Reversible Dehydration Behavior Reveals Coordinatively Unsaturated Metal Sites in Microporous Aluminum Phosphonates

Incorporation of the same ligand into three different aluminum phenylenediphosphonates (Al­(H₂O)­(O₃PC₆H₄PO₃H) (<b>1</b>), Al₄(H₂O)₂­(O₃PC₆H₄PO₃)₃ (<b>2</b>), and Al₄(H₂O)₄­(O₃PC₆H₄PO₃)_2.84­(OH)_0.64 (<b>3</b>)) was accomplished by varying the synthetic conditions. The compounds have different sorption properties; however, all exhibit reversible dehydration behavior. The structures of the hydrated and dehydrated phases were determined from powder X-ray diffraction data. Compounds <b>2</b> and <b>3</b> were found to be microporous, while compound <b>1</b> was found to be nonporous. The stability of the dehydrated phase and the resulting porosity was found to be influenced by the change in the structure upon loss of water

Rational Design of Porous Coordination Polymers Based on Bis(phosphine)MCl₂ Complexes That Exhibit High-Temperature H₂ Sorption and Chemical Reactivity

MCl₂ complexes of a new <i>p</i>-carboxylated 1,2-bis­(diphenylphosphino)­benzene ligand are effectively utilized as tetratopic building blocks to prepare isostructural porous coordination polymers with accessible reactive metal sites (M = Pd, Pt). The crystalline materials exhibit unusual and fully reversible H₂ sorption at 150 °C. Post-synthetic reactivity is also possible, in which Pt–Cl bonds can be activated to provide organometallic species in the pores

The Challenge of Palladium-Catalyzed Aromatic Azidocarbonylation: From Mechanistic and Catalyst Deactivation Studies to a Highly Efficient Process

Azidocarbonylation of iodoarenes with CO and NaN₃, a novel Heck-type carbonylation reaction, readily occurs in an organic solvent–H₂O biphasic system to furnish aroyl azides at room temperature and 1 atm. The reaction is catalyzed by Xantphos-Pd and exhibits high functional group tolerance. The catalyst deactivation product, [(Xantphos)­PdI₂], can be reduced in situ with PMHS to Pd(0) to regain catalytic activity. In this way, the catalyst loading has been lowered to 0.2% without any losses in selectivity at nearly 100% conversion to synthesize a series of aroyl azides in 80–90% isolated yield on a gram scale. Alternatively, the ArCON₃ product can be used without isolation for further transformations in situ, e.g., to isocyanates, ureas, benzamides, and iminophosphoranes. A detailed experimental and computational study has identified two main reaction pathways for the reaction. For both routes, Ar–I oxidative addition to Pd(0) is the rate-determining step. In the presence of CO in excess, the Ar–I bond is activated by the less electron-rich Pd center of a mixed carbonyl phosphine complex. Under CO-deficient conditions, a slightly lower energy barrier pathway is followed that involves Ar–I oxidative addition to a more reactive carbonyl-free (Xantphos)­Pd⁰ species. Mass transfer in the triphasic liquid–liquid–gas system employed for the reaction plays an important role in the competition between these two reaction channels, uniformly leading to a common aroyl azido intermediate that undergoes exceedingly facile ArCO–N₃ reductive elimination. Safety aspects of the method have been investigated

The Challenge of Palladium-Catalyzed Aromatic Azidocarbonylation: From Mechanistic and Catalyst Deactivation Studies to a Highly Efficient Process

Azidocarbonylation of iodoarenes with CO and NaN₃, a novel Heck-type carbonylation reaction, readily occurs in an organic solvent–H₂O biphasic system to furnish aroyl azides at room temperature and 1 atm. The reaction is catalyzed by Xantphos-Pd and exhibits high functional group tolerance. The catalyst deactivation product, [(Xantphos)­PdI₂], can be reduced in situ with PMHS to Pd(0) to regain catalytic activity. In this way, the catalyst loading has been lowered to 0.2% without any losses in selectivity at nearly 100% conversion to synthesize a series of aroyl azides in 80–90% isolated yield on a gram scale. Alternatively, the ArCON₃ product can be used without isolation for further transformations in situ, e.g., to isocyanates, ureas, benzamides, and iminophosphoranes. A detailed experimental and computational study has identified two main reaction pathways for the reaction. For both routes, Ar–I oxidative addition to Pd(0) is the rate-determining step. In the presence of CO in excess, the Ar–I bond is activated by the less electron-rich Pd center of a mixed carbonyl phosphine complex. Under CO-deficient conditions, a slightly lower energy barrier pathway is followed that involves Ar–I oxidative addition to a more reactive carbonyl-free (Xantphos)­Pd⁰ species. Mass transfer in the triphasic liquid–liquid–gas system employed for the reaction plays an important role in the competition between these two reaction channels, uniformly leading to a common aroyl azido intermediate that undergoes exceedingly facile ArCO–N₃ reductive elimination. Safety aspects of the method have been investigated

CF₃–Ph Reductive Elimination from [(Xantphos)Pd(CF₃)(Ph)]

CF₃–Ph reductive elimination from [(Xantphos)­Pd­(Ph)­(CF₃)] (<b>1</b>) and [(<i>i</i>-Pr-Xantphos)­Pd­(Ph)­(CF₃)] (<b>2</b>) has been studied by experimental and computational methods. Complex <b>1</b> is cis in the solid state and predominantly cis in solution, undergoing degenerate cis–cis isomerization (Δ<i>G</i>^≠_exp = 13.4 kcal mol^–1; <i>Δ<i>G</i></i>^≠_calc = 12.8 kcal mol^–1 in toluene) and slower cis–trans isomerization (<i>Δ<i>G</i></i>_calc = +0.9 kcal mol^–1; <i>Δ<i>G</i></i>^≠_calc = 21.9 kcal mol^–1). In contrast, <b>2</b> is only trans in both solution and the solid state with <b>trans-2</b> computed to be 10.2 kcal mol^–1 lower in energy than <b>cis-2</b>. Kinetic and computational studies of the previously communicated (<i>J. Am. Chem. Soc</i>. <b>2006</b>, <i>128</i>, 12644), remarkably facile CF₃–Ph reductive elimination from <b>1</b> suggest that the process does not require P–Pd bond dissociation but rather occurs directly from <b>cis-1</b>. The experimentally determined activation parameters (Δ<i>H</i>^≠ = 25.9 ± 2.6 kcal mol^–1; <i>Δ<i>S</i></i>^≠ = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (Δ<i>H</i>^≠_calc = 24.8 kcal mol^–1; Δ<i>G</i>^≠_calc = 25.0 kcal mol^–1). Δ<i>G</i>^≠_calc for CF₃–Ph reductive elimination from <b>cis-2</b> is only 24.0 kcal mol^–1; however, the overall barrier relative to <b>trans-2</b> is much higher (Δ<i>G</i>^≠_calc = 34.2 kcal mol^–1) due to the need to include the energetic cost of trans–cis isomerization. This is consistent with the higher thermal stability of <b>2</b> that decomposes to PhCF₃ only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of <b>1</b>. A steady slight decrease in <i>k</i>_obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]:<b>1</b> = 5, 10, and 20. Specific molecular interactions between <b>1</b> and Xantphos are not involved in this kinetic effect (NMR, <i>T</i>₁ measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of <i>cis</i>-[(η¹-Xantphos)₂Pd­(Ph)­(CF₃)] that, by computation, is predicted to access reductive elimination of CF₃–Ph with Δ<i>G</i>^≠_calc = 22.8 kcal mol^–1

CF₃–Ph Reductive Elimination from [(Xantphos)Pd(CF₃)(Ph)]

CF₃–Ph reductive elimination from [(Xantphos)­Pd­(Ph)­(CF₃)] (<b>1</b>) and [(<i>i</i>-Pr-Xantphos)­Pd­(Ph)­(CF₃)] (<b>2</b>) has been studied by experimental and computational methods. Complex <b>1</b> is cis in the solid state and predominantly cis in solution, undergoing degenerate cis–cis isomerization (Δ<i>G</i>^≠_exp = 13.4 kcal mol^–1; <i>Δ<i>G</i></i>^≠_calc = 12.8 kcal mol^–1 in toluene) and slower cis–trans isomerization (<i>Δ<i>G</i></i>_calc = +0.9 kcal mol^–1; <i>Δ<i>G</i></i>^≠_calc = 21.9 kcal mol^–1). In contrast, <b>2</b> is only trans in both solution and the solid state with <b>trans-2</b> computed to be 10.2 kcal mol^–1 lower in energy than <b>cis-2</b>. Kinetic and computational studies of the previously communicated (<i>J. Am. Chem. Soc</i>. <b>2006</b>, <i>128</i>, 12644), remarkably facile CF₃–Ph reductive elimination from <b>1</b> suggest that the process does not require P–Pd bond dissociation but rather occurs directly from <b>cis-1</b>. The experimentally determined activation parameters (Δ<i>H</i>^≠ = 25.9 ± 2.6 kcal mol^–1; <i>Δ<i>S</i></i>^≠ = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (Δ<i>H</i>^≠_calc = 24.8 kcal mol^–1; Δ<i>G</i>^≠_calc = 25.0 kcal mol^–1). Δ<i>G</i>^≠_calc for CF₃–Ph reductive elimination from <b>cis-2</b> is only 24.0 kcal mol^–1; however, the overall barrier relative to <b>trans-2</b> is much higher (Δ<i>G</i>^≠_calc = 34.2 kcal mol^–1) due to the need to include the energetic cost of trans–cis isomerization. This is consistent with the higher thermal stability of <b>2</b> that decomposes to PhCF₃ only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of <b>1</b>. A steady slight decrease in <i>k</i>_obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]:<b>1</b> = 5, 10, and 20. Specific molecular interactions between <b>1</b> and Xantphos are not involved in this kinetic effect (NMR, <i>T</i>₁ measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of <i>cis</i>-[(η¹-Xantphos)₂Pd­(Ph)­(CF₃)] that, by computation, is predicted to access reductive elimination of CF₃–Ph with Δ<i>G</i>^≠_calc = 22.8 kcal mol^–1

CF₃–Ph Reductive Elimination from [(Xantphos)Pd(CF₃)(Ph)]

CF₃–Ph reductive elimination from [(Xantphos)­Pd­(Ph)­(CF₃)] (<b>1</b>) and [(<i>i</i>-Pr-Xantphos)­Pd­(Ph)­(CF₃)] (<b>2</b>) has been studied by experimental and computational methods. Complex <b>1</b> is cis in the solid state and predominantly cis in solution, undergoing degenerate cis–cis isomerization (Δ<i>G</i>^≠_exp = 13.4 kcal mol^–1; <i>Δ<i>G</i></i>^≠_calc = 12.8 kcal mol^–1 in toluene) and slower cis–trans isomerization (<i>Δ<i>G</i></i>_calc = +0.9 kcal mol^–1; <i>Δ<i>G</i></i>^≠_calc = 21.9 kcal mol^–1). In contrast, <b>2</b> is only trans in both solution and the solid state with <b>trans-2</b> computed to be 10.2 kcal mol^–1 lower in energy than <b>cis-2</b>. Kinetic and computational studies of the previously communicated (<i>J. Am. Chem. Soc</i>. <b>2006</b>, <i>128</i>, 12644), remarkably facile CF₃–Ph reductive elimination from <b>1</b> suggest that the process does not require P–Pd bond dissociation but rather occurs directly from <b>cis-1</b>. The experimentally determined activation parameters (Δ<i>H</i>^≠ = 25.9 ± 2.6 kcal mol^–1; <i>Δ<i>S</i></i>^≠ = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (Δ<i>H</i>^≠_calc = 24.8 kcal mol^–1; Δ<i>G</i>^≠_calc = 25.0 kcal mol^–1). Δ<i>G</i>^≠_calc for CF₃–Ph reductive elimination from <b>cis-2</b> is only 24.0 kcal mol^–1; however, the overall barrier relative to <b>trans-2</b> is much higher (Δ<i>G</i>^≠_calc = 34.2 kcal mol^–1) due to the need to include the energetic cost of trans–cis isomerization. This is consistent with the higher thermal stability of <b>2</b> that decomposes to PhCF₃ only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of <b>1</b>. A steady slight decrease in <i>k</i>_obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]:<b>1</b> = 5, 10, and 20. Specific molecular interactions between <b>1</b> and Xantphos are not involved in this kinetic effect (NMR, <i>T</i>₁ measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of <i>cis</i>-[(η¹-Xantphos)₂Pd­(Ph)­(CF₃)] that, by computation, is predicted to access reductive elimination of CF₃–Ph with Δ<i>G</i>^≠_calc = 22.8 kcal mol^–1

Dihydrogen Bonding in Complex (PP₃)RuH(η¹‑BH₄) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies

Combining variable-temperature infrared and NMR spectroscopic studies with quantum-chemical calculations (density functional theory (DFT) and natural bond orbital) allowed us to address the problem of competition between MH (M = transition metal) and BH hydrogens as proton-accepting sites in dihydrogen bond (DHB) and to unravel the mechanism of proton transfer to complex (PP₃)­RuH­(η¹-BH₄) (<b>1</b>, PP₃ = κ⁴-P­(CH₂CH₂PPh₂)₃). Interaction of complex <b>1</b> with CH₃OH, fluorinated alcohols of variable acid strength [CH₂FCH₂OH, CF₃CH₂OH, (CF₃)₂CHOH (HFIP), (CF₃)₃COH], and CF₃COOH leads to the medium-strength DHB complexes involving BH bonds (3–5 kcal/mol), whereas DHB complexes with RuH were not observed experimentally. The two proton-transfer pathways were considered in DFT/M06 calculations. The first one goes via more favorable bifurcate complexes to BH_term and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP₃)­RuH_eq(H₂)]⁺[OR]⁻. The second pathway starts from the less-favorable complex with RuH ligand but shows a lower activation barrier (23.5 kcal/mol for HFIP) and eventually leads to the final product via the isomerization of intermediate [(PP₃)­RuH_ax(H₂)]⁺[OR]⁻. The B–H_br bond breaking is the common key step of all pathways investigated