Crystal structure of (6,9-diacetyl-5,10,15,20-tetraphenylsecochlorinato)nickel(II)

The structure of the title secochlorin nickel complex features two crystallographically independent molecules related by pseudo-A lattice centering. Both feature a noticeable in-plane deformation in the A1g mode and prominent out-of-plane deformation in the B1u (ruffling) mode.

The title compound 1Ni, a secochlorin nickel complex, was prepared by diol cleavage of trans-dihydroxychlorin 2Ni, itself made in a multi-step process from cis-diol chlorin 3Ni (Fig. 1).It has been used in the preparation of a number of porphyrinoids containing non-pyrrolic heterocycles (Banerjee et al., 2012;Sharma et al., 2016).

Structural commentary
1Ni crystallized in a centrosymmetric triclinic setting in space group P1, with two crystallographically independent molecules (Z 0 = 2) (Fig. 2).The structure of 1Ni confirms its rare secochlorin bisketone connectivity that was previously derived spectroscopically and by means of subsequent reactions (Banerjee et al., 2012;Sharma et al., 2016).Bond lengths and angles are in the expected ranges.Noteworthy, however, are the C-N bonds around the cleavage site at N1, which show a clear asymmetry present in both molecules.The lengths for N1-C3 are 1.3687 (15) and 1.3661 (15) A ˚in the two molecules 1Ni-1 and 1Ni-2, while the two N1-C20 bonds are substantially longer at 1.3879 (15) and 1.3997 (15) A ˚.For most of the other bonds of the secochlorin skeleton no such clear differentiation is observed.
The two crystallographically independent molecules of the secochlorinato nickel complex 1Ni are related by pseudo-A lattice centering (Fig. 3).Exact translational symmetry is broken by different torsion angles for one of the acetyl groups   (O1, C1, C2).In molecule 1, the N-C-C-O torsion angle between the non-pyrrolic N atom N1 and keto-oxygen O1 is 39.90 (19) � ; in molecule 2 the same torsion angle is À 37.75 (17) � .The same torsion angles for the methyl C atom C1 instead of O1 are 143.45 ( 13) � (molecule 1) and À 130.33 ( 13) � (molecule 2).The acetyl groups of the two molecules are thus rotated with respect to each other, by ca 78 and 86 � for keto O and methyl C atoms, respectively.The different values for C and O atoms are caused by a slight nonplanarity of the acetyl group of molecule 1, with C1 being offset from the plane of the C3, C2 and O1 by 0.230 (5) A ˚.This rotation results in a significantly different O1� � �O2 distance in the two molecules: 2.7997 (17) A ˚in molecule 1, and 3.2286 (16) A ˚in molecule 2. It also places the methyl groups in the two molecules on opposite sides of their neighboring phenyl rings of C23 to C28, which in turn induces different orientations of this ring between the two molecules [C3-C4-C23-C24 = À 130.42 (14) and 115.13 (15) � for molecules 1 and 2, respectively].The different orientation of the phenyl ring prevents an intramolecular C-H� � �O hydrogen bond between C28 and O2 that is present in molecule 1 to be formed in molecule 2. A second C-H� � �O hydrogen bond (between C42 and O1) is present for both molecules (see Table 1 for numerical details).The phenyl group on the opposite end of the molecule (C35-C40) is also slightly modulated, but much less so than the acetyl group and C23-C28 (torsion angles only differ by ca 29 � ).The remaining atoms of the two molecules are related by close to perfect translational symmetry, and the conformations of their secochlorin cores are thus qualitatively and quantitatively very similar to each other.
Importantly, the structure determination of 1Ni also allows for a detailed analysis of the conformation of the macrocycle.For both molecules 1Ni-1 and 1Ni-2 the conformations of the chromophores deviate greatly from planarity.On account of the small central nickel(II) ion inducing strain into the macrocycle, nickel(II) porphyrin and chlorin complexes tend to be non-planar, adopting commonly a ruffled conformation (Kingsbury & Senge, 2021).This is because the ruffled deformation mode effectively shortens the Ni-N bond lengths, without distorting the near-ideal planar coordination geometry of the four nitrogen donors around the metal.This is also the case here, and the N-Ni bond lengths vary only within a small margin, with values between 1.8845 (10) and 1.9128 (10) A ˚, and the coordination of the nickel atoms is close to perfectly square planar.To compare 1Ni to other related compounds and obtain a qualitative and quantitative analysis of its conformation, we performed a normal-coordinate structural decomposition (NSD) analysis of the macrocycle conformation of the two molecules of 1Ni-1 and 1Ni-2, of the starting diol nickel complex 2Ni and of its aldehyde analogue 5Ni (Fig. 4) (Jentzen et al., 1997;Kingsbury & Senge, 2021).
Both molecules 1Ni-1 and 1Ni-2 show in-plane compressed conformations, with the most prominent in-plane deformation in the A 1g mode.This is combined with drastically ruffled outof-plane deformations with significant waving and propellering contributions.These deformation modes are typical for nickel porphyrins and chlorins (Kingsbury & Senge, 2021) and their large extent was previously observed in secochlorins (Bru ¨ckner et al., 2005).The macrocycle conformation aligns the two ketone functionalities to be arranged antiparallel to each other.Qualitatively and quantitatively, the conformation of bisketone 1Ni is very similar to that of bisaldehyde 5Ni,

Figure 4
Bar diagrams of the in-plane (a) and out-of-plane (b) macrocycles NSD deformation analyses of the compounds 1Ni-1, 1Ni-2, 2Ni, and 5Ni; analyses by the porphyrin NSD online tool (Kingsbury & Senge, 2021), based on the method of Shelnutt (Jentzen et al., 1997).Stick representations of the X-ray single-crystal structures are shown below (c through e); for 1Ni, only 1Ni-1 is shown; all H atoms attached to sp 3hybridized carbon atoms, all meso-aryl groups, as well as all disorder and solvents removed for clarity.Next to the stick structures are the in-plane skeletal plots of the porphyrin core of the compounds indicated (black trace), compared to that of a benchmark planar porphyrin [meso-tetraphenylporphyrinato]copper(II) (red trace), as well as their out-of-plane skeletal plots.
showing that the additional alkyl substituents in 1Ni do not have any significant steric influence.While parent chlorin diol complex 2Ni is also ruffled, it is much less so, with much smaller waving and propellering deformations as well.Accordingly, the average N-Ni bond length in 2Ni is longer (1.908A ˚) compared to the corresponding bond lengths found in 1Ni (1.901A ˚) or 5Ni (1.893A ˚).The changes observed upon ring-opening align with what was previously observed (Bru ¨ckner et al., 2005).

Supramolecular features
In addition to the intramolecular C-H� � �O bonds within each molecule, there are also a small number of weak C-H� � �O hydrogen bond-like interactions present that connect molecules of 1Ni with each other (Fig. 5; see Table 1 for numerical values and symmetry operators).Some weak C-H� � �� interactions are also present (not shown), but these are too weak to be classified as directional and they are unlikely to have any strong structure-determining effects.Instead, intermolecular interactions in the structure of 1Ni are dominated by non-directional dispersion interactions (van der Waals interactions).This is quantitatively confirmed by analysis of the Hirshfeld surfaces of the two molecules (Spackman & Byrom, 1997;Crystal Explorer, Spackman et al., 2021)

Database survey
The structures most closely related to title compound 1Ni are its aldehyde analogue 5Ni [CSD (Groom et al., 2016) code GUBWAB;Bru ¨ckner et al., 1999], as well as two related bisketone derivatives in which the C atoms of the cleaved pyrrole have been annelated to the adjacent phenyl rings to form an indaphyrin (meso-diphenylindaporphyrinato)platinum(II) (CSD entry SUNXAB; Lau et al., 2009) and its bishydroxylated indachlorin counterpart (CSD entry OJEHAO; Samankumara et al., 2015).

Synthesis and crystallization
The title compound 1Ni was prepared by classic lead(IV)induced diol cleavage of trans-dihydroxychlorin 2Ni.This diol was made in two steps from cis-diol 3Ni: oxidation to dione 4Ni (Daniell et al., 2003), followed by double methyl-Grignard addition (Banerjee et al., 2012).Crystals of 1Ni were grown by slow evaporation of a solution of 1Ni in CHCl 3 /hexane to dryness.The spectroscopic data of 1Ni have been reported previously (Banerjee et al., 2012).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Two crystallographically independent molecules are present in the asymmetric unit of the structure.A common atom-naming scheme combined with residue numbers 1 and 2 were used for the two molecules, which are related by pseudo-A lattice centering.Exact translational symmetry is broken by different torsion angles of one of the acetyl groups (O1, C1, C2), of the adjacent phenyl group Packing view showing intra-and intermolecular C-H� � �O hydrogen bonds in 1Ni.Molecules 1Ni-1 are to the left and right at (x, 0, z) and (x, 1, z), molecules 1Ni-2 at the center at (x, 1 2 , z).Note that all C-H� � �O hydrogen bonds are formed between molecules 1, and between molecules 2, thus forming C-H� � �O bonded layers, while all interactions between molecules 1 and 2 are purely van der Waals in nature.C23 to C28), and of the phenyl group trans across the molecule (C35 to C40).
C-H bond distances were constrained to 0.95 A ˚for aromatic moieties, and to 0.98 A ˚for CH 3 moieties.Methyl CH 3 groups were allowed to rotate but not to tip to best fit the experimental electron density.U iso (H) values were set to a multiple of U eq (C) with 1.5 for CH 3 and 1.2 for C-H units, respectively.

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.

Figure 1
Figure 1Synthetic pathway to secochlorin nickel(II) complexes 1Ni and 5Ni by oxidative diol cleavage of a precursor diol.

Figure 3
Figure 3 View down [011] (slightly offset) showing the pseudo-A lattice centering relating the two molecules with each other (front: molecule 1Ni-1; back: molecule 1Ni-2).For clarity only selected labels for molecule 1 are shown.

Figure 6
Figure 6 Selected fingerprint plots highlighting the dominance of interactions involving H atoms in the structure of 1Ni.Shown are d e versus d i plots (Crystal Explorer; Spackman et al., 2021) of H� � �H interactions (left) and O� � �H/H� � �O interactions (right) for molecule 1Ni-1 (plots for molecule 1Ni-2 are equivalent).

for 1Ni-2), but
distances are rather long and the interactions are non-directional.The deformation modes of the molecules also prevent any effective �-stacking interactions to be established in the structure, which is confirmed by the low number for C� � �C contacts on the Hirshfeld surface (3.6 and 3.0% for the two molecules).

Table 2
Experimental details.