Designing Metal‐Free Catalysts by Mimicking Transition‐Metal Pincer Templates

Abstract

Whereas nature often promotes reactions by utilizing cheap and abundant light transition metals (TMs; e.g., Fe, Mn, Ni, Cu, and Zn) in enzyme active sites, the majority of man-made catalysts are based on precious heavy TMs (e.g., Ru, Rh, Ir, Pd, and Pt), despite high costs, limited availability, and contamination problems. Development of cheap, green, and effective catalysts (without precious TMs or even TMs at all) is at the forefront of chemical research. The recent discovery of metal-free reversible hydrogen activation by Stephan and co-workers facilitates direct catalytic hydrogenation.[1] On the basis of the frustrated Lewis pair (FLP) principle, [1,2,3] we designed metal-free hydrogenation catalysts computationally.[4] The metal-free catalyst 1 [4b, c] and the well-known metal-ligand bifunctional hydrogenation catalyst 2 [5] have related electronic and geometric structural features (Scheme 1). Indeed, increasing experimental evidence demonstrates the resemblance between the reactivity of TM-free compounds and TM complexes.[6] Herein, we computationally explore the design of metal-free catalysts by directly mimicking TM catalysts.The M05-2X DFT functional [7] was specifically developed to target nonbonding interactions. We previously calibrated the good performance of the functional [8] in describing weak bonding interactions and applied it to understand the catalytic role of N-heterocyclic carbenes in the metal-free transformation of carbon dioxide into methanol.[9] In our design of FLP-based hydrogen-activation molecules [4a] and hydrogenation catalysts, [4b] we found that the energetic results given by M05-2X were in good agreement with those provided by CCSD(T) [4a] or MP2 calculations.[4b] M05-2X (as implemented in the Gaussian 03 program) [10] was thus selected for use in all of the DFT calculations. All of the geometries were optimized and characterized as minima or transition states (TSs) at the M05-2X/6-31GA C H T U N G T R E N N U N G (d,p) level. The energies were then refined by using M05-2X/6-311 p) wave functions of the TSs were confirmed to be stable, which indicates the reliability of our single-referencebased calculations. The harmonic frequencies at the same level were employed for zero-point energy corrections and thermal and entropy corrections at 298.15 K and 1 atm. Bulky solvation effects were simulated by using the IEFPCM model [11] with benzene as a representative solvent. The corresponding free energies are discussed below, unless otherwise specified. For clarity, we use Lewis structure drawings to depict the electronic structures of molecules in the main text. The optimized geometries and their Cartesian coordinates are provided in the Supporting Information.We selected a new type of pincer catalyst (3 in Scheme 2) recently developed by Milstein and co-workers [12] as a template to derive TM-free analogues. Such TM pincer catalysts have intriguing electronic structures [13] (see below) and exhibit high s-bond activation reactivity (e.g., involving H À H, [12a, b] C À H, [12b]...

The activation strain model and molecular orbital theory

“…Conveniently, there is no special computer code required to perform activation strain analyses: all necessary quantities can be computed using any of the regular quantum‐chemical software packages available. As a result, the activation strain model has been applied by various research groups, on a range of chemical processes, such as nucleophilic substitution, cycloaddition, oxidative addition, isomerization, and many other processes from organic and organometallic chemistry.…”

Reversible Hydrogen Activation by a Pyridonate Borane Complex: Combining Frustrated Lewis Pair Reactivity with Boron‐Ligand Cooperation

“…[2] Lately,t ransition-metal systems with ap yridine-based pincer ligand capable of dihydrogen activation via ar eversible de-aromatization sequence attracted considerable attention. [5,6] Specifically, H 2 activation by methylpyridine complexes was investigated computationally (Scheme 1). [3,4] Inspired by this mode of action, Wang et al examined in at heoretical investigation whether the concept of metal-ligand cooperation can be transferred to metal-free systems.…”

“…[3,4] Inspired by this mode of action, Wang et al examined in at heoretical investigation whether the concept of metal-ligand cooperation can be transferred to metal-free systems. [5,6] Specifically, H 2 activation by methylpyridine complexes was investigated computationally (Scheme 1). This study led to the conclusion that for the simple case of amethylpyridine·BH 3 complex the de-aromatized form is too unstable to allow reversible hydrogen activation.…”

Element‐Ligand Cooperativity with p‐Block Elements

“…By moving from the d‐block to boron, Milstein and co‐workers found an analog concept that relies on aromaticity shifting and BLC. Inspired by quantum‐theoretical work, they synthesized pyridine‐coordinated amino‐borane 7 (Figure a) . Exposure of 7 to elevated temperatures induced the liberation of H 2 and a concomitant shift in aromaticity from the pyridine ring into the five‐membered boracycle of 8 , as revealed by the computation of NICS values.…”