Metal–Silicon Triple Bonds: The Molybdenum Silylidyne Complex [Cp(CO)₂Mo≡Si‐R]

Abstract

Carbene transfer from the silylidene complex [Cp(CO)2MoSi(C6H3-2,6-Trip2)(Im-Me4)] (Trip=2,4,6-triisopropylphenyl, Im-Me4=tetramethylimidazol-2-ylidene) to a triarylborane leads to the silylidyne complex [Cp(CO)2Mo≡Si(C6H3-2,6-Trip2)], which contains the first MoSi triple bond (see picture). Transition metal alkylidyne complexes are among the most important organometallic compounds. The great utility of these compounds in organometallic and organic chemistry was highlighted by E. O. Fischer and R. R. Schrock in their Nobel lectures.1, 2 Numerous stoichiometric and catalytic transformations have been accomplished with these complexes3 since their discovery in 1973.4 In comparison, silicon analogues of the alkylidyne complexes are presently unknown,5, 6 thereby emphasizing the marked difference between silicon and carbon in forming multiple bonds. A series of compounds featuring metal–germanium, metal–tin, or metal–lead triple bonds have been obtained by reacting carbonyl metalates7 or dinitrogen and homoleptic phosphane complexes of molybdenum and tungsten8 with organo Group 14 element(II) halides.9 Extension of this efficient process to silicon analogues was hampered to date by the absence of any suitable organosilicon(II) halide precursors. In fact, the isolation of transition metal silylidyne complexes is one of the most challenging unresolved targets in organosilicon chemistry. Recent studies in our group have shown that N-heterocyclic carbenes can be used to stabilize aryl silicon(II) chlorides.10 The synthetic potential of these compounds is demonstrated herein by the isolation of the first complex featuring a metal–silicon triple bond. Heating a toluene solution of SiRCl(Im-Me4) (1: R=C6H3-2,6-Trip2, Im-Me4=tetramethylimidazol-2-ylidene)10 with Li[CpMo(CO)3] 11 at 100 °C was accompanied by a color change from yellow over brown-green to brown. Monitoring of the reaction progress by IR spectroscopy revealed a rapid conversion of 1 into the silylidene complex 2, which was isolated after recrystallization from a toluene/hexane mixture as a dark-brown, air-sensitive solid in 51 % yield (Scheme 1).12 Stepwise synthesis of the silylidyne complex 3. Complex 2 was fully characterized, and its molecular structure was determined by single-crystal X-ray diffraction (Figure 1).13, 14 The three-legged piano-stool complex features a MoSi double bond (2.345 Å), which lies in the range of MoSi bonds reported for molybdenum arylsilylidene complexes (d(Mo–Si)=2.288(2)–2.3872(7) Å).5b, 15, 16 The silylidene ligand has a trigonal planar coordinated silicon center (sum of angles at Si=357.0°) and it adopts an upright conformation, with the m-terphenyl group pointing towards the cyclopentadienyl ring.17 The angles at silicon differ markedly; the Mo-Si-CAr angle is considerably widened to 145.3° owing to the large steric demand of the m-terphenyl substituent; the CAr-Si-Ccarbene angle is lowered to 100.4°, which reflects the low tendency of silicon for hybridization.18 The SiCcarbene bond of 2 (1.944 Å) compares well with that of 1 (1.963(2) Å)10 and is only slightly longer than the SiCAr bond of 2 (1.920 Å) and 1 (1.937(2) Å), thus indicating the presence of a rather strong CcarbeneSi donor–acceptor interaction.19 DIAMOND plot of the molecular structure of the silylidene complex 2. Thermal ellipsoids are set at 50 % probability, and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°] (values in square brackets are of the second independent molecule of 2 found in the asymmetric unit): Mo1–Si1 2.3474(6) [2.3430(6)], Mo1–C49 1.917(2) [1.906(2)], Mo1–C50 1.936(2) [1.938(3)], Si1–C1 1.918(2) [1.922(2)], Si1–C37 1.943(2) [1.945(2)]; Mo-Si1-C1 145.71(6) [144.92(6)], Mo-Si1-C37 111.08(6) [111.58(6)], C1-Si-C37 99.64(8) [101.12(8)], Si1-Mo1-C49 90.67(6) [91.36(7)], Si1-Mo1-C50 82.43(7) [82.53(8)], C49-Mo1-C50 78.80(9) [80.2(1)]. Further structural information is provided by the IR and NMR spectra of 2. The IR spectrum of 2 in toluene has two ν(CO) absorption bands at considerably lower wavenumbers (1859 and 1785 cm−1) than those of 3 (1937 and 1875 cm−1 in toluene), which indicates that the silylidene ligand in 2 is a considerably weaker π-acceptor ligand than the silylidyne ligand in 3. The 1H and 13C{1H} NMR spectra reveal a Cs symmetric structure of the silylidene complex in solution, and that rotation of the m-terphenyl substituent about the Si–CAr bond is fast on the NMR timescale at room temperature. The most distinctive signal in the 13C{1H} NMR spectrum of 2 is that of the silicon-bonded Ccarbene atom (δ=165.3 ppm), which appears considerably upfield to that of Im-Me4 (δ=212.7 ppm),20 but at similar position to that of 1 (δ=166.7 ppm).10 The 29Si{1H} NMR spectrum of 2 has a characteristic downfield-shifted signal at δ=201.8 ppm, which compares well with those of the molybdenum arylsilylidene complexes [(η5-C5Me5)(dmpe)Mo(H){Si(R)Ar}] (R=H, Cl, Me; Ar=Ph, Mes; δ=182–250 ppm).5b, 15a All the spectroscopic data and bonding parameters suggest that 2 is a new type of silylidene complex, which is best described by the zwitterionic structure depicted in Scheme 1. The bond dissociation enthalpy Do(0) and Gibbs free dissociation energy ΔGDo(298) required to cleave the SiCcarbene bond of 2 was calculated to be 62.1 and 1.2 kJ mol−1, respectively.21, 22 Both values are smaller than the respective values of 1 (Do(0)=94.3 kJ mol−1, ΔGDo(298)=28.1 kJ mol−1),10 which suggests that dissociation of the N-heterocyclic carbene Im-Me4 from 2 might occur to some extent at elevated temperatures and in the presence of a suitable carbene trapping agent may lead to the silylidyne complex 3. Indeed, reaction of 2 with one equivalent of the triarylborane B(C6H4-4-Me)3 in refluxing o-xylene afforded selectively the silylidyne complex 3 and the carbene–borane adduct Im-Me4⋅B(C6H4-4-Me)3 (4-Me; Scheme 1).12 Complex 3 was easily separated from the adduct 4-Me upon fractional crystallization from pentane and was isolated as a brick-red, air-sensitive solid in 53 % yield. The molecular structure of 3 was determined by single-crystal X-ray diffraction (Figure 2).13 The almost Cs-symmetric three-legged piano-stool complex23 is isostructural with the germanium analogue [Cp(CO)2Mo≡GeC6H3-2,6-Trip2].7b It features an almost linearly coordinated silicon center (Mo-Si-CAr=173.49(8)°) and a very short MoSi bond (2.2241(7) Å), which is 12 pm shorter than the MoSi double bond of 2. The MoSi bond length of 3 compares well with the calculated MoSi bond lengths of the hypothetical silylidyne complexes [Cp(CO)2MoSiR] (R=H: 2.213 Å, R=Me: 2.229 Å)24 and the MoSi bond length of [(η5-C5Me5)(dmpe)(H)MoSiMes][B(C6F5)4] (2.219(2) Å).5b DIAMOND plot of the molecular structure of the silylidyne complex 3. Thermal ellipsoids are set at 50 % probability, and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Mo–Si 2.2241(7), Mo–C37 1.968(3), Mo–C38 1.973(3), Si–C1 1.859(2); Mo-Si-C1 173.49(8), Si-Mo-C37 90.56(7), Si-Mo-C38 89.63(8), C37-Mo-C38 87.0(1). The IR and NMR spectra support the structure of complex 3. The 29Si{1H} NMR spectrum of 3 has a distinctive signal that is considerably downfield (at δ=320.1 ppm) to that of 2 (δ=201.8 ppm). The IR spectrum of 3 in toluene has two ν(CO) bands (1937 and 1875 cm−1) that appear at almost the same position as those of the germylidyne complex [Cp(CO)2Mo≡Ge-C6H3-2,6-Mes2] (1930 and 1875 cm−1 in nujol),7a but at considerably lower wavenumbers than those of the alkylidyne complex [Cp(CO)2MoCC6H3-2,6-Me2] (1992 and 1919 cm−1 in CH2Cl2).25 This result indicates that metal–carbonyl back-bonding is stronger in the complexes [Cp(CO)2Mo≡ER] (E=Si, Ge). It also suggests that silylidyne and germylidyne ligands have a similar σ-donor/π-acceptor ratio, which is however larger than that of alkylidyne ligands. Further evidence for the stronger metal–carbonyl back-bonding in [Cp(CO)2Mo≡EC6H3-2,6-Trip2] (E=Si, Ge) is provided by the downfield-shifted 13C NMR signal of the carbonyl ligands (E=Si (3): δ=231.1 ppm, E=Ge: δ=231.4 ppm7b) than that of [Cp(CO)2Mo≡CC6H3-2,6-Me2] (δ=228.7 ppm).25 The thermochemical parameters for the carbene transfer reaction of 2 with the borane B(C6H4-4-Me)3 to give 3 and 4-Me were computed.21, 22 Formation of the silylidyne complex 3 is an exergonic process (ΔGR°(298)=−39.0 kJ mol−1), which is favored by both the reaction enthalpy (ΔHR°(298)=−21.8 kJ mol−1) and the reaction entropy (ΔSR°(298)=57.7 Jmol−1 K−1). The negative reaction enthalpy results from the higher bond dissociation enthalpy Do(0) of the carbene–borane adduct 4-Me (88.4 kJ mol−1) than that of 2 (62.1 kJ mol−1) and suggests that triarylboranes, such as B(C6H4-4-Me)3, should be useful N-heterocyclic carbene abstracting agents owing to the formation of a rather strong BCcarbene bond. The isolation of the silylidyne complex 3 shows the potential of the carbene adduct 1 as a source for the generation of unprecedented compounds featuring silicon multiple bonds. Studies are currently in progress to explore this potential and the chemistry of the silylidyne complex 3. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.