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- Title
Quantitative Assessment of Ligand Substituent Effects on σ‐ and π‐Contributions to Fe−N Bonds in Spin Crossover Fe<sup>II</sup> Complexes.
- Authors
Bondì, Luca; Garden, Anna L.; Totti, Federico; Jerabek, Paul; Brooker, Sally
- Abstract
The effect of para‐substituent X on the electronic structure of sixteen tridentate 4‐X‐(2,6‐di(pyrazol‐1‐yl))‐pyridine (bppX) ligands and the corresponding solution spin crossover [FeII(bppX)2]2 complexes is analysed further, to supply quantitative insights into the effect of X on the σ‐donor and π‐acceptor character of the Fe‐NA(pyridine) bonds. EDA‐NOCV on the sixteen LS complexes revealed that neither ΔEorb,σ π (R2=0.48) nor ΔEorb,π (R2=0.31) correlated with the experimental solution T1/2 values (which are expected to reflect the ligand field imposed on the iron centre), but that ΔEorb,σ correlates well (R2=0.82) and implies that as X changes from EDG→EWG (Electron Donating to Withdrawing Group), the ligand becomes a better σ‐donor. This counter‐intuitive result was further probed by Mulliken analysis of the NA atomic orbitals: NA(px) involved in the Fe−N σ‐bond vs. the perpendicular NA(pz) employed in the ligand aromatic π‐system. As X changes EDG→EWG, the electron population on NA(pz) decreases, making it a better π‐acceptor, whilst that in NA(px) increases, making it a better σ‐bond donor; both increase ligand field, and T1/2 as observed. In 2016, Halcrow, Deeth and co‐workers proposed an intuitively reasonable explanation of the effect of the para‐X substituents on the T1/2 values in this family of complexes, consistent with the calculated MO energy levels, that M→L π‐backdonation dominates in these M−L bonds. Here the quantitative EDA‐NOCV analysis of the M−L bond contributions provides a more complete, coherent and detailed picture of the relative impact of M−L σ‐versus π‐bonding in determining the observed T1/2, refining the earlier interpretation and revealing the importance of the σ‐bonding. Furthermore, our results are in perfect agreement with the ΔE(HS‐LS) vs. σp (X) correlation reported in their work.
- Subjects
SPIN crossover; LIGAND field theory; ATOMIC orbitals; IRON; ELECTRONIC structure
- Publication
Chemistry - A European Journal, 2022, Vol 28, Issue 22, p1
- ISSN
0947-6539
- Publication type
Academic Journal
- DOI
10.1002/chem.202104314