A DFT computational study of spin crossover in iron(iii) and iron(ii) tripodal imidazole complexes. A comparison of experiment with calculations

B3LYP* functionals were used to model the sixteen iron(ii) ( ¹A, LS and ⁵T, HS) and iron(iii) (²T, LS and ⁶A, HS) complexes of the 1: 3 Schiff base condensate of tris(2-aminoethyl)amine and imidazole-4-carboxaldehyde, H₃L ¹, and its deprotonated forms, [H₂L¹] ^1-, [HL¹]^2-, and [L¹]^3-. This ligand system is unusual in that [FeH₃L¹] ³⁺, [FeH₃L¹]²⁺ and [FeL ¹]^- all exhibit a spin crossover between 100-300 K. This makes these complexes ideal for a hybrid DFT computational approach and provides an opportunity to refine the value of the exact exchange admixture parameter, c₃, and to predict properties of partially protonated complexes that are not experimentally available. The accepted value of 0.20 is larger than the value of ∼0.13 that was found to best reproduce experimental data in terms of spin state predictions. With iron(iii) B3LYP calculations showed that all of the complexes were low spin at 298 K with the exception of [FeH ₃L¹]³⁺ which is spin crossover in agreement with experimental results. It was also shown for iron(iii) that the ligand field increased as the number of protons decreased. In contrast all of the iron(ii) complexes were close to the spin crossover region regardless of protonation state. Experimental structures are fairly well modeled by this system in regard to the key structural indicators of spin state, which are the bite and trans angles. The calculated iron to nitrogen atom distances are always larger in the high spin form than the low spin form but all iron to nitrogen bond distances are larger than the experimental values. In general non-bonded interactions are not well modeled by this methodology.