Rh2(II,II) complexes are novel in their ability to act as single molecule photocatalysts for the production of H2. One characteristic contributing to successful photocatalytic H2 production in these complexes is high molar extinction coefficients spanning from the ultraviolet to near-infrared region which effectively harvest solar irradiation. However, some Rh2(II,II) complexes, such as cis-[Rh2(DTolF)2(np)2]2+ (DtolF = N,N′-di(p-tolyl)formamidinate, np = 1,8-Naphthyridine), have been shown to have short-lived excited states which result in lower H2 turnover numbers (TON) despite increased absorption efficacy. The work herein describes the synthesis and characterization of a series of cis-[Rh2(DtolF)2(bpnp)(L)]2+ (bpnp = 2,7-dipyridin-2-yl[1,8]naphthyridine) complexes where L = pyridazine, cinnoline, or benzo(c)cinnoline(BNCN). In particular, the studied complexes were made to exploit the excited state tunability of Rh2(II,II) complexes through axial capping of the Rh2 metallic backbone with bpnp while simultaneously constricting the Rh-Rh bond length by using ligands with decreased N-N bond lengths. Crystallographic data shows that the Rh-Rh bond length changes with ligand identity from 2.4466(7) Å in cis-[Rh2(DtolF)2(np)2]2+ to 2.434(2) Å in [Rh2(DtolF)2(bpnp)(BNCN)]2+, a decrease of 0.013 Å. In addition, the molar absorptivity maxima of the compounds became increasingly red-shifted as the bridging ligands increased in conjugation, shifting from 566 nm for cis-[Rh2(DTolF)2(np)2]2+ to 714 nm for [Rh2(DtolF)2(bpnp)(BNCN)]. This will result in better overlap with solar irradiation and should lead to more efficient photocatalysis. Data revealed that capping the axial sites and reducing the Rh-Rh bond length also resulted in increased excited state triplet lifetimes, with τ = 0.5 ns for cis-[Rh2(DTolF)2(np)2]2+ to τ= 1 ns for [Rh2(DtolF)2(bpnp)(BNCN)]2+. Preliminary mechanistic studies indicate these complexes are able to photocatalyticly produce H2 via the protonation of the nitrogen groups on the diazine bridging ligands, with modest TON of 16 for [Rh2(DtolF)2(bpnp)(BNCN)]2+. Further studies are ongoing to modify excited state properties via ligand modification to improve H2 turnover numbers.

Rh2(II,II) single molecule photocatalyst with red light absorption

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