The nature and importance of C-H···F-C interactions is a topical yet controversial issue, and the development of spectroscopic methods to probe such contacts is therefore warranted. A series of Group 4 bis(benzyl) complexes supported by (σ-aryl)-2-phenolate-6-pyridyl [O,C,N-R(1)] ligands bearing a fluorinated R(1) group (CF(3) or F) in the vicinity of the metal has been prepared. The X-ray crystal structure of the CF(3)-substituted Hf derivative features intramolecular C-H···F-C and Hf···F-C contacts. All complexes have been characterized by multinuclear NMR spectroscopy. The (1)H and (13)C NMR spectra of [M(O,C,N-CF(3))(CH(2)Ph)(2)] derivatives display coupling (assigned to (1h)J(HF) and (2h)J(CF) for Ti; (3)J(HF) and (2)J(CF) (through M···F) for Hf and Zr) between the benzyl CH(2) and CF(3) moieties. [(1)H,(19)F]-HMBC NMR experiments have been performed for the M-[O,C,N-R(1)] complexes and their [O,N,C] counterparts, revealing significant scalar coupling across the C-H···F-C interactions for Ti-[O,C,N] and [O,N,C] species.

The observation of weak intramolecular C−H•••F−C interactions in group 4 (σ-aryl)-chelating complexes using NMR spectroscopic and neutron diffraction studies was recently reported. In this work, a new titanium(IV) catalyst precursor supported by a tridentate pyridine-2-phenolate-6-(σ-aryl) ligand, featuring a metal center surrounded by multiple CF 3 substituents, has been synthesized. The nature of intramolecular interactions in the bis(benzyl) complex in solution was probed using multinuclear NMR spectroscopic experiments (including [ 1 H, 19 F]-HMQC and -HMBC), which reveal scalar coupling across C−H•••F−C interactions between methylene hydrogens and the proximal CF 3 group on the σ-aryl (but not the phenolate) moiety. High activities are observed for ethylene polymerization at different temperatures, which exceed those for the t Bu-phenolate congener.

Our interest in chelating σ-aryl ancillary ligands was motivated by their potential to impart unusual reactivity, since we envisioned that σ-donors with minimal π-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 (1)H-(19)F 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. [(1)H,(19)F]-HMBC NMR experiments were conducted to probe the observed (1)H-(19)F 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 (1)H-(19)F 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-...

The synthesis, spectroscopic and structural characterization, and olefin polymerization behavior of a family of pyridine‐2‐phenolate‐6‐arylmethine [O,N,CH(Ar)] Group 4 catalysts bearing a four‐membered N,C(sp3)‐donor metallacycle are described. The racemic [O,N,CH(Ar)] complexes adopt C1 symmetry, as confirmed by NMR spectroscopy and X‐ray crystallography. Advanced NMR experiments have been conducted to probe for possible C−H⋅⋅⋅F−C interactions within fluorinated derivatives. Use of a bis‐pyridyl ligand (Ar=py) resulted in the formation of C2v‐symmetric [O,NCHN] complexes containing a six‐membered partially delocalized N,N‐donor chelate. All complexes have been evaluated as ethylene polymerization catalysts. Notably, the Ti derivatives in conjunction with [Ph3C][B(C6F5)4]/iBu3Al display excellent catalytic efficiencies (TOF over 4×105 h−1 [atm C2H4]−1) at 22 °C, and are considerably more active than previously reported Ti−[O,N,C(σ‐aryl)] relatives. DFT calculations have been performed to gain insights into catalytic behavior. These studies indicate that although the ethylene assimilation process (comprising initial ethylene insertion into the Ti−C(methine) bond of the four‐membered chelate) is accessible, there exists a kinetic preference for normal chain propagation for some catalysts. The DFT results for the Ti−[O,N,CH(Ar)] (Ar=Ph) catalyst are consistent with the narrow molecular weight distributions of the polymers produced (Mw/Mn down to 2.2), suggesting close to single‐site character.

A novel μ-C4R2H2 core structure (formed by an unprecedented regioselective, redox-neutral C(sp(2))-C(sp(2)) coupling process) in binuclear group 4 complexes displays adaptable coordination and accommodates different metal sizes, and is sufficiently robust to promote interesting catalytic reactivity at the bimetallic centers.