Alkyne coupling reactions mediated by organolanthanides. Probing the mechanism by metal and alkyne variation

This contribution addresses, using the non-redox-active ion La[sup 3+], the constraints under which lanthanide M-C[triple bond]CR functionalities undergo facile coupling to yield binuclear M[sub 2]([mu]-RC[sub 4]R) complexes. The reaction of Cp[prime][sub 2]LaCHTMS[sub 2] (Cp[prime] = [eta][sup 5]-Me[sub 5]C[sub 5]) with PhCCH at room temperature yields the coupled product (Cp[prime][sub 2]La)[sub 2]([mu]-PhC[sub 4]Ph) (monoclinic space group C2/m, [alpha] = 15,600(2) A, b = 14.318 (2) A, c = 15.368(2) A, [beta] = 114.17(1)[degrees], Z = 2, R = 0.050, R[sub w] = 0.063) plus CH[sub 2]TMS[sub 2]. Reaction of Cp[prime][sub 2]LaCHTMS[sub 2] with t-BuCCH at 0[degrees]C, yields the uncoupled dimer (Cp[prime][sub 2]LaC[sub 2]-t-Bu)[sub 2] (plus CH[sub 2]TMS[sub 2]), which, in toluene solution at 50 and 60[degrees]C, undergoes clean unimolecular conversion to the coupled dimer (Cp[prime][sub 2]La)[sub 2]([mu]-t-BuC[sub 4]-t-Bu) (monoclinic space group P2[sub 1]/n, a = 11.232(2) A, b = 14.199(3) A, c = 15.309(4) A, [beta] = 103.35(2)[degrees], Z = 2, R = 0.027, R[sub w] = 0.034). These results argue that excursions in formal metal oxidation state (+3 = +2) are not important along the reaction coordinate, that acetylene aryl substituents are not necessary for the coupling process to occur, and that the immediate kinetic precursor to the coupled product is an uncoupled dimer. 18 refs., 3 figs., 4 tabs.