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 [FeII(L10)2(NCS)2]

Abstract

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^tBu (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 μ_1,5-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.