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 all 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 H-2, this set of transformations amounts to a stoichiometric cycle for selectively converting H-2 and CO into a C-2 compound under mild conditions.