A novel family of heteroleptic 1D chain spin-crossover complexes and its DFT modelling. How the formation of polynuclear chains modifies the ligand-field effects on spin transition.

Reactions of the ditopic ligands 1-(5-(2-pyridyl)tetrazol-1-yl)-3-(tetrazol-2-yl)propane (5-p1tz2tz), 1-(5-(2-pyridyl)tetrazol-2-yl)-3-(tetrazol-2-yl)propane (5-p2tz2tz), and 1-(3-(2-pyridyl)-1,2,4-triazol-1-yl)-3-(tetrazol-1-yl)propane (3-p1tr1tz) with iron(II) tetrafluoroborate afforded a series of one-dimensional coordination polymers: [Fe(5-p1tz2tz)2](BF4)2 (1), [Fe(5-p2tz2tz)2](BF4)2 (2), [Fe(3-p1tr1tz)2](BF4)2·CH3CN (3) and [Fe(3-p1tr1tz)2](BF4)2·CH3OH (4). The first coordination spheres of all complexes are composed of two chelating pyridylazole units and two tetrazole rings. The two complexes involving pyridyltetrazole-form 1D chains with trans-coordinated monotetrazoles. They exhibit gradual spin transitions centred at approximately 350 K. For the pyridyl-1,2,4-triazole ligand, two polymorphs were isolated. One compound is a structural analogue of pyridyltetrazole complexes and exhibits high-spin down to 10 K. Modifying the synthetic procedure yields a 1D zig-zag chain with almost the same molecular volume, but with cis-coordinated monodentate tetrazole. This complex exhibits an abrupt spin-crossover accompanied by a hysteresis loop (T1/2 = 114 K, T1/2 = 131 K). Density functional theory (DFT) modelling of the above systems, combined with results for known mononuclear and polynuclear complexes of 1-propyltetrazole revealed how the spin-transition energies vary from the mononuclear species to the corresponding 1D chains.