A Systematic Density Functional Study of the Zero-Field Splitting in Mn(II) Coordination Compounds

This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in Mn(II) coordination complexes. Eighteen experimentally well characterized four-, five-, and six-coordinate complexes of the general formula [Mn(L)nL‘2] with L‘ = Cl, Br, I, NCS, or N3 (L = an oligodentate ligand) are considered. Several DFT-based approaches for the prediction of the ZFSs are compared. For the estimation of the spin−orbit coupling (SOC) part of the ZFS, it was found that the Pederson−Khanna (PK) approach is more successful than the previously proposed quasi-restricted orbitals (QRO)-based method. In either case, accounting for the spin−spin (SS) interaction either with or without the inclusion of the spin-polarization effects improves the results. This argues for the physical necessity of accounting for this important contribution to the ZFS. On average, the SS contribution represents ∼30% of the axial D parameters. In addition to the SS part, the SOC contributions of d−d spin flip (αβ) and ligand-to-metal charge transfer excited states (ββ) were found to dominate the SOC part of the D parameter; the observed near cancellation between the αα and βα parts is discussed in the framework of the PK model. The calculations systematically (correlation coefficient ∼0.99) overestimate the experimental D values by ∼60%. Comparison of the signs of calculated and measured D values shows that the signs of the calculated axial ZFS parameters are unreliable once E/D > 0.2. Finally, we find that the calculated D and E/D values are highly sensitive to small structural changes. It is observed that the use of theoretically optimized geometries leads to a significant deterioration of the theoretical predictions relative to the experimental geometries derived from X-ray diffraction. The standard deviation of the theoretical predictions for the D values almost doubles from ∼0.1 to ∼0.2 cm-1 upon using quantum chemically optimized structures. We do not find any noticeable improvement in considering basis sets larger than standard double- (SVP) or triple-ζ (TZVP) basis sets or using functionals other than the BP functional

Molecule-Bridged Mixed-Valent Intermediates Involving the Ru^I Oxidation State

The diruthenium(II) complexes {(μ-L)[RuCl(Cym)]2}(PF6)n, Cym = p-cymene = 4-isopropyltoluene, L = 2,2‘-azobispyridine = abpy and n = 1, or L = 2,5-bis(1-phenyliminoethyl)pyrazine = bpip and n = 2, were synthesized and characterized by NMR (n = 2) or EPR spectroscopy (n = 1). Whereas the (n = 1) species are ligand radical-bridged RuIIRuII complexes, the three-electron reduction under loss of both chloride ions produces the ions {(μ-L)[Ru(Cym)]2}+, which could be identified as RuI(4d7)-containing mixed-valent species (Ru0RuI or RuIRuII) through UV−vis−NIR spectroelectrochemistry (intervalence charge-transfer bands around 1500 nm) and EPR (rhombic g tensor anisotropy). The weak metal−metal interaction of the dσ electrons from the eg set is responsible for the small electrochemical coupling with comproportionation constants Kc ≈ 102

Systematic Theoretical Study of the Zero-Field Splitting in Coordination Complexes of Mn(III). Density Functional Theory versus Multireference Wave Function Approaches

This paper presents a detailed evaluation of the performance of density functional theory (DFT) as well as complete active space self-consistent field (CASSCF)-based methods (CASSCF and second-order N-electron valence state perturbation theory, NEVPT2) to predict the zero-field splitting (zfs) parameters for a series of coordination complexes containing the Mn(III) ion. The physical origin of the experimentally determined zfs’s was investigated by studying the different contributions to these parameters. To this end, a series of mononuclear Mn(III) complexes was chosen for which the structures have been resolved by X-ray diffraction and the zfs parameters have been accurately determined by high-field EPR spectroscopy. In a second step, small models have been constructed to allow for a systematic assessment of the factors that dominate the variations in the observed zfs parameters and to establish magnetostructural correlations. Among the tested functionals, the best predictions have been obtained with B3LYP, followed by the nonhybrid BP86 functional, which in turn is more successful than the meta-hybrid GGA functional TPSSh. For the estimation of the spin−orbit coupling (SOC) part of the zfs, it was found that the coupled perturbed SOC approach CP is more successful than the Pederson−Khanna method. Concerning the spin−spin interaction (SS), the restricted open-shell Kohn−Sham (ROKS) approach led to a slightly better agreement with the experiment than the unrestricted KS (UKS) approach. The ab initio state-averaged CASSCF (SA-CASSCF) method with a minimal active space and the most recent implementation that treats the SOC and SS contributions on an equal footing provides the best predictions for the zfs. The analysis demonstrates that the major contribution to the axial zfs parameter (D) originates from the SOC interaction but that the SS part is far from being negligible (between 10 and 20% of D). Importantly, the various excited triplet ligand field states account for roughly half of the value of D, contrary to popular ligand field models. Despite covering dynamic correlation contributions to the transition energies, NEVPT2 does not lead to large improvements in the results as the excitation energies of the Mn(III) d−d transitions are already fairly accurate at the SA-CASSCF level. For a given type of coordination sphere (e.g., elongated or compressed octahedron), the magnetic anisotropy of the Mn(III) ion, D, does not appear to be highly sensitive to the nature of the ligands, while the E/D ratio is notably affected by all octahedral distortions. Furthermore, the introduction of different halides into the coordination sphere of Mn(III) only leads to small effects on D. Nevertheless, it appears that oxygen-based ligands afford larger D values than nitrogen-based ligands

Structural Characterization and Electronic Properties Determination by High-Field and High-Frequency EPR of a Series of Five-Coordinated Mn(II) Complexes

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)2] (terpy = 2, 2‘:6‘,2‘ ‘-terpyridine; X = I- (1), Br- (2), Cl- (3), or SCN- (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1−4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95−285 GHz) (D ≪ gβB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm-1 with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed

Structural Characterization and Electronic Properties Determination by High-Field and High-Frequency EPR of a Series of Five-Coordinated Mn(II) Complexes

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)2] (terpy = 2, 2‘:6‘,2‘ ‘-terpyridine; X = I- (1), Br- (2), Cl- (3), or SCN- (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1−4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95−285 GHz) (D ≪ gβB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm-1 with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed

Structural Characterization and Electronic Properties Determination by High-Field and High-Frequency EPR of a Series of Five-Coordinated Mn(II) Complexes

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)2] (terpy = 2, 2‘:6‘,2‘ ‘-terpyridine; X = I- (1), Br- (2), Cl- (3), or SCN- (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1−4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95−285 GHz) (D ≪ gβB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm-1 with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed

Syntheses, X-ray Structures, Solid State High-Field Electron Paramagnetic Resonance, and Density-Functional Theory Investigations on Chloro and Aqua Mn^II Mononuclear Complexes with Amino-Pyridine Pentadentate Ligands

The two pentadentate amino-pyridine ligands L52 and L53 (L52 and L53 stand for the N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine and the N-methyl-N,N′,N′-tris(2-pyridylmethyl)propane-1,3-diamine, respectively) were used to synthesize four mononuclear MnII complexes, namely [(L52)MnCl](PF6) (1(PF6)), [(L53)MnCl](PF6) (2(PF6)), [(L52)Mn(OH2)](BPh4)2 (3(BPh4)2), and [(L53)Mn(OH2)](BPh4)2 (4(BPh4)2). The X-ray diffraction studies revealed different configurations for the ligand L5n (n = 2, 3) depending on the sixth exogenous ligand and/or the counterion. Solid state high-field electron paramagnetic resonance spectra were recorded on complexes 1−4 as on previously described mononuclear MnII systems with tetra- or hexadentate amino-pyridine ligands. Positive and negative axial zero-field splitting (ZFS) parameters D were determined whose absolute values ranged from 0.090 to 0.180 cm−1. Density-functional theory calculations were performed unraveling that, in contrast with chloro systems, the spin−spin and spin−orbit coupling contributions to the D-parameter are comparable for mixed N,O-coordination sphere complexes

Structural Characterization and Electronic Properties Determination by High-Field and High-Frequency EPR of a Series of Five-Coordinated Mn(II) Complexes

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)2] (terpy = 2, 2‘:6‘,2‘ ‘-terpyridine; X = I- (1), Br- (2), Cl- (3), or SCN- (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1−4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95−285 GHz) (D ≪ gβB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm-1 with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed

Syntheses, X-ray Structures, Solid State High-Field Electron Paramagnetic Resonance, and Density-Functional Theory Investigations on Chloro and Aqua Mn^II Mononuclear Complexes with Amino-Pyridine Pentadentate Ligands

The two pentadentate amino-pyridine ligands L52 and L53 (L52 and L53 stand for the N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine and the N-methyl-N,N′,N′-tris(2-pyridylmethyl)propane-1,3-diamine, respectively) were used to synthesize four mononuclear MnII complexes, namely [(L52)MnCl](PF6) (1(PF6)), [(L53)MnCl](PF6) (2(PF6)), [(L52)Mn(OH2)](BPh4)2 (3(BPh4)2), and [(L53)Mn(OH2)](BPh4)2 (4(BPh4)2). The X-ray diffraction studies revealed different configurations for the ligand L5n (n = 2, 3) depending on the sixth exogenous ligand and/or the counterion. Solid state high-field electron paramagnetic resonance spectra were recorded on complexes 1−4 as on previously described mononuclear MnII systems with tetra- or hexadentate amino-pyridine ligands. Positive and negative axial zero-field splitting (ZFS) parameters D were determined whose absolute values ranged from 0.090 to 0.180 cm−1. Density-functional theory calculations were performed unraveling that, in contrast with chloro systems, the spin−spin and spin−orbit coupling contributions to the D-parameter are comparable for mixed N,O-coordination sphere complexes

Lithium/Sulfur Cell Discharge Mechanism: An Original Approach for Intermediate Species Identification

The lithium/sulfur battery is a promising electrochemical system that has a high theoretical capacity of 1675 mAh g^–1, but its discharge mechanism is well-known to be a complex multistep process. As the active material dissolves during cycling, this discharge mechanism was investigated through the electrolyte characterization. Using high-performance liquid chromatography, UV–visible absorption, and electron spin resonance spectroscopies, we investigated the electrolyte composition at different discharge potentials in a TEGDME-based electrolyte. In this study, we propose a possible mechanism for sulfur reduction consisting of three steps. Long polysulfide chains are produced during the first reduction step (2.4–2.2 V vs Li⁺/Li), such as S₈^2– and S₆^2–, as evidenced by UV and HPLC data. The S₃^•– radical can also be found in solution because of a disproportionation reaction. S₄^2– is produced during the second reduction step (2.15–2.1 V vs Li⁺/Li), thus pointing out the gradual decrease of the polysulfide chain lengths. Finally, short polysulfide species, such as S₃^2–, S₂^2–, and S^2–, are produced at the end of the reduction process, i.e., between 2.1 and 1.9 V vs Li⁺/Li. The precipitation of the poorly soluble and insulating short polysulfide compounds was evidenced, thus leading to the positive electrode passivation and explaining the early end of discharge