Technetium(V) and rhenium(V) complexes of biguanide derivatives. Crystal structures

Abstract

Biguanidine ligands HLn (HL1 = 1,1-dimethylbiguanide, HL2 = 1-phenethylbiguanide, HL3 = 1-phenylbiguanide) formed disubstituted cationic oxo- and nitrido-complexes [MO(HLn)2]3+ (M = Tc or Re) and [TcN(HLn)2(H2O)]2+. They are characterised by the presence of a network of N–H· · ·X (X = Cl or H2O) intra- and inter-molecular hydrogen bonds. The imido precursor of ReV [Re(NMe)(PPh3)2Cl3] formed monosubstituted complexes [Re(NMe)(HL1,2)(PPh3)Cl]2+. In alkaline solutions deprotonation of ligands occurs and monocationic, disubstituted oxo- and imido-species [MO(Ln)2]+ (M = Tc or Re), [Re(NCH3)(L1,3)2]+ and neutral nitrido complexes [TcN(Ln)2] are obtained. Elemental analyses, FT-IR and NMR spectroscopy and conductivity measurements are consistent with the proposed formulations. Crystal structures of [TcO(L1)2]+ and [TcN(HL1)2(H2O)]2+ were determined. The former shows a square pyramidal geometry in which the C–N bond distances are equivalent and indicative of π delocalisation on the chelate ring. The latter displays a pseudo-octahedral geometry with a water molecule trans to the TcN multiple bond. The C–N bond distances inside the ligands (1.30 and 1.38 Å) are consistent with single and double bond character, and less π delocalisation through the whole ligands.