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 (T-1/2) in CDCl3 solution of five spin-crossover (SCO)-active [Fe-II(L-azine)(2)(NCBH3)(2)] complexes and the DFT-calculated (and observed) N-15 NMR chemical shift (delta(NA)) of the five different azine-substituted 1,2,4-triazole ligands employed, L-azine = 4-(4-methylphenyl)-3-pheny1-5-(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 delta(NA) values for a family of 16 literature ligands, known as bpp(X,Y) [X,Y-substituted 2,6-(pyrazol-1-yl)pyridines], which have produced 16 SCO-active [Fe-II(bpp(X,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 delta(NA) and the observed T-1/2. 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 T-1/2 that would result from that modification. Achieving such easily predictable tuning of T-1/2, 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.