Redetermination of cyclo-tetrakis(μ-5,10,15,20-tetra-4-pyridylporphyrinato)tetrazinc(II) dimethylformamide octasolvate trihydrate at 100 K

The structure of the title compound, [Zn4(C40H24N8)4]·8C3H7NO·3H2O, has been redetermined at 100 K. The redetermination is of significantly higher precision and gives further insight into the disorder of pyridyl groups and solvent molecules. The molecules of (5,10,15,20-tetra-4-pyridylporphyrinato)zinc(II) (ZnTPyP) form homomolecular cyclic tetramers by coordination of a peripheral pyridyl group to the central Zn atom of an adjacent symmetry-related molecule. The tetramer so formed exhibits molecular S 4 symmetry and is located about a crystallographic fourfold rotoinversion axis. Severely disordered dimethylformamide and water molecules are present in the crystal, the contributions of which were omitted from refinement. Intermolecular C—H⋯N hydrogen bonding is observed.

The structure of the title compound, [Zn 4 (C 40 H 24 N 8 ) 4 ]Á-8C 3 H 7 NOÁ3H 2 O, has been redetermined at 100 K. The redetermination is of significantly higher precision and gives further insight into the disorder of pyridyl groups and solvent molecules. The molecules of (5,10,15,20-tetra-4-pyridylporphyrinato)zinc(II) (ZnTPyP) form homomolecular cyclic tetramers by coordination of a peripheral pyridyl group to the central Zn atom of an adjacent symmetry-related molecule. The tetramer so formed exhibits molecular S 4 symmetry and is located about a crystallographic fourfold rotoinversion axis. Severely disordered dimethylformamide and water molecules are present in the crystal, the contributions of which were omitted from refinement. Intermolecular C-HÁ Á ÁN hydrogen bonding is observed.
The small dark red plate-shaped crystals of the title compound were subjected to diffraction experiments using a Bruker AXS X8 PROSPECTOR diffractometer equipped with an INCOATEC microfocus X-ray source (IµS) for Cu radiation (Graf, 2008). Such microfocus X-ray sources use multilayer mirrors to focus the X-ray beam onto the crystal and, therefore, lead to a significant reduction of the background and an increase in diffracted intensities. It has already been demonstrated that the Mo IµS gives data of significantly higher quality than a 2 kW Mo fine focus sealed tube, when small crystals are examined (Schulz et al., 2009). The data collection presented here, using the Cu IµS, resulted in intensity data of surprisingly good quality and, hence, indicated a re-refinement of the crystal structure. The crystals investigated in the original work were significantly larger than those examined in the present study and split on cooling to 100 K. For this reason, the data were collected at 200 K with a Cu rotating anode system at that time. Using small crystals has the advantage that these are less likely to split on flash cooling.
The molecular structure of [ZnTPyP] 4 is depicted in Fig. 1. The asymmetric unit contains one ZnTPyP unit (Fig 2.) and the S 4 symmetric tetramer is generated by crystallographic fourfold rotoinversion symmetry. One peripheral pyridyl group binds to the central Zn atom of an adjacent symmetry related ZnTPyP unit. Zn1 is pentacoordinated and is displaced from the N 4 mean plane by 0.3196 (9) Å. The coordination geometry parameters about Zn1 are given in Table 1. The three remaining pyridyl groups are non-coordinating. Even at 100 K, the pyridyl groups attached to C5 and C15 show elongated ellipsoids, which cause a checkCIF B level alert (Spek, 2009) due to large U eq (max)/U eq (min) ratio. This reveals that the disorder is rather of static than dynamic nature. Attempts were made to describe the electron density of the pyridyl ring attached to C15 (Fig. 3) by a split model. However, the refinement results could not be improved thereby. Thus, both pyridyl rings were finally described with large displacement parameters.
In the crystal, the [ZnTPyP] 4 entities are stacked into columns located at x = 1/4, y = 1/4 and x = 3/4, y = 3/4 (Fig 4). The stacking propagates via C β -H···N py interactions (see Table 2) by translational symmetry in the c axis direction. Within a column, the distance between the centroids of the pyridyl rings attached to C5 and C15 iii is 4.0714 (1) Å. Adjacent columns of [ZnTPyP] 4 are arranged with an offset of c/2 (ca 7.49 Å). Interstitial channels are formed parallel to the c axis direction centred at x = 1/4, y = 3/4 and x = 3/4, y = 1/4 ( Despite intensive efforts, the disordered solvent molecules filling the voids within the columns of [ZnTPyP] 4 and the interstitial channels could not be modeled reasonably with the data collected at 100 K. Nevertheless, residual electron density was visible in a difference Fourier synthesis calculated for the solvent regions ( Fig. 6) with phases based on the model using COOT (Emsley et al., 2010). For the visualization of the surface of the (difference) electron density using a three-dimensional mesh, the electron densities should be read into COOT in terms of structure factors. To obtain a structure factor (.fcf) file containg the informations necessary for the calculation of electron density maps and suitable for COOT, the LIST 6 instruction of SHELXL-97 was used. The atomic model of the framework was read into COOT by means of the SHELXL-97. res file. The visual inspection of the difference electron density map indicates that four molecules of dimethylformamide (DMF) plus one water molecule are located within the voids in the columns approximately centred at (1/4,1/4,0), whereas another four molecules of DMF and two water molecules are clustered around the 4 2 screw axes running through the interstitial channels parallel to the c axis direction. The compound can, therefore, probably best be described as The compound was originally formulated as being a pure DMF solvate (Seidel et al., 2010).
To improve the fit of the model to the data and, hence, the precision of the main part of the structure, the contributions of the disordered solvent molecules were removed from the diffraction data with PLATON / SQUEEZE (van der Sluis & Spek, 1990;Spek, 2009). SQUEEZE estimated the electron counts in the voids within the columns and interstitial channels of [ZnTPyP] 4 to be 182 and 207, respectively. These values are relatively close to those based on the proposed chemical formula (178 and 196).

Experimental
Small dark red plate-shaped crystals of the title compound were obtained similarly as reported previously (Seidel et al., 2010); 12 mg of ZnTPyP (Aldrich) and 11 mg of [Pd(NO 3 ) 2 (en)] (en = 1,2-diaminoethane) were placed in an ampoule and 4 ml of DMF were added. The ampoule was sealed and placed in a heater. The sample was heated to 150 °C in 24 h and held for five days at this temperature. Subsequently, the sample was cooled down to room temperature in 100 h. Noteworthy, the crystals of the title compound were accompanied by crystals of the triclinic phase, containing a polymeric one-dimensional ladder structure of ZnTPyP, as observed previously (Seidel et al., 2010).

Refinement
For the final refinement, the contributions of severely disordered DMF and water molecules of crystallization were removed from the diffraction data with PLATON / SQUEEZE (van der Sluis & Spek, 1990;Spek, 2009), see comment. H atoms were placed at geometrically calculated positions and refined with constrained C-H bond length of 0.95 Å and U iso (H) = 1.2 U eq (C) allowing them to ride on the parent C atom.       -tetrakis(µ-5,10,15,20-tetra-4-pyridylporphyrinato)