HOMOLEPTIC ALKYLISOCYANIDE COMPLEXES OF MOLYBDENUM(0) AND TUNGSTEN(0), USEFUL PRECURSORS FOR THE SYNTHESIS OF LOW-VALENT DIALKYLAMINOCARBYNE COMPLEXES

A large scale, high yield synthesis of the homoleptic alkylisocyanide complexes M(RNC)6 (R = Et, tBu) and carbonyl-free diethylaminocarbyne complexes [(EtNC)P5MCNEt2]BF4 starting from M(CO)6 (M = Mo, W) is reported. The synthetic procedure begins with the conversion of M(CO)6 into fact-M(CO)3(MeCN)3, followed by substitution of the acetonitrile ligands in fac-M(CO)3(MeCN)3 by RNC to give the isocyanide complexes fac-M(CO)3(RNC)3 (1a-2b) (1: M = Mo, 2: M = W; a: R = Et, b R = tBu). Compounds 1a-2b are then oxidatively decarbonylated with Br2 to give the seven-coordinate MII complexes (M(CO)2(RNC)3(Br)2 (3a-4b (3: M = Mo, 4: M = W). Subsequent reaction of 3a-4b with an excess of RNC results in the elimination of the residual CO ligands and formation of the ionic compounds [M(RNC)6Br]Br (5a-6b) (5: M = Mo, 6: M = W). Compounds 5a-6b are then reduced with Na/Hg to yield (M(RNC)6 (7a-8b) (7: M = Mo, 8: M = W), the isoelectronic congeners of M(CO)6. Reaction of the ethylisocyanide derivatives M(EtNC)6 (7a, 8a) with Et3OBF4 leads finally to [(EtNC)5MCNEt2]BF4 (9a: M = Mo, 10a: M = W). This route to electron-rich, isocyanide-substituted diethylaminocarbyne complexes is compared with the established procedure involving the stepwise decarbonylation of the Fischer-type carbyne complex trans-I(CO)4WCNEt2 with RNC. © 1990.