Oxidative Grafting for Catalyst Synthesis in Surface Organometallic Chemistry.

The development of new methods of catalyst synthesis with the potential to generate active site structures orthogonal to those accessible by traditional protocols is of great importance for discovering new materials for addressing challenges in the evolving energy and chemical economy. In this work, the generality of oxidative grafting of organometallic and well-defined molecular metal precursors onto redox-active surfaces such as manganese dioxide (MnO2) and lithium manganese oxide (LiMn2O4) is investigated. Nine molecular metal precursors are explored, spanning groups 4-11 and each of the three periods of the transition metal series. The byproducts of the oxidative grafting reaction, a mixture of protodemetalation and ligand homocoupling for several organometallic precursors, was found to provide insights into the mechanism of the grafting reaction, suggesting oxidation of both the metal d-orbitals, as well as the metal-carbon σ-bonds, resulting in ejection of the ligand radical fragment. Analysis of the supported structures and oxidation state by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that several of the chemisorbed metal ions are intercalated into interstitial vacancies of the surface structure while other complexes form intact molecular fragments on the surface. Proof of concept for the use of this metalation protocol to generate diverse, metal-dependent catalytic performance is demonstrated by the application of these materials in the conversion of cyclohexane to K/A oil (cyclohexanol and cyclohexanone) with tert-butyl hydroperoxide, as well as in the low-temperature (T ≤ 50 °C) oxidation of carbon monoxide to carbon dioxide.

Sulfated Zirconium Metal-Organic Frameworks as Well-Defined Supports for Enhancing Organometallic Catalysis.

Understanding heterogeneous catalysts is a challenging pursuit due to surface site nonuniformity and aperiodicity in traditionally used materials. One example is sulfated metal oxides, which function as highly active catalysts and as supports for organometallic complexes. These applications are due to traits such as acidity, ability to act as a weakly coordinating ligand, and aptitude for promoting transformations via radical cation intermediates. Research is ongoing about the structural features of sulfated metal oxides that imbue the aforementioned properties, such as sulfate geometry and coordination. To better understand these materials, metal-organic frameworks (MOFs) have been targeted as structurally defined analogues. Composed of inorganic nodes and organic linkers, MOFs possess features such as high porosity and crystallinity, which make them attractive for mechanistic studies of heterogeneous catalysts. In this work, Zr6-based MOF NU-1000 is sulfated and characterized using techniques such as single crystal X-ray diffraction in addition to diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The dynamic nature of the sulfate binding motif is found to transition from monodentate, to bidentate, to tridentate depending on the degree of hydration, as supported by density functional theory (DFT) calculations. Heightened Brønsted acidity compared to the parent MOF was observed upon sulfation and probed through trimethylphosphine oxide physisorption, ammonia sorption, in situ ammonia DRIFTS, and DFT studies. With the support structure benchmarked, an organoiridium complex was chemisorbed onto the sulfated MOF node, and the efficacy of this supported catalyst was demonstrated for stoichiometric and catalytic activation of benzene-d6 and toluene with structure-activity relationships derived.

Effects of Protonation State on Electrocatalytic CO2 Reduction by a Cobalt Aminopyridine Macrocyclic Complex.

A critical component in the reduction of CO2 to CO and H2O is the delivery of 2 equiv of protons and electrons to the CO2 molecule. The timing and sequencing of these proton and electron transfer steps are essential factors in directing the activity and selectivity for catalytic CO2 reduction. In previous studies, we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO2 to CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant (kobs) and the number of pendant amine hydrogen bond donors minus one, suggesting the presence of a deprotonated active catalytic state. Herein, we investigate the feasibility of these proposed deprotonated complexes toward CO2 reduction. Two deprotonated derivatives, Co(L4-) and Co(L2-), of the tetraamino macrocycle Co(L) were independently synthesized and structurally characterized revealing extensive delocalization of the negative charge upon deprotonation. 1H nuclear magnetic resonance spectroscopy and ultraviolet-visible titration studies confirm that under catalytic conditions, the active form of the catalyst gradually becomes deprotonated, supporting thus the ndonor - 1 relationship with kobs. Electrochemical studies of Co(L4-) reveal that this deprotonated analogue is competent for electrocatalysis upon addition of an exogenous weak acid source, such as 2,2,2-trifluoroethanol, resulting in faradaic efficiencies for CO2-to-CO conversion identical to those observed with the fully protonated derivative (>98%).

Electronically Modified Cobalt Aminopyridine Complexes Reveal an Orthogonal Axis for Catalytic Optimization for CO2 Reduction.

The design of effective electrocatalysts for carbon dioxide reduction requires understanding the mechanistic underpinnings governing the binding, reduction, and protonation of CO2. A critical aspect to understanding and tuning these factors for optimal catalysis revolves around controlling the electronic environments of the primary and secondary coordination sphere. Herein we report a series of para-substituted cobalt aminopyridine macrocyclic catalysts 2-4 capable of carrying out the electrochemical reduction of CO2 to CO. Under catalytic conditions, complexes 2-4, as well as the unsubstituted cobalt aminopyridine complex 1, exhibit icat/ip values ranging from 144 to 781. Complexes 2 and 4 exhibit a pronounced precatalytic wave suggestive of an ECEC mechanism. A Hammett analysis reveals that ligand modifications with electron-donating groups enhance catalysis (ρ < 0), indicative of positive charge buildup in the transition state. This trend also extends to the CoI/0 potential, where complexes possessing more negative E(CoI/0) reductions exhibit greater icat/ip values. The reported modifications offer a synthetic lever to tune catalytic activity, orthogonal to our previous study of the role of pendant hydrogen bond donors.

Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO2 Reduction.

The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO2, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO2, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO2 substrate.

Proton-Assisted Reduction of CO2 by Cobalt Aminopyridine Macrocycles.

We report here the efficient reduction of CO2 to CO by cobalt aminopyridine macrocycles. The effect of the pendant amines on catalysis was investigated. Several cobalt complexes based on the azacalix[4](2,6)pyridine framework with different substitutions on the pendant amine groups have been synthesized (R = H (1), Me (2), and allyl (3)), and their electrocatalytic properties were explored. Under an atmosphere of CO2 and in the presence of weak Brønsted acids, large catalytic currents are observed for 1, corresponding to the reduction of CO2 to CO with excellent Faradaic efficiency (98 ± 2%). In comparison, complexes 2 and 3 generate CO with TONs at least 300 times lower than 1, suggesting that the presence of the pendant NH moiety of the secondary amine is crucial for catalysis. Moreover, the presence of NH groups leads to a positive shift in the reduction potential of the Co(I/0) couple, therefore decreasing the overpotential for CO2 reduction.

Synthesis of a cobalt(IV) ketimide with a squashed tetrahedral geometry.

Oxidation of [Li(THF)]2[Co(N=C(t)Bu2)4] with 1 equiv. of I2 generates Co(N=C(t)Bu2)4 in 85% yield. In the solid-state, this complex exhibits a squashed tetrahedral structure about the Co center. DFT calculations reveal this geometry arises, in part, to maximize ketimide-to-cobalt π-donation.