Dinuclear copper(II) complexes of two homologous pyrazine-based bis(terdentate) diamide ligands

Article abstract of DOI:10.1002/ejic.200400908

The 1:1 reactions of the new bis(terdentate) diamide ligand N, N′-bis(2-pyridylmethyl)pyrazine-2,5-dicarboxamide (H₂L ¹) and its higher homologue N,N′-bis[2-(2-pyridyl)ethyl] pyrazine-2,5-dicarboxamide (H₂L²) with Cu(BF ₄)₂·4H₂O in the absence of added base have consistently afforded dicopper(II) complexes of the doubly deprotonated ligands (L¹)^2- and (L²)^2-. The complex [Cu^II₂(L²)(H₂O) ₂(MeCN)₂](BF₄)₂ (2a) has been structurally characterised. Subsequently, reactions employing a correct stoichiometric 2:1 metal-to-ligand ratio in MeCN have afforded bulk samples of the dinuclear complexes. The compounds [Cu^II₂(L ¹)(MeCN)₂(H₂O)₂)(BF ₄)₂·H₂O (1a·H₂O) and [Cu^II₂(L²)(H₂O)₄(BF ₄)₂]-2H₂O (2b-2H₂O) have been structurally characterised. While complex 1a-H₂O of the lower ligand homologue exhibits very weak antiferromagnetic spin coupling (J = -0.24 cm ^-1), complex 2b of the higher ligand homologue exhibits very weak ferromagnetic spin coupling (J = +0.67 cm^-1). EPR studies have been carried out on polycrystalline powders and frozen DMF solutions of 1a-H ₂O and 2b·2H₂O. The EPR spectra of the polycrystalline powders indicate the presence of dipolar broadening and weak intermolecular exchange, while those of the frozen DMF solutions are characteristic of dipolar-coupled Cu^II pairs within the dinuclear molecules, with no evidence of intraor intermolecular exchange. The spectral simulations confirm that the binuclear structure and the Cu...Cu distances are retained in frozen solution. Dinuclear SiF₂^2- containing compounds, [Cu^II₂(L¹)(H₂O) ₄](SiF₆) (1b) and [[Cu^II₂(L ¹)(H₂O)₂(μSiF₆))·4H ₂O]_∞ (1c-4H₂O). were obtained serendipitously, in nearly quantitative yield, by the 2:1 reaction of Cu(BF ₄)₂· 4H₂O with H₂L ¹ in H₂O. The unexpected SiF₆^2- anions were generated in the course of the reaction by partial hydrolysis of the BF₄^- anions employed, thus forming traces of HF which reacted with the glassware.