Highly oxidizing excited states of Re and Tc complexes

University of Wyoming, Ларами, Wyoming, United States
Journal of the American Chemical Society (Impact Factor: 11.44). 01/2007; 128(51):16494-5. DOI: 10.1021/ja067114g
Source: PubMed

ABSTRACT Like the Re analogue, the ligand-to-metal charge transfer (LMCT) excited-state of [Tc(dmpe)3]2+ (dmpe is bis-1,2-(dimethylphosphino)ethane) is luminescent in solution at room temperature. Surprisingly, both [M(dmpe)3]2+* species have extremely large excited-state potentials (ESPs) as oxidants-the highest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon of visible light (calculated for M = Re(Tc); E1/2* = +2.61(2.52) V versus SCE; lambdamax = 526(585) nm). Using a Rehm-Weller analysis with a series of aromatic hydrocarbons as electron-transfer quenchers, E1/2(Re2+*/Re+) has been determined to be 2.58 V, in good agreement with the calculated value. Both [M(dmpe)3]2+* species are quenched by chloride ion and both can function as excited-state oxidants in water solution.

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    ABSTRACT: The ligand-to-metal charge transfer (LMCT) excited state luminescence of [Tc(dmpe)3](2+) (dmpe is 1,2-bis-(dimethylphosphino)ethane) has been measured in solution at room temperature and is compared to its Re analogue. Surprisingly, both [M(dmpe)3](2+)* (M = Re, Tc) species have extremely large excited-state potentials (ESPs) as oxidants, the highest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon of visible light (calculated for M = Tc; E°'* = +2.48 V versus SCE; λmax = 585 nm). Open shell time-dependent density functional theory (TDDFT) calculations support the assignment of the lowest energy transition in both the technetium and rhenium complexes to be a doublet-doublet process that involves predominantly LMCT (dmpe-to-metal) character and is in agreement with past assignments for the Re system. As expected for highly oxidizing excited state potentials, quenching is observed for the excited states of both the rhenium and technetium complexes. Stern-Volmer analysis resulted in quenching parameters for both the rhenium and technetium complexes under identical conditions and are compared using Rehm-Weller analysis. Of particular interest is the fact that both benzene and toluene are oxidized by both the Re and Tc systems.
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