<p>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 <i>N,N’</i>-di-<i>tert</i>-butyl-2,11-diaza[3.3](2,6)pyridinophane (<sup>tBu</sup>N4): [(N2S2)Pd<sup>II</sup>(MeCN)]<sub>2</sub>(OTf)<sub>4</sub>
(<b>1</b>), [(N2S2)Pd<sup>II</sup>Me]<sub>2</sub>(OTf)<sub>2</sub>
(<b>2</b>), [(N2S2)Pd<sup>II</sup>Cl](OTf)
(<b>3</b>), [(N2S2)Pd<sup>II</sup>X](OTf)<sub>2</sub>
(X = tBuNC <b>4</b>, PPh<sub>3</sub> <b>5</b>), [(N2S2)Pd<sup>II</sup>Me(PPh<sub>3</sub>)](OTf)
(<b>6</b>), and [(<sup>tBu</sup>N4)Pd<sup>II</sup>X<sub>2</sub>](OTf)<sub>2</sub>
(X = MeCN <b>8</b>, tBuNC <b>9</b>).
In addition, a stable Pd(I) dinuclear species, [(N2S2)Pd<sup>I</sup>(m-tBuNC)]<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub>
(<b>7</b>),
was isolated upon the electrochemical reduction of <b>4</b> and structurally characterized. Moreover, the (<sup>tBu</sup>N4)Pd<sup>I</sup>
intermediates, formed from the chemical reduction of [(<sup>tBu</sup>N4)Pd<sup>II</sup>X<sub>2</sub>](OTf)<sub>2</sub>
(X = MeCN <b>8</b>, tBuNC <b>9</b>) complexes,
were investigated by EPR spectroscopy, X-ray absorption spectroscopy (XAS), and
DFT calculations, and compared with the analogous (N2S2)Pd<sup>I</sup> systems.
Upon probing the stability of
Pd(I) species under various ligand environments (N2S2 and <sup>tBu</sup>N4), it
is apparent that the presence of soft ligands such as tBuNC and PPh<sub>3</sub>
significantly improves the stability of Pd(I) species, which should make the
isolation of mononuclear Pd(I) species possible.</p>