MECHANISTIC ASPECTS OF THE ELECTROCATALYTIC OXIDATIVE CLEAVAGE OF 1,2-DIOLS BY ELECTROGENERATED PB(IV)

The electrochemical oxidation of Pb(II) to Pb(IV) in acetonitrile solu tion containing benzoic acid and pyridine is possible at a basal-plane pyrolytic-graphite electrode and associated with a rapid ligand excha nge at the metal center. The Pb(IV) species generated under these cond itions is shown to react with diols such as 1,2:5,6-di-O-isopropyliden e-D-mannitol, 1,2-ethanediol, cis- and trans-1,2-cyclopentanediol, whi ch undergo a two-electron oxidation associated with carbon-carbon bond cleavage. Voltametric data obtained by both channel flow cell and rot ating disk electrode experiments are analyzed by numerical simulation. Consistent results for a second-order EC' (electrocatalytic) reaction pathway were obtained. Voltammetric data obtained by systematically v arying the concentration of pyridine and benzoic acid reveal a complex mechanism with a distinct trend in reaction rate for each diol expres sed in terms of apparent fractional reaction orders when analyzed in t erms of a chemically oversimplified EC' mechanism. This behavior is gi ven mechanistic significance by analysis of the data using numerical s imulation employing the following ''branched'' ECrevCrevCirrev' reacti on scheme, which allows all the experimental results to be rationalize d (benz = benzoic acid, py = pyridine): Pb(II)reversible arrow Pb(IV) + 2e(-); Pb(IV)(benz)(h)(py)(i) + diol reversible arrow Pb(IV)(benz)(h )(diol)(py)(i) with rate constants k(1) and k(-1) for the forward and reverse reactions, respectively; Pb(IV)(benz)(h+j)(diol)(py)(i-k) + k( py) reversible arrow Pb(IV)(benz)(h)(diol)(py)(i) + j(benz) with rate constants k(2) and k(-2) for the forward and reverse reactions, respec tively; Pb(IV)(benz)(h)(diol)(py)(i) --> Pb(II) + products with rate c onstant k(i). Here, the chemical processes are associated with appropr iate rate constants, k(n), and the equilibrium constant of a process i s given by K-n = k(n)/k(-n) (n = 1, 2). The ''true'' second-order rate constants K(j)k(i) obtained for the electrocatalytic cleavage of 1,2: 5,6-di-O-isopropylidene-D-mannitol (k =j = 1), K(1)k(f) = (36 +/- 7) x 10(3) M-1 s(-1), 1,2-ethanediol (k = 1; j = 2), K(1)k(f) = (70 +/- 14 ) x 10(3) M-1 s(-1), trans-1,2-cyclopentanediol (k = 1; j = 0), K(1)k( f) = (180 +/- 36) X 10(3) M-1 s(-1), and cis-1,2-cyclopentanediol (k = j = 0), K(1)k(f) = (280 +/- 56) x 10(3) M-1 s(-1) are similar to with in 1 order of magnitude. The effect of the diffusion of coexisting spe cies coupled via fast preequilibria to the irreversible chemical proce ss is discussed in respect to the physical meaning of the rate constan ts determined by applying a simplified mechanistic scheme.