Abstract
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.