Crystal structure of (methanol-κO)[5,10,15,20-tetrakis(2-aminophenyl)porphyrinato-κ4 N]zinc(II)–chloroform–methanol (1/1/1)

The synthesis and crystal structure of 5,10,15,20-tetrakis α,α,α,α 2-aminophenyl zinc(II) porphyrin is reported, which is a potent starting material for the synthesis of several other picket fence porphyrins.

In the crystal structure of the title compound, [Zn(C 44 H 32 N 8 )(CH 3 OH)]Á-CHCl 3 ÁCH 3 OH, the Zn II cation is coordinated by four porphyrin N and one methanol O atom within a slightly distorted square-pyramidal environment and is shifted out of the porphyrin plane towards the direction of the methanol molecule. The methyl group of the coordinating methanol molecule is disordered over two sets of sites. The porphyrin backbone is nearly planar and the phenyl rings are almost perpendicular to the porphyrin plane. As is typical for picket-fence porphyrins, all four ortho substituents of the mesophenyl groups (here the amino groups) are facing to the same side of the porphyrin molecule. In the crystal structure, two neighbouring porphyrin complexes form centrosymmetric dimers that are connected via O-HÁ Á ÁN hydrogen bonding. With the aid of additional N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonding, these dimers are stacked into columns parallel to [010] that are finally arranged into layers parallel to (001). Between these layers channels are formed where chloroform solvent molecules are located that are connected to the porphyrin complexes by weak C-HÁ Á ÁCl hydrogen bonding. There are additional cavities in the structure where some small residual electron density is found, indicating the presence of disordered methanol molecules, but a reasonable model could not be refined. Therefore the contribution of the electron density associated with the methanol solvent molecule was removed with the SQUEEZE procedure [Spek (2015). Acta Cryst. C71, 9-18] in PLATON. Nevertheless, the given chemical formula and other crystal data take into account the methanol solvent molecule.

Chemical context
Picket-fence porphyrins have been widely used as model compounds for the investigation of oxygen binding to hemoproteins (Collman et al., 1975(Collman et al., , 1976Tabushi et al., 1985;Schappacher et al., 1989). With bulky substituents in the orthopositions of the meso-substituents, their rotation is hindered, leading to only one side of the porphyrin being accessible for axial coordination in the all-isomer. In 1973, Collman et al. for the first time reported this behaviour on the prototype picket-fence porphyrin 5,10,15,20-tetrakis ,,, 2-pivalamidophenyl porphyrin (Collman et al., 1973). Afterwards, the first crystal structure of a picket-fence porphyrin was published (Collman et al., 1975). Since that time, several different substituted picket-fence porphyrins have been reported (Collman et al., 1983(Collman et al., , 1998Lee et al., 2010;Yu et al., 2015). In general, there is a risk of isomerization to the other atropisomers, but with the incorporation of zinc(II) the rotational barrier for the meso-substituents is increased, as ISSN 2056-9890 reported by Freitag & Whitten (1983). Therefore, harsher reaction conditions could be used to introduce substituents in the ortho-positions without atropisomerization. We became interested in this class of compounds as receptors for oxo anions. We synthesized the title compound in a four-step synthesis using 2-nitrobenzaldehyde and pyrrole as starting material ( Fig. 1) as the key precursor for further functionalizations. Surprisingly, no crystal structure of this compound has been reported. We inserted Zn II into the porphyrin to stabilize its planar geometry and thus to prevent atropisomerization. Single crystals could be obtained from a methanol/chloroform solution of the zinc(II) porphyrin complex, and were characterized by single-crystal X-ray diffraction.

Structural commentary
The asymmetric unit of the solvated title compound, [Zn(C 44 H 32 N 8 )(CH 3 OH)]ÁCHCl 3 ÁCH 3 OH, consists of one Zn II cation, one substituted porphyrin, one methanol, as well as one chloroform solvent molecule, all of them located in general positions (Fig. 2). The contribution of an additional methanol solvent molecule to the electron density was removed with the SQUEEZE procedure in PLATON (Spek, 2015). The methyl group of the methanol molecule is disordered over two positions and was refined using a split model. All four amino groups are located on the same side of the porphyrin moiety, which shows that the ,,, isomer was obtained. The porphyrin backbone is nearly planar, the largest deviation from the mean least-squares plane amounts to 0.189 (3) Å . All phenyl rings are nearly perpendicular to the porphyrin plane, with dihedral angles of 85.86 (9), 74.90 (7), 67.75 (6) and 85.17 (7) .
The zinc(II) cation is coordinated by four porphyrin N atoms that are located in the basal plane, and the metal coordination is completed by the O atom of a methanol molecule in apical position leading to an overall squarepyramidal environment (Fig. 3). The Zn-N distances range from 2.050 (2) to 2.060 (2) Å and correspond to literature values ( Table 1). As expected, the apical Zn-O distance of 2.143 (2) Å is slightly longer ( Table 1). All angles around the Zn II cation scatter between 88.96 (8) and 89.73 (8) for basal groups and between 92.93 (8) and 98.66 (9) involving the apical group, which shows that the coordination polyhedron is slightly distorted ( Table 1). The Zn II cation is located 0.1876 (9) Å above the mean plane formed by Zn1, N1, N2, N3 and N4 and is shifted towards the direction of the methanol O atom.

Supramolecular features
In the crystal structure of the title compound, each two neighbouring porphyrin complexes form dimers that are located on centers of inversion. The methanol molecules are directed into the cavity of the dimer and are linked to the symmetry-related complex by intermolecular O-HÁ Á ÁN hydrogen bonding (Fig. 4, Table 2). These dimers are stacked into columns extending parallel to [001] (Fig. 4). The columns are connected by weak N-HÁ Á ÁN and additional C-HÁ Á ÁN interactions into layers parallel to (001). Between the layers channels are formed, in which the chlorofom solvate molecules are embedded. The solvent molecules are linked to the porpyhrine complexes by intermolecular C-HÁ Á ÁCl hydrogen bonding (Fig. 4, Table 2).

Figure 3
Molecular structure of a discrete complex in a view into the porphyrin plane. The disordered methyl group is shown with the major component.
2-aminophenyl porphyrin, a crystal structure was published by Zimmer et al. (2002). A crystal structure for the tetrakis ,,, 2-aminophenyl porphyrin has not been reported so far.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-H hydrogen atoms were treated with calculated positions (methyl H atoms were allowed to rotate but not to tip) and were refined with U iso (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model with C-H = 0.95 Å for aromatic and 0.98 Å for methyl H atoms. The N-H and O-H hydrogen atoms were located in a difference map. Their bond lengths were set to ideal values, and finally they were refined with fixed bond lengths of N-H = 0.88 Å and O-H = 0.84 Å with U iso (H) = 1.5U eq (O,N) using a riding model. The methyl group of the methanol molecule is disordered over two sets of sites and was refined using a split model with restraints for the bond lengths (SADI). After initial refinement of the s.o.f. it was fixed at 60:40 in the final refinement cycles. There were two weak residual electron density peaks that are located near centres of inversion, indicating for a disordered methanol solvent molecule. However, a reasonable structural model could not be refined and therefore the contribution of this molecule to the electronic density data was removed with the SQUEEZE procedure in PLATON (Spek, 2015). The volume of the solventaccessible voids amounts to 68.7 Å 3 , and the number of electrons within the voids to 16.2, indicating that one methanol molecule per formula unit is present. The given chemical formula and other crystal data take into account this methanol solvent molecule.   SHELXL2014 (Sheldrick, 2015); molecular graphics: XP (Sheldrick, 2008) and DIAMOND (Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010). 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (