Synthesis, crystal structure and Hirshfeld surface analysis of the two-dimensional hydrogen-bonded network [TCNQ-H₂]²⁺[AsF₆]₂⁻

The structure of 2-[4-(di­cyano­meth­yl)cyclo­hexa-2,5-dien-1-yl]propane­bis­(nitrilium) bis­(hexa­fluorido­arsenate), C₁₂H₆N₄²⁺·2AsF₆⁻, has ortho­rhom­bic (Cmce) symmetry. The compound exhibits a layer structure, which is formed by hydrogen bonds between the semi-protonated nitrile groups. Unexpectedly, no H⋯F contacts are observed. Instead, the [AsF₆]⁻ anions show C⋯F contacts to the positively polarized carbon atoms of the dication with distances in the range 2.871 (2)–3.154 (2) Å.

1. Chemical context

Tetra­cyano­quinodi­methane (TCNQ) is widely used in organic semiconductors and in charge-transfer components (Torrance, 1979; Jérome, 2004; Phan et al., 2015; Potember et al., 1979). The weakly oxidizing properties and the stability of its radical anion and diamagnetic dianion have led to a large number of structurally characterized [TCNQ]^–· and [TCNQ]^2– salts (Singh et al., 2016). The oxidation potential of TCNQ can be dramatically increased to ca. 0.9 V vs Fc^+/0 by the coordination of the Lewis acid B(C₆F₅)₃ to every nitrile group (Albrecht et al., 2022). Although the treatment of those nitriles with such electrophiles forming Lewis acid–base adducts is a common approach, the protonation of nitriles requires superacids such as HF/EF₅ (E = As, Sb) or HSO₃F/SbF₅ (Olah & Kiovsky, 1968). A few years ago, the crystal structures of some protonated nitriles were reported, including e.g. [H₃CCN-H]⁺[AsF₆]⁻ and [H₅C₆CN-H]⁺[AsF₆]⁻ (Haiges et al., 2016). Protonations of cyano­metalates are on the other hand much more common due to the stronger basicity coming from the negative charge of the complex. Superacids can be used to convert octa­cyano­metalates to their respective homoleptic hydrogen isocyanide complexes [M(CNH)₈]⁴⁺([SbF₆]⁻)₄ (M = Mo, W; Sellin et al., 2020).

In this work we investigated the reactivity of TCNQ with HF/AsF₅. Instead of the expected fourfold protonation, we observed a di-periodic layered hydrogen-bonded network between diprotonated TCNQ moieties. In the following, its solid-state structure will be discussed.

2. Structural commentary

[TCNQ-H₂]²⁺[AsF₆]⁻₂ crystallizes in the ortho­rhom­bic space group Cmce. The packing is best described as a distorted NaCl structure with a close-packed [TCNQ-H₂]²⁺ network and one ([AsF₆]⁻)₂ moiety in each distorted octa­hedral void (Fig. 2). The asymmetric unit consists of a {CH–C₂–CNH} and an {AsF₄} unit (Fig. 1), whereas H01 is disordered over the mirror plane (−x, y, z) with a SOF of 0.5 giving the overall formula C₁₂H₆N₄As₂F₁₂. The inter­molecular distances between the nitrile groups are 2.542 (2) Å. Upon coordination of electrophiles to a C—N group, contraction of the C—N bond is expected due to electrostatic effects. Comparison to the structure of non-protonated TCNQ (Krause et al., 2015) reveals that the C—N bond of TCNQ shortens by 0.02 Å to 1.130 (2) Å upon semi-protonation. The six-membered ring in the dication is clearly identified as a quinoidal system [bond lengths C3—C4: 1.450 (2) and C4—C4′: 1.342 (3) Å].

Figure 1 The mol­ecular structure of the title compound. Displacement ellipsoids shown at the 50% probability level. Hydrogen atoms displayed in mint have an site-occupancy factor (SOF) of 0.5.

Figure 2 Packing of the title compound in the unit cell. Displacement ellipsoids shown at the 50% probability level. Colour code: arsenic – purple; fluorine – light green; nitro­gen – blue; carbon – dark grey; ordered hydrogen – white; hydrogen in PART −1 – mint.

3. Supra­molecular features

In the solid-state structures of the literature-known protonated nitriles, CN—H⋯F contacts are the dominant motif regarding cation–anion inter­actions (Haiges et al., 2016). However, in this structure, this motif is not observed. Instead, strong, symmetric CN—H⋯NC hydrogen bonds are formed (Table 1, Fig. 3). However, the complete absence of H⋯F contacts in a crystal structure of a protonated nitrile is very surprising. Instead, the [AsF₆]⁻ anions are located directly over and under the electron-deficient π-system of the [TCNQ-H₂]²⁺ moiety (Fig. 4). Three fluorine atoms of the [AsF₆]⁻ anion point directly to the electron deficient carbon atoms, leading to three stronger C⋯F contacts [F1—As01—C2—C3 torsion angle = 0.0 (1)°]. The other three fluorine atoms are found in a staggered geometry towards the dication [F2—As01—C2—C3 torsion angle = 61.6 (1)°], allowing more, but weaker, C⋯F contacts (Table 2). These inter­molecular C⋯F contacts can also be visualized with a Hirshfeld surface (Hirshfeld et al., 1977) in CrystalExplorer (Turner et al., 2017). This surface shows at the F1 and F4 sites three strong inter­actions (red) and, at the other sites, multiple smaller inter­actions for F2 and F3 (pink). Still, the C⋯F contacts are rather long in the present structure (shortest C⋯F contact = 2.894 Å). A salt of triprotonated 1,3,5-tri­cyano­benzene [C₆(CNH)₃H₃]³⁺[Sb₂F₁₁]⁻₂[SbF₆]⁻ (Nitzer et al., 2022a) has C⋯F contacts in the same range, as does triprotonated 1,3,5-tri­carb­oxy­benzene [C₆(CO₂H₂)₃H₃]³⁺[SbF₆]⁻₃ (Nitzer et al., 2022b). Besides the protonated aromatic compounds, de-electronated/oxidized aromatic compounds also have similarly short C⋯F contacts, e.g. the hexa­fluoro­benzene radical–cation salt [C₆F₆]⁺[Sb₂F₁₁]⁻ (Shorafa et al., 2009), the hexa­methyl­benzene dication (Malischewski et al., 2017a,b) and the cyclo­penta­dienium radical–cation salt [C₅(C₆F₅)₅]⁺[Sb₃F₁₆]⁻ (Schulte et al., 2024) and related perhalogenated dimers (Rupf et al., 2020).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N01—H01⋯N01^i	0.86 (4)	1.69 (4)	2.544 (2)	177 (7)
Symmetry code: (i) .

Table 2
F⋯C contacts between the [AsF₆]⁻ counter-ions and the [TCNQ-H₂]²⁺ moieties (Å)

site 1 (F1+F4)		site 2 (F2+F3)
C1⋯F4	2.893 (2)	C1⋯F2	3.097 (2)	C1⋯F3	3.053 (2)
C2⋯F4	3.152 (2)	C2⋯F3	3.091 (3)	C2⋯F2	3.064 (2)
C3⋯F1	2.869 (3)	C3⋯F2	3.115 (2)
C4⋯F1	3.124 (2)	C4⋯F2	3.155 (2)

Figure 3 The di-periodic hydrogen-bonded network of the diprotonated TCNQ moieties. Displacement ellipsoids are shown at the 50% probability level. Colour code: arsenic – purple; fluorine – light green; nitro­gen – blue; carbon dark grey; ordered hydrogen – white; hydrogen in PART −1 – mint.

Figure 4 Top: Hirshfeld surface of the [AsF6]^− anion. Colour ranges from red (short contacts) over white to blue (long contacts). Bottom: Contacts of the [AsF6]^− anions with the π-system of the diprotonated TCNQ. Displacement ellipsoids shown at the 50% probability level. Colour code: arsenic – purple; fluorine – light green; nitro­gen – blue; carbon dark grey; ordered hydrogen – white; disordered hydrogen in PART −1 – mint.

4. Database survey

A survey of the CSD (version 5.46, update June 2024; Groom et al., 2016) gave 1451 hits including the TCNQ moiety. While many charge-transfer salts are known, the poor basicity of the TCNQ leads to a rare role as Lewis base with only 235 hits in the CSD and 226 of them metals. In contrast, only five non-metal coordinations are known [all to B(C₆F₅)₃]. While the neutral TCNQ moiety coordinates only two B(C₆F₅)₃, the [TCNQ]^-/2- coordinates B(C₆F₅)₃ on all four nitrile functions. There are 74 hits for the –CN—H motif, but only 15 of them refer to protonated nitriles (C—CN—H).

5. Synthesis and crystallization

1.0 mL of anhydrous hydrogen fluoride, 1.0 mL of sulfur dioxide and arsenic penta­fluoride (64 mg, 0.4 mmol, 4.0 eq.) were condensed on TCNQ (20 mg, 0.1 mmol, 1.0 eq.) at 77 K. The solution was slowly warmed to room temperature leading to a clear orange solution. The solution was then slowly cooled down to 195 K over the course of a few days to afford yellow crystalline blocks suitable for single crystal X-ray diffraction analysis in ca. 80% yield.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3.

Table 3
Experimental details

Crystal data
Chemical formula	C12H6N4^2+·2AsF6^−
Mr	584.05
Crystal system, space group	Orthorhombic, Cmce
Temperature (K)	100
a, b, c (Å)	11.3963 (4), 20.9925 (9), 7.6198 (3)
V (Å^3)	1822.94 (12)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	3.79
Crystal size (mm)	0.36 × 0.18 × 0.11
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.635, 0.745
No. of measured, independent and observed [I > 2σ(I)] reflections	3524, 985, 976
Rint	0.013
(sin θ/λ)max (Å^−1)	0.626
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.019, 0.047, 1.12
No. of reflections	985
No. of parameters	84
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.40, −0.43
Computer programs: APEX3 and SAINT (Bruker, 2018), SHELXT2014/5 (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b) and OLEX2Dolomanov et al., 2009).

Supporting information

Computing details

2-[4-(Dicyanomethyl)cyclohexa-2,5-dien-1-yl]propanebis(nitrilium) bis(hexafluoridoarsenate)

Crystal data

C12H6N4^2+·2AsF6^−	Dx = 2.128 Mg m^−3
Mr = 584.05	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmce	Cell parameters from 9847 reflections
a = 11.3963 (4) Å	θ = 3.3–26.4°
b = 20.9925 (9) Å	µ = 3.79 mm^−1
c = 7.6198 (3) Å	T = 100 K
V = 1822.94 (12) Å^3	Block, yellow
Z = 4	0.36 × 0.18 × 0.11 mm
F(000) = 1120

Data collection

Bruker APEXII CCD diffractometer	976 reflections with I > 2σ(I)
φ and ω scans	Rint = 0.013
Absorption correction: multi-scan (SADABS-2016/1; Krause et al., 2015)	θmax = 26.4°, θmin = 3.3°
Tmin = 0.635, Tmax = 0.745	h = −12→14
3524 measured reflections	k = −26→12
985 independent reflections	l = −9→9

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.019	All H-atom parameters refined
wR(F^2) = 0.047	w = 1/[σ^2(Fo^2) + (0.0178P)^2 + 4.242P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max = 0.001
985 reflections	Δρmax = 0.40 e Å^−3
84 parameters	Δρmin = −0.43 e Å^−3
0 restraints

Special details

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.

Refinement. Hydrogen atoms at the nitrogen nuclei were refined in PART -1

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
As01	0.500000	0.62594 (2)	0.48213 (3)	0.01821 (10)
F2	0.60728 (10)	0.59600 (6)	0.61714 (15)	0.0336 (3)
F1	0.500000	0.55815 (8)	0.3601 (2)	0.0402 (5)
F4	0.39438 (12)	0.65848 (7)	0.34763 (15)	0.0399 (3)
F3	0.500000	0.69499 (8)	0.6063 (2)	0.0392 (5)
C2	0.500000	0.63170 (11)	−0.0275 (3)	0.0133 (5)
C3	0.500000	0.56661 (11)	−0.0157 (3)	0.0130 (4)
C4	0.39021 (14)	0.53182 (8)	−0.0085 (2)	0.0147 (3)
H4	0.3230 (18)	0.5540 (9)	−0.014 (2)	0.014 (5)*
C1	0.39634 (15)	0.66974 (8)	−0.0308 (2)	0.0153 (3)
N01	0.32043 (14)	0.70440 (7)	−0.0314 (2)	0.0212 (3)
H01	0.274 (4)	0.7358 (18)	−0.014 (6)	0.017 (11)*	0.5

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As01	0.02267 (16)	0.02017 (15)	0.01179 (14)	0.000	0.000	0.00001 (9)
F2	0.0290 (6)	0.0535 (8)	0.0182 (5)	0.0138 (6)	−0.0012 (5)	0.0026 (5)
F1	0.0777 (14)	0.0240 (8)	0.0189 (8)	0.000	0.000	−0.0067 (7)
F4	0.0408 (7)	0.0594 (9)	0.0194 (6)	0.0183 (6)	−0.0044 (5)	0.0050 (6)
F3	0.0748 (14)	0.0217 (8)	0.0211 (8)	0.000	0.000	−0.0023 (7)
C2	0.0162 (11)	0.0125 (11)	0.0113 (10)	0.000	0.000	−0.0006 (8)
C3	0.0159 (11)	0.0139 (11)	0.0092 (10)	0.000	0.000	−0.0006 (8)
C4	0.0115 (8)	0.0166 (8)	0.0160 (8)	0.0022 (7)	0.0000 (6)	0.0002 (6)
C1	0.0208 (9)	0.0103 (7)	0.0150 (8)	−0.0040 (7)	0.0005 (6)	0.0001 (6)
N01	0.0226 (8)	0.0144 (7)	0.0267 (8)	0.0042 (7)	0.0015 (6)	0.0005 (6)

Geometric parameters (Å, º)

As01—F2	1.7170 (11)	C2—C1	1.426 (2)
As01—F2^i	1.7170 (11)	C3—C4^i	1.450 (2)
As01—F1	1.6999 (16)	C3—C4	1.450 (2)
As01—F4^i	1.7221 (12)	C4—C4^ii	1.342 (3)
As01—F4	1.7221 (12)	C4—H4	0.90 (2)
As01—F3	1.7312 (17)	C1—N01	1.130 (2)
C2—C3	1.369 (3)	N01—H01	0.86 (4)
C2—C1^i	1.426 (2)
F2^i—As01—F2	90.80 (8)	F4^i—As01—F3	89.61 (6)
F2^i—As01—F4^i	178.08 (7)	F4—As01—F3	89.61 (6)
F2—As01—F4^i	90.23 (6)	C3—C2—C1^i	124.05 (10)
F2^i—As01—F4	90.24 (6)	C3—C2—C1	124.05 (10)
F2—As01—F4	178.08 (7)	C1^i—C2—C1	111.9 (2)
F2^i—As01—F3	88.79 (6)	C2—C3—C4	120.34 (11)
F2—As01—F3	88.80 (6)	C2—C3—C4^i	120.34 (11)
F1—As01—F2	91.22 (6)	C4^i—C3—C4	119.3 (2)
F1—As01—F2^i	91.22 (6)	C3—C4—H4	118.3 (13)
F1—As01—F4^i	90.37 (6)	C4^ii—C4—C3	120.34 (11)
F1—As01—F4	90.37 (6)	C4^ii—C4—H4	121.4 (13)
F1—As01—F3	179.98 (8)	N01—C1—C2	173.94 (18)
F4^i—As01—F4	88.68 (9)	C1—N01—H01	166 (3)
C2—C3—C4—C4^ii	−178.0 (2)	C1—C2—C3—C4	1.0 (3)
C4^i—C3—C4—C4^ii	1.4 (4)	C1^i—C2—C3—C4	178.36 (18)
C1—C2—C3—C4^i	−178.36 (18)	C1^i—C2—C3—C4^i	−1.0 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N01—H01···N01^iii	0.86 (4)	1.69 (4)	2.544 (2)	177 (7)

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