AAAA-DDDD Quadruple Hydrogen-Bond Arrays Featuring NH···N and CH···N Hydrogen Bonds

The
X-ray crystal structure of a previously reported extremely
strong quadruple NH···N AAAA-DDDD hydrogen-bond array
[5·4] (Ka = 1.5 × 106 M–1 in CH3CN; Ka > 3 × 1012 M–1 in CH2Cl2) features four short
linear hydrogen bonds. Changing the two benzimidazole groups of the
DDDD unit to triazole groups replaces two of the NH···N
hydrogen bonds with CH···N interactions (complex [5·6]), but only reduces the association
constant in CH3CN by 2 orders of magnitude (Ka = 2.6 × 104 M–1 in
CH3CN; Ka > 1 × 107 M–1 in CH2Cl2). Related
complexes without the triazole groups range in Ka from 18 to 270 M–1 in CH3CN,
suggesting that the CH···N interactions can be considered
part of a strong AAAA-DDDD quadruple hydrogen-bonding array. The NH···N/CH···N
AAAA-DDDD motif can be repeatedly switched “on” and
“off” in CDCl3 through successive additions
of acid and base.