Tantalum Carbene and Imide Complexes. Synthesis, Characterization, and Pathways of Formation

This dissertation focuses on two different types of organometallic compounds, carbenes and imides. The first project deals with the archetypal Schrock carbene, and the second project studies complexes that contain metal-nitrogen bonds, both amides and imides.

A summary of the research in this dissertation is discussed in Chapter 1. Chapter 2 begins the studies of the archetypal Schrock carbene (Bu^tCH₂)₃Ta=CHBu^t. The studies include the synthesis of deuterated compounds (Bu^tCD₂)₃TaCl₂ and Bu^tCD₂Li, observation and identification of the intermediate, Ta(CD₂Bu^t)₅, and kinetic studies of the conversion of Ta(CD₂Bu^t)₅ to (Bu^tCD₂)₃Ta=CDBu^t, giving the activation parameters and a kinetic isotope effect for the conversion. The work here confirms that the pentaneopentyltantalum is the precursor to the archetypal Schrock carbene.

Chapter 3 studies the effects of isotopic substitution on NMR chemical shifts of complexes in Chapter 2. Conformations of (Bu^tCD₂)₃TaCl₂ and Ta(CD₂Bu^t)₅ have also been investigated.

Chapter 4 begins the study of compounds containing metal-nitrogen bonds. Guanidinate imides Ta(NMe₂)(=NSiMe₃)[RNC(NMe₂)NR]₂ (R = Cy, Prⁱ) have been prepared from the reactions of Ta(NMe₂)₄[N(SiMe₃)₂] with two equivalents of carbodiimides, RN=C=NR. The two guanidinate imides have been characterized by NMR spectroscopy and elemental analysis. In addition, the structure of Ta(NMe₂)(=NSiMe₃)[CyNC(NMe₂)NCy]₂ has been studied by single crystal X-ray diffraction. Under heating, Ta(NMe₂)₄[N(SiMe₃)₂] undergoes an unprecedented elimination of Me₃Si-NMe₂, converting the amide ligand –N(SiMe₃)₂ to the imide ligand =NSiMe₃ to give an intermediate Ta(NMe₂)₃(=NSiMe₃). In the presence of CyN=C=NCy, the carbodiimide captures the intermediate to give another intermediate Ta(NMe₂)₂(=NSiMe₃)[CyNC(NMe₂)NCy]. Subsequent second carbodiimide insertion leads to the formation of the final product Ta(NMe₂)(=NSiMe₃)[CyNC(NMe₂)NCy]₂. The remaining amide ligand, –NMe₂, in Ta(NMe₂)(=NSiMe₃)[CyNC(NMe₂)NCy]₂ and Ta(NMe₂)(=NSiMe₃)[PrⁱNC(NMe₂)NPrⁱ]₂ gives two separate resonances in the proton NMR spectrum at room temperature indicating inequivalence of the two methyl groups. The interconversion of the methyl groups in the former has been studied with variable-temperature NMR.

Chapter 5 studies the synthesis and characterization of metal cage complexes [(Me₂N)₃MO]₄ (M = Nb, Ta). Single crystal X-ray diffraction studies show a cubane-like structure with M-O bridges. Variable-temperature NMR of the inequivalent amide methyl groups –NMe_AMe_B has also been carried out to find the activation parameters for the exchange.