2-Dimethylamino-1-(2-ethoxy-2-oxoethyl)-3-methyl-3,4,5,6-tetrahydropyrimidin-1-ium tetraphenylborate

Isolated guanidinium ions and tetraphenylborate ions are present in the crystal structure of the title compound, C11H22N3O2 +·C24H20B−. In the guanidinium ion, the dihedral angle between the N/C/N and C/C/C planes being 49.9 (1)°. The six-membered ring exhibits a half-chair conformation. The C—N bond lengths in the cation range between 1.3335 (16) and 1.3552 (16) Å, indicating charge delocalization on the CN3 plane. In the crystal, the cations are connected by C—H⋯O hydrogen bonds, generating a chain along the c axis.

Isolated guanidinium ions and tetraphenylborate ions are present in the crystal structure of the title compound, C 11 H 22 N 3 O 2 + ÁC 24 H 20 B À . In the guanidinium ion, the dihedral angle between the N/C/N and C/C/C planes being 49.9 (1) . The six-membered ring exhibits a half-chair conformation. The C-N bond lengths in the cation range between 1.3335 (16) and 1.3552 (16) Å , indicating charge delocalization on the CN 3 plane. In the crystal, the cations are connected by C-HÁ Á ÁO hydrogen bonds, generating a chain along the c axis.

Ioannis Tiritiris and Willi Kantlehner Comment
Tetrahydropyrimidine derivatives very often show pharmacologic activity. Prominent members are the cyclic amidines oxantel (Kraouti et al., 1993) andpyrantel (Kraouti et al., 1995), which are showing an anthelmintic effect against intestinal nematode infestations in humans and animals. 1-Methyl-2-dimethylamino-1,4,5,6-tetrahydropyrimidine (Tiritiris & Kantlehner, 2012), a cyclic guanidine derivative synthesized by us recently, could be used as a new candidate for preparing potentially pharmacologically active compounds in this field. By alkylation of the free nitrogen of the guanidine base, various cyclic guanidinium salts have been obtained and characterised (Tiritiris & Kantlehner, 2012). One of them, is the here presented title compound (Fig. 1). In the crystal structure of the salt,  (Fig. 1). The carbon atom C6 is not in the ring plane, the angle between the planes N3/C1/N2 and C5/C6/C7 is 49.9 (1)°. Finally, weak C-H···O hydrogen bonds between methylene hydrogen atoms and carbonyl oxygen atoms of neighbouring guanidinium ions have been determined [d(H···O) = 2.44 Å] (Tab. 1). The cations are connected by C-H···O hydrogen bonds, generating a chain (Fig. 2). The anions are packed inbetween these chains using van der Waals interactions, only.

Experimental
The title compound was obtained by reaction of 1-methyl-2-dimethylamino-1,4,5,6-tetrahydropyrimidine with bromoacetic acid ethyl ester in acetonitrile at room temperature. After evaporation of the solvent the crude 2-dimethylamino-3ethoxycarbonylmethyl-1-methyl-1,4,5,6-tetrahydropyrimidinium-bromide (I) was washed with diethylether and dried in vacuo. 1.05 g (3.4 mmol) of (I) was dissolved in 20 mL acetonitrile and 1.16 g (3.4 mmol) of sodium tetraphenylborate in 10 mL acetonitrile was added. After stirring for one h at room temperature, the precipitated sodium bromide was filtered off. The title compound crystallised from a saturated acetonitrile solution after several days at 273 K, forming colourless single crystals. Yield: 1.43 g (78.8%).

Refinement
The hydrogen atoms of the methyl groups were allowed to rotate with a fixed angle around the C-N bond to best fit the experimental electron density, with U(H) set to 1.5 U eq (C) and d They were included in the refinement in the riding model approximation, with U(H) set to 1.2 U eq (C).

Figure 1
The structure of the title compound with atom labels and 50% probability displacement ellipsoids. All H atoms have been omitted for clarity.   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.