Propargylic Cations Stabilized on Nickel−Molybdenum and Nickel−Tungsten Bonds

Abstract

The heterobimetallic complexes [(η-C 5 H 5 )(OC)Ni-M(CO) 3 (η-C 5 H 4 Me)] (Ni-M, M ) Mo, W) react with 3-methoxy-3-methyl-1-butyne [HCtCMe 2 (OMe)] to afford the enantiomericwhich have chiral dimetalatetrahedrane cores, and the metallacycles [(η-C 5 H 5 )Ni{µ-η 3 (Ni),η 1 -formed via highly regioselective alkyne-CO coupling. All complexes react with HBF 4 ‚Et 2 O. The Ni-Mo µ-alkyne complex generated the propargylic cationic species [(η-C 5 H 5 )Ni(µ-HC 2 CMe 2 )-Mo(CO) 2 (η-C 5 H 4 Me)] + BF 4 -(Ni-Mo) by protonation, followed by rapid methanol elimination. Attempted protonation of the Ni-W µ-alkyne complex led to intractable products. However cationic Ni-W (µ-HC 2 CMe 2 ) complexes, and their Ni-Mo analogues, are accessible by direct protonation with HBF 4 ‚Et 2 O of the metallacycles [(η-C 5 H 5 )Ni{µ-η 3 (Ni),η 1 (M)-C(O)-C(R)-C(H)}M(CO) 2 (η-C 5 H 4 Me)] [Ni-M, M ) Mo, W; R ) CMe 2 (OMe)]. Proton migration and CO and MeOH elimination reactions rapidly follow to generate heterobimetallic µ-HC 2 CMe 2 cationic complexes. An intermediate was isolated from the slower Ni-W reaction. Spectroscopic data suggest that the stabilized HC 2 CMe 2 + ions are π-coordinated to the group 6 metal atoms and not to the nickel. Differences in dynamic behavior are observed between the Ni-Mo and Ni-W cations: rotation about the C-CMe 2 bond occurs at ambient temperature for the Ni-Mo allenyl complex on the 1 H NMR time scale, but the Ni-W allenyl species is static.

Some reactions of Ni–Mo and Ni–W propargylic cations with nucleophiles

“…Three products (2a, 3a and 4a, Chart 1) were isolated from this reaction but the yields of all three were low owing to significant decomposition. The l-alkyne complex 2a was readily characterized by comparison of its spectroscopic data with those of an authentic sample [10]. The addition of methoxide to 1a to give the l-alkyne complex 2a is thus, at least partly, reversible.…”

“…Nevertheless, the insoluble tan solid that formed in this reaction [10] did react slowly with OBu -(+ OH -) + Scheme 2. The reaction of 1a with wet t butoxide ions.…”

“…The metallacycle [CpNi{l-g 3 (Ni),g 1 (W )-C(O)-C(R)-C(H)}W(CO) 2 Cp 0 ] [Ni-W, R ¼ CMe 2 (OMe)] undergoes elimination of methanol and decarbonylation over a 2 h period when treated with HBF 4 Á Et 2 O to afford the Ni-W propargylic cation [CpNi{l-g 2 (Ni),g 3 (W )-HC 2 CMe 2 }-W(CO) 2 Cp 0 ] þ BF À 4 (Ni-W, 1b) [10]. When a thf solution of 1b prepared this way was treated with NaOMe, the only recovered products were the cyclopentadiene-substituted l-alkyne complexes 3b and 4b (see Scheme 4).…”

“…Carbocations (propargylic cations) have also been stabilized over Mo-Mo, and W-W centers. Heterobimetallic examples are also known [6][7][8][9], but prior to our recent report [10], all such bimetallic l-HC 2 CR 0 2 þ species had been anchored onto Co-Mo or Co-W centers. To our knowledge no other heterobimetallic combinations have yet been reported.…”

“…1 The bridging organic ligand can also be considered to be a cationic l-propargylic (allenyl) species. In these complexes, the carbocation is spanning a Ni-Mo or a Ni-W bond [10]. Complex 1a was isolated by protonation of the l-alkyne complex 2a [CpNi{l-g 2 ,g 2 -HC 2 CMe 2 -(OMe)} Mo(CO) 2 Cp 0 ] (Ni-Mo) with HBF 4 Á Et 2 O, followed by MeOH loss, as shown in Eq.…”