Synthesis and characterization of phosphine adducts of thorium borohydride, Th(BH4)4: Crystal structures of Th(BH4)4(PEt3)2 and Th(BH4)4(Me2PCH2CH2PMe2)2

Addition of tertiary phosphines to Th(BH4)4(Et2O)2 yields the new Lewis base adducts, Th(BH4)4(PMe3)2, Th(BH4)4(PEt3)2, and Th(BH4)4(dmpe)2, where dmpe = 1,2-bis(dimethylphosphino)ethane. If one considers the BH4− groups to occupy one coordination site, then Th(BH4)4(PEt3)2 adopts a trans-octahedral geometry, and Th(BH4)4(dmpe)2 adopts a trigonal dodecahedral geometry with the dmpe ligands bridging between the “inner” sites. In the PMe3 and PEt3 complexes, all four BH4− groups are κ3, whereas in the dmpe complex two of the BH4− groups are κ2 and two are κ3. In the dmpe complex, the Th⋯B distances to the κ2 and κ3BH4− groups are 2.91 and 2.69 Å, respectively. All of the Lewis base adducts of Th(BH4)4 are volatile and may be sublimed in vacuum. They have been characterized by infrared and 1H, 11B, and 31P NMR spectroscopy. The results show that thorium complexes of unidentate phosphines can be made and are stable enough to isolate and characterize. 31P NMR coordination chemical shifts of thorium phosphine complexes are on the order of 30–45 ppm. The compound Th(BH4)4(dmpe)2 is the first thorium complex to contain κ2BH4− groups.

Three phosphine complexes of thorium(IV) borohydride have been prepared – Th(BH4)4(PMe3)2, Th(BH4)4(PEt3)2, and Th(BH4)4(dmpe)2, where dmpe = 1,2-bis(dimethylphosphino)ethane, of which two have been structurally characterized. The results include the first thorium complex of a unidentate phosphine, and the first thorium complex of a bidentate borohydride ligand.Figure optionsDownload as PowerPoint slideResearch highlights
► Surprisingly strong interaction of thorium(IV) with a unidentate phosphine.
► Preparation of thorium complexes with coordination numbers as high as 14.
► First example of bidentate BH4 groups in thorium chemistry.
► Thorium borohydrides are difficult to convert into hydrides by treatment with phosphines.