Jonathan S. Vilardo, Mark A. Lockwood, Linda G. Hanson, Janet R. Clark, Bernardeta C. Parkin, Phillip E. Fanwick and Ian P. Rothwell
The trichloride compounds
[Ta(OC6HPh2-2,6-R2-3,5)Cl3]
(1: R = H a, Ph b, Me c, Pri d or
But e) have been obtained by treating
[Ta2Cl10] with the corresponding
3,5-disubstituted-2,6-diphenylphenols Ia–Ie. The solid-state
structures of 1c and 1d show a square-pyramidal structure with an axial
aryloxide ligand. The reaction of 1 with
LiCH2SiMe3 (3 equivalents) led to the isolation of
the tris(alkyls)
[Ta(OC6HPh2-2,6-R2-3,5)2
(CH2SiMe3)3] (4a–4d) except in
the case of the 3,5-di-tert-butyl derivative 1e which generated
the alkylidene compound
[Ta(OC6H3Ph2-2,6-But-3,5)
2(CHSiMe3)(CH2SiMe3
)] 6e. The alkylidenes 6a–6d can be produced by photolysis of the
corresponding tris(alkyls) 4a–4d. The alkylidenes 6a–6d
undergo intramolecular cyclometallation of the aryloxide ligand
(addition of an aromatic C–H bond to the tantalum alkylidene) at
a rate which is extremely dependent on the meta substituents on
the phenoxide nucleus. Kinetic studies show that conversion of
6a–6d into monometallated 7a–7d is first order with the
phenyl, methyl and isopropyl substituents slowing the ring closure down
by factors of 20, 90 and 360 respectively. The tert-butyl
substituent completely shuts down cyclometallation of the adjacent
phenyl ring. It is argued that bulky substituents inhibit rotation of
the ortho-phenyl ring into a conformation necessary for
C–H bond activation. Structural analysis of the torsion angles
between ortho-phenyl and phenoxy rings has been carried out.
The use of 1H NMR chemical shifts has been demonstrated to be
a valuable tool to probe the average conformations adopted in
solution.