TPSSh Functionals Research Articles

Hydrogenation of hexene catalyzed by a ruthenium (II) complex with N‐heterocyclic carbene ligands

AbstractIn this study, we investigated the mechanism of the inactivated hexene hydrogenation reaction catalyzed by a ruthenium (II) complex containing “N‐heterocyclic carbene” (NHC) ligands, specifically SIMes and CBA, using DFT calculations. Our focus was on RuH(OSO2CF3)(CO)(SIMes)(CBA), which exhibits excellent catalytic behavior. We tested the B3LYP‐D3, cam‐B3LYP, and TPSSh functionals. The hydrogenation reaction is initiated by the release of SIMes rather than CBA due to the lower associated dissociation energy. Our findings indicate a reaction mechanism consisting of two consecutive steps, each involving one hydrogen atom migration. The first step, considered as the kinetically limiting transition state, exhibits a Gibbs free activation barrier of 12.9 kcal mol−1. This step involves two asynchronous processes. The first one describes the migration of the ruthenium hydride to the internal carbon of the olefine function, transitioning from π to σ coordination mode, which promotes the formation of a bond between ruthenium and the terminal olefinic carbon. The second process involves the oxidation of ruthenium from Ru(II) to Ru(IV). This oxidation is crucial as it enables the decomposition of the H2 molecule into two hydrogen atoms bonded to the ruthenium atom. The geometrical structures of the Hidden Reaction Intermediate Ru(II) complex and the quasi‐transition state of the second process have been determined by means of the RIRC technique. The second step entails the migration of one of the newly formed hydrides of the Ru(IV) complex to the terminal olefinic carbon, resulting in the release of hexane with a weak activation Gibbs free energy of .8 kcal mol−1. Lastly, we explored the use of dichloromethane as a solvent, considering the PCM model. The presence of the solvent significantly decreases the energy dissociation of SIMes from 17.9 to 9.0 kcal mol−1, providing notable benefits.

Gas-Phase Interaction of CO, CO2, H2S, NH3, NO, NO2, and SO2 with Zn12O12 and Zn24 Atomic Clusters.

Atmospheric pollutants pose a high risk to human health, and therefore it is necessary to capture and preferably remove them from ambient air. In this work, we investigate the intermolecular interaction between the pollutants such as CO, CO2, H2S, NH3, NO, NO2, and SO2 gases with the Zn24 and Zn12O12 atomic clusters, using the density functional theory (DFT) at the meta-hybrid functional TPSSh and LANl2Dz basis set. The adsorption energy of these gas molecules on the outer surfaces of both types of clusters has been calculated and found to have a negative value, indicating a strong molecular-cluster interaction. The largest adsorption energy has been observed between SO2 and the Zn24 cluster. In general, the Zn24 cluster appears to be more effective for adsorbing SO2, NO2, and NO than Zn12O12, whereas the latter is preferable for the adsorption of CO, CO2, H2S, and NH3. Frontier molecular orbital (FMO) analysis showed that Zn24 exhibits higher stability upon adsorption of NH3, NO, NO2, and SO2, with the adsorption energy falling within the chemisorption range. The Zn12O12 cluster shows a characteristic decrease in band gap upon adsorption of CO, H2S, NO, and NO2, suggesting an increase in electrical conductivity. Natural bond orbital (NBO) analysis also suggests the presence of strong intermolecular interactions between atomic clusters and the gases. This interaction was recognized to be strong and noncovalent, as determined by noncovalent interaction (NCI) and quantum theory of atoms in molecules (QTAIM) analyses. Overall, our results suggest that both Zn24 and Zn12O12 clusters are good candidate species for promoting adsorption and, thus, can be employed in different materials and/or systems for enhancing interaction with CO, H2S, NO, or NO2.

New Ferrocene-Based Metalloligand with Two Triazole Carboxamide Pendant Arms and Its Iron(II) Complex: Synthesis, Crystal Structure, 57Fe Mössbauer Spectroscopy, Magnetic Properties and Theoretical Calculations

The new ferrocene-based metalloligand bis (N-4-[3,5-di-(2-pyridyl)-1,2,4-triazoyl])ferrocene carboxamide (L) was prepared through derivatization of 1,1′-ferrocenedicarboxylic acid with 4-amino-3,5-di(pyridyl)-4H-1,2,4-triazole. The composition and purity of L in the solid state was determined with elemental analysis, FT-IR spectroscopy, and its crystal structure with single-crystal X-ray analysis, which revealed that the substituted cyclopentadienyl rings adopt the antiperiplanar conformation and the crystal structure of L is stabilized by O–H···N and N–H···O hydrogen bonds. The molecular properties of L in solution were investigated with NMR and UV-VIS spectroscopies, and cyclic voltammetry disclosed irreversible redox behavior providing one oxidation peak at E1/2 = 1.133 V vs. SHE. Furthermore, the polymeric FeII complex {Fe(L)(C(CN)3)2}n (1) was prepared and characterized with elemental analysis, FT-IR spectroscopy, 57Fe Mössbauer spectroscopy, and magnetic measurements. The last two methods confirmed that a mixture of low- and high-spin species is present in 1; however, the spin crossover properties were absent. The presented study was also supported by theoretical calculations at the DFT/TD-DFT level of theory using TPSS and TPSSh functionals.

Far Infrared spectrum of naphthalene in the 5–15 THz range: Experimental and theoretical study

The Fourier Transform Infrared technique was employed to measure the terahertz spectrum of naphthalene at room temperature. Naphthalene exhibited six absorption bands in the 5–15 THz range, two of which have not been previously reported. The additional bands appeared at 5.80 and 6.37 THz. Density Functional Theory modeling with PBE0 and TPSSh functionals gave vibrational modes which matched well with the experimental bands despite the slight shifts in energies. The TPSSh functional was found to perform slightly better than the PBEO functional. Comparison of the naphthalene spectrum with the spectrum of terra preta suggested the existence of cyclic hydrocarbons in terra preta.

Geometrical benchmarking and analysis of redox potentials of copper(I/II) guanidine-quinoline complexes: Comparison of semi-empirical tight-binding and DFT methods and the challenge of describing the entatic state (part III).

Copper guanidine-quinoline complexes are an important class of bioinorganic complexes that find utilization in electron and atom transfer processes. By substitution of functional groups on the quinoline moiety the electron transfer abilities of these complexes can be tuned. In order to explore the full substitution space by simulations, the accurate theoretical description of the effect of functional groups is essential. In this study, we compare three different methods for the theoretical description of the structures. We use the semi-empirical tight-binding method GFN2-xTB, the density functional TPSSh and the double-hybrid functional B2PLYP. We evaluate the methods on five different complex pairs (Cu(I) and Cu(II) complexes), and compare how well calculated energies can predict the redox potentials. We find even though B2PLYP and TPSSh yield better accordance with the experimental structures. GFN2-xTB performs surprisingly well in the geometry optimization at a fraction of the computational cost. TPSSh offers a good compromise between computational cost and accuracy of the redox potential for real-life complexes.

Hyperfine Coupling Constants in Local Exact Two-Component Theory

We present a highly efficient implementation of the electron-nucleus hyperfine coupling matrix within the one-electron exact two-component (X2C) theory. The complete derivative of the X2C Hamiltonian is formed, that is, the derivatives of the unitary decoupling transformation are considered. This requires the solution of the response and Sylvester equations, consequently increasing the computational costs. Therefore, we apply the diagonal local approximation to the unitary decoupling transformation (DLU). The finite nucleus model is employed for both the scalar potential and the vector potential. Two-electron picture-change effects are modeled with the (modified) screened nuclear spin-orbit approach. Our implementation is fully integral direct and OpenMP-parallelized. An extensive benchmark study regarding the Hamiltonian, the basis set, and the density functional approximation is carried out for a set of 12-17 transition-metal compounds. The error introduced by DLU is negligible, and the DLU-X2C Hamiltonian accurately reproduces its four-component "fully" relativistic parent results. Functionals with a large amount of Hartree-Fock exchange such as CAM-QTP-02 and ωB97X-D are generally favorable. The pure density functional r2SCAN performs remarkably and even outperforms the common hybrid functionals TPSSh and CAM-B3LYP. Fully uncontracted basis sets or contracted quadruple-ζ bases are required for accurate results. The capability of our implementation is demonstrated for [Pt(C6Cl5)4]- with more than 4700 primitive basis functions and four rare-earth single-molecule magnets: [La(OAr*)3]-, [Lu(NR2)3]-, [Lu(OAr*)3]-, and [TbPc2]-. Here, the results with the spin-orbit DLU-X2C Hamiltonian are in an excellent agreement with the experimental findings of all Pt, La, Lu, and Tb molecules.

The best density functional theory functional for the prediction of 1 H and 13 C chemical shifts of protonated alkylpyrroles.

The prediction of 13 C chemical shifts can be challenging with density functional theory (DFT). In this study 39 different functionals and three different basis sets were tested on three neutral alkylpyrroles and their corresponding protonated species. The calculated shielding constants were compared to experimental data and results from previous calculations at the MP2. We find that the meta-hybrid functional TPSSh with either the Pople style basis set 6-311++G(2d,p) or the polarization consistent basis set pcSseg-1 gives the best results for the 13 C chemical shifts, whereas for the 1 H chemical shifts it is the TPSSh functional with either the 6-311++G(2d,p) or pcSseg-2 basis set. Including an explicit solvent molecule hydrogen bonded to NH in the alkylpyrroles improves the results slightly for the 13 C chemical shifts. On the other hand, for 1 H chemical shifts the opposite is true. Compared to calculations at the MP2 level none of the DFT functionals can compete with MP2 for the 13 C chemical shifts but for the 1 H chemical shifts the investigated DFT functionals are shown to give better agreement with experiment than MP2 calculations.

Quantum Chemical Study of the Structures and Stability of Copper(II) Bis(diketonate) Dimers

The quantum-chemical study of the copper diketonate dimers ([Cu(Acac)2]2 and [Cu(Acac)Hfac)]2) is performed using various functionals (B3LYP, TPSSh, PBE0, and B2PLYP) and basis sets (6-311++G(d,p) and Def2-TZVP) and taking into account dispersion interactions (D3BJ). The calculations using the B3LYP and TPSSh functionals combined with the Def2-TZVP basis set give the best agreement with experiment. The Basis Set Superposition Errors introduce no substantial changes to the stabilization energies of the considered systems. The obtained results show that the quantum-chemical study of similar associates of transition metals should be performed without empirical D3BJ dispersion corrections.

Effect of the Current Dependence of Tau-Dependent Exchange-Correlation Functionals on Nuclear Shielding Calculations.

Exchange-correlation functionals that depend on the local kinetic energy τ are widely used in many fields. This includes meta-generalized gradient approximation (GGA) functionals and their global hybrid versions as well as local hybrid functionals with τ-dependent local mixing functions to determine position-dependent exact-exchange admixture. Under the influence of an external magnetic field, τ becomes dependent on the gauge of the magnetic vector potential and should thus be extended to a gauge-invariant formulation. The currently most widely used extension for nuclear shielding calculations is that suggested by Maximoff and Scuseria (Maximoff, S. N.; Scuseria, G. E. Chem. Phys. Lett. 2004, 390, 408). Using the recent first implementation of local hybrids in this framework, we have found unphysical paramagnetic contributions, which are most clearly identified for atoms but are also present in molecules. These τMS artifacts are small for the TPSS or TPSSh functionals, significantly deshielding in the case of nonhydrogen nuclei for the M06-L and M06 functionals and significantly shielding in those cases for the first-generation τ-dependent local hybrids LH07t-SVWN and LH12ct-SsifPW92. We have therefore implemented an extension of a linear-response nuclear shielding code to the proper current-density functional version of τ suggested by Dobson (Dobson, J. F. J. Chem. Phys. 1993, 98, 8870). Using τD eliminates the gauge dependence as well as the unphysical contributions introduced by τMS. A first evaluation of this implementation for a set of main-group nuclear shieldings against CCSD(T) benchmark data indicates rather small effects for the TPSS and TPSSh functionals but significant changes for the other functionals studied: the previously observed remarkable performance of the highly parameterized meta-GGA functional M06-L is found to be the result of error compensation with the lack of an explicit current dependence. Results for the M06 functional are improved somewhat but are still overall of relatively low accuracy. In contrast, too low proton shieldings found recently for τ-dependent local hybrids are improved significantly within the τD current-density functional framework while preserving the outstanding performance of these functionals for other nuclei.

The active E4 structure of nitrogenase studied with different DFT functionals.

The present study concerns the technical aspects of obtaining the energetics for the E4 state of nitrogenase, the enzyme that fixes N2 in nature. EPR experiments have shown that the critical E4 structure that activates N2 should contain two bridging hydrides in the FeMo‐cofactor. It is furthermore in equilibrium with a structure where the two hydrides have been released and N2 binds. These observations led to the suggestion that E4 should have two bridging hydrides and two protonated sulfides. It is important to note that the structure for E4 has not been determined, but only suggested. For a long time, no DFT study led to the suggested structure, independent of which functional was used. However, in two recent DFT studies a good agreement with the experimental suggestion was claimed to have been obtained. In one of them the TPSS functional was used. That was the first out of 11 functionals tried that led to the experimentally suggested structure. In the second of the recent DFT studies, a similar conclusion was reached using the TPSSh functional. The conclusions in the recent studies have here been studied in detail, by calculating a critical energetic value strongly implied by the same EPR experiments. Both the TPSS and TPSSh functionals have been used. The present calculations suggest that those DFT functionals would not lead to agreement with the experimental EPR results either.

Расчет стандартных электродных потенциалов электрохимического восстановления ванадиевых соединений: подбор функционала плотности

A systematic benchmark study was performed for the first time to investigate the performance of density functional theory for calculation of reduction potentials of vanadium compounds. Six density functionals of different types were selected for testing: local OLYP and M06L, global hybrid O3LYP and B3LYP, as well as, meta-hybrid functionals TPSSh and M06. Local and hybrid functionals with a relatively high contribution of Hartree-Fock exchange showed unsatisfactory results. In particular, the widely used hybrid functional B3LYP for the transformation VIII→VII occurring in the vanadium redox flow battery yields a negative value of the standard potential instead of a positive one. Among the tested functionals the smallest deviation from the experimental data provides the meta-hybrid functional TPSSh with a 10% contribution of the Hartree-Fock exchange. The computational protocol to calculate redox potentials of vanadium compounds is suggested.

Benchmark Study of Ground-State Raman Spectra in Conjugated Molecules.

In this work, we compare calculated and experimental off-resonance Raman spectra for a series of substituted benzene and thiophene derivatives in the ground state. We benchmark a variety of approximate density functionals following the "Jacob's Ladder" hierarchy of chemical accuracy, as well as second-order Møller-Plesset perturbation theory. The Raman calculations at all levels of theory consistently overestimate the intensities of high-frequency vibrations relative to the lower frequency region of the spectrum. Although all methods provide comparable estimates of relative intensities for all compounds, the hybrid B3LYP and TPSSh functionals perform 5-19% better than the other methods in terms of mean unsigned error and 2-24% better in the standard deviation of the error. Both functionals also give band positions accurate to within about ±10 cm-1 of the experimental frequencies.

Comparative study of electronic-structure methods for platinum-containing molecules: bond lengths and bond dissociation energies

We compare various electronic structure methods including a broad range of density functionals with experimental data on bond lengths and bond dissociation energies available for di- and triatomic platinum-containing molecules. In total we employ 54 GGA, 17 meta-GGA, 36 hybrid, 103 mixed GGA/meta GGA, 17 hybrid, 7 meta hybrid, 10 range-separated hybrid and 5 double hybrid density functionals. Furthermore, the performance of ab initio methods including Hartree-Fock, Møller-Plesset perturbation theory up to fourth order as well as coupled cluster theory up to perturbatively approximated triple excitations, i.e. CCSD(T), is also investigated. In the case of bond lengths, the smallest mean average deviation from experimental values yielding 0.3 pm is found for the hybrid density functional TPSSh. Interestingly, neither recent double hybrid functionals nor ab initio methods result in similar, commensurable accuracies. For the investigated bond dissociation energies, the GGA functional TPSSVWN5 is the closest to experiment with deviations of 6.97 kcal/mol. Finally, we address various possible sources of errors that may explain the large mean average deviation from experiment in the case of CCSD(T) (8.87 kcal/mol), including the effect of basis set size, the influence of the multireference character of the molecular wave function, the quality of the HF determinant as reference wave function and the influence of core electron correlation.Graphical abstract

Fully numerical Hartree‐Fock and density functional calculations. I. Atoms

AbstractAlthough many programs have been published for fully numerical Hartree‐Fock (HF) or density functional (DF) calculations on atoms, we are not aware of any programs that support hybrid DFs, which are popular within the quantum chemistry community due to their better accuracy for many applications, or that can be used to calculate electric properties. Here, we present a variational atomic finite element solver in the HelFEM program suite that overcomes these limitations. A basis set of the type is used, where B n(r) are finite element shape functions and are spherical harmonics, which allows for an arbitrary level of accuracy. HelFEM supports nonrelativistic HF and DF calculations even with hybrid functionals, which are not available in other commonly available program packages. Hundreds of functionals at the local spin‐density approximation (LDA), generalized‐gradient approximation (GGA), as well as the meta‐GGA levels of theory are included through an interface with the Libxc library. Electric response properties are achievable via finite field calculations. We introduce an alternative grid that yields faster convergence to the complete basis set limit than commonly used alternatives. We also show that high‐order Lagrange interpolating polynomials yield the best convergence, and that excellent agreement with literature HF limit values for electric properties, such as static dipole polarizabilities, can be achieved with the present approach. Dipole moments and dipole polarizabilities at finite field are also reported with the PBE, PBEh, TPSS, and TPSSh functionals. Finally, we show that a recently published Gaussian basis set is able to reproduce absolute HF and DF energies of neutral atoms, cations, as well as anions within a few dozen microhartrees.

Structural diradical character.

A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. As diradical and closed-shell electronic structures differ in their distribution of single, double, triple, and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones (for which the D3 dispersion correction was included). Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation. © 2018 Wiley Periodicals, Inc.

Performance of density functionals for the structure and energetics of (M–O)0,± (M=Al, Si, Sc–Zn)

ABSTRACTWe report the results of the performance of 20 exchange–correlation functionals of density functional theory (DFT) in the structure (Metal–Oxygen bond length) and energetical properties (bond dissociation energy, adiabatic ionisation energy, and adiabatic electron affinity) of twelve metal monoxides (M–O, M=Al, Si, Sc–Zn). The calculated results show that the selected DFT functionals have the ability to reproduce the M–O bond length with a mean deviation of 0.01–0.05 Å, the energy values are reproduced with a mean deviation of 0.20–1.00 eV. In general, the functionals with significant HF exchange show decent performance in the calculation of bond length and harmonic vibrational frequency. These functionals show poor performance in energetics. Our calculated results show that the M06-L, B3LYP, and TPSSh functionals give good performance in both structure and energetical properties of metal monoxides. These functionals are recommended for the studies of structure and energetics in metal oxide systems. Further, our studies indicate that M06-L can be used for the studies in larger molecular systems. Among the 20 DFT functionals, the recently developed N12 functional gives poor performance in the studies of metal monoxides. Hence this functional is not recommended for the studies of structure and energetics in metal oxide systems.

Benchmarking Density Functional Methods for Calculation of State Energies of First Row Spin-Crossover Molecules.

A systematic study of the performance of several density functional methodologies to study spin-crossover (SCO) on first row transition metal complexes is reported. All functionals have been tested against several mononuclear systems containing first row transition metal complexes and exhibiting spin-crossover. Among the tested functionals, the hybrid meta-GGA functional TPSSh with a triple-ζ basis set including polarization functions on all atoms provides the best results across different metals and oxidation states, and its performance in both predicting the correct ground state and the right energy window for SCO to occur is quite satisfactory. The effect of some additional contributions, such as zero-point energies, relativistic effects, and intramolecular dispersion interactions, has been analyzed. The reported strategy thus expands the use of the TPSSh functional to other metals and oxidation states other than FeII, making it the method of choice to study SCO in first row transition metal complexes. Additionally, the presented results validate the potential use of the TPSSh functional for virtual screening of new molecules with SCO, or its use in the study of the electronic structure of such systems.

Quantum-Chemical Investigation of Molecular Structure and Thermodynamic Properties of Spiropyran Molecules

Spiropyrans are heterocyclic organic compounds known for their photochromic properties which are of perspective use for technology. They can be potentially used as the elements of molecular machines, optical data recording, modulated light filters and numerous hybrid materials. The key characteristic of such compounds is an ability to enter a photochromic reaction, taking place upon irradiation with UV light (see Figures 1 and 2). To develop more compounds which can possibly be of practical use the most important problem is to investigate their chemical properties both theoretically and experimentally. To determine the optimal geometric structures for both spiro- and mesocyanine forms of 1,3,3,6’-tetramethyl-8’-formylsporoindoline-2,2’-[2H]-chromene-3 and 3,8’-dimethyl-6’formylspiro-(-4-oxo-3,4-dihydro-2H-1,3)-benzoxasine-2,2’-[2H]-chromene-7 (objects 1 and 2 respectively) using the DFT approach, calculations of quantum chemistry were used. All the computations were carried out in GAMESS software. In this research we used B3LYP, CAM-B3LYP, wB97, M11 and TPSSh functionals in conjunction with 6-311++G(d,p) basis set. In our studies we were able to compare experimental gas-phase IR spectra with the quantum-chemically computed one. This allowed us to consider the TPSSh functional as the best model for these compounds due to the most accurate geometrical structures and thermodynamic functions obtained with its use. In addition, it was possible to compute the photoinduced products of the isomerization reaction (see Figure) of our objects and their UV/Vis spectra which were also compared with the experimental spectra of thin films. Comparison allowed us to consider high accuracy of computed data. However, an attempt to find the optimal molecular structure for the cis-isomer of the second object was unsuccessful. Except for the B3LYP functional, none of the listed above could provide us with the reliable structure and, therefore, other properties. For this reason, it was only possible to compute the reliable reaction enthalpies for the one precursor molecule. The reaction enthalpies listed in the Table 1 are considered to be optimal for using such a compound as a photosensitive switch element because it is bigger than 10 kJ/mole but no more than 100 kJ/mole. From this it follows, that spontaneous switching probability is acceptably small, although at the same time such an amount of energy is practically feasible. Reaction ∆r H(298.15 K), kJ/mol156.392-11.783-68.17

Spin states of Mn(III) meso-tetraphenylporphyrin chloride assessed by density functional methods.

The present work assessed several exchange-correlation functionals (including GGA, meta-GGA and hybrid functionals), in combination with a variety of basis sets and effective core potentials (ECP) for their ability to predict the ground spin state of Mn(III) meso-tetraphenylporphyrin chloride complex, labeled Mn(III)TPPCl, for which experimental data support the quintet high spin state. Geometry optimization of Mn(III)TPPCl was performed for three possible spin states (singlet state, LS; triplet state, IS; and quintet state, HS) at the TPSSh level using the LANL2DZ ECP for Mn and the 6-311G(d) basis set for C, N, Cl and H. Afterwards, single-point energy calculations were conducted by applying 18 exchange-correlation functionals (BLYP, B3LYP, PW91, BPW91, BP86, OLYP, OPBE, OPW91, O3LYP, PBE0, PBEh1PBE, HSEH1PBE, TPSS, TPSSh, M06L, M06, M062X and M06HF). The influence of the basis set for the metal center was assessed using a smaller group of functionals and varying between the Pople basis set 6-31G(d), its newer formulation m6-31G(d) and the larger Def2-QZVP basis set. All functionals in combination with Pople basis sets predict the quintet state as the ground spin state. In addition, the BLYP, BP86, BPW91, PW91, PBEh1PBE, TPSS and TPSSh functionals predicted the IS lying at most ~60kJmol-1 above the HS, which agrees with the reference data. Results including Def2-QZVP basis set were inconsistent, since only BLYP and B3LYP predict HS as the ground spin state. The recommended methodology for the treatment of such systems seems to be exchange-correlations functionals with few or none Hartree-Fock exchange and modest size basis sets. Graphical Abstract MnTPPCl molecule and the energy ordering of its spin states assessed by 18 functionals.

DFT Analysis of Spin Crossover in Mn(III) Complexes: Is a Two-Electron S = 2 to S = 0 Spin Transition Feasible?

Six-coordinate, rigorously octahedral d4 Mn(III) spin crossover (SCO) complexes are limited by symmetry to an S = 1 (intermediate spin, IS) to S = 2 (high spin, HS) transition. In order to realize the potential S = 0 to S = 2 transition, a lower symmetry and/or change in coordination number is needed, which we explore here computationally. First, a number of complexes are analyzed to develop a reliable and relatively fast DFT protocol for reproducing known Mn(III) spin state energetics. The hybrid meta-GGA functional TPSSh with a modest split valence plus polarization basis set and an empirical dispersion correction is found to predict correctly the ground spin state of Mn(III) complexes, including true low-spin (LS) S = 0 systems, with a range of donor sets including the hexadentate [N4O2] Schiff base ligands. The electronic structure design criteria necessary for realizing a ΔS = 2 SCO transition are described, and a number of model complexes are screened for potential SCO behavior. Five-coordinate trigonal-bipyramidal symmetry fails to yield any suitable systems. Seven-coordinate, approximately pentagonal bipyramidal symmetry is more favorable, and when a known pentadentate macrocyclic donor is combined with π-acceptor axial ligands, a novel Mn(III) complex, [Mn(PABODP)(PF3)2]3+ (PABODP = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), is predicted to have the right spin state energetics for an S = 0 to S = 2 transition. Successful synthesis of such a complex could provide the first example of a ΔS = 2 SCO transition for d4 Mn(III). However, the combination of a rigid macrocycle and a high coordination number dilutes the stereochemical activity of the d electrons, leading to relatively small structural changes between HS and LS systems. It may therefore remain a challenge to realize strong cooperative effects in Mn(III) systems.