Four dipicolylamine (DPA) derivatives bearing methylphosphonic or methylphosphinic acid (P–R; R = H, Me, CH2PO2H2) groups were synthesized. Their acid–base and coordination properties were studied by potentiometry, UV-Vis and NMR measurements. The phosphonate derivative shows increased basicity (log K1 = 8.39), whereas the phosphinate derivatives show decreased basicity (log K1 = 6–7) compared to the parent dipicolylamine. Consequently, the stability constants of the phosphonate complexes are 3 to 4 orders of magnitude higher than those of the phosphinate complexes. All ligands show excellent selectivity for Zn(II) over Ca(II) and Mg(II) ions. The structures of several Cu(II) and Ni(II) complexes in the solid state were determined by X-ray diffraction analysis. The complexes mostly show dimeric or polymeric structures and the two metal ions induce a different coordination geometry of the DPA group. The coordination geometry is always (pseudo)octahedral. The DPA fragment is bound in mer geometry in the Cu(II) complexes, whereas the Ni(II) complexes have fac geometry. In conclusion, the phosphonate and phosphinate derivatives of DPA are efficient complexing agents for divalent transition metal ions and the DPA-phosphinate grouping is a suitable fragment for sensing Zn(II) ions.
A recent study (Sci. Adv. 2017, 3, e1602833) has shown that FH⋅⋅⋅OH2 hydrogen bond in a HF⋅H2O pair substantially shortens, and the H−F bond elongates upon encapsulation of the cluster in C70 fullerene. This has been attributed to compression of the HF⋅H2O pair inside the cavity of C70. Herein, we present theoretical evidence that the effect is not caused by a mere compression of the H2O⋅HF pair, but it is related to a strong lone-pair–π (LP–π) bonding with the fullerene cage. To support this argument, a systematic electronic structure study of selected small molecules (HF, H2O, and NH3) and their pairs enclosed in fullerene cages (C60, C70, and C90) has been performed. Bonding analysis revealed unique LP–πcage interactions with a charge-depletion character in the bonding region, unlike usual LP–π bonds. The LP–πcage interactions were found to be responsible for elongation of the H−F bond. Thus, the HF appears to be more acidic inside the cage. The shortening of the FH⋅⋅⋅OH2 contact in (HF⋅H2O)@C70 originates from an increased acidity of the HF inside the fullerenes. Such trends were also observed in other studied systems.