RESUMO
A computational inverse design method suitable to assist the development and optimization of molecular catalysts is introduced. Catalysts are obtained by continuous optimization of "alchemical" candidates in the vicinity of a reference catalyst with well-defined reaction intermediates and rate-limiting step. A NiII-iminoalkoxylate catalyst for aqueous CO/CO2 conversion is found with improved performance relative to a NiII-iminothiolate reference complex, previously reported as a biomimetic synthetic model of CO dehydroxygenase. Similar energies of other intermediates and transition states along the reaction mechanism show improved scaling relations relative to the reference catalyst. The linear combination of atomic potential tight-binding model Hamiltonian and the limited search of synthetically viable changes in the reference structure enable efficient minimization of the energy barrier for the rate-limiting step (i.e., formation of [LNiII(COOH)]-), bypassing the exponential scaling problem of high-throughput screening techniques. The reported findings demonstrate an inverse design method that could also be implemented with multiple descriptors, including reaction barriers and thermodynamic parameters for reversible reactivity.
RESUMO
The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°(298â¯K) = -3.1 kcal/mol measured by (1)H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru-hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N-H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru-H hydride transfer to CO2 followed by rotation of the resulting N-H-stabilized formate to a Ru-O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.
RESUMO
[2.2]Paracyclophanes with chiral ketimine side chains constitute a class of highly versatile and enantioselective ligands for catalytic carbon-carbon bond forming reactions. Proper matching of the side chain and [2.2]paracyclophane configurations induces chiral cooperativity, which is key to high selectivities. Here we show that the absolute configuration of both chirotropic elements may be fully resolved by CD spectroscopy and time-dependent density functional calculations. Different ketimine side chain conformations of the diastereomers perturb the planar chiral [2.2]paracyclophane chromophore. This leads to characteristic changes in the measured CD spectra and the specific rotation allowing for the simultaneous assignment of the absolute configuration of both chiral elements. Our results give rise to a simple rule relating sign and magnitude of the specific rotation and the first band of the CD spectra to the absolute configuration of both chiral elements. We infer a tautomeric equilibrium between an ortho-hydroquinone-imine and an ortho-quinone-enamine from strong solvatochromism observed in the CD spectra.
