ALTERNATING FERROMAGNETIC AND ANTIFERROMAGNETIC INTERACTIONS IN UNUSUAL COPPER(II) CHAINS

The preparation, the X-ray crystal structure, and the magnetic properties of four 2,2'-bipyrimidine (C6H6N4, bpm)containing copper(II) complexes of formula [Cu-2(bpm)(2)(H2O)(2)(OH)(2)(NO3)(2)].4H(2)O (1), [Cu-2(bpm)(OH)(2)-(NO3)(2)].2H(2)O (2), [Cu-2(bpm)(H2O)(2)(OH)(2)](NO3)(2) (3), and [CU2(bpm)(H2O)(2)(OH)(2)(NO3)(2)].2H2O (4) are reported. Crystallographic data are as follows: 1, triclinic system, space group P $($) over bar$$ 1, a = 7.703(1) Angstrom, b = 7.885(1) Angstrom, c = 11.172(2) Angstrom, alpha = 87.29(1)degrees, beta = 86.85(1)degrees, gamma = 78.17(1)degrees, Z = 1, V = 662.7(2) Angstrom(3); 2, monoclinic system, space group C2/c, a = 33.619(7) Angstrom, b = 13.040(3) Angstrom, c = 7.040(1) Angstrom, beta = 101.90(3)degrees, Z = 8, V = 3019.8(10) Angstrom(3); 3, monoclinic system, space group P(2)1/n, a = 9.316(2) Angstrom, b = 8.286(2) Angstrom, c = 9.693(2) Angstrom, beta = 90.52(2)degrees, Z = 2, V = 748.2(3) Angstrom(3); 4, triclinic system, space group P $($) over bar$$ 1, a = 7.564(2) Angstrom, b = 8.032(2) Angstrom, c = 8.238(2) Angstrom, alpha = 61.33(2)degrees, beta = 88.66(2)degrees, beta = 88.66(2)degrees, gamma = 75.97(2)degrees, Z = 1, V = 423.5(2) Angstrom(3). The structure of 1 is made up of discrete, centrosymmetric bis(mu-hydroxo) copper(II) dimers with bpm as terminal ligand, weakly coordinated water molecules, unidentate nitrate anions, and molecules of water of crystallization. The copper environment in 1 is distorted octahedral with two bridging hydroxo groups and two nitrogen atoms of bpm building:the equatorial plane whereas two oxygen atoms from water and nitrate groups occupy the axial positions. The Cu(1)-OH-Cu(1a) bridging angle is 95.7(1)degrees, and the intramolecular Cu(1)...Cu(1a) separation is 2.881(1) Angstrom. The structures of 2-4 consist of cationic one-dimensional chains of copper(II) ions alternatively bridged by bpm and two hydroxo groups. The electroneutrality is achieved through unidentate (2 and 4) and uncoordinated (3) nitrate groups. Coordinated (3 and 4) and crystallization (2 and 4) water molecules are also present. The metal environment in 2 and 3 is square pyramidal, whereas it can be described as distorted octahedral in 4. Copper atoms in 2-4 have in common the occurrence of two nitrogen atoms of bpm and two oxygen hydroxo groups in the equatorial plane, whereas the axial positions are occupied by oxygen atoms of nitrate (2), of water (3), and of both ligands (4). The angles at the hydroxo bridge for 2-4 vary in the range 95.0(1)-96.1(2)degrees, and they are very close to that of 1. The metal-metal separations through the double-hydroxo bridge [2.886(1) (2), 2.854(1) (3), and 2.860(1) Angstrom (4)1 are much shorter than that through bis-chelating bpm [5.471(1) and 5.474(1) (2), 5.461(2) (3), and 5.452(2) Angstrom (4)]. The magnetic properties of 1-4 have been investigated in the temperature range 4.2-300 K. A strong intramolecular ferromagnetic coupling (114 cm(-1) for the singlet-triplet energy gap) is observed in 1. Alternating antiferromagnetic (-J = 145-135 cm(-1)) interactions through the bpm bridge and ferromagnetic (alpha J = 160-97. 5 cm(-1)) interactions through the hydroxo bridge are obtained for 2-4 by analyzing the magnetic susceptibility data with the Hamiltonian ($) over cap H = -J Sigma(($) over cap S-2i($) over cap S-2i-1 - alpha ($) over cap S-2i($) over cap S-2i+1). These values are discussed in the light of the structural features and correlated with previously reported bpm- and double hydroxide-bridged copper(II) complexes.