Spin-crossover in dimeric hydrogen-bonded iron(II) 2-(pyrazolyl)-pyridine and 2-(imidazolyl)-pyridine complexes

Five new hydrogen-bonded solvated iron(II) complexes of pyrazolyl- and imidazolyl-based N,N-chelating ligands have been synthesised. Water to ligand-NH hydrogen-bonded bridges occur in the pseudo-dimeric complexes {cis-[Fe(pypzH)2(NCX)2]2(μ-OH2)(H2O)2} · H2O · MeOH (where X = S or Se), and in the chain complex {cis-[Fe(pypzH)2(NCS)2](μ-OH2)}n. A “half” spin-crossover (Tc = 125 K) was observed in the dimeric X = Se complex by means of magnetic measurements and no thermal hysteresis occurred between 4 and 300 K. The crystal structure at 123 K showed Fe–N distances consistent with the magnetism. Each Fe in the dimeric unit was structurally equivalent in the HS–LS state. Removal of the solvate molecules led to HS–HS behaviour over the temperature range 4–300 K. The pseudo-dimer with X = S also showed HS–HS behaviour as did the monomeric analogue cis-[Fe(pypzH)2(NCS)2]H2O and a structurally different methanol-bridged dimer {cis-[Fe(pyimH)2(NCS)2]2(μ-MeOH)2} · 2MeOH (pypzH = 2-(1H-pyrazol-3-yl)-pyridine; pyimH = 2-(1H-imidazol-2-yl)-pyridine).

Graphical abstractA family of iron(II) cis-[FeL2(NCX)2]2(μ-solvate) complexes has been made and structurally characterised, where L is a bidentate pyrazolyl- or imidazolyl-pyridine and X = S, Se. Solvate (water and methanol) molecules hydrogen-bond to NH groups on neighbouring L groups thus forming discrete pseudo-dimeric species, one of which, with X = Se, displays a ‘half’ spin-crossover. This spin transition is absent upon desolvation.Figure optionsDownload full-size imageDownload as PowerPoint slide