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 6d1 Th(III) square planar aryloxide complexes, as exemplified by [Th(OArMe )4 ]1- (OArMe = OC6 H2 t Bu2 -2,6-Me-4), were explored by synthetic and reduction studies of a series of related Th(IV) tetrakis(aryloxide) complexes, Th(OArR )4 (OArR = OC6 H2 t Bu2 -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 ThBr4 (DME)2 (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(OArH )4 (OArH = OC6 H3 t Bu2 -2,6), Th(OArtBu )4 (OArtBu = OC6 H2 t Bu3 -2,4,6), Th(OArOMe )4 (OArOMe = OC6 H2 t Bu2 -2,6-OMe-4), and Th(OArPh )4 (OArPh = OC6 H2 t Bu2 -2,6-Ph-4). Th(OArH )4 can be reduced by KC8 , 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(OArMe )4 ]1- . Hence, the para position of the aryloxide ligand does not have to be alkylated to obtain the Th(III) complexes. Furthermore, reduction of Th(OArOMe )4 , Th(OArtBu )4 , and Th(OArPh )4 with KC8 in THF generated purple solutions with EPR and UV-visible spectra that are similar to those of the previously reported Th(III) anion, [Th(OArMe )4 ]1- . Although many of these reduction reactions did not produce single crystals suitable for study by X-ray diffraction, reduction of Th(OArH )4 , Th(OArtBu )4 , and Th(OArOMe )4 with Li provided X-ray quality crystals whose structures had square planar coordination geometries. Reduction of Th(OArPh )4 with Li also gave a product with EPR and UV-visible spectra that matched those of [Th(OArMe )4 ]1- , but X-ray quality crystals of the reduction product were too unstable to provide data. Neither Th(Odipp)4 (THF)2 (Odipp = OC6 H3 i Pr2 -2,6) nor Th(Odmp)4 (THF)2 (Odmp = OC6 H3 Me2 -2,6) could be reduced to Th(III) products under similar conditions. Reduction of U(OArH )3 (THF) with KC8 in the presence of 2.2.2-cryptand (crypt) was examined for comparison and formed [K(crypt)][U(OArH )4 ], which has a tetrahedral arrangement of the aryloxide ligands. Moreover, no further reduction was observed when either [K(crypt)][U(OArH )4 ] or [K(crown)(THF)2 ][U(OArH )4 ] were treated with KC8 or Li.