A Simple Method of Predicting Spin State in Solution.

A simple method, using density functional theory (DFT), of predicting spin-state in advance of synthesis is reported. Specifically, an excellent correlation is observed between the switching temperatures (T1/2 ) in CDCl3 solution of five spin-crossover (SCO)-active [FeII (Lazine )2 (NCBH3 )2 ] complexes and the DFT-calculated (and observed) 15 N NMR chemical shift (δNA ) of the five different azine-substituted 1,2,4-triazole ligands employed, Lazine = 4-(4-methylphenyl)-3-phenyl-5-(azine)-1,2,4-triazole, where azine = pyridine, pyridazine, 4-pyrimidine, pyrazine, and 2-pyrimidine. To test the generality of this finding, DFT was also employed to readily predict the δNA values for a family of 16 literature ligands, known as bppX,Y [X,Y-substituted 2,6-(pyrazol-1-yl)pyridines], which have produced 16 SCO-active [FeII (bppX,Y )2 ](Z)2 complexes (Z = BF4 or in one case PF6 ) in (CD3 )2 CO solution: again an excellent correlation was found between the computed δNA and the observed T1/2 . These correlations represent a key advance in the field, as they allow a simple DFT calculation on a modified ligand to be used to reliably predict, before synthesis of the ligand or complex, the T1/2 that would result from that modification. Achieving such easily predictable tuning of T1/2 , and hence of spin-state, is a significant step forward in the field of SCO and also has big implications in many other fields in which spin-state is key, including catalysis, metallo-enzyme modeling studies, and host-guest chemistry.