Why Does Alkylation of the N–H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?

Molecular metal/NH bifunctional Noyori-type catalysts are remarkable in that they are among the most efficient artificial catalysts developed to date for the hydrogenation of carbonyl functionalities (loadings up to ∼10^–5 mol %). In addition, these catalysts typically exhibit high CO/CC chemo- and enantioselectivities. This unique set of properties is traditionally associated with the operation of an unconventional mechanism for homogeneous catalysts in which the chelating ligand plays a key role in facilitating the catalytic reaction and enabling the aforementioned selectivities by delivering/accepting a proton (H⁺) via its N–H bond cleavage/formation. A recently revised mechanism of the Noyori hydrogenation reaction (Dub, P. A. et al. <i>J. Am. Chem. Soc</i>. <b>2014</b>, <i>136</i>, 3505) suggests that the N–H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N–H···O hydrogen-bonding interactions (HBIs). The present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N–H functionality within M/NH bifunctional Noyori-type catalysts leads to detrimental catalytic activity. The purpose of this work is to demonstrate that decreasing the strength of this HBI, ultimately to the limit of its complete absence, are conditions under which the same alkylation may lead to beneficial catalytic activity

A Tertiary Carbon–Iron Bond as an Fe^ICl Synthon and the Reductive Alkylation of Diphosphine-Supported Iron(II) Chloride Complexes to Low-Valent Iron

Ligand-induced reduction of ferrous alkyl complexes via homolytic cleavage of the alkyl fragment was explored with simple chelating diphosphines. The reactivities of the sodium salts of diphenylmethane, phenyl­(trimethylsilyl)­methane, or diphenyl­(trimethylsilyl)­methane were explored in their reactivity with (py)₄FeCl₂. A series of monoalkylated salts of the type (py)₂FeRCl were prepared and characterized from the addition of 1 equiv of the corresponding alkyl sodium species. These complexes are isostructural and have similar magnetic properties. The double alkylation of (py)₄FeCl₂ resulted in the formation of tetrahedral high-spin iron complexes with the sodium salts of diphenylmethane and phenyl­(trimethylsilyl)­methane that readily decomposed. A bis­(cyclohexadienyl) sandwich complex was formed with the addition of 2 equiv of the tertiary alkyl species sodium diphenyl­(trimethylsilyl)­methane. The addition of chelating phosphines to (py)₂FeRCl resulted in the overall transfer of Fe­(I) chloride concurrent with loss of pyridine and alkyl radical. (dmpe)₂FeCl was synthesized via addition of 1 equiv of sodium diphenyl­(trimethylsilyl)­methane, whereas the addition of 2 equiv of the sodium compound to (dmpe)₂FeCl₂ gave the reduced Fe(0) nitrogen complex (dmpe)₂Fe­(N₂). These results demonstrate that iron–alkyl homolysis can be used to afford clean, low-valent iron complexes without the use of alkali metals

Why Does Alkylation of the N–H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?

Molecular metal/NH bifunctional Noyori-type catalysts are remarkable in that they are among the most efficient artificial catalysts developed to date for the hydrogenation of carbonyl functionalities (loadings up to ∼10^–5 mol %). In addition, these catalysts typically exhibit high CO/CC chemo- and enantioselectivities. This unique set of properties is traditionally associated with the operation of an unconventional mechanism for homogeneous catalysts in which the chelating ligand plays a key role in facilitating the catalytic reaction and enabling the aforementioned selectivities by delivering/accepting a proton (H⁺) via its N–H bond cleavage/formation. A recently revised mechanism of the Noyori hydrogenation reaction (Dub, P. A. et al. <i>J. Am. Chem. Soc</i>. <b>2014</b>, <i>136</i>, 3505) suggests that the N–H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N–H···O hydrogen-bonding interactions (HBIs). The present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N–H functionality within M/NH bifunctional Noyori-type catalysts leads to detrimental catalytic activity. The purpose of this work is to demonstrate that decreasing the strength of this HBI, ultimately to the limit of its complete absence, are conditions under which the same alkylation may lead to beneficial catalytic activity

A Tertiary Carbon–Iron Bond as an Fe^ICl Synthon and the Reductive Alkylation of Diphosphine-Supported Iron(II) Chloride Complexes to Low-Valent Iron

Ligand-induced reduction of ferrous alkyl complexes via homolytic cleavage of the alkyl fragment was explored with simple chelating diphosphines. The reactivities of the sodium salts of diphenylmethane, phenyl­(trimethylsilyl)­methane, or diphenyl­(trimethylsilyl)­methane were explored in their reactivity with (py)₄FeCl₂. A series of monoalkylated salts of the type (py)₂FeRCl were prepared and characterized from the addition of 1 equiv of the corresponding alkyl sodium species. These complexes are isostructural and have similar magnetic properties. The double alkylation of (py)₄FeCl₂ resulted in the formation of tetrahedral high-spin iron complexes with the sodium salts of diphenylmethane and phenyl­(trimethylsilyl)­methane that readily decomposed. A bis­(cyclohexadienyl) sandwich complex was formed with the addition of 2 equiv of the tertiary alkyl species sodium diphenyl­(trimethylsilyl)­methane. The addition of chelating phosphines to (py)₂FeRCl resulted in the overall transfer of Fe­(I) chloride concurrent with loss of pyridine and alkyl radical. (dmpe)₂FeCl was synthesized via addition of 1 equiv of sodium diphenyl­(trimethylsilyl)­methane, whereas the addition of 2 equiv of the sodium compound to (dmpe)₂FeCl₂ gave the reduced Fe(0) nitrogen complex (dmpe)₂Fe­(N₂). These results demonstrate that iron–alkyl homolysis can be used to afford clean, low-valent iron complexes without the use of alkali metals

Early-Lanthanide(III) Acetonitrile–Solvento Adducts with Iodide and Noncoordinating Anions

Dissolution of LnI₃ (Ln = La, Ce) in acetonitrile (MeCN) results in the highly soluble solvates LnI₃(MeCN)₅ [Ln = La (<b>1</b>), Ce (<b>2</b>)] in good yield. The ionic complex [La­(MeCN)₉]­[LaI₆] (<b>4</b>), containing a rare homoleptic La³⁺ cation and anion, was also isolated as a minor product. Extending this chemistry to NdI₃ results in the consistent formation of the complex ionic structure [Nd­(MeCN)₉]₂[NdI₅(MeCN)]­[NdI₆]­[I] (<b>3</b>), which contains an unprecedented pentaiodide lanthanoid anion. Also described is the synthesis, isolation, and structural characterization of several homoleptic early-lanthanide MeCN solvates with noncoordinating anions, namely, [Ln­(MeCN)₉]­[AlCl₄]₃ [Ln = La (<b>5</b>), Ce (<b>6</b>), Nd (<b>7</b>)]. Notably, complex <b>6</b> is the first homoleptic cerium MeCN solvate reported to date. All reported complexes were structurally characterized by X-ray crystallography, as well as by IR spectroscopy and CHN elemental analysis. Complexes <b>1</b>–<b>3</b> were also characterized by thermogravimetric analysis coupled with mass spectrometry to further elucidate their bulk composition in the solid-state

Early-Lanthanide(III) Acetonitrile–Solvento Adducts with Iodide and Noncoordinating Anions

Dissolution of LnI₃ (Ln = La, Ce) in acetonitrile (MeCN) results in the highly soluble solvates LnI₃(MeCN)₅ [Ln = La (<b>1</b>), Ce (<b>2</b>)] in good yield. The ionic complex [La­(MeCN)₉]­[LaI₆] (<b>4</b>), containing a rare homoleptic La³⁺ cation and anion, was also isolated as a minor product. Extending this chemistry to NdI₃ results in the consistent formation of the complex ionic structure [Nd­(MeCN)₉]₂[NdI₅(MeCN)]­[NdI₆]­[I] (<b>3</b>), which contains an unprecedented pentaiodide lanthanoid anion. Also described is the synthesis, isolation, and structural characterization of several homoleptic early-lanthanide MeCN solvates with noncoordinating anions, namely, [Ln­(MeCN)₉]­[AlCl₄]₃ [Ln = La (<b>5</b>), Ce (<b>6</b>), Nd (<b>7</b>)]. Notably, complex <b>6</b> is the first homoleptic cerium MeCN solvate reported to date. All reported complexes were structurally characterized by X-ray crystallography, as well as by IR spectroscopy and CHN elemental analysis. Complexes <b>1</b>–<b>3</b> were also characterized by thermogravimetric analysis coupled with mass spectrometry to further elucidate their bulk composition in the solid-state

Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions

Cobalt­(II) alkyl complexes of aliphatic PNP pincer ligands have been synthesized and characterized. The cationic cobalt­(II) alkyl complex [(PNHP^Cy)­Co­(CH₂SiMe₃)]­BAr^F₄ (<b>4</b>) (PNHP^Cy = bis­[(2-dicyclohexylphosphino)­ethyl]­amine) is an active precatalyst for the hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols. To elucidate the possible involvement of the N–H group on the pincer ligand in the catalysis via a metal–ligand cooperative interaction, the reactivities of <b>4</b> and [(PNMeP^Cy)­Co­(CH₂SiMe₃)]­BAr^F₄ (<b>7</b>) were compared. Complex <b>7</b> was found to be an active precatalyst for the hydrogenation of olefins. In contrast, no catalytic activity was observed using <b>7</b> as a precatalyst for the hydrogenation of acetophenone under mild conditions. For the acceptorless dehydrogenation of 1-phenylethanol, complex <b>7</b> displayed similar activity to complex <b>4</b>, affording acetophenone in high yield. When the acceptorless dehydrogenation of 1-phenylethanol with precatalyst <b>4</b> was monitored by NMR spectroscopy, the formation of the cobalt­(III) acetylphenyl hydride complex [(PNHP^Cy)­Co^III(κ²-O,C-C₆H₄C­(O)­CH₃)­(H)]­BAr^F₄ (<b>13</b>) was detected. Isolated complex <b>13</b> was found to be an effective catalyst for the acceptorless dehydrogenation of alcohols, implicating <b>13</b> as a catalyst resting state during the alcohol dehydrogenation reaction. Complex <b>13</b> catalyzed the hydrogenation of styrene but showed no catalytic activity for the room temperature hydrogenation of acetophenone. These results support the involvement of metal–ligand cooperativity in the room temperature hydrogenation of ketones but not the hydrogenation of olefins or the acceptorless dehydrogenation of alcohols. Mechanisms consistent with these observations are presented for the cobalt-catalyzed hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols

Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions

Cobalt­(II) alkyl complexes of aliphatic PNP pincer ligands have been synthesized and characterized. The cationic cobalt­(II) alkyl complex [(PNHP^Cy)­Co­(CH₂SiMe₃)]­BAr^F₄ (<b>4</b>) (PNHP^Cy = bis­[(2-dicyclohexylphosphino)­ethyl]­amine) is an active precatalyst for the hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols. To elucidate the possible involvement of the N–H group on the pincer ligand in the catalysis via a metal–ligand cooperative interaction, the reactivities of <b>4</b> and [(PNMeP^Cy)­Co­(CH₂SiMe₃)]­BAr^F₄ (<b>7</b>) were compared. Complex <b>7</b> was found to be an active precatalyst for the hydrogenation of olefins. In contrast, no catalytic activity was observed using <b>7</b> as a precatalyst for the hydrogenation of acetophenone under mild conditions. For the acceptorless dehydrogenation of 1-phenylethanol, complex <b>7</b> displayed similar activity to complex <b>4</b>, affording acetophenone in high yield. When the acceptorless dehydrogenation of 1-phenylethanol with precatalyst <b>4</b> was monitored by NMR spectroscopy, the formation of the cobalt­(III) acetylphenyl hydride complex [(PNHP^Cy)­Co^III(κ²-O,C-C₆H₄C­(O)­CH₃)­(H)]­BAr^F₄ (<b>13</b>) was detected. Isolated complex <b>13</b> was found to be an effective catalyst for the acceptorless dehydrogenation of alcohols, implicating <b>13</b> as a catalyst resting state during the alcohol dehydrogenation reaction. Complex <b>13</b> catalyzed the hydrogenation of styrene but showed no catalytic activity for the room temperature hydrogenation of acetophenone. These results support the involvement of metal–ligand cooperativity in the room temperature hydrogenation of ketones but not the hydrogenation of olefins or the acceptorless dehydrogenation of alcohols. Mechanisms consistent with these observations are presented for the cobalt-catalyzed hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols

Preparation and Reactivity of the Versatile Uranium(IV) Imido Complexes U(NAr)Cl₂(R₂bpy)₂ (R = Me, ^<i>t</i>Bu) and U(NAr)Cl₂(tppo)₃

Uranium tetrachloride undergoes facile reactions with 4,4′-dialkyl-2,2′-bipyridine, resulting in the generation of UCl₄(R₂bpy)₂, R = Me, ^<i>t</i>Bu. These precursors, as well as the known UCl₄(tppo)₂ (tppo = triphenylphosphine oxide), react with 2 equiv of lithium 2,6-di-isopropylphenylamide to provide the versatile uranium­(IV) imido complexes, U­(NDipp)­Cl₂(L)_<i>n</i> (L = R₂bpy, <i>n</i> = 2; L = tppo, <i>n</i> = 3). Interestingly, U­(NDipp)­Cl₂(R₂bpy)₂ can be used to generate the uranium­(V) and uranium­(VI) bisimido compounds, U­(NDipp)₂­X­(R₂bpy)₂, X = Cl, Br, I, and U­(NDipp)_2­I₂(^<i>t</i>Bu₂bpy), which establishes these uranium­(IV) precursors as potential intermediates in the syntheses of high-valent bis­(imido) complexes from UCl₄. The monoimido species also react with 4-methylmorpholine-N-oxide to yield uranium­(VI) oxo-imido products, U­(NDipp)­(O)­Cl₂(L)_<i>n</i> (L = ^<i>t</i>Bu₂bpy, <i>n</i> = 1; L = tppo, <i>n</i> = 2). The aforementioned molecules have been characterized by a combination of NMR spectroscopy, X-ray crystallography, and elemental analysis. The chemical reactivity studies presented herein demonstrate that Lewis base adducts of uranium tetrachloride function as excellent sources of U­(IV), U­(V), and U­(VI) imido species

Switchable Phase Behavior of [HBet][Tf₂N]–H₂O upon Neodymium Loading: Implications for Lanthanide Separations

Task-specific ionic liquids (TSILs) present an opportunity to replace traditional organic solvents for metal dissolution or separation. To date, a thorough investigation of the physical properties of biphasic TSIL–H₂O systems and the effects of metal loading is lacking. In this work, the change in the liquid–liquid equilibrium of [HBet]­[Tf₂N]–H₂O upon the addition of Nd­(III) is investigated by cloud-point measurements. The addition of Nd(III), drops the upper critical solution temperature by over 20 °C. Further investigation of the [HBet]­[Tf₂N]–Nd(III) system led to the formation of single crystals of [Nd₂(H₂O)₈(μ₂-Bet)₂(μ₃-Bet)₂]­[(Cl)₂(Tf₂N)₄] from the TSIL phase