Evidence for the Formation of Anionic Zerovalent Group 10

Article pubs.acs.org/Organometallics

Evidence for the Formation of Anionic Zerovalent Group 10 Complexes as Highly Reactive Intermediates Alexander Seyboldt,† Barbara Wucher,† Silvia Hohnstein,† Klaus Eichele,† Frank Rominger,‡ Karl W. Törnroos,§ and Doris Kunz*,† †

Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 250, D-69120 Heidelberg, Germany § Department of Chemistry, University of Bergen, Allégaten 41, 5007 Bergen, Norway ‡

S Supporting Information *

ABSTRACT: The in situ generated CNC pincer lithium complex [Li(bimca)] (2) (bimca = bis(3-methylimidazolin-2ylidene)carbazolide) reacts with M = Pt(0), Pd(0), and Ni(0) precursors under formal oxidative addition to (bimca)hydrido M(II) complexes 3. This unusual reaction involves proton abstraction that can derive from various sources including the ligand itself. Mechanistic considerations are given. The respective [M(bmica)Cl] complexes 4 have been prepared from the hydrido complexes 3 and subjected to reduction in order to identify a proposed zerovalent anionic [M(bimca)]− complex. In the case of M = Pt, only the (bimca)hydrido Pt(II) complex is observed, whereas, for [Pd(bimca)Cl], a dimeric Pd(0) complex [Pd2(bimca)2]K2 (6) bearing an anionic carbazole moiety is formed in the solid state. NMR DOSY experiments show that, in solution, this dimer dissociates to the monomeric anionic complex [Pd(bimca)]− (5b). We conclude that such anionic zerovalent complexes are plausible intermediates in the synthesis of group 10 metal(II) complexes from the respective metal(0) species with anionic CNC ligands.

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INTRODUCTION The reaction of anionic ligands with electron-rich zerovalent group 10 metals would lead to extremely electron-rich and reactive zerovalent anionic group 10 complexes. Palladium complexes of this type are discussed as the reactive intermediate in the oxidative addition step to arylhalides in C−C crosscoupling reactions and were generated electrochemically by Jutand and Amatore.1 The in situ generation of an anionic Pt(0) complex by deprotonation of a Pt(II) hydrido complex was reported by Reger and Ding,2 and the isolation of an anionic Pt(0) pincer complex of type A (Chart 1, M = Pt, L =

carbene, X = N-carbazole). The so-called bimca (bis(3methylimidazolin-2-ylidene)carbazolide) ligand shows a strong electron-donating character due to the N-heterocyclic carbene moieties, which is reflected in the very low CO stretching frequency of its Rh complexes and their high nucleophilicity.4 In this work, we describe the synthesis of goup 10 metal hydrido complexes and the mechanism of their formation that proceeds presumably via anionic, zerovalent group 10 metal complexes when starting from [Li(bimca)] (2) as well as the independent generation of such [M(bimca)]− complex anions.

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RESULTS AND DISCUSSION Reaction of [Li(bimca)] with M-d10 Species. We reacted [Li(bimca)] (2), in situ generated from the bisimidazolium salt 1 and 3 equiv methyllithium, with a substoichiometric amount of [Pt(PPh3)4] at 40 °C for 7 days under an inert atmosphere (Scheme 1). Surprisingly, we did not observe formation of an anionic Pt(0) complex, but rather the hydrido-Pt(II) complex 3a, which could be isolated in 53% yield. Due to the long reaction time, we suspected slow diffusion of water into the reaction mixture being the source of the hydrido ligand upon deprotonation. However, in a sealed NMR tube, the reaction in THF-d8 took place as well, leading to the nondeuterated product 3a exclusively. Therefore, other proton sources like the

Chart 1

phosphine, X = CAryl) obtained by reduction of the respective Pt(II) chloride complex has recently been reported by Milstein and co-workers,3 albeit not yet structurally characterized. In the case of anionic pincer ligands, complexes of type A or B, or an intermediate bonding situation of the counterion can be envisaged (Chart 1). A few years ago, we introduced the highly electron-donating carbazole based anionic CNC pincer ligand (L = N-heterocyclic © XXXX American Chemical Society

Received: August 19, 2014

A

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Organometallics Scheme 1. Syntheses of Group 10 Metal(II) Hydride Complexes from Metal(0) Precursora

a

2*HBF4 is exemplary for a mixture of 2, 1, and 2*HBF4.

Using only 2 equiv of methyllithium for deprotonation of bisimidazolium salt 1, the reaction also proceeded well with [Pt(PPh3)4]; however, no pronounced acceleration of the reaction was observed. This shows that the rate-determining step of the reaction is not the C−H activation step when using [Li(bimca)] (2). As no intermediate could be observed during the course of the reactions, we suppose that formation of anionic species like A or B is the rate-determining step. In the case of in situ formation of 2*HBF4, the anionic M(0) complex would bear a κ2 chelating ligand moiety (via C and N) plus one imidazolium moiety. A M(0) complex containing a κ2 bound bimca ligand must be a very basic compound, due to the high electron-donating ability of the carbene moiety and an additional coordination of the anionic carbazole nitrogen. Therefore, a deprotonation of the remaining imidazolium moiety is our preferred mechanism as it should be the fastest reaction, albeit a C−H activation as demonstrated by Thomas et al. cannot be excluded so far.12 Using the fully protonated bisimidazolium salt 1 under the same conditions did not lead to formation of the hydrido complexes in the case of Pd and Pt. Even prolonged heating at 100 °C did not result in a reaction in the case of platinum and lead to decomposition of the metal precursor in the case of palladium. In an optimized synthesis of the complexes, a mixture of 2 and 2*HBF4 is generated by using only 2 equiv of base and the metal precursor is added. The mixture is then stirred for 4 days at ambient temperature using [Ni(COD)2] as reactant and for 4 days at 40 °C in the case of [Pd(PPh3)4] or [Pt(PPh3)4]. Except for the very sensitive [Ni(bimca)H] complex (3c) that decomposes after a few days in the glovebox freezer (−30 °C), the raw product was purified at neutral Alox to obtain the Pt-H complex 3a in 80% yield and the Pd-H complex 3b in 73% yield. All complexes can easily be identified by their 1H NMR spectrum with the characteristic signal of the hydride ligand at δ = −17.56 (Ni), −10.77 (Pd), and −12.59 (Pt). The latter signal shows the expected 195Pt satellites with a coupling of 1JPtH =

bimca ligand itself or the released triphenylphosphine ligand have to be taken into account. Usually, the synthesis of group 10 hydrido complexes is accomplished by ligand exchange of the respective divalent halide complexes.5 Preparation of a group 10 hydrido complex from zerovalent complexes in a formal oxidative addition step was demonstrated by Stone and co-workers for Pt complexes with perfluoroarenes under C−H activation,6 but also shown for NH bonds by Jonas and Wilke7 already back in 1969 for Ni(0), by Maitlis and co-workers8 at 100 °C for Pt(0) for carbazole, and by Ozerov et al. for PNP pincer ligands with Pt(0), Pd(0), and Ni(0).9 We conducted this experiment also with [Pd(PPh3)4] and with [Ni(COD)2]. The reaction proceeds easier with the Pd(0) than the Pt(0) complex. After stirring at 40 °C for 4 days, the Pd-H complex could be obtained in a yield of 45% after workup over neutral Alox. In the case of the Ni(COD)2, 30% of the extremely sensitive Ni-H complex 3c could be isolated in a small-scale experiment (33 μmol) after stirring for 5 days at room temperature.10 After generation of [Li(bimca)] complex 2, the reaction with a metal(0) species would lead to a very basic anionic species A or B (Chart 1). However, if deprotonation of the bisimidazolium salt 1 was incomplete and 2*HBF4 was formed, deprotonation of the remaining imidazolium moiety could occur very easily (either intra- or intermolecularly) after formation of an anionic M(0) species. This would lead to M(II)-hydrido complexes by concomitant formation of the carbene moiety. According to the literature, carbazole (pKa = 19.9)11a is more acidic than an imidazolium salt (pKa = 24)11b in DMSO. In a 1H NMR experiment, the deprotonation of imidazolium salt 1 was carried out with only 2 equiv of methyllithium. The spectrum showed the signals for the fully deprotonated species 2 as the main product in solution; however, a white precipitate remained. 7Li NMR showed only one signal at −0.51 ppm. We, therefore, assume that [Li(bimca)] (2) and non- or partially deprotonated species like 2*HBF4 coexist in the suspension. B

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raw data by applying the Squeeze procedure in PLATON.14 The pincer and the hydrido ligand form a square planar coordination around the metal center. For compound 3a, the Pt−H distance was fixed at 1.590 Å, and for 3b at 1.565 Å, based upon DFT calculations. The carbene metal bonds show typical lengths of 2.022(6) and 2.032(3) Å for Pt and Pd, respectively, and metal nitrogen bond lengths of 2.010(5) Å (Pt−N) and 2.019(2) Å (Pd−N). Following a literature9 procedure, the hydrido complexes 3 could easily be converted into the respective chloride complexes 4a and 4b by reaction in chloroform at 50 °C for 11 days (Pt) or 16 h (Pd) in high yields after isolation. The Ni complex 4c could be obtained at room temperature after in situ generation of Ni-H complex 3c and addition of chloroform after a reaction time of only 10 min in a 48% isolated yield. Compared to the 13C NMR spectrum of the hydrido complexes 3, the carbene signals of 4 are shifted upfield by 10−15 ppm (159.9 (Pt) and 163.8 ppm (Pd)), and the carbene signal of NiCl complex 4c is detected at 161.5 ppm. All complexes are thermally very stable and decompose above 345 °C (Pt), 340 °C (Pd), and 312 °C (Ni). Proof for the mononuclear structure of the Pt-Cl complex 4a and the Pd-Cl complex 4b was obtained from single-crystal Xray structure analyses (Figure 2). The asymmetric unit of the

1166 Hz. Additional 195Pt satellites can be observed at the doublets (4JHH = 2.2 Hz) of the imidazolinylidene moiety at 7.47 ppm (H-4′) (4JPtH = 26 Hz) and 8.35 ppm (H-5′) (shoulder). A 1H,195Pt HSQC experiment reveals the Pt signal at −4190 ppm. In the 13C NMR spectrum, 195Pt satellites are observed for the carbon signals of the imidazolinylidene-moiety C4′ (3J = 30 Hz), C5′ (3J = 30 Hz) as well as for the carbazol signals C1/C8 (3J = 25.5 Hz) and C4a/C5a (3J = 16.5 Hz). Satellites at the carbene signal at 170.8 ppm are not well resolved. The carbene signal of Pd-H complex 3b is detected at 179.1 ppm, while the signal was not observed in the case of NiH complex 3c as a consequence of paramagnetic impurities. Formation of the hydrido complexes 3 was also proven by IR spectroscopy, showing characteristic M-H valence bands at 2045 cm−1 (Pd) and 2202 cm−1 (Pt). These stretching frequencies seem to be very high in comparison to PCP pincer hydrido complexes Pd (1730 cm−1) and Pt (1917 cm−1) reported by Moulton and Shaw.5b In addition, in NMR spectroscopy, the 1JPtH coupling constant is much lower (902 Hz) in their PtH(PCP) complex. However, these differences can be explained by the lower trans-influence of the carbazolid fragment. This is in accordance with a hydrido trialkylphosphine Pt complex bearing a carbazolide, indolide, or pyrrolide ligand, reported by Maitlis and Garcı ́a and co-workers.8 These complexes show stretching frequencies between 2136 and 2147 cm−1 and 1JPtH coupling constants between 912.6 and 1001 Hz. Whether the additional strength of the Pt−H bond in complex 3a is an effect of the geometric orientation of the carbazolide ligand,13 the bulkiness of the carbene moieties or an electronic influence of the NHC ligands can so far not be clarified. Apart from the Ni complex 3c, the complexes are highly temperature stable. Pt complex 3a decomposes above 332 °C before melting and Pd complex 3b above 330 °C. Proof for the assigned monomeric structure of complexes 3a and 3b was provided by X-ray crystal structure analyses (Figure 1). The yellow crystals of the two compounds are isomorphous, crystallizing in space group P21212. They suffer from inversion twinning and contain channels with disordered pentane solvent, the scattering contribution of which was eliminated from the

Figure 2. X-ray crystal structure of the Pd(II)-Cl complex 4b. Anisotropic displacement parameters are given at the 50% probability level; hydrogen atoms have been omitted for clarity. The Pt(II)-Cl complex 4a shows an analogous molecular structure.

Pd crystal contains two independent molecules. Both molecules show a square planar geometry with a tetrahedral distortion as Cl departs from the plane by 1.138(7) Å (Pt) and by 1.250(3) and 1.256(3) Å (Pd) in the two unique molecules, respectively. This distortion most likely results from the steric congestion caused by the N-methyl substituents. Such a structural motif is also observed in our rhodium and iridium carbonyl complexes [M(bimca)(CO)].4 The N-heterocyclic carbene moieties are tilted, showing a C1−Pd−C6 angle of 166.65(7)° (166.36(8)° in the second molecule), while the Cl−Pd−N1 angle measures 156.82(5)° (on average) and is tilted toward the opposite side of the coordination plane. For the Pt-complex 4a, the situation is similar. The C1−Pt−C6 angle measures 164.8(2)° and the Cl−Pt−N1 angle is 160.8(1)°. The M-Cl complexes 4a and 4b can be converted back to the respective hydrido complexes 3a and 3b by ligand exchange with NaBH4.

Figure 1. X-ray crystal structure of the hydrido Pd(II) complex 3b. Anisotropic displacement parameters are given at the 50% probability level; hydrogen atoms have been omitted for clarity, except for the hydrido ligand. The crystal structure of the hydrido Pt(II) complex 3a is isomorphous. C

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Organometallics Reduction of M-Cl Complexes. To see if an anionic metal(0) species can be obtained independently by reduction, the M-Cl complexes were subjected to reduction. While reaction with Na metal did not lead to a clean reaction, reduction with KC8 gave a new product in the case of the Pd complex. After stirring a suspension of KC8 and complex 4b in tetrahydrofuran for 20 min, the graphite was removed by filtration, and the deep orange filtrate was analyzed by NMR spectroscopy. The 1H NMR spectrum shows a symmetric product in 21% yield (internal standard) with the proton signals of the NHC moieties at 6.65 and 6.81 ppm. They are shifted upfield by at least 0.8 and 1.5 ppm compared to the PdH complex, and the carbazol signals at 7.15 and 8.02 ppm are upfield shifted by 0.7 and 0.1 ppm. This already indicates a strong electron-donating effect. In the 13C NMR spectrum, the carbene signal is detected at 197.5 ppm, which is a downfield shift of 18 ppm compared to the PdH complex 3b. This value is typical for Pd(0) coordinated by only two imidazolin-2-ylidene moieties.15 The simple signal set precludes an asymmetric dimeric Pd(0)/Pd(II) dinuclear complex, as described by Milstein et al.16 However, the collapse of the pincer-type coordination of the ligand and proof for the successful reduction forming a new complex was obtained by X-ray structure analysis of single crystals obtained from a saturated solution of the complex in THF. The molecular structure of complex 6 (Figure 3) shows a dimeric structure via an inversion center, in whichin contrast

N10)). An additional coordination site of the potassium is occupied by one THF molecule. Due to the unsymmetrical coordination around potassium, the NMR spectrum should show a second signal set for the bimca ligand. Dissolving the crystals in THF-d8 produced the initial NMR spectrum. A fluxional structure in solution that results in an average symmetric molecule on the NMR time scale could be the reason for this phenomenon or the dissociation of the dimer into two monomers and rearrangement to a monomeric pincer type complex 5b. This would also explain the clean reformation of the Pd-H complex 3b upon protonation with 1 equiv of trifluoromethanesulfonic acid in 75% yield (NMR experiment). A putitative equilibrium could not be frozen by cooling the NMR sample to −80 °C during the measurement. To distinguish between a possible monomer 5b and the dimer 6, we conducted a diffusion ordered (DOSY) NMR experiment and determined the 1H spin−lattice relaxation times via inversion recovery. A solution of the reduction product in THF-d8 shows T1 values of 2.24 and 1.08 s for the peaks at δ 8.02 (carbazole-H) and 3.47 (N-CH3). These values arewithin the erroridentical to the respective signals at δ 7.83 and 4.16 of the pincer Pd(II)-hydride complex 3b (1.02 and 1.11 s). The DOSY experiment also offers almost identical diffusion coefficients for both complexes of 7.94 × 10−10 m2 s−1 (3b) and of 7.07 × 10−10 m2 s−1 (5b). Therefore, we conclude that the dinuclear Pd(0) complex 6 bearing the anionic carbazole moieties dissociates into the anionic pincer Pd(0) complex 5b in solution. The exact bonding situation of complex 5b (Scheme 2) remains unclear, but from the very similar diffusion coefficient of 5b and 3b, it is likely that complex 5b forms a solvent separated contact ion pair in solution (vide infra for DFT calculations of the [M(bimca)]− anions). Scheme 2. Syntheses and Reduction of M(II)Cl Complexes and Formation of Anionic Pd(0) Species 5b and 6

Figure 3. X-ray crystal structure of the anionic Pd(0) complex 6. Anisotropic displacement parameters are given at the 50% probability level. For clarity reasons, the t-butyl substituents of the carbazole moieties are drawn in stick style and the hydrogen atoms and one cocrystallized THF molecule are omitted.

to Milstein et al.both Pd atoms are reduced to Pd(0). The NHC moieties of each bimca ligand are oriented perpendicular to the carbazolide moiety and coordinate pairwise to one Pd atom, thus coordinating both Pd atoms linearly with carbene− Pd bond lengths of 2.021(2) Å (C1−Pd1) and 2.004(2) Å (C6−Pd1#, # −x + 1, −y + 1, −z + 1). The anionic carbazolide nitrogen coordinates to potassium (2.715(2) Å (K−N1)) that also shows one shorter and one longer contact to each Pd atom (3.2376(5) Å (K−Pd1), 3.6166(5) Å (K−Pd1#)) and to the πsystems of both carbene moieties (3.132(2) Å (K−C1), 3.797(2) Å (K−N5); 3.034(2) Å (K−C6), 3.252(2) Å (K− D

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be the formation of the anionic complex A or B, as no intermediate is observed. Therefore, it is plausible that, under these conditions (low concentration), the active species are the Li analogues of complexes 5a, 5b, and the respective Ni complex. Theoretical Treatment. 16 e− ML3 complexes like [Pt(PPh3)3] usually adopt a trigonal planar geometry; however, this is not possible for complexes of type 5 due to the pincer geometry of these anionic zerovalent complexes. For a Tshaped d10 ML3 complex, it is expected that, in addition to the weakly antibonding dz2 orbital, the antibonding dx2−y2 orbital will be occupied. In contrast to the dx2−y2 orbital of a square planar complex, this orbital is stabilized due to removal of one ligand along the y-axis and mixing with the py orbital that leads to a large coefficient on the opposite side of the ligand on the yaxis.19 This also confirms the expected strong basicity of the complex at this trans-position. In case there was no bond between the nitrogen and the palladium atom, so that the negative charge was located only on the nitrogen atom, a linear ML2 d10 arrangement would lead to an antibonding dz2 orbital as the HOMO. DFT calculations of all [M(bimca)]− fragments of complexes 5a−c (tBu groups are substituted by H) reveal very nicely the ML3 bonding situation with the dx2−y2 orbital as the HOMO (Figure 4). The geometrical parameters of the

If the reaction time for the reduction of 4b with 2 equiv of KC8 is reduced to only 3 min (Scheme 3) or if the less reactive Scheme 3. Formation of the Pd(I)-Pd(I) Complex 7 by Reduction of Pd Chlorido Complex 4b

lithium graphite is used, the signals of an unsymmetrical species 7 are observed in the 1H NMR spectrum. The reduced symmetry of 7 is indicated by the double signal set with characteristic signals for the N-methyl groups at 3.05 and 3.95 ppm, as well as six signals of equal intensity in the aromatic region at δ 6.90, 7.04, 7.20, 7.25, 7.65, and 7.74. A hydride signal is not observed in the respective region. In the 13C NMR spectrum, the carbene signals are found at similar chemical shifts of 179.0 and 181.0 ppm. These values lie in between those of Pd(0) species 5b (197.5 ppm) and Pd(II) chlorido complex 3b (163.8 ppm). Therefore, we ascribe the structure of species 7 to an unsymmetric Pd(I)-Pd(I) species, analogous to that found by Mani and co-workers for the Pd(I)-Pd(I) complex with a pyrrole-bridged diphosphine PNP pincer ligand,16 rather than a mixed valence Pd(0)/Pd(II) dinuclear complex as reported by Milstein et al. for a PCP ligand.17a For the latter case, we would expect a larger difference for the chemical shifts of the carbene signals. Due to its reduced symmetry, a symmetric Pd(I)-Pd(I) PNP pincer complex as found by Ozerov and Hall can also be excluded.17b A Pd(I)Pd(I) complex with an N-heterocyclic carbene ligand was previously described by Gardiner and co-workers.18 As a potential alternative route to complex 7, 1 equiv of the Pd-K complex 5b was reacted with 1 equiv of complex 4b. An immediate color change to brown was observed, but the 1H NMR showed only the signals of the Pd-H complex 3b (in 63% yield) and no signals of complex 7. Reduction of the Pt-Cl complex under identical conditions in THF-d8 does not yield a detectable analogous platinum(0) compound. Instead, only formation of the Pt-H complex 3a is observed in 68% yield along with 5% of a still unknown complex. An explanation for this can be that, if a very active Pt(0) species is formed (similar to that generated by addition of [Li(bimca)] to [Pt(PPh3)4)], it might be too basic and would react immediately upon deprotonation of an sp2 carbon to form the observed Pt-H complex. Mechanistic Considerations. These results show that anionic bimca M(0) complexes indeed can be discussed as intermediates in the oxidative addition of [Li(bimca)] to the respective M(II)-hydrido complexes. The reduction of Pt-Cl complex 4a leads only to Pt-H complex 3a, showing that a possible intermediate K[Pt(bimca)] 5a is so reactive (i.e., basic) that proton abstraction of the ligand (of a second molecule) is possible. The molecular structure of such a K[Pt(bimca)] complex 5a still remains to be discovered, but it could be analogous to the Pd-complexes 5b or 6. When reacting [Li(bimca)] (2) and the metal(0) precursor to obtain the hydrido complexes 3, the rate-limiting step would

Figure 4. Highest occupied molecular orbital (HOMO) of the anionic [Pd(bimca)]− part of complex 5b (DFT) (t-Bu groups are substituted by H).

complexes resemble the structures of the respective [M(bimca)Cl] 4a−c complexes, but with even stronger deviations from planarity (torsion of the imidazole vs the carbazol plane as well as the bending of the carbazol moiety; see the Supporting Information). Most striking are the elongated metal−N bonds, especially for the Pt and Pd complexes (M−N = 2.24 (H-5a), 2.20 (H-5b), and 1.93 Å (H-5c)) compared to the respective calculated chloride complexes (M−N = 2.02 (H-4a), 2.02 (H4b), and 1.87 Å (H-4c); X-ray: 1.99 Å (4b)). This elongation can be explained by a weakening of the bond by occupation of the antibonding dx2−y2 orbital (HOMO) (Figure 4). In contrast, the M−C(Carbene) bonds in the anionic complexes 5 are shortened compared to the M-Cl complexes 4 (M−C(mean) = 1.99 (H-5a), 2.02 (H-5b), and 1.87 Å (H-5c) vs 2.04 (H-4a), 2.05 (H-4b), and 1.92 Å (H-4c); X-ray: 2.04 Å (4b)).

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CONCLUSION We have shown that the reaction of [Li(bimca)] (2) with zerovalent group 10 metal complexes leads to formation of the respective M(II) hydrido pincer complexes 3a−c, due to the E

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Article

Organometallics

been accounted for in the refinement, by including the Pt-H moiety but not refining also its bimca ligand part. The distribution between the main [Pt(bimca)Cl] component and the minor [Pt(bimca)H] precursor refines to 0.877(5) and 0.122(5), respectively. As for compound 3a, the Pt−H distance was fixed at 1.6 Å. The crystals of the dimeric anionic Pd(0) complex 6 contain two coordinated THF molecules, both disordered, as well as two THF solvent molecules, one being disordered. A third solvent THF molecule was identified, but disordered beyond refinement and its scattering contribution, therefore, eliminated from the raw data by applying the SQUEEZE routine (39 e− in 153 Å3). Synthesis of Bisimidazolium Salt 1. We4a already reported the synthesis of this compound. Additional 1H NMR data of compound 1 in THF-d8: 1H NMR (THF-d8, 400.11 MHz): δ 1.49 (s, 18H, 10-H), 4.09 (s, 6H, 12-H), 7.68 (s, 4H), 7.87 (s, 2H), 8.43 (s, 2H), 9.55 (s, 2H, 2′-H), 10.31 (br s, 1H, NH). Deprotonation of Bisimidazolium Salt 1 with 3 equiv of Methyllithium, in Situ Generation of [Li(bimca)] (2). In an NMR tube, a suspension of 1 (20.0 mg, 32.5 μmol) and dodecahydrotriphenylene as internal standard (12.0 mg, 49.9 μmol) in 0.5 mL of THF-d8 was treated with solid methyllithium containing 0.03 equiv of diethyl ether (2.4 mg, 99 μmol) at room temperature and stirred for 5 min to give a pale yellow solution of Li(bimca) complex 2 that shows blue fluorescence in daylight. The NMR tube was then sealed. The quantitative yield is observed based on the internal standard. 7Li NMR (THF-d8, 97.21 MHz): δ −0.64 ppm. The 1H NMR and 13C NMR spectra are identical to the literature.4a Deprotonation of Bisimidazolium Salt 1 with 2 equiv of Methyllithium. In an NMR tube, a suspension of 1 (20.0 mg, 32.5 μmol) and dodecahydrotriphenylene as internal standard (3.6 mg, 14.9 μmol) in 0.5 mL of THF-d8 was treated with solid methyllithium containing 0.03 equiv of diethyl ether (1.6 mg, 66 μmol) at room temperature and stirred for 5 min to give a pale yellow suspension. The NMR tube was then sealed. 7Li NMR (THF-d8, 97.21 MHz): δ −0.51 ppm. The 1H NMR spectrum shows the signals for the fully deprotonated species [Li(bimca)] (2) in a yield of 30% (9.1 μmol) as reported in the literature4a as well as the signals of the nondeprotonated bisimidazolium salt 1 (14.5%, 4.7 μmol) and an unknown species (340 °C (dec.). MS(FAB+): m/z 579.2 [M]+. Synthesis of [SP-4-3]-[1,1′-(3,6-Di-tert-butylcarbazol-9-id1,8-diyl-κN)-bis(3-methyl-1H-imidazolin-2-yliden-κ 2 C 2 )chloridonickel(II)] [Ni(bimca)Cl] (4c). Bisimidazolium salt 1 (100 mg, 162 μmol) was suspended in 15 mL of THF and deprotonated with 7.2 mg (0.324 mmol) of methyllithium (2 equiv). To this suspension of imidazolium salt and [Li(bimca)] (2), [Ni(COD)2] (44.6 mg, 162 μmol) was added, and the reaction mixture was stirred for 4 days at room temperature. Then, 3 mL of chloroform was added. The orange solution immediately turned brown, and the mixture was stirred for another 10 min. After evaporating the solvent, the residual solid was extracted with 30 mL of toluene and any undissolved material was removed by filtration. After evaporation of the solvent, the residue was dissolved in 1 mL of THF and filtered over a column charged with silica gel. Afterward, the solvent was removed in vacuo and the solid was washed twice with 2.5 mL of pentane. The product, complex 4c, was obtained as an orange brown solid in a 48% yield (41.0 mg, 76.9 μmol). 1H NMR (THF-d8, 400.13 MHz): δ 1.50 (s, 18 H, H-10), 4.36 (s, 6H, H-12), 7.38 (d, 4JHH = 1.0 Hz, 2H, H-4‘), 7.66 (d, 4JHH = 1.1 Hz, 2H, H-2/7), 8.03 (d, 4JHH = 1.1 Hz, 2H, H-4/5), 8.05 (d, 4JHH = 1.0 Hz, 2H, H-5‘). 13C-{1H}-NMR (THF-d8): δ 32.7 (C10), 35.7 (C11), 40.3 (C12), 110.1 (C2/7), 115.0 (C4/5), 116.2 (C5‘), 124.9 (C1/8), 126.3 (C4‘), 128.3 (C1a/8a), 136.5 (C4a/5a), 140.2 (C3/6), 161.5 (C2‘). IR (KBr, cm−1): 2961 (s), 1638 (m), 1451 (m), 1409 (m), 1362 (m), 1301 (m), 1262 (s), 1091 (s), 801 (m), 747 (m), 717, (m), 679 (m). Mp: >312 °C (dec.). MS(FAB+): m/z 531.1 [M]+, 496.1 [M − Cl]+. Reaction of [Pt(bimca)Cl] (4a) with NaBH4; Synthesis of [Pt(bimca)H] (3a). [Pt(bimca)Cl] (4a) (10.0 mg, 14.9 μmol) was dissolved in 1 mL of THF. To this solution, 1.0 mg (26 μmol) of NaBH4 was added. After stirring the reaction mixture for 16 h at room temperature, the undissolved NaBH4 was filtered off. The filtrate was dried in vacuo. After washing the residue with 2 mL of pentane and drying in vacuo, the product was obtained as a yellow solid. The NMR data are identical to those of the independently prepared [Pt(bimca)H] (3a).

1.56 (s, 18H, H-10), 4.03 (s, 6H, H-12), 7.16 (dd, 4J195PtH = 12.8 Hz, 3 JHH = 2.2 Hz, 2H, H-4′), 7.85 (d, 4JHH = 1.4 Hz, 2H, H-2/7), 8.18 (d, 4 JHH = 1.4 Hz, 2H, H-4/5), 8.34 (d, 3JHH = 2.2 Hz, 195PtH shoulder, 4 J195PtH < 8.2 Hz, 2H, H-5′). 13C {1H} NMR (THF-d8, 100.61 MHz): δ 32.6 (C10), 35.5 (C11), 41.6 (C12), 110.9 (C2/7), 114.9 (C4/5), 115.6 (C5′, 3J195Pt,13C = 30 Hz), 122.4 (C4′, 3J195Pt,13C = 30.0 Hz), 125.6 (C1/8, 3J195Pt,13C = 25.5 Hz), 127.2 (C4a/5a, 3J195Pt,13C = 16.5 Hz), 134.4 (C3/6), 138.4 (C1a/8a), 170.8 (C2′). 195Pt(H, Pt HETCOR) δ −4190. IR (KBr, cm−1): 2952 (s), 2864 (m), 2202 (m, Pt-H), 1637 (m), 1450 (s), 1401 (m), 1363 (s), 1325 (m), 1303 (m), 1269 (s), 1235 (m), 1202 (w), 1173 (w), 1077 (w), 896 (w), 841 (w), 783 (w), 750 (w), 714 (w), 685 (w), 645 (w). Mp: >332 °C (dec.). Anal. Calcd for C28H33N5Pt + 0.8 equiv pentane: C, 55.51; H, 6.20; N, 10.11. Found: C, 55.80; H, 6.11; N, 9.85. FT-ICR HR-ESI+ (MeOH): m/z 633.2308 [M − H]+, calculated 633.2305. Synthesis of [SP-4-3]-[1,1′-(3,6-Di-tert-butylcarbazol-9-id1,8-diyl-κN)-bis(3-methyl-1H-imidazolin-2-yliden-κ 2 C 2 )hydridopalladium(II)] [Pd(bimca)H] (3b). Imidazolium salt 1 (200 mg, 330 μmol) was suspended in 30 mL of THF and deprotonated with 16.0 mg (0.728 mmol) of methyllithium (2.1 equiv) containing 0.03 equiv of diethyl ether. To this suspension of imidazolium salt 1 and [Li(bimca)] (2), [Pd(PPh3)4] (254 mg, 220 μmol) was added, and the reaction mixture was stirred for 4 days at 40 °C. The solvent was removed in vacuo, the green residue was extracted with 50 mL of toluene, and the extract was filtered. After removing the solvent in vacuo, the orange solid was dissolved in 3 mL of THF and filtered over a column charged with neutral Alox. First, unreacted [Pd(PPh3)4] was eluted with pure THF from the column; then the product was collected with a mixture of THF:acetonitrile (4:1). After removing the solvents, the residue was washed twice with 2.5 mL of pentane. After drying in vacuo, product 3b was obtained as a yellow solid in 73% yield (87.6 mg, 0.161 mmol). Crystals suitable for X-ray diffraction were obtained by overlaying a saturated solution of the compound in THF with pentane. 1H NMR (THF-d8, 400.13 MHz): δ −10.77 (s, 1H, PdH), 1.54 (s, 18H, H-10), 4.16 (s, 6H, H-12), 7.45 (d, 3JHH = 2.0 Hz, 2H, H-4′), 7.83 (d, 4JHH = 1.6 Hz, 2H, H-2/7), 8.15 (d, 4JHH = 1.6 Hz, 2H, H-4/5), 8.32 (d, 3JHH = 2.0 Hz, 2H, H-5′). 13C {1H} NMR (THFd8, 100.61 MHz): δ 32.9 (C11), 35.6 (C10), 41.2 (C12), 110.8 (C2/ 7), 115.0 (C4/5), 117.0 (C5′), 122.5 (C4′), 125.8 (C1/8), 127.4 (C4a/5a), 136.2 (C3/6), 138.2 (C1a/8a), 179.1 (C2′). IR (KBr, cm−1): 3178 (w), 3118 (w), 2960 (s), 2863 (m), 2045 (m, Pd-H), 1584 (m), 1481 (m), 1447 (s), 1361 (s), 1324 (m), 1304 (m), 1261 (s), 1071 (s), 1024 (m), 839 (w), 803 (w), 750 (m), 707 (m), 678 (m) 535 (w). Mp: >300 °C (dec.). Anal. Calcd for C28H33N5Pd + 0.5 equiv C5H12: C, 62.93; H, 6.75; N, 12.03. Found: C, 63.08; H, 6.39; N, 11.90. FT-ICR HR-ESI+ (CH3CN/THF): m/z 544.170009 [M − H]+, calculated 544.169799. Synthesis of [SP-4-3]-[1,1′-(3,6-Di-tert-butylcarbazol-9-id1,8-diyl-κN)-bis(3-methyl-1H-imidazolin-2-yliden-κ 2 C 2 )hydridonickel(II)] [Ni(bimca)H] (3c). Bisimidazolium salt 1 (200 mg, 330 μmol) was suspended in 30 mL of THF and deprotonated with 16.0 mg (0.728 mmol) of methyllithium (2.1 equiv) containing 0.03 equiv of diethyl ether. To this suspension of imidazolium salt 1 and [Li(bimca)] (2), [Ni(COD)2] (90.8 mg, 330 μmol) was added, and the reaction mixture was stirred for 4 days at room temperature. After evaporating the solvent, the residue was extracted with toluene (40 mL) and the solid was filtered off. The toluene was then removed in vacuo. The product complex 3c, obtained as a brown solid, is very unstable so that further purification attempts only led to decomposition. 1H NMR (THF-d8, 400.13 MHz): δ −17.56 (s, 1H, Ni-H), 1.53 (s, 18H, H-10), 4.09 (br s, 6H, H-12), 7.36 (br s, 2H, H4′), 7.70 (br s, 2H, H-2/7), 8.07 (br s, 2H, H-4/5), 8.11 (br s, 2H, H5′). 13C {1H} NMR (THF-d8, 100.61 MHz): δ 32.9 (C11), 35.7 (C10), 40.2 (C12), 109.7 (C2/7), 114.7 (C4/5), 115.7 (C5′), 125.0 (C4′), 126.2 (C1/8), 129.1 (C4a/5a), 129.8 (C3/6), 136.6 (C1a/8a). The signal for the carbene carbon (C2′) was not observed. MS(FAB+): m/z 497.2 [M − hydride]+. Synthesis of [SP-4-3]-[1,1′-(3,6-Di-tert-butylcarbazol-9-id1,8-diyl-κN)-bis(3-methyl-1H-imidazolin-2-yliden-κ 2 C 2 )chloridoplatinum(II)] [Pt(bimca)Cl] (4a). [Pt(bimca)H] (3a) (50.0 G

DOI: 10.1021/om500836m Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Treatment of the Reduced Species 5b/6 with [Pd(bimca)Cl] (4b) To Prepare the Unsymmetrical Species 7 Independently. A vial was charged with 2.4 mg (35 mmol) of C8K in a glovebox, and a solution of 5.0 mg (17 μmol) of [Pd(bimca)Cl] (4b) and 3.5 mg (15 μmol) of dodecahydrotriphenylene in 0.6 mL of THF-d8 was added. After stirring the reaction mixture for 20 min, the graphite was filtered off and the deep orange colored solution of complex 5b/6 was examined by NMR spectroscopy (41.4% yield; 7.1 μmol). Then, 4.1 mg of [Pd(bimca)Cl] (4b) was added and the orange solution turned immediately into brown. Signals observed in the 1H NMR spectrum are identical to the signals of independently prepared [Pd(bimca)H] (3b) in a 63% (4.5 μmol) yield. Furthermore, signals of a still unknown species are detected (