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Silsesquioxane Models for Silica Surface Silanol Sites with Adjacent Siloxide Functionalites and Olefin Polymerization Catalysts Thereof
Author(s) -
Duchateau Robbert,
Dijkstra Tessa W.,
van Santen Rutger A.,
Yap Glenn P. A.
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400206
Subject(s) - silanol , silsesquioxane , chemistry , siloxane , polymer chemistry , polymerization , cationic polymerization , lewis acids and bases , hydrogen bond , monomer , ring opening polymerization , crystallography , catalysis , medicinal chemistry , organic chemistry , molecule , polymer
Incompletely condensed silsesquioxanes of the type R 7 Si 7 O 9 (O{SiR′ 2 O} n )OH (R = c ‐C 5 H 9 , c ‐C 6 H 11 ; R′ = Me, Ph; n = 1–4), containing a siloxane ring of variable size and rigidity and a remaining silanol, are described. Compared with a truly isolated silanol [R 7 Si 8 O 12 (OH)], solution and solid state FT‐IR spectra of these compounds show a $\tilde \nu $ OH shift of approximately 150 cm −1 to lower frequency, which suggests hydrogen bonding of the silanol with the internal siloxane ring. In agreement with this, the relative ion pair acidities of the silanols in THF, determined by UV/Vis, were lowered by 0.8–1.2 compared with a truly isolated silanol. Density functional theory (DFT) calculations on these systems confirm the presence of intramolecular hydrogen bonding. Possible interaction of the silyl ether functionalities with Lewis acidic metal sites was studied for the neutral gallium‐substituted systems and cationic titanium silsesquioxane complexes, models for an immobilized titanium olefin polymerization catalyst. The electron donating capability of the siloxide functionalities in 1 , 6 , and 7 is not sufficient to satisfy the electron deficiency of the corresponding gallium silsesquioxane species, which form dimeric structures with a bridging siloxide unit rather than Lewis base adducts with coordinated siloxide functionalities. Metallation of 1 and 4 with [Cp′′Ti(CH 2 Ph) 3 ] (Cp′′ = η 5 ‐1,3‐C 5 H 3 (SiMe 3 ) 2 ) in a 1:1 ratio afforded monomeric titanasilsesquioxanes. To probe the effect of the neighboring siloxane ring on the highly Lewis acidic titanium center, the catalytic activities of the corresponding cationic half‐sandwich complexes were tested in 1‐hexene polymerization. Compared with the catalyst system based on the isolated silanol [( c ‐C 5 H 9 ) 7 Si 8 O 12 OH], the presence of a neighboring siloxane ring causes considerable retardation of the polymerization process but also improves the stability of the catalyst.