Mononuclear Manganese Complexes Research Articles

Cooperative effects of ligands and Counteranions in mononuclear Manganese(III) Schiff-base spin crossover complexes

The flexible hexadentate Schiff-base ligands (R-sal-N-1,5,8,12)2− can be easily obtained by modifying the different substituents at the 5 or 3,5 positions on the phenyl rings of the salicylaldehyde. We report in this paper the syntheses, crystal structures and magnetic properties of three new mononuclear manganese(III) complexes [Mn(R-sal-N-1,5,8,12)]Y (R = 5-H, Y = BF4 (1); R = 3,5-diCl, Y = SbF6 (2); R = 5-tBu, Y = ClO4 (3)). All the complexes have an analogous monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand in a distorted octahedral geometry. Magnetic susceptibility measurements demonstrated that complex 1 maintains a high-spin (HS) state between 2 and 400 K due to the closing packing, while complex 2 shows a cooperative and complete one-step thermo-induced spin state switching with T1/2 of 194 K. As for complex 3, single-crystral X-ray structural studies show the existence of a large amount of HS manganese(III) ions at 100 K and it exhibits a surprisingly gradual SCO process with a hysteresis width of 6 K at lower temperature.

Spin-Symmetry Breaking and Hyperfine Couplings in Transition-Metal Complexes Revisited Using Density Functionals Based on the Exact-Exchange Energy Density.

A small set of mononuclear manganese complexes evaluated previously for their Mn hyperfine couplings (HFCs) has been analyzed using density functionals based on the exact-exchange energy density─in particular, the spin symmetry breaking (SSB) found previously when using hybrid functionals. Employing various strong-correlation corrected local hybrids (scLHs) and strong-correlation corrected range-separated local hybrids (scRSLHs) with or without additional corrections to their local mixing functions (LMFs) to mitigate delocalization errors (DE), the SSB and the associated dipolar HFCs of [Mn(CN)4]2-, MnO3, [Mn(CN)4N]-, and [Mn(CN)5NO]2- (the latter with cluster embedding) have been examined. Both strong-correlation (sc)-correction and DE-correction terms help to diminish SSB and correct the dipolar HFCs. The DE corrections are more effective, and the effects of the sc corrections depend on their damping factors. Interestingly, the DE-corrections reduce valence-shell spin polarization (VSSP) and thus SSB by locally enhancing exact-exchange (EXX) admixture near the metal center and thereby diminishing spin-density delocalization onto the ligand atoms. In contrast, sc corrections diminish EXX admixture locally, mostly on specific ligand atoms. This then reduces VSSP and SSB as well. The performance of scLHs and scRSLHs for the isotropic Mn HFCs has also been analyzed, with particular attention to core-shell spin-polarization contributions. Further sc-corrected functionals, such as the KP16/B13 construction and the DM21 deep-neural-network functional, have been examined.

Questing for homoleptic mononuclear manganese complexes with monodentate O-donor ligands.

Compounds containing Mn-O bonds are of utmost importance in biological systems and catalytic processes. Nevertheless, mononuclear manganese complexes containing all O-donor ligands are still rare. Taking advantage of the low tendency of the pentafluoroorthotellurate ligand (teflate, OTeF5) to bridge metal centers, we have synthesized two homoleptic manganese complexes with monomeric structures and an all O-donor coordination sphere. The tetrahedrally distorted MnII anion, [Mn(OTeF5)4]2-, can be described as a high spin d5 complex (S = 5/2), as found experimentally (magnetic susceptibility measurements and EPR spectroscopy) and using theoretical calculations (DFT and CASSCF/NEVPT2). The high spin d4 electronic configuration (S = 2) of the MnIII anion, [Mn(OTeF5)5]2-, was also determined experimentally and theoretically, and a square pyramidal geometry was found to be the most stable one for this complex. Finally, the bonding situation in both complexes was investigated by means of the Interacting Quantum Atoms (IQA) methodology and compared to that of hypothetical mononuclear fluoromanganates. Within each pair of [MnXn]2- (n = 4, 5) species (X = OTeF5, F), the Mn-X interaction is found to be comparable, therefore proving that the similar electronic properties of the teflate and the fluoride are also responsible for the stabilization of these unique species.

Mononuclear Mn(III) complex with a terminal azide supported by phenol-based tetradentate N2O2 donor ligand: Synthesis, structure, properties, and catechol oxidase activity

Mixed ligand octahedral Mn(III) complex [Mn(L)(CH3OH)(N3)] (1) with sterically constrained phenol-based tetradentate N2O2 ligand, N,Nʹ-dimethyl-N,Nʹ-bis (2-hydroxy-3,5-dichlorobenzyl)-ethylenediamine (H2L) in association with azide ion and methanol as ancillary ligand has been synthesized. Synthesis, structure, properties, and catecholase activity of the mononuclear manganese complex (1) are described. Complex (1) crystallizes in triclinic space group P-1 with a = 10.5431(5) Å, b = 11.3731(6) Å and c = 22.9638(12) Å. The crystal structure reveals that complex (1) is an Mn (III) complex with flexible phenol-based ligand in association with azide as a terminal monodentate ligand. Electrochemical study of the mononuclear manganese complex (1) gives two quasi reversible oxidation peaks (E11/2≈ 0.18 V and E21/2≈ 0.23 V) in the positive potential range versus Ag/AgCl reference in acetonitrile due to ligand centered phenolate/phenoxyl couple. Complex (1) shows a strong emission band at 440 nm. The complex exhibits catechol oxidase activity in methanol. Oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone in methanol has been accelerated in presence of reported manganese complex (1) with a high Kcat value of 1.43 × 104h−1.

Designing of eight-coordinate manganese(II) complexes as bio-active materials: Synthesis, X-ray crystal structures, spectroscopic, DFT, and molecular docking studies

Two new manganese(II) complexes based on bisaroylhydrazone derivatives: [MnII(L1)2(NO3)2] (1) and [MnII(L2)2(NO3)2] (2) were synthesized via the reaction of manganese (II) nitrate tetrahydrate with bisaroylhydrazone derivative in acetonitrile at room temperature. The synthesized organic ligands and manganese (II) complexes underwent thorough characterization, and their structures were elucidated using various techniques. The X-ray crystallography results confirmed that compounds 1 and 2 are mononuclear manganese (II) complexes, where the Mn(II) ion is eight-fold coordinated within a distorted pseudo-dodecahedral coordination environment. The coordination is achieved through the binding of the metal ion to N and O atoms of two bisaroylhydrazone molecules (bi-dentate ligands) and two nitrate ions. Furthermore, density functional theory (DFT) calculations were conducted to optimize the geometry of the complexes, generate molecular electrostatic potential (MEP) maps, and analyze vibrational spectra. Additionally, time-dependent DFT (TD-DFT) calculations were carried out to investigate their electronic transitions. In order to calculate the structure–activity relationship and binding energies for the most suitable conformation, bond between Mn(II) complexes and enzymes has been examined using molecular modeling software. In addition, in vitro enzyme inhibition and antimicrobial activity have been investigated. Moreover, pharmacokinetics and enzyme kinetic mechanism studies were performed. Molecular docking results suggested that our complexes have a high affinity for binding to the two selected enzymes. Enzymatic kinetics research findings strongly suggested that our molecules are noncompetitive urease inhibitors, suggesting that those substances may be able to withstand significant structural modification regardless of the design of the active site cavity and could be used as a lead candidate for the creation of novel urease inhibitors. Compound 1 showed a competitive inhibition for α-glucosidase enzyme, while compound 2 exhibited a noncompetitive inhibition. Antimicrobial activity study of L1, L2, 1, and 2 against some selected pathogenic bacteria showed good activity with respect to their MIC values. The ADME profile also demonstrated information on the compounds general properties, with some of the examined metrics showing a parameter from the expected range.

Spin-state switching: chemical modulation and the impact of intermolecular interactions in manganese(III) complexes.

A series of mononuclear manganese(III) complexes [Mn(X-sal2-323)](ReO4) (X = 5 Cl, 1; X = 5 Br, 2; X = 3,5 Cl, 3; X = 3,5 Br, 4; and X = 5 NO2, 5), containing hexadentate ligands prepared using the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and 5- or 3,5-substituted salicylaldehyde, has been synthesized. Variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical analyses, and theoretical calculations have been used to explore the role of various ligand substituents in the spin-state switching behavior of the prepared manganese(III) complexes. All five complexes consist of an analogous distorted octahedral monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand and ReO4- as the counter anion. However, a disordered water molecule was detected in complex 4. Complexes 1 (X = 5 Cl) and 5 (X = 5 NO2) show gradual and complete spin-state switching between the high-spin (HS) (S = 2) and the low-spin (LS) (S = 1) state with T1/2 values of 146 and 115 K respectively, while an abrupt and complete transition at 95 K was observed for complex 2 (X = 5 Br). Alternatively, complex 3 (X = 3, 5 Cl) exhibits an incomplete and sharp transition between the HS and LS states at 104 K, while complex 4 (X = 3, 5 Br) (desolvated) remains almost LS up to 300 K and then displays gradual and incomplete SCO at a higher temperature. The nature of the spin-state switch and transition temperature suggest that the structural effect (cooperativity) plays a more significant role in comparison with the electronic effect coming from various substituents (Cl, Br, and NO2), which is further supported by the detailed structural, electrochemical, and theoretical studies.

Stable manganese(II) complex with chlorido-substituted hexadentate Schiff base: Syntheses, crystal structure and spectral properties

The new mononuclear manganese(II) complex with chlorido-substituted hexadentate Schiff base (H23,5-2Cl-sal-N(1,5,8,12) = N,N′-Bis[3-(3,5-dichloridosalicylideneamino)propyl]-ethane-1,2-diamine) were synthesized and its crystal structure was solved using single-crystal X-ray diffraction. The complex crystallizes in a triclinic system with a P1¯ space group. Manganese(II) central atom is coordinated by two atoms of oxygen and four atoms of nitrogen of the Schiff base. The supramolecular structure of the complex and the intermolecular interactions were analyzed using Hirshfeld surface analysis. The complex was characterized by UV–vis and infrared spectra, and the presence of a high-spin Mn(II) state (S = 5/2) was revealed by EPR spectroscopy in the solid state.

Mononuclear manganese complexes as hydrogen evolving catalysts.

Molecular hydrogen (H2) is one of the pillars of future non-fossil energy supply. In the quest for alternative, non-precious metal catalysts for hydrogen generation to replace platinum, biological systems such as the enzyme hydrogenase serve as a blueprint. By taking inspiration from the bio-system, mostly nickel- or iron-based catalysts were explored so far. Manganese is a known oxygen-reducing catalyst but has received much less attention for its ability to reduce protons in acidic media. Here, the synthesis, characterization, and reaction mechanisms of a series of four mono-nuclear Mn(I) complexes in terms of their catalytic performance are reported. The effect of the variation of equatorial and axial ligands in their first and second coordination spheres was assessed pertaining to their control of the turnover frequencies and overpotentials. All four complexes show reactivity and reduce protons in acidic media to release molecular hydrogen H2. Quantum chemical studies were able to assign and interpret spectral characterizations from UV–Vis and electrochemistry and rationalize the reaction mechanism. Two feasible reaction mechanisms of electrochemical (E) and protonation (C) steps were compared. Quantum chemical studies can assign peaks in the cyclic voltammetry to structural changes of the complex during the reaction. The first one-electron reduction is essential to generate an open ligand-based site for protonation. The distorted octahedral Mn complexes possess an inverted second one-electron redox potential which is a pre-requisite for a swift and facile release of molecular hydrogen. This series on manganese catalysts extends the range of elements of the periodic table which are able to catalyze the hydrogen evolution reaction and will be explored further.

Sheet-like 2D Manganese(IV) Complex with High Photothermal Conversion Efficiency.

We report a stable, water-soluble, mononuclear manganese(IV) complex [MnIV(H2L)]·5H2O (Mn-HDCL) that acts as an efficient photothermal material. This system is based on a hexahydrazide clathrochelate ligand (L/HDCL) and is obtained via an efficient one-pot templated synthesis that avoids the need for harsh reaction conditions. Scanning tunneling microscopy images reveal that Mn-HDCL exists as a 2D sheet-like structure. In Mn-HDCL, the manganese(IV) ion is trapped within the cavity of the cage-like ligand. This effectively shields the Mn(IV) ion from the external environment while providing adequate water solubility. As a result of orbital transitions involving the coordinated manganese(IV) ion, as well as metal-to-ligand charge transfer effects, Mn-HDCL possesses a large extinction coefficient and displays a photothermal performance comparable to single-wall carbon nanotubes in the solid state. A high photothermal conversion efficiency (ca. 71%) was achieved in aqueous solution when subjected to near-infrared 730 nm laser photo-irradiation. Mn-HDCL is paramagnetic and provides a modest increase in the T1-weighted contrast of magnetic resonance images both in vitro and in vivo. Mn-HDCL was found to target tumors passively and allow tumor margins to be distinguished in vivo in a mouse model. In addition, it also exhibited an efficient laser-triggered photothermal therapy effect in vitro and in vivo. We thus propose that Mn-HDCL could have a role to play as a tumor-targeting photothermal sensitizer.

Novel isatin-based complexes of Mn(II) and Cu(II) ions: Characterization, homogeneous catalysts for sulfides oxidation, bioactivity screening and theoretical implementations via DFT and pharmacokinetic studies

Two mononuclear manganese(II) and copper(II) complexes were prepared from the coordination of 3,5-di-tert-butyl isatin hydrazone derivative (HLOEt) with either 2 : 1 or 1 : 1 M ratio for Mn(II) or Cu(II) ion, within octahedral and square planar geometries (Mn(LOEt)2 and CuLOEt), respectively. The catalytic potential of both M(II)-complexes was evaluated in the oxidation of sulfides, i.e. diphenyl sulfide (DPS) and methylphenyl sulfide (MPS), by tert-butyl hydroperoxide (tBuOOH) in acetonitrile. The chemo- and regio-selectivity of the redox system was determined by formation of mono-oxoproducts, diphenyl sufoxide (DPO) and methylphenyl sulfoxide (MPO), with detection of further products of mainly dioxide-product of diphenyl sufone (DPN) and methylphenyl sulfone (MPN). With Mn(LOEt)2, the reaction gave 55 and 54% (ee, 79% of S and 21% of R) yields of DPO and MPO, respectively. The catalytic activity of CuLOEt was slightly more progressed than that of Mn(LOEt)2, which awarded 52 and 56% (ee, 83% of S and 17% of R) percentages of DPO and MPO, respectively. The higher catalytic reactivity of CuLOEt catalyst was based on their geometrical structures to facilitate the approach of the oxidant and substrate in the redox systems. Their reactivity was studied against some common strains of bacteria and fungi. Moreover, their bio-potential was explored as anticancer reagents versus some human cancer cell lines. Both M(II)-complexes exhibited high biological action against the microorganism and cancer growth. The effect of M(II) ion in thier complexes was examined through the interaction of with ctDNA (calf thymus DNA). Such interaction was tested spectroscopically and via changes in ctDNA viscosity. The reactivity was measured by the binding constant values (Kb = 3.19, 13.85 and 15.39 × 108 mol−1 dm3 for HLOEt, Mn(LOEt)2 and CuLOEt, respectively), which reflects high interaction with ctDNA. Furthermore, the Gibbs’ free energy values supported their high potential against ctNDA, with −1.24, −1.36 and −1.37 kJ mol−1, respectively. Furthermore, HLOEt, CuLOEt and Mn(LOEt)2 were optimized by DFT/B3LYP method under either of 6-31G* of SDD basis set to estimate the essential physical characteristics. On the other hand, in silico approaches were utilized through online software as Swiss-ADME and Pharmacophore beside the molecular docking program. Pharmacokinetic, drug-likeness and the interaction with iso-enzymes were investigated for CuLOEt complex versus receptor tyrosine kinase (2a91, 2src) and human oestrogen receptor (2iok, 4rj3), as well as, liver cancer cells via 6jxt and (PCSK9) Proprotein Convertase Subtilisin/Kexin type 9 (4nmx, 4ne9). Only the proteins 4nmx and 4ne9 from liver cancer cells were found to be effectively inhibited by CuLOEt complex.

Mononuclear Manganese(IV) Complexes Derived from Polyfunctional Fumaryldihydrazone

A series of mononuclear manganese(IV) complexes of disalicylaldehyde fumaroyldihydrazone (slfhH4) were synthesized by reacting manganese acetate tetrahydrate with dihydrazone in absence and presence of various pyridine bases in methanol medium. Pyridine (py) and its derivatives like 2-picoline (2-pic), 3-picoline (3-pic), 4-picoline (4-pic), 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) were added to the reaction mixture of metal salt and dihydrazone to explore the binding possibilities of these molecules to manganese center. The elemental analysis, mass spectral and thermal studies were used to derive their composition. Study of magnetic moment, molar conductances, electronic, EPR and infrared spectroscopy have led to interpretation of their tentative structures. Magnetic moment and EPR studies correspond to their Mn(IV) oxidation state. The dihydrazone functions as tetradentate ligand chelating the manganese(IV) ion present in octahedral geometry with anti-cis configuration in enol form. Electrochemical and antimicrobial studies were also performed.

Impact of counter anions on spin-state switching of manganese(III) complexes containing an azobenzene ligand.

Four mononuclear manganese(III) complexes coordinated with photo-active hexadentate azobenzene ligands, [Mn(5azo-sal2-323)](X) (X = Cl, 1; X = BF4, 2; X = ClO4, 3; X = PF6, 4), were prepared. The impact of various counter anions on the stabilization and switching of the spin state of the manganese(III) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, and spectroelectrochemical studies, along with theoretical calculations. All four complexes consisted of an isostructural monocationic distorted octahedral MnN4O2 coordination environment offered by the hexadentate ligand and Cl-, BF4-, ClO4-, and PF6- as counter anions respectively. Complex 1 with a spherical Cl- counter anion showed a reversible and gradual spin-state switching between low-spin (LS) (S = 1) and high-spin (HS) (S = 2) states above 400 K, where non-covalent cation-anion interactions played a significant role in stabilizing the LS state. While, irrespective of the shape of the counter anion, complexes 2-4 remained in the HS state throughout the measured temperature range of 300-2 K, where strong π-π interaction between the azobenzene motifs among cationic units played a substantial role in stabilizing the HS state. Furthermore, magnetic data analyses revealed significantly large zero-field splitting in the S = 1 state for 1 (D = 19.4 cm-1, E/D = 0.008) in comparison with that in the S = 2 state for 2-4 (D = 3.99-4.97 cm-1, E/D = 0.002-0.195). Spectroelectrochemical investigations revealed the quasi-reversible reduction and oxidation of the manganese(III) center to manganese(II) and manganese(IV), respectively. A detailed theoretical calculation at the DFT and CASSCF level of theory was carried out to better understand the magneto-structural correlation.

Multiple-Site Concerted Proton-Electron Transfer in a Manganese-Based Complete Functional Model for [FeFe]-Hydrogenase.

The active site of [FeFe]-hydrogenase (H2 ase) is preorganized with an amine (azadithiolate) as a proton relay and a [4Fe4S] subunit as an electron reservoir, which together lower the overpotential for proton reduction and hydrogen oxidation by multiple-site concerted proton-electron transfer (MS-CPET). Herein, we report a mononuclear manganese complex, fac-[Mn(CO)3 (6-(2-hydroxyphenol)-2-pyridine-2-quinoline) Br] (1), as a rare model to fully mimic the functions of the H2 ase. In 1, a redox-active bidentate ligand with a pendent phenol replicates the roles of the electron reservoir and the proton relay in the enzyme. Experimental and theoretical studies revealed two consecutive MS-CPET processes in the catalytic cycle, in each of which an electron stored in the reductive ligand and a proton at the proximal phenol moiety are transferred to the Mn center in a concerted way. By virtue of this mechanism, complex 1 exhibited a low overpotential comparable to that of natural enzyme in electrochemical hydrogen production using phenol as a proton source.

Multiple‐Site Concerted Proton–Electron Transfer in a Manganese‐Based Complete Functional Model for [FeFe]‐Hydrogenase

AbstractThe active site of [FeFe]‐hydrogenase (H2ase) is preorganized with an amine (azadithiolate) as a proton relay and a [4Fe4S] subunit as an electron reservoir, which together lower the overpotential for proton reduction and hydrogen oxidation by multiple‐site concerted proton–electron transfer (MS‐CPET). Herein, we report a mononuclear manganese complex, fac‐[Mn(CO)3(6‐(2‐hydroxyphenol)‐2‐pyridine‐2‐quinoline) Br] (1), as a rare model to fully mimic the functions of the H2ase. In 1, a redox‐active bidentate ligand with a pendent phenol replicates the roles of the electron reservoir and the proton relay in the enzyme. Experimental and theoretical studies revealed two consecutive MS‐CPET processes in the catalytic cycle, in each of which an electron stored in the reductive ligand and a proton at the proximal phenol moiety are transferred to the Mn center in a concerted way. By virtue of this mechanism, complex 1 exhibited a low overpotential comparable to that of natural enzyme in electrochemical hydrogen production using phenol as a proton source.

Computational Study on the Mechanisms and Origins of Selectivity in Hydroarylation of 1,3-Diyne Alcohol Catalyzed by Di- and Mononuclear Manganese Complexes

Computational Study on the Mechanisms and Origins of Selectivity in Hydroarylation of 1,3-Diyne Alcohol Catalyzed by Di- and Mononuclear Manganese Complexes

Effect of ligand substituents and tuning the spin-state switching in manganese(iii) complexes.

Three mononuclear manganese(iii) complexes based on flexible hexadentate ligands obtained from the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and salicylaldehyde or salicylaldehyde with substitutions at the 5 or 3,5 positions, namely [Mn(X-sal2-323)](BPh4) (X = 5 H, 1; X = 5 Br, 2, and X = 3,5 Br, 3) have been synthesized. The impact of ligand substituents has been studied by variable temperature single-crystal X-ray diffraction analyses, and magnetic, spectroscopic and electrochemical investigations. The complexes have an analogous monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand in a distorted octahedral geometry. Complex 1 remains in the high spin state over the entire temperature range, while complex 2 shows a reversible and complete two-step thermo-induced spin-state switching. An incomplete spin-state switching from a high spin to an intermediate high-spin low-spin (1 : 1) state was observed for complex 3. Single-crystal X-ray structural studies show the presence of three different spin states in 2 during the occurrence of the spin-state switching process. Electrochemical investigations showed that the reduced state of manganese(iii) centers in 3 is easily accessible in comparison to complexes 1 and 2.

Synthesis, Crystal Structures, Characterization and Catalytic Property of Manganese(II) Complexes Derived from Hydrazone Ligands

A new bromido-coordinated mononuclear manganese(II) complex [MnL1Br2(OH2)] (1), and a new nitrato-coordinated mononuclear manganese(II) complex [Mn(L2)2(ONO2)(OH2)]NO3 (2), with the hydrazone ligands 4-hydroxy-N’-(pyridin-2-ylmethylene)benzohydrazide (HL1) and N’-(pyridin-2-ylmethylene)isonicotinohydrazide (HL2), have been synthesized and structurally characterized by physico-chemical methods and single crystal X-ray determination. Single crystal structural analysis shows that the Mn atom in complex 1 is in octahedral coordination, and that in complex 2 is in pentagonal bipyramidal coordination. The catalytic property for epoxidation of styrene by the complexes was evaluated.

Field-Induced Slow Magnetic Relaxation of a Six-Coordinate Mononuclear Manganese(II) and Cobalt(II) Oxamate Complexes.

Two air-stable, isostructural, mononuclear six-coordinate manganese(II) and cobalt(II) oxamate complexes, [M(4-HOpa)2(H2O)2] [4-HOpa = N-4-hydroxyphenyloxamate; M= Mn2+ (1) or Co2+ (2)], exhibit field-induced slow magnetic relaxation. A bottleneck process is observed throughout the temperature range of 2-20 K for 1, while for 2, it dominates only at low temperatures (2-4 K). Additionally, the Raman process [n = 6.9(2)] is responsible for an increase in the relaxation time at higher temperatures to 2.

Exploitation of the electron deficient outer O4 compartment of a compartmental Schiff base to act as H-bond acceptors in forming a self-assembled dimer of a manganese(III) complex: A joint experimental and theoretical venture

A mononuclear manganese(III) complex, [MnL1(DMSO)(OH2)]ClO4, where H2L1 = (2,2-dimethyl-(1,3-propylene))-bis(3-ethoxysalicylideneimine) is a tetradentate Schiff base ligand, has been synthesized and characterized by elemental and spectral analysis and single crystal X-ray diffraction. The most electron rich part of the cationic manganese(III) complex corresponds to the region where the four oxygen atoms (two from the ethoxy arms and two from the phenol groups) are located (−47 kcal/mol) and the most electron deficient part is located at the hydrogen atoms of the coordinated water molecule (+65 kcal/mol). This large value is due to the effect of the coordination of the oxygen atom to the manganese(III) center that increases the acidity of the water protons, thereby facilitating the formation of a H-bond mediated dimer. It is further stabilized by π⋯π stacking interactions involving the aromatic rings. The dimerization energy is very large confirming the relevance of this combination of interactions. The contribution of the H-bonds is also estimated, which confirms that the formation of the dimer is basically governed by the H-bonds.

The syntheses, structures, and magnetic properties of two mononuclear manganese(II) complexes involving in situ hydrothermal decarboxylation

Abstract Two new mononuclear compounds [Mn(3-Br-pydc)(H2O)3] (1) and {[Mn(5-Br-pyc)(bipy)(H2O)(Cl)]·2H2O (2) (3-Br-H2pydc = 3-Br-pyridine-2,6-dicarboxylic acid, 5-Br-Hpyc = 5-Br-pyridine-2-carboxylic acid, bipy = 2,2′-bipyridine) have been synthesized by traditional solution reaction and hydrothermal reaction, respectively. In both compounds, the MnII center is six-coordinated in a distorted octahedral geometry, formed by one tridentate chelate 3-Br-pydc dianion and three water molecules in 1, while the coordination sphere consists of one bidentate chelate 5-Br-pyc anion, one bipy, one water molecule, and one chloride anion in 2 (MnNO5 for 1 and MnN3O2Cl for 2). O–H⋯O hydrogen bonds, Br⋯O halogen bonds, and/or π-π stacking assist in the construction of the three-dimensional (3D) network structures of 1 and 2. Notably, the 5-Br-Hpyc ligand was generated in situ by decarboxylation of the 3-Br-H2pydc precursor under hydrothermal conditions. Variable-temperature magnetic susceptibility data in the 2–300 K temperature range indicate weak antiferromagnetic coupling in both 1 and 2.