Tantalum μ-Alkylidyne Compounds Containing o-Phenyl- and o-(1-Naphthyl)phenoxides: Probing Molecular Structure and Dynamics

The new o -(1-naphthyl)phenols 2-(1-naphthyl)-4,6-di- tert -butylphenol, 2,6-bis(1-naphthyl)phenol, 2,6-bis(1-naphthyl)-3,5-dialkylphenol (alkyl = Ph, Me, Bu t ), and 2-(1-naphthyl)-3,5,6-triphenylphenol have been synthesized. The reaction of these phenols and their o -phenyl counterparts with the compound [Ta 2 (μ-CSiMe 3 ) 2 (CH 2 SiMe 3 ) 4 ] has been investigated. This reaction produces the monosubstitution products [(ArO)(Me 3 SiCH 2 )Ta(μ-CSiMe 3 ) 2 Ta(CH 2 SiMe 3 ) 2 ] at rates which are strongly dependent on the nature of the phenol substituents. The NMR spectroscopic properties of the resulting derivatives can be used to probe the phenoxide structure. Nonchiral phenoxides yield singlets for the CH 2 SiMe 3 methylene protons and one set of μ-CSiMe 3 resonances, whereas the presence of a chiral phenoxide generates diastereotopic CH 2 SiMe 3 protons and nonequivalent μ-CSiMe 3 groups. The solid-state structure of the 2-(1-naphthyl)-3,5,6-triphenylphenoxide and 2,6-bis(1-naphthyl)phenoxide derivatives shows that the 1,3-dimetallacyclobutadiene core remains intact with Ta−Ta distances of 2.8943(6) and 2.8886(7) Å, respectively. The rate expression for substitution of the first alkyl group in [Ta 2 (μ-CSiMe 3 ) 2 (CH 2 SiMe 3 ) 4 ] ( 11) by 2-phenyl-4,6-di- tert -butylphenol ( 5 ) in C 6 D 6 solvent is first order in both [ 11 ] and [ 5 ] with a second-order rate constant of [1.40(7)] × 10 - 4 mol - 1 L - 1 s - 1 at 30(1) °C. Use of the deuterated phenol ArOD gave a rate constant of [0.25(3)] × 10 - 4 mol - 1 L - 1 s - 1, hence demonstrating the primary kinetic isotope effect k H / k D = 5.6(5). Competition reactions have yielded the relative rates of substitution of 11 by various phenols.