High- and Mid-Valent Tantalum and Mono(peralkylcyclopentadienyl)tantalum Complexes of the Bicyclic Guanidinate Hexahydropyrimidopyrimidinate

Ta­(hpp)­Cl₄, Ta­(hpp)₂Cl₃, and (η-C₅Me₄R)­Ta­(hpp)­Cl₃ (R = Me, Et; hpp = anion of 1,3,4,6,7,8-hexahydro-2<i>H</i>-pyrimido­[1,2-<i>a</i>]­pyrimidine, hppH) are readily formed in good to excellent yields by addition of (hpp)­SiMe₃ to TaCl₅, Ta­(hpp)­Cl₄, or (η-C₅Me₄R)­TaCl₄, respectively, in dichloromethane. Ta­(hpp)­Cl₄ in solution exhibits three CH₂ multiplets in the proton NMR spectrum, consistent with a mirror plane that bisects the hpp ligand. The six-coordinate, yellow Ta­(hpp)­Cl₄ possesses a distorted octahedral solid-state structure with chelating hpp and Ta–N distances of 2.038(1) and 2.059(1) Å. Ta­(hpp)₂Cl₃ is fluxional in solution by proton NMR spectroscopy, with three CH₂ multiplets at room temperature that split into additional multiplets at −50 °C, consistent with the six multiplets expected from the solid-state structure. In the solid state, bright yellow Ta­(hpp)₂Cl₃ has an approximately pentagonal bipyramidal structure with both hpp chelates coordinated in the pentagonal plane. Ta–N distances are 2.114(3) and 2.110(3) Å for one hpp ligand and 2.112(3) and 2.117(3) Å for the other. The proton and carbon-13 solution NMR spectra of (η-C₅Me₄R)­Ta­(hpp)­Cl₃ are consistent with a static solution structure at room temperature, with the two coordinated nitrogens in different chemical environments as shown by six different methylene resonances. The solid-state structure of bright pinkish-red (η-C₅Me₅)­Ta­(hpp)­Cl₃ is a four-legged piano stool with axial ligation, with one hpp nitrogen coordinated in the square plane and one nitrogen in the axial position opposite the cyclopentadienyl centroid. Addition of (hpp)­SiMe₃ to the organoditantalum­(III) complex (η-C₅Me₄R)₂Ta₂(μ-Cl)₄ or Na/Hg reduction of (η-C₅Me₄R)­Ta­(hpp)­Cl₃ in benzene yields the first mid-valent tantalum hpp adduct, paramagnetic red (η-C₅Me₄R)­Ta­(hpp)­Cl₂. This Ta­(IV) four-legged piano stool structure in the solid state has an hpp chelate with Ta–N distances of 2.118(3) and 2.091(3) Å. Intraligand bond distances and angles for the hpp ligands, proton and carbon-13 NMR solution spectra, EPR spectra, and UV–vis spectra for the new Ta­(V) and Ta­(IV) hpp complexes are discussed