Complementary hydrogen bonding in a new tridentate Schiff base ligand: X-ray, DFT and solution NMR studiesElectronic supplementary information (ESI) available: unit cell packing diagram for 3 (Fig. S1) and 1H NMR spectra of 3 as a function of the concentration of added H2O in CDCl3 solution (Fig. S2). See http://www.rsc.org/suppdata/nj/b3/b305946d/

Molecular Recognition: Preorganization of a Bis(pyrrole) Schiff Base Derivative for Tight Dimerization by Hydrogen Bonding

“…[37] The free-energy change for the dimerization process at 25 8C in CDCl 3 is thus exergonic and highly favorable: DG 298 = À16.6(4) kJ mol À1 . Importantly, the value of K D is three orders of magnitude higher than that measured for the monoA C H T U N G T R E N N U N G (pyrrole) system 8 [14] under identical conditions. Our interpretation of this rather intriguing result is that a) the basic geometry of the supramolecular synthon 3 is probably near-ideal for a tight two-point hydrogenbonding interaction, b) two molecular recognition motifs (as opposed to only one) lead to enhanced binding, and c) the conformation of 4 is in fact preorganized for self-assembly in this system.…”

“…The major (nonlinear) change clearly occurs in the concentration range below 0.3 m. This marked change in line-width is due primarily to the formation of the hydrogen-bonded dimer. As noted previously for the monoA C H T U N G T R E N N U N G (pyrrole) derivative 8, [14] hydrogen-bond formation in the dimeric structure localizes the pyrrole N À H protons and diminishes their exchange rate until, on the NMR timescale, the slow exchange limit is reached. At concentrations higher than 0.3 m, it is evident that further more-gradual narrowing of the line-width occurs with a constant slope.…”

“…The simplest model accounting for the self-assembly of 4 in fluid solution is one based on a dimerization process involving two monomers (M) combining to give the hydrogen-bonded dimer (D, Figure 5). If we assume that the observed chemical shift for the pyrrole N À H protons is the equilibrium-weighted average of the chemical shifts of the monomer and dimer, then a modified form of the usual dimerization equation [14] that takes into account nonspecific changes in the chemical shift with increasing solute concentration (i.e., perturbations not due to the dimerization process itself, such as monotonic changes in the dielectric constant of the medium) can be used to fit the concentration dependence of the signal (Equation (1) …”

“…For example, the monoA C H T U N G T R E N N U N G (pyrrole) Schiff base derivative N-[(1E)-1H-pyrrol-2-ylmethylene)]benzene-1,2-diamine (8) self-assembles by complementary (pyrrole)NÀH···NH 2 Ar hydrogen bonding both in solution and in the solid state. [14] Crystal engineering applications of metal-free, pyrrole-imine Schiff bases are clearly at a very early stage of development, but have the potential to yield some useful new supramolecular architectures. Herein, we report the X-ray structural characterization of 4 (a tight H-bonded dimer), NMR studies which delineate the process of self-assembly in fluid solution, and gas-phase AM1 simulations of the electronic structures of the monomeric and dimeric species.…”

“…Finally, the self-association constant K D measured from the fit of the data for the imine C À H proton resonance (Figure 6 a) is experimentally equivalent to that measured from the concentration dependence of the pyrrole N À H resonance, thus implying an internally consistent method for quantifying the self-association constant in this system. Molecular simulations: Semi-empirical quantum mechanics simulations using methods such as AM1 are not only computationally efficient, but are widely recognized as being sufficiently accurate for simulations on medium to large hydrogen-bonded supramolecular species with both strong [11,14,38] and weak [39,40] hydrogen bonds. Using this method, our computational objectives were to: 1) fully map the region of conformational space encompassing the X-ray conformations for 4 in an effort to further understand the marked nonplanarity of each monomer conformation observed in the crystal lattice, 2) determine the structures and energies of the global and local minima accessible to 4, 3) determine the in vacuo enthalpy of association of the dimer- …”