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

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journal contribution
posted on 03.07.2018, 17:23 authored by Kyle E. Rosenkoetter, Michael K. Wojnar, Bronte J. Charette, Joseph W. Ziller, Alan F. Heyduk
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 13 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 pKa 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.