Ferromagnetism in malonato-bridged copper(II) complexes. Synthesis, crystal structures, and magnetic properties of {[Cu(H2O)(3)][Cu(mal)(2)(H2O)]}(n)and {[Cu(H2O)(4)](2)[Cu(mal)(2)(H2O)]}[Cu(mal)(2)(H2O)(2)]{[Cu(H2O)(4)][Cu(mal)(2)(H2O)(2)]} (H(2)mal = malonic acid)

Three malonato-bridged copper(II) complexes of the formulas {[Cu(H2O)(3)][C u(C3H2O4)(2)(H2O)]}(n) (1), {[Cu-(H2O)(4)](2)[Cu(C3H2O4)(2)H2O)]}[Cu(C3H2O4 )(2)(H2O)(2)]{[Cu(H2O)(4)] [Cu(C3H2O4)(2)(H2O)(2)]} (2), and [Cu(H2O)(4)][C u-(C3H2O4)(2)(H2O)(2)] (3) (C3H2O4 = malonate dianion) have been prepared, and the structures of the two former have been solved by X-ray diffraction methods. The structure of compound 3 was already known. Complex 1. crystall izes in the orthorhombic space group Pcab, Z = 8, with unit cell parameters of a 10.339(1) Angstrom, b = 13.222(2) Angstrom, and c = 17.394(4) Angstro m. Complex 2 crystallizes in the monoclinic space group P2/c, Z = 4, with u nit cell parameters of a = 21.100(4) Angstrom, b = 21.088(4) Angstrom, c = 14.007(2) Angstrom, and beta = 115.93(2)degrees Complex 1 is a chain compou nd with a regular alternation of aquabis(malonato)copper(II) and triaquacop per(LT) units developing along the z axis. The aquabis(malonato)copper(n) u nit acts as a bridging ligand through two slightly different trans-carboxyl ato groups exhibiting an anti-syn coordination mode. The four carboxylate o xygens, in the basal plane, and the one water molecule, in the apical posit ion, describe a distorted square pyramid around Cu1, whereas the same metal surroundings are observed around Cu2 but with three water molecules and on e carboxylate oxygen building the equatorial plane and a carboxylate oxygen from another malonato filling the apical site. Complex 2 is made up of dis crete mono-, di-, and trinuclear copper(YT) complexes of the formulas [Cu(C 3H2O4)(2)(H2O)(2)](2-), {[Cu(H2O)(4)] [Cu(C3H2O4)(2)(H2O)(2)]}, and {[Cu(H2 O)(4)](2)[Cu(C3H2O4)(2)(H2O)]}(2+), , respectively, which coexist in a sing le crystal. The copper environment in the mononuclear unit is that of an el ongated octahedron with four carboxylate oxygens building the equatorial pl ane and two water molecules assuming the axial positions. The neutral dinuc lear unit contains two types of copper atoms, one that is six-coordinated, as in the mononuclear entity, and another that is distorted square pyramida l with four water molecules building the basal plane and a carboxylate oxyg en in the apical position. The overall structure of this dinuclear entity i s nearly identical to that of compound 3. Finally, the cationic trimer cons ists of an aquabis(malonato)copper(ll) complex that acts as a bismonodentat e ligand through two cis-carboxylato groups (anti-syn coordination mode) to ward two tetraaqua-copper(II) terminal units. The environment of the copper atoms is distorted square pyramidal with four carboxylate oxygens (four wa ter molecules) building the basal plane of the central (terminal) copper at om and a water molecule (a carboxylate oxygen) filling the axial position. The magnetic properties of 1-3 have been investigated in the temperature ra nge 1.9-290 K. Overall, ferromagnetic behavior is observed in the three cas es: two weak, alternating intrachain ferromagnetic interactions (J - 3.0 cm (-1) and alpha J = 1. 9 cm(-1) with (H) over cap = -J Sigma(i)[(S) over cap(2i).(S) over cap(2i-1 ) + alpha (S) over cap(2i).(S) over cap(2i+1)]) occur in 1, whereas the mag netic behavior of 2 is the sum of a magnetically isolated spin doublet and ferromagnetically coupled di- (J(3) = 1.8 cm(-1) from the magnetic study of the model complex 3) and trinuclear (J = 1.2 cm(-1) with (H) over cap = -J ((S) over cap(1).(S) over cap(2) + (S) over cap(1).(S) over cap(3)) copper (II) units. The exchange pathway that accounts for the ferromagnetic coupli ng, through an anti-syn carboxylato bridge, is discussed in the light of th e available magneto-structural data.