title: 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 (T1/2 ) 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 T1/2 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.
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.|Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK.
authors.forename: Laurence J
authors.initials: LJ
authors.lastname: Kershaw Cook
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Rafal
authors.initials: R
authors.lastname: Kulmaczewski
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Rufeida
authors.initials: R
authors.lastname: Mohammed
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Stephen
authors.initials: S
authors.lastname: Dudley
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.
authors.forename: Simon A
authors.initials: SA
authors.lastname: Barrett
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK.|Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, UK.
authors.forename: Marc A
authors.initials: MA
authors.lastname: Little
authors.affiliation: Inorganic Computational Chemistry Group, Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK. r.j.deeth@warwick.ac.uk.|School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, EH9 3FJ, UK. r.j.deeth@warwick.ac.uk.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Deeth
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK. m.a.halcrow@leeds.ac.uk.
authors.forename: Malcolm A
authors.identifier: http://orcid.org/0000-0001-7491-9034
authors.initials: MA
authors.lastname: Halcrow
keywords: N ligands; density functional calculations; iron; spin state; substituent effects