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

Abstract

The kinetics of the redox reaction between [Et,N] [NO,] and [MoOC13(0PPh,),] have been studied in CH,CI2 solution at temperatures from 3.2 to 25 "C. This reaction proceeds in three observable steps when [NO,-] = .[Mov]. The first step involves co-ordination of the nitrate ion via an S , 1 (limiting) mechanism involving loss of a triphenylphosphine oxide molecule; at 25 "C, kobs. = 40 f 1 s-l and the corresponding activation parameters are AH$ = 9.7 f 0.5 kcal rn0l-l and ASS = -18.4 f 1.7 cal K -l mol-l. The data obtained for the second and third steps strongly suggest that they involve intramolecular substitution a t molybdenum(v) and intermolecular substitution at molybdenum(v1) 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(vt) 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.INTERACTION between a molybdenum(v) centre and a nitrate ion has been proposed to occur immediately prior to the final step in the electron-transfer sequence of the reduction path of the nitrate reductase enzymes.1.2 These enzymes, which require molybdenum both for their formation and a ~t i v i t y , ~ are responsible for the first step in nitrate assimilation and for nitrate respiration, converting nitrate to nitrite with the molybdenum apparently changing between the oxidation states MoV andAs part of a study of the chemistry of molybdenum related to that of the molybdenumcontaining enzymes, we report details of a kinetic investigation of the reaction between nitrate ions and the molybdenum(v) complex [MoOCl,(OPPh,),] .t EXPERIMENTAL

Model for the active sites of oxo-transfer molybdoenzymes: reactivity, kinetics, and catalysis

“…These observations establish catalysis of reaction 20 (R = Me). In the absence of any catalyst, Me2SO and Ph3P do not react for at least 1 h at 189 °C.44 It was demonstrated by 31P NMR R2SO + Ph3P -R2S + Ph3PO (20) spectroscopy that none of the sulfoxides in Table I underwent detectable reaction with excess Ph3P in DMF solutions at room temperature. If phosphine oxidation is rate-limiting under these conditions, the consumption of Ph3P should be first order in both phosphine and Mo02(LNS2).…”

The inorganic biochemistry of molybdoenzymes

“…So far as the mechanisms of the enzymes is concerned, as much data has come from studies on model compounds as from those on the enzymes. Garner and coworkers early concluded (Garner et al 1975) that nitrate must coordinate in an equatorial position, cis to an oxo ligand. The viability of oxo transfer mechanism has been elegantly demonstrated by Berg & Holm (19856).…”

Electrochemistry and Spectroscopy of Catalytically Active Molybdenum-Catechol Complexes

“…MoO(ONO)(cat) 2 2 " -» Mo0 2 (cat) 2 2 " + products (14) Consistent with similar catalytic reactions of Mo(V) in nonaqueous solvents (36,37), the rate-determining step is presumed to involve rearrangement of coordinated N0 2 " from a nitrogen-bonded to an oxygen-bonded species. If this change is accomplished so that an oxy gen atom of N0 2~ becomes located cis to the oxo group on Mo(V), then Electrochemical and Spectrochemical Studies of Biological Redox Components Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/26/15.…”