Dinuclear, tetranuclear and one-dimensional pyrazine-based copper(II) complexes: preparation, X-ray structure and magnetic properties

The preparation, crystal structures and magnetic properties of the copper(II) complexes of formula [Cu-2(tppz)(H2O)(2)(CF3SO3)(4)] (1), [Cu(tppz)(CrO4)](n)center dot 3nH(2)O (2) and [Cu-4(tppz)(4)(H2O)(4)(MoO4)(2)](CF3SO3)(4)center dot 7H(2)O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl) pyrazine] are reported. Complex 1 is a dinuclear species where tppz adopts a bis-tridentate bridging mode with an intramolecular copper-copper separation of 6.5221(6) A. Each copper(II) ion in 1 has an elongated octahedral geometry with three tppz nitrogen atoms and a water molecule in the equatorial positions and two triflate oxygen atoms occupying the axial sites. Complex 2 is a uniform copper(II) chain where the copper atoms are bridged by two oxygen atoms of the chromate group (mu(1,2)-CrO4), the value of the intrachain metal-metal separation being 4.2614(5) angstrom. Each copper(II) ion in 2 exhibits a somewhat distorted square pyramidal geometry with three nitrogen atoms from a tridentate tppz ligand and a chromate oxygen atom in the basal plane and an oxygen atom of a symmetry-related chromate group in the apical position. Complex 3 is a cyclic tetranuclear compound where peripheral [Cu(tppz)(H2O)](2+) units are connected through two molybdate groups, each one adopting the m1,1,2-MoO4 bridging mode. Regular alternating five [Cu(1) and Cu(4)] and six [Cu(2) and Cu(3)] coordinated copper(II) ions occur in 3 with distorted square pyramidal CuN3O2 and elongated octahedral CuN3O3 surroundings, respectively. The four copper atoms in 3 have in common the presence of a molybdate-oxygen and three tppz-nitrogen atoms in the basal [Cu(1) and Cu(4)]/equatorial [Cu(2) and Cu(3)] planes and a water molecule in the apical [Cu(1) and Cu(4)]/one axial [Cu(2) and Cu(3)] positions whereas the remaining axial position is occupied by a molybdate-oxygen [Cu(2) and Cu(3)]. The shorter intramolecular copper-copper separation is 3.371(1) angstrom [Cu(2)center dot center dot center dot Cu(3)], the other ones varying in the range 5.537(1)-6.301(1) angstrom. The investigation of the magnetic properties of 1-3 in the temperature range 1.9-295 K has shown the occurrence of intermediate antiferro-[J = -48.0(1) cm(-1) with (H) over cap = -J (S) over cap (1)center dot(S) over cap (2) (1)], weak ferro- [J = +0.28(1) cm(-1) with (H) over cap = -J Sigma(i) (S) over cap (i)center dot(S) over cap (i) + I (2)] and coexistence of weak ferro-and antiferromagnetic interactions [J(1) = + 2.38(2) cm(-1), J(2) = +0.56(2) cm(-1) and J(3) = -1.53(2) cm(-1) with (H) over cap = -J(1) ((S) over cap (2)center dot(S) over cap (3)) + J(2) ((S) over cap (1)center dot(S) over cap (3) + (S) over cap (2)center dot(S) over cap (4)) + J(3) ((S) over cap (1)center dot(S) over cap (2) + (S) over cap (3)center dot(S) over cap (4)) (3)] between the copper(II) ions across bis-tridentate tppz (1), mu(1,2)-CrO4 (2), di-mu(1,1)-MoO4 and mu(1,2)-MoO4 (3) bridges. The values of the magnetic interactions are analyzed in terms of simple orbital symmetry considerations and compared with those reported for parent ystems.