Coordination and Hydrogenation of 1,3-Cyclohexadiene by Niobium and Tantalum Aryl Oxide Compounds:  Relevance to Catalytic Arene Hydrogenation

• Published in: Journal of the American Chemical Society Vol. 119; no. 15; pp. 3490 - 3499
• Main Authors: Visciglio, Valerie M., Clark, Janet R., Nguyen, Mindy T., Mulford, Douglas R., Fanwick, Phillip E., Rothwell, Ian P.
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 16.04.1997; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; ARYLOXIDE COMPOUNDS; HOMOGENEOUS HYDROGENATION; AROMATIC-HYDROCARBONS; REGIOSELECTIVE REDUCTIONS; (ETA-5-PENTAMETHYLCYCLOPENTADIENYL)RHODIUM TRIS(ACETONITRILE) DICATION; MOLECULAR-STRUCTURE; CRYSTAL-STRUCTURE; METAL CENTERS; DONOR LIGANDS; HYDRIDE COMPOUNDS
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja964073n

The sodium amalgam (2 Na per M) reduction of hydrocarbon solutions of the chloro, aryl oxide compounds [M(OC6H3Pri 2-2,6)2Cl3]2 (1) and [M(OC6H3Pri 2-2,6)3Cl2] (2) (a, M = Nb; b, M = Ta) in the presence of 1,3-cyclohexadiene leads to formation of the η4-cyclohexadiene derivatives [M(OC6H3Pri 2-2,6)2Cl(η4-C6H8)] (3) and [M(OC6H3Pri 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(OC6H3Pri 2-2,6)2(Cl)(H)2(PMe2Ph)2] and [Ta(OC6H3But 2-2,6)2(Cl)(H)2(PMe2Ph)] leads to the dehydrogenation product [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri 2-2,6)(Cl)(PMe2Ph)2] (7) and the cyclohexyl compound [Ta(OC6H3But-CMe2CH2)(OC6H3But 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) Å, V = 2881.0(9) Å3, 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), c = 21.914(3) Å, β = 103.69(1)°, V = 3967(2) Å3, D c = 1.180 g cm-3, Z = 4; for 4b at −57 °C:  TaO3C42H59. M = 792.88, space group P21/c (no. 14), a = 11.452(2), b = 16.175(3), c = 21.765(3) Å, β = 103.52(1)°, V = 3919(2) Å3, 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) Å, V = 2095(6) Å3, D c = 1.358 g cm-3, Z = 2.