Role of Alkali Metal Cation Size in the Energy and Rate of Electron Transfer to Solvent-Separated 1:1 [(M+)(Acceptor)] (M+ = Li+, Na+, K+) Ion Pairs Academic Article uri icon

abstract

  • The effect of cation size on the rate and energy of electron transfer to [(M+)(acceptor)] ion pairs is addressed by assigning key physicochemical properties (reactivity, relative energy, structure, and size) to an isoelectronic series of well-defined M+−acceptor pairs, M+ = Li+, Na+, K+. A 1e- acceptor anion, α-SiVVW11O405- (1, a polyoxometalate of the Keggin structural class), was used in the 2e- oxidation of an organic electron donor, 3,3‘,5,5‘-tetra-tert-butylbiphenyl-4,4‘-diol (BPH2), to 3,3‘,5,5‘-tetra-tert-butyldiphenoquinone (DPQ) in acetate-buffered 2:3 (v/v) H2O/t-BuOH at 60 °C (2 equiv of 1 are reduced by 1e- each to 1red, α-SiVIVW11O406-). Before an attempt was made to address the role of cation size, the mechanism and conditions necessary for kinetically well behaved electron transfer from BPH2 to 1 were rigorously established by using GC−MS, 1H, 7Li, and 51V NMR, and UV−vis spectroscopy. At constant [Li+] and [H+], the reaction rate is first order in [BPH2] and in [1] and zeroth order in [1red] and in [acetate] (base) and is independent of ionic strength, μ. The dependence of the reaction rate on [H+] is a function of the constant, Ka1, for acid dissociation of BPH2 to BPH- and H+. Temperature dependence data provided activation parameters of ΔH⧧ = 8.5 ± 1.4 kcal mol-1 and ΔS⧧ = −39 ± 5 cal mol-1 K-1. No evidence of preassociation between BPH2 and 1 was observed by combined 1H and 51V NMR studies, while pH (pD)-dependent deuterium kinetic isotope data indicated that the O−H bond in BPH2 remains intact during rate-limiting electron transfer from BPH2 and 1. The formation of 1:1 ion pairs [(M+)(SiVW11O405-)]4- (M+1, M+ = Li+, Na+, K+) was demonstrated, and the thermodynamic constants, KM1, and rate constants, kM1, associated with the formation and reactivity of each M+1 ion pair with BPH2 were calculated by simultaneous nonlinear fitting of kinetic data (obtained by using all three cations) to an equation describing the rectangular hyperbolic functional dependence of kobs values on [M+]. Constants, KM1red, associated with the formation of 1:1 ion pairs between M+ and 1red were obtained by using KM1 values (from kobs data) to simultaneously fit reduction potential (E1/2) values (from cyclic voltammetry) of solutions of 1 containing varying concentrations of all three cations to a Nernstian equation describing the dependence of E1/2 values on the ratio of thermodynamic constants KM1 and KM1red. Formation constants, KM1, and KM1red, and rate constants, kM1, all increase with the size of M+ in the order KLi1 = 21 < KNa1 = 54 < KK1 = 65 M-1, KLi1red = 130 < KNa1red = 570 < KK1red = 2000 M-1, and kLi1 = 0.065 < kNa1 = 0.137 < kK1 = 0.225 M-1 s-1. Changes in the chemical shifts of 7Li NMR signals as functions of [Li51] and [Li61red] were used to establish that the complexes M+1 and M+1red exist as solvent-separated ion pairs. Finally, correlation between cation size and the rate and energy of electron transfer was established by consideration of KM1, kM1, and KM1red values along with the relative sizes of the three M+1 pairs (effective hydrodynamic radii, reff, obtained by single-potential step chronoamperometry). As M+ increases in size, association constants, KM1, become larger as smaller, more intimate solvent-separated ion pairs, M+1, possessing larger electron affinities (q/r), and associated with larger kM1 values, are formed. Moreover, as M+1 pairs are reduced to M+1red during electron transfer in the activated complexes, [BPH2, M+1], contributions of ion pairing energy (proportional to −RT ln(KM1red/KM1) to the standard free energy change associated with electron transfer, ΔG°et, increase with cation size:  −RT ln(KM1red/KM1) (in kcal mol-1) = −1.2 for Li+, −1.5 for Na+, and −2.3 for K+.

publication date

  • May 11, 2001