Oxidative Reactivity of (N2S2)PdRX Complexes (R = Me, Cl; X = Me, Cl, Br): Involvement of Palladium(III) and Palladium(IV) Intermediates

2016-02-19T06:45:46Z (GMT) by Jia Luo Nigam P. Rath Liviu M. Mirica
A series of (N2S2)­PdRX complexes (N2S2 = 2,11-dithia[3.3]­(2,6)­pyridinophane; R = X = Me, <b>1</b>; R = Me, X = Cl, <b>2</b>; R = Me, X = Br, <b>3</b>; R = X = Cl, <b>4</b>) were synthesized, and their structural and electronic properties were investigated. X-ray crystal structures show that for the corresponding Pd­(II) complexes the N2S2 ligand adopts a κ<sup>2</sup> conformation, with the pyridine N donors binding in the equatorial plane. Cyclic voltammetry (CV) studies suggest that the Pd­(III) oxidation state is accessible at moderate redox potentials. In situ EPR, ESI-MS, UV–vis, and low-temperature electrochemical studies were employed to detect the formation of Pd­(III) species during the oxidation of Pd­(II) precursors. In addition, the [(N2S2)­Pd<sup>IV</sup>Me<sub>2</sub>]­(PF<sub>6</sub>)<sub>2</sub> ([<b>1</b><sup>2+</sup>]­(PF<sub>6</sub>)<sub>2</sub>) complex was isolated by oxidation of <b>1</b> with 2 equiv of FcPF<sub>6</sub>, and its structural characterization reveals an octahedral Pd­(IV) center. The reversible Pd<sup>IV/III</sup> redox couple for the Pd­(IV) species supports the observed formation of the Pd­(III)–dimethyl species upon chemical reduction of <b>1</b><sup>2+</sup>. In addition, reactivity studies reveal ethane, MeCl, and MeBr elimination upon one-electron oxidation of <b>1</b> (as well as the one-electron reduction of <b>1</b><sup>2+</sup>), <b>2</b>, and <b>3</b>, respectively. Mechanistic studies suggest the initial formation of a Pd­(III) species, followed by methyl group transfer/disproportionation and subsequent reductive elimination from a Pd­(IV) intermediate, although a halogen radical pathway cannot be completely excluded during C–halide bond formation. Interestingly, computational results suggest that the N2S2 ligand stabilizes to a greater extent the Pd­(IV) vs the Pd­(III) oxidation state, likely due to steric rather than electronic effects.