A Thermodynamic Analysis of Rhenium(I)−Formyl C−H Bond Formation via Base-Assisted Heterolytic H_2 Cleavage in the Secondary Coordination Sphere
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(PB:-κ1-P)(CO)5]n (n = 0, +1), where PB: 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 complexes, and several of the free-base formyl complexes can be protonated, generating observable [ReI(PBH-κ1-P)(CHO)(CO)4]n complexes. Intramolecular hydrogen bonding is evident for one of the complexes, providing additional stabilization to the protonated formyl complex. By measuring both the hydricity of the formyl, ΔG°H–, and its pKa, the overall free energy of H2 cleavage is calculated from an appropriate cycle and found to be thermodynamically uphill in all cases (in the best case by only about 8 kcal/mol), although significantly dependent upon the properties of the supporting ligand.