Probing the generality of spin crossover complex T1/2vs. ligand 15N NMR chemical shift correlations: towards predictable tuning

Four new bidentate 5-(Z-pyridine)-4-(4-methyl-phenyl)-3-phenyl-1,2,4-triazole ligands L-pytZ (meta-Z = CF3, Br, F, Me) and the corresponding family of [Fe-II(L-pytZ)(2)(NCBH3)(2)] complexes, in addition to the literature unsubstituted analogue [Fe-II(L-pytH)(2)(NCBH3)(2)], are prepared and studied herein. Single crystal structure determinations on all four new complexes confirmed the expected octahedral coordination, with trans-NCBH3 co-ligands. Solid-state variable temperature magnetic studies of air-dried crystals showed that [Fe-II(L-3(pytCF))(2)(NCBH3)(2)] is SCO active with a hysteresis loop at 208 K (T-1/2 down arrow = 203 K, T-1/2 up arrow = 213 K; Delta T-1/2 = 10 K); [Fe-II(L-pytBr)(2)(NCBH3)(2)] is not SCO active until heated above RT, while [Fe-II(L-pytF)(2)(NCBH3)(2)] and [Fe-II(L-pytMe)(2)(NCBH3)(2)] are SCO active close to RT (T-1/2 = 290 and 300 K respectively). Solution phase variable temperature Evans NMR method studies in CDCl3 showed that four of the complexes were SCO active close to RT (T-1/2 = 279-294 K) whilst [Fe-II(L-3(pytCF))(2)(NCBH3)(2)] was mostly LS at RT (T-1/2 similar to 374 K). These solution phase T-1/2 values, and those for four literature families of bi- (five L-azine), tri- (fourteen bpp(X,Y), twelve pybox(X)) or tetra-dentate (seven pytacn(X)) ligands, which feature para (X) pyridine or meta (Y) pyrazole ring substituents, are used, along with the calculated N-15 NMR chemical shift of the coordinating azine/azole nitrogen (N-A) in the ligand (for all forty-two ligands; using a refined protocol), to test the generality of the previously reported correlation of delta N-A chemical shift in the free ligand with the solution T-1/2 for the respective iron(ii) complex. Moderately good to excellent correlations of delta N-A with T-1/2 were observed for each of the ligand families with a para substituent (R-2 = 0.69-0.96), whereas there is no correlation when meta substituents are modified (R-2 = 0.15-0.37), probably because the electronic impact of this is too small. Finally, delta N-A also shows promise as an easily calculated measure of the electronic effect of any substituent, in contrast to the Hammett constant (sigma(+)(p)) which is not available for all possible substituents.

Automated summary

Four new bidentate ligands and their corresponding iron complexes were prepared and studied, showing different SCO activities at various temperatures. NMR studies in solution further confirmed the properties of the ligands and complexes. The correlation between NMR chemical shifts and SCO behavior was explored to understand the electronic effects of different substituents.