pubs.acs.org/Organometallics Published on Web 09/23/2009 r 2009 American Chemical Society Organometallics 2009, 28, 5877–5882 5877 DOI: 10.1021/om9004675 Indirect Substituent Effects upon the Olefin Polymerization Reactivity of Titanium(IV) Chelating σ-Aryl Catalysts Ka-Ho Tam, † Michael C. W. Chan,* ,† Hiromu Kaneyoshi, ‡ Haruyuki Makio, ‡ and Nianyong Zhu § † Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China, ‡ R&D Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan, and § Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China Received June 3, 2009 The surprising impact of substituents that are seemingly remote from the active site upon the olefin polymerization characteristics of group 4 catalysts supported by pyridine-2-phenolate-6-(σ-aryl) ancillary ligands are described. Excellent ethylene polymerization activities (up to 22 kg mmol -1 h -1 (mol/L C2) -1 ) with borate cocatalysts are observed at 100 °C. On the basis of an X-ray crystal structure, a steric origin is proposed for the effect, which influences polymerization behavior by modulating catalyst conformation. However, excessively bulky substituents adjacent to the catalytic center can counteract this effect and give diminished efficiencies. Introduction The variation of ligand substituents in metallocene and postmetallocene complexes and the resultant effects upon catalytic reactivity and material properties in olefin polym- erization have been regularly explored and utilized. 1 Steri- cally demanding groups adjacent to the catalytic center can provide protection from chain transfer/termination processes 2 as well as confer stereoselectivity in R-olefin polymerization reactions by enantiomorphic site control. 3 Electron-withdrawing substituents are routinely incorpo- rated to enhance the electrophilicity of the metal center, giving improved activities. Therefore, significant catalytic consequences for “distal” substituents located away from the active site are unusual. Nevertheless, several reports of distal substituent effects in (post)metallocenes and constrained- geometry catalysts have appeared, but the reported substituent(s) can invariably interact with the incoming olefin 4 and cases of genuinely remote substituent effects are rare. 5 Furthermore, substantial changes in catalytic reactivity for isomers which differ only in the subtle arrange- ment of peripheral substituents would be unexpected. We recently reported the existence of intramolecular C-H 333 F-C contacts (based on evidence from multinuclear NMR spectroscopy and a neutron diffraction study) in F-functionalized pyridine-2-phenolate-6-(σ-aryl) [O,N,C] group 4 catalysts, 6 which constitute synthetic models of weak nonco- valent interactions between a noninnocent ligand and the poly- mer chain in olefin polymerization. 7 A key consideration for these complexes is that the σ-aryl moiety should impart greater electrophilicity at the metal. Crucially, the substituent ortho to the M-C(aryl) bond is in the proximity of but does not interact with the metal center, and nonfluorinated catalysts bearing Me and halogen substituents were subsequently investigated. 8 In our desire to elucidate structure/reactivity relationships and enhance catalytic performance for the Ti(IV) [O,N,C] system, in con- junction with the trityl cocatalyst [Ph 3 C][B(C 6 F 5 ) 4 ], we wish to describe some substituent effects which may be surprising at first glance but are revealed to impact upon the conformation and reactivity of the catalytic center. *To whom correspondence should be addressed. E-mail: mcwchan@ cityu.edu.hk. (1) (a) Resconi, L.; Chadwick, J. C.; Cavallo, L. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Boch- mann, M., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 4, pp 1005-1166. (b) Fujita, T.; Makio, H. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. 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Ethylene polymerization activities in conjunction with MAO were reported.