Crystal structure of cis-7,8-dihydroxy-5,10,15,20-tetraphenylchlorin and its zinc(II)–ethylenediamine complex

Normal mode structural decomposition (NSD) shows the title chlorin compounds to have considerable saddling deformation from planarity.

The title chlorin, 2 Ph H 2 , hydrogen-bonded to dimethylaminopyridine (DMAP), C 44 H 32 N 4 O 2 ÁC 7 H 10 N 2 , and its corresponding zinc(II) complex, 2 Ph Zn, axially coordinated to ethylenediamine (EDA), [Zn(C 44 H 30 N 4 O 2 )]ÁC 2 H 8 N 2 , were isolated and crystallized by adventitious reduction of the corresponding osmate esters by DMAP and EDA, respectively. Known since 1996 and, inter alia, used for the preparation of a wide range of (planar and non-planar) chlorin analogues (so-called pyrrole-modified porphyrins), their conformational analyses in the solid state are important benchmarks. Both macrocycles are only modestly distorted from planarity and both are slightly more non-planar than the corresponding dimethoxy-derivative, but less planar than a free-base mesopentafluorophenyl-based osmate ester. NSD analyses provide quantitative and qualitative analyses of the distortion modes. One origin of the non-planarity is presumably the avoidance of the eclipsed configuration of the two vic-cis diols on the pyrroline moiety; the resulting deformation of the pyrroline translates in some cases into the macrocycle. The structure of 2 Ph H 2 features voids making up ca 26% of the unit-cell volume filled with highly disordered solvate molecules (chloroform and hexanes). 2 Ph Zn crystallized with a 13.6 (4)% occupied solvate methanol molecule.

Chemical context
The study of synthetic chlorins as functional, spectroscopic, or structural models for nature's premiere light-harvesting pigment chlorophyll is one of the central aspects in contemporary porphyrinoid chemistry (Flitsch, 1988;Liu et al., 2018;Taniguchi & Lindsey, 2017;Lindsey, 2015). Because of the facility of the synthesis of a wide range of meso-tetraarylporphyrins, their conversion to chlorins has been widely studied (Flitsch, 1988;Taniguchi & Lindsey, 2017).
We contributed to the field the description of the OsO 4mediated dihydroxylation of meso-tetraarylporphyrins 1 Ar M, generating the corresponding chlorin diols 2 Ar M ( Fig. 1) (Brü ckner & Dolphin, 1995a;Brü ckner et al., 1998). Depending on the stoichiometric ratio of OsO 4 used and whether the porphyrin metal complex or free base is used, the reaction may also lead to the regioselective formation of tetrahydroxymetalloisobacteriochlorins or tetrahydroxybacteriochlorins, respectively (Brü ckner & Dolphin, 1995b;Samankumara et al., 2010;Hyland et al., 2012;Bruhn & Brü ckner, 2015). Chlorin diols 2 Ar H 2 have shown efficacy as photosensitizers in photodynamic therapy (Macalpine et al., 2002) or are substrates toward their oxidation to the corresponding diones (Starnes et al., 2000(Starnes et al., , 2001Daniell et al., 2003). Importantly, chlorin diols 2 Ar M are the starting materials for the generation of a wide range of planar and non-planar chlorin analogues (so-called pyrrole-modified porphyrins) (Brü ckner, 2016;Sharma et al., 2017;Hewage et al., 2019;Brü ckner et al., 2020;Luciano et al., 2020;Wu et al., 2020), whereby the parent chlorin diols 2 Ph H 2 and 2 Ph Zn generally serve as spectroscopic benchmarks. Since the conformation of a porphyrinic macrocycle greatly influences its electronic structure, the structural characterization of the benchmark compounds 2 Ph H 2 and 2 Ph Zn is important. Curiously, however, even though these fundamental compounds are known since 1996, crystals suitable for single X-ray crystal structure analyses could not be grown to date. However, related derivatives, such as osmate ester 3 F H 2 (Hewage et al., 2019), a number of tetrahydroxybacteriochlorins and isobacteriochlorins (Samankumara et al., 2010), and a number of alkylated diol free base and metal complexes 4 Ar M (M = 2H, Ni, Cu, Zn, Pd) (Samankumara et al., 2010;Sharma et al., 2017) could be structurally characterized.
In due course of working with the intermediate osmate esters and attempts to form crystals of the amine adducts, we inadvertently reduced the osmate ester and the long-sought parent free base meso-phenyl chlorin diol 2 Ph H 2 , as 2 Ph H 2 ÁDMAP hydrogen-bonded to DMAP (4-dimethylaminopyridine) and the zinc(II) complex 2 Ph Zn, in the form 2 Ph ZnÁEDA in which the metal is axially coordinated to ethylenediamine (EDA), crystallized in single-crystal X-ray diffraction quality.

Structural commentary
The structures of both 2 Ph H 2 ÁDMAP and 2 Ph ZnÁEDA confirm the cis-vic stereochemistry of the diol functionality and the near-perpendicular arrangement of the meso-phenyl groupsstructural features well known for these types of meso-arylchlorin diols (Hewage et al., 2019;Samankumara et al., 2010;Sharma et al., 2017) or meso-arylporphyrinoids, in general (Senge, 2000)  X-ray structure of 2 Ph H 2 ÁDMAP with the atom-labeling scheme for non-H atoms. 50% probability ellipsoids.

Figure 1
Synthetic pathways towards 2 Ph H 2 ÁDMAP and 2 Ph ZnÁEDA and their methoxy ethers. conformation of 2 Ph H 2 ÁDMAP using a normal mode structural decomposition (NSD) analysis (Kingsbury & Senge, 2021;Shelnutt et al., 1998) shows that its chromophore exhibits a considerable saddling distortion. In comparison, the dimethoxy derivative 4 Ph H 2 (Samankumara et al., 2010) is more planar, with only very modest distortions evenly spread over a number of distortion modes (Fig. 4a). In 4 Ph H 2 , both methoxy substituents point toward the outside, whereas the corresponding hydroxy groups in 2 Ph H 2 ÁDMAP point in opposite directions, with only the hydrogen-bonded (to DMAP) hydroxy group pointing outwards. A slight deformation of the pyrroline moiety in 2 Ph H 2 ÁDMAP alleviates the steric interactions between the two hydroxy groups [26.65 (13) O-C-C-O torsion angle] that would be otherwise forced to be eclipsed. The corresponding torsion angle in 4 Ph H 2 is slightly smaller [17.23 (17) ; Samankumara et al., 2010]. This vic--cis-substituents-induced pyrroline deformation was also observed previously (Sharma et al., 2017;Hewage et al., 2019).
The out-of-plane plots (Kingsbury & Senge, 2021) of the two free-base chlorins 2 Ph H 2 ÁDMAP and 4 Ph H 2 also illustrate the qualitative and quantitative differences in the conformations of the two (Fig. 5a).
The saddling deformation is more pronounced in the corresponding zinc(II) complexes but the deformation modes observed in either of the complexes are very similar ( Fig. 4b  and 5b). This (small) B 2u deformation mode is typical for penta-coordinated, square-pyramidal porphyrinoid zinc(II) complexes (Kingsbury & Senge, 2021). The differences in conformation quality and quantity is only minimal between the parent compound 2 Ph ZnÁEDA and its p-aryl-substituted and methylated analogue 4 CF3 ZnÁpy. In addition, both molecules carry their axial ligand on the same hemisphere defined by the macrocycle the diol/dimethoxy moieties are located. Nonetheless, there are differences. For instance, a smaller O-C-C-O torsion angle was observed in the diol zinc complex 2 Ph ZnÁEDA [O-C -C -O dihedral angle = 7.86 (17) ], whereas the corresponding angle in the dimethoxy derivative 4 CF3 Zn is 28.1 (4) (Sharma et al., 2017).
In neither the free base nor the zinc complex of the diol chlorins are any significant in-plane deformations observed. The change in the macrocycle conformation upon methylation and/or hydrogen bonding to an amine acceptor reiterates the conformational malleability of the chlorin chromophore (Kratky et al., 1985), as previously also shown in the varying conformations of a range of transition-metal complexes (Sharma et al., 2017).

Supramolecular features
The dominant supramolecular interactions in both 2 Ph H 2 ÁDMAP and 2 Ph ZnÁEDA are hydrogen-bonding inter- Normal mode Structural Decomposition (NSD) analysis (Kingsbury & Senge, 2021) of (a), the chromophore conformations of dihydroxychlorin 2 Ph H 2 ÁDMAP (hydrogen-bonded to DMAP) in comparison to the conformation of the chromophore of dimethoxychlorin 4 Ph H 2 (Samankumara et al., 2010), and (b), the equivalent chromophore conformation analysis of 2 Ph ZnÁEDA in comparison to the closely related dimethoxy derivative 4 CF3 Zn (Sharma et al., 2017).
actions between the hydroxyl functions of the chlorin molecules, and the DMAP and EDA bases incorporated into the crystal structure.
In 2 Ph H 2 ÁDMAP one of the hydroxyl groups acts as a donor towards the DMAP with O1-H1OÁ Á ÁN5 = 2.6968 (14) Å . O1 in turn acts as acceptor for an O-HÁ Á ÁO bond originating from O2 of a neighboring molecule. A symmetry-equivalent interaction (by inversion) connects the other two oxygen atoms of the same two molecules with each other, creating an inversion-symmetric dimer (Fig. 6). A number of additional interactions that augment the strong hydrogen bonds, among them C-HÁ Á ÁO, C-HÁ Á ÁN and C-HÁ Á Á interactions, are listed in the hydrogen-bonding Table 1.
The structure of 2 Ph H 2 ÁDMAP also contains 647 Å 3 (ca 26% of the unit-cell volume) of solvent-accessible voids occupied by highly disordered solvent molecules that could not be properly modeled or refined (Fig. 7). The content of these voids, presumably chloroform and hexane, the crystallization solvents, were instead included in the model via reverse-Fourier-transform methods using the SQUEEZE routine (van der Sluis & Spek, 1990;Spek, 2015) as imple-mented in the program PLATON (Spek, 2020), and added as additional not-model-based structure-factor contributions. The procedure corrected for 162 electrons within the solventaccessible voids.
Hydrogen bonding in 2 Ph ZnÁEDA is similar to that of 2 Ph H 2 ÁDMAP, but more complex. In contrast to the DMAP molecule in 2 Ph H 2 ÁDMAP, the amino NH 2 groups of the ethylene diamine in 2 Ph ZnÁEDA can act as both hydrogenbond acceptors as well as hydrogen-bond donors. One of the two amine moieties of the EDA base is axially coordinated to the zinc center of the chlorin complex, and is thus not available as a hydrogen-bond acceptor. The partially occupied methanol molecule also takes part in hydrogen-bonding interactions, and the disorder of the not-metal-coordinated amino group further complicates the hydrogen-bonding network of 2 Ph ZnÁEDA.

Figure 7
Solvent-accessible voids in 2 Ph H 2 ÁDMAP. The void volume is 647 Å 3 , or ca 26% of the unit-cell volume.

Figure 8
Hydrogen bonding and packing of 2 Ph ZnÁEDA. 50% probability ellipsoids. Symmetry code: (i) 1 À x, 1 À y, 1 À z. 50% ellipsoids for fully occupied and major occupancy non-H atoms. Others in capped stick mode. Phenyl and pyrrole H atoms are omitted for clarity.
creates an N-HÁ Á ÁO bond that provides an additional connection within the dimer to create a 3D hydrogen-bonding network between the two molecules ( Fig. 8).
Several 'terminal' hydrogen bonds or hydrogen-bond-like interactions cap off the not yet used acidic and basic atoms, which are listed in the hydrogen-bonding Table 2 (interactions not shown). The second amine H atom of the metal-coordinated NH 2 group is engaged in an N-HÁ Á Á interaction towards the -density of C29 of the phenyl ring of a neighboring molecule. The major moiety of the disordered amino group hydrogen bonds with the partially occupied methanol molecule. However, this interaction is not always present, as the occupancy of the MeOH molecule is only 13.6 (4)%, while that of the amino group is 88.2 (12)%. The second amino H atom is not involved in any directional interactions. One of the H atoms of the minor amino moiety might be engaged in another N-HÁ Á Á interaction towards the -density of C43 and C43 of a phenyl ring of the second dimer molecule, but the exact positions of the amino H atoms are not determined accurately given the low occupancy of the amino fragment [11.8 (12)%]. The same is true for the position of the methanol hydroxyl H atom, which appears to be engaged in a weak O-HÁ Á Á interaction with the porphyrinic -system of a molecule at À1 + x, y, z. O3, the methanol oxygen atom, acts as acceptor for a C-HÁ Á ÁO interaction originating from a phenyl C atom of a molecule not part of the dimer. The HÁ Á ÁO distance is unusually short for a C-HÁ Á ÁO interaction, 2.53 Å , which could be an artifact of the low occupancy of the methanol molecule.  (Luciano et al. 2020)] or contain other (sterically encumbering) substituents at the pyrrolic -positions or on the meso-aryl groups. Most metallochlorins contain also a different metal than zinc(II). Only a few compounds are structurally closely related to 2 Ph H 2 ÁDMAP or 2 Ph ZnÁEDA. Among them is the parent non-hydroxylated chlorin zinc chelate [5,10,15,20-tetraphenylchlorinato]zinc(II)Ápyridine complex (HPORZN10; Spaulding et al., 1977), the bis--n-butylated free base and zinc(II) chlorins (QAKLUJ and QAKMAQ, respectively; Senge et al., 2000), free base 5,10,15,20-tetraphenyl-7-hydroxychlorin (SAZSAP; Samankumara et al., 2010), thenitrated analogue of 2 Ph H 2 (TIPBIF; Worlinsky et al., 2013), dimethoxy derivatives 4 Ph H 2 (SAZROC; Samankumara et al., 2010)
While crystallizing the osmate esters in CH 2 Cl 2 and layering with the non-solvent hexane in the presence of DMAP (for 3 Ph H 2 ) or by allowing a solution of the ester in CH 2 Cl 2 /MeOH to slowly evaporate in the presence of EDA (for 3 Ph Zn), both osmate esters adventitiously reduced and diols 2 Ph H 2 ÁDMAP and 2 Ph ZnÁEDA crystallized, respectively. The spectroscopic data of both known chromophores are as described previously (Brü ckner et al., 1998).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-H bond distances were constrained to 0.95 Å for aromatic and alkene C-H groups, and to 1.00, 0.99 and 0.98 Å for aliphatic C-H, CH 2 and CH 3 groups, respectively. Positions of N-H and NH 2 hydrogen atoms were refined. N-H distances within NH 2 groups in 2 Ph ZnÁEDA were restrained to 0.88 (2) Å and H-N-H and H-N-C angles were restrained to be similar to each other. Methyl CH 3 and hydroxyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. The hydroxyl H atom of the partially occupied methanol molecule in 2 Ph ZnÁEDA was restrained to hydrogen bond to a porphyrin N atom of a neighboring complex. U iso (H) values were set to a multiple of U eq (C/O/N) with 1.5 for CH 3 and OH, and 1.2 for C-H, CH 2 , N-H and NH 2 units, respectively.
In the structure of 2 Ph ZnÁEDA, disorder of the not-metalcoordinated amino group of the ethylene diamine molecule is observed and a methanol solvate molecule is partially occu-  Table 2 Hydrogen-bond geometry (Å , ) for 2 Ph Zn.  (14) pied. The C-N bonds were restrained to be similar in length. A partially occupied methanol molecule is located nearby the major disordered amino group and hydrogen-bonded to it. The hydroxyl H atom was restrained to hydrogen bond to a porphyrin N atom of a neighboring complex. Subject to these conditions, the occupancy ratio for the amino groups refined to 0.882 (12): 0.118 (12), and the occupancy rate for the methanol molecule refined to 0.136 (4). The occupancy of the methanol molecule is not correlated with the disorder of the amino group (the major 88% occupied amino group is hydrogen-bonded to the 14% occupied methanol molecule). The structure of 2 Ph H 2 ÁDMAP contains 647 Å 3 of solventaccessible voids occupied by highly disordered solvate molecules (presumably chloroform and hexane, the crystallization solvents). The residual electron-density peaks are not arranged in an interpretable pattern and no unambiguous disorder model could be developed. The structure factors were instead augmented via reverse-Fourier-transform methods using the SQUEEZE routine (van Sluis & Spek, 1990;Spek, 2015), as implemented in the program PLATON (Spek, 2020). The resultant .fab file containing the structurefactor contribution from the electron content of the void space was used in together with the original hkl file in the further refinement. The SQUEEZE procedure accounted for 162 electrons within the solvent-accessible voids. 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. Refinement. The structure contains 647 Ang3 of solvent accessible voids occupied by highly disordered solvate molecules (presumably chloroform and hexane, the crystallization solvents). The residual electron density peaks are not arranged in an interpretable pattern and no unambiguous disorder model could be developed. The structure factors were instead augmented via reverse Fourier transform methods using the SQUEEZE routine (P. van der Sluis & A.L. Spek (1990). Acta Cryst. A46, 194-201) as implemented in the program Platon. The resultant FAB file containing the structure factor contribution from the electron content of the void space was used in together with the original hkl file in the further refinement. (The FAB file with details of the Squeeze results is appended to this cif file). The Squeeze procedure corrected for 162 electrons within the solvent accessible voids.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x (18) (7) 0.0290 (2)  C15 0.69121 (11) 0.40654 (7) 0.70330 (7) 0.0266 (2)  C16 0.66552 (11) 0.38612 (7) 0.63837 (7) 0.0256 (2)    0.0550 (9) 0.0366 (7) 0.0508 (9) −0.0091 (6) 0.0013 (7  173.89 (10) C47-C48-C49-N5 0.9 (2) N4-C19-C20-C1 6.04 (19) Hydrogen-bond geometry (Å, º) 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. Refinement. The not metal coordinated amino group of an ethylene diamine ligand was refined as disordered. The C-N bonds were restrained to be similar in length. Amine H atom positions were refined and N-H distances were restrained to 0.88 (2) Angstrom. Equivalent H···H and C···H distances were restrained to be similar to each other. Subject to these conditions the occupancy ratio refined to 0.882 (12) to 0.118 (12). A partially occupied methanol molecule is located nearby the major disordered amino group and H-bonded to it. The hydroxyl H atom was restrained to hydrogen bond to a porphyrin N atom of a neighboring complex. Subject to these conditions the occupancy rate refined to 0.136 (4).