Photochemistry of dirhodium(II,II) diphosphazane tetrachloride complexes

The dirhodium complex Rh2(dfpma)3Cl4 (dfpma=bis(difluorophosphine)methyl-amine), in THF and in the presence of excess dfpma, undergoes photoreduction (λexc>304 nm) to Rh2(dfpma)3(η1-dfpma)2 via the two-electron mixed-valence compound, Rh2(dfpma)3Cl2(η1-dfpma). Conversion of Rh2(dfpma)3Cl4 to Rh2(dfpma)3Cl2(η1-dfpma) is quantitative with a quantum efficiency of φp334=0.05(1); the subsequent conversion of Rh2(dfpma)3Cl2(η1-dfpma) to Rh2(dfpma)3(η1-dfpma)2 also proceeds quantitatively, though at a reduced quantum efficiency (φp365=0.006(2)). The photochemistry of Rh2 diphosphazanes has been expanded to include dirhodium cores coordinated by bis(bis(trifluoroethoxy)phosphine)methylamine, MeN[P(OEtF3)2]2 (OEtF3=OCH2CF3). The Rh2{MeN[P(OEtF3)2]2}3Cl4, Rh2{MeN[P(OEtF3)2]2}3Cl2L (where L=CO or CNBut) and Rh2{MeN[P(OEtF3)2]2}3(CNBut)2 compounds have been prepared and the Rh2(II,II) complex has been structurally characterized by X-ray diffraction analysis. Increased steric congestion of the bridging MeN[P(OEtF3)2]2 ligands about the dirhodium core distinguishes this Rh2 diphosphazane series from its dfpma counterparts. The two-electron Rh2(II,0)Cl2 mixed-valence intermediate, which is crucial to sustaining four-electron photoreactivity in the Rh2 dfpma series, is susceptible to ligand loss when three MeN[P(OEtF3)2]2 ligands span the bimetallic core, engendering a new photochemistry for this class of Rh2 diphosphazane compounds.