A Diverse Array of C–C Bonds Formed at a Tantalum Metal Center

• Published in: Inorganic chemistry 2021-07, Vol.60 (13), p.9912-9931
• Main Authors: Fostvedt, Jade I, Boreen, Michael A, Bergman, Robert G, Arnold, John
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c01159

We demonstrate the formation of a diverse array of organic and organometallic products containing newly formed C–C bonds via successive methyl transfers from di-, tri-, and tetramethyl Ta­(V) precursors to unsaturated small molecule substrates under mild conditions. The reactions of Ta­(V) methyl complexes 1–X [H2B­(MesIm)2]­TaMe3X (X = Me, Cl; Im = imidazole, Mes = 2,4,6-trimethylphenyl) with CO led to oxo enolate Ta­(V) products, in which the enolate ligands were constructed from Ta-Me groups and two equivalents of CO. Similarly, the reaction of 1-Me with CNXyl yielded an imido enamine Ta­(V) product. Surprisingly, 1-Cl reacted with CNXyl (1 equiv) at the borate backbone of the [H2B­(MesIm)2] ligand with concomitant methyl transfer from the metal center to form a new, dianionic scorpionate ligand that supported a Ta­(V) dimethyl chloro complex (6). Treatment of 1-Cl with further CNXyl led to an azaallyl scorpionate complex, and an imido isocyanide scorpionate complex, along with propene and xylyl ketenimine. Complex 6 reacted with CO to yield a pinacol scorpionate complex 10a new reaction pathway in early transition metal chemistry. Mechanistic studies revealed that this proceeded via migratory insertion of CO into a Ta-Me group, followed by methyl transfer to form an η2-acetone intermediate. Elimination of acetone furnished a CO-stabilized Ta­(III) intermediate capable of rebinding and subsequently coupling two equivalents of CO-derived acetone to form the pinacol ligand in 10.