Ring-strain release in neutral and dicationic 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin: crystal structures of C44H26Br4N4 and C44H28Br4N4 2+·2ClO4 −·3CH2Cl2

The different degrees of ring folding of a neutral and dicationic porphyrin are described and discussed.

Two porphyrin complexes were studied to determine the effects of protonation on ring deformation within the porphyrin. The porphyrin 7,8,17,10,15,C 44 H 26 Br 4 N 4 , was selected because the neutral species is readily doubly protonated to yield a dication, which was crystallized here with perchlorate counter-ions as a dichloromethane trisolvate, C 44 H 28 Br 4 N 4 2+ Á2ClO 4 À Á3CH 2 Cl 2 . The centrosymmetric neutral species is observed to have a mild 'ruffling' of the pyrrole rings and is essentially planar throughout; intramolecular N-HÁ Á ÁN hydrogen bonds occur. In contrast, the dication exhibits considerable deformation, with the pyrrole rings oriented well out of the plane of the porphyrin, resulting in a 'saddle' conformation of the ring. The charged species forms N-HÁ Á ÁO hydrogen bonds to the perchlorate anions, which lie above and below the plane of the porphyrin ring. Distortions to the planarity of the pyrrole rings in both cases are very minor. The characterization of the neutral species represents a low-temperature redetermination of the previous room-temperature analyses [Zou et al. (1995). Acta Cryst. C51, 760-761; Rayati et al. (2008). Polyhedron, pp. 2285Polyhedron, pp. -2290, which showed disorder and physically unrealistic displacement parameters.

Chemical context
Ring folding in porphyrins has long been of interest with characteristics such as ruffling, doming and saddling resulting in strain relief about the ring. In particular, the interactions within the constrained environment of the tetra-pyrrole core predominantly affect the orientation of the pyrrole rings. Two porphyrin molecules were studied to examine the effects of protonation of the pyrrole nitrogen atoms upon the overall geometry of the porphyrin ring systems. The porphyrin: 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin (I), H 2 TPPBr 4 was adopted for this study. It readily accepts two protons forming a dicationic species (II), [H 4 TPPBr 4 ] 2+ . The neutral porphyrin (I) has previously been reported in two different, room-temperature determinations (Zou et al., 1995;Rayati et al., 2008). However, those two structures display disorder that is not present in the low-temperature determination provided herein.

Structural commentary
The neutral porphyrin (I) was found to crystallize about the center of symmetry at the origin (Fig. 1). Distinctly different, the dicationic porphyrin (II) was found to crystallize with one ISSN 2056-9890 complete porphyrin dication, two perchlorate ions and three molecules of dichloromethane solvent of crystallization in the asymmetric unit (Fig. 2). Thus, the geometry of (I) is influenced by symmetry, while the geometry of (II) is independent of such constraints. In both studies, we elected to use the metacarbon atoms of the porphyrin ring (labeled as CMn in the Figures; n = atom number) as the basis for an arbitrary mean plane for analyzing distortions.
The neutral compound (I) exhibits very mild 'ruffling' of the pyrrole rings. The two independent pyrrole rings form periplanar angles of 3.0 (3) and 6.5 (3) with the four porphyrin meta-carbon atoms (Table 1). This is largely influenced by the lack of steric hindrance of the two hydrogen atoms within the core of the porphyrin ring (Fig. 3). This lack of hindrance is also reflected in the intramolecular N-HÁ Á ÁN hydrogen bonds formed in the core that have typical DÁ Á ÁA distances Structure and labeling scheme of (I). Atomic displacement parameters are depicted at 50% probability. H atoms are depicted as spheres of an arbitrary radius. [Symmetry code: (i) Àx, Ày, Àz.]

Figure 2
Structure and labelling scheme of (II). Atomic displacement parameters are depicted at 50% probability. H atoms are depicted as spheres of an arbitrary radius. Hydrogen bonds are represented as light-blue dashed lines. Table 1 Pyrrole periplanar angles ( ).
Angles with respect to the mean four atom meta-carbon plane. A 'negative' angle represents an arbitrary orientation with the pyrrole N atom below the mean porphyrin plane. Table 2). However, these intramolecular hydrogen bonds are not well directed, as demonstrated by the relatively constrained N-HÁ Á ÁN angles. The pyrrole rings experience very little distortion, with the greatest deviation from the mean-plane being À0.018 (3) Å for CB2 (Table 3). The ruffling of the ring is reflected more so in the deviations of the bromine and ipso-carbon atoms of the phenyl groups from the mean plane (Table 4). It should be noted that due to the center of symmetry, the transannular pairs of pyrrole rings are tilted in opposite directions with respect to the mean plane.

Pyrrole Ring
Presumably this also plays a role in reducing steric hindrance of the pyrrole hydrogen atoms.
In contrast the dicationic porphyrin (II) relieves strain by adopting a 'saddled' conformation ( Fig. 4). In this fashion, steric repulsion between the four hydrogen atoms intruding on the core of the porphyrin is significantly reduced. Furthermore, due to the presence of charge-balancing perchlorate anions, each pair of transannular pyrrole nitrogen atoms form hydrogen bonds to one oxygen atom of either perchlorate anion (N1/N3Á Á ÁO21, N2/N4Á Á ÁO25, Fig. 2, Table 5).
Surprisingly, the pyrrole rings in (II) do not adopt any crystallographic symmetry. Crystallographically, each pair of rings oriented 'up' and 'down' (arbitrarily defined) form different angles with respect to the meta-carbon plane. Inspection of the structure shows that the bromo-pyrrole rings are inclined in the same fashion (we have arbitrarily defined this as 'down' or a negative periplanar angle with regards to the pyrrole nitrogen atoms with respect to the porphyrin mean plane). In contrast with (I), the pyrrole rings in (II) form angles AE30 with respect to the mean porphyrin plane (Table 1). Compared with (I) wherein one bromine atom is deformed 'above' the pyrrole plane and the other 'below', the bromine atoms in (II) are all oriented out of the mean plane of their respective pyrrole rings in the same fashion (i.e. all of the deviations from the mean pyrrole plane are negative). The atoms of the pyrrole rings are essentially co-planar with the largest deviation from the mean plane for any pyrrole atom being À0.027 (7) Å for CA4 (Table 3) Table 4 Deviations of peripheral atoms from mean meta-carbon plane for (I) and (II) (Å ).

Figure 3
View through the porphyrin plane of (I) showing ring 'ruffling'. H atoms, except pyrrole H atoms, have been omitted for clarity. Table 2 Hydrogen-bond geometry (Å , ) for (I).

Figure 4
View through the porphyrin plane of (II) demonstrating ring 'saddling'. H atoms, except pyrrole H atoms, have been omitted for clarity. Table 5 Hydrogen-bond geometry (Å , ) for (II). Comparing bond distances around the neutral and dicationic porphyrin ring systems reveals little change in the bond distances associated with the pyrrole rings or backbone (see CIF files). Thus, in either a neutral or charged state the porphyrin consists largely of delocalized bonds, rather than the single-bond/double-bond formalism.

Supramolecular features
The neutral compound (I) packs with typical van der Waals contacts. Potential close contacts from C16 to the pyrrole of an adjacent molecule have the shortest heavy-atom contact around 3.45 Å .
In contrast, compound (II) is formed with hydrogen bonds from the pyrrole nitrogen atoms to perchlorate oxygen atoms (Fig. 2, Table 5 for details). Remaining intermolecular contacts throughout the structure are all usual van der Waals interactions.

Database survey
Inspection of the Cambridge Structure Database (Version 5.38 plus 1 update; Groom et al., 2016) Zou et al., 1995) are room-temperature determinations of the low-temperature structure (I) reported herein. Examination of those two structures reveals several underlying problems, such as disorder and unreasonable atomic displacement parameters that are not present in this study. The third compound that incorporates H 2 TPPBr 4 is a co-crystallant with C60 fullerene (TUBPAJ: Karunanithi & Bhyrappa, 2015). To the best of our knowledge, the dicationic species (II) has not been structurally characterized in any form.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 6. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. Cbound hydrogen atoms were included in geometrically calculated positions. N-bound hydrogen atoms were initially located from a difference Fourier map and subsequently   (Flack, 1983) Absolute structure parameter -À0.032 (14) included using a riding model. U iso (H) = 1.2U eq (C/N); C-H distances were set at 0.95 Å and N-H set at 0.88 Å for (I) and (II). Due to the age of the data and an infelicity in data archiving, only the printed structure-factor tables and final residuals file were available. Data were reconstituted from these tables into an h k l F (F) format file and the atomic models refined against these to result in the structures contained herein. It was not considered reasonable to attempt to resynthesize and recrystallize the compounds and collect new intensity data. For both compounds, data collection: MADNES (Pflugrath & Messerschmidt, 1989); cell refinement: MADNES (Pflugrath & Messerschmidt, 1989); data reduction: MADNES (Pflugrath & Messerschmidt, 1989); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010

sup-1
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.17 e Å −3 Δρ min = −1.41 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.