Synthesis, structure and properties of [Re^VL(O)Cl₃], [Re^VL(NR)Cl₃], [Re^IIIL(OPPh₃)Cl₃], and [Re^IIIL(PPh₃)Cl₃] [L = 2-(arylazo)-1-methylimidazole, R = aryl]

The reaction of KReO₄ with L [2-(arylazo)-1-methylimidazole, with aryl = Ph (L¹), C₆H₄Me-p (L²) or C₆H₄Cl-p (L³)] in concentrated HCl afforded [Re^VL(O)Cl₃] 1. Aromatic amines and PPh₃ smoothly converted 1 into [Re^VL(NR)Cl₃] 2 and [Re^IIIL(OPPh₃)Cl₃] 3 respectively. Treatment of 3 with PPh₃ yielded [Re^IIIL(PPh₃)Cl₃] 4. Complexes of type 3 and 4 display large paramagnetic shifts of ¹H NMR lines which spread over ≈60 ppm. Structure determination of [ReL¹(O)Cl₃] 1a, [ReL²(NC₆H₄Me-p)Cl₃] 2a, [ReL³(OPPh₃)Cl₃] 3c and [ReL³(PPh₃)Cl₃] 4c has revealed meridional geometry for all except 4c which is facial. In the latter Re-azo and Re-PPh₃ back bonding is maximized. The metal atom is displaced away from the equatorial plane by ≈0.3 Å towards the oxo ligand in 1a and the imido ligand in 2a. The imidazole nitrogen is co-ordinated trans to oxo, imido, Ph₃PO and chloride ligands in 1a, 2a, 3c and 4c, respectively. The azo N=N distance is lengthened by ≥0.05 Å as a result of direct (3c, 4c) or indirect (1a, 2a) Re-azo back bonding. Azo reduction potential values are consistent with the low-lying nature of the azo(π*) orbital. The metal reduction potentials follow the trends: Re^VI-Re^V, 1 > 2 (imido better donor than oxo); Re^IV-Re^III, 4 > 3 (stabilization of t₂ by Re^III-PPh₃ back bonding).