Synthesis, Properties, and Structure of Poly(silyl)pyridines. The Phantom of Intramolecular Si−N Bonding

Abstract

2- and 3-silylpyridine ( 2, 3 ) have been prepared from the corresponding bromopyridines either directly by metathesis reaction with KSiH 3 or via the lithiated pyridines and their reaction with halosilanes (in poorer yield). 3-Silylquinoline 4 was obtained from 3-lithioquinoline and tetra(ethoxy)silane followed by reduction with LiAlH 4 . 4-Silylpyridine ( 5 ), 2,6-disilylpyridine ( 9 ), 5/6-methyl-2-silylpyridine ( 6, 7 ), and 2-bromo-6-silylpyridine ( 8 ) were also obtained via the KSiH 3 route, and traces of pentasilylpyridine ( 15 ) have been detected among the products of the analogous reaction with pentabromopyridine. These compounds 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- trimethylsilyl pyridine ( 16 ) have been determined by low-temperature (“in situ”) X-ray diffraction techniques. In all cases the silyl substituents in the 2-position show bending toward the nitrogen atom of the ring (with angles N−C−Si much smaller than 120°), suggesting a coordinative bonding of nitrogen to silicon. The structures are in excellent agreement with results of ab initio calculations (MP2/6-31G*) carried out for 2-silyl-, 3-silyl-, and 2,6-disilylpyridine. These calculations indicate, however, that the distortions are not due to direct (exocyclic) Si−N donor−acceptor bonding (as also suggested in the literature for trimethylsilyl pyridines), but originate from the changes in the electronic configuration of the heterocycle (as compared to benzene). This conclusion is supported by a review of structural data for methylpyridines and finally for pyridine itself: all these compounds show small N−C−X angles (X = H, C, Si in ortho -position to N) and similar distortions of the heterocycle. Ab initio quantum chemical calculations for pyridine (in the literature) and for 2,6-dimethylpyridine (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 pyridines. The physical properties of the compounds are best interpreted through the effect of the heteroatom N on the structure of the arene ring. In the crystal structures there is also no evidence for inter molecular donor−acceptor interactions, e.g., as described for the pentameric [Me 3 Si−NH 2 ] 5 .