2-Acetyl-1,1,3,3-tetramethylguanidine

In the molecule of the title compound, C7H15N3O, the central C atom is surrounded in a nearly ideal trigonal–planar geometry by three N atoms. The C—N bond lengths in the CN3 unit are 1.3353 (13), 1.3463 (12) and 1.3541 (13) Å, indicating an intermediate character between a single and a double bond for each C—N bond. The bonds between the N atoms and the terminal C-methyl groups all have values close to that of a typical single bond [1.4526 (13)–1.4614 (14) Å]. In the crystal, the guanidine molecules are connected by weak C—H⋯O and C—H⋯N hydrogen bonds, generating layers parallel to the ab plane.

In the molecule of the title compound, C 7 H 15 N 3 O, the central C atom is surrounded in a nearly ideal trigonal-planar geometry by three N atoms. The C-N bond lengths in the CN 3 unit are 1.3353 (13), 1.3463 (12) and 1.3541 (13) Å , indicating an intermediate character between a single and a double bond for each C-N bond. The bonds between the N atoms and the terminal C-methyl groups all have values close to that of a typical single bond [1.4526 (13)-1.4614 (14) Å ]. In the crystal, the guanidine molecules are connected by weak C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds, generating layers parallel to the ab plane.

Comment
The preparation and properties of various acylguanidines have been described in the literature several years ago (Matsumoto & Rapoport, 1968). While in acylguanidines it can be distinguished between the acylamino and and acylimino form, the increase in pK a going from acylimino-to acylaminoguanidines was explained by conjugation of the guanidine part with the acetyl group. The here presented acylimino type title compound was described in the literature as a colorless liquid (Kessler & Leibfritz, 1970), but quite recently it was possible to obtain single crystals and to elucidate its hitherto unknown crystal structure. According to the structure analysis, the C-N bond lengths of the CN 3 unit are: C1 -N3 = 1.3353 (13) (Table 1), generating chains along (100) (Fig. 2).

Experimental
The title compound was obtained by heating two equivalents of N′,N′,N′′,N′′-tetramethylguanidine with one equivalent acetyl chloride in acetonitrile for 2 h under reflux (Kessler & Leibfritz, 1970). After cooling at room temperature the precipitated N′,N′,N′′,N′′-tetramethylguanidinium chloride was filtered off and the solvent was removed. The residue was redissolved in diethylether and the insoluble part was filtered off. After evaporation of the solvent a colorless liquid has been obtained. The title compound crystallized spontaneously after several days during standing at room temperature,

Refinement
The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C-N or C-C bond to best fit the experimental electron density, with U(H) set to 1.5 U eq (C) and d(C-H) = 0.98 Å.

Figure 1
The structure of the title compound with atom labels and 50% probability displacement ellipsoids.

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