NMR Investigation of the Interaction of Vanadate with Carbasilatranes in Aqueous Solutions

Reaction of vanadate with carbasilatranes [methoxy{N,N‘,N‘ ‘-2,2‘,3-[bis(1-methylethanolato)(propyl)]amino}silane
(1), methoxy{N,N‘,N‘ ‘-2,2‘,3-[bis(1-ethanolethanolato)(propyl)]amino}silane (2), and {N,N‘,N‘ ‘-2,2‘,2-[bis(ethanolato)(glycolpropyl ether)]amino}silane (3)] in aqueous solution results in the formation of vanadosilicates and five-coordinated chelate vanadium(V) complexes as evidenced by 51V, 1H, and 13C NMR spectroscopy. Chiral
carbasilatrane S,S-1 was characterized in the solid state by X-ray diffraction, revealing a trigonal bipyramidal geometry
around the metal ion, with one unidentate methoxy group and one atrane nitrogen atom at the axial positions and
one carbon and two atrane oxygen atoms at the equatorial plane of the bipyramid. Crystal data (Mo Kα; 100(2)
K) are as follows:  orthorhombic space group P212121; a = 8.8751(6), b = 9.7031(7), c = 14.2263(12) Å; Z = 4.
The complexation of vanadium either with 1 or 2 is stereoselective yielding ∼94% of the complex containing ligand
in the S,R-configuration. The lower ability of the S,S- and R,R-diastereoisomers of 1 and 2 to ligate vanadate was
attributed to stereochemical factors, dictating a square pyramidal geometry for the chelated complexes. A dynamic
process between the vanadium chelate complexes and the respective carbasilatranes was evaluated by 2D {1H}
EXSY NMR spectroscopy. These spectra show that the vanadate complexes with the open carbasilatranes exchange
more slowly with the free ligand compared to the respective alcohol aminate complexes.