Synthesis and characterization of phosphine adducts of thorium borohydride, Th(BH 4) 4: Crystal structures of Th(BH 4) 4(PEt 3) 2 and Th(BH 4) 4(Me 2PCH 2CH 2PMe 2) 2

Abstract

Addition of tertiary phosphines to Th(BH 4) 4(Et 2O) 2 yields the new Lewis base adducts, Th(BH 4) 4(PMe 3) 2, Th(BH 4) 4(PEt 3) 2, and Th(BH 4) 4(dmpe) 2, where dmpe = 1,2-bis(dimethylphosphino)ethane. If one considers the BH 4 − groups to occupy one coordination site, then Th(BH 4) 4(PEt 3) 2 adopts a trans-octahedral geometry, and Th(BH 4) 4(dmpe) 2 adopts a trigonal dodecahedral geometry with the dmpe ligands bridging between the “inner” sites. In the PMe 3 and PEt 3 complexes, all four BH 4 − groups are κ 3, whereas in the dmpe complex two of the BH 4 − groups are κ 2 and two are κ 3. In the dmpe complex, the Th⋯B distances to the κ 2 and κ 3 BH 4 − groups are 2.91 and 2.69 Å, respectively. All of the Lewis base adducts of Th(BH 4) 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(BH 4) 4(dmpe) 2 is the first thorium complex to contain κ 2 BH 4 − groups.