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

• Published in: Journal of the American Chemical Society Vol. 123; no. 22; pp. 5292 - 5307
• Main Authors: Grigoriev, Vladimir A, Cheng, Danny, Hill, Craig L, Weinstock, Ira A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.06.2001
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja010074q

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-SiVVW11O40 5- (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 1 red, α-SiVIVW11O40 6-). 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 [1 red] 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, K a 1 , 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+)(SiVW11O40 5-)]4- (M+ 1, M+ = Li+, Na+, K+) was demonstrated, and the thermodynamic constants, K M 1 , and rate constants, k M 1 , 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 k obs values on [M+]. Constants, K M 1 red , associated with the formation of 1:1 ion pairs between M+ and 1 red were obtained by using K M 1 values (from k obs data) to simultaneously fit reduction potential (E 1/2) values (from cyclic voltammetry) of solutions of 1 containing varying concentrations of all three cations to a Nernstian equation describing the dependence of E 1/2 values on the ratio of thermodynamic constants K M 1 and K M 1 red . Formation constants, K M 1 , and K M 1 red , and rate constants, k M 1 , all increase with the size of M+ in the order K Li 1 = 21 < K Na 1 = 54 < K K 1 = 65 M-1, K Li 1 red = 130 < K Na 1 red = 570 < K K 1 red = 2000 M-1, and k Li 1 = 0.065 < k Na 1 = 0.137 < k K 1 = 0.225 M-1 s-1. Changes in the chemical shifts of 7Li NMR signals as functions of [Li5 1] and [Li6 1 red] were used to establish that the complexes M+ 1 and M+ 1 red exist as solvent-separated ion pairs. Finally, correlation between cation size and the rate and energy of electron transfer was established by consideration of K M 1 , k M 1 , and K M 1 red values along with the relative sizes of the three M+ 1 pairs (effective hydrodynamic radii, r eff, obtained by single-potential step chronoamperometry). As M+ increases in size, association constants, K M 1 , become larger as smaller, more intimate solvent-separated ion pairs, M+ 1, possessing larger electron affinities (q/r), and associated with larger k M 1 values, are formed. Moreover, as M+ 1 pairs are reduced to M+ 1 red during electron transfer in the activated complexes, [BPH2, M+ 1], contributions of ion pairing energy (proportional to −RT ln(K M 1 red /K M 1 ) to the standard free energy change associated with electron transfer, ΔG°et, increase with cation size:  −RT ln(K M 1 red / K M 1 ) (in kcal mol-1) = −1.2 for Li+, −1.5 for Na+, and −2.3 for K+.