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Steric Factors Affecting the Brønsted Acidity of Aluminosilsesquioxanes
Author(s) -
SkowronskaPtasinska Maria D.,
Duchateau Robbert,
van Santen Rutger A.,
Yap Glenn P. A.
Publication year - 2001
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(20011)2001:1<133::aid-ejic133>3.0.co;2-u
Subject(s) - chemistry , steric effects , stereochemistry , organic chemistry
By reacting AlMe 3 with two equivalents of ( c‐ C 5 H 9 ) 7 Si 7 O 9 (OH) 2 OSiR 3 , the corresponding Brønsted acidic aluminosilsesquioxanes [( c ‐C 5 H 9 ) 7 Si 7 O 11 (OSiR 3 )]Al[( c ‐C 5 H 9 ) 7 Si 7 O 10 (OH)OSiR 3 ] [SiR 3 = SiMe 3 ( 1a ), SiMePh 2 ( 1b )] are obtained. These complexes readily react with triethylamine to yield the corresponding ammonium salts {[( c ‐C 5 H 9 ) 7 Si 7 O 11 (OSiR 3 )] 2 Al} − {HNEt 3 } + ( 2a , b ). Hydrogen bonding between the acidic SiO(H)→Al proton and the pendant silyl ether function is effectively reduced by increasing the steric bulk of the silyl ether substituents, resulting in a higher acidity for 1b compared to that of 1a . With the silsesquioxane ligand ( c ‐C 5 H 9 ) 8 Si 8 O 11 (OH) 2 , which lacks pendant silyl ether functions, the acidic proton cannot satisfactorily be stabilized and this renders the putative Brønsted acid [( c ‐C 5 H 9 ) 8 Si 8 O 13 ]Al[( c ‐C 5 H 9 ) 8 Si 8 O 12 (OH)] unstable. In the absence of proton acceptors, the disproportionation product [( c ‐C 5 H 9 ) 8 Si 8 O 13 ] 3 Al 2 ( 3 ) is formed instead of the Brønsted acid. However, in the presence of triethylamine, the initially formed Brønsted acid readily transfers its proton to the amine, affording the ammonium salt {[( c ‐C 5 H 9 ) 8 Si 8 O 13 ] 2 Al} − {HNEt 3 } + ( 4 ).