Spin crossover in hydrogen-bonded frameworks of Fe^II complexes with organodisulfonate anions

We present here the syntheses, crystal structures, and thermal and magnetic properties of a series of mononuclear Fe²⁺ spin crossover (SCO) complexes of the formula [Fe(bamp)₂]·Anion·Solv (Anion = NDS^2-, Solv = 2H₂O, 1_NDS; Anion = BPDS^2-, Solv = 4.4H₂O, 2_BPDS; Anion = ABDS^2-, Solv = Et₂O and H₂O, 3_ABDS; Anion = DNDS^2-, Solv = MeCN, 4_DNDS; bamp = 2,6-pyridinedimethanamine, H₂NDS = 1,5-naphthalenedisulphonic acid, H₂BPDS = 4,4'-biphenyldisulphonic acid, H₂ABDS = 4,4'-azobenzenedisulfonic acid, H₂DNDS = 4,4'-dinitrostilbene-2,2'-disulfonic acid). The structures and SCO properties of these complexes can be finely modified by organodisulfonate couteranions. Single-crystal X-ray analyses revealed that all these compounds are hydrogen-bonded three-dimensional frameworks constructed from the charge-assisted hydrogen bonds between bamp donors, organodisulfonate acceptors, and/or crystallized solvent molecules. SCO behavior was observed in all four complexes and has been evidenced by detailed structural and magnetic investigations. While 1_NDS exhibits a sharp cooperative SCO transition with a transition temperature T_1/2 of 247 K, 2_BPDS, 3_ABDS, and 4_DNDS undergo more gradual SCO transitions with T_1/2 values of 176, 171 and 158 K, respectively. Magneto-structural relationship studies revealed that the tunable SCO properties, including the trend of the transition temperatures and the cooperativity of the SCO transition, are mainly attributable to the size of the organodisulfonate anions. This study shows that in order to exhibit cooperative SCO properties, "efficient" hydrogen bonds directly connecting the SCO centers, rather than those between the SCO centers and the innocent neighboring groups, are preferred.