Crystal structure of 5-tert-butyl-10,15,20-triphenylporphyrin

In the title free base porphyrin, the neighbouring N⋯N distances in the center of the ring vary from 2.818 (8) to 2.998 (8) Å and the phenyl rings are tilted from the 24-atom mean plane at angles varying between 62.42 (2) to 71.63 (2)°. The free base porphyrin is characterized by a significant degree of ruffled (B 1u) distortion with contributions from domed (A 2u) and wave [Eg(y) and Eg(x)] modes.

In the title free base porphyrin, C 42 H 34 N 4 , the neighbouring NÁ Á ÁN distances in the center of the ring vary from 2.818 (8) to 2.998 (8) Å and the phenyl rings are tilted from the 24-atom mean plane at angles varying between 62.42 (2)-71.63 (2) . The NH groups are involved in intramolecular bifurcated N-HÁ Á Á(N,N) hydrogen bonds. The C a -C m -C a angles vary slightly for the phenyl rings, between 124.19 (18)-126.17 (18) . The largest deviation from the mean plane of the 24-atom macrocycle is associated with the meso carbon at the substituted tert-butyl position, which is displaced from the mean plane by 0.44 (2) Å . The free base porphyrin is characterized by a significant degree of ruffled (B 1u ) distortion with contributions from domed (A 2u ) and wave [E g (y) and E g (x)] modes. In the crystal, molecules are linked by a number of weak C-HÁ Á Á interactions, forming a three-dimensional framework. The structure was refined as a two-component inversion twin.

Chemical context
Unsymmetrically meso-substituted porphyrins are of interest for a wide range of potential applications including non-linear optics (Notaras et al., 2007;Zawadzka et al., 2009), photodynamic therapy , and sensor and device applications (Scheicher et al., 2009). The synthesis of unsymmetrical porphyrin systems, such as the title compound, has been well documented (Senge et al., 2010;Senge, 2011). The title compound was first synthesized as part of a study on the identification of stable porphomethenes and porphodimethenes using sterically hindered aldehydes . This was achieved through acid-catalyzed condensation of pyrroles with aldehydes. It was later synthesized as part of this publication through the bromination of 5-tert-butylporphyrin following a reported literature procedure for similar compounds (Fazekas et al., 2008) and subsequent Suzuki cross-coupling with phenylboronic acid, in excellent yield.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The distance of neighbouring NÁ Á ÁN atoms in the center of the ring shows a slight elongation of the porphyrin core along direction C5 to C15 [N1Á Á ÁN2 2.818 (8) Å , N2Á Á ÁN3 2.998 (8) Å , N3Á Á ÁN4 2.830 (8) Å , N4Á Á ÁN1 2.994 (7) Å ]. The NH groups are involved in intramolecular bifurcated N-HÁ Á Á(N,N) hydrogen bonds ( Table 1). The angles between the alpha carbons (C a ) and the meso carbon atoms (C m ) can be used to determine the structural differences between similar porphyrins and differences within the individual porphyrin structure. In the title compound, the C a -C m -C a angles vary slightly with the C a -C m (tert-butyl)-C a angle of 120.55 (18) at C5 representing the smallest. This is due to the nature of the tert-butyl substitution present. This angle is similar to that observed in the dication, 5,10,15,20-tetrakis(tert-butyl)-22H + ,24H + -porphyrindiium ditrifluoroacetate (Senge, 2000), with an average C a -C m (tert-butyl)-C a angle of 119.53 and 5-tert-butylporphyrin published (Ryppa et al., 2005), which shows an C a -C m (tert-butyl)-C a angle of 119.86 . The C a -C m (phenyl)-C a angle of the title compound at C10 and C20 are quite similar at 126.03 (18) and 126.17 (18) , respectively. The C a -C m (phenyl)-C a angles in 5,10,15,20-tetraphenylporphyrin, with an average angle of 125.35 (Silvers & Tulinsky, 1967), are comparable to that of the title compound, however, the C a -C m (phenyl)-C a angle at C15 of the title compound is smaller [124.19 (18) ].
The tilt angles of the phenyl meso-substituents are 67.62 (2) (C10), 71.63 (2) (C15) and 62.42 (2) (C20). These angles are larger than the tilt angles observed in 5,10,15,20tetraphenylporphyrin, which are ca 60 (Silvers & Tulinsky, 1967). The tilt of the pyrrole rings against the 24-atom plane are 9.93 (2) (N1), 172.68 (6) (N2), 0.17 (2) (N3) and 3.45 (1) (N4), with the highest deviation from the mean plane associated with the pyrrole rings closest to the tert-butyl group at C5. The pyrrole ring N2 shows the largest deviation and this is visible in the overall conformation of the macrocycle rings ( Fig. 2). A conformational analysis (Senge et al., 2015) was performed using the NSD (normal structural decomposition) method developed by Shelnutt and co-workers (Jentzen et al., 1997). The conformation is characterized by a significant degree of ruffled (B 1u ) distortion with contributions from domed (A 2u ) and wave [E g (y) and E g (x)] modes (Fig. 3). Contributions are also evident in the B 2g in-plane distortion. A comparison with 5-tert-butylporphyrin (Ryppa et al., 2005) reveals a relatively similar composition of distortion modes for both compounds. This indicates that the tert-butyl group is the predominant contributor to the macrocycle distortion. There is, however, a noticeable difference between the NSD of both structures with regards to the B 1u and E g (y) out-of-plane distortions. The title compound exhibits similar contributions from both these modes whereas the free base 5-tert-butylporphyrin shows significantly more contributions in the B 1u compared to the E g (y) distortions. This can also be seen in the in-plane distortions as both compounds show significant contributions from the B 2g and smaller contributions from the A 1g mode, the title compound shows much larger contributions towards the B 1g in-plane distortions compared to that of the 5-tert-butylporphyrin. The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The bifurcated N-HÁ Á Á(N,N) hydrogen bonds are shown as dashed lines (see Table 1). Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Side view of the structure of the title compound looking down the C5 meso-position, showing the tilt angle of the macrocycle rings. Displacement ellipsoids are drawn at the 50% probability level.

Supramolecular features
In the crystal, the four molecules stack with a 90 rotation with regards to the tert-butyl-substituted group. The centroidcentroid distance of the 24-atom mean planes of the porphyrin rings are between 8.762 (2) and 7.758 (2) Å . The rings that stack above each other are separated by 8.762 (2) Å and the rings that are orientated in an edge-on packing are separated by a centroid-centroid distance of 7.758 (2) Å (Fig. 4). The orientation of the molecules in the unit cell shows that the C bhydrogen atoms between the tert-butyl group at C5 and the phenyl group at C10 are pointing towards the center of the neighbouring ring. Molecules are linked by a number of weak C-HÁ Á Á interactions (Table 1), forming a three-dimensional framework. There are no solvent molecules contained within the overall structure, as seen in Fig. 5.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.36, update November 2014; Groom & Allen, 2014) gave several hits for unsymmetrically meso-substituted porphyrins similar to the title compound. Speck et al. (1997) reported the structure of 5-(3,5-di-tert-butylmuconic acid anhydride)-10,15,20-triphenylporphyrin in which they reported phenyl tilt angles of 59.07-78.15 from the 24-atom mean plane, with the largest deviation on the phenyl group opposite the alternative meso-substituted position. In this structure there was a larger variance of the C a -C m (phenyl)-C a angle of 123.88-125.51 and a C a -C m (C5)-C a larger than the title compound of 127.35 . Senge et al. (1999) published the structure of 5-(2,5-dimethoxybenzyl)-10,15,20-triphenylporphyrin. The tilt angle of the phenyl rings from the 24-atom mean plane was larger and more varied compared to the title compound (73.47-87.56 ). The C a -C m (phenyl)-C a angle is similar to the title compound with an angle range of 125.46-125.78 . The structure of 5-(3,5-dihydroxyphenyl)-10,15,20triphenylporphyrin pyridine clathrate has been reported by Tanaka et al. (2001). This compound displayed a phenyl tilt angle of 65.87-73.97 from the 24-atom mean plane and all NSD analysis of the title compound and comparison with 5-tertbutylporphyrin. NSD gives a graphical representation of the displacements along the lowest frequency coordinates that best simulate the structures.

Figure 4
Unit cell of the title compound viewed along the a axis, showing four complete molecular units.  equal to each other (123.78-123.95 ). The C a -C m (phenyl)-C a opposite the unsubstituted meso position is smaller than the C a -C m (H)-C a angle, 126.20 and 127.93 , respectively. The phenyl tilt angle from the 24-atom mean plane shows a larger tilt angle (73.56-78.16 ) associated with the phenyl rings. However, there is a narrower variance in these angle than in the title compound. Ryppa et al. (2005) published the structure of 5-tert-butylporphyrin which presents C a -C m (H)-C a angles of 129.00-129.23 for the C10 and C15 positions and 125.23 for the C15 position which are all larger than in the title compound. The C a -C m (tert-butyl)-C a angle (C5 in both structures) are of similar size at 119.86 (120.28 for the title compound). The overall pyrrole tilt against the mean 24-atom plane shows similar results to that of the title compound. The pyrrole rings (N1 and N2) closest to the tertbutyl meso substitute show significantly higher tilts (11.68 and 14.33 , respectively) compared to the pyrrole rings (N3 and N4) closest to the unsubstituted position at C15 (4.04 and 5.26 , respectively). Yang et al. (2011) reported the structure ethyl 8-(10,15,20-triphenylporphyrin-5-yl)-1-naphthoate exhibiting a phenyl tilt angle from the 24-atom mean plane of 59.13 for the phenyl opposite the naphthanote substitute and between 74.91-76.38 for the other phenyl groups. A similar angle for all C a -C m -C a is observed, 125.36-125.82 . Ma et al. (2013) published the structure of 2-hydroxyphenyl 8-(10,15,20triphenylporphyrin-5-yl)-1-naphthoate which exhibited a C a -C m (phenyl)-C a angle of 124.36-124.68 similar to the title compound and a C a -C m (naphthoate)-C a angle of 125.25 which is slightly larger compared to the title compound. The tilt angle of the phenyl rings from the 24-atom mean plane is 60.46-83.15 which shows a larger variance than for the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The structure was refined as a twocomponent inversion twin. The NH and C-bound H atoms were placed in their expected calculated positions and refined using a standard riding model: N-H = 0.88 Å , C-H = 0.95-0.98 Å , with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (N,C) for other H atoms.