The magnetic anisotropy associated with a pentagonal bipyramidal coordination sphere is examined on the basis of experimental and theoretical investigations; effects of crystal field, structural distortion, and second coordination sphere are discussed.

Trinuclear systems of formula [{Cr(L N3O2Ph )(CN) 2 } 2 M-(H 2 L N3O2R )] (M = Mn II and Fe II , L N3O2R stands for pentadentate ligands) were prepared in order to assess the influence of the bending of the apical MÀ N�C linkages on the magnetic anisotropy of the Fe II derivatives and in turn on their Single-Molecule Magnet (SMM) behaviors. The cyanido-bridged [Cr 2 M] derivatives were obtained by assembling trans-dicyanido Cr III complex [Cr(L N3O2Ph )(CN) 2 ] À and divalent pentagonal bipyramid complexes [M II (H 2 L N3O2R )] 2 + with various R substituents (R = NH 2 , cyclohexyl, S,S-mandelic) imparting different steric demand to the central moiety of the complexes. A comparative examination of the structural and magnetic properties showed an obvious effect of the deviation from straightness of the MÀ N�C alignment on the slow relaxation of the magnetization exhibited by the [Cr 2 Fe] complexes. Theoretical calculations have highlighted important effects of the bending of the apical CÀ NÀ Fe linkages on both the magnetic anisotropy of the Fe II center and the exchange interactions with the Cr III units.

Each enantiomer of a chiral pentagonal bipyramidal (PBP) FeII complex ([FeLN5R,R Cl(MeOH)]Cl·H2O, 1 R, or [FeLN5S,S Cl(MeOH)]Cl·H2O, 1 S) has been obtained using either the R or the S stereoisomer of the macrocyclic pentadentate ligand formed in a template-condensation reaction of the tetraamine N,N′-Bis-{(1R,2R)-[2-(amino)]cyclohexyl}-1,2-diaminoethane, or its (1S,2S) enantiomer, and 2,5-diacetylpyridine, respectively LN5R,R and LN5S,S . Subsequently, the 1-D coordination polymers [FeLN5R,R {FeCl4}], 2 R, and [FeLN5S,S {FeCl4}], 2 S, were formed by reacting respectively 1 R or 1 S with 1 equiv of FeCl2(H2O)4. These chain compounds consist of an alternation of PBP and tetrahedral FeII units sharing a Cl atom. Theoretical calculations reveal large magnetic anisotropy for each Fe center but of different type with axial zero-field splitting parameter D of respectively −30 cm–1 and 15 cm–1. The magnetic behavior for the 1-D compound revealed canted antiferromagnetic Fe–Fe interactions (J = −6 cm–1) and SCM behavior characterized by Δ/k B = 42 K with pre-exponential factor τ0 = 2.17 × 10–10 s. It was noted that this SCM behavior was accompanied by a magnetic order leading to a weak ferromagnet (i.e., canted antiferromagnet). The preparations, crystal structures (CCDC references 2214580–2214583), spectroscopic data, magnetic behaviors, and theoretical investigations are reported.

A pentadentate-[N 3 O 2 ] ligand, H 2 L NMe2 , formed by condensation of diacetyl pyridine and dimethylcarbamoyl hydrazide (i. e. Me 2 NCONHNH 2 ), is reported to yield mononuclear pentagonal bipyramidal (PBP) metal complexes with Cr III , Mn II , Fe II , Co II , and Ni II , that exhibit good solubility in a wide range of solvents as compared to the classically used H 2 L R ligands. With Cu II , dinuclear complexes were obtained. The potassium salt of the deprotonated ligand, K 2 L NMe2 , was also characterized. TheThe magnetic behaviors of the PBP derivatives were assessed, especially the zero-field splitting (ZFS) characteristics for the Cr III , Fe II , Co II , and Ni II derivatives. The ZFS characteristics were also determined from ab initio theoretical calculations. The obtained values confirm those extracted from magnetic measurements.