Investigating Steric and Electronic Effects in the Synthesis of Square Planar 6d 1 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 = OC H Bu -2,6-Me-4), were explored by synthetic and reduction studies of a series of related Th(IV) tetrakis(aryloxide) complexes, Th(OAr ) (OAr = OC H Bu -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 = OC H Bu -2,6), Th(OAr ) (OAr = OC H Bu -2,4,6), Th(OAr ) (OAr = OC H Bu -2,6-OMe-4), and Th(OAr ) (OAr = OC H Bu -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 = OC H Pr -2,6) nor Th(Odmp) (THF) (Odmp = OC H Me -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.