To increase the supramolecular cooperativity in Fe(II) spin crossover materials based on N1-substituted tetrazoles, a series of ω-(1H-tetrazol-1-yl) carboxylic acids with chain-lengths of C2–C4 were synthesized. Structural characterization confirmed the formation of a strong hydrogen-bond network, responsible for enhanced cooperativity in the materials and thus largely complete spin-state transitions for the ligands with chain lengths of C2 and C4. To complement the structural and magnetic investigation, electronic spectroscopy was used to investigate the spin-state transition. An initial attempt to utilize the bifunctional coordination ability of the ω-(1H-tetrazol-1-yl) carboxylic acids for preparation of mixed-metallic 3d–4f coordination polymers resulted in a novel one-dimensional gadolinium-oxo chain system with the ω-(1Htetrazol-1-yl) carboxylic acid acting as μ2-η2:η1 chelating–bridging ligand.

"Bifunctional Fe(II) Spin Crossover-complexes based on ω-(1H-tetrazol-1-yl) carboxylic acids”, W. Zeni, M. Seifried, C. Knoll, J.M. Welch, G. Giester, B. Stöger, W. Artner, M. Reissner, D. Müller, P. Weinberger, Dalton Trans., 49 (2020) 17183 – 17193
http://dx.doi.org/10.1039/d0dt03315d

Criteria for a technologically relevant spin crossover (SCO) material include temperature and abruptness. A series of Fe(II) – 1,3-bis((1H tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane SCO complexes with various anions (BF₄⁻, ClO₄⁻, and PF₆⁻) designed using a structure–property based concept is reported. All complexes feature abrupt SCO-behavior with T_1/2 between 170 K and 187 K. These materials demonstrate that without stabilizing the effects of incorporated solvents or a hydrogen bond-network, the observed cooperativity during high-spin–low-spin transition is anion independent and originates only from the rigidity and internal strain of the propellane-moiety in the ligand. Spectroscopy and structural investigations of these materials are supported by quantum chemical calculations.

 „Cooperativity in spin crossover materials as ligand’s responsibility – investigations of the Fe(II) – 1,3-bis((1H-tetrazol-1-yl)methyl)bicycle [1.1.1]pentane system“, C. Knoll, D. Müller, M. Seifried, G. Giester, J. Welch, W. Artner, K. Hradil, M. Reissner, P. Weinberger, Dalton Trans., 2018, 47, 5553–5557
http://dx.doi.org/10.1039/c8dt00781k

1-(3-Halopropyl)-1H-tetrazoles and their corresponding FeII spin-crossover complexes have been investigated in a combined experimental and theoretical study. Halogen substitution was found to positively influence the spin transition, shifting the transition temperature about 70 K towards room temperature. Halogens located at the w position were found to be too far away from the coordinating tetrazole moiety to have an electronic impact on the spin transition. The subtle variation of the steric demand of the ligand in a highly comparable series was found to have a comparatively large impact on the spin-transition behavior, which highlights the sensitivity of the effect to subtle structural changes.

„Halogenated Alkyltetrazoles for the Rational Design of FeII Spin-Crossover Materials: Fine-Tuning of the Ligand Size“, D. Müller, C. Knoll, M. Seifried, J. Welch, G. Giester, M. Reissner, P. Weinberger, Chem. Eur. J. 2018, 24, 5271–5280
https://doi.org/10.1002/chem.201704656