5-Formyl-2-furan­boronic acid at 100 K

The furan ring in the title compound, C₅H₅BO₄, is planar and each of the formyl and boronic groups makes a dihedral angle of ca 3° with this ring. The geometry of the furan ring is somewhat different to that found for substituted and unsubstituted furan structures. The mol­ecules are connected to each other in the bc plane by C—H...O and O—H...O hydrogen bonds.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](C-C) = 0.003 Å
• R factor = 0.047
• wR factor = 0.132
• Data-to-parameter ratio = 10.1

checkCIF/PLATON results

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Alert level C
PLAT029_ALERT_3_C _diffrn_measured_fraction_theta_full Low .......       0.99
PLAT250_ALERT_2_C Large U3/U1 ratio for average U(i,j) tensor ....       2.08

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   2 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   1 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion

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S1. Comment

Arylboronic acids, ArB(OH)₂, have been known since the end of the nineteenth century. However, attention has recently been paid to these compounds owing to their new applications (Cuthbertson, 1998). The main synthetic application is Suzuki coupling: reaction of aryl halides with arylboronic acids (Miyaura et al., 1981; Miyaura & Suzuki, 1995; ChemFiles 2003). Other applications are asymmetric synthesis using chiral boronic acids (Currie et al., 2000) or analytical use as molecular sensors (Ward et al., 2002). These compounds are also used in medicine, e.g. for boron neutron-capture therapy (BNCT) (Soloway et al., 1998) or as virus enzyme inhibitors (Priestley & Decicco, 2000). There are only a few examples of the crystal structures of boronic acids. In these structures, the B(OH)₂ group is attached to the phenyl ring (Feulner et al., 1990; Gainsford et al., 1995; Scouten et al., 1994), pyridine ring (Parry et al., 2002) or five-membered cyclopentadienyl ring (Norrild & Sotofte, 2001).

We present here the crystal structure of 5-formyl-2-furanboronic acid, (I), in which the boronic acid group is a substitutent of the furan ring. The B—O bond lengths are different, the B6—O7 bond being shorter than the B6—O8 bond by about 0.02A%. Similar differences have been reported for 4-carboxy-2-nitrobenzeneboronic acid (Soundararajan et al., 1993) and L-p-boronophenylalanine (Shull et al., 2000). The bond angles around atom B6 are distorted from the value of 120°; the O8–B6–C2 angle is 3.8° greater than 120°, whereas the O7–B6–C2 angle is 4.9° less than 120° and O7–B6–O8 is closest to the expected value. A similar geometry of the B(OH)₂ acid group is observed in 2-bromo-5-pyridylboronic acid structures (Parry et al., 2002). The formyl and B(OH)₂ groups in (I) are essentially coplanar with the furan ring. The dihedral angles of these planes with the furan ring are less than 3°. The five atoms of the furan ring are coplanar. The geometry of the furan ring in (I) is somewhat different to that of both unsubstituted furan rings [(II); Fourme, 1972] and substituted 5-nitro-2-furancarboxylic acid [(III); Alcock et al., 1996]. The C3—C4 bond length is similar to that found in (III) but it is shorter than that observed in (II) by ca 0.25 Å. The remaining bond lengths viz. C—C_α (C2—C3 and C4—C5) and O—C_α (O1—C2 and O1—C5) are slightly different from those observed in (II) and (III). The O—C_α—C and C_α—O—C_α angles in the furane ring of (I) are closer to those in (II) than in (III).

The molecular network in the crystal consists of two nearly linear strong O–H···O and one weak C–H···O hydrogen bonds. The O7–H7···O8 hydrogen bond forms dimers of (I); this is characteristic for arylboronic acids in solid state (Alcock et al., 1996; Feulner et al., 1990; Gainsford et al., 1995; Scouten et al., 1994).

S2. Experimental

5-formyl-2-furanboronic acid was obtained from Aldrich.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2002); cell refinement: CrysAlis RED (Oxford Diffraction, 2002); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXTL (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL; software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. The packing diagram of the title compound, showing the hydrogen-bonding scheme (dashed lines).

(I)

Crystal data

C5H5BO4	F(000) = 288
Mr = 139.90	Dx = 1.542 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 3391 reflections
a = 3.7550 (8) Å	θ = 3.3–25.5°
b = 7.758 (2) Å	µ = 0.13 mm^−1
c = 20.694 (4) Å	T = 100 K
β = 91.31 (3)°	Cube, colourless
V = 602.7 (2) Å^3	0.20 × 0.18 × 0.16 mm
Z = 4

Data collection

Xcalibur diffractometer	764 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.089
Graphite monochromator	θmax = 25.5°, θmin = 3.3°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −4→3
ω scans	k = −9→9
3391 measured reflections	l = −25→25
1126 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)] = 0.047	Hydrogen site location: difference Fourier map
wR(F^2) = 0.132	All H-atom parameters refined
S = 0.95	w = 1/[σ^2(Fo^2) + (0.0811P)^2] where P = (Fo^2 + 2Fc^2)/3
1126 reflections	(Δ/σ)max < 0.001
111 parameters	Δρmax = 0.16 e Å^−3
0 restraints	Δρmin = −0.21 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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
O1	0.4972 (4)	0.36605 (18)	0.16766 (6)	0.0377 (5)
C2	0.3680 (6)	0.3652 (3)	0.10461 (10)	0.0370 (6)
C3	0.2287 (6)	0.5230 (3)	0.09057 (11)	0.0435 (6)
C4	0.2760 (6)	0.6270 (3)	0.14538 (11)	0.0439 (6)
C5	0.4390 (6)	0.5284 (3)	0.19130 (10)	0.0368 (5)
B6	0.4052 (7)	0.2001 (3)	0.06251 (12)	0.0404 (6)
O7	0.2772 (5)	0.2141 (2)	0.00177 (7)	0.0545 (6)
O8	0.5605 (5)	0.0522 (2)	0.08392 (8)	0.0516 (6)
C9	0.5554 (6)	0.5719 (3)	0.25553 (11)	0.0442 (6)
O10	0.7116 (5)	0.4754 (2)	0.29327 (7)	0.0550 (5)
H9	0.502 (6)	0.697 (4)	0.2692 (11)	0.058 (7)*
H3	0.128 (5)	0.556 (3)	0.0477 (11)	0.046 (6)*
H4	0.209 (7)	0.745 (4)	0.1508 (13)	0.070 (8)*
H7	0.325 (9)	0.114 (5)	−0.0258 (16)	0.094 (11)*
H8	0.617 (7)	0.048 (4)	0.1224 (14)	0.065 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0528 (9)	0.0329 (8)	0.0270 (8)	0.0032 (7)	−0.0068 (6)	−0.0006 (6)
C2	0.0429 (12)	0.0398 (12)	0.0280 (11)	−0.0003 (10)	−0.0049 (9)	−0.0004 (9)
C3	0.0530 (14)	0.0414 (13)	0.0357 (13)	0.0039 (11)	−0.0082 (10)	0.0020 (10)
C4	0.0522 (14)	0.0354 (12)	0.0439 (13)	0.0038 (11)	−0.0056 (11)	0.0002 (10)
C5	0.0446 (12)	0.0320 (12)	0.0338 (12)	−0.0012 (9)	−0.0002 (9)	−0.0044 (9)
B6	0.0509 (15)	0.0411 (14)	0.0289 (13)	0.0032 (12)	−0.0022 (10)	0.0002 (10)
O7	0.0849 (13)	0.0459 (10)	0.0319 (9)	0.0161 (9)	−0.0150 (8)	−0.0052 (7)
O8	0.0854 (13)	0.0411 (10)	0.0276 (9)	0.0128 (9)	−0.0132 (8)	−0.0028 (7)
C9	0.0568 (15)	0.0404 (13)	0.0353 (13)	−0.0046 (11)	−0.0015 (11)	−0.0054 (10)
O10	0.0816 (13)	0.0501 (11)	0.0326 (9)	−0.0018 (9)	−0.0123 (8)	0.0002 (7)

Geometric parameters (Å, º)

O1—C5	1.371 (2)	C5—C9	1.430 (3)
O1—C2	1.382 (2)	B6—O7	1.340 (3)
C2—C3	1.360 (3)	B6—O8	1.357 (3)
C2—B6	1.557 (3)	O7—H7	0.98 (4)
C3—C4	1.400 (3)	O8—H8	0.82 (3)
C3—H3	0.99 (2)	C9—O10	1.222 (3)
C4—C5	1.355 (3)	C9—H9	1.03 (3)
C4—H4	0.95 (3)
C5—O1—C2	106.6 (2)	C4—C5—C9	130.0 (2)
C3—C2—O1	108.8 (2)	O1—C5—C9	120.0 (2)
C3—C2—B6	131.3 (2)	O7—B6—O8	121.1 (2)
O1—C2—B6	119.9 (2)	O7—B6—C2	115.1 (2)
C2—C3—C4	107.7 (2)	O8—B6—C2	123.8 (2)
C2—C3—H3	124.0 (14)	B6—O7—H7	114.5 (19)
C4—C3—H3	128.2 (14)	B6—O8—H8	116.8 (19)
C5—C4—C3	106.8 (2)	O10—C9—C5	125.6 (2)
C5—C4—H4	125.0 (16)	O10—C9—H9	119.5 (14)
C3—C4—H4	128.1 (17)	C5—C9—H9	114.9 (14)
C4—C5—O1	110.0 (2)
C5—O1—C2—C3	−1.0 (2)	C2—O1—C5—C9	−178.0 (2)
C5—O1—C2—B6	177.8 (2)	C3—C2—B6—O7	−0.3 (4)
O1—C2—C3—C4	1.0 (3)	O1—C2—B6—O7	−178.8 (2)
B6—C2—C3—C4	−177.6 (2)	C3—C2—B6—O8	178.4 (2)
C2—C3—C4—C5	−0.7 (3)	O1—C2—B6—O8	−0.1 (4)
C3—C4—C5—O1	0.1 (3)	C4—C5—C9—O10	−177.5 (2)
C3—C4—C5—C9	178.4 (2)	O1—C5—C9—O10	0.7 (4)
C2—O1—C5—C4	0.5 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O7—H7···O8^i	0.98 (4)	1.82 (4)	2.799 (2)	173 (3)
O8—H8···O10^ii	0.82 (3)	1.93 (3)	2.729 (2)	164 (3)
C3—H3···O7^iii	0.99 (2)	2.55 (2)	3.355 (3)	139 (2)

Symmetry codes: (i) −x+1, −y, −z; (ii) −x+3/2, y−1/2, −z+1/2; (iii) −x, −y+1, −z.