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-2H-pyrimido­[1,2-a]­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.