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

Abstract

The X-ray crystal structure of a previously reported extremely strong quadruple NH center dot center dot center dot N AAAA-DDDD hydrogen-bond array [5.4] (K-a 1.5 x 10(6) M-1 in CH3CN; K-a > 3 x 10(12) 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 center dot center dot center dot N hydrogen bonds with CH center dot center dot center dot N interactions (complex [5.6]), but only reduces the association constant in CH3CN by 2 orders of magnitude (K-a = 2.6 x 10(4) M-1 in CH3CN; K-a > 1 x 10(7) M-1 in CH2Cl2). Related complexes without the triazole groups range in K-a from 18 to 270 M-1 in CH3CN, suggesting that the CH center dot center dot center dot N interactions can be considered part of a strong AAAA-DDDD quadruple hydrogen-bonding array. The NH center dot center dot center dot N/CH center dot center dot center dot N AAAA-DDDD motif can be repeatedly switched "on" and "off" in CDCl3 through successive additions of acid and base.