LIGAND INDUCED DINITROGEN CLEAVAGE AND FUNCTIONALIZATION BY GROUP FOUR METALLOCENES

Abstract

The reactivity of the dihafnocene dinitrogen compound towards a variety of E-H (E = Si, B, C) bonds was explored. For E = Si or B, 1,2-addition of the bond across the dinitrogen fragment was observed giving hafnium diazenido hydride complexes which underwent further ligand-induced N-N bond cleavage. For E = C, addition of nitrogen heterocycles with sp2 C-H bonds leads to precoordination of the substrate to one metal center followed by addition across the dinitrogen unit to give hafnium diazenido alkyl complexes.

The 1,2,4-trimethylcyclopentadienyl hafnium platform was investigated in ligand-induced N2 cleavage reactions, providing a unique example of a dihafnium nitride following CO-induced N2 cleavage, whose electronic structure was explored computationally. Additional N-C and C-C bond forming reactions were investigated by addition of excess carbon monoxide, affording hafnium "oxamidide" compounds. The smaller steric profile of the cyclopentadienyl ligand allowed access to unique tetrametallic clusters. This N2 chemistry was extended to isocyanides, providing hafnium carbodiimidyl isocyanide complexes and mixed carbodiimidyl isocyanate complexes which proved competent for additional C-C coupling reactions. The divergent reactivity of isocyanides versus carbon monoxide for ligand-induced N2 cleavage in other systems was also explored.

The chemistry of the CO and N2 derived base-free dihafnium nitride toward a number of small molecule substrates was investigated. Activated alkynes, mono-substituted allenes, and heterocummulenes underwent cycloaddition to forge new N-C bonds, reactivity reminiscent of early metal-imido complexes. Nitriles underwent preferential insertion into the Hf-N bond rather than cycloaddition and could be used to enable cyclization cascades which produced complex small molecule architectures from N2 and CO. N-C bond formation could also be enabled by addition of alkyl triflates and chlorosilanes; these triggered cascade chemistry involving the pendent terminal isocyanate ligand, the mechanism of which was elucidated by deuterium labeling experiments. The base-free nitrido complex was also promiscuous in C-H bond activation chemistry; reaction with terminal alkynes, bulky allenes, dihydrogen and carbonyl containing substrates afforded μ2-NH fragments and the corresponding Hf-X species, consistent with the potent basicity of the nitrido fragment.

The electronic structure of the anionic ansa-zirconocene dinitrogen complex bearing an adamantyl substituted ligand was explored by X-ray crystallography, EPR spectroscopy, isotope labeling experiments, and DFT calculations. The complex is best described as a resonance between Zr(III) and Zr(IV) valence states with rare examples of [N2]1- and [N2]3- ligands. The similar spectroscopic signatures between this complex and the purported monomeric, side-on bound zirconium N2 complex reported by Lappert prompted reinvestigation of this species. Single crystal X-ray diffraction revealed that it was in fact the end-on anionic zirconium dinitrogen complex. The ability of both of these complexes to undergo redox-induced N2 hapticity changes was explored.

The dinitrogen chemistry of titanocene complexes bearing variably disubstituted cyclopentadienyl ligands was explored. Variation of the size and distribution of the Cp substituents allowed the isolation of rare examples of side-on bound titanocene dinitrogen complexes, as well as a unique trimetallic cluster. The electronic structures of these complexes were investigated by magnetic and spectroscopic measurements as well as DFT calculations.

Preliminary investigations into the chemistry of cobalt complexes supported by tridentate, meridionally coordinating bis(phosphino)pyridine pincer ligands are additionally described.