Isolation of an Arsenic Diradicaloid with a Cyclic C As -Core.

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C As -diradicaloid {(IPr)CAs} (6) (IPr = C{N(Dipp)CH} ; Dipp = 2,6-iPr C H ). Treatment of (IPr)CH (1) with AsCl affords the Lewis adduct {(IPr)CH }AsCl (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl (3) and {(IPr)CAsCl} (5 a) or [{(IPr)CAs} Cl]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 C As 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 . The diradical character of 6 amounts to 20% (CASSCF, complete active space self consistent field) and 28% (DFT). Treatments of 6 with (PhSe) and Fe (CO) give rise to {(IPr)CAs(SePh)} (7) and {(IPr)CAs} Fe(CO) (8), respectively.