The magnetic and structural elucidation of 3,5-bis(2-pyridyl)-1,2,4-triazolate-bridged dinuclear iron(II) spin crossover compounds

Variable temperature magnetic susceptibility, Mössbauer spectroscopic and X-ray crystallographic studies are described on two structurally similar families of dinuclear iron(II) spin crossover (SCO) complexes of formula [Fe(NCX)(py)]2(μ-L)2, where L is either a 3,5-bis(2-pyridyl)-pyrazolate bridging ligand, bpypz−, examples of which have been earlier reported by Kaizaki and coworkers, or a corresponding 3,5-bis(2-pyridyl)-1,2,4-triazolate, bpytz−. Compounds synthesised were [Fe(NCS)(py)]2(μ-bpypz)2 (1), [Fe(NCSe)(py)]2(μ-bpypz)2 (2), [Fe(NCS)(py)]2(μ-bpytz)2 (3), [Fe(NCSe)(py)]2(μ-bpytz)2 (4), [Fe(NCBH3)(py)]2(μ-bpytz)2 (5). The crystal and molecular structures of 1 and 3 are very similar in their HS–HS forms (HS = high spin d6). In contrast to reported SCO behaviour for precipitated samples of 1, also repeated here, crystals of 1 show only HS–HS behaviour with no spin crossover transition. Complex 3 likewise displays HS–HS magnetism, with very weak antiferromagnetic coupling. Compound 5 displays a well resolved two-step, full spin transition from HS–HS to LS–LS states while compound 2 shows a one step transition. The Mössbauer data for 2 and 5 show unusual features at low temperatures.

The structurally similar families of dinuclear iron(II) complexes of type [Fe(NCX)(py)]2(μ-L)2, where L− is a bridging pyrazolate or triazolate dinucleating ligand, and X = S, Se, BH3, display different spin-crossover behaviour that is fundamentally related to the different ligand field strengths provided by the bridging azolate and apical NCX− ligands.Figure optionsDownload as PowerPoint slide