Direct and remarkably efficient conversion of methane into acetic acid catalyzed by amavadine and related vanadium complexes. A synthetic and a theoretical DFT mechanistic study

Vanadium(IV or V) complexes with N,O- or O,O-ligands, i.e., VO{N(CH2CH2O)3}], CaV(HIDPA)2] (synthetic amavadine), CaV(HIDA)2], or Bu4N]2V(HIDA)2] HIDPA, HIDA = basic form of 2,2'-(hydroxyimino)dipropionic or -diacetic acid, respectively], VO(CF3SO3)2], BaVO(nta)(H2O)]2 (nta = nitrilotriacetate), VO(ada)(H2O)] (ada = N-2-acetamidoiminodiacetate), VO(Hheida)(H2O)] (Hheida = 2-hydroxyethyliminodiacetate), VO(bicine)] bicine = basic form of N,N-bis(2-hydroxyethyl)glycine], and VO(dipic)(OCH2CH3)] (dipic = pyridine-2,6-dicarboxylate), are catalyst precursors for the efficient single-pot conversion of methane into acetic acid, in trifluoroacetic acid (TFA) under moderate conditions, using peroxodisulfate as oxidant. Effects on the yields and TONs of various factors are reported. TFA acts as a carbonylating agent and CO is an inhibitor for some systems, although for others there is an optimum CO pressure. The most effective catalysts (as amavadine) bear triethanolaminate or (hydroxyimino)dicarboxylates and lead, in a single batch, to CH3COOH yields > 50% (based on CH4) or remarkably high TONs up to 5.6 x 103. The catalyst can remain active upon multiple recycling of its solution. Carboxylation proceeds via free radical mechanisms (CH3* can be trapped by CBrCl3), and theoretical calculations disclose a particularly favorable process involving the sequential formation of CH3*, CH3CO*, and CH3COO* which, upon H-abstraction (from TFA or CH4), yields acetic acid. The CH3COO* radical is formed by oxygenation of CH3CO* by a peroxo-V complex via a V{eta1-OOC(O)CH3} intermediate. Less favorable processes involve the oxidation of CH3CO* by the protonated (hydroperoxo) form of that peroxo-V complex or by peroxodisulfate. The calculations also indicate that (i) peroxodisulfate behaves as a source of sulfate radicals which are methane H-abstractors, as a peroxidative and oxidizing agent for vanadium, and as an oxidizing and coupling agent for CH3CO* and that (ii) TFA is involved in the formation of CH3COOH (by carbonylating CH3*, acting as an H-source to CH3COO*, and enhancing on protonation the oxidizing power of a peroxo-VV complex) and of CF3COOCH3 (minor product in the absence of CO).