No 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...

Ka band ESEEM spectroscopy was used to determine the hyperfine (hfi) and nuclear quadrupole (nqi) interaction parameters for the oxo-17O ligand in [Mo 17O(SPh)4]-, a spectroscopic model of the oxo-Mo(V) centers of enzymes. The isotropic hfi constant of 6.5 MHz found for the oxo-17O is much smaller than the values of approximately 20-40 MHz typical for the 17O nucleus of an equatorial OH(2) ligand in molybdenum enzymes. The 17O nqi parameter (e2qQ/h = 1.45 MHz, eta approximately = 0) is the first to be obtained for an oxo group in a metal complex. The parameters of the oxo-17O ligand, as well as other magnetic resonance parameters of [Mo 17O(SPh)4]- predicted by quasi-relativistic DFT calculations, were in good agreement with those obtained in experiment. From the electronic structure of the complex revealed by DFT, it follows that the SOMO is almost entirely molybdenum d(xy) and sulfur p, while the spin density on the oxo-17O is negative, determined by spin polarization mechanisms. The results of this work will enable direct experimental identification of the oxo ligand in a variety of chemical and biological systems.

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes.

No abstract

Using photodetachment photoelectron spectroscopy (PES) in the gas phase, we investigated the electronic structure and chemical bonding of six anionic [Mo(V)O](3+) complexes, [MoOX(4)](-) (where X = Cl (1), SPh (2), and SPh-p-Cl (3)), [MoO(edt)(2)](-) (4), [MoO(bdt)(2)](-) (5), and [MoO(bdtCl(2))(2)](-) (6) (where edt = ethane-1,2-dithiolate, bdt = benzene-1,2-dithiolate, and bdtCl(2) = 3,6-dichlorobenzene-1,2-dithiolate). The gas-phase PES data revealed a wealth of new electronic structure information about the [Mo(V)O](3+) complexes. The energy separations between the highest occupied molecular orbital (HOMO) and HOMO-1 were observed to be dependent on the O-Mo-S-C(alpha) dihedral angles and ligand types, being relatively large for the monodentate ligands, 1.32 eV for Cl and 0.78 eV for SPh and SPhCl, compared to those of the bidentate dithiolate complexes, 0.47 eV for edt and 0.44 eV for bdt and bdtCl(2). The threshold PES feature in all six species is shown to have the same origin and is due to detaching the single unpaired electron in the HOMO, mainly of Mo 4d character. This result is consistent with previous theoretical calculations and is verified by comparison with the PES spectra of two d(0) complexes, [VO(bdt)(2)](-) and [VO(bdtCl(2))(2)](-). The observed PES features are interpreted on the basis of theoretical calculations and previous spectroscopic studies in the condensed phase.

Gas-phase photoelectron spectroscopy and density functional theory have been utilized to investigate the interactions between the p orbitals of dithiolate ligands and d orbitals of titanium in bent titanocene complexes as minimum molecular models of active site features of pyranopterin Mo/W enzymes. The compounds Cp(2)Ti(S-S) [where (S-S) is 1,2-ethenedithiolate (S(2)C(2)H(2)), 1, 1,2-benzenedithiolate (bdt), 2, or 1,3-propanedithiolate (pdt), 3, and Cp(-) is cyclopentadienyl] provide access to a formal 16-electron d(0) electronic configuration at the metal. A "dithiolate-folding-effect" involving an interaction of metal and sulfur orbitals is demonstrated in complexes with arene- and enedithiolates. This effect is not observed for the alkanedithiolate in complex 3.

Reaction of VO(acac)(2) with 1,2-dithiols in the presence of triethylamine gives pentacoordinate oxovanadium complexes [HNEt(3)](2)[VO(bdt)(2)] (1), [HNEt(3)](2)[VO(tdt)(2)] (2), and [HNEt(3)](2)[VO(bdtCl(2))(2)] (3) (where H(2)bdt = 1,2-benzenedithiol, H(2)tdt = 3,4-toluenedithiol, and H(2)bdtCl(2) = 3,6-dichloro-1,2-benzenedithiol). Compounds 1-3 have been characterized by IR, UV/visible, EPR, and mass spectroscopies. The X-ray crystal stuctures of 1 and 2 show hydrogen-bonding interactions between the terminal oxo atom and triethylammonium counterions and between ligand sulfur atoms and the counterions. These interactions are comparable with those found at the active sites of mononuclear molybdenum enzymes.