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Complexity in seemingly simple sodium magnesiate systems† J. Francos,a B. J. Fleming,a P. García-Álvarez,b A. R. Kennedy,a K. Reilly,a G. M. Robertson,a S. D. Robertsona and C. T. O’Hara*a A systematic study both in the solid- and solution-state, was carried out for a series of sodium magnesiates containing the utility amide ligand 1,1,1,3,3,3-hexamethyldisilazide (HMDS). The first complex considered is the donor-free bisamido monoalkyl polymeric complex [Na(μ-HMDS)2Mg(nBu)]∞ 1. The reactivity of 1 with common tertiary bidentate donors including N,N,N’,N’-tetramethylethylenediamine (TMEDA) or its chiral relative (1R,2R)-tetramethylcyclohexyldiamine [(R,R)-TMCDA] is detailed. Surprisingly, the products of these reactions are not simple diamine adducts but are solvent separated sodium magnesiate systems [(TMEDA)2·Na]+[Mg(HMDS)3]− 2 and [{(R,R)-TMCDA}2·Na]+[Mg(HMDS)3]− 3. By concentrating on the likely equilibria which may give rise to formation of 2, a potential intermediate complexed ion pair
Received 28th March 2014, Accepted 24th June 2014 DOI: 10.1039/c4dt00921e www.rsc.org/dalton
[{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4 was isolated. Additionally, the novel “inverse magnesiates” [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5 and [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)-TMCDA}]∞ 6, were obtained by reacting solutions of composition “NaMg(HMDS)(nBu)2” (a likely by-product in the formation of 2 from 1), with TMEDA or (R,R)-TMCDA. The structure and nature of these bimetallic complexes have been determined using a combination of X-ray crystallographic studies and multinuclear NMR spectroscopy.
Introduction Organomagnesium reagents are amongst the first discovered classes of organometallics. Synonymous with magnesium, Grignard pioneered much of the early organomagnesium chemistry.1 He focused on the direct insertion of magnesium metal into an organic halide to synthesise complexes with the formulation RMgX (where R is an alkyl or aryl group; and X is a halogen). This discovery culminated in him being awarded the Nobel Prize in Chemistry in 1912. The utilisation of organomagnesium reagents has increased at a rapid rate,2 having found many applications throughout various fields in organic chemistry.3 In addition, the solid-state structure of many Grignard reagents have been determined over the past few decades to give an insight into the structural complexity and diversity of the systems. The solution structure of Grignard reagents is complicated as they are inherently kinetically unstable in solution and as a consequence of the Schlenk equi-
a WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK. E-mail: [email protected]; Tel: +44 (0)141 548 2667 b Departamento de Química Orgánica e Inorgánica-IUQOEM, Universidad de OviedoCSIC, E-33071 Oviedo, Spain † Electronic supplementary information (ESI) available: Full experimental data, X-ray data and NMR spectra. CCDC 993041–993045. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00921e
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librium (Scheme 1) they often exist as mixtures alongside their associated disproportionation products (i.e., R2Mg/MgX2). In addition, (akin to their solid state chemistry) they can exist as different oligomeric aggregates. To emphasize the complexity in such an apparently simple system, the ultimate structure of each reagent has to be treated on an individual basis as they “depend on the nature of R, the nature of X, the properties of the coordinating solvent, concentration and temperature.”3 Organomagnesiums are generally less reactive than their lithium counterparts. A way to enhance their reactivity is to formally combine them with an alkali metal organometallic or alkali metal amide complex to give rise to an alkali metal magnesiate complex. In a seminal paper published by Wittig in 1951,4 alongside an analogous “zincate” example, the synthesis of an “ate” complex of formulation LiMgPh3 was reported. This was produced by combining the two homometallic species, namely Ph2Mg and PhLi. It was shown to deliver unique reactivities when reacted with benzalacetophenone, giving predominantly the 1,4-addition product, contrasting with the 1,2-adduct formed on reaction with monometallic PhLi alone.4 More recently, magnesiate chemistry has developed at a frantic pace, both in terms of structural chemistry and
Scheme 1 The generalised Schlenk equilibrium, where R = alkyl or aryl and X = Br or Cl.
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reactivity.5 Magnesiate complexes generally display a unique synergic chemistry that is distinct from the reactivities of their parent complexes. For instance, they have been shown to effect deprotonative metallation regioselectively on a range of organic substrates (such as benzene,6,7 toluene,6,8 furan,9 metallocenes10,11 and alkynes12) at ambient temperatures. These mixed-metal reactions have been termed alkali-metalmediated magnesiations (AMMMg).13–15 Mongin and coworkers have comprehensively studied the use of magnesiates for the deprotonation of a range of molecules including, furans, pyridines, thiophenes and oxazoles.16–25 When lithium chloride is added to certain Grignard reagents, a twofold synthetic advantage can often be observed. Firstly, an enhancement of reactivity (cf., the Grignard reagent), and secondly, a greater functional group tolerance (cf., a conventional lithium reagent) in a multitude of deprotonation and metal–halide exchange reactions occurs. This class of lithium magnesiates, discovered by Knochel, have been termed “turbo Grignard” reagents.26,27 Magnesium reagents exhibiting the formula R2NMgCl·LiCl (turbo Hauser reagents) were found to perform especially well in the area of deprotonative metallation.28–30 For example, in 2006 Knochel utilised TMPMgCl·LiCl31,32 (TMP = 2,2,6,6-tetramethylpiperidide) to metallate (magnesiate) a wide range of heterocycles with great control in regioselectivity. In this work, we initially focus on the bis(amido)-monoalkyl sodium magnesiate NaMg(HMDS)2nBu. By adding donor molecules it is revealed that akin to Grignard reagents, their structural chemistry is considerably more complex than expected.
Results and discussion Synthesis The syntheses of sodium magnesiates 1–6 are summarised in Scheme 2. Several novel sodium magnesiates were prepared during this study. Due to the importance of other sodium
Scheme 2
Syntheses of 1–6.
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magnesiates [e.g., ‘NaMg(TMP)2nBu’]33 in deprotonative metalation reactions, the synthesis of donor-free NaMg(HMDS)2nBu was the initial focus. This was achieved by combining n-butylsodium,34 HMDS(H) and di-n-butylmagnesium in hexane in a 1 : 2 : 1 stoichiometric ratio. Slow cooling of the resultant solution yielded polymeric sodium magnesiate [NaMg(μ-HMDS)2nBu]∞ 1 in high crystalline yield (81%). Diamines, particularly N,N,N′,N′-tetramethylethylenediamine (TMEDA) have proven to be an essential additive in the field of alkali metal mediated magnesiation as they act as coordination blockers, thus preventing polymerization (and often insolubility) of the alkali metal magnesiate.35–43 As a consequence, it was decided to study the coordination chemistry of 1 with TMEDA and its chiral relative (1R,2R)-tetramethylcyclohexyldiamine [(R,R)-TMCDA], in an effort to break up the polymer and prepare a lower aggregated donor solvate of 1. However, the only crystalline product obtained from the former reaction was the tris(amido) solvent separated ion pair [(TMEDA)2·Na]+[Mg(HMDS)3]− 2. An identical reaction pathway occurred when TMEDA was substituted for (R,R)-TMCDA to produce 3 (Scheme 2). These reactions suggest that in the presence of bidentate donor, 1 is subject to a ligand reorganisation (vide infra). By employing two molar equivalents of TMEDA [with respect to sodium (or magnesium) reagent] a loosely-contacted ion pair structure, the bis(amido) mono-alkyl species [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4 was formed (Scheme 2). When nBuNa, nBu2Mg, HMDS(H) and TMEDA are combined in a 1 : 1 : 1 : 1 ratio, the surprising product was polymeric [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5. In keeping with terminology introduced by Hevia et al.44 for a related zinc-containing compound, 5 can be considered as the first example of an inverse magnesiate (vide infra). Complex 5 can be rationally prepared by altering the starting material ratio from 1 : 1 : 1 : 1 to 2 : 1 : 2 : 1. By replacing TMEDA with (R,R)-TMCDA a second inverse magnesiate [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)-TMCDA}]∞ 6 can be rationally prepared (Scheme 2). Structural studies Sodium magnesiate [Na(μ-HMDS)2Mg(nBu)]∞ 1 is polymeric in the solid state (the polymer is generated by the action of the n glide plane) and crystallizes in the P21/n space group (Fig. 1). The sodium atom adopts a highly distorted trigonal planar arrangement, and bonds to two N atoms of bridging HMDS ligands [bond distances, 2.486(2) and 2.451(2) Å]. Considering that the polymer consists of dinuclear units, the coordination sphere of the Na atom is completed by an intermolecular interaction to the anionic C butyl centre [Na1–C13 bond distance, 2.781(2) Å]. In addition, the Na atom is close to a single methyl group from both HMDS units (mean distance of 2.913 Å) which may contribute to stabilising the Na centre further. The Na1–C13–Mg1* bridge angle is close to linearity [164.9(1)°] allowing the propagation of a one-dimensional chain-like polymer. Akin to the sodium, the magnesium atom adopts a distorted trigonal planar arrangement, bonding to both bridging N atoms and the C atom of the butyl anion [Mg1–N1 2.077(2), Mg1–N2 2.063(2), Mg1–C13 2.155(2) Å].
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Fig. 2 A representation of [(TMEDA)2·Na]+[Mg(HMDS)3]− 2. Key bond distances (Å) and angles (°): Mg1–N1 2.017(2), Mg1–N2 2.024(2), Mg1– N3 2.030(2), Na1–N4 2.562(2), Na1–N5 2.472(2), N1–Mg1–N2 119.77(7), N1–Mg1–N3 121.04(7), N2–Mg1–N3 119.19(7), N5–Na1–N4 74.57(6).
Fig. 1 Molecular structure of [Na(HMDS)2Mg(nBu)]∞ 1, showing (top) asymmetric unit; (bottom) polymer propagation. Key bond distances (Å) and angles (°): Mg1–N2 2.063(2), Mg1–N1 2.077(2), Mg1–C13 2.155(2), Na1–N1 2.486(2), Na1–N2 2.451(2), Na1–C13 2.781(2), Na1–C5 2.935(2), Na1–C10 2.891(2), N2–Mg1–N1 106.13(7), N2–Mg1–C13 129.48(9), N1– Mg1–C13 124.32(9), Mg1–N2–Na1 85.38(6), Na1–N1–Mg1 84.19(6), N2– Na1–N1 84.16(6), N2–Na1–C13 133.57(7), N1–Na1–C13 142.00(7).
Complex 1 can be compared with another alkali metal magnesiate polymer, the potassium magnesiate [KMg(TMP)2(nBu)]∞.33 ˉ space The solvent-separated ion pair 2 crystallises in the P1 group (Fig. 2). The almost perfectly trigonal planar [Mg(HMDS)3]− anion of 2 is well known in magnesium chemistry, and its structural parameters are essentially identical to those in previously published complexes.45–50 Likewise the [Na·(TMEDA)2]+ cation is common and a search of the Cambridge Crystallographic Database shows that the cation appears in ten other complexes and the key dimensions of 2 are generally identical to these within experimental error.51–60 The structure of the (R,R)-TMCDA-containing ion pair 3 has been published.46 Complex 4 crystallising in the Pna21 space group, is perhaps best described as a loosely contacted ion pair structure; however, akin to 2 the Na atom within the structure is bound to two TMEDA ligands (Fig. 3). The coordination sphere of Na is completed by a long Na–Cbutyl contact [2.986(5) and 3.001(5) Å for the two crystallographic independent units] which bridges to the Mg atom giving rise to a square pyramidal arrangement. Surprisingly (and unlike in 1) the alkyl group is the only bridge between the two metals rather than an amide group, which is most often considered to be a better
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bridging ligand. Di-TMEDA-solvated dinuclear 4 can be compared with the dinuclear building blocks of polymer 1; the chelation has a dramatic effect on altering the comparable Na–C and Mg–C distances and Na–C–Mg angles. For instance, the Na–Cbutyl bond distance [2.781(2) and 2.986(5) Å in 1 and 4 respectively] is predictably shorter in 1 due to the Na atom’s lower coordination number, and the Na–C–Mg angle in 1 [164.94(1) and 148.0(2)° for 1 and 4 respectively] is considerably wider than that in 4. Reflecting the similar Mg coordination geometries, the Mg–C bonds in 1 and 4 [2.155(2) and 2.170(4) Å respectively] are identical within experimental error. Complex 5 crystallises in the P21/n space group (Fig. 4 and 5). It is polymeric ( propagating along the crystallographic b axis through the action of the 21 screw axis) and can be described as an inverse magnesiate [akin to its zincate relative [{Li(HMDS)}2Zn(Me)2·(TMEDA)]∞].44 To explain this nomenclature, it is best to first consider normal “ate” complexes. In general, they are most commonly associated with a bimetallic
Fig. 3 Molecular structure of one of the two crystallographicallyunique units of [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4. Key bond distances (Å) and angles (°): Mg1–N1 2.036(3), Mg1–N2 2.024(3), Mg1–C1 2.170(4), Na1–C1 2.986(5), Na1–N3 2.505(3), Na1–N6 2.533(4), Na1–N5 2.538(4), Na1–N4 2.583(4) N2–Mg1–N1 122.63(14), N2–Mg1–C1 117.8(2), N1– Mg1–C1 119.6(2), Mg1–C1–Na1 148.0(2).
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Returning to 5, the Na and Mg are linked via a Na–CH2– Mg–CH2–Na arrangement [Na2–C5 2.734(2); Mg1–C5 2.151(2); Mg1–C1 2.153(2) and Na1–C1 2.706(4) Å] propagating the polymeric 1D chain (Fig. 5). Surprisingly, these Mg–C bond distances are considerably shorter than the mean Mg–Cbridging bond distance in the aforementioned NHC complex (2.270 Å)67 and are still shorter (and by inference, stronger) than the mean Mg–Cterminal distance in the bis(imino) adduct (2.175 Å).66 Complex 6 is the second example of an inverse magnesiate, and incorporates the chiral diamine (R,R)-TMCDA (Fig. 6). Its structure (Fig. 6 and 7) and geometrical parameters are essentially identical to those of 5. Fig. 4 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5, showing asymmetric unit. Key bond distances (Å) and angles (°): Mg1–N1 2.212(2), Mg1–N2 2.2141(19), Mg1–C5 2.151(2), Mg1–C1 2.153(2), Na2– C5 2.734(2), Na2–N4 2.385(2), Na2–N3 2.386(2), Na1–N3 2.390(2), Na1– N4 2.370(2), C5–Mg1–C1 124.1(1), C5–Mg1–N1 109.81(9), C1–Mg1–N1 109.01(9), C5–Mg1–N2 112.10(9), C1–Mg1–N2 111.50(9), Mg1–C5–Na2 165.1(1), N4–Na2–C5 136.96(8), N3–Na2–C5 121.48(7), N4–Na2–N3 101.49(6), Na2–N3–Na1 78.12(6), Na1–N4–Na2 78.52(6), N4–Na1–C1 130.12(7), N3–Na1–C1 128.06(7).
system, whereby one metal has a higher Lewis acidity than the second, thus the former metal captures the more Lewis basic ligands (i.e., anions). In 1–4, this is indeed the case as the magnesiates are formally formed by the most Lewis acidic metal (Mg2+) capturing the third anion from Na. For 5 however, it seems that the normal Lewis acidic/basic assignments are reversed, i.e., the di-n-butylmagnesium is now acting as a Lewis base to solvate the NaHMDS dimer (and concomitantly the NaHMDS dimer is acting as a Lewis acid) – hence the nomenclature inverse magnesiate. Compound 5 is a co-complex of dimeric (NaHMDS)2 and monomeric [nBu2Mg(TMEDA)]. Dimers of NaHMDS are known in the literature when monodentate donors such as THF,61,62 adamantylnitrile,63 t-butylnitrile,63,64 TEMPO49 and pyrimidine65 are incorporated. nBu2Mg·(TMEDA) is the other building block of 5. To the best of our knowledge “nBu2Mg(TMEDA)” has never been isolated as a single entity although two adducts of n Bu2Mg are known including a monomeric bis(imino) adduct66 and a tetranuclear N-heterocyclic carbene species.67 Other examples where a formal “nBu2Mg” fragment can be found in the molecule are the mixed alkyl/alkoxide sodium (or potassium) magnesium complexes [{(TMEDA)·M}2Mg2(nBu)4(OtBu)2] (M = Na or K).68
Fig. 5 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5, showing polymer expansion.
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Discussion, rationale and solution studies Thus far in this paper, the syntheses and solid-state structural data for several new sodium magnesiates have been presented. Despite containing essentially the same, building blocks, these complexes have shown a wide structural diversity. In this
Fig. 6 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)TMCDA}]∞ 6, showing asymmetric unit. Key bond distances (Å) and angles (°): Mg1–N1 2.178(2), Mg1–N2 2.215(2), Mg1–C5 2.156(4), Mg1– C1 2.161(4), Na2–C5 2.7159(30), Na2–N4 2.407(2), Na2–N3 2.389(2), Na1–N3 2.405(2), Na1–N4 2.382(2), C5–Mg1–C1 120.8(1), C5–Mg1–N1 112.0(1), C1–Mg1–N1 109.3(1), C5–Mg1–N2 116.9(1), C1–Mg1–N2 109.9(1), Mg1–C5–Na2 163.6(1), N4–Na2–C5 120.29(9), N3–Na2–C5 136.41(9), N4–Na2–N3 103.24(8), Na2–N3–Na1 76.46(7), Na1–N4–Na2 76.56(7), N4–Na1–C1 130.01(9), N3–Na1–C1 123.45(9).
Fig. 7 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)TMCDA}]∞ 6, showing polymer expansion.
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section, the potential rationale for their formation will be discussed. Akin to Grignard reagents, the structural chemistry of these ‘simple’ compounds appears more complex than meets the eye. As structure is inextricably linked to reactivity, it is important to have an understanding of the processes that may result in structural changes. The first reaction attempted was the formation of donor-free [NaMg(HMDS)2nBu]∞ 1 (Scheme 2). This reaction proceeded as expected and the ultimate polymeric formation of 1 was not a surprise given recent literature precedent.33,69 1H NMR studies confirmed the expected 2 : 1 HMDS : nBu ratio (ESI Fig. S1†). It was then decided to try to deaggregate 1 by adding TMEDA. This was expected to yield (TMEDA)·NaMg(HMDS)2nBu; however, as summarised in Scheme 2, the reaction took a different course and the only X-ray quality crystalline product isolated was the amide-rich (nBu-depleted) solvent-separated ion pair 2, although an amorphous solid also precipitated from solution. The formation of 2 is completely reproducible and can be explained by a potential ligand reorganisation process in hydrocarbon solution (Scheme 3).41,70,71 To examine this possibility, the solid material extracted from the reaction solution upon cooling (containing crystalline and amorphous material) was dissolved in cyc-C6D12 and analysed by 1H and 13C NMR spectroscopy. The nBu CH2 and SiCH3 H-atoms were used as handles to determine the number/nature of different solution species present. The 1H NMR spectra repeatedly showed that both HMDS and nBu resonances (in an approximate 3 : 1 ratio) were present (Fig. S2†). As the initial HMDS : nBu ratio in the reaction mixture is 2 : 1, this is consistent with the co-precipitation of approximately 40% of “NaMgHMDS(nBu)2” by-product along with crystalline 2 (Scheme 3). A potential pathway for this solution reorganisation is shown in Scheme 4. As only a 1 : 1 ratio of TMEDA : Na is initially employed in the reaction, it suggests that to form 100% of “(TMEDA)2·Na+”, stoichiometrically 50% of 1 remains unchanged in solution. It is envisaged that 1 could undergo dynamic exchange with [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4 to give 2 and “[NaMg(HMDS)nBu2]”. In an effort to investigate the processes involved in this ligand reorganisation, we decided to try to halt the process by adding an additional equivalent of TMEDA, thus fully satisfying sodium’s need to form “(TMEDA)2·Na+”. This relatively minor modification resulted in the desired change in the reaction course, and the only crystalline product obtained from this solution was 4. In an effort to test whether the potential reorganisation pathway shown in Scheme 4 is viable, equimolar quantities of 1 and 4 were reacted in hexane for two hours. The volatiles were then removed in vacuo and the solid residues were dissolved in C6D6. This solution was subjected to 1H NMR spectroscopy
Scheme 3 to 1.
Scheme 4 Potential reorganisation pathway to form 2, by adding TMEDA to 1.
and the resultant spectrum showed resonances, which were essentially identical to those obtained when 1 is reacted with an equivalent of TMEDA. We next focused on lowering the quantity of HMDS(H) in the sodium magnesiate reactions in an effort to rationally prepare the predicted by-product [“NaMg(HMDS)nBu2”] in Scheme 3. Unfortunately, due to the high solubility of the product, no crystalline material precipitated from the reaction. The cyc-C6D121H NMR spectrum of this mixture obviously contains a 1 : 2 HMDS to nBu ratio, but interestingly the chemical shift of the nBu-CH2 resonance is close to that observed for the mixture of “isolated crystals of 2” (Fig. S2b†). On addition of TMEDA, the expected donor-solvated complex was not obtained; instead, the inverse magnesiate 5 was isolated in a reproducible manner. Indeed, by altering the amine from TMEDA to (R,R)-TMCDA, it was possible to isolate a second example of an inverse magnesiate 6. The addition of donor to “NaMg(HMDS)nBu2” appears to cleave the heteroleptic magnesiate into its homometallic components [NaHMDS and n Bu2Mg·(donor)] which then recombine to form 5 or 6 [extruding a molar equivalent of nBu2Mg·(donor)] (Scheme 5). As discussed previously, the inverse magnesiates can be rationally prepared by increasing the quantity of NaHMDS from one to two molar equivalents. It could be envisaged that 5 in solution could deaggregate to its homometallic components. 1H NMR spectroscopic analysis of NaHMDS, n Bu2Mg·TMEDA and crystalline 5 in C6D6 show that only minor changes in chemical shift between the potential equilibrium products and 5 are observed (Fig. S4c†). To assess whether this deaggregation is occurring we performed a DOSY NMR study of a C6D6 solution of 5.72 This analysis shows that all the cross points for all ligand (nBu, HMDS, TMEDA) resonances are approximately in the same line of the second dimension, which is consistent with the absence of deaggregation of
Potential equilibrium which results when TMEDA is added
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Scheme 5
Potential pathway for the formation of inverse magnesiate 5.
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Fig. 8
1
H DOSY NMR spectrum of a C6D6 solution of 5.
the inverse magnesiate to its corresponding monomeric counterparts [i.e. nBu2Mg(TMEDA) and Na(HMDS)] (Fig. 8). This is in keeping with the scenario observed for the previously reported inverse zincate structure.44
Experimental General methods All reactions were performed under a protective argon atmosphere using standard Schlenk techniques. Hexane was dried by heating to reflux over sodium benzophenone ketyl and distilled under nitrogen prior to use. nBuNa was prepared according to the literature procedure.34 Na(HMDS) was used as an equivalent of a 1 : 1 mixture of nBuNa and HMDS(H) and n Bu2Mg was purchased from Sigma Aldrich Chemicals and used as received. (R,R)-TMCDA was prepared according to the literature procedure and distilled prior to use.73 TMEDA was distilled over CaH2 prior to use. 1H and 13C NMR spectra were recorded on a Bruker DPX 400 MHz spectrometer. All 13C NMR spectra were proton decoupled. Elemental analyses were attempted using a Perkin-Elmer 2400 elemental analyzer; however, due to the extreme air sensitivity of the compounds satisfactory analyses could not be obtained. Crystal structure determinations Measurements were made at 123 K on Agilent Xcalibur or Gemini diffractometers using graphite monochromated Mo Kα radiation (λ = 0.71073). The structures were solved by direct methods and refined on F2 with all unique data. Tables S1–3 (ESI†) gives further details. All non-hydrogen atoms were refined anisotropically, except for a few atoms in disordered groups, and most H atoms were constrained with a riding model (except for CH2 groups bonded to more than one metal centre, for which appropriate soft restrains were required). Programs used were Oxford Diffraction CrysAlisPro for data resolution and SHELX/SHELXTL for structure solution, refinement, and molecular graphics.74
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Synthesis of [Na(μ-HMDS)2Mg(nBu)]∞ (1). nBuNa (2 mmol, 0.16 g) was added to a Schlenk tube under inert atmosphere, and suspended in 8 mL hexane. 4 mmol HMDS(H) were then added and the mixture was allowed to stir for 1 hour. 2 mL (2 mmol) nBu2Mg (1 M, in heptane) were then added forming a white suspension after stirring. After adding toluene (2 mL), the mixture was gently heated and allowed to cool overnight in a Dewar flask filled with hot water, which afforded a large crop of colourless crystalline material (0.73 g, 81%). 1H NMR (400.13 MHz, 298 K, C6D6): δ ( ppm) 0.08 (2H, t, Mg–CH2, Bu), 0.21 (36H, s, Si(CH3)3), 1.26 (3H, t, 3JHH = 7.2 Hz, CH3, Bu), 1.79 (2H, m, CH2, Bu), 2.05 (2H, m, CH2, Bu). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ ( ppm) 6.1 (Si(CH3)3), 12.1 (CH3, Bu), 14.6 (Mg–CH2, Bu), 32.2 (CH2, Bu), 33.2 (CH2, Bu). Synthesis of [Na(TMEDA)2]+[Mg(HMDS)3]− (2). To a suspension of 1 (0.5 g, 1.17 mmol) in hexane (10 mL), was added TMEDA (0.17 mL, 1.17 mmol). After stirring for 1 hour, the solution was heated giving a clear solution, and the Schlenk tube was slowly cooled overnight in a hot water Dewar affording a large amount of colourless crystalline material plus amorphous solid (0.28 g of 60% pure 2). 1H NMR (400.13 MHz, 298 K, C6D12): δ ( ppm) 0.13 (54H, s, Si(CH3)3), 2.25 (24H, s, N(CH3)2, TMEDA), 2.36 (8H, s, NCH2, TMEDA). 13 C{1H} NMR (100.62 MHz, 298 K, C6D12): δ ( ppm) 7.0 (Si(CH3)3), 45.8 (N(CH3)2, TMEDA), 57.5 (NCH2, TMEDA). Only the NMR spectroscopic resonances corresponding to 2 are indicated. Synthesis of [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] (4). [NaMg(μ-HMDS)2(nBu)]∞ 1 (0.5 g, 1.17 mmol) was suspended in hexane (10 mL), before adding TMEDA (0.35 mL, 2.34 mmol). After stirring for 1 hour, the solution was gently warmed to give a clear solution. The mixture was cooled slowly overnight in Dewar flask filled with hot water, affording a crop of colourless crystalline material (0.13 g, 17%). The low isolated yield of the compound can be accounted for due its high solubility in hydrocarbon solvent. 1H NMR (400.13 MHz, 298 K, C6D6): δ ( ppm) −0.63 (2H, m, Mg–CH2, Bu), 0.40 (36H, s, Si(CH3)3), 1.17 (3H, t, 3JHH = 7.5 Hz, CH3, Bu), 1.62–1.67 (2H, m, CH2, Bu), 1.92 (26H, m, N(CH3)2, TMEDA and CH2, Bu), 1.96 (8H, s, NCH2, TMEDA). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ ( ppm) 6.3 (Si(CH3)3), 6.8 (Mg–CH2, Bu), 13.0 (CH3, Bu), 14.3 (CH2, Bu), 31.8 (CH2, Bu), 45.7 (N(CH3)2, TMEDA), 57.4 (NCH2, TMEDA). Synthesis of [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ (5). n Bu2Mg (1 mL of a 1 M solution in heptane, 1 mmol) was added to a suspension of Na(HMDS) (0.37 g, 2 mmol) in hexane (5 mL), producing a slightly cloudy solution. The addition of TMEDA (0.12 mL, 1 mmol), stirring for 1 hour, and gentle heating gave a clear solution. Cooling the mixture to −30 °C, produced X-ray quality colourless crystals of 5 after six days (0.25 g, yield 43%). 1H NMR (400.13 MHz, 298 K, C6D12): δ (ppm) −0.66 (4H, t, 3JHH = 8 Hz, Mg–CH2, Bu), 0.07 (36H, s, Si(CH3)3), 0.91 (6H, t, 3JHH = 7.2 Hz, CH3, Bu), 1.28–1.38 (4H, m, CH2, Bu), 1.60–1.65 (4H, m, CH2, Bu), 2.32 (12H, s, N(CH3)2, TMEDA), 2.40 (4H, s, NCH2, TMEDA). 13C{1H} NMR (100.62 MHz, 298 K, C6D12): δ (ppm) 5.7 (Si(CH3)3), 10.6 (Mg–
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CH2, Bu), 13.9 (CH3, Bu), 31.3 (CH2, Bu), 32.0 (CH2, Bu), 46.1 (N(CH3)2, TMEDA), 57.6 (NCH2, TMEDA). Synthesis of [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)-TMCDA}]∞ (6). nBu2Mg (1 mL of a 1 M solution in heptane, 1 mmol) was added to a suspension of Na(HMDS) (0.37 g, 2 mmol) in hexane (5 mL), producing a slightly cloudy solution. The addition of (R,R)-TMCDA (0.19 mL, 1 mmol), stirring for 1 hour, and gentle heating gave a clear solution. Cooling the mixture to −30 °C, produced X-ray quality colourless crystals of 6 after two days. (0.45 g, yield 66%). 1H NMR (400.13 MHz, 298 K, C6D12): δ ( ppm) −0.62 (4H, m, Mg–CH2, Bu),0.07 (36H, s, Si(CH3)3), 0.90 (6H, t, 3JHH = 7.2 Hz, CH3, Bu), 1.15–1.18 (4H, m, CH2, TMCDA), 1.30–1.36 (4H, m, CH2, Bu), 1.58–1.65 (4H, m, CH2, Bu), 1.82–1.97 (m, 4H, CH2, TMCDA), 2.35 (12H, s, N(CH3)2, TMCDA), 2.44 (2H, s, NCH, TMCDA). 13C{1H} NMR (100.62 MHz, 298 K, C6D12): δ ( ppm) 5.6 (Si(CH3)3), 8.3 (Mg– CH2, Bu), 14.0 (CH3, Bu), 21.3 (2 × CH2, TMCDA), 21.8 (CH2, TMCDA), 24.6 (CH2, TMCDA), 31.5 (CH2, Bu), 32.8 (CH2, Bu), 44.5 (N(CH3), TMCDA), 64.0 (NCH, TMCDA).
Conclusions In summary, a new family of bimetallic sodium–magnesium compounds incorporating the HMDS amido ligand were synthesised and characterised, showing a wide range of structures and solution-equilibria.
Acknowledgements We gratefully acknowledge the support of the EPSRC (J001872/1 and L001497/1) for the award of a Career Acceleration Fellowship to CTOH. PGA thanks the Spanish MEC (RYC-2012-10491).
Notes and references 1 V. Grignard, C. R. Acad. Sci. Paris, 1900, 130, 1322. 2 H. G. Richey Jr., Grignard Reagents New Developments, John Wiley & Sons Ltd, Chichester, 2000. 3 Z. Rappoport and I. Marek, in The Chemistry of Organomagnesium Compounds, ed. Z. Rappoport and I. Marek, John Wiley & Sons, Chichester, 2008. 4 G. Wittig, F. J. Meyer and G. Lange, Liebigs Ann. Chem., 1951, 571, 167. 5 R. E. Mulvey, F. Mongin, M. Uchiyama and Y. Kondo, Angew. Chem., Int. Ed., 2007, 46, 3802. 6 D. R. Armstrong, A. R. Kennedy, R. E. Mulvey and R. B. Rowlings, Angew. Chem., Int. Ed., 1999, 38, 131. 7 E. Hevia, D. J. Gallagher, A. R. Kennedy, R. E. Mulvey, C. T. O’Hara and C. Talmard, Chem. Commun., 2004, 2422. 8 P. C. Andrikopoulos, D. R. Armstrong, D. V. Graham, E. Hevia, A. R. Kennedy, R. E. Mulvey, C. T. O’Hara and C. Talmard, Angew. Chem., Int. Ed., 2005, 44, 3459. 9 D. V. Graham, E. Hevia, A. R. Kennedy, R. E. Mulvey, C. T. O’Hara and C. Talmard, Chem. Commun., 2006, 417.
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Complexity in seemingly simple sodium magnesiate systems† J. Francos,a B. J. Fleming,a P. García-Álvarez,b A. R. Kennedy,a K. Reilly,a G. M. Robertson,a S. D. Robertsona and C. T. O’Hara*a A systematic study both in the solid- and solution-state, was carried out for a series of sodium magnesiates containing the utility amide ligand 1,1,1,3,3,3-hexamethyldisilazide (HMDS). The first complex considered is the donor-free bisamido monoalkyl polymeric complex [Na(μ-HMDS)2Mg(nBu)]∞ 1. The reactivity of 1 with common tertiary bidentate donors including N,N,N’,N’-tetramethylethylenediamine (TMEDA) or its chiral relative (1R,2R)-tetramethylcyclohexyldiamine [(R,R)-TMCDA] is detailed. Surprisingly, the products of these reactions are not simple diamine adducts but are solvent separated sodium magnesiate systems [(TMEDA)2·Na]+[Mg(HMDS)3]− 2 and [{(R,R)-TMCDA}2·Na]+[Mg(HMDS)3]− 3. By concentrating on the likely equilibria which may give rise to formation of 2, a potential intermediate complexed ion pair
Received 28th March 2014, Accepted 24th June 2014 DOI: 10.1039/c4dt00921e www.rsc.org/dalton
[{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4 was isolated. Additionally, the novel “inverse magnesiates” [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5 and [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)-TMCDA}]∞ 6, were obtained by reacting solutions of composition “NaMg(HMDS)(nBu)2” (a likely by-product in the formation of 2 from 1), with TMEDA or (R,R)-TMCDA. The structure and nature of these bimetallic complexes have been determined using a combination of X-ray crystallographic studies and multinuclear NMR spectroscopy.
Introduction Organomagnesium reagents are amongst the first discovered classes of organometallics. Synonymous with magnesium, Grignard pioneered much of the early organomagnesium chemistry.1 He focused on the direct insertion of magnesium metal into an organic halide to synthesise complexes with the formulation RMgX (where R is an alkyl or aryl group; and X is a halogen). This discovery culminated in him being awarded the Nobel Prize in Chemistry in 1912. The utilisation of organomagnesium reagents has increased at a rapid rate,2 having found many applications throughout various fields in organic chemistry.3 In addition, the solid-state structure of many Grignard reagents have been determined over the past few decades to give an insight into the structural complexity and diversity of the systems. The solution structure of Grignard reagents is complicated as they are inherently kinetically unstable in solution and as a consequence of the Schlenk equi-
a WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK. E-mail: [email protected]; Tel: +44 (0)141 548 2667 b Departamento de Química Orgánica e Inorgánica-IUQOEM, Universidad de OviedoCSIC, E-33071 Oviedo, Spain † Electronic supplementary information (ESI) available: Full experimental data, X-ray data and NMR spectra. CCDC 993041–993045. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00921e
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librium (Scheme 1) they often exist as mixtures alongside their associated disproportionation products (i.e., R2Mg/MgX2). In addition, (akin to their solid state chemistry) they can exist as different oligomeric aggregates. To emphasize the complexity in such an apparently simple system, the ultimate structure of each reagent has to be treated on an individual basis as they “depend on the nature of R, the nature of X, the properties of the coordinating solvent, concentration and temperature.”3 Organomagnesiums are generally less reactive than their lithium counterparts. A way to enhance their reactivity is to formally combine them with an alkali metal organometallic or alkali metal amide complex to give rise to an alkali metal magnesiate complex. In a seminal paper published by Wittig in 1951,4 alongside an analogous “zincate” example, the synthesis of an “ate” complex of formulation LiMgPh3 was reported. This was produced by combining the two homometallic species, namely Ph2Mg and PhLi. It was shown to deliver unique reactivities when reacted with benzalacetophenone, giving predominantly the 1,4-addition product, contrasting with the 1,2-adduct formed on reaction with monometallic PhLi alone.4 More recently, magnesiate chemistry has developed at a frantic pace, both in terms of structural chemistry and
Scheme 1 The generalised Schlenk equilibrium, where R = alkyl or aryl and X = Br or Cl.
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reactivity.5 Magnesiate complexes generally display a unique synergic chemistry that is distinct from the reactivities of their parent complexes. For instance, they have been shown to effect deprotonative metallation regioselectively on a range of organic substrates (such as benzene,6,7 toluene,6,8 furan,9 metallocenes10,11 and alkynes12) at ambient temperatures. These mixed-metal reactions have been termed alkali-metalmediated magnesiations (AMMMg).13–15 Mongin and coworkers have comprehensively studied the use of magnesiates for the deprotonation of a range of molecules including, furans, pyridines, thiophenes and oxazoles.16–25 When lithium chloride is added to certain Grignard reagents, a twofold synthetic advantage can often be observed. Firstly, an enhancement of reactivity (cf., the Grignard reagent), and secondly, a greater functional group tolerance (cf., a conventional lithium reagent) in a multitude of deprotonation and metal–halide exchange reactions occurs. This class of lithium magnesiates, discovered by Knochel, have been termed “turbo Grignard” reagents.26,27 Magnesium reagents exhibiting the formula R2NMgCl·LiCl (turbo Hauser reagents) were found to perform especially well in the area of deprotonative metallation.28–30 For example, in 2006 Knochel utilised TMPMgCl·LiCl31,32 (TMP = 2,2,6,6-tetramethylpiperidide) to metallate (magnesiate) a wide range of heterocycles with great control in regioselectivity. In this work, we initially focus on the bis(amido)-monoalkyl sodium magnesiate NaMg(HMDS)2nBu. By adding donor molecules it is revealed that akin to Grignard reagents, their structural chemistry is considerably more complex than expected.
Results and discussion Synthesis The syntheses of sodium magnesiates 1–6 are summarised in Scheme 2. Several novel sodium magnesiates were prepared during this study. Due to the importance of other sodium
Scheme 2
Syntheses of 1–6.
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magnesiates [e.g., ‘NaMg(TMP)2nBu’]33 in deprotonative metalation reactions, the synthesis of donor-free NaMg(HMDS)2nBu was the initial focus. This was achieved by combining n-butylsodium,34 HMDS(H) and di-n-butylmagnesium in hexane in a 1 : 2 : 1 stoichiometric ratio. Slow cooling of the resultant solution yielded polymeric sodium magnesiate [NaMg(μ-HMDS)2nBu]∞ 1 in high crystalline yield (81%). Diamines, particularly N,N,N′,N′-tetramethylethylenediamine (TMEDA) have proven to be an essential additive in the field of alkali metal mediated magnesiation as they act as coordination blockers, thus preventing polymerization (and often insolubility) of the alkali metal magnesiate.35–43 As a consequence, it was decided to study the coordination chemistry of 1 with TMEDA and its chiral relative (1R,2R)-tetramethylcyclohexyldiamine [(R,R)-TMCDA], in an effort to break up the polymer and prepare a lower aggregated donor solvate of 1. However, the only crystalline product obtained from the former reaction was the tris(amido) solvent separated ion pair [(TMEDA)2·Na]+[Mg(HMDS)3]− 2. An identical reaction pathway occurred when TMEDA was substituted for (R,R)-TMCDA to produce 3 (Scheme 2). These reactions suggest that in the presence of bidentate donor, 1 is subject to a ligand reorganisation (vide infra). By employing two molar equivalents of TMEDA [with respect to sodium (or magnesium) reagent] a loosely-contacted ion pair structure, the bis(amido) mono-alkyl species [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4 was formed (Scheme 2). When nBuNa, nBu2Mg, HMDS(H) and TMEDA are combined in a 1 : 1 : 1 : 1 ratio, the surprising product was polymeric [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5. In keeping with terminology introduced by Hevia et al.44 for a related zinc-containing compound, 5 can be considered as the first example of an inverse magnesiate (vide infra). Complex 5 can be rationally prepared by altering the starting material ratio from 1 : 1 : 1 : 1 to 2 : 1 : 2 : 1. By replacing TMEDA with (R,R)-TMCDA a second inverse magnesiate [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)-TMCDA}]∞ 6 can be rationally prepared (Scheme 2). Structural studies Sodium magnesiate [Na(μ-HMDS)2Mg(nBu)]∞ 1 is polymeric in the solid state (the polymer is generated by the action of the n glide plane) and crystallizes in the P21/n space group (Fig. 1). The sodium atom adopts a highly distorted trigonal planar arrangement, and bonds to two N atoms of bridging HMDS ligands [bond distances, 2.486(2) and 2.451(2) Å]. Considering that the polymer consists of dinuclear units, the coordination sphere of the Na atom is completed by an intermolecular interaction to the anionic C butyl centre [Na1–C13 bond distance, 2.781(2) Å]. In addition, the Na atom is close to a single methyl group from both HMDS units (mean distance of 2.913 Å) which may contribute to stabilising the Na centre further. The Na1–C13–Mg1* bridge angle is close to linearity [164.9(1)°] allowing the propagation of a one-dimensional chain-like polymer. Akin to the sodium, the magnesium atom adopts a distorted trigonal planar arrangement, bonding to both bridging N atoms and the C atom of the butyl anion [Mg1–N1 2.077(2), Mg1–N2 2.063(2), Mg1–C13 2.155(2) Å].
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Fig. 2 A representation of [(TMEDA)2·Na]+[Mg(HMDS)3]− 2. Key bond distances (Å) and angles (°): Mg1–N1 2.017(2), Mg1–N2 2.024(2), Mg1– N3 2.030(2), Na1–N4 2.562(2), Na1–N5 2.472(2), N1–Mg1–N2 119.77(7), N1–Mg1–N3 121.04(7), N2–Mg1–N3 119.19(7), N5–Na1–N4 74.57(6).
Fig. 1 Molecular structure of [Na(HMDS)2Mg(nBu)]∞ 1, showing (top) asymmetric unit; (bottom) polymer propagation. Key bond distances (Å) and angles (°): Mg1–N2 2.063(2), Mg1–N1 2.077(2), Mg1–C13 2.155(2), Na1–N1 2.486(2), Na1–N2 2.451(2), Na1–C13 2.781(2), Na1–C5 2.935(2), Na1–C10 2.891(2), N2–Mg1–N1 106.13(7), N2–Mg1–C13 129.48(9), N1– Mg1–C13 124.32(9), Mg1–N2–Na1 85.38(6), Na1–N1–Mg1 84.19(6), N2– Na1–N1 84.16(6), N2–Na1–C13 133.57(7), N1–Na1–C13 142.00(7).
Complex 1 can be compared with another alkali metal magnesiate polymer, the potassium magnesiate [KMg(TMP)2(nBu)]∞.33 ˉ space The solvent-separated ion pair 2 crystallises in the P1 group (Fig. 2). The almost perfectly trigonal planar [Mg(HMDS)3]− anion of 2 is well known in magnesium chemistry, and its structural parameters are essentially identical to those in previously published complexes.45–50 Likewise the [Na·(TMEDA)2]+ cation is common and a search of the Cambridge Crystallographic Database shows that the cation appears in ten other complexes and the key dimensions of 2 are generally identical to these within experimental error.51–60 The structure of the (R,R)-TMCDA-containing ion pair 3 has been published.46 Complex 4 crystallising in the Pna21 space group, is perhaps best described as a loosely contacted ion pair structure; however, akin to 2 the Na atom within the structure is bound to two TMEDA ligands (Fig. 3). The coordination sphere of Na is completed by a long Na–Cbutyl contact [2.986(5) and 3.001(5) Å for the two crystallographic independent units] which bridges to the Mg atom giving rise to a square pyramidal arrangement. Surprisingly (and unlike in 1) the alkyl group is the only bridge between the two metals rather than an amide group, which is most often considered to be a better
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bridging ligand. Di-TMEDA-solvated dinuclear 4 can be compared with the dinuclear building blocks of polymer 1; the chelation has a dramatic effect on altering the comparable Na–C and Mg–C distances and Na–C–Mg angles. For instance, the Na–Cbutyl bond distance [2.781(2) and 2.986(5) Å in 1 and 4 respectively] is predictably shorter in 1 due to the Na atom’s lower coordination number, and the Na–C–Mg angle in 1 [164.94(1) and 148.0(2)° for 1 and 4 respectively] is considerably wider than that in 4. Reflecting the similar Mg coordination geometries, the Mg–C bonds in 1 and 4 [2.155(2) and 2.170(4) Å respectively] are identical within experimental error. Complex 5 crystallises in the P21/n space group (Fig. 4 and 5). It is polymeric ( propagating along the crystallographic b axis through the action of the 21 screw axis) and can be described as an inverse magnesiate [akin to its zincate relative [{Li(HMDS)}2Zn(Me)2·(TMEDA)]∞].44 To explain this nomenclature, it is best to first consider normal “ate” complexes. In general, they are most commonly associated with a bimetallic
Fig. 3 Molecular structure of one of the two crystallographicallyunique units of [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4. Key bond distances (Å) and angles (°): Mg1–N1 2.036(3), Mg1–N2 2.024(3), Mg1–C1 2.170(4), Na1–C1 2.986(5), Na1–N3 2.505(3), Na1–N6 2.533(4), Na1–N5 2.538(4), Na1–N4 2.583(4) N2–Mg1–N1 122.63(14), N2–Mg1–C1 117.8(2), N1– Mg1–C1 119.6(2), Mg1–C1–Na1 148.0(2).
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Returning to 5, the Na and Mg are linked via a Na–CH2– Mg–CH2–Na arrangement [Na2–C5 2.734(2); Mg1–C5 2.151(2); Mg1–C1 2.153(2) and Na1–C1 2.706(4) Å] propagating the polymeric 1D chain (Fig. 5). Surprisingly, these Mg–C bond distances are considerably shorter than the mean Mg–Cbridging bond distance in the aforementioned NHC complex (2.270 Å)67 and are still shorter (and by inference, stronger) than the mean Mg–Cterminal distance in the bis(imino) adduct (2.175 Å).66 Complex 6 is the second example of an inverse magnesiate, and incorporates the chiral diamine (R,R)-TMCDA (Fig. 6). Its structure (Fig. 6 and 7) and geometrical parameters are essentially identical to those of 5. Fig. 4 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5, showing asymmetric unit. Key bond distances (Å) and angles (°): Mg1–N1 2.212(2), Mg1–N2 2.2141(19), Mg1–C5 2.151(2), Mg1–C1 2.153(2), Na2– C5 2.734(2), Na2–N4 2.385(2), Na2–N3 2.386(2), Na1–N3 2.390(2), Na1– N4 2.370(2), C5–Mg1–C1 124.1(1), C5–Mg1–N1 109.81(9), C1–Mg1–N1 109.01(9), C5–Mg1–N2 112.10(9), C1–Mg1–N2 111.50(9), Mg1–C5–Na2 165.1(1), N4–Na2–C5 136.96(8), N3–Na2–C5 121.48(7), N4–Na2–N3 101.49(6), Na2–N3–Na1 78.12(6), Na1–N4–Na2 78.52(6), N4–Na1–C1 130.12(7), N3–Na1–C1 128.06(7).
system, whereby one metal has a higher Lewis acidity than the second, thus the former metal captures the more Lewis basic ligands (i.e., anions). In 1–4, this is indeed the case as the magnesiates are formally formed by the most Lewis acidic metal (Mg2+) capturing the third anion from Na. For 5 however, it seems that the normal Lewis acidic/basic assignments are reversed, i.e., the di-n-butylmagnesium is now acting as a Lewis base to solvate the NaHMDS dimer (and concomitantly the NaHMDS dimer is acting as a Lewis acid) – hence the nomenclature inverse magnesiate. Compound 5 is a co-complex of dimeric (NaHMDS)2 and monomeric [nBu2Mg(TMEDA)]. Dimers of NaHMDS are known in the literature when monodentate donors such as THF,61,62 adamantylnitrile,63 t-butylnitrile,63,64 TEMPO49 and pyrimidine65 are incorporated. nBu2Mg·(TMEDA) is the other building block of 5. To the best of our knowledge “nBu2Mg(TMEDA)” has never been isolated as a single entity although two adducts of n Bu2Mg are known including a monomeric bis(imino) adduct66 and a tetranuclear N-heterocyclic carbene species.67 Other examples where a formal “nBu2Mg” fragment can be found in the molecule are the mixed alkyl/alkoxide sodium (or potassium) magnesium complexes [{(TMEDA)·M}2Mg2(nBu)4(OtBu)2] (M = Na or K).68
Fig. 5 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ 5, showing polymer expansion.
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Discussion, rationale and solution studies Thus far in this paper, the syntheses and solid-state structural data for several new sodium magnesiates have been presented. Despite containing essentially the same, building blocks, these complexes have shown a wide structural diversity. In this
Fig. 6 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)TMCDA}]∞ 6, showing asymmetric unit. Key bond distances (Å) and angles (°): Mg1–N1 2.178(2), Mg1–N2 2.215(2), Mg1–C5 2.156(4), Mg1– C1 2.161(4), Na2–C5 2.7159(30), Na2–N4 2.407(2), Na2–N3 2.389(2), Na1–N3 2.405(2), Na1–N4 2.382(2), C5–Mg1–C1 120.8(1), C5–Mg1–N1 112.0(1), C1–Mg1–N1 109.3(1), C5–Mg1–N2 116.9(1), C1–Mg1–N2 109.9(1), Mg1–C5–Na2 163.6(1), N4–Na2–C5 120.29(9), N3–Na2–C5 136.41(9), N4–Na2–N3 103.24(8), Na2–N3–Na1 76.46(7), Na1–N4–Na2 76.56(7), N4–Na1–C1 130.01(9), N3–Na1–C1 123.45(9).
Fig. 7 Molecular structure of [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)TMCDA}]∞ 6, showing polymer expansion.
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section, the potential rationale for their formation will be discussed. Akin to Grignard reagents, the structural chemistry of these ‘simple’ compounds appears more complex than meets the eye. As structure is inextricably linked to reactivity, it is important to have an understanding of the processes that may result in structural changes. The first reaction attempted was the formation of donor-free [NaMg(HMDS)2nBu]∞ 1 (Scheme 2). This reaction proceeded as expected and the ultimate polymeric formation of 1 was not a surprise given recent literature precedent.33,69 1H NMR studies confirmed the expected 2 : 1 HMDS : nBu ratio (ESI Fig. S1†). It was then decided to try to deaggregate 1 by adding TMEDA. This was expected to yield (TMEDA)·NaMg(HMDS)2nBu; however, as summarised in Scheme 2, the reaction took a different course and the only X-ray quality crystalline product isolated was the amide-rich (nBu-depleted) solvent-separated ion pair 2, although an amorphous solid also precipitated from solution. The formation of 2 is completely reproducible and can be explained by a potential ligand reorganisation process in hydrocarbon solution (Scheme 3).41,70,71 To examine this possibility, the solid material extracted from the reaction solution upon cooling (containing crystalline and amorphous material) was dissolved in cyc-C6D12 and analysed by 1H and 13C NMR spectroscopy. The nBu CH2 and SiCH3 H-atoms were used as handles to determine the number/nature of different solution species present. The 1H NMR spectra repeatedly showed that both HMDS and nBu resonances (in an approximate 3 : 1 ratio) were present (Fig. S2†). As the initial HMDS : nBu ratio in the reaction mixture is 2 : 1, this is consistent with the co-precipitation of approximately 40% of “NaMgHMDS(nBu)2” by-product along with crystalline 2 (Scheme 3). A potential pathway for this solution reorganisation is shown in Scheme 4. As only a 1 : 1 ratio of TMEDA : Na is initially employed in the reaction, it suggests that to form 100% of “(TMEDA)2·Na+”, stoichiometrically 50% of 1 remains unchanged in solution. It is envisaged that 1 could undergo dynamic exchange with [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] 4 to give 2 and “[NaMg(HMDS)nBu2]”. In an effort to investigate the processes involved in this ligand reorganisation, we decided to try to halt the process by adding an additional equivalent of TMEDA, thus fully satisfying sodium’s need to form “(TMEDA)2·Na+”. This relatively minor modification resulted in the desired change in the reaction course, and the only crystalline product obtained from this solution was 4. In an effort to test whether the potential reorganisation pathway shown in Scheme 4 is viable, equimolar quantities of 1 and 4 were reacted in hexane for two hours. The volatiles were then removed in vacuo and the solid residues were dissolved in C6D6. This solution was subjected to 1H NMR spectroscopy
Scheme 3 to 1.
Scheme 4 Potential reorganisation pathway to form 2, by adding TMEDA to 1.
and the resultant spectrum showed resonances, which were essentially identical to those obtained when 1 is reacted with an equivalent of TMEDA. We next focused on lowering the quantity of HMDS(H) in the sodium magnesiate reactions in an effort to rationally prepare the predicted by-product [“NaMg(HMDS)nBu2”] in Scheme 3. Unfortunately, due to the high solubility of the product, no crystalline material precipitated from the reaction. The cyc-C6D121H NMR spectrum of this mixture obviously contains a 1 : 2 HMDS to nBu ratio, but interestingly the chemical shift of the nBu-CH2 resonance is close to that observed for the mixture of “isolated crystals of 2” (Fig. S2b†). On addition of TMEDA, the expected donor-solvated complex was not obtained; instead, the inverse magnesiate 5 was isolated in a reproducible manner. Indeed, by altering the amine from TMEDA to (R,R)-TMCDA, it was possible to isolate a second example of an inverse magnesiate 6. The addition of donor to “NaMg(HMDS)nBu2” appears to cleave the heteroleptic magnesiate into its homometallic components [NaHMDS and n Bu2Mg·(donor)] which then recombine to form 5 or 6 [extruding a molar equivalent of nBu2Mg·(donor)] (Scheme 5). As discussed previously, the inverse magnesiates can be rationally prepared by increasing the quantity of NaHMDS from one to two molar equivalents. It could be envisaged that 5 in solution could deaggregate to its homometallic components. 1H NMR spectroscopic analysis of NaHMDS, n Bu2Mg·TMEDA and crystalline 5 in C6D6 show that only minor changes in chemical shift between the potential equilibrium products and 5 are observed (Fig. S4c†). To assess whether this deaggregation is occurring we performed a DOSY NMR study of a C6D6 solution of 5.72 This analysis shows that all the cross points for all ligand (nBu, HMDS, TMEDA) resonances are approximately in the same line of the second dimension, which is consistent with the absence of deaggregation of
Potential equilibrium which results when TMEDA is added
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Scheme 5
Potential pathway for the formation of inverse magnesiate 5.
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Fig. 8
1
H DOSY NMR spectrum of a C6D6 solution of 5.
the inverse magnesiate to its corresponding monomeric counterparts [i.e. nBu2Mg(TMEDA) and Na(HMDS)] (Fig. 8). This is in keeping with the scenario observed for the previously reported inverse zincate structure.44
Experimental General methods All reactions were performed under a protective argon atmosphere using standard Schlenk techniques. Hexane was dried by heating to reflux over sodium benzophenone ketyl and distilled under nitrogen prior to use. nBuNa was prepared according to the literature procedure.34 Na(HMDS) was used as an equivalent of a 1 : 1 mixture of nBuNa and HMDS(H) and n Bu2Mg was purchased from Sigma Aldrich Chemicals and used as received. (R,R)-TMCDA was prepared according to the literature procedure and distilled prior to use.73 TMEDA was distilled over CaH2 prior to use. 1H and 13C NMR spectra were recorded on a Bruker DPX 400 MHz spectrometer. All 13C NMR spectra were proton decoupled. Elemental analyses were attempted using a Perkin-Elmer 2400 elemental analyzer; however, due to the extreme air sensitivity of the compounds satisfactory analyses could not be obtained. Crystal structure determinations Measurements were made at 123 K on Agilent Xcalibur or Gemini diffractometers using graphite monochromated Mo Kα radiation (λ = 0.71073). The structures were solved by direct methods and refined on F2 with all unique data. Tables S1–3 (ESI†) gives further details. All non-hydrogen atoms were refined anisotropically, except for a few atoms in disordered groups, and most H atoms were constrained with a riding model (except for CH2 groups bonded to more than one metal centre, for which appropriate soft restrains were required). Programs used were Oxford Diffraction CrysAlisPro for data resolution and SHELX/SHELXTL for structure solution, refinement, and molecular graphics.74
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Synthesis of [Na(μ-HMDS)2Mg(nBu)]∞ (1). nBuNa (2 mmol, 0.16 g) was added to a Schlenk tube under inert atmosphere, and suspended in 8 mL hexane. 4 mmol HMDS(H) were then added and the mixture was allowed to stir for 1 hour. 2 mL (2 mmol) nBu2Mg (1 M, in heptane) were then added forming a white suspension after stirring. After adding toluene (2 mL), the mixture was gently heated and allowed to cool overnight in a Dewar flask filled with hot water, which afforded a large crop of colourless crystalline material (0.73 g, 81%). 1H NMR (400.13 MHz, 298 K, C6D6): δ ( ppm) 0.08 (2H, t, Mg–CH2, Bu), 0.21 (36H, s, Si(CH3)3), 1.26 (3H, t, 3JHH = 7.2 Hz, CH3, Bu), 1.79 (2H, m, CH2, Bu), 2.05 (2H, m, CH2, Bu). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ ( ppm) 6.1 (Si(CH3)3), 12.1 (CH3, Bu), 14.6 (Mg–CH2, Bu), 32.2 (CH2, Bu), 33.2 (CH2, Bu). Synthesis of [Na(TMEDA)2]+[Mg(HMDS)3]− (2). To a suspension of 1 (0.5 g, 1.17 mmol) in hexane (10 mL), was added TMEDA (0.17 mL, 1.17 mmol). After stirring for 1 hour, the solution was heated giving a clear solution, and the Schlenk tube was slowly cooled overnight in a hot water Dewar affording a large amount of colourless crystalline material plus amorphous solid (0.28 g of 60% pure 2). 1H NMR (400.13 MHz, 298 K, C6D12): δ ( ppm) 0.13 (54H, s, Si(CH3)3), 2.25 (24H, s, N(CH3)2, TMEDA), 2.36 (8H, s, NCH2, TMEDA). 13 C{1H} NMR (100.62 MHz, 298 K, C6D12): δ ( ppm) 7.0 (Si(CH3)3), 45.8 (N(CH3)2, TMEDA), 57.5 (NCH2, TMEDA). Only the NMR spectroscopic resonances corresponding to 2 are indicated. Synthesis of [{(TMEDA)2·Na}(μ-nBu)Mg(HMDS)2] (4). [NaMg(μ-HMDS)2(nBu)]∞ 1 (0.5 g, 1.17 mmol) was suspended in hexane (10 mL), before adding TMEDA (0.35 mL, 2.34 mmol). After stirring for 1 hour, the solution was gently warmed to give a clear solution. The mixture was cooled slowly overnight in Dewar flask filled with hot water, affording a crop of colourless crystalline material (0.13 g, 17%). The low isolated yield of the compound can be accounted for due its high solubility in hydrocarbon solvent. 1H NMR (400.13 MHz, 298 K, C6D6): δ ( ppm) −0.63 (2H, m, Mg–CH2, Bu), 0.40 (36H, s, Si(CH3)3), 1.17 (3H, t, 3JHH = 7.5 Hz, CH3, Bu), 1.62–1.67 (2H, m, CH2, Bu), 1.92 (26H, m, N(CH3)2, TMEDA and CH2, Bu), 1.96 (8H, s, NCH2, TMEDA). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ ( ppm) 6.3 (Si(CH3)3), 6.8 (Mg–CH2, Bu), 13.0 (CH3, Bu), 14.3 (CH2, Bu), 31.8 (CH2, Bu), 45.7 (N(CH3)2, TMEDA), 57.4 (NCH2, TMEDA). Synthesis of [{Na(μ-HMDS)}2Mg(μ-nBu)2·(TMEDA)]∞ (5). n Bu2Mg (1 mL of a 1 M solution in heptane, 1 mmol) was added to a suspension of Na(HMDS) (0.37 g, 2 mmol) in hexane (5 mL), producing a slightly cloudy solution. The addition of TMEDA (0.12 mL, 1 mmol), stirring for 1 hour, and gentle heating gave a clear solution. Cooling the mixture to −30 °C, produced X-ray quality colourless crystals of 5 after six days (0.25 g, yield 43%). 1H NMR (400.13 MHz, 298 K, C6D12): δ (ppm) −0.66 (4H, t, 3JHH = 8 Hz, Mg–CH2, Bu), 0.07 (36H, s, Si(CH3)3), 0.91 (6H, t, 3JHH = 7.2 Hz, CH3, Bu), 1.28–1.38 (4H, m, CH2, Bu), 1.60–1.65 (4H, m, CH2, Bu), 2.32 (12H, s, N(CH3)2, TMEDA), 2.40 (4H, s, NCH2, TMEDA). 13C{1H} NMR (100.62 MHz, 298 K, C6D12): δ (ppm) 5.7 (Si(CH3)3), 10.6 (Mg–
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CH2, Bu), 13.9 (CH3, Bu), 31.3 (CH2, Bu), 32.0 (CH2, Bu), 46.1 (N(CH3)2, TMEDA), 57.6 (NCH2, TMEDA). Synthesis of [{Na(μ-HMDS)}2Mg(μ-nBu)2·{(R,R)-TMCDA}]∞ (6). nBu2Mg (1 mL of a 1 M solution in heptane, 1 mmol) was added to a suspension of Na(HMDS) (0.37 g, 2 mmol) in hexane (5 mL), producing a slightly cloudy solution. The addition of (R,R)-TMCDA (0.19 mL, 1 mmol), stirring for 1 hour, and gentle heating gave a clear solution. Cooling the mixture to −30 °C, produced X-ray quality colourless crystals of 6 after two days. (0.45 g, yield 66%). 1H NMR (400.13 MHz, 298 K, C6D12): δ ( ppm) −0.62 (4H, m, Mg–CH2, Bu),0.07 (36H, s, Si(CH3)3), 0.90 (6H, t, 3JHH = 7.2 Hz, CH3, Bu), 1.15–1.18 (4H, m, CH2, TMCDA), 1.30–1.36 (4H, m, CH2, Bu), 1.58–1.65 (4H, m, CH2, Bu), 1.82–1.97 (m, 4H, CH2, TMCDA), 2.35 (12H, s, N(CH3)2, TMCDA), 2.44 (2H, s, NCH, TMCDA). 13C{1H} NMR (100.62 MHz, 298 K, C6D12): δ ( ppm) 5.6 (Si(CH3)3), 8.3 (Mg– CH2, Bu), 14.0 (CH3, Bu), 21.3 (2 × CH2, TMCDA), 21.8 (CH2, TMCDA), 24.6 (CH2, TMCDA), 31.5 (CH2, Bu), 32.8 (CH2, Bu), 44.5 (N(CH3), TMCDA), 64.0 (NCH, TMCDA).
Conclusions In summary, a new family of bimetallic sodium–magnesium compounds incorporating the HMDS amido ligand were synthesised and characterised, showing a wide range of structures and solution-equilibria.
Acknowledgements We gratefully acknowledge the support of the EPSRC (J001872/1 and L001497/1) for the award of a Career Acceleration Fellowship to CTOH. PGA thanks the Spanish MEC (RYC-2012-10491).
Notes and references 1 V. Grignard, C. R. Acad. Sci. Paris, 1900, 130, 1322. 2 H. G. Richey Jr., Grignard Reagents New Developments, John Wiley & Sons Ltd, Chichester, 2000. 3 Z. Rappoport and I. Marek, in The Chemistry of Organomagnesium Compounds, ed. Z. Rappoport and I. Marek, John Wiley & Sons, Chichester, 2008. 4 G. Wittig, F. J. Meyer and G. Lange, Liebigs Ann. Chem., 1951, 571, 167. 5 R. E. Mulvey, F. Mongin, M. Uchiyama and Y. Kondo, Angew. Chem., Int. Ed., 2007, 46, 3802. 6 D. R. Armstrong, A. R. Kennedy, R. E. Mulvey and R. B. Rowlings, Angew. Chem., Int. Ed., 1999, 38, 131. 7 E. Hevia, D. J. Gallagher, A. R. Kennedy, R. E. Mulvey, C. T. O’Hara and C. Talmard, Chem. Commun., 2004, 2422. 8 P. C. Andrikopoulos, D. R. Armstrong, D. V. Graham, E. Hevia, A. R. Kennedy, R. E. Mulvey, C. T. O’Hara and C. Talmard, Angew. Chem., Int. Ed., 2005, 44, 3459. 9 D. V. Graham, E. Hevia, A. R. Kennedy, R. E. Mulvey, C. T. O’Hara and C. Talmard, Chem. Commun., 2006, 417.
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