Detection and Characterization of Mononuclear Pd(I) Complexes

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 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 ( tBu N4): [(N2S2)Pd II (MeCN)] 2 (OTf) 4 ( 1 ), [(N2S2)Pd II Me] 2 (OTf) 2 ( 2 ), [(N2S2)Pd II Cl](OTf) ( 3 ), [(N2S2)Pd II X](OTf) 2 (X = tBuNC 4 , PPh 3 5 ), [(N2S2)Pd II Me(PPh 3 )](OTf) ( 6 ), and [( tBu N4)Pd II X 2 ](OTf) 2 (X = MeCN 8 , tBuNC 9 ). In addition, a stable Pd(I) dinuclear species, [(N2S2)Pd I (m-tBuNC)] 2 (ClO 4 ) 2 ( 7 ), was isolated upon the electrochemical reduction of 4 and structurally characterized. Moreover, the ( tBu N4)Pd I intermediates, formed from the chemical reduction of [( tBu N4)Pd II X 2 ](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)Pd I systems. Upon probing the stability of Pd(I) species under various ligand environments (N2S2 and tBu N4), it is apparent that the presence of soft ligands such as tBuNC and PPh 3 significantly improves the stability of Pd(I) species, which should make the isolation of mononuclear Pd(I) species possible.