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

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-H...O 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-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.