New p-tolylimido rhenium(v) complexes with carboxylate-based ligands: synthesis, structures and their catalytic potential in oxidations with peroxides

Abstract

Novel p-tolylimido rhenium(V) complexes trans-(Cl,Cl)-[Re(p-NC₆H₄CH₃)Cl₂(pyz-2-COO)(PPh₃)]·MeCN (1), trans-(Cl,Cl)-[Re(p-NC₆H₄CH₃)Cl₂(pyz-2-COO)(PPh₃)] (2), trans-(Br,Br)-[Re(p-NC₆H₄CH₃)Br₂(pyz-2-COO)(PPh₃)] (3), cis-(Cl,Cl)-[Re(p-NC₆H₄CH₃)Cl₂(ind-3-COO)(PPh₃)]·2MeOH (4), 2[Re(p-NC₆H₄CH₃)Cl₂(ind-3-COO)(PPh₃)]·MeCN (5), 2[Re(p-NC₆H₄CH₃)Br₂(ind-3-COO)(PPh₃)]·MeOH (6) and 2[Re(p-NC₆H₄CH₃)Br₂(ind-3-COO)(PPh₃)]·MeCN (7) were obtained in the reactions of [Re(p-NC₆H₄CH₃)X₃(PPh₃)₂] (X = Cl, Br) with pyrazine-2-carboxylic (pyz-2-COOH or PCA) and indazole-3-carboxylic (ind-3-COOH) acids. The compounds were identified by elemental analysis, IR, ¹H, ¹³C and ³¹P NMR spectroscopy and X-ray crystallography. To get a deeper understanding of the structural and bonding properties of the imido rhenium(V) complexes, calculations at the DFT level were undertaken for trans-(Cl,Cl)-[Re(p-NC₆H₄CH₃)Cl₂(pyz-2-COO)(PPh₃)] and cis-(Cl,Cl)-[Re(p-NC₆H₄CH₃)Cl₂(ind-3-COO)(PPh₃)]. Complexes 1, 3, 4 and 6 exhibited high catalytic activity in oxidation of alkanes with H₂O₂ and tert-butyl hydroperoxide (TBHP) and of alcohols with TBHP. The selectivity parameters measured in the reactions with linear and branched alkanes indicated that the processes with H₂O₂ or TBHP proceed with the participation of hydroxyl or tert-butoxyl radicals, respectively. The composition of isomers from oxygenation of methylcyclohexane corresponds to the existence of some steric hindrance around the reaction centers.