TERNARY ALKALINE-EARTH METAL-COMPLEX IONS IN THE M(2+) BORATE/TARTRATE SYSTEM AS STUDIED BY B-11 NMR/

In aqueous alkaline solution, the berate ion (B(OH)(4)(-)) is Capable of forming both 1:1 and 1:2 complexes with polyhydroxy carboxylate lig ands such as L-(2R,3R)-tartrate or meso-(2R,3S)-tartrate (T2-). The mo de of coordination is pH dependent and has been shown in base to be ex clusively via the 2,3-diol linkage on the ligand. Equilibria in this s ystem can be described by the following two equations: B(OH)(4)(-) + T 2- <-> BT3- + 2H(2)O (K-1); BT3- + T2- <-> BT25- + 2H(2)O (K-2) B-11 N MR was used to study both these equilibria and the binding of alkaline earth metal ions by the several borotartrate complex ions. Metal ion specificity (Mg2+, Ca2+, Sr2+) was studied in the M(2+)/B(OH)(4)(-)/L- tartrate systems, while the effect of absolute ligand configuration (L -tartrate vs meso-tartrate) was studied in the Ca2+/B(OH)(4)(-)/tartra te systems. The main conclusions of this work are (1) Mg2+ binds prefe rentially to free tartrate, T2- and shows little if any interaction wi th BT3- and BT25-, (2) Ca2+ and Sr2+ bind preferentially (by 3 orders of magnitude) to BT25- (L-tartrate) with Ca2+ binding being selectivel y favored by a factor of 5, and (3) absolute ligand (tartrate) configu ration has no significant effect on Ca2+ binding to BT25-. Stability c onstants for the various complexation reactions are reported as well a s a proposed Ca2+ and Sr2+ binding site in the MBT(2)(3-) complex ions .