Subtle Size Effects in C-H Activation Reactions of Lanthanum and Praseodymium Tetramethylaluminates by Neutral Trinitrogen Bases

The reaction of Pr(AlMe(4))(3) with 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) leads to the formation of two isomeric products by C-H activation of the AlMe(4)(-) ligands: [(eta(3)-TMTAC)Pr{eta(4)-(Me(3)AlCH(2)AlMe(2)CH(2)AlMe(3))}] (1), containing a linear tris(aluminate) trianion, and [(eta(3)-TMTAC)Pr(eta(4)-{CH(AlMe(3))(3)})] (2), containing a branched tris(aluminate) trianion. The analogous reaction with the more bulkily substituted 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane (TtBuTAC) leads selectively to [(eta(3)-TtBuTAC)Pr{mu(4)-(Me(3)AlCH(2)AlMe(2)CH(2)AlMe(3))}] (3). The reaction of La(AlMe(4))(3) with TtBuTAC affords [(eta(3)-TtBuTAC)La{eta(4)-(Me(3)AlCH(2)AlMe(2)CH(2)AlMe(3))}] (4). All compounds were characterised by elemental analysis and crystal-structure determination. Isomers 1 and 2 could not be separated and form a cocrystalline product with a 1/2 ratio of 1:2 with two slightly different structures of 2 (2a and 2b). In compounds 2, 3 and 4 the tris(aluminate) ligand [Me(3)AlCH(2)AlMe(2)CH(2)AlMe(3)](3-) is bonded to the lanthanide ions in an.4-mode through two CH(2) units and two terminal Me groups.