Influence of Selenocyanate Ligands on the Transition Temperature and Cooperativity of bapbpy-Based Fe(II) Spin-Crossover Compounds

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.