Chelating σ-Aryl Post-Metallocenes : Probing Intramolecular [C−H···F−C] Interactions and Unusual Reaction Pathways

Research output: Journal Publications and Reviews (RGC: 21, 22, 62)21_Publication in refereed journalpeer-review

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Original languageEnglish
Pages (from-to)1580 - 1590
Journal / PublicationAccounts of Chemical Research
Volume48
Issue number6
Online published20 May 2015
Publication statusPublished - 16 Jun 2015

Abstract

ConspectusOur interest in chelating σ-aryl ancillary ligands was motivated by their potential to impart unusual reactivity, since we envisioned that σ-donors with minimal-donation would create a catalytic center with enhanced electrophilicity. We developed a family of Group 4 post-metallocene catalysts supported by pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands bearing a fluorinated moiety in the vicinity of the metal. Notable features of these meta-substituted tris(hetero)aryl frameworks include their coordination geometry and inherent rigidity. For the first time, the elusive C-H···F-C interaction was manifested as NMR-discernible 1H-19F coupling in solution and characterized by a neutron diffraction study. Their existence carries implications for catalyst design and in the context of weak attractive ligand-polymer interactions (WALPI), since they substantiate the practical viability of the ortho-F···H(β) ligand-polymer interactions proposed for living Group 4 fluorinated bis(phenoxyimine) catalysts. In metal-catalyzed olefin polymerization reactions, the notion of noncovalent interactions between an active ancillary ligand and the growing polymer chain is new. These interactions must be fragile and transient in nature, otherwise the intrinsic chain propagation process would be disrupted, and inherently tunable attractive forces such as hydrogen bonds are ideally suited to this role. The nature, relevance, and usability of extremely weak hydrogen bonds such as C-H···F-C has been a topical yet controversial area of research.We subsequently prepared a series of Group 4 complexes supported by fluorinated (σ-aryl)-2-phenolate-6-pyridyl [O,C,N] ligands. [1H,19F]-HMBC NMR experiments were conducted to probe the observed 1H-19F coupling, and specifically separate contributions from scalar (J) coupling and cross-correlation (CR) interference. For the first time, a significant scalar component was confirmed for the 1H-19F coupling in Ti-[O,C,N] and [O,N,C] complexes, which occurs with chemical connectivity across intramolecular C-H···F-C interactions. This result is important because the applicability of weak attractive ligand-polymer interactions in catalysis is feasible only if the observed coupling and hence the noncovalent interaction is genuine. The verified intramolecular C-H···F-C contacts in these complexes can therefore be considered as synthetic models for ligand-polymer interactions in olefin polymerization processes. Significantly, reports concerning late transition metal systems have appeared that hint at the generality of the WALPI concept for modulating polymerization reactions.We evaluated the olefin polymerization reactivity of Ti-[O,N,C] catalysts through judicious substitution. DFT calculations, which revealed diverse kinetically competitive reaction pathways and active sites (including unusual ethylene-assimilated species) in addition to normal chain propagation, were also employed to rationalize polymerization efficiencies. Further developments in catalytic applications of multidentate σ-aryl ligand systems and novel reactivity of the corresponding complexes can be envisaged.