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

Abstract

2H-pyrimido[1,2-a]pyrimidine, hppH) are readily formed in good to excellent yields by addition of (hpp)SiMe 3 to TaCl 5 , Ta(hpp)Cl 4 , or (η-C 5 Me 4 R)TaCl 4 , respectively, in dichloromethane. Ta(hpp)Cl 4 in solution exhibits three CH 2 multiplets in the proton NMR spectrum, consistent with a mirror plane that bisects the hpp ligand. The six-coordinate, yellow Ta(hpp)Cl 4 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) 2 Cl 3 is fluxional in solution by proton NMR spectroscopy, with three CH 2 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) 2 Cl 3 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-C 5 Me 4 R)Ta(hpp)Cl 3 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-C 5 Me 5 )Ta(hpp)Cl 3 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)SiMe 3 to the organoditantalum(III) complex (η-C 5 Me 4 R) 2 Ta 2 (μ-Cl) 4 or Na/Hg reduction of (η-C 5 Me 4 R)Ta(hpp)Cl 3 in benzene yields the first mid-valent tantalum hpp adduct, paramagnetic red (η-C 5 Me 4 R)Ta(hpp)Cl 2 . 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.

Guanidinates as Alternative Ligands for Organometallic Complexes

“…As was the case for group 4 metals [120], guanidinato ligands stabilize organometallic tantalum species in a medium or low oxidation state [129][130][131]. For example, mixed cyclopentadienyl-guanidinato species of Ta(IV) give rise to N≡N bond cleavage to provide the bimetallic bis(µ-nitrido) complex, {Cp*Ta[N(iPr)C(NMe 2 )N(iPr)](µ-N)} 2 57.…”