Tris[(1-methylimidazol-2-yl)methyl]amine–boric acid (1/1)

Cocrystallization of a poly­imidazole compound with boric acid results in the formation of the title compound, C₁₅H₂₁N₇·B(OH)₃, which has an extensive hydrogen-bonding network. The O...N(im) separations (im is imidazole) range from 2.6991 (15) to 2.7914 (14) Å, with O—H...N angles ranging from 170.6 (18) to 175 (2)°. In addition, symmetry-related boric acid mol­ecules form intermolecular hydrogen bonds, with an O...O distance of 2.7582 (14) Å, and symmetry-related imidazole groups form π–π stacks, with a centroid-to-centroid separation of 3.533 Å.

S1. Comment

The donor-acceptor properties of B(OH)₃ are well known (Farmer, 1982; Coddington & Taylor, 1989). In neutral aqueous solutions, B(OH)₃ generally behaves as a Lewis acid (Farmer, 1982), forming B(OH)₄^- anions rather than the conjugate base BO(OH)₂^-, which has been observed in the solid state in [Cu₂{BO(OH)₂}(OH)₃] (Behm & Baerlocher, 1985) and (Et₄N)₂[BO(OH)₂]₂B(OH)₃·5H₂O (Freyhardt & Wiebcke, 1994). Boric acid is also known to co-crystallize with urea, forming supramolecular structures containing complex intermolecular hydrogen-bonding networks (Li et al., 1999). These compounds and their derivatives are of interest as host lattices for the inclusion (Li & Mak, 1997a) of small molecules and ions (Li & Mak, 1997b).

With this in mind, the present study explores the potential use of boric acid in the crystallization of heterocyclic polyamine compounds, such as the polyimidazole ligand tris[(1-methylimidazol-2-yl)methyl]amine (tmima). As with many polyimidazole compounds (Chen et al., 1994), tmima is hygroscopic and does not crystallize readily using conventional crystallization methods. Here, we describe a general method for crystallizing N-alkylated imidazole compounds in the presence of boric acid, and the X-ray crystal structure of the resulting tmima-boric acid complex, the title compound, (I). \sch

Compound (I) crystallizes with one tmima and boric acid molecule per asymmetric unit, and the structures of the independent tmima and boric acid molecules are shown in Fig. 1. The tmima molecule has approximate threefold symmetry, with the imidazole pendants oriented in a paddle-wheel arrangement around the amine atom N1. The imidazole C—C and C—N distances and C—C—N and C—N—C angles are normal and consistent with other imidazole compounds (Richardson et al., 1988). The boric acid molecule, on the other hand, is planar and has local C_{3 h} symmetry. The B—O distances and O—B—O angles are consistent with the values reported for other boric acid structures (Andrews et al., 1983), and all three B(OH)₃ H atoms are involved in hydrogen bonding.

The packing arrangement in (I) (Fig. 2) shows the presence of an extended intermolecular hydrogen-bonding network between tmima and boric acid molecules in the ac plane and projected along the crystallographic b axis. These layers are slightly less than one unit cell deep along the a axis, and are separated by hydrophobic boundaries composed of imidazole N—CH₃ and ring H atoms (deposited Fig. 3).

The two types of hydrogen-bonding interactions are illustrated in Fig. 2. The first involves bonds between boric acid molecules and different imidazole pendants of neighboring tmima molecules. The O1···N6 [2.7914 (14) Å] and O3'···N2 [2.6991 (15) Å] separations, as well as the O1—H1O···N6 [170.6 (18)°] and O3'-H3O'···N2 [175 (2)°] angles, indicate strong hydrogen bonding between different tmima pendants and boric acid molecules (Table 1). A second type of hydrogen bonding occurs between symmetry-related B(OH)₃ molecules. The resulting eight-membered ring formed via two O···H bonds is planar, and the O2···O3' [2.7582 (14) Å] separations and O2—H2O···O3' [173 (2)°] angles indicate strong hydrogen bonding between B(OH)₃ molecules.

Fig. 2 also shows the intermolecular π–π stacking interactions between tmima molecules. Imidazole rings containing atom N2 form π–π stacks around inversion centers. The separation between imidazole ring centroids is 3.533 Å, indicative of strong π–π interactions between the imidazole rings. Curiously, the third imidazole pendant, containing atom N4, is not involved in hydrogen bonding, nor in π–π stacking, but rather is oriented towards the methyl groups of neighboring tmima molecules, which are part of the hydrophobic boundary between hydrogen-bonding layers. The closest H···H contact between layers is 2.560 Å.

In conclusion, we have established the effectiveness of boric acid for inducing crystallization of the polyimidazole compound tmima. The crystal structure shows an interesting hydrogen-bonding network between imidazole and boric acid molecules along the crystallographic b axis, as well as π–π stacking interactions between imidazole groups.

S2. Experimental

Tris[(1-methylimidazol-2-yl)methyl]amine (tmima) was prepared following the procedure of Oberhausen et al. (1990). Crystallization of tmima was induced by addition of boric acid (0.052 g, 0.84 mmol) dissolved in ethyl acetate (1 ml) containing H₂O (5%) and tmima (0.25 g, 0.84 mmol). The resulting solution was carefully layered with diethyl ether and large colorless single crystals of (I) formed after 12 h at 298 K.

S3. Refinement

The imidazole and methylene H atoms were calculated and included as fixed contributions, with U_iso(H) = 1.2U_eq(C). The methyl groups were calculated and allowed to ride (the torsion angle which defines the orientation was allowed to refine) on the attached C atom, and these atoms were assigned U_iso(H) = 1.5U_eq(C). Fixed C—H distances were in the range 0.95–0.99 Å. Is this added text OK? The H atoms of the boric acid molecule were located in difference Fourier maps and refined isotropically.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS86 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and SHELXTL (Bruker, 2001); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the two molecular components of (I), with 50% probability displacement ellipsoids. Methyl and methylene H atoms have been omitted for clarity.
	Fig. 2. A packing diagram for (I), displaying the hydrogen-bonding interactions between imidazole rings and boric acid molecules, and the π–π stacking interactions between imidazole rings containing N2.

Tris[(1-methylimidazol-2-yl)methyl]amine boric acid

Crystal data

C15H21N7·B(HO)3	F(000) = 768
Mr = 361.22	Dx = 1.285 Mg m^−3 Dm = 1.28 Mg m^−3 Dm measured by pycnometry
Monoclinic, P21/c	Melting point = 458–459 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 16.2812 (17) Å	Cell parameters from 6545 reflections
b = 10.001 (1) Å	θ = 2.4–28.1°
c = 12.2564 (13) Å	µ = 0.09 mm^−1
β = 110.72 (2)°	T = 100 K
V = 1866.6 (3) Å^3	Plate, colorless
Z = 4	0.24 × 0.21 × 0.08 mm

Data collection

Bruker SMART APEX CCD area-detector diffractometer	4361 independent reflections
Radiation source: fine-focus sealed tube	3542 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ω scans	θmax = 28.3°, θmin = 2.4°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −21→20
Tmin = 0.90, Tmax = 0.99	k = −13→13
16065 measured reflections	l = −15→16

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)] = 0.042	Hydrogen site location: mixed
wR(F^2) = 0.110	H atoms treated by a mixture of independent and constrained refinement
S = 1.03	w = 1/[σ^2(Fo^2) + (0.044P)^2 + 0.7596P] where P = (Fo^2 + 2Fc^2)/3
4361 reflections	(Δ/σ)max = 0.028
250 parameters	Δρmax = 0.31 e Å^−3
0 restraints	Δρmin = −0.21 e Å^−3

Special details

Experimental. Data were collected with a Bruker SMART APEX CCD-based diffractometer using /w scans of width 0.3° and 25 s duration at a crystal-to-detector distance of 4.908 cm. Intensity decay over the course of the data collection was evaluated by re-collecting the first 50 frames of data at the end of the experiment. No significant decay was noted.

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F².

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

	x	y	z	Uiso*/Ueq
N1	0.25207 (7)	0.49024 (10)	0.18171 (9)	0.0174 (2)
N2	0.45825 (7)	0.60347 (11)	0.32134 (10)	0.0215 (2)
N3	0.37941 (7)	0.57300 (11)	0.43335 (10)	0.0221 (3)
N4	0.11603 (8)	0.68132 (12)	−0.03682 (10)	0.0237 (3)
N5	0.17575 (7)	0.78254 (11)	0.13423 (10)	0.0203 (2)
N6	0.18253 (7)	0.17898 (11)	0.23441 (9)	0.0185 (2)
N7	0.10750 (7)	0.35553 (11)	0.25492 (10)	0.0200 (2)
C1	0.33864 (8)	0.43420 (13)	0.25123 (12)	0.0195 (3)
H1A	0.3694	0.4054	0.1985	0.023*
H1B	0.3306	0.3547	0.2945	0.023*
C2	0.39287 (8)	0.53528 (13)	0.33503 (11)	0.0180 (3)
C3	0.48762 (9)	0.68846 (14)	0.41549 (13)	0.0263 (3)
H3	0.5345	0.7504	0.4296	0.032*
C4	0.43990 (9)	0.67071 (14)	0.48517 (12)	0.0271 (3)
H4	0.4469	0.7166	0.5557	0.032*
C5	0.31387 (10)	0.52060 (17)	0.47802 (13)	0.0315 (3)
H5A	0.2685	0.5884	0.4693	0.047*
H5B	0.3421	0.4979	0.5607	0.047*
H5C	0.2869	0.4403	0.4341	0.047*
C6	0.25984 (9)	0.58995 (13)	0.09688 (11)	0.0199 (3)
H6A	0.2623	0.5435	0.0267	0.024*
H6B	0.3150	0.6412	0.1319	0.024*
C7	0.18362 (8)	0.68345 (13)	0.06253 (11)	0.0188 (3)
C8	0.06228 (9)	0.78394 (14)	−0.02761 (13)	0.0260 (3)
H8	0.0078	0.8070	−0.0859	0.031*
C9	0.09816 (9)	0.84731 (14)	0.07649 (13)	0.0245 (3)
H9	0.0744	0.9214	0.1038	0.029*
C10	0.23511 (10)	0.81162 (14)	0.25287 (12)	0.0272 (3)
H10A	0.2165	0.7612	0.3087	0.041*
H10B	0.2335	0.9076	0.2683	0.041*
H10C	0.2950	0.7856	0.2611	0.041*
C11	0.19142 (8)	0.38282 (13)	0.12017 (11)	0.0188 (3)
H11A	0.2214	0.3224	0.0822	0.023*
H11B	0.1400	0.4223	0.0585	0.023*
C12	0.16115 (8)	0.30447 (13)	0.20224 (11)	0.0170 (3)
C13	0.14029 (8)	0.14923 (14)	0.31105 (11)	0.0212 (3)
H13	0.1434	0.0657	0.3492	0.025*
C14	0.09369 (9)	0.25665 (14)	0.32418 (12)	0.0233 (3)
H14	0.0586	0.2623	0.3717	0.028*
C15	0.06665 (10)	0.48741 (14)	0.23549 (14)	0.0283 (3)
H15A	0.1051	0.5510	0.2159	0.042*
H15B	0.0574	0.5170	0.3065	0.042*
H15C	0.0101	0.4828	0.1710	0.042*
B1	0.40567 (9)	0.06126 (14)	0.34655 (13)	0.0180 (3)
O1	0.34868 (6)	0.10075 (9)	0.24053 (8)	0.0208 (2)
O2	0.37670 (6)	0.03776 (10)	0.43659 (9)	0.0227 (2)
O3	0.49262 (6)	0.04577 (10)	0.36186 (8)	0.0216 (2)
H1O	0.2957 (13)	0.1165 (19)	0.2418 (17)	0.043 (5)*
H2O	0.4206 (14)	0.016 (2)	0.4986 (19)	0.051 (6)*
H3O	0.5056 (13)	0.067 (2)	0.299 (2)	0.054 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0165 (5)	0.0172 (5)	0.0185 (5)	−0.0018 (4)	0.0063 (4)	0.0004 (4)
N2	0.0195 (5)	0.0225 (6)	0.0224 (6)	−0.0017 (4)	0.0071 (5)	−0.0015 (4)
N3	0.0220 (6)	0.0262 (6)	0.0176 (5)	0.0018 (5)	0.0065 (5)	0.0006 (4)
N4	0.0231 (6)	0.0255 (6)	0.0212 (6)	−0.0025 (5)	0.0063 (5)	0.0021 (5)
N5	0.0220 (6)	0.0183 (5)	0.0204 (6)	−0.0006 (4)	0.0071 (5)	0.0009 (4)
N6	0.0160 (5)	0.0195 (5)	0.0190 (5)	0.0002 (4)	0.0050 (4)	0.0000 (4)
N7	0.0175 (5)	0.0197 (5)	0.0242 (6)	0.0012 (4)	0.0092 (5)	−0.0015 (4)
C1	0.0179 (6)	0.0176 (6)	0.0233 (7)	0.0011 (5)	0.0078 (5)	−0.0009 (5)
C2	0.0173 (6)	0.0185 (6)	0.0183 (6)	0.0023 (5)	0.0062 (5)	0.0016 (5)
C3	0.0236 (7)	0.0228 (7)	0.0277 (7)	−0.0029 (5)	0.0031 (6)	−0.0040 (5)
C4	0.0288 (7)	0.0277 (7)	0.0200 (7)	0.0032 (6)	0.0027 (6)	−0.0056 (5)
C5	0.0319 (8)	0.0427 (9)	0.0248 (7)	0.0020 (7)	0.0161 (6)	0.0048 (6)
C6	0.0209 (6)	0.0208 (6)	0.0202 (6)	−0.0026 (5)	0.0100 (5)	−0.0002 (5)
C7	0.0221 (6)	0.0183 (6)	0.0180 (6)	−0.0043 (5)	0.0096 (5)	0.0013 (5)
C8	0.0212 (7)	0.0260 (7)	0.0280 (7)	−0.0003 (5)	0.0051 (6)	0.0060 (6)
C9	0.0237 (7)	0.0203 (7)	0.0302 (7)	0.0032 (5)	0.0103 (6)	0.0043 (5)
C10	0.0321 (8)	0.0227 (7)	0.0230 (7)	−0.0014 (6)	0.0051 (6)	−0.0037 (5)
C11	0.0185 (6)	0.0194 (6)	0.0182 (6)	−0.0022 (5)	0.0063 (5)	−0.0020 (5)
C12	0.0132 (6)	0.0193 (6)	0.0172 (6)	−0.0010 (5)	0.0036 (5)	−0.0029 (5)
C13	0.0198 (6)	0.0233 (7)	0.0201 (6)	−0.0012 (5)	0.0066 (5)	0.0029 (5)
C14	0.0216 (6)	0.0281 (7)	0.0241 (7)	−0.0012 (5)	0.0128 (6)	0.0007 (5)
C15	0.0264 (7)	0.0211 (7)	0.0401 (8)	0.0056 (6)	0.0152 (7)	−0.0018 (6)
B1	0.0187 (7)	0.0151 (6)	0.0207 (7)	−0.0015 (5)	0.0076 (6)	−0.0031 (5)
O1	0.0184 (5)	0.0241 (5)	0.0213 (5)	0.0034 (4)	0.0086 (4)	0.0010 (4)
O2	0.0176 (5)	0.0311 (5)	0.0200 (5)	0.0006 (4)	0.0072 (4)	0.0019 (4)
O3	0.0168 (4)	0.0291 (5)	0.0196 (5)	0.0011 (4)	0.0072 (4)	0.0015 (4)

Geometric parameters (Å, º)

N1—C1	1.475 (2)	C5—H5C	0.98
N1—C6	1.478 (2)	C6—C7	1.491 (2)
N1—C11	1.474 (2)	C6—H6A	0.99
N2—C2	1.324 (2)	C6—H6B	0.99
N2—C3	1.376 (2)	C8—C9	1.358 (2)
N3—C2	1.352 (2)	C8—H8	0.95
N3—C4	1.373 (2)	C9—H9	0.95
N3—C5	1.458 (2)	C10—H10A	0.98
N4—C7	1.321 (2)	C10—H10B	0.98
N4—C8	1.379 (2)	C10—H10C	0.98
N5—C7	1.360 (2)	C11—C12	1.490 (2)
N5—C9	1.373 (2)	C11—H11A	0.99
N5—C10	1.462 (2)	C11—H11B	0.99
N6—C12	1.324 (2)	C13—C14	1.357 (2)
N6—C13	1.379 (2)	C13—H13	0.95
N7—C12	1.356 (2)	C14—H14	0.95
N7—C14	1.373 (2)	C15—H15A	0.98
N7—C15	1.458 (2)	C15—H15B	0.98
C1—C2	1.488 (2)	C15—H15C	0.98
C1—H1A	0.99	B1—O1	1.361 (2)
C1—H1B	0.99	B1—O2	1.365 (2)
C3—C4	1.354 (2)	B1—O3	1.369 (2)
C3—H3	0.95	O1—H1O	0.88 (2)
C4—H4	0.95	O2—H2O	0.87 (2)
C5—H5A	0.98	O3—H3O	0.90 (2)
C5—H5B	0.98
C1—N1—C6	111.13 (10)	N4—C7—N5	111.65 (12)
C1—N1—C11	110.42 (10)	N4—C7—C6	126.07 (12)
C6—N1—C11	109.91 (10)	N5—C7—C6	122.26 (12)
C2—N2—C3	105.56 (11)	N4—C8—C9	110.47 (12)
C2—N3—C4	107.15 (11)	C9—C8—H8	124.8
C2—N3—C5	127.23 (12)	N4—C8—H8	124.8
C4—N3—C5	125.62 (12)	N5—C9—C8	106.01 (12)
C7—N4—C8	104.97 (12)	C8—C9—H9	127.0
C7—N5—C9	106.89 (11)	N5—C9—H9	127.0
C7—N5—C10	127.38 (11)	N5—C10—H10A	109.5
C9—N5—C10	125.64 (12)	N5—C10—H10B	109.5
C12—N6—C13	105.39 (11)	H10A—C10—H10B	109.5
C12—N7—C14	107.28 (11)	N5—C10—H10C	109.5
C12—N7—C15	126.60 (12)	H10A—C10—H10C	109.5
C14—N7—C15	125.98 (11)	H10B—C10—H10C	109.5
N1—C1—C2	110.66 (10)	N1—C11—C12	111.08 (10)
N1—C1—H1A	109.5	N1—C11—H11A	109.4
C2—C1—H1A	109.5	C12—C11—H11A	109.4
N1—C1—H1B	109.5	N1—C11—H11B	109.4
C2—C1—H1B	109.5	C12—C11—H11B	109.4
H1A—C1—H1B	108.1	H11A—C11—H11B	108.0
N2—C2—N3	111.12 (11)	N6—C12—N7	111.15 (11)
N2—C2—C1	124.73 (12)	N6—C12—C11	125.76 (11)
N3—C2—C1	124.11 (12)	N7—C12—C11	123.09 (11)
N2—C3—C4	109.96 (12)	N6—C13—C14	110.14 (12)
C4—C3—H3	125.0	C14—C13—H13	124.9
N2—C3—H3	125.0	N6—C13—H13	124.9
N3—C4—C3	106.21 (12)	N7—C14—C13	106.04 (12)
C3—C4—H4	126.9	C13—C14—H14	127.0
N3—C4—H4	126.9	N7—C14—H14	127.0
N3—C5—H5A	109.5	N7—C15—H15A	109.5
N3—C5—H5B	109.5	N7—C15—H15B	109.5
H5A—C5—H5B	109.5	H15A—C15—H15B	109.5
N3—C5—H5C	109.5	N7—C15—H15C	109.5
H5A—C5—H5C	109.5	H15A—C15—H15C	109.5
H5B—C5—H5C	109.5	H15B—C15—H15C	109.5
N1—C6—C7	110.61 (10)	O1—B1—O2	120.52 (12)
N1—C6—H6A	109.5	O1—B1—O3	119.10 (12)
C7—C6—H6A	109.5	O2—B1—O3	120.38 (12)
N1—C6—H6B	109.5	B1—O1—H1O	112.1 (13)
C7—C6—H6B	109.5	B1—O2—H2O	109.8 (14)
H6A—C6—H6B	108.1	B1—O3—H3O	113.7 (13)
C11—N1—C1—C2	−166.31 (10)	C10—N5—C7—C6	−2.3 (2)
C6—N1—C1—C2	71.45 (13)	N1—C6—C7—N4	−103.72 (14)
C3—N2—C2—N3	0.00 (15)	N1—C6—C7—N5	75.22 (15)
C3—N2—C2—C1	177.70 (12)	C7—N4—C8—C9	0.43 (15)
C4—N3—C2—N2	−0.10 (15)	N4—C8—C9—N5	−0.44 (16)
C5—N3—C2—N2	−179.73 (12)	C7—N5—C9—C8	0.28 (14)
C4—N3—C2—C1	−177.81 (12)	C10—N5—C9—C8	−176.63 (12)
C5—N3—C2—C1	2.6 (2)	C1—N1—C11—C12	74.16 (13)
N1—C1—C2—N2	−103.28 (14)	C6—N1—C11—C12	−162.88 (10)
N1—C1—C2—N3	74.12 (16)	C13—N6—C12—N7	0.05 (14)
C2—N2—C3—C4	0.10 (16)	C13—N6—C12—C11	179.51 (12)
N2—C3—C4—N3	−0.16 (16)	C14—N7—C12—N6	−0.26 (15)
C2—N3—C4—C3	0.16 (15)	C15—N7—C12—N6	−176.14 (12)
C5—N3—C4—C3	179.79 (13)	C14—N7—C12—C11	−179.73 (12)
C11—N1—C6—C7	80.37 (13)	C15—N7—C12—C11	4.4 (2)
C1—N1—C6—C7	−157.09 (10)	N1—C11—C12—N6	−110.92 (14)
C8—N4—C7—N5	−0.24 (14)	N1—C11—C12—N7	68.48 (15)
C8—N4—C7—C6	178.78 (12)	C12—N6—C13—C14	0.18 (14)
C9—N5—C7—N4	−0.02 (15)	N6—C13—C14—N7	−0.34 (15)
C10—N5—C7—N4	176.82 (12)	C12—N7—C14—C13	0.36 (15)
C9—N5—C7—C6	−179.09 (11)	C15—N7—C14—C13	176.27 (13)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O···N6	0.88 (2)	1.92 (2)	2.7914 (14)	170.6 (18)
O2—H2O···O3^i	0.87 (2)	1.90 (2)	2.7582 (14)	173 (2)
O3—H3O···N2^ii	0.90 (2)	1.81 (2)	2.6991 (15)	175 (2)

Symmetry codes: (i) −x+1, −y, −z+1; (ii) −x+1, y−1/2, −z+1/2.