A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

Abstract

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2]2+ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T 1/2 ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T 1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding.

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2]2+ Derivatives

“…Electron‐donating methyl substituents could reduce Fe→ L R backbonding into the oxazolinyl rings, which would weaken the ligand field and favour the high‐spin state as observed. A comparable influence of ligand substituents on Fe→ligand π‐bonding has also been seen in the [Fe(bpp) 2 ] 2+ series of complexes …”

“…The influence of chirality on the SCO properties of such compounds has only recently begun to be investigated and, importantly, all data published thus far have been measured in the solid state . The effect of ligand structure on molecular spin states is better quantified in solution, in which the spin state of a complex is not influenced by lattice effects …”

Towards the Molecular Design of Spin‐Crossover Complexes of 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…Tailoring it for the above applications relies on proper chemical modifications of the ligands identified mostly through systematic screening of solutions (by NMR spectroscopy that is behind the popular Evans method) of closely related iron(II) complexes with bi‐ and tridentate or higher denticity ligands. The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents . The contradictory electronic effects have been recently reconciled in a solution‐state study of 2,6‐bis(pyrazol‐1‐yl)pyridines (1‐bpp; Scheme ), which are among the most popular ligands to induce an SCO behaviour in iron(II) complexes, and to control it with a judicious choice of substituents .…”

“…The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents . The contradictory electronic effects have been recently reconciled in a solution‐state study of 2,6‐bis(pyrazol‐1‐yl)pyridines (1‐bpp; Scheme ), which are among the most popular ligands to induce an SCO behaviour in iron(II) complexes, and to control it with a judicious choice of substituents . By modifying their electron‐donating or electron‐withdrawing ability, the spin state of the iron(II) ion can now be fine‐tuned by this versatile family of tridentate ligands…”

“…The contradictory electronic effects have been recently reconciled in a solution‐state study of 2,6‐bis(pyrazol‐1‐yl)pyridines (1‐bpp; Scheme ), which are among the most popular ligands to induce an SCO behaviour in iron(II) complexes, and to control it with a judicious choice of substituents . By modifying their electron‐donating or electron‐withdrawing ability, the spin state of the iron(II) ion can now be fine‐tuned by this versatile family of tridentate ligands…”