Structural and Stereochemical Aspects of the Group 4 Metal Chemistry of Constrained-Geometry 2-(Indenyl)phenoxide Ligation

The reactivity of 2-(inden-3-yl)-4,6-di-tert-butylphenol (1) and its 2-methyl (2), 1,2-dimethyl (3), 2,4,7-trimethyl (4), and 1,2,4,7-tetramethyl (5) derivatives toward group 4 metal dialkylamides has been examined. Reaction of 1 equiv of the phenols 1−4 with the tetrakis(dialkylamido) compounds [M(NR2)4] (M = Ti, R = Me; M = Zr, R = Me, Et; M = Hf, R = Et) has allowed isolation of a series of bis(dialkylamido) compounds: e.g., [Ti(OC6H2{η5-Ind}-2-But 2-4,6)(NMe2)2]. In these compounds the parent phenol was found to be deprotonated at both the phenolic OH and indenyl ring, leading to the elimination of 2 equiv of dialkylamine. Structural studies of eight of these derivatives allow the variation of metal and methyl substitution to be assessed. The metal coordination of the series of compounds is best described as pseudo-tetrahedral, with the indenyl ring occupying one site of a three-legged piano-stool geometry. The M−C(indenyl) distances are consistent with an essentially η5 coordination for all bis(dialkylamido) compounds structurally characterized. The M−O−C angles for the chelates fall in the very narrow range of 126−129°. In the solution NMR spectra, nonequivalent dialkylamido ligands give rise to sharp, well-separated resonances. It was found possible to introduce two chelating indenylphenoxide ligands into the coordination sphere of titanium, zirconium, or hafnium either by treatment of the tetra(dialkylamides) with 2 equiv of the parent phenol and overall loss of 4 equiv of amine or using metathetical exchange of the dilithio salt of the indenylphenol with the corresponding metal tetrachloride. The combination of the three chiral elements (two planar chiral indenyl rings and an axially chiral metal center) within [M(OC6H2{η n -Ind}-2-But 2-4,6)2] generates three distinct diastereoisomers. Two of the enantiomer pairs, (S,pR,pR)/(R,pS,pS) and (R,pR,pR)/(S,pS,pS), contain a C 2 axis leading to equivalent chelates and one set of ligand signals. In both of these diastereomers there are rac arrangements of indenyl ligands. The only difference between the two lies in the chelation of the aryloxide ligands to the metal center. The third enantiomer pair (S,pR,pS)/(R,pR,pS) has no symmetry element and has two equal-intensity sets of ligand resonances (meso indenyl rings). Spectroscopic evidence for all three enantiomer pairs has been obtained, while two forms have been structurally characterized. The compound [Ti(OC6H2{η n -Ind}-2-But 2-4,6)2] was found to p