Isolation of an Arsenic Diradicaloid with a Cyclic C 2 As 2 -Core.

Herein, we report on the synthesis, characterization, and reactivity studies of the first cyclic C 2 As 2 -diradicaloid {(IPr)CAs} 2 (6) (IPr = C{N(Dipp)CH} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ). Treatment of (IPr)CH 2 (1) with AsCl 3 affords the Lewis adduct {(IPr)CH 2 }AsCl 3 (2). Compound 2 undergoes stepwise dehydrochlorination to yield {(IPr)CH}AsCl 2 (3) and {(IPr)CAsCl} 2 (5 a) or [{(IPr)CAs} 2 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 2 As 2 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 Fe 2 (CO) 9 give rise to {(IPr)CAs(SePh)} 2 (7) and {(IPr)CAs} 2 Fe(CO) 4 (8), respectively.