An exciting discovery resulted from treating 4 with excess Na[HBEt3] to generate anionic methylidene complex 7, [Na][Tp'(OC)2W=CH2] as per the following figure:
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 Ca 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 as per the follwing figure:
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 Ca 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