Crystal structure and Hirshfield analysis of the 4-(dimethylamino)pyridine adduct of 4-methoxyphenylborane

The asymmetric unit contains two independent molecules, which exhibit coplanar, mostly sp 2-hybridized methoxy and dimethylamino substituents on their respective aromatic rings, consistent with π-donation into the aromatic systems. The B—H groups exhibit an intramolecular close contact with a C—H group of the pyridine ring, which may be evidence of electrostatic attraction between the hydridic B—H and the electropositive aromatic C—H.


Chemical context
Monoorganoboranes (RBH 2 ) have been the focus of chemical research for over fifty years, most notably for their use in the indispensable hydroboration reaction, which permits reduction of olefins, carbonyl compounds and others (Brown & Krishnamurthy, 1979;Crudden & Edwards, 2003.) Such boranes are often isolated as their Lewis base adducts, in which the base donates a lone pair into the vacant p orbital of the sp 2 borane. Among the most common class of Lewis bases for the formation of borane adducts are amines. Amine boranes are widely used as hydroboration reagents (Clay & Vedejs, 2005), precursors for borenium cation synthesis (De Vries et al., 2012), frustrated Lewis pairs (Stephan, 2015), and have been investigated as hydrogen-storage materials (Campbell et al., 2010). We have synthesized the zwitterionic title compound by hydride removal from sodium 4-methoxyphenylborohydride with chlorotrimethylsilane in the presence of 4-dimethylaminopyridine. This compound is slightly unusual, as examples of monoorganoboranes with heteroatoms on the organic substitituent are limited.

Supramolecular features
The molecules within the asymmetric unit exhibit weak C-HÁ Á Á (arene) interactions between two of the hydrogen atoms of the aminomethyl group and the methoxyphenyl group of a neighboring molecule (see Table 1) as well as a C-HÁ Á Á(arene) interaction between one of the pyridine hydrogen atoms and the same methoxyphenyl ring (Fig. 4). The molecular structure of the other independent molecule of the title compound with displacement ellipsoids drawn at the 50% probability level.

Figure 3
An overlay of the two independent molecules. Table 1 Hydrogen-bond geometry (Å , ).
Cg is the centroid of the C1 0 -C6 0 ring.

Figure 1
The molecular structure of one of the independent molecules of the title compound with displacement ellipsoids drawn at the 50% probability level.

Hirshfield analysis
The weak intermolecular interactions of the title compound were explored by Hirshfield analysis. Hirshfield surfaces were generated using Crystal Explorer 3.1 (McKinnon et al., 2007;Spackman & Jayatilaka, 2009). The space within a crystal is partitioned so that the ratio of promolecule to procrystal is equal to 0.5, generating continuous surfaces that permit the visualization of weak interactions. The d norm values illustrate whether the intermolecular contact is shorter or longer than the van der Waals radii. Red areas of the Hirshfield surface indicate negative d norm values contacts closer than the van der Waals radii. This analysis lends further support to the weak C-H Á Á Á (arene) interactions described in the previous section ( Fig. 5.)

Database survey
A search of the Cambridge Structural Database (Version 5.37, update February 2017; Groom et al., 2016) for DMAP-borane adducts yielded only two structures: VOGJEI (Chu, et al., 2014) and JUDQAA (Lesley et al., 1998) (Franz et al., 2011), and GEBNAE (Jacobs et al., 2012). In all four of these structures, the B-N bonds are approximately perpendicular to the plane of the arene rings. In all six cases, the boron atom is tetrahedral and displays structural features consistent with sp 3 hybridization. Additionally, the C-B and B-N bonds are all within the range for formal C-B and C-N single bonds.

Synthesis and crystallization
In a nitrogen-filled glove box, sodium 4-methoxyphenylborohydride (97mg, 0.67 mmol) and 4-dimethylaminopyridine (82 mg, 0.67 mmol) were combined in a 20 mL vial containing a stir bar and dissolved in anhydrous THF (4 mL). The solution was cooled to 247 K in the freezer and chlorotrimethylsilane (73 mg, 0.67 mmol) was added dropwise via syringe. The reaction was allowed to come to 295 K and was stirred for 1 h. The solvent was then removed in vacuo and the residue was washed with anhydrous diethyl ether (4 mL), followed by extraction with anhydrous dichloromethane (4 mL). Hirshfield surface mapped over d norm . Red areas highlight intermolecular contacts shorter than the sum of the van der Waals radii.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were refined in calculated positions (C-H = 0.95 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2eq(C) for other H atoms. The B-bound H atoms were located in a difference-Fourier map and freely refined. Methyl H atoms were refined without restrictions on rotation around the C-C bonds, HFIX 138 in SHELXL (Sheldrick, 2015).

Funding information
Funding for this research was provided by: Eastern Washington University Faculty Grants for Creative Works. program(s) used to solve structure: SHELXS97 (Sheldrick 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). 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.