Thorium(IV) and Uranium(IV)
Halide Complexes Supported by Bulky β‑Diketiminate
Ligands
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Abstract
The coordination behavior of the bulky β-diketiminate
ligands <i>N</i>,<i>N</i>′-bis(2,6-diisopropylphenyl)pentane-2,4-diiminate
(L<sup>Me</sup>) and <i>N</i>,<i>N</i>′-bis(2,6-diisopropylphenyl)-2,2–6,6-tetramethylheptane-3,5-diiminate
(L<sup>tBu</sup>) toward ThX<sub>4</sub>(THF)<sub>4</sub> (X = Br,
I) and UCl<sub>4</sub> has been investigated. The reaction between
K[L<sup>Me</sup>] and ThX<sub>4</sub>(THF)<sub>4</sub> (X = Br, I)
afforded the mono(β-diketiminate)thorium(IV) halide complexes
(L<sup>Me</sup>)ThX<sub>3</sub>(THF) (X = Br (<b>7</b>), I (<b>8</b>)). The same reaction carried out with the more sterically
demanding K[L<sup>tBu</sup>] gave (L<sup>tBu</sup>)ThBr<sub>3</sub>(THF) (<b>9</b>) and (L<sup>tBu</sup>)ThI<sub>3</sub> (<b>11</b>). All attempts to install two β-diketiminate ligands
on thorium(IV) were unsuccessful, giving the mono(β-diketiminate)thorium(IV)
halide complex and unreacted K[L<sup>Me</sup>] or K[L<sup>tBu</sup>]. However, complex <b>9</b> was shown to react with smaller
anions such as K[C<sub>5</sub>H<sub>4</sub>Me] to give the mixed-ligand
methylcyclopentadienyl β-diketiminate complex (L<sup>tBu</sup>)Th(C<sub>5</sub>H<sub>4</sub>Me)Br<sub>2</sub> (<b>10</b>).
Complexes <b>7</b>–<b>11</b> represent rare examples
of thorium complexes featuring only one β-diketiminate ligand,
and complexes <b>9</b>–<b>11</b> are the first
examples of thorium and halide complexes supported by the L<sup>tBu</sup> framework. In a similar manner, both K[L<sup>Me</sup>] and K[L<sup>tBu</sup>] were shown to react with UCl<sub>4</sub> to give the corresponding
mono(β-diketiminate)uranium(IV) chloride complexes (L<sup>Me</sup>)UCl<sub>3</sub>(THF) (<b>12</b>) and (L<sup>tBu</sup>)UCl<sub>3</sub> (<b>13</b>). Complex <b>13</b> represents the
first example of a uranium complex featuring the L<sup>tBu</sup> framework.
Efforts to prepare the bis(β-diketiminate)uranium(IV) complex
(L<sup>Me</sup>)<sub>2</sub>UCl<sub>2</sub> by reacting 2 equiv of
K[L<sup>Me</sup>] with UCl<sub>4</sub> led instead to the interesting
cationic diuranium complex [{(L<sup>Me</sup>)(Cl)U}<sub>2</sub>(μ-Cl)<sub>3</sub>][Cl] (<b>14</b>). Complexes <b>7</b>–<b>14</b> have been characterized by a combination of <sup>1</sup>H and <sup>13</sup>C{<sup>1</sup>H} NMR spectroscopy, elemental analysis,
electrochemistry, and UV–visible–near-IR spectroscopy.
Several complexes have also been characterized by X-ray crystallography,
and a discussion of their structures is presented. NMR spectroscopy
and the X-ray structures demonstrate that the β-diketiminate
ligand is symmetrically bound to the actinide metal in the L<sup>Me</sup> complexes and is asymmetrically bound to the actinide metal in the
L<sup>tBu</sup> complexes. In all cases the actinide(IV) metal centers
lie out of the plane of the β-diketiminate ligand NCCCN backbone
by ∼1–2 Å. The electronic spectroscopy data on
K[L<sup>Me</sup>], (L<sup>Me</sup>)ThI<sub>3</sub>(THF) (<b>8</b>), and (L<sup>Me</sup>)UCl<sub>3</sub>(THF) (<b>12</b>) suggest
relatively weak metal–(β-diketiminate) ligand bonding
interactions, although small perturbations in the characteristics
of the β-diketiminate π–π* bands with changes
in the the metal ion are consistent with some metal–ligand
orbital interactions. This new class of mono(β-diketiminate)thorium
and -uranium halide complexes promises to provide a robust platform
for developing new chemistry of the actinides