. Chem. Phys., 1989, 90, 2154–2161; (b) C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1990, 94, 5523–5527. (a) A. E. Reed, L. A. Agrtiss and F. Weinhold, Chem. Rev., 1988, 88, 899–926; (b) F. Weinhold, In The Encyclopedia of Computational Chemistry, ed. P. v. R. Schleyer, John Wiley & Sons. M. L. Steigerwald and W. A. Goddard III, J. Am. Chem. Soc., 1984, 106, 308–311. L. Maron and O. Eisenstein, J. Am. Chem. Soc., 2001, 123, 1036–1039. H. J. Heeres, J. Renkema, M. Booij, A. Meetsma and J. H. Teuben, Organometallics, 1988, 7, 2495–2502. T. Cuenca, R. Gómez, P. Gómez-Sal, G. M. Rodriguez and P. Royo, Organometallics, 1992, 11, 1229–1234. I. Vidal, S. Melchor and J. A. Dobado, J. Phys. Chem. A, 2008, 112, 3414–3423. A. Yahia, M. U. Kramer, J. Okuda and L. Maron, J. Organomet. Chem., 2010, 695, 2789–2793. L. Maron, L. Perrin and O. Eisenstein, Dalton Trans., 2003, 4313–4318. L. Perrin, L. Maron and O. Eisenstein, Inorg. Chem., 2002, 41, 4355–4362. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Cryst., 1976, A32, 751–767.
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Computational insights into carbon–carbon homocoupling reactions mediated by organolanthanide(III) complexes† Christos E. Kefalidis,* Lionel Perrin‡ and Laurent Maron* Homocoupling of terminal alkynes into trienediyl complexes by alkyl samarocenes is known experimentally. By means of computational techniques, we investigated the mechanism of this reaction in detail. The overall reaction sequence is: σ-bond metathesis, dimerisation of metallocenes, and homocoupling of two acetylides into trienediyl. We show that the rate-determining step corresponds to the homocoupling of two anionic acetylides. This coupling takes place at a bis-samarocene dimer complex in
Received 17th October 2013, Accepted 17th December 2013
which the bridging mode of the two acetylide moieties is critical for the reaction to proceed. The limited energy barrier for the homocoupling of the carbanions originates from a synergistic effect of the two
DOI: 10.1039/c3dt52937a
samarium centres within the dimer. Variation of the steric demand of both substrates and lanthanocenes
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allowed rationalising all the experimental data available for these systems.
Introduction The reactivity of lanthanide(III) complexes proceeds frequently at a fixed oxidation state – and among others – via concerted σbond metathesis, insertion, or α-/β-elimination.1–3 It has been shown both experimentally and computationally that C–C bond coupling cannot be achieved by σ-bond metathesis at a single lanthanide(III) centre since it requires the presence of a carbon atom at the β-position of the 4-center transition state.4 This limitation has been circumvented by the use of Ln(II) reagents, and mainly illustrated by Sm(II) complexes. The versatility of the coupling reaction induced by a Sm(II) Single Electron Transfer (SET) has been strongly illustrated by the chemistry of SmI2.5–9 This chemistry has been expanded by the use of Cp*2Sm(THF)n (Cp* = η5-C5Me5, n = 0, 2).10–14 However, C–C homocoupling reactions were achieved using lanthanocene(III) complexes with terminal alkynes as substrates. Of utmost importance, this dimerisation of terminal alkynes appeared in 1990 from the group of Evans.15 In that study the phenylacetylene was used as the substrate, reacting with different mono- (in II or III oxidation state) or bimetallic (in III oxidation state) samarocene complexes to afford a new class of trienediyl complexes of samarium of the type
Université de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3dt52937a ‡ Present address: Université Lyon 1, CNRS, UMR 5246, ICBMS, 43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France.
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{(Cp*2SmIII)2(µ-η2:η2-Ph–CvCvCvC–Ph)}.15 It should be noted that the latter complex was also previously prepared from the oxidation of two Cp*2Sm(THF)2 molecules with the diyne Ph– CuC–CuC–Ph.16 This was the first example in the organolanthanide(III) chemistry of forming a carbon–carbon bond by coupling two anionic ligands. X-ray characterisation of this bimetallic complex, along with the absence of characteristic bands of triple bonds in the infra-red region,16 allowed identification of the same dimeric product obtained by different synthetic routes.15 This reactivity was extended to include other terminal alkynes HCuCR (where R = CH2CH2Ph, CH2CH2CHMe2, and CHMe2), producing the same type of dinuclear complex.17 However, starting from hindered acetylides like [Cp*2Sm(CuCtBu)(THF)], the homocoupling reaction could not be observed. In contrast the uncoupled dimers [Cp*2Sm(CuCtBu)]2 were characterised as products.17 Similarly, the use of different ancillary ligands than Cp*, for example CpMe (η5-C5H4Me) or CptBu (η5-C5H4tBu), also led to the formation of the corresponding uncoupled binuclear complexes.18,19 In 1993, Teuben’s group reported a related study using early lanthanide alkyls of the type Cp*2LnCH(SiMe3)2. These precursors, upon the reaction with terminal alkynes HCuCR, gave bis-µ-acetylides [Cp*2Ln(CuCR)]2 (where Ln = Ce, R = Me or tBu; Ln = La, R = Me). These complexes were shown to be unstable in solution. They easily afford the corresponding trienediyl bimetallic species of the type {[Cp*2Ln]2(µ-η2:η2-RC4R)} upon a homo C–C coupling step. These reactions were characterised by NMR techniques and their molecular structures were determined by X-ray diffraction.20 Independently, Marks et al.21 reported the same year a related study
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Dresden–Koln type, using large core potentials (RECP),25,26 augmented by an f polarisation function (exp = 1.000). The Si atoms were represented by the same type of RECP27 and augmented by a d polarisation function (exp = 0.284).28 For the remaining atoms the all-electron double zeta basis set 6-31G(d,p) was used.29 Enthalpy energies were obtained at T = 298.15 K based on the harmonic approximation. Intrinsic Reaction Paths (IRPs) were traced from the various transition structures to verify the reactant to product linkage.30 Natural population analysis (NPA) was performed using Weinhold’s methodology.31
Results and discussion Scheme 1 Proposed mechanisms for the formation of trienediyls from the acetylide monomers.
using the same alkyl precursor of lanthanum, but with different terminal alkynes HCuCR (R = tBu and Ph). The outcome of the reaction was again the formation of the same type of trienediyl bimetallic complex. These authors proposed based on kinetic studies that the dimer [Cp*2La(µ2-CuCR)]2 is the direct kinetic precursor of the trienediyl complex. Consequently, they speculate about two different mechanisms as possible routes for this process: a concerted one and an insertive one as shown in Scheme 1a.21 Alternatively, Evans and coworkers proposed that a heterolytic cleavage of Sm-CCPh is likely to occur to form an anionic phenylacetylide fragment and a cationic samarocene one. By forming the dimer, the coupling reaction will occur then followed by a rearrangement of the samarocene fragments in order to give the final trienediyl product (Scheme 1b).17 Having in mind the previous mechanistic scenarios, there is a need to complete and give a clearer picture of this type of reactivity, which remains elusive up to now. The underlying mechanistic issue is the interplay between two lanthanocenes III complexes that mediate the homocoupling of two carbanions. In this report, we address this mechanistic aspect for this important class of reactions in lanthanide chemistry. We will also highlight and explain the importance of stereo-electronic effects in the metallocenes and acetylides that govern the reaction in order to rationalise the experimental observations. This will be done by considering, explicitly, experimental and model systems in our mechanistic density functional theory investigations.
As mentioned in the Introduction, among the different synthetic routes that have been followed to obtain diphenylbutatrienediyl derivatives, the first one was via the classical lanthanide reaction of Sm(III) alkyl complexes with phenylacetylene (following equation).11,15
We will consider this reaction as a baseline for our mechanism exploration. The above transformation of two monomers of phenylacetylene to give the corresponding trienediyl complex has been proposed to be a two-step reaction. The first corresponds to a σ-bond metathesis (σ-BM) between the terminal acetylenic C–H bond and the Sm–C bond of (Cp*)2Ln–CH(SiMe3)2. It affords the corresponding phenylalkynide with the simultaneous formation of bis(trimethylsilyl)methane CH2(SiMe3)2, as is shown in Fig. 1. The second refers to the C–C homocoupling step.
Computational section The molecular electronic structure calculations required for the depiction of equilibrium and transition state molecular structures were performed at the density functional theory (DFT) level using the B3PW9122,23 hybrid functional as implemented in the Gaussian program code.24 The three lanthanides (La, Ce, and Sm) were described by the corresponding scalar relativistic pseudopotentials of the Stuttgart–
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Fig. 1 Enthalpic reaction profile for the σ-bond metathesis of the Sm alkyl complex (in kcal mol−1). Figures in parentheses refer to the Cp ligand, while numbers in blue correspond to natural charges.
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σ-Bond metathesis As shown by the alternation of the natural charges of the four atoms involved in a kite-shaped geometry, the transition state associated with the metallation of the terminal alkyne is indeed a classical σ-BM one (Fig. 1). This is the most important criteria for characterizing such a transition state as σ-BM.4,32,33 In particular, the energy barrier that is needed to pass from the separated reactants to the alkynide intermediate is of only 7.1 kcal mol−1. From a thermodynamic point of view, the reaction is exothermic, with the resulting phenylalkynide intermediate that is more stable with respect to the entrance channel by 29.1 kcal mol−1. Based on these energetics, this reaction proceeds almost spontaneously. It is noteworthy that by moving from the Cp* ligand to the less bulky Cp ligand, the difference in the activation barrier is about 1 kcal mol−1 and the exothermicity is in the same range. The latter shows that the substituents on cyclopentadienyl ligands on samarocene do not play any role on both the thermodynamics and kinetics of the reaction. The combination of a low activation barrier and high exothermicity is also found to be true for the La or Ce analogue complexes. These analogues are characterised by a similar chemical environment (both ligands and substrates), and their corresponding enthalpy profiles are similar, as shown in Fig. 2 (ΔH# = 5.0 and 5.3 kcal mol−1, and ΔrH = −25.7 and 27.1 kcal mol−1, respectively for La and Ce). It should be noted that for all combinations of alkyne with cerium or lanthanum precursors experimental results are available,20,21 except in the case of cerium phenylacetylene reactivity which was considered in the present study for the sake of comparison. In order to confirm the adequacy between experimental results and computed energetics, we investigated well-established reactions from the literature with a variety of substrates using the [Ln]CH(TMS)2 precursor alkyl complexes of La and Ce.20,21 In particular, we replaced the phenyl substituent with a methyl one and kept intact the metallocene fragment. Though this modification strongly modifies both steric and electronic properties of the alkynides, we found that activation
Fig. 2
barriers do not dramatically change, as well as the thermodynamics of the reaction. The same thermodynamic and kinetic trends are observed for the bulky tBu substituent. In general, the formation of the phenylalkynide complex in all cases is more exothermic than the tbutyl- and methyl-acetylide analogues. The latter difference in thermodynamics is due to the ability of the phenyl group to accept more density from the more electron rich Sm than the La and the Ce one, resulting in the stabilization of the corresponding monomer. In all cases, the activation barriers are close to 5 kcal mol−1, with the tBu being systematically higher by almost 2 kcal mol−1 compared to the other substituents (Fig. 2). It is worth noting that the computed geometry of the (Cp*)2CeCH(SiMe3)2 is in very good agreement with the molecular structure which is determined by single-crystal X-ray analysis.34 Alkynides homocoupling The second and definite step of the proposed mechanism corresponds to the homo C–C coupling of the two anionic moieties to afford the final C–C coupled product. This commences by the formation of the dimer as a result of the weak intermolecular interaction between the π-system of the alkynylide ligand of one monomer and the Sm centre of the second monomer. This doubly bridged interaction only requires 2.4 kcal mol−1 to be established. This low endothermicity is likely attributable to the steric demand that is apparent in the dimeric structure between the phenylacetylide groups and the methyl groups of the cyclopentadienyl ligands. This is further clarified by the change in some important distances and angles as the dimer forms. In particular, the Sm–Cacetylide bond distance is increased substantially by almost 0.15 Å, being 2.583 and 2.588 Å in the dimer, and 2.436 Å in the monomer. To a lesser extent, the bond distance of the CuC of the alkynides is increased by only 0.007 Å, being almost of the same value for the C–Cphenyl bond as well. Moreover, the ∠SmCC moves from linearity by almost 20°, while the ∠CCCphenyl decreases by almost 7°. It should be noted at this point that the geometries obtained computationally for the trienediyl bimetallic complexes are in excellent agreement with
Enthalpic reaction profiles for the σ-bond metathesis of a series of La and Ce alkyl complexes (in kcal mol−1).
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the X-ray one.15 In conclusion, the energy needed for all the distortions within the phenylacetylide fragment is overcompensated by the energy gained from the electrostatic attractions between the two monomers. As previously mentioned, the final trienediyl dinuclear complex results from the C–C homo coupling of the two terminal anionic charged phenylacetylide moieties. Interestingly, similar transition states were postulated to proceed in the respective bis-µ-alkynyl titanocene complexes35 and were confirmed computationally by Dobado et al.36 In the present study, the energy barrier for this step is 10.7 kcal mol−1 relatively to the precursor dimer. This reaction formally couples two terminal alkynyls and is the rate-determining step of the entire mechanism. The surprisingly low energy barrier for the homocoupling of two negatively charged moieties, as phenylacetylide is, needs further explanation. This can be done by inspecting the geometrical changes on passing from the precursor dimer to the transition state, in conjunction with the changes in electron density of the atoms involved in the coupling. For the C–C coupling to proceed, the electronic density of the negatively charged α-carbons of the alkynides has to be reduced. This is achieved by the electrophilic assistance from the β-carbon of each triple bond that accumulates negative charge due to its interaction with the second samarium centre. The natural population analysis shows the aforementioned statement. The charge distribution is depicted in Fig. 3. More precisely, the α-carbons of the alkynyls on the dimer acquire a negative charge of −0.45 |e| each, whereas each of the β-carbons has a negative charge of −0.15 |e|. At the transition state the corresponding charges are almost equal, being −0.32 |e| for all but one which is −0.35 |e| (Fig. 3). The NPA analysis is in line with the changes in some important geometrical features of the active site of the reactivity. In particular, there is a substantial decrease of the distance between the β-carbons of the alkynyls with respect to the vicinal Sm centers, being 2.724 and 2.561 Å at the transition state. The corresponding distances are respectively 3.184 and 3.268 Å in the dimer (denoted as Sm1–C*α and Sm2–Cα respectively in Table 1). The noticeable
Fig. 3 Enthalpic reaction profile, in kcal mol−1, for the C–C coupling step starting from the Cp*2Sm(CCPh) monomer.
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shortening of the C–C distance is directly related to the newly formed bond. In contrast, there is no significant change in the Sm–Calkynide distance. All the above theoretical observations account for the low barrier for this process compared to that reported in the literature.37 Finally, C–C coupling is exothermic by 24.5 kcal mol−1 and the overall exothermicity is 51.1 kcal mol−1 with respect to starting materials.
Acetylide substituent effects It has been experimentally highlighted that the coupling of alkynides into trienediyl depends on the steric properties of both the acetylide and metallocene substituents. We will focus now on the ligand effects (R groups) on the C–C coupling step as it is indeed the rate-determining step from samarocene alkyl complexes and terminal alkynes to trienediyl. In order to examine the effect introduced by the R substituents, we have chosen methylacetylide as a model reactive ligand, as well as the experimentally used tbutylacetylide. The choice of these two substrates is also made, apart from the fact that they introduce different steric as well as electronic effects compared to the phenyl substituent, based on the availability in the literature of analogous systems, for which experimental results are accessible for direct comparisons.17,20,21 From a thermodynamic point of view, whereas the dimerisation of Cp*2Sm(CuCMe) is almost athermic, this process requires 7.7 kcal mol−1 to proceed in the case of the more hindered Cp*2Sm(CuCtBu) (Fig. 4). The same trend is observed concerning the activation barrier, with an increase of 6.2 kcal mol−1 with respect to the precursor dimer. When R = Me, the activation barrier is found to be almost 3 kcal mol−1 higher than for phenylacetylide, but two times higher when R is the tbutylacetylide relative to the same phenylacetylide. However, from the computed kinetic and thermodynamic parameters, the coupling could be expected to occur for all substrates considered, though the coupling of tBuCuCH should be slower. This is not experimentally the case for the tbutyl substituent.15 The explanation for this controversy is given by the nature of the “real” precursor that is Cp*2Sm(CuCtBu)(THF), in which a THF molecule binds the Sm center. Such a THF molecule, which usually inhibits the reactivity in lanthanide chemistry, requires here +13.6 kcal mol−1 to decoordinate from the Sm center. Having this in mind and adding another 7.7 kcal mol−1 that are needed for the formation of the dimer, as well as the 20 kcal mol−1 to overcome the activation barrier, leads to an overall barrier of 41.3 kcal mol−1. This barrier makes the process kinetically not feasible. In addition, with respect to the THF adduct, the whole process is endothermic by 7.2 kcal mol−1, which is large enough to prevent this reaction from occurring. Experimentally, when the same reaction conditions were applied for the case of phenylacetylene, the reaction occurred only after heating the system at 120° for 3 days affording the trienediyl complex. The corresponding sum of energy that is required in this case is much lower than for tbutyl, being 26.4 kcal mol−1, which explains the experimentally observed low reactivity.15
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Table 1
CuCR R = Me Reactant TS Product R = tBu Reactant TS Product R = Ph Reactant TS Product
Dalton Transactions Selected interatomic distances (Å) and angles (°) for the C–C coupling step of different substituent alkynides using samarium complexes
Sm1–C1
Sm1–C′1
Sm1–C′2
Sm2–C1
Sm2–C′1
Sm2–C2
C1–C′1
Sm1–Sm2
Sm1–C1–C2
C′1–C1–C2
2.590 2.580 3.168
2.961 3.001 2.784
3.090 2.603 2.497
3.019 2.927 2.776
2.586 2.538 3.155
3.012 2.714 2.494
3.152 2.023 1.329
4.619 5.155 5.803
166.0 153.1 141.2
−125.9 −125.6 −157.6
2.713 2.597 3.407
2.695 2.992 2.836
3.635 2.698 2.522
2.720 3.000 2.750
2.695 2.584 3.098
3.554 2.685 2.539
3.318 1.991 1.324
4.275 5.235 5.893
133.2 158.1 147.1
−174.9 −121.7 −159.1
2.583 2.554 3.223
3.002 3.004 2.805
3.184 2.724 2.507
3.041 3.004 2.806
2.588 2.561 3.230
3.268 2.689 2.503
3.202 2.017 1.315
4.623 5.200 5.900
159.9 157.3 140.6
−138.0 −120.9 −159.3
Fig. 4 Enthalpic reaction profile, in kcal mol−1, for the C–C coupling step starting from Cp*2Sm(CCR) monomers (where R = Me or tBu).
In terms of the binding mode, a close inspection of some critical geometrical features reveals important insights into the observed reactivity (Table 1). In particular, the most relevant distances in the dimers are very similar for the methyl and phenyl substituents, with a few exceptions, like the Sm2⋯C2 that is longer by almost 0.25 Å for phenylacetylide with respect to the methylacetylide case. This
4524 | Dalton Trans., 2014, 43, 4520–4529
is mainly due to the larger repulsions that are developed between the phenyl group and the Cp* ligands, with respect to the corresponding one in the case of the methyl substituent. It should be noted that in both cases the dimer can be characterised as two discrete monomeric fragments; this is not the case for tbutylacetylides which correspond to a bimetallic system. In the latter case, the terminal carbon atoms of the bulky alkynides adopt an almost ideal 4-center symmetric bridging coordination mode (d(Sm1–C1) = 2.713 Å, and d(Sm2–C1) = 2.720 Å). Also, the CuC bonds of both tbutylacetylides are almost perpendicular to the intermetallic axis (∠C′1–C1–C2 = −174.9°) as a consequence of the bulkiness of the tbutyl substituents that induces steric effects with the methyl substituents of the cyclopentadienyl ligands. This bonding situation also results in a shorter intermetallic distance (4.275 Å) compared to the others (∼4.620 Å) whose importance will be highlighted later. In contrast with the substantial changes found in this set of three dinuclear complexes, no major variation could be pointed out in the geometries of the corresponding transition states. However, in the case of the tbutyl substituent, passing from the bimetallic complex to the transition state leads to significant variations, in contrast with the other two cases. More particularly, the Sm1–C′2 and Sm2–C2 distances are dramatically shortened by almost 1.0 Å. The corresponding difference in the other two cases under study is 0.3–0.6 Å. As
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Dalton Transactions
we have shown previously, it is mandatory that the β-carbons of the alkynides have to develop a strong interaction with neighbouring metals in the transition state; however, in the methyl and the phenyl case the β-carbons are much more close to the metals, so they do not need to vary their geometry dramatically, which is not the case in the tbutyl one, with the latter being translated in the extra energy required. In the meantime, the approach of the two bulky groups in the transition state with respect to the methyl groups of the Cp* ligands also affects the energy of the activation barrier. These two observations can serve as an explanation for the higher calculated activation barrier of this process for the tbutyl case compared to the other two cases. The same trend is followed when the same series of substrates are used CuC–R (R = Me, tBu, Ph), but with different lanthanides as cerium and lanthanum (Fig. 5). Apart from the combination of Ln = Ce and R = Ph, all other complexes were experimentally studied and the final trienediyl complexes were characterized by X-rays. In general, the computational protocol used herein is giving geometries very close to the experimental one, allowing further DFT analysis. Firstly, in terms of kinetics, there is an increase in the corresponding activation barriers in both cases compared to the samarocene one in a range of 1.3 to 4.0 kcal mol−1. The largest difference was found for the methyl substituents and the smallest for the tbutyl substituents meaning that steric effects play the most important role in the CC coupling step. In addition, starting from the same alkynide, differences in calculated activation barriers between the La and Ce cases are not significant (Fig. 5). This indicates that the metal plays an almost negligible role in the reactivity, as expected and previously suggested experimentally17,20,21 and theoretically.4,33,38,39 From the thermodynamic point of view, it should be noted that the formation of the corresponding uncoupled bimolecular intermediates from the approach of two monomeric fragments corresponds to a relatively high exothermic process, being in contrast with what was observed computationally for the samarocene analogues, apart from the methyl case which is only slightly exothermic. This is in line with the
Fig. 5
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experimental observations that demonstrate the existence in solution of {Cp*2Ln (CCR)}2 dimeric species, where Ln = La and R = Me, tBu, Ph, and Ln = Ce and R = Me, tBu, while the opposite is true for the uncoupled {Cp*2Sm(CCPh)}2 type of precursors.17,20,21 Moreover, the thermodynamics of this step refers to an exothermic reaction, nevertheless not as exothermic as in the case of samarocene analogues. Table 2 summarises the most important bond distances as well as critical angles of the C–C coupling step of lanthanum and cerium complexes. The first observation is the increase of the Ln1–C1 and Ln2–C′1 sigma bonds compared to the samarium one by more than 0.1 Å, as expected based on the differences in ionic radii between trivalent lanthanum (1.160 Å), trivalent cerium (1.143 Å), and trivalent samarium (1.079 Å).40 However, this increase is observed when the comparison is made between the phenyl and methyl substituents of the alkynides. In the case of the tbutyl bulky groups, the difference is not significant since the steric effects induced by these two substituents have governed the geometry around metals. It is also worth noting that independent from the metal centre, Ce or La, the most critical geometrical features, is almost identical for the tbutyl and phenyl cases and differentiate from the less bulky methyl. Another interesting feature is that the dimerisation of the alkynide monomers does not lead to bis-µ dimers but to bimetallic intermediates in which bridging acetylides are σ-bonded to one Sm and σ-bonded to the other. The latter conclusion is extracted by careful inspection of the corresponding geometries. In particular, in all the reactants, the alkynide terminal carbon atom is separated by an almost equal distance of the two lanthanide centres (Table 2). Likewise, for the cases of Ce and La having tBu and Ph as alkynide substituents, the triple bond is almost perpendicular to the intermetallic axis. The latter is clear by the ∠Ln–C1–C2 being about 136° and the angle of C′1–C1–C2 which is about 170°, as well as the long distances of La1–C′2 and La2–C′2. Moreover, the NPA analysis for all the species that are involved in this process is in close resemblance with what is shown previously in the samarium case (see ESI Fig. S1†).
Enthalpic reaction profiles in kcal mol−1 for the C–C coupling step of {Cp*2Ln(CCR)}2 intermediates (where Ln = La, Ce, and R = Me, tBu, Ph).
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Table 2 Selected interatomic distances (Å) and angles (°) for the C–C coupling step of different substituent alkynides using lanthanum and cerium complexes
CuCR R = Me Reactant TS Product R = tBu Reactant TS Product R = Ph Reactant TS Product
La1–C1 Ce1–C1
La1–C′1 Ce1–C′1
La1–C′2 Ce1–C′2
La2–C1 Ce2–C1
La2–C′1 Ce2–C′1
La2–C2 Ce2–C2
C1–C′1 C1–C′1
La1–La2 Ce1–Ce2
La1–C1–C2 Ce1–C1–C2
C′1–C1–C2 C′1–C1–C2
2.735 2.705 2.641 2.626 3.079 3.091
2.811 2.813 3.074 3.057 2.837 2.823
3.355 3.318 2.690 2.672 2.591 2.571
2.854 2.878 3.000 2.982 2.834 2.820
2.721 2.699 2.617 2.600 3.077 3.087
3.271 3.175 2.767 2.753 2.587 2.567
3.325 3.292 2.025 2.025 1.341 1.339
4.456 4.465 5.306 5.271 5.764 5.763
153.7 158.6 152.1 152.3 140.1 140.4
−147.4 −141.2 −126.0 −125.9 −154.0 −153.7
2.770 2.755 2.657 2.644 3.084 3.089
2.783 2.766 3.061 3.047 2.821 2.805
3.561 3.554 2.740 2.724 2.654 2.633
2.808 2.790 3.057 3.043 2.819 2.803
2.761 2.745 2.649 2.635 3.093 3.100
3.594 3.589 2.759 2.747 2.647 2.626
3.397 3.380 1.985 1.986 1.330 1.328
4.403 4.374 5.371 5.341 5.763 5.754
135.8 135.1 155.5 155.7 136.1 136.3
−171.7 −172.5 −123.4 −123.3 −157.8 −158.5
2.761 2.742 2.635 2.616 3.120 3.132
2.785 2.772 3.074 3.061 2.850 2.838
3.591 3.572 2.768 2.759 2.608 2.585
2.815 2.804 3.077 3.062 2.849 2.834
2.747 2.730 2.642 2.623 3.118 3.127
3.553 3.526 2.762 2.748 2.608 2.586
3.333 3.313 1.974 1.991 1.327 1.325
4.443 4.420 5.376 5.335 5.823 5.822
137.2 138.3 155.3 155.7 139.4 139.7
−167.9 −167.1 −122.1 −121.8 −154.5 −155.2
Cyclopentadienyl substituent effects We examined the steric effects of the cyclopentadienyl ligands on the C–C coupling step, by replacing all their methyl substituents but one by hydrogen atoms leading to CpMe, or totally giving unsubstituted Cp. In both cases we limited our study to tbutylacteylide as a substrate, since there are experimental data reported on {CpMe2Sm(CCtBu)}2 by Evans’ group (Fig. 6).18 Experimentally, as soon as the tbutylacetylide complex is formed, it yields the bimetallic species with the acetylides being in the µ-fashion. Hence, varying the substituents of the cyclopentadienyl ligands allows stoppage of the reaction at the bimetallic complex and consequently prevents the C–C coupling. The computed thermodynamics of the reaction shows that it is highly exothermic (41.1 kcal mol−1) and gives a direct explanation to the experimental observation. It should be noted that the formation of the hypothetical product of C–C coupling corresponds to an endothermic process, which is 8.9 kcal mol−1 less stable than the bimetallic species, as is shown in Fig. 6. This, along with the high energy barrier of 36.8 kcal mol−1, excludes any possible formation of the 1,4-di-tert-butylbutatrienyl derivative. However the exact origin of this high activation
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Fig. 6 Enthalpic reaction profile, in kcal mol−1, for the C–C coupling step starting from CpMe2Sm(CCtBu) monomers. Figures in parentheses refer to the Cp ligands.
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Table 3 Selected interatomic distances (Å) and angles (°) for the C–C coupling step of tbutylalkynides using samarium complexes bearing different cyclopentadienyl ligands
CpR C5H5 Reactant TS Product C5H5Me Reactant TS Product C5Me5 Reactant TS Product
Sm1–C1
Sm1–C′1
Sm1–C′2
Sm2–C1
Sm2–C′1
Sm2–C2
C1–C′1
Sm1–Sm2
Sm1–C1–C2
C′1–C1–C2
2.574 2.483 2.753
2.576 2.878 2.669
3.471 2.592 2.520
2.578 2.859 2.672
2.573 2.477 2.833
3.470 2.637 2.497
3.421 1.981 1.351
3.851 4.983 5.293
136.0 155.3 137.4
−175.5 −125.3 −150.5
2.578 2.494 2.812
2.600 2.878 2.674
3.290 2.617 2.514
2.601 2.872 2.675
2.579 2.489 2.825
3.283 2.629 2.511
3.435 1.981 1.345
3.875 5.000 5.327
150.1 156.6 139.1
−160.3 −124.2 −150.7
2.713 2.597 3.407
2.695 2.992 2.836
3.635 2.698 2.522
2.720 3.000 2.750
2.695 2.584 3.098
3.554 2.685 2.539
3.318 1.991 1.324
4.275 5.235 5.893
133.2 158.1 147.1
−174.9 −121.7 −159.1
barrier is unclear. In order to gain insights into the nature of this discrepancy we have considered the Cp ligand as a borderline case. From the geometrical point of view, replacement of all the methyl groups of Cp* by H atoms leads to an expected significant decrease of the intermetallic distance, both for the reactants and the products, as is shown in Table 3. Additionally, in [Cp2Sm(CCtBu)]2, the bulky alkynides bridge almost symmetrically the two lanthanide centres, while a slight distortion from symmetry was identified in the case of the CpMe. In the latter case, the tbutylalkynide groups are bent due to the steric congestion induced by the methyl substituents of the cyclic ligands. This is highlighted by the ∠Ln–C1– C2, being 150.1°, as well as the angle of C′1–C1–C2 which is −160°. Nevertheless, there is no conclusive observation from the geometrical viewpoint which clarifies the difference in kinetics between the Cp or CpMe with the Cp* supporting ligands. For that purpose, we computed isodesmic reactions in which Cp and Cp*, or CpMe and Cp* are exchanged as anions. This formal exchange has been computed for the corresponding reactant, transition state and product of the C–C coupling reaction, as depicted in Scheme 2. For the reactant, the exchange of CpMe with Cp* is favouring the less bulky CpMe by 72.9 kcal mol−1. The same trend is computed for the transition state: the stabilisation energy is 56.1 kcal mol−1 being lower by 16.8 kcal mol−1 with respect to the reactants one. This justifies the observed difference in activation barriers initially computed. For the products, the isodesmic balance is 49.9 kcal mol−1 in favour of CpMe; this stabilisation is even lower and reflects the endothermicity of the C–C coupling step of tbutyl-alkynides with CpMe as ligands (+8.9 kcal mol−1). This trend is also evident for the pair of Cp and Cp* ligands;
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however, the transition state is stabilised by only 1.9 kcal mol−1 in comparison with the CpMe case. These trends suggest that a stronger Sm–acetylide bond is developed in Cp- and CpMe-based than in Cp*-based complexes especially for the reactants, and to a lesser extent for the corresponding transition states. This is in line with the shortening of the Sm–C1 bond distances along the Cp*, CpMe and Cp series (Table 3). This most likely results from a better orbital overlap between the alkynides and the samarium centres in less encumbered complexes. Consequently, the steric bulk induced by the Cp* ligand destabilizes the reactant in favour of the kinetics of the homocoupling reaction. The analysis of the NPA charges for the three ancillary ligands (Scheme 3) reveals that there is a charge transfer from the Cp-type ligand to the π* of the alkynide ligands, in a kind of “push–pull” effect. This confers a radical character to the transition state that favours the formation of the C–C bond, as already demonstrated in this study. Interestingly, it is found that the less substituted the Cp ring the less charge transfer to the π* is observed. This is in line with the greater donation ability of the methyl substituted Cp ligand.
Conclusions This computational study on the homocoupling of acetylides mediated by two lanthanide(III) centres clearly highlights the interplay of stereoelectronic effects of both metallocenes and substituents in order to process the coupling of two carbanions. The set of ligands and substrates considered herein leads to some optimal geometrical parameters for the reaction
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Scheme 2 Enthalpy energy differences for some isodesmic reactions of CpMe with Cp* anions, in kcal mol−1. Figures in parentheses correspond to the isodesmic reaction of Cp neat with Cp* anions.
Scheme 3
Selected natural charges for the reactants and transition states bearing Cp, or CpMe or Cp* as ancillary ligands.
to proceed. Among them, the ideal intermetallic distance threshold for C–C coupling to proceed is close to 4.2 Å. In addition, we have shown that each lanthanide plays two complementary roles. For each acetylide motif, one lanthanide σbinds the acetylide terminal carbon atom whereas the second lanthanide brings electrophilic assistance via π-coordination of the triple bond. At one lanthanide centre, for symmetry reasons, the weakening of the Ln–acetylide σ-bond is compensated by the π-bonding to the second acetylide triple bond in which density is accumulated. This synergistic effect is mandatory for the reaction to proceed with a realistic energy barrier. The mechanism described here is closer to the early picture given by Marks21 but shares common features with the polarization of the triple bond as proposed by Evans.17
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In terms of ancillary ligand effects, we have shown that steric constraints raise the energy of reactants, and hence facilitate the reaction. Hence, the right combination of the sterics, induced by the ancillary cyclopentadienyl ligands, and the substrate along with the metal is essential for an efficient route of forming the desired trienediyls.
Acknowledgements CINES and CalMip are acknowledged for generous grant of computing time. LM is also grateful to the Humboldt Foundation.
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