ABSTRACT
Ligands are an essential part of organometallic compounds, which impose a dominant control over both chemical and physical properties of the resulting metal complexes.1−12 Cyclopentadienyl (Cp), a six-electron π-ligand, has long been a ubiquitous component of metallocenes, and it is hard to imagine what organometallic chemistry would be without this ligand.13 In recent decades, various types of ligands have been synthesized for different purposes, among which early-transition metal complexes bearing linked cyclopentadienyl-amido ligands, regularly referred to as constrainedgeometry complexes (CGCs), stand out and have been found wide interests in both academia and industry since their ™rst description in 1990.14,15 For example, a rare earth metal CGC is recently reported to be an active catalyst for the hydroamination reaction of carbodiimides,16 which is compatible with both primary and secondary amines via an insertion-protonation pathway. On the other hand, catalytic systems based on group 4 metal compounds give access to a large array of polymers with unique material properties and considerable commercial values.4-12,17-29
In this connection, replacement of a uninegative cyclopentadienyl (Cp−) in the classical constrained-geometry ligand by an isolobal, dinegative dicarbollide ion C B H2 9 11
2−, can reduce the overall charge of the resulting metallocene by one unit but leave the gross structural and metal frontier orbital properties unchanged. Such new metal/charge combinations would have an impact on the properties of resultant metal complexes. With this in mind, we synthesized constrainedgeometry titanacarborane monoamides [σ:η1:η5-(OCH2)(R2NCH2)C2B9H9]Ti(NR2) (R = Me, Et), studied their reactivity, and explored their catalytic activity in C-N bond forming/breaking reactions. The results are summarized in this chapter.
