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 (R1–O–C2H4–)3N [R1 = –CH2–P(O)Ph2 (P1), –C2H4– P(O)Ph2 (P2), -o-C6H4P(O)Ph2 (P3) and -o-C6H4–CH2–P(O)Ph2 (P4)], one bi-podand (R2–O–C2H4–)2N–CH3 [R2 = -o-C6H4–CH2–P(O)Ph2 (P5)] and one mono-podand [R2–O–(CH2–CH2–O)3–R2 (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+)3L, (M+)2L, 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+L2 complexes with K+. Formation of poly-nuclear (M+)nL complexes of tri-podands in solution has been confirmed by electro-spray mass spectrometry. At relatively small concentrations of the ligand (C 0L), P1 binds Na+ much better than Li+, whereas P4 and P5 display a remarkable Li+/Na+ selectivity; at large C0L 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 M+ 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.