138753-72-7Relevant academic research and scientific papers
Early-transition-metal-mediated [2 + 2 + 2] cycloadditions: Formation and fragmentation of a reactive metallacyclopentadiene and its direct conversion to η6-arene and η2-pyridine complexes of tantalum
Smith, David P.,Strickler, Jamie R.,Gray, Steven D.,Bruck, Michael A.,Holmes, Renee S.,Wigley, David E.
, p. 1275 - 1288 (2008/10/08)
The kinetic product from reducing Ta(DIPP)2Cl3(OEt2) (where DIPP = O-2,6-C6H3iPr2) in the presence of HC≡CCMe3 is the α,α′ metallacyclopentadiene complex (DIPP)2ClTa(CCMe3=CHCH=CCMe3) (1). Thermolysis of 1 provides the α,β′ isomer (DIPP)2ClTa(CCMe3=CHCCMe3=CH) (2). Kinetic and mechanistic studies of the 1 → 2 isomerization are presented which lead to the proposal that the rearrangement proceeds by the disruption of the metallacycle and the formation of an intermediate Ta(V) metallacyclopropene (DIPP)2ClTa(HC=CCMe3) adduct, followed by its reaction with free HC≡CCMe3 to reform the metallacycle. Compound 1 crystallizes in the monoclinic space group P21/n (No. 14) with a = 12.130 (2) A?, b = 18.541 (3) A?, c = 15.844 (3) A?, β = 95.48 (1)°, and V = 3547.1 A?3 with Z = 4 and ρcalcd = 1.38 g cm-3. The final R = 0.029 and Rw = 0.033 for 6835 reflections (6456 unique). Complex 1 assumes a TBP structure in the solid state with metallacyclic Cα carbons occupying equatorial sites. Complex 2 (but not 1) engages in cycloaddition chemistry with HC≡CCMe3 to afford the η6-arene complex (η6-1,3,5-C6H3tBu 3)Ta(DIPP)2Cl (6). 6 can be alkylated with MeMgBr to afford purple crystals of (η6-1,3,5-C6H3tBu 3)Ta(DIPP)2(CH3) (7). Compound 7 crystallizes in the monoclinic space group P21/n (No. 14) with a = 13.555 (1) A?, b = 20.607 (1) A?, c = 14.595 (1) A?, β = 97.91 (6)°, V = 4037.8 A?3 with Z = 4 and ρcalcd = 1.31 g cm-3. The final R = 0.024 and Rw = 0.026 for 7686 reflections (6769 unique). The arene ligand in 7 exhibits a diene-diyl distortion and thus resembles a purported intermediate in the [2 + 2 + 2] cycloaddition of alkynes, the 7-metallanorbornadiene. Complex 2 (but not 1) forms adducts with THF and N≡CCMe3, viz. (DIPP)2Cl(THF)Ta(CCMe3=CHCCMe3=CH) (5) and (DIPP)2Cl(Me3CC≡N)Ta-(CCMe3=CHCCMe 3=CH) (8). Compound 8 undergoes a cycloaddition reaction upon warming to room temperature to afford (η2(N,C)-2,4,6-NC5H2tBu 3)Ta(DIPP)2Cl (9). Compound 9 crystallizes in the orthorhombic space group Pca21 (No. 29) with a = 20.674 (2) A?, b = 10.087 (5) A?, c = 19.908 (5) A?, and V = 4151.6 A?3 with Z = 4 and ρcalcd = 1.31 g cm-3. The final R = 0.037 and Rw = 0.041 for 4153 reflections (3799 unique). The η2-pyridine ligand in 9 is severely distorted, and spectroscopic and electrochemical studies on 9 reveal that the metal is in a higher effective oxidation state than the metal in the η6-arene complex 6.
