Chains and channels in polynuclear copper(II) complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) as bridging ligand; syntheses, crystal structures and magnetic properties

Abstract

The preparation and crystal structures for three Cu(II) polynuclear, chain complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) as bridging ligand are reported, [Cu(dpp)(H 2O) 2] n (NO 3) 2 n ·2 n/3H 2O ( 1), [Cu(dpp)(H 2O) 2] n (CF 3SO 3) 2 n ( 2), and [Cu(dpp)(H 2O) 2] n (BF 4) 2 n ·2 nH 2O ( 3). For the latter compound the crystal structure at four different temperatures have been studied. Variable-temperature magnetic susceptibility data and ESR measurements of 1– 3 and of the related copper(II) chain [Cu(dpp)(H 2O) 2] n (ClO 4) 2 n ·2 nH 2O ( 4) (whose structure was previously reported) have been performed. Compounds 1 and 2 crystallize in the same trigonal space group, R 3 ̄ c; 3 is orthorhombic, space group Pbca. Complexes 1 and 2 are built of linear dpp-bridged chains which extend along threefold screw axes. The copper atom has in each case an elongated octahedral geometry with pyrazine nitrogen atoms in axial positions. The prominent feature of the crystal packing is the supramolecular arrangement of six chains around a threefold inversion axis, creating channels housing the counter ions, and in the case of 1, also crystal water. In 3 the chain is zig–zag shaped and extends along a twofold screw axis. Counter ions and crystal water are situated in channels formed between four symmetry related chains. At room temperature (r.t.) the X-ray results show a copper ion with a compressed octahedral coordination geometry, pyrazine and pyridyl nitrogen atoms binding in equatorial and axial positions, respectively. The low temperature X-ray studies of 3 show significant changes in the copper coordination geometry, strongly suggesting that the apparent compressed geometry at r.t. is due to a dynamic Jahn–Teller distortion. The Cu⋯Cu separations across the bridging dpp at r.t. are, 7.133(1) ( 1), 7.228(1) ( 2), 7.005(1) ( 3) and 7.002(2) Å ( 4). X-band ESR spectra of 1 and 2 exhibit the pattern of Cu(II) in elongated geometry ( g ∣∣> g ⊥>2.0), whereas those of 3 and 4 exhibit inverse ( g ⊥> g ∣∣>2.0) patterns with a shoulder in the perpendicular signal. Complexes 1– 4 exhibit a Curie law behaviour with very weak intrachain antiferromagnetic coupling, the relevant magnetic parameters being J=−0.27 cm −1, g=2.11 for 1, J=−0.17 cm −1, g=2.09 for 2, J=−1.38 cm −1, g=2.15 for 3, and J=−1.36 cm −1, g=2.14 for 4 (the Hamiltonian being H ̂ =− JS A S B).