SYNTHESIS, PROPERTIES, AND STRUCTURE OF POLY(SILYL)PYRIDINES - THE PHANTOM OF INTRAMOLECULAR SI-N BONDING

2- and 3-silylpyridine (2, 3) have been prepared from the correspondin g bromopyridines either directly by metathesis reaction with KSiH3 or via the lithiated pyridines and their reaction with halosilanes (in po orer yield). 3-Silylquinoline 4 was obtained from 3-lithioquinoline an d tetra(ethoxy)silane followed by reduction with LiAlH4. 4-Silylpyridi ne (5), 2,6-disilylpyridine (9), 5/6-methyl-2-silylpyridine (6, 7), an d 2-bromo-6-silylpyridine (8) were also obtained via the KSiH3 route, and traces of pentasilylpyridine (15) have been detected among the pro ducts of the analogous reaction with pentabromopyridine. These compoun ds and the di- and tripyridylsilanes 10-13 have been characterized by their analytical and spectroscopic data. The crystal structures of 6, 9, and 5-methyl-2-trimethylsilylpyridine (16) have been determined by low-temperature (''in situ'') X-ray diffraction techniques. In all cas es the silyl substituents in the 2-position show bending toward the ni trogen atom of the ring (with angles N-C-Si much smaller than 120 degr ees), suggesting a coordinative bonding of nitrogen to silicon. The st ructures are in excellent agreement with results of ab initio calculat ions (MP2/6-31G) carried out for 2-silyl-, 3-silyl-, and 2,6-disilylp yridine. These calculations indicate, however, that the distortions ar e not due to direct (exocyclic) Si-N donor-acceptor bonding (as also s uggested in the literature for trimethylsilylpyridines), but originate from the changes in the electronic configuration of the heterocycle ( as compared to benzene). This conclusion is supported by a review of s tructural data for methylpyridines and finally for pyridine itself: al l these compounds show small N-C-X angles (X = H, C, Si in ortho-posit ion to N) and similar distortions of the heterocycle. Ab initio quantu m chemical calculations for pyridine (in the literature) and for 2,6-d imethylpyridine (this work) further corroborate the findings for these analogues. In summary therefore, there is no special ''silicon effect '' in the molecular and electronic structure of ortho-silylated pyridi nes. The physical properties of the compounds are best interpreted thr ough the effect of the heteroatom N on the structure of the arene ring . In the crystal structures there is also no evidence for intermolecul ar donor-acceptor interactions, e.g., as described for the pentameric [Me3Si-NH2](5).