Acta Crystallographica Section B

Structural Science

Volume 62, Part 5 (October 2006)

research papers

Acta Cryst. (2006). B62, 850-863 [ doi:10.1107/S0108768106020933 ]

-Bond cooperativity and anticooperativity effects in resonance-assisted hydrogen bonds (RAHBs)

V. Bertolasi, L. Pretto, G. Gilli and P. Gilli

Abstract: Bond cooperativity effects, which are typical of `resonant' chains or rings of [pi] -conjugated hydrocarbons, can also occur in hydrogen-bonded systems in the form of [sigma] -bond and [pi] -bond cooperativity or anticooperativity. [sigma] -Bond cooperativity is associated with the long chains of O-HO bonds in water and alcohols while [sigma] -bond anticooperativity occurs when the cooperative chain is interrupted by a local defect reversing the bond polarity. [pi] -Bond cooperativity is the driving force controlling resonance-assisted hydrogen bonds (RAHBs), while [pi] -bond anticooperativity has never been considered so far and is investigated here by studying couples of hydrogen-bonded [beta] -enolone and/or [beta] -enaminone six-membered rings fused through a common C=O or C-C bond. The effect is studied by X-ray crystal structure determination of five compounds [(2Z)-1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione enol (1), (2Z)-1-(2-hydroxy-5-chlorophenyl)-3-phenyl-1,3-propanedione enol (2), (2Z)-1-(2-hydroxy-5-methylphenyl)-3-phenyl-1,3-propanedione enol (3), (2Z)-1-(2-hydroxy-4-methyl-5-chlorophenyl)-3-phenyl-1,3-propanedione enol (4) and dimethyl(2E)-3-hydroxy-2-{[(4-chlorophenyl)amino]carbonyl}pent-2-enedioate (5)] and by extensive analysis of related fragments found in the CSD (Cambridge Structural Database). It is shown that fusion through the C=O bond is always anticooperative and such to weaken the symmetric O-HOH-O and N-HOH-N bonds formed, but not the asymmetric O-HOH-N bond. Fusion through the C-C bond may produce either cooperative or anticooperative hydrogen bonds, the former being more stable than the latter and giving rise to a unique resonance-assisted ten-membered ring running all around the two fused six-membered rings, which can be considered a type of tautomerism never described before.

Keywords: resonance-assisted hydrogen bond; hydrogen-bond cooperativity and anticooperativity; prototropic tautomerism.


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