Synthesis, X-ray structure, electrochemical, and EPR studies of a pentacoordinated mn(II) tetramethylcyclam complex

Crystal structure of 1,4,8,11-tetra-methyl-1,4,8,11-tetra-azonia-cyclo-tetra-decane bis-(perchlorate) dichloride from synchrotron X-ray data

In the title salt, [C₁₄H₃₆N₄]⁴⁺·2ClO₄⁻·2Cl⁻, the macrocyclic cations lie about an inversion center. In the crystal, N–H⋯Cl, C–H⋯Cl and C–H⋯O hydrogen bonds connect the cations and anions, forming a three-dimensional network.

The crystal structure of title salt, C₁₄H₃₆N₄⁴⁺·2ClO₄⁻·2Cl⁻, has been determined using synchrotron radiation at 220 K. The structure determination reveals that protonation has occurred at all four amine N atoms. The asymmetric unit contains one half-cation (completed by crystallographic inversion symmetry), one perchlorate anion and one chloride anion. A distortion of the perchlorate anion is due to its involvement in hydrogen-bonding inter­actions with the cations. The crystal structure is consolidated by inter­molecular hydrogen bonds involving the 1,4,8,11-tetra­methyl-1,4,8,11-tetra­azonia­cyclo­tetra­decane N—H and C—H groups as donor groups, and the O atoms of the perchlorate and chloride anion as acceptor groups, giving rise to a three-dimensional network.

Structural, Spectroscopic, Electrochemical, and Magnetic Properties for Manganese(II) Triazamacrocyclic Complexes

We report the synthesis of [Mn(tacud)2](OTf)2 (1) (tacud = 1,4,8-triazacycloundecane), [Mn(tacd)2](OTf)2 (2) (tacd = 1,4,7-triazacyclodecane), and [Mn(tacn)2](OTf)2 (3) (tacn = 1,4,7-triazacyclononane). Electrochemical measurements on the MnIII/II redox couple show that complex 1 has the largest anodic potential of the set (E 1/2 = 1.16 V vs NHE, ΔE p = 106 mV) compared to 2 (E 1/2 = 0.95 V, ΔE p = 108 mV) and 3 (E 1/2 = 0.93 V, ΔE p = 96 mV). This is due to the fact that 1 has the fewest 5-membered chelate rings and thus is least stabilized. Magnetic studies of 1-3 revealed that all complexes remain high spin throughout the temperature range investigated (2 - 300 K). X-band EPR investigations in methanol glass indicated that the manganese(II) centers for 2 and 3 resided in a more distorted octahedral geometric configuration compared to 1. To ease spectral interpretation and extract ZFS parameters, we performed high-frequency high-field EPR (HFEPR) at frequencies above 200 GHz and a field of 7.5 T. Simulation of the spectral data yielded g = 2.0013 and D = -0.031 cm-1 for 1, g = 2.0008, D = -0.0824 cm-1, |E/D| = 0.12 for 2, and g = 2.00028, D = -0.0884 cm-1 for 3. These results are consistent with 3 possessing the most distorted geometry. Calculations (PBE0/6-31G(d)) were performed on 1-3. Results show that 1 has the largest HOMO-LUMO gap energy (6.37 eV) compared to 2 (6.12 eV) and 3 (6.26 eV). Complex 1 also has the lowest HOMO energies indicating higher stability.

A combination of super-resolution fluorescence and magnetic resonance imaging using a Mn(ii) compound

Two manganese(ii) complexes supported by terpyridyl-based ligands were synthesized; they showed an enhanced fluorescence, including a two-photon signal and magnetic contrast, and were used in multi-modal imaging.

Crystal structure and spectroscopic characterization of a cobalt(II) tetraazamacrocycle: completing a series of first-row transition-metal complexes

The tetraazamacrocyclic ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (TMC) has been used to bind a variety of first-row transition metals but to date the crystal structure of the cobalt(II) complex has been missing from this series. The missing cobalt complex chlorido(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane-κ⁴N)cobalt(II) chloride dihydrate, [CoCl(C₁₄H₃₂N₄)]Cl·2H₂O or [Co^IICl(TMC)]Cl·2H₂O, crystallizes as a purple crystal. This species adopts a distorted square-pyramidal geometry in which the TMC ligand assumes the trans-I configuration and the chloride ion binds in the syn-methyl pocket of the ligand. The Co^II ion adopts an S = 3/2 spin state, as measured by the Evans NMR method, and UV-visible spectroscopic studies indicate that the title hydrated salt is stable in solution. Density functional theory (DFT) studies reveal that the geometric parameters of [Co^IICl(TMC)]Cl·2H₂O are sensitive to the cobalt spin state and correctly predict a change in spin state upon a minor perturbation to the ligand environment.

Determination and prediction of the magnetic anisotropy of Mn ions

EPR spectroscopy combined with quantum chemistry for the investigation of the magnetic anisotropy of Mn^II, Mn^III and Mn^IV.

Electronic structure aspects of the complete O2 transfer reaction between Ni(II) and Mn(II) complexes with cyclam ligands

This work explores the electronic structure aspects involving the complete intermolecular O2 transfer between Ni(ii) and Mn(ii) complexes, both containing N-tetramethylated cyclams (TMC). The energy of the low-lying states of reactants, intermediates and products is established at the CASSCF level and also the DDCI level when possible. The orthogonal valence bond analysis of the wave functions obtained from CASSCF and DDCI calculations indicates the dominant superoxide nature of all the adducts participating in the reaction, and consequently that the whole reaction can be described as the transfer of the superoxide O2(-) between Ni(ii) and Mn(ii) complexes, without any additional change in the electronic structure of the fragments.

On the reaction mechanism of the complete intermolecular O2 transfer between mononuclear nickel and manganese complexes with macrocyclic ligands

The recently described intermolecular O2 transfer between the side-on Ni-O2 complex [(12-TMC)Ni-O2](+) and the manganese complex [(14-TMC)Mn](2+), where 12-TMC and 14-TMC are 12- and 14-membered macrocyclic ligands, 12-TMC=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane and 14-TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long-range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed. The calculations confirm a two-step reaction, with a first rate-determining bimolecular step and predict the exothermic character of the global process. The relative stability of the products and the reverse barrier are in line with the fact that no reverse reaction is experimentally observed. An intermediate with a μ-η(1):η(1)-O2 coordination and two transition states are identified on the triplet PES, slightly below the corresponding stationary points of the quintet PES, suggesting an intersystem crossing before the first transition state. The calculated activation parameters and the relative energies of the two transition sates and the products are in very good agreement with the experimental data. The calculations suggest that a superoxide anion is transferred during the reaction.

Crystals in which some metal atoms are more equal than others: inequalities from crystal packing and their spectroscopic/magnetic consequences

Crystal structures of the heterometallic compounds CrCrFe(dpa)(4)Cl(2) (1), CrCrMn(dpa)(4)Cl(2) (2), and MoMoMn(dpa)(4)Cl(2) (3) (dpa = 2,2'-dipyridylamide) show disorder in the metal atom positions such that the linear M(A)[quadruple bond]M(A)···M(B) array for a given molecule in the crystal is oriented in one of two opposing directions. Despite the fact that the direct coordination sphere of the metals in the two crystallographically independent orientations is identical, subtle differences in some metal-ligand bond distances are observed in 1 and 3 due to differences in the orientation of a solvent molecule of crystallization. The Fe(II) and Mn(II) ions serve as sensitive local spectroscopic probes that have been interrogated by Mössbauer spectroscopy and high-field EPR spectroscopy, respectively. The subtle differences in the two independent Fe and Mn sites in 1 and 3 unexpectedly give rise to unusually large differences in the measured Fe quadrupole splitting (ΔE(Q)) in 1 and Mn zero-field splitting (D) in 3. Variable-temperature/single-crystal EPR spectroscopy has allowed us to determine that the temperature-dependent D tensors in 3 are oriented along the metal-metal axis and that they show significantly different dynamic behavior with temperature. The differences in ΔE(Q) and D are reproduced by density functional calculations on truncated models for 1 and 3 that lack the quadruply bonded M(A)[quadruple bond]M(A) groups, though the magnitude of the calculated effect is not as large as that observed experimentally. We suggest that the large observed differences in ΔE(Q) and D for the individual sites could be due to the influence of the strong diamagnetic anisotropy of the quadruply bonded M[quadruple bond]M unit.

Copper(II) complexes of a hexadentate mixed-donor N3S3 macrobicyclic cage: facile rearrangements and interconversions

The potentially hexadentate mixed-donor cage ligand 1-methyl-8-amino-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]eicosane (AMME-N(3)S(3)sar; sar=sarcophagine) displays variable coordination modes in a complex with copper(II). In the absence of coordinating anions, the ligand adopts a conventional hexadentate N(3)S(3) binding mode in the complex [Cu(AMME-N(3)S(3)sar)](ClO(4))(2) that is typical of cage ligands. This structure was determined by X-ray crystallography and solution spectroscopy (EPR and NIR UV/Vis). However, in the presence of bromide ions in DMSO, clean conversion to a five-coordinate bromido complex [Cu(AMME-N(3)S(3)sar)Br](+) is observed that features a novel tetradentate (N(2)S(2))-coordinated form of the cage ligand. This copper(II) complex has also been characterized by X-ray crystallography and solution spectroscopy. The mechanism of the reversible interconversion between the six- and five-coordinated copper(II) complexes has been studied and the reaction has been resolved into two steps; the rate of the first is linearly dependent on bromide ion concentration and the second is bromide independent. Electrochemistry of both [Cu(AMME-N(3)S(3)sar)](2+) and [Cu(AMME-N(3)S(3)sar)Br](+) in DMSO shows that upon reduction to the monovalent state, they share a common five-coordinated form in which the ligand is bound to copper in a tetradentate form exclusively, regardless of whether a six- or five-coordinated copper(II) complex is the precursor.

Characterizations of chloro and aqua Mn(II) mononuclear complexes with amino-pyridine ligands. Comparison of their electrochemical properties with those of Fe(II) counterparts

The solution behavior of mononuclear Mn(II) complexes, namely, [(L(5)(2))MnCl](+) (1), [(L(5)(3))MnCl](+) (2), [(L(5)(2))Mn(OH(2))](2+) (3), [(L(5)(3))Mn(OH(2))](2+) (4), and [(L(6)(2))Mn(OH(2))](2+) (6), with L(5)(2/3) and L(6)(2) being penta- and hexadentate amino-pyridine ligands, is investigated in MeCN using EPR, UV-vis spectroscopies, and electrochemistry. The addition of one chloride ion onto species 6 leads to the formation of the complex [(L(6)(2))MnCl](+) (5) that is X-ray characterized. EPR and UV-vis spectra indicate that structure and redox states of complexes 1-6 are maintained in MeCN solution. Chloro complexes 1, 2, and 5 show reversible Mn(II)/Mn(III) process at 0.95, 1.02, and 1.05 V vs SCE, respectively, whereas solvated complexes 3, 4, and 6 show an irreversible anodic peak around 1.5 V vs SCE. Electrochemical oxidations of 1 and 5 leading to the Mn(III) complexes [(L(5)(2))MnCl](2+) (7) and [(L(6)(2))MnCl](2+) (8) are successful. The UV-vis signatures of 7 and 8 show features associated with chloro to Mn(III) LMCT and d-d transitions. The X-ray characterization of the heptacoordinated Mn(III) species 8 is also reported. The analogous electrochemical generation of the corresponding Mn(III) complex was not possible when starting from 2. The new mixed-valence di-mu-oxo [(L(5)(2))Mn(muO)(2)Mn(L(5)(2))](3+) species (9) can be obtained from 3, whereas the sister [(L(5)(3))Mn(muO)(2)Mn(L(5)(3))](3+) species can not be generated from 4. Such different responses upon oxidations are commented on with the help of comparison with related Mn/Fe complexes and are discussed in relation with the size of the metallacycle formed between the diamino bridge and the metal center (5- vs 6-membered). Lastly, a comparison between redox potentials of the studied Mn(II) complexes with those of Fe(II) analogues is drawn and completed with previously reported data on Mn/Fe isostructural systems. This gives us the opportunity to get some indirect insights into the metal specificity encountered in enzymes among which superoxide dismutase is the archetypal model.

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 L5(2) and L5(3) (L5(2) and L5(3) 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 Mn(II) complexes, namely [(L5(2))MnCl](PF6) (1(PF6)), [(L5(3))MnCl](PF6) (2(PF6)), [(L5(2))Mn(OH2)](BPh4)2 (3(BPh4)2), and [(L5(3))Mn(OH2)](BPh4)2 (4(BPh4)2). The X-ray diffraction studies revealed different configurations for the ligand L5(n) (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 Mn(II) 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.

Properties of a new 4-imidazolyl derivative of a 14-membered tetraazamacrocyclic chelating agent

A new bis-N,N'-(5-methylimidazol-4-ylmethyl) derivative of a 14-membered tetraazamacrocycle, L1, has been synthesized. The protonation constants of this compound and the stability constants of its complexes with divalent first-row transition metal ions and Fe3+ were determined at 298.2 K in aqueous 0.10 mol dm(-3) KNO3. Compound L1 exhibits high overall basicity, which is mainly conferred by the imidazolyl groups. The complexes of the divalent first row-transition metal ions of L1 follow the Irving-Williams order of stability with the maximum for Cu2+ as expected, but a steep fall of constants is verified for the Mn2+, Fe2+ and Co2+, in one side, and for the Zn2+ complexes, in the other side. Additionally, L1 shows a large affinity for Fe3+, and the relative stability constants for its Cd2+ and Pb2+ complexes indicate that L1 may be useful for the complexometric determination of these two toxic metal ions in solutions containing both metal ions. These studies together with NMR, UV-vis and EPR spectroscopic data indicated the presence of mononuclear complexes, which adopt distorted pyramidal or octahedral geometries depending on the metal centre. The X-ray crystal structure of [Cu(HL1)](PF6)2(NO3).H2O showed that the coordination sphere of the copper centre can be described as a distorted square pyramid with the basal plane defined by three nitrogen donors of the macrocycle backbone and one nitrogen atom from one imidazolyl pendant arm. The apical position is occupied by the nitrogen atom of the macrocycle trans to the pyridine ring. To achieve this coordination environment, the macrocycle is folded along the axis defined by the two N atoms contiguous to the pyridine ring. The free methylimidazolyl arm points away from the metal centre leading to an intramolecular Cu[dot dot dot]N distance of 5.155(1) A.

Synthesis, Structure, and Characterization of New Mononuclear Mn(II) Complexes. Electrochemical Conversion into New Oxo-Bridged Mn2(III,IV) Complexes. Role of Chloride Ions

Two Mn(II) complexes are isolated and X-ray characterized, namely, cis-[(L(2))Mn(II)(Cl)(2)] (1) and [(L(3))Mn(II)Cl(OH(2))](ClO(4)) (2(ClO(4))), where L(2) and L(3) are the well-known tetradentate N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine and N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)propane-1,3-diamine ligands, respectively. The crystal structure reveals that whereas the ligand L(2) is in the cis-alpha conformation in complex 1, the ligand L(3) is in the more unusual cis-beta conformation in 2. EPR spectra are recorded on frozen solutions for both complexes and are characteristic of Mn(II) species. Electrochemical behaviors are investigated on acetonitrile solution for both complexes and show that cation 2 exists as closely related Mn(II) species in equilibrium. For both complexes exhaustive bulk electrolyses of acetonitrile solution are performed at oxidative potential in various experimental conditions. In the presence of 2,6-lutidine and after elimination of chloride ligands, the formation of the di-mu-oxo mixed-valent complexes [(L(2))Mn(III)(mu-O)(2)Mn(IV)(L(2))](3+) (3a) and [(L(3))Mn(III)(mu-O)(2)Mn(IV)(L(3))](3+) (4) is confirmed by UV-vis and EPR spectroscopies and cyclic voltammetry. In addition crystals of 4(ClO(4))(3) were isolated, and the X-ray structure reveals the cis-alphaconformation of L(3). In the absence of 2,6-lutidine and without elimination of the exogenous chloride ions, the electrochemical oxidation of 1 leads to the formation of the mononuclear Mn(III) complex, namely, [(L(2))Mn(III)(Cl)(2)](+) (5), as confirmed by UV-vis as well as parallel mode EPR spectroscopy and cyclic voltammetry. In the same conditions, the electrochemical oxidation of complex 2 is more intricate, and a thorough analysis of EPR spectra establishes the formation of the binuclear mono-mu-oxo mixed-valent [(L(3))ClMn(III)(mu-O)Mn(IV)Cl(L(3))](3+) (6) complexes. Electrochemical conversion of Mn(II) complexes into mixed-valent Mn(2)(III,IV) oxo-bridged complexes in the presence of 2,6-lutidine is discussed. The role of the chloride ligands as well as that of L(3) in the building of oxo bridges is discussed. Differences in behavior between L(2) and L(3) are commented on.

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 << gbetaB). 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.

Catalytic Asymmetric Sulfoxidation by Chiral Manganese Complexes: Acetylacetonate Anions as Chirality Switches

Three manganese(II) complexes, namely [Mn(1)(ClO(4))(2)] (3), [Mn(1)(acac)(2)] (4), and [Mn(2)(1)(acac)(4)] (5), were isolated from solutions of Mn(ClO(4))(2) or Mn(acac)(2), and an easily accessible diimine ligand (1S,2S)-N,N'-bis-pyridin-2-ylmethylene-cyclohexane-1,2-diamine (1). Their structure was determined by X-ray crystallography, and these complexes proved to be catalysts for asymmetric sulfide oxidation by H(2)O(2). Enantiomeric excesses ranging from 5% to 62% were obtained with a variety of aryl alkyl sulfides. We also observed an interesting "chirality switch" effect by the achiral acac anion reversing the enantioselectivity of the complex [Mn(1)(ClO(4))(2)] from the S to the R sulfoxide enantiomer.