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Tohoku University, Japan

Published in issue 10, 2011 of Dalton Transactions

Image reproduced with permission of Kenneth Kam-Wing Lo Articles in the issue include: PERSPECTIVES: Pyrazolin-4-ylidenes: a new class of intriguing ligands Yuan Han and Han Vinh Huynh, Dalton Trans., 2011, DOI: 10.1039/C0DT01037E Solvent induced molecular magnetic changes observed in single-crystal-to-single-crystal transformation Zheng-Ming Hao and Xian-Ming Zhang, Dalton Trans., 2011, DOI: 10.1039/C0DT00979B, ARTICLES: Negative thermal expansion emerging upon structural phase transition in ZrV2O7 and HfV2O7 Yasuhisa Yamamura, Aruto Horikoshi, Syuma Yasuzuka, Hideki Saitoh and Kazuya Saito Dalton Trans., 2011, DOI: 10.1039/C0DT01087A Preparation of surface molecularly imprinted Ru-complex catalysts for asymmetric transfer hydrogenation in water media Zhihuan Weng, Satoshi Muratsugu, Nozomu Ishiguro, Shin-ichi Ohkoshi and Mizuki Tada Dalton Trans., 2011, DOI: 10.1039/C0DT00950D Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research

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Synthesis and characterisation of hydrido molybdenum and tungstencomplexes having a hemilabile tridentate Si,Si,O-ligand: observation ofstepwise hydrosilylation of a nitrile to form an N-silylimine on the metalcentre†

Takashi Komuro, Rockshana Begum, Rikima Ono and Hiromi Tobita*

Received 18th August 2010, Accepted 11th November 2010DOI: 10.1039/c0dt01047b

Photoinduced decarbonylation of Cp*M(CO)3Me (M = Mo and W, Cp* = h5-C5Me5) in the presence ofxantsilH2 [xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)] in pentane gave bis(silyl)hydridocomplexes Cp*M(k2Si,Si-xantsil)(CO)2(H) (1a: M = Mo and 1b: M = W) through two-fold Si–Hoxidative addition and methane elimination. Further irradiation of 1a,b in toluene afforded tridentatexantsil complexes Cp*M(k3Si,Si,O-xantsil)(CO)(H) (2a: M = Mo and 2b: M = W) via CO dissociation.Reactions of complexes 2a,b with nitriles led to stoichiometric hydrosilylation at the C N triple bond.Thus, reaction of 2a,b with t-BuCN at room temperature afforded N-silyliminoacyl complexes 3a,b,through insertion of a nitrile into the M–Si bond, and the products slowly isomerised to thecorresponding N-silylimine complexes 4a,b via intramolecular hydrogen migration. On the other hand,reaction of 2a,b with PhCN afforded N-silylimine complexes 5a,b directly. The molecular structures of1a, 3a and 5b were determined by X-ray crystallography, revealing that complex 3a has a3-centre–2-electron (3c–2e) Mo–Si–H bond.

Introduction

Coordinatively-unsaturated hydrido(silyl) complexes are proposedas crucial intermediates in metal-mediated hydrosilylation oforganic molecules to produce organosilicon compounds.1 In hy-drosilylation reactions, these complexes are commonly generatedby the reaction of transition-metal complexes with hydrosilanes.2

The vacant coordination site on the metal centre can accommodatea substrate such as an alkene, and then insertion of the alkene intoa metal–hydrogen bond (Chalk–Harrod mechanism) or a metal–silicon bond (modified Chalk–Harrod mechanism) gives the hy-drosilylation product.1a Hydrosilylation reactions of other organicsubstrates are also important, but their mechanisms have not yetbeen well understood. To elucidate the detailed mechanisms ofhydrosilylation, it is fundamentally important to investigate thereactivity of coordinatively-unsaturated hydrido(silyl) complexestoward these organic substrates.

Hydrosilylation of nitriles to give N-silylimines or N,N-bis(silyl)amines is considered to be more difficult than that of otherunsaturated organic compounds such as alkenes, alkynes, ketones,

Department of Chemistry, Graduate School of Science, Tohoku University,Sendai, 980-8578, Japan. E-mail: tobita@m.tohoku.ac.jp; Fax: +81 22 7956543; Tel: +81 22 795 6539† CCDC reference numbers 789738 (1a), 789739 (3a) and 789740 (5b).For crystallographic data in CIF or other electronic format see DOI:10.1039/c0dt01047b

aldehydes and imines, because their C N bonds are relativelyinert.3,4 Thus, there is still only a limited number of examplesof catalytic3 or stoichiometric4 nitrile-hydrosilylation promotedby transition-metal complexes. In these hydrosilylation reactions,complexes having a metal–silicon bond play a crucial role. Forexample, Murai and coworkers discovered the hydrosilylation ofnitriles to N,N-bis(silyl)amines, catalyzed by Co2(CO)8, in which(trialkylsilyl)cobalt complex (R3Si)Co(CO)4 was proposed as theactive catalyst.3b,c However, most nitrile-hydrosilylation reactionsrequire high-temperature conditions, which make mechanisticstudies difficult. Also, little attention has been paid to the use of16-electron hydrido(silyl) complexes as active species for nitrile-hydrosilylation.

In this work, we found that reactions of bis(silyl)hydridomolybdenum and tungsten complexes, which can generate 16-electron species, with nitriles led to hydrosilylation of theirC N bonds to give N-silylimine complexes at room temperature.Our group has recently developed transition-metal complexeshaving a xanthene-based bis(silyl) chelate ligand xantsil [(9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)], which can easilygenerate coordinatively-unsaturated complexes.5 The xantsil lig-and can act as a bidentate (k2Si,Si) or a tridentate (k3Si,Si,O)ligand. As the oxygen atom on the xanthene backbone of thetridentate xantsil ligand is only weakly coordinated to the metalcentre, dissociation of the oxygen readily occurs to convertthe k3Si,Si,O-xantsil complex to the coordinatively-unsaturated

2348 | Dalton Trans., 2011, 40, 2348–2357 This journal is © The Royal Society of Chemistry 2011

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k2Si,Si-xantsil complex. Our group has recently reported a16-electron ruthenium k3Si,Si,O-xantsil complex Ru(k3Si,Si,O-xantsil)(CO)(PCy3) that is a precursor of a formally 14-electronbis(silyl) complex: The ruthenium complex reacted with two equiv-alents of CO to afford the tricarbonyl(k2Si,Si-xantsil) complexRu(k2Si,Si-xantsil)(CO)3(PCy3).5d,f In this work, we employed thishemilabile nature6 of the xantsil ligand to generate 16-electronbis(silyl)hydrido molybdenum and tungsten complexes.

Here we report the synthesis of hydrido(k3Si,Si,O-xantsil) com-plexes of molybdenum and tungsten, and also the reactions of thexantsil complexes with nitriles, which resulted in hydrosilylation ofnitrile via: (1) formation of N-silyliminoacyl complexes throughinsertion of a nitrile molecule into the metal–silicon bond, and(2) isomerisation of the iminoacyl complexes to N-silyliminecomplexes via migration of hydrogen. A part of this work hasbeen published as a preliminary report.7

Experimental

General procedures

All manipulations were carried out under dry nitrogen in aglovebox or using a standard high-vacuum line and standardSchlenk techniques. Benzene-d6, hexane, pentane, toluene andnitriles RCN (R = t-Bu and Ph) were dried over CaH2 andvacuum transferred, then stored under nitrogen over 4 A molecularsieves in a glovebox. Toluene-d8 was dried over potassium mirrorand vacuum transferred into an NMR tube. Cp*M(CO)3Me(M = Mo8 and W9) were prepared according to the literaturemethod.

Physical measurements

1H, 13C{1H} and 29Si{1H}NMR spectra were recorded on a BrukerAVANCE-300 or AVANCE-600 Fourier transform spectrometer.Chemical shifts were reported in parts per million. Couplingconstants (J) and line widths at half-height (Dn1/2) are givenin Hz. 29Si{1H} NMR measurements for products except 2band 5b were performed using the DEPT pulse sequence. The29Si{1H} NMR measurements for 2b and 5b were carried outusing an inverse gate decoupling pulse sequence. The residualproton (C6D5H, 7.15 ppm; C6D5CD2H, 2.09 ppm) and thecarbon resonances (C6D6, 128.0 ppm; C6D5CD3, 20.4 ppm)of deuterated solvents were used as internal references for 1Hand 13C resonances, respectively. Aromatic proton or carbon isabbreviated to ArH or ArC. 29Si{1H} NMR chemical shifts werereferenced to SiMe4 (0 ppm) as an external standard. The NMRspectra were measured at room temperature unless otherwiseindicated. Infrared spectra were recorded with a KBr pellet usinga HORIBA FT-730 spectrometer. Mass spectra were recorded ona Shimadzu QP5050 or Hitachi M-2500S spectrometer operatingin the electron impact (EI) mode. Measurement of some NMRand mass spectra and elemental analysis were performed at theResearch and Analytical Center for Giant Molecules, TohokuUniversity.

Synthesis of Cp*Mo(j2Si,Si-xantsil)(CO)2(H) (1a)

A Pyrex sample tube (20 mm o.d.) with a Teflon vacuum stopcockwas charged with a solution of Cp*Mo(CO)3Me (20 mg, 0.061

mmol) and xantsilH2 (20 mg, 0.061 mmol) in pentane (ca. 2mL). The solution was degassed by conventional freeze–pump–thaw cycles on a vacuum line. The mixture was irradiatedwith a 450 W medium pressure Hg lamp for 23 min at ca.7 ◦C. During the photoreaction, the mixture was degassed after2, 10 and 18 min of irradiation by a freeze–pump–thaw cycle.After removal of volatiles, the residue was washed with pentaneseveral times and dried under vacuum to give compound 1a (29 mg,0.047 mmol, 78%) as a colourless powder (found: C, 60.78; H, 6.74.C31H40O3Si2Mo requires C, 60.76; H, 6.58%); nmax/cm-1 1909 s(COsym) and 1849 m (COasym); dH (300 MHz; C6D6) 7.42 (2H, dd,JHH 7.5 and 1.5, ArH), 7.24 (2H, dd, JHH 7.5 and 1.5, ArH), 7.05(2H, t, JHH 7.5, ArH), 1.57 (3H, s, 9-CMe), 1.35 (3H, s, 9-CMe),1.21 (6H, s, SiMe), 1.16 (15H, s, C5Me5), 0.95 (6H, br s, SiMe) and-5.52 [1H, s, JSiH (satellite) 37, Mo–H]; dC (75.5 MHz; C6D6) 232.9(CO), 160.2, 134.6, 133.0, 129.4, 125.4 and 123.4 (ArC), 103.8(C5Me5), 36.5 (9-CMe2), 31.1 and 22.9 (9-CMe2), 10.3 (C5Me5),8.3 and 4.1 (SiMe2); dSi (59.6 MHz; C6D6) 15.5; m/z (EI) 614(24%, M+), 586 (46%, M+ - CO) and 558 (100%, M+ - 2CO).

Synthesis of Cp*W(j2Si,Si-xantsil)(CO)2(H) (1b)

A mixture of Cp*W(CO)3Me (20 mg, 0.048 mmol) and xantsilH2

(16 mg, 0.049 mmol) in pentane (ca. 2 mL) was irradiated asdescribed for 1a. Workup similar to that for 1a yielded 1b (27 mg,0.039 mmol, 81%) as a colourless powder (found: C, 53.17; H,5.90. C31H40O3Si2W requires C, 53.14; H, 5.75%); nmax/cm-1 1901 s(COsym) and 1843 m (COasym); dH (300 MHz; C6D6) 7.43 (2H, dd,JHH 7.5 and 1.5, ArH), 7.24 (2H, dd, JHH 7.5 and 1.5, ArH),7.06 (2H, t, JHH 7.5, ArH), 1.57 (3H, s, 9-CMe), 1.35 (3H, s,9-CMe), 1.24 (6H, s, SiMe), 1.21 (15H, s, C5Me5), 1.10 (6H,br s, SiMe) and -3.95 [1H, s, JWH (satellite) 83, JSiH (satellite)25, W–H]; dC (75.5 MHz; C6D6) 223.7 [JWC (satellite) 145, CO],160.3, 134.6, 133.1, 128.9, 125.3 and 123.4 (ArC), 102.7 (C5Me5),36.4 (9-CMe2), 31.1 and 22.9 (9-CMe2), 10.2 (C5Me5), 9.1 and2.8 (SiMe2); dSi (59.6 MHz; C6D6) 5.9 [JWSi (satellite) 14]; m/z(EI) 700 (37%, M+), 672 (100%, M+ - CO) and 644 (54%, M+ -2CO).

Synthesis of Cp*Mo(j3Si,Si,O-xantsil)(CO)(H) (2a)

A solution of 1a (17 mg, 0.028 mmol) in toluene (ca. 1 mL) wasplaced in a Pyrex sample tube (20 mm o.d.) with a Teflon vacuumstopcock and degassed by freeze–pump–thaw cycles on a vacuumline. The solution was irradiated with a 450 W medium pressure Hglamp for 50 min at ca. 7 ◦C. During the photoreaction, the mixturewas degassed after 20 and 40 min of irradiation by a freeze–pump–thaw cycle. After removal of volatiles, the residue was washed withhexane and dried under vacuum to give compound 2a (14 mg,0.024 mmol, 86%) as an orange powder (found: C, 61.27; H, 6.89.C30H40O2Si2Mo requires C, 61.62; H, 6.89%); nmax/cm-1 1819 s(CO); dH (300 MHz; C6D6) 7.38 (2H, dd, JHH 6.1 and 2.5, ArH),6.90–7.00 (4H, m, ArH), 1.60 (15H, s, C5Me5), 1.35 (3H, s, 9-CMe),1.24 (3H, s, 9-CMe), 0.94 (6H, s, SiMe), 0.91 (6H, s, SiMe) and-2.27 [1H, s, JSiH (satellite) 12, Mo–H]; dC (75.5 MHz; C6D6) 236.6(CO), 160.6, 138.7, 133.4, 130.6, 125.2 and 123.9 (ArC), 101.4(C5Me5), 35.5 (9-CMe2), 33.2 and 24.2 (9-CMe2), 11.3 (C5Me5),10.4 and 5.3 (SiMe2); dSi (59.6 MHz; C6D6) 32.9; m/z (EI) 586(22%, M+) and 558 (100%, M+ - CO).

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 2348–2357 | 2349

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Synthesis of Cp*W(j3Si,Si,O-xantsil)(CO)(H) (2b)

Compound 2b was prepared by the photoreaction of 1b (33 mg,0.047 mmol) in a manner similar to that for 2a. An orange powderof the product (29 mg, 0.043 mmol, 92%) was isolated (found:C, 53.67; H, 6.08. C30H40O2Si2W requires C, 53.57; H, 5.99%);nmax/cm-1 1815 s (CO); dH (300 MHz; C6D6) 7.42 (2H, dd, JHH 7.2and 1.6, ArH), 6.95 (2H, t, JHH 7.2, ArH), 6.88 (2H, dd, JHH 7.2and 1.6, ArH), 1.69 (15H, s, C5Me5), 1.35 (3H, s, 9-CMe), 1.24(3H, s, 9-CMe), 0.98 (6H, s, SiMe), 0.92 (6H, br s, SiMe) and 0.78(1H, s, W–H);10 dH (600 MHz; toluene-d8; 203 K) 7.42 (1H, br d,JHH 6.7, ArH), 7.31 (1H, br d, JHH 6.5, ArH), 6.95 (1H, br t, JHH

7.4, ArH), 6.92 (1H, br t, JHH 7.8, ArH), 6.81 (1H, br d, JHH 7.2,ArH), 6.75 (1H, br d, JHH 7.1, ArH), 1.64 (15H, s, C5Me5), 1.45(3H, s, SiMe), 1.27 (3H, s, 9-CMe), 1.224 and 1.216 (3H ¥ 2, s ¥ 2,9-CMe, SiMe), 0.90 (3H, s, SiMe), 0.53 (3H, br s, SiMe) and 0.23(1H, br s, W–H);10 dC (75.5 MHz; toluene-d8) 231.5 (CO), 162.4,141.1, 133.6, 130.9, 125.8 and 123.9 (ArC), 100.0 (C5Me5), 35.7(9-CMe2), 33.1 and 24.4 (9-CMe2), 11.3 (C5Me5), 10.6 (SiMe) and5.6 (br, Dn1/2 ca. 10, SiMe); dSi (119 MHz; toluene-d8; ca. 298 K)33.8 (br, Dn1/2 ca. 150); dSi (119 MHz; toluene-d8; 203 K) 45.9 and23.6; m/z (EI) 672 (24%, M+) and 644 (100%, M+ - CO).

Synthesis of Cp*Mo[j4Si,H ,N ,C-xantsil(H){N C(t-Bu)}](CO)(3a)

A toluene (6 mL) solution of 2a (49 mg, 0.084 mmol) and t-BuCN(24 mg, 0.29 mmol) was stirred at room temperature under anatmosphere of dinitrogen. The solution immediately turned fromorange to reddish purple. After 30 min of stirring, volatiles wereremoved under reduced pressure. The residue was washed withpentane three times and dried under vacuum to give compound3a (46 mg, 0.069 mmol, 82%) as a reddish purple powder (found:C, 62.86; H, 7.30; N, 2.11. C35H49NO2Si2Mo requires C, 62.94;H, 7.39; N, 2.10%); nmax/cm-1 1794 s (CO) and 1634 m (CN); dH

(300 MHz; C6D6) 7.70 (1H, dd, JHH 6.9 and 1.5, ArH), 7.23–7.32(2H, m, ArH), 6.96–7.12 (3H, m, ArH), 1.84 (15H, s, C5Me5), 1.51(3H, s, 9-CMe), 1.27 (3H, s, 9-CMe), 1.00 (3H, s, SiMe), 0.92 (3H, s,SiMe), 0.84 (3H, s, SiMe), 0.82 (9H, s, t-Bu), 0.47 (3H, s, SiMe)and -1.72 [1H, s, JSiH (satellite) 64, Mo–H–Si]; dSi (59.6 MHz;C6D6) -1.8 and -8.4; m/z (EI) 669 (94%, M+), 641 (81%, M+ -CO), 583 (100%, M+ - CO - t-BuH) and 558 (75%, M+ - CO -t-BuCN). Because of the thermal instability of 3a in C6D6 at roomtemperature, we could not measure the 13C{1H} NMR spectrumof 3a.

Synthesis of Cp*W[j4Si,H ,N ,C-xantsil(H){N C(t-Bu)}](CO)(3b)

The title compound was synthesised by a procedure similar to thatfor 3a using a toluene (6 mL) solution of 2b (40 mg, 0.059 mmol)and t-BuCN (24 mg, 0.29 mmol) (reaction period: 100 min). Theresulting purple solution was evaporated to remove volatiles. Theresidue was washed with pentane three times and dried undervacuum to give compound 3b (21 mg, 0.028 mmol, 47%) as apurple powder. The combined washings were concentrated andcooled at -30 ◦C to give a second crop (14 mg, 0.019 mmol, 31%).Total yield: 35 mg, 0.046 mmol, 78% (found: C, 55.51; H, 6.49;N, 1.74. C35H49NO2Si2W requires C, 55.62; H, 6.53; N, 1.85%);nmax/cm-1 1792 s (CO) and 1605 m (CN); dH (300 MHz; C6D6)

7.74 (1H, dd, JHH 6.4 and 2.4, ArH), 7.25–7.33 (2H, m, ArH),6.96–7.11 (3H, m, ArH), 1.89 (15H, s, C5Me5), 1.54 (3H, s, 9-CMe), 1.31 (3H, s, 9-CMe), 1.20 (3H, s, SiMe), 1.04 (3H, s, SiMe),1.03 (1H, s, W–H–Si),10 0.83 (3H, s, SiMe), 0.81 (9H, s, t-Bu)and 0.46 (3H, s, SiMe); dSi (59.6 MHz; C6D6) -2.9 and -9.2; m/z(EI) 755 (100%, M+) and 727 (60%, M+ - CO). Because of thethermal instability of 3b in C6D6 at room temperature, we couldnot measure the 13C{1H} NMR spectrum of 3b.

Synthesis of Cp*Mo[j3Si,N ,C-xantsil{N C(H)(t-Bu)}](CO)(4a)

A toluene (3 mL) solution of 3a (22 mg, 0.033 mmol) was stirred atroom temperature for 4 days under an atmosphere of dinitrogen.The solution turned from reddish purple to dark cyan. Volatileswere removed under reduced pressure. Compound 4a (21 mg,0.031 mmol, 95%) was obtained as a blue–green powder (found:C, 63.23; H, 7.32; N, 1.93. C35H49NO2Si2Mo requires C, 62.94;H, 7.39; N, 2.10%); nmax/cm-1 1867 s (CO); dH (300 MHz; C6D6)7.41–7.52 (2H, m, ArH), 7.29 (1H, br d, JHH 7.7, ArH), 7.25 (1H,dd, JHH 7.6 and 1.7, ArH), 7.09 (1H, t, JHH 7.4, ArH), 7.03 (1H,t, JHH 7.5, ArH), 3.91 (1H, s, N CH), 1.60 (3H, s, 9-CMe), 1.32(15H, s, C5Me5), 1.17 (12H, s, t-Bu, 9-CMe), 0.97 (3H, s, SiMe),0.78 (3H, s, SiMe), 0.67 (3H, s, SiMe) and 0.45 (3H, s, SiMe); dC

(75.5 MHz; C6D6) 245.9 (CO), 159.6, 155.9, 135.1, 133.6, 131.6,131.2, 126.8, 125.3, 124.4, 123.2 and 123.1 (ArC, two of the signalsare overlapped at 133.6 ppm), 105.5 (C5Me5), 74.3 (N CH), 39.5,35.0 and 33.9 (CMe3, 9-CMe2, 9-CMe), 30.4 (CMe3), 24.5 (9-CMe), 11.0 (C5Me5), 6.6, 2.9, 2.1 and -0.5 (SiMe2); dSi (59.6 MHz;C6D6) 31.7 and 0.7; m/z (EI) 669 (87%, M+), 641 (77%, M+ -CO), 583 (100%, M+ - CO - t-BuH) and 558 (73%, M+ - CO -t-BuCN).

Synthesis of Cp*W[j3Si,N ,C-xantsil{N C(H)(t-Bu)}](CO) (4b)

A toluene (3 mL) solution of 3b (20 mg, 0.026 mmol) wasstirred at room temperature for 4 days under an atmosphereof dinitrogen. The solution turned from purple to violet. Afterremoval of volatiles under reduced pressure, compound 4b (19 mg,0.025 mmol, 95%) was obtained as a violet powder (found: C,55.28; H, 6.34; N, 1.92. C35H49NO2Si2W requires C, 55.62; H,6.53; N, 1.85%); nmax/cm-1 1868 s (CO); dH (300 MHz; C6D6) 7.50(1H, br d, JHH 6.6, ArH), 7.42 (1H, br d, JHH 6.2, ArH), 7.29 (1H,br d, JHH 7.6, ArH), 7.25 (1H, dd, JHH 7.6 and 1.6, ArH), 7.08–7.15(1H, m, ArH), 7.02 (1H, t, JHH 7.4, ArH), 3.89 (1H, s, N CH),1.61 (3H, s, 9-CMe), 1.39 (15H, s, C5Me5), 1.20 (9H, s, t-Bu), 1.17(3H, s, 9-CMe), 0.96 (3H, s, SiMe), 0.80 (3H, s, SiMe), 0.64 (3H, s,SiMe) and 0.49 (3H, s, SiMe); dC (75.5 MHz; C6D6) 239.1 (CO),159.7, 156.0, 136.4, 134.0, 133.5, 131.7, 131.2, 126.9, 125.4, 124.3,123.3 and 123.1 (ArC), 104.1 (C5Me5), 68.9 (N CH), 39.4, 35.0and 33.9 (CMe3, 9-CMe2, 9-CMe), 31.3 (CMe3), 24.5 (9-CMe),11.0 (C5Me5), 6.3, 2.6 and -1.1 (SiMe2, two of the signals areoverlapped at 2.6 ppm); dSi (59.6 MHz; C6D6) 22.1 and 3.7; m/z(EI) 755 (100%, M+) and 727 (59%, M+ - CO).

Synthesis of Cp*Mo[j3Si,N ,C-xantsil{N C(H)(Ph)}](CO) (5a)

A toluene (4.5 mL) solution of 2a (30 mg, 0.051 mmol) andPhCN (25 mg, 0.24 mmol) was stirred at room temperature for2 h under an atmosphere of dinitrogen. The solution immediately

2350 | Dalton Trans., 2011, 40, 2348–2357 This journal is © The Royal Society of Chemistry 2011

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turned from orange to dark yellow. After removal of volatilesunder reduced pressure, the residue was washed with pentanethree times and dried under vacuum to give compound 5a (24 mg,0.035 mmol, 68%) as a khaki powder. The combined washings wereconcentrated and cooled at -30 ◦C to give a second crop (2 mg,0.003 mmol, 6%). Total yield: 26 mg, 0.038 mmol, 74% (found:C, 64.55; H, 6.81; N, 2.14. C37H45NO2Si2Mo requires C, 64.60; H,6.59; N, 2.04%); nmax/cm-1 1745 s (CO); dH (300 MHz; C6D6) 7.96(1H, dd, JHH 7.0 and 1.6, ArH), 7.74 (2H, br d, JHH 7.3, ArH),7.16–7.26 (3H, m, ArH), 7.03–7.12 (3H, m, ArH), 6.96–7.03 (2H,m, ArH), 3.92 (1H, s, N CH), 1.71 (15H, s, C5Me5), 1.59 (3H, s, 9-CMe), 1.52 (3H, s, 9-CMe), 0.59 (3H, s, SiMe), 0.57 (3H, s, SiMe),-0.04 (3H, s, SiMe) and -0.77 (3H, s, SiMe); dC (75.5 MHz; C6D6)227.9 (CO), 162.9, 162.1, 160.6, 144.3, 142.0, 138.6, 135.3, 134.6,131.1, 127.3, 126.0, 125.2, 123.5, 123.3 and 119.8 (ArC, one of thesignals is overlapped with that of benzene-d6), 104.9 (C5Me5), 71.0(N CH), 37.8 (9-CMe2), 28.6 and 22.6 (9-CMe2), 10.5 (C5Me5),5.5, 5.0, 2.3 and 1.9 (SiMe2); dSi (59.6 MHz; C6D6) 36.5 and 13.7;m/z (EI) 689 (27%, M+), 661 (100%, M+ - CO) and 583 (53%, M+

- CO - PhH).

Synthesis of Cp*W[j3Si,N ,C-xantsil{N C(H)(Ph)}](CO) (5b)

The title compound was synthesized by a procedure similar to thatfor 5a using a toluene (3 mL) solution of 2b (20 mg, 0.030 mmol)and PhCN (14 mg, 0.14 mmol) (reaction period: 90 min). The re-sulting reddish purple solution was evaporated to remove volatiles.The residue was washed with pentane three times and dried undervacuum to give compound 5b (13 mg, 0.017 mmol, 56%) as apurple powder. The combined washings were concentrated andcooled at -30 ◦C to give a second crop (5 mg, 0.006 mmol, 22%).Total yield: 18 mg, 0.023 mmol, 78% (found: C, 57.10; H, 6.01;N, 1.86. C37H45NO2Si2W requires C, 57.28; H, 5.85; N, 1.81%);nmax/cm-1 1911 s (CO); dH (300 MHz; C6D6) 7.47 (1H, dd, JHH 7.1and 1.5, ArH), 7.32 (1H, dd, JHH 7.7 and 1.6, ArH), 7.29 (1H, dd,JHH 7.7 and 1.4, ArH), 7.16–7.25 (2H, m, ArH), 6.83–6.94 (3H, m,ArH), 6.72–6.80 (1H, m, ArH), 6.42 (2H, br, Dn1/2 17 Hz, ArH),3.93 (1H, br, Dn1/2 19 Hz, N CH), 1.56 (3H, s, 9-CMe), 1.51(15H, s, C5Me5), 1.42 (3H, s, 9-CMe), 1.20 (3H, br s, SiMe), 0.86

(6H, s, SiMe ¥ 2) and 0.50 (3H, s, SiMe); m/z (EI) 775 (18%, M+),747 (20%, M+ - CO) and 325 (100%, xantsilH+). Because of somedynamic behaviour of 5b in C6D6 at room temperature, we couldnot observe all of the 13C and 29Si signals in the 13C{1H} and29Si{1H} NMR spectra of 5b. The observed signals are as follows:dC (75.5 MHz; C6D6) 157.5, 156.8, 150.8, 133.6, 132.5, 131.6, 130.3,127.0, 126.8, 124.5, 124.0 123.4 and 123.2 (ArC), 99.9 (C5Me5),35.3 (9-CMe2), 11.9 (br, SiMe), 11.2 (C5Me5), 9.9 (SiMe), 3.7 (br,SiMe) and 2.8 (SiMe); dSi (119 MHz; toluene-d8; ca. 296 K) 10.6.

X-Ray crystal structure determination

Selected crystallographic data for 1a, 3a and 5b are summarisedin Table 1. Intensity data for the analysis were collected ona Rigaku RAXIS-RAPID imaging plate diffractometer withgraphite-monochromated Mo-Ka radiation (l = 0.71069 A) undera cold nitrogen stream (T = 150 K). Numerical absorptioncorrections were applied to the data. The structures were solvedby the Patterson method using the DIRDIF-99 program11 andrefined by full matrix least-squares techniques on all F 2 data withSHELXL-97.12 Anisotropic refinement was applied to all non-hydrogen atoms. The metal–hydrido hydrogen in 1a, the hydrogenatom that participates in the h2-Si–H ligand in 3a and the hydrogenon the imine carbon C(2) in 5b were found on the difference Fouriermap and refined isotropically. Other hydrogen atoms were putat calculated positions. The correct absolute structure of 1a wasconfirmed by refinement of the Flack parameter13 [-0.02(4)]. Thestructure of 3a was solved as a racemic twin [Flack parameter:0.35(3)] in the acentric space group P212121. Crystallographic dataare available as a CIF file.†

Results and discussion

Synthesis and characterisation of hydrido(j2Si,Si-xantsil)complexes of molybdenum and tungsten

Irradiation (l > 300 nm) of a 1 : 1 mixture of Cp*M(CO)3Me (M =Mo and W) and the ligand precursor xantsilH2 in pentane gaveCp*M(k2Si,Si-xantsil)(CO)2(H) (1a: M = Mo and 1b: M = W) in

Table 1 Crystallographic data for 1a, 3a and 5b

Compound 1a 3a 5b

Formula C31H40O3Si2Mo C35H49NO2Si2Mo C37H45NO2Si2WFormula weight 612.75 667.87 775.77Crystal system Monoclinic Orthorhombic MonoclinicSpace group P21 (no. 4) P212121 (no. 19) P21/n (no. 14)a/A 10.0607(3) 11.2178(3) 21.5695(7)b/A 8.3977(4) 14.7149(5) 8.7738(2)c/A 17.5907(6) 20.4190(6) 21.5198(7)b/◦ 95.1177(7) 124.2959(12)V/A3 1480.26(10) 3370.54(18) 3364.49(17)Z 2 4 4Dcalcd/g cm-3 1.375 1.316 1.532F(000) 640 1408 1568m(Mo-Ka)/mm-1 0.554 0.491 3.538Reflections collected 13385 29007 25937Unique reflections (Rint) 6606 (0.0436) 7699 (0.0549) 7306 (0.0680)Refined parameters 349 389 403R1, wR2 (all data) 0.0491, 0.1171 0.0339, 0.0878 0.0402, 0.1177R1, wR2 [I > 2 s (I)] 0.0421, 0.1099 0.0323, 0.0848 0.0342, 0.1023GOF 1.196 1.277 1.141

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78% (1a) and 81% (1b) isolated yields as colourless powders [eqn(1)]. A possible formation mechanism of 1a,b is as follows: (1) Si–H oxidative addition on the vacant coordination site generated byphoto-dissociation of one carbonyl ligand in Cp*M(CO)3Me, (2)reductive elimination of methane and (3) second Si–H oxidativeaddition to produce complexes 1a,b.

(1)

Complexes 1a,b were characterised by spectroscopy and ele-mental analysis. Two SiMe2 moieties in 1a,b are equivalent in the1H, 13C{1H} and 29Si{1H} NMR spectra in C6D6 solution. The 1HNMR spectra of 1a,b show one singlet of the hydrido ligand at d-5.52 (1a) and -3.95 (1b) with 29Si satellites [JSiH = 37 (1a) and 25(1b) Hz]. Since JSiH values are generally below 20 Hz for classicalhydrido(silyl) complexes with 2c–2e bonds,2 the JSiH values of 1a,bsuggest the presence of non-classical interaction between siliconand the hydrido hydrogen.2,14 The 29Si{1H} NMR spectrum showsone signal at d 15.5 (1a) and 5.9 (1b) which is shifted downfieldfrom that of xantsilH2 (d -22.2)5e due to the coordination to themetal. The IR spectra of 1a,b exhibit two bands for a symmetricand an antisymmetric CO stretching vibration in the terminal COregion, where the intensity of the former band [1909 (1a) and 1901(1b) cm-1] is stronger than that of the latter one [1849 (1a) and1843 (1b) cm-1]. This supports the mutually cis arrangement of thetwo carbonyl ligands in 1a,b.

The molecular structure of 1a was confirmed by X-ray crys-tal structure analysis (Fig. 1, Table 2) having a piano-stoolgeometry with cis arrangement of two carbonyl ligands. Thexantsil ligand in 1a is coordinated to the molybdenum centrein a bidentate k2Si,Si-fashion. The Mo–Si distances [2.6765(13)and 2.6815(13) A] are at the longer end of the usual Mo–Sisingle-bond distances [2.4866(4)–2.670(2) A]15 and considerablylonger than those of the bis(silyl)hydrido molybdenum complexCp*Mo(SiH2Ph)2(dmpe)(H) [2.504(3) and 2.531(3) A].16 Similar

Fig. 1 Crystal structure of 1a. Thermal ellipsoids are drawn at the 50%probability level, and all the hydrogen atoms except the hydrido hydrogenatom H(1) are omitted for clarity.

Table 2 Selected bond distances (A) and angles (◦) in 1a

Mo–Si(1) 2.6765(13) Mo–Si(2) 2.6815(13)Mo–C(1) 1.953(6) Mo–C(2) 1.965(6)Mo–H(1) 1.80(8)

Si(1)–Mo–Si(2) 94.08(4) Si(1)–Mo–C(1) 118.01(16)Si(1)–Mo–C(2) 67.59(16) Si(2)–Mo–C(1) 66.37(16)Si(2)–Mo–C(2) 118.17(16) C(1)–Mo–C(2) 72.2(2)

elongations of metal–silicon bonds are known in the k2Si,Si-xantsil complexes of group 8 metals cis-[M(xantsil)(CO)4] (M =Fe, Ru and Os), which is attributable to the steric requirementof the xantsil ligand on coordination to the metal centre.5e Thehydrido hydrogen H(1) was found on the difference Fourier mapand located between two silicon atoms of the xantsil ligand.The Si ◊ ◊ ◊ H(1) interatomic distances [Si(1) ◊ ◊ ◊ H(1): 1.93(8) A andSi(2) ◊ ◊ ◊ H(1): 2.06(8) A] are in the range of Si ◊ ◊ ◊ H distances (1.9–2.1 A) of hydrido(silyl) complexes with significant silyl–hydridointeractions.14a This suggests the presence of a non-classical Si–Hinteraction in 1a, which is consistent with the above discussionon the JSiH value of 1a. Similar Si–H interligand interactionswere discovered by Nikonov and his coworkers in bis(silyl)hydridoniobium complexes.14a,b

Synthesis and characterisation of hydrido(j3Si,Si,O-xantsil)complexes

Further irradiation of 1a,b in toluene led to dissociation of a COligand, and subsequent coordination of oxygen in the xanthenebackbone afforded tridentate xantsil complexes Cp*M(k3Si,Si,O-xantsil)(CO)(H) (2a: M = Mo and 2b: M = W), in 86% (2a) and92% (2b) isolated yields, as orange powders [eqn (2)].

(2)

The photo-induced CO dissociation from 1a,b is reversible. Ina sealed NMR tube, a C6D6 solution of 2a,b in the presence ofone equivalent of CO (which had been quantitatively formed viaphotolysis of 1a,b) displayed slow conversion into 1a,b, almostquantitatively, upon standing at room temperature for 1 month[eqn (2)]. This clearly indicates that the xantsil ligand in 2a,bis hemilabile, and the xantsil oxygen dissociates easily at roomtemperature.

The crystal structure of 2b determined by single-crystal X-rayanalysis has already been reported in the preliminary communica-tion, revealing the k3Si,Si,O-coordination of the xantsil ligand.7

The 1H NMR spectra of 2a,b in C6D6 show the resonances of thehydrido ligand at unusually low field [d -2.27 (2a) and 0.78 (2b)]presumably due to the large magnetic anisotropy around the metalcentre. The JSiH value of the hydrido signal of 2a (12 Hz) is smallerthan that of 1a (37 Hz) and less than 20 Hz, indicating that non-classical interaction between silicon and the hydrido hydrogen in2a is negligible.2 In the 29Si{1H} NMR spectrum of 2a in C6D6

at room temperature, a signal was observed at d 32.9. Thus, thetwo SiMe2 moieties in 2a are equivalent in the 29Si{1H} NMR

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spectrum. On the other hand, the 29Si{1H} NMR spectrum of2b in toluene-d8 at room temperature (ca. 25 ◦C) showed a broadsignal at d 33.8. Upon cooling of the solution to -70 ◦C, two sharpresonances were observed at d 45.9 and 23.6. This is attributableto some dynamic behaviour in the solution of 2b on the NMRtime scale. The 29Si signals of 2a,b are shifted downfield comparedto those of 1a,b. The IR spectra of 2a,b show one CO stretchingband at 1819 (2a) and 1815 (2b) cm-1, which is shifted to a lowerwavenumber region compared to those of 1a,b. The molecularformulas of 2a,b were also confirmed by elemental analysis andmass spectroscopy.

Reaction of hydrido(j3Si,Si,O-xantsil) complexes 2a,b witht-BuCN: formation of N-silyliminoacyl complexes

Complexes 2a,b can become a good precursor of 16-electron silylcomplex because of the hemilability of the xantsil ligand. Thisprompted us to examine their reactions with nitriles.17 The reactionof 2a,b with t-BuCN (3–5 equiv.) in toluene at room temperaturefor 30 (2a) or 100 (2b) min led to colour change of the solution fromorange to reddish purple (for 2a) and from orange to purple (for2b). From the reaction mixture, h2-N-silyliminoacyl complexesCp*M[k4Si,H,N,C-xantsil(H){N C(t-Bu)}](CO) (3a: M = Moand 3b: M = W) [eqn (3)] were isolated as a reddish purple (3a)or purple (3b) powder in 82% (3a) and 78% (3b) yields. In thisreaction, one nitrile molecule was inserted into the M–Si bond of2a,b.

(3)

The 1H NMR spectra of 3a,b in C6D6 show four inequivalentSiMe signals at d 1.00, 0.92, 0.84 and 0.47 for 3a and d 1.20,1.04, 0.83 and 0.46 for 3b. A signal assignable to the hydrogenof the h2-Si–H ligand was observed at d -1.72 with 29Si satellites[JSiH = 64 Hz] for 3a and at d 1.03 for 3b.10 The JSiH value of 3a isconsiderably larger than those of 1a (37 Hz) and 2a (12 Hz) andis in the range (38–65.4 Hz) of those in the complexes of the typeCp¢Mn(CO)L(h2-H–SiHPh2) (Cp¢ = C5H4Me, C5Me5; L = CO,isocyanide, phosphine).18 The 29Si{1H}NMR spectra of 3a,b showtwo signals at d -1.8 and -8.4 (for 3a) and d -2.9 and -9.2 (for 3b)as expected for the structures. In the 1H-29Si HMQC NMR spectraof 3a,b, one cross-peak is observed between the 29Si signal at higherfield [d -8.4 (3a) and -9.2 (3b)] and the hydrido proton resonance[d -1.72 (3a) and 1.03 (3b)], respectively, indicating that the 29Sisignal is assignable to the silicon that strongly interacts with thehydrido hydrogen in 3a,b. This 29Si signal is considerably upfieldshifted compared to those of 2a,b [d 32.9 (2a) and 33.8 (2b)], whichpossibly reflects a weaker metal–silicon bonding interaction in 3a,bcompared with that in 2a,b. These observations are consistentwith the existence of a 3c–2e M–Si–H bond in 3a,b. On theother hand, the other 29Si signal at d -1.8 (3a) and -2.9 (3b)can be assigned to the silicon composing the N-silyliminoacylligand. These chemical shifts are close to that of the 29Si signal ofthe N-silylimine (t-Bu)CH N(SiMe2Ph) (d -3.1),3f in which the

substituents on silicon are similar to those in 3a,b. The IR spectraof 3a,b show one band each at 1634 (3a) and 1605 (3b) cm-1,assignable to the C N stretching mode of the h2-N-silyliminoacylligand, as well as one CO stretching band at 1794 (3a) and1792 (3b) cm-1. The nC N values are comparable with those ofthe h2-N-silyliminoacyl tungsten complexes Cp*W(CO)2{k2C,N-C(R) NSiR¢3} [R = Me, Et, i-Pr, t-Bu; R¢3 = (p-Tol)2Me, (p-Tol)3, Et3] (1591–1637 cm-1).19 These spectroscopic data supportthe molecular structures of 3a,b.

The crystal structure of 3b has already been reported in a pre-liminary communication.7 This time, single-crystal X-ray analysiswas performed for 3a (Fig. 2, Table 3), revealing that complex3a involves not only a Mo–N–C three-membered ring but alsoa 3c-2e Mo–Si–H bond. Complex 3a has a pseudo three-leggedpiano-stool geometry around the molybdenum centre, where thelegs are composed of an h2-N-silyliminoacyl, an h2-Si–H and a COligand. The N-silyliminoacyl and h2-Si–H ligands, interconnectedby a xanthene backbone, form a unique 9-membered chelate ring.The Mo–N, Mo–C(2) and N–C(2) bond distances in the Mo–N–C three-membered ring are 2.236(2), 2.073(3) and 1.262(4) A,respectively. These distances are similar to those found in h2-N-alkyliminoacyl molybdenum complexes,20 and this fact confirmsthe h2-iminoacyl structure. The Mo–Si(2) bond distance [2.6659(7)A] is similar to those in k2Si,Si-xantsil complex 1a [2.6765(13) and2.6815(13) A]. The hydrogen atom H(1) of the h2-Si–H ligand,which was found on the difference Fourier map and refined,

Fig. 2 Crystal structure of 3a. Thermal ellipsoids are drawn at the 50%probability level, and all the hydrogen atoms except the hydrogen atomH(1) in the h2-Si–H ligand are omitted for clarity.

Table 3 Selected bond distances (A) and angles (◦) in 3a

Mo–Si(2) 2.6659(7) Mo–N 2.236(2)Mo–C(1) 1.917(3) Mo–C(2) 2.073(3)Mo–H(1) 1.71(4) Si(1)–N 1.747(2)Si(2)–H(1) 1.68(4) N–C(2) 1.262(4)

Si(2)–Mo–N 94.46(6) Si(2)–Mo–C(1) 65.45(8)Si(2)–Mo–C(2) 107.82(7) Si(2)–Mo–H(1) 37.8(13)N–Mo–C(1) 110.65(11) N–Mo–C(2) 33.79(10)C(1)–Mo–C(2) 87.78(12) Mo–Si(2)–H(1) 38.6(14)Mo–N–Si(1) 145.45(13) Mo–N–C(2) 65.99(15)Mo–C(2)–N 80.22(17) Mo–C(2)–C(3) 147.1(2)Mo–H(1)–Si(2) 104(2) Si(1)–N–C(2) 148.1(2)N–C(2)–C(3) 131.5(3)

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bridges over the Mo–Si(2) bond [Mo–H(1)–Si(2): 104(2)◦]. TheSi(2)–H(1) bond distance [1.68(4) A] is in the normal range(1.6–1.9 A) of those in h2-silane complexes.14a These structuralfeatures confirm the Mo–Si–H 3c–2e bond. Although we couldnot locate the hydrido hydrogen of tungsten complex 3b in the X-ray analysis,7 the geometrical similarity between 3a and 3b suggeststhe presence of a W–Si–H 3c–2e interaction in 3b.

Conversion of N-silyliminoacyl complexes into N-silyiminecomplexes via intramolecular hydrogen migration

Complexes 3a,b were thermally unstable in toluene evenat room temperature, and were quantitatively isomerisedto N-silylimine complexes Cp*M[k3Si,N,C-xantsil{N C(H)(t-Bu)}](CO) (4a: M = Mo and 4b: M = W) through migrationof hydrogen in the h2-Si–H ligand to the iminoacyl carbon [eqn(4)]. Thus, stirring a toluene solution of 3a,b for 4 days at roomtemperature resulted in a colour change of the solution fromreddish purple to dark cyan (for 3a) and from purple to violet (for3b), and removal of volatiles from the solution resulted in isolationof 4a and 4b as a blue–green or violet powder, respectively, in both95% yields.

(4)

The following spectroscopic data clearly demonstrate the for-mation of h2-N-silylimine complex 4a,b via hydrogen migration.The 1H NMR spectra of 4a,b show a new characteristic signalassignable to the imine proton at d 3.91 (4a) and 3.89 (4b), whereasthere is no 1H NMR signal for an h2-Si–H or a hydrido ligand.In the 13C{1H} NMR spectra of 4a,b, one signal assignable toan imine carbon appears at d 74.3 (4a) and 68.9 (4b). These 1Hand 13C chemical shifts are typical of h2-imine complexes.21,22 The29Si{1H} NMR spectra of 4a,b show two signals at d 31.7 and 0.7(4a) and d 22.1 and 3.7 (4b). The signals at lower field [d 31.7(4a) and 22.1 (4b)] can be assigned to the silicon of the silyl ligandbecause the chemical shifts are close to those of 2a,b [d 32.9 (2a)and 33.8 (2b)]. The IR spectra of 4a,b show one CO stretchingband at 1867 (4a) and 1868 (4b) cm-1, while no characteristic bandassignable to a C N stretching vibration is observed. The nCO

values are shifted to a higher wavenumber region by 73 (4a) and76 (4b) cm-1 compared to those of 3a,b, implying that the backdonation to the CO ligand is weaker in 4a,b than in 3a,b.

Reaction of hydrido(j3Si,Si,O-xantsil) complexes 2a,b with PhCNto form N-silylimine complexes

To explore the substituent effect of nitrile on its reaction with 2a,b,we next examined the reaction of complexes 2a,b with PhCN. Thus,stirring of a mixture of 2a,b and PhCN (5 equiv.) in toluene atroom temperature for 2 h (2a) or 90 min (2b) gave N-silyliminecomplexes Cp*M[k3Si,N,C-xantsil{N C(H)(Ph)}](CO) (5a: M =Mo and 5b: M = W) directly in 74% (5a) and 78% (5b) isolatedyields, as khaki (5a) or purple (5b) powders [eqn (5)]. This reaction

probably proceeds through the generation of an N-silyliminoacylintermediate (complexes C in Scheme 1), which then isomerise to5a,b much faster than 3a,b.

(5)

Scheme 1 Possible formation mechanism of N-silyliminoacyl complexes3a,b and N-silylimine complexes 4a,b and 5a,b.

The NMR spectroscopic data for 5a are consistent with themolecular structure involving the h2-N-silylimine ligand. The1H signal of imine proton and the 13C signal of imine carbonwere observed at d 3.92 and 71.0, respectively. These chemicalshifts are similar to those of 4a. The 29Si{1H} NMR spectrumof 5a shows two signals at d 36.5 and 13.7, where the formersignal is attributable to the silicon of the silyl ligand. On theother hand, the 1H NMR spectrum of 5b in C6D6 at roomtemperature shows a broad signal assignable to the imine protonat d 3.93. Some broad SiMe and ArH signals are also observed.These broadenings of 1H signals are attributable to some dynamicbehaviour in solution at room temperature on the NMR time scale.A likely process is interconversion between two diastereomersvia inversion of one of two chiral centres on tungsten and theimine carbon.22g,23 Due to this fluctionality, we could not observesignals assignable to the imine carbon in the 13C{1H} NMRspectrum at room temperature. The 29Si{1H} NMR spectrum of5b in toluene-d8 at room temperature (ca. 23 ◦C) shows onlyone signal at d 10.6, where a signal attributable to the othersilicon in 5b is possibly too broadened to be observed becauseof the fluctionality.23 Thus, 5b was further characterised by X-raycrystallography.

The molecular structure of N-silylimine complex 5b having aW–N–C three-membered ring was unequivocally determined bythe single-crystal X-ray analysis (Fig. 3, Table 4). Complex 5badopts a pseudo three-legged piano-stool geometry: the tungstencentre possesses an h5-C5Me5, an h2-N-silylimine, a silyl and a

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Table 4 Selected bond distances (A) and angles (◦) in 5b

W–Si(2) 2.5991(13) W–N 1.948(3)W–C(1) 1.974(5) W–C(2) 2.175(4)Si(1)–N 1.755(4) N–C(2) 1.430(6)

Si(2)–W–N 89.05(11) Si(2)–W–C(1) 82.21(14)Si(2)–W–C(2) 93.24(12) N–W–C(1) 90.12(18)N–W–C(2) 40.12(17) C(1)–W–C(2) 130.23(18)W–N–Si(1) 155.1(2) W–N–C(2) 78.5(2)W–C(2)–N 61.4(2) W–C(2)–C(3) 123.0(3)Si(1)–N–C(2) 126.3(3) N–C(2)–C(3) 119.1(4)

Fig. 3 Crystal structure of 5b. Thermal ellipsoids are drawn at the 50%probability level, and all the hydrogen atoms except the hydrogen atomH(1) on the imine carbon C(2) are omitted for clarity.

CO ligand. The W–N–C three-membered ring is nearly coplanarwith the CO ligand, where the nitrogen atom is located at theproximal position to CO. A similar arrangement is reported byTempleton and co-workers for the related carbonyl(h2-imine) com-plexes W(CO)(acac)2(h2-PhN CHPh)22f and W(CO)(acac)2(h2-MeN CMePh),22g but the position of the nitrogen atom in theh2-imine ligand in these complexes is distal to the CO ligand.The W–Si(2) bond distance of 2.5991(13) A is within the range(2.53–2.63 A)24 of typical tungsten–silicon single bonds. The W–N bond distance [1.948(3) A] in the W–N–C three-membered ringis considerably shorter than that of N-silyliminoacyl complex 3b[2.235(5) A],7 whereas the W–C(2) bond distance [2.175(4) A] islonger than that of 3b [2.063(6) A].7 The C–N bond distance in 5b[1.430(6) A] is longer than those of N-silyliminoacyl complex 3a,b[1.262(4) (3a) and 1.257(9) A (3b)7]. The sum of the bond anglesaround the imine nitrogen is 359.9(4)◦, indicating the planarity ofthe bonds around nitrogen. These geometric features of the W–N–C three-membered ring suggest that the ring is best described as ametallaaziridine containing nitrogen-to-tungsten p-donation.22b,f ,g

Thus, the h2-N-silylimine ligand in 5b acts as a 4-electron donorto satisfy the 18-electron rule.

Although we tentatively propose that the coordination geometryin molybdenum complex 5a is analogous to that in tungsten

complex 5b, there are some marked differences in the 1H NMRand IR data between them: (1) The 1H signals of the SiMe groupsin 5a (d 0.59, 0.57, -0.04 and -0.77) are substantially upfieldshifted compared to those in 5b (d 1.20, 0.86 and 0.50 in 1 : 2 : 1intensity ratio), indicating the presence of magnetic shielding bythe ring current on the aromatic rings in 5a. (2) In the IR spectraof 5a,b, the CO stretching band of 5a (1745 cm-1) is shown ata much lower wavenumber (166 cm-1 lower) than that of 5b(1911 cm-1).25 Therefore, we cannot rule out the possibility that5a has a molecular structure that is significantly different fromthat of 5b. Further structural characterisation of 5a by X-raycrystallographic analysis is underway.

Possible formation mechanism of N-silyliminoacyl complexes 3a,band N-silylimine complexes 4a,b and 5a,b

A possible formation mechanism of N-silyliminoacyl complexes3a,b and N-silylimine complexes 4a,b and 5a,b is depicted inScheme 1. This reaction possibly proceeds through the followingsteps: (1) Dissociation of the xantsil oxygen ligand in 2a,bgenerates 16-electron bis(silyl)hydrido complexes A. (2) Side-on coordination of nitrile RCN (R = t-Bu and Ph) on themetal centre affords nitrile complexes B. (3) Insertion of nitrileinto the M–Si bond gives N-silyliminacyl complexes 3a,b andC. (4) 1,2-Migration of the hydrogen of the h2-Si–H moietyin 3a,b and C to the iminoacyl carbon produces N-silyliminecomplexes 4a,b and 5a,b. Importantly, in this reaction sequence,a nitrile molecule is hydrosilylated to an N-silylimine ligand. Thisresult suggests that 16-electron hydrido(silyl) complexes can bean intermediate in nitrile-hydrosilylation to N-silylimine via N-silyliminoacyl complexes.

The related migratory insertion of a nitrile C N bond intoa metal–silyl bond to give an h2-N-silyliminoacyl complex hasrecently been discovered by Bergman, Brookhart and co-workersfor rhodium complexes26 and by Nakazawa and co-workers for aniron complex.27 In these reactions, the resulting N-silyliminoacylcomplexes easily isomerise to the silylisocyanide complexes viacleavage of a C–C bond. Thus, the N-silyliminoacyl complexesact as intermediates in the C–C bond activation of nitriles.On the other hand, in our case, the produced N-silyliminoacylcomplexes 3a,b are thought to be stable against C–C bondactivation. This assumption is strongly supported by our recentwork on the thermally induced C–C bond cleavage of structurallyclosely related N-silyliminoacyl tungsten complexes that proceedsabove 120 ◦C.19b Thus, instead of the C–C bond activation, 3a,bundergo isomerisation via hydrogen migration to give N-silyliminecomplexes 4a,b.

The isomerisation of N-silyliminoacyl complexes C to N-silylimine complexes 5a,b is much faster than that of 3a,b to4a,b, implying that the thermal stability of PhCN derivativesC is considerably lower than that of t-BuCN derivatives 3a,b.The difference in the thermal stability between 3a,b and C isattributable to both steric and electronic factors: (1) The bulkiert-Bu group on the iminoacyl carbon in 3a,b inhibits the hydrogenmigration from metal to iminoacyl carbon (steric factor), and(2) the higher electrophilicity of the phenyl-substituted iminoacylcarbon in C lowers the activation barrier for the migration of anucleophilic hydrido ligand (electronic factor).

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Conclusions

In this work, we synthesised molybdenum and tungsten xantsilcomplexes 1a,b and 2a,b by photoreaction. The k3Si,Si,O-xantsilligand in complexes 2a,b was found to be hemilabile, and thus 2a,bcan generate 16-electron hydrido(silyl) complexes under mild con-ditions. The reactions of 2a,b with nitriles RCN (R = t-Bu and Ph)led to N-silylimine complexes 4a,b and 5a,b through stoichiometrichydrosilylation. In the reaction with t-BuCN, N-silyliminoacylcomplexes 3a,b were isolated as intermediates. These findingssuggest that hydrosilylation of nitriles mediated by a 16-electronhydrido(silyl) complex can proceed through insertion of nitrileinto the metal–silicon bond to give N-silyliminoacyl complexesfollowed by migration of the hydrido hydrogen atom. Furtherstudy on applications of this stoichiometric hydrosilylation tocatalytic reactions is in progress.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research(Nos. 18350027, 18064003 and 20750040) from the Ministry ofEducation, Culture, Sports, Science and Technology of Japan. Oneof the authors (T.K.) is grateful to JGC-S Scholarship Foundationfor financial support. We are grateful to Mr. Takeyoshi Kondo(Tohoku University) for his help with the NMR spectroscopicanalysis. We also acknowledge the Research and Analytical Centerfor Giant Molecules, Tohoku University for spectroscopic andanalytical measurements.

Notes and references

1 (a) B. Marciniec, Appl. Organomet. Chem., 2000, 14, 527 and referencestherein; (b) I. Ojima, Z. Li and J. Zhu, in The Chemistry of OrganicSilicon Compounds, ed. Z. Rappoport and Y. Apeloig, Wiley, NewYork, 1998, vol. 2, pp. 1687–1792; (c) I. Ojima, in The Chemistry ofOrganic Silicon Compounds, ed. S. Patai and Z. Rappoport, Wiley, NewYork, 1989, vol. 1, pp. 1479–1526; (d) B. Marciniec, H. Maciejewski, C.Pietraszuk and P. Pawluc, in Hydrosilylation: A Comprehensive Reviewon Recent Advances, ed. B. Marciniec, Springer, 2009.

2 J. Y. Corey and J. Braddock-Wilking, Chem. Rev., 1999, 99, 175 andreferences therein.

3 (a) R. J. P. Corriu, J. J. E. Moreau and M. Pataud-Sat, J. Organomet.Chem., 1982, 228, 301; (b) T. Murai, T. Sakane and S. Kato, TetrahedronLett., 1985, 26, 5145; (c) T. Murai, T. Sakane and S. Kato, J. Org. Chem.,1990, 55, 449; (d) N. P. Reddy, Y. Uchimaru, H.-J. Lautenschlager andM. Tanaka, Chem. Lett., 1992, 45; (e) J. Kim, Y. Kang, J. Lee, Y. K.Kong, M. S. Gong, S. O. Kang and J. Ko, Organometallics, 2001, 20,937; (f) P. Deglmann, E. Ember, P. Hofmann, S. Pitter and O. Walter,Chem.–Eur. J., 2007, 13, 2864; (g) A. Y. Khalimon, R. Simionescu, L. G.Kuzmina, J. A. K. Howard and G. I. Nikonov, Angew. Chem., Int. Ed.,2008, 47, 7701; (h) D. V. Gutsulyak and G. I. Nikonov, Angew. Chem.,Int. Ed., 2010, 49, 7553.

4 (a) M. Tanabe and K. Osakada, Organometallics, 2001, 20, 2118; (b) H.Hashimoto, I. Aratani, C. Kabuto and M. Kira, Organometallics, 2003,22, 2199; (c) T. Watanabe, H. Hashimoto and H. Tobita, J. Am. Chem.Soc., 2006, 128, 2176.

5 (a) H. Tobita, K. Hasegawa, J. J. G. Minglana, L.-S. Luh, M. Oakazakiand H. Ogino, Organometallics, 1999, 18, 2058; (b) J. J. G. Minglana,M. Okazaki, H. Tobita and H. Ogino, Chem. Lett., 2002, 406; (c) M.Okazaki, J. J. G. Minglana, N. Yamahira, H. Tobita and H. Ogino,Can. J. Chem., 2003, 81, 1350; (d) M. Okazaki, N. Yamahira, J. J. G.Minglana and H. Tobita, Organometallics, 2004, 23, 4531; (e) J. J. G.Minglana, M. Okazaki, K. Hasegawa, L.-S. Luh, N. Yamahira, T.Komuro, H. Ogino and H. Tobita, Organometallics, 2007, 26, 5859;(f) M. Okazaki, N. Yamahira, J. J. G. Minglana, T. Komuro, H.Ogino and H. Tobita, Organometallics, 2008, 27, 918; (g) H. Tobita,

N. Yamahira, K. Ohta, T. Komuro and M. Okazaki, Pure Appl. Chem.,2008, 80, 1155.

6 C. S. Slone, D. A. Weinberger and C. A. Mirkin, Prog. Inorg. Chem.,1999, 48, 233.

7 R. Begum, T. Komuro and H. Tobita, Chem. Commun., 2006, 432.8 R. B. King and M. B. Bisnette, J. Organomet. Chem., 1967, 8, 287.9 K. A. Mahmoud, A. J. Rest, H. G. Alt, M. E. Eichner and B. M. Jansen,

J. Chem. Soc., Dalton Trans., 1984, 175.10 We could not determine the JSiH and JWH values of the hydrido signal

of 2b and 3b because the 29Si and 183W satellites are obscured by othersignals.

11 P. T. Beurskens, G. Beurskens, R. de Gelder, S. Garcia-Granda, R. O.Gould, R. Israel and J. M. M. Smits, The DIRDIF-99 program system,Crystallography Laboratory, University of Nijmegen, The Netherlands,1999.

12 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,64, 112.

13 H. D. Flack, Acta Crystallogr., Sect. A: Found. Crystallogr., 1983, 39,876.

14 (a) Z. Lin, Chem. Soc. Rev., 2002, 31, 239 and references therein; (b) G. I.Nikonov, J. Organomet. Chem., 2001, 635, 24; (c) S. Lachaize andS. Sabo-Etienne, Eur. J. Inorg. Chem., 2006, 2115; (d) G. J. Kubas,J. Organomet. Chem., 2001, 635, 37.

15 Based on a survey of the Cambridge Structural Database, CSD version5.31, November 2009.

16 B. V. Mork, T. D. Tilley, A. J. Schultz and J. A. Cowan, J. Am. Chem.Soc., 2004, 126, 10428.

17 We have also examined the reaction of 2b with 4-(dimethylamino)pyridine, giving a W–Si–N–C four-memberedmetallacycle: R. Begum, T. Komuro and H. Tobita, Chem. Lett., 2007,36, 650.

18 U. Schubert, Adv. Organomet. Chem., 1990, 30, 151.19 (a) E. Suzuki, T. Komuro, M. Okazaki and H. Tobita, Organometallics,

2007, 26, 4379; (b) E. Suzuki, T. Komuro, Y. Kanno, M. Okazaki andH. Tobita, Organometallics, 2010, 29, 1839.

20 (a) R. D. Adams and D. F. Chodosh, Inorg. Chem., 1978, 17, 41; (b) T.Yoshida, K. Hirotsu, T. Higuchi and S. Otsuka, Chem. Lett., 1982,11, 1017; (c) J. E. Hill, P. E. Fanwick and I. P. Rothwell, Polyhedron,1992, 11, 2825; (d) S. J. Dossett, S. Li, D. F Mullica, E. L. Sappenfieldand F. G. A. Stone, Organometallics, 1994, 13, 822; (e) A. Pizzano, L.Sanchez, M. Altmann, A. Monge, C. Ruiz and E. Carmona, J. Am.Chem. Soc., 1995, 117, 1759; (f) C. J. Adams, K. M. Anderson, I. M.Bartlett, N. G. Connelly, A. G. Orpen, T. J. Paget, H. Phetmung andD. W. Smith, J. Chem. Soc., Dalton Trans., 2001, 1284; (g) J. H. Shin,W. Savage, V. J. Murphy, J. B. Bonanno, D. G. Churchill and G. Parkin,J. Chem. Soc., Dalton Trans., 2001, 1732.

21 For selected examples of molybdenum h2-imine complexes, see: (a) J.Okuda and G. E. Herberich, Organometallics, 1987, 6, 2331; (b) T. M.Cameron, C. G. Ortiz, K. A. Abboud, J. M. Boncella, R. T. Baker andB. L. Scott, Chem. Commun., 2000, 573; (c) Z. J. Tonzetich, A. J. Jiang,R. R. Schrock and P. Muller, Organometallics, 2007, 26, 3771.

22 For selected examples of tungsten h2-imine complexes, see: (a) K. W.Chiu, R. A. Jones, G. Wilkinson, A. M. R. Galas and M. B. Hursthouse,J. Am. Chem. Soc., 1980, 102, 7978; (b) K. W. Chiu, R. A. Jones, G.Wilkinson, A. M. R. Galas and M. B. Hursthouse, J. Chem. Soc.,Dalton Trans., 1981, 2088; (c) M. A. Lockwood, P. E. Fanwick and I. P.Rothwell, Chem. Commun., 1996, 2013; (d) A. Dimitrov, S. Seidel and K.Seppelt, Eur. J. Inorg. Chem., 1999, 95; (e) B. D. Ward, G. Orde, E. Clot,A. R. Cowley, L. H. Gade and P. Mountford, Organometallics, 2004, 23,4444; (f) A. B. Jackson, C. K. Schauer, P. S. White and J. L. Templeton,J. Am. Chem. Soc., 2007, 129, 10628; (g) A. B. Jackson, C. Khosla,H. E. Gaskins, P. S. White and J. L. Templeton, Organometallics, 2008,27, 1322.

23 Tentative low temperature NMR spectroscopic measurements for 5bin toluene-d8 indicate that the dynamic behaviour is attributable tointerconversion between two diastereomers. The 1H NMR spectrum of5b at -70 ◦C shows two sets of signals assignable to the diastereomers.Two singlet signals assignable to imine protons of the diastereomersare observed at d 4.83 and 3.50 in the intensity ratio of 0.6 : 1, whichcorresponds to the molar ratio of the diastereomers. Similarly, the29Si{1H} NMR spectrum of 5b at -90 ◦C shows four signals at d 15.3,11.2, 10.9 and 4.5, but that at room temperature (ca. 23 ◦C) shows onlyone signal at d 10.6 due to the dynamic behaviour.

24 M. Okazaki, E. Suzuki, N. Miyajima, H. Tobita and H. Ogino,Organometallics, 2003, 22, 4633.

2356 | Dalton Trans., 2011, 40, 2348–2357 This journal is © The Royal Society of Chemistry 2011

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25 The unusual lower-wavenumber shift of the CO stretching frequency of5a is possibly attributable to s–p* back-bonding from the silyl ligandto the CO ligand. This type of silicon-to-carbon back-bonding wasdiscovered and reported by Berry and co-workers for the zirconiumsilanimine carbonyl complex Cp2Zr[h2-Me2Si N(t-Bu)](CO), wherethe carbonyl stretching frequency (1797 cm-1) is considerably lower-wavenumber shifted compared with other zirconocene carbonyl com-

plexes: L. J. Procopio, P. J. Carroll and D. H. Berry, Polyhedron, 1995,14, 45.

26 (a) F. L. Taw, P. S. White, R. G. Bergman and M. Brookhart, J. Am.Chem. Soc., 2002, 124, 4192; (b) F. L. Taw, A. H. Mueller, R. G. Bergmanand M. Brookhart, J. Am. Chem. Soc., 2003, 125, 9808.

27 H. Nakazawa, T. Kawasaki, K. Miyoshi, C. H. Suresh and N. Koga,Organometallics, 2004, 23, 117.

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