Unexpected structural complexity of thorium coordination polymers and polyoxo cluster built from simple formate ligands

Abstract

A simple binary synthetic approach with variable [HCOOH]/[Th(IV)] ratios and the addition of water resulted in the self-assembly of six novel thorium formate complexes with well-controlled structures ranging from 0D clusters and 2D layered networks to 3D frameworks. Specifically, small [HCOOH]/[Th(IV)] promoted the formation of the hexanuclear Th₆O₄(OH)₄(HCOO)₁₂ core, a secondary building unit for the construction of 3D [Th₆Na(μ₃-O)₄(μ₃-OH)₄(HCOO)₁₄]·[NH₂(CH₃)₂]·(DMF)₃(H₂O)₅ (Th-SINAP-1), 2D [Th₆(μ₃-O)₄(μ₃-OH)₄(HCOO)₁₂(DMF)₂]·(H₂O)₁₀ (Th-SINAP-2), and 0D [Th₆(μ₃-O)₄(μ₃-OH)₄(HCOO)₁₂(H₂O)₆]·G (Th-SINAP-3). The large [HCOOH]/[Th(IV)] inhibited the olation/oxolation reaction, affording 3D frameworks of [Th₂(HCOO)₉]·[NH₂(CH₃)₂]·(DMF)(H₂O)₂ (Th-SINAP-4), [Th₂(HCOO)₉]·[NH₂(CH₃)₂]·(DMF)(H₂O)₄ (Th-SINAP-5), and [Th₂(HCOO)₈(H₂O)(DMF)]·(H₂O)₃ (Th-SINAP-6) with monomeric Th(IV) cations as the metal nodes. The addition of water not only affected the hydrolysis of Th(IV) to form nucleophilic hydroxo/oxo-aquo Th species, but also influenced the overall solid-state packing via the competition between H₂O, HCOO⁻, OH⁻/O²⁻, and DMF for coordination with Th⁴⁺ cations. This unexpected structural complexity of thorium formates opens a new avenue for the targeted design and synthesis of atypical coordination polymers, e.g. open-framework materials of actinides, for applications in diverse fields such as radionuclide adsorption.