METAL HYDRIDE ALKYNYL -] METAL VINYLIDENE REARRANGEMENTS OCCURRING IN BOTH SOLID-STATE AND SOLUTION - ROLE OF THE 1-ALKYNE SUBSTITUENT IN DETERMINING THE RELATIVE STABILITY OF PI-ALKYNE, HYDRIDE ALKYNYL, AND VINYLIDENE FORMS AT COBALT

The 16-electron fragment [(PP3)Co]+ obtained in situ from the dinitrogen complex [(PP3)Co(N2)]BPh4 (2) reacts in tetrahydrofuran with terminal alkynes, HC = CR, yielding pi-alkyne adducts of the type [(PP3)Co(pi-HC = CR)]+ and vinylidene complexes [(PP3)Co{C = C(H)R}]+ as kinetic and thermodynamic products, respectively (R = H, Ph, n-C3H7, n-C5H11, CMe3, SiMe3; PP3 = P(CH2CH2PPh2)3). Between the pi-alkyne and vinylidene forms, the system evolves through a third species, namely the Co(III) hydride alkynyl complexes [(PP3)Co(H)(C = CR)]+, which are thermodynamically favored over the pi-alkyne complexes and disfavored over the vinylidene ones. The sequence Co(pi-HC = CR) --> Co(H)(C = CR) --> Co{C = C(H)R} is controlled by the temperature, so that by a judicious choice of this parameter it is possible to freeze the reactions at any of the three steps. Spectroscopic evidence is provided for the formation of the pi-alkyne adducts whereas both the hydride alkynyl and vinylidene complexes have been isolated in the solid state and completely characterized by spectroscopic and X-ray diffraction techniques. The conversion of the hydride alkynyl complexes to the vinylidene isomers is affected inter alia by the nature of the alkyne substituent. In particular, the temperature at which the conversion occurs increases in the order SiMe3 > Ph > H >> CMe3 > n-C3H7 congruent-to n-C5H11. Accordingly, the electronic effects seem to prevail over the steric ones in governing the tautomeric rearrangement. Kinetic and thermodynamic studies show that (i) the hydride alkynyl to vinylidene rearrangement is first order in the Co(III) hydride alkynyl complexes and (ii) the rearrangement most likely proceeds via a dissociative intramolecular 1,3-hydrogen shift pathway. The hydride alkynyl to vinylidene rearrangement occurs also in the solid state at relatively low temperatures (from 303 K for R = CMe3 to 363 K for R = SiMe3) and depends again on the nature of the 1-alkyne terminal substituent.