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

The reaction of [Re^vO(OEt)X₂(PPh₃) ₂] (X = Cl, Br, I) with 2-(aryIazo)-1-methylimidazoles (aryl = Ph(L¹), C₆H₄Me-p(L²) or C ₆H₄Cl-p(L³)) as well as 2-(p-tolylazo)-1-benzylimidazole (L⁴) in toluene has afforded the orange coloured bis chelates of type [Re¹¹X₂(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 [ReOCl ₃(PPh₃)₂] with 2-(arylazo)pyridines (aryl = Ph(L⁵), C₆H₄Cl-_p(L⁶)). Structure determination of [ReCl₂(L²)₂], [ReI₂(L⁴)₂] and [ReCl₂(L ⁶)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-N^a 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 [ReX ₂(L)₂]. In facial [Re(L₃)]⁺ the net back-bonding is strong enough to offset the disadvantage of steric crowding.