IntroductionThe supramolecular chemistry of anions in solution1and in the solid state,2and in particular their molecular recognition properties and role in self­assembly processes,3is an area of continuing interest and research effort. Pyrrolic and poly­pyrrolic anion receptors are an important subset of anion recognition agents. These systems may be charged4or neutral5and coordinate anionsviahydrogen bond donation from a number of pyrrole rings6or from pyrrole in combination with other hydrogen bond donor groups.7Whilst these receptors are extremely efficient and selective in their anion binding properties, simple pyrroles interact only weakly with anions in solution.8In order to prepare an anion complex of pyrrole itself wehave crystallized tetramethylammonium chloride from pyrrole. The resulting complex has been crystallographically characterized, revealing the formation of pyrrole NH⋯anion hydrogen bonds in the solid state. Whilst the crystal structure of pyrrole reveals the formation of NH⋯π hydrogen bonds,9we believe this is the first time a hydrogen bonded anion complex of a single pyrrole ring has been synthesized and characterized.The asymmetric unit of structureI, shown inFig. 1, is comprised of a single pyrrole, co­crystallized with half a chloride anion and a partial tetramethylammonium cation situated on the mirror plane of the space group.Asymmetric unit and numbering scheme ofI(i = −x, 1 − y,z − 1/2).The supramolecular structure of the crystal depicted inFig. 2shows the chloride anion to be hydrogen bonded to two pyrrole heterocycles, in addition to a presumably weaker interaction to the tetramethylammonium cation, as demonstrated by the intermolecular separations inTable 1.Coordination environment around the chloride anion (i = 1/2 − x, 1/2 − y, 1/2 + z; ii = −1/2 + x, 1/2 − y, 1/2 + z; iii = x, 1 − y, 1/2 + z).Hydrogen bond and donor–H distances and angles inID–H⋯Ad(D–H)/Åd(H⋯A)/Åd(D⋯A)/Å∠(D–H⋯A)/°Symmetry operations: i = 1 − x, 2 − y, −1/2 + z; ii = x + 1/2, −y + 3/2,z + 1/2; iii = 1/2 − x, 1/2 − y, 1/2 + z.N1–H1N⋯Cl1i0.918(4)2.327(4)3.241(3)174(4)C5ii–H5Aii⋯Cl10.9802.713(4)3.656(7)161(3)C7–H7B⋯Centroidiiipyrrole0.9802.600(5)3.433(5)143(4)Comparison of the pyrrole–chloride interactions inIwith those observed in the chloride complex ofmeso­octamethylcalix[4]pyrrole8(CCDC code = TEQKIJ9) reveals a marked similarity in the donor–acceptor separation [i.e.it is in the range 3.264(7)–3.331(7) Å in the macrocyclic complex and is 3.241(9) Å inI] and also in the angle between adjacent donors and the anion [inmeso­octamethylcalix[4]pyrrole it is 60.36(8)°, compared to 61.73(15)° inI]. Furthermore, a C–H⋯π interaction is observed between a methyl proton of the tetramethylammonium group and the ring centroid of the pyrrole, a similar interaction to the NH⋯π hydrogen bond observed in the crystal structure of pyrrole10and a CH⋯π interaction in the dichloromethane solvate ofmeso­tetraspirocyclohexylcalix[4]pyrrole.8The dihedral angle of 61.73(15)° allows the two coordinating pyrrole units to form a cleft in which the tetramethylammonium group is accommodated. Investigation of the supramolecular structure of the extended solid shows the tetramethylammonium molecules to be positioned between two pyrrole–chloride–pyrrole complex clefts, giving rise to a structure reminiscent of a honeycomb framework depicted inFig. 3. When viewed parallel to thecaxis, the cavities produced by the encapsulating pyrrole framework are occupied by two tetramethylammonium moieties arranged in a head-to-tail fashion.Honeycomb­like array, parallel to thecaxis, adopted by the supramolecular structure. Colour code: pyrrole red, tetramethylammonium blue, chloride green. Click image orhereto access a 3D representation. (The 3D structure shows a degree of disorder for the tetramethylammonium moieties.)