Crystal structure of cyclo-tris(μ-3,4,5,6-tetrafluoro-o-phenylene-κ2 C 1:C 2)trimercury–tetracyanoethylene (1/1)

The crystal structure and thermal properties of a mixed-stack donor–acceptor complex of trimeric perfluoro-o-phenylene mercury with tetracyanoethylene in an 1:1 ratio were studied by X-ray diffraction and TGA methods.

The title compound, [Hg 3 (C 6 F 4 ) 3 ]ÁC 6 N 4 , contains one molecule of tetracyanoethylene B per one molecule of mercury macrocycle A, i.e., A*B, and crystallizes in the monoclinic space group C2/c. Macrocycle A and molecule B both occupy special positions on a twofold rotation axis and the inversion centre, respectively. The supramolecular unit [A*B] is built by the simultaneous coordination of one of the nitrile N atoms of B to the three mercury atoms of the macrocycle A. The HgÁ Á ÁN distances range from 2.990 (4) to 3.030 (4) Å and are very close to those observed in the related adducts of the macrocycle A with other nitrile derivatives. The molecule of B is almost perpendicular to the mean plane of the macrocycle A at the dihedral angle of 88.20 (5) . The donoracceptor HgÁ Á ÁN interactions do not affect the C N bond lengths [1.136 (6) and 1.140 (6) Å ]. The trans nitrile group of B coordinates to another macrocycle A, forming an infinite mixed-stack [A*B] 1 architecture toward [101]. The remaining N atoms of two nitrile groups of B are not engaged in any donoracceptor interactions. In the crystal, the mixed stacks are held together by intermolecular C-FÁ Á ÁC N secondary interactions [2.846 (5)-2.925 (5) Å ].

Chemical context
Trimeric perfluoro-o-phenylene mercury (A) is a versatile Lewis acid that is applied for complexation with different substrates, in particular, for the obtaining of charge-transfer complexes based on donor-acceptor intermolecular interactions (Hasegawa et al., 2004). Importantly, some physical properties of the guest substrates can change upon complexation. For example, unusual optical properties of the organic molecules in supramolecular complexes with macrocycle A have previously been observed (Haneline et al., 2002;Elbjeirami et al., 2007;Filatov et al., 2009Filatov et al., , 2011. Moreover, using complexation with A, the stabilization of different organic (diphenylpolyynes; Taylor & Gabbaï, 2006;Taylor et al., 2008) and metal-organic (nickelocene; Haneline & Gabbaï, 2004a) molecules was achieved under ambient conditions. In this paper, a complex of A with tetracyanoethylene (B) -an unstable dienophilic (-electron donor and -electron acceptor) compound -[Hg 3 (C 6 F 4 ) 3 ]ÁC 6 N 4 , (I), was prepared and studied by X-ray diffraction analysis to get a deeper understanding of the complexation process.

Structural commentary
Complex (I) contains one molecule of tetracyanoethylene B per one molecule of the mercury macrocycle A, i.e., C 18 F 12 Hg 3 ÁC 6 N 4 (A*B), and crystallizes in the monoclinic ISSN 2056-9890 space group C2/c. Both macrocycle A and the molecule of B occupy special positions on a twofold rotation axis and inversion centre, respectively. The supramolecular unit of (I) is built by the simultaneous coordination of the nitrile N1 nitrogen atom of B to the three mercury atoms of the macrocycle A (Fig. 1) (Tikhonova et al., 2000) and 7,7,8,8-tetracyanoquinodimethane (II) [3.102 (11)-3.134 (11) Å ] (Haneline & Gabbaï, 2004b). Thus, the N1 nitrogen atom is essentially equidistant to the three Lewis acidic sites of the macrocycle A. The molecule of B is almost perpendicular to the mean plane of macrocycle A, making a dihedral angle of 88.20 (5) . It is very important to point out that the donor-acceptor HgÁ Á ÁN interactions do not affect the C N bond lengths [1.136 (6) and 1.140 (6) Å ].
Taking into account the intrinsic C i symmetry of B, the trans nitrile group of this molecule coordinates to another macrocycle A, forming an infinite mixed-stack [A*B] 1 architecture (Fig. 2). The remaining nitrogen atoms of the two nitrile groups of B are not engaged in any donor-acceptor interactions.

Comparison with compound (II)
It is interesting to note that the crystal structures of (I) and (II) are very similar. In both complexes, the guest molecules of tetracyanoethylene B and tetracyanoquinodimethane C are arranged perpendicularly to macrocycle A, with the same coordination mode of the trans nitrile groups to the three mercury atoms (Fig. 4). However, the supramolecular unit in

Figure 2
The infinite mixed-stack [A*B] 1 architecture of (I). Dashed lines indicate the intermolecular secondary HgÁ Á ÁN interactions.

TGA analysis
Despite complexes (I) and (II) being structural analogs, they are substantially different in their chemical stability. The crystalline complex (II) decomposes over a few days, while complex (I) is stable in the solid state for several months under ambient conditions. As free B decomposes rapidly upon reaction with moisture to produce toxic hydrogen cyanide, the high chemical stability of complex (I) is surprising. Moreover, the thermal stability of complex (I) has been studied by thermogravimetric analysis (TGA) which revealed that, upon complexation, tetracyanoethylene is stable to higher temperatures (Fig. 5). So, the free compound B starts to decompose at 363 K, but, being incorporated into the supramolecular complex (I), B is stable up to 393 K. Complex (I) decomposes in two different steps. The first step of a 18.3% weight loss is attributed to molecule B because the much lower decomposition temperature of this molecule compared to macrocycle A. Consequently, the second weight loss of 81.7% is attributed to decomposition of macrocycle A. The complete decomposition of the free B is complete at 445 K; however, its final decomposition temperature is equal to 467 K within the supramolecular complex (I). Final decomposition of complex (I) occurs at 573 K, and is likely due decomposition of macrocycle A.
It is known that tetracyanoethylene is used not only as a component of charge-transfer complexes for organic electronics, but also in the preparation of organic magnets (Kao et al., 2012). Consequently, the increase of its thermal stability attracts special attention in the manufacturing of organic materials. The complexation method described here could help to solve this problem.

Synthesis and crystallization
Trimeric perfluoro-o-phenylene mercury was synthesized according to the procedure described previously (Sartori & Golloch, 1968), and purified by recrystallization in dichloromethane (Filatov et al., 2009). Tetracyanoethylene was acquired from TCI America. All solvents were HPLC grade   The supramolecular structure of complex (II) ([A*C*A*1.5D] 1 ). Dashed lines indicate the intermolecular secondary HgÁ Á ÁN interactions. and used without any further purification. Thermogravimetric analysis was performed with a Hitachi STA7200 SII Nano-Technology instrument (an aluminum crucible (45 mL) was used; heating rate was 10 K min À1 ).
Stoichiometric amounts of trimeric perfluoro-o-phenylene mercury (63.8 mg, 59.6 mmol) and tetracyanoethylene (7.7 mg, 59.6 mmol) were dissolved in dichloromethane in separate tubes using ultrasonication. The contents of the tubes were mixed carefully, and then left for slow evaporation of the solvents. Complex (I) was obtained as yellow prismatic crystals, m.p. = 499-500 K.