The effect of ring substitution on the geometry of the silatrane skeleton: the crystal structures of 1-phenyl-3,7,10-trimethylsilatrane and 1-p-tolyl-carbasilatrane

The crystal structures of 1-phenyl-3,7,10-trimethylsilatrane (I) and 1-p-tolyl-carbasilatrane (II) were determined by X-ray diffraction. Both compounds crystallize with two independent molecules in the asymmetric unit. I (d(Si ← N) = 2.175(6) Å) is the minor stereoisomer fraction of the diastereomer mixture possessing approximate C3 symmetry. The conformation of the silatrane moiety is different from that observed in the major (asymmetric) structure of p-tolyl-trimethylsilatrane. The mean Si ← N distance in II (2.29(2) Å) is identical with the bond length observed for the phenyl derivative. The dative bond lengths in various silatranes and silatrane analogs are compared.