Bond cooperativity effects, which are typical of `resonant' chains or rings of π-conjugated hydrocarbons, can also occur in hydrogen-bonded systems in the form of σ-bond and π-bond cooperativity or anticooperativity. σ-Bond cooperativity is associated with the long chains of O—H...O bonds in water and alcohols while σ-bond anticooperativity occurs when the cooperative chain is interrupted by a local defect reversing the bond polarity. π-Bond cooperativity is the driving force controlling resonance-assisted hydrogen bonds (RAHBs), while π-bond anticooperativity has never been considered so far and is investigated here by studying couples of hydrogen-bonded β-enolone and/or β-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—H...O...H—O and N—H...O...H—N bonds formed, but not the asymmetric O—H...O...H—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.
