Intramolecular hydrogenation of aryloxide ligands at niobium metal centers: stereochemical consequences of reaction regiochemistry

Article abstract of DOI:10.1016/0277-5387(95)85012-0

The sodium amalgam reduction (2Na per Nb) of hydrocarbon solutions of the niobium compounds [Nb(OC6H3Ph2-2,6)3Cl2], 2a (OC6H3Ph2-2,6=2,6-diphenylphenoxide) and [Nb(OC6H2Ph3-2,4,6)3Cl2], 2b (OC6H2Ph3-2,4,6=2,4,6-triphenylphenoxide) under 1 atm of H2 leads to the deep-red, η4-cyclohexadiene complexes [Nb(OC6H3Ph-η4-C6H7)(OC6H3Ph2-2,6)2], 3a and [Nb(OC6H2Ph2-η4-C6H7)(OC6H2Ph3-2,4,6)2], 3b, respectively. The spectroscopic properties of 3a and 3b are consistent with one of the aryloxide ligands having undergone 1,2-hydrogenation of an ortho-phenyl substituent, leading to a cyclohexadiene group which is strongly coordinated to the metal center. This is confirmed by a single crystal X-ray diffraction analysis of 3a, showing the coordination sphere about niobium to consist of two, normal 2,6-diphenylphenoxide ligands and one chelating, partially hydrogenated aryloxide group. The η4-bonding of the chelated cyclohexadiene group is slightly distorted with Nb-C distances of 2.27(1), 2.27(2), 2.37(1) and 2.40(1) ?. Hydrocarbon solutions of 3a and 3b will react further with hydrogen (200-1200 psi) to produce upon hydrolysis 2-cyclohexyl-4-phenylphenol and 2,6-dicyclohexylphenol (for 3a) and 2-cyclohexyl-4,6-diphenylphenol and 2,6-dicyclohexyl-4-phenylphenol (for 3b). The intramolecular hydrogenation of aryl-phenoxide groups can also be achieved by treating mixtures of the chloride compounds [Nb(OC6H3Ph2-2,6)nCl5-n] or [Nb(OC6HPh4-2,3,5,6)nCl5-n] (n=2,3) with (5-n) equivalents of Bu(n)Li under hydrogen. Spectroscopic ((1)H, (13)C NMR) analysis of the phenols obtained in all cases indicated that exclusive hydrogenation of ortho-phenyl groups occurs with no evidence of attack on either the meta- or para-phenyl substituents or upon the central phenoxy ring. The use ofD2 as reagent gas leds to dicyclohehylphenols containing up to 11 deute rium atoms per cyclohexyl ring due to the presence of H/D exchange processes. Analysis of the stereochemical positions of the remaining protons in the cyclohexyl rings indicates that the hydrogenation of the phenyl rings occurs via two regiochemically distinct reaction pathways. Crystal data at 183 K for NbCl3O2C72H54, 1c: [Nb(OC6HPh4-2,3,5,6)2Cl3].2C6H6, a=12.561(2), b=13.810(3), c=17.836(3) ?, α=87.37(1), β=72.63(1), γ=76.85(1)°, Z=2 in space group P1-; at 294 K for NbO3C36H50, 3a: [Nb(OC6H3Ph-η4-C6H7)(OC6H3Ph2)2].C7H7, a=9.339(1), b=13.107(1), c=20.681(2) ?, α=93.807(8), β=94.109(9), γ=105.992(9)°, Z=2 in space group P1-. The trichloride compound 1cadopts a square pyramidal geometry about the mononuclear niobium metal center with one axial and one basal aryloxide ligand.