Tellurium electrodeposition from tellurium(II) and (IV) chloride salts in dichloromethane

<p>Electrodeposition of elemental Te from [NⁿBu₄]₂[TeCl₆] in a weakly coordinating solvent, dichloromethane, has been investigated. The reduction of the Te(IV) complex to deposit Te shows a very large (&gt; 1 V) overpotential on clean Pt macrodisc electrodes and microelectrodes that is significantly reduced once Te has been deposited on the electrode. Corresponding studies show that there is no significant overpotential for the deposition of Te from the Te(II) complex, [NEt₄]₂[TeI₄], under the same conditions, suggesting that the barrier for electrodeposition from [TeCl₆]²⁻ arises because of the instability of the intermediate Te(III) species. Density functional theory (DFT) calculations with the M06–2X density functional and treatment of solvation using the polarizable continuum model confirm that the possible Te(III) complexes, [TeCl₅]²⁻, [TeCl₄]⁻ and [TeCl₃], are all unstable with respect to disproportionation, confirming that the Te(III) intermediate is a thermodynamic barrier to the electrodeposition of Te(0) from [TeCl₆]²⁻. We propose that this thermodynamic barrier is overcome for the reaction at the Te electrode surface by a catalytic mechanism in which the transient Lewis acidic [TeCl₅]⁻ complex forms a Lewis acid/base adduct ([TeCl₅]⁻ —Te_surface) through p-orbital overlap with the Te on the electrode surface. This mechanism is shown to be consistent with the results of DFT calculations for the speciation equilibria for the Te(IV) chloride complexes and with the results of experiments at different chloride concentration. Our results suggest that Lewis acid/base adduct formation at the electrode surface may also play a role in electrodeposition of other p-block elements.</p>