Chelating Aryloxide Ligands in the Synthesis of Titanium, Niobium, and Tantalum Compounds:  Electrochemical Studies and Styrene Polymerization Activities

Abstract

The chelating trisphenol ligands tris(2-hydroxyphenyl)amine (1H 3 ) and tris(2-hydroxy-4,6-dimethylbenzyl)amine (2H 3 ) proved to be excellent precursors for the chelating phenoxides, and the latter has been used to prepare a series of cyclopentadienylmetal derivatives of early transition metals. For niobium and tantalum, reactions with CpMCl 4 lead to the compounds CpMCl(1) and CpMCl( 2). An X-ray diffraction study of CpNbCl(1) establishes a pseudo-octahedral structure with a trans disposition of the η 5 -cyclopentadienyl ring and the nitrogen atom of the chelating ligand. Similar reactions of CpTiCl 3 lead to the CpTi(1) and CpTi(2) analogues. Electrochemical experiments provide useful information on the reduction potentials of the compounds, from which it is clear that ligand 2 is a stronger donor than is 1. At the same time, it appears that chelate ring size is important; while the reduction of complexes containing 1 are largely reversible, those of complexes containing 2 are irreversible. This is interpreted to mean that the six-membered rings in the latter are opening during reduction, a process involving formal loss of an aryloxide from the metal center. In an attempt to correlate this solution reactivity with catalytic efficiency in a bond-forming process, the compounds were screened for activity as styrene polymerization catalysts in the presence of methylaluminoxane cocatalyst. While the niobium and tantalum analogues were inactive, the titanium compounds of 1 showed high activity and appreciable selectivity for the preparation of syndiotactic polystyrene.(1) (a) Wesleyan University. (b) Yale University.(2) (a) Bradley, D. C.

Titanium complexes supported by a sterically encumbering N-anchored tris-arylphenoxide ligand

“…The AgOTf step is needed in order to promote anion exchange of 1-OTf with a salt such as Li[B(C 6 In conclusion, we have demonstrated that the sterically encumbering ligand (O 3 N) 3À can stabilize neutral and cationic titanium(IV) systems. All these compounds appear resistant to hydrolysis and coordination of polar substrates, which could be the result of the sterically protecting Ad groups.…”

“…2 and exposes a neutral Ti(IV) center confined in a similar environment to the structure of 1-Cl. 6 It is clear from the molecular structure that the OTf À group is coordinated to the metal center (Ti-O OTf , 2.031(2) Å ), and that the metal center is in the mean plane defined by the O-atoms of the N-anchored ligand ($0.18 Å ). As with 1-Cl, the depth of the cavity provided by the adamantyl groups was estimated at $4.14 Å .…”

“…An obvious strategy to avoid this problem is to incorporate steric groups proximal to the hydroxyl motif, thereby disfavoring intermolecular interactions [2][3][4]. However, several examples of mononuclear systems supported by non-encumbering N-anchored tris-arylphenoxide ligands have been scarcely reported, thus hinting that electronic factors might also play a role in formation of mono versus polynuclear compounds [6]. Recent reports by Kol [2] Brown [4] and others [3,5,6] have demonstrated that the (O 3 N) 3À framework is highly compatible with Ti(IV), and depending on the steric nature of the tripodal ligand, these systems can have considerable resistance to hydrolysis.…”

“…However, several examples of mononuclear systems supported by non-encumbering N-anchored tris-arylphenoxide ligands have been scarcely reported, thus hinting that electronic factors might also play a role in formation of mono versus polynuclear compounds [6]. Recent reports by Kol [2] Brown [4] and others [3,5,6] have demonstrated that the (O 3 N) 3À framework is highly compatible with Ti(IV), and depending on the steric nature of the tripodal ligand, these systems can have considerable resistance to hydrolysis. Given the high Ti-O bond enthalpy ($673 kJ/mol) [7] and stability of the N-anchored tris-arylphenoxide core, we decided to investigate the chemistry of Ti(IV) complexes supported with the sterically demanding ligand (O 3 N) 3À (O 3 N 3À = tris(2-O,3-Ad,5-tBubenzyl)amine, Ad = 1-adamantyl, Scheme 1).…”

“…Unfortunately, compound 1-Cl failed to react with nucleophiles such as LiMe or salts such as Na[B(3,5-(CF 3 ) 2 C 6 …”