Role of Multiple Vanadium Centers on Redox Buffering and Rates of Polyvanadomolybdate-Cu(II)-Catalyzed Aerobic Oxidations.

A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA 4 H 5 [PMo 6 V 6 O 40 ] ( PV 6 Mo 6 ) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number ( x = 0-4 and 6) in TBA salts of PV x Mo 12- x O 40 (3+ x )- ( PVMo ) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to -2000 mV vs Fc/Fc + under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo /Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV 3 Mo 9 and PV 4 Mo 8 are 8.9 and 48 s -1 , respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo 12 is more positive than that of PVMo 11 (-236 and -405 mV vs Fc/Fc + ); however, the initial reduction rates are 1.06 × 10 -4 s -1 and 0.036 s -1 for PMo 12 and PVMo 11 , respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo 11 and PV 2 Mo 10 , where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.