Detection and Characterization of Mononuclear Pd(I) Complexes Supported by N2S2 and N4 Tetradentate Ligands

• Published in: Inorganic chemistry 2020-11, Vol.59 (21), p.15659-15669
• Main Authors: Luo, Jia, Tran, Giang N, Rath, Nigam P, Mirica, Liviu M
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.0c01938

Palladium is a versatile transition metal used to catalyze a large number of chemical transformations, largely due to its ability to access various oxidation states (0, I, II, III, and IV). Among these oxidation states, Pd­(I) is arguably the least studied, and while dinuclear Pd­(I) complexes are more common, mononuclear Pd­(I) species are very rare. Reported herein are spectroscopic studies of a series of Pd­(I) intermediates generated by the chemical reduction at low temperatures of Pd­(II) precursors supported by the tetradentate ligands 2,11-dithia[3.3]­(2,6)­pyridinophane (N2S2) and N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane (tBuN4): [(N2S2)­PdII(MeCN)]2(OTf)4 (1), [(N2S2)­PdIIMe]2(OTf)2 (2), [(N2S2)­PdIICl]­(OTf) (3), [(N2S2)­PdIIX]­(OTf)2 (X = tBuNC 4, PPh3 5), [(N2S2)­PdIIMe­(PPh3)]­(OTf) (6), and [(tBuN4)­PdIIX2]­(OTf)2 (X = MeCN 8, tBuNC 9). In addition, a stable Pd­(I) dinuclear species, [(N2S2)­PdI(μ-tBuNC)]2(ClO4)2 (7), was isolated upon the electrochemical reduction of 4 and structurally characterized. Moreover, the (tBuN4)­PdI intermediates, formed from the chemical reduction of [(tBuN4)­PdIIX2]­(OTf)2 (X = MeCN 8, tBuNC 9) complexes, were investigated by EPR spectroscopy, X-ray absorption spectroscopy (XAS), and DFT calculations and compared with the analogous (N2S2)­PdI systems. Upon probing the stability of Pd­(I) species under different ligand environments, it is apparent that the presence of soft ligands such as tBuNC and PPh3 significantly improves the stability of Pd­(I) species, which should make the isolation of mononuclear Pd­(I) species possible.