Unconventional hydrogen and dihydrogen bonded supramolecular array of a 2,6-dioxa-9,16-diaza-1,3(1,2),4(1,4)- tribenzenacycloheptadecaphane-borane adduct

The crystal structure of the title borane-oxaazacyclophane adduct consists of a tri-dimensional supramolecular array sustained by unconventional N-H···H-B and C-H···H-B dihydrogen bonding and weak C-H··· π interactions. The interchain organization in the crystal structure is characterized by weak C-H···H-B dihydrogen bonds.


Introduction
The importance of hydrogen bonds in many physical, chemical, and biological processes is broadly accepted nowadays.Typical hydrogen bonds are those where the hydrogen atom lies between two electronegative atoms, described as X-H•••Y (X, Y = O, N, F).Hydrogen bonds that involve donors and/or acceptors different from those described previously are called unconventional hydrogen bonds.Different types of unconventional hydrogen bonds, such as those where the acceptors correspond to a π electron density, a metal-hydrogen bond and where the donors are C-H, have been described. 1Experimental and theoretical studies have been performed to understand the nature of these unconventional hydrogen bonds, 1a-b, 2 because they play key roles to elucidate some physical, chemical, and biological properties. 3he dihydrogen bond, 1a-b described as X-H•••H-Y (X = O, N, Y = B or transition metal) is possibly the most surprising unconventional hydrogen bond due to its nature and ability to influence the structure, reactivity, and selectivity in both solution and solid state. 4everal structural and energetic similarities have been observed between the conventional hydrogen bond and the dihydrogen bond.The H•••H distance in X-H•••H-Y systems typically ranges from 1.7 to 2.4 Å.The interaction energy also lies within the range of typical hydrogen bonds, 3-10 kcal/mol.The linearity of normal hydrogen bonds is preserved in unconventional hydrogen bonds.The H•••H-X angles generally lie within 160-180°.However, the Y-H•••H angles are found to be strongly bent, falling in the range of 95-130°.2c The C-H•••π interaction is another class of unconventional hydrogen bonds that is gaining attention because of its role as the driving force in determining crystal packing, molecular conformation, and alignment of liquid crystals. 5This interaction is characterized by an inter-or intra-molecular distance in the range of 2.6-3.0Å.Furthermore, the C-H bond points close to the center of an aromatic ring and the angle between the C-H bond and the center of the aromatic ring is close to linearity.2d Despite being the weakest hydrogen bond, the C-H•••π interaction can influence the molecular recognition pattern in a host-guest system and should, therefore, be taken into account.2b On the other hand, the H 3 NBH 3 complex is perhaps the most studied complex with dihydrogen bonds.It exhibits multiple N-H•••H-B intermolecular interactions in the solid state with H•••H distance of 2.02 Å. 6 Besides, it has been proposed that some physical properties of this compound come from the dihydrogen bonds. 7orane-amine adducts are widely used in modern synthetic organic chemistry because of their capability to carry out reductions of aldehydes and ketones, reductive aminations, olefin hydroborations, and amide reductions.Complexes of amine-boranes have also been utilized as chiral transfer agents, and as a protective device against nitrogen lone pair oxidation. 8Usually, borane-amine complexes are readily formed by reduction of amides, imides, and imines, or by treatment of the corresponding amine with diborane or borane.
We are interested in the synthesis, characterization, and molecular recognition studies of synthetic and semi-synthetic macrocycles. 9During the course of our research, in attempting to obtain one of these macrocyclic compounds, we obtained a borane adduct; hence, the aim of the present work is to emphasize the relevance of unconventional hydrogen bonds, such as protonhydride and C-H•••π, in organizing interactions in the borane-azacyclophane complex in the solid state.

Results and Discussion
In order to prepare macrocycle 2, we reacted the Schiff base 1 with LiBH 4 in THF, 9b Scheme 1.The reaction was monitored by TLC (heptane:acetone, 60:40) until a single product was observed and the starting material disappeared.Then, water was added and generation of gas was observed.Purification by chloroform-water extraction produced a stable white powder with a very good yield.
As expected, the IR-spectrum of the product showed N-H stretching (3268 cm -1 ), and the characteristic band for imine group (1644 cm -1 ) was not observed.Besides, four unexpected bands were present at 2390, 2271, 1603, and 1590 cm -1 .First we thought that LiBH 4 was present but the 1 H NMR spectrum of this product was much more complex than that we had expected for the macrocycle 2; we then concluded that this compound should be the borane-oxaazacyclophane 3 (see Figure 1) having its nitrogen atoms as chiral centers and therefore hydrogen atoms on the adjacent carbon are now diastereotopic.To confirm the assigned structure, the 11   It is important to point out that compound 3 is air-and hydrolysis-stable even in basic media.This increased stability may be attributed to some structural and electronic effects present in the molecule.
We were able to get single crystals of compound 3, and the crystalline structure is shown in Figure 2. In this figure, the 17-membered oxaazacyclophane with two BH 3 groups, coordinated to its nitrogen atoms, can be seen.The internal ring cavity dimension is 8.53 x 5.14 Å.

The unconventional N-H•••H-B and C-H•••H-B dihydrogen bonds and the C-H•••π interactions
play an important role in determining the supramolecular array in the crystal structure of the oaxaazacyclophane-borane adduct and they all influence the stability of this compound to hydrolysis.

Experimental Section
General Procedures.All solvents and reagents were used as received without further purification. 1H NMR and 13 C NMR spectra were recorded with Varian INOVA 400 and 500 or Bruker AVANCE-400 spectrometers using TMS and BF 3 -OEt 2 as standard references.Infrared spectra were obtained in solid KBr using a FT-IR Perkin Elmer Spectrum GX.Cold electrospray mass spectrometric measurements were performed with a JEOL JMS 700 equipment, cooling the supply tube with dry ice.Elemental analyses were carried out on a Perkin-Elmer series II 2400 instrument.Elemental analysis for carbon was not satisfactory.Elemental analyses of boron derivatives are particularly ARKAT USA, Inc. complicated because of the production of incombustible residues and are, therefore, not always in the established limits of exactitude, especially with respect to carbon. 11X-ray diffraction studies were performed with a BRUKER-AXS APEX diffractometer with a CCD area detector (0.71073 Å, monochromator: graphite).Frames were collected via ω-rotation at 10 s per frame (SMART 12a ).The measured intensities were reduced to F2 and corrected for absorption with SADABS (SAINT-NT 12b ).Structure solution, refinement, and data output were achieved with the SHELXTL-NT.Non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed in geometrically calculated positions using a riding model.The molecular and crystal structures were created by the BRUKER SHELXTL and MERCURY software packages.12c,13 Details of the crystal structure determination and solution refinement: colorless prism with dimensions 0.525 × 0.5 × 0.275 mm, M = 345.87,Triclinic, P-1, a=10.3152(10), b=10.9207(11), c=15.6942(16)Å, α = 110.262(2),β = 98.274(2), γ = 101.512(2)°,V = 1580.9(3)Å 3 , Z = 2, T = 293(2) K, ρ calc = 1.213,F(000) = 612, µ = 0.318 mm -1 , λ = 0.71073 Å; 4128 total independent reflections measured, 3046 reflections observed with I ≥ 2σ(I); number of parameters refined = 367, R = 0.0893, goodness of fit = 1.197.Crystallographic data for 3 have been deposited at the Cambridge Crystallographic Data Center as CCDC No 650762.Copies of the data can be obtained free of charge upon application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (e-mail: deposit@ccdc.cam.ac.uk).
B NMR spectrum was recorded and a broad signal (3764 Hz ) at δ = -17 ppm was observed.The 11 B-1 H coupled spectrum gave an even broader signal (5621 Hz ).

Figure 5 .
Figure 5. Intermolecular array in the lattice of 3.