Di- and Mononuclear Iron Complexes of N,C,S-Tridentate Ligands Containing an Aminopyridyl Group: Effect of the Pendant Amine Site on Catalytic Properties for Proton Reduction

A series of diiron complexes of N,C,S-tridentate ligands containing a 6-, 5-, or 4-amino-2-pyridyl group, [{Fe­(μ-L-κ<sup>3</sup><i>N,C,S</i>)­(CO)<sub>2</sub>}­Fe­(CO)<sub>3</sub>] (<b>2</b>, L = <i>o</i>-apyBPT; <b>3</b>, L = <i>m</i>-apyBPT; <b>4</b>, L = <i>p</i>-apyBPT), was synthesized: apyBPT is a doubly deprotonated form of 3′-(amino-2″-pyridyl)-1,1′-biphenyl-2-thiol. Complexes <b>2</b>–<b>4</b> were converted to the mononuclear iron­(II) complexes <i>trans</i>-[Fe­(L-κ<sup>3</sup><i>N,C,S</i>)­(CO)­(PMe<sub>2</sub>Ph)<sub>2</sub>] (<b>6</b>, L = <i>o</i>-apyBPT; <b>7</b>, L = <i>m</i>-apyBPT; <b>8</b>, L = <i>p</i>-apyBPT). In <b>2</b> and <b>6</b>, the <i>o</i>-amino group is close to Fe bound to the aminopyridyl group. Cyclic voltammograms of <b>2</b>–<b>4</b> exhibit two consecutive one-electron reduction events, and catalytic current for proton reduction appears in the presence of acetic acid. The reduction potentials of <b>2</b>–<b>4</b> are similar to each other, while the overpotential for proton reduction with <i>o</i>-amino complex <b>2</b> is ca. 0.2 V lower than those with <b>3</b> and <b>4</b>. In the mononuclear complexes <b>6</b>–<b>8</b>, the redox potentials for the Fe<sup>III</sup>/Fe<sup>II</sup> couple are dependent on the position of the amino group in the pyridine ring, which is described by electronic and steric effects of the amino group. Such effects on the redox potentials are suppressed in the diiron complexes because the reduction occurs at the diiron core with π-accepting CO ligands, which is supported by DFT calculations. The lower overpotential in <b>2</b> compared with <b>3</b> and <b>4</b> is attributed to the concerted effect of the amino group proximal to the iron center. The amino group probably acts as a proton acceptor and assists the formation of the H–H bond from a hydride on the iron centers and a proton bound to the amino group.