Louis Messerle, PhD , University of Iowa
In the first project period three specific tasks were accomplished: (1) Syntheses of new tantalum hpp complexes were developed in order to optimize synthetic protocols with the inexpensive hpp ligand before the study of tantalum chemistry of C2-symmetric dialkylated derivatives. We prepared, characterized spectroscopically, and determined the solid-state structures of pseudo-octahedral Ta(hpp)Cl4, pentagonal bipyramidal Ta(hpp)2Cl3, pinkish-red Cp*Ta(hpp)Cl3, and the first mid-valent Ta hpp complex, orange-red paramagnetic Cp*Ta(hpp)Cl2. These syntheses proceed cleanly when distilled hpp transfer reagent (hpp)SiMe3 is used instead of in situ-generated (hpp)SiMe3 or Li(hpp). The hpp ligand(s) chelate by both nitrogens to the Ta center in each complex. Both hpp ligands in the pentagonal bipyramidal Ta(hpp)2Cl3 coordinate in the equatorial plane in the solid state. The least-squares plane of the hpp ligand in Cp*Ta(hpp)Cl3 is perpendicular to the plane of the Cp* ligand, with one nitrogen in a pseudo-equatorial position along with the three chlorine atoms, while the other nitrogen is in a pseudo-axial position. Unlike the fluxional Ta(hpp)2Cl3, Cp*Ta(hpp)Cl3 is static on the proton NMR spectroscopic time scale at room temperature. Cp*Ta(hpp)Cl2 has a four-legged piano-stool structure with the hpp and Cp* planes roughly parallel. The Ta-N distances in Cp*Ta(hpp)Cl2 are shorter than those found in Cp*Ta(hpp)Cl3, whereas the opposite would have been expected, to a first approximation, given the larger covalent radius of Ta(IV) over Ta(V), so other electronic factors in the disparate core structures are operative. In all complexes the bridgehead carbon-to-bridgehead nitrogen distances are short and the bridgehead nitrogens are planar, consistent with strong delocalization of the bridgehead nitrogen lone pair into the guanidinate π-system and hpp-to-metal charge transfer. Ta(hpp)2Cl3, a plausible precursor to the unknown Ta2(hpp)4 with a postulated Ta–Ta triple bond, can be reduced with sodium amalgam under argon to a dinitrogen-sensitive mid-valent tantalum complex. (2) An optimized multi-step enantioselective synthesis of the C2-symmetric 4,10-dimethylhppH from L-alanine was executed, involving a reductive amination step and a later crystallization method for separating intermediate diasteromers. The final cyclization step from the chiral triamine precursor [H2N(CH2)2C*HMe]2NH to 4,10-Me2hppH was greatly improved by the use of a guanidinium salt C1 reagent over CS2 or (MeS)2CS. (3) A seven-step organic synthetic route from nitro-xylene to a tricyclic bis(formamidinium) dication with N-isopropyl groups was developed and the structure of the unusual dication with a iPrNC+HNC+HNiPr base was determined by single-crystal X-ray diffractometry. The bis(formamidinium) dication can be deprotonated with Li bases to form at low temperature a lithiated dicarbene based on the symmetry of the resulting NMR spectra and a single, broad low-field carbon-13 NMR resonance at δ232.