Carbene character in a series of neutral PC carbene P cobalt(I) complexes: Radical carbenes versus nucleophilic carbenes

Cobalt(I) complexes supported by a series of PCcarbeneP pincer ligands of varying donicity, differing in the aryl group linking the phosphine arms with the anchoring carbon donor, are described. Addition of the proligands to cobalt bromide results in the formation of a series of cobalt(II) tetrahedral complexes, Ln-1, which serve as excellent precursors to the corresponding PCalkylP and PCcarbeneP complexes. The square-planar cobalt PCcarbeneP complexes L2R-3-X (X = Cl, Br) are readily synthesized by addition of a bulky aryloxide radical to the corresponding PCalkylP complex L1-2-Br, via addition of L2R to ClCo(PPh3)3 in the presence of trityl radical, or by addition of NaHBEt3 and trityl radical to isolated L2R-1. For the L2NMe2 PCcarbeneP complexes, salt metathesis reactions with CsOH·H2O, LiCH2TMS, or LiNH2 result in the corresponding hydroxo, alkyl, and amine complexes L2NMe2-3-R (R = OH, CH2TMS, NH2). Reaction of L2NMe2-3-OH with benzoic acid affords the κ2-O2CPh derivative The nature of the carbene bond in either ligand platform and the effects of the X-type capping ligand on the Co═C bond are explored using CASSCF and CASPT2 calculations and show that triplet structures are relatively more stable for the less electron donating ligand L1 while singlet Co(I) carbenes dominate for the more electron rich L2 derivatives. For L2NMe2 complexes, the effect of the trans ligand X was also probed. π donors imbue the carbene with singlet character, while the strongly σ donating alkyl derivative exhibits significant triplet character.