Rhenium(VII) and -(V) imido and oxo complexes containing 1,4,7-triazacyclononane; synthesis, electrochemistry and crystal structures

Abstract

A new class of cationic rhenium(VII) imido and oxo complexes with the macrocyclic ligand 1,4,7-triazacyclononane (L) was prepared. Depending on the reaction conditions, [(LH) Re(N 1Bu) 2(OSiMe 3)] + ( 1) [(LH) denotes deprotonation at N], [LRe(N 1Bu) 2O] 4 ( 2) and/or [LRe(N 1Bu) 2(NH 1Bu)] 2+ ( 4) can be obtained by the treatment of Re(N 1Bu) 3(OSiMe 3) with L. Reaction of 2·BPh 4 with trifluoromethanesulfonic anhydride gives [(LH)Re(N 1Bu) 2(CF 3SO 3)]BPh 4 ( 3·BPh 4). Interaction of [Re(N 1Bu) 2 (OSiMe 3)(μ-O)] 2 with L yields [LRe(N 1Bu)O 2] + ( 5). In contrast to the t-butylimido analogues, only [LRe(NAr) 2O]Cl ( 6·Cl) is isolated from the reaction of Re(NAr) 3Cl with excess L. Complexes 2, 5 and 6 are stable in 0.1 M CF 3CO 2H. Electrochemically, they undergo proton-coupled two electron reductions at pH 1 with E pv (cathodic peak potential) of −0.30. −0.29 and −0.12V versus SCE respectively. Constant potential electrolysis of 6 at −0.3V affords a green solution which is rapidly oxidised in air to give [LRe(NAr)O 2] + ( 7). The reduction of 5·ClO 4 by zinc in the presence of oxalic acid and CF 4CO 2H affords the rhenium (V) species [LRe(N 1Bu)(oxalate)]ClO 4 ( 8·ClO 4) and [LRe(N 1Bu)(CF 3CO 2) 2]ClO 4 ( 9·ClO 4) respectively the molecular structures of 2·ClO 4, 3·BPh 4, 5·ClO 4, 6·Cl·2H 2O, and 8·ClO 4 have been determined by X-ray crystal analysis. The Re-N(L) bonds which are trans to imido and oxo ligands are comparatively long (2.344−2.243 Å). The range of Re-N(imido) and Re-O(oxo) distances (1.67–1.78 Å and 1.71–1.76 Å respectively) can be rationalised by the oxo/imido ligand ratio in the Re(VII) complexes.