Synthesis, crystal structure and magnetic properties of the Re(ii) complexes NBu4[Re(NO)Br4(L)] (L = pyridine and diazine type ligands)

Abstract

Four novel Re(II) complexes of formula NBu₄[Re(NO)Br₄(L)] [NBu₄⁺ = tetra-n-butylammonium cation and L = pyridine (1), pyrazine (2), pyrimidine (3), pyridazine (4)] have been prepared by a substitution reaction involving NBu₄[Re(NO)Br₄(EtOH)] and L. Their crystal structures have been determined by single crystal X-ray diffraction. They are all mononuclear complexes whose structure is made up of [Re(NO)Br₄L]⁻ anions and NBu₄⁺ cations. Each Re(II) ion is six-coordinate with four bromide ligands, a linear nitrosyl group and one monodentate nitrogen donor L building a tetragonally distorted octahedral surrounding. The Re–Br bond distances cover a narrow range [2.5048(8)–2.5333(5) Å] and they are longer than the Re–NO [1.688(15)–1.736(3) Å] and Re–N bonds [2.219(4)–2.234(3) Å]. The magnetic properties of 1–4 have been investigated in the temperature range 1.9–295 K. They behave like quasi magnetically isolated spin doublets with very weak antiferromagnetic interactions through intermolecular Br⋯Br contacts. Their magnetic properties are discussed through a deep analysis of the influence of the ligand field, spin–orbit coupling, tetragonal distortion and covalence effects. The values of the temperature-independent paramagnetism for 1–4 are also substantiated and compared to those previously reported in related systems.