Aluminium porphyrins catalyse the hydrogenation of CO2 with H2

Boron-based frustrated Lewis pairs (FLPs) have become well-established catalysts for the hydrogenation of a wide range of functional groups. Conversely, aluminium-based FLP hydrogenation catalysts are less common, especially for CO2 reduction. They are mostly confined to the hydrogenation of imines, alkenes, and alkynes even though aluminium is much more abundant than boron and forms structurally related compounds. Moreover, aluminium forms penta- and hexa-coordinated complexes, which remain untested in FLP hydrogenation catalysis. Herein, we demonstrate that cationic, hexa-coordinated diaqua-meso-tetraphenylporphyrin aluminium complexes [Al(TPP)(OH2)2]X and [Al( t BuTPP)(OH2)2]X (X = Cl-, OTf-, ClO4 -) form FLPs with nitrogen bases, activate H2, and reductively couple CO2 to amines, yielding N-formylamines and water. Our experimental results and DFT analysis indicate that H2 activation involves the formation of an FLP, base-promoted CO2 reduction and formate salt elimination from the FLP, as proposed for transition metal-catalyzed N-formylations. These similarities in the reaction mechanism and structure of aluminium complexes brings Al-based FLPs closer to transition metal catalysis and may enable us to apply this knowledge to ligand design to enhance main group metal-promoted hydrogenations.

Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements

The change in sign of the interaction force constant between element-hydrogen stretching modes of trans-dihydrides of the d block and p block elements is analyzed for the first time. As the transition metal M approaches group 12, the higher energy symmetric trans-H-M-H vibration ν_sym approaches the energy of the antisymmetric vibration ν_asym. Crossing to group 13 elements E, the trans-H-E-H vibration ν_sym increasingly drops below ν_asym. This reversal is attributed to the d orbital that participates in the H-M-H bonding but is nonbonding in the H-E-H compounds. DFT calculations are used to probe the energetics of isoelectronic triatomic [H-M-H]ⁿ⁺ and [H-E-H]^n- to reveal this trend and also to demonstrate that the magnitude of these interactions (ν_gap) increases down groups 11, 12, and 14 but remains fairly constant for group 13. They are also used to show that this reversal is seen in the transition state for hydride transfer to CO₂ from the model compounds trans-NiH₂(porphyrin) and trans-EH₂(porphyrin), E = Si and Ge in their singlet states.

Hydrosilyation of CO2 using a silatrane hydride: structural characterization of a silyl formate compound

The silatrane hydride compound, [N(CH2CH2O)3]SiH, reacts with CO2 in the presence of the [tris(2-pyridylthio)methyl]zinc hydride complex, [Tptm]ZnH, to afford the silyl formate and methoxide derivatives, [N(CH2CH2O)3]SiO2CH and [N(CH2CH2O)3]SiOCH3. The molecular structure of [N(CH2CH2O)3]SiO2CH has been determined by X-ray diffraction, thereby demonstrating that the formate ligand adopts a distal conformation in which the uncoordinated oxygen atom resides with a trans-like disposition relative to silicon. Density functional theory calculations indicate that the atrane motif of [N(CH2CH2O)3]SiO2CH is flexible, such that the energy of the molecule changes relatively little as the Si···N distance varies over the range 2.0–3.0 Å.

Ionic liquid [DBUH][BO2]: an excellent catalyst for chemical fixation of CO2 under mild conditions

The basic IL [DBUH][BO2] was easily synthesized and used for the chemical fixation of CO₂ at atmospheric pressure and room temperature.