What happens when (1H-benzimidazol-2-ylmethyl)-N-phenyl amine is added to copper(II) acetate? Spectroscopic, magnetic, and DFT studies

On performing a reaction between (1H-benzimidazol-2-ylmethyl)-N-phenyl amine (H₂L) and copper(II) acetate in methanol, a green "paddle-wheel" copper(II) complex, [Cu₂(CH ₃COO)₄(H₂L)₂] (1), was separated after about 5 min. The two Cu atoms are separated by 2.852 Å and bridged by four bidentate acetate groups with Cu-O distance of 2.012-2.043 Å as calculated by DFT/B3LYP method. As the reflux continues, the reaction mixture was rapidly turned brown, and a polymeric complex, {[Cu₂(CH ₃COO)₃(H₂O)₂(HL)]}_n (2), was separated. In complex 2, the benzimidazole ligand serves as a bridge between dimeric Cu₂(CH₃COO)₃(H₂O) ₂ units via the benzimidazole moiety. The anionic effect was established by comparing complexes (1) and (2) with [Cu(H₂L) ₂(OH₂)₂Cl]·Cl (3). Variable temperature magnetic susceptibility measurements of compound 2 showed occurrence of large anti-ferromagnetic interactions caused by the presence of syn-syn conformation of bridged acetate groups in terms of direct interaction, and π-π super exchange pathway of the benzimidazolate moiety.