Synthesis and Structure of Heteroleptic [C₅Me₄(SiMe₂^<i>t</i>Bu)]^1– Uranium(III) Complexes

The U(III) metallocene chemistry of the [C5Me4(SiMe2tBu)]1– ligand was explored for comparison with that of (C5Me5)1–. Reactions between UI3, UI3(dioxane)1.5, or UI3(THF)4 and 2 equiv of LiC5Me4(SiMe2tBu) in THF yielded the dihalide salt, [Li(THF)4]{[C5Me4(SiMe2tBu)]2UI2}, rather than a neutral metallocene monohalide. In order to obtain neutral and alkali-metal-free complexes comparable to known uranium(III) compounds with (C5Me5)1– ligands, the potassium salt, KC5Me4(SiMe2tBu), was synthesized. The neutral mono- and bis-cyclopentadienyl complexes, [C5Me4(SiMe2tBu)]UI2(THF)3 and [C5Me4(SiMe2tBu)]2UI(THF), were generated from the reactions between UI3(dioxane)1.5 and 1 and 2 equiv of KC5Me4(SiMe2tBu) in THF, respectively. [C5Me4(SiMe2tBu)]2UI(THF) readily oxidizes to the U(IV) complex, [C5Me4(SiMe2tBu)]2UI2, which can also be made directly from UI4(dioxane)2 and 2 equiv of KC5Me4(SiMe2tBu). The borohydride analog, [C5Me4(SiMe2tBu)]2U(BH4)(THF), was obtained starting from U(BH4)3(THF)2 and is less reducing and less air-sensitive than its iodide analog, [C5Me4(SiMe2tBu)]2UI(THF). The alkyl derivative, [C5Me4(SiMe2tBu)]2U[CH(SiMe3)2], can be synthesized from [C5Me4(SiMe2tBu)]2UI(THF) and 0.5 equiv of LiCH(SiMe3)2 in diethyl ether, but [C5Me4(SiMe2tBu)]2U(μ-I)2Li(THF)2 is also formed as a byproduct. The ansa-metallocene uranium complex, [Me4C5(SiMe2)O(SiMe2)C5Me4]UI(THF), was crystallographically identified from a reaction containing a partially hydrolyzed sample of the [C5Me4(SiMe2tBu)]1– ligand. In general, uranium complexes of the [C5Me4(SiMe2tBu)]1– ligand are more soluble in organic solvents than the analogous (C5Me5)1– complexes.