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

David A. Leigh,Alexandra M. Z. Slawin,Patrick I. T. Thomson,Craig C. Robertson

doi:10.1021/ja404504m

Abstract

The X-ray crystal structure of a previously reported extremely strong quadruple NH···N AAAA-DDDD hydrogen-bond array [5·4] (K(a) = 1.5 × 10(6) M(-1) in CH3CN; K(a) > 3 × 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···N hydrogen bonds with CH···N interactions (complex [5·6]), but only reduces the association constant in CH3CN by 2 orders of magnitude (K(a) = 2.6 × 10(4) M(-1) in CH3CN; K(a) > 1 × 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···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.

Highlights

While individual hydrogen bonds are generally weak and have short lifetimes, their enthalpy of formation is additive, meaning that multipoint hydrogen-bonding arrays[1] can very effectively hold together supramolecular assemblies and materials.[2]Secondary electrostatic interactions between adjacent hydrogen bonds have a significant influence on the stability of a supramolecular complex.[3]
The four NH···N hydrogen bonds are all close to linear, in contrast to the staggered arrangement observed in the X-ray structure of an AAA-DDD hydrogenbond array.4d The phenyl groups of 5 are slightly twisted away from the plane formed by the fused pyridine rings (Figure 2b)
The N−H groups of the benzimidazole groups of DDDD partner 4 still partake in short, close-to-linear hydrogen bonds despite being in five-membered rings which are not optimal for presenting the NH groups parallel to each other

Summary

■ INTRODUCTION

While individual hydrogen bonds are generally weak and have short lifetimes, their enthalpy of formation is additive, meaning that multipoint hydrogen-bonding arrays[1] can very effectively hold together supramolecular assemblies and materials.[2]. C−H groups are generally much weaker hydrogen-bond donors than hydrogens bound to heteroatoms, CH···N/O interactions can play important roles in molecular recognition and assembly processes,[6] including protein−protein interactions,[7] anion recognition,[8] and extended crystal lattices.[9] We recently reported[5] an extremely strong (Ka > 3 × 1012 M−1 in CH2Cl2) AAAA-DDDD quadruple hydrogen-bonding array ([5·4], Figure 1) based on four NH···N intercomponent hydrogen bonds.[10] Here we report the X-ray crystal structure of [5·4] and the effect on the strength of binding of the quadruple hydrogen-bond array of replacing two of the NH···N hydrogen bonds with CH···N interactions

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES