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

• Published in: Angewandte Chemie 2016-03, Vol.128 (13), p.4399-4403
• Main Authors: Kershaw Cook, Laurence J., Kulmaczewski, Rafal, Mohammed, Rufeida, Dudley, Stephen, Barrett, Simon A., Little, Marc A., Deeth, Robert J., Halcrow, Malcolm A.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Chemical synthesis; Chemistry; Coordination compounds; Dichtefunktionalrechnungen; Eisen; Electronegativity; Iron; Ligands; Mathematical analysis; Metal ions; Metals; N-Liganden; Organic chemistry; Pyridines; Reduction; Spinzustände; Substituenteneffekte; Synthesis (chemistry); Trends
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.201600165

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. Höhen und Tiefen: Der Low‐spin‐Zustand des abgebildeten Komplexes wird durch elektronenziehende Substituenten X an den Pyridylringen stabilisiert, durch elektronenschiebende Substituenten Y an den Pyrazolylringen hingegen schon. Dichtefunktionalrechnungen bestätigen diese Befunde, die sich mit konkurrierenden Einflüssen der Substituenten an den Pyridyl‐ and Pyrazolylliganden auf die σ‐ und π‐Fe‐L‐Bindung begründen.