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 Re^V [Re(NMe)(PPh₃)₂Cl₃] formed monosubstituted complexes [Re(NMe)(HL^1,2)(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)(L^1,3)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.