Synthesis, characterization, properties and structures of rhenium-(ii), -(iii), and -(v) complexes containing amino acids conjugated with a 2-aminothiazoleligand

Abstract

The one-step reaction of KReO₄ with benzoylhydrazine in the presence of concentrated HCl and PPh₃ afforded [Re^V(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)(L^2,5)(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.