Substituent Size Effects in Lewis Base Induced Reductions in Organolanthanide Chemistry

Bojer D, Neumann B, Stammler H-G, Mitzel NW. Substituent Size Effects in Lewis Base Induced Reductions in Organolanthanide Chemistry. Chemistry. 2011;17(22):6239-6247.The reaction of the tripodal 1,3,5-trialkyl-1,3,5-triazacyclohexanes (L= cyclo-[N(R) CH2](3), R= Et, iPr, tBu), with [Sm(AlMe4)(3)] resulted in the formation of divalent samarium complexes of the constitution [{LnSm(AlMe4)(2)}(m)] (n, m= 1,2) under ethane extrusion. These compounds were characterised by single-crystal X-ray diffraction and elemental analyses. Simultaneous occurrence of Lewis base induced reduction and C-H-activation reactions is observed. The ratio of products depends on the bulkiness of the N-alkyl substituent R. The reaction of [Sm(AlMe4)(3)] with 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) in benzene afforded the inversion-symmetric dimer [{(TiPTAC)(eta(3)-AlMe4) Sm}(2)(mu(2)-AlMe4)(2)], whereas in toluene the pseudo-samarocene [(TiPTAC)(2)Sm(eta(1)-AlMe4)(2)] was obtained. The trisaluminate [(TiPTAC)-Sm{(mu 2-Me)(Me2Al)}(2)(mu(3)-CH2)(2)AlMe2)] was found to be the C-H-activation product. In the case of the particular bulky 1,3,5-tri-tert-butyl-1,3,5-triazacy clohexane (TtBuTAC), the reaction led to the formation of the dimeric [{(TtBuTAC)(eta(3)-AlMe4) Sm}(2)(mu(2)-AlMe4)(2)] even in toluene in comparably high yields. The decrease of the steric demand to ethyl groups in 1,3,5-triethyl- 1,3,5-triazacyclohexane (TETAC) afforded the samarocene-like [(TETAC)(2)Sm(eta(1)-AlMe4)(2)] in lower yields. The resulting divalent samarium compounds are found to be stable with respect to reagents like dinitrogen, conjugated olefins and polycyclic aromatic systems