Neutron and X‐ray Diffraction and Spectroscopic Investigations of Intramolecular [CH⋅⋅⋅FC] Contacts in Post‐Metallocene Polyolefin Catalysts: Modeling Weak Attractive Polymer–Ligand Interactions

Abstract

A family of Group 4 post-metallocene catalysts, supported by fluorine-functionalized tridentate ligands with the fluorine substituent in the locality of the metal center, is described. For the first time, the contentious C-H...F-C interaction has been characterized by a neutron diffraction study, which has allowed the position of the hydrogen atoms to be accurately determined. The nature of the weak intramolecular C-H...F-C contacts in these complexes in solution and the solid state was probed by using multinuclear NMR spectroscopy in tandem with neutron and X-ray crystallography. Evidence is presented to demonstrate that the spectroscopic C-H...F-C coupling occurs "through-space" rather than "through-bond" or by MF coordination. The titanium catalysts exhibit excellent activities and high co-monomer incorporation in olefin polymerization. The observed intramolecular C-H...F-C interactions are important with regards to potential applications in polyolefin catalysis because they substantiate the proposed ortho-F...H(beta) ligand-(polymer chain) contacts derived from DFT calculations for the remarkable fluorinated phenoxyimine Group 4 catalysts. Compared with agostic and co-catalyst...metal contacts, weak attractive noncovalent interactions between a polymer chain and a judiciously designed "active" ligand is a new concept in polyolefin catalysis.

Scalar Coupling Across [CH⋅⋅⋅FC] Interactions in (σ‐Aryl)‐Chelating Post‐Metallocenes

“…Researchers at Symyx and Dow have developed a class of pyridylamido [N,N,C]-Hf isotactic propylene polymerization catalysts (which are amide congeners of the [O,N,C] system) through the use of high-throughput screening, and described their application in tandem with Zr bis(phenoxyimine) catalysts for the commercial-scale production of multiblock polyolefin materials by a novel chain-shuttling polymerization process. [29] In contrast, our olefin polymerization studies on Ti-[O,N,C] catalysts often revealed multi-site behavior, [24,30] which was ascribed to a strained TiÀCA C H T U N G T R E N N U N G (aryl) linkage and subsequent olefin insertion therein to afford additional, modified catalytic entities. [31] Detailed studies on the [N,N,C]-Hf system (C = 1-naphthyl) have similarly described favorable olefin insertion into the Hf À CA C H T U N G T R E N N U N G (aryl) bond, which is weakened as a steric consequence of the naphthyl moiety, to generate "monomer-inserted" catalysts.…”

“…[37] The Hf metal center in 3 is coordinated by the [O,C,N-CF 3 ] and benzyl ligands in a distorted trigonal-bipyramidal geometry with axial O and N and equatorial CA C H T U N G T R E N N U N G (aryl) and C-A C H T U N G T R E N N U N G (benzyl) atoms (Figure 3). Like previous [O,N,C] structures [23] and in particular the neutron structure [24] of 6 (bearing a CF 3 group in the equivalent R 1 position), the two benzyl units in 3 protrude towards the central s-aryl ring in an unusual anti,anti configuration, which differs from the syn,anti arrangement typically found in tridentate post-metallocene complexes. [38] Although effects from crystal packing cannot be discounted, we have previously ascribed the anti,-anti configuration to severe steric congestion in the equatorial plane between the tBu and CF 3 substituents ( Figure S7 in the Supporting Information), and as a consequence of this, we contend that rotation of the HfÀC(methylene) bonds is greatly impeded (this conclusion is strongly supported by -coordination of the benzyl ligands, reflecting the reduced electrophilicity of the Hf core, and this is also indicated in solution by the relevant NMR spectroscopic data.…”

“…[31] Detailed studies on the [N,N,C]-Hf system (C = 1-naphthyl) have similarly described favorable olefin insertion into the Hf À CA C H T U N G T R E N N U N G (aryl) bond, which is weakened as a steric consequence of the naphthyl moiety, to generate "monomer-inserted" catalysts. [32] The (s-aryl)-2-phenolate-6-pyridyl [O,C,N] ligand set was thus designed [33] to obviate the complication for the Ti- [24] 5 and 6). [23,24] (e.g., 5; see below): 1) the upfield of the two 1 H NMR peaks for the diastereotopic CH 2 hydrogen atoms, which conventionally appear as a doublet of doublets, is observed as an overlapping doublet of quartets (which collapses to a doublet upon 19 F-decoupling; Figure S1 in the Supporting Information); 2) the H nuclei is the HMBC experiment, [35] and for complexes 1-4 containing a single 19 F resonance, the experiment can be performed without the evolution of its chemical shifts to provide simple one-dimensional Figure S4 and S5, respectively, in the Supporting Information).…”

“…[32] The (s-aryl)-2-phenolate-6-pyridyl [O,C,N] ligand set was thus designed [33] to obviate the complication for the Ti- [24] 5 and 6). [23,24] (e.g., 5; see below): 1) the upfield of the two 1 H NMR peaks for the diastereotopic CH 2 hydrogen atoms, which conventionally appear as a doublet of doublets, is observed as an overlapping doublet of quartets (which collapses to a doublet upon 19 F-decoupling; Figure S1 in the Supporting Information); 2) the H nuclei is the HMBC experiment, [35] and for complexes 1-4 containing a single 19 F resonance, the experiment can be performed without the evolution of its chemical shifts to provide simple one-dimensional Figure S4 and S5, respectively, in the Supporting Information). Analogous [ 1 H, 31 P]-HMBC experiments have been employed to detect scalar coupling across NH···OP hydrogen bonds in a flavoprotein [26] and artificial arginine receptor.…”

“…[22] Due to the emerging importance and functionality of fluorine substituents, it is desirable to develop additional spectroscopic techniques to probe fluorinated moieties and elucidate their surrounding environment and connectivity. Our previous work on CF 3 -substituted Group 4 complexes supported by phenolate-pyridine-(s-aryl) [O,N,C] ligands yielded evidence, based on multinuclear NMR coupling plus X-ray [23] and neutron diffraction studies, [24] for the presence of intramolecular CÀH···FÀC contacts. These results indicated that such weak CÀH···FÀC interactions are experimentally feasible, provided support for the DFT-derived o-F···H (b) contacts proposed by Fujita, [3] and could be relevant to the potential application of non-covalent interactions between a non-innocent ligand and the polymer chain in olefin-polymerization reactions.…”