Linear Polymeric Structure Research Articles

Synthesis, characterization, and structural analysis of hetero Ni(II)–Na(I) and Homo Ni(II)–Ni(II) dinuclear complexes with a flexible tridendate schiff base ligand

Two different novel [Ni(II)–Na(I)]n and Ni(II)–Ni(II)] complexes have been synthesized successfully using a flexible tridentate Schiff base ligand. These complexes are characterized by IR and single crystal X–ray analysis and the optical properties of these complexes have been studied by UV spectroscopy (solution and solid). The [Ni(II)–Na(I)]n complexes are repeatedly connected with another Ni(II)–Na(I) unit to yield a linear polymeric chain. The Ni(II)–Ni(II) complex shows the nature of supramolecular dimeric cluster. These results are evidenced by x-ray structural analysis. Both as-obtained complexes are shown in different structures, such as hetro metal [Ni(II)–Na(I)]n] and homo metal [Ni(II)–Ni(II)]coordination environments with the same ligand. The first one forms a linear polymeric structure whereas the second one forms a cluster. The more intense band at 2104 cm1 is assigned to the NCS ligand of compound 1. A sharp band at 1068 cm1 together with a band at 625 cm1 in compound 2 is assigned to the ClO4 stretching vibrations and these bands are evidence for the presence of ionic perchlorate (non–coordinated). The electronic spectra of compounds 1 and 2 are recorded in ACN solvent to exhibit a broad peak around 380 nm due to d-d transition.

Constructing a solid-state supramolecular polymer based on host-guest recognition between perethylated pillar[5]arene and tetrathiafulvalene.

Herein we present a novel linear supramolecular polymeric structure formed in both the solution and solid state, utilizing the host-guest recognition motif between perethylated pillar[5]arene and tetrathiafulvalene.

Induction and rationalization of supramolecular chirality in a Zn(II)porphyrin dimer using chiral substrates: Role of substrates in the architectural selectivity

A flexible Zn(II)porphyrin dimer has been utilized as an efficient host for the selective binding of several diols and diamines as substrates. Stepwise formation of a 1:1 host-guest polymer and 1:2 host-guest monomer via intermolecular self-assembling and disassembling processes, has been demonstrated with a series of diamine/diols. Transfer of chirality from the molecular to the supramolecular level has been rationalized with structural elucidation. The formation of the 1:1 host-guest polymer displays a bisignate CD couplet. X-ray structure demonstrates the presence of both clockwise and anticlockwise conformers, however, the conformer formed as per pre-organization in the substrate chirality contributes more towards the overall CD couplet in solution. The addition of the substrate in excess leads to the inversion of the CD couplet due to the formation of the monomeric complex. Architectural selectivity has been illustrated for the 1:1 host-guest complexation by simply tuning the length of the substrates. Crystallographic characterizations revealed linear 1D polymeric structures for the 1:1 host-guest complexes with longer diamines while shorter substrates stabilize cyclic dimer via intermolecular H-bonding between the urea bridge (cis-NH-CO-) of the two porphyrin dimers. Interestingly, the molecular structure of 1:1 host-guest complexes with diols displays a linear polymer despite the shorter length of the substrates. Along with weaker coordination of the alcoholic oxygen towards the Zn(II) ion, the presence of bulky substituents at the coordinating sites of the diols renders the formation of intermolecular H-bonding interactions. Thus, the nature of metal ion and their binding affinity towards the substrate, host-guest size complementarity, and the bulk of the substituents contribute cumulatively towards the architectural selectivity. The cyclic dimers display larger upfield shifts of the substrate protons in the 1H NMR spectra as compared to the linear polymers. Moreover, the computational calculations further substantiate the experimental observations.

Iminophosphonamido-Supported Plumbylenes and Plumbyliumylidenes: Synthesis and Properties.

A series of low-valent lead(II) species, i.e., plumbylenes and plumbyliumylidenes supported by an N,N'-di-tert-butyliminophosphonamide ligand, have been synthesized. Chloroplumbylene [Ph2P(NtBu)2PbCl] (1) was readily prepared by the reaction of the corresponding lithium iminophosphonamide and PbCl2 in THF. The substitution reaction of 1 with KN(SiMe3)2 in THF afforded the corresponding aminoplumbylene [Ph2P(NtBu)2PbN(SiMe3)2] (2) in the form of colorless crystals. The structures of these plumbylenes in the solid state and in solution were exhaustively characterized using multinuclear NMR spectroscopy and X-ray diffraction analysis. In the crystalline state, 1 adopts a nearly linear polymeric structure in the direction of the c axis via Pb-Cl bonds, with alternating four-membered PbN2P rings. The chloride-abstraction reaction of 1 using Na[B(C6F5)4] in fluorobenzene proceeded efficiently at ambient temperature to furnish plumbyliumylidene [Ph2P(NtBu)2Pb:]+ (3+), which was isolated as the air-sensitive yellow borate salt 3[B(C6F5)4]. Plumbyliumylidene 3[B(C6F5)4] acts as a Lewis acid catalyst for the hydroboration of benzophenone and benzaldehyde at catalyst loadings of as low as 0.1 mol % under ambient conditions. Furthermore, the details of the reaction mechanism are discussed on the basis of the results of DFT calculations.

Dendrite-Free and Long-Cycling Lithium Metal Battery Enabled by Ultrathin, 2D Shield-Defensive, and Single Lithium-Ion Conducting Polymeric Membrane.

Polymeric membranes are considered as promising materials to realize safe and long-life lithium metal batteries (LMBs). However, they are usually based on soft 1D linear polymers and thus cannot effectively inhibit piercing of lithium dendrites at high current density. Herein, single lithium-ion conducting molecular brushes (GO-g-PSSLi) are successfully designed and fabricated with a new 2D "soft-hard-soft" hierarchical structure by grafting hairy lithium polystyrenesulfonate (PSSLi) chains on both sides of graphene oxide (GO) sheets. The ultrathin GO-g-PSSLi membrane is further constructed by evaporation-induced layer-by-layer self-assembly of GO-g-PSSLi molecular brushes. Unlike conventional soft 1D linear polymeric structure, the rigid 2D extended aromatic structure of intralayer GO backbones can bear the shield effect of preventing the dendrites possibly generated at high current density from piercing. More importantly, such a shield effect can be significantly strengthened by layer-by-layer stacking of 2D molecular brushes. On the other hand, the 3D interconnected interlayer channels and the soft single lithium-ion conducting PSSLi side-chains on the surface of channels provide rapid lithium-ion transportation pathways and homogenize lithium-ion flux. As a result, LMBs with GO-g-PSSLi membrane possess long-term reversible lithium plating/striping (6months) at high current density.

Synthesis, crystal structure, DFT studies and Hirshfeld surface analysis of Manganese(II) and Cadmium(II) coordination polymers of 2-aminopyridine and dicyanamide

Mn(II) and Cd(II) metal salts reacted with dicyanamide and 2-aminopyridine resulted in the formation of the ligand-rich linear polymeric chain structures [M(dca)2(2-Ampy)2]n (dca = dicyanamide, 2-Ampy = 2-Aminopyridine; M = Mn (1) or Cd (2)), of the same dimensionality, in which the metal centers are μ1,5-bridged by the dca anions. The complexes were characterized by elemental analysis, IR spectroscopy, UV–vis spectroscopy, thermal analysis and single crystal X-ray structure analysis. These coordination polymers crystallize in the Monoclinic space group Cc. The 3D crystal packing structure results from a combination of N—H/H—-N interactions followed by CH—π stacking interactions. Hirshfeld surface (HS) analysis of the polymers was undertaken to investigate further the intermolecular interactions. DFT calculations on 1 and 2 demonstrated a good correlation between the experimental and theoretical data. The in vitro antimicrobial activity of the manganese complex against selected micro-organisms is also reported.

Synthesis of 4-{(E)-[(3-methyl-5-sulphanyl-4H-1,2,4-triazol-4-yl)imino]methyl} benzaldehyde and its complexes with Ag(I), Hg(II), Zn(II), Co(II), Ni(II) ions and its characterisation

A novel Schiff base 4-{(E)-[(3-methyl-5-sulphanyl-4H-1,2,4-triazol-4-yl)imino]methyl}benzaldehyde (MSTB) and its complexes with Ag(I), Hg(II), Cu(II), Ni(II) and Co(II) have been prepared. MSTB is prepared by the condensation reaction between triazole and terephthaldehyde. In complexes, MSTB molecules are found to link with the metal ion, through thiolsulphur after deprotonation and the nitrogen of the azomethine group. The composition of the complexes are found to be ML where M =Ag, ML2 where M=Zn, Hg and ML2(H2O)2 where M= Co, Ni. Linear polymeric structure is proposed for Ag(I) complex, tetrahedral structure is proposed for Zn(II) and Hg(II) complexes and octahedral structure is proposed for Co(II) and Ni(II) complexes.

Biological properties of a new mixed lanthanide(III) complex incorporating a dypiridinium ylide

A new mixed lanthanide(III) complex [LaNd(μ2-DPY)(μ4-SO4)2(Et3N)]Br2·2H2OMeOH (La-Nd-DPY) was obtained by the reaction of N,N′-diphenacyl-4,4′-dipyridinium dibromide (DPB) with a mixture of La(III) and Nd(III) sulfates in a 2:1 M ratio, in the presence of triethylamine (Et3N). The method used for the synthesis of La-Nd-DPY promoted the in situ transformation of the pro-ligand DPB into the dipyridinium ylide-based ligand. Fourier transform infrared spectroscopy (FTIR), elemental analysis (EA), thermogravimetric analysis (TGA) and mass spectrometry (MS) concurred to propose a linear polymeric structure for La-Nd-DPY, in which both La(III) and Nd(III) ions are six-coordinated. Cyclic voltammetry was also used to assess the redox potential of the mixed complex. Scanning electron microscopy (SEM) showed quite uniform and homogeneous fibrillary net-like morphology and also confirmed, by means of EDX analysis, the presence of both lanthanide (III) ions in the mixed complex. UV–vis absorption spectroscopy demonstrated that the mixed Ln complex is stable up to 1 h in DMSO and up to 72 h under the physiological conditions used for cell culture. The cytotoxic activity of La-Nd-DPY in cancer cell lines of ovarian (A2780), breast (MCF7) and prostate (PC3) origins and in multicellular tumor spheroids derived from PC3 cells was evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and acid phosphatase (APH) assays. Compared to cisplatin this new complex showed improved toxicity, with IC50 values at least 10 times higher, even at 24 h of exposure. The cytotoxic effects seemed to be mediated by reactive oxygen species (ROS) generation but not apoptosis, confirmed by a caspase-3/7 activation and Hoechst nuclei staining assays. The mechanism of cell death observed on the cytotoxicity assays is probably due to other pathways of cell death, which deserve to be further investigated.

The Self-Assembly of [{Ag3(C≡CtBu)2}n]n+ Building Units into a Template-Free Cuboctahedron and Anion-Encapsulating Silver Cages.

We describe the controlled synthesis of silver acetylide clusters based on a simple polymeric [Ag3L2]+ (L = -C≡CtBu) building block. A linear one-dimensional polymeric structure shows alternating pyramidal motifs and is the basic repeating unit forming discrete molecular cages (pentamers [X⊂Ag15L10]4+ and hexamers [PF6⊂Ag18L12]5+) obtained by incorporating suitable templates. These motifs and a rare template-free cuboctahedral [Ag12L8]4+ cluster (tetramer) were crystallographically characterized.

A one-dimensional polymeric mixed-valent Mn(II)Mn(III) complex with a macrocyclic compartmental ligand: Structure, properties and catecholase like activity

Reaction of 2,6-diformyl-4-methylphenol, 1,3-diaminopropane, Mn(ClO4)2∙6H2O and CH3COONa∙3H2O in 1:1:1:1 mole ratio in methanol afford a linear coordination polymer of formula {[MnIIMnIIIL(OAc)2]ClO4}n (L2− represents the Robson-type ligand) in 64% yield. The complex has been characterized by elemental (CHN) analysis, magnetic susceptibility, spectroscopic (IR, UV–Vis and EPR) and solution electrical conductivity measurements. The linear polymeric structure and the mixed-valent character of the complex have been confirmed by single crystal X-ray crystallography. In the repetitive dinuclear unit, both metal centers are in N2O4 coordination spheres assembled by the compartmental macrocyclic N2O2N2-donor L2−, an intra-dinuclear bridging acetate and an inter-dinuclear bridging acetate. The coordination geometry around the manganese(II) center (S = 5/2) is severely distorted octahedral, while that around the manganese(III) center (S = 2) is tetragonally elongated octahedral. Variable temperature magnetic susceptibility measurements indicate a weak intra-dinuclear ferromagnetic (J = 1.12(3) cm−1) interaction as well as a weak inter-molecular antiferromagnetic (zJ′ = −0.679 (3) cm−1) interaction. Cyclic voltammogram of the complex displays a metal centered quasi-reversible reduction at −0.03 V and an irreversible ligand centered reduction at −1.16 V (vs Ag/AgCl). The complex efficiently catalyzes the aerobic oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylbenzoquinone with a turnover number (kcat) 1.2 × 104 h−1.

Synthesis, Characterization, Crystal Structure Determination and Luminescence Study of New Linear Zn(II) Coordination Polymers with 4,7-Phenanthroline Ligand

Two linear coordination polymers containing Zn(II) and 4,7-phenanthroline (4,7-phen) ligand have been synthesized and structurally characterized by single-crystal X-ray diffraction method. Complexes {[Zn(η2-4,7-phen)X2] ° CH3CN}n (X = Br for 1 and X = I for 2) were synthesized from the reaction of one equivalent of ZnBr2(1) and ZnI2(2) in acetonitrile with one equivalent of 4,7-phen ligand in methanol. Crystals of title complexes suitable for X-ray diffraction experiments have been obtained by slow evaporation of the resulted solution at room temperature. Both complexes have linear polymeric structures which extend parallel to the b axis and contain Zn(II) centers coordinated to two bridging 4,7-phen ligands, and two coordinated halogen molecules, showing a distorted tetrahedral geometry around Zn atoms. In addition, π⋯π (in 1), C-H⋯N (in 1 and 2), C-H⋯Br (in 1) and C-H⋯I (in 2) hydrogen bonding interactions play important role on stabilization of the crystal packing. The luminescence properties of the free ligand as well as the complexes 1 and 2 were investigated in solution.

Selectins and Immune Cells in Acute Myocardial Infarction and Post-infarction Ventricular Remodeling: Pathophysiology and Novel Treatments.

The glycosciences aim to understand the impact of extracellular and intracellular carbohydrate structures on biological function. These glycans primarily fall into three major groups: lipid-linked carbohydrates that are referred to as glycosphingolipids or simply glycolipids; relatively short carbohydrate chains that are often O- or N-linked to proteins yielding common glycoproteins; and extended linear polymeric carbohydrate structures that are referred to as glycosaminoglycans (GAGs). Whereas, the impact of such carbohydrate structures has been extensively examined in cancer biology, their role in acute and chronic heart disease is less studied. In this context, a growing body of evidence indicates that glycans play an important role in immune mediated cell recruitment to damaged heart tissue to initiate wound healing and repair after injury. This is particularly important following ischemia and reperfusion that occurs in the heart in the setting of acute myocardial infarction. Here, immune system-mediated repair of the damaged myocardium plays a critical role in determining post-infarction ventricular remodeling, cardiac function, and patient outcome. Further, alterations in immune cell activity can promote the development of heart failure. The present review summarizes our current understanding of the phases of immune-mediated repair following myocardial infarction. It discusses what is known regarding glycans in mediating the recruitment of circulating immune cells during the early inflammatory stage of post-infarction repair, with focus on the selectin family of adhesion molecules. It offers future directions for research aimed at utilizing our knowledge of mechanisms underlying immune cell recruitment to either modulate leukocyte recruitment to the injured tissue or enhance the targeted delivery of biologic therapeutics such as stem cells in an attempt to promote repair of the damaged heart.

The structural diversity of heterocycle-fused potassium cyclopentadienides.

Cyclopentadienides of d- and f-elements are highly important complexes with undoubted potential for practical applications. Annelation of a heterocyclic fragment with an η5-ring results in substantial improvement of the catalytic properties of these compounds, called “heterocenes”; the investigation of metal coordination with these specific ligands is a highly important problem. We prepared potassium derivatives 5–8 of heterocycle-annelated cyclopentadienes with different structures – derivatives of cyclopenta[1,2-b:4,3-b′]dithiophene (1), indeno[2,1-b]indole (2), indeno[1,2-b]indole (3), and indeno[1,2-b]indolizine (4) and studied the crystal and molecular structures of these salts by X-ray diffraction. We found that heterocycle-fused cyclopentadienides demonstrate remarkable diversity in metal–ligand coordination modes and crystal packing, with formation of two-dimensional polymeric (5), linear polymeric (6), tetrameric (7) and monomeric (8) structures. The NMR spectral data and results of DFT modeling indicate an increase in electron density in the cyclopentadienyl fragment, and this effect was found to be larger in the derivative of the new indolizine ligand precursor 4. The results of our study will be used in the design of next-generation catalysts of α-olefin polymerization.

Synthesis, characterization, crystal structure determination and luminescence study of new linear coordination polymers based on Mercury(II), Cadmium(II) and 4,7-phenanthroline ligand

Three novel coordination polymers containing Hg(II) and/or Cd(II) with 4,7-phenanthroline ligand (4,7-phen) have been synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes [Hg(η2-4,7-phen)Br2]n (1), [Hg(η2-4,7-phen)I2]n (2) and [Cd(η2-4,7-phen)I2]n (3) were prepared from the reaction of one equivalent of 4,7-phenanthroline ligand with one equivalent of HgBr2 (1), HgI2 (2) and CdI2 (3), in methanol, respectively. Suitable crystals of title compounds were obtained for X-ray diffraction measurement by methanol diffusion into a DMSO solution. All compounds have linear polymeric structures which extend parallel to the c axis and contain Hg(II) and Cd(II) centers coordinated by two bridging 4,7-phenanthroline ligands, and two coordinated halogen molecules, completing the Hg(II) and Cd(II) coordination spheres, and resulting in a distorted tetrahedral geometry. Furthermore, π-π (in 1–3) and M-I⋯π (in 2 and 3) interactions play crucial role on stabilization of the crystal packing. The luminescence properties of the free ligand as well as the complexes 1–3 were investigated in solution. Thermal stabilities of these complexes were also studied by TGA/DTA analyses.

Formation of a Hexanuclear Octatetraenyl Organocopper(I) Aggregate via Oxidation of Spiro Butadienyl Organocuprate

Spiro butadienyl tetraorganocuprates 2a,b were synthesized from butadienyl dilithio reagent and 0.5 equiv of copper(I) salt. The oxidation reaction of 2a,b with I2 or p-benzoquinone afforded the octatetraenyl organocopper(I) aggregate 3. This result demonstrated that the butadienyl skeletons rather than the copper(I) atom were oxidized preferentially. Single-crystal X-ray structural analysis revealed that the high-order organocuprates 2a,b have linear or zigzag polymeric structures, while the hexanuclear octatetraenyl organocopper(I) aggregate 3 consists of a core Cu6 structure, which forms a regular hexagon.

Cross-Linked and Biodegradable Polymeric System as a Safe Magnetic Resonance Imaging Contrast Agent.

Owing to the low efficacy of clinically used small-molecule gadolinium (Gd)-based magnetic resonance imaging (MRI) agents, we designed and explored biodegradable macromolecular conjugates as MRI contrast agents. The linear polymeric structure and core-cross-linked formulation possessed different characteristics and features, so we prepared and comparatively studied the two kinds of Gd-based N-(2-hydroxypropyl) methacrylamide (HPMA) polymeric systems (the core-cross-linked pHPMA-DOTA-Gd and the linear one) using the clinical agent diethylene-triamine pentaacetic acid-Gd(III) (DTPA-Gd) as a control. This study was aimed to find the optimal polymeric formulation as a biocompatible and efficient MRI contrast agent. The high molecular weight (MW, 181 kDa) and core-cross-linked copolymer was obtained via the cross-linked block linear copolymer and could be degraded to low-MW segments (29 kDa) in the presence of glutathione (GSH) and cleaned from the body. Both core-cross-linked and linear pHPMA-DOTA-Gd copolymers displayed 2-3-fold increased relaxivity (r1 value) than that of DTPA-Gd. Animal studies demonstrated that two kinds of macromolecular systems led to much longer blood circulation time, higher tumor accumulation, and much higher signal intensity compared with the linear and clinical ones. Finally, in vivo and in vitro toxicity studies indicated that the two macromolecular agents had great biocompatibility. Therefore, we performed preliminary but important studies on the Gd-based HPMA polymeric systems as biocompatible and efficient MRI contrast agents and found that the biodegradable core-cross-linked pHPMA-DOTA-Gd copolymer might have greater benefits for the foreground.

Polyaceticzirconium for zirconia continuous fibers: Polymeric evolution process and the relationship between polymeric structure and rheological behavior

Abstract Polyaceticzirconium (PEZ) sol, which is the precursor of zirconia (ZrO2) continuous fibers, has been synthesized by means of the reaction between zirconium oxychloride octahydrate (ZrOCl2·8H2O) and potassium acetate (CH3COOK) in methanol. The polymeric structures and the rheological behaviors of the sols with different molar ratios of ZrOCl2·8H2O/CH3COOK are different and would affect the spinnability and stability significantly. Herein, the polymeric structures and rheological behaviors of the PEZ sols with the molar ratios of 1:1, 1:1.5, 1:1.8 and 1:2 were investigated by Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance spectrum (1H-NMR), thermogravimetry and differential thermal analysis (TG/DTA) and rheological measurements. The results showed the carboxyl group acted as a chelating ligand instead of bidentate ligand, and the water molecule coordinated to Zr4+ was deprotoned gradually with the decrease of the molecular ratios. The different polymeric structures lead to a different rheological behaviors of the sols. It is divided into three types, the Newton fluid with a low viscosity produced by the charged linear polymeric structures, the shear thinning fluid with a high viscosity produced by the three-dimensional polymeric structures and the shear thinning fluids with a proper viscosity produced by the linear twining polymeric structures. Through the experimental guidance, we successfully synthesized a sol with suitable viscosity and stability, and used it to fabricate high quality ZrO2 fibers.

Radical Nature of C-Lignin

The recently discovered lignin composed of caffeoyl alcohol monolignols or C-lignin is particularly intriguing given its homogeneous, linear polymeric structure and exclusive benzodioxane linkage between monomers. By virtue of this simplified chemistry, the potential emerges for improved valorization strategies with C-lignin relative to other natural heterogeneous lignins. To better understand caffeoyl alcohol polymers, we characterize the thermodynamics of the radical recombination dimerization reactions forming the benzodioxane linkage and the bond dissociation into radical monolignol products. These properties are also predicted for the cross-coupling of caffeoyl alcohol with the natural monolignols, coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, in anticipation of polymers potentially enabled by genetic modification. The average BDEs for the C-lignin benzodioxane α- and β-bonds are 56.5 and 63.4 kcal/mol, respectively, with similar enthalpies for heterodimers. The BDE of the α-bond within...

Synthesis of Dipyridinogermole–Copper Complex as Soluble Phosphorescent Material

The reaction of diphenyldipyridinogermole with [Cu2I2(PPh3)3] in THF afforded a complex with high thermal stability. Crystals with linear polymeric structures (determined by single-crystal X-ray di...

Self-assembly of organotin(IV) 4-fluorobenzeneseleninato complexes: 1D polymeric chain, helical double-chain polymer and tetranuclear cage

Four new organotin complexes, [Me3SnO2SeC6H4-F-p]n (1), [Ph3SnO2SeC6H4-F-p] (2), [Me2Sn(O2SeC6H4-F-p)2]n (3), and [(PhSn)4(μ2-O)2(O2SeC6H4-F-p)8] (4) have been synthesized via reactions of 4-fluorobenzeneseleninato ligand and the corresponding R3SnCl or R2SnCl2 (R = Me, Ph) with sodium ethoxide in methanol. All the complexes have been characterized by X-ray crystallography, elemental analysis, FT-IR, and NMR (1H, 13C, 119Sn) spectroscopy. The structural analysis indicates that complex 1 adopts a 1D infinite zigzag chain structure, complex 2 displays a 1D infinite linear polymeric chain structure, complex 3 is 1D helical double-chain polymer containing eight-membered Sn2O4Se2 inorganic ring, and complex 4 shows a tetranuclear cage structure containing two Sn–O–Sn motifs connected by bridging [p-F-C6H4SeO2]- ligands.