Three one-dimensional systems with end-to-end dicyanamide bridges between copper(ii) centres: structural and magnetic properties

Abstract

The preparation, crystal structures and magnetic properties of three different copper(II) chains of formula [Cu(pyim)(H₂O)(dca)]_n(NO₃)_n (1), [Cu(dpa)(dca)₂]_n (2) and [Cu(bpa)(dca)₂]_n (3) [pyim = 2-(2-pyridyl)imidazole, dca = dicyanamide anion, dpa = 2,2′-dipyridylamine and bpa = 1,2-bis(4-pyridyl)ethane] are reported. A chain structure with single, symmetrical, end-to-end dca bridges is found in compound 1. This bridging mode, with two short, equatorially coordinated Cu–N bonds has not been previously observed in the [Cu(dca)X_n] (X = coligand) family of compounds. The copper atom in 1 has a distorted square pyramidal geometry with a bidentate pyim ligand and two nitrile nitrogen atoms from two dca groups in the equatorial positions and a water molecule occupying the axial site. In compound 2, the single dca bridge is of the asymmetric end-to-end type. The copper environment in 2 is also distorted square pyramidal with a bidentate dpa ligand and two nitrile nitrogen atoms (one from a terminally bound dca and the other from a bridging dca) in the equatorial positions and a nitrile nitrogen atom from another bridging dca occupying the axial site. Compound 3 also features a uniform chain structure. In this case double, asymmetric end-to-end dca bridges and a bis-monodenate bpa bridge in the gauche conformation connect the neighbouring copper atoms. The copper(II) coordination geometry in 3 is elongated octahedral with two trans coordinated bpa nitrogen atoms and two dca nitrile nitrogens in the equatorial sites and two nitrile nitrogen atoms from two other dca groups occupying the axial positions. The copper–copper separations across the dca bridges are 7.7011(9) (1), 7.689(2) (2) and 7.095(3) Å (3). Magnetic susceptibility measurements for 1–3 in the temperature range 1.9–290 K reveal the occurrence of weak intrachain antiferromagnetic interactions [J = −0.35 (1), −0.10 (2) and −0.22 cm⁻¹ (3) the Hamiltonian being defined as Ĥ = −J Σ_iŜ_i·Ŝ_{i + 1}]. The efficiency of the dca bridge to mediate magnetic interactions in copper(II) complexes is analyzed and discussed in the light of the available magneto-structural data.