Cyclic carbene complexes of molybdenum and tungsten. Crystal and molecular structure of [Mol(CO)2{CO(CH2)2CH2}(η-C5H5)

Abstract

The cyclic carbene complex [Mol(CO)2{[graphic omitted]H2}(η-C5H5)] is formed during reaction of [Mo(CO)3(η-C5H5)]– with I[CH2]3l or of [Mo{(CH2)3Br}(CO)3(η-C5H5)] with Lil. Crystals of this complex are Monoclinic, space group P21/c, with a= 6.372(2), b= 14.168(5), c= 14.390(3)A, β= 98.12(2)°, and Z= 4. The molecule contains mutually trans carbonyl ligands and a cyclic carbene ligand in which all but one of the ring carbon atoms are coplanar with the metal. Similarly, reactions of [Mo{(CH2)3Br}(CO)3(η-C5H5)] with SPh– or CN– provide [MoX(CO)2{[graphic omitted]H2}(η-C5H5)](X = SPh or CN). The cyclisation is tolerant to functionality on the cyclopentadienyl ring, thus reaction of [Mo(CO)3(η-C5H4R)]– with I[CH2]3l produces [Mol(CO)2{[graphic omitted]H2}(η-C5H4R)][R = Me, C(O)Me, or SiMe3]. The tungsten analogue [W{(CH2)3Br}(CO)3(η-C5H5)] undergoes carbene formations with l– and CN– to form [WX(CO)2{[graphic omitted]H2}(η-C5H5)](X = l or CN). Formation of the carbene complexes is likely to proceed by migration of the (CH2)3Br alkyl group to an adjacent CO group together with co-ordination of the incoming anion. This is followed by a ring closure reaction eliminating Br– and isomerisation so that in all cases the two CO groups are mutually trans in the isolated products. While [Mol(CO)2{[graphic omitted]H2}(η-C5H5)] does not react with transition-metal anions to form dinuclear carbene complexes by elimination of l–, the salt [Mo(CO)2(PPh3){[graphic omitted]H2}(η-C5H5)]Br reacts with [W(CO)3(η-C5H5)]– to give the unusual acyl complex [(η-C5H5)(Ph3P)(OC)2MoC(O)(CH2)3W(CO)3(η-C5H5)].