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

Abstract

The reaction of [ReVO(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 [ReIIX2(L)2]. The green coloured tris chelates [ReI(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], [ReI2(L4)2] and [ReCl2(L6)2] has revealed that the isomeric geometry for the XX–NhNh–NaNa donor sites (Nh, heterocyclic nitrogen; Na, 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 ReIII/ReII response of [ReIIX2(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 [ReI(L)3]+ the ReII/ReI 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–Nh distance by ∼0.1 A and the average N–N length is longer by ∼0.1 A 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.