Transformations of Group 7 Carbonyl Complexes: Possible Intermediates in a Homogeneous Syngas Conversion Scheme

A variety of C−H and C−C bond forming reactions of group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C2+ compounds. The metal formyl complexes M(CO)3(PPh3)2(CHO) (M = Mn, Re) are substantially stabilized by coordination of boranes BX3 (X = F, C6F5) in the form of novel boroxycarbene complexes M(CO)3(PPh3)2(CHOBX3), but these boron-stabilized carbenes do not react with hydride sources to undergo further reduction to metal alkyls. The related manganese methoxycarbene cations [Mn(CO)5−x(PPh3)x(CHOMe)]+ (x = 1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atmosphere of CO. The resulting acyls, cis- and trans-Mn(PPh3)(CO)4(C(O)CH2OMe), can be alkylated to form the cationic carbene complex [Mn(PPh3)(CO)4(C(OR)CH2OMe)]+, which undergoes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or diethyl ether adducts of [Mn(PPh3)(CO)4]+. The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh3)(CO)4(CH2COOMe) and is protonolyzed to yield methyl acetate and [Mn(PPh3)(CO)4]+; addition of L (L = PPh3, CO) to the manganese cation regenerates [Mn(PPh3)(CO)4(L)]+. Since the original formyl complex can be obtained by the reaction of [Mn(PPh3)(CO)5]+ with [PtH(dmpe)2]+, which in turn can be generated from H2, this set of transformations amounts to a stoichiometric cycle for selectively converting H2 and CO into a C2 compound under mild conditions.