The structures of two distinct polymorphic forms of
N-(2,6-difluorophenyl)formamide, C
7H
5F
2NO, have been studied using single crystals obtained under different crystallizing conditions. The two forms crystallize in different space groups,
viz. form (I
a) in the orthorhombic
Pbca and form (I
b) in the monoclinic
P2
1 space group. Each polymorph crystallizes with one complete molecule in the asymmetric unit and they have a similar molecular geometry, showing a
trans conformation with the formamide group being out of the plane of the aromatic ring. The packing arrangements of the two polymorphs are quite different, with form (I
a) having molecules that are stacked in an alternating arrangement, linked into chains of N—H

O hydrogen bonds along the crystallographic
a direction, while form (I
b) has its N—H

O hydrogen-bonded molecules stacked in a linear fashion. A theoretical study of the two structures allows information to be gained regarding other contributing interactions, such as π–π and weak C—H

F, in their crystal structures.
Supporting information
CCDC references: 749727; 749728
N-(2,6-difluorophenyl)formamide was synthesized following a known
procedure (Ugi et al., 1965). Commercially available
2,6-difluoro-N-phenylaniline (Aldrich, purity > 95%) was heated in a
tenfold excess of formic acid for a period of 15 h at 363 K. The excess formic
acid was then removed under vacuum to give a white solid, which was treated
with dilute hydrochloric acid (0.1 M HCl, 10 ml) and ethyl acetate (60 ml). The organic layer was separated from the aqueous layer, dried over
magnesium sulfate and filtered. Colourless needle-shaped crystals of
N-(2,6-difluorophenyl)formamide were grown from the filtrate The
compound was obtained in good yields (over 80%). The purity of the compound
was confirmed by NMR spectroscopy using a Bruker 300 MHz instrument. It was
found to exist in solution (C6D6) as a mixture of cis- and
trans-isomers in a ratio of 2:1.
The second polymorph of N-(2,6-difluorophenyl)formamide (Ib)
could only be found as an impurity during the preparation of
2,6-difluoro-N-phenylthioamide. Efforts to produce (Ib)
experimentally by sublimation of (Ia) or by seeding a solution of
(Ia) using crystals of 2,6-difluoro-N-phenylthioamide were not
successful.
With the exception of the H atoms involved in hydrogen bonding (i.e.
H1), all H atoms were positioned geometrically, with C—H = 0.95 Å, and
allowed to ride on their parent atoms with Uiso(H) =
1.2Ueq(C). H1 was located in the difference map and refined freely.
For both compounds, data collection: SMART (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997), PLATON (Spek, 2009) and DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: WinGX (Farrugia, 1999).
(Ia)
N-(2,6-difluorophenyl)formamide
top
Crystal data top
C7H5F2NO | F(000) = 640 |
Mr = 157.12 | Dx = 1.532 Mg m−3 |
Orthorhombic, Pbca | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ac 2ab | Cell parameters from 9351 reflections |
a = 8.5031 (15) Å | θ = 2.9–28.0° |
b = 11.387 (2) Å | µ = 0.14 mm−1 |
c = 14.075 (3) Å | T = 298 K |
V = 1362.8 (4) Å3 | Needle, colourless |
Z = 8 | 0.5 × 0.16 × 0.1 mm |
Data collection top
CCD area-detector diffractometer | 1006 reflections with I > 2σ(I) |
ω scans | Rint = 0.039 |
Absorption correction: multi-scan (SADABS; Bruker, 2004) | θmax = 28.0°, θmin = 2.9° |
Tmin = 0.933, Tmax = 0.986 | h = −10→11 |
8426 measured reflections | k = −11→15 |
1637 independent reflections | l = −18→18 |
Refinement top
Refinement on F2 | H atoms treated by a mixture of independent and constrained refinement |
Least-squares matrix: full | w = 1/[σ2(Fo2) + (0.0457P)2 + 0.1294P] where P = (Fo2 + 2Fc2)/3 |
R[F2 > 2σ(F2)] = 0.037 | (Δ/σ)max < 0.001 |
wR(F2) = 0.100 | Δρmax = 0.15 e Å−3 |
S = 1.02 | Δρmin = −0.12 e Å−3 |
1637 reflections | Extinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
105 parameters | Extinction coefficient: 0.017 (2) |
1 restraint | |
Crystal data top
C7H5F2NO | V = 1362.8 (4) Å3 |
Mr = 157.12 | Z = 8 |
Orthorhombic, Pbca | Mo Kα radiation |
a = 8.5031 (15) Å | µ = 0.14 mm−1 |
b = 11.387 (2) Å | T = 298 K |
c = 14.075 (3) Å | 0.5 × 0.16 × 0.1 mm |
Data collection top
CCD area-detector diffractometer | 1637 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2004) | 1006 reflections with I > 2σ(I) |
Tmin = 0.933, Tmax = 0.986 | Rint = 0.039 |
8426 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.037 | 1 restraint |
wR(F2) = 0.100 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.02 | Δρmax = 0.15 e Å−3 |
1637 reflections | Δρmin = −0.12 e Å−3 |
105 parameters | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
C1 | 0.93365 (16) | 0.40677 (13) | 0.65042 (10) | 0.0403 (4) | |
C2 | 0.84099 (19) | 0.50268 (15) | 0.67277 (11) | 0.0485 (4) | |
C3 | 0.7354 (2) | 0.55047 (15) | 0.60991 (13) | 0.0631 (5) | |
H3 | 0.6726 | 0.6137 | 0.6276 | 0.076* | |
C4 | 0.7244 (2) | 0.50308 (19) | 0.52029 (13) | 0.0671 (6) | |
H4 | 0.6543 | 0.5355 | 0.4768 | 0.081* | |
C5 | 0.8151 (2) | 0.40870 (18) | 0.49392 (12) | 0.0603 (5) | |
H5 | 0.8073 | 0.3768 | 0.4333 | 0.072* | |
C6 | 0.91744 (17) | 0.36285 (14) | 0.55948 (11) | 0.0471 (4) | |
C7 | 0.99516 (18) | 0.31247 (15) | 0.79871 (12) | 0.0509 (4) | |
H7 | 1.073 | 0.2816 | 0.8378 | 0.061* | |
N1 | 1.04056 (16) | 0.35676 (13) | 0.71596 (10) | 0.0473 (4) | |
O1 | 0.86048 (13) | 0.30868 (11) | 0.82797 (8) | 0.0641 (4) | |
F1 | 0.85624 (14) | 0.55027 (9) | 0.76039 (7) | 0.0722 (4) | |
F2 | 1.00861 (12) | 0.27001 (9) | 0.53586 (7) | 0.0677 (4) | |
H1 | 1.131 (2) | 0.3511 (15) | 0.7007 (12) | 0.052 (5)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1 | 0.0338 (7) | 0.0430 (8) | 0.0441 (8) | −0.0044 (7) | 0.0018 (6) | 0.0028 (7) |
C2 | 0.0496 (9) | 0.0459 (9) | 0.0499 (10) | −0.0017 (8) | 0.0037 (7) | 0.0015 (7) |
C3 | 0.0556 (11) | 0.0563 (11) | 0.0773 (12) | 0.0088 (9) | 0.0041 (9) | 0.0188 (9) |
C4 | 0.0541 (11) | 0.0812 (13) | 0.0662 (12) | −0.0061 (10) | −0.0088 (9) | 0.0341 (11) |
C5 | 0.0575 (10) | 0.0774 (13) | 0.0459 (9) | −0.0174 (10) | −0.0031 (8) | 0.0077 (9) |
C6 | 0.0416 (9) | 0.0490 (9) | 0.0508 (9) | −0.0085 (8) | 0.0077 (7) | −0.0008 (8) |
C7 | 0.0385 (7) | 0.0614 (10) | 0.0528 (9) | 0.0088 (8) | −0.0062 (7) | 0.0031 (8) |
N1 | 0.0302 (7) | 0.0595 (9) | 0.0523 (8) | 0.0027 (7) | 0.0015 (6) | −0.0013 (7) |
O1 | 0.0412 (6) | 0.0923 (10) | 0.0587 (7) | 0.0095 (6) | 0.0023 (5) | 0.0214 (6) |
F1 | 0.0870 (8) | 0.0617 (7) | 0.0680 (7) | 0.0153 (6) | 0.0011 (5) | −0.0175 (5) |
F2 | 0.0703 (7) | 0.0668 (7) | 0.0662 (7) | 0.0036 (5) | 0.0099 (5) | −0.0183 (5) |
Geometric parameters (Å, º) top
C1—C6 | 1.381 (2) | C4—H4 | 0.93 |
C1—C2 | 1.383 (2) | C5—C6 | 1.372 (2) |
C1—N1 | 1.4147 (19) | C5—H5 | 0.93 |
C2—F1 | 1.3534 (17) | C6—F2 | 1.3524 (18) |
C2—C3 | 1.373 (2) | C7—O1 | 1.2178 (19) |
C3—C4 | 1.375 (3) | C7—N1 | 1.327 (2) |
C3—H3 | 0.93 | C7—H7 | 0.93 |
C4—C5 | 1.374 (3) | N1—H1 | 0.800 (18) |
| | | |
C6—C1—C2 | 116.10 (14) | C6—C5—C4 | 118.18 (16) |
C6—C1—N1 | 121.51 (14) | C6—C5—H5 | 120.9 |
C2—C1—N1 | 122.39 (14) | C4—C5—H5 | 120.9 |
F1—C2—C3 | 119.42 (15) | F2—C6—C5 | 119.73 (15) |
F1—C2—C1 | 117.96 (14) | F2—C6—C1 | 116.98 (14) |
C3—C2—C1 | 122.62 (16) | C5—C6—C1 | 123.29 (16) |
C2—C3—C4 | 118.70 (17) | O1—C7—N1 | 125.73 (15) |
C2—C3—H3 | 120.7 | O1—C7—H7 | 117.1 |
C4—C3—H3 | 120.7 | N1—C7—H7 | 117.1 |
C5—C4—C3 | 121.09 (16) | C7—N1—C1 | 122.58 (14) |
C5—C4—H4 | 119.5 | C7—N1—H1 | 118.9 (12) |
C3—C4—H4 | 119.5 | C1—N1—H1 | 118.3 (12) |
| | | |
C6—C1—C2—F1 | −178.35 (13) | C4—C5—C6—C1 | 0.0 (2) |
N1—C1—C2—F1 | 0.9 (2) | C2—C1—C6—F2 | 179.14 (12) |
C6—C1—C2—C3 | 1.7 (2) | N1—C1—C6—F2 | −0.1 (2) |
N1—C1—C2—C3 | −179.02 (15) | C2—C1—C6—C5 | −0.8 (2) |
F1—C2—C3—C4 | 178.27 (15) | N1—C1—C6—C5 | 179.91 (14) |
C1—C2—C3—C4 | −1.8 (2) | O1—C7—N1—C1 | 0.3 (3) |
C2—C3—C4—C5 | 0.9 (3) | C6—C1—N1—C7 | −120.59 (17) |
C3—C4—C5—C6 | −0.1 (3) | C2—C1—N1—C7 | 60.2 (2) |
C4—C5—C6—F2 | −179.93 (14) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.801 (17) | 2.050 (17) | 2.843 (2) | 170 |
Symmetry code: (i) x+1/2, y, −z+3/2. |
(Ib)
N-(2,6-difluorophenyl)formamide
top
Crystal data top
C7H5F2NO | F(000) = 160 |
Mr = 157.12 | Dx = 1.577 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2yb | Cell parameters from 8124 reflections |
a = 4.468 (2) Å | θ = 2.3–28° |
b = 8.486 (3) Å | µ = 0.14 mm−1 |
c = 8.881 (1) Å | T = 123 K |
β = 100.698 (5)° | Needle, colourless |
V = 330.88 (19) Å3 | 0.35 × 0.09 × 0.04 mm |
Z = 2 | |
Data collection top
CCD area-detector diffractometer | 710 reflections with I > 2σ(I) |
ω scans | Rint = 0.040 |
Absorption correction: multi-scan (SADABS; Bruker, 2004) | θmax = 28°, θmin = 2.3° |
Tmin = 0.951, Tmax = 0.994 | h = −5→5 |
8100 measured reflections | k = −11→11 |
843 independent reflections | l = −11→11 |
Refinement top
Refinement on F2 | H atoms treated by a mixture of independent and constrained refinement |
Least-squares matrix: full | w = 1/[σ2(Fo2) + (0.0431P)2 + 0.0279P] where P = (Fo2 + 2Fc2)/3 |
R[F2 > 2σ(F2)] = 0.031 | (Δ/σ)max < 0.001 |
wR(F2) = 0.076 | Δρmax = 0.19 e Å−3 |
S = 1.08 | Δρmin = −0.21 e Å−3 |
843 reflections | Absolute structure: Flack (1983) |
104 parameters | Absolute structure parameter: 10 (10) |
2 restraints | |
Crystal data top
C7H5F2NO | V = 330.88 (19) Å3 |
Mr = 157.12 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 4.468 (2) Å | µ = 0.14 mm−1 |
b = 8.486 (3) Å | T = 123 K |
c = 8.881 (1) Å | 0.35 × 0.09 × 0.04 mm |
β = 100.698 (5)° | |
Data collection top
CCD area-detector diffractometer | 843 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2004) | 710 reflections with I > 2σ(I) |
Tmin = 0.951, Tmax = 0.994 | Rint = 0.040 |
8100 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.031 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.076 | Δρmax = 0.19 e Å−3 |
S = 1.08 | Δρmin = −0.21 e Å−3 |
843 reflections | Absolute structure: Flack (1983) |
104 parameters | Absolute structure parameter: 10 (10) |
2 restraints | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
C1 | 0.6039 (5) | 0.6745 (3) | 0.7482 (3) | 0.0243 (5) | |
C2 | 0.3661 (5) | 0.6227 (3) | 0.6355 (2) | 0.0264 (5) | |
C3 | 0.2352 (5) | 0.4765 (3) | 0.6376 (3) | 0.0324 (5) | |
H3 | 0.0704 | 0.4459 | 0.5594 | 0.039* | |
C4 | 0.3490 (6) | 0.3746 (3) | 0.7564 (3) | 0.0352 (6) | |
H4 | 0.2618 | 0.2729 | 0.7597 | 0.042* | |
C5 | 0.5883 (6) | 0.4198 (3) | 0.8699 (3) | 0.0351 (6) | |
H5 | 0.6677 | 0.3497 | 0.951 | 0.042* | |
C6 | 0.7092 (5) | 0.5677 (3) | 0.8635 (3) | 0.0294 (5) | |
C7 | 0.5730 (5) | 0.9590 (3) | 0.7401 (2) | 0.0270 (5) | |
H7 | 0.6802 | 1.0553 | 0.7371 | 0.032* | |
N1 | 0.7345 (4) | 0.8263 (2) | 0.7452 (2) | 0.0269 (4) | |
F1 | 0.2649 (3) | 0.72061 (17) | 0.51670 (14) | 0.0383 (4) | |
F2 | 0.9449 (3) | 0.61452 (19) | 0.97244 (16) | 0.0425 (4) | |
O1 | 0.2991 (3) | 0.9680 (2) | 0.73884 (19) | 0.0319 (4) | |
H1 | 0.930 (7) | 0.846 (4) | 0.751 (3) | 0.057 (10)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1 | 0.0223 (10) | 0.0211 (10) | 0.0299 (11) | 0.0005 (8) | 0.0061 (9) | −0.0020 (9) |
C2 | 0.0262 (11) | 0.0245 (11) | 0.0271 (11) | 0.0053 (10) | 0.0017 (9) | −0.0013 (10) |
C3 | 0.0299 (12) | 0.0266 (12) | 0.0396 (13) | −0.0037 (11) | 0.0035 (10) | −0.0101 (11) |
C4 | 0.0370 (13) | 0.0205 (12) | 0.0503 (17) | −0.0034 (10) | 0.0141 (12) | −0.0066 (11) |
C5 | 0.0421 (14) | 0.0255 (13) | 0.0385 (13) | 0.0070 (10) | 0.0091 (11) | 0.0075 (11) |
C6 | 0.0265 (12) | 0.0288 (13) | 0.0314 (12) | 0.0037 (10) | 0.0018 (9) | −0.0002 (11) |
C7 | 0.0249 (8) | 0.0196 (10) | 0.0350 (12) | −0.0057 (9) | 0.0022 (9) | 0.0002 (10) |
N1 | 0.0196 (9) | 0.0239 (10) | 0.0360 (11) | −0.0029 (8) | 0.0021 (8) | 0.0006 (9) |
F1 | 0.0433 (8) | 0.0319 (8) | 0.0337 (8) | 0.0020 (6) | −0.0085 (6) | 0.0014 (6) |
F2 | 0.0381 (8) | 0.0415 (9) | 0.0402 (8) | −0.0007 (7) | −0.0127 (6) | 0.0058 (7) |
O1 | 0.0237 (7) | 0.0212 (8) | 0.0507 (10) | −0.0005 (7) | 0.0066 (7) | −0.0010 (8) |
Geometric parameters (Å, º) top
C1—C6 | 1.383 (3) | C4—H4 | 0.95 |
C1—C2 | 1.389 (3) | C5—C6 | 1.372 (4) |
C1—N1 | 1.416 (3) | C5—H5 | 0.95 |
C2—F1 | 1.353 (2) | C6—F2 | 1.351 (3) |
C2—C3 | 1.373 (3) | C7—O1 | 1.224 (3) |
C3—C4 | 1.385 (4) | C7—N1 | 1.334 (3) |
C3—H3 | 0.95 | C7—H7 | 0.95 |
C4—C5 | 1.381 (4) | N1—H1 | 0.88 (3) |
| | | |
C6—C1—C2 | 115.8 (2) | C6—C5—C4 | 118.8 (2) |
C6—C1—N1 | 121.7 (2) | C6—C5—H5 | 120.6 |
C2—C1—N1 | 122.5 (2) | C4—C5—H5 | 120.6 |
F1—C2—C3 | 119.0 (2) | F2—C6—C5 | 120.0 (2) |
F1—C2—C1 | 117.9 (2) | F2—C6—C1 | 116.8 (2) |
C3—C2—C1 | 123.1 (2) | C5—C6—C1 | 123.2 (2) |
C2—C3—C4 | 118.5 (2) | O1—C7—N1 | 125.9 (2) |
C2—C3—H3 | 120.7 | O1—C7—H7 | 117 |
C4—C3—H3 | 120.7 | N1—C7—H7 | 117 |
C5—C4—C3 | 120.5 (2) | C7—N1—C1 | 123.11 (17) |
C5—C4—H4 | 119.8 | C7—N1—H1 | 111 (2) |
C3—C4—H4 | 119.8 | C1—N1—H1 | 126 (2) |
| | | |
C6—C1—C2—F1 | −177.06 (18) | C4—C5—C6—C1 | −0.2 (4) |
N1—C1—C2—F1 | 2.2 (3) | C2—C1—C6—F2 | 178.58 (19) |
C6—C1—C2—C3 | 1.4 (3) | N1—C1—C6—F2 | −0.7 (3) |
N1—C1—C2—C3 | −179.4 (2) | C2—C1—C6—C5 | −0.7 (3) |
F1—C2—C3—C4 | 177.3 (2) | N1—C1—C6—C5 | −179.9 (2) |
C1—C2—C3—C4 | −1.2 (3) | O1—C7—N1—C1 | 0.8 (4) |
C2—C3—C4—C5 | 0.1 (4) | C6—C1—N1—C7 | −124.1 (2) |
C3—C4—C5—C6 | 0.5 (4) | C2—C1—N1—C7 | 56.7 (3) |
C4—C5—C6—F2 | −179.5 (2) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.88 (3) | 1.97 (3) | 2.807 (3) | 158 |
Symmetry code: (i) x+1, y, z. |
Experimental details
| (Ia) | (Ib) |
Crystal data |
Chemical formula | C7H5F2NO | C7H5F2NO |
Mr | 157.12 | 157.12 |
Crystal system, space group | Orthorhombic, Pbca | Monoclinic, P21 |
Temperature (K) | 298 | 123 |
a, b, c (Å) | 8.5031 (15), 11.387 (2), 14.075 (3) | 4.468 (2), 8.486 (3), 8.881 (1) |
α, β, γ (°) | 90, 90, 90 | 90, 100.698 (5), 90 |
V (Å3) | 1362.8 (4) | 330.88 (19) |
Z | 8 | 2 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.14 | 0.14 |
Crystal size (mm) | 0.5 × 0.16 × 0.1 | 0.35 × 0.09 × 0.04 |
|
Data collection |
Diffractometer | CCD area-detector diffractometer | CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Bruker, 2004) | Multi-scan (SADABS; Bruker, 2004) |
Tmin, Tmax | 0.933, 0.986 | 0.951, 0.994 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 8426, 1637, 1006 | 8100, 843, 710 |
Rint | 0.039 | 0.040 |
(sin θ/λ)max (Å−1) | 0.660 | 0.661 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.037, 0.100, 1.02 | 0.031, 0.076, 1.08 |
No. of reflections | 1637 | 843 |
No. of parameters | 105 | 104 |
No. of restraints | 1 | 2 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.15, −0.12 | 0.19, −0.21 |
Absolute structure | ? | Flack (1983) |
Absolute structure parameter | ? | 10 (10) |
Selected geometric parameters (Å, º) for (Ia) topC1—N1 | 1.4147 (19) | C7—N1 | 1.327 (2) |
C7—O1 | 1.2178 (19) | | |
| | | |
C6—C1—N1 | 121.51 (14) | O1—C7—N1 | 125.73 (15) |
C2—C1—N1 | 122.39 (14) | C7—N1—C1 | 122.58 (14) |
| | | |
C2—C1—N1—C7 | 60.2 (2) | | |
Hydrogen-bond geometry (Å, º) for (Ia) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.801 (17) | 2.050 (17) | 2.843 (2) | 170 |
Symmetry code: (i) x+1/2, y, −z+3/2. |
Selected bond and torsion angles (º) for (Ib) topC6—C1—N1 | 121.7 (2) | O1—C7—N1 | 125.9 (2) |
C2—C1—N1 | 122.5 (2) | C7—N1—C1 | 123.11 (17) |
| | | |
C2—C1—N1—C7 | 56.7 (3) | | |
Hydrogen-bond geometry (Å, º) for (Ib) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.88 (3) | 1.97 (3) | 2.807 (3) | 158 |
Symmetry code: (i) x+1, y, z. |
The use of fluorine atoms or the induction of fluorine-containing moieties into organic compounds has been shown to be useful in modulating physical, chemical and biological properties of target compounds (Thayer, 2006; Zheng et al., 2007; Ravikumar et al., 2003). Formamides have also been used as simple theoretical and experimental models for important chemical and biological compounds as was mentioned in our previous publications (Omondi et al., 2005, 2008; Omondi, Levendis et al., 2009 and references therein); and those with at least a carbon–fluorine bond would probably be just as useful, if not more useful, in the sense that they fall into the category of organic compounds that are commonly found in pharmaceuticals and agrochemicals (Thayer, 2006).
The primary molecular packing of formamides is dominated by N—H···O hydrogen bonds (Ferguson et al., 1998; Boeyens et al., 1988; Godwa et al., 2000). As a part of the study on polymorphism and phase transformations in 2,6-disubstituted N-arylformamides (Omondi et al., 2005) in which the effect of chloromethyl exchange and the role of weak interactions on their structural and thermal properties was investigated, N-(2,6-difluorophenyl)formamide [(I) in the scheme] was also found to exist in two structural phases [forms (Ia) and (Ib)] (Fig. 1) similar to those of 2,6-dichloro-N-phenylformamide [(II) in the scheme] and 2-chloro-6-methyl-N-phenylformamide [(III) in the scheme] (Omondi et al., 2005). The crystals of the two forms of compounds (II) and (III) were obtained at different temperatures, orthorhombic form (IIa) and (IIIa) at room temperature from solution and monoclinic form (IIb) and (IIIb) at high temperature by sublimation. Recently, we have also reported on the crystal structure of 2,6-dibromo-N-phenylformamide (Omondi, Lemmerer et al., 2009) which forms hydrogen-bonded chains similar to forms (IIb) and (IIIb) (Omondi et al., 2005), compound (Ib) in this report and 2,6-dimethyl-N-phenylformamide (Omondi et al., 2005). In a related study, 2,6-disubstituted-N-phenylthioamides (Omondi, Lemmerer et al., 2009) were found to exist in only one known phase, but adopted a cis conformation, different from that of the 2,6-disubstituted-N-phenylformamides.
An overlay of the two structures (Ia) and (Ib) (Fig. 2) reveals similar conformations for the two polymorphic forms. Only one case, that of N-phenylformamide, is known to exist as a cis- and trans-conformer in one crystal (Omondi et al., 2008). Thioacetanilide (Michta et al., 2008) was also found to have four independent molecules, all with a trans conformation, in the asymmetric unit. The angle between the planes defined by the aryl ring C1–C6 and the formamide group C1—N1—C7—O1 is larger in form (Ia) [60.2 (2)°] compared with form (Ib) [56.7 (3)°]. This is different from what has been observed in the previously reported two forms of compounds (II) and (III) (Omondi et al., 2005) and also for the two forms of N-2,6-dichloroacetanilide (Nagarajan et al., 1986), where the structures of crystals of 'form a' have a lower value of this torsion angle compared with those of 'form b'. Bond distances and angles (Tables 1 and 3) for both polymorphs are comparable with those of similar structures from the literature (Omondi et al., 2005 and references therein).
A comparison of the cell parameters of form (Ia) with the room-temperature orthorhombic phases of (II) and (III) [(IIa) and (IIIa)] reveals that the three compounds are isostructural and isomorphous with variations in the cell dimensions. The three structures have similar packing patterns. However, compounds (Ib), (IIb) and (IIIb) are not isostructural. Hydrogen-bonding patterns for the two polymorphic forms are shown in Figs. 3, 4, 6 and 7. The hydrogen-bonding distances and angles are given in Tables 2 and 4.
In the crystal, (Ia) has molecules linked by the N—H···O hydrogen bonds forming chains of molecules related by a glide plane in the crystallographic a direction. This results in the formamide molecules pointing in alternating directions (Fig. 3). Adjacent N—H···O hydrogen-bonded chains are held together through π–π interactions [Cg···Cg(-x + 2, -y + 1, -z + 1) = 3.903 (5) Å]. Joining of molecules by the N—H···O and π–π intermolecular interactions results in (010) sheets. Neighbouring sheets interact with each other through very weak C—H···F [H3···F2(-x + 3/2, y - 1/2, z) = 2.685 (1) Å] interactions (Fig. 4).
The crystals of form (Ib) were found to exist in the batch of crystals of 2,6-diflouro-N-phenylthioamide. Since 2,6-diflouro-N-phenylformamide is a starting material in the synthesis of 2,6-difluoro-N-phenylthioamide, it was assumed that unconverted 2,6-difluoro-N-phenylformamide from the reaction crystallized out under the influence of 2,6-difluoro-N-phenylthioamide, thereby obtaining (Ib). Attempts to grow crystals of (Ib) under controlled conditions were not successful as only the starting crystals of 2,6-difluoro-N-phenylformamide and 2,6-difluoro-N-phenylthioamide were obtained in their original forms. Attempts to convert (Ia) to (Ib) by sublimation of a powder of (Ia) in a similar manner to converting (IIa) and (IIIa) to (IIb) and (IIIb), respectively, were also unsuccessful. Due to limited amounts of (Ib), further studies (thermal and crystallographic) were not possible. Fig. 5 shows crystals of (Ib) in a batch of crystals of 2,6-difluoro-N-phenylthioamide.
Hydrogen-bonded chains in compound (Ib) are very similar to those of (IIb) and (IIIb) (Omondi et al., 2005). In these structures, the molecules are stacked with the aryl rings linearly arranged on top of one another and related by translation along a short crystallographic axis [the a axis for (Ib) and (IIb) and b axis for (IIIb)]. This results in the molecules being parallel to each other, forming chains through N—H···O hydrogen bonding along the crystallographic a direction (Fig. 6). Neighbouring N—H···O hydrogen-bonded chains in (Ib) are further connected through C—H···F intermolecular interactions [F2···H5(-x + 2, y + 1/2, -z + 2) = 2.647 (2) Å, F1···H3(x + 1, y, z) = 2.446 (2) Å] (Fig. 7). The second C—H···F interaction in (Ib) is shorter [2.446 (2) Å] than the lower limits set by Rowland & Taylor (1996) at 2.54 Å for normalized hydrogen positions. This would be an indication that the interaction is only secondary and therefore exists as a result of the close proximity of neighbouring molecules caused by the N—H···O, the longer C—H···F, and possibly a C—H···π, intermolecular interactions [C2—F1···π (-x + 1, y + 1/2, -z + 1), C2···π = 3.825 (2) Å and C2—F1···π = 143°].
The stability of the two polymorphs was assessed on the basis of the different intermolecular interactions involved in their crystal packing. Estimation and description of lattice energies by summation of potential energies between interacting atoms (or atom–atom interaction energies) were carried out using the ZipOpec module of the OPIX program suite (Gavezzotti, 2003) described by the UNI force field (Filippini & Gavezzotti, 1994) in a similar manner as was done for (II) and (III) (Omondi et al., 2005). Values of -91.3 and -89.9 kJ mol-1 were obtained for the lattice energies of forms (Ia) and (Ib), respectively.
In addition to lattice energies, ZipOpec calculates molecule–molecule interaction energies to identify which molecular arrangements contribute most to the overall lattice stabilization. For compound (Ia), the most stabilizing interaction is between molecules involved in the formation of the N—H···O chain (-35.2 kJ mol-1), followed by molecules arranged in a π–π interaction configuration (-21.9 kJ mol-1). The third most stabilizing interaction (-9.1 kJmol-1) brings neighbouring F and H atoms into close proximity to form C—H···F interactions. In (Ib), the most stabilizing interaction is again between molecules involved in the N—H···O chain formation (-36.9 kJ mol-1). The next most stabilizing geometries contribute -12.6 and -12.4 kJ mol-1 and involve molecules interacting via C—H···F and C—F···π interactions, respectively, towards lattice stability. As we mentioned previously (Omondi et al., 2005), it seems like the π–π interaction configuration in (Ia) which is not present in (Ib) contributes to the preferential formation of (Ia) at room temperature.
After standing for several weeks and even after heating (as observed by DSC [differential scanning calorimetry]), compound (Ib) does not seem to revert to (Ia), unlike the analogue of (III) (Omondi et al., 2005). Should a transformation of (Ia) to (Ib) be found, we would speculate that the mechanism for such a transformation is similar to that proposed for the polymorphs of (II) and (III). In this case, C—H···F intermolecular interactions would play a similar role to Cl···Cl interactions in (II) and (III). The phase transformation of (IIa) and (IIIa) involves rotation of the aryl group, leaving the N—H···O hydrogen-bonding chain intact with the aryl rings stacked along the short axis. The transformation of (IIa) was said to be entropically driven as it reverts back to form (IIa) in large part because of the stabilizing π–π interactions, whereas there was no reverse change for compound (III) probably due to inhibition by intermolecular C—H···O interactions present in (IIIb) but not present in (IIb).