Four-Electron Photochemistry of Dirhodium Fluorophosphine Compounds

Abstract

The singly bonded dirhodium(II) complex, Rh2(dfpma)3Br4 (dfpma = bis(difluorophosphine)methylamine, CH3N(PF2)2), photoreacts when irradiated with UV or visible light. The mixed-valence LRh0RhΙΙX2 species, Rh2(dfpma)3Br2(L), is obtained quantitatively when THF solutions containing Rh2(dfpma)3Br4 and excess L = dfpma or PR3 are photolyzed (λexc > 436 nm). The photoreaction quantum yield is ∼10-2 over the near-UV absorption manifold, decreasing slightly as the excitation wavelength is extended into the visible region (φp 313-405 = 0.022(3), φp 436-468 = 0.012(1)). Continued irradiation with excitation wavelengths coincident with the absorption manifold of the LRh0RhΙΙX2 complex results in a second two-electron elimination reaction to give the LRh0Rh0L dimer, Rh2(dfpma)3L2, again in quantitative yield but with an attenuated quantum efficiency (φp 313-365 = 0.0035(3), φp 405-436 = 0.0017(3)). For photoreactions performed in THF, the bromine photoproduct is HBr, which may be trapped with 2,6-lutidine. NMR experiments reveal the production of 2 equiv of HBr for each two-electron transformation of the dirhodium photoreagent. The wavelength dependence of φp and the results of extended Hückel calculations are consistent with the photoreaction occurring from an excited state of dσ* parentage. The ability to preserve the same dσ* electronic structure across the four-electron series allows us to overcome the barriers traditionally associated with metal−halogen bond cleavage and consequently to design a four-electron photoreaction among discrete molecular species.

Chemistry of Personalized Solar Energy

“…This photoefficiency is the same as that measured independently for the photoconversion of Rh 2 0,II (dfpma) 3 Cl 2 L to Rh 2 0,0 (dfpma) 3 L 2 (Φ p = 7 × 10 −3 ). 57 These observations suggested to us that the activation of the Rh—X bond is determinant to overall photocatalytic activity. An increase in the quantum efficiency for hydrogen photocatalysis is therefore equated to increasing the photoefficiency for halogen elimination from the binuclear metal core.…”

“…(5) may be driven to the right by designing ligands that incorporate antithetical design elements: two π-accepting moieties with a π-donor bridgehead (A–D–A) 57–61 or two π-donating moieties with a π-accepting bridgehead (D–A–D). 62–64 The A–D–A ligand motif is achieved by placing an amine bridgehead between two electron-deficient phosphines (PR F 2 ) or phosphites (P(OR F ) 2 ).…”

“…62–64 The A–D–A ligand motif is achieved by placing an amine bridgehead between two electron-deficient phosphines (PR F 2 ) or phosphites (P(OR F ) 2 ). The A–D–A ligands, bis(difluorophosphino)methylamine (dfpma, CH 3 N(PF 2 ) 2 ) and bis(bistrifluoroethoxyphosphino)methylamine (tfepma, CH 3 N[P(OCH 2 CF 3 ) 2 ] 2 ) shown in Chart 1, exhibit a particular proclivity for promoting the internal disproportionation of binuclear M 2 I,I cores to M 2 0,II cores 65 for the metals of rhodium 57,58 and iridium. 59–61,66 X-ray crystal structures reveal a pronounced asymmetry in the diphosphazane framework upon ligation to a bimetallic core derived from the asymmetric stabilization of metals that differ by two in their formal oxidation.…”

Driving Multi-electron Reactions with Photons: Dinuclear Ruthenium Complexes Capable of Stepwise and Concerted Multi-electron Reduction

“…are multi-electron processes and require not only the appropriate driving force but also the appropriate redox stoichiometry if the energetically most favorable reaction pathway is to be used. Clearly, viable artificial photosynthetic systems will require entities/catalysts capable of driving MET and/or PCMET reactions (Cukier and Nocera 1998;Heyduk et al 1999;Heyduk and Nocera 2001). Unfortunately, few molecular 'artificial' photocatalysts are capable of meeting the multi-electron stoichiometries of small molecule activation reactions because photoexcitation is typically a 1-photon, 1-electron process (Watts 1991).…”

Supramolecular Complexes as Photoinitiated Electron Collectors: Applications in Solar Hydrogen Production

“…Metal-metal bonded mixed-valence systems that undergo photochemical multielectron photochemistry, which are able to convert hydrohalic acids to hydrogen using light and a halogen trap have been reported by Nocera et al [55][56][57][58]. The approach takes advantage of twoelectron mixed-valence compounds, M n þ Á Á ÁM n þ 2 , to drive multielectron chemistry.…”