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

A new class of cationic rhenium(VII) imido and oxo complexes wilh the macrocyclic ligand 1,4,7-triazacyclononane (L) was prepared. Depending on the reaction conditions, [(L - H)Re(NtBu)2(OSiMe3)]+(1) [(L-H) denotes deprotonation at N], [LRe(NtBu)2O] (2) and/or [LRe(NtBu)2(NHtBu)]2+(4) can be obtained by the treatment of Re(NtBu)3(OSiMe3) with L. Reaction of 2·BPh4with trifluoromethanesulfonic anhydride gives [(L - H)Re(NtBu)2(CF3SO3)]BPh4(3-BPh4). Interaction of [Re(NtBu)2(OSiMe3)(μ-O)]2with L yields [LRe(NtBu)O2]+(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 CF3CO2H. Electrochemically, they undergo proton-coupled two electron reductions at pH 1 with Epc; (cathodic peak potential) of -0.30. -0.29 and -0.12 V versus SCE respectively. Constant potential electrolysis of 6 at -0.3 V affords a green solution which is rapidly oxidised in air to give [LRe(NAr)O2]+(7). The reduction of 5-ClO4by zinc in the presence of oxalic acid and CF3CO2H affords the rhenium(V) species [LRe(NtBu)(oxalate)]ClO4(8·ClO4) and [LRe(NtBu)(CF3CO2)2]ClO4(9·ClO4) respectively. The molecular structures of 2·ClO4,3·BPh4, 5·ClO4, 6·Cl·2H2O. and 8·ClO4have 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.