Hyperfine Interact (2012) 205:75–79 DOI 10.1007/s10751-011-0412-7 Weak cooperativity in selected iron(II) 1D coordination polymers Marinela M. Dîrtu · Damien Gillard · Anil D. Naik · Aurelian Rotaru · Yann Garcia Published online: 16 November 2011 © Springer Science+Business Media B.V. 2011 Abstract The spin crossover behaviour of a new class of Fe II coordination polymers [Fe(phtptrz) 3 ]I 2 (1), [Fe(phtptrz) 3 ](ReO 4 ) 2 •CH 3 OH (2) and [Fe(phtptrz) 3 ]TaF 7 • 6H 2 O(3) based on a novel ligand 4-(3 ′ -N-phtalimido-propyl)-1,2,4-triazole (phtptrz), were investigated by temperature dependent 57 Fe Mössbauer spectroscopy and mag- netic susceptibility measurements. The adverse effect of bulky substituent on 1,2,4- triazole, favorable supramolecular interactions and influence of increasing anion size on spin crossover profile is discussed. 1 and 2 show thermally induced spin conversions of gradual and incomplete nature with associated thermochromism, and transition temperatures T 1/2 ∼ 163 K and 137 K, respectively. A spin state crossover is also identified for 3. Keywords 57 Fe Mössbauer spectroscopy · Coordination polymers · Spin-transition · 1,2,4-triazole 1 Introduction Cooperativity factors which determine the importance in iron(II) spin crossover (SCO) materials performances have long been the subject matter of intensive research with targeted fascinating applications in sensor and memory devices [1, 2]. Abruptness, completeness and hysteresis width/shape of magnetic profiles are key elements to probe strength of cooperative interactions in SCO complexes which can be influenced by ligand design, anion geometry and charge, crystal defects, solvent M. M. Dîrtu · D. Gillard · A. D. Naik · Y. Garcia (B ) Institute of Condensed Matter and Nanosciences, MOST – Inorganic Chemistry, Université Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium e-mail: ann.garcia@uclouvain.be A. Rotaru Department of Electrical Engineering and Computer Science, “Stefan cel Mare” University, University Street, 13, Suceava 720229, Romania