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Nitric Oxide Reactivity of Fluorophore Coordinated Carboxylate-Bridged Diiron(II) and Dicobalt(II) Complexes

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posted on 23.08.2004, 00:00 by Scott A. Hilderbrand, Stephen J. Lippard
The synthesis, structural characterization, and NO reactivity of carboxylate-bridged dimetallic complexes were investigated. The diiron(II) complex [Fe2(μ-O2CArTol)4(Ds-pip)2] (1), where O2CArTol = 2,6-di(p-tolyl)benzoate and Ds-pip = dansyl-piperazine, was prepared and determined by X-ray crystallography to have a paddlewheel geometry. This complex reacts with NO within 1 min with a concomitant 4-fold increase in fluorescence emission intensity ascribed to displacement of Ds-pip. Although the diiron complex reacts with NO, as revealed by infrared spectroscopic studies, its sensitivity to dioxygen renders it unsuitable as an atmospheric NO sensor. The air-stable dicobalt(II) analogue was also synthesized and its reactivity investigated. In solution, the dicobalt(II) complex exists as an equilibrium between paddlewheel [Co2(μ-O2CArTol)4(Ds-pip)2] (2) and windmill [Co2(μ-O2CArTol)2(O2CArTol)2(Ds-pip)2] (3) geometric isomers. Conditions for crystallizing pure samples of each of these isomers are described. Reaction of 2 with excess NO proceeds by reductive nitrosylation giving [Co(μ-O2CArTol)2(NO)4] (5), which is accompanied by release of the Ds-pip fluorophore that is N-nitrosated in the process. This reaction affords an overall 9.6-fold increase in fluorescence emission intensity, further demonstrating the potential utility of ligand dissociation as a strategy for designing fluorescence-based sensors to detect nitric oxide in a variety of contexts.