Hydrogen Bonded Complexes with Tee Aa*dd, Aa*ddd, and Aaa*dd Motifs: Ted3 Role of Three Centered (bifurcated) Hydrogen Bonding

The stabilities of hydrogen bonded complexes containing the AA,DD, AA.DDD, and AAAeDD motifs were measured in chloroform. X-ray analysis of the l-6 and l-7 complex and solution studies support the formation of an unsymmetrical bifurcated hydrogen bonding motif. Many factors affect the stability of multiply hydrogen bonded complexes,1 including the number? strength, and geometry of the individual hydrogen bonds, and the extent of solvent competition for the donor and acceptor sites. Recent attention was focussed on the importance of the arrangement of the hydrogen bonds. In the context of base-pairing, it was suggested that both A-T and G-C base-pairs benefit from “n-cooperativity” wherein the cyclic arrangement of hydrogen bonds allows each to strengthen the other (Figure lA).s xCooperativity favors the alternating arrangement of hydrogen bonds found in AD-DA and DAD-ADA complexes. Another type of polarization, shown in Figure lB, would disfavor AADD and AAAeDDD complexes. Recently, Jorgensen noted that ADA-DAD complexes contain four repulsive secondary electrostatic interactions, whereas AADaDDA and MA-DDD complexes contain zero and four attractive secondary electrostatic interactions, respectively (Figure lC).4 We reported several new multiply hydrogen bonded complexes, including the first example of a AAA.DDD complex, whose bsoc were consistent with the secondary electrostatic interaction model.5 Herein we report on the role of extra “over-hanging” donor and acceptor group& in hydrogen bonded complexes with the AA.DDD and A&Y-DD motifs (i.e., 1.3 and 5.2). A B C +.7. . ” *’ ‘N -\yf+ A +-index -4 +-X +-‘. .f A --+ 0 +-X +-X +-+4 Figure 1. A. rc-cooperativity. B. Polarization of hydrogen bonds in a DDD4AA complex. C. Jorgensen’s secondary electrostatic interaction m&L , primary hydrogen bond, _, attractive, and a-------w , repulsive secondary electrostatic interaction. With the exception of 5,’ the compounds used in this study (l-4, Chart) were prepared by known methods.* All compounds gave a correct elemental analysis, and had spectral data