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)43+, LnZn5(picHA)53+,
LnZn8(picHA)83+, LnZn12(picHA)123+, LnZn16(picHA)163+, Ln2Zn3(picHA)44+, 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)42+ intermediate, which is monitored using 1H 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)43+, LnZn5(picHA)53+, and LnZn8(picHA)83+ effectively reach equilibrium after 24 h at room temperature.
In contrast, LnMCs that must disrupt the Zn5L42+ structure to assemble, such as the LnZn16L163+, reach equilibrium after heating for
24 h at 65 °C. A pathway for LnMC assembly is presented where
the Zn5L42+ 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.