2 research outputs found
Mixed N‑Heterocyclic Carbene–Bis(oxazolinyl)borato Rhodium and Iridium Complexes in Photochemical and Thermal Oxidative Addition Reactions
In
order to facilitate oxidative addition chemistry of <i>fac</i>-coordinated rhodiumÂ(I) and iridiumÂ(I) compounds, carbene–bisÂ(oxazolinyl)Âphenylborate
proligands have been synthesized and reacted with organometallic precursors.
Two proligands, PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>(Im<sup><i>t</i>Bu</sup>H) (HÂ[<b>1</b>]; Ox<sup>Me2</sup> = 4,4-dimethyl-2-oxazoline;
Im<sup><i>t</i>Bu</sup>H = 1-<i>tert</i>-butylimidazole)
and PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>(Im<sup>Mes</sup>H) (HÂ[<b>2</b>]; Im<sup>Mes</sup>H = 1-mesitylimidazole), are deprotonated
with potassium benzyl to generate KÂ[<b>1</b>] and KÂ[<b>2</b>], and these potassium compounds serve as reagents for the synthesis
of a series of rhodium and iridium complexes. Cyclooctadiene and dicarbonyl
compounds {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup><i>t</i>Bu</sup>}ÂRhÂ(η<sup>4</sup>-C<sub>8</sub>H<sub>12</sub>) (<b>3</b>), {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂRhÂ(η<sup>4</sup>-C<sub>8</sub>H<sub>12</sub>) (<b>4</b>), {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂRhÂ(CO)<sub>2</sub> (<b>5</b>), {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂIrÂ(η<sup>4</sup>-C<sub>8</sub>H<sub>12</sub>) (<b>6</b>), and {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂIrÂ(CO)<sub>2</sub> (<b>7</b>) are synthesized along with To<sup>M</sup>MÂ(η<sup>4</sup>-C<sub>8</sub>H<sub>12</sub>) (M = Rh (<b>8</b>); M
= Ir (<b>9</b>); To<sup>M</sup> = trisÂ(4,4-dimethyl-2-oxazolinyl)Âphenylborate).
The spectroscopic and structural properties and reactivity of this
series of compounds show electronic and steric effects of substituents
on the imidazole (<i>tert</i>-butyl vs mesityl), effects
of replacing an oxazoline in To<sup>M</sup> with a carbene donor,
and the influence of the donor ligand (CO vs C<sub>8</sub>H<sub>12</sub>). The reactions of KÂ[<b>2</b>] and [MÂ(ÎĽ-Cl)Â(η<sup>2</sup>-C<sub>8</sub>H<sub>14</sub>)<sub>2</sub>]<sub>2</sub> (M
= Rh, Ir) provide {Îş<sup>4</sup>-PhBÂ(Ox<sup>Me2</sup>)Â<sub>2</sub>Im<sup>Mes<sup>′</sup></sup>CH<sub>2</sub>}ÂRhÂ(ÎĽ-H)Â(ÎĽ-Cl)ÂRhÂ(η<sup>2</sup>-C<sub>8</sub>H<sub>14</sub>)<sub>2</sub> (<b>10</b>) and {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂIrHÂ(η<sup>3</sup>-C<sub>8</sub>H<sub>13</sub>) (<b>11</b>). In the former
compound, a spontaneous oxidative addition of a mesityl <i>ortho</i>-methyl to give a mixed-valent dirhodium species is observed, while
the iridium compound forms a monometallic allyl hydride. Photochemical
reactions of dicarbonyl compounds <b>5</b> and <b>7</b> result in C–H bond oxidative addition providing the compounds {Îş<sup>4</sup>-PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes<sup>′</sup></sup>CH<sub>2</sub>}ÂRhHÂ(CO) (<b>12</b>) and {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂIrHÂ(Ph)ÂCO (<b>13</b>).
In <b>12</b>, oxidative addition results in cyclometalation
of the mesityl <i>ortho</i>-methyl similar to <b>10</b>, whereas the iridium compound reacts with the benzene solvent to
give a rare crystallographically characterized <i>cis</i>-[Ir]Â(H)Â(Ph) complex. Alternatively, the rhodium carbonyl <b>5</b> or iridium isocyanide {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂIrÂ(CO)ÂCN<sup><i>t</i></sup>Bu (<b>15</b>)
reacts with PhSiH<sub>3</sub> in the dark to form the silyl compound
{PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂRhHÂ(SiH<sub>2</sub>Ph)ÂCO (<b>14</b>) or {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}ÂIrHÂ(SiH<sub>2</sub>Ph)ÂCN<sup><i>t</i></sup>Bu (<b>17</b>). These examples demonstrate the enhanced
thermal reactivity of {PhBÂ(Ox<sup>Me2</sup>)<sub>2</sub>Im<sup>Mes</sup>}-supported iridium and rhodium carbonyl compounds in comparison to trisÂ(oxazolinyl)Âborate,
trisÂ(pyrazolyl)Âborate, and cyclopentadienyl-supported compounds
Organometallic Complexes of Bulky, Optically Active, <i>C</i><sub>3</sub>‑Symmetric Tris(4<i>S</i>‑isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (To<sup>P</sup>*)
A bulky, optically active monoanionic
scorpionate ligand, trisÂ(4<i>S</i>-isopropyl-5,5-dimethyl-2-oxazolinyl)Âphenylborate
(To<sup>P</sup>*), is synthesized from the naturally occurring amino
acid l-valine as its lithium salt, LiÂ[To<sup>P</sup>*] (<b>1</b>). That compound is readily converted to the thallium complex
TlÂ[To<sup>P</sup>*] (<b>2</b>) and to the acid derivative HÂ[To<sup>P</sup>*] (<b>3</b>). Group 7 tricarbonyl complexes To<sup>P</sup>*MÂ(CO)<sub>3</sub> (M = Mn (<b>4</b>), Re (<b>5</b>))
are synthesized by the reaction of MBrÂ(CO)<sub>5</sub> and LiÂ[To<sup>P</sup>*] and are crystallographically characterized. The ν<sub>CO</sub> bands in their infrared spectra indicate that Ď€ back-donation
in the rhenium compounds is greater with To<sup>P</sup>* than with
non-methylated trisÂ(4<i>S</i>-isopropyl-2-oxazolinyl)Âphenylborate
(To<sup>P</sup>). The reaction of HÂ[To<sup>P</sup>*] and ZnEt<sub>2</sub> gives To<sup>P</sup>*ZnEt (<b>6</b>), while To<sup>P</sup>*ZnCl (<b>7</b>) is synthesized from LiÂ[To<sup>P</sup>*] and ZnCl<sub>2</sub>. The reaction of To<sup>P</sup>*ZnCl and
KO<i>t</i>Bu followed by addition of PhSiH<sub>3</sub> provides
the zinc hydride complex To<sup>P</sup>*ZnH (<b>8</b>). Compound <b>8</b> is the first example of a crystallographically characterized
optically active zinc hydride. We tested its catalytic reactivity
in the cross-dehydrocoupling of silanes and alcohols, which provided
Si-chiral silanes with moderate enantioselectivity