Cyanopyridine adducts of SiF₄ and SiCl₄

Abstract The formation of cyanopyridine (CN-py) adducts of silicon tetrahalides was investigated for 3- and 4-cyanopyridine in combination with SiF 4 and SiCl 4 . Whereas bubbling of SiF 4 through toluene solutions of 3-CN-py and 4-CN-py afforded white precipitates, which should possess the respective composition SiF 4 (CN-py) 2 , addition of SiCl 4 did not cause any precipitation. Upon storage of the toluene solution of SiCl 4 and 4-CN-py at 6 °C for several weeks, some crystals of the composition SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py) ⋅ (toluene) were obtained. The use of SiCl 4 as the solvent (i.e. SiCl 4 in large excess) and dissolving 4-CN-py therein gave access to a crystalline adduct of the composition SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py). 3-CN-py instead recrystallized from SiCl 4 without forming an adduct with the silane. Computational analyses (B2T-PLYP level) of the single-point energy differences between starting materials Si X 4 ( X = F, Cl) and ‘pyridine’ (‘pyridine’ = pyridine, 3-CN-py, 4-CN-py) and their adducts Si X 4 (‘pyridine’) 2 revealed the tendencies toward adduct formation to decrease in the order SiF 4 > SiCl 4 as well as pyridine >> 4-CN-py > 3-CN-py. For SiCl 4 with 4- and 3-CN-py, the energy of adduct formation (−7.0 and −5.7 kcal mol −1 , respectively) is easily compensated by entropy effects at room temperature. Whereas the former explains as to why cyanopyridines and SiCl 4 may co-exist without noticeable adduct formation, the crystal structures of the adducts SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py) ⋅ (toluene) and SiCl 4 (4-CN-py) 2 ⋅ 2 (4-CN-py) reveal the additional stabilization of these solids by co-crystallization with 4-cyanopyridine, which eventually enabled the isolation of the SiCl 4 (4-CN-py) 2 moiety in a solid. Partial decomposition (hydrolysis) during attempts of recrystallization of SiF 4 (4-CN-py) 2 and SiF 4 (3-CN-py) 2 afforded crystals of the ionic compounds [4-CN-PyH] + [SiF 5 (4-CN-py)] – and [3-CN-PyH] + 2 [SiF 6 ] 2– , respectively.