Michael R. Hyde

Crystal and electronic structure and reactivity of mononuclear halogeno-oxomolybdenum(V) complexes☆

Abstract The results of an integrated series of preparative, structural, spectroscopic and kinetic studies for a selection of mononuclear halogenooxomolybdenum(V) complexes are described and discussed.

Oxygen transfer reactions involving certain oxomolybdenum complexes

Abstract Kinetic data have been obtained for the oxygen transfer reactions between MoOCl3L2 (where L is Ph3PO or (Me2N)3PO) and NO3−, MoOCl3-(OPPh3)2 and N2O4/NO2 or NO2 and MoO2(S2CNEt2)2 and Ph3P in non-aqueous solutions using stopped flow techniques. These data are interpreted and their relevance to the oxygen transfer reactions catalysed by certain of the molybdoenzymes is considered.

The Cr2+ and V2+ reduction of µ-carboxylato-dicobalt(III) ammine complexes. Part V. The mechanism of reduction of µ-maleato- and µ-fumarato-complexes

The Cr2+ and V2+ reductions of binuclear cobalt(III) complexes [(NH3)4Co·µ(NH2, maleato)·Co(NH3)4]4+, [(NH3)3Co·µ(OH,OH, maleato)·Co(NH3)3]3+, and [(NH3)3Co·µ(OH,OH,fumarato)·Co(NH3)3]3+, referred to as µ(am), µ(hhm), and µ(hhf) respectively, proceed by a slow/fast reaction sequence. Kinetic data have been obtained for the first stages with Cr2+ as reductant and at 25 °C, I= 1.0M(LiClO4), second-order rate constants (l mol–1 s–1), enthalpies of activation (kcal mol–1), and entropies of activation (cal K–1 mol–1), are respectively for µ(am), 0.59, 5.5, and –41.3; µ(hhm), 1.87, 4.3, and –43.0; and µ(hhf), 2.64, 2.8, and –47.1. Rate constants are independent of [H+] in the range 0.02–1.00M. Product analyses are consistent with inner-sphere mechanisms involving remote attack. With V2+ as reductant kinetic data for the first stage are respectively for µ(am), 1.46, 9.8, and –24.9; µ(hhm), 0.164, 5.8, and –42.3; and µ(hhf), 0.099, 75. and –38.0. No dependence on [H+] was observed except in the V2+ reduction of µ(hhm), when significant contributions were obtained from an [H+]–1 term with kinetic parameters, 0.45 (s–1), 10.6, and –29.3.

Kinetics and mechanism of oxidation of trichloro-oxobis(triphenylphosphine oxide)molybdenum(V) by nitrate in dichloromethane

The kinetics of the redox reaction between [Et4N][NO3] and [MoOCl3(OPPh3)2] have been studied in CH2Cl2 solution at temperatures from 3·2 to 25 °C. This reaction proceeds in three observable steps when [NO3–] > [MoV]. The first step involes co-ordination of the nitrate ion via an SN1 (limiting) mechanism involving loss of a loss of a triphenylphosphine oxide molecule; at 25 °C, kobs.= 40 ± 1 s–1 and the corresponding activation parameters are ΔH‡= 9·7 ± 0·5 kcal mol–1 and ΔS‡=–18·4 ± 1·7 cal K–1 mol–1. The data obtained for the second and third steps strongly suggest that they involve intramolecular substitution at molybdenum(V) and inter-molecular substitution at molybdenum(VI) centres, respectively. Rapid non-rate-determining electron transfer from molybdenum(V) to nitrate is proposed to occur between these two latter steps, ke.t. 1 s–1 at 25 °C, giving a dioxomolybdenum(VI) complex and nitrogen dioxide. The kinetic results imply that a specific orientation of the nitrato-group with respect to the oxomolybdenum(V) centre is necessary prior to rapid electron transfer.

Kinetics and mechanism of halide-substitution reactions of trichloro-oxobis(triphenylphosphine oxide)molybdenum(V)

A detailed study of the substitution reactions of chloride and bromide ions with trichloro-oxobis(triphenyl-phosphine oxide)molybdenum(V), [MoOCl3(OPPh3)2], [MoOCl3(OPPh3)2]+ X–→[MoOCl3X(OPPh3)]–+ Ph3PO, has been accomplished in dichloromethane solution over the temperature range 1–25 °C. The rate of substitution was found to be independent of halide-ion concentration and, for constant [X–](X = Cl or Br), to be inversely dependent on [Ph3PO]. These data are consistent with an SN1 (limiting) mechanism, with essentially identical values for the activation parameters for both chloride and bromide substitution: ΔH‡= 10·6 ± 0·8 and 11·4 ± 0·2 kcal moh–1 and ΔS‡=–15·4 ± 2·9 and –12·5 ± 0·8 cal K–1 mol–1 at 298 K, respectively. For both reactions the rate-determining step is considered to involve loss of the Ph3PO ligand trans to the oxo-group in [MoOCl3(OPPh3)2]. Subsequent to halide substitution, exchange of the substituted halide with the equatorial Ph3PO occurs. The reactions of Cl– with [MoOCl3Br(OPPh3)] and of several nucleophiles with [MoOCl4]– in dichloromethane solution at 25 °C have been studied and the data obtained shown to provide support for the proposed mechanism of halide substitution of [MoOCl3(OPPh3)2].

Kinetics and mechanism of the oxygen-transfer reaction between bis-(diethyldithiocarbamato)dioxomolybdenum(VI) and triphenylphosphine

The kinetics of the oxygen-transfer reaction from [MoO2(S2CNEt2)2] to PPh3 in acetonitrile solution have been monitored using stopped-flow techniques at temperatures between 15 and 45 °C. The rate law d[MoIV]/dt=k2[MoVI][PPh3], with k2(25 °C)= 1.1 ± 0.3 dm3 mol–1 s–1, describes the kinetic data obtained, with activation parameters at 25 °C of ΔH‡= 8.4 ± 0.5 kcal mol–1 and ΔS‡=–30 ± 1.6 cal K–1 mol–1.

Kinetics and mechanism of substitution reactions of trichloro-oxobis-[tris(dimethylamino)phosphine oxide]molybdenum(V) in dichloromethane solution

Kinetic data for the reactions [MoCl3O{P(NMe2)3O}2]+[NEt4]X [graphic omitted] [NEt4][MoCl3XO{P(NMe2)3O}]+ P(NMe2)3O (X = Cl, Br, or NO3) in dichloromethane solution are reported over the temperature range 7–35 °C. These data are consistent with an SN1 (limiting) mechanism, in which the dissociative loss of a P(NMe2)3O molecule is the rate-determining step, with kx(25 °C)= 9.1 ± 1.5 s–1, ΔHX‡= 18.5 ± 0.6 kcal mol–1, and ΔSX‡= 8.2 ± 2.3 cal K–1 mol–1. It is considered that the reaction proceeds by the initial loss of the P(NMe2)3O ligand trans to the oxo-group. This is then followed by a rapid isomerisation resulting in overall cis substitution. In the case of X = NO3 this then leads to a rapid redox reaction producing NO2 and the cis-dioxomolybdenum(VI) complex. Comparisons of the substitution reactivity of the complexes [MoCl3OL2][L = PPh3O or P(NMe2)3O] are described.

The Cr2+ and V2+ reduction of -carboxylato dicobalt(III) ammine complexes. Part VI. Mechanism of the reduction of -p-formlybenzoato- and -o-formylbenzoato-complexes

The kinetic of the Cr2+ and V2+ reductios of the µ-p-formylbenzoato- and µ-o-formylbenzoato-di-µ-hydorxobis[triamminecobalt(III)] complexes. µ(pfb) and µ(ofb) respectively, have been studied, I= 1.0M(LiCLO4), by stopped-flow spectrophotometry. The reactions are first order in oxidant and reductant, and no [H+]-dependence is observed, [H+]= 0.06–0.60M. Reduction of the first cobalt (III) centre is rate determiniing. At 25 °C with Cr2+ as reductant kcr= 122.5 l mol–1 s–1, ΔH‡= 1.4 ± 0.3 kcal mol–1, and ΔS‡=–44.2 ± 0.9 cal K–1 mol–1 for the µ(pfb) complex, and kCr= 31.6l l mol–1 s–1. ΔH‡=–3.6 ± 0.7 kcal mol–1, and ΔS‡=–63.7 ± 0.9 cal K–1 mol–1 for the µ(ofb) complex. It is concluded that inner-sphere Cr2+ reduction via the remote aldehyde function is occurring in both cases, and that the primary chromium(III) product formed in the initial step aquates rapidly to give [Cr(H2O)6]3+. Kinetic data (25 °C) with V2+ as reductant are kV= 0.148 l mol–1 s–1ΔH‡= 7.8 ± 1.9 kcal mol–1, ΔS‡=–36.4 ± 0.6 cal K–1 mol–1 for µ(pfb), and kV= 0.235 l mol–1 s–1, ΔH‡= 6.1 ± 0.5 kcal mol–1, and ΔS‡=–41.0 ± 1.5 cal K–1 mol–1 for µ(ofb).

Kinetics and mechanism of nitrite reduction by trichloro-oxobis(triphenylphosphine oxide)molybdenum(V)

In the presence of excess of nitrite ion the complex [MoOCl3(OPPh3)2] is completely oxidised to MoVI in CH2Cl2. The reaction proceeds by three observable stages. Data for the first stage correspond to co-ordination of nitrite at molybdenum(V), via a limiting SN1 mechanism, involving loss of a triphenylphosphine oxide molecule. At 25 °C, k1= 51·8 ± 2·1 s–1, ΔH1‡= 12·6 ± 0·3 kcal moh–1, and ΔS1‡=–8·2 ± 1·0 cal K–1 mol–1. The data for the second stage of reaction are taken to indicate chelation and/or isomerisation of co-ordinated nitrito-ligand into a position cis to the oxo-group, followed by rapid non-rate-determining electron transfer. The third and final stage of reaction appears to involve co-ordination of the available nitrite to the primary molybdenum(VI) product, and subsequent rearrangement of these molybdenum(VI) products.