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

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-OEt 2 ), concomitant with alkane formation. The connectivity in compound 4-Et 2 O 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.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

“…234,269,274,275 As already discussed, the exocyclic methylene group can coordinate to Lewis acidic metal centres (Schemes 59 and 67), 260 or lead to the formation of f borane adducts (Scheme 68). For instance, the #BDI ligand is protonated back to BDI by ammonium salts, as shown in Scheme 66.…”

“…79,269 These degradation products result from double C-H activation of the isopropyl methines by the vCH t Bu alkylidene moiety (which is elliminated as CMe 4 ) and intramolecular cross-metathesis (see section 5) forming a Ti(imine) dimer supported by a BDIderived anilido-olefinic chelate. For instance, the Ti(IV) alkylidene complex (BDI*)Ti-(vCH t Bu)(CH 2 SiMe 3 ) decomposes rapidly in solution to from several products, from which the doubly cyclometallated [{2-(CMe 2 )6-( i Pr)C 6 H 3 ]NC(Me)} 2 CH]Ti(CH 2 SiMe 3 ) species and the dimer [Ti(vNAr){(Ar)NC(Me)CHC(μ-CH 2 )vCH t Bu}] 2 (Ar = Dipp) were isolated (Scheme 80top).…”

Phosphorus‐Atom Transfer from Phosphaethynolate to an Alkylidyne

“…Other than this one example with a strained Zr−C bond, the chemistry of the [PCO] − anion with reactive early‐transition metals possessing metal‐carbon multiple bonds has remained unexplored. Hence, we desired to obtain some insight into the reactivity between [PCO] − and a highly polarized and basic alkylidyne moiety with a redox‐active metal center, namely the complexes ( Me nacnac)V≡C t Bu(OTf) ( 1 ) [21] and ([ArNC(CH 2 )]CH[C(CH 3 )NAr])V≡C t Bu(OEt 2 ) ( 3 ) [22] . In spite of the hard nature and oxophilicity of vanadium center in 1 and 3 , the V≡C t Bu fragment in these systems engages in a [2+2]‐cycloaddition involving the P−C multiple bond, followed by a decarbonylation step that furnishes a V‐carbonyl and side‐bound phosphaalkyne.…”

“…To understand if salt metathesis is required in the transfer of [PCO] − to the V‐center, we turned our attention to the etherate‐alkylidyne complex 3 [22] . Following a similar protocol to prepare 2 , we introduced Na(OCP)(dioxane) 2.5 to 3 , which resulted in the formation of a salt‐like complex in nearly quantitative yield when the reaction mixture was treated with the crown‐ether 15‐crown‐5 to encapsulate the Na + counter ion.…”

Formation and Redox Interconversion of Niobium Methylidene and Methylidyne Complexes

“…Although methylidynes are known ligands for group 6and 7 transition metals, [1,2] there are no reported methylidyne complexes with group 5t ransition metals.T his contrasts examples of substituted alkylidynes,w here the hydrogen is replaced with am ore sterically encumbering group. [3,4] Our group is particularly interested in this archetypal moiety since tantalum methylidenes and methylidynes,s upported on as ilicon oxide surface,h ave been implicated in the dehydrocoupling of methane to ethane and ethylene. [5] Therefore, we decided to explore niobium systems since this metal has more accessible low-valent states than tantalum and could allow us to probe redox processes involving the methylidyne ligand-such as a-elimination and a-migration.…”