AAA?DDD Triple Hydrogen Bond Complexes

Experiment and theory both suggest that the AAA−DDD pattern of hydrogen bond acceptors (A) and donors (D) is the arrangement of three contiguous hydrogen bonding centers that results in the strongest association between two species. Murray and Zimmerman prepared the first example of such a system (complex 3•2) and determined the lower limit of its association constant (Ka) in CDCl3 to be 105 M−1 by 1H NMR spectroscopy (Murray, T. J. and Zimmerman, S. C. J. Am. Chem. Soc. 1992, 114, 4010−4011). The first cationic AAA−DDD pair (3•4+) was described by Bell and Anslyn (Bell, D. A. and Anslyn, E. A. Tetrahedron 1995, 51, 7161−7172), with a Ka > 5 × 105 M−1 in CH2Cl2 as determined by UV−vis spectroscopy. We were recently able to quantify the strength of a neutral AAA−DDD arrangement using a more chemically stable AAA−DDD system, 6•2, which has an association constant of 2 × 107 M−1 in CH2Cl2 (Djurdjevic, S., Leigh, D. A., McNab, H., Parsons, S., Teobaldi, G., and Zerbetto, F. J. Am. Chem. Soc. 2007, 129, 476−47...