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Three heterometallic aggregates, [(Co II ) 2 (Gd III ) 2 ( t BuPO 3 ) 2 (O 2 C t Bu) 2 (HO 2 C t Bu) 2 (NO 3 ) 4 ]·NEt 3 (1), [(Co II ) 2 (Co III ) 2 (Gd III ) 3 (μ 3 -OH) 2 ( t BuPO 3 ) 2 (O 2 C t Bu) 9 (deaH) 2 (H 2 O) 2 ] (2), and (Co III ) 2 (Gd III ) 5 (μ 2 -OH)(μ 3 -OH) 2 ( t BuPO 3 ) 2 (O 2 C t Bu) 10 (HO 2 C t Bu)(deaH) 2 ]·MeOH (3), were successfully isolated in reactions of [Co 2 (μ-OH 2 )(O 2 C t Bu) 4 ]·(HO 2 C t Bu) 4 , Gd(NO 3 ) 3 ·6H 2 O, t Bu-PO 3 H 2 , and diethanolamine (deaH 3 ) by varying the stoichiometry of the reactants and/or changing the solvent. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. The metal-oxo core of these complexes displays a hemicubane or a defective dicubane-like view. Bond valence sum calculations and bond lengths indicate the presence of Co II centers in compound 1, mixed valent Co centers (Co II /Co III ) in compound 2, and only Co III centers in compound 3 as required for the charge balances and supported by the magnetic measurements. Magnetic studies reveal significant magnetic entropy changes for complexes 1-3 (-ΔS m values of 28.14, 25.06, and 29.19 J kg -1 K -1 for 3 K and 7 T, respectively). This study shows how magnetic refrigeration can be affected by anisotropy, magnetic interactions (ferro- or antiferromagnetic), the metal/ligand ratio, and the content of Gd III in the molecule.

Two heterometallic Co(III)-Gd(III) nanomagnets (Co2Gd6 and Co2Gd9) with defective dicubane-like cores were isolated from the same set of reactants by varying the reaction conditions. These are the first examples of cobalt(III)-gadolinium(III) phosphonate compounds and a rare class of compounds with large 4f ratio among the reported 3d-4f complexes. Magnetic studies reveal large magnetic entropy changes for both complexes (-ΔSm = 27.81 and 33.07 J kg(-1) K(-1), respectively at 3 K and 7 T).

The synthesis, structural characterization, and magnetic property studies of five new transition metal (M = Co, Ni) phosphonate-based cages are reported. Three substituted phenyl and benzyl phosphonate ligands [RPO3H2; R1 = p-tert-butylbenzyl, R2 = p-tert-butylphenyl, R3 = 3-chlorobenzyl] were synthesized and employed to seek out high-nuclearity cages. Complexes 1-3 are quasi-isostructural and feature a dodecanuclear metal-oxo core having the general molecular formula of [M12(μ3-OH)4 (O3PR)4(O2C(t)Bu)6 (HO2C(t)Bu)6(HCO3)6] {M = Co, Ni and R = R1 for 1 (Co12), R2 for 2, 3 (Co12, Ni12)}. The twelve metal centers are arranged at the vertices of a truncated tetrahedron in a manner similar to Keggin ion. Complex 4 is an octanuclear nickel phosphonate cage [Ni8(μ3-OH)4 (OMe)2(O3PR1)2 (O2C(t)Bu)6(HO2C(t)Bu)8], and complex 5 represents a pentadecanuclear cobalt phosphonate cage, [Co15(chp)8(chpH) (O3PR3)8(O2C(t)Bu)6], where chpH = 6-chloro-2-hydroxypyridine. Structural investigation reveals some interesting geometrical features in the molecular cores, which may provide new models in single molecular magnetic materials. Magnetic property measurements of compounds 1-5 indicate the coexistence of both antiferromagnetic and ferromagnetic interactions between magnetic centers for all cages.

A novel octadecanuclear copper pyrazolate-phosphonate nanocage with a bowl-shaped arrangement of the copper(II) centers in the asymmetric unit is reported. Characterization of intermediates in both solid and solution states aids to propose the mechanism of such a giant aggregation. Magnetic studies affirm the presence of antiferromagnetic interactions between the adjacent copper(II) centers. Extensive supramolecular interactions result in a framework structure.

This report describes the synthesis, characterization, and magnetic properties of two novel phosphonate-based Co(II) cages. Structural investigation reveals some interesting geometrical features in the molecular core that may provide new models in single molecular magnetic materials.