Synthesis and crystal structure of bis(9-mesityl-9,10-dihydro-10-aza-9-borabenzo[h]quinolinato-κ2 N 1,N 10)zinc(II)

The NH-deprotonation of a 10-aza-9-borabenzo[h]quinoline yields a bidentate ligand that was used for the synthesis of a 2:1 coordination complex with zinc. Its crystal packing is dominated by intense intra- and intermolecular π–π stacking interactions.


Chemical context
1,2-Azaborinine is an aromatic six-membered ring that consists of a polar boron-nitrogen unit and a butadienyl moiety, making it an isoelectronic congener of benzene.Its strikingly similar geometry in conjunction with a significantly altered electron distribution has promoted research on monoand polycyclic aromatic hydrocarbons (PAHs) with a BN substitution pattern.Several studies highlighted the BNinduced tailored adjustment of chemical, physical and optical properties, enabling the application of such heteroaromatics for instance as white-emitting layers in organic light-emitting diodes (Hoffmann et al., 2021), as reversible hydrogen storage materials (Campbell et al., 2010) or as building blocks in pharmaceuticals with increased bioavailability (Zhao et al., 2017).Relatively few reports made use of the selectively deprotonable NH group (pK a ' 24) to introduce electrophilic functional groups or metal atoms (Pan et al., 2004;Lamm et al., 2011;Baggett & Liu, 2017;Lindl et al., 2023).
In a previous study (Appiarius et al., 2021), a BN-substituted benzo[h]quinoline (HL), containing one 1,2-azaborininyl-and one pyridyl subunit with both nitrogen atoms beneficially preorganized for chelation was presented.In the context of this communication, we report on the synthesis and crystal structure of the 2:1 coordination complex of ligand L with zinc(II).

Structural commentary
The molecular structure of the title compound (C 40 H 36 B 2 N 4 Zn, ZnL 2 ) is illustrated in Fig. 1.The coordination complex crystallizes in the monoclinic C2/c centrosymmetric space group with one zinc(II) cation and one ligand molecule in the asymmetric unit, being completed by the application of inversion symmetry at Zn II .The latter is fourfold coordinated by two types of N donors, namely the azaborinine and pyridine subunits comprised in the BN-benzo[h]quinoline.This results in a significantly distorted tetrahedral configuration [bond angle N1-Zn1-N2 84.72 (4) � ; all other N-Zn-N bond angles > 118 � , see Table 1].The bond lengths within the 1,2-azaborinine motif of the ligand [B1-N1: 1.4245 (17) A ˚, B1-C11: 1.5315 (19) A ˚, N1-C1: 1.3580 (14) A ˚] are in characteristic ranges (Paetzold et al., 2004;Pan et al., 2009), confirming electron delocalization and an elevated aromatic character.The N-Zn bond lengths [N1-Zn1: 1.9606 (10) A ˚, N2-Zn1: 2.0527 (10) A ˚] are in excellent agreement with bis(2-(2 0 -pyridyl)pyrrolyl)zinc [N pyrrole -Zn 1.9513 (18) A ˚, N pyridine -Zn 2.0444 (18) A ˚; Wang et al., 2009], supporting the electronic similarities of 1,2-azaborinine and pyrrole (Davies et al., 2017).This contrasts with zinc complexes involving the geometrically similar but uncharged 1,10-phenanthroline ligand (N pyridine -Zn 2.13-2.20A ˚) with higher coordination numbers of the central ion.All aromatic rings within the BN-PAH ligand are close to planar, with an average torsion angle of 2.2 � and a maximum deviation of an atom from the mean aromatic plane of 0.0217 (8) A ˚.In contrast, the Zn II ion is located 0.365 (2) A out of the mean N1-C1-C2-N2 plane and points in the direction of the mesityl ring of the second ligand unit.The 1,2azaborinine motif and the attached planar mesityl group [maximum deviation from the mean aromatic plane: 0.0125 (9) A ˚] are oriented almost perpendicularly to each other, with an angle between their mean planes of 79.41 (4) � .

Supramolecular features and Hirshfeld surface analysis
A Hirshfeld surface (Hirshfeld, 1977, Fig. 2) and the respective two-dimensional fingerprint plots (Fig. 3) were generated using CrystalExplorer21.5 (Spackman et al., 2021) to analyze the intermolecular interactions.No close atom contacts involving the boron and zinc heteroatoms and a negligible participation of the nitrogen atom (N� � �H: 1.5%, C� � �N: 0.4%) were found.Therefore, the intermolecular interactions were almost exclusively caused by van der Waals forces Molecular structure of ZnL 2 with atom labeling.The image was generated with ORTEP-3 for Windows (Farrugia, 2012).Non-hydrogen atoms as displacement ellipsoids drawn at the 50% probability level.Hydrogen atoms were omitted for clarity.[Symmetry code:
involving carbon and hydrogen.In particular, close H� � �H contacts and aromatic interactions dominate the overall intermolecular interactions in a crystal.The 'wings' at the top left (d i ' 1.05 A ˚and d e ' 1.60 A ˚) and their pseudosymmetrical counterparts at the bottom right of the twodimensional fingerprint plot correspond to C-H� � �� interactions.These are also mapped by several red spots on the Hirshfeld surface (Spackman & McKinnon, 2002).Moreover, considerable �-� stacking interactions are apparent by the light coloring of the Hirshfeld surface around the PAH backbone and intense C� � �C contacts (5.8%).The crystal packing shows that each aromatic ligand has one ligand unit of another molecule in close proximity, so that pairs of almost parallel but slightly displaced sheets in two dimensions result (Fig. 4).In particular, the phenyl and pyridyl subunits of neighboring molecules show a significant overlap, with an offset of only 1.181 (2) A ˚and a minimum interplanar distance of 3.3826 (13) A ˚. On the other hand, the PAH scaffolds and the mesityl �-planes of the inverse ligand units are aligned almost coplanar [mesityl-pyridine interplanar angle: 1.46 (5) � ] with a similarly small minimum interplanar distance [3.3996 (10) A ˚]. Therefore, intramolecular �-� stacking contributes significantly to the overall stabilizing forces.We assume that the discussed, unusual off-plane position of the zinc ion and the increased angle between the mean mesityl plane and the B1-C12 bond [7.93 ( 8) � ] also derives from this favorable stacking geometry.

Database survey
A survey of the Cambridge Structural Database (WebCSD version 1.9.32, accessed in July 2023; Groom et al., 2016) revealed that 654 crystal structures of six-membered carbocycles with 1-aza-2-bora substitution patterns have been reported.Among these, 101 structures comprising B-mesityl substituents have been deposited, which involves aromatic 1,2azaborinine subunits for the most part.The crystal structures of 13 compounds with 1,2-azaborinine substructures and nitrogen-metal bonds have been published, of which different lithium solvates as well as potassium, beryllium, aluminum, gallium and tin complexes are included in one publication (Lindl et al., 2023).Moreover, one study describes several complexes of a bidentate ligand with aluminum (Appiarius et al., 2023).However, there are only three reports of 1,2-azaborinines with N-transition-metal bonds, including zirconium (refcode JIZQEP; Pan et al., 2008), ruthenium (refcode DOXBEY; Pan et al., 2008) and iridium (refcode NEZXAV; Baschieri et al., 2023).Also, the structure of a 6-pyridyl-1,2azaborinine has been reported, which is structurally similar to HL and was used for the preparation of a dimesitylboron complex (refcode WUGMIW; Baggett et al., 2015).The search query for coordination complexes of zinc with 1,2-azaborinine ligands did not yield any results.

Synthesis and crystallization
The synthesis of ZnL 2 is shown in Fig. 5.Under argon at 298 K, 9,10-dihydro-9-mesityl-10-aza-9-borabenzo[h]quinoline (HL, 29.8 mg, 100 mmol, 1.00 equiv., prepared according to Appiarius et al., 2021) was dissolved in THF (1.5 mL).A solution of lithium bis(trimethylsilyl)amide (1.0 M in THF, 120 mL, 1.20 equiv.) was added, before a solution of diethylzinc (15% w/w in hexanes, 230 mL, 2.00 equiv.) was added via a syringe.The mixture was heated to 428 K for 17 h while stirring.In a glove box, the volatiles were removed under reduced pressure.The residue was extracted with n-hexane (3 � 2 mL) and the solvent was removed.The crude product was dissolved in THF (500 mL) and n-hexane was allowed to diffuse into this solution over the course of 3 d.The light-yellow product (6.4 mg, 19%) was obtained as air-sensitive crystals suitable

Figure 5
Reaction scheme for the synthesis of ZnL 2 .
for X-ray diffraction analysis by repeating this process twice.

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.

Figure 3
Figure 3 Two-dimensional fingerprint plots, showing the contributions of the individual elements in close atom contacts.

Figure 4
Figure 4 Section of the crystal packing, showing the �-� stacking interactions propagating in two dimensions.BN rings are shown in orange, mesityl rings in yellow, phenyl and pyridyl rings in blue.

Table 2
Experimental details.
Computer programs: APEX2 and SAINT