Early transition metal aryloxide chemistry: New insights into arene C-H bond activation
Abstract
The facile cyclometallation of the 2,6-di-tert-butylphenoxide ligand with tantalum-aryl complexes has been shown to go via two different reaction pathways. Kinetic and isotopic labeling studies have provided evidence for both a heterolytic, multicenter pathway as well as a benzyme (o-phenylene) pathway. The use of various substituents on the phenyl leaving groups has led to further proof that these two pathways are controlling the C-H bond activation. Similar reactivity is seen with the 2,6-diphenylphenoxide ligand at Group 6 metal centers. Both molybdenum and tungsten have been seen to form orthometallated complexes with this ligand. Another mode of chelation has also been noted. One of the phenyl substituents of the chelated phenoxide can be bond to the metal center in a hexahapto-fashion. The reduction chemistry of the tungstentetrachloride, cis-WCl$\sb4$(OAr-2,6Ph$\sb2)\sb2$, has produced a variety of products. Under mild conditions simple coordinated complexes are formed. In contrast under more vigorous conditions, the various low valent tungsten complexes contain either a metallacyclic ligand or an $\eta\sp6$-arene bound ligand. The interconversion of the $\eta\sp6$-arene complex to the cyclometallated complex has also been noted. Most of these compounds have been isolated and characterized structurally and spectroscopically, and various mechanistic insights have been discussed. The reactivity of one of these low valent, $\eta\sp6$-arene complexes W(OC$\sb6$H$\sb3$Ph-$\eta\sp6$-C$\sb6$H$\sb{5}$)(OAr-2,6Ph$\sb2$)(dppm) is quite rich, especially when reacted with various small molecules. Reaction of this compound with acetylenes led to the formation of a new type of metallacyclic complex, a metallacyclopentatriene. Analogous mono- and bisacetylene compounds have also been prepared, but the interconversion of these complexes to the metallacyclopentatriene has proved futile. Again these complexes have been isolated and characterized and their unusual structures have been discussed in detail.
Degree
Ph.D.
Advisors
Rothwell, Purdue University.
Subject Area
Chemistry
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