The dioxo-Mo(VI) complexes LM0O2X [L = hydrotris(3,5-dimethylpyrazol-1 -yl)borate (La), hydrotris(3isopropylpyrazol-1 -yl)borate (Lb), hydrotris(3,5-dimethyl-1,2,4-triazol-1 -yl)borate (Lc); X = Cl, Br, NCS, OMe, OEt, OPh, SPr', SPh, SCH2Ph] have been synthesized and characterized by spectroscopic and structural methods.The infrared spectra of the complexes exhibit ( 2) bands at 940-920 and 910-890 cm-1, and the NMR spectra are indicative of molecular C, symmetry in solution. The X-ray crystal structures of three complexes are reported. LaMo02(SPh): monoclinic space group P2\lc, a = 18.265( 6) Á, b = 8.110(3) Á, c = 18.299(3) Á, ß = 117.06(2)°, V = 2414(1) Á* 123 with Z = 4. LbMo02(OMe): monoclinic space group Cllc, a = 30.365(4) Á, b = 8.373(1) Á, c = 19.646( 2) Á, ß = 113.28(1)°, V = 4588(1) Á3 5with Z = 8. LcMo02(SPh): orthorhombic space group P2,2,2,, a = 7.9302(13) k,b = 16.627(2) k,c= 17.543(2) k,V= 2313.1(9) Á3 with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043,0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries. The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMovC>2X]-. The requirements for reversible, one-electron electrochemical reduction of dioxo-Mo(VI) complexes appear to be (i) minimal conformational change, restricting substitution trans to the oxo groups, upon reduction and (ii) a steric or electrostatic barrier to the close approach and dinucleation of the reduced species. A number of oxo-hydroxo-Mo(V) complexes of the type LMovO-(OH)X were generated by protonation of the anions [LMov02X]~. Chemical reduction by Bun4NSH results in the sequential generation of [LMov02X]-and [LMovOSX]-anions (except for X = OPh, SPh, and SPr1, when only [LMov02X]is formed). The Mo(V) complexes have been characterized by EPR spectroscopy.

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The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Me, Et, Pr(i)(), Ph] were prepared by reacting MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) complexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr(i)(), Ph) were prepared by oxidation of the oxo-Mo(IV) complexes or by reaction of LMoO(2)Cl with NaS(2)PR(2). Oxygen atom transfers from Me(2)SO to LMoO(S(2)PR(2)) were first-order with respect to Me(2)SO and complex; the overall second-order rate constants at 40 degrees C range from 9.0(1) x 10(-)(5) M(-)(1).s(-)(1) for LMoO(S(2)PMe(2)) to 2.08(5) x 10(-)(4) M(-)(1).s(-)(1) for LMoO(S(2)PPr(2)); activation parameters were in the ranges DeltaH() = 63(1) to 73(1) kJ.mol(-)(1), DeltaS() = -88(1) to -111(1) J.K(-)(1).mol(-)(1), and DeltaG() = 100(2) kJ.mol(-)(1) for LMoO(S(2)PMe(2)) to 98(2) kJ.mol(-)(1) for LMoO(S(2)PPr(2)). Oxygen atom transfer from pyridine N-oxide to LMoO(S(2)PPr(2)) was also second-order with a rate constant of 1.54(5) x 10(-)(3) M(-)(1).s(-)(1) at 40 degrees C, DeltaH() = 62(1) kJ.mol(-)(1), DeltaS() = -90(1) J.K(-)(1).mol(-)(1), and DeltaG() = 90(1) kJ.mol(-)(1). The second-order rate laws and large negative entropies of activation are consistent with associative mechanisms for the above reactions. Oxygen atom transfer from LMoO(2)(S(2)PPr(2)) to PPh(3) was first-order with respect to reactants, with an overall second-order rate constant of 2.5(3) x 10(-)(4) M(-)(1).s(-)(1) at 30 degrees C. In toluene at 40 degrees C, all the above complexes catalyzed the oxidation of PPh(3) by Me(2)SO, with turnover rates of ca. 0.9 mol of PPh(3)/(mol of catalyst/h). Reduction of LMoO(2)(S(2)PR(2)) by SH(-) led to the generation of the dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2)-S)](-), which were slowly converted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2)-S)](-). Dioxygen reacted with [LMoOS(S(2)PPr(2))](-) to produce the oxothio-Mo(VI) complex LMoOS(S(2)PPr(2)-S). The (hydroxo)oxo-Mo(V) complexes LMoO(OH)(S(2)PR(2)-S) were formed upon reduction of LMoO(2)(S(2)PR(2)) with PPh(3) in wet (3-5 M H(2)O) tetrahydrofuran or upon ferrocenium oxidation of LMoO(S(2)PR(2)) in wet tetrahydrofuran. In dry solvents, LMoO(S(2)PR(2)) were oxidized to the corresponding cations, [LMoO(S(2)PR(2)-S,S')](+), which reacted with water to form LMoO(OH)(S(2)PR(2)). The Mo(V) complexes have been characterized by EPR spectroscopy.

The pterin-containing molybdoenzymes catalyze the net exchange of an oxygen atom between water and substrate and there is evidence to support the involvement of oxygen atom transfer (OAT or oxo transfer) in the reactions of dimethyl sulfoxide reductases (DMSOR), sulfite oxidase, and nitrate reductase. [2][3][4][5][6][7] Accordingly, many model studies have focused on OAT reactions, most notably the reduction of dimethyl sulfoxide by oxo-Mo-(IV) complexes and the oxidation of tertiary phosphines by dioxo-Mo(VI) complexes. 3 Indeed, Schultz et al. 4 have shown that the DMSOR from Rhodobacter sphaeroides couples these reactions during catalysis of OAT from Me 2 SO to the watersoluble tertiary phosphine 1,3,5-triaza-7-phosphatricyclo[3.3.1.1 3,7 ]-decane. As well, the crystal structure of Me 2 S-soaked crystals of oxidized DMSOR from R. capsulatus has revealed the presence of an Mo-bound Me 2 SO molecule formed upon incomplete OAT from Mo to Me 2 S. 5 Putative oxo(phosphine oxide) intermediates formed during the oxidation of phosphines by enzyme or model systems have never been detected or isolated.The reactions of LMo VI O 2 X (L ) hydrotris(3,5-dimethylpyrazol-1-yl)borate) with PPh 3 are second-order and produce OPPh 3 and "LMo IV OX", which may be trapped, e.g., as LMo IV OX-(solvent) or LMo IV OX (as monodentate X becomes bidentate). 8-10 An associative mechanism has been proposed for the overall OAT reaction. 9 However, it is not clear whether the intermediate, LMoOX(OPPh 3 ), gives the product by an associative or dissociative mechanism. The reaction of LMoO 2 (SPh) with phosphines led Hall and co-workers 6 to examine the reaction of MoO 2 (NH 3 ) 2 -(SH) 2 with PMe 3 by computational methods. In the first step of the reaction, nucleophilic attack of PMe 3 on a π* ModO orbital perpendicular to the MoO 2 unit and at an Mo-O‚‚‚P angle of ca. 130°takes place. This results in a transition state with a weakened Mo-O bond (1.83 Å), an O-P interaction (2.43 Å), and a P-OModO torsion angle of 89.7°; the remaining ModO bond becomes stronger consistent with a "spectator oxo" function. 11 The OPMe 3 ligand then rotates about the Mo-O bond, breaking the Mo-O π interaction to generate an intermediate with Mod O ) 1.67 Å, Mo-O ) 2.18 Å, O-P ) 1.53 Å, and P-O-Mod O torsion ) 0.5°. The intermediate was 68.9 kcal‚mol -1 lower in energy than the reactants. At this stage replacement of OPMe 3 by water is predicted to take place by an associatiVe mechanism.Here, we report the detection of oxo(phosphine oxide) intermediates in the OAT reactions of LMoO 2 X and PPh 3 by fast atom bombardment mass spectrometry (FABMS) and the use of this technique to assess the stability of the intermediates and examine the kinetics of decay for unstable species. 12 Also, we report the isolation and characterization of L Pr Mo IV O(OPh)(OPEt 3 ) (L Pr ) hydrotris(3-isopropylpyrazol-1-yl)borate), the first stable oxo-(phosphine oxide) complex to be synthesized by incomplete OAT in a molybdoenzyme model system.Intermediates in the reactions of LMoO ...

Cobaltocene reduction of LMoVIO2(SPh) complexes yielded extremely dioxygen-sensitive cobaltocenium salts of the cis-dioxo-Mo(V) radical anions cis-[LMoVO2(SPh)]-, while sodium acenaphthalenide reduction of LaMoVIO2(SPh) yielded a complex sodium salt containing [LaMoVO2(SPh)]- [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (La), hydrotris(3-isopropylpyrazol-1-yl)borate (Lb), or hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Lc)]. Crystals of [CoCp2][LcMoVO2(SPh)]·toluene were orthorhombic, space group Pbca, with a = 17.695(3) Å, b = 19.490(3) Å, c = 21.925(4) Å, V = 7561(2) Å3 for Z = 8. The structure, refined using a full-matrix least-squares procedure and 2269 data, converged with R = 0.067 (R w = 0.067). In the distorted octahedral anion, the metrical parameters of the cis-MoVO2 fragment [Mo−O = 1.742(9) Å, O−Mo−O = 112.1(4)°] are significantly larger than those of LcMoVIO2(SPh) [average Mo−O = 1.700(6) Å, O−Mo−O = 103.9(2)°]. 18O-Substitution of [CoCp2][La,bMoVO2(SPh)] permitted the assignment of bands at ca. 870 and 770 cm-1 to the νs and νas modes, respectively, of the cis-MoVO2 fragment. Freshly prepared solutions of [CoCp2][LMoVO2(SPh)] exhibited a broad EPR signal (g, 1.92; W 1/2, 20 G) characteristic of a cis-[MoVO2]+ complex. The signal was stable when L = Lc but when L = La or Lb it was replaced by a proton-coupled signal characteristic of a cis-[MoVO(OH)]2+ center. The complex LaMoVO(OH)(SPh) was isolated as a coprecipitate with its conjugate base salt [CoCp2][LaMoVO2(SPh)]. It was characterized by infrared bands at 915 and 535 cm-1, assigned to ν(MoO) and ν(MoOH) modes, respectively. The anions were readily converted to air-stable LMoVO(OSiMe3)(SPh) upon reaction with Me3SiCl and reacted with dioxygen to regenerate LMoVIO2(SPh). The paper reports the first isolation in substance of compounds containing novel cis-[MoVO2]+ and cis-[MoVO(OH)]2+ centers and the first crystallographic characterization of a cis-dioxo-Mo(V) species.

Intermediates in the oxygen atom transfer from Mo(VI) to P(III), [Tp(iPr)MoOX(OPR3)] (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate; X = Cl-, phenolates, thiolates), have been isolated from the reactions of [Tp(iPr)MoO2X] with phosphines (PEt3, PMePh2, PPh3). The green, diamagnetic oxomolybdenum(IV) complexes possess local C(1) symmetry (by NMR spectroscopy) and exhibit IR bands assigned to nu(Mo==O) (approximately 950 cm(-1)) and nu(P==O) (1140-1083 cm(-1)) vibrations. The X-ray crystal structures of [Tp(iPr)MoOX(OPEt3)] (X = OC6H4-2-sBu, SnBu), [Tp(iPr)MoO(OPh)(OPMePh2)], and [Tp(iPr)MoOCl(OPPh3)] have been determined. The monomeric complexes exhibit distorted octahedral geometries, with coordination spheres composed of tridentate fac-Tp(iPr) and mutually cis monodentate terminal oxo, phosphoryl (phosphine oxide), and monoanionic X ligands. The electronic structures and stabilities of the complexes have been probed by computational methods, with the three-dimensional energy surfaces confirming the existence of a low-energy steric pocket that restricts the conformational freedom of the phosphoryl ligand and inhibits complete oxygen atom transfer. The reactivity of the complexes is also briefly described.

The oxygen-atom-transfer (OAT) reactivity of [LiPrMoO2(OPh)] (1, LiPr=hydrotris(3-isopropylpyrazol-1-yl)borate) with the tertiary phosphines PEt3 and PPh2Me in acetonitrile was investigated. The first step, [LiPrMoO2(OPh)]+PR3-->[LiPrMoO(OPh)(OPR3)], follows a second-order rate law with an associative transition state (PEt3, DeltaH not equal=48.4 (+/-1.9) kJ mol-1, DeltaS not equal=-149.2 (+/-6.4) J mol-1 K-1, DeltaG not equal=92.9 kJ mol-1; PPh2Me, DeltaH not equal=73.4 (+/-3.7) kJ mol-1, DeltaS not equal=-71.9 (+/-2.3) J mol-1 K-1, DeltaG not equal=94.8 kJ mol-1). With PMe3 as a model substrate, the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide-coordinated intermediate were calculated. The latter, 95 kJ mol-1, is in good agreement with the experimental values. An unexpectedly large O-P-C angle calculated for the TS suggests that there is significant O-nucleophilic attack on the P--C sigma* in addition to the expected nucleophilic attack of the P on the Mo==O pi*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [LiPrMoO(OPh)(OPR3)]+MeCN-->[LiPrMoO(OPh)(MeCN)]+OPR3, follows a first-order rate law in MeCN. A dissociative interchange (Id) mechanism, with activation parameters of DeltaH not equal=93.5 (+/-0.9) kJ mol-1, DeltaS not equal=18.2 (+/-3.3) J mol-1 K-1, DeltaG not equal=88.1 kJ mol-1 and DeltaH not equal=97.9 (+/-3.4) kJ mol-1, DeltaS not equal=47.3 (+/-11.8) J mol-1 K-1, DeltaG not equal=83.8 kJ mol-1, for [LiPrMoO(OPh)(OPEt3)] (2 a) and [LiPrMoO(OPh)(OPPh2Me)] (2 b), respectively, is consistent with the experimental data. Although gas-phase calculations indicate that the Mo--OPMe3 bond is stronger than the Mo--NCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [LiPrMoO(OPh)(MeCN)] (3).