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

Abstract The bidentate, monoanionic dimethyloxazoline-phenol ligand HL1 was used to synthesize the corresponding oxidorhenium(V) complex [ReOCl(L1)2] (2). Ligand HL1 is equipped with two electron-donating tert-butyl substituents. The HdmozR class of ligands has so far enabled the stereoselective synthesis of the so-called N,N-trans isomers of oxidorhenium(V) complexes. In contrast, when precursor complex [ReOCl3(OPPh3)(SMe2)] (P1) and HL1 are reacted, in addition to the expected N,N-trans isomer (trans- 2), also the N,N-cis isomer (cis-2) is formed. So far, this isomerism has only been observed for the nonmethylated phenol-oxazoline ligand Hoz, resulting in mixtures of complexes N,N-cis/trans [ReOCl(oz)2] (cis/trans- 1). For trans- 2, the catalytic properties in oxyanion (perchlorate and nitrate) reduction were studied. Due to the slow kinetics in the latter, the two cationic complexes [ReO(L1)2]X (X = SO3CF3̅, 3a; X = O2CCF3̅, 3b) were synthesized. Cationic triflate complex 3a showed the highest conversion rates in perchlorate reduction compared to 3b and chlorido complex trans- 2, corresponding to the weakness of the coordinating anion. A targeted synthesis of the dioxidorhenium(VI) complex [ReO2(L1)2] (4), the product of nitrite (NO2̅) reduction, allowed for mechanistic and electrochemical investigations. The solid-state structures of complexes cis- 2, trans- 2, 3b, and 4 were characterized by single-crystal X-ray diffraction.