Conformation and crystal packing sensitivity to substitution of thioxo- and oxo-tetrazane derivatives

Abstract

The synthesis of a series of six thioxo-tetrazane derivatives is reported. The crystal structures of five of these new products and the corresponding oxo-compounds (except the 4,6-dimethylpyrimidyl derivative) including two new crystal structures are also described and compared. The structural data show the importance of the substituent for the intramolecular arrangement while the heteroatom (S, O) does not play any role at this stage: the 2-pyridyl, 4,6-dimethylpyrimidyl and 8-quinolyl groups occupy a pseudo-equatorial position related to the existence of an intramolecular hydrogen bond where the substituent acts as an acceptor. 2-Hydroxyphenyl moieties occupy a pseudo-axial position related to the existence of an intramolecular hydrogen bond where the substituent acts as a donor. In the sole example where an intramolecular hydrogen bond is not present (2-imidazolyl derivatives), the preference for the pseudo-axial conformation is determined by the hyperconjugative effect. In contrast with the intramolecular arrangement, the crystal packing strongly depends on the choice of the heteroatom. It shows noticeable differences related to the variable involvement of CS and CO functions as hydrogen-bond acceptors and the occurrence of π stacking between the molecules. It thus appears that the sulfur atom of the thioxotetrazanes is always involved in hydrogen bonding whereas this is not the case for the oxygen atom of the oxotetrazane analogues. This result contrasts with the statistical treatment of the crystal structures of the Cambridge Structural Database in the urea/thiourea series and could be related to the steric hindrance imposed by the two methyl groups borne by the nitrogen atoms located in the α positions of the CX function (X = S, O).