Zwitterionic and Cationic Titanium and Vanadium Complexes Having Terminal M−C Multiple Bonds. The Role of the β-Diketiminate Ligand in Formation of Charge-Separated Species

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Abstract

Treatment of the neopentylidene complex ([ArNC(Me)]2CH)Ti═CHtBu(OTf) (1) with a strong base such as an alkyl reagent (lithium or potassium salt) results in deprotonation of the β-diketiminate β-methyl group to form complex (ArN(Me)CCHC(CH2)NAr)Ti═CHtBu(THF) (2) along with liberation of the alkane. Likewise, ligand deprotonation of ([ArNC(Me)]2CH)V≡CtBu(OTf) (3), in THF or Et2O, results in formation of the alkylidyne−solvent adduct (ArNC(Me)CHC(CH2)NAr)V≡CtBu(L) (L = THF, 4-THF; L = Et2O, 4-OEt2), concomitant with alkane formation. The connectivity in compound 4-Et2O has been established by single-crystal X-ray diffraction studies. Compounds 2 and 4-THF react with B(C6F5)3 to produce the first examples of terminal titanium alkylidene and vanadium alkylidyne zwitterions, namely, the borane adducts (ArNC(Me)CHC(CH2B(C6F5)3)NAr)Ti═CHtBu(THF) (5) and (ArNC(Me)CHC(CH2B(C6F5)3)NAr)V≡CtBu(THF) (6), respectively. The solid state structure for each zwitterion was also obtained. Compounds 2 and 4-THF react readily with [HNMe2Ph][B(C6F5)4] to produce the discrete salts [([ArNC(Me)]2CH)Ti═CHtBu(THF)][B(C6F5)4] (7) and [([ArNC(Me)]2CH)V≡CtBu(THF)][B(C6F5)4] (8), respectively, via protonation of the methylene group in the N,N′-chelating ligand ArNC(Me)CHC(CH2)NAr2− to re-form the β-diketiminate scaffold. The reactivity of 7 allows for spectroscopic elucidation only at low temperatures, while complex 8 has been fully characterized including a single-crystal X-ray structure.

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