Binuclear Manganese Complexes Research Articles

Facile Synthesis of Rigid Binuclear Manganese Complexes for Magnetic Resonance Angiography and SLC39A14-Mediated Hepatic Imaging.

Manganese(II)-based contrast agents (MBCAs) are potential candidates for gadolinium-free enhanced magnetic resonance imaging (MRI). In this work, a rigid binuclear MBCA (Mn2-PhDTA2) with a zero-length linker was developed via facile synthetic routes, while the other dimer (Mn2-TPA-PhDTA2) with a longer rigid linker was also synthesized via more complex steps. Although the molecular weight of Mn2-PhDTA2 is lower than that of Mn2-TPA-PhDTA2, their T1 relaxivities are similar, being increased by over 71% compared to the mononuclear Mn-PhDTA. In the presence of serum albumin, the relaxivity of Mn2-PhDTA2 was slightly lower than that of Mn2-TPA-PhDTA2, possibly due to the lower affinity constant. The transmetalation reaction with copper(II) ions confirmed that Mn2-PhDTA2 has an ideal kinetic inertness with a dissociation half-life of approximately 10.4 h under physiological conditions. In the variable-temperature 17O NMR study, both Mn-PhDTA and Mn2-PhDTA2 demonstrated a similar estimated q close to 1, indicating the formation of monohydrated complexes with each manganese(II) ion. In addition, Mn2-PhDTA2 demonstrated a superior contrast enhancement to Mn-PhDTA in in vivo vascular and hepatic MRI and can be rapidly cleared through a dual hepatic and renal excretion pattern. The hepatic uptake mechanism of Mn2-PhDTA2 mediated by SLC39A14 was validated in cellular uptake studies.

Synthesis, Structural Characterization, Reactivity and Bioactivity Studies of Some Binuclear Salen Type Schiff Base Complexes of Manganese(III)

Four new phenoxy bridged binuclear Salen type Schiff base complexes of manganese(III), [MnLX]2 with tetradentate Schiff base, H2L [H2L = N,N′-O-phenylene bis(salicylaldimine)] containing anionic co-ligands, X (X = benzoate, p-hydroxy benzoate, p-aminobenzoate, 3,5-dinitrobenzoate) have been synthesized from manganese(II) acetate, Schiff base (H2L) and the corresponding co-ligands in methanol medium. Schiff base ligand (H2L) was obtained in situ by condensation of salicylaldehyde and o-phenylenediamine in aqueous methanol. Elemental analyses, FT-IR, molar conductance, UV-visible spectroscopic studies and magnetic moment measurements at room temperature were used to characterize the binuclear Schiff base complexes. The IR spectra of [MnLX]2 (X = benzoate, p-hydroxy benzoate, p-aminobenzoate, 3,5-dinitrobenzoate) complexes showed the characteristic absorptions due to coordinated azomethine nitrogen and phenolic oxygen atoms of the Schiff base. In addition, IR spectra of the compounds also suggest coordination of the co-ligands namely benzoate, p-hydroxy benzoate, p-aminobenzoate, 3,5-dinitrobenzoate in the respective complexes. A consistent presence of a band at ca. 755 cm-1 in all the binuclear complexes arises due to (Mn-O-Mn) moiety originated from interactions of phenoxy oxygen and manganese atoms, resulted in dimeric structure of the compounds. The magnetic moment values ranging from 4.74-4.91 B.M. per manganese centre for the complexes suggest presence of Mn(III) ion in the complexes. The overall coordination geometry around the manganese centres is distorted octahedral. Redox behaviour of the synthesized complexes was ascertained from cyclic voltametric studies. The Schiff base complexes, [Mn(L)X]2 (X = p-aminobenzoate, 3,5-dinitrobenzoate) have demonstrated their catalytic potentials in oxidizing selective organic substrates namely cyclohexene and styrene by hydrogen peroxide as oxidant. The oxidized products were characterized as trans-cyclohexane 1,2-diol and styrene epoxide, respectively. Complex 4 demonstrated a considerable level of effectiveness against a wide variety of pathogenic microorganisms.

Binuclear manganese(II) complexes based on pyridyl-containing dialkylphosphine oxides

New manganese(II) complexes with Mn2Cl2 and more rare Mn2O2 cores were obtained based on the diisopropyl(pyridin-2-yl)phosphine oxide (L2) and diethyl(pyridin-2-yl)phosphine oxide (L3). The crystal structures and sample purity of the compounds were determined by single-crystal and powder X-ray diffraction. The luminescence, the electrocatalytic activity in hydrogen evolution reaction, and the cytotoxicity against the M−HeLa and HuTu80 cancer cell lines and Chang liver normal cell lines were investigated. It was found that manganese(II) complex 3 with the Mn2O2 core was the effective catalyst in the hydrogen evolution reaction and exhibited high cytotoxicity against the cancer HuTu80 cell line.

Highly Luminescent Nonclassical Binuclear Manganese(II) Complex Scintillators for Efficient X-ray Imaging.

Scintillators can be widely applied to the fields of radioactivity detection, space exploration, and medical diagnosis and have attracted great attention. Recently, manganese(II) complexes with high-efficiency phosphorescent emission are expected to be good candidates for new scintillator materials. Herein, we design and synthesize a series of novel nonclassical binuclear neutral manganese(II) complexes with unique coordination modes, in which the crystal structures of CP1 and CP2 contain both four-coordinated and six-coordinated manganese(II) centers and CP3 is formed as a binuclear five-coordinated dichloro-bridged dimer. The single crystals of CP1-CP3 exhibit excellent stability, which can be attributed to their nonionic structures. They all exhibit intense red emission under UV and X-ray irradiation. Among the three manganese(II) complexes, CP1 demonstrates the best luminescence efficiency and X-ray scintillation performance with a high photoluminescence quantum yield (PLQY) of 91.9%, a relative light yield of 21037 photons MeV-1, and a detection limit of 34.45 nGyair s-1. Moreover, X-ray imaging based on CP1 scintillator screen demonstrates a spatial resolution of nearly 6 lp mm-1. As far as we know, this is the first report about stable binuclear neutral manganese(II) complexes for X-ray imaging and opens a new avenue for exploring novel scintillator materials.

Manganese(II) Bromide Coordination toward the Target Product and By-Product of the Condensation Reaction between 2-Picolylamine and Acenaphthenequinone

A heteroleptic binuclear manganese complex was obtained and characterized by single-crystal X-ray diffraction. Manganese ions coordinate with the target product and by-product of the condensation reaction between 2-picolylamine and acenaphthenequinone are characterized by different geometries in the resulting complex.

Synthesis, characterization, DNA binding and cleavage studies, in-vitro antimicrobial, cytotoxicity assay of new manganese(III) complexes of N-functionalized macrocyclic cyclam based Schiff base ligands

A series of binuclear manganese(III) complexes were synthesized from the macrocyclic ligands 1,8-[bis(3-formyl-2-hydroxy-5-methyl)benzyl]-1,4,8,11-tetraazacyclotetradecane (L1) and 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-1,4,8,11-tetraazacyclotetradecane (L2) by a Schiff base condensation with the appropriate aliphatic or aromatic diamines, MnCl2·4H2O and triethylamine. All the complexes were characterized by elemental and spectral analysis. A broad band around 600–650 nm indicates a d–d transition of the metal ion, which is characteristic of the Mn3+ ion in a 5 or 6 coordination environment. The ESR spectrum of [Mn2L2b] displays a broad band without splitting, with g = 2.12, which shows there is an antiferromagnetic interaction between the two manganese ions. An irreversible reduction in the cathodic potential region and a quasi reversible oxidation in the anodic potential region were observed for all the complexes. The kinetic activity of the binuclear manganese(III) complexes were found in the range of 2.21 × 10−3 to 8.14 × 10−3 min−1. The enhanced DNA binding affinity of the complexes [Mn2L1c] and [Mn2L1e] is due to the existence of the electron donating CH3 group which leads to a hydrophobic interaction with the hydrophobic DNA surface. The existence of the electron withdrawing Br atom in complexes [Mn2L2a] and [Mn2L2d] leads to a lesser DNA binding affinity. The fluorescence quenching of the binuclear manganese(III) complexes at 620 nm (d-d transition) indicates the strong coordination of the metal ions with the N and O atoms of the ligand. The 3D fluorescence spectrum of the [Mn2L2d] complex has been quenched more compared to [Mn2L2a], which may be due to the planarity of aromatic system. DNA cleavage by the manganese(III) complexes begins at a low concentration (25 μM) and reaches a maximum cleavage with a successive increase in concentration (100 μM). All the complexes were screened for antimicrobial activity and cytotoxicity.

Interpretation of Exchange Interaction through Orbital Entanglement

Recently, the analysis of single-orbital entropy and mutual information has been introduced as a tool for the investigation of contributions to the exchange (J) coupling between open-shell metal ions [Stein et al. J. Phys. Chem. Lett. 2019, 10, 6762-6770]. Here, we show that this analysis may lead to an incorrect interpretation of the J-coupling mechanism. Instead, we propose an orbital-entanglement analysis that is based on the two-electron density and that provides a coherent picture of the contributing exchange pathways, which seems fully consistent with the available J values. For this purpose, we used a prototypical bis-μ-oxo binuclear manganese complex ([Mn2O2(NH3)8]4+) and demonstrated that its antiferromagnetism (J < 0), calculated by using the active space composed of all valence pO and dMn orbitals, correlates well with the largest elements in the differential low-spin vs high-spin entanglement map. These elements correspond to interactions between the pairs of dMn orbitals mediated by the oxo-bridging out-of-plane p orbitals, representing the π superexchange pathway. We also show that the reduction of active space to manifold of the singly occupied magnetic orbitals does not lead to discrepancy between the calculated J values and entanglement maps. This contrasts to analysis of mutual information, which suggests the "direct" dMn-dMn interactions to play a dominant role for the J coupling, irrespective of the size of active space as well as of the antiferromagnetism expected. The failure is attributed to the large contribution of spin entanglement contained in the mutual information of the low-spin state, which may be regarded as the origin of the different complexity of its wave function and electron density.

Binuclear iminopyridine-bridged 3d late transition metal complexes with o-semiquinones

Four novel binuclear high spin manganese(II), iron(II), cobalt(II) and nickel(II) complexes containing 1,2-bis(pyridin-2-ylmethylene)-para-phenylenediamine as a bridging ligand have been synthesized and investigated. Molecular structures of manganese and cobalt complexes were established by single crystal X-ray structural analysis. For all the complexes the magnetic susceptibility measurements have been performed. According to the obtained magnetic data the energies of intramolecular magnetic exchange interactions strongly depend on the electron structure of metal ion. In addition, all the obtained complexes demonstrate very weak antiferromagnetic metal-metal interaction.

Boron Cluster Anions [B10X10]2– (X = H, Cl) in Manganese(II) Complexation with 2,2'-Bipyridyl

The complexation of manganese(II) with 2,2'-bipyridyl in the presence of the [B10H10]2– and [B10Cl10]2– boron cluster anions has been studied in acetonitrile. The effect of the ratio of reagents on the composition and structure of final complexes has been shown. Mono- and binuclear manganese(II) complexes [(Mn(Bipy)3]2+ and [Mn2(Bipy)4Cl2]2+ have been synthesized with the boron cluster anions as counterions. The X-ray diffraction studies have been performed for complexes [Mn(Bipy)3][B10Cl (I)10] (I), [Mn(Bipy)3][B10Cl10]0.5[B10Cl9H]0.5 · CH3CN (III· CH3CN), and [Mn2(Bipy)4(µ-Cl)2][B10Cl10] · 2CH3CN (IV ⋅ 2CH3CN) (CIF files CCDC nos. 1868278 (I), 1868279 (III · CH3CN), 1868280 (IV · 2CH3CN)). Thermal properties of complex I have been studied.

UNO DMRG CAS CI calculations of binuclear manganese complex Mn(IV)2 O2 (NHCHCO2 )4 : Scope and applicability of Heisenberg model.

Both direct exchange and super-exchange interactions cooperate to realize inter-spin magnetic interaction in binuclear manganese complex Mn(IV)2 O2 (NHCHCO2 )4 with a di-μ-oxo path. We revisited this spin system using DMRG CAS methods and CAS selection procedures. Our results indicate that our previous "dynamically extended spin polarization" (DE-SP) procedure for organic polyradicals and so forth does not work well. Thus, we have examined another selection procedure, the "dynamically extended super-exchange" (DE-SE) procedure. DMRG CASCI [18,18] by UB3LYP(HS)-UNO(DE-SE) can realize antiferromagnetic J values similar to experimental ones (-87 cm-1 ). In addition, all J values between all spin states (HS[septet],IS[quintet],IS[triplet],LS[singlet])were also shown to be correct under sufficiently large M values. © 2018 Wiley Periodicals, Inc.

Electrogenic reactions in Mn-depleted photosystem II core particles in the presence of synthetic binuclear Mn complexes

An electrometrical technique was used to investigate electron transfer between synthetic binuclear manganese (Mn) complexes, designated M − 2 and M − 3, and the redox-active neutral tyrosine radical (YZ•) in proteoliposomes containing Mn-depleted photosystem II (PS II) core particles in response to single laser flashes. In the absence of Mn-containing compounds, the observed flash-induced membrane potential (ΔΨ) decay was mainly due to charge recombination between the reduced primary quinone acceptor QA− and the oxidized YZ•. More significant slowing down of the ΔΨ decay in the presence of lower concentrations of M − 2 and M − 3 associated with electron donation from Mn in the Mn-binding site to YZ• indicates that these synthetic compounds are more effective electron donors than MnCl2. The exponential fitting of the kinetics of additional electrogenic components of ΔΨ rise in the presence of Mn-containing compounds revealed the following relative amplitudes (A) and lifetimes (τ): for MnCl2 - A∼ 3.5, τ∼150 μs, for M − 2 - A∼5%, τ∼1.4 ms, and for M − 3 - A∼5.5%, τ∼150 μs. This suggests that the efficiency of the manganese complexes in electron donation depends on the chemical nature of ligands. The experiments with EDTA-treated samples indicated that the ligands for M − 2 and M − 3 are required for their tight binding with the PS II reaction center. The obtained results demonstrate the importance of understanding the molecular mechanism(s) of flash-induced electrogenic reduction of the tyrosine radical YZ• by synthetic Mn complexes capable of splitting water into oxygen and reducing equivalents.

Synthesis, characterization and structure assessment of mononuclear and binuclear low-spin manganese(II) complexes derived from oxaloyldihydrazones, 1,10-phenanthroline and 2,2′-bipyridine

Low-spin manganese (II) complexes [MnII(H2slox) (bipy)] (1), [MnII(H2slox) (phen)] (2), [MnII(H2slox) (phen)].1.5.H2O (2a), [MnII(H2nph) (bipy)].H2O (3) and [MnII(H2nph) (phen)].H2O (4) were synthesized from oxaloyldihydrazones in methanolic-water medium. The stoichiometry of the complexes was established by analytical, molecular weight and thermoanalytical data. Based on the data obtained from molar conductance, UV–visible, Infrared spectral, magnetic moment and electron paramagnetic resonance spectroscopic studies, the structures of the complexes have been established. From molar conductance it is suggested that all the complexes are non-electrolyte in DMF medium. The complex (2a) is monomeric in nature whereas the complexes (1) to (4) are dimeric based on the molecular weight data. The effective magnetic moment of complexes (1) to (4) show metal-metal interaction while complex (2a) has no metal-metal interaction. The complexes (1)–(4) show two quasi-reversible metal centred electron transfer reaction involving MnII/MnI/Mn0 redox reactions in DMF medium.

Synthesis, Characterization, and Cytotoxic Activities of a Schiff Base Ligand and Its Binuclear Copper(II) and Manganese(III) Complexes

A novel symmetrical N 2 O 2 type Schiff base ( 1 ) and its copper (II) ( 2 ) and manganese (III) ( 3 ) complexes were synthesized and characterized by spectroscopic, analytical, and magnetic susceptibility studies. Spectroscopic and magnetic susceptibility studies suggested that copper and manganese ions are in 2+ and 3+ states and their complexes have a binuclear double stranded helical structure in the form of 2:2 (metal to ligand) stoichiometry . Cytotoxic effects of the ligand and its metal complexes against MCF-7 (human breast cancer cell line), DLD-1 (human colorectal cancer cell line), ECC-1 (human endometrium cancer cell line), DU-145 (human prostate cancer line), MDA-MB231 (human breast cancer cell line), PC-3 (human prostate cancer line) and HEK293 (normal cells) were evaluated by determining their cellular viability using the colorimetric 3-(4,5- dimethylthiazole-2-yl)-2,5-biphenyl tetrazolium bromide (MTT) assay. It has been found that cytotoxicity of the ligand was significantly enhanced towards cancer cells and declined towards normal HEK293 cells by metal chelation. Copper complex yielded better results in comparison with manganese complex. Particularly, c opper complex showed a selective cytotoxicity, harming the cancerous cell lines while not impairing the normal cells, which is considered as the key to the future of cytotoxic therapy.

Structure, Attributive and Bacterial Effectiveness Studies for Complexes, Co(II), Hg(II), Ni(II), Cu(II) and Mn(II) with Ligand kind (N2O2)

Binuclear copper, nickel, cobalt, manganese and mercury complexes of the Schiff base H2L (C40H28N2O4) obtained by condensation of 2-benzoyl benzoic acid with benzidine. The Schiff base and their complexes have been subjected to[ FT-IR, elemental analysis ,UV-Vis, 1H and 13C NMR ] spectral studies, molar conductivity, magnetic moment and HPLC measurements. All the complexes showed tetrahedral geometries with the general structure [M2(L)2]. Generality of the synthesized components offer antibacterial efficiency to (Staphylococcu saureus), (Escherichia coli),(Bacillus subtilis) and(Pseudomonas aeruginosa).

Novel binuclear manganese(III), cobalt(III) and chromium(III) complexes for the alternating ring-opening copolymerization of cyclohexene oxide and maleic anhydride

Several unsymmetrical Schiff-base mononuclear and binuclear manganese(III), cobalt(III) and chromium(III) complexes have been designed, synthesized and applied to the alternating ring-opening copolymerization of cyclohexene oxide and maleic anhydride with the optimized conditions. Experimental results show that these complexes are effective in the catalytic copolymerization. Among the six complexes, the binuclear complexes exhibited better catalytic activity than mononuclear complexes under the same conditions. The highest molecular weight (24,600gmol−1) of copolymer with a narrow polydispersity (1.07) and higher conversion of the monomer (95%) were achieved when using binuclear chromium(III) complex, followed by the manganese(III) complex and the cobalt(III) complex showed relatively lower activity at the same conditions. It is due to maybe existed an intramolecular bimetallic synergistic effect in the binuclear catalysts system different from the mononuclear catalysts system.

A hydroperoxo-rebound mechanism of alkane oxidation with hydrogen peroxide catalyzed by binuclear manganese(IV) complex in the presence of an acid with involvement of atmospheric dioxygen

The binuclear manganese(IV) complex [Mn2L2O3][PF6]2 (L=1,4,7-trimethyl-1,4,7-triazacyclononane) (compound 1) catalyzes very efficiently the oxygenation of alkanes with H2O2 only if a carboxylic (acetic, oxalic) acid is present as a co-catalyst. The oxidation produces mainly alkyl hydroperoxide which slowly decomposes in the course of the reaction to afford the corresponding alcohol and ketone. Real concentrations of the three products (alkyl hydroperoxide, alcohol and ketone) were estimated at each given moment by comparing the chromatograms before and after reduction of the samples with PPh3. Selectivity parameters and kinetics of the cyclohexane oxidation are described. The mode of the initial rate W0 dependence on the initial concentration of cyclohexane should reflect a concurrence between the alkane and acetonitrile for the oxidizing species generated in the H2O2 decomposition. It was concluded that hydroxyl radicals do not participate in this process. The cyclohexane oxidation by the systems ‘18O2-hydrogen peroxide-1-oxalic acid’ and ‘16O2-hydrogen peroxide-H218O-1-oxalic acid’ were carried out. Cyclohexanol produced in the reaction in an 18O2 atmosphere after reduction with PPh3 (which was transformed into 18OPPh3) was found to contain up to 56% of 18O pointing that the respective cyclohexyl hydroperoxide also contained labeled oxygen. No traces of labeled cyclohexanol were detected in the reaction under 16O2 in the samples reduced with PPh3. All experimental data are in an excellent agreement with an unusual catalytic cycle proposed for the alkane oxygenation. The transformation of alkane into the corresponding alkyl hydroperoxide proceeds via generation of alkyl radicals R which rapidly react with atmospheric molecular oxygen. Unlabeled alkyl hydroperoxide can be partially formed via the rebound mechanism R+HOO–Mn→R–OOH+Mn, where Mn is a fragment of the binuclear intermediate species.

Resonance Light Scattering Study on the Formation of a Manganese (II) Coordination Supramolecular Polymer and Its Analytical Application

The formation of a manganese (II) coordination supramolecular polymer was studied by resonance light scattering spectra for manganese (II) detection. Bis-sulfosalophen (a ditopic tetradentate Schiff base ligand) and bis-phenanthroline-glutaraldehyde (a ditopic bidentate ligand) were prepared for the construction of the supramolecular polymer. In a procedure of manganese (II) detection, manganese (II) ion reacts with bis-sulfosalophen to form a binuclear manganese (II) complex. The binuclear complex then self-assembles with bis-phenanthroline-glutaraldehyde to form the supramolecular polymer, resulting in the production of strong resonance light scattering signal. The amount of manganese was detected by measuring the resonance light scattering intensity. Under optimal conditions, a linear range was found to be 0.5–50.0 ng/mL, with a detection limit of 0.1 ng/mL. The method has been successfully applied to determine manganese in vegetable and tea samples with relative standard deviations of less than 5% and recoveries of 95.8–105.7%.

Highly efficient aerobic epoxidation of cyclic olefins in mild conditions by a novel binuclear manganese(II) complex containing N-(4-nitrophenyl)picolinamide ligand

A novel binuclear Mn(II) complex based on a para-nitro substituted amidic ligand (N-(4-nitrophenyl)picolinamide) has been synthesized and characterized by X-ray crystallography. This complex shows a high degree of conversion and epoxide selectivity in Mukaiyama aerobic epoxidation reactions of cyclic olefins in mild conditions.

Synthesis, characterization, biological evaluation and docking studies of macrocyclic binuclear manganese(II) complexes containing 3,5-dinitrobenzoyl pendant arms

A series of bis(phenoxo) bridged binuclear manganese(II) complexes of the type [Mn2L1–3](ClO4)2 (1–3) containing 3,5-dinitrobenzoyl pendant-arms have been synthesized by cyclocondensation of 2,6-diformyl-4-R-phenols (where R=CH3, C(CH3)3 or Br) with 2,2′-3,5-dinitrobenzoyliminodi(ethylamine) trihydrochloride in the presence of manganese(II) perchlorate. The IR spectra of complexes indicate the presence of uncoordinated perchlorate anions. The UV–Vis spectra of complexes suggest the distorted octahedral geometry around manganese(II) nuclei. The EPR spectra of Mn(II) complexes show a broad signal with g value 2.03–2.04, which is characteristic for octahedral high spin Mn2+ complex. The observed room temperature magnetic moment values of the Mn(II) complexes (5.60–5.62B.M.) are less than the normal value (5.92B.M.), indicating weak antiferromagnetic coupling interaction between the two metal ions. Electrochemical studies of the complexes show two distinct quasi-reversible one electron transfer processes in the cathodic (E1pc=−0.73 to −0.76V, E2pc=−1.30 to −1.36V), and anodic (E1pa=1.02–1.11V, E2pa=1.32–1.79V) potential regions. Antibacterial efficacy of complexes have been screened against four Gram (−ve) and two Gram (+ve) bacterial strains. The DNA interaction studies suggest that these complexes bind with CT-DNA by intercalation, giving the binding affinity in the order 1>2>3. All the complexes display significant cleavage activity against circular plasmid pBR322 DNA. Docking simulation was performed to insert complexes into the crystal structure of EGFR tyrosine kinase and B-DNA at active site to determine the probable binding mode.

Synthesis and magnetic properties of manganese carbonyl complexes with different coordination modes of 3,4,5-triaryl-1,2-diphospholide ligands.

The set of complexes bis-(μ:η(1),η(1)-3,4,5-triaryl-1,2-diphosphacyclopentadienyl)-bis-(tetracarbonyl manganese(i)) (aryl = C6H5 (), p-FC6H4 (), p-ClC6H4 ()) undergo an irreversible rearrangement to mononuclear 3,4,5-triaryl-1,2-diphosphacymantrenes (). According to quantum-chemical calculations binuclear complexes can be considered to be products of kinetic control and mononuclear species are thermodynamically favorable compounds. The antiferromagnetic intramolecular interaction observed for complexes can be effectively tuned by using substituents in the para-position of the arene ring, whereas mononuclear 1,2-diphosphacymantrenes are diamagnetic.