HYDROGEN BONDING IN MODEL COMPLEXES OF PROTON SPONGES

Krzysztof Wozniak

Chemistry Department, The University of Warsaw, ul. Pasteura 1, 02 093 Warszawa, Poland.

Keywords: hydrogen bonding, proton sponges, weak interactions

The aromatic diamine 1,8-bis(dimethylamino)naphthalene (DMAN) is a parent molecule of a class of compounds with extremely high basicity constants (pKa values up to 16.3) and proton affinities, known as 'proton sponges' [1]. When in contact with an acid, proton sponges sequester the acidic protons and form ionic complexes with very strong [N-H...N]⁺ hydrogen bonds. Properties of hydrogen bonding in proton sponges are used in discussion of the role of hydrogen bonding in enzymatic catalysis [2,3]. The properties this unusual hydrogen bonding on electronic and structural factors will be presented in this contribution.

In particular, the results of X-ray diffraction, Solid-State NMR and Electron Spectroscopy (ESCA) studies for the series of proton sponge complexes will be discussed as well as the details of experimental electron density distribution in the model DMAN molecule and in its complex with 1,2-dichloromaleic acid [4].

The acid salt formed by DMAN with 1,2-dichloromaleic acid contains two strong, asymmetric hydrogen bonds [N-H...N]⁺ and [O-H...O]^- [4,5]. For the cation, ab initio MO calculations [6] predict an asymmetric H bond to be 25 kJ/mol more stable than a symmetric one. This is confirmed by experimental results. Directionality of the H-bond acceptor lone pairs towards the hydrogen nuclear position is observed in Laplacian distributions of the charge density in both cation and anion, obtained from X-ray and neutron diffraction data. On protonation of DMAN, density in the bonds to the carbon at the 9-position increases, while the density in the C-N bonds decreases. The properties of electron density at critical points for C-H...O interactions correlate very well with structural parameters describing these weak hydrogen boonds.

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