Linear pentanuclear nickel(II) and tetranuclear copper(II) complexes with pyrazine-modulated tripyridyldiamine ligand: Synthesis, structure and properties

Abstract

By using tripyridyldiamine ligand, N,N’-di(pyrazin-2-yl)pyridine-2,6-diamine (H2dpzpda), linear pentanuclear nickel(II) [Ni5(µ5-dpzpda)4Cl2] (1) and tetranuclear copper(II) [Cu4(Hdpzpda)2(CH3COO)6] (2) complexes were first synthesized and structurally characterized. This pentanickel linear metal chain is helically wrapped by four syn-syn-syn-syn type dpzpda2- ligands. There are two types of NiNi distances existing in the complex. The terminal NiNi distances bonded to the axial ligand are longer (2.3877(8) Å) affected by the axial ligands. The inner NiNi distances are very short and remain constant (2.3071(6) Å). Two terminal Ni(II) ions bonded to the axial ligands are in a square-pyramidal (NiN4Cl) environment and exhibit long NiN bonds (2.097(4) Å), which are consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions display short NiN (1.8915(8)–1.903(4) Å) bond distances, which are consistent with a square planar (NiN4), the diamagnetic arrangement of a low-spin Ni(II) configuration. Temperature-dependent magnetic research revealed an antiferromagnetic interaction between the two terminal atoms through a superexchange pathway along metal cores with a parameter of about −51.0 cm–1. Observation of the first oxidation peak in the cyclic voltammograms of 1 at +1.206 V revealed that this complex is the most resistant to oxidation among all known pentanuclear nickel strings with unmodulated and modulated oligo-α-pyridylamido ligands. The structure of 2 consists of four copper atoms linearly placed with a copper(II) acetate dimer in the centre. In complex 2 ligand H2dpzpda coordinates to the Cu(II) atoms as a tetradentate ligand in an all anti conformation. Copper complex 2 is built to a 1-D supramolecular chain structure through intermolecular hydrogen bond and strong π-π interaction (3.19 Å) between the pyridine and pyrazine rings. From the magnetic susceptibility data, 2 shows strong antiferromagnetic coupling, which was satisfactorily fitted using Hamiltonian H = J1(S1S2 + S3S4) + J2S2S3. The best fitting result were obtained with the parameters J1 = 10.64 cm−1, J2 = −175.36 cm−1, g = 2.11.