High Nuclearity (Octa‑, Dodeca‑, and Pentadecanuclear) Metal (M = CoII, NiII) Phosphonate Cages: Synthesis, Structure, and Magnetic Behavior

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(O2CtBu)6 (HO2CtBu)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 (O2CtBu)6(HO2CtBu)8], and complex 5 represents a pentadecanuclear cobalt phosphonate cage, [Co15(chp)8(chpH) (O3PR3)8(O2CtBu)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.