Probing the generality of spin crossover complex T½vs. ligand 15N NMR chemical shift correlations: towards predictable tuning

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^pytCF₃)₂(NCBH₃)₂] is SCO active with a hysteresis loop at 208 K (T_1/2↓ = 203 K, T_1/2↑ = 213 K; ΔT_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^pytCF₃)₂(NCBH₃)₂] was mostly LS at RT (T_1/2 ∼ 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 ¹⁵N 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 δN_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 δN_A with T_1/2 were observed for each of the ligand families with a para substituent (R² = 0.69–0.96), whereas there is no correlation when meta substituents are modified (R² = 0.15–0.37), probably because the electronic impact of this is too small. Finally, δN_A also shows promise as an easily calculated measure of the electronic effect of any substituent, in contrast to the Hammett constant (σ_p⁺) which is not available for all possible substituents.