USE OF AN ARYLDIAMINE PINCER LIGAND IN THE STUDY OF TANTALUM ALKYLIDENE-CENTERED REACTIVITY - TANTALUM-MEDIATED ALKENE SYNTHESIS VIA REDUCTIVE REARRANGEMENTS AND WITTIG-TYPE REACTIONS

The potentially terdentate, monoanionic aryldiamine ligand {C6H3(CH2NMe2)2-2,6}- provides an excellent tool for the study of tantalum alkylidene-centered reactivity. Transmetallation of [TaCl3(CH-t-Bu)(THF)2] with 1/2[Li{C6H3-(CH2NMe2)2-2,6}]2 affords the alkylidene complex [TaCl2{C6H3(CH2NMe2)2-2,6}(CH-t-Bu)] (1) in 90% yield. Complex 1 belongs to the space group Cc with a = 12.983 (2) angstrom, b = 12.384 (1) angstrom, c = 11.969 (1) angstrom, beta = 95.43 (1)-degrees, and Z = 4. The hexacoordinate tantalum center in 1 has an irregular ligand array in which the aryldiamine ligand adopts a new pseudofacial N,C,N-coordination mode. A Wittig-type reaction of the alkylidene complex 1 with acetone or benzaldehyde affords [TaCl2{C6H3(CH2NMe2)2-2,6}(mu-O)]2 (2) in virtually quantitative yield. A similar reaction of 1 with PhCH=NR affords [TaCl2{C6H3(CH2NMe2)2-2,6}(NR)] (R = Me, SiMe3). Complex 2 belongs to the space group Pc with a = 10.800 (4) angstrom, b = 11.804 (3) angstrom, c = 12.539 (4) angstrom, beta = 108.67 (3)-degrees, and Z = 2. Complex 2 is a dimer with bridging oxo functions and has a distorted pentagonal bipyramidal ligand array around each tantalum center. Complex 1 with ethene and propene gives a rearrangement reaction that affords alkene adducts [TaCl2{C6H3(CH2NMe2)2-2,6}(H2C=CHR)] (R = CH2-t-Bu and Me, respectively). [TaCl2{C6H3(CH2NMe2)2-2,6}(H2C=CHCH2-t-Bu)] belongs to the space group P1BAR with a = 8.320 (1) angstrom, b = 9.041 (2) angstrom, c = 14.054 (3) angstrom, a = 94.53 (2)-degrees, beta = 90.75 (2)-degrees, gamma = 92.86 (2)-degrees, and Z = 2. The overall structure is distorted pentagonal bipyramidal. The chirality in the coordinated alkene determines the conformation of both five-membered Ta-C-C-C-N chelate rings. NMR experiments show the alkene complexes to be rigid in solution; the bonding of the alkene to the metal center is best described as resulting in a metallacyclopropane unit.