Molecular Structures and Photophysical Properties of Dirhodium Fluorophosphine Complexes

Abstract

The excited state properties of a series of singly bonded dirhodium compounds, consisting of Rh(0)(2), Rh(0)Rh(II)X(2), and Rh(II)(2)X(4) (X = Cl and Br) cores coordinated by three bis(difluorophosphino)methylamine ligands, have been investigated. The newly synthesized complexes with X = Br have been structurally characterized. The mixed-valence complex Rh(2)[&mgr;-CH(3)N(PF(2))(2)](3)Br(2)[(PF(2))CH(3)N(PF(2))] crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 13.868(7) Å, b = 16.090(5) Å, c = 11.614(5) Å, V = 1591(3) Å(3), and Z = 4; the structure was refined to values of R = 0.052 and R(w) = 0.062. Orange crystals of Rh(2)[&mgr;-CH(3)N(PF(2))(2)](3)Br(4) are monoclinic with a C2/c space group: a = 14.62(6) Å, b = 12.20(2) Å, c = 14.33(1) Å; beta = 106.0(2) degrees; V = 2457(11) Å(3); Z = 4; and R = 0.058 and R(w) = 0.056. Crystalline solids and low-temperature glasses of each member of the chloride and bromide series exhibit long-lived red luminescence. Excitation profiles and temperature dependencies of the emission bandwidths and lifetimes for all complexes are characteristic of luminescence originating from a dsigma excited state. Efficient nonradiative decay is observed upon the thermal population of an excited state proximate to the lowest energy emissive excited state of these complexes. The nonradiative decay rate constant of the upper excited state is 10(2)-10(3) and 10(3)-10(4) greater than that of the emissive excited state for complexes with X = Cl and Br, respectively.

Four-Electron Photochemistry of Dirhodium Fluorophosphine Compounds

“…The two-electron mixed-valence complex Rh 2 (dfpma) 3 X 2 (L) (L = PF 3 or η 1 -dfpma; X = Cl, Br, or I) readily undergoes oxidation and reduction to afford symmetric congeners, Rh 2 (dfpma) 3 X 4 and Rh 2 (dfpma) 3 (L) 2 , respectively. Electronic absorption and luminescence spectra of the LRh 0 Rh 0 L, LRh 0 Rh II X 2 , and X 2 Rh II Rh II X 2 dfpma complexes are consistent with each possessing a lowest energy excited state of dσ* parentage . We now show that the dσ* excited state allows us to interconvert among LRh 0 Rh 0 L, LRh 0 Rh II X 2 , and X 2 Rh II Rh II X 2 cores, enabling us to rationally design a system capable of supporting four-electron photochemistry among discrete molecular species.…”

“…Significant LMCT character of the higher energy absorption bands is revealed by their red shift of 3500 and 7600 cm -1 upon substitution of chloride by bromide and bromide by iodide, respectively. The low-energy absorption band red shifts to a lesser extent along the same halide series (825 cm -1 for Cl - /Br - and 3600 cm -1 for Br - /I - ), consistent with greater metal-based character of a dπ* → dσ* transition . Coordinating solvents do not affect the electronic structure of the compounds, as evidenced by the similarity between the absorption spectra obtained for compounds dissolved in CH 2 Cl 2 and THF.…”

“…The coordination environment of the Rh 0 center is defined by an equatorial plane of three phosphorus atoms from the bridging dfpma ligands, axially capped by Rh II and a phosphorus atom from a donor phosphine ligand. We previously established dfpma as an adequate axial donor ligand of the Rh 0 center with the isolation and structural characterization of the bromide complex, Rh 2 (dfpma) 3 Br 2 (η 1 -dfpma) . However, the fully reduced LRh 0 Rh 0 L complex has remained conspicuously absent from a complete homologous series featuring η 1 -dfpma as a capping ligand.…”

“…Electronic absorption and luminescence spectra of the LRh 0 Rh 0 L, LRh 0 Rh II X 2 , and X 2 Rh II Rh II X 2 dfpma complexes are consistent with each possessing a lowest energy excited state of dσ* parentage. 37 We now show that the dσ* excited state allows us to interconvert among LRh 0 Rh 0 L, LRh 0 Rh II X 2 , and X 2 Rh II -Rh II X 2 cores, enabling us to rationally design a system capable of supporting four-electron photochemistry among discrete molecular species.…”

“…Valence orbitals for all atoms were used in the basis set. The extended Hückel calculations of the LRh 0 Rh 0 L and Br 2 Rh II Rh II Br 2 complexes were performed by using the atomic positions taken from the published crystal structures of Rh 2 (dfpma) 3 (PF 3 ) 2 36b and Rh 2 (dfpma) 3 Br 4 , respectively. For the mixed-valence compound, an idealized molecule, Rh 2 (dfpma) 3 Br 2 (PF 3 ), was constructed with the atomic positions obtained from the crystal structure of Rh 2 (dfpma) 3 Br 2 (η 1 -dfpma).…”