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A new donor-acceptor biradical complex, Tp

The complex [TEA][Tp*Mo

Dichalcogenolene platinum(II) diimine complexes, (LE,E′)Pt(bpy), are characterized by charge-separated dichalcogenolene donor (LE,E′) → diimine acceptor (bpy) ligand-to-ligand charge transfer (LL′CT) excited states that lead to their interesting photophysics and potential use in solar energy conversion applications. Despite the intense interest in these complexes, the chalcogen dependence on the lifetime of the triplet LL′CT excited state remains unexplained. Three new (LE,E′)Pt(bpy) complexes with mixed chalcogen donors exhibit decay rates that are dominated by a spin–orbit mediated nonradiative pathway, the magnitude of which is proportional to the anisotropic covalency provided by the mixed-chalcogen donor ligand environment. This anisotropic covalency is dramatically revealed in the 13C NMR chemical shifts of the donor carbons that bear the chalcogens and is further probed by S K-edge XAS. Remarkably, the NMR chemical shift differences also correlate with the spin–orbit matrix element that connects th...

New bis(ene-1,2-dithiolato)-oxido-alcoholato molybdenum(VI) and -oxido-thiolato molybdenum(VI) anionic complexes, denoted as [Mo(VI)O(ER)L2](-) (E = O, S; L = dimethoxycarboxylate-1,2-ethylenedithiolate), were obtained from the reaction of the corresponding dioxido-molybdenum(VI) precursor complex with either an alcohol or a thiol in the presence of an organic acid (e.g., 10-camphorsulfonic acid) at low temperature. The [Mo(VI)O(ER)L2](-) complexes were isolated and characterized, and the structure of [Mo(VI)O(OEt)L2](-) was determined by X-ray crystallography. The Mo(VI) center in [Mo(VI)O(OEt)L2](-) exhibits a distorted octahedral geometry with the two ene-1,2-dithiolate ligands being symmetry inequivalent. The computed structure of [Mo(VI)O(SR)L2](-) is essentially identical to that of [Mo(VI)O(OR)L2](-). The electronic structures of the resulting molybdenum(VI) complexes were evaluated using electronic absorption spectroscopy and bonding calculations. The nature of the distorted O(h) geometry in these [Mo(VI)O(EEt)L2](-) complexes results in a lowest unoccupied molecular orbital wave function that possesses strong π* interactions between the Mo(d(xy)) orbital and the cis S(p(z)) orbital localized on one sulfur donor from a single ene-1,2-dithiolate ligand. The presence of a covalent Mo-S(dithiolene) bonding interaction in these monooxido Mo(VI) compounds contributes to their low-energy ligand-to-metal charge transfer transitions. A second important d-p π bonding interaction derives from the ∼180° O(oxo)-Mo-E-C dihedral angle involving the alcoholate and thiolate donors, and this contributes to ancillary ligand contributions to the electronic structure of these species. The formation of [Mo(VI)O(OEt)L2](-) and [Mo(VI)O(SEt)L2](-) from the dioxidomolybdenum(VI) precursor may be regarded as a model for the active-site formation process that occurs in the dimethyl sulfoxide reductase family of pyranopterin molybdenum enzymes.

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO(SPh)2((i)Pr2Dt(0)) (where (i)Pr2Dt(0) = N,N'-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° "envelope"-type fold of the five-membered dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ∼18 000 cm(-1) is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL'CT) transition that also possesses Mo(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo(x(2)-y(2)) orbital character is derived from a strong pseudo Jahn-Teller effect, which drives the large fold-angle distortion to yield a double-well potential in the electronic ground state.

A detailed electron paramagnetic resonance (EPR) and computational study of a key paramagnetic form of xanthine oxidase (XO) has been performed and serves as a basis for developing a valence-bond description of C-H activation and transition-state (TS) stabilization along the reaction coordinate with aldehyde substrates. EPR spectra of aldehyde-inhibited XO have been analyzed in order to provide information regarding the relationship between the g, (95,97)Mo hyperfine (A(Mo)), and (13)C hyperfine (A(C)) tensors. Analysis of the EPR spectra has allowed for greater insight into the electronic origin of key delocalizations within the Mo-O(eq)-C fragment and how these contribute to C-H bond activation/cleavage and TS stabilization. A natural bond orbital analysis of the enzyme reaction coordinate with aldehyde substrates shows that both Mo═S π → C-H σ* (ΔE = 24.3 kcal mol(-1)) and C-H σ → Mo═S π* (ΔE = 20.0 kcal mol(-1)) back-donation are important in activating the substrate C-H bond for cleavage. Additional contributions to C-H activation derive from O(eq) lp → C-H σ* (lp = lone pair; ΔE = 8.2 kcal mol(-1)) and S lp → C-H σ* (ΔE = 13.2 kcal mol(-1)) stabilizing interactions. The O(eq)-donor ligand that derives from water is part of the Mo-O(eq)-C fragment probed in the EPR spectra of inhibited XO, and the observation of O(eq) lp → C-H σ* back-donation indicates a key role for O(eq) in activating the substrate C-H bond for cleavage. We also show that the O(eq) donor plays an even more important role in TS stabilization. We find that O(eq) → Mo + C charge transfer dominantly contributes to stabilization of the TS (ΔE = 89.5 kcal mol(-1)) and the Mo-O(eq)-C delocalization pathway reduces strong electronic repulsions that contribute to the classical TS energy barrier. The Mo-O(eq)-C delocalization at the TS allows for the TS to be described in valence-bond terms as a resonance hybrid of the reactant (R) and product (P) valence-bond wave functions.

New dioxomolybdenum(VI) complexes, (Et(4)N)(Ph(4)P)[Mo(VI)O(2)(S(2)C(2)(CO(2)Me)(2))(bdt)] (2) and (Et(4)N)(Ph(4)P)[Mo(VI)O(2)(S(2)C(2)(CO(2)Me)(2))(bdtCl(2))](4)(S(2)C(2)(CO(2)Me)(2) = 1,2-dicarbomethoxyethylene-1,2-ditholate, bdt = 1,2-benzenedithiolate, bdtCl(2) = 3,6-dichloro-1,2-benzenedithiolate), that possess at least one ene-1,2-dithiolate ligand were synthesized by the reaction of their mono-oxo-molybdenum(IV) derivatives, (Et(4)N)(2)[Mo(IV)O(S(2)C(2)(CO(2)Me)(2))(bdt)] (1) and (Et(4)N)(2)[Mo(IV)O(S(2)C(2)(CO(2)Me)(2))(bdtCl(2))] (3), with Me(3)NO. Additionally, the bis(ene-1,2-dithiolate)Mo(VI)O(2) complex, (Et(4)N)(Ph(4)P)[Mo(VI)O(2)(S(2)C(2)(CO(2)Me)(2))(2)] (6), was isolated. Complexes 2, 4, and 6 were characterized by elemental analysis, negative-ion ESI mass spectrometry, and IR spectroscopy. X-ray analysis of 4 and 6 revealed a Mo(VI) center that adopts a distorted octahedral geometry. Variable-temperature (1)H NMR spectra of (CD(3))(2)CO solutions of the Mo(VI)O(2) complexes indicated that the Mo centers isomerize between Delta and Lambda forms. The electronic structures of 2, 4, and 6 have been investigated by electronic absorption and resonance Raman spectroscopy and bonding calculations. The results indicate very similar electronic structures for the complexes and considerable pi-delocalization between the Mo(VI)O(2) and ene-1,2-dithiolate units. The similar oxygen atom transfer kinetics for the complexes results from their similar electronic structures.

The effect of sterically encumbering ligands on the electronic structure of oxomolybdenum tetrathiolate complexes was determined using a combination of electronic absorption and magnetic circular dichroism spectroscopies, complimented by DFT bonding calculations, to understand geometric and electronic structure contributions to reduction potentials. These complexes are rudimentary models for a redox-active metalloenzyme active site in a protein matrix and allow for detailed spectroscopic probing of specific oxomolybdenum-thiolate interactions that are directly relevant to Mo-S(cysteine) bonding in pyranopterin molybdenum enzymes. Data are presented for three para-substituted oxomolybdenum tetrathiolate complexes ([PPh4][MoO(p-SPhCONHCH3)4], [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2CN)3)4], and [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2COOCH2CH3)3)4]). The Mo(V/IV) reduction potentials of the complexes in DMF are -1213, -1251, and -1247 mV, respectively. The remarkably similar electronic absorption and magnetic circular dichroism spectra of these complexes establish that the observed reduction potential differences are not a result of significant changes in the electronic structure of the [MoOS4]- cores as a function of the larger ligand size. We provide evidence that these reduction potential differences result from the driving force for a substantial reorganization of the O-Mo-S-C dihedral angle upon reduction, which decreases electron donation from the thiolate sulfurs to the reduced molybdenum center. The energy barrier to favorable O-Mo-S-C geometries results in a reorganizational energy increase, relative to [MoO(SPh)4](-/2-), that correlates with ligand size. The inherent flexible nature of oxomolybdenum-thiolate bonds indicate that thiolate ligand geometry, which controls Mo-S covalency, could affect the redox processes of monooxomolybdenum centers in pyranopterin molybdenum enzymes.

Deconvoluting the different contributions of thiolate and ene-1,2-dithiolate donors to the underlying electronic structure of the Mo site in sulfite oxidase (SO) has proven to be a difficult task. One way in which these differences might be illuminated is by selectively substituting Se for S in model complexes which possess multiple sulfur donor ligand environments. Here we report the synthesis and structures of new oxo-Mo(V) complexes as effective models for the one-electron reduced active site of SO. We have used the tridentate heteroscorpionate ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) in order to model the constrained cysteinyl sulfur (S(Cys)) ligand environment observed in the crystal structure of the enzyme, and benzene-1,2-dithiol (bdt) as a mimic of the ene-1,2-dithiolate chelate. [(L3S)MoO(bdt)] and [(L3S)MoO(SPh)(2)] have been structurally characterized by X-ray crystallography, and as such, [(L3S)MoO(bdt)] is only the second known model compound that closely approximates the active site structure of reduced forms of SO. Additionally, benzenethiol (SPh) and benzeneselenol (SePh) have been used to perturb the equatorial ligand environment of [(L3S)MoO(bdt)].) This has provided much needed insight into the electronic structure of the one-electron reduced SO site and has allowed for increased understanding of the individual roles played by these different thiolate donors in the oxidative half-reaction of the enzyme. Interestingly, the EPR spectra of [(L3S)MoO(bdt)], [(L3S)MoO(SPh)(2)], and [(L3S)MoO(SePh)(2)] closely resemble that of both high pH (hpH) and low pH (lpH) SO, except for the fact that the magnitude of g(1) is found to be consistently higher in the model spectra compared to that of the enzyme. It is suggested that this derives from an increase in Mo-S covalency in the models relative to hpH and lpH SO.