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

Abstract

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 S_N1 (limiting) mechanism, in which the dissociative loss of a P(NMe₂)₃O molecule is the rate-determining step, with k_x(25 °C)= 9.1 ± 1.5 s^–1, ΔH_X^‡= 18.5 ± 0.6 kcal mol^–1, and ΔS_X^‡= 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.