Synthesis and Reduction of Heteroleptic Bis(cyclopentadienyl) Uranium(III) Complexes

Heteroleptic U­(III) complexes supported by bis­(cyclopentadienyl) frameworks have been synthesized to examine their suitability as precursors to U­(II) complexes. The newly synthesized (C5Me5)2U­(OC6H2 tBu2-2,6-Me-4), (C5Me5)2U­(OC6H2Ad2-2,6-tBu-4) (Ad = 1-adamantyl), (C5Me5)2U­(C5H5), and (C5Me5)2U­(C5Me4H) are compared with (C5Me5)2U­[N­(SiMe3)2], (C5Me5)2U­[CH­(SiMe3)2], and (C5Me5)­U­[N­(SiMe3)2]2. An improved synthesis of (C5Me5)2U­(μ-Ph)2BPh2 was developed, which was used to synthesize (C5Me5)2U­(C5Me4H). Since the X-ray crystal structure of (C5Me5)2U­(OC6H2 tBu2-2,6-Me-4) contained two very different molecules in the asymmetric unit with 115.7(5)° and 166.0(5)° U–O–Cipso angles, the (C5Me4H)2U­(OC6H2 tBu2-2,6-Me-4) and (C5Me5)2Ce­(OC6H2 tBu2-2,6-Me-4) analogues were synthesized and characterized by X-ray diffraction for comparison. Electrochemical studies in THF with a 100 mM [ n Bu4N]­[BPh4] supporting electrolyte showed U­(IV)/U­(III) and U­(III)/U­(II) redox couples for all the heteroleptic complexes except (C5Me5)2U­(C5H5). Chemical reduction of all heteroleptic compounds formed dark blue solutions characteristic of U­(II) when reacted with KC8 at −78 °C, but none formed isolable U­(II) complexes. The targeted U­(II) complexes, [(C5Me5)2U­(OC6H2 tBu2-2,6-Me-4)]1–, {(C5Me5)2U­[CH­(SiMe3)2]}1–, [(C5Me5)2U­(C5H5)]1–, and [(C5Me5)2U­(C5Me4H)]1–, were analyzed by density functional theory, and a 5f36d1 electron configuration was found to be the ground state in each case.