A Base-Stabilized Silyliumylidene Cation as a Ligand for Rhodium

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A Base-Stabilized Silyliumylidene Cation as a Ligand for Rhodium and Tungsten Complexes Hui-Xian Yeong, Yongxin Li, and Cheuk-Wai So* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore S Supporting Information *

ABSTRACT: The reaction of the base-stabilized silyliumylidene triflate [LSi(DMAP)]OTf (1·OTf; L = PhC(NtBu)2, DMAP = 4-dimethylaminopyridine) with [Rh(cod)Cl]2 and W(CO)5THF afforded [{L(DMAP)Si}2{μ-Rh(μ-Cl)2Rh(cod)}](OTf)2 (2) and [L(DMAP)Si→W(CO)5]OTf (3), respectively. Their crystal structures determined by X-ray crystallography show that the silyliumylidene cation acts as a two-electron donor coordinating to the rhodium and tungsten atoms. hortly after the isolation of the first stable N-heterocyclic carbene (NHC),1 the syntheses of stable silicon analogues (NHSis) and their subsequent reactivities with transition metals were investigated extensively.2,3 NHSis comprise a lone pair of electrons and vacant p orbital on the low-valent silicon atom. As a result, they can act as both donor and weaker acceptor and serve as ancillary ligands in transition-metal complexes, in which there is a synergy electron transfer between the lowvalent silicon atom and transition metal. Thus, the resulting metal−silicon bonds in transition-metal−NHSi complexes are typically shorter than the corresponding single bonds. Since transition-metal−silylene complexes have been postulated as catalytic intermediates,4 the application of transition-metal− NHSi complexes toward small molecule activation and catalytic transformations has also been investigated recently. For example, [L′Si→Ni(CO)3] (L′ = CH{(CCH 2)NAr}{CMeNAr)}, Ar = 2,6-iPr2C6H3) is capable of activating the S−H and N−H bonds of small molecules.5 The catalytic activities of transition-metal−NHSi complexes include that of [{1,1′-Cp2Fe(LSi)2}CoCp] (L = PhC(NtBu)2, Cp = η5cyclopentadienyl) in the [2 + 2 + 2] cyclotrimerization of phenylacetylene and acetonitrile,6 [{HCN(tBu)}2Si→μ-PdPPh3] in Suzuki reactions,4g and [(dmpe)2Fe←Si(H)L] (dmpe = 1,2-bis(dimethylphosphino)ethane) as a precatalyst in the hydrosilylation of ketones.7 Furthermore, the chemistry of transition-metal−NHSi complexes has been extended to the stabilization of otherwise highly reactive silylene species within the coordination sphere of an organometallic fragment. For example, Roesky et al. have reported that a silicon(II) fluoride moiety can be stabilized by coordinating with M(CO)5 complexes (M = Cr, M, W).8 Reactive silicon(II) hydride

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moieties were also synthesized by coordinating with Cp2Ti, Ni(CO)3, and (dmpe)2Fe.7,9 Similar to NHSi, a silyliumylidene cation of composition [RSi:]+, which consists of two vacant orbitals and a lone pair of electrons on the low-valent silicon cation, could serve as a ligand in a transition-metal complex, but a silyliumylidene cation should possess enhanced π-accepting properties compared with an NHSi due to its stronger Lewis acidity. Although stable derivatives of the silyliumylidene cation, which contain sterically hindered substituents with electron-donor moieties, were synthesized, their coordination chemistry toward transition-metal complexes is still unknown. Recently, we isolated a stable silyliumylidene cation [LSi(DMAP)]+ (1), which is stabilized by an amidinate ligand and 4-dimethylaminopyridine (DMAP).11d This result prompted our interest to investigate its reactivity toward transition metals.10 Herein, we describe the synthesis of transition-metal− silyliumylidene complexes [{L(DMAP)Si}2{μ-Rh(μ-Cl)2Rh(cod)}]OTf2 (2; cod = 1,5-cyclooctadiene) and [L(DMAP)SiW(CO)5]OTf (3). Treatment of two molar equivalents of 1·OTf with [Rh(cod)Cl]2 in toluene for 60 h afforded [{L(DMAP)Si}2{μ-Rh(μ-Cl)2Rh(cod)}](OTf)2 (2; Scheme 1). Compound 2 was formed by the displacement of one cod molecule by two silyliumylidene cations. It was crystallized from DME as air- and moisture-sensitive orange crystals in low yield (20%). It is stable in solution and the solid state at room temperature in an inert atmosphere. Compound 2 has been characterized by Received: March 5, 2014 Published: June 27, 2014 3646

dx.doi.org/10.1021/om500232f | Organometallics 2014, 33, 3646−3648

Organometallics

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Scheme 1. Synthesis of 2

elemental analysis, spectroscopic methods, and X-ray crystallography. The 1H and 13C NMR spectra of 2 display resonances due to the amidinate, DMAP, and cod ligands. The 13C NMR spectrum also exhibits a quartet at δ = 122.04 ppm (1JC−F = 324.3 Hz) for the CF3SO3− anion. The 29Si NMR spectrum exhibits a doublet at δ = 40.54 ppm (1JSi−Rh = 117.9 Hz), with coupling to the rhodium atom. This is an expected downfield shift compared with that of compound 1·OTf (δ = −82.3 ppm),11 as the lone pair electrons of the silicon cations are donated to the Rh atom. Compound 2 was analyzed by X-ray crystallography (Figure S1; see the Supporting Information), but the quality of data is rather poor (R1 (I > 2(σ)I) = 0.1014; wR2 (all data) = 0.3043) as the crystal is twinned and the molecules in the asymmetric unit are all disordered. As a result, its bond lengths and angles are dubious and their comparison with literature data cannot be discussed. Compound 1·OTf can also displace the THF molecule in W(CO)5THF to afford [L(DMAP)Si→W(CO)5]OTf (3; Scheme 2), which was crystallized from toluene as air- and

Figure 1. ORTEP drawing of 3 (30% thermal ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Si1−N1 1.825(4), Si1−N2 1.821(4), Si1−N3 1.839(4), Si1− W1 2.497(1), W1−C1 2.030(5), W1−C2 2.058(5), W1−C3 2.018(5), W1−C4 2.054(5), W1−C5 2.029(5), C1−O1 1.138(5), C2−O2 1.130(6), C3−O3 1.145(6), C4−O4 1.134(5), C5−O5 1.138(5); N1−Si1−N2 71.57(16), N1−Si1−N3 100.09(16), N2−Si1−N3 101.13(16), N1−Si1−W1 124.82(12), N2−Si1−W1 126.26(12), N3−Si1−W1 121.08(13).

In conclusion, the reactivity of the base-stabilized silyliumylidene cation [LSi(DMAP)]+ with [Rh(cod)Cl]2 and W(CO)5THF yielded the first examples of transition-metal−silyliumylidene complexes 2 and 3, respectively.

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Scheme 2. Synthesis of 3

EXPERIMENTAL SECTION

General Procedure. All manipulations were carried out under an inert atmosphere of argon gas using standard Schlenk techniques. THF, DME, and toluene were dried over and distilled over a Na/K alloy prior to use. C6D6 was dried over and distilled over K metal prior to use. 1·OTf was prepared as described in the literature.11 [Rh(cod)Cl]2 and W(CO)6 were purchased from Aldrich Chemicals and used without further purification. W(CO)5THF was prepared by the UV irradiation of W(CO)6 in THF as described in the literature.13 The 1H, 13C, 19F, and 29Si NMR spectra were recorded on a JEOL ECA 400 spectrometer. The chemical shifts δ are relative to SiMe4 for 1 H, 13C, and 29Si, and CFCl3 for 19F. Elemental analyses were performed by the Division of Chemistry and Biological Chemistry, Nanyang Technological University. Melting points were measured in sealed glass tubes and were not corrected. Compound 2. Toluene (16.4 mL) was added to a mixture of 1· OTf (0.174 g, 0.328 mmol) and [Rh(cod)Cl]2 (0.0819 g, 0.166 mmol) at ambient temperature. The reaction mixture was stirred for 60 h. Volatiles were removed in vacuo, and the residue was extracted with DME. After filtration and concentration of the filtrate, 2 was obtained as yellow crystals. The solvent residues in the crystals were then removed in vacuo. Yield: 0.0470 g (20%). mp 175 °C; 1H NMR (399.5 MHz, C6D6, 23.9 °C): δ = 1.42 (s, 36H, t-Bu), 1.44−1.50 (m, 4H, CH2 of cod), 2.13−2.26 (m, 4H, CH2 of cod), 2.75 (s, 12H, NMe2), 4.40 (br s, 4H, CH of cod), 6.81−6.97 (m, 6H, Ph and DMAP), 7.16−7.23 (m, 2H, Ph), 7.54−7.65 (m, 6H, Ph and DMAP), 8.57 ppm (d, 3JHH = 6.83 Hz, 4H, DMAP); 13C NMR (99.55 MHz, C6D6, 24.8 °C): δ = 30.70 (CH2 of cod), 31.13 (CMe3), 39.62 (NMe2), 55.24 (CMe3), 78.14 (d, 1JC−Rh = 14.30 Hz, CH of cod), 109.56 (Ph), 122.04 (q, 1JC−F = 324.3 Hz, CF3), 128.39, 128.73, 128.85, 130.07, 130.68, 141.86, 156.76 (Ph and DMAP), 176.14 ppm (NCN); 29 Si{1H} NMR (79.49 MHz, C6D6, 24.9 °C): δ = 40.54 ppm (d, 1JSi−Rh = 117.9 Hz); 19F{1H} NMR (375.94 MHz, C6D6, 24.0 °C): δ = −77.77 ppm; Anal. Calcd for C54H78Cl2F6N8O6Rh2S2Si2: C, 44.84; H, 5.44; N, 7.75. Found: C, 44.76; H, 5.04; N, 7.69.

moisture-sensitive pale-yellow crystals in moderate yield (67%). It is stable in solution and the solid state at room temperature in an inert atmosphere. Compound 3 has been characterized by elemental analysis, spectroscopic methods, and X-ray crystallography. The 1H and 13C NMR spectra of 3 display resonances due to the amidinate, DMAP, and carbonyl ligands. The 13C NMR spectrum also exhibits a quartet at δ = 122.59 ppm (1JC−F = 322.2 Hz) for the CF3SO3− anion. The 29Si NMR spectrum exhibits a singlet at δ = 51.61 ppm. Similary to 2, this is an expected downfield shift compared with that of compound 1· OTf.11 Additionally, the 29Si NMR resonance of 3 is the intermediate of [L(Cl)Si→W(CO)5] (δ = 52.99 ppm) and [L(F)Si→W(CO)5] (δ = 41.79 ppm).8 The molecular structure of 3 (Figure 1) shows that the SiII cation is tetracoordinated, which is similar to that of 2. The tungsten atom adopts an octahedral geometry, with coordination to one silicon cation and five carbonyl groups. The Si1− W1 bond (2.497(1) Å) is comparable with typical Si−W bonds (2.388(6)−2.708(3) Å).12 3647

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Organometallics

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Compound 3. A solution of W(CO)5THF (32 mL, 0.320 mmol) was added to a stirring solution of 1·OTf (0.170 g, 0.320 mmol) in THF (16 mL) at ambient temperature. The resulting yellow solution was stirred for 3 days. Volatiles were removed in vacuo, and the residue was extracted with toluene. After filtration and concentration of the filtrate, 3 was obtained as pale-yellow crystals. Yield: 0.184 g (67%). mp 237 °C; 1H NMR (395.9 MHz, C6D6, 24.3 °C): δ = 1.05 (s, 18H, t-Bu), 2.46 (s, 12H, NMe2), 6.84−6.89 (m, 3H, Ph and DMAP), 6.99−7.03 (m, 2H, Ph), 7.52 (t, 3JHH = 8.16 Hz, 1H, Ph), 8.50 (d, 3JHH = 7.68 Hz, 2H, DMAP), 8.96 ppm (d, 3JHH = 7.72 Hz, 1H, DMAP); 13C NMR (100.6 MHz, C6D6, 24.9 °C): δ = 30.73 (CMe3), 39.54 (NMe2), 55.63 (CMe3), 109.01 (Ph), 122.59 (q, 1JC−F = 322.2 Hz, CF3), 127.28, 127.55, 129.40, 129.69, 131.63, 143.54, 157.42 (Ph and DMAP), 178.38 (NCN), 196.87, 198.02 ppm (CO); 29Si{1H} NMR (79.49 MHz, C6D6, 24.8 °C): δ = 51.61 ppm; 19F{1H} NMR (372.50 MHz, C6D6, 24.3 °C): δ = −77.71 ppm; FT-IR (Nujol, cm−1): 1921 (s), 1991 (s), 2070 (s) cm−1; Anal. calcd for C28H33F3N4O8SSiW: C, 39.35; H, 3.89; N, 6.56. Found: C, 39.12; H, 3.67; N, 6.31. X-ray Data Collection and Structural Refinement. Intensity data for compound 3 was collected using a Bruker APEX II diffractometer. The crystal was measured at 103(2) K. The structure was solved by direct phase determination (SHELXS-97) and refined for all data by full-matrix least-squares methods on F2.14 All nonhydrogen atoms were subjected to anisotropic refinement. The hydrogen atoms were generated geometrically and allowed to ride in their respective parent atoms; they were assigned appropriate isotopic thermal parameters and included in the structure-factor calculations.

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(3) For a review of reactions of NHSis with transition metals, please see: Blom, B.; Stoelzel, M.; Driess, M. Chem.Eur. J. 2013, 19, 40−62. (4) (a) Lewis, K. M.; Rethwisch, D. G. Catalyzed Direct Reactions of Silicon; Elsevier: Amsterdam, The Netherlands, 1993. (b) Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351−1374. (c) Curtis, M. D.; Epstein, P. S. Adv. Organomet. Chem. 1981, 19, 213−232. (d) Gauvin, F.; Harrod, J. F.; Woo, H. G. Adv. Organomet. Chem. 1998, 42, 363− 405. (e) Corey, J. Y. Adv. Organomet. Chem. 2004, 51, 1−52. (f) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37−46. (g) Fürstner, A.; Krause, H.; Lehmann, C. W. Chem. Commun. 2001, 22, 2372−2373. (h) Calimano, E.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 11161− 11173. (i) Prévost, M.; Woerpel, K. A. J. Am. Chem. Soc. 2009, 131, 14182−14183. (j) Ohmura, T.; Masuda, K.; Takase, I.; Suginome, M. J. Am. Chem. Soc. 2009, 131, 16624−16625. (k) Zhang, M.; Liu, X.; Shi, C.; Ren, C.; Ding, Y.; Roesky, H. W. Z. Anorg. Allg. Chem. 2008, 634, 1755−1758. (l) Someya, C. I.; Haberberger, M.; Wang, W.; Enthaler, S.; Inoue, S. Chem. Lett. 2013, 42, 286−288. (5) Meltzer, A.; Inoue, S.; Präsang, C.; Driess, M. J. Am. Chem. Soc. 2010, 132, 3038−3046. (6) (a) Wang, W.; Inoue, S.; Enthaler, S.; Driess, M. Angew. Chem. 2012, 124, 6271−6275;(b) Angew. Chem., Int. Ed. 2012, 51, 6167− 6171. (7) Blom, B.; Enthaler, S.; Inoue, S.; Irran, E.; Driess, M. J. Am. Chem. Soc. 2013, 135, 6703−6713. (8) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Wolf, H.; Stalke, D. J. Am. Chem. Soc. 2012, 134, 2423−2428. (9) (a) Blom, B.; Driess, M.; Gallego, D.; Inoue, S. Chem.Eur. J. 2012, 18, 13355−13360. (b) Stoelzel, M.; Präsang, C.; Inoue, S.; Enthaler, S.; Driess, M. Angew. Chem. 2012, 124, 411−415; Angew. Chem., Int. Ed. 2012, 51, 399−403. (10) Recently, Inoue et al. reported the synthesis of diplatinum and dipalladium silyliumylidene-phosphide complexes through several rearrangement processes of a base-stabilized phosphasilene with [(η2-C2H4)Pt(PPh3)2] and [Pd(PPh3)4], respectively. See: Breit, N. C.; Szilvási, T.; Suzuki, T.; Gallego, D.; Inoue, S. J. Am. Chem. Soc. 2013, 135, 17958 Until now, the coordination chemistry of a stable silyliumylidene cation toward transition metals is still unknown. (11) (a) Jutzi, P.; Mix, A.; Rummel, B.; Schoeller, W. W.; Neumann, B.; Stammler, H.-G. Science 2004, 305, 849−851. (b) Driess, M.; Yao, S.; Brym, M.; Wüllen, C. van Angew. Chem. 2006, 118, 6882−6885; Angew. Chem., Int. Ed. 2006, 45, 6730−6733. (c) Xiong, Y.; Yao, S.; Inoue, S.; Irran, E.; Driess, M. Angew. Chem., Int. Ed. 2012, 51, 10074− 10077. (d) Yeong, H.-X.; Xi, H.-W.; Li, Y.; Lim, K. H.; So, C.-W. Chem.Eur. J. 2013, 19, 11786−11790. (12) (a) Rosenberg, L.; Fryzuk, M. D.; Rettig, S. J. Organometallics 1999, 18, 958−969. (b) Junold, K.; Baus, J. A.; Burschka, C.; Tacke, R. Angew. Chem., Int. Ed. 2012, 51, 7020−7023. (13) Rosa, A.; Baerends, E. J.; van Gisbergen, S. J. A.; van Lenthe, E.; Groeneveld, J. A.; Snijders, J. G. J. Am. Chem. Soc. 1999, 121, 10356− 10365. (14) Sheldrick, G. M. SHELXL-97; Universität Göttingen: Göttingen, Germany, 1997.

ASSOCIATED CONTENT

* Supporting Information S

Ball-and-stick structure representing the structure of 2 along with a list of Cartesian coordinates, a table giving crystallographic data for 3, and a CIF file giving X-ray data for 3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS This work was supported by the Academic Research Fund Tier 1 (RG 22/11). REFERENCES

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