Synthesis and reactivity of air-stable isolable anionic Group 6 molybdenum and tungsten carbene complexes with a metal-centred negative charge

The first air-stable anionic carbene complexes with a metal-centred negative charge [TpMe2(CO)2MC(CN)2]− (3a, M=Mo; 3b, M=W), [TpMe2,4Cl(CO)2MoC(CN)2]− (4), [Tpi-Pr,4Br(CO)2MoC(CN)2]− (5) and [TpMe2(CO)2MoC(CN)R]− (11a–d, R=substituted alkyl) have been synthesised by addition of cyanide anion to the corresponding chlorocarbyne complexes 1, 6, 7 or to the alkylcarbyne complexes 9a–d. Carbene complexes 3 and 11 are stereochemically rigid with the plane of the carbene ligand coinciding with the molecular mirror plane. The electron-rich carbene carbons in these complexes show some of the most highly shielded values of δCcarbene so far observed. The dicyanocarbene complexes (3) did not give stable products with electrophiles but the cyano(alkyl)carbene complexes (11b–d) could be doubly alkylated at the CN nitrogen to give the cationic dialkylaminoalkyne complexes [TpMe2(CO)2Mo{η2(C,C)Me2N–CC–R}]+ (12). Complex 11a (R=C(CN)2CH3) eliminates [CH3C(CN)2]− in the presence of MeI and yields purple TpMe2(CO)2MC–CN (2a), the first example of a cyanocarbyne complex and a presumed intermediate in the formation of 3a.