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(OC6H3Pr2i-2,6)(2)Cl-3](2) (1) and [M(OC6H3Pr2i-2,6)(3)Cl-2] (2) (a, M = Nb; b, M = Ta) in the presence of 1,3-cyclohexadiene leads to formation of the eta(4)-cyclohexadiene derivatives [M(OC6H3Pr2i-2,6)(2)Cl(eta(4)-C6H8)] (3) and [M(OC6H3Pr2i-2,6)(3)(eta(4)-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-Cy-2-2,6)(2)(H)(3)(PMe2Ph)(2)] leads to the interesting products [Ta(OC6H3Cy2-2,6)(2)(eta(1)-C6H10-eta(4)-C6H7)] (5) and [Ta(OC6HPh2-3,5-Cy-2-2,6)(2)(eta(1)-C6H10-eta(4)-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(OC6H3Pr2i-2,6)(2)(Cl)(H)(2)(PMe2Ph)(2)] and [Ta(OC6H3Bu2t-2,6)(2)(Cl)(H)(2)(PMe2Ph)] leads to the dehydrogenation product [Ta(OC6H3Pri-eta(2)-CMe=CH2)(OC6H3Pr2i-2,6)(Cl)(PMe2Ph)(2)] (7) and the cyclohexyl compound [Ta(OC6H3But-CMe2CH2)(OC6H3Bu2t-2,6)(Cl)(C6H11)] (8), respectively. The mechanistic implications of these stoichiometric and catalytic reactions are discussed. Crystal data for 3a at 20 degrees C: NbClO2C30H42. M = 563.03, space group P-nma (no. 62), a = 12.237(1), b = 21.633(1), c = 10.883(2) Angstrom, V = 2881.0(9) Angstrom(3), D-c = 1.298 g cm(-3), Z = 4; for 4a at 20 degrees C: NbO3C42H59. M = 704.84, space group P2(1)/c (no. 14), a = 11.562(1), b = 16.117(2), c = 21.914(3) Angstrom, beta = 103.69(1)degrees, V = 3967(2) Angstrom(3), D-c = 1. 180 g cm(-3), Z = 4; for 4b at -57 degrees C: TaO3C42H59 M = 792.88, space group P2(1)/c (no. 14), a = 11.452(2), b = 16.175(3), c = 21.765(3) Angstrom, beta = 103.52(1)degrees, V = 3919(2) Angstrom(3), D-c = 1.343 g cm(-3), Z = 4; for 5 at 20 degrees C: TaO2C48H67. M = 857.02, space group P2(1) (no. 4), a = 10.559(9), b = 15.828(10), c = 13.266(12) Angstrom, V = 2095(6) Angstrom(3), D-c = 1.358 g cm(-3), Z = 2.