Photochemistry of dirhodium(II,II) diphosphazane tetrachloride complexes

The dirhodium complex Rh-2(dfpma)(3)Cl-4 (dfpma = bis(difluorophosphine)methyl-amine), in THF and in the presence of excess dfpma, undergoes photoreduction (lambda(exc) > 304 nm) to Rh-2(dfpma)(3)(eta(1)-dfpma)(2) via the two-electron mixed-valence compound, Rh-2(dfpma)(3)Cl-2(eta(1)-dfpma). Conversion of Rh-2(dfpma)(3)Cl-4 to Rhz(dfpma)(3)Cl-2(eta(1)-dfpma) is quantitative with a quantum efficiency of phi(p)(334) = 0.05(1); the subsequent conversion of Rh-2(dfpma)(3)Cl-2(eta(1)-dfpma) to Rh-2(dfpma)(3)(eta(1)-dfpma)(2) also proceeds quantitatively, though at a reduced quantum efficiency (phi(p)(365) = 0.006(2)). The photochemistry of Rh-2 diphosphazanes has been expanded to include dirhodium cores coordinated by bis(bis(trifluoroethoxy)phosphine)methylamine, MeN[P(OEtF3)(2)](2), (OEtF3 = OCH2CF3). The Rh-2{MeN[P(OEtF3)(2)](2)}(3)Cl-4, Rh-2{MeN[P(OEtF3)(2)](2)}(3)Cl2L (where L = CO or CNBut) and Rh-2{MeN[P(OEtF3)(2)](2)}(3)(CNBut)(2) compounds have been prepared and the Rh-2(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 Rh-2 diphosphazane series from its dfpma counterparts. The two-electron Rh-2(II,0)Cl-2 mixed-valence intermediate, which is crucial to sustaining four-electron photoreactivity in the Rh-2 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 Rh-2 diphosphazane compounds. (C) 2000 Elsevier Science S.A. All rights reserved.