Crystal structure of bis(mesityl)(pyrrol-1-yl)borane

In the crystal structure of the title compound, the dimesitylboron group acts to reduce the delocalization of the nitrogen atom’s lone pair into the pyrrole ring, with increases in the two N—C bond lengths compared to pyrrole itself. The N—B bond is 1.44125 (15) Å long.

In the crystal structure of the title compound, C 22 H 26 BN, the B atom acts to reduce the delocalization of the nitrogen lone-pair electron density into the pyrrole ring, so that the two N-C bonds increase in length to 1.4005 (14) and 1.3981 (14) Å . The N-B bond length is 1.4425 (15) Å , which is longer than a typical N-B bond because the nitrogen lone pair is not fully available to participate in the bond.

Chemical context
The structure of the title compound 1 is of interest because of the effect of the bis(mesityl)boron group on the electronic structure of the pyrrole, since the nitrogen lone pair, which is essential to the heterocycle's aromatic 6 system, now has the possibility of being donated to the boron atom. This molecule has been investigated previously for its fluorescence, and shows a large Stokes shift, which involves twisted intramolecular charge transfer (TICT) of the excited state, by rotation about the N-B bond (Brittelli & Eaton, 1989;Cornelissen & Rettig, 1994;Cornelissen-Gude & Rettig,1999). It has been used recently in the preparation of conductive polymers (Wildgoose et al. 2019).

Structural commentary
The crystal structure of bis(mesityl)(pyrrol-1-yl)borane 1 was determined at 120 K and the molecular structure is shown in Fig. 1. The bonding geometries at both the boron and nitrogen atoms are almost planar, with an angle of 16.13 (8) between these two bonding planes. The N and B atoms lie 0.0351 (11) and 0.0285 (13) Å , respectively, out of the planes defined by their three attached atoms. The planes of the two mesityl groups lie at 58.53 (3) and 61.46 (4) to the boron atom's bonding plane, and so there is limited donation of theirelectron densities to boron. These dispositions are controlled by the need to maintain separations between their two adja-cent pairs of ortho methyl groups [H 3 CÁ Á ÁCH 3 = 3.610 (3) and 3.736 (3) Å ], and is supported by the widening of the C-B-C bond angle [125.18 (9) ], compared to the two N-B-C bond angles [118.30 (10) and 116.42 (9) ]. The mesityl groups' planes lie at 77.14 (4) to each other, and at 69.18 (3) and 67.06 (4) to the pyrrole ring's best plane. The hydrogen atoms of three methyl groups (C12, C13 and C21) were modelled in two orientations. The positions and displacement parameters of hydrogen atoms on the pyrrole ring were refined, and for those attached to the C atoms, the H-C -C angle showed widening to 131-132 , similar to that in pyrrole (Goddard et al., 1997;Lee & Boo, 1996).
The N-B bond is 1.4425 (15) Å long. This is ca 0.04 Å longer than in similar compounds where the nitrogen atom is attached to two sp 3 carbon atoms [ROCRAD (two molecules; Morawitz et al., 2008), TAYYAV (Araki et al., 2012), UWUFID (Smith et al., 2016), YOMKAM (Khasnis et al., 1995); T 173 K, N-B range 1.388 (2)-1.412 (3) Å , average 1.40 Å ] and where the nitrogen lone pair is fully available for donation to boron. Compared to the molecular geometry of pyrrole itself, as determined by X-ray crystallography at 103 K (Goddard et al., 1997) and by calculation at the B3LYP-631G * level (Lee & Boo, 1996), the most notable difference is in the increase of the two N-C bond lengths to 1.4005 (14) and 1.3981 (14) Å from 1.365 (2) Å (experimental) and 1.376 Å (calculated) ( Table 1). The C -C bond lengths are 1.3536 (16) and 1.3514 (17) Å and the C -C bond length is 1.4290 (17) Å . Thus, in contrast to pyrrole, the N-C and C -C bonds are no longer similar in length, due to a reduction in the contribution of the nitrogen atom's lone pair to the electronic system of the pyrrole ring. When the mesityl groups are replaced by pentafluorophenyl groups in derivative 2, the N-B bond is considerably shorter than in 1 [1.4094 (9) cf. 1.4425 (15) Å ] due to greater lone-pair donation from nitrogen towards the more electron-deficient boron. Consequently, compared to 1, the pyrrole ring shows slightly longer N-C bonds [1.4033 (6) Å ] and a longer C -C bond [1.4418 (9) Å ], though similar lengths for the C -C bonds [1.3553 (6) Å ] (Table 1). For comparison, the effect of boron on the pyrrole ring in 1 is similar to that when the pyrrole nitrogen atom is substituted with a carbonyl group to form an amide (Table 1).

Supramolecular features
The molecules are packed in layers in the ab plane (Fig. 2). There are no particularly short intermolecular contacts, consistent with the low density of the crystal (1.132 g cm À3 ). Within a layer, the molecules are related by centres of symmetry and translations along a and b. Adjacent layers are related by the twofold screw and n-glide planes. The four shortest intermolecular CÁ Á ÁH distances are in the range 2.81-2.83 Å . Two of these involve the meta-C atom, C18, with a research communications Acta Cryst. (2023). E79, 50-53 Sahin and Wallis C 22 H 26 BN 51 Table 1 Bond lengths (Å ) for 1 and related compounds. Notes: (a) RUVQII (Goddard et al., 1997); (b) Lee & Boo (1996)

Figure 2
The crystal-packing arrangement for 1, viewed down the b axis, with the shortest CÁ Á ÁH separations (2.81-2.83 Å ) shown in blue. Both orientations of the H atoms on methyl groups C12, C13 and C21 are shown.

Figure 1
The molecular structure of 1, with anisotropic displacement parameters drawn at the 50% probability level. Only the more populated orientations for methyl groups C12, C13 and C21 are shown.

Database survey
The structure of the analogue of 1 bearing two 2 0 -thienyl groups in the pyrrole's 2-and 5-positions (XEQVUM; Taniguchi et al., 2013) shows a larger angle (35.1 ) between the bonding planes at nitrogen and boron due to avoidance of steric interactions between the thiophene and mesityl groups, and has a longer N-B bond [1.472 (7)

Refinement
Crystal data and details of data collection and structure refinement are summarized in Table 2. Pyrrole H-atom posi-tions and displacement parameters were refined. All other H atoms were refined using a riding model with C-H bonds fixed at 0.95 Å for hydrogens attached to phenyl carbon atoms and at 0.98 Å for methyl hydrogen atoms. Three methyl groups were refined in two orientations (C12, C13 and C21). The isotropic atomic displacement parameters of the H atoms were set at 1.2U eq of the parent atom for aromatic groups and at 1.5U eq for methyl groups.  Extinction correction: SHELXL2018/3 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0035 (4)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (