Coordinative NB bond formation and hydrogen-bonded molecular ladders in 2,5-bis[(N→B)-(2-aminoethoxy)phenylboryl]thiophene

The molecular structure of the title compound, C₂₀H₂₄B₂­N₂O₂S, is characterized by a twofold rotation axis passing through the S atom and the midpoint of the C—C single bond in the thio­phene ring. A coordinative NB bond is present in the boroxazolidine ring and a single N—H...O hydrogen bond [H...O 1.93 (3) Å, N...O 2.829 (3) Å and N—H...O 172 (2)°] links the mol­ecules into a molecular ladder.

S1. Comment

2-Aminoethoxyborinates, readily available from diorganoborinic acids and 2-aminoalcohols, have found various uses [see, for example, references cited in Höpfl, Farfán et al. (1998)]. The crystal structures of several of these important compounds have been reported, including those of R₂BOCH₂CH₂NH₂, (I), e.g. where R is 2-thienyl (Low et al., 2000) or R is p-XC₆H₄ (X is H, F or Me; Rettig & Trotter, 1973, 1974, 1976). Similar to (I) is R₂BOCH₂CHR'NHR'', (II), where, for example, R is Ph, and R' and R'' are (CH₂)₄ (Höpfl, Farfán et al., 1998), and also R₂BOCH₂CH₂NR'₂, (III), where, for example, R is Ph and R' is (CH₂)₅ (Höpfl, Farfán et al., 1998).

Invariably, the B centre in these compounds is four-coordinate as a result of intramolecular N→ B interactions, which give rise to five-membered chelate rings (boroxazolidine rings). The N→ B coordination greatly increases the hydrolytic stability of the B—O bond in such diorganoborinates (Zimmermann, 1963), to the extent that these compounds can be used as reagents and bioactive materials in aqueous media, unlike simple R₂BOR' compounds, which would hydrolyse back to diorganoborinic acids. Indications of the strengths of the N→ B interactions have been provided by the N→ B bond lengths determined in crystallographic studies, as well as from variable-temperature NMR spectroscopic studies in solution (e.g. Höpfl, Farfán et al., 1998) and ab initio calculations at the HF/6–31G** level (Höpfl, Galván et al., 1998).

From such studies, it is clear that substitution both at B but especially at N can affect the strength of the N→ B bond. When the amino group is primary, as in (I), the molecules are linked into chains via intermolecular N—H···O hydrogen bonds (Low et al., 2000). Similar N—H···O hydrogen bonding should also arise in (II); however, no mention has been made of this in the literature. Such classical hydrogen bonding cannot occur in the tertiary amino derivatives, and so (III), in condensed phases, would remain essentially molecular. Compounds containing two 2-aminoethoxyborinato groups have attracted less structural attention. We address this deficiency by reporting the structure of 2,5-bis[(N—B)-(2-aminoethoxy)phenylboryl]thiophene, (IV). \sch

Compound (IV), in addition to having two B centres, has two different organic substituents at each B centre, which results in chirality at B on N→ B interaction, and primary amino groups, which results in the formation of hydrogen-bonded molecular aggregates. In (IV), there is a twofold axis passing through the S atom (in special position 1/2, y, 1/4) and the midpoint of the Cβ-Cβ bond [C1—C1ⁱ in the crystallographic numbering scheme; symmetry code: (i) 1 - x, y, 1/2 - z] of the thiophene ring. Please check added symmetry code.

The molecular and crystallographic symmetry of (I) coincide such that the asymmetric unit contains one half molecule. Both B centres in the same molecule have the same chirality, i.e. molecules have either (R,R) or (S,S) configurations. As a consequence of the centrosymmetric space group, there are equal numbers of the (R,R) and (S,S) enantiomers. These are linked alternately into chains via N—H···O hydrogen bonds, involving both NH and O centres of each molecule. The overall result is a molecular ladder running in the direction of the c axis. There are no interactions between the chains.

The B centres have slightly distorted tetrahedral geometries, with bond angles at B ranging around the ideal tetrahedral angle of 109.5°, between 100.3 (2) and 114.0 (3)°. The N→ B bond length of 1.642 (4) Å in (IV) at 150 K is slightly shorter than those [both 1.654 (3) Å] in (I) with R = 2-thienyl (Low et al., 2000) and (I) with R = Ph (Rettig & Trotter, 1973, 1976), both at 298 K. All these are only slightly longer than the sum of the covalent radii (1.51 Å) but considerably shorter than the sum of the van der Waals radii (3.18 Å; Spek, 2001).

The B—O distance in (IV) at 150 K [1.477 (4) Å] is similar to those determined at 298 K for (I) with R = 2-thienyl [1.479 (3) Å; Low et al., 2000] and (I) with R = Ph [1.480 (3) Å; Rettig & Trotter, 1973, 1976]. The differences in the N→ B bond lengths are considered to be primarily consequences of the temperature difference, rather than any electronic effects of the substituents.

A comparison of N→ B and B—O bond lengths in (I), determined at 298 K, with those of (V) (Höpfl, Farfán et al., 1998) clearly indicates the greater impact of substitution at N compared with that at other positions: N→ B = 1.654 (3), 1.648 (3) and 1.73 (1) Å in (I), (V) with R = H and (V) with R = Me, respectively, and B—O = 1.479, 1.481 (3) and 1.45 (1) Å in (I), (V) with R = H and (V) with R = Me, respectively. As expected, a decrease in the N→ B bond length occurs with an increase in the B—O bond length.

The planar thiophene and phenyl rings are inclined to each other by 72.54 (8)° in (IV), and the boroxazolidene rings adopt envelope conformations, with flaps at C3 and C3ⁱ.

S2. Experimental

The title compound was prepared according to the procedure of Coutts & Musgrave (1970) and was recrystallized from chloroform.

S3. Refinement

The low goodness-of-fit value and the simplistic weighting scheme appear to be consequences of an over-estimation of the uncertainties associated with the diffracted intensities. The amine H atoms were freely refined with isotropic displacement parameters. The remaining H atoms were initially placed in calculated positions and refined isotropically, and thereafter allowed to ride on their attached atoms with C—H distances of 0.95 Å (Csp² atoms) and 0.99 Å (Csp³ atoms).

Computing details

Data collection: MADNES (Pflugrath & Messerschmidt, 1989); cell refinement: MADNES; data reduction: ABSMAD (Karaulov, 1992); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and PLATON (Spek, 2001); software used to prepare material for publication: SHELXL97 and PLATON.

Figures

	Fig. 1. The atomic arrangement in the molecule of (IV). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii [symmetry code: (i) 1 - x, y, 1/2 - z]. Please check added symmetry code.
	Fig. 2. Part of the crystal structure of (IV), showing formation of the molecular ladder in the direction of the c axis.

2,5-bis[(N—B)-(2-aminoethoxy)phenylboryl]thiophene

Crystal data

C20H24B2N2O2S	Dx = 1.251 Mg m^−3
Mr = 378.10	Melting point = 498–499 K
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.71073 Å
a = 11.484 (3) Å	Cell parameters from 5000 reflections
b = 18.385 (6) Å	θ = 2.1–24.9°
c = 9.509 (3) Å	µ = 0.18 mm^−1
V = 2007.7 (11) Å^3	T = 150 K
Z = 4	Lozenge, colourless
F(000) = 800	0.22 × 0.20 × 0.16 mm

Data collection

Delft Instruments FAST diffractometer with Oxford Cryosystems low-temperature device (Cosier & Glazer, 1986)	738 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.123
Graphite monochromator	θmax = 24.9°, θmin = 2.1°
Detector resolution: 9.091 pixels mm^-1	h = −10→13
area detector scans	k = −19→20
7298 measured reflections	l = −10→9
1620 independent reflections

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)] = 0.040	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.075	H atoms treated by a mixture of independent and constrained refinement
S = 0.63	w = 1/[σ^2(Fo^2)] where P = (Fo^2 + 2Fc^2)/3
1620 reflections	(Δ/σ)max = 0.008
141 parameters	Δρmax = 0.22 e Å^−3
0 restraints	Δρmin = −0.23 e Å^−3

Special details

Experimental. Please note cell_measurement_ fields are not relevant to area-detector data; the entire data set is used to refine the cell, which is indexed from all observed reflections in a 10 degree phi range. Absence of crystal decay in the X-ray beam was confirmed by checking equivalent reflections at the beginning and end of data collection, which lasted about 8 h.

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. The unit-cell and intensity data were collected on a Delft Instruments FAST diffractometer using the routines ENDEX, REFINE and MADONL in the MADNES software (Pflugrath & Messerschmidt, 1989) and processed using ABSMAD (Karaulov, 1992); detailed procedures are described by Darr, J. A., Drake, S. R., Hursthouse, M. B. & Malik, K. M. A. (1993). Inorg. Chem. 32, 5704–5708. A l l non-H atoms were refined with anisotropic displacement parameters.

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

	x	y	z	Uiso*/Ueq
S1	0.5000	−0.01244 (6)	0.2500	0.0291 (3)
O1	0.79473 (16)	−0.00550 (11)	0.21691 (15)	0.0287 (6)
N1	0.7124 (3)	−0.02854 (16)	−0.0105 (3)	0.0285 (7)
H1A	0.646 (3)	−0.0478 (15)	−0.028 (2)	0.034 (11)*
H1B	0.745 (2)	−0.0154 (16)	−0.096 (3)	0.048 (11)*
B1	0.7217 (3)	0.0334 (2)	0.1135 (3)	0.0235 (9)
C1	0.5530 (2)	0.1179 (2)	0.2121 (2)	0.0299 (8)
H1	0.5912	0.1616	0.1852	0.030 (9)*
C2	0.5962 (3)	0.05165 (17)	0.1807 (2)	0.0219 (8)
C3	0.8626 (3)	−0.05877 (17)	0.1464 (3)	0.0355 (9)
H3A	0.8913	−0.0959	0.2134	0.027 (8)*
H3B	0.9301	−0.0360	0.0990	0.041 (9)*
C4	0.7815 (3)	−0.09243 (18)	0.0404 (3)	0.0379 (9)
H4A	0.8253	−0.1154	−0.0375	0.048 (10)*
H4B	0.7305	−0.1293	0.0848	0.028 (8)*
C5	0.7814 (3)	0.10478 (18)	0.0496 (3)	0.0272 (8)
C6	0.7335 (3)	0.14601 (19)	−0.0586 (3)	0.0366 (9)
H6	0.6637	0.1296	−0.1017	0.049 (10)*
C7	0.7829 (4)	0.2091 (2)	−0.1053 (3)	0.0465 (10)
H7	0.7474	0.2356	−0.1795	0.050 (10)*
C8	0.8849 (3)	0.2346 (2)	−0.0443 (4)	0.0563 (12)
H8	0.9197	0.2785	−0.0760	0.046 (10)*
C9	0.9346 (4)	0.1949 (2)	0.0629 (4)	0.0582 (12)
H9	1.0044	0.2114	0.1059	0.099 (15)*
C10	0.8839 (3)	0.1325 (2)	0.1070 (3)	0.0396 (9)
H10	0.9201	0.1062	0.1810	0.032 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.0238 (6)	0.0366 (8)	0.0269 (6)	0.000	0.0016 (5)	0.000
O1	0.0242 (11)	0.0448 (17)	0.0170 (10)	0.0091 (11)	−0.0010 (9)	0.0008 (9)
N1	0.0217 (17)	0.044 (2)	0.0194 (17)	−0.0034 (16)	0.0011 (15)	−0.0016 (13)
B1	0.026 (2)	0.032 (3)	0.0121 (18)	0.004 (2)	−0.0021 (17)	−0.0018 (15)
C1	0.0233 (18)	0.033 (2)	0.033 (2)	−0.0016 (18)	0.0055 (15)	0.0014 (15)
C2	0.0239 (19)	0.030 (2)	0.0118 (15)	−0.0040 (16)	−0.0031 (13)	0.0028 (14)
C3	0.031 (2)	0.042 (3)	0.0337 (19)	0.005 (2)	0.0025 (18)	0.0105 (18)
C4	0.049 (3)	0.029 (3)	0.036 (2)	0.009 (2)	0.000 (2)	0.0022 (17)
C5	0.021 (2)	0.041 (2)	0.0196 (17)	0.0030 (18)	0.0034 (15)	−0.0009 (15)
C6	0.041 (2)	0.038 (3)	0.0305 (19)	−0.010 (2)	0.0046 (18)	0.0003 (17)
C7	0.056 (3)	0.043 (3)	0.041 (2)	0.004 (2)	0.010 (2)	0.0086 (19)
C8	0.053 (3)	0.047 (3)	0.069 (3)	−0.017 (3)	0.022 (2)	0.004 (2)
C9	0.040 (3)	0.056 (4)	0.079 (3)	−0.009 (2)	0.007 (2)	0.005 (2)
C10	0.028 (2)	0.044 (3)	0.047 (2)	−0.007 (2)	0.0017 (18)	0.0065 (19)

Geometric parameters (Å, º)

S1—C2	1.744 (3)	C3—H3B	0.9900
S1—C2^i	1.744 (3)	C4—H4A	0.9900
O1—C3	1.420 (3)	C4—H4B	0.9900
O1—B1	1.477 (4)	C5—C6	1.391 (3)
N1—C4	1.497 (4)	C5—C10	1.394 (4)
N1—B1	1.642 (4)	C6—C7	1.366 (4)
N1—H1A	0.86 (3)	C6—H6	0.9500
N1—H1B	0.92 (3)	C7—C8	1.388 (4)
B1—C5	1.600 (4)	C7—H7	0.9500
B1—C2	1.612 (4)	C8—C9	1.377 (4)
C2—C1	1.348 (4)	C8—H8	0.9500
C1—C1^i	1.415 (5)	C9—C10	1.353 (4)
C1—H1	0.9500	C9—H9	0.9500
C3—C4	1.505 (4)	C10—H10	0.9500
C3—H3A	0.9900
C2—S1—C2^i	95.0 (2)	H3A—C3—H3B	108.8
C3—O1—B1	109.30 (19)	N1—C4—C3	102.8 (3)
C4—N1—B1	106.1 (2)	N1—C4—H4A	111.2
C4—N1—H1A	102 (2)	C3—C4—H4A	111.2
B1—N1—H1A	119.0 (18)	N1—C4—H4B	111.2
C4—N1—H1B	105.8 (18)	C3—C4—H4B	111.2
B1—N1—H1B	114.9 (18)	H4A—C4—H4B	109.1
H1A—N1—H1B	108 (2)	C6—C5—C10	115.1 (3)
O1—B1—C5	114.0 (3)	C6—C5—B1	124.0 (3)
O1—B1—C2	110.1 (2)	C10—C5—B1	120.9 (3)
C5—B1—C2	111.2 (3)	C7—C6—C5	122.6 (3)
O1—B1—N1	100.3 (2)	C7—C6—H6	118.7
C5—B1—N1	108.9 (2)	C5—C6—H6	118.7
C2—B1—N1	111.8 (3)	C6—C7—C8	120.1 (4)
C1—C2—B1	127.2 (3)	C6—C7—H7	120.0
C1—C2—S1	107.1 (2)	C8—C7—H7	120.0
B1—C2—S1	125.1 (2)	C9—C8—C7	118.7 (4)
C2—C1—C1^i	115.43 (18)	C9—C8—H8	120.6
C2—C1—H1	122.3	C7—C8—H8	120.6
C1^i—C1—H1	122.3	C10—C9—C8	120.0 (4)
O1—C3—C4	105.1 (2)	C10—C9—H9	120.0
O1—C3—H3A	110.7	C8—C9—H9	120.0
C4—C3—H3A	110.7	C9—C10—C5	123.5 (4)
O1—C3—H3B	110.7	C9—C10—H10	118.2
C4—C3—H3B	110.7	C5—C10—H10	118.2
C3—O1—B1—C5	−91.2 (3)	B1—N1—C4—C3	−22.3 (3)
C3—O1—B1—C2	143.0 (2)	O1—C3—C4—N1	38.5 (3)
C3—O1—B1—N1	25.1 (3)	O1—B1—C5—C6	173.8 (2)
C4—N1—B1—O1	−0.3 (3)	C2—B1—C5—C6	−60.9 (3)
C4—N1—B1—C5	119.6 (3)	N1—B1—C5—C6	62.7 (4)
C4—N1—B1—C2	−117.1 (3)	O1—B1—C5—C10	−10.3 (4)
O1—B1—C2—C1	112.8 (3)	C2—B1—C5—C10	115.0 (3)
C5—B1—C2—C1	−14.6 (4)	N1—B1—C5—C10	−121.4 (3)
N1—B1—C2—C1	−136.6 (3)	C10—C5—C6—C7	−0.1 (4)
O1—B1—C2—S1	−57.1 (3)	B1—C5—C6—C7	176.0 (3)
C5—B1—C2—S1	175.52 (18)	C5—C6—C7—C8	−0.1 (5)
N1—B1—C2—S1	53.5 (3)	C6—C7—C8—C9	0.2 (5)
C2^i—S1—C2—C1	0.12 (13)	C7—C8—C9—C10	−0.1 (5)
C2^i—S1—C2—B1	171.7 (3)	C8—C9—C10—C5	−0.1 (6)
B1—C2—C1—C1^i	−171.7 (3)	C6—C5—C10—C9	0.2 (5)
S1—C2—C1—C1^i	−0.3 (4)	B1—C5—C10—C9	−176.0 (3)
B1—O1—C3—C4	−41.3 (3)	C2—C1—C1^i—C2^i	0.5 (5)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1B···O1^ii	0.93 (3)	1.91 (3)	2.829 (3)	172 (2)

Symmetry code: (ii) x, −y, z−1/2.