N-Heterocyclic Carbene Stabilized Dichlorosilylene Transition-Metal Complexes of V ( I ) , Co ( I ) , and Fe ( 0 )

N-heterocyclic carbenes (NHCs) have proven highly versatile σ-donor ligands for transition-metal (TM) complexes and as effective Lewis bases to stabilize reactive main group element species and as organocatalysts on their own right. Use of NHCs as spectator ligands, particularly as alternatives to phosphines (R3P), resulted in a second generation of Grubbs catalysts. This is one of the remarkable developments in organometallic chemistry. Similarly, chemistry of TM silylene complexes has attracted considerable attention during the last two decades. TM silylene complexes have been proposed as intermediates in catalytic hydrosilylation, dehydrosilylation of organosilicon compounds, redistribution of substituents on organosilicon compounds, and deoligomerization of disilanylmetal complexes to monosilyl derivatives. In general, silylenes are compounds with a neutral divalent silicon atom, and therefore, they are highly reactive species to be isolated at normal laboratory conditions. Coordination to a TM center offers a convenient synthetic approach to trap or to generate such highly reactive species. Remarkable contributions to this field have been made by Tilley and others. Availability of the first stable N-heterocyclic silylene (NHSi) and its application as a ligand for TM complexes introduced a direct method to prepare TM silylene complexes. 20 Among acyclic silylenes, dichlorosilylene (SiCl2) is one of the extremely reactive species and has academic and industrial importance. SiCl2 readily polymerizes to (SiCl2)n or decomposes to Si and SiCl4. 21 Therefore access to TM complexes containing SiCl2 as a ligand is rare and based on indirect multistep methods. Moreover, these reactions often lead to a mixture of several products. In general, silylenes (e.g., SiX2, X = halogen, H, alkyl, or aryl) are divalent neutral silicon species with the lone pair of electrons as the highest occupied molecular orbital (HOMO) and an empty p-orbital as the lowest unoccupied molecular orbital (LUMO) in the singlet ground (A) state. Therefore, silylenes can in principle behave as Lewis acids as well as Lewis bases and are known to possess an ambiphilic nature. Very recently, we isolated the first Lewisbase stabilized dichlorosilylene IPr 3 SiCl2 (1) (IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) in very good yield by reductive dehydrochlorination of HSiCl3 using NHC. This method was the basis for developing silylene chemistry on a broader scale, without using alkali metals for the reduction. However even more important was the increase in the yield of silylene. Compound 1 consists of a three coordinate silicon atom containing a stereoactive lone pair of electrons. Therefore, 1 should serve as a convenient and readily available source of a neutral σ-donor ligand for TM complexes. In 1, NHC is a Lewis base, and SiCl2 behaves as a Lewis acid. Ambiphilic nature of SiCl2, in which it behaves as a Lewis acid as well as a Lewis base at the same time, has been already demonstrated in our previous reports.We have investigated the properties of 1 as a Lewis base, and its oxidative addition to organic substrates, functionalization of NHC, and use of 1 as a σ-donor ligand for the TM complexes [Co(CO)3{SiCl2(IPr)}2][CoCl3(THF)] A 19 and Ni(CO)2{SiCl2(IPr)}2 B. 20 Formation of an ionic (A) and a neutral (B) bis-silylene complex further convinced us to continue our studies on the chemistry of TM complexes containing 1 as a ligand. A survey of literature reveals that there are only a few complexes, such as (CO)5VSiH3, (η -C5H5)2V(SiCl3)2, (η-C5H5)V(N Bu)(NHBu)Si(SiMe3)3, and (η -C5H5)V(L)(SiHRR0) (L = 1,2-bis(dimethylphosphino)ethane; R = Ph or

' INTRODUCTION N-heterocyclic carbenes (NHCs) have proven highly versatile σ-donor ligands for transition-metal (TM) complexes 1,2 and as effective Lewis bases to stabilize reactive main group element species 3,4 and as organocatalysts on their own right. 5 Use of NHCs as spectator ligands, particularly as alternatives to phosphines (R 3 P), resulted in a second generation of Grubbs catalysts. 6 This is one of the remarkable developments in organometallic chemistry. Similarly, chemistry of TMÀsilylene complexes has attracted considerable attention 7 during the last two decades. TMÀsilylene complexes have been proposed as intermediates in catalytic hydrosilylation, 8 dehydrosilylation of organosilicon compounds, 9 redistribution of substituents on organosilicon compounds, 10 and deoligomerization of disilanylmetal complexes to monosilyl derivatives. 11 In general, silylenes are compounds with a neutral divalent silicon atom, and therefore, they are highly reactive species to be isolated at normal laboratory conditions. 12 Coordination to a TM center offers a convenient synthetic approach to trap or to generate such highly reactive species. 13 Remarkable contributions to this field have been made by Tilley and others. 7,14 Availability of the first stable N-heterocyclic silylene (NHSi) 15 and its application as a ligand for TM complexes introduced a direct method to prepare TMÀsilylene complexes. 16À20 Among acyclic silylenes, dichlorosilylene (SiCl 2 ) is one of the extremely reactive species and has academic and industrial importance. SiCl 2 readily polymerizes to (SiCl 2 ) n or decomposes to Si and SiCl 4 . 21 Therefore access to TM complexes containing SiCl 2 as a ligand is rare and based on indirect multistep methods. 22 Moreover, these reactions often lead to a mixture of several products. In general, silylenes (e.g., SiX 2 , X = halogen, H, alkyl, or aryl) are divalent neutral silicon species with the lone pair of electrons as the highest occupied molecular orbital (HOMO) and an empty p-orbital as the lowest unoccupied molecular orbital (LUMO) in the singlet ground ( 1 A) state. Therefore, silylenes can in principle behave as Lewis acids as well as Lewis bases and are known to possess an ambiphilic nature. 23 Very recently, we isolated the first Lewisbase stabilized dichlorosilylene 4 IPr 3 SiCl 2 (1) (IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) in very good yield by reductive dehydrochlorination of HSiCl 3 using NHC. This method was the basis for developing silylene chemistry on a broader scale, without using alkali metals for the reduction. However even more important was the increase in the yield of silylene. Compound 1 consists of a three coordinate silicon atom containing a stereoactive lone pair of electrons. Therefore, 1 should serve as a convenient and readily available source of a neutral σ-donor ligand for TM complexes. In 1, NHC is a Lewis base, and SiCl 2 behaves as a Lewis acid. Ambiphilic nature of SiCl 2 , in which it behaves as a Lewis acid as well as a Lewis base at the same time, has been already demonstrated in our previous reports. 19,20,23 We have investigated the properties of 1 as a Lewis base, 23 and its oxidative addition to organic substrates, 24 functionalization of NHC, 25 and use of 1 as a σ-donor ligand for the TM complexes [Co(CO) 3 {SiCl 2 (IPr)} 2 ][CoCl 3 (THF)] A 19 and Ni(CO) 2 {SiCl 2 (IPr)} 2 B. 20 Formation of an ionic (A) and a neutral (B) bis-silylene complex further convinced us to continue our studies on the chemistry of TM complexes containing 1 as a ligand. A survey of literature reveals that there are only a few complexes, such as (CO) 5 VSiH 3 , (η 5 -C 5 H 5 ) 2 V(SiCl 3 ) 2 , (η 5 -C 5 H 5 )V(N t Bu)(NH t Bu)Si(SiMe 3 ) 3 , and (η 5 -C 5 H 5 )V(L)-(SiHRR 0 ) (L = 1,2-bis(dimethylphosphino)ethane; R = Ph or Received: May 24, 2011 ABSTRACT: Reactions of N-heterocyclic carbene stabilized dichlorosilylene IPr 3 SiCl 2 (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with (η 5 -C 5 H 5 )V(CO) 4 , (η 5 -C 5 H 5 )Co(CO) 2 , and Fe 2 (CO) 9 afford dichlorosilylene complexes IPr 3 SiCl 2 3 V(CO) 3 (η 5 -C 5 H 5 ) (2), IPr 3 SiCl 2 3 Co(CO)-(η 5 -C 5 H 5 ) (3), and IPr 3 SiCl 2 3 Fe(CO) 4 (4), respectively. Complexes 2À4 are stable under an inert atmosphere, are soluble in common organic solvents, and have been characterized by elemental analysis and multinuclear ( 1 H, 13 C, and 29 Si) NMR spectroscopy. Molecular structures of 2À4 have been determined by single crystal X-ray crystallographic studies and refined with nonspherical scattering factors.

ARTICLE
Mes, and R 0 = Ph or H), containing vanadiumÀsilicon bonds which have been characterized by X-ray crystallographic studies. 26 Moreover, to the best of our knowledge no vanadiumÀsilylene complex has been isolated so far. The lack of progress in this area is due to the paramagnetic properties of most vanadium complexes. 26 Therefore, we became interested in exploring the chemistry of TM, especially vanadium, complexes using 1 as a ligand. Ligand substitution reactions are essential for the application of TM organometallic compounds as homogeneous catalysts. Compared to the TMÀcarbene complexes, TMÀsilylene complexes derived from stable silylenes are still elusive. Herein, we report on a convenient access to TMÀsilylene complexes IPr 3 SiCl 2 3 V(CO) 3 (η 5 -C 5 H 5 ) (2), IPr 3 SiCl 2 3 Co(CO)-(η 5 -C 5 H 5 ) (3), and IPr 3 SiCl 2 3 Fe(CO) 4 (4) by ligand substitution reaction. Complexes 2À4 have been characterized by elemental analyses and NMR spectroscopy. Molecular structures of 2À4 have been established by single crystal X-ray crystallography. Complex 2 is the first vanadiumÀsilylene complex which is structurally characterized by single crystal X-ray crystallography. The quality of the X-ray structures in terms of accuracy and precision has been improved by using nonspherical scattering factors.
Complexes 2À4 are crystalline solids, soluble in common organic solvents, and stable under an inert atmosphere. These complexes were characterized by elemental analyses and 1 H, 13 C, and 29 Si NMR spectroscopic studies. 1 H NMR spectra of 2À4 show two sets of resonances for methyl protons (CHMe 2 ) of isopropyl groups, whereas methine protons appear as a multiplet. Imidazole backbone protons exhibit a singlet and show solvent dependence. Complexes 2 and 3 each show a singlet for cyclopentadienyl (C 5 H 5 ) protons. 13 C{ 1 H} NMR spectra of 2À4 exhibit usual resonances for the IPr ligand. In the 13 C NMR spectra (in CD 2 Cl 2 ) of 2 and 3, a resonance at δ 90.78 and 82.17 ppm, respectively, may be assigned for each cyclopentadienyl group. Complex 2 exhibits a broad 29 Si NMR signal at δ 88.70 ppm due to the presence of paramagnetic vanadium. Similar broadening can also be seen in the 13 C{ 1 H} NMR spectrum of 2 for carbonyl groups (δ 259.20À263.03 ppm).
However, resonances for the IPr ligand in 2 exhibit without any broadening. 51 V NMR resonance for 2 appears at δ À1593.46 ppm, which is consistent with those observed for analogues vanadium complexes. 26 For the carbonyl group, 3 and 4 show 13 C NMR resonances between δ 191 and 219 ppm. 29 Si NMR spectra of 3 and 4 exhibit a resonance at 31.86 and 59.20 ppm, which is consistent with those observed for TMÀsilylene complexes. 19,20 Single Crystal X-ray Structure Determination. Single Crystal X-ray Structures. Molecular structures of compounds 2À4 were established by single-crystal X-ray crystallographic studies and corresponding ORTEP-representations are shown in Figures 1À3. Crystallographic data for 2À4 are summarized in Table 1. All crystals were measured on a Bruker three-circle diffractometer equipped with a SMART 6000 CCD area detector and a CuKR rotation anode. Integrations were performed with SAINT. 27 Intensity data for all compounds were corrected for absorption and scaled with SADABS. 28 Structures were solved by direct methods and initially refined by full-matrix least-squares methods on F 2 with the program SHELXL-97, 29 utilizing anisotropic displacement parameters for nonhydrogen atoms. The structural model was improved by a subsequent refinement with nonspherical scattering factors, 30 which was initiated by converged IAM parameter values. The scattering-factor model used in this refinement was based on the Hansen and Coppens multipole formalism. 31 For compounds 2 and 4 hydrogen atoms were included in the model by constraints via a riding model,and in case of 3, hydrogen atoms were refined freely. Instead of adjusting the respective multipole parameters to the experimental data, which requires Bragg data to a high resolution, multipole parameters were predicted from theoretical calculations on each whole molecule, using the density functional theory (DFT) functional B3LYP and 6-31 g* as the basis set. This procedure is different to the fragmentbased invariom approach. 32 Prior to refinement with XDLSM as part of the XD suite, 33 input files were processed with the program InvariomTool. 32 The criterion for observed reflections was [I > 3σ (I)]. Only positional and displacement parameters of nonhydrogen atoms were adjusted in the nonspherical atom refinement, so that the number of parameters was not increased in comparison to the IAM. Bond distances to hydrogen atoms were set to values from geometry optimization. 34 These aspherical atom refinements share the benefits of a conventional charge density refinement. For all compounds, parameter precision (as indicated by parameter standard deviations) and figures of merit improve. Anisotropic displacement parameters (ADPs) become deconvoluted from electron density, and an interpretable electron density model was obtained ( Figures 1À3). Calculated deformation densities show the expected electron density accumulations in bonding regions. Valence-shell charge concentrations (VSCC) of σ-donation can also be localized by calculating and interpreting the Laplacian of the electron density. These results are presented for compounds 2À4 (Figures 1À3).
Complex 2 is the first example of a vanadiumÀsilylene complex that has been characterized by single crystal X-ray
The molecular structure of complex 4 is shown in Figure 3a. Complex 4 crystallizes in orthorhombic space group Fdd2. The geometry around the iron atom is distorted trigonal bipyramidal with three equatorial positions occupied by carbonyl groups and each of the two axial positions by a carbonyl group and a silylene ligand. The silicon center is four coordinate and features a distorted tetrahedral geometry. The SiÀFe bond distance in 4 (2.2315(13) Å) is comparable to those observed in silyleneÀ iron complexes. 17,22 The average SiÀCl bond length (av 2.0916(16)°) in 4 is slightly shorter than those observed for 1 (av 2.1664(16) Å) and 2 (av 2.1192(11)°). The SiÀC-(carbene) bond distance (1.949(4) Å) is shorter than that obtained for 1 (1.985(4) Å) and 2 (1.984(3) Å). The axial SiÀFeÀC bond angle is almost linear (177.01(19)°).
' ASSOCIATED CONTENT b S Supporting Information. Crystallographic data for complexes 2À4 as a CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.