Unravelling the Mechanism of CO2 Activation: Insights Into Metal-Metal Cooperativity and Spin-Orbit Coupling with {3d-4f} Catalysts.

Converting CO2 into useful chemicals using metal catalysts is a significant challenge in chemistry. Among the various catalysts reported, transition metal lanthanide hybrid {3d-4f} complexes stand out for their superior efficiency and site selectivity. However, unlike transition metal catalysts, understanding the origin of this efficiency in lanthanides poses a challenge due to their orbital degeneracy, rendering the application of DFT methods ineffective. In this study, we employed a combination of density functional theory (DFT) and ab initio CASSCF/RASSI-SO calculations to explore the mechanism of CO2 conversion to cyclic carbonate using a 3d-4f heterometallic catalyst for the first time. This work unveils the importance of 3d and 4f metal cooperativity and the role of individual spin-orbit states in dictating the overall efficiency of the catalyst.

Rebound or Cage Escape? The Role of the Rebound Barrier for the Reactivity of Non-Heme High-Valent FeIV =O Species.

Owing to their high reactivity and selectivity, variations in the spin ground state and a range of possible pathways, high-valent FeIV =O species are popular models with potential bioinspired applications. An interesting example of a structure-reactivity pattern is the detailed study with five nonheme amine-pyridine pentadentate ligand FeIV =O species, including N4py: [(L1 )FeIV =O]2+ (1), bntpen: [(L2 )FeIV =O]2+ (2), py2 tacn: [(L3 )FeIV =O]2+ (3), and two isomeric bispidine derivatives: [(L4 )FeIV =O]2+ (4) and [(L5 )FeIV =O]2+ (5). In this set, the order of increasing reactivity in the hydroxylation of cyclohexane differs from that with cyclohexadiene as substrate. A comprehensive DFT, ab initio CASSCF/NEVPT2 and DLPNO-CCSD(T) study is presented to untangle the observed patterns. These are well reproduced when both activation barriers for the C-H abstraction and the OH rebound are taken into account. An MO, NBO and deformation energy analysis reveals the importance of π(pyr) → π*xz (FeIII -OH) electron donation for weakening the FeIII -OH bond and thus reducing the rebound barrier. This requires that pyridine rings are oriented perpendicularly to the FeIII -OH bond and this is a subtle but crucial point in ligand design for non-heme iron alkane hydroxylation.

Enhancing Spin-Transport Characteristics, Spin-Filtering Efficiency, and Negative Differential Resistance in Exchange-Coupled Dinuclear Co(II) Complexes for Molecular Spintronics Applications.

Single-molecule spintronics, where electron transport occurs via a paramagnetic molecule, has gained wide attention due to its potential applications in the area of memory devices to switches. While numerous organic and some inorganic complexes have been employed over the years, there are only a few attempts to employ exchange coupled dinuclear complexes at the interface, and the advantage of fabricating such a molecular spintronics device in the observation of switchable Kondo resonance was demonstrated recently in the dinuclear [Co2(L)(hfac)4] (1) complex (Wagner et al., Nat. Nanotechnol. 2013, 8, 575-579). In this work, employing an array of theoretical tools such as density functional theory (DFT), the ab initio CASSCF/NEVPT2 method, and DFT combined with nonequilibrium Green Function (NEGF) formalism, we studied in detail the role of magnetic coupling, ligand field, and magnetic anisotropy in the transport characteristics of complex 1. Particularly, our calculations not only reproduce the current-voltage (I-V) characteristics observed in experiments but also unequivocally establish that these arise from an exchange-coupled singlet state that arises due to antiferromagnetic coupling between two high-spin Co(II) centers. Further, the estimated spin Hamiltonian parameters such as J, g values, and D and E/D values are only marginally altered for the molecule at the interface. Further, the exchange-coupled state was found to have very similar transport responses, despite possessing significantly different geometries. Our transport calculations unveil a new feature of the negative differential resistance (NDR) effect on 1 at the bias voltage of 0.9 V, which agrees with the experimental I-V characteristics reported. The spin-filtering efficiency (SFE) computed for the spin-coupled states was found to be only marginal (∼25%); however, if the ligand field is fine-tuned to obtain a low-spin Co(II) center, a substantial SFE of 44% was noted. This spin-coupled state also yields a very strong NDR with a peak-to-valley ratio (PVR) of ∼56 - a record number that has not been witnessed so far in this class of compounds. Additionally, we have established further magnetostructural-transport correlations, providing valuable insights into how microscopic spin Hamiltonian parameters can be associated with SFE. Several design clues to improve the spin-transport characteristics, SFE and NDR in this class of molecule, are offered.

Origin of Unprecedented Formation and Reactivity of FeIV═O Species via Oxygen Activation: Role of Noncovalent Interactions and Magnetic Coupling.

Emulating the capabilities of the soluble methane monooxygenase (sMMO) enzymes, which effortlessly activate oxygen at diiron(II) centers to form a reactive diiron(IV) intermediate Q, which then performs the challenging oxidation of methane to methanol, poses a significant challenge. Very recently, one of us reported the mononuclear complex [(cyclam)FeII(CH3CN)2]2+ (1), which performed a rare bimolecular activation of the molecule of O2 to generate two molecules of FeIV═O without the requirement of external proton or electron sources, similar to sMMO. In the present study, we employed the density functional theory (DFT) calculations to investigate this unique mechanism of O2 activation. We show that secondary hydrogen-bonding interactions between ligand N-H groups and O2 play a vital role in reducing the energy barrier associated with the initial O2 binding at 1 and O-O bond cleavage to form the FeIV═O complex. Further, the unique reactivity of FeIV═O species toward simultaneous C-H and O-H bond activation process has been demonstrated. Our study unveils that the nature of the magnetic coupling between the diiron centers is also crucial. Given that the influence of magnetic coupling and noncovalent interactions in catalysis remains largely unexplored, this unexplored realm presents numerous avenues for experimental chemists to develop novel structural and functional analogues of sMMO.

Synthesis, Characterization, and Reactivity of Bispidine-Iron(IV)-Tosylimido Species.

Reported are the synthesis and detailed studies of the iron(IV)-tosylimido complexes of two isomeric pentadentate bispidine ligands (bispidines are 3,7-diazabicyclo[3.3.1]nonane derivatives). This completes a series of five tosylimido complexes with comparable pentadentate amine/pyridine ligands, where the corresponding [(L)FeIV═O]2+ oxidants have been studied in detail. The characterization of the two new complexes in solution (UV-vis-NIR, Mössbauer, HR-ESI-MS) shows that these oxidants have an intermediate spin (S = 1) electronic ground state. The reactivities have been studied as oxidants in C-H activation at 1,3-cyclohexadiene and nitrogen atom transfer to thioanisole. For the latter substrate, the entire set of data for the five ligands and for both nitrogen and oxygen atom transfer is now available and the interesting observation is that oxygen atom transfer is, as expected, generally faster than nitrogen atom transfer, with the exception of the two ligands that have four and three pyridine groups oriented parallel to the Fe-O and Fe-N axes. A thorough DFT analysis indicates that this is due to steric effects in the case of the [(L)FeIV═O]2+ species, which are less important in the [(L)FeIV═NTs]2+ compounds due to partial electron transfer from the thioanisole substrate to the iron(IV)-tosylimido oxidant.

Hourglass-Shaped Homo- and Heteronuclear Nonanuclear Lanthanide Clusters: Structures, Magnetism, Photoluminescence, and Theoretical Analysis.

Synthesis of nonameric cationic clusters [Dy9(acac)16(µ3-OH)8(µ4-OH)2]OH·6H2O (1), [Dy8Tb (acac)16(µ3-OH)8(µ4-OH)2]OH·2H2O (2), and [Gd9(acac)16(µ3-OH)8(µ4-OH)2]OH·6H2O (3) (acac = acetylacetonate) is reported. The emission spectrum of 1 shows Dy(III) ion characteristic bands assignable to the 4F9/2 → 6HJ (J = 15/2 to 9/2) transitions. Emission due to both Dy(III) and Tb(III) ions is observed for 2 in the visible range, with Tb(III) specific bands appearing due to the 5D4 → 7FJ (J = 6, 4, and 3) transitions. Cluster 3 exhibits a significant magnetocaloric effect (MCE), with -ΔSm values increasing with decrease in temperature and increase in field, reaching -ΔSmmax = 20.98 J kg-1 K-1 at 2 K and 9 T. Isotropic magnetic coupling constants (Js) in 3 derived from density functional theory (DFT) calculations reveal that the exchange interactions are antiferromagnetic and weak. Compound 3 possesses S = 7/2 ground state arising from the central Gd(III) ion along with several nested excited states due to competing antiferromagnetic interactions that yield reasonably large MCE values. Utilizing computed exchange coupling interactions, we have performed ab initio CASSCF/RASSI-SO/POL_ANISO calculations on antiferromagnetic 1 and 2 to estimate the exchange interactions using the Lines model. For 2, Dy(III)···Tb(III) exchange interactions were extracted for the first time and were found to be weakly antiferromagnetically coupled.

Unusual Stabilisation of Remarkably Bent Tetra-Cationic Tetra-radical Intermolecular Fe(III) µ-Oxo Tetranuclear Complexes.

A hitherto unknown series of air stable, π-conjugated, remarkably bent tetra-cation tetra-radical intermolecular Fe(III) µ-oxo tetranuclear complex, isolated from the dication diradical diiron(III) porphyrin dimers, has been synthesised and spectroscopically characterised along with single crystal X-ray structure determination of two such molecules. These species facilitate long-range charge/radical delocalisation through the bridge across the entire tetranuclear unit manifesting an unusually intense NIR band. Assorted spin states of Fe(III) centres are stabilised within these unique tetranuclear frameworks: terminal six-coordinate iron centres stabilise the admixed intermediate spin states while the central five-coordinate iron centres stabilise the high-spin states. Variable temperature magnetic susceptibility measurements indicated strong antiferromagnetic coupling for the Fe(III)-O-Fe(III) unit while the exchange interactions between the Fe centres and the porphyrin π-cation radicals are weaker as supported both by magnetic data and DFT calculations. The nature of orbital overlap between the SOMOs of Fe(III) and π* orbital of the porphyrin was found to rationalise the observed exchange coupling, establishing such a complex magnetic exchange in this tetranuclear model with a significant bioinorganic relevance.

A Theoretical Perspective to Decipher the Origin of High Hydrogen Storage Capacity in Mn(II) Metal-Organic Framework.

Herein, we report a detailed periodic DFT investigation of Mn(II)-based [(Mn4 Cl)3 (BTT)8 ]3- (BTT3- =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) to explore various hydrogen binding pockets, nature of MOF…H2 interactions, magnetic coupling and, H2 uptake capacity. Earlier experiments found an uptake capacity of 6.9 wt % of H2, with the heat of adsorption estimated to be ∼10 kJ/mol, which is one among the highest for any MOFs reported. Our calculations unveil different binding sites with computed binding energy varying from -6 to -15 kJ/mol. The binding of H2 at the Mn2+ site is found to be the strongest (site I), with H2 found to bind Mn2+ ion in a η2 fashion with a distance of 2.27 Šand binding energy of -15.4 kJ/mol. The bonding analysis performed using NBO and AIM reveal a strong donation of σ (H2 ) to the dz 2 orbital of the Mn2+ ion responsible for such large binding energy. The other binding pockets, such as -Cl (site II) and BTT ligands (site III and IV) were found to be weaker, with the binding energy decreasing in the order I>II>III>IV. The average binding energy computed for these four sites put together is 9.6 kJ/mol, which is in excellent agreement with the experimental value of ∼10 kJ/mol. We have expanded our calculations to compute binding energy for multiple sites simultaneously, and in this model, the binding energy per site was found to decrease as we increased the number of H2 molecules suggesting electronic and steric factors controlling the overall uptake capacity. The calculated adsorption isotherm using the GCMC method reproduces the experimental observations. Further, the magnetic coupling computed for the unbound MOF reveals moderate ferromagnetic and strong antiferromagnetic coupling within the tetrameric {Mn4 } unit leading to a three-up-one-down spin configuration as the ground state. These were then coupled ferromagnetically to other tetrameric units in the MOF network. The magnetic coupling was found to alter only marginally upon gas binding, suggesting that both exchange interaction and the spin-states are unlikely to play a role in the H2 uptake. This is contrary to the O2 uptake studied lately, where strong dependence on exchange-coupling/spin state was witnessed, suggesting exchange-coupling/magnetic field dependent binding as a viable route for gas separation.

Does the Spin State and Oriented External Electric Field Boost the Efficiency of Fe(II) Pincer Catalyst toward CO2 Hydrogenation Reaction?

In this study, we have explored the catalytic reactivities of four PNP-pincer supported Fe(II) complexes, namely, [(iPrPNMeP)FeH2(CO)] (1), [(iPrPNMeP)FeH(CO)(BH4)] (2), [(iPrPNHP)FeH2(CO)] (3), and [(iPrPNMeP)FeH(BH4)] (4) (iPrPNMeP = MeN{CH2CH2(PiPr2)}2 and iPrPNHP = HN{CH2CH2(PiPr2)}2) toward reductive CO2 hydrogenation for formate production. Our density functional theory and ab initio complete active space self-consistent field study have identified three fundamental steps in this catalytic transformation: (i) anchoring of the CO2 molecule in the vicinity of the metal using noncovalent interactions, (ii) catalyst regeneration via H2 cleavage, and (iii) formate rebound step leading to catalytic poisoning. The variations in the catalytic efficiency observed among these catalysts were attributed to either easing of steps (i) and (ii) or the hampering step (iii). This can be achieved in various chemical/non-chemical ways, for instance, (a) incorporation of strong-field ligands such as CO facilitating single-state reactivity and eliminating two-state reactivity that generally enhances the rate and (b) inclusion of Lewis acids such as LiOTf and strong bases found to either avoid catalytic poisoning or ease the H-H cleavages, to enhance the rate of reaction (c) evading mixing of excited open-shell singlet states to the ground closed-shell singlet state that hampers the catalytic regeneration. We have probed the role of oriented external electric fields (OEEFs) in the entire mechanistic profile for the best and worst catalyst, and our study suggests that imposing OEEFs opposite to the reaction axis (z-axis) fastens the catalytic regeneration step and, at the same time, hampers catalytic poisoning. The application of OEEFs is found to regulate the energetics of various spin states and can hamper two-state reactivity, therefore increasing the efficiency. Thus, this study provides insights into the CO2 hydrogenation mechanism where the role of bases/Lewis acid, ligand design, spin states, and electric field in a particular direction has been established and is, therefore, likely to pave the way forward for a new generation of catalysts.

Mechanism of Hydroboration of CO2 Using an Fe Catalyst: What Controls the Reactivity and Product Selectivity?

Using a combination of density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations, various elementary steps in the mechanism of the reductive hydroboration of CO2 to two-electron-reduced boryl formate, four-electron-reduced bis(boryl)acetal, and six-electron-reduced methoxy borane by the [Fe(H)2(dmpe)2] catalyst were established. The replacement of hydride by oxygen ligation after the boryl formate insertion step is the rate-determining step. Our work unveils, for the first time, (i) how a substrate steers product selectivity in this reaction and (ii) the importance of configurational mixing in contracting the kinetic barrier heights. Based on the reaction mechanism established, we have further focused on the effect of other metals, such as Mn and Co, on rate-determining steps and on catalyst regeneration.

Probing the Origins of Puzzling Reactivity in Fe/Mn-Oxo/Hydroxo Species toward C-H Bonds: A DFT and Ab Initio Perspective.

Activation of C-H bonds using an earth-abundant metal catalyst is one of the top challenges of chemistry, where high-valent Mn/Fe-oxo(hydroxo) biomimic species play an important role. There are several open questions related to the comparative oxidative abilities of these species, and a unifying concept that could accommodate various factors influencing reactivity is lacking. To shed light on these open questions, here, we have used a combination of density functional theory (DFT) (B3LYP-D3/def2-TZVP) and ab initio (CASSCF/NEVPT2) calculations to study a series of high-valent metal-oxo species [Mn+H3buea(O/OH)] (M = Mn and Fe, n = II to V; H3buea = tris[(N'-tert-butylureaylato)-N-ethylene)]aminato towards the activation of dihydroanthracene (DHA). The H-bonding network in the ligand architecture influences the ground state-excited state gap and brings several excited states of the same spin multiplicity closer in energy, which triggers reactivity via one of those excited states, reducing the kinetic barriers for the C-H bond activation and rationalizing several puzzling reactivity trends observed in various high-valent Mn/Fe-oxo(hydroxo) species.

Fluxionality Modulating the Magnetic Anisotropy in Lanthanoarene [(ηCnRn)2Ln(II/III)] (n = 4-8) Single-Ion Magnets.

Lanthanoarenes have emerged as the best bet for the futuristic application of single-ion magnets in information storage devices. While dysprosocenium molecules with various substituents at the arene ring exhibit a very large blocking temperature, the corresponding Er(III) analogues do not, and this is reversed if the size of the arene ring is eight. Using a combination of ab initio CASSCF and DFT-based molecular dynamics (MD) study, we have explored 25 Dy(III)/Er(III)/Ho(II)/Tb(II)/Dy(II) arene complexes with the ring size varying from 4 to 8 to understand the differences observed and decipher the correlation of structure to the spin dynamics behavior. Among the oxidation state of +2 complexes studied, Tb(II) exhibits the highest barrier, with the Cp-Tb-Cp angle being linear. Further, one of the four-membered arene model studied exhibits a very large barrier of 1442 cm-1, suggesting a potential high-blocking SIM. While bulky substituents at the arene ring help increase the axiality and the CR-Ln-CR angle, this also fetches several agostic C-H···Ln interactions, which injects transverse anisotropy. Furthermore, MD coupled with the CASSCF study reveals that the fluxional behavior of the arene ring generates several rotational conformers that are even accessible at lower temperatures offering a shortcut to the magnetization relaxation process. The importance of structural fluctuations in controlling the magnetic anisotropy by choosing apt metal-ion/ring partners and the corresponding substituents has been highlighted to offer clues to the futuristic SIM design.

Tuning Magnetic Anisotropy in Co(II) Tetrahedral Carbazole-Modified Phosphine Oxide Single-Ion Magnets: Importance of Structural Distortion versus Heavy-Ion Effect.

Three mononuclear cobalt(II) tetrahedral complexes [Co(CzPh2PO)2X2] (CzPh2PO = (9H-carbazol-9-yl)diphenylphosphine oxide and X = Cl (1), Br (2), I (3)) have been synthesized using a simple synthetic approach to examine their single-ion magnetic (SIM) behavior. A detailed study of the variation in the dynamic magnetic properties of the Co(II) ion in a tetrahedral ligand field has been carried out by the change of the halide ligand. The axial zero-field splitting parameter D was found to vary from -16.4 cm-1 in 1 to -13.8 cm-1 in 2 and +14.6 cm-1 in 3. All the new complexes exhibit field-induced SIM behavior. The results obtained from ab initio CASSF calculations match well with the experimental data, revealing how halide ions induce a change in the D value as we move from Cl- to I-. The ab initio calculations further reveal that the change in the sign of D is due to the multideterminant characteristics of the ground state wave function of 1 and 2, while single-determinant characteristics are instead observed for 3. To gain a better understanding of the relationship between the structural distortion and the sign and magnitude of D values, magnetostructural D correlations were developed using angular relationships, revealing the importance of structural distortions over the heavy halide effect in controlling the sign of D values. This study broadens the scope of employing electronically and sterically modified phosphine oxide ligands in building new types of air-stable Co(II) SIMs.

Bis-Olefin Based Crystalline Schlenk Hydrocarbon Diradicals with a Triplet Ground State.

A modular approach for the synthesis of isolable crystalline Schlenk hydrocarbon diradicals from m-phenylene bridged electron-rich bis-triazaalkenes as synthons is reported. EPR spectroscopy confirms their diradical nature and triplet electronic structure by revealing a half-field signal. A computational analysis confirms the triplet state to be the ground state. As a proof-of-principle for the modular methodology, the 4,6-dimethyl-m-phenylene was further utilized as a coupling unit between two alkene motifs. The steric conjunction of the 4,6-dimethyl groups substantially twists the substituents at the nonbonding electron bearing centers relative to the central coupling m-phenylene motif. As a result, the spin delocalization is decreased and the exchange coupling between the two unpaired spins, hence, significantly reduced. Notably, 108 years after Schlenk's m-phenylene-bis(diphenylmethyl) synthesis as a diradical, for the first time we were able to isolate its derivative with the same spacer, i.e. m-phenylene, between two radical centers in a crystalline form.

Analysis of the Magnetic Coupling in a Mn(II)-U(V)-Mn(II) Single Molecule Magnet.

[{Mn(TPA)I}{UO2(Mesaldien)}{Mn(TPA)I}]I formula (here TPA=tris(2-pyridylmethyl)amine and Mesaldien=N,N'-(2-aminomethyl)diethylenebis(salicylidene imine)) reported by Mazzanti and coworkers (Chatelain et al. Angew. Chem. Int. Ed. 2014, 53, 13434) is so far the best Single Molecule Magnet (SMM) in the {3d-5f} class of molecules exhibiting barrier height of magnetization reversal as high as 81.0 K. In this work, we have employed a combination of ab initio CAS and DFT methods to fully characterize this compound and to extract the relevant spin Hamiltonian parameters. We show that the signs of the magnetic coupling and of the g-factors of the monomers are interconnected. The central magnetic unit [UV O2 ]+ is described by a Kramers Doublet (KD) with negative g-factors, due to a large orbital contribution. The magnetic coupling for the {Mn(II)-U(V)} pair is modeled by an anisotropic exchange Hamiltonian: all components are ferromagnetic in terms of spin moments, the parallel component JZ twice larger as the perpendicular one J⊥ . The spin density distribution suggests that spin polarization on the U(V) center favors the ferromagnetic coupling. Further, the JZ /J⊥ ratio, which is related to the barrier height, was found to correlate to the corresponding spin contribution of the g-factors of the U(V) center. This correlation established for the first time offers a direct way to estimate this important ratio from the corresponding gS -values, which can be obtained using traditional ab initio packages and hence has a wider application to other {3d-5f} magnets. It is finally shown that the magnetization barrier height is tuned by the splitting of the [UV O2 ]+ 5 f orbitals.

The Decisive Role of Spin States and Spin Coupling in Dictating Selective O2 Adsorption in Chromium(II) Metal-Organic Frameworks.

Invited for the cover of this issue are Sourav Pal, Gopalan Rajaraman and co-workers at the Indian Institute of Technology Bombay, the Bhabha Atomic Research Centre and the Indian Institute of Science Education and Research. The image depicts how a mixture of atmospheric gases such as CO2 , H2 , N2 and O2 can be selectively separated using a Cr metal-organic framework where spin state and spin coupling play a crucial role. Read the full text of the article at 10.1002/chem.202104526.

Deciphering the Role of Anions and Secondary Coordination Sphere in Tuning Anisotropy in Dy(III) Air-Stable D5h SIMs.

Precise control of the crystal field and symmetry around the paramagnetic spin centre has recently facilitated the engineering of high-temperature single-ion magnets (SIMs), the smallest possible units for future spin-based devices. In the present work, we report a series of air-stable seven coordinate Dy(III) SIMs {[L2 Dy(H2 O)5 ][X]3 ⋅L2 ⋅n(H2 O), n = 0, X = Cl (1), n=1, X = Br (2), I (3)} possessing pseudo-D5h symmetry or pentagonal bipyramidal coordination geometry with high anisotropy energy barrier (Ueff ) and blocking temperature (TB ). While the strong axial coordination from the sterically encumbered phosphonamide, t BuPO(NHi Pr)2 (L), increases the overall anisotropy of the system, the presence of high symmetry significantly quenches quantum tunnelling of magnetization, which is the prominent deactivating factor encountered in SIMs. The energy barrier (Ueff ) and the blocking temperature (TB ) decrease in the order 3>2>1 with the change of anions from larger iodide to smaller strongly hydrogen-bonded chloride in the secondary coordination sphere, albeit the local coordination geometry and the symmetry around the Dy(III) display only slight deviations. Ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations provide deeper insights into the dynamics of magnetic relaxation in addition to the role of the secondary coordination sphere in modulating the anisotropy of the D5h systems, using diverse models. Thus, in addition to the importance of the crystal field and the symmetry to obtain high-temperature SIMs, this study also probes the significance of the secondary coordination sphere that can be tailored to accomplish novel SIMs.

The Decisive Role of Spin States and Spin Coupling in Dictating Selective O2 Adsorption in Chromium(II) Metal-Organic Frameworks.

The coordinatively unsaturated chromium(II)-based Cr3 [(Cr4 Cl)3 (BTT)8 ]2 (Cr-BTT; BTT3- =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) has been shown to exhibit exceptional selectivity towards adsorption of O2 over N2 /H2 . Using periodic density functional theory (DFT) calculations, we attempted to decipher the origin of this puzzling selectivity. By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas-bound MOFs [(Cr4 (X)4 Cl)3 (BTT)8 ]3- (X=O2 , N2 , and H2 ), we unequivocally established the role of spin states and spin coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The binding of O2 to the MOF follows an electron-transfer mechanism resulting in a CrIII superoxo species (O2 .- ) with a very strong antiferromagnetic coupling between the two centers, whereas N2 /H2 are found to weakly interact with the metal center and hence only slightly perturb the associated coupling constants. Although the gas-bound and unbound MOFs have an S=0 ground state (GS), the nature of spin the configurations and the associated magnetic exchanges are dramatically different. The binding energy and the number of oxygen molecules that can favorably bind to the Cr center were found to vary with respect to the spin state, with a significant energy margin (47.6 kJ mol-1 ). This study offers a hitherto unknown strategy of using spin state/spin couplings to control gas adsorption selectivity in MOFs.

Can you break the oxo-wall? A multiconfigurational perspective.

The concept of the "oxo-wall" was conceived about 60 years ago by Harry B. Gray, and has been found to be related to the non-existence of high-valent M-oxo species in the +IV oxidation state in a tetragonal geometry beyond group 8 in the periodic table. Several efforts have been made in the past decades to test and find examples that violate this theory. Several claims of violation in the past were attributed to the difference in the geometries/coordination number and, therefore, these are not examples of true violation. In recent years, substantial efforts have been undertaken to synthesise a true CoIVO species with various ligand architectures. CoIVO and CoIII-O˙ are electromers and, while they are interchangeably used in the literature; the former violates the oxo-wall while the latter does not. The possibility that these two species could exist in various proportions similar to resonating structures has not been considered in detail in this area. Furthermore, there have been no attempts to quantify such mixing. In this direction, we have employed density functional theory (DFT) and ab initio CASSCF/NEVPT2 methods to quantify the co-existence of CoIVO and CoIII-O˙ isomeric species. By thoroughly studying six different metal-oxo species, we affirm that the nature of such electromeric mixing is minimal/negligible for FeIVO and MnIVO species - both are pre-oxo-wall examples. By studying four different ligand architectures with Co-oxo species, our results unveil that the mixing of CoIVO ↔ CoIII-O˙ is substantial in all geometries, with dominant CoIVO species favourable for the S = 3/2 intermediate spin state. The percentage of the CoIII-O˙ species is enhanced substantially for the S = 1/2 low-spin state. We have attempted to develop a tool to estimate the percentage of the CoIII-O˙ species using various structural parameters. Among those tested, a linear relationship between % of CoIII-O˙ and a bond length based ratio is found (, where d(Co-O) and d(Co-Nax) are the axial Co-O and Co-Nax bond lengths in Å, respectively). It is found that the higher the Rd, the greater the CoIII-O˙ character will be and the geometrically portable correlation developed offers a way to qualitatively compute the % of CoIII-O˙ character for unknown geometries.

Deciphering the Role of Symmetry and Ligand Field in Designing Three-Coordinate Uranium and Plutonium Single-Molecule Magnets.

Actinide single-molecule magnets (SMMs) have gained paramount interest in molecular magnetism as they offer a larger barrier height of magnetization (Ueff) reversal compared to the lanthanide analogue, thanks to their greater metal-ligand covalency. However, the reported actinide SMMs to date yield a relatively smaller Ueff as there is no established design principle to enhance Ueff values. To address this issue, we have employed ab initio CASSCF/CASPT2/NEVPT2 calculations to study a series of three-coordinate U3+ and Pu3+ SMMs. To begin with, we have studied two experimentally characterized U3+ ion-field-induced SMMs, namely, planar [U{N(SiMe2tBu)2}3] (1) and pyramidal [U{N(SiMe3)2}3] (2) complexes reported earlier. Both the complexes were found to stabilize mJ = |±1/2⟩ as the ground state with a very strong quantum tunneling of magnetization (QTM), rendering them unsuitable for SMMs. Our calculations reveal that in the pyramidal geometry (such as in 2), the energy of the 5f26d1 state is lowered compared to the planar geometry (as in 1), resulting in a slightly better SMM characteristic in the former. To unravel the effect of symmetry in magnetic properties, ab initio calculations were performed on two reported T-shaped complexes [U(NSiiPr2)2(I)] (3) and [U(NHAriPr6)2I] (4, AriPr6 = 2,6-(2,4,6-iPr3C6H2)2C6H3). Quite interestingly, mJ = |±9/2⟩ is found to be the ground state for both the complexes with a blocking barrier exceeding 900 cm-1. Furthermore, to decipher the effect of the transuranic element in magnetic anisotropy, ab initio calculations were extended to the Pu analogue of 2, [Pu{N(SiMe3)2}3] (5), which yields a record-breaking blocking barrier of ∼1933 cm-1. Among the three-coordinate geometries studied, the pyramidal geometry was found to offer substantial magnetic anisotropy for Pu3+ ions, while a T-shaped geometry is best suited for U3+ ions. While the chosen theoretical protocols' overestimation of barrier height cannot be avoided, these values are still several orders of magnitude larger than the Ueff values reported for any actinide SMMs and unveil a design principle for superior three-coordinate actinide-based SMMs.