Conformation and coordination of β-diketonates in mixed-ligand copper(II) chelates of 1,2-diamines. Theoretical approach and experimental verification. Crystal structure of [bis(N,N′-dimethyl-1,2-diaminoethane) bis(3-cyano-2,4-pentanedionate)copper(II)]di

The reaction of the bis(2,4-pentanedionato)copper(II), Cu(acac)2, and its substituted derivatives (Cu(NC-acac)2, Cu(O2N-acac)2, and Cu(tfac)2), with 1,2-diaminoethane (en), 1,2-diaminopropane (pn) and certain N-substituted derivatives of 1,2-diaminoethane, enR, is reported. The reaction products were found to depend on the reaction conditions, the extent of N-substitution of the diamine and the nature of the β-diketonate anion, β−. The [Cu(enR)β2] addition compounds are not always sufficiently stable and in most instances the tetragonal species [Cu(enR)2β2] prevail as the final product. The 1,2-diamine molecules in [Cu(enR)2β2] form chelate rings attaining the gauche conformation while the β-diketonato anions essentially confer electrical neutrality. Density functional theory (DFT) calculations suggest that the active sites of the β-diketonato anion and its conformation depend on their ability to create hydrogen bonds and on the substituents in 3-position. The Fukui indices of chemical reactivity favor the carbonyl oxygens as binding sites of most anions while in the case of the 3-cyano-2,4-pentanedionato anion, NC-acac−, bonding through the cyano nitrogen is envisaged with the S(E, Z) conformer having the lowest energy. These findings are in accord with experimental data and further proof is given by single crystal X-ray diffraction analysis of the structure of [Cu(MeenMe)2(NC-acac)2] · 2H2O (MeenMe denoting N,N′-dimethyl-1,2-diaminoethane). In this compound the NC-acac− attains the S(E, Z) conformation and interacts with the square-planar entity [Cu(MeenMe)2]2+ through the cyano nitrogen, while intermolecular hydrogen bonding involving the water molecules leads to supramolecular structure.

The products of the reaction of the bis(2,4-pentanedionato)copper(II), Cu(acac)2, and cognate compounds, Cuβ2, with 1,2-diaminoethane (en), and certain of its N-substituted derivatives have been investigated and are discussed in the light of DFT calculations. Further, the crystal structure of [Cu(Me-en-Me)2(NC-acac)2] · 2H2O is reported.Figure optionsDownload as PowerPoint slide