A Thermodynamic Analysis of Rhenium(I)–Formyl C–H Bond Formation via Base-Assisted Heterolytic H2Cleavage in the Secondary Coordination Sphere

Abstract

Conversion of synthesis gas, a mixture of carbon monoxide and hydrogen, into value-added Cn≥2 products requires both C–H and C–C bond-forming events. Our group has developed a series of molecular complexes, based on group 7 (manganese and rhenium) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, we explore a new mode of H2 activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H2 to form a formyl C–H bond at a rhenium-bound carbonyl. A series of cationic rhenium(I) complexes of the type [ReI(P∼B:-κ1-P)(CO)5]n (n = 0, +1), where P∼B: is a phosphine ligand with a tethered strong base, are prepared and characterized; measurement of their protonation equilibria demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepared in good yield by hydride delivery to the parent pentacarbonyl comple...