Triazolopyridines as ligands: structural diversity in iron(II), cobalt(II), nickel(II) and copper(II) complexes of 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L-10) and spin crossover in [Fe-II(L-10)(2)(NCS)(2)]

The coordination chemistry of the ligand 3-(2-pyridyl)-[1,2,4] triazolo[4,3-a] pyridine (L-10) has been investigated and iron(II), cobalt(II), nickel(II) and copper(II) complexes featuring diverse structural motifs have been prepared. In the 2 : 1-type complexes [Co-II(L-10)(2)(MeOH)(2)](ClO4)(2) (20), [Ni-II(L-10)(2)(MeOH)(2)](ClO4)(2) (21), [Cu-II(L-10)(2)(MeOH)(2)](ClO4)(2) (22), [Co-II(L-10)(2)(H2O)(2)](ClO4)(2) (23) and [Cu-II(L-10)(2)(ClO4)(2)] (24) the metal centres are N4O2 octahedrally coordinated with two N-2,N-pyr bidentate ligands L-10 in the equatorial positions. In the N-6 octahedral 4 : 1-type complex [Co-II(L-10)(4)](ClO4)(2)center dot H2O (25) both axially coordinating N-1 unidentate and equatorially bound N-2, N-pyr bidentate ligands L-10 are observed. The N-6 octahedral 3 : 1-type complex [Fe-II(L-10)(3)](OTf)(2)center dot 1.5MeCN center dot 0.13H(2)O center dot 0.87MeO(t)Bu (27) features three N-2,N-pyr bidentate ligands L-10 in the mer configuration. The two closely related N-6 octahedral complexes [Fe-II(L-10)(2)(NCS)(2)] (29) and [Fe-II(L-10)(2)(dca)(2)] (30) have fundamentally different structures. While complex 29 features two equatorially bound N-2,N-pyr bidentate ligands L-10 and axial NCS- co-ligands, complex 30 is a one-dimensional doubly mu(1,5)-dicyanamido-bridged polymer with N-1 unidentate ligands L-10 in the axial positions. Temperature-dependent magnetic susceptibility measurements of the iron(II) complexes 28 and 29 have shown the 3 : 1-type complex [Fe-II(L-10)(3)](ClO4)(2)center dot H2O (28) to be in the low-spin state over the range 300-2 K and the 2 : 1-type complex 29 to be a spin crossover compound with T-1/2 = 269 K whereas the dicyanamido-bridged complex 30 remains in the high-spin state even down to 113 K, according to X-ray diffraction data. A single end-to-end bridging NCS- co-ligand is found in the N4S square-pyramidal complex [Cu-II(L-10)(NCS)(2)] (31) which shows Curie-Weiss behaviour over the range 300-2 K. A brief review of the coordination chemistry of triazolopyridines is given.