Unveiling key factors governing the catalytic performance of half-metallocene complexes for photochemical CO2 reduction and hydrogen production: relativistic effects and charge delocalization

초록

Unraveling how transition-metal identity and ligand electronics cooperatively govern multi-electron management is central to designing selective photocatalysts for CO2 reduction and hydrogen evolution. Herein, we employ a series of half-metallocene complexes, [Cp*M-III(4,4 '-R-2-bpy)Cl](+) (M = Ir, Rh, Co; R = H, Me, OMe, tBu, CF3), to establish a unified framework connecting orbital interactions to catalytic outcomes. Cyclic voltammetry reveals that bipyridine functionalization induces systematic shifts in reduction potentials (E-pc) while modulating the anodic-cathodic peak separation (Delta Epa-pc), which serves as a quantitative metric for two-electron storage capacity. Under homogeneous photolysis with an Ir(iii) photosensitizer, the 5d Ir series preferentially catalyzes CO2-to-formate conversion (<bold>Ir-tBu</bold>, TONformate = 804), whereas 4d Rh analogues channel reactivity toward H-2 evolution, with <bold>Rh-CF3</bold> achieving exceptional activity and selectivity (TONhydrogen = 1534; 92.9%). Mechanistic analysis indicates that these divergent pathways are governed by the accessibility of critical reduced intermediates-one-electron-reduced metal-hydrides for Ir(iii) and Cp*-protonated species for Rh(iii)-which correlates directly with Delta Epa-pc rather than E-pc alone. Conversely, 3d Co complexes display negligible Delta Epa-pc values and consistently poor activity due to an inability to stabilize two-electron-reduced species (TERS). Density functional theory (DFT) calculations substantiate these trends, demonstrating that relativistic radial expansion across 3d(Co) < 4d(Rh) < 5d(Ir) enhances d-pi* orbital mixing and LUMO delocalization. Electron-withdrawing substituents (CF3) lower the ligand pi* acceptor levels, compressing the d-pi* energy gap to further reinforce charge delocalization. Overall, this work delivers predictive design principles for tailoring ligand-pi* energetics and metal-ligand orbital overlap to achieve high reactivity and product control in solar fuels catalysis.

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