ARTICLE pubs.acs.org/crystal
Hydrogen Bonding in Pyridinium Picrates: From Discrete Ion Pairs to 3D Networks Vladimir Stilinovic* and Branko Kaitner Department of Chemistry, Laboratory of General and Inorganic Chemistry, Faculty of Science, Horvatovac 102a, University of Zagreb, 10002 Zagreb, Croatia
bS Supporting Information ABSTRACT: A series of 20 picrates of pyridine derivatives were synthesized and their crystal structures studied by X-ray diffraction. The characteristic pyridinium-picrate synthon I was found to be present in 15 structures. The geometry of the pyridiniumpicrate ion pair was found to be governed by additional C H 3 3 3 O and N H 3 3 3 O hydrogen bonds with the o-nitro groups of the picrate. If additional hydrogen donors are present on the pyridine cation, hydrogen bonds between these donors and nitro groups of picrate anions of neighboring ion pairs connect the ion pairs into 1D, 2D, or 3D supramolecular architectures. Synthon I was absent in five structures, in two cases as a result of protonation of an amino substituent on the pyridine ring, and in two cases as a result of insertion of a water molecule between the pyridinium cation and the picrate anion.
’ INTRODUCTION One of the most prominent challenges of crystal engineering is the rational synthesis of multicomponent solids.1 This can be achieved by utilizing specific interactions between molecules’ functional groups. Although many types of interactions have been used in achieving this goal, hydrogen bonding has been and has remained the most common “tool” in crystal engineering.2 The energy, and therefore the strength, of a hydrogen bond greatly depends on the ability of the hydrogen acceptor to attract the proton to itself, but also on the strength of the donor hydrogen bond. It has been demonstrated that the strongest hydrogen bonds are those that are (approximately) symmetrical, i.e., where the hydrogen atom is placed centrally between the “donor” and the “acceptor” due to the similarity of their affinities toward the proton.3 If the hydrogen acceptor is a noticeably stronger base than the (deprotonated) donor, a proton transfer to the acceptor may occur.4 This leads to a charge separation and the formation of a rather strong hydrogen bond between a protonated cation and an anion. The strength of such hydrogen bonds as well as their predictability makes them quite useful in design and synhesis of molecular solids.5 As a measure for the relative proton affinities of the donor and the acceptor atoms, the pKa values of DH and AH+ are commonly used,6 and a number of semiempirical rules for prediction of both salt/cocrystal formation, as well as the hydrogen bond strength, have been derived.7 Particularly interesting are the cases with polytopic hydrogen donors and acceptors, i.e. molecules with several potential hydrogen donors or acceptors of different acidities (or basicities). In such systems a hydrogen donor has a choice between several acceptors (and vice versa).8 Generally, the assembly of such molecules will follow the “rule” pointed out by M. Etter—the strongest donor will form a bond with the strongest acceptor, the second strongest donor with the second acceptor, etc.1a There r 2011 American Chemical Society
are, however, many exceptions in which the connectivity will not be such as would be expected, since steric or packing effects can render the expected hydrogen bonding connectivity energetically less favorable.9 The picrate anion (pic) is an example of a polytopic hydrogen acceptor. It has one phenoxy oxygen and three nitro groups in a rigid molecular configuration which can allow for specific bonding of different hydrogen acceptors. Crystallization of picric acid (Hpic) with amines will almost necessarily lead to formation of picrates, since it is a strong organic acid (pKa of 0.38 in dilute aqueous solution at 25 °C)10 which ensures proton transfer to the amine. This in turn should lead to the formation of a rather strong hydrogen bond between the protonated cation and the picrate anion, leaving at the same time the nitro groups free to interact with other potential hydrogen donors in the structure.11 If the hydrogen donor bonded to the picrate anion is a protonated pyridine, supramolecular synthon I (Scheme 1) is expected to be present. This synthon can be found in almost all structures of picrates of pyridine and its derivatives deposited in the CSD to date.12,13 Such specific bonding of pyridinium cation to picrate anion can enable further bonding of different hydrogen donors to other bonding sites on the picrate anion, leading to self-assembly of ion pairs into hydrogen bonded networks, or sometimes to multicomponent crystals, such as that obtained by cocrystallization of pyridinium picrate with 1-aminonaphthalene.14 Our study of pyridinium picrates consisted of testing the reliability of the supramolecular synthon I, as well as ascertaining its applicability for further control of supramolecular chemistry, by investigating the crystal structures of 20 picrates of pyridine Received: May 30, 2011 Revised: July 15, 2011 Published: July 20, 2011 4110
dx.doi.org/10.1021/cg200684x | Cryst. Growth Des. 2011, 11, 4110–4119
Crystal Growth & Design derivatives with substituents of varying hydrogen donating and sterical properties (Scheme 2). The results of such a study should prove valuable for better understanding of the supramolecular chemistry of not only picrates but also phenolates in general and o-nitrophenolates in particular.
’ RESULTS AND DISCUSSION The pyridine derivatives chosen for this study (Scheme 2) can be divided into three groups: molecules with no additional hydrogen donating functional groups (1 7, 19, 20), carboxylic acids and amides (8 11), and aminopyridines (12 18). These molecules cover a wide range of possibilities as a result of the varying number, position, bulkiness, and electron donating or subtracting potential of substituents, as well as hydrogen donating potential. The structures are studied from two aspects— the geometric characteristics and reliability of synthon I and the supramolecular architectures attainable in the substituited pyridine picric acid systems. Geometry of the Pyridinium Picrate Ion Pair. Overall, synthon I was found in 15 structures, and the geometric parameters describing the geometry of the synthon I are given in Table 1. This includes all the picrates of pyridine derivatives with no additional hydrogen donating functionalities, most compounds Scheme 1
ARTICLE
derived from aminopyridines (13, 14, 16, 17), and the picrates of acids 9 and 10. All of these structures comprise pyridinium picrate ion pairs (Figure 1). The average hydrogen bond distance (d(N 3 3 3 O)) is 2.63 Å, which indicates that the hydrogen bonds connecting the ion pairs are quite strong. The only compounds in which this distance is somewhat larger are 4Hpic and 9Hpic 3 H2O. In the case of 4Hpic, one might be tempted to associate this with the sterical hindrance due to the two ortho-methyl groups; however, that would be in contrast with the fact that the hydrogen bonds in other stericaly hindered pyridinium picrates (2Hpic and 19Hpic) are more than 0.1 Å shorter than those in 4Hpic. It is therefore likely that this increase in hydrogen bond length is due to specific packing effects in the crystal structure of this compound. An interesting feature apparent from Table 1 is that the dihedral angles between the planes of the cation and anion aromatic rings for six ion pairs are very low (
