Self-recognition in a flexible bis(pyrrole) Schiff base derivative: formation of a one-dimensional hydrogen-bonded polymer

Abstract

The title Schiff base compound, N,N'-bis(pyrrol-2-ylmethylene)propane-1,2-diamine, C(13)H(16)N(4), forms an interesting supramolecular structure (a one-dimensional ladder-like polymer) in the solid state that is based on the existence of complementary intermolecular N-H.N=C hydrogen bonds between the monomer units. The polymer axis is collinear with the c axis of the orthorhombic unit cell. Quantum-chemical AM1 calculations clearly indicate that self-recognition in this system by hydrogen bonding is favoured on electrostatic grounds, since the partial atomic charge on the H atom of the pyrrole NH group (0.274 e) complements the partial atomic charge of the N atom of the C=N group (-0.239 e) on a neighbouring molecule.

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

“…As might be anticipated from a reasonably good data set, these mean distances and angles compare favorably with those reported for similar derivatives of 1. [3,6,11] The structural parameters for all four molecules (A-D) are similar, which obviates the need for further discussion of the key bond distances and angles for molecules A, C, and D.…”

“…DÀH···A Av. D···A AWIXIN [12] 7, BP 2.26 (33) 154 (3) 2.910(4) EMIHEN [11] 6, BP 2.066 (6) 166 (2) 2.942(6) EWOLUX [d], [21] 9, MP 2.198 158 2.999 EWOMAE [d], [21] 10, MP 2.172 171 3.023 QOSMAM [6] 5, BP 2.10 (3) 156 (1) 2…”

“…[10] To date, studies of the coordination chemistry of analogues of 2 have not been paralleled to the same extent by structural and spectroscopic studies of the metal-free species. However, two independent articles [11,12] published in 2003 showed, in more than passing detail, that the pyrroleimine functional group combination in 3 (Scheme 1 is a potentially useful supramolecular synthon comprising a strong H-bond donor (pyrrole N À H group) and a strong H-bond acceptor (imine nitrogen). Efficient self-recognition or selfassembly through complementary hydrogen bonding in this type of system is now known to lead to dimerization (3 a) Supporting Information for this article is available on the WWW under http://www.chemeurj.org/ or from the author.…”

“…[10] Although the simplest aromatic derivative of 1 with X = C 6 H 4 (1,2-diaminobenzene-based spacer, compound 4; Scheme 2) has been synthesized before, no crystal structure of the free base has been reported. [3,6] Surprisingly, only three derivatives of 1 have been crystallographically characterized, namely 4,5-dimethyl-N,N'-bis(1H-pyrrol-2-ylmethylene)benzene-1,2-diamine (5), [6] N,N'-bis(1H-pyrrol-2-ylmethylene)propane-1,2-diamine (6), [11] and N,N'-(1,2-cyclohexylene)bis(1H-pyrrol-2-ylmethyleneamine) (7). [3] Self-assembly occurs in 5 and 6 through hydrogen bonding, as depicted in 3/3 a (Scheme 1).…”

“…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- …”