149130-47-2Relevant articles and documents
Perturbation of1JC,FCoupling in Carbon-Fluorine Bonds on Coordination to Lewis Acids: A Structural, Spectroscopic, and Computational Study
Skeel, Brighton A.,Boreen, Michael A.,Lohrey, Trevor D.,Arnold, John
, p. 17259 - 17267 (2020)
A lithiated m-terphenyl ligand bearing fluorine atoms at the ortho positions of the flanking aryl rings was synthesized and characterized using single crystal X-ray diffraction, variable-temperature multinuclear NMR spectroscopy, and computational methods. Changes in 1JC,F on coordination to lithium as a spectroscopic observable parametrizing the strength of the C-F···Li interaction are described, and a general, qualitative relationship between C-F bond lengths, Δ1JC,F values, and the extent of C-F bond activation as a result of Lewis acid coordination is proposed.
Highly electrophilic olefin polymerization catalysts. Quantitative reaction coordinates for fluoroarylborane/alumoxane methide abstraction and ion-pair reorganization in group 4 metallocene and 'constrained geometry' catalysts
Deck, Paul A.,Beswick, Colin L.,Marks, Tobin J.
, p. 1772 - 1784 (2007/10/03)
Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)2 (L = Cp, 1,2-Me2Cp, Me5Cp; L2 = Me2Si(Me4Cp)((t)BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)3 were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3+CH3B(C6F5)3- contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution: for M = Zr, ΔH = -23.1(3), -24.3(4), and -36.7(5) kcal mol-1 for L = Cp, Me2Cp, and Me5Cp, respectively for M = Hf and L = 1,2-Me2Cp, ΔH = -20.8(5) kcal mol-1. 'Constrained geometry' complexes (L2 = Me2Si(Me4Cp)((t)BuN)) exhibit similar exothermicities, with ΔH = -22.6(2), -23.9(4), and -19.3(6) kcal mol-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = -10.9(3) and -8.9(4) kcal mol-1 for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)2M-(CH3)2 (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)2BAr (AT = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2-Cp)2MCH3+ CH3B(C6F5)2Ar- ion pairs, are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, ΔH = -18.7(7) and -15.2(8) kcal mol-1 with ΔS = -42(2) and -35(3) e.u:; for Ar = Ph, ΔH = -14.8(8) and -13.3(6) kcal mol-1 with ΔS = -31(2) and -39(2) e.u.; for Ar = 3,5-C6H3Me2, ΔH = -10.8(6) and -12.7(5) with ΔS = -19(2) and -36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (AH = 2-6 kcal mol-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)2MCH3+CH3B(C6F5)2Ar- complexes measured in toluene solution (25°C, 1 atm) follow a trend in metal (Zr > Hf)as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph ~3,5-C6H3-Me2). Polymerization activities correlate roughly with MCH3+ 13C NMR chemical shifts and enthalpies of methide abstraction.
