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

• Published in: Inorganic chemistry 2018-08, Vol.57 (16), p.9728-9737
• Main Authors: Rosenkoetter, Kyle E, Wojnar, Michael K, Charette, Bronte J, Ziller, Joseph W, Heyduk, Alan F
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.8b00618

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 pK 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.