A theoretical investigation to evaluate structural and conformational effects of ligands in transition metal complexes useful for the preparation of ultra-high.

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A theoretical investigation to evaluate structural and conformational effects of ligands in transition metal complexes useful for the preparation of ultra-high molecular weight poly(ethylene)s K. Patela, K. Vankab, S. Chikkalia*, S. Sivaramc aPolymer Science & Engineering Division, bPhysical & Materials Chemistry Division, CSIR-National Chemical Laboratory, cIndian Institute of Science Education Dr. Homi Bhabha Road, Pune-411008. India , * e-mail id: s.chikkali@ncl.res.in Background Results UHMW-PE can be defined as linear polyethylenes with molecular weight greater than ~2-3 million g/mol. Difficult to prepare such PE’s Ring inversion (axial/equatorial as function of substituent) Energy difference between axial and equatorial conformers vary with the nature of substituent group, and with sterically more demanding substituent enforce the conformation exclusively to the equatorial position in case of ligand and complexes . Kinetic chain length kp>>>>>>>>>>>> ktr or kt Inherent transfer reaction limit Mw Catalyst (β-H) (Nature of Transition metal) Co-catalyst (Organoaluminium) Features of known catalyst Metallocene Ti >>Zr >>Hf Fluxional behavior (Possible structural isomers) Di Cp > mono Cp Bulky substituent on cyclohexyl ring restrict fluxional behaviour, and stabilized Oh complexes in C2 cis-imine symmetry di Cp mono Cp Bite angle in di Cp > mono Cp Increased bite angle can reduces β-H transfer Di Cp produces higher MW than mono Cp (open framework) Schiff Base Complexes Ligand effect (Steric and electronic) Buried volume (%Vbur) Objectives %VBur gives a measure of the space occupied by the ligand in the first coordination sphere of the metal. To investigate the role of steric and/or conformational effect of the ligand on kp/ktr for ethylene polymerization in a Schiff base metal complexes. The following structural variants were chosen for study. Complex (substituent X on Cy) H Me tBu Ph Buried volume (% VBar) 69.1 70.8 71.3 70.6 Steric mapping H Me tBu Ph 3.00 1.50 Steric bulk Increase Ti Ti Ti Ti 0.00 -1.50 H Me Et Pr iPr Bu tBu Ph -3.00 To estimate ground state energies, symmetry and geometry, electronic and steric effects of given ligand framework and their metal complexes. Conclusions Rank of metal complexes in the order of increasing steric crowding around metal centre Methods The gometry optimizations were conducted employing DFT with the Turbomole 7.0 (basis set TZVP) suite of programs with Perdew, Burke, and Ernzerhof (PBE)9 function The resolution of identity (RI) along with the multipole accelerated resolution of identity (marij) approximations was employed for an accurate and efficient treatment of the electronic Coulomb term. Buried volume (%VBur) was calculated using SambVca 2. (A Web Tool for Analyzing Catalytic Pockets with Topographic Steric Maps) H Ph Me Et Pr iPr tBu Increasing steric crowding Increasing rate of kp/ktr Experimentally Zr complexes with phenoxy-imine ligande framework having 2-methyl cyclohexyl group found advantageous in obtaining higher MW as compared to cyclohexyl. group. . Acknowledgement KP gratefully acknowledges financial support from DST India Young Scientist Scheme. SHC acknowledges DST References: 1. S. M. Kurtz, The UHMWPE Handbook.; Elsevier Academic Press: New York, 2004 2. H. H. Brintzinger, et al, Angew. Chem. Int. Ed. Engl. 34, 1143, 1995. 3. H. Makio, H. Terao, A. Iwashita, T. Fujita., Chem. Rev. 111, 2363, 2011