Artificial Photosynthesis

Kinetics and Thermodynamics of Small Molecule Binding to Pincer-PCP Rhodium(I) Complexes

The kinetics and thermodynamics of the binding of several small molecules (L = N₂, H₂, D₂, and C₂H₄) to the coordinatively unsaturated pincer-PCP rhodium(I) complexes, Rh[^tBu₂PCH₂(C₆H₃)CH₂P^tBu₂] (1) and Rh[^tBu₂P(CH₂)₅P^tBu₂] (2), in organic solvents (n-heptane, toluene, THF and cyclohexane-d₁₂) have been investigated by a combination of kinetic flash photolysis methods, NMR equilibrium studies, and density functional theory (DFT) calculations. Using various gas mixtures and monitoring by NMR until equilibrium was established, the relative free energies of binding of N₂, H₂ and C₂H₄ in cyclohexane-d₁₂ were found to increase in the order, C₂H₄ < N₂ < H₂. Time-resolved infrared (TRIR) and UV-visible transient absorption spectroscopy revealed that 355 nm excitation of 1–L and 2–L results in the photoejection of ligand L. The subsequent mechanism of binding of L to 1 and 2 to regenerate 1–L and 2–L is determined by the structure of the PCP ligand framework and the nature of the solvent. In both cases, the primary transient is a long-lived, unsolvated species. For 2, we propose this to be an agostically-stabilized intermediate that is in equilibrium with a more reactive, non-stabilized form, which reacts with L at diffusion-controlled rates to regenerate 2–L. For 1, a similar mechanism is proposed to occur, but with an additional parallel reaction pathway that involves the direct reaction of the less-reactive form of 1 with L. Experiments in the more coordinating solvent, THF revealed the binding of THF to 1 to generate 1–THF, and its subsequent reaction with L, as a competing pathway.

Inorg. Chem. 2013, 52, 4160-4172, DOI: 10.1021/ic300672g.