Interesting copper(ii)-assisted transformations of 2-acetylpyridine and 2-benzoylpyridine

Abstract

The reactions of various copper(II) sources with 2-acetylpyridine, (py)(me)CO, and 2-benzoylpyridine, (py)(ph)CO, under strongly basic conditions have been studied and novel ligand transformations have been discovered. Reaction of Cu(ClO₄)₂·6H₂O and (py)(me)CO in the presence of NBu₄ⁿOMe (1 : 1 : 1) in CHCl₃ gave a mixture of [Cu₂Cl₂(HL_A)₂](ClO₄)₂ (1) and [Cu₂Cl₂(L_B)₂(ClO₄)₂] (2), where HL_A is 3-hydroxy-1,3-di(pyridin-2-yl)-butane-1-one and L_B is the zwitterionic-type ligand 3-hydroxy-1-methyl-3-(pyridin-2-yl)-3H-indolizin-4-ium. The ligand HL_A is formed through an aldol reaction-type mechanism, while the formation of L_B takes place via an intramolecular nucleophilic attack of the remote 2-pyridyl nitrogen atom on the positive carbonyl carbon of HL_A, after the transformation of the latter through deprotonation and dehydration. The Cu^II ions in 1 are bridged by two 2.1111 HL_A ligands resulting in a long Cu^II⋯Cu^II distance (5.338 Å); the metal ions in 2 are triply bridged by the alkoxide oxygen atoms of the two 2.21 L_B ligands and one 2.1100 perchlorato group. The absence of α-hydrogens in (py)(ph)CO leads the reactivity of this ligand in the presence of Cu^II to different pathways. The Cu(ClO₄)₂·6H₂O/(py)(ph)CO/NBu₄ⁿOMe reaction mixture in MeOH/H₂O (25 : 1 v/v) gave the dinuclear cationic complex [Cu₂{(py)(ph)CO}₂(L_C)₂](ClO₄)₂ (3), where L_C⁻ is the anion of (methoxy)(phenyl)(pyridin-2-yl)methanol formed in situ via the nucleophilic addition of MeO⁻ to the carbonyl carbon of (py)(ph)CO upon Cu^II coordination. The Cu^II ions in the cation are doubly bridged by the deprotonated oxygen atoms of the two L_C⁻ ligands. Replacement of Cu(ClO₄)₂·6H₂O with Cu(NO₃)₂·3H₂O and NBu₄ⁿOMe with NMe₄OH and the decrease of the H₂O concentration in the above reaction system yielded the tetranuclear coordination cluster [Cu₄(OMe)₂(NO₃)₄{(py)(ph)CO}₂(L_C)₂] (4). The Cu^II centres in this complex define a parallelogram. Two parallel sides of the parallelogram are each supported by deprotonated oxygen atoms belonging to a 2.21 L_C⁻ ligand and a 2.2 MeO⁻ group. The metal ions that define each of the other two sides are singly bridged by an oxygen atom of a 2.210 nitrato group. No bridging exists between the Cu^II ions that define the two diagonals of the parallelogram. Replacement of MeOH with EtOH in the reaction system that gave 4 resulted in the dinuclear complex [Cu₂(NO₃)₂(L_D)₂)(EtOH)] (5), L_D⁻ being the anion of (ethoxy)(phenyl)(pyridin-2-yl)methanol. The Cu^II ions are doubly bridged by the alkoxide oxygen atoms of the two 2.21 L_D⁻ ligands. The 1 : 1 : 1 Cu(NO₃)₂·3H₂O/(py)(ph)CO/NMe₄OH reaction system in CH₃NO₂ gave the dinuclear complex [Cu₂(NO₃)₂(L_E)₂] (6), where L_E⁻ is the anion of 2-nitro-1-phenyl-1-(pyridin-2-yl)ethanol. The OH⁻ ion abstracts one of the methyl hydrogens of CH₃NO₂, and once the carbanion ⁻:CH₂NO₂ is formed it attacks the positive (δ+) carbonyl carbon of (py)(ph)CO; as the carbanion forms the new C–C bond, the π electrons of the carbonyl group of the original ligand are transferred completely to oxygen forming the alkoxide-type ligand L_E⁻. The Cu^II ions are doubly bridged by the alkoxide oxygen atoms of the two 2.21 L_E⁻ ligands. Simplified mechanistic views of the Cu^II-assisted formation of the transformed ligands are proposed. Dc magnetic susceptibility studies in the 2–300 K range for the representative complexes 3–6 reveal the presence of very strong antiferromagnetic Cu^II⋯Cu^II exchange interactions in the dinuclear complexes 3, 5, and 6 and within the dimeric {Cu₂(OMe)(NO₃){(py)(ph)CO}(L_C)}⁺ subunits of 4. The strong antiferromagnetic coupling is discussed in terms of the large Cu–O–Cu angles (101.0–102.9°) in the dinuclear, planar {Cu₂O₂} units/subunits of 3–6.