Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer

Abstract

The control of the solution electrochemical potential as well as pH impacts products in redox reactions, but the former gets far less attention. Redox buffers facilitate the maintenance of potentials and have been noted in diverse cases, but they have not been a component of catalytic systems. We report a catalytic system that contains its own built-in redox buffer. Two highly synergistic components (a) the tetrabutylammonium salt of hexavanadopolymolybdate TBA 4 H 5 [PMo 6 V 6 O 40 ] (PV 6 Mo 6 ) and (b) Cu(ClO 4 ) 2 in acetonitrile catalyze the aerobic oxidative deodorization of thiols by conversion to the corresponding nonodorous disulfides at 23 °C (each catalyst alone is far less active). For example, the reaction of 2-mercaptoethanol with ambient air gives a turnover number (TON) = 3 × 10 2 in less than one hour with a turnover frequency (TOF) of 6 × 10 −2 s −1 with respect to PV 6 Mo 6 . Multiple electrochemical, spectroscopic, and other methods establish that (1) PV 6 Mo 6 , a multistep and multielectron redox buffering catalyst, controls the speciation and the ratio of Cu(II)/Cu(I) complexes and thus keeps the solution potential in different narrow ranges by involving multiple POM redox couples and simultaneously functions as an oxidation catalyst that receives electrons from the substrate; (2) Cu catalyzes two processes simultaneously, oxidation of the RSH by PV 6 Mo 6 and reoxidation of reduced PV 6 Mo 6 by O 2 ; and (3) the analogous polytungstate-based system, TBA 4 H 5 [PW 6 V 6 O 40 ] (PV 6 W 6 ), has nearly identical cyclic voltammograms (CV) as PV 6 Mo 6 but has almost no catalytic activity: it does not exhibit self-redox buffering.

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

“…(4) While many O 2 -based organic substrate oxidation processes catalyzed by vanadopolymolybdates, PV x Mo 12– x O 40 (3+ x )– ( PVMo ), are well-studied by other groups and some commercialized, none of these studies have noted the self-redox-buffering phenomenon. This dynamic redox self-buffering chemistry is demonstrated in our previous article, and this one might lead to faster rates for other POM-catalyzed selective oxidation of other organic compounds.…”

“…This approach has been used for PV 6 Mo 6 in our previous study. 52 The V IV -to-Mo VI intervalence charge transfer (IVCT) band at 550 nm 69,84,85 was used to calibrate the extinction coefficient versus the number of electrons in the reduced states of PV 4 Mo 8 (Figure S9). We found that ascorbic acid has limited ET ability in acetonitrile: It can only reduce the vanadium atoms but not the molybdenum atoms in PVMo.…”

“…The overall mechanism of eq was proposed in a previous work . The key step of POM as a redox buffering catalyst is the fast and reversible ET reaction in eq , where n is the number of electrons in the reduced POM. Cu ( II ) false( RS − false) 2 + POM ( n ) ⇄ Cu ( I ) false( RS • false) 2 + POM ( n + 1 ) , n = 0 − 5 …”

“… 55 , 57 We note that in some H 2 O 2 -based oxidations catalyzed by divanadium-substituted polyoxotungstates, peroxo-vanadium intermediates have been proposed. 61 , 62 In our previous work, 52 we conducted a series of experiments to show that free vanadium has little effect on the PV 6 Mo 6 /Cu catalytic system in acetonitrile. Thus, we assume that in our system, the intact POM is the catalytically significant unit, and we explain the activity tendency from two vantages: (1) the redox buffer effect of PVMo —the difference in electrochemical properties dictates different potential buffer ranges; (2) the difference in ET rates of V and Mo in the PVMo Keggin complexes.…”

“…In our previous work, we have found that PV 6 Mo 6 can accept up to six electrons in the Cu/ PVMo catalytic system for air-based thiol oxidation. 52 Thus, the redox properties of PVMo in acetonitrile at very negative potentials are important. However, trace amounts of water in acetonitrile can have a significant effect on the electrochemical behavior of reduced POMs.…”