New rhenium(I) and rhenium(II) species assembled by stereospecific azopyridine chelation

Abstract

The spontaneous reaction of [ReO(OEt)I₂(PPh₃)₂] with 2,2'-azobipyridine (L) in acetonitrile, and of [ReOCl₃(PPh₃)₂] with L in toluene, proceeds stereospecifically furnishing the green tris-chelate [Re^IL₃]I (1) and the red bis-chelate [Re^IICl₂L₂] (2), respectively. A structure determination reveals that the coordination geometry of 1 is facial and that of 2 is cis(Cl,Cl)-trans(N^ρ,N^ρ)-cis(N^α,N^α) (N^ρ is a pyridine nitrogen and Na is an azo nitrogen). The isomer preference is exclusive in both cases and no other isomer has been observed. The average N-N distance in both compounds is about 1.35 Å, signifying the presence of strong d(Re)-π(azo) back-bonding which plays a crucial role in controlling isomer specificity. The bis-chelate 2 is a one-electron paramagnet and displays a six-line EPR spectrum in fluid solution with g = 2.080 and A = 320 G. The ¹H NMR spectrum of diamagnetic 1 is consistent with the facial geometry. In acetonitrile solution 1 displays an Re^II/Re^I couple and 2 an ReIII/ReII couple, the reduction potentials being 0.60 and 0.50V vs. SCE respectively. The violet-coloured imide complex [Re^VCl₃(NC₆H4Cl-ρ)L] (3) is also reported; it displays an Re^VI/Re^V redox wave at 1.33 V vs. SCE. The stabilization of the lower rhenium oxidation states in 1 and 2 is consistent with the decrease of metal reduction potential with progressive L chelation.