Substituent Size Effects in Lewis Base Induced Reductions in Organolanthanide Chemistry

• Published in: Chemistry : a European journal 2011-05, Vol.17 (22), p.6239-6247
• Main Authors: Bojer, Daniel, Neumann, Beate, Stammler, Hans-Georg, Mitzel, Norbert W.
• Format: Article
• Language: English
• Subjects: Chemistry; Crystals; CH activation; Lewis bases; nitrogen; reduction; samarium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201002707

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)(η3‐AlMe4)Sm}2(μ2‐AlMe4)2], whereas in toluene the pseudo‐samarocene [(TiPTAC)2Sm(η1‐AlMe4)2] was obtained. The trisaluminate [(TiPTAC)Sm{(μ2‐Me)(Me2Al)}2(μ3‐CH2)2AlMe2)] 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‐triazacyclohexane (TtBuTAC), the reaction led to the formation of the dimeric [{(TtBuTAC)(η3‐AlMe4)Sm}2(μ2‐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)2Sm(η1‐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. Divalent samarium complexes similar to 1 have been prepared through the reaction between triazacyclohexane derivatives and [Sm(AlMe4)3] and characterised by single‐crystal X‐ray diffraction and elemental analyses. A base‐induced reduction and a CH activation co‐exist in the formation of these species; the relative abundances of these reaction channels can be tuned by varying the steric bulk of the substituent at the triazacyclohexane unit.