Isolation of an Arsenic Diradicaloid with a Cyclic C2As2-Core

Abstract

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C2As2-diradicaloid {(IPr)CAs}2 (6) (IPr = C{N(Dipp)CH}2; Dipp = 2,6-iPr2C6H3). Treatment of (IPr)CH2 (1) with AsCl3 affords the Lewis adduct {(IPr)CH2}AsCl3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl2 (3) and {(IPr)CAsCl}2 (5 a) or [{(IPr)CAs}2Cl]OTf (5 b). Reduction of 5 a (or 5 b) with magnesium turnings gives 6 as a red crystalline solid in 90% yield. Compound 6 featuring a planar C2As2 ring is diamagnetic and exhibits well resolved NMR signals. DFT calculations reveal a singlet ground state for 6 with a small singlet-triplet energy gap of 8.7 kcal mol−1. The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe)2 and Fe2(CO)9 give rise to {(IPr)CAs(SePh)}2 (7) and {(IPr)CAs}2Fe(CO)4 (8), respectively.