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

The sodium amalgam (2 Na per M) reduction of hydrocarbon solutions of the chloro, aryl oxide compounds [M(OC 6 H 3 Pr i 2 -2,6) 2 Cl 3 ] 2 ( 1 ) and [M(OC 6 H 3 Pr i 2 -2,6) 3 Cl 2 ] ( 2 ) ( a, M = Nb; b, M = Ta) in the presence of 1,3-cyclohexadiene leads to formation of the η 4 -cyclohexadiene derivatives [M(OC 6 H 3 Pr i 2 -2,6) 2 Cl(η 4 -C 6 H 8 )] ( 3 ) and [M(OC 6 H 3 Pr i 2 -2,6) 3 (η 4 -C 6 H 8 )] ( 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 PMe 2 Ph 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(OC 6 H 3 Cy 2 -2,6) 2 (H) 3 (PMe 2 Ph) 2 ] and [Ta(OC 6 HPh 2 -3,5-Cy 2 -2,6) 2 (H) 3 (PMe 2 Ph) 2 ] leads to the interesting products [Ta(OC 6 H 3 Cy 2 -2,6) 2 (η 1 -C 6 H 10 -η 4 -C 6 H 7 )] ( 5) and [Ta(OC 6 HPh 2 -3,5-Cy 2 -2,6) 2 (η 1 -C 6 H 10 -η 4 -C 6 H 7 )] ( 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(OC 6 H 3 Pr i 2 -2,6) 2 (Cl)(H) 2 (PMe 2 Ph) 2 ] and [Ta(OC 6 H 3 Bu t 2 -2,6) 2 (Cl)(H) 2 (PMe 2 Ph)] leads to the dehydrogenation product [Ta(OC 6 H 3 Pr i -η 2 -CMe CH 2 )(OC 6 H 3 Pr i 2 -2,6)(Cl)(PMe 2 Ph) 2 ] ( 7 ) and the cyclohexyl compound [Ta(OC 6 H 3 Bu t -CMe 2 CH 2 )(OC 6 H 3 Bu t 2 -2,6)(Cl)(C 6 H 11 )] ( 8 ), respectively. The mechanistic implications of these stoichiometric and catalytic reactions are discussed. Crystal data for 3a at 20°C: NbClO 2 C 30 H 42 . 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) Å 3, D c = 1.298 g cm - 3, Z = 4; for 4a at 20 °C: NbO 3 C 42 H 59 . M = 704.84, space group P 2 1 / c (no. 14), a = 11.562(1), b = 16.117(2), c = 21.914(3) Å, β = 103.69(1)°, V = 3967(2) Å 3, D c = 1.180 g cm - 3, Z = 4; for 4b at −57 °C: TaO 3 C 42 H 59 . M = 792.88, space group P 2 1 / c (no. 14), a = 11.452(2), b = 16.175(3), c = 21.765(3) Å, β = 103.52(1)°, V = 3919(2) Å 3, D c = 1.343 g cm - 3, Z = 4; for 5 at 20 °C: TaO 2 C 48 H 67 . M = 857.02, space group P2 1 (no. 4), a = 10.559(9), b = 15.828(10), c = 13.266(12) Å, V = 2095(6) Å 3, D c = 1.358 g cm - 3, Z = 2.