Electron-Donating Ligand in Oxidorhenium(V) Chemistry: Consequences for Isomerism and Catalyst Properties

The bidentate, monoanionic dimethyloxazoline-phenol ligand H<b>L1</b> was used to synthesize the corresponding oxidorhenium(V) complex [ReOCl(<b>L1</b>)₂] (<b>2</b>). Ligand H<b>L1</b> is equipped with two electron-donating <i>tert-</i>butyl substituents. The HdmozR class of ligands has so far enabled the stereoselective synthesis of the so-called <i>N</i>,<i>N</i>-<i>trans</i> isomers of oxidorhenium(V) complexes. In contrast, when precursor complex [ReOCl₃(OPPh₃)(SMe₂)] (<b>P1</b>) and H<b>L1</b> are reacted, in addition to the expected <i>N</i>,<i>N</i>-<i>trans</i> isomer (<i>trans-</i><b>2</b>), also the <i>N</i>,<i>N</i>-<i>cis</i> isomer (<i>cis</i>-<b>2</b>) is formed. So far, this isomerism has only been observed for the nonmethylated phenol-oxazoline ligand Hoz, resulting in mixtures of complexes <i>N</i>,<i>N</i>-<i>cis/trans</i> [ReOCl(oz)₂] (<i>cis/trans-</i><b>1</b>). For <i>trans-</i><b>2</b>, the catalytic properties in oxyanion (perchlorate and nitrate) reduction were studied. Due to the slow kinetics in the latter, the two cationic complexes [ReO(<b>L1</b>)₂]X (X = SO₃CF₃̅, <b>3a</b>; X = O₂CCF₃̅, <b>3b</b>) were synthesized. Cationic triflate complex <b>3a</b> showed the highest conversion rates in perchlorate reduction compared to <b>3b</b> and chlorido complex <i>trans-</i><b>2</b>, corresponding to the weakness of the coordinating anion. A targeted synthesis of the dioxidorhenium(VI) complex [ReO₂(<b>L1</b>)₂] (<b>4</b>), the product of nitrite (NO₂̅) reduction, allowed for mechanistic and electrochemical investigations. The solid-state structures of complexes <i>cis-</i><b>2</b>, <i>trans-</i><b>2</b>, <b>3b</b>, and <b>4</b> were characterized by single-crystal X-ray diffraction.