Investigation of metal–dithiolate fold angle effects: Implications for molybdenum and tungsten enzymes

Abstract

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur -orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin Mo͞W enzymes. The compounds (Tp*)MoO(bdt) (compound 1), Cp 2Mo(bdt) (compound 2), and Cp 2Ti(bdt) (compound 3) [where Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is 5 -cyclopentadienyl] provide access to three different electronic configurations of the metal, formally d 1 , d 2 , and d 0 , respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A ''dithiolate-folding-effect'' involving an interaction of the metal in-plane and sulfur-orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes. C oordination by the sulfur atoms of one or two ene-1,2-dithiolate (dithiolene) ligands of the novel substituted pyranopterin-dithiolate (''molybdopterin''; ref. 1) is a common structural feature of mononuclear molybdenum-containing enzymes (2-4). These enzymes catalyze a wide range of oxidation͞reduction reactions in carbon, sulfur, and nitrogen metabolism. Fig. 1 shows the structure of the active site of sulfite oxidase, a representative example (5, 6) of the coordination of the pyranopterin-dithiolate (hereafter abbreviated S 2 pdt; ref. 7). These structural results raise fundamental questions about the role of the S 2 pdt coordination in the overall catalytic cycle of molybdenum enzymes (8). The unusual ability of ene-1,2-dithiolate ligands to stabilize metals in multiple oxidation states has been recognized since the compounds were first investigated (9). Proposed roles for the S 2 pdt ligand include functioning as an electron transfer conduit from the metal to other prosthetic groups (10) and as a modulator of the oxidation͞reduction potential of the metal site (10). During catalysis, the metal center is proposed to pass through the M(VI͞V͞IV) oxidation states, i.e., the Mo d electron count changes from d 0 to d 1 to d 2 . Thus, studies of discrete metal dithiolate complexes encompassing these and related electron configurations may provide insight concerning metal thiolate bonding and reactivity in enzymes.Previous structural studies of model molybdenum complexes of the type (Tp*)MoE(1,2-dithiolate) [where E is O or NO, Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, and the 1,2-dithiolates are bdt (1,2-benzenedithiolate), bdtCl 2 (3,6-dichlorobenzenedithiolate), and qdt (2,3-quinoxalinedithiolate)] have shown that the fold angle of the dithiolate metallacycle along the S⅐⅐⅐S vector (Fig. 2) varies in a way that depends on the occupation of a d orbital that is in the equatorial pl...

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