Overdestabilization vs Overstabilization in the Theoretical Analysis of f-Orbital Covalency.

The complex nature of the f-orbital electronic structures and their interaction with the chemical environment pose significant computational challenges. Advanced computational techniques that variationally include scalar relativities and spin-orbit coupling directly at the molecular orbital level have been developed to address this complexity. Among these, variational relativistic multiconfigurational multireference methods stand out for their high accuracy and systematic improvement in studies of f-block complexes. Additionally, these advanced methods offer the potential for calibrating low-scaling electronic structure methods such as density functional theory. However, studies on the Cl K-edge X-ray absorption spectra of the [Ce(III)Cl6]3- and [Ce(IV)Cl6]2- complexes show that time-dependent density functional theory with approximate exchange-correlation kernels can lead to inaccuracies, resulting in an overstabilization of 4f orbitals and incorrect assessments of covalency. In contrast, approaches utilizing small active space wave function methods may understate the stability of these orbitals. The results herein demonstrate the need for large active space, multireference, and variational relativistic methods in studying f-block complexes.

Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research.

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Molecular Geometry and Electronic Structure of Copper Corroles.

Copper corroles are known for their unique multiconfigurational electronic structures in the ground state, which arise from the transfer of electrons from the π orbitals of the corrole to the d-orbital of copper. While density functional theory (DFT) provides reasonably good molecular geometries, the determination of the ground spin state and the associated energetics is heavily influenced by functional choice, particularly the percentage of the Hartree-Fock exchange. Using extended multireference perturbation theory methods (XMS-CASPT2), the functional choice can be assessed. The molecular geometries and electronic structures of both the unsubstituted and the meso-triphenyl copper corroles were investigated. A minimal active space was employed for structural characterization, while larger active spaces are required to examine the electronic structure. The XMS-CASPT2 investigations conclusively identify the ground electronic state as a multiconfigurational singlet (S0) with three dominant electronic configurations in its lowest energy and characteristic saddled structure. In contrast, the planar geometry corresponds to the triplet state (T0), which is approximately 5 kcal/mol higher in energy compared to the S0 state for both the bare and substituted copper corroles. Notably, the planarity of the T0 geometry is reduced in the substituted corrole compared with that in the unsubstituted one. By analyzing the potential energy surface (PES) between the S0 and T0 geometries using XMS-CASPT2, the multiconfigurational electronic structure is shown to transition toward a single electron configuration as the saddling angle decreases (i.e., as one approaches the planar geometry). Despite the ability of the functionals to reproduce the minimum energy structures, only the TPSSh-D3 PES is reasonably close to the XMS-CASPT2 surface. Significant deviations along the PES are observed with other functionals.

The Influence of Phosphorus Substituents on the Structures and Solution Speciation of Trivalent Uranium and Lanthanide Phosphinodiboranates.

Here, we report the mechanochemical synthesis and characterization of homoleptic uranium and lanthanide phosphinodiboranates with isopropyl and ethyl substituents attached to phosphorus. M(H3BPiPr2BH3)3 complexes with M = U, Nd, Sm, Tb, and Er were prepared by ball milling UI3(THF)4, SmBr3, or MI3 with three equivalents of K(H3BPiPr2BH3). M(H3BPEt2BH3)3 with M = U and Nd were prepared similarly using K(H3BPEt2BH3), and the complexes were purified by extraction and crystallization from Et2O or CH2Cl2. Single-crystal XRD studies revealed that all five M(H3BPiPr2BH3)3 crystallize as dimers, despite the significant differences in metal radii across the series. In contrast, Nd(H3BPEt2BH3)3 with smaller ethyl substituents crystallized as a coordination polymer. Crystals of U(H3BPEt2BH3)3 were not suitable for structural analysis, but crystals of U(H3BPMe2BH3)3 isolated in low yield by solution methods were isostructural with Nd(H3BPEt2BH3)3. 1H and 11B NMR studies in C6D6 revealed that all of the complexes form mixtures of monomer and oligomers when dissolved, and the extent of oligomerization was highly dependent on metal radius and phosphorus substituent size. A comprehensive analysis of all structurally characterized uranium and lanthanide phosphinodiboranate complexes reported to date, including those with larger Ph and tBu substituents, revealed that the degree of oligomerization in solution can be correlated to differences in B-P-B angles obtained from single-crystal XRD studies. Density functional theory calculations, which included structural optimizations in combination with conformational searches using tight binding methods, replicated the general experimental trends and revealed free energy differences that account for the different solution and solid-state structures. Collectively, these results reveal how steric changes to phosphorus substituents significantly removed from metal coordination sites can have a significant influence on solution speciation, deoligomerization energies, and the solid-state structure of homoleptic phosphinodiboranate complexes containing trivalent f-metals.

Importance of Dispersion in the Molecular Geometries of Mn(III) Spin-Crossover Complexes.

The computational investigation of the molecular geometries of a pair of manganese(III) spin-crossover complexes is reported. For the geometry of the quintet high-spin state, density functionals significantly overestimate Mn-Namine bond distances, although the geometry for the triplet intermediate-spin state is well described. Comparisons with several wave function-based methods demonstrate that this error is due to the limited ability of commonly used density functionals to recover dispersion beyond a certain extent. Among the methods employed for geometry optimization, restricted open-shell Møller-Plesset perturbation theory (MP2) appropriately describes the high-spin geometry but results in a slightly shorter Mn-O distance in both spin states. On the other hand, extended multistate complete active space second-order perturbation theory (XMS-CASPT2) provides a good description of the geometry for the intermediate-spin state but also sufficiently recovers dispersion, performing well for the high-spin state. Despite the fact that the electronic structure of both spin states is dominated by one-electron configuration, XMS-CASPT2 offers a balanced approach, leading to molecular geometries with much better agreement with experiment than MP2 and DFT. A scan along the Mn-Namine bond demonstrates that for these complexes coupled cluster methods (i.e., DLPNO-CCSD(T)) also yield bond distances in agreement with experiment while multiconfiguration pair density functional theory (MC-PDFT) is unable to recover dispersion well enough, analogous to single-reference DFT.

Multireference Electron Correlation Methods: Journeys along Potential Energy Surfaces.

Multireference electron correlation methods describe static and dynamical electron correlation in a balanced way and, therefore, can yield accurate and predictive results even when single-reference methods or multiconfigurational self-consistent field theory fails. One of their most prominent applications in quantum chemistry is the exploration of potential energy surfaces. This includes the optimization of molecular geometries, such as equilibrium geometries and conical intersections and on-the-fly photodynamics simulations, both of which depend heavily on the ability of the method to properly explore the potential energy surface. Because such applications require nuclear gradients and derivative couplings, the availability of analytical nuclear gradients greatly enhances the scope of quantum chemical methods. This review focuses on the developments and advances made in the past two decades. A detailed account of the analytical nuclear gradient and derivative coupling theories is presented. Emphasis is given to the software infrastructure that allows one to make use of these methods. Notable applications of multireference electron correlation methods to chemistry, including geometry optimizations and on-the-fly dynamics, are summarized at the end followed by a discussion of future prospects.

CASPT2 molecular geometries of Fe(II) spin-crossover complexes.

Using fully internally contracted (FIC)-CASPT2 analytical gradients, geometry optimizations of spin-crossover complexes are reported. This approach is tested on a series of Fe(II) complexes with different sizes, ranging from 13 to 61 atoms. A combination of active space and basis set choices are employed to investigate their role in determining reliable molecular geometries. The reported strategy demonstrates that a wave function-based level of theory can be used to optimize the geometries of metal complexes in reasonable times and enables one to treat the molecular geometry and electronic structure of the complexes using the same level of theory. For a series of smaller Fe(II) SCO complexes, strong field ligands in the LS state result in geometries with the largest differences between DFT and CASPT2; however, good agreement overall is observed between DFT and CASPT2. For the larger complexes, moderate sized basis sets yield geometries that compare well with DFT and available experimental data. We recommend using the (10e,12o) active space since convergence to a minimum structure was more efficient than with truncated active spaces despite having similar Fe-ligand bond distances.

Samarium Diiodide Acting on Acetone-Modeling Single Electron Transfer Energetics in Solution.

Samarium diiodide is a versatile single electron transfer (SET) agent with various applications in organic chemistry. Lewis structures regularly insinuate the existence of a ketyl radical when samarium diiodide binds a carbonyl group. The study presented here investigates this electron transfer by the means of computational chemistry. All electron CASPT2 calculations with the inclusion of scalar relativistic effects predict an endotherm electron transfer from samarium diiodide to acetone. Energies calculated with the PBE0-D3(BJ) functional and a small core pseudopotential are in good agreement with CASPT2. The calculations confirm the experimentally measured increase of the samarium diiodide reduction potential through the addition of hexamethylphosphoramide also known as HMPA.

Quantifying the Influence of Covalent Metal-Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes.

Qualitative differences in the reactivity of trivalent lanthanide and actinide complexes have long been attributed to differences in covalent metal-ligand bonding, but there are few examples where thermodynamic aspects of this relationship have been quantified, especially with U3+ and in the absence of competing variables. Here we report a series of dimeric phosphinodiboranate complexes with trivalent f-metals that show how shorter-than-expected U-B distances indicative of increased covalency give rise to measurable differences in solution deoligomerization reactivity when compared to isostructural complexes with similarly sized lanthanides. These results, which are in excellent agreement with supporting DFT and QTAIM calculations, afford rare experimental evidence concerning the measured effect of variations in metal-ligand covalency on the reactivity of trivalent uranium and lanthanide complexes.

Structural Characterization of the [CuOR]2+ Core.

Formal Cu(III) complexes bearing an oxygen-based auxiliary ligand ([CuOR]2+, R = H or CH2CF3) were stabilized by modulating the donor character of supporting ligand LY (LY = 4-Y, N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Y = H or OMe) and/or the basicity of the auxiliary ligand, enabling the first characterization of these typically highly reactive cores by NMR spectroscopy and X-ray crystallography. Enhanced lifetimes in solution and slowed rates of PCET with a phenol substrate were observed. NMR spectra corroborate the S = 0 ground states of the complexes, and X-ray structures reveal shortened Cu-ligand bond distances that match well with theory.

Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal-Organic Framework.

Significant advances have been made in the synthesis of chemically selective environments within metal-organic frameworks, yet materials development and industrial implementation have been hindered by the inability to predictively control crystallite size and shape. One common strategy to control crystal growth is the inclusion of coordination modulators, which are molecular species designed to compete with the linker for metal coordination during synthesis. However, these modulators can simultaneously alter the pH of the reaction solution, an effect that can also significantly influence crystal morphology. Herein, noncoordinating buffers are used to independently control reaction pH during metal-organic framework synthesis, enabling direct interrogation of the role of the coordinating species on crystal growth. We demonstrate the efficacy of this strategy in the synthesis of low-dispersity single-crystals of the framework Co2(dobdc) (dobdc4-= 2,5-dioxido-1,4-benzenedicarboxylate) in a pH 7-buffered solution using cobalt(II) acetate as the metal source. Density functional theory calculations reveal that acetate competitively binds to Co during crystallization, and by using a series of cobalt(II) salts with carboxylate anions of varying coordination strength, it is possible to control crystal growth along the c-direction. Finally, we use zero length column chromatography to show that crystal morphology has a direct impact on guest diffusional path length for the industrially important hydrocarbon m-xylene. Together, these results provide molecular-level insight into the use of modulators in governing crystallite morphology and a powerful strategy for the control of molecular diffusion rates within metal-organic frameworks.

Computational Spectroscopy of the Cr-Cr Bond in Coordination Complexes.

We report the accurate computational vibrational analysis of the Cr-Cr bond in dichromium complexes using second-order multireference complete active space methods (CASPT2), allowing direct comparison with experimental spectroscopic data both to facilitate interpreting the low-energy region of the spectra and to provide insights into the nature of the bonds themselves. Recent technological development by the authors has realized such computation for the first time. Accurate simulation of the vibrational structure of these compounds has been hampered by their notorious multiconfigurational electronic structure that yields bond distances that do not correlate with bond order. Some measured Cr-Cr vibrational stretching modes, ν(Cr2), have suggested weaker bonding, even for so-called ultrashort Cr-Cr bonds, while others are in line with the bond distance. Here, we optimize geometries and compute ν(Cr2) with CASPT2 for three well-characterized complexes, Cr2(O2CCH3)4(H2O)2, Cr2(mhp)4, and Cr2(dmp)4. We obtain CASPT2 harmonic ν(Cr2) modes in good agreement with experiment at 282 cm-1 for Cr2(mhp)4 and 353 cm-1 for Cr2(dmp)4, compute 50Cr and 54Cr isotope shifts, and demonstrate that the use of the so-called IPEA shift leads to improved Cr-Cr distances. Additionally, normal mode sampling was used to estimate anharmonicity along ν(Cr2), leading to an anharmonic mode of 272 cm-1 for Cr2(mhp)4 and 333 cm-1 for Cr2(dmp)4.

Sulfur-Containing Analogues of the Reactive [CuOH]2+ Core.

With the aim of drawing comparisons to the highly reactive complex LCuOH (L = bis(2,6-diisopropylphenylcarboxamido)pyridine), the complexes [Bu4N][LCuSR] (R = H or Ph) were prepared, characterized by spectroscopy and X-ray crystallography, and oxidized at low temperature to generate the species assigned as LCuSR on the basis of spectroscopy and theory. Consistent with the smaller electronegativity of S versus O, redox potentials for the LCuSR-/0 couples were ∼50 mV lower than for LCuOH-/0, and the rates of the proton-coupled electron transfer reactions of LCuSR with anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine at -80 °C were significantly slower (by more than 100 times) than the same reaction of LCuOH. Density functional theory (DFT) and time-dependent DFT calculations on LCuZ (Z = OH, SH, SPh) revealed subtle differences in structural and UV-visible parameters. Further comparison to complexes with Z = F, Cl, and Br using complete active space (CAS) self-consistent field and localized orbital CAS configuration interaction calculations along with a valence-bond-like interpretation of the wave functions showed differences with previously reported results ( J. Am. Chem. Soc. 2020, 142, 8514), and argue for a consistent electronic structure across the entire series of complexes, rather than a change in the nature of the ligand field arrangement for Z = F.

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks.

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as 'phase-change' adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg(2+) within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.

Plausible Emergence and Self Assembly of a Primitive Phospholipid from Reduced Phosphorus on the Primordial Earth.

How life arose on the primitive Earth is one of the biggest questions in science. Biomolecular emergence scenarios have proliferated in the literature but accounting for the ubiquity of oxidized (+ 5) phosphate (PO43-) in extant biochemistries has been challenging due to the dearth of phosphate and molecular oxygen on the primordial Earth. A compelling body of work suggests that exogenous schreibersite ((Fe,Ni)3P) was delivered to Earth via meteorite impacts during the Heavy Bombardment (ca. 4.1-3.8 Gya) and there converted to reduced P oxyanions (e.g., phosphite (HPO32-) and hypophosphite (H2PO2-)) and phosphonates. Inspired by this idea, we review the relevant literature to deduce a plausible reduced phospholipid analog of modern phosphatidylcholines that could have emerged in a primordial hydrothermal setting. A shallow alkaline lacustrine basin underlain by active hydrothermal fissures and meteoritic schreibersite-, clay-, and metal-enriched sediments is envisioned. The water column is laden with known and putative primordial hydrothermal reagents. Small system dimensions and thermal- and UV-driven evaporation further concentrate chemical precursors. We hypothesize that a reduced phospholipid arises from Fischer-Tropsch-type (FTT) production of a C8 alkanoic acid, which condenses with an organophosphinate (derived from schreibersite corrosion to hypophosphite with subsequent methylation/oxidation), to yield a reduced protophospholipid. This then condenses with an α-amino nitrile (derived from Strecker-type reactions) to form the polar head. Preliminary modeling results indicate that reduced phospholipids do not aggregate rapidly; however, single layer micelles are stable up to aggregates with approximately 100 molecules.

Chelating Borohydrides for Lanthanides and Actinides: Structures, Mechanochemistry, and Case Studies with Phosphinodiboranates.

In this Forum Article, we review the development of chelating borohydride ligands called aminodiboranates (H3BNR2BH3-) and phosphinodiboranates (H3BPR2BH3-) for the synthesis of trivalent f-element complexes. The advantages and history of using mechanochemistry to prepare molecular borohydride complexes are described along with new results demonstrating the mechanochemical synthesis of M2(H3BPtBu2BH3)6, where M = U, Nd, Tb, Er, and Lu (1-5). Multinuclear NMR, IR, and single-crystal X-ray diffraction data are reported for 1-5 alongside complementary density functional theory calculations to reveal differences in their structure and reactivity with and without tetrahydrofuran. The results demonstrate how mechanochemistry can be used to access f-element complexes with chelating borohydrides in improved and reproducible yields, which is an important step toward investigating the properties of lanthanide and actinide phosphinodiboranate complexes with different phosphorus substituents. The relevance of these results is contextualized by a discussion of structural factors known to influence the volatility of f-element borohydrides and applications that require the development of volatile f-element complexes.

Response to "Impact of Zeolite Structure on Entropic-Enthalpic Contributions to Alkane Monomolecular Cracking: An IR Operando Study".

This is a response to the paper published by S. A. Kadam, H. Li, R. F. Wormsbacher, A. Travert, Chem. Eur. J. 2018, 24, 5489. Key consistencies between our reported results and those reported in this work are also highlighted.

Ligand-Centered Borenium Reactivity in Triaminoborane-Bridged Diphosphine Complexes.

Borenium ions (i.e., three-coordinate boron cations) are known to promote a wide variety of stoichiometric and catalytic reactions because of their high Lewis acidity. As demonstrated by the growing number of chemically reactive borane ligands, there is considerable interest in developing ligands with highly electrophilic boron sites that promote multisite reactivity in metal complexes. However, there are currently few examples of ligand-centered borenium ions, especially with ligands that form coordination complexes with a wide range of metals. Here we report borenium-like reactivity on a highly versatile diphosphine ligand. Treating (PhTBDPhos)NiCl2 (1) with strong Bronsted acids such as HBF4·Et2O, HOTf, or HNTf2 resulted in fluoride or chloride abstraction from BF4- or NiCl2, respectively, to form trans N-H and B-X bonds on the ligand backbone. HCl addition to the bridgehead N-B bond is reversible, and the reactivity depends on the identity of the supporting counteranions, as observed when treating [(PhTBDPhos)NiCl]2X2, where X = NTf2- (3), OTf- (4), or BArF4- (5), with HCl. The reaction of 4 with HNTf2 instead of HCl yielded NMR evidence of the latent borenium cation in solution and showed how poor nucleophiles such as triflate bind to the borenium ion in the solid state. Remarkably, replacing the chloride ligands in 1 with chelating and less-labile thiolates or catecholates, or changing the phosphorus substituents (phenyl to isopropyl), attenuates the reactivity on the ligand backbone. Density functional theory was used to quantify the reaction free energies, and the theoretical results corroborate the experimental observations. Given the broad utility of diphosphines in homogeneous catalysis and the known benefits of strong Lewis acid promotors in many catalytic reactions, we anticipate that the results will provide new opportunities for dual-site reactivity involving boron ligands and metals.

Magnetic Anisotropy from Main-Group Elements: Halides versus Group 14 Elements.

Precise modulation of the magnetic anisotropy of a transition-metal center would affect physical properties ranging from photoluminescence to magnetism. Over the past decade, exerting nuanced control over ligand fields enabled the incorporation of significant magnetic anisotropy in a number of mononuclear transition-metal complexes. An alternate approach to increasing spin-orbit coupling relies upon using heavy diamagnetic main-group elements as sources of magnetic anisotropy. Interacting first-row transition metals with main-group elements enables the transfer of magnetic anisotropy to the paramagnetic metal center without restricting coordination geometry. We sought to study the effect of covalency on this anisotropy transfer by probing the effect of halides in comparison to early main-group elements. Toward that end, we synthesized a series of four isostructural heterobimetallic complexes, with germanium or tin covalently bound to a triplet spin Fe(II) center. These complexes are ligated by a halide (Br- or I-) in the apical position to yield a series of complexes with variation in the mass of the main-group elements. This series enabled us to interrogate which electronic structure factors influence the heavy-atom effect. Using a suite of approaches including magnetometry, computation, and Mössbauer spectroscopy, we probed the electronic structure and the spin-orbit coupling, as parametrized by axial zero-field splitting across the series of complexes, and found an increase in zero-field splitting from -11.8 to -17.9 cm-1 by increasing the axial ligand mass. Through direct comparison between halides and group 14 elements, we observe a greater impact on magnetic anisotropy from the halide interaction. We attribute this counterintuitive effect to a larger spin population on the halide elements, despite greater covalency in the group 14 interactions. These results recommend modification of the intuitive design principle of increasing covalency toward a deeper focus on the interactions of the spin-bearing orbitals.