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 rhodium­(I) and iridium­(I) compounds, carbene–bis­(oxazolinyl)­phenylborate proligands have been synthesized and reacted with organometallic precursors. Two proligands, PhB­(Ox^Me2)₂(Im^<i>t</i>BuH) (H­[<b>1</b>]; Ox^Me2 = 4,4-dimethyl-2-oxazoline; Im^<i>t</i>BuH = 1-<i>tert</i>-butylimidazole) and PhB­(Ox^Me2)₂(Im^MesH) (H­[<b>2</b>]; Im^MesH = 1-mesitylimidazole), are deprotonated with potassium benzyl to generate K­[<b>1</b>] and K­[<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 {PhB­(Ox^Me2)₂Im^<i>t</i>Bu}­Rh­(η⁴-C₈H₁₂) (<b>3</b>), {PhB­(Ox^Me2)₂Im^Mes}­Rh­(η⁴-C₈H₁₂) (<b>4</b>), {PhB­(Ox^Me2)₂Im^Mes}­Rh­(CO)₂ (<b>5</b>), {PhB­(Ox^Me2)₂Im^Mes}­Ir­(η⁴-C₈H₁₂) (<b>6</b>), and {PhB­(Ox^Me2)₂Im^Mes}­Ir­(CO)₂ (<b>7</b>) are synthesized along with To^MM­(η⁴-C₈H₁₂) (M = Rh (<b>8</b>); M = Ir (<b>9</b>); To^M = tris­(4,4-dimethyl-2-oxazolinyl)­phenylborate). 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^M with a carbene donor, and the influence of the donor ligand (CO vs C₈H₁₂). The reactions of K­[<b>2</b>] and [M­(μ-Cl)­(η²-C₈H₁₄)₂]₂ (M = Rh, Ir) provide {κ⁴-PhB­(Ox^Me2)­₂Im^{Mes^′}CH₂}­Rh­(μ-H)­(μ-Cl)­Rh­(η²-C₈H₁₄)₂ (<b>10</b>) and {PhB­(Ox^Me2)₂Im^Mes}­IrH­(η³-C₈H₁₃) (<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 {κ⁴-PhB­(Ox^Me2)₂Im^{Mes^′}CH₂}­RhH­(CO) (<b>12</b>) and {PhB­(Ox^Me2)₂Im^Mes}­IrH­(Ph)­CO (<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]­(H)­(Ph) complex. Alternatively, the rhodium carbonyl <b>5</b> or iridium isocyanide {PhB­(Ox^Me2)₂Im^Mes}­Ir­(CO)­CN^<i>t</i>Bu (<b>15</b>) reacts with PhSiH₃ in the dark to form the silyl compound {PhB­(Ox^Me2)₂Im^Mes}­RhH­(SiH₂Ph)­CO (<b>14</b>) or {PhB­(Ox^Me2)₂Im^Mes}­IrH­(SiH₂Ph)­CN^<i>t</i>Bu (<b>17</b>). These examples demonstrate the enhanced thermal reactivity of {PhB­(Ox^Me2)₂Im^Mes}-supported iridium and rhodium carbonyl compounds in comparison to tris­(oxazolinyl)­borate, tris­(pyrazolyl)­borate, and cyclopentadienyl-supported compounds

Organometallic Complexes of Bulky, Optically Active, <i>C</i>₃‑Symmetric Tris(4<i>S</i>‑isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (To^P*)

A bulky, optically active monoanionic scorpionate ligand, tris­(4<i>S</i>-isopropyl-5,5-dimethyl-2-oxazolinyl)­phenylborate (To^P*), is synthesized from the naturally occurring amino acid l-valine as its lithium salt, Li­[To^P*] (<b>1</b>). That compound is readily converted to the thallium complex Tl­[To^P*] (<b>2</b>) and to the acid derivative H­[To^P*] (<b>3</b>). Group 7 tricarbonyl complexes To^P*M­(CO)₃ (M = Mn (<b>4</b>), Re (<b>5</b>)) are synthesized by the reaction of MBr­(CO)₅ and Li­[To^P*] and are crystallographically characterized. The ν_CO bands in their infrared spectra indicate that π back-donation in the rhenium compounds is greater with To^P* than with non-methylated tris­(4<i>S</i>-isopropyl-2-oxazolinyl)­phenylborate (To^P). The reaction of H­[To^P*] and ZnEt₂ gives To^P*ZnEt (<b>6</b>), while To^P*ZnCl (<b>7</b>) is synthesized from Li­[To^P*] and ZnCl₂. The reaction of To^P*ZnCl and KO<i>t</i>Bu followed by addition of PhSiH₃ provides the zinc hydride complex To^P*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