A zwitterion produced by a strong intra­molecular N→B inter­action in 1-hy­droxy-2-(pyridin-2-yl­carbon­yl)benzo[d][1,2,3]di­aza­borinine

2-Acyl­ated 2,3,1-benzodi­aza­borines can display unusual structures and reactivities. The crystal structure analysis of the boron heterocycle obtained by condensing 2-formyl­phenyl­boronic acid and picolinohydrazide reveals it to be an N→B-chelated zwitterionic tetracycle (systematic name: 1-hy­droxy-11-oxo-9,10,17λ⁵-tri­aza-1λ⁴-bora­tetra­cyclo­[8.7.0.0^2,7.0^12,17]hepta­deca-3,5,7,12,14,16-hexa­en-17-ylium-1-uide), C₁₃H₁₀BN₃O₂, produced by the intra­molecular addition of the Lewis basic picolinoyl N atom of 1-hy­droxy-2-(pyridin-2-ylcarbon­yl)benzo[d][1,2,3]di­aza­borinine to the boron heterocycle B atom acting as a Lewis acid. Neither of the other two pyridinylcarbonyl isomers (viz. nicotinoyl and isonicotino­yl) are able to adopt such a structure for geometric reasons. A favored yet reversible chelation equilibrium provides an explanation for the slow D₂O exchange observed for the OH resonance in the ¹H NMR spectrum, as well as for its unusual upfield chemical shift. Deuterium exchange may take place solely in the minor open (unchelated) species present in solution.

S1. Introduction

As part of a recent broader investigation into the unusual structures and reactivities of 2-acyl­ated 2,3,1-benzodi­aza­borines, we prepared and characterized boron heterocycles obtained by condensing 2-formyl­phenyl­boronic acid and three isomeric pyridine-based acyl hydrazides (picolinoyl, nicotinoyl, and isonicotinoyl) (see Fig. 1). The title compound, (Ia), obtained using picolinoyl hydrazide, clearly stood out as unique, not only when compared with its two constitutional isomers but also with the nearly four dozen other compounds synthesized. The products obtained using nicotinoyl or isonicotinoyl hydrazides, viz. (Ib) and (Ic), respectively, were established by high-field NMR and single-crystal X-ray crystallography (Kanichar et al., 2014) to be anhydro dimeric boron heterocycles with no exchangeable OH or NH groups. The product obtained using picolinoyl hydrazide, on the other hand, was found to have a D₂O-exchangeable OH resonance at δ 4 p.p.m. in the ¹H NMR spectrum, far upfield from the usual δ 9–10 p.p.m. range observed for those in all other monomeric 1-hy­droxy-2,3,1-di­aza­borines [2,3,1-benzodi­aza­borines or 1,2,3-di­aza­borines] we have studied thus far (Groziak et al., 1997; Robinson et al., 1998; Robinson & Groziak, 1999). The abnormal upfield chemical shift of the OH peak in (Ia) is characteristic of a more shielded local environment with a sizeable electron density, such as that adjacent to a tetra­hedral anionic borate B atom.

In addition, the OH signal in the ¹H NMR spectrum of (Ia) was found to be reduced by deuterium exchange (D₂O added to a (CD₃)₂SO solution) only very slowly. In fact, a reasonable rate of exchange was achieved only upon heating for several minutes at ca 340 K with a heat gun. This observation suggests the existence of an equilibrium in which the ²H-for-¹H exchange takes place only in the minor unchelated structure (see Fig. 1). Likely, the rate of loss of H⁺ from major chelated form (Ia) is substanti­ally diminished by the energetically unfavorable development of adjacent negative charges (i.e. on the O and the B atom) in the transition state for Brønsted acid dissociation of the OH group. The identity of product (Ia), as confirmed by crystallography, is shown in the Scheme.

S2. Experimental

Using picolinic acid hydrazide synthesized from ethyl picolinate (Aldrich) according to the literature method of Yale et al. (1953), the title compound was prepared by condensation with 2-formyl­phenyl­boronic acid (Frontier Scientific) in 50% aqueous EtOH at room temperature (yield 82%, m.p. > 533 K) (Fig. 1). FT–IR, UV, ¹H NMR, and ¹³C NMR as previously reported [see compound (16b) in Kanichar et al. (2014)].

S2.1. Synthesis and crystallization

Crystals of the title compound were grown by slow evaporation from a 50% aqueous ethanol solution.

S2.2. Refinement

The hy­droxy H atom was located from the difference map and its position was allowed to refine. All other H atoms are placed in calculated positions. Displacement parameters for the H atoms were set at 1.2 times the U_eq value of the parent atom. Crystal data, data collection and structure refinement details are summarized in Table 1.

S3. Results and discussion

In the solid-state structure of (Ia) (Fig. 2), the lack of planarity of the 6–6–5–6 tetra­cyclic ring system is due to the inclusion of a tetra­substituted sp³-hybridized ring junctional B atom. Because of the lack of stereofacial selectivity in chelation, molecules of (Ia) in the crystal are present as pairs of enanti­omers. The inter­molecular hydrogen-bonding scheme is shown in Fig. 3, and features a connection between the acceptor carbonyl O atom the the donor OH group on the B atom (Table 2). Infinite chains of these connections are aligned in anti­parallel directions within the structure.

The O2—B1 bond length in (Ia) [1.4235 (9) Å] (Table 3) is significantly longer than that [1.357 (3) Å] in a representative unchelated 1-hy­droxy-2,3,1-benzodi­aza­borine, such as the 1-methyl­ated version (Robinson et al., 1998), but this is typical of the change from an sp²-hybridized trigonal–planar B atom to an sp³-hybridized tetra­hedral anionic borate one (Höpfl, 1999). The length of the intra­molecular N1—B1 bond [1.6189 (10) Å] is close to those [1.687 (6) and 1.678 (6) Å] of the two N→B chelates in 1,3,5-tri­phenyl­boroxine (II), derived from the 1,1-di­methyl­hydrazone of 2-formyl­phenyl­boronic acid (Robinson et al., 1996). Inter­estingly, it is more than 1 Å shorter than those [2.750 (7) and 2.721 (10) Å, respectively] in the 2-formyl­phenyl­boronic acid 2,4-di­nitro­phenyl­hydrazones (IVa) and (IVb) we reported previously (Groziak & Robinson, 2002), despite the fact that the same type of N atom (imine) is involved in an intra­molecular chelating inter­action forming the same five-membered size ring. As a point of reference, the sum of the van der Waals radii of nitro­gen and boron is 2.91 Å (Dvorak et al., 1992). Clearly, the N/O→B inter­actions in (Ia), (II), and (III) are strong and produce substantial charge separation, whereas those in (IVa) and (IVb) do not.

An indirect approach to assessing the build-up of negative charge on the B atom of (Ia) is to calculate Höpfl's THCDA (tetra­hedral character) value using the values of all six bond angles around the B atom (Höpfl, 1999). Doing this, we find that the THCDA of (Ia) is 67.9%, a fairly high value. Taken together with the relativly short N1—B1 bond length [1.6189 (10) Å], a parameter which correlates inversely quite well with THCDA, the crystal structure data describes a molecule with substantial sp³-hybridization of, and a commensurate build-up of negative charge at, the B atom.

In conclusion, molecule (Ia) reveals that the B atom of a 1-hy­droxy-2,3,1-benzodi­aza­borine can act as a Lewis acid toward a judiciously positioned pyridine-type N atom, and that an intra­molecular chelating connection between these two sites can form a very stable N→B zwitterionic heterocycle. It is noteworthy that molecule (Ia) can be viewed as a chemical model for the enzyme inhibition effected by certain di­aza­borine anti­bacterial agents (Baldock et al., 1996), which form a covalent B—O bond with the 2'-hy­droxy group of the NAD cofactor's ribose unit, producing an anionic borate species at the active site.

Computing details

Data collection: Apex2 (Bruker, 2013); cell refinement: SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXT (Sheldrick, 2015); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: SHELXTL, XP (Sheldrick, 2008); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015).

Figures

	Fig. 1. Boron heterocycles produced from condensation reactions.
	Fig. 2. A view of the title molecule, with displacement ellipsoids drawn at the 50% probability level.
	Fig. 3. Infinite chains of O—H···O interactions as viewed down the b axis.

1-Hydroxy-11-oxo-9,10,17λ⁵-triaza-1λ⁴-boratetracyclo[8.7.0.0^2,7.0^12,17]heptadeca-3,5,7,12,14,16-hexaen-17-ylium-1-uide

Crystal data

C13H10BN3O2	F(000) = 520
Mr = 251.05	Dx = 1.450 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 12.6613 (9) Å	Cell parameters from 9955 reflections
b = 8.0054 (6) Å	θ = 3.1–33.3°
c = 12.8345 (9) Å	µ = 0.10 mm^−1
β = 117.8900 (9)°	T = 90 K
V = 1149.79 (14) Å^3	Block, colourless
Z = 4	0.49 × 0.47 × 0.41 mm

Data collection

Bruker APEXII diffractometer	4160 independent reflections
Radiation source: fine-focus sealed tube	3852 reflections with I > 2σ(I)
Detector resolution: 8.3 pixels mm^-1	Rint = 0.016
ω scans	θmax = 32.6°, θmin = 3.1°
Absorption correction: multi-scan (SADABS; Bruker, 2013)	h = −19→19
Tmin = 0.717, Tmax = 0.747	k = −12→12
19673 measured reflections	l = −19→19

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	Hydrogen site location: mixed
wR(F^2) = 0.103	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0626P)^2 + 0.3289P] where P = (Fo^2 + 2Fc^2)/3
4160 reflections	(Δ/σ)max = 0.001
176 parameters	Δρmax = 0.53 e Å^−3
0 restraints	Δρmin = −0.26 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.72288 (5)	0.43841 (7)	1.07877 (5)	0.01279 (11)
O2	0.39677 (5)	0.19933 (7)	0.78509 (5)	0.01185 (11)
H2	0.3456 (14)	0.1705 (19)	0.7135 (13)	0.031 (4)*
N1	0.47321 (5)	0.48828 (7)	0.80120 (5)	0.00929 (11)
N2	0.60795 (5)	0.27628 (7)	0.91166 (5)	0.01022 (11)
N3	0.68326 (6)	0.13698 (8)	0.94552 (6)	0.01352 (12)
C1	0.38418 (6)	0.58487 (9)	0.72459 (6)	0.01177 (13)
H1	0.3261	0.5396	0.6518	0.014*
C2	0.37680 (7)	0.75140 (9)	0.75174 (6)	0.01313 (13)
H2A	0.3150	0.8209	0.6969	0.016*
C3	0.46076 (7)	0.81522 (9)	0.86000 (7)	0.01324 (13)
H3	0.4572	0.9292	0.8788	0.016*
C4	0.55020 (6)	0.71138 (9)	0.94100 (6)	0.01164 (13)
H4	0.6071	0.7517	1.0161	0.014*
C5	0.55285 (6)	0.54788 (8)	0.90785 (6)	0.00926 (12)
C6	0.63918 (6)	0.41399 (8)	0.97945 (6)	0.00947 (12)
C7	0.68720 (7)	0.06540 (9)	0.85675 (7)	0.01471 (14)
H7	0.7313	−0.0358	0.8729	0.018*
C8	0.63098 (6)	0.12357 (9)	0.73395 (6)	0.01323 (13)
C9	0.67320 (7)	0.06208 (11)	0.65779 (8)	0.01824 (15)
H9	0.7338	−0.0210	0.6846	0.022*
C10	0.62642 (8)	0.12274 (11)	0.54320 (8)	0.01945 (15)
H10	0.6551	0.0815	0.4917	0.023*
C11	0.53732 (7)	0.24428 (10)	0.50434 (7)	0.01701 (14)
H11	0.5058	0.2867	0.4264	0.020*
C12	0.49410 (7)	0.30404 (9)	0.57945 (6)	0.01332 (13)
H12	0.4330	0.3864	0.5516	0.016*
C13	0.53912 (6)	0.24491 (9)	0.69477 (6)	0.01083 (12)
B1	0.49478 (7)	0.29145 (9)	0.78914 (7)	0.00970 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0110 (2)	0.0135 (2)	0.0093 (2)	−0.00167 (17)	0.00085 (18)	0.00028 (17)
O2	0.0102 (2)	0.0128 (2)	0.0107 (2)	−0.00248 (17)	0.00334 (18)	−0.00150 (17)
N1	0.0087 (2)	0.0096 (2)	0.0083 (2)	0.00085 (18)	0.00293 (19)	0.00057 (18)
N2	0.0089 (2)	0.0084 (2)	0.0103 (2)	0.00142 (18)	0.00188 (19)	0.00035 (18)
N3	0.0110 (3)	0.0101 (3)	0.0151 (3)	0.00293 (19)	0.0025 (2)	0.0016 (2)
C1	0.0107 (3)	0.0130 (3)	0.0099 (3)	0.0028 (2)	0.0034 (2)	0.0020 (2)
C2	0.0135 (3)	0.0116 (3)	0.0143 (3)	0.0032 (2)	0.0064 (2)	0.0032 (2)
C3	0.0150 (3)	0.0095 (3)	0.0160 (3)	0.0010 (2)	0.0079 (2)	0.0012 (2)
C4	0.0126 (3)	0.0096 (3)	0.0121 (3)	−0.0009 (2)	0.0053 (2)	−0.0004 (2)
C5	0.0085 (3)	0.0097 (3)	0.0086 (3)	−0.0004 (2)	0.0032 (2)	0.0005 (2)
C6	0.0085 (3)	0.0097 (3)	0.0093 (3)	−0.0007 (2)	0.0034 (2)	0.0010 (2)
C7	0.0119 (3)	0.0114 (3)	0.0174 (3)	0.0030 (2)	0.0040 (2)	0.0002 (2)
C8	0.0113 (3)	0.0121 (3)	0.0154 (3)	0.0012 (2)	0.0054 (2)	−0.0018 (2)
C9	0.0156 (3)	0.0178 (3)	0.0228 (4)	0.0028 (3)	0.0103 (3)	−0.0040 (3)
C10	0.0197 (3)	0.0218 (4)	0.0220 (4)	−0.0003 (3)	0.0140 (3)	−0.0052 (3)
C11	0.0193 (3)	0.0195 (3)	0.0153 (3)	−0.0021 (3)	0.0107 (3)	−0.0026 (3)
C12	0.0135 (3)	0.0143 (3)	0.0123 (3)	0.0002 (2)	0.0062 (2)	−0.0008 (2)
C13	0.0095 (3)	0.0105 (3)	0.0115 (3)	−0.0002 (2)	0.0042 (2)	−0.0016 (2)
B1	0.0087 (3)	0.0093 (3)	0.0090 (3)	0.0008 (2)	0.0023 (2)	−0.0002 (2)

Geometric parameters (Å, º)

O1—C6	1.2339 (8)	C4—C5	1.3816 (9)
O2—B1	1.4235 (9)	C4—H4	0.9500
O2—H2	0.872 (15)	C5—C6	1.4994 (9)
N1—C1	1.3411 (8)	C7—C8	1.4690 (11)
N1—C5	1.3516 (8)	C7—H7	0.9500
N1—B1	1.6189 (10)	C8—C9	1.4045 (10)
N2—C6	1.3442 (9)	C8—C13	1.4149 (10)
N2—N3	1.3978 (8)	C9—C10	1.3909 (12)
N2—B1	1.5597 (10)	C9—H9	0.9500
N3—C7	1.2969 (10)	C10—C11	1.3935 (12)
C1—C2	1.3920 (10)	C10—H10	0.9500
C1—H1	0.9500	C11—C12	1.3967 (10)
C2—C3	1.3927 (10)	C11—H11	0.9500
C2—H2A	0.9500	C12—C13	1.3965 (10)
C3—C4	1.3974 (10)	C12—H12	0.9500
C3—H3	0.9500	C13—B1	1.5991 (10)
B1—O2—H2	112.4 (10)	N3—C7—H7	116.4
C1—N1—C5	120.80 (6)	C8—C7—H7	116.4
C1—N1—B1	127.78 (6)	C9—C8—C13	120.69 (7)
C5—N1—B1	111.17 (5)	C9—C8—C7	118.98 (7)
C6—N2—N3	120.11 (6)	C13—C8—C7	120.28 (6)
C6—N2—B1	116.24 (6)	C10—C9—C8	120.07 (7)
N3—N2—B1	123.16 (6)	C10—C9—H9	120.0
C7—N3—N2	112.36 (6)	C8—C9—H9	120.0
N1—C1—C2	120.03 (6)	C9—C10—C11	119.68 (7)
N1—C1—H1	120.0	C9—C10—H10	120.2
C2—C1—H1	120.0	C11—C10—H10	120.2
C1—C2—C3	119.46 (6)	C10—C11—C12	120.34 (7)
C1—C2—H2A	120.3	C10—C11—H11	119.8
C3—C2—H2A	120.3	C12—C11—H11	119.8
C2—C3—C4	119.91 (7)	C13—C12—C11	121.19 (7)
C2—C3—H3	120.0	C13—C12—H12	119.4
C4—C3—H3	120.0	C11—C12—H12	119.4
C5—C4—C3	117.55 (6)	C12—C13—C8	118.00 (6)
C5—C4—H4	121.2	C12—C13—B1	127.58 (6)
C3—C4—H4	121.2	C8—C13—B1	114.34 (6)
N1—C5—C4	122.15 (6)	O2—B1—N2	111.74 (6)
N1—C5—C6	110.67 (6)	O2—B1—C13	117.43 (6)
C4—C5—C6	127.18 (6)	N2—B1—C13	105.25 (5)
O1—C6—N2	130.47 (6)	O2—B1—N1	108.94 (5)
O1—C6—C5	123.20 (6)	N2—B1—N1	95.50 (5)
N2—C6—C5	106.27 (6)	C13—B1—N1	115.71 (6)
N3—C7—C8	127.15 (7)
C6—N2—N3—C7	−140.75 (7)	C9—C10—C11—C12	−0.63 (12)
B1—N2—N3—C7	30.86 (9)	C10—C11—C12—C13	0.40 (12)
C5—N1—C1—C2	3.60 (10)	C11—C12—C13—C8	0.65 (11)
B1—N1—C1—C2	177.29 (6)	C11—C12—C13—B1	−176.07 (7)
N1—C1—C2—C3	−1.56 (11)	C9—C8—C13—C12	−1.48 (11)
C1—C2—C3—C4	−1.06 (11)	C7—C8—C13—C12	175.86 (7)
C2—C3—C4—C5	1.61 (10)	C9—C8—C13—B1	175.67 (7)
C1—N1—C5—C4	−3.06 (10)	C7—C8—C13—B1	−6.99 (10)
B1—N1—C5—C4	−177.71 (6)	C6—N2—B1—O2	−109.07 (7)
C1—N1—C5—C6	176.93 (6)	N3—N2—B1—O2	79.01 (8)
B1—N1—C5—C6	2.28 (7)	C6—N2—B1—C13	122.40 (6)
C3—C4—C5—N1	0.40 (10)	N3—N2—B1—C13	−49.51 (8)
C3—C4—C5—C6	−179.60 (6)	C6—N2—B1—N1	3.85 (7)
N3—N2—C6—O1	−7.92 (11)	N3—N2—B1—N1	−168.07 (6)
B1—N2—C6—O1	179.90 (7)	C12—C13—B1—O2	85.05 (9)
N3—N2—C6—C5	169.28 (6)	C8—C13—B1—O2	−91.77 (8)
B1—N2—C6—C5	−2.90 (8)	C12—C13—B1—N2	−149.91 (7)
N1—C5—C6—O1	177.72 (6)	C8—C13—B1—N2	33.27 (8)
C4—C5—C6—O1	−2.29 (11)	C12—C13—B1—N1	−45.93 (10)
N1—C5—C6—N2	0.26 (8)	C8—C13—B1—N1	137.25 (6)
C4—C5—C6—N2	−179.75 (7)	C1—N1—B1—O2	−62.41 (9)
N2—N3—C7—C8	5.37 (11)	C5—N1—B1—O2	111.78 (6)
N3—C7—C8—C9	160.50 (8)	C1—N1—B1—N2	−177.65 (6)
N3—C7—C8—C13	−16.88 (12)	C5—N1—B1—N2	−3.47 (7)
C13—C8—C9—C10	1.28 (12)	C1—N1—B1—C13	72.49 (9)
C7—C8—C9—C10	−176.09 (7)	C5—N1—B1—C13	−113.33 (6)
C8—C9—C10—C11	−0.21 (13)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1^i	0.872 (15)	1.911 (15)	2.7605 (7)	164.4 (14)

Symmetry code: (i) x−1/2, −y+1/2, z−1/2.