9-[(Z)-2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)ethenyl]-9H-carbazole

The title compound has a polarized π-system due to resonance between N—C(H)=C(H)—B and ionic N+=C(H)—C(H)=B− canonical structures. In comparison with the previously reported E-isomer, the reduced planarity of Z-isomer results in a decrease of the contribution of the N+=C(H)—C(H)=B− canonical structure.


Structure description
The title compound, C 20 H 22 BNO 2 , has a hybrid -conjugated system comprising an N-C(H) C(H)-B unit (Fig. 1). The insertion of a -conjugated system in the N-B bond can give a highly polarized -system as a result of the contribution of an ionic canonical structure, N + C(H)-C(H) B À . However, the contribution of the ionic canonical structure is very small when p-phenylene is inserted into the N-B bond (Yuan et al., 2006). By contrast, there is a significant contribution of the ionic canonical structure when a C C bond is inserted into the N-B bond (Onuma et al., 2015). The crystal structure of one isomer of the C C bond-inserted system, namely 9-[(E)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethenyl]-9H-carbazole has been reported (Hatayama & Okuno, 2012), which is an E-isomer of the title compound. In this work, the preparation of the Zisomer is reported as is a comparison of the crystal structures of the isomers.
The dihedral angles between the C13/C14/H13/H14 plane (r.m.s. deviation 0.0333 Å ) and the N1/C1/C12/C13 plane (r.m.s. deviation 0.0423 Å ) and B1/O1/O2/C14 plane (r.m.s. deviation 0.0082 Å ) are 45.86 (8) and 37.47 (8) , respectively. The relatively large angles data reports result in steric repulsion between carbazolyl and Bpin (pin = pinacolato) residues. The equivalent dihedral angles for the two independent molecules in the E-isomer are 19.37 (3) and 10.74 (6) and 5.70 (11) and 9.74 (9) , respectively (Hatayama & Okuno, 2012). In comparison with the Z-isomer, the Eisomer has a more planar conformation. The C C bond length of the olefinic unit in the Z-isomer is ca 0.01 Å shorter than those of the E-isomer in spite of the steric repulsion. This is presumably because the reduced planarity of the Z-isomer decreases the contribution of the N + C(H)-C(H) B À canonical structure.
In conclusion, structural analyses of both isomers of the hybrid -system afford an important insight showing the discussed dihedral angles play a crucial role for contribution of the ionic canonical structure.
Single crystals of the title compound suitable for X-ray crystallographic analysis were prepared by recrystallization from its hexane solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.

Figure 1
The molecular structure of the title compound showing the atomnumbering scheme with displacement ellipsoids drawn at the 50% probability level and H atoms shown as arbitrary spheres. 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.

data-2
IUCrData (2021). 6, x210142 Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt). The C-bound H atoms were placed at ideal positions and were refined as riding on their parent C atoms. U iso (H) values of the H atoms were set at 1.2U eq (parent atom for C sp2 ) and 1.5 U eq (parent atom for C sp3 ).