TY - JOUR
T1 - A unified treatment of the relationship between ligand substituents and spin state in a family of iron(II) complexes
AU - Kershaw Cook, Laurence J.
AU - Kulmaczewski, Rafal
AU - Mohammed, Rufeida
AU - Dudley, Stephen
AU - Barrett, Simon A.
AU - Little, Marc A.
AU - Deeth, Robert J.
AU - Halcrow, Malcolm A.
PY - 2016/3/18
Y1 - 2016/3/18
N2 - 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. Highs and lows: The low-spin state of the complex shown is stabilized by electron-withdrawing pyridyl "X" substituents, but also by electron-donating pyrazolyl "Y" substituents. DFT calculations reproduce these results, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L σ and π bonding.
AB - 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. Highs and lows: The low-spin state of the complex shown is stabilized by electron-withdrawing pyridyl "X" substituents, but also by electron-donating pyrazolyl "Y" substituents. DFT calculations reproduce these results, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L σ and π bonding.
KW - density functional calculations
KW - iron
KW - N ligands
KW - spin state
KW - substituent effects
UR - http://www.scopus.com/inward/record.url?scp=84961775195&partnerID=8YFLogxK
UR - http://dx.doi.org/10.1002/anie.201600165
UR - http://dx.doi.org/10.1002/anie.201600165
U2 - 10.1002/anie.201600165
DO - 10.1002/anie.201600165
M3 - Article
AN - SCOPUS:84961775195
VL - 55
SP - 4327
EP - 4331
JO - Angewandte Chemie-International Edition
JF - Angewandte Chemie-International Edition
SN - 1433-7851
IS - 13
ER -