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 [ReVL(O)Cl3] 1. Aromatic amines and PPh3 smoothly converted 1 into [ReVL(NR)Cl3] 2 and [ReIIIL(OPPh3)Cl3] 3 respectively. Treatment of 3 with PPh3 yielded [ReIIIL(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: ReVI–ReV, 1 > 2 (imido better donor than oxo); ReIV–ReIII, 4 > 3 (stabilization of t2 by ReIII–PPh3 back bonding)