A combination of reaction coordinate computations, resonance Raman spectroscopy, spectroscopic computations, and hydrogen bonding investigations have been used to understand the importance of substrate orientation along the xanthine oxidase reaction coordinate. Specifically, 4-thiolumazine and 2,4-dithiolumazine have been used as reducing substrates for xanthine oxidase to form stable enzyme-product charge transfer complexes suitable for spectroscopic study. Laser excitation into the near-infrared molybdenum-to-product charge transfer band produces rR enhancement patterns in the high frequency in-plane stretching region that directly probe the nature of this MLCT transition and provide insight into the effects of electron redistribution along the reaction coordinate between the transition state and the stable enzyme-product intermediate, including the role of the covalent Mo–O–C linkage in facilitating this process. The results clearly show that specific Mo-substrate orientations allow for enhanced electronic coupling and facilitate strong hydrogen bonding interactions with amino acid residues in the substrate binding pocket.

The correlation of electron transfer with molecular conductance (g: electron transport through single molecules) by Nitzan and others has contributed to a fundamental understanding of single-molecule electronic materials. When an unsymmetric, dipolar molecule spans two electrodes, the possibility exists for different conductance values at equal, but opposite electrode biases. In the device configuration, these molecules serve as rectifiers of the current and the efficiency of the device is given by the rectification ratio (RR = gforward/greverse). Experimental determination of the RR is challenging since the orientation of the rectifying molecule with respect to the electrodes and with respect to the electrode bias direction is difficult to establish. Thus, while two different values of g can be measured and a RR calculated, one cannot easily assign each conductance value as being aligned with or opposed to the molecular dipole, and calculations are often required to resolve the uncertainty. Herein, we describe the properties of two isomeric, triplet ground state biradical molecules that serve as constant-bias analogs of single-molecule electronic devices. Through established theoretical relationships between g and electronic coupling, H2, and between H2 and magnetic exchange coupling, J (g ∝ H2 ∝ J), we use the ratio of experimental J-values for our two isomers to calculate a RR for an unsymmetric bridge molecule with known geometry relative to the two radical fragments of the molecule and at a spectroscopically-defined potential bias. Our experimental results are compared with device transport calculations.

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO(SPh)2(iPr2Dt0) (where iPr2Dt0 = 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(x2–y2) 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.

New bis(ene-1,2-dithiolato)-oxido–alcoholato molybdenum(VI) and -oxido–thiolato molybdenum(VI) anionic complexes, denoted as [MoVIO(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 [MoVIO(ER)L2]– complexes were isolated and characterized, and the structure of [MoVIO(OEt)L2]– was determined by X-ray crystallography. The Mo(VI) center in [MoVIO(OEt)L2]– exhibits a distorted octahedral geometry with the two ene-1,2-dithiolate ligands being symmetry inequivalent. The computed structure of [MoVIO(SR)L2]– is essentially identical to that of [MoVIO(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 Oh geometry in these [MoVIO(EEt)L2]– complexes results in a lowest unoccupied molecular orbital wave function that possesses strong π* interactions between the Mo(dxy) orbital and the cis S(pz) orbital localized on one sulfur donor from a single ene-1,2-dithiolate ligand. The presence of a covalent Mo–Sdithiolene 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° Ooxo–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 [MoVIO(OEt)L2]– and [MoVIO(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.

A combination of electron paramagnetic resonance (EPR) spectroscopy and computational approaches has provided insight into the nature of the reaction coordinate for the one-electron reduction of nitrite by the mitochondrial amidoxime reducing component (mARC) enzyme. The results show that a paramagnetic Mo(V) species is generated when reduced enzyme is exposed to nitrite, and an analysis of the resulting EPR hyperfine parameters confirms that mARC is remarkably similar to the low-pH form of sulfite oxidase. Two mechanisms for nitrite reduction have been considered. The first shows a modest reaction barrier of 14 kcal/mol for the formation of ·NO from unprotonated nitrite substrate. In marked contrast, protonation of the substrate oxygen proximal to Mo in the Mo(IV)–O–N–O substrate-bound species results in barrierless conversion to products. A fragment orbital analysis reveals a high degree of Mo–O(H)–N–O covalency that provides a π-orbital pathway for one-electron transfer to the substrate and defines orbital constraints on the Mo–substrate geometry for productive catalysis in mARC and other pyranopterin molybdenum enzymes that catalyze this one-electron transformation.

The torsional dependence of donor–bridge–acceptor (D–B–A) electronic coupling matrix elements (HDA, determined from the magnetic exchange coupling, J) involving a spin SD = 1/2 metal semiquinone (Zn-SQ) donor and a spin SA = 1/2 nitronylnitroxide (NN) acceptor mediated by the σ/π-systems of para-phenylene and methyl-substituted para-phenylene bridges and by the σ-system of a bicyclo[2.2.2]octane (BCO) bridge are presented and discussed. The positions of methyl group(s) on the phenylene bridge allow for an experimentally determined evaluation of conformationally dependent (π) and conformationally independent (σ) contributions to the electronic and magnetic exchange couplings in these D–B–A biradicals at parity of D and A. The trend in the experimental magnetic exchange couplings are well described by CASSCF calculations. The torsional dependence of the pairwise exchange interactions are further illuminated in three-dimensional, “Ramachandran-type” plots that relate D–B and B–A torsions to both electronic and exchange couplings. Analysis of the magnetic data shows large variations in magnetic exchange (J ≈ 1–175 cm–1) and electronic coupling (HDA ≈ 450–6000 cm–1) as a function of bridge conformation relative to the donor and acceptor. This has allowed for an experimental determination of both the σ- and π-orbital contributions to the exchange and electronic couplings.

The recently synthesized and isolated low-coordinate FeV nitride complex has numerous implications as a model for high-oxidation states in biological and industrial systems. The trigonal [PhB(tBuIm)3FeV≡N]+ (where (PhB(tBuIm)3– = phenyltris(3-tert-butylimidazol-2-ylidene)), (1) low-spin d3 (S = 1/2) coordination compound is subject to a Jahn–Teller (JT) distortion of its doubly degenerate 2E ground state. The electronic structure of this complex is analyzed by a combination of extended versions of the formal two-orbital pseudo Jahn–Teller (PJT) treatment and of quantum chemical computations of the PJT effect. The formal treatment is extended to incorporate mixing of the two e orbital doublets (30%) that results from a lowering of the idealized molecular symmetry from D3h to C3v through strong “doming” of the Fe–C3 core. Correspondingly we introduce novel DFT/CASSCF computational methods in the computation of electronic structure, which reveal a quadratic JT distortion and significant e–e mixing, thus reaching a new level of synergism between computational and formal treatments. Hyperfine and quadrupole tensors are obtained by pulsed 35 GHz ENDOR measurements for the 14/15N-nitride and the 11B axial ligands, and spectra are obtained from the imidazole-2-ylidene 13C atoms that are not bound to Fe. Analysis of the nitride ENDOR tensors surprisingly reveals an essentially spherical nitride trianion bound to Fe, with negative spin density and minimal charge density anisotropy. The four-coordinate 11B, as expected, exhibits negligible bonding to Fe. A detailed analysis of the frontier orbitals provided by the electronic structure calculations provides insight into the reactivity of 1: JT-induced symmetry lowering provides an orbital selection mechanism for proton or H atom transfer reactivity.

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A molecular orbital analysis provides new insight into the mechanism of Mo/Cu carbon monoxide dehydrogenase, and reveals electronic structure contributions to reactivity that are remarkably similar to the structurally related molybdenum hydroxylases. A calculated reaction barrier of ∼12 kcal mol−1 is in excellent agreement with experiment.

Mo K-edge X-ray absorption spectroscopy has been used to probe as-isolated structures of the MOSC family proteins pmARC-1 and HMCS-CT. The Mo K-edge near-edge spectrum of HMCS-CT is shifted ∼2.5 eV to lower energy compared to the pmARC-1 spectrum, which indicates that as-isolated HMCS-CT is in a more reduced state than pmARC-1. Extended X-ray absorption fine structure analysis indicates significant structural differences between pmARC-1 and HMCS-CT, with the former being a dioxo site and the latter possessing only a single terminal oxo ligand. The number of terminal oxo donors is consistent with pmARC-1 being in the MoVI oxidation state and HMCS-CT in the MoIV state. These structures are in accord with oxygen-atom-transfer reactivity for pmARC-1 and persulfide bond cleavage chemistry for HMCS-CT.

Magnetic circular dichroism (MCD) and electronic absorption spectroscopies have been used to probe the electronic structure of the classical paramagnetic metal–metal-bonded complexes [Re2X4(PMe3)4]+ (X = Cl, Br). A violation of the MCD sum rule is observed that indicates the presence of ground-state contributions to the MCD intensity. The z-polarized δ → δ* band in the near-IR is formally forbidden in MCD but gains intensity through a combination of ground- and excited-state mechanisms to yield a positive C term.

Transient absorption and emission spectroscopic studies on a series of diimineplatinum(II) dichalcogenolenes, LPtL′, reveal charge-separated dichalcogenolene → diimine charge-transfer (LL′CT) excited-state lifetimes that display a remarkable and nonperiodic dependence on the heteroatoms of the dichalcogenolene ligand. Namely, there is no linear relationship between the observed lifetimes and the principle quantum number of the E donors. The results are explained in terms of heteroatom-dependent singlet–triplet (S–T) energy gaps and anisotropic covalency contributions to the M–E (E = O, S, Se) bonding scheme that control rates of intersystem crossing. For the dioxolene complex, 1-O,O′, E(T2) > E(S1) and rapid nonradiative decay occurs from S1 to S0. However, E(T2) ≤ E(S1) for the heavy-atom congeners, and this provides a mechanism for rapid intersystem crossing. Subsequent internal conversion to T1 in 3-S,S produces a long-lived, emissive triplet. The two LPtL′ complexes with mixed chalcogen donors and 5-Se,Se show lifetimes intermediate between those of 1-O,O′ and 3-S,S.

The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form MoIV-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase. These MoIV-product complexes display an intense metal-to-ligand charge-transfer (MLCT) band in the near-infrared region of the spectrum. Optical pumping into this MLCT band yields resonance Raman spectra of the Mo site that are devoid of contributions from the highly absorbing FAD and 2Fe2S clusters in the protein. The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes. This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.

The nature of the electronic coupling in transition metal Donor–Acceptor and Donor–Bridge–Acceptor systems has become a subject of interest in the last decade due to the importance of electronic coupling in molecular electronics. Here we review the synthesis, structure, magnetism, and spectroscopy of donor–acceptor biradicals as they pertain to understanding the electronic origins and magnitude of the electronic coupling matrix element that figures prominently in electron transfer (transport). Stable D–A systems may be thought of as ground state analogues of charge separated states generated in photoinduced electron transfer processes and as model systems for understanding single molecule mediated electron transport between biased nanoelectrodes. The strong coupling (adiabatic) regime will be covered in this review as well as the methodology for probing the magnitude of the electronic coupling in this limit. D–A biradicals can facilitate long-range ferromagnetic exchange between localized spins mediated by delocalized electrons, and this is a new phenomenon in molecular systems. The use of the VBCI approach to develop a complete electronic structure description of strong electronic coupling in D–A and D–B–A biradicals, the origin of long-range biradical ferromagnetic exchange, and the relationship of these biradicals to molecular electronic materials will form the basis of the review.Highlights► Molecular, conformational and electronic structure of biradical ligands presented. ► Valence Bond Configuration Interaction (VBCI) model presented. ► VBCI model used to relate magnetic exchange to electronic coupling. ► Bridge effects on electronic coupling of DBA biradical ligands reviewed. ► Mixed-valent DBA paramagnetic ligands promote long-range electron correlation.

A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on TpCum,MeZn(SQ-m-Ph-NN) (1-meta) a donor–bridge–acceptor (D–B–A) biradical that possesses a cross-conjugated meta-phenylene (m-Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of 1-meta. The results reveal important excited-state contributions to the ground-state singlet–triplet splitting in this cross-conjugated D–B–A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D–B–A biradical possessing a para-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D–B–A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the m-Ph bridged D–A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D–B–A molecules where, in the case of 1-meta, the magnetic exchange in the active electron approximation is expected to be J ∼ 0 cm–1. We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D–B–A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated.

The preparation and characterization of three new donor–bridge–acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), β = 0.39 Å–1 and oligo(2,5-thiophene), β = 0.22 Å–1. This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D–B energy gap, Δ, rather than bridge–bridge electronic couplings, HBB. This is supported by the fact that the HBB values extracted from the experimental data for oligo(para-phenylene) (HBB = 11 400 cm–1) and oligo(2,5-thiophene) (12 300 cm–1) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern HDA and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.

We report here an ENDOR study of an S = 1/2 intermediate state trapped during reduction of the binuclear Mo/Cu enzyme CO dehydrogenase by CO. ENDOR spectra of this state confirm that the 63,65Cu nuclei exhibits strong and almost entirely isotropic coupling to the unpaired electron, show that this coupling atypically has a positive sign, aiso = +148 MHz, and indicate an apparently undetectably small quadrupolar coupling. When the intermediate is generated using 13CO, coupling to the 13C is observed, with aiso = +17.3 MHz. A comparison with the couplings seen in related, structurally assigned Mo(V) species from xanthine oxidase, in conjunction with complementary computational studies, leads us to conclude that the intermediate contains a partially reduced Mo(V)/Cu(I) center with CO bound at the copper. Our results provide strong experimental support for a reaction mechanism that proceeds from a comparable complex of CO with fully oxidized Mo(VI)/Cu(I) enzyme.

Spectroscopic and kinetic studies indicate that oxo-carboxylato-molybdenum(VI) bis-dithiolene complexes, (MoVIO(p-X-OBz)L2), have been generated at low temperature as active site structural models for the type II class of pyranopterin molybdenum DMSOR family enzymes. A DFT analysis of low energy charge transfer bands shows that these complexes possess a Mo–Sdithiolene π-bonding interaction between the Mo(dxy) redox active molecular orbital and a cis S(pz) donor orbital located on one of the dithiolene ligands.

Variable-temperature electronic absorption and resonance Raman spectroscopies are used to probe the excited state electronic structure of TpCum,MeZn(SQ-Ph-NN) (1), a donor-bridge-acceptor (D-B-A) biradical complex and a ground state analogue of the charge-separated excited state formed in photoinduced electron transfer reactions. Strong electronic coupling mediated by the p-phenylene bridge stabilizes the triplet ground state of this molecule. Detailed spectroscopic and bonding calculations elucidate key bridge distortions that are involved in the SQ(π)SOMO → NN-Ph (π*)LUMO D → A charge transfer (CT) transition. We show that the primary excited state distortion that accompanies this CT is along a vibrational coordinate best described as a symmetric Ph(8a) + SQ(in-plane) linear combination and underscores the dominant role of the phenylene bridge fragment acting as an electron acceptor in the D-B-A charge transfer state. Our results show the importance of the phenylene bridge in promoting (1) electron transfer in D-Ph-A systems and (2) electron transport in biased electrode devices that employ a 1,4-phenylene linkage. We have also developed a relationship between the spin density on the acceptor, as measured via the isotropic NN nitrogen hyperfine interaction, and the strength of the D → A interaction given by the magnitude of the electronic coupling matrix element, Hab.

The electronic structure of a genuine paramagnetic des-oxo Mo(V) catalytic intermediate in the reaction of dimethyl sulfoxide reductase (DMSOR) with (CH3)3NO has been probed by electron paramagnetic resonance (EPR), electronic absorption, and magnetic circular dichroism (MCD) spectroscopies. EPR spectroscopy reveals rhombic g- and A-tensors that indicate a low-symmetry geometry for this intermediate and a singly occupied molecular orbital that is dominantly metal centered. The excited-state spectroscopic data were interpreted in the context of electronic structure calculations, and this has resulted in a full assignment of the observed MCD and electronic absorption bands, a detailed understanding of the metal–ligand bonding scheme, and an evaluation of the Mo(V) coordination geometry and Mo(V)–Sdithiolene covalency as it pertains to the stability of the intermediate and electron-transfer regeneration. Finally, the relationship between des-oxo Mo(V) and des-oxo Mo(IV) geometric and electronic structures is discussed relative to the reaction coordinate in members of the DMSOR enzyme family.

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, 95,97Mo hyperfine (AMo), and 13C hyperfine (AC) tensors. Analysis of the EPR spectra has allowed for greater insight into the electronic origin of key delocalizations within the Mo–Oeq–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 Oeq 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 Oeq-donor ligand that derives from water is part of the Mo–Oeq–C fragment probed in the EPR spectra of inhibited XO, and the observation of Oeq lp → C–H σ* back-donation indicates a key role for Oeq in activating the substrate C–H bond for cleavage. We also show that the Oeq donor plays an even more important role in TS stabilization. We find that Oeq → Mo + C charge transfer dominantly contributes to stabilization of the TS (ΔE = 89.5 kcal mol–1) and the Mo–Oeq–C delocalization pathway reduces strong electronic repulsions that contribute to the classical TS energy barrier. The Mo–Oeq–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.

A model system for the molybdenum cofactor has been developed that illustrates the noninnocent behavior of an N-heterocycle appended to a dithiolene chelate on molybdenum. The pyranopterin of the molybdenum cofactor is modeled by a quinoxalyldithiolene ligand (S2BMOQO) formed from the reaction of molybdenum tetrasulfide and quinoxalylalkyne. The resulting complexes TEA[Tp*MoX(S2BMOQO)] [1, X = S; 3, X = O; TEA = tetraethylammonium; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate] undergo a dehydration-driven intramolecular cyclization within quinoxalyldithiolene, forming Tp*MoX(pyrrolo-S2BMOQO) (2, X = S; 4, X = O). 4 can be oxidized by one electron to produce the molybdenum(5+) complex 5. In a preliminary report of this work, evidence from X-ray crystallography, electronic absorption and resonance Raman spectroscopies, and density functional theory (DFT) bonding calculations revealed that 4 possesses an unusual asymmetric dithiolene chelate with significant thione–thiolate character. The results described here provide a detailed description of the reaction conditions that lead to the formation of 4. Data from cyclic voltammetry, additional DFT calculations, and several spectroscopic methods (IR, electronic absorption, resonance Raman, and electron paramagnetic resonance) have been used to characterize the properties of members in this suite of five Mo(S2BMOQO) complexes and further substantiate the highly electron-withdrawing character of the pyrrolo-S2BMOQO ligand in 2, 4, and 5. This study of the unique noninnocent ligand S2BMOQO provides examples of the roles that the N-heterocycle pterin can play as an essential part of the molybdenum cofactor. The versatile nature of a dithiolene appended by heterocycles may aid in modulating the redox processes of the molybdenum center during the course of enzyme catalysis.

We have obtained low-temperature magnetic circular dichroism (MCD) spectra for ferric cyano complexes of the wild type and E546N mutant of a human inducible nitric oxide synthase (iNOS) oxygenase/flavin mononucleotide (oxyFMN) construct. The mutation at the FMN domain has previously been shown to modulate the MCD spectra of the l-arginine-bound ferric iNOS heme (Sempombe, J.; et al. J. Am. Chem. Soc.2009, 131, 6940–6941). The addition of l-arginine to the wild-type protein causes notable changes in the CN–-adduct MCD spectrum, while the E546N mutant spectrum is not perturbed. Moreover, the MCD spectral perturbation observed with l-arginine is absent in the CN– complexes incubated with N-hydroxy-l-arginine, which is the substrate for the second step of NOS catalysis. These results indicate that interdomain FMN–heme interactions exert a long-range effect on key heme axial ligand–substrate interactions that determine substrate oxidation pathways of NOS.

New square-pyramidal bis(ene-1,2-dithiolate)MoSe complexes, [MoIVSe(L)2]2−, have been synthesised along with their terminal sulfido analogues, [MoIVS(L)2]2−, using alkyl (LC4H8), phenyl (LPh) and methyl carboxylate (LCOOMe) substituted dithiolene ligands (L). These complexes now complete three sets of MoIVO, MoIVS and MoIVSe species that are coordinated with identical ene-1,2-dithiolate ligands. The [alkyl substituted Mo(S/Se)(LC4H8)2]2− complexes were reported in prior investigations (H. Sugimoto, T. Sakurai, H. Miyake, K. Tanaka and H. Tsukube, Inorg. Chem. 2005, 44, 6927, H. Tano, R. Tajima, H. Miyake, S. Itoh and H. Sugimoto, Inorg. Chem. 2008, 47, 7465). The new series of complexes enable a systematic investigation of terminal chalcogenido and supporting ene-1,2-dithiolate ligand effects on geometric structure, electronic structure, and spectroscopic properties. X-ray crystallographic analysis of these (Et4N)2[MoEL2] (E = terminal chalocogenide) complexes reveals an isostructural Mo centre that adopts a distorted square pyramidal geometry. The ME bond distances observed in the crystal structures and the ν(ME) vibrational frequencies indicate that these bonds are weakened with an increase in L→Mo electron donation (LCOOMe < LPh < LC4H8), and this order is confirmed by an electrochemical study of the complexes. The 77Se NMR resonances in MoSeL complexes appear at lower magnetic fields as the selenido ion became less basic from MoSeLC4H8, MoSeLPh and MoSeLCOOMe. Electronic absorption and resonance Raman spectroscopies have been used to assign key ligand-field, MLCT, LMCT and intraligand CT bands in complexes that possess the LCOOMe ligand. The presence of low-energy intraligand CT transition in these MoELCOOMe compounds directly probes the electron withdrawing nature of the -COOMe substituents, and this underscores the complex electronic structure of square pyramidal bis(ene-1,2-dithiolate)-MoIV complexes that possess extended dithiolene conjugation.

A new monoanionic dithiolene ligand is found in Tp*MoO(S2BMOQO). A combination of X-ray crystallography, electronic absorption spectroscopy, resonance Raman spectroscopy, and bonding calculations reveal that the monoanionic dithiolene ligand possesses considerable thiolate-thione character resulting from an admixture of an intraligand charge transfer excited state into the ground state wave function. The unusual dithiolene exhibits a highly versatile donor−acceptor character that dramatically affects the Mo(IV/V) redox couple and points to a potentially noninnocent role of the pterin fragment in pyranopterin Mo enzymes.

A monooxomolybdenum(VI) model complex for the oxidized active site in the DMSOR family of molybdoenzymes has been synthesized and structurally characterized. The compound was obtained from the desoxomolybdenum(IV) derivative by clean oxygen-atom transfer from an amine N-oxide in a manner similar to that observed in the enzyme. A combination of electronic absorption and resonance Raman spectroscopies, coupled with the results of bonding and excited-state calculations, has been used to provide strong support for a highly covalent Mo(dxy)−S(dithiolene) π*-bonding interaction in the molybdenum(VI) complex. It is proposed that the resulting Mo−S covalency facilitates electron-transfer regeneration of the catalytically competent DMSOR MoIV active site.

Computations and EPR spectroscopy are used to probe the spin distribution of donor−bridge−acceptor (D−B−A) biradical complexes: TpCum,MeZn(SQ-NN) (1), TpCum,MeZn(SQ-1,4-Ph-NN) (2), TpCum,MeZn(SQ-2,5-TP-NN) (3), and TpCum,MeZn(SQ-2,5-Xyl-NN) (4) (SQ = orthosemiquinone and NN = nitronylnitroxide). These complexes are ground-state analogs of the charge-separated excited states formed in photoinduced electron transfer reactions. The intraligand magnetic exchange interaction (J) in these complexes is mediated by the bridges and has been found to stabilize the triplet ground states of 1 and 2. Detailed spectroscopic and bonding calculations have been used to elucidate the role of the bridge fragment (B) and its conformation relative to donor (SQ) and acceptor (NN) on spin density distributions. The computed results correlate well with experimental nitrogen hyperfine coupling constants.

The nitric oxide synthase (NOS) ouput state for NO production is a complex of the flavin mononucleotide (FMN)-binding domain and the heme domain, and thereby it facilitates the interdomain electron transfer from the FMN to the catalytic heme site. Emerging evidence suggests that interdomain FMN−heme interactions are important in the formation of the output state because they guide the docking of the FMN domain to the heme domain. In this study, notable effects of mutations in the adjacent FMN domain on the heme structure in a human iNOS bidomain oxygenase/FMN construct have been observed by using low-temperature magnetic circular dichroism (MCD) spectroscopy. The comparative MCD study of wild-type and mutant proteins clearly indicates that a properly docked FMN domain contributes to the observed l-Arg perturbation of the heme MCD spectrum in the wild-type protein and that the conserved surface residues in the FMN domain (E546 and E603) play key roles in facilitating a productive alignment of the FMN and heme domains in iNOS.

Electronic paramagnetic resonance (EPR), electronic absorption, and magnetic circular dichroism spectroscopies have been performed on YedY, a SUOX fold protein with a Mo domain that is remarkably similar to that found in chicken sulfite oxidase, Arabidopsis thaliana plant sulfite oxidase, and the bacterial sulfite dehydrogenase from Starkeya novella. Low-energy dithiolene → Mo and cysteine thiolate → Mo charge-transfer bands have been assigned for the first time in a Mo(V) form of a SUOX fold protein, and the spectroscopic data have been used to interpret the results of bonding calculations. The analysis shows that second coordination sphere effects modulate dithiolene and cysteine sulfur covalency contributions to the Mo bonding scheme. In particular, a more acute Ooxo−Mo−SCys−C dihedral angle results in increased cysteine thiolate S → Mo charge transfer and a large g1 in the EPR spectrum. The spectrosocopic results, coupled with the available structural data, indicate that these second coordination sphere effects may play key roles in modulating the active-site redox potential, facilitating hole superexchange pathways for electron transfer regeneration, and affecting the type of reactions catalyzed by sulfite oxidase family enzymes.

New dioxomolybdenum(VI) complexes, (Et4N)(Ph4P)[MoVIO2(S2C2(CO2Me)2)(bdt)] (2) and (Et4N)(Ph4P)[MoVIO2(S2C2(CO2Me)2)(bdtCl2)](4)(S2C2(CO2Me)2 = 1,2-dicarbomethoxyethylene-1,2-ditholate, bdt = 1,2−benzenedithiolate, bdtCl2 = 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, (Et4N)2[MoIVO(S2C2(CO2Me)2)(bdt)] (1) and (Et4N)2[MoIVO(S2C2(CO2Me)2)(bdtCl2)] (3), with Me3NO. Additionally, the bis(ene-1,2-dithiolate)MoVIO2 complex, (Et4N)(Ph4P)[MoVIO2(S2C2(CO2Me)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 MoVI center that adopts a distorted octahedral geometry. Variable-temperature 1H NMR spectra of (CD3)2CO solutions of the MoVIO2 complexes indicated that the Mo centers isomerize between Δ and Λ 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 π-delocalization between the MoVIO2 and ene-1,2-dithiolate units. The similar oxygen atom transfer kinetics for the complexes results from their similar electronic structures.

The preparation and characterization of new model complexes for the molybdenum cofactor are reported. The new models are distinctive for the inclusion of pterin-substituted dithiolene chelates and have the formulation Tp∗MoX(pterin-R-dithiolene) (Tp∗ = tris(3,5,-dimethylpyrazolyl)borate), X = O, S, R = aryl. Syntheses of Mo(4+) and (5+) complexes of two pterin-dithiolene derivatives as both oxo and sulfido compounds, and improved syntheses for pterinyl alkynes and [Et4N][Tp∗MoIV(S)S4] reagents are described. Characterization methods include electrospray ionization mass spectrometry, electrochemistry, infrared spectroscopy, electron paramagnetic resonance and magnetic circular dichroism. Cyclic voltammetry reveals that the Mo(5+/4+) reduction potential is intermediate between that for dithiolenes with electron-withdrawing substituents and simple dithiolates chelates. Electron paramagnetic resonance and magnetic circular dichroism of Mo(5+) complexes where X = O, R = aryl indicates that the molybdenum environment in the new models is electronically similar to that in Tp∗MoO(benzenedithiolate).

A molecular orbital analysis provides new insight into the mechanism of Mo/Cu carbon monoxide dehydrogenase, and reveals electronic structure contributions to reactivity that are remarkably similar to the structurally related molybdenum hydroxylases. A calculated reaction barrier of ∼12 kcal mol−1 is in excellent agreement with experiment.

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Spectroscopic and kinetic studies indicate that oxo-carboxylato-molybdenum(VI) bis-dithiolene complexes, (MoVIO(p-X-OBz)L2), have been generated at low temperature as active site structural models for the type II class of pyranopterin molybdenum DMSOR family enzymes. A DFT analysis of low energy charge transfer bands shows that these complexes possess a Mo–Sdithiolene π-bonding interaction between the Mo(dxy) redox active molecular orbital and a cis S(pz) donor orbital located on one of the dithiolene ligands.

The correlation of electron transfer with molecular conductance (g: electron transport through single molecules) by Nitzan and others has contributed to a fundamental understanding of single-molecule electronic materials. When an unsymmetric, dipolar molecule spans two electrodes, the possibility exists for different conductance values at equal, but opposite electrode biases. In the device configuration, these molecules serve as rectifiers of the current and the efficiency of the device is given by the rectification ratio (RR = gforward/greverse). Experimental determination of the RR is challenging since the orientation of the rectifying molecule with respect to the electrodes and with respect to the electrode bias direction is difficult to establish. Thus, while two different values of g can be measured and a RR calculated, one cannot easily assign each conductance value as being aligned with or opposed to the molecular dipole, and calculations are often required to resolve the uncertainty. Herein, we describe the properties of two isomeric, triplet ground state biradical molecules that serve as constant-bias analogs of single-molecule electronic devices. Through established theoretical relationships between g and electronic coupling, H2, and between H2 and magnetic exchange coupling, J (g ∝ H2 ∝ J), we use the ratio of experimental J-values for our two isomers to calculate a RR for an unsymmetric bridge molecule with known geometry relative to the two radical fragments of the molecule and at a spectroscopically-defined potential bias. Our experimental results are compared with device transport calculations.

New square-pyramidal bis(ene-1,2-dithiolate)MoSe complexes, [MoIVSe(L)2]2−, have been synthesised along with their terminal sulfido analogues, [MoIVS(L)2]2−, using alkyl (LC4H8), phenyl (LPh) and methyl carboxylate (LCOOMe) substituted dithiolene ligands (L). These complexes now complete three sets of MoIVO, MoIVS and MoIVSe species that are coordinated with identical ene-1,2-dithiolate ligands. The [alkyl substituted Mo(S/Se)(LC4H8)2]2− complexes were reported in prior investigations (H. Sugimoto, T. Sakurai, H. Miyake, K. Tanaka and H. Tsukube, Inorg. Chem. 2005, 44, 6927, H. Tano, R. Tajima, H. Miyake, S. Itoh and H. Sugimoto, Inorg. Chem. 2008, 47, 7465). The new series of complexes enable a systematic investigation of terminal chalcogenido and supporting ene-1,2-dithiolate ligand effects on geometric structure, electronic structure, and spectroscopic properties. X-ray crystallographic analysis of these (Et4N)2[MoEL2] (E = terminal chalocogenide) complexes reveals an isostructural Mo centre that adopts a distorted square pyramidal geometry. The ME bond distances observed in the crystal structures and the ν(ME) vibrational frequencies indicate that these bonds are weakened with an increase in L→Mo electron donation (LCOOMe < LPh < LC4H8), and this order is confirmed by an electrochemical study of the complexes. The 77Se NMR resonances in MoSeL complexes appear at lower magnetic fields as the selenido ion became less basic from MoSeLC4H8, MoSeLPh and MoSeLCOOMe. Electronic absorption and resonance Raman spectroscopies have been used to assign key ligand-field, MLCT, LMCT and intraligand CT bands in complexes that possess the LCOOMe ligand. The presence of low-energy intraligand CT transition in these MoELCOOMe compounds directly probes the electron withdrawing nature of the -COOMe substituents, and this underscores the complex electronic structure of square pyramidal bis(ene-1,2-dithiolate)-MoIV complexes that possess extended dithiolene conjugation.