173601-98-4Relevant academic research and scientific papers
Coordination and hydrogenation of 1,3-cyclohexadiene by niobium and tantalum aryl oxide compounds: Relevance to catalytic arene hydrogenation
Visciglio, Valerie M.,Clark, Janet R.,Nguyen, Mindy T.,Mulford, Douglas R.,Fanwick, Phillip E.,Rothwell, Ian P.
, p. 3490 - 3499 (1997)
The sodium amalgam (2 Na per M) reduction of hydrocarbon solutions of the chloro, aryl oxide compounds [M(OC6H3Pr(i)2-2,6)2Cl3]2 (1) and [M(OC6H3Pr(i)2-2,6)3C12] (2) (a, M = Nb; b, M = Ta) in the presence of 1,3-cyclohexadiene leads to formation of the η4-cyclohexadiene derivatives [M(OC6H3Pr(i)2-2,6)2Cl(η4-C6H8)] (3) and [M(OC6H3Pr(i)2-2,6)3(η4-C6H8)] (4). Spectroscopic studies of compounds 3 and 4 show in all cases a strongly bound cyclohexadiene ligand which does not readily undergo displacement (NMR) with added reagents such as PMe2Ph and cyclohexene. Single crystal X-ray diffraction analyses of 3a and the isomorphous pair 4a and 4b show in all three cases a geometry about the metal center best described as three-legged piano stool. Compound 4a will catalyze the disproportionation of 1,3-cyclohexadiene into cyclohexene and benzene as well as the hydrogenation of 1,3-cyclohexadiene and cyclohexene into cyclohexane. Mechanistic studies clearly show that cyclohexene is not released during the conversion of 1,3-cyclohexadiene to cyclohexane catalyzed by 4a. In contrast, solutions of 3a will convert 1,3-cyclohexadiene slowly to cyclohexene prior to conversion to cyclohexane. The addition of 1,3-cyclohexadiene to the trihydride compounds [Ta(OC6H3Cy2-2,6)2(H)3(PMe2Ph)2] and [Ta(OC6HPh2-3,5-Cy2-2,6)2(H)3(PMe2Ph)2] leads to the interesting products [Ta(OC6H3Cy2-2,6)2(η1-C6H10-η4-C6H7)] (5) and [Ta(OC6HPh2-3,5-Cy2-2,6)2(η1 C6H10-η4-C6H7)] (6) which, based upon structural studies of 5 contain a partially hydrogenated non-Diels-Alder dimer of 1,3-cyclohexadiene. The addition of 1,3-cyclohexadiene to the dihydride compounds [Ta(OC6H3Pr(i)22,6)2(Cl)(H)2(PMe2Ph)2] and [Ta(OC6H3Bu(t)2-2,6)2(Cl)(H)2(PMe2Ph)] leads to the dehydrogenation product [Ta(OC6H3Pr(i)-η2-CMe=CH2)(OC6H3Pr(i)2-2,6)(Cl)(PMe2Ph)2] (7) and the cyclohexyl compound [Ta(O6H3Bu(t)-CMe2CH2)(OC6H3Bu(t)2-2,6)(Cl)(C6H11)] (8), respectively. The mechanistic implications of these stoichiometric and catalytic reactions are discussed. Crystal data for 3a at 20°C: NbClO2C30H42. M = 563.03, space group P(nma) (no. 62), a = 12.237(1), b = 21.633(1), c = 10.883(2) A?, V = 2881.0(9) A?3, D(c) = 1.298 g cm-3, Z = 4; for 4a at 20 °C: NbO3C42H59. M = 704.84, space group P21/c (no. 14), a = 11.562(1), b = 16.117(2), e = 21.914(3) A?, β = 103.69(1)°, V = 3967(2) A?3, D(c)= 1.180 g cm-3, Z = 4; for 4b at -57 °C: TaO3C42H59. M = 792.88, space group R21/c (no. 14), a = 11.452(2), b = 16.175(3), c = 21.765(3) A?, β = 103.52(1)°, V = 3919(2) A?3, D(c) = 1.343 g cm-3, Z = 4; for 5 at 20 °C: TaO2C48H67. M = 857.02, space group P21 (no. 4), a = 10.559(9), b = 15.828(10), c = 13.266(12) A?, V = 2095(6) A?3, D(c) = 1.358 g cm-3, Z = 2.
