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The goal of the proposed research is to prepare new dinucleating, monoanionic bicyclic guanidinate and neutral N-heterobicyclic dicarbene transition metal ligands that will support new classes of metal-metal multiply-bonded complexes.  The ligand architectures are based on the bicyclic guanidinate hpp^–, the deprotonated form of hexahydropyrimidopyrimidine or 1,5,7-triazabicyclo[4.4.0]dec-5-ene, that has been used to prepare a wide variety of dinuclear transition metal complexes such as the novel, highly-reducing W₂(hpp)₄ with a W–W quadruple bond.  Several NHC-like dicarbenes are known, but none have an architecture that would support dinucleation of two transition metals in close proximity. 

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.