Structural Features of Tetraazathiapentalenes Fused with Pyrimidine and/or Pyridine Rings. Experimental Evaluation of the Nature of Hypervalent N−S−N Bond by Restricted Internal Rotation of the Pyrimidine Ring

A series of neutral (8−10),
monomethylated (12−14), and dimethylated
(17) 10-S-3 sulfuranes, derivatives
of tetraazathiapentalenes fused with pyrimidine and/or pyridine ring,
were prepared. These molecules are planar,
and bond energies of the hypervalent N−S−N bond were evaluated by
the temperature dependent restricted rotation
of the pyrimidine ring caused by cleavage of one of the S−N bonds.
The bond length is longer, and the energy is
lower for the S−N bond fused with more electron-withdrawing
heterocycles.