Intramolecular activation of aromatic C–H bonds at tantalum(V) metal centers: evaluating cyclometallation ‘resistant’ and ‘immune’ aryloxide ligation

Abstract

The trichloride compounds [Ta(OC 6 HPh 2 -2,6-R 2 -3,5)Cl 3 ] (1: R = H a, Ph b, Me c, Pr i d or Bu t e) have been obtained by treating [Ta 2 Cl 10 ] 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 LiCH 2 SiMe 3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC 6 HPh 2 -2,6-R 2 -3,5) 2 (CH 2 SiMe 3 ) 3 ] (4a-4d) except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC 6 H 3 Ph 2 -2,6-Bu t -3,5) 2 (᎐ ᎐ CHSiMe 3 )(CH 2 SiMe 3 )] 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 1 H NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.

Rare Earth Complexes of Bulky 2,6-Diphenylphenolates Containing Additional, Potentially Buttressing 3,5-Substituents

“…Complexes with THF or DME lose solvent on being heated in sealed capillaries under N 2 hence melting points have little significance (see examples below). 2,6‐Diphenylphenol was purchased from Aldrich, while HOAr R reagents were prepared by previously reported procedures (R = Me,6b,19 t Bu,20 Ph21). Rare earth metals were obtained as powders or chunks from Rhône−Poulenc, Phoenix, USA.…”

Synthesis and characterization of new, modified terphenyl ligands: Increasing the rotational barrier for flanking rings

“…Attempts to extend the range of such compounds to include boron or silicon species such as ArBBAr or ArSiSiAr have encountered problems with activation of the substituents on the flanking aryls that yield products incorporating boron [34][35][36] or silicon [37][38][39][40][41][42][43] in the rings rather than the desired multiply bonded species. However, in parallel work by Rothwell and coworkers on related 2,6-diarylphenoxide ligands [44], it has been shown that the introduction of alkyl groups on the central aryl ortho to the flanking aryl rings confers lower susceptibility, if not immunity, to undesirable reactions of this type. This is because rotation of the flanking ring into the plane of the central ring, which facilitates undesirable activation, is made more difficult.…”

Synthesis and Characterization of the Very Bulky Phenols Ar*OH and Ar′OH (Ar* = C6H3‐2,6‐Trip2, Trip = C6H2‐2,4,6‐iPr3; Ar′ = C6H3‐2,6‐Dipp2, Dipp = C6H3‐2,6‐iPr2) and Their Lithium and Sodium Derivatives (LiOAr′)2 and (NaOAr*)2

“…For the phenyl-substituted -OC 6 H 3 -2,6-Ph 2 activation of the ortho phenyls by close approach of the phenyl CH groups to the reactive center can occur by rotation of the ortho phenyl ring so that it is almost coplanar with the central phenoxide moiety. Recent work by Rothwell and coworkers has shown that when large meta substituents are also present at the central ring, as in -OC 6 H-2,6-Ph 2 -3,5-tBu 2 , [7] resistance or immunity to the cyclometallation reaction is induced because the ortho phenyl rings cannot become coplanar with the central ring due to the steric effects of the tBu substituents. In related work we have pursued the use of very bulky terphenyl ligands in the stabilization of several new compound classes.…”