Nonchromophoric Halide Ligand Variation in Polyazine-Bridged Ru(II),Rh(III) Bimetallic Supramolecules Offering New Insight into Photocatalytic Hydrogen Production from Water

The new bimetallic complex [(Ph(2)phen)(2)Ru(dpp)RhBr2(Ph(2)phen)](PF6)(3) (1) (Ph2phen = 4,7-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine) was synthesized and characterized to compare with the Cl analogue [(Ph(2)phen)(2)Ru(dpp)RhCl2(Ph(2)phen)](PF6)(3) (2) in an effort to better understand the role of halide coordination at the Rh metal center in solar H-2 production schemes. Electrochemical properties of complex 1 display a reversible Ru-II/III oxidation, and cathodic scans indicate multiple electrochemical mechanisms exist to reduce Rh(III) by two electrons to Rh(I) followed by a quasi-reversible dpp(0/-) ligand reduction. The weaker sigma-donating ability of Br- vs Cl- impacts the cathodic electrochemistry and provides insight into photocatalytic function by these bimetallic supramolecules. Complexes 1 and 2 exhibit identical light-absorbing properties with UV absorption dominated by intraligand (IL) pi -> pi* transitions and visible absorption by metal-to-ligand charge transfer (MLCT) transitions to include a lowest energy Ru(d pi) -> dpp(pi*) 1MLCT transition (lambda(abs) = 514 nm; epsilon = 16 000 M-1 cm-(1)). The relatively short-lived, weakly emissive Ru(d pi) -> dpp(pi*) (MLCT)-M-3 excited state (t tau = 46 ns) for both bimetallic complexes is attributed to intramolecular electron transfer from the 3MLCT excited state to populate a low-energy Ru(d pi) -> Rh(d sigma*) triplet metal-to-metal charge transfer (3MMCT) excited state that allows photoinitiated electron collection. Complex 1 outperforms the related Cl- bimetallic analogue 2 as a H-2 photocatalyst despite identical light-absorbing and excited-state properties. Additional H-2 experiments with added halide suggest ion pairing plays a role in catalyst deactivation and provides new insight into observed differences in H-2 production upon halide variation in Ru(II),Rh(III) supramolecular architectures.