Synthesis, characterization, properties and structures of rhenium-(ii), -(iii), and -(v) complexes containing amino acids conjugated with a 2-aminothiazole ligand
The one-step reaction of KReO4 with benzoylhydrazine in the presence of concentrated HCl and PPh3 afforded [ReV(NNC(Ph)O)(PPh3)2Cl2] 1 which was converted with carbon monoxide into the dinitrogen Re(I) complex [ReN2(CO)2(PPh3)2Cl] 2 in good yield. A new class of ligands, of general formula HLn (n = 1–5), was obtained by conjugation of N-protected amino acids and benzoic acid with 2-aminothiazole. Complex 2 reacted with HLn to give neutral Re(II) chelate complexes [Re(Ln)(CO)(PPh3)2Cl] 5–9 (n = 1–5), by substitution of N2 and CO groups. The molecular structure of complex 9 was determined by X-ray diffraction. When reactions between [Re(NNC(Ph)O)(PPh3)2Cl2] 1 and HLn (n = 2, 5) were carried out in different stoichiometric ratios, neutral mono- and di-substituted Re(III) organodiazenido complexes [Re(NNCOPh)(L2,5)(PPh3)2Cl] 10, 11 and [Re(NNCOPh)(L5)2(PPh3)] 12 were obtained. In these complexes the organodiazenido Re–N–N moiety adopts a linear conformation, as has been authenticated by X-ray diffraction analysis for 11 and 12. Treatment of Re(I) dinitrogen precursors [ReN2(P)4Cl] (P = PMe2Ph, 3 and PMePh2, 4) with ligands resulted in interesting dinuclear oxo-(μ-oxo)-Re(V) species [Re2(O)3(Ln)2(P)2] (P = PMe2Ph, n = 1, 13; n = 2, 14; n = 4, 15. P = PMePh2, n = 2, 16). Suitable crystals for an X-ray analysis have been obtained for complex 14. The structure represents one of the few cases where, for steric reasons, the Re–O–Re bridging unit is not linear but rather bent. All the compounds have been characterized by elemental analyses, IR and NMR spectroscopy. The Re(II) complexes 5–7 and 9 were further characterized by electrochemical measurements.