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

Abstract

The first air-stable anionic carbene complexes with a metal-centred negative charge [Tp Me2(CO) 2MC(CN) 2] − ( 3a, M=Mo; 3b, M=W), [Tp Me2,4Cl(CO) 2MoC(CN) 2] − ( 4), [Tp i-Pr,4Br (CO) 2MoC(CN) 2] − ( 5) and [Tp Me2(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 δC carbene 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 [Tp Me2(CO) 2Mo{η 2( C, C)Me 2N–CC–R}] + ( 12). Complex 11a (R=C(CN) 2CH 3) eliminates [CH 3C(CN) 2] − in the presence of MeI and yields purple Tp Me2(CO) 2MC–CN ( 2a), the first example of a cyanocarbyne complex and a presumed intermediate in the formation of 3a.