Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand

Abstract

Double deprotonation of bis(2-mercapto-4-methylphenyl)amine ([SNS]H₃) followed by addition to NiCl₂(PR₃)₂ in air-free conditions afforded [SN(H)S]Ni(PR₃) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR₃)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = ¹/₂ complex, [SNS]Ni(PCy₃) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh₃)}₂ ({3b}₂) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = ¹/₂ monomers analogous to 3a. Complexes 1-3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol^-1, respectively, using the corresponding p K_a and E°' values. Consistent with these BDFE values, TEMPO^• reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.