The accurate electron density distribution and magnetic properties of two metal−organic polymeric magnets, the quasi-onedimensional (1D) Cu(pyz)(NO 3 ) 2 and the quasi-two-dimensional (2D) [Cu(pyz) 2 (NO 3 )]NO 3 •H 2 O, have been investigated by high-resolution single-crystal X-ray diffraction and density functional theory calculations on the whole periodic systems and on selected fragments. Topological analyses, based on quantum theory of atoms in molecules, enabled the characterization of possible magnetic exchange pathways and the establishment of relationships between the electron (charge and spin) densities and the exchange-coupling constants. In both compounds, the experimentally observed antiferromagnetic coupling can be quantitatively explained by the Cu−Cu superexchange pathway mediated by the pyrazine bridging ligands, via a σ-type interaction. From topological analyses of experimental charge-density data, we show for the first time that the pyrazine tilt angle does not play a role in determining the strength of the magnetic interaction. Taken in combination with molecular orbital analysis and spin density calculations, we find a synergistic relationship between spin delocalization and spin polarization mechanisms and that both determine the bulk magnetic behavior of these Cu(II)-pyz coordination polymers.

Magnetisation decay measurements are commonly being used to characterise very slow relaxation in single-molecule magnets. We explore measurement protocol and data analysis to define the best practise.

Isolated dysprosocenium cations, [Dy(CpR)2]+ (CpR = substituted cyclopentadienyl), have recently been shown to exhibit superior single-molecule magnet (SMM) properties over closely related complexes with equatorially-bound ligands. However, gauging the crossover point at which the CpR substituents are large enough to prevent equatorial ligand binding, but small enough to approach the metal closely and generate strong crystal field splitting, has required laborious synthetic optimization. We therefore created the computer program, AtomAccess, to predict the accessibility of a metal binding site and its ability to accommodate additional ligands. Here we apply AtomAccess to rapidly screen a series of derivatized dysprosium metallocene fragments of varying steric bulk in silico, allowing us to identify the crossover point for equatorial coordination in [Dy(CpR)2]+ cations, and hence predict a cation that is at the cusp of stability without equatorial interactions, viz. [Dy(Cpttt)(Cp*)]+ (Cpttt = C5H2tBu3-1,2,4, Cp* = C5Me5). Upon synthesizing this cation we found it crystallizes as either a contact ion-pair, [Dy(Cpttt)(Cp*){Al[OC(CF3)3]4-k-F}], or separated ion-pair polymorph, [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4]C6H6. These complexes, together with their precursors and yttrium analogs, have been characterized by NMR and ATR-IR spectroscopy, elemental analysis, powder and single crystal X-ray diffraction, SQUID magnetometry and ab initio calculations. We find that the contact ion-pair shows inferior SMM properties to the separated ion-pair, as expected, due to faster Raman and quantum tunneling of magnetization relaxation processes. The experimental verification of the predicted crossover point for equatorial coordination in this work indicates that programs like AtomAccess have significant potential to boost efficiency in exploratory synthetic chemistry.

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The hexagonal-bipyramidal lanthanide(III) complex [Dy(O t Bu)Cl(18-C-6)][BPh 4 ] (1; 18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane ether) displays an energy barrier for magnetization reversal (U eff ) of ca. 1000 K in a zero direct-current field. Temperature-dependent X-ray diffraction studies of 1 down to 30 K reveal bending of the Cl−Ln−O t Bu angle at low temperature. Using ab initio calculations, we show that significant bending of the O−Dy−Cl angle upon cooling from 273 to 100 K leads to a ca. 10% decrease in the energy of the excited electronic states. A thorough exploration of the temperature and field dependencies of the magnetic relaxation rate reveals that magnetic relaxation is dictated by five mechanisms in different regimes: Orbach, Raman-I, quantum tunnelling of magnetization, and Raman-II, in addition to the observation of a phonon bottleneck effect.