Magnetic study of a one-dimensional Mn(II) coordination polymer dealing with π–π stacking

Aluminium 8-Hydroxyquinolinate N-Oxide as a Precursor to Heterometallic Aluminium-Lanthanide Complexes

A reaction in anhydrous toluene between the formally unsaturated fragment [Ln(hfac)3] (Ln3+ = Eu3+, Gd3+ and Er3+; Hhfac = hexafluoroacetylacetone) and [Al(qNO)3] (HqNO = 8-hydroxyquinoline N-oxide), here prepared for the first time from [Al(OtBu)3] and HqNO, affords the dinuclear heterometallic compounds [Ln(hfac)3Al(qNO)3] (Ln3+ = Eu3+, Gd3+ and Er3+) in high yields. The molecular structures of these new compounds revealed a dinuclear species with three phenolic oxygen atoms bridging the two metal atoms. While the europium and gadolinium complexes show the coordination number (CN) 9 for the lanthanide centre, in the complex featuring the smaller erbium ion, only two oxygens bridge the two metal atoms for a resulting CN of 8. The reaction of [Eu(hfac)3] with [Alq3] (Hq = 8-hydroxyquinoline) in the same conditions yields a heterometallic product of composition [Eu(hfac)3Alq3]. A recrystallization attempt from hot heptane in air produced single crystals of two different morphologies and compositions: [Eu2(hfac)6Al2q4(OH)2] and [Eu2(hfac)6(µ-Hq)2]. The latter compound can be directly prepared from [Eu(hfac)3] and Hq at room temperature. Quantum mechanical calculations confirm (i) the higher stability of [Eu(hfac)3Al(qNO)3] vs. the corresponding [Eu(hfac)3Alq3] and (ii) the preference of the Er complexes for the CN 8, justifying the different behaviour in terms of the Lewis acidity of the metal centre.

Magnetostructural correlation for rational design of Mn(II) hybrid-spin complexes

The magnetic properties of a series of manganese(II) diacetylacetonate and dihexafluoroacetylacetonate hybrid-spin complexes with neutral pyridine-based organic radicals were characterized theoretically by DFT calculations. Three stable radicals, in which a radical group is bound in either para or meta position with respect to the pyridine nitrogen atom, were considered. The correct stable structures and multiplets of the complexes were obtained by full geometry optimization starting from an ideal structure. A total of three important geometry descriptors of the complexes were monitored and related to their magnetic characteristics. These structural parameters are (i) the torsion angle governing the conjugation of the organic radical m-PyNO (anti versus gauche), (ii) the coordination geometry of the acetyl acetonate ligands around the metal ion (square versus rhombic), and (iii) the relative orientation of the organic radical with respect to the acetyl acetonate plane (parallel versus perpendicular). It was found that the magnetic properties are not sensitive to the orientation of the radicals with respect to the equatorial plane but do depend on the conformation of the organic radicals. Even a spin switch between the ferromagnetic (S = (7)/(2)) and antiferromagnetic (S = (3)/(2)) ground state was found to be feasible for one of the complexes upon variation of the organic radical geometry, namely, the dihedral angle between the organic radical moiety and the pyridine ring. The pattern of molecular orbital overlap was determined to be the key factor governing the exchange coupling in the modeled systems. Bonding π-type overlap provides antiferromagnetic coupling in all complexes of the para radicals. In the meta analogues, the spins are coupled through the σ orbitals. A low-spin ground state occurs whenever a continuous σ-overlap pathway is present in the complex. Ferromagnetic interaction requires σ-π orthogonality of the pyridine atomic orbitals and/or π-antibonding Mn-pyridine natural orbital overlap. Using an estimate of the donor-acceptor energy stabilization, the affinity of a given Mn(II) d-orbital to mix with the sp(2) orbital from pyridine can be predicted.

catena-Poly[[[diaqua-bis(8-hydroxy-quinoline N-oxide-κO)cobalt(II)]-μ-2,5-dimethyl-pyrazine 1,4-dioxide-κO:O] bis-(perchlorate)]

In the title complex, {[Co(C₆H₈N₂O₂)(C₉H₇NO₂)(H₂O)₂](ClO₄)₂}_n, the Co^II ion lies on an inversion centre and is coordinated in a slightly distorted octa­hedral environment. The 2,5-dimethyl­pyrazine 1,4-dioxide ligand, which also lies on an inversion center, acts as a bridging ligand, linking symmetry-related Co^II ions [Co⋯Co = 8.669 (3) Å] and forming one-dimensional chains along the b axis. In the crystal structure, these chains are linked by inter­molecular aqua–perchlorate O—H⋯O hydrogen bonds, forming two-dimensional layers which are in turn connected into a three-dimensional network via π–π stacking inter­actions between quinoline rings, with a centroid–centroid distance of 3.580 (3) Å. An intermolecular O—H⋯Cl inter­action is also present.