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

Abstract

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 ⁰_L), P1 binds Na⁺ much better than Li⁺, whereas P4 and P5 display a remarkable Li⁺/Na⁺ selectivity; at large C⁰_L 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.