Investigating Steric and Electronic Effects in the Synthesis of Square Planar 6d Th(III) Complexes.

The factors affecting the formation and crystal structures of unusual 6d Th(III) square planar aryloxide complexes, as exemplified by [Th(OAr)] (OAr = OCHBu-2,6-Me-4), were explored by synthetic and reduction studies of a series of related Th(IV) tetrakis(aryloxide) complexes, Th(OAr) (OAr = OCHBu-2,6-R-4). Specifically, electronic, steric, and countercation effects were explored by varying the aryloxide ligand, the alkali metal reducing agent, and the alkali metal chelating agent. Salt metathesis reactions between ThBr(DME) (DME = 1,2-dimethoxyethane) and 4 equiv of the appropriate potassium aryloxide salt were used to prepare a series of Th(IV) aryloxide complexes in high yields: Th(OAr) (OAr = OCHBu-2,6), Th(OAr) (OAr = OCHBu-2,4,6), Th(OAr) (OAr = OCHBu-2,6-OMe-4), and Th(OAr) (OAr = OCHBu-2,6-Ph-4). Th(OAr) can be reduced by KC, Na, or Li in the absence or presence of 2.2.2-cryptand (crypt) or 18-crown-6 (crown) to form dark purple solutions that have EPR and UV-visible spectra similar to those of the square planar Th(III) complex, [Th(OAr)]. Hence, the position of the aryloxide ligand does not have to be alkylated to obtain the Th(III) complexes. Furthermore, reduction of Th(OAr), Th(OAr), and Th(OAr) with KC in THF generated purple solutions with EPR and UV-visible spectra that are similar to those of the previously reported Th(III) anion, [Th(OAr)]. Although many of these reduction reactions did not produce single crystals suitable for study by X-ray diffraction, reduction of Th(OAr), Th(OAr), and Th(OAr) with Li provided X-ray quality crystals whose structures had square planar coordination geometries. Reduction of Th(OAr) with Li also gave a product with EPR and UV-visible spectra that matched those of [Th(OAr)], but X-ray quality crystals of the reduction product were too unstable to provide data. Neither Th(Odipp)(THF) (Odipp = OCHPr-2,6) nor Th(Odmp)(THF) (Odmp = OCHMe-2,6) could be reduced to Th(III) products under similar conditions. Reduction of U(OAr)(THF) with KC in the presence of 2.2.2-cryptand (crypt) was examined for comparison and formed [K(crypt)][U(OAr)], which has a tetrahedral arrangement of the aryloxide ligands. Moreover, no further reduction was observed when either [K(crypt)][U(OAr)] or [K(crown)(THF)][U(OAr)] were treated with KC or Li.