We have demonstrated the effect of a solvent at the second coordination sphere on slow relaxation of magnetization for hepta-coordinated cobalt­(II) complexes with the formulas [Co­(H4L)­(DMF)­(H2O)]­(NO3)2·(DMF) (1), [Co­(H4L)­(MeOH)­(H2O)]­(NO3)2·(MeOH) (2), and [Co­(H4L)­(DEF)­(H2O)]­(NO3)2 (3) (H4L = 2,2′-(pyridine-2,6-diylbis­(ethan-1-yl-1-ylidene))­bis­(N-phenylhydrazinecarboxamide). Structural analysis reveals that the presence of lattice solvent molecule in 1 and 2 dramatically changes the crystal packing and noncovalent interactions as compared to 3 where no solvent molecule is present in the crystal lattice. The dc and ac magnetic susceptibility measurements reveal the presence of easy-plane magnetic anisotropy for all the complexes, and field induced slow relaxation behavior has been observed above 2 K for 1 and 2 in contrast to 3 due to the availability of the solvent molecules in the crystal lattice. The ab initio calculations further support the sign of D and the negligible effect of the first co-ordination sphere, as almost similar D values were obtained for all the complexes. The field and temperature dependence of relaxation time confirm that quantum tunnelling of magnetization (QTM) plays a major role in slow magnetic relaxation, and thermal dependence like an optical or acoustic Raman pathway is also important. To further analyze the effect of dipole–dipole interaction on slow magnetic relaxation, a dilution experiment has been performed.

We report the synthesis and structural characterization of a unique Co-MOF-1 with the formula [Co 2 (dpmndi)(bdc) 2 )]•DMF; (dpmndi = N,N′-bis(4-pyridylmethyl)-1,4,5,8-naphthalenediimide and bdc = benzene-1,4dicarboxylate) along with an isonicotinate based Co-MOF-2 having the formula of [Co 3A single crystal X-ray analysis reveals that Co-MOFs 1 and 2 exhibits (5)-connected and (8)-connected three-dimensional framework architecture having tts and hex topology, respectively. Importantly, Co-MOF-1 possess microsized cage with an internal cavity of 8.6 Å. The gas sorption analysis was also performed on both Co-MOFs 1 and 2, and results show high uptake for N 2 and CO 2 compared to H 2 and CH 4 . Moreover, both Co-MOFs 1 and 2 display high value of isosteric heat of adsorption (Q st ) for CO 2 gas. In addition, Co-MOF-1 also exhibits the promising solvatochromic behavior.

We report a new class of four dimeric Co(II) complexes [Co 2 (bbpen)(X) 2 ] (H 2 bbpen = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)ethylenediamine) [X − = SCN (1), Cl (2), Br (3), and I (4)] with different coordination geometry of two Co(II) centers (trigonal-prismatic and pseudo-tetrahedral) and their magnetic study. Interestingly, the two Co(II) centers show two different types of magnetic anisotropy. State of the art ab initio CASSCF analysis reveals that the six-coordinate or the trigonalprismatic Co(II) center possesses a consistently large negative axial zero-field splitting (negative D) parameter (∼−60 cm −1 ), while the four-coordinate or the pseudo-tetrahedral Co(II) center exhibits a range of D values from +13 to −23 cm −1 . Ab initio calculations employing the lines model were used to estimate the magnetic exchange as both the Co(II) centers possess significant magnetic anisotropy. All the complexes display rare ferromagnetic interaction, and the strength of this interaction decreases as the ligand field on the pseudo-tetrahedral Co(II) center decreases from SCN − > Cl − > Br − > I − .

A co-operative ligand approach leads to beautiful two-dimensional and three-dimensional architectures with interesting magnetic properties.