Ligand Field Research Articles

Bio‐evaluation of pyrazolone‐furan derived Schiff base Cr(III), Co(II), Ni(II), Cu(II), and Zn(II) nano‐sized complexes: Design, spectral characterization, antimicrobial efficacy, antioxidant activity, apoptotic activity, and cell cycle analysis

A tridentate pyrazolone‐furan Schiff's base ligand, [(E)‐4‐(((5‐[2‐fluorophenyl]furan‐2‐yl)methylene)amino)‐1,5‐dimethyl‐2‐phenyl‐1,2‐dihydro‐3H‐pyrazol‐3‐one] (PFSB), was produced by condensation between compounds 5‐(2‐fluorophenyl)furan‐2‐carbaldehyde and 4‐amino‐1,5‐dimethyl‐2‐phenyl‐1,2‐dihydro‐3H‐pyrazol‐3‐one. Spectroscopic characterization evidence suggested the tridentate nature (ONO) of pyrazolone‐furan Schiff's base ligand (PFSB) and coordinated to the metal (II/III) moieties through nitrogen and oxygen donor sites giving tetra‐ and/or hexa‐coordinated geometry to complexes. The complexes have been supported by elemental analyses, molar conductance, magnetic moments, spectral analyses, EPR, ligand field parameters, XRD, and thermogravimetric measurements. Theoretical optimization employing the density functional theory (DFT) was accomplished on the molecular structures of the compounds in question, and subsequently, the quantum chemical descriptors were computed. Cr(III), Co(II), Ni(II), and Zn(II) complexes were allocated octahedral coordination, while the Cu(II) complex was allocated square‐planar coordination. The SEM, TEM, XRD, AFM, and EDX of the studied complexes unveiled distinct and strong diffraction peaks, signifying their crystalline nature, and providing evidence of their particle sizes being within the nano‐size. The crystal sizes calculated for all complexes were determined to be ranging from 27.73 to 76.39 nm. However, the Cr(III) complex exhibited a particle size exceeding 100 nm. The interactions between metal complexes and calf thymus DNA, and their potential for mimicking insulin activity, were investigated in a controlled lab setting by measuring their ability to inhibit alpha‐amylase. The anti‐microbial potential of the ligand (PFSB) and complexes (C1‐C5) were tested. The viscosity and UV–Vis absorption determinations were utilized to assess the calf thymus DNA (CT‐DNA) interaction with the nano‐sized metal (II/III) chelates. Our flow cytometry data indicate significant levels of apoptosis and cell cycle arresting in both liver and breast cancer cell lines.

Group IV complexes with nitrogen based ligands in the ring opening polymerization of cyclic esters

The ring opening polymerization of cyclic esters has attracted much interest for the relevance of polyesters as sustainable alternatives to petroleum-based plastics. Since their first discovery, group IV metal complexes have been demonstrated to be active in the different fields of the homogenous catalysis. This brief review focused the light on the ring-opening polymerization of cyclic esters promoted by group IV complexes supported by ligand with anionic nitrogen atoms. The structures and catalytic activity of the complexes are discussed. The aim of this work is to demonstrate how this field of nitrogen-based ligands to support group IV metals deserves to be further investigated to obtain increasingly efficient catalysts in this polymerization reaction, which remains poorly researched.

1,1,1-Trifluoro-5,5-dimethyl-2,4-hexanedione based ternary complexes of Eu(III): Synthesis, structural and luminescence investigations

In this current study, we have synthesized four europium(III) complexes utilizing a tri-fluorinated β-diketone moiety: 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (TFDH) with auxiliary ligand: 2,2′- bipyridine and its derivatives. FTIR, UV–Vis and luminescence spectroscopic characterizations were implemented for evaluation of the bonding and luminous characteristics in synthesized europium(III) complexes. Eight-coordination environment around Eu3+ ion has been demonstrated by 1H NMR spectral data. Photoluminescence (PL) study of these complexes has been carried out in powdered form. Characteristic red emitting behavior of Eu(III) ion was observed corresponds to the transition of 5D0→7F2 in PL spectra of Eu(III) complexes. The CIE (Commission Internationale de l'Eclairage) chroma coordinates 1931 and CCT (Correlated Color Temperature) values were derived from the information obtained from the emission spectra. The Judd-Ofelt intensity parameters (Ωτ) and excited state deactivation time (τ) in solid state have also been estimated. In the synthesized Eu(III) complexes, the europium ion was present in a highly polarizable ligand field, as demonstrated by the higher values of Ω2. Values of band gap i.e. optical and electronic as well as the red luminous nature of prepared complexes showed up their utilization in display devices.

Three coordinated first row-transition metal complexes, [M{N(SiMe3)2}3]−1/0: Structure, bonding, and magnetic properties

Structure, bonding, and magnetic properties of [M{N(SiMe3)2}3]−1/0 (M = first row-transition metal, Sc-Cu in + 2/+3 oxidation state) complexes are studied. The atoms in molecules (AIM) analysis shows that non-covalent interactions between the methyl hydrogens are prevalent. Energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) analysis shows that the ligand is bonded to the metal ions through σ and π bonding. In addition, the reasonable contribution from the dispersion interaction supports van der Waals' type of interaction. The highest value of magnetic anisotropy (D) is calculated in the NiII complex with a value of −382.945 (−234.145) cm−1 at the CASSCF (NEVPT2) level of theory. It is significantly higher than the previously reported values of −14.644 (−15.631) and −63.691 (−64.000) cm−1 in similar CrII and MnIII complexes, respectively. Ab Initio Ligand Field Theory (AILFT) is utilized to calculate the splitting of the d-orbitals in all the metal complexes. In the NiII complexes, the blocking barrier (Ueff) is found to be 127 cm−1. Thus, it has a high potential to be utilized as a single ion magnet (SIM).

Correlating Valence and 2p3d RIXS Spectroscopies: A Ligand-Field Study of Spin-Crossover Iron(II).

The molecular spin-crossover phenomenon between high-spin (HS) and low-spin (LS) states is a promising route to next-generation information storage, sensing applications, and molecular spintronics. Spin-crossover complexes also provide a unique opportunity to study the ligand field (LF) properties of a system in both HS and LS states while maintaining the same ligand environment. Presently, we employ complementing valence and core-level spectroscopic methods to probe the electronic excited-state manifolds of the spin-crossover complex [FeII(H2B(pz)2)2phen]0. Light-induced excited spin-state trapping (LIESST) at liquid He temperatures is exploited to characterize magnetic and spectroscopic properties of the photoinduced HS state using SQUID magnetometry and magnetic circular dichroism spectroscopy. In parallel, Fe 2p3d RIXS spectroscopy is employed to examine the ΔS = 0, 1 excited LF states. These experimental studies are combined with state-of-the-art CASSCF/NEVPT2 and CASCI/NEVPT2 calculations characterizing the ground and LF excited states. Analysis of the acquired LF information further supports the notion that the spin-crossover of [FeII(H2B(pz)2)2phen]0 is asymmetric, evidenced by a decrease in eπ in the LS state. The results demonstrate the power of cross-correlating spectroscopic techniques with high and low LF information content to make accurate excited-state assignments, as well as the current capabilities of ab initio theory in interpreting these electronic properties.

Unravelling the Highest Magnetic Anisotropy Among all the nd-Shells in [WCp2]0 Metallocene.

Single-molecule magnets (SMMs) with a large magnetization reversal barrier are predominated by the lanthanide systems due to their strong spin-orbit coupling (SOC). However, the transition metals have also emerged as potential contenders and the largest magnetic anisotropy has been identified for a cobalt system among any d-series-based SMMs (Bunting et al. Science 2018, 362, eaat7319). In this work, we have explored the magnetic anisotropy in highly axial ligand field systems of metallocene, having different d-subshell (3d4, 4d4, and 5d4). The wave function-based multireference methods including static and dynamic electron correlations have been employed to investigate the zero-field splitting (ZFS) parameters. Here, we report exceptionally large magnetic anisotropy for a 5d complex of [WCp2]0 with the highest energy barrier that is nearly twice as high as the previous record value for the Co complex. We have also observed that the axial ZFS parameter (D) is increasing down the group in the order of 3d < 4d < 5d, pertaining to a large SOC.

Engineering Spin State in Spinel Co3O4 nanosheets by V‐Doping for Bidirectional Catalysis of Polysulfides in Lithium–Sulfur Batteries

AbstractThe slow bidirectional conversion and fast shuttling of lithium polysulfides (LiPSs) remain the main obstacles that inhibit the practical application of lithium sulfur batteries (LSBs). Here, by engineering the in‐spin state of spinel oxides, V doping‐induced hexagonal nanosheets (V‐Co3O4 NS) with intermediate spin state (IS, t2g5 eg1) can form stable bond order toward LiS* based on the ligand field theory and molecular orbital theory, thus effectively accelerating the sulfur conversion kinetics. Owing to these merits, the V‐Co3O4 NS modified separator base battery achieves a high areal capacity of 15.37 mAh cm−2 with sulfur loading of 15.4 mg cm−2, and it displays a low‐capacity attenuation of 0.015% each cycle at 3.0 C for 1900 cycles.

Ligand Field Sensitive Spin Acceleration in the Iron-Catalyzed [2 + 2] Cycloaddition of Unactivated Alkenes and Dienes.

Redox-active pyridine(diimine) (PDI) iron catalysts promote the reversible [2 + 2] cycloaddition of alkenes and dienes to cyclobutane derivatives that have applications ranging from fuels to chemically recyclable polymers. Metallacycles were identified as key intermediates, and spin crossover from the singlet to the triplet surface was calculated to facilitate the reductive coupling step responsible for the formation of the four-membered ring. In this work, a series of sterically and electronically differentiated PDI ligands was studied for the [2 + 2] cycloaddition of ethylene and butadiene to vinylcyclobutane. Kinetic studies revealed that the fastest and slowest turnover were observed with equally electron-deficient supporting ligands that either feature phenyl-substituted imine carbon atoms (MeBPDI) or a pyrazine core (MePZDI). While the oxidative cyclization was comparatively slow for both catalysts, the rate of reductive coupling─determined by stoichiometric 13C2H4 labeling studies─correlated with the turnover frequencies. Two-state density functional theory studies and the distinct electronic structures of related (iPrBPDI) and (iPrPZDI) iron methyl complexes revealed significantly different ligand field strengths due to either diminished ligand σ-donation (MeBPDI) or promoted metal π-backbonding (MePZDI). Spin acceleration, leading to fast reductive coupling and catalytic turnover, was promoted in the case of the weaker ligand field and depends on both the nature and position of the electron-withdrawing group. This study provides strong evidence for the role of two-state reactivity in C(sp3)-C(sp3) bond formation and insights on how ligand design either promotes or inhibits spin acceleration in earth-abundant metal catalysis.

Magneto-Structural Correlation of Five-Coordinate Trigonal Bipyramidal High Spin Cobalt(II) Complexes.

Here, we combined magnetometry, multi-frequency electronic paramagnetic resonance, and wave function based ab initio calculations to investigate magnetic properties of two high spin Co(II) complexes Co(BDPRP) (BDPRP=2,6-bis((2-(S)-di(4-R)phenylhydroxylmethyl-1-pyrrolidi-nyl)methyl)pyridine, R=H for 8; R=tBu for 9). Complexes 8 and 9 featuring effective D3h symmetry were found to possess D=24.0 and 32.0 cm-1, respectively, in their S=3/2 ground states of . Ligand field analyses revealed that the low-lying d-d excited states make either positive or vanishing contributions to D. Hence, total positive D values were measured for 8 and 9, as well as related D3h high spin Co(II) complexes. In contrast, negative D values are usually observed for C3v congeners. In-depth analyses suggested that lowering symmetry from D3h to C3v induces orbital mixing between and and admixes excited state into the ground state. Both factors turn the total D value progressively negative with the increasing distance (δ) of the Co(II) center out of the equatorial plane. Therefore, δ determines the sign and magnitude of final D values of five-coordinate trigonal bipyramidal S=3/2 Co(II) complexes as measured for a series of such species with varying δ.

Oxygen-Donor Metalloligands Induce Slow Magnetization Relaxation in Zero Field for a Cobalt(II) Complex with {CoO4} Motif.

Most 3d metal-based single-molecule magnets (SMMs) use N-ligands or ligands with even softer donors to impart a particular coordination geometry and increase the zero-field splitting parameter |D|, while complexes with hard O-donor ligands showing slow magnetization relaxation are rare. Here, we report that a diamagnetic NiII complex of a tetradentate ligand featuring two N-heterocyclic carbene and two alkoxide-O donors, [LO,ONi], can serve as a {O,O'}-chelating metalloligand to give a trinuclear complex [(LO,ONi)Co(LO,ONi)](OTf)2 (2) with an elongated tetrahedral {CoIIO4} core, D = -74.3 cm-1, and a spin reversal barrier Ueff = 86.9 cm-1 in the absence of an external dc field. The influence of diamagnetic NiII on the electronic structure of the {CoO4} unit in comparison to [Co(OPh)4]2- (A) has been probed with multireference ab initio calculations. These reveal a contrapolarizing effect of the NiII, which forms stronger metal-alkoxide bonds than the central CoII, inducing a change in ligand field splitting and a 5-fold increase in the magnetic anisotropy in 2 compared to A, with an easy magnetization axis along the Ni-Co-Ni vector. This demonstrates a strategy to enhance the SMM properties of 3d metal complexes with hard O-donors by modulating the ligand field character via the coordination of diamagnetic ions and the benefit of robust metalloligands in that regard.

Tracking Coordination Environment and Reaction Intermediates in Homogeneous and Heterogeneous Epoxidation Catalysts via Ti L2,3-Edge Near-Edge X-ray Absorption Fine Structures.

Ti-based molecules and materials are ubiquitous and play a major role in both homogeneous and heterogeneous catalytic processes. Understanding the electronic structures of their active sites (oxidation state, local symmetry, and ligand environment) is key to developing molecular-level structure-property relationships. In that context, X-ray absorption spectroscopy (XAS) offers a unique combination of elemental selectivity and sensitivity to local symmetry. Commonly, for early transition metals such as Ti, K-edge XAS is applied for in situ characterization and subsequent structural analysis with high sensitivity toward tetrahedral species. Ti L2,3-edge spectroscopy is in principle complementary and offers specific opportunities to interrogate the electronic structure of five-and six-coordinated species. It is, however, much more rarely implemented because the use of soft X-rays implies ultrahigh vacuum conditions. Furthermore, the interpretation of the data can be challenging. Here, we show how Ti L2,3-edge spectroscopy can help to obtain unique information about both homogeneous and heterogeneous epoxidation catalysts and develop a molecular-level relationship between spectroscopic signatures and electronic structures. Toward this goal, we first establish a spectral library of molecular Ti reference compounds, comprising various coordination environments with mono- and dimeric Ti species having O, N, and Cl ligands. We next implemented a computational methodology based on multiplet ligand field theory and maximally localized Wannier orbitals benchmarked on our library to understand Ti L2,3-edge spectroscopic signatures. We finally used this approach to track and predict the spectra of catalytically relevant intermediates, focusing on Ti-based olefin epoxidation catalysts.

Electronic Structure and Ligand Effects on the Activation and Cleavage of N2 on a Molybdenum Center.

Dinitrogen fixation under ambient conditions remains a challenge in the field of catalytic chemistry due to the inertness of N2. Nitrogenases and heterogeneous solid catalysts have displayed remarkable performance in the catalytic conversion of dinitrogen to ammonia. By introduction of molybdenum centers in molecular complexes, one of the most azophilic metals of the transitional metal series, moderate ammonia yields have been attained. Here, we present a combined multiconfigurational/density functional theory study that addresses how ligand fields of different strengths affect the binding and activation of dinitrogen on molybdenum atoms. First, we explored with MRCI computations the diatomic Mo-N and triatomic Mo-N2 molecular systems. Then, we performed a systematic examination on the stabilization effects introduced by external NH3 ligands, before we explore model neutral and charged complexes with different types of ligands (H2O, NH3, and PH3) and their consequences on the N2 binding and activation.

Impact of mixed heavy metal cations (Ba and Bi) on the structure, optical and ionizing radiation shielding parameters of Bi2O3–BaO–Fe2O3–SrO–B2O3 glass matrix

The impacts of the Bi2O3/BaO ratio on the microstructure, optical, ligand field, and radiation shielding features of BSrFeBaBi glass matrix. The Bi2O3 content of the BSrFeBaBi glasses was increased from 0 to 20 mol%. The density increased from 3.77 g/cm3 to 5.00 g/cm3 and the molar volume increased from 24.74 cm3/mol to 31.17 cm3/mol with increasing Bi2O3/BaO ratio. The optical analysis confirmed that the transparency of BSrFeBaBi glasses in the visible region is high. Moreover, the presence of Fe cations in the octahedral coordination (FeO6 units) was confirmed. Furthermore, the fundamental absorption edge of BSrFeBaBi glasses shifts towards lower energies with additional Bi2O3/BaO concentrations, confirming the reduced behavior of the optical band gap. Also, ligand field splitting values decreased while Racah parameters increased. The obtained results demonstrate a tendency of the bonds between the Fe ions and their surroundings towards higher degree of ionic nature. Furthermore, the Bi2O3/BaO ratio demonstrated an enhanced radiation shielding behavior. Finally, BSrFeBaBi glasses exhibit favorable attributes for use in the radiation shielding domain due to their efficient radiation shielding features and transparency.

Theoretical Photoelectron Spectroscopy of Quadruple-Bonded Dimolybdenum(II,II) and Ditungsten(II,II) Paddlewheel Complexes: Performance of Common Density Functional Theory Methods.

We have revisited the gas-phase photoelectron spectra of quadruple-bonded dimolybdenum(II,II) and ditungsten(II,II) paddlewheel complexes with modern density functional theory methods and obtained valuable calibration of four well-known exchange-correlation functionals, namely, BP86, OLYP, B3LYP*, and B3LYP. All four functionals were found to perform comparably, with discrepancies between calculated and experimental ionization potentials ranging from <0.1 to ∼0.5 eV, with the lowest errors observed for the classic pure functional BP86. All four functionals were found to reproduce differences in ionization potentials (IPs) between analogous Mo2 and W2 complexes, as well as large, experimentally observed ligand field effects on the IPs, with near-quantitative accuracy. The calculations help us interpret a number of differences between analogous Mo2 and W2 complexes through the lens of relativistic effects. Thus, relativity results in not only significantly lower IPs for the W2 complexes but also smaller HOMO-LUMO gaps and different triplet states relative to their Mo2 counterparts.

Mössbauer, optical and structural properties of [formula omitted] ion in borate glass

Lanthanum-bearing iron lithium borate glass is a quaternary system for oxide glasses and was prepared via the melt-quenching method. The present article correlates the structure, optical, ligand field and Mössbauer data on iron lithium borate glass containing La3+. The density was measured, while the molar volume was calculated. Other physical parameters are well-described. With increasing the La2O3 content within the glass network, infrared spectra analysis reveals structural modifications such as the increase in BO4 units and the decline in both BO3 units and NBO bonds content. Furthermore, optical absorption spectra were measured. The absorption spectra disclose a plethora of electronic transitions that are related to Fe3+ in tetrahedral and octahedral sites, however, Fe2+ phase is not observed in optical spectra, but it has a clear signature in Mössbauer spectra. Besides, the glass absorption edges undergo a clear blue shift, reflecting an increased band gap energy (1.96–2.28 eV). The decline in NBO bonds justifies this trend. Bewitchingly, the values of crystal field splitting are increased, while the values of Racah parameters are decreased. This trend is justified by the decline in NBO bonds and increases electron localization around Fe cations. Mössbauer spectra confirm the existence of Fe3+ in tetrahedral and octahedral sites, while Fe2+ exists in only a tetrahedral state. With increasing La2O3 content, the isomer shift of Fe3+ in tetrahedral sites changes to be 0.312–0.329 mm/s, while the isomer shift of octahedral Fe3+ is 0.424–0.456 mm/s. These findings coincide with optical data. While the isomer shift of tetrahedral Fe2+ is 0.902–0.911 mm/s. Our results of structural, optical and ligand field associated with Mössbauer spectra open more vistas toward the utility of these samples in the optics realm.

Introducing Weakly Ligated Tris(trifluoromethyl)copper(III)

AbstractTris(trifluoromethyl)copper(III), Cu(CF3)3, was prepared for the first time and fully characterized in the form of weakly coordinated highly reactive solvent neutral adducts (DMF)2Cu(CF3)3 and (MeCN)Cu(CF3)3. Free Cu(CF3)3 is an elusive molecule of distorted T‐shaped geometry with Lewis acid properties, which is stabilized by solvation. Substitution of solvent molecules with monoanionic ligands in mild conditions afforded the first Cu(III)(CF3)3 carboxylates [Cu(CF3)3(OAc)]− and [Cu(CF3)3(OBz)]− and Cu(III) nitrito complex [Cu(CF3)3(ONO)]−. An improved synthesis of valuable Grushin's trifluoromethylation reagents via the intermediacy of (MeCN)Cu(CF3)3 was developed. (DMF)2Cu(CF3)3 demonstrated promising properties in photoinduced radical C‐H trifluoromethylation. Oxidation state theoretical studies showed the Cu(CF3)3 compounds to be borderline between classical Cu(III) and inverted ligand field Cu(I) assignment.

Two-Dimensional Spin-Crossover Molecular Solid Solutions with Tunable Transition Temperatures across 90 K.

Spin-crossover (SCO) materials exhibit remarkable potential as bistable switches in molecular devices. However, the spin transition temperatures (Tc) of known compounds are unable to cover the entire ambient temperature spectrum, largely limiting their practical utility. This study reports an exemplary two-dimensional SCO solid solution system, [FeIII(H0.5LCl)2-2x(H0.5LF)2x]·H2O (H0.5LX = 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, X = F or Cl, x = 0 to 1), in which the adjacent layers are adhered via hydrogen bonding. Notably, the Tc of this system can be fine-tuned across 90 K (227-316 K) in a linear manner by modulating the fraction x of the LF ligand. Elevating x results in strengthened hydrogen bonding between adjacent layers, which leads to enhanced intermolecular interactions between adjacent SCO molecules. Single-crystal diffraction analysis and periodic density functional theory calculations revealed that such a special kind of alteration in interlayer interactions strengthens the FeIIIN2O2S2 ligand field and corresponding SCO energy barrier, consequently resulting in increased Tc. This work provides a new pathway for tuning the Tc of SCO materials through delicate manipulation of molecular interactions, which could expand the application of bistable molecular solids to a much wider temperature regime.

Conformational polymorphs of the homoleptic scorpionate sandwich complex [(TpPh,Me)2Ni], TpPh,Me=hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate: Ligand substituent effects include intramolecular non-covalent interactions

The homoleptic scorpionate sandwich complex [(TpPh,Me)2Ni] (1), where TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate, crystallizes as a mixture of two polymorphs. A major monoclinic fraction crystallizes as well-formed parallelepipeds and a minor triclinic fraction forms irregularly shaped, anhedral crystals. These exhibit distinct melting points (288 and 294 °C, respectively) as observed by differential scanning calorimetry. We obtained X-ray crystal structures of both polymorphs at 100 K. The complex adopts different conformations between the two lattices. These are supported by distinctive ensembles of intramolecular non-covalent interactions, specifically π-stacking and perpendicular C–H···π contacts between opposing 3-phenylpyrazolyl arms on the co-ligands. The conformation found in the major monoclinic polymorph consists of a counter-rotation of the ligands that reduces the ideal complex symmetry from D3d to D3 and results in three-fold pair-wise association of 3-phenylpyrazolyl arms. In contrast, an alternate conformation of 1 in the minor triclinic polymorph consists of two triads, with an ideal complex symmetry of C2h. Also affected are the tetragonality of the ligand fields and interligand N–Ni–N′ bond angles. A review of sandwich complex structures [(TpR)2M], where R is an aromatic substituent, shows these conformations are adopted broadly. Hence, substituent effects in second-generation scorpionate ligands are not merely steric in origin.

FeN4 Environments upon Reduction: A Computational Analysis of Spin States, Spectroscopic Properties, and Active Species.

FeN4 motifs, found, for instance, in bioinorganic chemistry as heme-type cofactors, play a crucial role in man-made FeNC catalysts for the oxygen reduction reaction. Such single-atom catalysts are a potential alternative to platinum-based catalysts in fuel cells. Since FeNC catalysts are prepared via pyrolysis, the resulting materials are amorphous and contain side phases and impurities. Therefore, the geometric and electronic nature of the catalytically active FeN4 site remains to be clarified. To further understand the behavior of FeN4 centers in electrochemistry and their expected spectroscopic behavior upon reduction, we investigate two FeN4 environments (pyrrolic and pyridinic). These are represented by the model complexes [Fe(TPP)Cl] and [Fe(phen2N2)Cl], where TPP = tetraphenylporphyrin and phen = 1,10-phenanthroline. We predict their Mössbauer, UV-vis, and NRV spectral data using density functional theory as windows into their electronic structure differences. By varying the axial ligand, we further show how well small chemical changes in both complexes can be discerned. We find that the differences in ligand field strength in pyrrolic and pyridinic coordination result in different spin ground states, which in turn leads to distinct Mössbauer spectroscopic properties. As a result, pyrrolic nitrogen donors with a weaker ligand field are predicted to show more pronounced spectroscopic differences under in situ and operando conditions, while pyridinic nitrogen donors are expected to show less pronounced spectroscopic changes upon reduction and/or ligand loss. We therefore suggest that a weaker ligand field leads to better detectability of catalytic intermediates in in situ and operando experiments.

Synthesis and Magnetic Properties of Bis-Halobenzene Decamethyldysprosocenium Cations.

The decamethyldysprosocenium cation, [Dy(Cp*)2]+ (Cp* = {C5Me5}), was a target single-molecule magnet (SMM) prior to the isolation of larger dysprosocenium cations, which have recently shown magnetic memory effects up to 80 K. However, the relatively short Dy···Cp*centroid distances of [Dy(Cp*)2]+, together with the reduced resonance of its vibrational modes with electronic states compared to larger dysprosocenium cations, could lead to more favorable SMM behavior. Here, we report the synthesis and magnetic properties of a series of solvated adducts containing bis-halobenzene decamethyldysprosocenium cations, namely [Dy(Cp*)2(PhX-κ-X)2][Al{OC(CF3)3}4] (X = F or Cl) and [Dy(Cp*)2(C6H4F2-κ2-F,F)(C6H4F2-κ-F)][Al{OC(CF3)3}4]. These complexes were prepared by the sequential reaction of [Dy(Cp*)2(μ-BH4)]∞ with allylmagnesium chloride and [NEt3H][Al{OC(CF3)3}4], followed by recrystallization from parent halobenzenes. The complexes were characterized by powder and single crystal X-ray diffraction, NMR and ATR-IR spectroscopy, elemental analysis, and SQUID magnetometry; experimental data were rationalized by a combination of density functional theory and ab initio calculations. We find that bis-halobenzene adducts of the [Dy(Cp*)2]+ cation exhibit highly bent Cp*···Dy···Cp* angles; these cations are also susceptible to decomposition by C-X (X = F, Cl, Br) activation and displacement of halobenzenes by O-donor ligands. The effective energy barrier to reversal of magnetization measured for [Dy(Cp*)2(PhF-κ-F)2][Al{OC(CF3)3}4] (930(6) cm-1) sets a new record for SMMs containing {Dy(Cp*)2} fragments, though all SMM parameters are lower than would be predicted for an isolated [Dy(Cp*)2]+ cation, as expected due to transverse ligand fields introduced by halobenzenes and the large deviation of the Cp*···Dy···Cp* angle from linearity promoting magnetic relaxation.