REVERSIBLE CARBON-CARBON BOND FORMATION IN ORGANOLANTHANIDE SYSTEMS - PREPARATION AND PROPERTIES OF LANTHANIDE ACETYLIDES [CP-ASTERISK-2LNCCR]N AND THEIR REARRANGEMENT PRODUCTS [CP-ASTERISK-2LN]2(MU-ETA(2)-ETA(2)-RC4R) (LN = LA, CE, R = ALKYL)

The early lanthanide carbyls Cp*2LnCH(SiMe3)21 (1, Ln = Ce; 2, Ln = La) react with terminal alkynes HC=CR (R = Me, t-Bu) to produce oligomeric acetylides [Cp*2LnC=CR]n (3, Ln = Ce, R = Me; 4, Ln = La, R = Me; 5, Ln = Ce, R = t-Bu). The acetylides are not stable in solution and rearrange to give the carbon-carbon coupled prodUCts, [Cp*2Ln]2(mu-eta2:eta2-RC4R) (6, Ln = Ce, R = Me; 7, Ln = La, R = Me; 8, Ln = Ce, R = t-Bu). The acetylide carbon-carbon coupling reaction is reversible in solution. Thermodynamic and kinetic parameters for this process were determined for the [Cp*2LaC=CMe]n/[Cp*2La]2(mu-eta2:eta2-MeC4Me) couple: DELTAG(theta) = 4.5 +/- 0.4 kJ/mol; k1 = 8.3(+/-0.4) x 10(-5) s-1; k-1 = 1.1(+/-0.7) x 10(-5) s-1, and DELTAG(double dagger) = 96.3(+/-0.1) kJ/mol (298 K). The molecular structures of 6 and 8 were determined by single-crystal X-ray diffraction and shown to consist of two Cp*2Ce units bridged by a conjugated RC4R group. Cell data for 6: monoclinic, P2(1)/n, a = 10.670(8) angstrom, b = 25.081(9) angstrom, c = 16.706(9) angstrom, beta = 102.41(7)degrees, D(calcd)(Z = 4) = 1.37 g.cm-3. Least-squares refinement based on 4179 reflections converged to R = 0.046 and R(w) = 0.053. Cell data for 8: monoclinic, P2(1)/n, a = 11.376(6) angstrom, b = 14.151(5) angstrom, c = 15.541(4) angstrom, beta = 103.92(4)degrees, D(calcd)(Z = 2) = 1.34 g.cm-3. Least-squares refinement based on 3502 reflections converged to R = 0.031 and R(w) = 0.040. Protolysis of 6 and 7 with 2,6-di-tert-butyl-4-methylphenol affords (E)-2-hexen-4-yne, exclusively. Contrastingly, a mixture of enynes and butatrienes is formed upon protolysis of 8. Both 5 and 8 catalyze the dimerization of tert-butylacetylene to 2,4-di-tert-butyl-1-buten-3-yne.