Synthesis, reactivity and structural studies of early transition metal alkyl and alkylidene complexes in pi-loaded environments

Abstract

Cyclopentadienyl bis(imido) complexes of molybdenum, prepared from Mo(=NAr)₂Cl₂(THF)₂ (Ar = 2,6-diisopropylphenyl) and one equivalent NaCp or LiCp’ [Cp=η⁵- C₅H₅ (1), Cp’ = η⁵- C₅H₄CH₃ (7)], are described. These complexes undergo metathesis with alkyl Gringard reagents to produce the alkyl derivatives CpMo(=NAr)₂R [R = Me (2), Et (3), ⁱPr (4), allyl (5)] or Cp’Mo(=NAr)₂Me (8). Complexes 1-8 do not exhibit reactivity similar to their isolobal tris(imido) analogs and are unreactive towards additional electrophiles and nucleophiles under a variety of conditions. An unprecedented terminal cyclopentadienylidene complex of tantalum has been formed from reacting CpTa(NEt)₂Cl₂ (11) and 2 equivalents MeLi. Deuterium labeling studies show that CpTa(NEt₂)₂(=CC₄H₄) (12) is formed via a pathway that involves intermolecular C-W bond activation of a C₅H₅ ligand. Complex 12 is observed to undergo thermally induced intramolecular β-H abstraction from an ancillary NEt₂ ligand to eliminate the organic imine EtN=CHMe (23) and produce the tantalocene hydride complex (η⁵- C₅H₅)₂Ta(η²-EtN=CHMe)H (22). [Zr(μ-NAr)Me₂(THF)]₂ (26), Zr(=NAr) Me₂py₂ (27) and W(=NAr’)₂Me₂py (28, Ar’ = 2,6-dimethylphenyl) represent the first dimethyl complexes of these respective classes of early transition metal imido compounds. Complexes 26-28 are observed to contain nucleophilic imido nitrogens. Thus, Zr(=NAr)Me₂py₂ (27) reacts with 2 equivalents of H₂NAr to produce the previously synthesized Zr(=NAr)(NHAr)₂py₂ but the reaction of 27 with HOAr’ results in the elimination of CH₄. W(=NAr’)₂Me₂py (28) reacts with HOAr’ and H₂NAr in reactions that releases an imido ligand from the complex as aniline. Complexes 26-28 react with H⁻ sources to provide unidentifiable organometallic products, CH₄, and in the case of W(=NAr’)₂Me₂py, H₂NAr. The insertion of ᵗBuN=C into one of the W-Me bonds of complex 28 to give the η²-iminoacyl product W(=NAr’)₂Me(η²-C(=NᵗBu)Mepy (31). The relevance of these complexes and reactions to the fundamental C-N bond scission and heterocycle degradation steps in hydrodenitrogenation catalysis will be described.