Kinetic and mechanistic studies on the reaction of dioxygen with hexa-aquamolybdenum(III)

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The reaction of hexa-aquamolybdenum(III) with O2in p-toluenesulphonic acid (Hpts) solutions, I= 2.0 M [Li(pts)], gives the di-µ-oxo-molybdenum(V) ion [Mo2O4(H2O)6]2+ as product: 2Mo3++ O2+ 2H2O → Mo2O42+ 4H+. Use of isotopically labelled O2followed by i.r. spectroscopy on the isolated Mo2O42+ has demonstrated that at least some 18O is taken up in the µ-oxo-positions. With Mo3+ in large excess an intense yellow intermediate MoO2Mo6+(or a related form) with a peak at 395 nm (ε= 2 300 M–1 cm–1) is observed. At 25 °C stopped-flow studies on the decay of this intermediate give rate constants k3(obs.) which can be expressed as in equation (i). The various constants are as defined in equations (ii)–(iv), where values indicated are for 2.0 M Hpts. k3(obs.)=K1K2k3[Mo3+]2//1 +K1[Mo3+]+K1K2[Mo3+]2(i), Mo3++ O2⇌ MoO23+(K1 360 M–1)(ii), MoO23++ Mo3+⇌ MoO2Mo6+(K2 535 M–1)(iii), MoO2Mo6++ 2H2O → Mo2O42++ 4H+(k3 0.23 s–1)(iv) Estimates of rate constants for reactions (ii) and (iii) were obtained from studies on the formation of MoO2Mo6+. All parameters were further refined by an iterative fitting procedure using the program KINSIM for multi-step reactions. The values of k1(180 M–1 s–1) and k2(42 M–1 s–1) are in excess of previous substitution rate constants for Mo3+. With O2 in excess of Mo3+(0.72 × 10–4 M) there was no evidence for MoO2Mo6+, and an alternative route for conversion of MoO23+ to Mo2O42+, which is first order in MoO23+(k4= 0.037 s–1) was identified. In the refinement this step was included alongside (ii)–(iv). The reactions of MoO2Mo6+(k3) and MoO23+(k4) exhibit [H+]–1 dependences consistent with an increase in hydrolysis of the metal ion.