Mapping Intermediates in Hydrogen Evolution with [Cp*Rh] Catalysts and Carbon Monoxide Release from [Mn(CO)3] Complexes

Abstract

Pentamethylcyclopentadienyl rhodium ([Cp*Rh]) and manganese tricarbonyl ([Mn(CO)3]) complexes represent workhorse families of compounds with key applications in catalytic hydrogen evolution and light-driven carbon monoxide (CO) release, respectively. Investigations with these model molecular systems are motivated by our limited knowledge concerning the fundamental mechanisms involved in storing energy in chemical bonds and releasing carbon monoxide using visible light. In Part I of this dissertation, synthetic, electrochemical, structural, and spectroscopic studies have been applied to map the elementary electron- and proton-transfer steps leading to catalytic dihydrogen evolution. Time-resolved pulse radiolysis and stopped-flow spectroscopic studies reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, followed by tautomerization to form [(η4-Cp*H)Rh(bpy)]+. Spectroscopic monitoring of the second proton transfer event reveals both the hydride and related Cp*H complex are involved in further reactivity and confirm that [(Cp*H)Rh] is not an off-cycle intermediate, but can be an active participant in catalytic H2 evolution. In Part II, synthetic, electrochemical, structural, and spectroscopic studies have been applied to elucidate the redox and electronic properties of a family of monometallic, as well as homo- and hetero-bimetallic complexes, bearing the [Mn(CO)]3 unit. In particular, we have elucidated the early intermediates involved in visible-light-driven speciation of a series of [Mn(CO)3(Rbpy)] complexes bearing 4,4′-disubstituted 2,2′-bipyridyl ligands (Rbpy, R = tBu, H, CF3, NO2). Ultrafast transient absorption spectroscopy measurements with UV-visible monitoring reveals loss of a CO ligand on the femtosecond timescale, followed by solvent coordination on the picosecond timescale. Taken together, the findings described in this dissertation of [Cp*Rh] and [Mn(CO)3] complexes provide a foundation for future investigations that could yield more detailed mechanistic insight into H2 generation and controlled CO release.