An exciting discovery resulted from treating 4 with excess Na[HBEt₃] to generate anionic methylidene complex 7, [Na][Tp'(OC)₂W=CH₂] (Scheme 1).

Complex 7 is the first anionic methylidene reported. The synthetic methodology for generating 7 by hydride addition to neutral Tp'(CO)₂WºC-H contrasts with routes that utilize a-hydrogen abstraction or hydride removal from neutral methyl precursors to generate other methylidene complexes. Addition of PhSSPh to complex 7 in solution generates the saturated tungsten product Tp'(CO)₂W(h²-CH₂SPh).¹ The generation of new anionic carbene complexes and the study of their reactivity is the focus of current research in our lab.

We are also using methylidyne complex 4, Tp'(CO)₂WºC-H, to elucidate other fundamental chemical properties of the metal carbon triple bond. We had previously prepared complex 4 in two ways: 1) through fluoride induced desilylation of silylcarbynes, and 2) via dimethylphenyl phosphine abstraction by iodomethane from the zwitterionic carbene complex Tp'(CO)₂W=C(H)(PMe₂Ph).^2,3 However, the chemistry of the parent WºC-H carbyne unit was difficult to access because of the low yields which plague both of these reactions. Dimerization of the carbyne monomer to form the vinylidene bridged dinuclear product, Tp'(CO)₂W(m-CCH₂)W(CO)₂Tp', also complicates using the monomer as a reagent.

The readily available phosphonium carbyne complex [Tp'(CO)₂WºC-PMe₂Ph]⁺ is susceptible to nucleophilic attack by hydride donor reagents at C_a to generate the neutral zwitterionic carbene complex Tp'(CO)₂W=C(H)(PMe₂Ph).^3,4 In light of this reactivity, we hypothesized that the cationic triphenylphosphonium carbyne analog [Tp'(CO)₂WºC-PPh₃][PF₆] (1) would provide a better leaving group (PPh₃) for the nucleophilic hydride substitution reaction than does [Tp'(CO)₂WºC-PMe₂Ph]⁺.

Indeed, treatment of [Tp'(CO)₂WºC-PPh₃][PF₆] (1) with Na[HBEt₃] in THF forms the methylidyne complex 4 in high yield. The mechanism for this reaction was studied using NMR and in situ IR techniques (Scheme 2).

The reaction proceeds via initial attack at carbonyl to form the formyl complex Tp'(CO)(C(O)H)WºC-PPh₃ (2). Hydride migration from the formyl carbon to the carbyne C_a generates the carbene intermediate Tp'(CO)₂W=C(PPh₃)(H) (3) which loses neutral PPh₃ and generates the desired methylidyne complex 4.

Treatment of the methylidyne complex with various reagents has provided some other exciting results. Complex 4 can be deprotonated with alkyl lithium reagents to provide the anionic terminal carbide Tp'(CO)₂WºC-Li (5) and protonated with HBF₄·Et₂O to yield the cationic a-agostic methylidene complex [Tp(CO)₂W=CH₂][BF₄] (6) (Scheme 1). The terminal carbide 5 adds electrophiles at the carbide carbon to generate Fischer carbynes of the type Tp'(CO)₂WºC-R (R = CH₃, SiMe₃, I, C(OH)Ph₂, CH (OH)Ph, and C(O)Ph).

1. Templeton, J. L. et al J. Am. Chem. Soc. 2001, accepted for publication.
2. Templeton, J. L. et al. In Transition Metal Carbyne Complexes; Kreissl, F. R. Ed.; Kluwer Academic Publishers: Netherlands, 1993, pp 201-218.
3. Templeton, J. L. et al J. Am. Chem. Soc. 1991, 113, 5057-5059.
4. Templeton, J. L. et al. Organometallics 1987, 6, 1987.