Enhanced Ion Anisotropy by Nonconventional Coordination Geometry: Single-Chain Magnet Behavior for a FeIIL2NbIV (CN) 8 Helical Chain Compound Designed with Heptacoordinate FeII

Nonconventional heptacoordination in combination with efficient magnetic exchange coupling is shown to yield a 1-D heteronuclear Fe (II) Nb (IV) compound with remarkable magnetic features when compared to other Fe (II) -based single chain magnets (SCM). Cyano-bridged heterometallic 3d-4d and 3d-5d chains are formed upon assembling Fe (II) bearing a pentadentate macrocycle as the blocking ligand with octacyano metallates, M (CN) (8) (4-) (M = Nb (IV), Mo (IV), W (IV) ). X-ray diffraction (single-crystal and powder) measurements reveal that the (H (2) O) Fe (L (1) ) M (CN) (8) Fe (L (1) ) (infinity) architectures consist of isomorphous 1-D polymeric structures based on the alternation of Fe (L (1) ) (2+) and M (CN) (8) (4-) units (L (1) stands for the pentadentate macrocycle). Analysis of the magnetic susceptibility behavior revealed cyano-bridged Fe-Nb exchange interaction to be antiferromagnetic with J = -20 cm (-1) deduced from fitting an Ising model taking into account the noncollinear spin arrangement. For this ferrimagnetic chain a slow relaxation of its magnetization is observed at low temperature revealing a SCM behavior with Delta/k (B) = 74 K and tau (0) = 4. 6 x 10 (-11) s. The M versus H behavior exhibits a hysteresis loop with a coercive field of 4 kOe at 1 K and reveals at 380 mK magnetic avalanche processes, i. e. , abrupt reversals in magnetization as H is varied. The origin of these characteristics is attributed to the combination of efficient Fe-Nb exchange interaction and significant anisotropy of the Fe (L (1) ) unit. High field EPR and magnetization experiments have revealed for the parent compound Fe (L (1) ) (H (2) O) (2) Cl (2) a negative zero field splitting parameter of D approximately = -17 cm (-1). The crystal structure, magnetic behavior, and Mossbauer data for Fe (L (1) ) (H (2) O) (2) Cl (2) are also reported.