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