Quantitative Assessment of Ligand Substituent Effects on σ‐ and π‐Contributions to Fe−N Bonds in Spin Crossover Fe^II Complexes.

Bondì, Luca; Garden, Anna L.; Totti, Federico; Jerabek, Paul; Brooker, Sally

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.

SPIN crossover; LIGAND field theory; ATOMIC orbitals; IRON; ELECTRONIC structure

Chemistry - A European Journal, 2022, Vol 28, Issue 22, p1

0947-6539

Academic Journal

10.1002/chem.202104314