Square-planar ruthenium(II) complexes: control of spin state by pincer ligand functionalization.

DOI: 10.1002/chem.201404282

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Square-Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization Bjorn Askevold,[b] Marat M. Khusniyarov,[b] Wolfgang Kroener,[c] Klaus Gieb,[c] Paul Mller,[c] Eberhardt Herdtweck,[d] Frank W. Heinemann,[b] Martin Diefenbach,[e] Max C. Holthausen,[e] Veacheslav Vieru,[f] Liviu F. Chibotaru,[f] and Sven Schneider*[a]

amido complexes exhibit unusual triplet (S = 1) ground states as confirmed by experimental and computational examination. However, essentially non-magnetic ground states arise for the two intermediate-spin complexes owing to unusually large zero-field splitting (D > + 200 cm1). The change in ground state electronic configuration is attributed to tailored pincer ligand-to-metal p-donation within the PNP ligand series.

Abstract: Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH2CH2PtBu2)2}], [RuCl{N(CHCHPtBu2)(CH2CH2PtBu2)}], and [RuCl{N(CHCHPtBu2)2}], in which the ruthenium(II) ions are in the extremely rare square-planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S = 0) ground state and energetically lowlying triplet (S = 1) state, the vinyl alkyl amido and the divinyl

Introduction

are bound to the two terminal donor groups of the pincer framework. In contrast, strong electronic effects can arise from exchange of the central pincer donor that is in the trans position with respect to the substrate binding site. For example, anionic PNP pincer ligands as derived from diaryl-, disilyl, or dialkyl amines or from dearomatized pyridine pincers introduce a potentially p-donating amido group with profound influence on reactivity.[2] However, systematic studies that examine the electronic structures of complexes as consequences of subtle variation of pincer donor properties are scarce. Within the square-planar coordination geometry, that is often imposed by bulky pincer ligands, complexes of ions with d6 metal valence electron configuration are particularly interesting, because three possible spin-states have to be considered, that is, low-spin (LS, S = 0), intermediate-spin (IS, S = 1), and high-spin (HS, S = 2). All three spin-states were reported depending on the metal ion and ligand field strength (Figure 1). In ideal D4h symmetry, the d-orbital manifold is split into non-bonding dxz, dyz, and dxy (eg and b2g), weakly antibonding dz2 (a1g), and strongly antibonding dx2y2 (b1g) orbitals, typically giving rise to an IS (eg)4(b2g)1(a1g)1(b1g)0 configuration (Figure 1, center).[3, 4] Strong p-donating ligands can sufficiently raise the dxz and dyz orbitals in energy to result in an electronic HS configuration (Figure 1, left).[5] However, 2nd and 3rd-row metal ions generally exhibit higher ligand field stabilization and smaller interelectronic repulsion.[6] Hence, HS/IS groundstate configurations are extremely rare. Caulton and co-workers reported the square-planar ruthenium(II) disilylamido PNP pincer complex [RuCl{LSitBu}] (1, see Scheme 1)[7] and some closely related examples.[8] For complex 1, an IS yet non-magnetic ground-state was assigned and attributed to zero-field splitting (ZFS)[9] with an unprecedentedly large axial ZFS

Tridentate, meridionally coordinating “pincer” ligands have attracted considerable attention in recent years owing to their widespread use in catalysis and stoichiometric bond activation reactions.[1] Reactivity control is often accomplished upon tuning the steric shielding imposed by the substituents that [a] Prof. S. Schneider Institut fr Anorganische Chemie Georg-August-Universitt, Tammannstr. 4 37077 Gçttingen (Germany) E-mail: [email protected] [b] Dr. B. Askevold, Dr. M. M. Khusniyarov, Dr. F. W. Heinemann Department Chemie und Pharmazie Universitt Erlangen-Nrnberg Egerlandstr. 1, 91058 Erlangen (Germany) [c] Dr. W. Kroener, Dr. K. Gieb, Prof. P. Mller Department of Physics and Interdisciplinary Center for Molecular Materials Universitt Erlangen-Nrnberg, Erwin-Rommel-Str. 1 91058 Erlangen (Germany) [d] Dr. E. Herdtweck Department Chemie, Technische Universitt Mnchen Lichtenbergstr. 4, 85748 Garching b. Mnchen (Germany) [e] Dr. M. Diefenbach, Prof. M. C. Holthausen Institut fr Anorganische und Analytische Chemie Goethe-Universitt, Max-von-Laue-Str. 7 60438 Frankfurt am Main (Germany) [f] V. Vieru, Prof. L. F. Chibotaru Theory of Nanomaterials Group and INPAC Institute of Nanoscale Physics and Chemistry Katholieke Universiteit Leuven Celestijnenlaan 200F, 3001 Heverlee (Belgium) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404282. Chem. Eur. J. 2014, 20, 1 – 12

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Full Paper emphasized a significant contribution of ruthenium(II) to the ground state.[13] We recently presented a related, square-planar ruthenium(II) dialkylamido complex, [RuCl{L1tBu}] (2, Scheme 1) and its use in small-molecule activation.[14, 15] The temperature-dependent solution NMR data was rationalized with an LS ground state and an energetically low-lying IS excited state. Stabilization of the LS configuration was attributed to the presence of one single-faced, strong p-donor (Figure 1, right). The strikingly different electronic structures of complexes 1 and 2 suggest the possible control by subtle tuning of the PNP pincer ligand electronic properties. Previously, we have presented the functionalization of {L1tBu} by pincer backbone dehydrogenation towards enamido {L3tBu} and dienamido {L4tBu} ligands (Figure 2).[16, 17] In the present study, we report a series of highly unusual, square-planar ruthenium(II) complexes stabilized by a monoanionic PNP ligand family, and discuss the subtle control of pincer donor properties as expressed in the ground state configuration of [RuCl(PNP)].

Figure 1. Schematic d-orbital ligand field splitting for representative square-planar complexes with d6 ions in HS (left), IS (center), and LS (right) configuration.

Figure 2. PNP chelating ligands related to HN(CH2CH2PR2)2.

Scheme 1. Structure of Caulton’s disilylamido pincer complex 1 and syntheses of hydrocarbylamido complexes in this study: a) DT, H2, THF, several weeks; b) 20 8C, KOtBu, THF;[14] c) 5 8C, [KN(SiMe3)2], benzene; d) 78 8C to RT, benzoquinone (1 equiv), THF; e) 78 8C to RT, benzoquinone (2 equiv), THF; f) 78 8C to RT, CO, THF.

Results Syntheses and spectroscopic characterization Thermolysis of the five-coordinate amine complex [RuCl2{HL1tBu}] (3)[14] over several weeks in THF results in H2 elimination from the ligand backbone (Scheme 1). The long reaction time could not be reduced by addition of potential hydrogen acceptors (e.g., acetone, H2C=CHtBu). However, the resulting imine complex [RuCl2{L2tBu}] (4) is selectively obtained in over 80 % yield and was fully characterized. Multinuclear NMR characterization of 4 is in agreement with a C1 symmetric, square-pyramidal molecular structure. Meridional arrangement of the {L2tBu} ligand is indicated by the mutual coupling constant (2JPP = 341 Hz) of the two 31P NMR spectroscopic signals. Dehydrogenation of the ligand backbone is confirmed by

parameter (D = + 273 cm1), which removes spin multiplet degeneracy. The origin and control of such unusual magnetic properties is of great current interest, as the anisotropy along the magnetic easy axis is directly proportional to the energetic barrier that separates the MS = S and MS = 0 states.[9–11] Beyond magnetic applications, the reactivity of a transient, squareplanar PCP ruthenium(II) pincer complex was attributed by Gusev to an IS ground state, based on DFT computations.[12] Recently, Trincado and Grtzmacher proposed a square-planar Ru intermediate in catalytic methanol reforming and &

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Full Paper a characteristic 1H NMR signal at d = 8.16 ppm.[16] The highfield shift of one CH3 1H NMR signal at d = 0.61 ppm suggests CH agostic interactions with the metal center at the vacant coordination site, which is confirmed by single-crystal X-ray diffraction (see below). This observation contrasts with [RuHCl{L2tBu}],[14] which is attributed to the weaker chloride versus hydride trans influence in 4. Complex 4 selectively reacts with [KN(SiMe3)2] to give the deep-purple enamido complex [RuCl{L3tBu}] (5) in 80 % yield (Scheme 1). Deprotonation of such chelating imine to enamide ligands has been described before for several related systems.[2d] The resulting enamido PNP ligand also represents an aliphatic analogue of the dearomatized pyridine-based PNP pincer ligand family that was popularized by Milstein et al.[2b,c] Alternatively, oxidation of amido complex 2 with one equivalent of benzoquinone as the hydrogen acceptor also affords 5. However, the product from double backbone dehydrogenation, that is, dienamido complex [RuCl{L4tBu}] (6), is also obtained as side product on this route and separation of 5 and 6 was not successful. Accordingly, oxidation of 2 with two equivalents of benzoquinone afforded the green dieneamide complex 6 as main product (Scheme 1). Isolation of analytically pure 6 for magnetic characterization (see below) required repeated recrystallization resulting in a low yield (16 %). However, spectroscopically pure (1H NMR spectroscopy) 6, which is suitable for most synthetic purposes, can be isolated in a considerably higher yield (around 40 %). Both complexes 5 and 6 are highly air-sensitive compounds that also need to be stored at low temperatures over an extended time. The IR spectra of 5 and 6 show characteristic absorptions for the backbone C=C double bonds at 1532 (5) and 1524 cm1 (6), respectively, and no indications for the presence of additional hydride ligands. The absence of signals in the 31P NMR spectra and the strongly shifted 1H NMR spectroscopic signals suggest paramagnetism for both complexes. Effective magnetic moments of 2.8 (5) and 2.7 mB (6), respectively, were obtained by the Evans method in solution at room temperature, which is in agreement with S = 1 (spin-only value: 2.83 mB). The 1 H NMR signals are comparatively sharp and their number and

intensities are in agreement with CS (5) and C2v (6) symmetric molecules on the NMR timescale. Curie plots (1H NMR chemical shift vs. T1) for 5 and 6 (the Supporting Information) are linear over the full temperature range (193–333 K) indicating the population of only one electronic state. The UV/Vis characterization of the square-planar complexes 2, 5, and 6 was augmented with TD-DFT results for these compounds in the singlet (S = 0) and triplet (S = 1) states. The experimental spectrum of 2 in THF is dominated by a strong peak at 326 nm (emax = 8720 m1 cm1) and a weak absorption band at 743 nm (emax = 64 m1 cm1). This observation is reasonably well resembled by the calculated spectrum for the S = 0 state that assigns the metal-to-ligand charge transfer (DFT: 300/318 nm) and d–d transitions (DFT: 797 nm), respectively (the Supporting Information), which is in agreement with our previous assignment of a low-spin ground-state based on NMR spectroscopic data.[14] In contrast, the number and absorption maxima of the peaks calculated for S = 1 do not match the experimental data. However, a shoulder at around 380 nm, which is not present in the computed spectrum for S = 0, possibly indicates a minor population of the IS configuration that exhibits a strong peak at 407 nm, according to DFT, which is also in agreement with our previous interpretation.[14] The UV/Vis spectra of 5 and 6 (Figure 3) are well resembled by the computed spectra in the triplet (S = 1) state, whereas the number of peaks and absorption maxima do not match for S = 0 in case of both compounds. The carbonyl complexes [Ru(Cl)(CO){L1tBu}] (7), [Ru(Cl) (CO){L3tBu}] (8), and [Ru(Cl)(CO){L4tBu}] (9) were prepared from the corresponding four-coordinate complexes as spectroscopic probes for the pincer ligand-donor properties (Scheme 1). Stirring of 2, 5, or 6 at low temperature (78 8C) under a CO atmosphere (1 bar) quantitatively yields carbonyl complexes 7–9, respectively. The three diamagnetic complexes exhibit NMR spectra that are in agreement with CS (7 and 9) and C1 (8) symmetry on the NMR timescale, respectively. The two 31P NMR signals of 8 (2JPP = 260 Hz) confirm meridional coordination of the L3 ligand. Characteristic signals in the olefinic region of the 1 H NMR spectra of 8 and 9 are assigned to the vinylic protons

Figure 3. Comparison of experimental (black) and computed (blue: S = 0, red: S = 1; quasi-relativistic TD-DFT B3LYP/ZORA including solvent effects) UV/Vis spectra of complexes 5 (left) and 6 (right) in THF, respectively (for complex 2, see the Supporting Information). Chem. Eur. J. 2014, 20, 1 – 12

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Full Paper (dNCH = 7.18 (8), 7.01 (9); dPCH = 3.95 (8), and 4.10 ppm (9)). The CO stretching frequencies are observed within a range of around 30 cm1 (nCO = 1888 (7), 1896 (8), and 1916 cm1 (9)), which indicates a decreasing metal to CO back-donation within the series 7 > 8 > 9. As a point of reference, compound 7 exhibits a CO stretching vibration that is very close to alkyl complex [RuCl(CO){CH(CH2CH2PtBu2)2}] (nCO = 1887 cm1).[18] This comparison indicates that the weaker expected N!Ru versus C!Ru s-donation is offset by the additional N!Ru p-donation.

amine and amido complexes. As indicated by NMR spectroscopy (see above), a short Ru···HtBu (2.135(1) ) contact at the vacant coordination site suggests a CH agostic interaction with the coordinatively unsaturated metal center in the solid state. The metal ion in complex 6 (measured at 100 K) exhibits square-planar coordination with an ideally linear N1Ru1Cl1 axis (180.0 8) and a typical PNP bite angle P1Ru1P2 of 165.14(2) 8 (Figure 5 and Table 1). The Ru1N1 bond length (1.994(2) ), which is strongly dependent on the electronic configuration (see below), compares better with that of square-planar disilylamide complex 1 (2.050(1) ) rather than that of parent dialkylamide 2 (1.890(2) ).[8a, 14] The planar coordination of the nitrogen atom (sum of angles 360.0 8) and the short C1C2 (1.346(2) ) and N1C1 (1.386(2) ) distances confirm dehydrogenation of both pincer backbone bridges. Comparison with the DFT computed structures of 2 and 6 in the singlet and triplet states (Table 1) indicates that particularly the RuN bond length seems to be a sensitive probe for the spin-state owing to the population of a RuN p*-orbital in the IS configuration (see below and Figure 1). Also, the computed structure of 16 exhibits considerable deviation from a squareplanar structure (NRuCl: 158.1 8) towards a sawhorse (cis-divacant octahedral) conformation, as typically observed for four-coordinate complexes of precious metal d6 ions.[20] Hence, to probe for a possible LS/IS spin-equilibrium, the temperature dependence of the structural parameters of 6 was further examined by single-crystal X-ray diffraction at several temperatures between 30 and 200 K.[22a] However, all bond lengths and angles of 6 were invariant within standard deviation of the structural model (the Supporting Information, Table S2).

Molecular structures The molecular structures of 4–7 were investigated by singlecrystal X-ray diffraction. However, satisfactory structural solutions were obtained only for 4 and 6, whereas crystallographic disorder in the cases of 5 and 7 only allowed for confirmation of the molecular constitution. The five-coordinate imine complex 4 exhibits square-pyramidal coordination of the ruthenium ion with the PNP ligand donor atoms in basal and one chloride in apical positions, respectively (Figure 4). The RuN1 distance in 4 (2.056(1) ) lies between the five-coordinate ruthenium(II) alkylamide [Ru(H)PMe3{L1iPr}] (2.023(1) ) and the amine complex [RuCl2{HL1tBu}] (2.13(2) ), attributed to the smaller coordination number of the imine versus amine nitrogen.[8a, 16b] Formation of an imine complex is also indicated by planar coordination of N1 (sum of bond angles: 359.4 8) and the N1C2 distance (1.28(1) ).[19] All other metal–ligand bond lengths compare well with our previous examples of PNP

Figure 4. Molecular structure of complex 4 from single-crystal X-ray diffraction (thermal ellipsoids drawn at the 50 % probability level); hydrogen atoms are omitted for clarity. Selected bond lengths [] and angles [8]: Ru1 Cl1 2.444(1), Ru1Cl2 2.355(1), Ru1N1 2.056(1), N1C2 1.28(1); N1Ru1Cl1 176.5(1), P1Ru1P2 179.8(2).

Figure 5. Left: Molecular structure of complex 6 from single-crystal X-ray diffraction (thermal ellipsoids drawn at the 50 % probability level); hydrogen atoms are omitted for clarity. Right: Packing diagram of 6 in the crystal (lines denote unit cell).

Table 1. Selected bond lengths and angles of divinyl amido complex 6 (at 100 K) in comparison with the disilyl (1)[7] and dialkyl (2)[14] amido analogues and with computed structures in the singlet and triplet states, respectively.

bond lengths [] RuCl RuN RuP CC/C=C bond angles [8] NRuCl PRuP

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1[7]

2[14]

2DFT (1A’, Cs)

2DFT (3A’, Cs)

6

6DFT (1A’, Cs)

6DFT (3B, C2)

2.361(1) 2.050(1) 2.384(4), 2.376(4) –

2.381(1) 1.890(2) 2.332(5), 2.324(6) 1.513(4), 1.517(4)

2.36 1.89 2.32 1.53

2.42 1.98 2.33 1.53

2.3720(5) 1.994(2) 2.3530(4) 1.346(2)

2.35 1.96 2.35 1.34

2.37 2.03 2.36 1.35

177.9(4) 175.1(2)

179.63(6) 168.13(2)

178.6 168.8

175.4 167.3

180.0 165.14(2)

158.1 165.8

180.0 164.5

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uniaxial and transversal anisotropy by introduction of axial (D) and rhombic (E/D) ZFS of the form given in Equation (1).

Complexes 5 and 6 were examined by SQUID magnetometry on powdered samples and, in case of 6, also as a single crystal. Although 5 and 6 exhibit very similar magnetic properties, the data for 5 could only be satisfactorily fitted assuming a slightly higher level of impurity (around 6 % with S = 1/2; see the Supporting Information) and is therefore not discussed in as much detail as 6. The temperature-dependent magnetic susceptibility data of 6 is depicted in Figure 6. The room temperature value

1 2 H ¼ gmB m0 SH þ D Sz SðS þ 1Þ þ EðS2x S2y Þ 3

The numerical best fit is given by the parameters gx = 2.18, gy = 2.25, gz = 2.65, D = 209 cm1, E = 46.4 cm1. Small amounts of an unidentified, paramagnetic impurity (S = 1=2 ; 1.6 %) were included in the fit to account for the increasing susceptibility at very low temperatures (Figure 6, top left), which should level off to a finite value for a pure j S = 1, MS = 0i ground state.[21] Such extraordinarily large D parameters were also reported for the analogous disilylamido complex 1 (D = + 273 cm1) and for diruthenium (RuII/RuII) and (RuII/RuIII) tetracarboxylato and tetraformamidato paddlewheel complexes from susceptibility measurements.[7, 22] The validity of our model for the magnetic data of 6 was further confirmed by an additional set of temperature-independent, field-dependent magnetization measurements, which were also well reproduced with the same parameter set (Figure 6, bottom left). Finally, the origin of the magnetic behavior, as being attributed to g- and D-anisotropy, was validated by angular resolved single-crystal magnetization measurements at low temperatures (Figure 6, bottom right). Note that in the crystalline state all molecules are aligned coplanar with respect to the squareplane, which defines the molecular geometry (Figure 5), suggesting distinct favorable directions for magnetization. A single micro-crystal was aligned by hand under the optical microscope along three approximately orthogonal axes. Since the error in the alignment of the micro-crystal is rather large, the angle was used as an additional fit parameter within some constraints in Equation (1) and all other parameters were kept constant. Hence, the complete set of data obtained both on powder and single-crystal measurements could be well fitted with a single set of parameters in a self-consistent way within the experimentally accessible range of fields and temperatures. Therefore, this data strongly suggests the physical relevance of these parameters as origin for the strong magnetic anisotropy and confirms the triplet (S = 1) ground state for 6.

Figure 6. Magnetic data of complex 6; lines are the best fits obtained using the spin Hamiltonian [Eq. (1)] and the fit parameters given in the text. Top left: Temperature-dependent magnetic susceptibility measured at different magnetic fields. Top right: cT-Product derived from these data. Bottom left: Magnetization data at different temperatures. Bottom right: Angle-resolved single-crystal magnetization measurement at 1.8 K.

of the cT product at 1.25 cm3 K mol1 is in good agreement with the expected value for a triplet spin-state (S = 1) ion. Furthermore, the cT product shows a significant drop at lower temperatures, which is not expected for a Curie–Weiss paramagnet. Several reasons could be responsible for this drop: 1) Intermolecular magnetic exchange coupling; 2) IS!LS spincrossover upon lowering the temperature, or 3) A large positive axial ZFS parameter, D, which lifts the MS microstate degeneracy and puts the j S = 1, MS = 0i state below j S = 1, MS = 1i resulting in a non-magnetic ground state.[9] The shortest intermolecular RuRu distances for both 5 and 6 are considerably larger than 7 , which renders intermolecular exchange interactions unlikely as origin for the magnetic anisotropy. Further, the variable-temperature (VT) single-crystal X-ray diffraction data of 6 (see above) does not indicate spin-crossover within the experimental temperature range (30–200 K). Nevertheless, a spin-crossover scenario based on the susceptibility data was modeled for 6 (the Supporting Information), yet leading to unphysical parameters, safely excluding spin-crossover. Finally, magnetic susceptibility data could be well fitted using a Heisenberg–Dirac–Van–Vleck Hamiltonian (Figure 6) including Chem. Eur. J. 2014, 20, 1 – 12

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Electronic structure calculations The square-planar complexes 2, 5, and 6 were examined by electronic structure calculations. Preliminary results reported for 2 (DFT, B3LYP/6-31 + G**) indicated a small singlet/triplet excitation energy.[14] Revaluation within the present comprehensive study using density functional theory with several functionals and referencing of these results using coupled cluster calculations on simpler, PR2-truncated models (R = Me, H; the Supporting Information) corroborate this view. Note that the variation of the functional shows the typical behavior, that is, a slight preference for the higher spin-state for hybrid functionals (B3LYP(V)-D/def2-TZVPP: D(ETES) = 0.6 kcal mol1) versus the lower spin-state with pure GGA functionals (B97D/ def2-TZVPP: D(ETES) = + 3.0 kcal mol1).[23] However, CCSD(T) 5


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Full Paper calculations for the PMe2 truncated model favor a singlet ground-state, as well, by D(ETES) = + 5.6 kcal mol1, as was previously proposed on the basis of solution NMR spectroscopic data.[14] In contrast, both 5 and 6 exhibit IS ground-states at all levels of theory, in analogy to Caulton’s disilylamido complex 1.[7] Furthermore, the S/T excitation energies show a clear trend to rise in the order 2!5!6, rendering 6 most stable as the S = 1 spin isomer. Hence, the assignment of singlet (2) and triplet ground states (6 and tentatively 5), respectively, from experimental structural, spectroscopic, and magnetic data are confirmed by the DFT computations. The DFT optimized geometries are in excellent agreement with the calculated metrics in the respective spin states, that is, S = 0 for 2 and S = 1 for 5 and 6 (Table 1). Particularly indicative for this evaluation are the Ru N bond lengths which differ by Dd = 0.07–0.09 for the respective spin isomer pairs. The generally shorter RuN distance in the LS state can be easily rationalized upon analysis of the frontier molecular orbitals. The order of molecular orbitals with predominant metal d character is the same for all three complexes in the respective spin states. Importantly, the LUMO in the LS configuration (Figure 7) exhibits strong RuN p* antibonding character by mixing of the dxz with N(px) (2) and additional C(p) contributions in case of unsaturated ligand backbones (5 and 6). Hence, a short RuN distance with considerable double bond character results for the LS state. Accordingly, in the triplet state the highest SOMO represents this RuN p* antibonding interaction (the Supporting Information), thus weakening the RuN bond. Using this structural parameter as a probe is less clear-cut in case of 6, as the experimental value is almost right between the computed ones. However, strong deviation from square-planar geometry (S = 1: NRuCl 180 8) towards sawhorse geometry was predicted for S = 0 (NRuCl 158.1 8; Table 1), at variance with the experimental finding.

Both fitting of the magnetic data and VT single-crystal X-ray diffraction of 6 (see above) disfavor a spin-equilibrium as explanation for the magnetic properties. To further confirm our model, and particularly the unusually large zero-field splitting parameter, multi-configurational self-consistent field calculations were performed. Initial CASSCF computations gave large positive ZFS parameters for all three complexes, 2, 5, and 6 and the computed values for 6 (D = 251 cm1, j E j = 67 cm1) are close to the results from fitting of the experimental magnetic data (D = 209 cm1, j E j = 46 cm1). Furthermore, non-perturbative computations using the CASSCF/CASPT2/SINGLE_ ANISO approach were performed for 6.[24] Within all approximations (see the Computational Methods) these predict a triplet ground state (the Supporting Information). The best approximation (large basis set/active space with the dynamical correlation energy included) shows that the first excited state is at DE = 3463 cm1, which is several times larger than the spin-orbit coupling constant of Ru2 + (zRuII 1000 cm1). This means that spin-orbit coupling for the ground term becomes operative in the second-order of perturbation theory. On the other hand, DE and z are within the same order of magnitude which, given the large value of z, explains why such a large D value is obtained. In fact, the estimation z2/DE = 290 cm1 is close to the ab initio calculated D parameter (266 cm1). The ab initio calculations deliver a reasonable agreement of the computed g values (gx = 1.97, gy = 2.07, gy = 2.71) and ZFS parameters (D = 266 cm1, j E j = 62 cm1) with the parameters extracted from magnetic data (gx = 2.18, gy = 2.25, gz = 2.65; D = 209 cm1, j E j = 46.4 cm1). The computed magnetic data fit very well the experimental data (Figure 8),[25] which supports the large zero-field splitting and indicates that the magnetic anisotropy is not attributable to unquenched orbital moment. The main magnetic and anisotropy axes in Z direction pass through the Ru-N-Cl axis due to the presence of the C2 axis of symmetry (Figure 9). Note that X and Y anisotropy axes do not coincide with the corresponding main magnetic axes within approximately 10 8, at variance to the simple ligand-field model. A similar observation has been recently documented for a NiII complex.[24]

Discussion The direct comparison of the ruthenium(II) amido (2), -enamido (5), and -dienamido (6) complexes demonstrates the intricate influence of the PNP pincer ligand donor properties on the electronic structure. Compounds 2, 5, and 6 were examined by several methods. We previously proposed for complex 2 an S = 0 ground state and a low-lying IS excited state to account for the NMR spectroscopic data in solution.[14] This interpretation was confirmed in the present study. Structural and UV/Vis spectroscopic data as well as the results of DFT and coupled cluster computations corroborate a LS ground state. In contrast to 2, the room temperature magnetic moment and UV/Vis data of the new complexes 5 and 6 in solution are in agreement with S = 1. However, in analogy to Caulton’s complex 1, a sharp drop in the solid-state magnetic moment at low temperatures is observed. This observation is attributed

Figure 7. DFT computed Kohn–Sham frontier orbitals of complexes 2, 5, and 6 in the singlet state and graphic representation of the respective LUMOs (B3LYP(V)-D/def2-TZVPP).

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Figure 8. Computed versus experimental data of magnetic susceptibility (left) and magnetization at 3.5 K (right, experimental data downscaled by 12 %)[25] for complex 6.

a clear trend concerning the S/T excitation energy, rising from close to zero (2) stepwise by around 6 kcal mol1, respectively, upon dehydrogenation of the pincer backbone to 5 and 6, in favor of the triplet state. Within the simplified 1-electron orbital picture, the LUMOs of the S = 0 states display a RuN p* antibonding contribution, which is in line with the strong dependence of the RuN bond length on multiplicity. Gradually increasing stabilization of this orbital within the series 2!5! 6 arises from mixing with C(p)-orbitals of one (5) or two (6) vinyl moieties (Figure 7). The decreasing HOMO/LUMO gap destabilizes the LS state with respect to the IS electromer. Hence, p-electron acceptance by the vinyl (and silyl) substituents reduces the degree of N!Ru p-donation in favor of an S = 1 ground-state for 5 and 6 (and 1), as opposed to LS 2. This interpretation is further supported by using CO as a spectroscopic probe. The five-coordinate CO complexes 7–9 exhibit a clear trend for the CO stretching vibrations, which shift over around 30 cm1 (Figure 10). The order is in line with decreasing N!Ru p-electron donation upon ligand dehydrogenation within the series 7!8!9. Accordingly, the squareplanar iridium(I) complexes [IrCO{L1iPr}] (nCO = 1908 cm1)[26] and [IrCO{L4tBu}] (nCO = 1937 cm1)[27] exhibit an almost identical shift, suggesting comparable electronic effects of the ligand field that are independent of the metal, oxidation state, and coordination geometries. Direct comparison of the ruthenium series 7–9 with the disilylamido pincer system is not possible, as [RuCl(CO){LSiR}] was not reported. However, the iridium complex [IrCO{N(LSiiPr)2}] (nCO = 1930 cm1)[28] comes close to [IrCO{L4tBu}] (Figure 10), suggesting similar donor properties of the divinyl- and disilylamido pincer ligands as anticipated on the basis of the triplet ground state of 1.

Figure 9. The orientation of ab initio calculated main magnetic axes (Xm, Ym, Zm) and main anisotropy axes (Xa, Ya, Za) for complex 6.

to large ZFS, which leads to an essentially non-magnetic ground state, whereas a IS/LS spin-transition is excluded for 6 on the basis of the powder and single-crystal magnetic data, VT structural parameters and computations at the DFT, CASSCF and CASPT2 levels of theory. The latter also confirm the second order contribution, that is, out-of-state angular momentum, that arises from splitting of the S = 1 ground term into three almost equidistant energy levels through mixing with excited states through spin-orbit coupling as origin for the unusually high magnetic anisotropy of 6 (and presumably 1 and 5). These results suggest that the square-planar coordination geometry, as enforced by the bulky pincer ligand, affords the accessibility of non-LS ground-states also for 4d metal ions with large spin-orbit coupling as a strategy to obtain high magnetic anisotropy. Furthermore, the preference for LS (2) versus IS (5, 6, and 1) ground-states suggests the subtle tunability of the pincer ligand field within this series. Our computations indicate Chem. Eur. J. 2014, 20, 1 – 12

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Conclusion The present work demonstrates that subtle functionalization of the pincer backbone can be utilized to tune the N!M p-donation. In this specific case, application to the extremely rare class of compounds with 4/5d6 ions in square-planar 7


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Full Paper SQUID magnetometer (MPMS-XL manufactured by QuantumDesign). Diamagnetic corrections for the sample holder were applied. Analysis of the magnetic data was performed with the computer program DAVE.[29]

Syntheses Preparation of [RuCl2{L2tBu}] (4): [RuCl2{HL1tBu}] (3, 0.150 g; 0.281 mmol) was suspended in THF (10 mL) and heated under reflux for four weeks. The solvent was removed under vacuum, and the resulting solid dissolved in methylene chloride and precipitated with pentane. The solution was filtered off and the pale-grey solid washed with pentane (2 10 mL) and dried under vacuum: Yield: 0.126 g (0.173 mmol, 84 %). 1H NMR (CD2Cl2, 230 K, 399.8 MHz): d = 8.16 (d, 3JHH = 21.7 Hz, 1 H, NCH), 4.23–4.07 (m, 1 H, NCH2), 3.78–3.74 (m, 1 H, NCH2), 3.36 (dd, 2JHH = 7.4, 2JHH = 17.3 Hz, 1 H, PCH2CH), 2.94–2.83 (m, 1 H, PCH2CH), 2.49 (m, 1 H, PCH2), 2.05 (m, 1 H, PCH2), 1.52 (d, 3JHP = 13.1 Hz, 9 H, CH3), 1.32 (d, 3JHP = 12.5 Hz, 9 H, CH3), 1.13 (d, 3JHP = 12.2 Hz, 9 H, CH3), 0.61 ppm (br s, 9 H, CH3); 13 1 C{ H} NMR (100.53 MHz): d = 176.1 (dd, 2JCP = 6.5, 3JCP = 1.0 Hz, NCH), 66.1 (dd, 2JCP = 5.3, 3JCP = 1.6 Hz, NCH2), 32.4 (dd, 1JCP = 11.9, 3 JCP = 2.3 Hz, PCH2CH), 29.0 (m, CH3), 23.2 ppm (dd, 1JCP = 15.7, 3 JCP = 1.7 Hz, PCH2), (PC(CH3)3) not detected due to low solubility; 31 1 P{ H} NMR (161.83 MHz): d = 62.0 (d, 2JPP = 341.2 Hz), 38.1 ppm (d, 2 JPP = 340.8 Hz); IR: n˜ = 1616 (C=N) cm1; elemental analysis calcd (%) for C20H43Cl2NP2Ru (531.49): C 45.20; H 8.15; N 2.64; found: 44.65; H 8.59; N 2.41.

Figure 10. Comparison of CO stretching vibrations of PNP pincer carbonyl compounds.

coordination geometry afforded the control of the electronic structure and ultimately the sign and size of the S/T separation energy, giving rise to unusually large magnetic anisotropy for examples with IS ground-states. In the light of the recent popularity of such pincer ligands in catalysis and sensing, this study provides guidelines for future pincer ligand design.

Preparation of [RuCl(L3tBu)] (5): A mixture of 4 (42.8 mg, 0.080 mmol) and [KN(SiMe3)2] (20.9 mg, 0.080 mmol, 1.0 equiv) was cooled to 5 8C and dissolved in benzene (5 mL). Immediately, the suspension turned dark blue. After stirring for 5 min, the solution was filtered of and the white residue extracted twice with benzene. The solvent was removed under vacuum affording 5 in high yield (32.7 mg, 0.066 mmol, 82 %). 1H NMR (C6D6, RT, 399.8 MHz): d = 459 (s, CH2, 2 H), 48.7 (s, CH, 1 H), 0.69 (s, CH3, 18 H), 7.69 (s, CH3, 18 H), 15.34 (s, CH2, 2 H), 275 ppm (s, CH, 1 H). No signal was found by 31P NMR spectroscopy; IR: n˜ = 1532 cm1 (C=C); elemental analysis calcd (%) for C20H42ClNP2Ru (495.02): C 48.53; H 8.55; N 2.83; found: C 48.94; H 8.40; N 2.85. Preparation of [RuCl{L4tBu}] (6): THF (5 mL) was added to a mixture of 3 (50.0 mg, 0.10 mmol) and benzoquinone (21.7 mg, 0.20 mmol, 2.0 equiv) at 78 8C. The suspension immediately turned dark purple. After stirring for 30 min, the reaction mixture was slowly warmed to RT. The color changes to black and the solvent is removed under vacuum after 30 min. The residue is extracted with Et2O (4 5 mL), the solvent removed under vacuum and the crude product extracted with pentanes (3 5 mL). Repeated crystallization form pentanes at 35 8C affords analytically pure, dark green 6. Yield: 7.8 mg (0.015 mmol, 16 %). 1H NMR (C6D6, RT, 399.8 MHz): d = 44.1 (s, CH, 2 H), 5.70 (s, CH3, 36 H), 189 ppm (s, CH, 2 H). No signal was found by 31P NMR spectroscopy; IR: n˜ = 1524 cm1 (C=C); elemental analysis calcd (%) for C20H40NClNP2Ru (493.01): C 48.72; H 8.18; N 2.84; found: C 48.41; H 8.19; N 2.85.

Experimental Section Materials and methods All experiments were carried out under an argon atmosphere by using Schlenk and glovebox techniques. Solvents were dried over Na/benzophenone/tetraglyme (benzene), Na/benzophenone (THF), distilled under argon and deoxygenated prior to use. Acetone, pentane and Et2O were dried by passing through columns packed with activated alumina. Deuterated solvents were obtained from Euriso-Top GmbH, dried over Na/K ([D8]THF, C6D6) or CaH2 (CD2Cl2), distilled by trap-to-trap transfer in vacuo, and degassed by three freeze-pump-thaw cycles, respectively. KOtBu (VWR) and 1,4-benzoquinone (Merck) were sublimed prior to use and ruthenium(III) chloride hydrate (ABCR) used as purchased. Compounds 2 and 3 were prepared according to published procedures.[14]

Analytical methods Elemental analyses were obtained from the Microanalytical Laboratory of Technische Universitt Mnchen. IR spectra were recorded as Nujol mulls between KBr plates. NMR spectra were recorded on a Bruker Avance III 400 spectrometer and calibrated to the residual proton resonance and the natural abundance 13C NMR resonance of the solvent (CD2Cl2 : dH = 5.32 and dC = 54.0 ppm; C6D6 : dH = 7.16 and dC = 128.1 ppm; [D8]THF: dH = 3.58 and dC = 67.57 ppm). 31 P NMR chemical shifts are reported relative to external phosphoric acid (dP = 0.0 ppm). Signal multiplicities are abbreviated as: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), or br (broad). The magnetic data was obtained with a commercial

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Preparation of [RuCl(CO){L1tBu}] (7): A solution of 2 (36.7 mg, 0.074 mmol) in THF (5 mL) was frozen at 77 K, evacuated, and backfilled with CO. The solution was slowly warmed to 50 8C and stirred for 15 min. The solvent is removed under vacuum at low temperature giving red 7. Yield: 38.1 mg (0.073 mmol, 98 %). 1 H NMR (C6D6, RT, 399.8 MHz): d = 3.04–2.85 (m, 2 H, NCH2), 2.68– 2.61 (m, 2 H, NCH2), 1.68–1.48 (m, 4 H, PCH2), 1.38 (A9XX’A’9, N = j 3 JHP + 5JHP j = 13.4 Hz, 18 H, CH3), 1.32 ppm (A9XX’A’9, N = j 3JHP + 5 JHP j = 13.4 Hz, 18 H, CH3); 13C{1H} NMR (C6D6, RT, 100.53 MHz): d = 214.4 (br, CO), 66.7 (AXX’A’, N = j 2JCP + 3JCP j = 10.2 Hz, NCH2), 37.0

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Full Paper 1calcd = 1.352 gcm3, T = 100(2) K, F(000) = 516; q range = 1.94– 29.248; data collected: 23235; independent data [Io > 2s(Io)/all data/Rint]: 3136/3136/0.0401; data/restraints/parameter: 3136/0/ 121; R1 [Io > 2s(Io)/all data]: 0.0226/0.0261; wR2 [Io > 2s(Io)/all data]: 0.0576/0.0605; GOF = 1.050; D1max/min : 0.828/0.677 e 3. For detailed information, see the Supporting information. CCDC-951657 (4), CCDC-1009809 (6@30 K), CCDC-1009810 (6@50 K), CCDC1009811 (6@60 K), CCDC-1009812 (6@80 K), CCDC-1009813 (6@100 K), CCDC-1009814 (6@150 K), and CCDC-1009815 (6@200 K) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

(A2XX’A’2, N = j 1JCP + 3JCP j = 16.8 Hz, PCCH3), 36.8 (A2XX’A’2, N = j 1 JCP + 3JCP j = 15.4 Hz, PCCH3), 29.6 (A6XX’A’6, N = j 1JCP + 3JCP j = 4.0 Hz, CH3), 29.2 (A6XX’A’6, N = j 1JCP + 3JCP j = 5.0 Hz, CH3), 24.9 ppm (AXX’A’, N = j 1JCP + 3JCP j = 16.6 Hz, PCH2); 31P{1H} NMR (C6D6, RT, 161.83 MHz): d = 87.8 ppm (s); IR: n˜ = 1888 cm1 (CO); elemental analysis calcd (%) for C21H44ClNOP2Ru (525.05): C 48.04; H 8.45; N 2.67; found: C 48.03; H 8.62; N 2.55. Preparation of [RuCl(CO){L3tBu}] (8): A solution of 5 (10 mg, 0.020 mmol) in THF was set under an atmosphere of CO at 50 8C and stirred for 15 min. Removal of the solvent under vacuum afforded the violet complex 8. Yield: 10.5 mg (0.020 mmol, 100 %). 1 H NMR (C6D6, RT, 399.8 MHz): d = 7.18 (dd, 3JHH = 40.0 Hz, 3JHP = 4.0 Hz, 1 H, NCH), 4.00–3.95 (m, 1 H, PCH), 3.16 (ddd, 2JHH = 36.0 Hz, 3 JHH = 8.0 Hz, 3JHH = 12.0, Hz, 1 H, NCH2), 2.98–2.89 (m, 1 H, NCH2), 1.46 (d, 3JHP = 13.7 Hz, 9 H, CH3), 1.38 (d, 3JHP = 12.9 Hz, 10 H, CH3 and PCH2), 1.24 (d, 3JHP = 13.0 Hz, 10 H, CH3 and PCH2), 1.17 ppm (d, 3 JHP = 12.9 Hz, 9 H, CH3); 13C{1H} NMR (100.53 MHz): d = 208.9 (dd, 2 JCP = 13.8 Hz, 2JCP = 10.6 Hz, CO), 169.4 (dd, 3JCP = 15.7 Hz, 3JCP = 2.5 Hz, NCH), 80.9 (d, 1JCP = 40.2 Hz, PCH), 61.8 (d, 2JCP = 5.4 Hz, NCH2), 40.3 (dd, 1JCP = 16.2 Hz, 3JCP = 3.0 Hz, PCCH3), 36.6 (dd, 1JCP = 12.0 Hz, 3JCP = 1.9 Hz, PCCH3), 36.0 (dd, 1JCP = 19.0 Hz, 3JCP = 3.6 Hz, PCCH3), 35.6 (dd, 1JCP = 11.3 Hz, 3JCP = 4.5 Hz, PCCH3), 29.8 (d, 2JCP = 3.6 Hz, CH3), 29.1 (d, 2JCP = 3.8 Hz, CH3), 28.9 (d, 2JCP = 3.7 Hz, CH3), 28.8 (d, 2JCP = 3.1 Hz, CH3), 25.1 ppm (d, 1JCP = 17.9 Hz, PCH2); 31 1 P{ H} NMR (161.83 MHz): d = 78.6 (d, 2JPP = 259.9 Hz), 72.0 ppm (d, 2 JPP = 259.8 Hz); IR: n˜ = 1896 (CO), 1542 cm1 (C=C); elemental analysis calcd (%) for C21H42ClNOP2Ru (523.03): C 48.22; H 8.09; N 2.68; found: C 48.19; H 7.62; N 2.62.

Computational methods DFT computations: Geometries, singlet-triplet splittings, and UV/ Vis spectra were computed using the Gaussian,[30] Molpro,[31] and ORCA[32] programme packages, respectively. Molecular structures were optimized at the density functional level of theory (DFT) employing the B3LYP(V)-D[33] hybrid functional, which incorporates the VWN(V)[34] local correlation functional, and makes use of Grimme’s empirical dispersion correction.[35] The def2-TZVPP basis set[36] was used for the description of the one-particle space. For Ru, the [Ar]3d10 core was replaced by the quasi-relativistic 28-electron small-core pseudo-potential (ECP28MWB in the common jargon) of Andrae et al.[37] An ultrafine grid with 99 radial shells and 590 angular points per shell was applied for the numerical integration and tight convergence criteria in the geometry optimization procedure. Hessians were computed analytically from harmonic frequency calculations to ensure that the obtained structures correspond to minima on the potential energy surface. For the calculation of the UV/Vis spectra, the B3LYP(V) functional within the time-dependent DFT framework[38] as implemented in ORCA was employed using the B3LYP(V)-D/def2-TZVPP geometries. Relativistic effects were treated at the all-electron level with the ZORA method[39] in conjunction with the corresponding re-contracted def2-TZVPP basis sets,[40] employing the model potential approach of van Wllen.[41] “Picture change effects”[42] were also included as a correction within the scalar relativistic framework. The density-fitting procedure was used within the RIJCOSX approximation[43] in conjunction with the corresponding def2-Coulomb fit basis sets.[44] Large integration grids (Grid6 and GridX6 in ORCA convention) were used. Solvent effects arising from tetrahydrofurane (THF) were accounted for through the implicit conductor-like screening model (COSMO).[45, 46] Coupled-cluster computations: Singlet-triplet excitation energies were computed based on B3LYP(V)-D/def2-TZVPP geometries at the explicitly correlated coupled-cluster level with single and double excitations and perturbative triple excitations (CCSD(T)F12),[47] utilizing the fixed-amplitude ansatz in conjunction with the CCSD(T)-F12b method.[48, 49] In open-shell calculations the spin-unrestricted coupled-cluster formalism based on restricted open-shell Hartree–Fock reference functions was used. The hierarchy of augmented correlation-consistent orbital basis sets, aug-cc-pVnZ (n = D, T), from Dunning[50] was employed for all non-metal atoms, and the aug-cc-pVnZ-PP set[51] along with the small-core relativistic pseudo-potential (ECP28MDF) of Peterson et al.[51] for the Ru center. The F12 methods produce nearly converged results already with basis sets of double-zeta quality.[49] The OptRI auxiliary basis sets[52] of respective size were used as the RI basis for the manyelectron integrals within the complementary auxiliary basis set (CABS) approach as implemented in Molpro. In the case of ruthenium, the aVnZ/JKfit auxiliary basis set (ABS) of Weigend (identical to

Preparation of [RuCl(CO){L4tBu}] (9): A solution of 6 (10 mg, 0.020 mmol) in THF was set under an atmosphere of CO at 50 8C and stirred for 15 min. After removal of the solvent under vacuum, compound 9 was crystallized form pentanes. Yield: 8.3 mg (0.020 mmol, 79 %). NMR (C6D6, RT, ppm) 1H NMR (C6D6, RT, 399.8 MHz): d = 7.00 (AMXX’, N = j 3JHH + 3JHP + 4JHP j = 42.4 Hz, 2 H, NCH), 4.10–4.07 (m, 2 H, PCH), 1.38 (A9XX’A’9, N = j 3JHP + 5JHP j = 14.1 Hz, 18 H, CH3), 1.21 ppm (A9XX’A’9, N = j 3JHP + 5JHP j = 13.7 Hz, 18 H, CH3); 13C{1H} NMR (100.53 MHz): d = 205.1 (br, CO), 163.8 (AXX’A’, N = j 2JCP + 3JCP j = 17.0 Hz, NCH), 86.0 (AXX’A’, N = j 1JCP + 3 JCP j = 37.6 Hz, PCH), 38.2 (A2XX’A’2, N = j 1JCP + 3JCP j = 19.2 Hz, PCCH3), 36.3 (A2XX’A’2, N = j 1JCP + 3JCP j = 20.6 Hz, PCCH3), 29.9 ppm (A6XX’A’6, N = j 1JCP + 3JCP j = 5.0 Hz, CH3), 28.6 ppm (A6XX’A’6, N = j 1 JCP + 3JCP j = 5.6 Hz, CH3); 31P{1H} NMR (161.83 MHz): d = 69.4 ppm (s); IR: n˜ = 1917 cm1 (CO), 1528 (C=C); elemental analysis calcd (%) for C21H40ClNOP2Ru (521.02): C 48.41; H 7.74; N 2.69; found: C 48.62; H 7.51; N 2.62.

X-ray crystal structure determinations Compound 4: Crystal data: Formula: C20H43Cl2NP2Ru; Mr = 531.46; crystal color and shape: red brown fragment, crystal dimensions: 0.05 0.05 0.05 mm; crystal system: monoclinic; space group: P21 (no. 4); a = 7.2261(3), b = 14.6172(7), c = 11.8768(6) , b = 90.517(2)8, V = 1254.44(10) 3, Z = 2, l(MoKa) = 0.71073 , m = 0.971 mm1, 1calcd = 1.407 g cm3, T = 123(1) K, F(000) = 556; q range = 1.71–25.39 8; data collected: 36893; independent data [Io > 2s(Io)/all data/Rint]: 4526/4512/0.045; data/restraints/parameter: 4526/1/247; R1 [Io > 2s(Io)/all data]: 0.0167/0.0168; wR2 [Io > 2s(Io)/ all data]: 0.0393/0.0393; GOF = 1.071; D1max/min : 0.46/0.28 e 3. Compound 6 (at 100 K): Crystal data: Formula: C20H40ClNP2Ru; Mr = 492.99; crystal color and shape: green block, crystal dimensions: 0.50 0.36 0.33 mm; crystal system: monoclinic; space group: P2/ c; a = 11.4118(3), b = 8.6063(2), c = 13.3913(3) , b = 112.947(1) 8, V = 1211.13(5) 3, Z = 2, l(MoKa) = 0.71073 , m = 0.894 mm1, Chem. Eur. J. 2014, 20, 1 – 12

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Full Paper def2-QZVPP/JKfit)[53] was used instead. For the density-fitting of the Fock and exchange matrices (JKfit) as well as of the remaining integrals (MP2fit), the respective aVnZ/JKfit and aVnZ/MP2fit auxiliary sets were employed. The JKfit auxiliary sets are always the def2QZVPP/JKfit ABS of Weigend.[53] The aVnZ/MP2fit ABS for the nonmetal atoms are from Weigend et al.,[54] that for ruthenium is due to Hill and Platts.[55] CASSCF/CASPT2 computations: The CASSCF/CASPT2/SO-RASSI/ SINGLE_ANISO calculations for complex 6 were performed with the Molcas 7.8 program package using the molecular geometry obtained from X-ray diffraction.[56] The Cholesky decomposition threshold was set to 1 108. Three different basis sets have been employed (the Supporting Information, Table S5) to check the stability of the results obtained. The calculations were carried out within different active spaces. All quintet, triplet, and singlet states were mixed by spin-orbit coupling within the RASSI program. To account for the dynamical correlation energy, CASPT2 calculations were done for the low-lying spin terms.[57] To avoid intruder states the imaginary shift was set to 0.1. On the basis of the obtained spin-orbit multiplets the magnetic properties were calculated with the SINGLE_ANISO program.[58]

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Acknowledgements S.S. thanks the Deutsche Forschungsgemeinschaft for a grant within the Emmy-Noether Program (SCHN950/1). M.M.K. is grateful to the Fonds der Chemischen Industrie and Deutsche Forschungsgemeinschaft. Quantum-chemical calculations were performed at the Center for Scientific Computing (CSC) Frankfurt on the LOEWE-CSC high-performance computer cluster. V.V. and L.C. acknowledge the support of the Flemish Science Foundation (FWO) and of INPAC and Methusalem programs of the University of Leuven. Keywords: computer chemistry · ligands properties · NMR spectroscopy · ruthenium

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FULL PAPER & Pincer Complexes B. Askevold, M. M. Khusniyarov, W. Kroener, K. Gieb, P. Mller, E. Herdtweck, F. W. Heinemann, M. Diefenbach, M. C. Holthausen, V. Vieru, L. F. Chibotaru, S. Schneider* && – && Square-Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization

The pincer makes a spin: Two examples of extremely rare ruthenium(II) complexes in a square-planar geometry are reported (see figure). Spectroscopic,

magnetic, and computational examination reveals that the ground-state spin multiplicity can be controlled by subtle PNP pincer backbone functionalization.

The electronic structures……within a series of squareplanar ruthenium(II) pincer complexes, with dialkyl-, vinyl alkyl-, and divinyl amido pincer backbones is compared in an experimental and computational study. While [RuCl{N(CH2CH2PtBu2)2}] exhibits a low-spin (S = 0) ground state and low-lying triplet (S = 1) excited state, [RuCl{N(CH2CH2PtBu2)(CHCHPtBu2)}] and [RuCl{N(CHCHPtBu2)2}] adopt intermediate-spin (S = 1) ground states with strong magnetic anisotropy due to large zero-field splitting (D > + 200cm–1). The change in ground-state electronic configuration is attributed to tailored pincer ligand-to-metal p-donation within the PNP ligand series.

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A high-spin square-planar Fe(ii) complex stabilized by a trianionic pincer-type ligand and conclusive evidence for retention of geometry and spin state in solution.

Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand.

O2 activation by metal-ligand cooperation with Ir(I) PNP pincer complexes.

Four-coordinate cobalt pincer complexes: electronic structure studies and ligand modification by homolytic and heterolytic pathways.

Bimetallic iron-iron and iron-zinc complexes of the redox-active ONO pincer ligand.

On the stability of a POCsp3OP-type pincer ligand in nickel(II) complexes.

Introducing a new azoaromatic pincer ligand. Isolation and characterization of redox events in its ferrous complexes.

Trianionic pincer and pincer-type metal complexes and catalysts.

Metal effects on ligand non-innocence in Group 5 complexes of the redox-active [ONO] pincer ligand.

Effects of Ligand Halogenation on the Electron Localization, Geometry and Spin State of Low-Coordinate (β-Diketiminato)iron Complexes.

Control of the spin state by charge and ligand substitution: two-step spin crossover behaviour in a novel neutral iron(II) complex.

Ligand-metal cooperating PC(sp3)P pincer complexes as catalysts in olefin hydroformylation.

Spin-state ordering in hydroxo-bridged diiron(III)bisporphyrin complexes.

Carbon dioxide hydrosilylation promoted by cobalt pincer complexes.

Predicting the spin state of paramagnetic iron complexes by DFT calculation of proton NMR spectra.

An NCN-pincer ligand dysprosium single-ion magnet showing magnetic relaxation via the second excited state.

Benzo annulated cycloheptatriene PCP pincer iridium complexes.

Crystal structures of a novel NNN pincer ligand and its dinuclear titanium(IV) alkoxide pincer complex.

Synthesis and Reactivity of Four- and Five-Coordinate Low-Spin Cobalt(II) PCP Pincer Complexes and Some Nickel(II) Analogues.

System with potential dual modes of metal-ligand cooperation: highly catalytically active pyridine-based PNNH-Ru pincer complexes.

Electron spin resonance studies of the solution structure of vanadyl amino acid complexes and mixed ligand complexes of oxalate.

Pincer-Type Platinum(II) Complexes Containing N-Heterocyclic Carbene (NHC) Ligand: Structures, Photophysical and Anion-Binding Properties, and Anticancer Activities.

Synthesis and catalytic activities of porphyrin-based PCP pincer complexes.

Spin crossover in Fe(II) and Co(II) complexes with the same click-derived tripodal ligand.