Complexation properties of heteroditopic cryptands towards Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ in aqueous solution: crystal structures of [(H₅L1)(ClO₄)₅]^.4 H₂O and [(NiL₂Cl)Cl]·5.5 H₂O·CH₃OH

Abstract

The protonation and metal ion (Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺) complexation abilities of three heteroditopic cryptands in solution are described and the relevant equilibrium constants are reported. The two distinct binding subunits of the ligands act cooperatively in the coordination of M(H₂O)²⁺ (M = metal ion) species. The cryptand L1 binds five protons in very acidic solution and crystallizes as [(H₅L1)(ClO₄)₅].4 H₂O in the monoclinic space group P2₁ln with a = 14.281(4), b = 14.313(7), c = 24.082(8) Å, β = 92.00(2)°, Z = 4, R = 0.079, and wR2 = 0.2010. The molecule shows an endo-endo conformation with a pseudo-threefold symmetry axis passing through the two bridgehead nitrogen atoms. All the cryptands (L1-L3) readily form inclusion complexes with Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ ions at the tren end of the cavity. The crystal structure of [(NiL2Cl)Cl].5.5 H₂O.CH₃OH is also reported [C2/c, a = 26.671(6), b = 16.784(4), c = 18.591(8) Å, β = 101.006(14)°, Z = 8, R = 0.091, and wR2 = 0.1424]. The Ni^II ion resides in a trigonal-bipyramidal coordination environment with the equatorial sites occupied by the three secondary nitrogen atoms of the cryptand and the axial sites occupied by a bridgehead nitrogen atom and a Cl^- ion.