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.
Amalgam reduction of the Nb(III) compounds
Cp‘2Nb(Cl)(L) (1, L = CO;
2, L = PMe3; 3, L =
CNtBu)
gives rise to the electron-rich Nb(II) radicals
Cp‘2Nb(L).
In one case (L = CO) the intermediate radical was
observed using ESR spectroscopy. The radicals undergo
a facile reaction with elemental mercury to give the
heterometallic compounds
[Cp‘2Nb(L)]2Hg, one of
which
(L = PMe3) was characterized crystallographically;
since
this process is reversible, the mercury adducts constitute
convenient sources of the niobium(II) radicals.
The niobium−mercury compounds
[Cp‘2Nb(L)]2Hg (Cp‘ =
η5-C5H4SiMe3, L
= CO (4), PMe3
(5), or CNtBu (6)) serve as
stable precursors to the short-lived Nb(II) radicals of
general
formula [Cp‘2Nb−L]. The homolysis of the
Nb−Hg bond may result from a slow thermal
reaction (generating low concentrations of radicals) or from a
photochemical process in which
mercury extrusion is more rapid. Since the Nb(II) species
show no evidence for Nb−Nb
bond formation, they are useful in the synthesis of a variety of
Nb(III) and Nb(IV) species.
Reactions of 4 with dimeric species such as
[CpFe(CO)2]2,
[CpNi(CO)]2, Co2(CO)8,
or RSe−SeR give rise to new Nb−M or Nb−Se compounds, while reactions with
potential π donors
such as formaldehyde or azobenzene lead to displacement of the ligand L
and formation of
the Nb(IV) complexes. Crystallographic and
variable-temperature NMR studies on the Nb−Fe compound
Cp‘2Nb(μ-CO)2Fe(CO)Cp
(10) are consistent with a low-energy fluxional
process
involving exchange of bridging and terminal carbonyl ligands.
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