The bicyclic guanidinate 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate (hpp–) has been recognized as an excellent stabilizing ligand for low oxidation state metals for about a decade. It has been used as a dinucleating anionic ligand in the synthesis of complexes containing the ‘paddlewheel’ structure motif (which often contain metal-metal multiple bonds) and is resistant to reductive cleavage by low-valent metal centers. One drawback to the ligand is that its metal complexes often have reduced solubility. This can be problematic in isolation and characterization. One solution to this problem is alkylation of the rings. Two derivatives of hpp–, tetramethyl-hppH (hpp*H) and tetraethyl-hppH (hpp”H) have published syntheses that were irreproducible by our research group.
New and easily reproducible syntheses for hpp*H and hpp”H are presented. The syntheses of (hpp*)SiMe3 and (hpp”)SiMe3, which are liquid silanes that can be isolated and purified for better stoichiometric control when transferring hpp–, hpp*–, and hpp” – onto metal centers, are also described. Chapter 3 details the synthesis of several high-valent mononuclear tantalum and zirconium complexes containing hpp– and hpp*– using these silanes. The solid-state structures of the Ta(hpp*) compounds are compared to those of the previously reported Ta(hpp) compounds.
In the course of this research, it was discovered that the M(hpp)/M(hpp*) [M = Zr, Ta] complexes were highly fluorescent, exhibiting intense Ligand-to-Metal Charge Transfer (LMCT). A series of Zr(hpp)(bpy) [bpy = 2,2’-bipyridine] complexes containing different bpy derivatives were synthesized to explore the possibility of Ligand-to-Ligand Charge Transfer, a very rare phenomenon for zirconium, and are also detailed in chapter 3. Chapter 4 describes the spectrofluorimetry of the synthesized Ta(hpp), Ta(hpp*), Zr(hpp), and Zr(hpp)(bpy) complexes.
Since hpp*– is known to change electronics at the metal center compared to hpp–, we wanted to investigate if its effects could be examined by spectroscopy. Chapter 5 describes the synthesis of several metal-oxo hpp complexes that were characterized via the M=O IR stretch. The comparison was made between MO(hpp)2L and MO(hpp*)2L [M = W, Re]. The M=O IR stretches were verified by 18O isotopic labeling. Attempts at synthesizing TaOCl3 and TaO(hpp)2L are also detailed.
In the last chapter (chapter 6), a few U(hpp) complexes were generated during our development of non-cyclopentadienyl hpp– coordination chemistry of uranium, and are discussed. Two of the complexes characterized via X-ray crystallography are diuranium complexes with UIV-UIV distances of 3.56 Å, possibly being the first compounds with U-U bonds. The new complex U(hpp*)4 is also described.