Influence of selenocyanate ligands on the transition temperature and cooperativity of bapbpy-based Fe(II)...

Coordination of the ligand bapbpy (1, bapbpy = N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine), of one of its four dimethyl-substituted analogues 2-5 (R2bapbpy = N,N'-di(methylpyrid-2-yl)-2,2'-bipyridine-6,6'-diamine), or of one of its three bis(iso)quinoline analogues 6-8 (R2bapbpy= N,N'-di(quinolyl)-2,2'-bipyridine-6,6'-diamine), to Fe(NCSe)2, afforded eight new iron(II) compounds of the type [Fe(R2bapbpy)(NCSe)2] (9-16). Three of these compounds (11, 13, and 16) were structurally characterized by single crystal X-ray diffraction, which showed similar molecular geometry and packing compared to their thiocyanate analogues. Magnetic susceptibility measurements were carried out for all iron compounds and revealed thermal spin-crossover (SCO) behavior for compounds 9, 11, 13, 15, and 16. Compounds 11, 13, 15, and 16 show an increased transition temperature compared to the thiocyanate analogues. [Fe(bapbpy)(NCSe)2] (9) shows a gradual, one-step SCO, whereas its thiocyanate analogue [Fe(bapbpy)(NCS)2] is known for its cooperative two-step SCO. To discuss the influence of S-to-Se substitution on the cooperativity of the SCO, heat capacity measurements were carried out for compounds 9, 11, 13, 15, and 16, and fitted to the Sorai domain model. The number n of like-spin SCO centers per interacting domain, which is a quantitative measure of the cooperativity of the spin transition, was found to be high for compounds 11 and 15, and low for compounds 9, 11, and 13. Compound 15 is one of the few known SCO compounds that is more cooperative than its thiocyanate analogue. Altogether, X-ray diffraction, calorimetry, and magnetic data give a consistent structure-property relationship for this family of compounds: hydrogen-bonding networks made of intermolecular N-H···Se interactions are of paramount importance for the cooperativity of the SCO.