Divergent Bimetallic Mechanisms in Copper(II)-Mediated C-C, N-N, and O-O Oxidative Coupling Reactions.

Copper-catalyzed aerobic oxidative coupling of diaryl imines provides a route for conversion of ammonia to hydrazine. The present study uses experimental and density functional theory computational methods to investigate the mechanism of N-N bond formation, and the data support a mechanism involving bimolecular coupling of Cu-coordinated iminyl radicals. Computational analysis is extended to CuII-mediated C-C, N-N, and O-O coupling reactions involved in the formation of cyanogen (NC-CN) from HCN, 1,3-butadiyne from ethyne (i.e., Glaser coupling), hydrazine from ammonia, and hydrogen peroxide from water. The results reveal two different mechanistic pathways. Heteroatom ligands with an uncoordinated lone pair (iminyl, NH2, OH) undergo charge transfer to CuII, generating ligand-centered radicals that undergo facile bimolecular radical-radical coupling. Ligands lacking a lone pair (CN and CCH) form bridged binuclear diamond-core structures that undergo C-C coupling. This mechanistic bifurcation is rationalized by analysis of spin densities in key intermediates and transition states, as well as multiconfigurational calculations. Radical-radical coupling is especially favorable for N-N coupling owing to energetically favorable charge transfer in the intermediate and thermodynamically favorable product formation.

Active Learning Configuration Interaction for Excited-State Calculations of Polycyclic Aromatic Hydrocarbons.

We present the active learning configuration interaction (ALCI) method for multiconfigurational calculations based on large active spaces. ALCI leverages the use of an active learning procedure to find important electronic configurations among the full configurational space generated within an active space. We tested it for the calculation of singlet-singlet excited states of acenes and pyrene using different machine learning algorithms. The ALCI method yields excitation energies within 0.2-0.3 eV from those obtained by traditional complete active-space configuration interaction (CASCI) calculations (affordable for active spaces up to 16 electrons in 16 orbitals) by including only a small fraction of the CASCI configuration space in the calculations. For larger active spaces (we tested up to 26 electrons in 26 orbitals), not affordable with traditional CI methods, ALCI captures the trends of experimental excitation energies. Overall, ALCI provides satisfactory approximations to large active-space wave functions with up to 10 orders of magnitude fewer determinants for the systems presented here. These ALCI wave functions are promising and affordable starting points for the subsequent second-order perturbation theory or pair-density functional theory calculations.

Using Redox-Active Ligands to Generate Actinide Ligand Radical Species.

Using a redox-active dioxophenoxazine ligand, DOPO (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazine-9-olate), a family of actinide (U, Th, Np, and Pu) and Hf tris(ligand) coordination compounds was synthesized. The full characterization of these species using 1H NMR spectroscopy, electronic absorption spectroscopy, SQUID magnetometry, and X-ray crystallography showed that these compounds are analogous and exist in the form M(DOPOq)2(DOPOsq), where two ligands are of the oxidized quinone form (DOPOq) and the third is of the reduced semiquinone (DOPOsq) form. The electronic structures of these complexes were further investigated using CASSCF calculations, which revealed electronic structures consistent with metals in the +4 formal oxidation state and one unpaired electron localized on one ligand in each complex. Furthermore, f orbitals of the early actinides show a sizable bonding overlap with the ligand 2p orbitals. Notably, this is the first example of a plutonium-ligand radical species and a rare example of magnetic data being recorded for a homogeneous plutonium coordination complex.

A Tunable Multivariate Metal-Organic Framework as a Platform for Designing Photocatalysts.

Catalysts for photochemical reactions underlie many foundations in our lives, from natural light harvesting to modern energy storage and conversion, including processes such as water photolysis by TiO2. Recently, metal-organic frameworks (MOFs) have attracted large interest within the chemical research community, as their structural variety and tunability yield advantages in designing photocatalysts to address energy and environmental challenges. Here, we report a series of novel multivariate metal-organic frameworks (MTV-MOFs), denoted as MTV-MIL-100. They are constructed by linking aromatic carboxylates and AB2OX3 bimetallic clusters, which have ordered atomic arrangements. Synthesized through a solvent-assisted approach, these ordered and multivariate metal clusters offer an opportunity to enhance and fine-tune the electronic structures of the crystalline materials. Moreover, mass transport is improved by taking advantage of the high porosity of the MOF structure. Combining these key advantages, MTV-MIL-100(Ti,Co) exhibits a high photoactivity with a turnover frequency of 113.7 molH2 gcat.-1 min-1, a quantum efficiency of 4.25%, and a space time yield of 4.96 × 10-5 in the photocatalytic hydrolysis of ammonia borane. Bridging the fields of perovskites and MOFs, this work provides a novel platform for the design of highly active photocatalysts.

Tuning Catalytic Sites on Zr6O8 Metal-Organic Framework Nodes via Ligand and Defect Chemistry Probed with tert-Butyl Alcohol Dehydration to Isobutylene.

Metal-organic frameworks (MOFs) have drawn wide attention as candidate catalysts, but some essential questions about their nature and performance have barely been addressed. (1) How do OH groups on MOF nodes act as catalytic sites? (2) What are the relationships among these groups, node defects, and MOF stability, and how do reaction conditions influence them? (3) What are the interplays between catalytic properties and transport limitations? To address these questions, we report an experimental and theoretical investigation of the catalytic dehydration of tert-butyl alcohol (TBA) used to probe the activities of OH groups of Zr6O8 nodes in the MOFs UiO-66 and MOF-808, which have different densities of vacancy sites and different pore sizes. The results show that (1) terminal node OH groups are formed as formate and/or acetate ligands present initially on the nodes react with TBA to form esters, (2) these OH groups act as catalytic sites for TBA dehydration to isobutylene, and (3) TBA also reacts to break node-linker bonds to form esters and thereby unzip the MOFs. The small pores of UiO-66 limit the access of TBA and the reaction with the formate/acetate ligands bound within the pores, whereas the larger pores of MOF-808 facilitate transport and favor reaction in the MOF interior. However, after removal of the formate and acetate ligands by reaction with methanol to form esters, interior active sites in UiO-66 become accessible for the reaction of TBA, with the activity depending on the density of defect sites with terminal OH groups. The number of vacancies on the nodes is important in determining a tradeoff between the catalytic activity of a MOF and its resistance to unzipping. Computations at the level of density functional theory show how the terminal OH groups on node vacancies act as Brønsted bases, facilitating TBA dehydration via a carbocation intermediate in an E1 mechanism; the calculations further illuminate the comparable chemistry of the unzipping.

Evidence of Alpha Radiolysis in the Formation of a Californium Nitrate Complex.

Well-characterized complexes of transplutonium elements are scarce because of the experimental challenges of working with these elements and the rarity of the isotopes. This leads to a lack of structural and spectroscopic data needed to understand the nature of chemical bonds in these compounds. In this work, the synthesis of Cf(DOPOq )2 (NO3 )(py) (DOPOq =2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate; py=pyridine) is reported, in which the nitrate anion is hypothesized to form through the α-radiolysis-induced reaction of pyridine and/or the ligand. Computational analysis of the electronic structure of the complex reveals that the CfIII -ligand interactions are largely ionic.

DFT Study on the Catalytic Activity of ALD-Grown Diiron Oxide Nanoclusters for Partial Oxidation of Methane to Methanol.

Using density functional theory (DFT), we studied the catalytic activity of iron oxide nanoclusters that mimic the structure of the active site in the soluble form of methane monooxygenase (sMMO) for the partial oxidation of methane to methanol. Using N2O as the oxidant, we consider a radical-rebound mechanism and a concerted mechanism for the oxidation of methane on either a bridging oxygen (Ob) or a terminal oxygen (Ot) active site. We find that the radical-rebound pathway is preferred over the concerted pathway by 40-50 kJ/mol, but the desorption of methanol and the regeneration of the oxygen site are found to be the highest barriers for the direct conversion of methane to methanol with these catalysts. As demonstrated by a population analysis, the Ox (x = b or t) site behaves as an oxygen radical during the H abstraction, and the [Fe+-Ox-] site behaves as a Lewis acid-base pair during the concerted C-H cleavage. Molecular orbital decomposition analysis further demonstrates electron transfer during the oxidation and reduction steps of the reaction. High-level multireference calculations were also carried out to further assess the DFT results. Understanding how these systems behave during the proposed reaction pathways provides new insights into how they can be tuned for methane partial oxidation.

Effects of Covalency on Anionic Redox Chemistry in Semiquinoid-Based Metal-Organic Frameworks.

Two iron-semiquinoid framework materials, (H2NMe2)2Fe2(Cl2 dhbq)3 (1) and (H2NMe2)4Fe3(Cl2 dhbq)3(SO4)2 (Cl2 dhbqn- = deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone) (2-SO4), are shown to possess electrochemical capacities of up to 195 mAh/g. Employing a variety of spectroscopic methods, we demonstrate that these exceptional capacities arise from a combination of metal- and ligand-centered redox processes, a result supported by electronic structure calculations. Importantly, similar capacities are not observed in isostructural frameworks containing redox-inactive metal ions, highlighting the importance of energy alignment between metal and ligand orbitals to achieve high capacities at high potentials in these materials. Prototype lithium-ion devices constructed using 1 as a cathode demonstrate reasonable capacity retention over 50 cycles, with a peak specific energy of 533 Wh/kg, representing the highest value yet reported for a metal-organic framework. In contrast, the capacities of devices using 2-SO4 as a cathode rapidly diminish over several cycles due to the low electronic conductivity of the material, illustrating the nonviability of insulating frameworks as cathode materials. Finally, 1 is further demonstrated to access similar capacities as a sodium-ion or potassium-ion cathode. Together, these results demonstrate the feasibility and versatility of metal-organic frameworks as energy storage materials for a wide range of battery chemistries.

Hydrogen Atom or Proton Coupled Electron Transfer? C-H Bond Activation by Transition-Metal Oxides.

The C-H bond activation in oxidative dehydrogenation of propane by heterobimetallic oxide clusters (first-row transition metals), deposited on the zirconium oxide node of the NU-1000 metal organic framework, was investigated by multireference wave function theory. The redox-active part of the systems studied has the composition (CoO)(MO)(OH)2 with M = Ti, Mn, Fe, Co, Ni, Cu, Zn. In this series, the energy of H transfer from propane to the metal oxide (ΔE) varies from -26 kcal/mol for M = Cu, Zn to 85 kcal/mol for M = Ti. This is accompanied by a change in the mechanism from hydrogen atom transfer, M2+(dn) O•- → M2+(dn) OH-, for M = Cu, Zn to proton coupled electron transfer, Mm+(dn) O2- → M(m-1)+(dn+1) OH-, for M = Ni, Co, Fe, Mn, Ti. Whereas for M = Ni (ΔE = -13 kcal/mol) Ni+III is reduced to Ni+II, for M = Co, Fe, Mn (ΔE = 1, 10, 6 kcal/mol, respectively) it is Co+III that is reduced to Co+II. For M = Ti, Ti maintains its +IV oxidation state and Co+II is reduced to Co+I.

Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides.

An isostructural family of f-element compounds (Ce, Nd, Sm, Gd; Am, Bk, Cf) of the redox-active dioxophenoxazine ligand (DOPOq; DOPO = 2,4,6,8-tetra- tert-butyl-1-oxo-1 H-phenoxazin-9-olate) was prepared. This family, of the form M(DOPOq)3, represents the first nonaqueous isostructural series, including the later actinides berkelium and californium. The lanthanide derivatives were fully characterized using 1H NMR spectroscopy and SQUID magnetometry, while all species were structurally characterized by X-ray crystallography and electronic absorption spectroscopy. In order to probe the electronic structure of this new family, CASSCF calculations were performed and revealed these systems to be largely ionic in contrast to previous studies, where berkelium and californium typically have a small degree of covalent character. To validate the zeroth order regular approximation (ZORA) method, the same CASSCF analysis using experimental structures versus UDFT-ZORA optimized structures does not exhibit sizable changes in bonding patterns. This shows that UDFT-ZORA combined with CASSCF could be a useful first approximation to predict and investigate the structure and electronic properties of actinides and lanthanides that are difficult to synthesize or characterize.

Theoretical Investigation of Plutonium-Based Single-Molecule Magnets.

The electronic structure of a plutonium-based single-molecule magnet (SMM) was theoretically examined by means of multiconfigurational electronic structure theory calculations, including spin-orbit coupling effects. All Pu 5f to 5f transitions for all possible spin states were computed, as well as ligand to metal charge transfer and Pu 5f to 6d transitions. Spin-orbit coupling effects were included a posteriori to accurately describe the electronic transitions. The spin-orbit coupled energies and magnetic moments were then used to compute the magnetic susceptibility curves. The experimental electronic structure and magnetic susceptibility curve were reproduced well by our calculations. A compound with a modified electron-donating ligand (namely a carbene ligand) was also investigated in an attempt to tune the electronic properties of the plutonium SMM, revealing a higher ligand field splitting of the 5f orbitals of Pu, which could in turn enhance the barrier against magnetic relaxation.

Spin-Forbidden Reactions: Adiabatic Transition States Using Spin-Orbit Coupled Density Functional Theory.

A spin-forbidden chemical reaction involves a change in the total electronic spin state from reactants to products. The mechanistic study is challenging because such a reaction does not occur on a single diabatic potential energy surface (PES), but rather on two (or multiple) spin diabatic PESs. One possible approach is to calculate the so-called "minimum energy crossing point" (MECP) between the diabatic PESs, which however is not a stationary point. Inclusion of spin-orbit coupling between spin states (SOC approach) allows the reaction to occur on a single adiabatic PES, in which a transition state (TS SOC) as well as activation free energy can be calculated. This Concept article summarizes a previously published application in which, for the first time, the SOC effects, using spin-orbit ZORA Hamiltonian within density functional theory (DFT) framework, are included and account for the mechanism of a spin-forbidden reaction in gold chemistry. The merits of the MECP and TS SOC approaches and the accuracy of the results are compared, considering both our recent calculations on molecular oxygen addition to gold(I)-hydride complexes and new calculations for the prototype spin-forbidden N2 O and N2 Se dissociation reactions.

Globularity-Selected Large Molecules for a New Generation of Multication Perovskites.

Perovskite solar cells (PSCs) use perovskites with an APbX3 structure, where A is a monovalent cation and X is a halide such as Cl, Br, and/or I. Currently, the cations for high-efficiency PSCs are Rb, Cs, methylammonium (MA), and/or formamidinium (FA). Molecules larger than FA, such as ethylammonium (EA), guanidinium (GA), and imidazolium (IA), are usually incompatible with photoactive "black"-phase perovskites. Here, novel molecular descriptors for larger molecular cations are introduced using a "globularity factor", i.e., the discrepancy of the molecular shape and an ideal sphere. These cationic radii differ significantly from previous reports, showing that especially ethylammonium (EA) is only slightly larger than FA. This makes EA a suitable candidate for multication 3D perovskites that have potential for unexpected and beneficial properties (suppressing halide segregation, stability). This approach is tested experimentally showing that surprisingly large quantities of EA get incorporated, in contrast to most previous reports where only small quantities of larger molecular cations can be tolerated as "additives". MA/EA perovskites are characterized experimentally with a band gap ranging from 1.59 to 2.78 eV, demonstrating some of the most blue-shifted PSCs reported to date. Furthermore, one of the compositions, MA0.5 EA0.5 PbBr3 , shows an open circuit voltage of 1.58 V, which is the highest to date with a conventional PSC architecture.

The ligand effect on the oxidative addition of dioxygen to gold(i)-hydride complexes.

The ligand effect on the recently uncovered feasible oxidative addition reaction of O2 on [LAuH] complexes has been investigated for a series of fifteen ligands. The activation barriers of this spin-forbidden reaction have been estimated at the minimum energy crossing points (MECP, relativistic scalar level) between the adiabatic triplet (reactants spin state) and singlet (product spin state) potential energy surfaces (PES) and calculated at the transition states by including Spin-Orbit Coupling (SOC) effects, as applied for the mechanistic study of this reaction in a previous study by us [Chem. Sci., 2016, 7, 7034-7039]. We find a sizeable effect of the ligand on the activation barriers, and some of the stronger electron donating phosphines are predicted to induce the highest catalyst efficiency. The inclusion of SOC effects lowers the activation barriers by about 3 kcal mol-1 systematically with respect to the MECP values independently of the ligand type. We used the Charge-Displacement (CD) analysis to quantify the net electron charge donation from the ligand L towards the metallic fragment AuH in the [LAuH] series, and surprisingly only a poor correlation was found between the net electron donor character of L and the activation barriers. Application of the CD-NOCV (Natural Orbitals for Chemical Valence) approach, which allows the quantification of the Dewar-Chatt-Duncanson (DCD) L-AuH bond components, suggests that the ligand effect on the activation barriers is not easily predictable on the basis of solely the electronic properties of the ligand and depends significantly on the ligand nature or carbene or phosphine type. We show that for both phosphine and carbene ligand subsets, however, the σ donation component of the L-AuH bond quantitatively accounts for the ligand effect on the activation energy barriers (a larger σ-donor capability of L correlates with a smaller activation barrier), whereas the π back-donation, strongly affected by geometrical rearrangement, is a poor reactivity descriptor (π acceptor properties of the ligand L in the linear [LAuH] complexes are not transferable to the trigonal [LAuH(O2)] transition state structures).

Modulating the Bonding Properties of N-Heterocyclic Carbenes (NHCs): A Systematic Charge-Displacement Analysis.

In view of their intensive use as ligands in many reactions catalyzed by transition-metal complexes, modulation of the bonding properties of N-heterocyclic carbenes (NHCs) on a rational basis is highly desirable, which should enable optimization of current applications or even promote new functions. In this paper, we provide a quantitative analysis of the chemical bond between a metal fragment AuCl and a series of 29 different NHCs in [(NHC)AuCl] complexes. NHCs electronic properties are modified through: i) variation of the groups attached to the NHC nitrogen atoms or backbone; ii) change of unsaturation/size of the NHC ring; iii) inclusion of paracyclophane moieties; or iv) heteroatom substitution on the NHC ring. For evaluating the donation and back-donation components of the Dewar-Chatt-Duncanson (DCD) model in the NHC-AuCl bond, we apply the charge-displacement (CD) analysis within the NOCV (natural orbitals for chemical valence) framework, a methodology that avoids the constraint of using symmetrized structures. We show that modulation of the NHC bonding properties requires substantial modification of their structure, such as, for instance, insertion of two ketone groups into the NHC backbone (which enhances the π back-donation bond component and introduces an effective electronic communication within the NHC ring) or replacement of a nitrogen atom in the ring with an sp3 or sp2 carbon atom (which increases and decreases the π back-donation bond component, respectively). We extend our investigation by quantitatively comparing the NHC electronic structures for a subset of 13 NHCs in [(NHC)PPh] adducts, the 31 P NMR chemical shift values of which are experimentally available. The latter have been considered as a suitable tool for measuring the NHCs π acceptor properties [Bertrand et al., Angew. Chem. Int. Ed. 2013, 52, 2939-2943]. We show that information obtained using the metal fragment can be transferred to the PPh moiety and vice versa. However, the 31 P NMR chemical shift values only qualitatively correlate with the π acceptor properties of the NHCs, with the stronger π acidic carbenes as the most outliners.

The gold(iii)-CO bond: a missing piece in the gold carbonyl complex landscape.

We investigate the Au(iii)-CO bond in the [(C∧N∧C)Au(iii)CO]+ complex via charge-displacement (CD) analysis, revealing surprising peculiarities with respect to the Au(i)-CO bond. It shows a large Au(iii) ← CO σ donation and, unexpectedly, an even larger Au(iii) → CO π back-donation. The overall bonding picture, including a marked anisotropy in the Au(iii) → CO in-plane and out-of-plane π back-donation components, is consistent with a CO carbon atom highly susceptible to nucleophilic attack.

Dioxygen insertion into the gold(i)-hydride bond: spin orbit coupling effects in the spotlight for oxidative addition.

O2 insertion into a Au(i)-H bond occurs through an oxidative addition/recombination mechanism, showing peculiar differences with respect to Pd(ii)-H, for which O2 insertion takes place through a hydrogen abstraction mechanism in the triplet potential energy surface with a pure spin transition state. We demonstrate that the spin-forbidden Au(i)-hydride O2 insertion reaction can only be described accurately by inclusion of spin orbit coupling (SOC) effects. We further find that a new mechanism involving two O2 molecules is also feasible, and this result, together with the unexpectedly high experimental entropic activation parameter, suggests the possibility that a third species could be involved in the rate determining step of the reaction. Finally, we show that the O2 oxidative addition into a Au(i)-alkyl (CH3) bond also occurs but the following recombination process using O2 is unfeasible and the metastable intermediate Au(iii) species will revert to reactants, thus accounting for the experimental inertness of Au-alkyl complexes toward oxygen, as frequently observed in catalytic applications. We believe that this study can pave the way for further theoretical and experimental investigations in the field of Au(i)/Au(iii) oxidation reactions, including ligand, additive and solvent effects.

Anomalous ligand effect in gold(I)-catalyzed intramolecular hydroamination of alkynes.

We analyzed the ligand electronic effect in a gold(I)-catalyzed intramolecular alkyne hydroamination, through a DFT and charge-displacement function (CDF) study. We found that, in the presence of π-electron conjugation between the alkyne and the nucleophilic functionality, electron poor ligands modify the coordination mode and the geometric parameters of the substrate in such a way that, contrary to expectations, the activation barrier of the nucleophilic attack increases. This remarkable effect is due to the competition between alkyne activation and nucleophile deactivation. The general relevance of these findings is highlighted.