Mol­ecular and crystal structure of 2,5-bis­[(4-fluoro­phen­yl)imino­meth­yl]furan

Lauer, M.K.; Balaich, G.J.; Iacono, S.T.; Weeks, N.J.

doi:10.1107/S2056989025005006

research communications

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 81| Part 7| July 2025| Pages 623-626

https://doi.org/10.1107/S2056989025005006

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Molecular and crystal structure of 2,5-bis[(4-fluorophenyl)iminomethyl]furan

Moira K. Lauer,^a ‡ Gary J. Balaich,^b Scott T. Iacono ^b and Nathan J. Weeks ^b ^*

^aDepartment of Chemistry and Physics, University of North Carolina at Pembroke, Pembroke, NC 28372, USA, and ^bDepartment of Chemistry & Chemistry Research Center, USAF Academy, Colorado, Springs, CO 80840, USA
^*Correspondence e-mail: [email protected]

Edited by M. Weil, Vienna University of Technology, Austria (Received 5 May 2025; accepted 2 June 2025; online 17 June 2025)

The title furan bis(imine) compound, 2,5-bis[(4-fluorophenyl)iminomethyl]furan, C₁₈H₁₂F₂N₂O, was synthesized by condensation of 2,5-furandicarboxaldehyde with two equivalents of 4-fluoroaniline. The molecular structure consists of a central furan ring symmetrically bound to nearly coplanar iminomethyl groups with N-bonded 4-fluorophenyl rings that are significantly tipped out of the plane of the furan ring. In the crystal structure, the furan ring lies on a twofold rotation axis in space group C2/c with the furan ring and imine groups of adjacent molecules participating in C—H⋯N interactions to give furan-ring-centered hydrogen-bonded chains extending along [010]. Further cohesion of the crystal structure is achieved by participation of the peripheral 4-fluorophenyl rings in C—H⋯F hydrogen bonding and edge-to-face C—H⋯π interactions, resulting in a tri-periodic network. The resulting supramolecular chains formed by C—H⋯F hydrogen bonding extend in a direction parallel to [101].

Keywords: crystal structure; furan; monomer; green chemistry.

CCDC reference: 2455881

1. Chemical context

The ongoing plastic pollution crisis and limited recycling strategies related to polyethylene terephthalate (PET) has led to significant research on the development of alternative materials possessing similar mechanical and gas barrier properties (Yoshida et al., 2016 ; Thiounn & Smith, 2020 ; Lauer & Smith, 2020 ). Polyethylene furanoate (PEF) may function as a drop-in replacement for PET plastics due to the structural similarity of the furanic core of PEF relative to the phenyl core of PET (Fei et al., 2020 ). Difluoro-terminated furanic monomers have already been developed and used to synthesize thermally robust (T_d-5% = 753–766 K) aryl ether ketones, but the synthesis of these monomers required the use of several deleterious reagents and yields were not published (Bao et al., 2019 ). However, 2,5-furandicarboxaldehyde-derived imines can be synthesized in a single step in an environmentally friendly solvent with high yields, low energy requirements, facile isolation, and in excellent purity. This green chemistry approach was used to synthesize the title compound, 2,5-bis[(4-fluorophenyl)iminomethyl]furan (Fig. 1), a possible candidate for the development of next-generation bio-based polymeric materials.

Figure 1
Single-step reaction of 2,5-bis[(4-fluorophenyl)iminomethyl]furan in the environmentally friendly solvent ethanol.

2. Structural commentary

Molecules of 2,5-bis[(4-fluorophenyl)iminomethyl]furan crystallize in space group C2/c with one half molecule per asymmetric unit. Bond lengths alternate long [C1—C1ⁱ = 1.411 (2) Å; symmetry code: (i) −x + 1, y, −z + $[{1\over 2}]$ ] – short [C1—C2 = 1.3678 (15) Å] – long [C2—C3 = 1.4353 (15) Å], as expected for the central furan ring symmetrically bound to the C atoms of two methanimine groups (Fig. 2). The furan ring lies on a twofold rotation axis with nearly co-planar methanimine groups [small O1—C2—C3—N1 torsion angle of −3.35 (15)°] bound through their N atoms to 4-fluorophenyl groups as the E isomer in a δ-cis conformation (Fig. 2). In the crystal structure of the non-fluorinated 2,5-bis(phenyliminomethyl)furan molecule, a similar core structure was reported, with the peripheral benzene rings significantly tipped out of the plane of the central furan ring at a reported torsional angle of 38° (Mallet et al., 2011 ). The title molecule displays a similar peripheral ring tip, with the planes of the 4-fluorophenyl groups tipped out of the plane of the central furan ring [34.38 (3)°] as well as the plane of the methanimine groups [39.03 (11)°].

Figure 2
Molecular structure of 2,5-bis[(4-fluorophenyl)iminomethyl]furan. The central furan ring lies on a twofold rotation axis in space group C2/c with the planes of the 4-fluorophenyl rings tipped out of the central furan ring plane by 34.38 (3)°. Displacement ellipsoids are shown at the 50% probability level, with H atoms of arbitrary size; non-labeled atoms are generated by symmetry operation −x + 1, y, −z + $[{1\over 2}]$ .

3. Supramolecular features

The crystal structure of 2,5-bis[(4-fluorophenyl)iminomethyl]furan is consolidated by a tri-periodic network consisting of C—H⋯N and C—H⋯F hydrogen bonds as well as weaker edge-to-face C—H⋯π interactions. Molecules pack head (N1) to tail (C1H), held in place by four furan-ring-centered C1—H1⋯N1 intermolecular hydrogen bonds [2.576 (14) Å, Table 1] per molecule, forming chains that run along [010] (Fig. 3). Further, the two 4-fluorophenyl rings of each molecule interact with the 4-fluorophenyl rings of adjacent molecules, forming four additional C—H⋯F hydrogen bonds [2.617 (14) Å, Table 1] per molecule that repeat in a direction parallel to [101] (Fig. 3). Adjacent molecules pack along [001] in a head-to-head orientation, resulting in the O atoms of the co-parallel central furan rings facing opposite directions with an interplane spacing of 3.2026 (11) Å but with their centroids (Cg1) offset by 3.1666 (16) Å (Fig. 4). Although the planes of the 4-fluorophenyl rings (corresponding centroid is Cg2) are co-parallel along [010] (Fig. 3), they are mutually tilted at an angle of 58.35 (5)° along [001], giving edge-to-face 4-fluorophenyl group C—H⋯π contacts that involve H6⋯Cg2 [2.6004 (4) Å] and H9⋯Cg2 [2.6384 (4) Å interactions], see Fig. 4. Data for the non-fluorinated 2,5-bis(phenyliminomethyl)furan gave a furan ring-to-furan ring interplane spacing of 3.3 Å with a reported C—H⋯π contact distance of 2.63 Å (Mallet et al., 2011). The C—H⋯F contact distance in the crystal structure for the title molecule [2.617 (14) Å, Table 1] is also consistent with the range of values reported for the perfluorophenyl compound, 2,5-bis(pentafluorophenyliminomethyl)furan [2.50 (4)–2.77 (5) Å; (Mallet et al., 2011]. Although π–π stacking interactions were observed in the packing pattern of 2,5-bis(pentafluorophenyliminomethyl)furan (Mallet et al., 2011), the incorporation of only one F atom on each peripheral ring in the title molecule produced a molecular and crystal structure that more closely resembles that of the non-fluorinated 2,5-bis(phenyliminomethyl)furan, and that is consolidated by C—H⋯N and C—H⋯F hydrogen bonds as well as edge-to-face C—H⋯π interactions.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C1—H1⋯N1ⁱ	0.970 (14)	2.576 (14)	3.5408 (14)	173.1 (10)
C8—H8⋯F1ⁱⁱ	0.945 (14)	2.617 (14)	3.2815 (13)	127.8 (10)
Symmetry codes: (i) ; (ii) $[-x+{\script{3\over 2}}, y-{\script{1\over 2}}, -z+{\script{3\over 2}}]$ .

Figure 3
Hydrogen-bonding motif with unit cell overlay for 2,5-bis[(4-fluorophenyl)iminomethyl]furan. Each molecule forms eight hydrogen bonds using C—H⋯N [2.576 (14) Å] and C—-H⋯F [2.617 (14) Å] interactions. Displacement ellipsoids are shown at the 50% probability level, with H atoms of arbitrary size. [Symmetry codes: (i) 1 − x, y, $[{3\over 2}]$ − z; (ii) x, 1 + y, z; (iii) 1 − x, 1 + y, $[{3\over 2}]$ − z; (iv) x, y − 1, z; (v) 1 − x, y − 1, $[{3\over 2}]$ − z; (vi) $[{1\over 2}]$ − x, y − $[{1\over 2}]$ , $[{1\over 2}]$ − z; (vii) $[{1\over 2}]$ + x, y − $[{1\over 2}]$ , 1 + z; (viii) $[{1\over 2}]$ − x, $[{1\over 2}]$ + y, $[{1\over 2}]$ − z; (ix) $[{1\over 2}]$ + x, $[{1\over 2}]$ + y, 1 + z.]

Figure 4
Details of the packing for 2,5-bis[(4-fluorophenyl)iminomethyl]furan. The view along [001] (a) shows the head-to-head arrangement of the central furan rings in addition to the edge-to-face interactions between peripheral 4-fluorophenyl groups. A portion of the view along [010] (b) depicts the offset furan ring-to-furan ring interplanar spacing as well as the C—H⋯π interactions [H6⋯Cg2 (2.6004 (4) Å] and H9⋯Cg2 [2.6384 (4) Å] that extend along [001]. Displacement ellipsoids are shown at the 50% probability level, with H atoms of arbitrary size.

4. Database survey

The crystal structures of the related compounds, 2,5-bis(phenyliminomethyl)furan [Cambridge Structural Database (CSD; Groom et al., 2016 ) deposition identifier EBEVIS] and 2,5-bis(pentafluorophenyliminomethyl)furan (CSD deposition number EBEVUE) were previously reported (Mallet et al., 2011). Compared to the title molecule, 2,5-bis(phenyliminomethylfuran) crystallizes in the same space group type (C2/c) with similar unit cell parameters (Mallet et al., 2011), giving nearly identical molecular and crystal structures. However, 2,5-bis(pentafluorophenyliminomethyl)furan crystallizes in space group P1, having a molecular structure with one methanimine arm in the δ-cis conformation and the other arm in the δ-trans conformation in addition to a packing pattern featuring π—π stacking interactions (Mallet et al., 2011). The consolidating effect of C—H⋯N hydrogen bonding was not discussed for the reported crystal structures of 2,5-bis(phenyliminomethyl)furan and 2,5-bis(pentafluorophenyliminomethyl)furan, but C—H⋯F interactions were noted for the structure of 2,5-bis(pentafluorophenyliminomethyl)furan (Mallet et al., 2011).

5. Synthesis and crystallization

To a well-stirred solution of 2,5-furandicarboxaldehyde (0.200 g, 1.6 mmol) in ethanol (20 ml) was added 4-fluoroaniline (0.394 g, 3.5 mmol), and the reaction mixture heated to 313 K. The reaction mixture was allowed to stir until all of the monosubstituted product had converted to the disubstituted product as determined by GC–MS. The reaction mixture was allowed to cool, diluted by half with water, filtered, and washed with water. After drying at 333 K under reduced pressure, a greenish-yellow-colored crystalline solid was obtained (0.402 g, 80%).

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms, except H1 and H8, were placed using a riding model with their positions constrained relative to their parent C atom using the appropriate HFIX command in SHELXL (Sheldrick, 2015b ). Hydrogen atoms H1 and H8 involved in C—H⋯N and C—H⋯F hydrogen bonding were placed from the electron-density map, and their C—H distances restrained (DFIX, C—H range 0.94–0.96 Å) at 0.95 Å with U_iso(H) = 1.2U_eq(C).

Table 2
Experimental details

Crystal data
Chemical formula	C₁₈H₁₂F₂N₂O
M_r	310.30
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	32.9033 (3), 6.02694 (5), 7.14998 (6)
β (°)	95.5021 (8)
V (Å³)	1411.35 (2)
Z	4
Radiation type	Cu Kα
μ (mm⁻¹)	0.93
Crystal size (mm)	0.21 × 0.11 × 0.07

Data collection
Diffractometer	XtaLAB Synergy, Dualflex, HyPix3000
Absorption correction	Gaussian (CrysAlis PRO; Rigaku OD, 2023 $[Rigaku OD (2023). Rigaku Oxford Diffraction, Yarnton, England.]$ )
T_min, T_max	0.671, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	13137, 1312, 1252
R_int	0.028
(sin θ/λ)_max (Å⁻¹)	0.605

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.029, 0.078, 1.05
No. of reflections	1312
No. of parameters	114
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.20, −0.17
Computer programs: CrysAlis PRO (Rigaku OD, 2023 $[Rigaku OD (2023). Rigaku Oxford Diffraction, Yarnton, England.]$ ), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009 ).

Supporting information

CCDC reference: 2455881

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989025005006/wm5760sup1.cif

Supporting information file. DOI: https://doi.org/10.1107/S2056989025005006/wm5760Isup3.mol

Supporting information file. DOI: https://doi.org/10.1107/S2056989025005006/wm5760sup4.docx

HKL. DOI: https://doi.org/10.1107/S2056989025005006/wm5760sup5.txt

Supporting information file. DOI: https://doi.org/10.1107/S2056989025005006/wm5760Isup5.cml

checkCIF report

Computing details top

2,5-Bis[(4-fluorophenyl)iminomethyl]furan top

Crystal data top

C₁₈H₁₂F₂N₂O	F(000) = 640
M_r = 310.30	D_x = 1.460 Mg m⁻³
Monoclinic, C2/c	Cu Kα radiation, λ = 1.54184 Å
a = 32.9033 (3) Å	Cell parameters from 10588 reflections
b = 6.02694 (5) Å	θ = 2.7–68.8°
c = 7.14998 (6) Å	µ = 0.93 mm⁻¹
β = 95.5021 (8)°	T = 100 K
V = 1411.35 (2) Å³	Rectangular prism, clear greenish yellow
Z = 4	0.21 × 0.11 × 0.07 mm

Data collection top

XtaLAB Synergy, Dualflex, HyPix3000 diffractometer	1312 independent reflections
Radiation source: micro-focus sealed X-ray tube, PhotonJet (Cu) X-ray Source	1252 reflections with I > 2σ(I)
Mirror monochromator	R_int = 0.028
Detector resolution: 10.0000 pixels mm^-1	θ_max = 68.9°, θ_min = 2.7°
ω scans	h = −31→39
Absorption correction: gaussian (CrysAlisPro; Rigaku OD, 2023)	k = −7→7
T_min = 0.671, T_max = 1.000	l = −8→8
13137 measured reflections

Refinement top

Refinement on F²	Hydrogen site location: mixed
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F² > 2σ(F²)] = 0.029	w = 1/[σ²(F_o²) + (0.0395P)² + 1.169P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.078	(Δ/σ)_max = 0.001
S = 1.05	Δρ_max = 0.20 e Å⁻³
1312 reflections	Δρ_min = −0.17 e Å⁻³
114 parameters	Extinction correction: SHELXL-2019/2 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00100 (16)
Primary atom site location: dual

Special details top

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
C1	0.51924 (3)	0.11462 (17)	0.30286 (14)	0.0162 (3)
F1	0.73109 (2)	0.88737 (12)	0.71765 (9)	0.0280 (2)
N1	0.57648 (3)	0.62961 (15)	0.42481 (12)	0.0158 (2)
O1	0.500000	0.46983 (17)	0.250000	0.0150 (3)
C2	0.52992 (3)	0.33229 (18)	0.33073 (14)	0.0152 (2)
C3	0.56719 (3)	0.42300 (18)	0.41997 (14)	0.0160 (2)
H3	0.586445	0.322231	0.480032	0.019*
C4	0.61637 (3)	0.68551 (17)	0.50427 (14)	0.0149 (2)
C5	0.62174 (3)	0.88471 (17)	0.60383 (14)	0.0165 (3)
H5	0.598685	0.974298	0.621945	0.020*
C6	0.66028 (3)	0.95307 (19)	0.67642 (14)	0.0185 (3)
H6	0.663913	1.088192	0.744385	0.022*
C7	0.69331 (3)	0.81989 (19)	0.64753 (15)	0.0189 (3)
C8	0.68949 (3)	0.62270 (19)	0.54995 (15)	0.0180 (3)
C9	0.65075 (3)	0.55601 (18)	0.47783 (14)	0.0163 (3)
H9	0.647485	0.420854	0.409678	0.020*
H1	0.5356 (4)	−0.012 (2)	0.3467 (17)	0.019 (3)*
H8	0.7127 (4)	0.537 (2)	0.5300 (19)	0.027 (4)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0153 (6)	0.0160 (5)	0.0175 (5)	0.0018 (4)	0.0024 (4)	0.0015 (4)
F1	0.0172 (4)	0.0342 (4)	0.0314 (4)	−0.0070 (3)	−0.0038 (3)	−0.0062 (3)
N1	0.0146 (5)	0.0164 (5)	0.0162 (4)	0.0003 (3)	0.0004 (3)	−0.0007 (3)
O1	0.0130 (5)	0.0137 (5)	0.0179 (5)	0.000	−0.0010 (4)	0.000
C2	0.0138 (5)	0.0168 (5)	0.0149 (5)	0.0023 (4)	0.0011 (4)	0.0014 (4)
C3	0.0153 (5)	0.0165 (5)	0.0160 (5)	0.0021 (4)	0.0011 (4)	0.0013 (4)
C4	0.0165 (5)	0.0146 (5)	0.0133 (5)	−0.0007 (4)	0.0004 (4)	0.0030 (4)
C5	0.0191 (6)	0.0143 (5)	0.0162 (5)	0.0016 (4)	0.0022 (4)	0.0025 (4)
C6	0.0240 (6)	0.0161 (5)	0.0155 (5)	−0.0030 (4)	0.0019 (4)	−0.0004 (4)
C7	0.0156 (5)	0.0236 (6)	0.0169 (5)	−0.0053 (4)	−0.0012 (4)	0.0022 (4)
C8	0.0161 (6)	0.0203 (6)	0.0177 (5)	0.0021 (4)	0.0021 (4)	0.0019 (4)
C9	0.0187 (5)	0.0152 (5)	0.0149 (5)	−0.0001 (4)	0.0009 (4)	0.0007 (4)

Geometric parameters (Å, º) top

C1—C1ⁱ	1.411 (2)	C4—C5	1.3983 (15)
C1—C2	1.3678 (15)	C4—C9	1.4020 (15)
C1—H1	0.970 (14)	C5—H5	0.9500
F1—C7	1.3573 (12)	C5—C6	1.3861 (15)
N1—C3	1.2819 (14)	C6—H6	0.9500
N1—C4	1.4198 (13)	C6—C7	1.3825 (16)
O1—C2ⁱ	1.3712 (12)	C7—C8	1.3778 (16)
O1—C2	1.3712 (12)	C8—C9	1.3880 (15)
C2—C3	1.4353 (15)	C8—H8	0.945 (14)
C3—H3	0.9500	C9—H9	0.9500

C1ⁱ—C1—H1	128.1 (8)	C6—C5—C4	120.82 (10)
C2—C1—C1ⁱ	106.43 (6)	C6—C5—H5	119.6
C2—C1—H1	125.5 (8)	C5—C6—H6	120.8
C3—N1—C4	116.75 (9)	C7—C6—C5	118.40 (10)
C2ⁱ—O1—C2	105.61 (12)	C7—C6—H6	120.8
C1—C2—O1	110.76 (10)	F1—C7—C6	118.43 (10)
C1—C2—C3	128.81 (10)	F1—C7—C8	118.77 (10)
O1—C2—C3	120.34 (10)	C8—C7—C6	122.79 (10)
N1—C3—C2	124.98 (10)	C7—C8—C9	118.29 (10)
N1—C3—H3	117.5	C7—C8—H8	120.8 (9)
C2—C3—H3	117.5	C9—C8—H8	120.9 (9)
C5—C4—N1	118.31 (9)	C4—C9—H9	119.6
C5—C4—C9	118.84 (10)	C8—C9—C4	120.86 (10)
C9—C4—N1	122.74 (10)	C8—C9—H9	119.6
C4—C5—H5	119.6

C1ⁱ—C1—C2—O1	0.69 (14)	C3—N1—C4—C9	37.67 (14)
C1ⁱ—C1—C2—C3	−175.88 (10)	C4—N1—C3—C2	−174.20 (9)
C1—C2—C3—N1	172.93 (11)	C4—C5—C6—C7	0.19 (15)
F1—C7—C8—C9	−179.59 (9)	C5—C4—C9—C8	0.43 (15)
N1—C4—C5—C6	−176.78 (9)	C5—C6—C7—F1	179.63 (9)
N1—C4—C9—C8	176.64 (9)	C5—C6—C7—C8	−0.02 (17)
O1—C2—C3—N1	−3.35 (15)	C6—C7—C8—C9	0.05 (17)
C2ⁱ—O1—C2—C1	−0.27 (6)	C7—C8—C9—C4	−0.26 (16)
C2ⁱ—O1—C2—C3	176.63 (11)	C9—C4—C5—C6	−0.39 (15)
C3—N1—C4—C5	−146.10 (10)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1···N1ⁱⁱ	0.970 (14)	2.576 (14)	3.5408 (14)	173.1 (10)
C8—H8···F1ⁱⁱⁱ	0.945 (14)	2.617 (14)	3.2815 (13)	127.8 (10)

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

Footnotes

‡Additional correspondence author, e-mail: [email protected].

Acknowledgements

The authors acknowledge the Air Force Office of Scientific Research (AFOSR) and the Defense Threat Reduction Agency (DTRA) for support through a memorandum of agreement with the US Air Force Academy.

Funding information

Funding for this research was provided by: Air Force Office of Scientific Research (grant No. 703-588-8487).

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