In Search of the [PhB(μ-NtBu)2]2As• Radical: Experimental and Computational Investigations of the Redox Chemistry of Group 15 Bis-boraamidinates

Abstract

DFT calculations for the group 15 radicals [PhB(μ-NtBu)2]2M• (M = P, As, Sb, Bi) predict a pnictogen-centered SOMO with smaller contributions to the unpaired spin density arising from the nitrogen and boron atoms. The reactions of Li2[PhB(μ-NR)2] (R = tBu, Dipp) with PCl3 afforded the unsolvated complex LiP[PhB(μ-NtBu)2]2 (1a) in low yield and ClP[PhB(μ-NDipp)2] (2), both of which were structurally characterized. Efforts to produce the arsenic-centered neutral radical, [PhB(μ-NtBu)2]2As•, via oxidation of LiAs[PhB(μ-NtBu)2]2 with one-half equivalent of SO2Cl2, yielded the Zwitterionic compound [PhB(μ-NtBu)2As(μ-NtBu)2B(Cl)Ph] (3) containing one four-coordinate boron center with a B−Cl bond. The reaction of 3 with GaCl3 produced the ion-separated salt, [PhB(μ-NtBu)2]2As+GaCl4− (4), which was characterized by X-ray crystallography. The reduction of 3 with sodium naphthalenide occurred by a two-electron process to give the corresponding anion [{PhB(μ-NtBu)2}2As]− as the sodium salt. Voltammetric investigations of 4 and LiAs[PhB(μ-NtBu)2]2 (1b) revealed irreversible processes. Attempts to generate the neutral radical [PhB(μ-NtBu)2]2As• from these ionic complexes via in situ electrolysis did not produce an EPR-active species.