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 (L¹⁰) and spin crossover in [Fe(II)(L¹⁰)₂(NCS)₂].

The coordination chemistry of the ligand 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L¹⁰) 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)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (20), [Ni(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (21), [Cu(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (22), [Co(II)(L¹⁰)₂(H₂O)₂](ClO₄)₂ (23) and [Cu(II)(L¹⁰)₂(ClO₄)₂] (24) the metal centres are N₄O₂ octahedrally coordinated with two N²,N(pyr) bidentate ligands L¹⁰ in the equatorial positions. In the N₆ octahedral 4 : 1-type complex [Co(II)(L¹⁰)₄](ClO₄)₂·H₂O (25) both axially coordinating N¹ unidentate and equatorially bound N²,N(pyr) bidentate ligands L¹⁰ are observed. The N₆ octahedral 3 : 1-type complex [Fe(II)(L¹⁰)₃](OTf)₂·1.5MeCN·0.13H₂O·0.87MeO(t)Bu (27) features three N²,N(pyr) bidentate ligands L¹⁰ in the mer configuration. The two closely related N₆ octahedral complexes [Fe(II)(L¹⁰)₂(NCS)₂] (29) and [Fe(II)(L¹⁰)₂(dca)₂] (30) have fundamentally different structures. While complex 29 features two equatorially bound N²,N(pyr) bidentate ligands L¹⁰ and axial NCS⁻ co-ligands, complex 30 is a one-dimensional doubly μ1,5-dicyanamido-bridged polymer with N¹ unidentate ligands L¹⁰ 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)(L¹⁰)₃](ClO₄)₂·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)(L¹⁰)(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.