CC-COUPLING REACTIONS AT LOW-VALENT MOLYBDENUM CENTERS - SELECTIVE FORMATION OF ETA-2-IMINOACYL, ETA-3-1-AZAALLYL AND ETA-2-1,4-DIAZABUTADIEN-2-YL COMPLEXES OF MOLYBDENUM FROM ETHYL ISOCYANIDE COMPLEXES

A stepwise, high-yield synthesis of low-valent molybdenum eta-2-iminoacyl, eta-3-azaallyl and eta-2-1,4-diazabutadien-2-yl complexes is reported starting from readily accessible ethyl isocyanide complexes. Thus reduction of cis/trans-Cp*Mo(CO)2(EtNC)I (Cp* = eta-5-C5Me5) (1a/1b) with sodium gives the Mo0-metallate Na[Cp*Mo(CO)2(EtNC)] (2) in quantitative yield. Reaction of 2 with RI yields (via alkylation of the metal center and insertion of the ethyl isocyanide ligand into the metal-alkyl bond exclusively the Mo(II)-iminoacyl complexes Cp*(CO)2Mo[eta-2-C(NEt)R] (3: R = Me; 4: R = Et). Complexes 3 and 4 undergo isomerization in refluxing toluene by H-migration into the 1-azaallyl compounds Cp*(CO)2Mo(eta-3-CH2-CH-NEt) (5) and Cp*(CO)2Mo[eta-3-CH(Me)-CH-NEt] (6), respectively. Treatment of 3 with RNC (R = Me, Et) results in the quantitative formation of the isocyanide insertion products Cp*(CO)2 activated Mo[C(NR)C(NEt)Me] (7: R = Me; 8: R = Et). The solid state structure of 8 was established by a single-crystal X-ray crystallographic study. A striking feature of the structure is the planarity of the four-membered metallaheterocycle resulting from (C,N)-coordination of the 1,4-diaza-3-methylbutadien-2-yl ligand to molybdenum.