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Photochemical and time-resolved spectroscopic studies of dirhodium hydrogen-generating species

Photochemical and time-resolved spectroscopic studies of diand mono-rhodium inorganic complexes were undertaken to elucidate the mechanism of photocatalytic hydrogen generation from hydrohalic acid solutions. Nanosecond transient absorption (NSTA) experiments on solutions of Rh2(dfpma)3PPh3(CO) (dfpma = bis(difluorophosphino)methylamine) in THF and methylene chloride revealed a long-lived (τ ~ 12 ms) intermediate species with an absorption maximum at 395 nm. This intermediate was shown to be quenched by PMe3 following pseudofirst-order kinetics characterized by a 4.3 * 10 M s second-order rate constant. Preliminary NSTA experiments undertaken in the presence of HCl showed behavior indicative of bulk reaction of Rh2(dfpma)3PPh3(CO) with HCl yielding Rh2(dfpma)3PPh3Cl2. Transient absorption experiments on Rh2(dfpma)3PPh3Cl2 revealed a long-lived (τ > 3 ms) transient species with an absorption max at 400 nm. NSTA experiments undertaken with Rh2(dfpma)3PPh3Cl2 in the presence of HCl showed no distinction from those performed in solvent alone. Steady-state photolysis experiments showed that the presence of dissolved CO greatly increases the rate of decomposition of Rh2(dfpma)3PPh3(CO) when irradiated. This coupled with NSTA data pointed to the possibility of a ligand re-arranged intermediate during the photolysis of Rh2(dfpma)3PPh3(CO). An improved synthesis of Rh2(dfpma)3Cl2 was found by allowing 1 eq. [Rh(COD)Cl]2 to react with 3 equivalents of dfpma. Subsequent thermal chemistry showed that Rh2(dfpma)3Cl2 did not react thermally with H2 or HCl. Photocatalytic studies showed that 2 days of irradiation in 0.1 M HCl/THF solution yielded only 3 – 5 eq. H2, as compared to 10 – 50 eq. of H2 when starting with Rh2(dfpma)3Cl4. This ruled out Rh2(dfpma)3Cl2 as the major catalytic species in that process. Preliminary investigations of (PPh3)3RhCl as a mononuclear catalyst capable of taking HCl to H2 and PPh3Cl2 in the presence of light determined the process to be complex and low-yielding. Thesis Supervisor: Daniel G. Nocera Title: Professor of Chemistry