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

We present here the syntheses, crystal structures, and thermal and magnetic properties of a series of mononuclear Fe spin crossover (SCO) complexes of the formula [Fe(bamp)]·Anion·Solv (Anion = NDS, Solv = 2HO, 1; Anion = BPDS, Solv = 4.4HO, 2; Anion = ABDS, Solv = EtO and HO, 3; Anion = DNDS, Solv = MeCN, 4; bamp = 2,6-pyridinedimethanamine, HNDS = 1,5-naphthalenedisulphonic acid, HBPDS = 4,4'-biphenyldisulphonic acid, HABDS = 4,4'-azobenzenedisulfonic acid, HDNDS = 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 exhibits a sharp cooperative SCO transition with a transition temperature T of 247 K, 2, 3, and 4 undergo more gradual SCO transitions with T 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.