Investigating Steric and Electronic Effects in the Synthesis of Square Planar 6d¹ Th(III) Complexes

The factors affecting the formation and crystal structures of unusual 6d¹ Th(III) square planar aryloxide complexes, as exemplified by [Th(OAr^Me)₄]^1- (OAr^Me = OC₆H₂Bu₂-2,6-Me-4), were explored by synthetic and reduction studies of a series of related Th(IV) tetrakis(aryloxide) complexes, Th(OAr^R)₄ (OAr^R = OC₆H₂Bu₂-2,6-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(OAr^H)₄ (OAr^H = OC₆H₃Bu₂-2,6), Th(OAr^tBu)₄ (OAr^tBu = OC₆H₂Bu₃-2,4,6), Th(OAr^OMe)₄ (OAr^OMe = OC₆H₂Bu₂-2,6-OMe-4), and Th(OAr^Ph)₄ (OAr^Ph = OC₆H₂Bu₂-2,6-Ph-4). Th(OAr^H)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(OAr^Me)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(OAr^OMe)₄, Th(OAr^tBu)₄, and Th(OAr^Ph)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(OAr^Me)₄]^1-. Although many of these reduction reactions did not produce single crystals suitable for study by X-ray diffraction, reduction of Th(OAr^H)₄, Th(OAr^tBu)₄, and Th(OAr^OMe)₄ with Li provided X-ray quality crystals whose structures had square planar coordination geometries. Reduction of Th(OAr^Ph)₄ with Li also gave a product with EPR and UV-visible spectra that matched those of [Th(OAr^Me)₄]^1-, but X-ray quality crystals of the reduction product were too unstable to provide data. Neither Th(Odipp)₄(THF)₂ (Odipp = OC₆H₃Pr₂-2,6) nor Th(Odmp)₄(THF)₂ (Odmp = OC₆H₃Me₂-2,6) could be reduced to Th(III) products under similar conditions. Reduction of U(OAr^H)₃(THF) with KC₈ in the presence of 2.2.2-cryptand (crypt) was examined for comparison and formed [K(crypt)][U(OAr^H)₄], which has a tetrahedral arrangement of the aryloxide ligands. Moreover, no further reduction was observed when either [K(crypt)][U(OAr^H)₄] or [K(crown)(THF)₂][U(OAr^H)₄] were treated with KC₈ or Li.