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

Preliminary reactions of the metal stabilized carbocationic species [(η-C5H5)Ni(μ-η2(Ni),η3(Mo)-HC2CMe2)Mo(CO)2(η-C5H4Me)]+ BF4− (Ni–Mo) with nucleophiles are reported. The Ni–Mo cationic propargylic complex undergoes nucleophilic attack by sodium methoxide to regenerate the neutral μ-alkyne complex [(η-C5H5)Ni{μ-η2,η2-HC2CMe2(OMe)}Mo(CO)2(η-C5H4Me)] (Ni–Mo), from which the stabilized carbocation was originally derived by protonation. The new complexes [(η-C5H5)Ni{μ-η2,η2-HC2CMe2(C5H5)}Mo(CO)2(η-C5H4Me)] (Ni–Mo), which exist as an inseparable mixture of 1(c)-1,3- and 2(c)-1,3-cyclopentadienyl isomers, were also obtained. When the Ni–Mo cations were treated with potassium t-butoxide, the alkyne isomers with pendant 1(c)-1,3- and 2(c)-1,3-cyclopentadienyl groups are also formed. The μ-hydroxyalkyne complex [(η-C5H5)Ni{μ-η2,η2-HC2CMe2(OH)}-Mo(CO)(η-C5H4Me)] (Ni–Mo) was also isolated concurrently, and presumably arises from nucleophilic attack of fortuitously present hydroxide ions in the BuO− reagent on the Ni–Mo cation. When NaBH4 was added to the Ni–Mo propargylic, nucleophilic attack by hydride resulted and the μ-iPrC2H heterobimetallic complex [(η-C5H5)Ni{μ-η2,η2-HC2Pri}Mo(CO)2(η-C5H4Me)] (Ni–Mo) was recovered in good yield. Small quantities of other side-products were isolated and characterized spectroscopically. Some tantalizing differences in reactivity were observed when the corresponding Ni–W stabilized carbocation was reacted with methoxide ions. When the not fully characterized solid formed by protonating [(η-C5H5)Ni(μ-η2,η2-{HC2CMe2)(OMe)}W(CO)2(η-C5H4Me)] (Ni–W) was treated with methoxide ions, regioisomers (1(c)-1,3- and 2(c)-1,3-cyclopentadienyl species) of composition [(η-C5H5)Ni{μ-η2,η2-HC2CMe2(C5H5)}W(CO)2(η-C5H4Me)] (Ni–W) were formed. Direct reaction of the pure cation [(η-C5H5Niμ-η2,η3-HC2CMe2)W(CO)2(η-C5H4Me)]+ (Ni–W) with methoxide also generated the same 1(c)-1,3- and 2(c)-1,3-cyclopentadiene-substituted alkyne complexes. Unlike the case with the Ni–Mo complexes, the initial μ-HC2CMe2(OMe) species was not regenerated.