When H2L (alpha,omega-bis((1,3-dimethyl-5-nitrosouracil-6-yl)amino)propane) in hydroethanolic basic medium is reacted with a copper(II) salt, the complex [Cu(L)(H2O)].3H2O (1) is obtained. This complex can be used as ligand for the preparation of heterodinuclear complexes. Thus, reaction of 1 with either [Ni(CTH)](ClO4)2 or a neutral aqueous solution of [Ni(OH)(Me3[12]N3)]2(ClO4)2 gives rise to the complexes [Cu(ClO4)(mu-L)Ni(CTH)](ClO4).H2O (2) and [Cu(ClO4)(mu-L1)Ni(Me3[12]N3)](ClO4).H2O (3) (H2L1 is obtained from H2L by elimination of a CO molecule from one of the pyrimidine rings, leading to an imidazole ring in 3; Me3[12]N3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene and CTH = d,l-5,5,7,12,12,14-hexamethyul-1,4,8,11-tetrazacyclotetradecane). The structures of these complexes have been solved by X-ray crystallographic methods. Complex 1 crystallizes in the triclinic system, space group P1BAR, with cell constants a = 8.208(1) angstrom, b = 10.468(1) angstrom, c = 13.335(2) angstrom, alpha = 81.74(1)-degree, beta = 81.42(1)-degree, gamma = 76.61(1)-degree and Z = 2. The structure consists of neutral [Cu(L)(H2O)] molecules and three lattice water molecules, which are involved in an extended network of hydrogen bonds. The CuN4O coordination polyhedron exhibits a distorted square-pyramidal geometry, in which two amino and two oximato nitrogen atoms at about 2 angstrom form the basal plane whereas the oxygen atom from a water molecule occupies the apical position at a longer distance of 2.284(2) angstrom. Crystals of 2 are monoclinic, space group P2(1)/c, with cell constants a = 18.046(3) angstrom, b = 12.541(1) angstrom, c = 19.471(4) angstrom, beta = 101.08(1)-degree, and Z = 4. The structure of 2 consists of one [Cu(ClO4)(mu-L)Ni(CTH)]+ cation, one perchlorate anion, and one nonbonded lattice water molecule. In the dinuclear cation, the coordination of the precursor 1 to the [Ni(CTH)]2+ moiety takes place through an exocyclic oxygen atom of the uracil ring and a syn-anti nitroso-oximato group disordered on two sets of crystallographic positions, one with a 78% occupancy and the other with a 22% occupancy. The intramolecular Cu...Ni distance is 4.972(1) angstrom. The Cu(II) ion occupying the inside site of the L2-ligand exhibits a distorted square-pyramidal coordination environment. The basal plane comprises of two amino and one oximate nitrogen atoms and either an oxygen (occupancy 0.78) or a nitrogen atom (occupancy 0.22) from the other oximato group. The fifth position is occupied by a disordered oxygen atom, from a perchlorate anion with occupancy factors of 0.56 and 0.44, respectively. The nickel(II) ion is in a distorted octahedral geometry. Crystals of 3 are orthorombic, space group Pna2(1), with cell constants a = 20.046(5) angstrom, b = 10.462(2) angstrom, c = 35.397(7) angstrom and Z = 8. The asymmetric unit of 3 contains two crystallographic independent heterodinuclear cations [Cu(ClO4)(mu-L1)Ni(Me3[12]N3)]+, two nonbonded perchlorate anions, and two lattice water molecules. Within each heterodinuclear cation copper(II) and nickel(II) are bibridged by one diatomic syn-syn group and one O-monatomic nitroso-oximato group from the pyrimidine and imidazole rings, respectively. The angle at the bridging oxygen are 120.1(8) and 119.3(7)-degrees for each one of the molecules whereas the intramolecular copper-nickel distances are 3.440(4) and 3.452(4) angstrom. The coordination geometry around each copper(II) is distorted square-pyramidal CuN3O2. The basal plane comprises of two amino and one oximato nitrogens and one oximato oxygen, whereas an oxygen atom of a disordered perchlorate anion occupies the apical position. The Ni(II) ion is in a distorted square pyramidal NiN3O2 environment. The basal plane is defined by two oximato oxygens and two nitrogens from the Me3[12]N3 ligand, the remaining nitrogen atom occupying the apical position. Magnetic susceptibility measurements in the temperature range 4.2-298 K revealed the occurrence of a weak intermolecular magnetic exchange interaction between copper(II) ions in 1 (2J = -2.3 cm-1), whereas 2 and 3 were found to exhibit a strong antiferromagnetic interaction between copper(II) and nickel(II) ions with J values of 123.6 cm-1 and larger than 500 cm-1, respectively. For 2, below 50 K only the S = 1/2 state is thermally populated, whereas for 3, the spin exchange interaction causes a complete or nearly complete spin coupling even at room temperature. The room temperature powder EPR spectrum of 2 exhibits a very large and intense signal independent of the temperature at approximately g = 2.2 and a broad signal at approximate g = 4.0, which vanishes when the temperature decreases. This is interpreted as being due to an axial ZFS in the excited quartet state. The powder EPR spectrum of 3 does not exhibit any signal at about g = 4, which may indicate that only the S = 1/2 state is thermally populated at room temperature, this is in good agreement with the magnetic data. The difference between the exchange coupling observed for 2 and 3 is discussed in terms of structural differences in the bridging network.
