[Pd(Fmes)2(tmeda)]: A Case of Intermittent C[BOND]H⋅⋅⋅F[BOND]C Hydrogen-Bond Interaction in Solution

Authors

  • Dr. Camino Bartolomé,

    1. IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid (Spain), Fax: (+34) 983-423013
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  • Dr. Fernando Villafañe,

    1. IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid (Spain), Fax: (+34) 983-423013
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  • Dr. Jose M. Martín-Alvarez,

    1. IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid (Spain), Fax: (+34) 983-423013
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  • Dr. Jesús M. Martínez-Ilarduya,

    1. IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid (Spain), Fax: (+34) 983-423013
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  • Prof. Dr. Pablo Espinet

    Corresponding author
    1. IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid (Spain), Fax: (+34) 983-423013
    • IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid (Spain), Fax: (+34) 983-423013
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  • Fmes=2,4,6-tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′-tetramethylethylenediamine.

Abstract

The X-ray structure of the title compound [Pd(Fmes)2(tmeda)] (Fmes=2,4,6-tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′-tetramethylethylenediamine) shows the existence of uncommon C[BOND]H⋅⋅⋅F[BOND]C hydrogen-bond interactions between methyl groups of the TMEDA ligand and ortho-CF3 groups of the Fmes ligand. The 19F NMR spectra in CD2Cl2 at very low temperature (157 K) detect restricted rotation for the two ortho-CF3 groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen-bond energy shows that it is too weak (about 7 kJ mol−1) to account for the rotational barrier observed (ΔH=26.8 kJ mol−1), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.

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