A Triple-Decker Heptadecanuclear (CuII)15(CrIII)2 Complex Assembled from Pentanuclear Metallacrowns

Reaction of the pentanuclear Cu-II metallacrown [Cu-5(ahpha)(4)](ClO4)center dot 4H(2)O (ahpha(2-) is the dianion of 3-amino-3-(hydroximino)propanehydroxamic acid), with Cr(C2O4)(3)(3-) led to the formation of a heptadecanuclear complex {[Cu-5(ahpha)(4)](3)-[Cr(C2O4)(3)](2)center dot 4H(2)O}-8H(2)O center dot 1/3(DMF) (1.8H(2)O center dot 1/3(DMF)). This compound contains three stacked Cu-5(ahpha)(4)(2+) building blocks, linked by axial bonds between Cu2+ ions of one Cu-5 metallacrown and hydroxamate oxygen atoms of the neighboring Cu-5 unit. Two Cr(C2O4)(3)(3-) anions are bonded to the two lateral Cu-5(ahpha)(4)(2+) cations through axial Cu-O(oxalate) bonds. The formation of 1 may be considered the first example of metallacrown trimerization caused by anion metathesis. The compound contains 10 x 13 angstrom voids (about 25% of crystal volume), filled with solvate water molecules. The magnetic properties (chi T-M vs. T) could be fitted as the superposition of the magnetism of 3 chi T-M(Cu-5(II)) and 2 chi T-M(Cr-III). Exchange interactions within the Cu(II)5 units were fit in the framework of a model based on the Hamiltonian H(Cu(II)5) = -2J(1)(S-1 center dot S-5 + S-2 center dot S-5 + S-3 center dot S-5 + S-4 center dot S-5) -2J(2)(S-1 center dot S-2 + S-2 center dot S-3 + S-3 center dot S-4 + S-4 center dot S-1), where S-5 represents the central Cu2+ ions' spins and the other spin operators correspond to the peripheral Cu2+ ions. With other possible interactions taken into account using a molecular field approach, the best fit corresponded to J(1) = -153(5) cm(-1), J(2) = -71(2) cm(-1) and zJ' = -0.058(4) cm(-1).