Trianionic Pincer Complexes of Niobium and Tantalum as Precatalysts for ROMP of Norbornene

Abstract

This report details the synthesis and characterization of a series of Nb complexes and one tantalum complex supported by the [CF3–ONO]3– trianionic pincer-type ligand. Access to the trianionic Nb dichloride complex, [CF3–ONO]­NbCl2(OEt2) (3-Et 2 O), allows for the synthesis of Nb-dialkyl complexes, [CF3–ONO]­NbR2 (where R = benzyl (5), neopentylsilyl (6), neophyl (7)). The sterically encumbered Nb-neophyl complex (7) is thermally stable and fails to convert to the alkylidene even in the presence of donor ligands. Complex 7, however, promotes catalytic ring-opening metathesis polymerization (ROMP) of norbornene, suggesting the plausible intermediacy of a Nb-alkylidene. The corresponding Ta analogue, [CF3–ONO]­Ta­(CH2C­(CH3)2(C6H5))2 (9), requires dramatically higher temperatures to initiate ROMP and provides poor yields of polymer. Complexes 5 and 6 also promote ROMP of norbornene. Characterization of all new complexes includes multinuclear NMR spectroscopy and combustion analysis. For complex 7, characterization also includes solid-state structure elucidation via a single crystal X-ray diffraction experiment.

Ring Opening Metathesis Polymerization (ROMP) of Norbornenes by (Arylimido)Niobium(V)–Alkylidene Catalysts, Nb(CHSiMe₃)(NAr)[OC(CF₃)₃](PMe₃)₂

“…ROMPs of NBE using the dialkyl complex with trianionic chelate ligand, Nb(CH2CMe2Ph)2[N{2-OC(CF3)2-4-MeC6H3}2}] (B) 30) , and the imido-alkylidene catalysts, Nb(CHSiMe3 31) , were reported in 2016. ROMP of NBE by C was also reported (85 turnovers after 120 h) 32) .…”

(Arylimido)niobium(V)–Alkylidenes, Nb(CHSiMe3)(NAr)[OC(CF3)3](PMe3)2, That Enable to Proceed Living Metathesis Polymerization of Internal Alkynes

“…High-oxidation-state early transition metal–alkylidene (called Schrock-type carbene) complexes attract considerable attention − because they play an essential role as catalysts in the olefin metathesis and Wittig-type coupling, − and some of these alkylidenes (and alkylidynes) are also known as the catalyst for alkane C–H bond activation. ,− In contrast to the number of reports in olefin metathesis active molybdenum–, tungsten–, and vanadium–alkylidene complexes (exemplified in Chart ), − the reported examples in olefin metathesis polymerization using niobium­(V)–dialkyl complexes as the precursors ( A and B , Chart ), , especially by isolated niobium­(V)–alkylidene complexes ( C , D , and E ), , have been limited, − whereas there are some reports for synthesis and reaction chemistry of niobium–alkylidene complexes. ,− …”

Stereospecific Ring‐Opening Metathesis Polymerization of Norbornene Catalyzed by Iron Complexes

“…Homogeneous complexes of other transition metals are also capable of performing ROMP of NBE, including thirdrow transition metals, such as Re [10] and Os; [11] second-row Zr [12] and Nb; [13] and first-row V. [14] Attempts to use complexes based on other first-row transition metals, such as Fe, Co, and Ni, in the reaction with NBE lead to the formation of saturated polymers through addition polymerization. [15] An alternative approach to obtain polyNBE is metal-free photoredox mediated ROMP, which uses an organic redox mediator and a vinyl ether initiator.…”