Chemistry of monovalent and bivalent rhenium: synthesis, structure, isomer specificity and metal redox of azoheterocycle complexes

Abstract

The reaction of [Re^VO(OEt)X₂(PPh₃)₂] (X = Cl, Br, I) with 2-(arylazo)-1-methylimidazoles (aryl = Ph(L¹), C₆H₄Me–p(L²) or C₆H₄Cl-p(L³)) as well as 2-(p-tolylazo)-1-benzylimidazole (L⁴) in toluene has afforded the orange coloured bis chelates of type [Re^IIX₂(L)₂]. The green coloured tris chelates [Re^I(L)₃]ReO₄ are formed when the ligand is used in excess. Similar bis and tris chelates have also been synthesized by reacting [ReOCl₃(PPh₃)₂] with 2-(arylazo)pyridines (aryl = Ph(L⁵), C₆H₄Cl-p(L⁶)). Structure determination of [ReCl₂(L²)₂], [Re^I₂(L⁴)₂] and [ReCl₂(L⁶)₂] has revealed that the isomeric geometry for the XX–N^hN^h–N^aN^a donor sites (N^h, heterocyclic nitrogen; N^a, azo nitrogen) is uniformly cis–trans–cis. In the structure of [Re(L)₃]ReO₄ the tris chelate has facial geometry. The isomer preference of both families is exclusive, no other isomer having been observed in any of the preparations. The ¹H NMR spectra of the tris chelates are consistent with the facial geometry. The bis chelates are one-electron paramagnets and display well-resolved EPR sextets in fluid solutions. The cyclic voltammetric Re^III/Re^II response of [Re^IIX₂(L)₂] occurs in the range 0.20–0.50 V vs. SCE in the case of the azoimidazole chelates and in the range 0.60–0.70 V in the case of the azopyridine chelates. In the case of [Re^I(L)₃]⁺ the Re^II/Re^I couple is observed near 0.50 and 0.90 V for the azoimidazole and azopyridine species respectively. The average Re–Na distance is generally shorter than the Re–N^h distance by ∼0.1 Å and the average N–N length is longer by ∼0.1 Å compared to that of uncoordinated azo function. Strong d(Re)–π*(azo) back-bonding characterize the present systems. Both back-bonding and steric factors stabilize the cis–trans–cis isomer of [ReX₂(L)₂]. In facial [Re(L₃)]⁺ the net back-bonding is strong enough to offset the disadvantage of steric crowding.