Crystal structures and magnetic properties of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz)-containing copper(II) complexes

Four new copper(II) complexes of formula [Cu2(tppz)(dca) 3(H2O)]·dca·3H2O (1), [Cu 5(tppz)(N3)10]n (2), {[Cu 2(tppz)-(N3)2][Cu2(N 3)6]}n (3), and [Cu(tppz)(N3) 2]·0.33H2O (4) [tppz = 2,3,5,6-etrakis(2-pyridyl)pyrazine and dca = dicyanamide anion] have been synthesized and structurally characterized by X-ray diffraction methods. The structure of complex 1 is made up of dinuclear tppz-bridged [Cu 2(tppz)(dca)3(H2O)]+ cations, uncoordinated dca anions, and crystallization water molecules. The copper-copper separation across bis-terdentate tppz is 6.5318(11) Å. Complex 2 is a sheetlike polymer whose asymmetric unit contains five crystallographically independent copper(II) ions. These units are building blocks in double chains in which the central part consists of a zigzag string of copper atoms bridged by double end-on azido bridges, and the outer parts are formed by dinuclear tppz-bridged entities which are bound to the central part through single end-on azido bridges. The chains are furthermore connected through weak, double out-of-plane end-on azido bridges, yielding a sheet structure. The intrachain copper-copper separations in 2 are 6.5610(6) Å across bis-terdentate tppz, 3.7174(5) and 3.8477(5) Å across single end-on azido bridges, and from 3.0955(5) to 3.2047(7) Å across double end-on azido bridges. The double dca bridge linking the chains into sheets yields a copper-copper separation of 3.5984(7) Å. The structure of 3 consists of centrosymmetric [Cu2(tppz)(N3) 2]2+ and [Cu2(N3)6] 2- units which are linked through axial Cu⋯N(azido) (single end-on and double end-to-end coordination modes) type interactions to afford a neutral two-dimensional network. The copper-copper separations within the cation and anion are is 6.5579(5) Å (across the bis-terdentate tppz ligand) and 3.1034(6) Å (across the double end-on azido bridges), whereas those between the units are 3.6652(4) Å (through the single end-on azido group) and 5.3508(4) Å (through the double end-to-end azido bridges). The structure of complex 4 is built of neutral [Cu(tppz)(N3) 2] mononuclear units and uncoordinated water molecules. The mononuclear units are grouped by pairs to give a rather short copper-copper separation of 3.9031(15) Å. The magnetic properties of 1-4 have been investigated in the temperature range 1.9-300 K. The magnetic behavior of complexes 1 and 4 is that of antiferromagnetically coupled copper(II) dimers with J = -43.7 (1) and -2.1 cm-1 (4) (the Hamiltonian being Ĥ = -JŜA·ŜB). An overall ferromagnetic behavior is observed for complexes 2 and 3. Despite the structural complexity of 2, its magnetic properties correspond to those of magnetically isolated tppz-bridged dinuclear copper-(II) units with an intermediate antiferromagnetic coupling (J = -37.5 cm-1) plus a ferromagnetic chain of hexanuclear double azido-bridged copper(II) units (the values of the magnetic coupling within and between the hexameric units being +61.1 and +0.0062 cm-1, respectively). Finally, the magnetic properties of 3 were successfully analyzed through a model of a copper(II) chain with regular alternating of three ferromagnetic interactions, J1 = +69.4 (across the double end-on azido bridges in the equatorial plane), J2 = +11.2 (through the tppz bridge), and J3 = +3.4 cm-1 (across the single end-on azido bridge).