Solvent dependent assembly of lanthanide metallacrowns using building blocks with incompatible symmetry preferences

Solvent dependence in the assembly of coordination driven macrocycles is a poorly understood phenomenon. This work presents the solvent dependent assembly of 8 lanthanide metallacrowns (LnMCs) in solution using picoline hydroxamic acid (picHA), Zn(II), and Ln(III) ions. ESI-MS and single-crystal X-ray crystallography reveal the selective assembly of LnZn4(picHA)4(3+), LnZn5(picHA)5(3+), LnZn8(picHA)8(3+), LnZn12(picHA)12(3+), LnZn16(picHA)16(3+), Ln2Zn3(picHA)4(4+), Ln2Zn7-9(picHA)8-10, and Ln4Zn4-5(picHA)8-9 complexes in five different solvents. The coordination preferences of the hard Ln(III) ion and relatively soft Zn(II) ion dictate the solvent selectivity in this system. The LnMCs assemble with open or closed Zn(II) and/or Ln(III) coordination sites based on the behavior of the solvent as an ancillary ligand. This structural promiscuity is attributed to the symmetry incompatible building blocks, which generate assemblies with substantial geometric strain such that no clear thermodynamic minimum exists between the different LnMCs. These LnMCs assemble from a Zn5(picHA)4(2+) intermediate, which is monitored using (1)H NMR and ESI-MS to assess the stability of the complexes and possible assembly pathways based on kinetic considerations. LnMC assemblies that can be generated through central metal substitution reactions such as the LnZn4(picHA)4(3+), LnZn5(picHA)5(3+), and LnZn8(picHA)8(3+) effectively reach equilibrium after 24 h at room temperature. In contrast, LnMCs that must disrupt the Zn5L4(2+) structure to assemble, such as the LnZn16L16(3+), reach equilibrium after heating for 24 h at 65 °C. A pathway for LnMC assembly is presented where the Zn5L4(2+) is the key intermediate based on these reaction data and shared structural motifs in the complexes. These results correlate solvent dependent assembly to the building block geometry, highlighting synthetic approaches for generating novel complexes.