Complexation of phosphoryl-containing mono-, bi- and tri-podands with alkali cations in acetonitrile. Structure of the complexes and binding selectivity

We present experimental and theoretical studies on new ionophores (L) which possess a high complexation ability for Li+ or Na+ cations. Four tri-podands (R-1-O-C2H4-)(3)N [R-1 = -CH2-P(O)Ph-2(P1), -C2H4-P(O)Ph-2 (P2), -o-C6H4P(O)Ph-2 (P3) and -o-C6H4-CH2-P(O)Ph-2 (P4)], one bi-podand (R-2-O-C2H4-)(2)N-CH3 [R-2 = -o-C6H4-CH2-P(O)Ph-2 (P5)] and one mono-podand [R-2-O-(CH2-CH2-O)(3)-R-2 (P6)] containing phosphine oxide terminal groups have been synthesised, Stability constants, enthalpies and entropies of their complexation with lithium, sodium and potassium thiocyanates have been determined in acetonitrile at 298 K by a calorimetric titration technique. We find that tri-podands form a variety of complexes [(M+)(3)L, (M+)(2)L, M+L and M+L2)], whereas the bi-and mono-podand form only M+L complexes with Li+ and Na+, and M+L and M+L, complexes with K+. Formation of poly-nuclear (M+)(n)L complexes of tri-podands in solution has been confirmed by electro-spray mass spectrometry, At relatively small concentrations of the ligand (C-L(0)), P1 binds Na+ much better than Li+, whereas P4 and P5 display a remarkable Li+/Na+ selectivity; at large C-L(0),the complexation selectivity decreases. X-Ray diffraction studies performed on monocrystals of complexes of NaNCS with tri-podands P2 and P3 show that Na+ is encapsulated inside a 'basket-like' pseudocavity, coordinating all donor atoms of the tri-podand, Molecular dynamics simulations on P2, P3 and P4 and on their 1:1 complexes with MC in acetonitrile solution suggest that the structures of M+L complexes in solution are similar to those found for P2 and P3 complexes in the solid state.