Reactivity Studies on [Cp′FeI]₂: Monomeric Amido, Phenoxo, and Alkyl Complexes

A series of monomeric mono­(cyclopentadienyl) iron amido, phenoxo, and alkyl complexes were synthesized, and their structure and reactivity are presented. The iron­(II) centers in these 14VE one-legged piano stool complexes are high spin (<i>S</i> = 2) in solid state and solution independent of solvent. The silylamide compound [Cp′FeN­(SiMe₃)₂] (<b>2a</b>, Cp′ = 1,2,4-(Me₃C)₃C₅H₂) is an excellent starting material for the reaction with more acidic substrates such as phenols. Sterically encumbered phenols 2,6-(Me₃C)₂(4-R)­C₆H₂OH (R = H, Me, and <i>t</i>Bu) were investigated. In all cases monomeric iron phenoxo half-sandwich complexes [Cp′FeOR′] (<b>4-R</b>) are initially formed. Rearrangement of <b>4-R</b> to the diamagnetic oxocyclohexadienyl complex [Cp′Fe­(η⁵-OC₆H₂R′₂R″)] (<b>5-R</b>) is observed for 2,6-(Me₃C)₂(4-R)­C₆H₂OH (R = H and Me) and the Gibbs free enthalpy of activation (Δ<i>G</i>^⧧) was determined. In contrast this rearrangement is inhibited when the 4-position is blocked by a <i>t</i>Bu group. Removing the steric bulk from the 2,6-positions leads to the formation of a μ-phenoxo dimer, [Cp′Fe­(μ-OC₆H₃<i>t</i>Bu₂-3,5)]₂ (<b>5</b>). Density functional theory (DFT) was used to further elucidate the structure–reactivity relationship in these molecules. The one-legged piano stool anilido complex [Cp′Fe­(NHC₆H₂<i>t</i>Bu₃-2,4,6)] (<b>7</b>) is not accessible via acid–base reaction between <b>2a</b> and H₂NC₆H₂<i>t</i>Bu₃-2,4,6, but can be prepared by conventional salt metathesis reaction from [Cp′FeI]₂ and [Li­(NHC₆H₂<i>t</i>Bu₃-2,4,6)­(OEt₂)]₂. In contrast, reaction of <b>2a</b> with Ph₂NH yields the bimetallic [Cp′Fe­(<i>N,C</i>-κ¹,η⁵-C₆H₅NPh)­Fe­(<i>N</i>-κ¹-NPh₂)­Cp′] (<b>8</b>) which combines two iron centers in the same oxidation state (+2), but different spin-states (<i>S</i> = 0 and <i>S</i> = 2) which is reflected in very different Cp­(cent)–Fe distances of 1.68 and 2.04 Å, respectively. A monomeric iron alkyl half-sandwich complex [Cp′FeCH­(SiMe₃)₂] (<b>9</b>) was prepared that exhibits no reactivity toward H₂, C₂H₄ or N₂O. This behavior might be rationalized by a spin-state induced reaction barrier. However, <b>9</b> reacts in the presence of CO to the iron acyl-complex [Cp′Fe­(CO)₂(C­(O)­CH­(SiMe₃)₂)] (<b>10</b>) and with a CO/H₂ mixture [Cp′Fe­(CO)₂]₂ (<b>11</b>) and CH₂(SiMe₃)₂ are formed

Reactivity Studies on [Cp′FeI]₂: Monomeric Amido, Phenoxo, and Alkyl Complexes

A series of monomeric mono­(cyclopentadienyl) iron amido, phenoxo, and alkyl complexes were synthesized, and their structure and reactivity are presented. The iron­(II) centers in these 14VE one-legged piano stool complexes are high spin (<i>S</i> = 2) in solid state and solution independent of solvent. The silylamide compound [Cp′FeN­(SiMe₃)₂] (<b>2a</b>, Cp′ = 1,2,4-(Me₃C)₃C₅H₂) is an excellent starting material for the reaction with more acidic substrates such as phenols. Sterically encumbered phenols 2,6-(Me₃C)₂(4-R)­C₆H₂OH (R = H, Me, and <i>t</i>Bu) were investigated. In all cases monomeric iron phenoxo half-sandwich complexes [Cp′FeOR′] (<b>4-R</b>) are initially formed. Rearrangement of <b>4-R</b> to the diamagnetic oxocyclohexadienyl complex [Cp′Fe­(η⁵-OC₆H₂R′₂R″)] (<b>5-R</b>) is observed for 2,6-(Me₃C)₂(4-R)­C₆H₂OH (R = H and Me) and the Gibbs free enthalpy of activation (Δ<i>G</i>^⧧) was determined. In contrast this rearrangement is inhibited when the 4-position is blocked by a <i>t</i>Bu group. Removing the steric bulk from the 2,6-positions leads to the formation of a μ-phenoxo dimer, [Cp′Fe­(μ-OC₆H₃<i>t</i>Bu₂-3,5)]₂ (<b>5</b>). Density functional theory (DFT) was used to further elucidate the structure–reactivity relationship in these molecules. The one-legged piano stool anilido complex [Cp′Fe­(NHC₆H₂<i>t</i>Bu₃-2,4,6)] (<b>7</b>) is not accessible via acid–base reaction between <b>2a</b> and H₂NC₆H₂<i>t</i>Bu₃-2,4,6, but can be prepared by conventional salt metathesis reaction from [Cp′FeI]₂ and [Li­(NHC₆H₂<i>t</i>Bu₃-2,4,6)­(OEt₂)]₂. In contrast, reaction of <b>2a</b> with Ph₂NH yields the bimetallic [Cp′Fe­(<i>N,C</i>-κ¹,η⁵-C₆H₅NPh)­Fe­(<i>N</i>-κ¹-NPh₂)­Cp′] (<b>8</b>) which combines two iron centers in the same oxidation state (+2), but different spin-states (<i>S</i> = 0 and <i>S</i> = 2) which is reflected in very different Cp­(cent)–Fe distances of 1.68 and 2.04 Å, respectively. A monomeric iron alkyl half-sandwich complex [Cp′FeCH­(SiMe₃)₂] (<b>9</b>) was prepared that exhibits no reactivity toward H₂, C₂H₄ or N₂O. This behavior might be rationalized by a spin-state induced reaction barrier. However, <b>9</b> reacts in the presence of CO to the iron acyl-complex [Cp′Fe­(CO)₂(C­(O)­CH­(SiMe₃)₂)] (<b>10</b>) and with a CO/H₂ mixture [Cp′Fe­(CO)₂]₂ (<b>11</b>) and CH₂(SiMe₃)₂ are formed

Synthesis, Structure, and Reactivity of a Thorium Metallocene Containing a 2,2′-Bipyridyl Ligand

The reduction of [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThCl₂ (<b>1</b>) with potassium graphite in the presence of 2,2′-bipyridine gives the purple thorium bipy metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th­(bipy) (<b>2</b>) in good yield. Complex <b>2</b> has been characterized by various spectroscopic techniques, elemental analysis, and single-crystal X-ray diffraction. Complex <b>2</b> is a good synthon for low-valent thorium, as shown by the reactivity with silver halides, trityl chloride, pyridine-<i>N</i>-oxide, RN₃, 9-diazofluorene, and diphenyl diselenide, yielding the halide metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThX₂ (X = Cl (<b>1</b>), F (<b>3</b>), Br (<b>4</b>)), oxo metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThO­(py) (<b>5</b>), imido metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThNR (R = <i>p</i>-tolyl (<b>6</b>), Ph₃C (<b>7</b>), Me₃Si (<b>8</b>), (9-C₁₃H₈)N (<b>9</b>)), and selenido complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]­Th­(SePh)₃(bipy) (<b>10a</b>), in quantitative conversions

Synthesis, Structure, and Reactivity of a Thorium Metallocene Containing a 2,2′-Bipyridyl Ligand

The reduction of [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThCl₂ (<b>1</b>) with potassium graphite in the presence of 2,2′-bipyridine gives the purple thorium bipy metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th­(bipy) (<b>2</b>) in good yield. Complex <b>2</b> has been characterized by various spectroscopic techniques, elemental analysis, and single-crystal X-ray diffraction. Complex <b>2</b> is a good synthon for low-valent thorium, as shown by the reactivity with silver halides, trityl chloride, pyridine-<i>N</i>-oxide, RN₃, 9-diazofluorene, and diphenyl diselenide, yielding the halide metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThX₂ (X = Cl (<b>1</b>), F (<b>3</b>), Br (<b>4</b>)), oxo metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThO­(py) (<b>5</b>), imido metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThNR (R = <i>p</i>-tolyl (<b>6</b>), Ph₃C (<b>7</b>), Me₃Si (<b>8</b>), (9-C₁₃H₈)N (<b>9</b>)), and selenido complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]­Th­(SePh)₃(bipy) (<b>10a</b>), in quantitative conversions

Thorium Oxo and Sulfido Metallocenes: Synthesis, Structure, Reactivity, and Computational Studies

The synthesis, structure, and reactivity of thorium oxo and sulfido metallocenes have been comprehensively studied. Heating of an equimolar mixture of the dimethyl metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThMe₂ (<b>2</b>) and the bis-amide metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th(NH-<i>p</i>-tolyl)₂ (<b>3</b>) in refluxing toluene results in the base-free imido thorium metallocene, [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThN(<i>p</i>-tolyl) (<b>4</b>), which is a useful precursor for the preparation of oxo and sulfido thorium metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThE (E = O (<b>5</b>) and S (<b>15</b>)) by cycloaddition–elimination reaction with Ph₂CE (E = O, S) or CS₂. The oxo metallocene <b>5</b> acts as a nucleophile toward alkylsilyl halides, while sulfido metallocene <b>15</b> does not. The oxo metallocene <b>5</b> and sulfido metallocene <b>15</b> undergo a [2 + 2] cycloaddition reaction with Ph₂CO, CS₂, or Ph₂CS, but they show no reactivity with alkynes. Density functional theory (DFT) studies provide insights into the subtle interplay between steric and electronic effects and rationalize the experimentally observed reactivity patterns. A comparison between Th, U, and group 4 elements shows that Th⁴⁺ behaves more like an actinide than a transition metal

Thorium Oxo and Sulfido Metallocenes: Synthesis, Structure, Reactivity, and Computational Studies

The synthesis, structure, and reactivity of thorium oxo and sulfido metallocenes have been comprehensively studied. Heating of an equimolar mixture of the dimethyl metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThMe₂ (<b>2</b>) and the bis-amide metallocene [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th(NH-<i>p</i>-tolyl)₂ (<b>3</b>) in refluxing toluene results in the base-free imido thorium metallocene, [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThN(<i>p</i>-tolyl) (<b>4</b>), which is a useful precursor for the preparation of oxo and sulfido thorium metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂ThE (E = O (<b>5</b>) and S (<b>15</b>)) by cycloaddition–elimination reaction with Ph₂CE (E = O, S) or CS₂. The oxo metallocene <b>5</b> acts as a nucleophile toward alkylsilyl halides, while sulfido metallocene <b>15</b> does not. The oxo metallocene <b>5</b> and sulfido metallocene <b>15</b> undergo a [2 + 2] cycloaddition reaction with Ph₂CO, CS₂, or Ph₂CS, but they show no reactivity with alkynes. Density functional theory (DFT) studies provide insights into the subtle interplay between steric and electronic effects and rationalize the experimentally observed reactivity patterns. A comparison between Th, U, and group 4 elements shows that Th⁴⁺ behaves more like an actinide than a transition metal

Stability and Dynamic Processes in 16VE Iridium(III) Ethyl Hydride and Rhodium(I) σ‑Ethane Complexes: Experimental and Computational Studies

Iridium­(I) and rhodium­(I) ethyl complexes, (PONOP)­M­(C₂H₅) (M = Ir (<b>1-Et</b>), Rh (<b>2-Et</b>)) and the iridium­(I) propyl complex (PONOP)­Ir­(C₃H₇) (<b>1-Pr</b>), where PONOP is 2,6-(<i>t</i>Bu₂PO)₂C₅H₃N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)­Ir­(H)­(R) (<b>1-(H)­(Et)</b>^<b>+</b> and <b>1-(H)­(Pr)</b>^<b>+</b>), respectively. Dynamic ¹H NMR characterization of <b>1-(H)­(Et)</b>^<b>+</b> establishes site exchange between the Ir–<i>H</i> and Ir–C<i>H</i>₂ protons (Δ<i>G</i>_exH^‡(−110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic ¹³C NMR spectroscopy, the exchange barrier between the α and β carbons (“chain-walking”) was measured (Δ<i>G</i>_exC^‡(−110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (−40 °C), to be compared with the reported barrier to methane loss in <b>1-(H)­(Me)</b>^<b>+</b> of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)­Rh­(EtH) (<b>2-(EtH)</b>^<b>+</b>), was prepared by protonation of <b>2-Et</b> at −150 °C. The barrier for ethane loss (Δ<i>G</i>_dec^‡(−132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, <b>2-(MeH)</b>^<b>+</b>, (Δ<i>G</i>_dec^‡(−87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of <b>2-(EtH)</b>^<b>+</b> is reported, a key feature of which is the upfield signal at −31.2 ppm for the coordinated CH₃ group in the ¹³C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (−132 °C) is observable by ¹³C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η²-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex

Stability and Dynamic Processes in 16VE Iridium(III) Ethyl Hydride and Rhodium(I) σ‑Ethane Complexes: Experimental and Computational Studies

Iridium­(I) and rhodium­(I) ethyl complexes, (PONOP)­M­(C₂H₅) (M = Ir (<b>1-Et</b>), Rh (<b>2-Et</b>)) and the iridium­(I) propyl complex (PONOP)­Ir­(C₃H₇) (<b>1-Pr</b>), where PONOP is 2,6-(<i>t</i>Bu₂PO)₂C₅H₃N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)­Ir­(H)­(R) (<b>1-(H)­(Et)</b>^<b>+</b> and <b>1-(H)­(Pr)</b>^<b>+</b>), respectively. Dynamic ¹H NMR characterization of <b>1-(H)­(Et)</b>^<b>+</b> establishes site exchange between the Ir–<i>H</i> and Ir–C<i>H</i>₂ protons (Δ<i>G</i>_exH^‡(−110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic ¹³C NMR spectroscopy, the exchange barrier between the α and β carbons (“chain-walking”) was measured (Δ<i>G</i>_exC^‡(−110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (−40 °C), to be compared with the reported barrier to methane loss in <b>1-(H)­(Me)</b>^<b>+</b> of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)­Rh­(EtH) (<b>2-(EtH)</b>^<b>+</b>), was prepared by protonation of <b>2-Et</b> at −150 °C. The barrier for ethane loss (Δ<i>G</i>_dec^‡(−132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, <b>2-(MeH)</b>^<b>+</b>, (Δ<i>G</i>_dec^‡(−87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of <b>2-(EtH)</b>^<b>+</b> is reported, a key feature of which is the upfield signal at −31.2 ppm for the coordinated CH₃ group in the ¹³C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (−132 °C) is observable by ¹³C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η²-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex

Stability and Dynamic Processes in 16VE Iridium(III) Ethyl Hydride and Rhodium(I) σ‑Ethane Complexes: Experimental and Computational Studies

Iridium­(I) and rhodium­(I) ethyl complexes, (PONOP)­M­(C₂H₅) (M = Ir (<b>1-Et</b>), Rh (<b>2-Et</b>)) and the iridium­(I) propyl complex (PONOP)­Ir­(C₃H₇) (<b>1-Pr</b>), where PONOP is 2,6-(<i>t</i>Bu₂PO)₂C₅H₃N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)­Ir­(H)­(R) (<b>1-(H)­(Et)</b>^<b>+</b> and <b>1-(H)­(Pr)</b>^<b>+</b>), respectively. Dynamic ¹H NMR characterization of <b>1-(H)­(Et)</b>^<b>+</b> establishes site exchange between the Ir–<i>H</i> and Ir–C<i>H</i>₂ protons (Δ<i>G</i>_exH^‡(−110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic ¹³C NMR spectroscopy, the exchange barrier between the α and β carbons (“chain-walking”) was measured (Δ<i>G</i>_exC^‡(−110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (−40 °C), to be compared with the reported barrier to methane loss in <b>1-(H)­(Me)</b>^<b>+</b> of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)­Rh­(EtH) (<b>2-(EtH)</b>^<b>+</b>), was prepared by protonation of <b>2-Et</b> at −150 °C. The barrier for ethane loss (Δ<i>G</i>_dec^‡(−132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, <b>2-(MeH)</b>^<b>+</b>, (Δ<i>G</i>_dec^‡(−87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of <b>2-(EtH)</b>^<b>+</b> is reported, a key feature of which is the upfield signal at −31.2 ppm for the coordinated CH₃ group in the ¹³C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (−132 °C) is observable by ¹³C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η²-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex

Small-Molecule Activation Mediated by a Uranium Bipyridyl Metallocene

Addition of potassium graphite (KC₈) to a solution of (η⁵-C₅Me₅)₂UCl₂ (<b>1</b>) and 2,2′-bipyridine (bipy) gives the uranium bipyridyl metallocene (η⁵-C₅Me₅)₂U­(bipy) (<b>2</b>) in good yield. In complex <b>2</b> a bipy radical anion is coordinated to a U­(III) atom, and it is therefore an ideal starting material for small-molecule activation: e.g., it serves as a synthetic equivalent for the (η⁵-C₅Me₅)₂U^II fragment on treatment with conjugated alkynes and a variety of heterounsaturated molecules such as imines, carbodiimide, organic azides, hydrazine, and azo derivatives. Alternatively, it may also react with aldehydes, ketones, nitriles, and α,β-unsaturated reagents such as <i>p</i>-ClPhCHO, (CH₂)₅CO, PhCN, and methyl methacrylate (MMA), forming the C–C bond coupling products (η⁵-C₅Me₅)₂U­[(bipy)­(<i>p</i>-ClPhCHO)] (<b>10</b>), (η⁵-C₅Me₅)₂U­[(bipy)­{(CH₂)₅CO}] (<b>11</b>), (η⁵-C₅Me₅)₂U­[(bipy)­(PhCN)] (<b>12</b>), (η⁵-C₅Me₅)₂U­[(bipy)­{CH₂C­(Me)­CO­(OMe)] (<b>13a</b>), and [(η⁵-C₅Me₅)₂U­{OC­(OMe)C­(Me)­CH₂–3-bipy}]₂ (<b>13b</b>), respectively, in quantitative conversion. Furthermore, addition of CuI to complex <b>2</b> induces a single-electron-transfer process to form the uranium­(III) iodide complex (η⁵-C₅Me₅)₂U­(I)­(bipy) (<b>14</b>)