Crystal structure of cis-1-phenyl-8-(pyridin-2-ylmethyl)dibenzo[1,2-c:2,1-h]-2,14-dioxa-8-aza-1-borabicyclo[4.4.0]deca-3,8-diene

The present work describes the synthesis and crystal structure of the new B-phenyloxazaborocine, C26H23BN2O2. The title compound adopts a zwitterionic form with a significant intramolecular N→B dative bond and intermolecular C—H⋯O interactions connecting molecules parallel to the b axis.

The title compound, C 26 H 23 BN 2 O 2 , was obtained as by product during synthetic attempts of a complexation reaction between the tripodal ligand H 2 L [N,Nbis(2-hydroxybenzyl)(pyridin-2-yl)methylamine] and manganese(III) acetate in the presence of NaBPh 4 . The isolated B-phenyl dioxazaborocine contains an N!B dative bond with a cis conformation. In the crystal, C-HÁ Á ÁO hydrogen bonds define chains parallel to the b-axis direction. A comparative analysis with other structurally related derivatives is also included, together with a rationalization of the unexpected production of this zwitterionic heterocycle.

Chemical context
As part of our research program directed at obtaining manganese complexes as bio-inspired mimetics with different nuclearity and properties (Ledesma et al., 2014(Ledesma et al., , 2015, we were interested in coordination reactions of the tripodal tetradentate ligand H 2 L, namely N,N-bis(2-hydroxybenzyl)(pyrid-2-yl)methylamine. We envisaged a systematic study comprising the use of several metal-to-ligand ratios, with the idea of varying the nuclearity of the resulting compounds. Unexpectedly, however, during consecutive attempts to obtain manganese complexes derived from H 2 L, we isolated the Bphenyl dioxazaborocine derivative, I. Here we report its synthesis and crystal structure and, in order to unravel its presence, we rationalize its production under the employed reaction conditions. A comparative analysis of its structural data with that of other dioxazaborocines is also presented. ISSN 2056-9890

Figure 1
The molecular structure of the title molecule, with displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

Figure 2
Detail of the intermolecular interactions in the title compound forming a dimeric system through C-HÁ Á ÁO hydrogen bonds (dashed lines). H atoms not involved in these hydrogen bonds are omitted [symmetry code: (i)= 1 À x, 2 À y, 2 À z].

Figure 3
Detail of C-HÁ Á ÁO interactions in the title compound (grey and orange dashed lines denote C2-H2Á Á ÁO1 i and C24-H24Á Á ÁO2 ii interactions, respectively). H atoms not involved in these interactions are omitted for clarity [symmetry codes: (i) 1 À x, 2 À y, 2 À z; (ii) x, À1 + y, z]. Woodgate et al., 1999). Specifically, two members of this selected group are structurally related to the title compound: II (MAWDET; Woodgate et al., 1999) and the recently described compound III (EROJIF; Geng & Wu, 2011) ( Fig. 4). Table 2 summarizes relevant bond lengths and angles for I compared with those observed in II and III. The intramolecular N-B bond lengths can vary, depending on the substituent groups to boron and nitrogen atoms. In particular, the covalent N1-B1 bond distance for I [1.674 (4) Å ] is in the range observed for III and II [1.641 (2)-1.674 (5) Å ]. The N-B bond distance for III is shorter than that in II, quite probably due to the extra oxygen atom bonded to the boron atom (from the -OCH 3 group).
The crystal structure of I shows that the phenyl group at the boron atom and the N-pyridin-2-ylmethyl substituent adopts a cis conformation around the N!B dative bond, in total agreement with that reported for II and III. The C21-N1-B1-C15 torsion angle assumes a value of 57.8 (3) . Analysis of the structural data for II showed the corresponding torsion angle (C37-N1-B1-C15) is 56.71 . In compound III, the corresponding angle (C13-N2-B1-O4) is 62.34 . These two examples display a cis geometry around the intramolecular N-B bond, in concordance with compound I (Fig. 5).
We have performed an analysis of the experimental data of compounds I--III and calculated the tetrahedral character (THC DA ) at the boron atom (Hö pfl et al., 1999), making use of the values of the six angles around the boron atom ( 1 -6 ).
The quite high value of 82.8% for I is in the range observed for compounds II and III. Altogether, this parameter and the measured N-B bond lengths can be considered a clear indication of sp 3 -hybridization of the boron atom and of a resident negative charge (Sarina et al., 2015). Therefore, we confirm that compound I adopts a zwitterionic form with a significant intramolecular N!B dative bond.
Based on previous observations (Barnes et al., 1998), we hypothesize that employing an aqueous solution of NaBPh 4 led to the unexpected isolation of I. It is well known that NaBPh 4 in the presence of oxygen leads to the production of phenylboronic acid PhB(OH) 2 and phenol. Then, the in situ generated phenylboronic acid (derived in turn from an excess of NaBPh 4 ) is capable of reacting with the tripodal ligand H 2 L, leading to the formation of compound I (Fig. 6).
Inspection of the reaction conditions already reported by Woodgate et al. (1999) indicates that compound II was obtained by reaction of phenylboronic acid and the corresponding tertiary amine. In turn, the authors reported that compound III was obtained unintentionally when using salicylaldehyde benzylamine and boron compounds (Geng et al., 2011). We hypothesize that, in the case of I, the use of NaBPh 4 determined the course of the reaction, leading to the formation of the zwitterionic heterocycle in the described reaction conditions. Comparison of the bonding environment at boron in I (title compound), II (MAWDET) and III (EROJIF). For clarity, only atoms closely involved in the N!B dative bonds are shown. Table 2 Structural data and calculated tetrahedral character THC DA (Å , ) for compounds I-III. (13)

Figure 4
Oxazaborocine compounds structurally related to the title compound.

Synthesis and crystallization
H 2 L (0.064 g; 0.2 mmol) was dissolved in methanol (4 mL), then solid manganese(III) acetate dihydrate (0.052 g; 0.2 mmol) was added. Immediately after, an excess of NaBPh 4 (0.2065 g, 0.60 mmol) in 2 mL of methanol/water was added to the reaction flask. The resulting dark-brown solution was sonicated at 313 K for 15 min and then stirred at reflux for additional 16 h (overnight). After cooling, the obtained precipitate was collected by filtration, washed with diethyl ether and dried in vacuo. Recrystallization from methanol gave colourless crystals of I suitable for X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were placed at calculated positions, with d(C-H) = 0.95À0.99 Å and U iso (H) = 1.2U eq (C).

Figure 6
Synthesis of the title compound.

Computing details
Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.31 e Å −3 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.