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-trimethylsilylpyridine ( 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 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 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 intermolecular donor-acceptor interactions, e.g., as described for the pentameric [Me 3 Si-NH 2 ] 5 .

Synthesis and Molecular Structures of 2-Trimethylsilyl-, 2-Trimethylgermyl-, and 2-Trimethylstannyl-pyridines

“…A s rep o rted p rev io u sly [18], th e stru ctu re o f 5m e th y l-2 -trim e th y lsily l-p y rid in e ( 1) sh o w s a tiltin g o f the trim e th y lsily l su b stitu en t to w ard s the n itro gen atom : T h e a n g les S i-C l-N an d S i-C l-C 2 are 115.8(2) and 1 2 4 .6 (2 )°, resp ectiv ely , m ean in g fo r b o th a d ev iatio n fro m th e 120° sta n d ard by ab o u t 5° in o p p o site d irec tio n s. T h e p o sitio n in g o f the m eth y l g ro u p has no su ch an o m alies, w ith th e an g les C 6 -C 4 -C 3 /C 5 clo se to 120° (Fig. 1).…”

“…T h e p rese n t resu lts are in full ag reem en t w ith re c e n t fin dings fo r the fully h y d ro g en -su b stitu ted p ro to ty p e s o f the silyl-pyridines [18]. E ven fo r th e h ea v ie r (and larger) group IV elem en ts, w here d o n o r-a c c e p to r interactions are m ore likely, the stru ctu ra l data are co m parable and show no c o n s e q u e n c e s o f co o rd in atio n interactio n s bey o n d sta n d ard tetraco o rd in a tio n .…”

“…The synthesis followed the literature procedure for other trimethylsilyl-pyridines [13,18]. A colourless liq uid is obtained in 48% yield (b.p.…”

“…and the n itro g e n d o n o r fu n ctio n [13 -17]. In a rec en t p ap e r from th is la b o ra to ry [18] it has been d em o n strate d th at the stru ctu ra l p e c u lia ritie s d o not h ave th e ir o rig in in p e rip h e ra l E -N d o n o r/a c c e p to r in te ractio n s (E = Si), b u t are ra th e r b ased on in trin sic stru ctu ral p ara m ete rs o f th e p y rid in e h e te ro c y cle. B o th ex p e rim en tal an d th e o re tic a l resu lts are c o n s iste n t w ith this in te rp re ta tio n [18], W e have ex ten d e d th e se stu d ies now to in clu d e rep rese n tativ e e x a m p le s w ith E = G e, Sn, to g e th er w ith a silico n co m p o u n d w ith a slig h tly differen t * Reprint requests to Prof. Dr. H. Schmidbaur.…”

Synthesis and Catalytic Performance of Organic–Inorganic Hybrid Mesoporous Material Having Basic Nanospace

“…Co. Ltd.; SiO 2 = 15.3 wt%, Na 2 O = 6.1 wt%, Al = 0.6 ppm, Fe = 0.4 ppm) was used. 3-(Triethoxysilyl)pyridine (3-PySi(OEt) 3 ) was synthesized according to the literature [21]. The pyridyl groups were anchored on FSM-16 (Py_FSM-16) by the post-modification method as follows: 3-PySi(OEt) 3 and FSM-16, which were previously evacuated at 473 K for 20 h, were refluxed in dehydrated toluene under a N 2 atmosphere for 24 h. After refluxing, the modified materials were separated by filtration, then washed with dry toluene, and evacuated at 353 K overnight.…”