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.