Abstract
The new bimetallic complex [(Ph2phen)2Ru(dpp)RhBr2(Ph2phen)](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 [(Ph2phen)2Ru(dpp)RhCl2(Ph2phen)](PF6)3 (2) in an effort to better understand the role of halide coordination at the Rh metal center in solar H2 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 σ-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) π → π* transitions and visible absorption by metal-to-ligand charge transfer (MLCT) transitions to include a lowest energy Ru(dπ) → dpp(π*) (1)MLCT transition (λ(abs) = 514 nm; ε = 16 000 M(-1) cm(-1)). The relatively short-lived, weakly emissive Ru(dπ) → dpp(π*) (3)MLCT excited state (τ = 46 ns) for both bimetallic complexes is attributed to intramolecular electron transfer from the (3)MLCT excited state to populate a low-energy Ru(dπ) → Rh(dσ*) triplet metal-to-metal charge transfer ((3)MMCT) excited state that allows photoinitiated electron collection. Complex 1 outperforms the related Cl(-) bimetallic analogue 2 as a H2 photocatalyst despite identical light-absorbing and excited-state properties. Additional H2 experiments with added halide suggest ion pairing plays a role in catalyst deactivation and provides new insight into observed differences in H2 production upon halide variation in Ru(II),Rh(III) supramolecular architectures.
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