ABSTRACT
Reactions of Cp2M (M¼Ti, Zr) ............................................................. 203 14.5 Conclusions.............................................................................................. 211 Acknowledgments................................................................................................. 211 References ............................................................................................................. 211
Our attempt in this chapter is to demonstrate the application of density functional theory (DFT) to real-life problems in transition metal organometallic chemistry through examples. Organometallic chemistry is an area where use of DFT to predict the structure, bonding, and reactivity has become complementary to experimental studies. A major part of the organometallic chemistry can be viewed profitably as
resulting from the replacement of small groups in the organic structures by the transition metal fragments. The transformation of the organic moiety as a result of the attachment of a metal fragment is so delicate and specific that seemingly similar fragments make large changes in the system. We have selected the reactions of the biscyclopentadienyl titanium (Cp2Ti) and biscyclopentadienyl zirconium (Cp2Zr) complexes as an example to demonstrate the dramatic changes brought in by these metallocenes. The in situ generated metallocenes (Cp2Ti, Cp2Zr), with d
2 valence electron count, have been playing a pivotal role in the stoichiometric and catalytic reactions [1]. In their many different forms, these metallocenes are extensively used as catalysts in olefin polymerization [2]. Their importance is attributed to their specific catalytic activity for the generation of stereoregular and stereospecefic polyolefins. Cp2Ti and Cp2Zr are also used in the synthesis of several precursors for the organometallic chemical vapor deposition of ceramic thin films [3]. Recently, Chirik and coworkers reported the formation of a tetramethylated cyclopentadienyl zirconium complex with side-on bound dinitrogen. This dinitrogen complex on hydrogenation gives ammonia [4a]. Pentamethylated cyclopentadienyl derivative of zirconium complex under similar reaction conditions forms end-on-bound dinitrogen complex and further reaction of this complex with H2 does not yield ammonia [4]. On the contrary, a similar reaction with dinitrogen complexes of Cp2Ti does not show any activity toward the addition of H2 across the TiN bonds [4d]. DFT calculations performed by Morokuma and coworkers explain these experimental observations [5]. The metallocenes (Cp2Ti, Cp2Zr) play an important role in the CC coupling and cleavage reactions of unsaturated molecules such as alkynes, olefins, acetylides, and vinylides [1,6,7]. The difference in the reactivity of Cp2Ti and Cp2Zr complexes toward CC coupling and cleavage reaction is dramatic. The systematic study of Rosenthal [7a,b], Erker [7c-e], and others [7f-o] reveals that, from similar starting materials, titanocene forms the CC coupled structure 1 (the two central carbon C2 and C3 are connected by a bond), whereas zirconocene favors the structure 2, where the coupling between the two central carbon atoms C2 and C3 is absent (Scheme 14.1) [7].
