Electronic structure and photochemical reactivity of binuclear metal complexes

Citation

Smith, David Charles (1989) Electronic structure and photochemical reactivity of binuclear metal complexes. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11052009-142520920

Abstract

A valence bond (VB) "weak coupling" model of the electronic structure for [Ir_2(TMB)_4](B(C_6H_5)_4)_2 is developed and generalized to the class of dimeric systems in which the metals are nonbonded, in a formal sense, and can be viewed as weakly coupled. With the VB model, the energies and widths of the previously observed optical absorption bands can be rationalized; in addition, plausible assignments are made for bands that were not interpreted satisfactorily or not observed in earlier work. The VB model does not change any of the molecular orbital-based interpretations of the thermal chemistry, photochemistry, or photophysics of these systems. Photophysical characterization of the ^(1,3)(dσ*pσ) excited state of Ir_2(TMB)_4^(2+) finds a system quite comparable to other binuclear d^8 complexes. Both fluorescence (λ_(max) 735 nm, τ ~ 70 ± 30 ps) and phosphorescence (λ_(max) 1080 nm, τ = 210 ± 20 ns) are observed. The relatively long lifetime of the ^3(dσ*pσ) excited state of Ir_2(TMB)_4^(2+) suggests that it should be able to participate in bimolecular photochemical reactions. The diradical-like structure of the excited state, an electron (or oxidizing hole) localized on the exterior of the M_2 unit (the dσ* orbital) and an electron localized in the interior of the dimer cage (the pσ orbital), implies that one-electron chemistry will be observed. Reactions of the ground state follow two-electron pathways, similar to those observed for mononuclear d^8 complexes. The ^3(dσ*pσ) excited state of Ir_2(TMB)_4^(2+) is found to be a powerful reductant, E^0(Ir_2(TMB)_4^(3+/3)(Ir_2(TMB)_4^(2+))*) ~ 1.0 V (SSCE). Excited-state electron- transfer quenching by pyridinium acceptors is observed to follow classical Marcus theory for outer-sphere electron transfer. No "inverted" behavior is found. The bimolecular electron-transfer reaction is highly nonadiabatic, K ~ 0.0001, because of the large donor-acceptor separation, ~ 8 Å. The results for Ir_2(TMB)_4^(2+) are discussed in comparison to those for [Ir(µ-pz)COD]_2. Ir_2(TMB)_4^(2+) is found to react photochemically with alkyl halides. Although the ^3(dσ*pσ) excited state is a good reductant, outer-sphere electron transfer seems unlikely (E^0(RX/RX-) < -1.5 V (SSCE)). An S_(RN)1 pathway has been suggested to explain the alkyl halide photoreduction reaction observed for metal complexes with E^0(M_2+/^3M_2*) < -1.5 V (SSCE); however, atom transfer to the ^3(dσ*pσ) excited state is the favored reaction mechanism for the alkyl halide photoreduction reaction of Ir_2(TMB)_4^(2+). The generality of this reaction is discussed. While there is some ambiguity as to the primary photoprocess for alkyl halide photoreactivity, ^3(dσ*pσ) excited-state hydrogen-atom transfer has been established as the mechanism of the reaction of Ir_2(TMB)_4^(2+) and a number of organic substrates. The atom-transfer reactivity of the ^3(dσ*pσ) excited state is attributed to the presence of a hole in the dσ* orbital, analogous to the 3nπ* state of organic ketones. Interaction of the oxidizing hole with the electron pair of the C-H bond is the presumed pathway. Electrochemical oxidation of Rh2_(TMB)_4^(2+) generates the d^8-d^7 species Rh_2(TMB)_4^(3+). This complex reacts with 1,4-cyclohexadiene to abstract a hydrogen atom mimicking the initial step of the ^3(dσ*pσ) photoreaction. The importance of this result is discussed in terms of energy storage systems and extension of the range of hydrocarbon oxidations with binuclear d^8 complexes. The d^7-d^7 dihydride product obtained from the photoreaction of Ir_2(TMB)_4^(2+) and 1,4-cyclohexadiene is isolated and characterized. In addition to NMR, UV-Vis, IR, and Raman spectra, the complex is characterized crystallographically. The reactivity of this complex is also discussed.

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