Crystal structure of (diethyl ether-κO)[5,10,15,20-tetrakis(2-isothiocyanatophenyl)porphyrinato-κ4 N]zinc diethyl ether solvate

The synthesis and crystal structure of 5,10,15,20-tetrakis α,α,α,α 2-isothiocyanatophenyl zinc(II) porphyrin are reported. The crystal structure consists of discrete porphyrin complexes that are located on a twofold rotation axis with the ZnII cation in a square-pyramidal coordination environment defined by the porphyrin N atoms at the basal sites and a diethyl ether molecule at the apical site.


Chemical context
Isothiocyanates serve as versatile starting materials for a variety of functional groups (Batey & Powell, 2000;Ding et al., 2011;Serra et al., 2014;Guo et al., 2010;Shin et al., 2000;Kosurkar et al., 2014;Alizadeh et al., 2016;Rao et al., 2015). Included in porphyrin scaffolds, isothiocyanates may serve as precursors for the synthesis of tetratopic ligands with fourfold symmetry. In the case where all four ortho-substituents of the meso-phenyl groups face the same side of the porphyrin plane, these porphyrins are denominated picket fence porphyrins. These compounds are widely used as model compounds for hemoproteins (Collman et al., 1975;Tabushi et al., 1985;Schappacher et al., 1989). With a bulky ortho-substituent and Zn II as the central metal cation, the rotational barriers are sufficiently high to isolate the different atropisomers (Freitag & Whitten, 1983). A variety of picket fence porphyrins has been reported (Collman et al., 1975;Mansour et al., 2017;Cormode et al., 2006;Le Maux et al., 1993;Wuenschell et al., 1992). In most cases, amides are used as functional groups in the ortho-positions of the meso-phenyl groups, which hampers further functionalization. The title compound now opens new avenues for the synthesis of functionalized picket fence porphyrins and is a promising starting material for the design of anion binding ligands. The title compound can be obtained in one step using a method reported by Jha et al. (Fig. 1), starting from the all-isomer of the amino derivative we have published previously (Jha et al., 2007;Leben et al., 2018). It is important to note that the reaction has to be carried out at 273 K, because at room temperature a mixture of the atropisomers is obtained. After dissolving the tetrakis(isothiocyanatophenyl) porphyrin in acetone and precipitating with diethyl ether, single crystals were obtained, which were characterized by single crystal X-ray diffraction.

Structural commentary
The asymmetric unit of the title compound, Zn(C 48 H 24 N 8 S 4 )(C 4 H 10 O)ÁC 4 H 10 O, comprises one Zn II cation, one half of the porphyrin molecule and one half of a coordinating diethyl ether molecule as well as one half of a diethyl ether solvate molecule. The complex porphyrin molecule and the coordinating diethyl ether molecule are located on a twofold rotation axis whereas the solvent diethyl ether molecule is in a general position and is equally disordered around a twofold rotation axis (Fig. 2). The four isothiocyanate substituents of the phenyl groups at the meso-positions point to the same side of the porphyrin moiety, which proves that the tetra-isomer has formed. The porphyrin plane is close to planar with a maximum deviation from the mean plane of 0.276 (3) Å . The phenyl rings are rotated out of the porphyrin plane by 63.16 (5) and 82.06 (6) . The Zn II cation is fivefold coordinated by the four N atoms of the porphyrin molecule in the basal positions and by one O atom of a diethyl ether molecule in the apical position, leading to a distorted squarepyramidal coordination environment (  Table 1). The Zn II cation is located 0.4052 (9) Å out of the mean porphyrin plane and is shifted towards the coordinating diethyl ether molecule (Fig. 4).

Supramolecular features
In the crystal structure of the title compound, each two discrete complexes form centrosymmetric pairs with the coordinating diethyl ether molecules pointing in opposite directions (Fig. 5)  The molecular entities of the title compound with the atom labelling and displacement ellipsoids drawn at the 50% probability level. Only one orientation of the disordered diethyl ether solvent is given. [Symmetry code: (i) Àx + 2, y, Àz + 3 2 .] Table 1 Selected geometric parameters (Å , ).

Figure 1
Reaction scheme for the synthesis of the title compound.
molecules, in which the disordered diethyl ether solvate molecules are embedded (Fig. 5). There are no notable intermolecular interactions between the molecular moieties in the crystal structure.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-H hydrogen atoms were positioned with idealized geometries (C-H = 0.95-0.99 Å ; methyl H atoms of the coordinating diethyl ether molecule were allowed to rotate but not to tip) and were refined with Molecular structure of the discrete complex in a view parallel to the porphyrin plane.  U iso (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model. The O atom of the diethyl ether solvate molecule is not located exactly on the twofold rotation axis and thus the complete molecule is equally disordered over two sets of sites because of symmetry. Therefore for each atom the occupancy was set to 0.5, and atoms were treated with SADI and SIMU commands (Sheldrick, 2015b) to achieve similar displacement ellipsoids.