Elucidation of the Thermochemical Properties of Triphenyl- or Tributyl-Substituted Si-, Ge-, and Sn-Centered Radicals by Means of Electrochemical Approaches and Computations

Abstract

Redox potentials of a number of triphenyl- or tributyl-substituted Si-, Ge-, or Sn-centered radicals, R(3)M(*), have been measured in acetonitrile, tetrahydrofuran, or dimethyl sulfoxide by photomodulated voltammetry or through a study of the oxidation process of the corresponding anions in linear sweep voltammetry. For the results pertaining to the Ph(3)M(*) series (including literature data for M = C), the order of reduction potentials follows Sn > Ge > C > Si, while for the two oxidation potentials, it is C > Si. The effect of the R group on the redox properties of R(3)Sn(*) is pronounced in that the reduction potential is more negative by 490 mV in tetrahydrofuran (390 mV in dimethyl sulfoxide) when R is a butyl rather than a phenyl group. The experimental trends have been substantiated through quantum chemical calculations, and they can be explained qualitatively by considering a combination of effects, such as charge capacity being most pronounced for the heavier elements, resonance stabilization present for the planar Ph(3)C(*) and all R(3)M(+)(), and finally a contribution from solvation. The solvation of R(3)M(-) is observed to be relatively strong because of a rather localized negative charge in the pyramidal geometry. However, there is no evidence in the calculations to support the existence of covalent interactions between solvent and anions. The solvation of R(3)M(+)() is relatively weak, which may be attributed to the planar geometry around the center atom, leading to more spread out charge than that for a pyramidal geometry. Although the calculated solvation energies based on the polarizable continuum model approach exhibit the expected trends, they are not able to reproduce the experimentally derived values on a detailed level for these types of ions. An evaluation of the general performance of the continuum model is provided on the basis of present and previous studies.

Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals

“…space is such that the silyl radicals 5 (with own reduction potentials E  -0.5…-1.4 V [17,18]) are instantaneously transformed into silyl anions, which is consistent with the results of Corriu [12].…”

“…In the following principal steps, provided an over-equimolar presence of DMF prior any ET, we 5 Dimerization of homologous Ph 3 Ge  radicals was postulated in [35]. However provided E 1/2 (Ph 3 Ge  /Ph 3 Ge -) = -0.61 V vs SCE [17], an increase in the driving force of their reduction to germyl anions at the potential of E = -2.1 V, required for the reductive cleavage of Ge-Cl bond, is by 510 12 times at the level of k s . Most probably, no radical coupling could therefore compete with their reduction under these conditions.…”

“…Indeed, the reduction of the majority of chlorosilanes is usually observed at -3.2…-2.4 V vs. SCE [9], whereas the E 1/2 of the couple Ph 3 Si  /Ph 3 Si -, estimated by photomodulated voltammetry, is -1.39 V vs SCE [17];…”

Complexes of germanium chlorides (RnGeCl4-n, n=0-3; R=Me, Ph) with redox active N-hetero­aromatic ligands: an electrochemical study

“…Standard redox potentials of bipy ⋅ [R 3 Ge ‧ / R 3 Ge -] systems have close values, E o ≅ -1.40 ± 0.05 V vs. SCE, which is ca. 0.8 V more negative compared to the only reported value of E 1/2 (Ph 3 Ge ‧ /Ph 3 Ge -) = -0.61 V determined by photomodulated voltammetry (Holm et al, 2005). In an earlier study on the oxidation of Ph 3 Ge -(as Ph 3 GeLi in HMPA/LiCl), an even lower value was proposed, E p = -0.29 V vs. SCE (Mochida et al, 1988).…”

Triarylmethyl and Related Radicals

“…14c The oxidation/reduction potentials of triphenyl-or tributyl-substituted silicon-, germanium-, or tincentered radicals 36 were measured in acetonitrile, tetrahydrofuran, or dimethylsulfoxide by photomodulated voltammetry and through oxidation of the corresponding anions using linear sweep voltammetry for comparison to those of the carbon analogues (Equation 1.13). 15 The order of reduction potentials follows Sn > Ge > C > Si, and is C > Si for the two oxidation potentials. Computational methods gave qualitative interpretation of the experimental results by considering a combination of effects, such as charge capacity, resonance stabilization, and solvation effects.…”