The class of dirhodium(II,II) complexes [Rh2(µ-BL)2(NN)2][BF4] (µ-BL = DTolF = p-ditolylformamidinate, acam = acetamidate, OAc = acetate; NN = dpq = dipyrido[3,2-f:20,30-h]quinoxaline, dppz = dipyrido[3,2-a:2',3'c]phenazine, dppn = benzo[i]dipyrido[3,2-a:2',3'-h]quinoxaline, phen = 1,10-phenanthroline) were investigated as catalysts for the generation of fuels and useful chemicals from abundant, inexpensive sources. When coupled to a light absorber that can transfer electrons to the Rh2(II,II) complexes, it was hypothesized that they would be able to reduced H+ to H2 and CO2 to molecules useful as fuels or in industry. As such, the ability of these complexes to electrocatalytically reduce each substrate was investigated.
When reduced electrochemically, the [Rh2(DTolF)2(NN)2][BF4] (1) complexes were shown to serve as a highly efficient and robust catalysts for the reduction of H+ to H2. Turnover frequencies (TOF) of 2.8 × 104, 2.6 × 104, and 5.9 × 104 s-1 were determined for 1.2, (NN = dppz) 1.3 (NN = dppn), and 1.4 (NN = phen), respectively, with overpotential values of 0.50 (1.2), 0.56 (1.3), and 0.64 V (1.4). Bulk electrolysis followed by headspace injection into a gas chromatograph confirmed the only product to be H2. The proposed catalytic mechanism proceeds through electrochemical generation of a Rh2II,I species that may be protonated to form a Rh2II,III-hydride; the latter undergoes subsequent reduction and protonation to release H2 gas.
Complexes 1.1 and 1.4 were shown to also electrocatalytically reduce CO2 in 3 M H2O to produce HCOOH and carbonate. However, ~70% of the electrons in these systems generate H2, such that 1 is not selective for CO2 in the presence of H+. [Rh2(OAc)2(phen)2][BF4] (2) and [Rh2(acam)2(phen)2][BF4] (3) generate greater current enhancement than 1.1 and 1.4 in the presence of CO2 and exhibit significantly different selectivity when compared to 1, with nearly 100% HCOOH production achieved for 3 without degradation.
The catalytic mechanism for both H+ and CO2 reduction is proposed to proceed through a Rh2II,III-hydride intermediate. This intermediate may be subsequently protonated and reduced to evolve H2. Alternatively, CO2 may insert into the metal hydride bond to yield a formato complex, which may be protonated and reduced to release HCOOH. The Rh2II,III-hydride intermediate, as well as the formato complex, have been successfully synthesized and isolated. The direct hydrogenation of 1.4 in methanol resulted in the paramagnetic species [HRh2(µ-DTolF)2(phen)2]2+ (1.4-H), which was characterized by infrared spectroscopy, electron paramagnetic resonance, and electrospray ionization mass spectrometry. The reaction of 1.4-H with CO2 yields what may be the corresponding formato complex, [(OCHO)Rh2(µ-DTolF)2(phen)2]2+ (1.4-OCHO), but further characterization of this species is required. These results provide support for the proposed catalytic mechanism.