Synthesis and Crystal Structures of 1,1′-methylene- Bis(imidazolidine-2,4-dione) and Alkali Metal Salts

Single-crystal structures of 1,1′-methylenebis(imidazolidine-2,4-dione) and its sodium and dipotassium salts were determined. Powder X-ray diffraction was also employed to characterize the bulk materials and those phases which did not yield single-crystals. These compounds are of interest for intumescent coatings.


Introduction
Intumescent coatings [1,2] are used as flame retardants for passive fire protection.Upon heating, they turn into an insulative foam.They typically consist of char-forming and gas-releasing constituents, often along with catalysts and binding resins.They should be non-toxic and inexpensive.It is also advantageous when they contain water as hydrates.The release of this water cools adjacent materials.We envisioned hydantoin derivatives as possible constituents of intumescent mixtures or intumescents OPEN ACCESS on their own.As a prerequisite, the decomposition temperature should be between 300 and 400 °C.The synthesis of methylene derivatives of hydantoin has been reported [3][4][5], and a crystal structure of a related methylene bis hydantoin tungstophosphate has been published previously [6].Preparation of some salts of 1,1′-methylene-bis(imidazolidine-2,4-dione) was mentioned in a patent [5].Salts were expected to have high melting or decomposition temperatures.Thus, we prepared a series of salts of the title compound (such as Na, K, Ca, Ba, tetramethylammonium, and tetramethylguanidinium) to be tested for their potential suitability as new intumescent materials.Single-crystals suitable for X-ray diffraction were obtained only from Na and K salts.

Results and Discussion
The preparation of the title compound 1 was essentially carried out according to the literature with minor modifications as described below.In our hands, it was not necessary to strictly limit the amount of water in the reaction mixture, in contrast to the patented method [5].The synthesis of the mono-valent and di-valent salts of 1,1′-methylenebis(imidazolidine-2,4-dione) was expected to be straightforward, using one or two equivalents of sodium hydroxide or potassium hydroxide, respectively (Figure 1).However, we encountered several unexpected difficulties.Reaction of the title compound with one equivalent of sodium hydroxide in the presence of ammonia (which is needed to get a clear solution) yielded 2a as an amorphous paste which was converted to a crystalline powder by prolonged stirring in ethanol.Surprisingly, this product was found by powder X-ray diffraction (PXRD) to consist of starting material 1 and disodium salt 2b.The disodium salt 2b was readily prepared by reaction of 1 with two equivalents of sodium hydroxide, but no single-crystals could be obtained.By serendipity, we found a single-crystal of the monosodium salt 2a instead in the bulk of the disodium salt.The potassium salts also posed some problems.The purity of the monopotassium salt 3a was better than that of the monosodium salt, but there was still contamination with the starting material.On the other hand, single-crystals of the dipotassium salt 3b were obtained which permitted the determination of their crystal structures.The crystallographic data and refinement details are summarized in Table 1.
Because of the absence of significant anomalous scattering effects [Flack parameter x around −2(2)], Friedel pairs were merged.All hydrogens were found, especially at nitrogen atoms, but were calculated because of a low data/parameter ratio.In the asymmetric unit, there is one complete molecule, half a molecule with C11 on a twofold rotation axis, and a disordered molecule with occupancy of 0.5 for each atom (Figure 2a).Moderate N-H ... O hydrogen bonds are observed (Figure 2b).The disordered molecules produce a partially overlapping chain by symmetry elements of a twofold rotation axis along a and a twofold screw axis along c.In order to obtain reasonable bond lengths O9, C14, C15, and C16 were refined with isotropic displacement parameters (Figure 2c).The hydrogen bond parameters are summarized in Table 2.  Powder X-ray diffractograms showed that the starting material was pure (Figure 3a,b).It can be clearly seen that the attempted synthesis of the monosodium salt gave only a mixture of starting material 1 and disodium salt 2b (Figure 3c).The disodium salt contained traces of the monosodium salt 2a (Figure 3d,e), no single-crystals were obtained.In the crystal structure of 2a, the sodium ions are coordinated to two oxygen atoms of the title ligand and four water molecules (Figure 4a).The sodium ions are alternately linked by one (O6) or two (O7) μ 2 -aqua bridges (Figure 4b).The O6 atom of the single μ 2 -aqua bridge is located on a twofold rotation axis in b direction.An inversion center is situated in the bis(μ 2 -aqua) moiety.Hydrogen bonds between ligands (N-H ... O) and between ligand and water molecules (O-H ... N and O-H ... O) are observed (Table 2, Figure 4b).Sodium ... oxygen contacts are summarized in Table 3.  Powder X-ray diffractograms showed that the bulk of the monopotassium salt 3a contained considerable amounts of starting material 1 (Figure 5a,b), whereas the bulk of the dipotassium salt 3b contained only traces of 1 (Figure 5b,c).Single-crystal and bulk material of the dipotassium salt were identical crystalline phases (Figure 5c,d).
The dipotassium salt 3b is a typical coordination polymer.In the crystal structure of 3b, there is half a ligand molecule in the asymmetric unit, C1 is located on a two-fold rotation axis in the direction of the b axis.Each potassium ion coordinates to two ligand molecules and four water molecules (Figure 6a).The potassium atoms are arranged in columns parallel to the b axis.They are linked by O1 of the ligand and two μ 2 -aqua bridges (O3 and O4), as shown in Figure 6b.Hydrogen bonds between ligand and water molecules (O-H ... N and O-H ... O) are observed (Table 2, Figure 6b).Potassium ... oxygen contacts are summarized in Table 3.Comparison of experimental and calculated PXRD data (Le Bail fit) of starting material 1 and dipotassium salt 3b are available in the Supporting Information (Figures S1 and S2).

Experimental Section
Hydantoin (purity 99%) was purchased from Alfa Aesar (Ward Hill, MA, USA).All other chemicals were obtained from Sigma-Aldrich, St. Louis, MO, USA (European affiliate Steinheim, Germany).Nuclear magnetic resonance (NMR) spectra were recorded with Bruker Avance DPX 300 (Billerica, MA, USA) and Bruker Avance II+ 600 spectrometers.IR spectra were obtained with a Bruker Alpha FT-IR instrument.Water content was determined by coulometric Karl Fischer titration using a Mettler Toledo C20 apparatus (Greifensee, Switzerland).
The powder X-ray diffraction patterns were measured with a Bruker D8 Discover diffractometer using Cu Kα radiation.The patterns were recorded applying a step size of 0.02° with 1 s measuring time per step in the angular range of 2° to 40°.
Single-crystal diffraction intensity data were recorded by  and  scans with a Nonius KappaCCD diffractometer (Bruker) (for 1) or by  scans with an Oxford Diffraction Gemini-R Ultra diffractometer (Agilent, Santa Clara, CA, USA) (for 2a and 3b) using Mo Kα radiation.All hydrogen atoms were identified in difference Fourier maps.Secondary CH 2 (C-H = 0.99 Å) were positioned geometrically and refined using a riding model.Hydrogen atoms of NH groups as well as of the water molecules were refined with restrained distances of 0.88(1) Å (for N-H) and 0.86(1) Å (for O-H), respectively.Furthermore, an additional H … H restraint (1.35 Å) was introduced to obtain an H-O-H angle of about 104°.The isotropic displacement parameters of all H atoms were set at U iso (H) = 1.2U eq (C,N,O).CCDC reference numbers: 950723-950725.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.tests have shown that the disodium and dipotassium salts possess promising intumescent properties.We plan to develop these compounds as constituents of intumescent paints [7].

Figure 4 .
Figure 4. (a) Coordination environment of the ion pair; (b) Hydrogen bonding of the sodium salt 2a.

Figure 6 .
Figure 6.(a) Coordination of the ion pair; (b) Packing of the dipotassium salt 3b.

Table 1 .
Crystal data and structure refinement details.