Mixed-Valent Cobalt Spin Clusters: a Hexanuclear Complex and a One-Dimensional Coordination Polymer Comprised of Alternating Hepta- and Mononuclear Fragments

Abstract

Reactions between Co(OAc)2 and 2-amino-2-methyl-1,3-propanediol (ampdH2) afford a hexanuclear complex [Co6(H2O)(MeOH)(OAc)6(ampd)4] (1) and a one-dimensional coordination polymer comprised of discrete heptanuclear complexes covalently bound to mononuclear Co centers [Co8(H2O)2(OAc)7(ampd)6]n (2). While 1 is obtained under ambient reaction conditions, the formation of 2 requires solvothermal methods. Both products have been characterized crystallographically and found to be mixed-valent, containing divalent and trivalent Co centers. Down to 30 K, the variable-temperature magnetic susceptibility data of 1 and 2 are dominated by the single-ion properties of high-spin Co(II) centers with distorted-octahedral coordination geometries. Below this temperature, the effect of intramolecular ferromagnetic exchange interactions becomes apparent. The ferromagnetic coupling in 1 has been analyzed in terms of an anisotropic exchange model, and inelastic neutron scattering data are consistent with the proposed model. Although the structure of 2 precludes a quantitative interpretation, the magnetic data suggest ferromagnetic exchange within the heptanuclear unit and negligible interactions along the chain between the hepta- and mononuclear fragments.