⁷Li, ⁶Li, ²³Na and ¹³³Cs multinuclear NMR studies of adducts formed with shift reagent, TmDOTP⁵

Interactions between Li⁺, Na⁺, Cs⁺, Ca²⁺ and Mg²⁺ and the shift reagent (SR), TmDOTP^5-, were studied by ⁷Li, ⁶Li, ²³Na and ¹³³Cs multinulear NMR spectroscopy. The unusually large paramagnetic shifts (>>100 ppm) observed in the cation NMR resonances at low alkali metal ion to TmDOTP^5- ratios indicated that the cations bind near the 4-fold symmetry axis of the complex. The geometric parameters for the adducts formed between ⁷Li, ⁶Li, ²³Na and ¹³³Cs cations and TmDOTP^5- were obtained from shift and relaxation rate data and compared with those calculated using MMX energy minimization techniques. It is found that the larger Cs⁺ ion lies closest to the 4-fold axis of symmetry and displays the largest binding constant. The smaller Li⁺ ion deviates most from the 4-fold symmetry axis and has a less favorable binding interaction than Cs⁺. Na⁺ lies somewhere between these two extremes but has the largest limiting shifts due to its favorable distance from the paramagnetic metal center. A significant reversal of the ²³Na shift upon addition of Ca²⁺ and Mg²⁺ indicate that these two divalent cations form very stable 1:1 adducts with TmDOTP^5- (log K (Ca²⁺) = 5.69 and log K₁ (Mg²⁺) = 3.85). An analysis of the Mg²⁺ titration data shows that the 2Mg²⁺:TmDOTP^5- adduct becomes dominant at high Mg²⁺/TmDOTP^5- ratio (log K₂ (Mg²⁺) = 2.17). The significant influence of NH₄⁺ on the TmDOTP^5--induced ⁶Li shifts suggests that the NH₄⁺ ion may form multiple H-bonds with the SR along the 4-fold axis of symmetry.