From Overcrowded Polycyclic Aromatic Enes to Semifullerenes

Abstract

Overcrowded polycyclic aromatic enes (1), e.g., bi-9H-fluoren-9-ylidene (2) and bi-4H-cyclopenta[def]-phenanthren-4-ylidene (3) are potential starting materials for the preparation of bowl-shaped fragments of fullerenes. Semiempirical MNDO-PM3 calculations of C₂₆ H_n and C₃₀ H_n (n = 12,14,16) species 2–14 are used to analyze energetic and steric effects on the dehydrocyclization and isomerization reactions of these molecules. the out-of-plane bending and pyramidalization in these species are ascribed to intramolecular overcrowding in the fjord and cove regions and to strain introduced by C5 rings in the PAH skeleton. Oxidative photocyclization reactions on Z-2,2′-bridged derivatives of 2 and 3 are briefly outlined.     

Syntheses and Crystal Structures of Di- and Trimethyltin Carboxylates: Ladders and 1D Zig–Zag Chains Polymers

A series of organotin(IV) carboxylates complexes; namely, [(Me₂Sn)₄O₂(RCOO)₄] (R = C₁₂H₁₅ 1, C₉H₁₁ 2, C₈H₈ClO 3, C₇H₉ 4) and [Me₃(RCOO)]_n (R = C₁₂H₁₅ 5, C₉H₁₁ 6, C₈H₈ClO 7, C₇H₉ 8) have been synthesized. All complexes were characterized by elemental analysis, FT-IR, and NMR (¹H, ¹³C and ¹¹⁹Sn) spectroscopy. Among them, the structures of complexes 1–3 and 5–8 were also determined by X-ray crystallography. The structural analysis showed that complexes 1–3 are the same tetranuclear monomer, and complexes 5–8 are the same 1D zigzag chain coordination polymer. Furthermore, each complex 1, 2 and 3, can form a supramolecular chain through weak intermolecular interactions.     

Incorporation of Chromium–Carbonyl Fragments to Bis-Spirocyclophosphazene and Spiro Polyphosphazene Containing a Benzylcyanide Spacer

The reaction of HOC₆H₄CH₂CN (1), N₃P₃(O₂C₁₂H₈)₂(OC₆H₄CH₂CN)₂ (2), and [NP(O₂C₁₂H₈)_{2 0.8}NP(OC₆H₄CH₂CN)_{2 0.18}] _n (3) with Cr(CO)₆ (4) in a di-n-buthylether/THF solvent mixture affords the new compounds Cr(CO)₃(NCCH₂C₆H₄OH)₃ (5), N₃P₃(O₂C₁₂H₈)₂[(OC₆H₄CH₂CN·Cr(CO)₅]₂ (6), and the copolymer [NP(O₂C₁₂H₈)_{2 0.8}NP(OC₆H₄CH₂CN)_{2 0.18}][Cr(CO)₅]_{0.132 n} (7), respectively. The coordination of the chromium carbonyl toward the nitrile moiety rather than the -link toward the benzene groups is indicated by spectroscopic data. Thermal effects of the incorporation of the organometallic fragment into the copolymer were investigated using differential scanning calorimetry and thermogravimetric analysis. Incorporation of the organometallic fragments into the polymer does not produce improved conductivity.     

Polymer and oligomer phosphazene cymantrene derivatives as solid state precursors of nanostructured manganese pyrophosphate

Pyrolysis under air at 800 °C, of the cyclic N₃P₃[OC₆H₄PPh₂·Mn(CO)₂(η-MeC₅H₄)]₆ (1) and the polymer {[NP(O₂C₁₂H₈)]_0.5[NP(OC₆H₄COOPr ⁿ )(OC₆H₄PPh₂·Mn(CO)₂(η-MeC₅H₄))]_0.5} _n (2) phosphazene with pendant OC₆H₄PPh₂·Mn(CO)₂(η-MeC₅H₄) units affords nanostructured Mn₂P₂O₇. The morphology of the pyrolytic products is highly dependent on the phosphazene support. Dense structures were observed from pyrolysis of cyclic (1) while most porous materials were obtained from pyrolysis of polymer (2). The new polymeric complex (2) was prepared from reaction of the phosphine containing polymer {[NP(O₂C₁₂H₈)]_0.5[NP(OC₆H₄COOPr ⁿ )(OC₆H₄PPh₂)]_0.5} _n and the photochemically generated [Mn(THF)(CO)₂(η-MeC₅H₄)] and was fully characterized.     

Ethylene polymerization by alkylidene‐bridged asymmetric dinuclear titanocene/MAO systems

Two asymmetric alkylidene‐bridged dinuclear titanocenium complexes (CpTiCl₂)₂(η⁵‐η⁵‐C₉H₆(CH₂)_nC₅H₄), 1 (n = 3) and 2 (n = 4) have been prepared by treating two equivalents of CpTiCl₃ with the corresponding dilithium salts of the ligands C₉H₇(CH₂)_nC₅H₅ (n = 3, 4). Additionally, Ti(η⁵:η⁵‐n‐BuC₅H₄C₅H₅)Cl₂ (3) and Ti(η⁵:η⁵‐n‐BuC₉H₆C₅H₅)Cl₂ (4) were synthesized as corresponding mononuclear complexes. All complexes were characterized by ¹H, ¹³C NMR, and IR spectroscopy. Homogenous ethylene polymerization catalyzation using those complexes has been conducted in the presence of methylaluminoxane (MAO). The influences of reaction parameters, such as [MAO]/[Cat] molar ratio, catalyst concentration, ethylene pressure, temperature, and time have been studied in detail. The results showed that the catalytic activities of both dinuclear titanocenes were higher than those of the corresponding mononuclear titanocenes. Although the two dinuclear complexes were different in only one [CH₂] unit, the catalytic activity of 2 was about 50% higher than that of 1; however, the molecular weight of polyethylene (PE) obtained by 2 was lower than that obtained from 1. The molecular weight distribution of PE produced by these dinuclear complexes reached 6.9 and 7.3, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3317–3323, 2006     

The Influence of Alkyl Chain Length and Steric Effect on the Stability Constants and Extractability of Co(II) Complexes with 1-Alkyl-2-Methylimidazoles

Using the liquid-liquid partition method, the formation of Co(II) complexes with 1-alkyl-2-methylimidazoles (where alkyl = C₄H₉ trough C₁₂H₂₅) at 25°C and at a fixed ionic strength of the aqueous phase (I = 0.5, (HL)NO₃, KNO₃) were studied. Dichloromethane, trichloromethane, and 2-ethylhexanol were used as diluents. The tetrahedral and octahedral complexes were formed. Stability constants (β_n) of the complexes as well partition ratios (P_n) of the extracted species were determined. It was shown that both β_n and P_n increased with an increasing 1-alkyl chain length. Tetrahedral complexes are more readily extractable by organic solvent. Their P_n values are the highest.     

On the presence of polytetrahydrofuran in the polyspiro‐phosphazenes [NP(O2C12H8)]n prepared from [NPCl2]n and 2,2′‐dihydroxybiphenyl in THF as solvent

Polydichlorophosphazene [NPCl₂]_n reacts with the diphenol 2,2′‐(HO)C₆H₄‐C₆H₄(OH) in THF in the presence of K₂CO₃ to give the polymers [NP(O₂C₁₂H₈) · x(OC₄H₈)]_n (1) that contain variable ammouts of polytetrahydrofuran (PTHF) with x ranging from 0.05 to 0.8. This PTHF content (x) depends on the method followed to prepare the THF solutions of [NPCl₂]_n used for the reactions with the biphenol and can be made negligibly small, forming these solutions in the presence of K₂CO₃. This reveals the presence in the [NPCl₂]_n of acidic species capable of catalyzing the ring opening polymerization of THF. Polyphosphazene [NP(O₂C₁₂H₈)]_n (2) was prepared completely free of PTHF using dioxane as solvent. A comparison of the thermal behavior and morphological data of the polymers 1, 2, PTHF (5), and mixtures of [NP(O₂C₁₂H₈)]_n + x(OC₄H₈)_m (4) revealed that the products 1 are strongly interacting polymer blends and ruled out the possibility of block copolymers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 568–576, 2000     

Random Copolymers with 2,2′-Dioxy-1,1′-biphenyl-phosphazene and Chiral 2,2′-dioxy-1,1′-binaphthylphosphazene Units Functionalized in the 6,6′-Positions with PPh<Subscript>2</Subscript> Groups as Precursors for Ru(arene) Catalysts

The chiral phosphazene copolymers {[NP(O₂C₁₂H₈)]_0.9[NP(O₂C₂₀H₁₂)]_0.1} (1) and {[NP(O₂C₁₂H₈)]_0.9[NP(O₂C₂₀H₁₀Br₂)]_0.1} _n (2) [(O₂C₁₂H₈) = 2,2′-dioxy-1,1′-biphenyl; (O₂C₂₀H₁₂) = R-2,2′-dioxy-1,1′-binaphthyl and (O₂C₂₀H₁₀Br₂) = R-6,6′-dibromo-2,2′-dioxy-1,1′-binaphthyl] were prepared by sequential substitution from [NPCl₂] _n and the corresponding dihydroxy-biphenyl or binaphthyl reagents in the presence of Cs₂CO₃ and K₂CO₃. The reaction of (2) with ^tBuLi in THF, followed by addition of PPh₂Cl and a treatment with SiHCl₃/PPh₃ to eliminate any oxidized OC₆H₄P(O)Ph₂ groups, gave the phosphine containing copolymer {[NP(O₂C₁₂H₈)]_0.9[NP(O₂C₂₀H₁₀[PPh₂]₂)]_0.1} _n (3), that was used as a chiral ligand to support [Ru(p-cymene)Cl] complexes. The resulting catalyst was active for hydrogen transfer from isopropyl alcohol to acetophenone but the placement of the Ru centers in the 6,6′-positions of the binaphthoxyphosphazene units induced no enantioselectivity. Dedicated to Professor Christopher Allen.     

Synthesis of Small‐Scale Boron‐Rich Nano‐Size Particles

The synthesis times required to produce high energy density compounds (C₂B₄H₂)_n and (C₂B₁₀H₄)_n by gas phase pyrolysis of the carboranes C₂B₄H₆ and C₂B₁₀H₁₂, respectively, have been measured at 1150–2000 K and carborane pressures of (0.3–3.0)⋅10⁻³ MPa. Kinetic model simulations of the synthesis have been performed. The temperatures, carborane pressures, and synthesis times required to produce small‐scale (C₂B₄H₂)_n and (C₂B₁₀H₄)_n particles of 10 to 30 nm diameter are determined.     

Ru(II)(η6-p-cymene) Complexes Supported on Linear Chiral and Achiral Poly(spirophosphazene-pyridine) Copolymers

The reaction in dichloromethane at room temperature of the polyspirophosphazene copolymer {[NP(O₂C₁₂H₈)]_0.7[NP(OC₅H₄N)₂]_0.3} _n (1) (O₂C₁₂H₈=2,2dioxybiphenyl) with [Ru₂( ⁶-p-cymene)₂Cl₄] gives the polymer supported organometallic complex {[NP(O₂C₁₂H₈)]_0.7[NP(OC₅H₄N-Ru( ⁶-p-cymene)Cl₂)₂]_0.3} _n (2). Similarly the chiral phosphazene {[NP(O₂C₂₀H₁₂)]_0.9[NP(OC₅H₄N)₂]_0.1} _n (3) (O₂C₂₀H₁₂=R-2,2-dioxy,11-binaphthyl) reacts with the appropriate amount of the Ru precursor to give the related polymeric complex {[NP(O₂C₂₀H₁₂)]_0.9[NP(OC₅H₄N)(OC₅H₄N-Ru( ⁶-p-cymene)Cl₂]_0.1} _n (4) as an orange solid. Carrying out the reaction of 3 with the Ru precursor in acetone at room temperature using 0.35 equivalents of Ru per pyridine site, gives the crosslinked yellow material with formula {[NP(O₂C₂₀H₁₂)]_0.9[NP(OC₅H₅N)₂]_0.1[Ru( ⁶-p-cymene)Cl₂]_0.07]} _n (5), that may contain cationic [Ru( ⁶-p-cymene)Cl]⁺ units trapped in the rigid interior of a chiral network.     

Synthesis, Characterization and Photoluminescence of Dimeric and Polymeric Metallaynes of Group 10–12 Metals Containing Conjugation-breaking Diphenylmethane Unit

A novel approach based on conjugation interruption was developed for a luminescent and thermally stable platinum(II) polyyne polymer trans-[–Pt(PBu₃)₂C≡C(C₆H₄)CH₂(C₆H₄)C≡C–] _n (1) containing the diphenylmethane chromophoric spacer. Particular attention was focused on the photophysical properties of this group 10 polymetallayne and comparison was made to its binuclear model complex trans-[Pt(Ph)(PEt₃)₂C≡C(C₆H₄)CH₂(C₆H₄)C≡CPt(Ph)(PEt₃)₂] (2) and their closest group 11 gold(I) and group 12 mercury(II) neighbors, [MC≡C(C₆H₄)CH₂(C₆H₄)C≡CM] (M = Au(PPh₃) (3), HgMe (4)). The regiochemical structures of these angular-shaped compounds were studied by various spectroscopic analyses. Upon photoexcitation, each of them emits an intense purple-blue fluorescence emission in the near UV region in dilute fluid solutions at room temperature. Harvesting of organic triplet emissions harnessed through the strong heavy-atom effects of group 10–12 transition metals was examined. These metal-containing phenyleneethynylenes spaced by the conjugation-breaking CH₂ unit were found to have high optical gaps and high-energy triplet states. The influence of metal and sp³-hybridized methylene conjugation-interrupters on the intersystem crossing rate and the spatial extent of the lowest singlet and triplet excitons was fully elucidated. Our investigations indicate that high-energy triplet states in these materials intrinsically give rise to very efficient phosphorescence with fast radiative decays. Dedicated to Professor Didier Astruc in recognition of his outstanding contribution to metallodendrimers and polymers.     

Neutral AuCl complexes supported in linear high molecular weight, poly-spirophosphazene-phosphine copolymers and its conversion to nanostructured gold materials

Summary The reaction under very mild conditions of the polyspirophosphazene copolymer with pendant diphenylphosphine groups {[NP(O₂C₁₂H₈)]_0.85[NP(OC₆H₄PPh₂)₂]_0.15}_n (I) (O₂C₁₂H₈= 2,2’-dioxybiphenyl) with [Au(THT)Cl] (THT = tetrahydrothiophene) gives the neutral polymeric complex {[NP(O₂C₁₂H₈)]_0.85[NP(OC₆H₄PPh₂-AuCl)₂]_0.15}_n (II). The new material has been characterized by spectrocopic and thermochemical methods (TGA and DSC). Pyrolysis of the Au polymer (I) in air at 800 °C gave gold nanostructured materials that were characterized by TEM, SEM-EDAX and X-ray diffraction. Nanoparticles in the range of 90 to 130 nm were seen. Dedicated to Professor Victor Riera, Departamento de Quimica Orgánica e Inorgánica, Universidad de Oviedo, Spain, on the occasion of his 70^thbirthday.     

Synthesis and characterization of soluble polyphosphazenes having pendent Cp* Fe(dppe) groups

Summary The reaction of HOC₆H₄CH₂CN , N₃P₃(O₂C₁₂H₈)₂(OC₆H₄CH₂CN)₂ and – [{NP(O₂C₁₂H₈)}_0.8 {NP (OC₆H₄CH₂CN)₂}_0.18 ]_n with Cp*Fe(dppe)I in dichloromethane solution and in the presence of TIPF₆ affords the new compounds [Cp*Fe(dppe)NCCH₂C₆H₄OH][PF₆]1 Cp* = C₅(CH₃)₅ [{N₃P₃ (O₂C₁₂H₈)₂(OC₆H₄CH₂CN•Cp*Fe(dppe))₂] [PF₆]₂ 2 and the copolymer [{NP{(O₂C₁₂H₈}_0.8 {NP(OC₆H₄CH₂CN•[Cp*Fe(dppe)][PF₆])₂}_O.8]_n 3 respectively. The σ coordination of the Cp*Fe(dppe) fragment toward the nitrile group is indicated by spectroscopic data. The copolymer 3 is soluble in several organic solvents with no significant cross-linking and has a Mw on the order of 1.260.000. Thermal effects of the incorporation of the organometallic fragment to the copolymer were investigated using differential scanning calorimetry (DSC) and therrnogravimetric analysis (TGA). Received: 7 May 2002/Revised version: 21 March 2003/ Accepted: 27 March 2003 Correspondence to C. Diaz     

Ethylene polymerization by 3‐oxa‐pentamethylene bridged asymmetric dinuclear titanocene/MAO system

An asymmetric 3‐oxa‐pentamethylene bridged dinuclear titanocenium complex (CpTiCl₂)₂(η⁵‐η⁵‐C₉H₆(CH₂CH₂OCH₂CH₂)C₅H₄) ( 1 ) has been prepared by treating two equivalents of CpTiCl₃ with the corresponding dilithium salts of the ligand C₉H₇(CH₂CH₂OCH₂ CH₂)C₅H₅. The complex 1 was characterized by ¹H‐, ¹³C‐NMR, and elemental analysis. Homogenous ethylene polymerization catalyzed using complex 1 has been conducted in the presence of methylaluminoxane (MAO). The influences ofreaction parameters, such as [MAO]/[Cat] molar ratio, catalyst concentration, ethylene pressure, temperature, and time have been studied in detail. The results show that the catalytic activity and the molecular weight (MW) of polyethylene produced by 1 /MAO decrease gradually with increasing the catalyst concentration or polymerization temperature. The most important feature of this catalytic system is the molecular weight distribution (MWD) of polyethylene reaching 12.4, which is higher than using common mononuclear metallocenes, as well as asymmetric dinuclear titanocene complexes like [(CpTiCl₂)₂(η⁵‐η⁵‐C₉H₆(CH₂)_nC₅H₄)] (n = 3, MWD = 7.31; n = 4, MWD = 6.91). The melting point of polyethylene is higher than 135°C, indicating highly linear and highly crystalline polymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis of Poly(propargyl esters) with Rhodium Catalysts and Their Characterization

Summary Propargyl esters (HC≡CCH₂OC(=O)R; 1: R = n-C₅H₁₁, 2: R = CH₃, 3: R = CHBrCH₃, 4: R = C₆H₅, 5: R = C(C₆H₅)₃) were polymerized by using (nbd)Rh⁺(η⁶-Ph-B^-Ph₃) (nbd = 2,5-norbornadiene) to produce poly(1)–poly(5) with molecular weights in the range of M_n = 4,900–40,000. Poly(1), poly(3) and poly(4) were readily soluble in common organic solvents such as toluene, THF and CHCl₃, and poly(2) showed similar solubility behavior except that it was insoluble in THF. Poly(5) did not dissolve in any organic solvent. Poly(1) was yellow oil, while poly(2)–poly(5) were yellow solids. Poly(1)–poly(4) exhibited UV-vis absorptions in a range of 300–425 nm, which are attributed to the conjugation of the main chain. All the polymers were thermally stable up to 150–200 °C.     

Synthesis of atactic polypropylene: Propylene polymerization reactions with TiCl4–Al(C2H5)2Cl/Mg(C4H9)2 catalyst

The Ziegler catalyst, a combination of TiCl₄ and Al(C₂H₅)₂Cl, is a very poor catalyst for propylene polymerization. However, the addition of Mg(C₄H₉)₂ to the Ziegler catalyst at a [Mg]:[Al] molar ratio < 0.5 leads to the formation of a potent catalyst system for homopolymerization of propylene. Polymers produced with the TiCl₄–Al(C₂H₅)₂Cl/Mg(C₄H₉)₂ catalyst are mostly atactic and amorphous, their minor partially crystalline isotactic components constitute merely 5–10% of the combined polymer material. Principal advantages of the TiCl₄–Al(C₂H₅)₂Cl/Mg(C₄H₉)₂ catalyst for the manufacture of atactic PP include high activity, low cost, and the ease of use: the catalyst is prepared in situ from three commercially available components. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47692.     

Studies of a Supported Titanium Catalyst System Using Magnesium Dichloride–Alcohol Adduct

Magnesium dichloride reacts with aliphatic alcohols [ROH; R = n-C₂H₅, n-C₃H₇, i-C₃H₇, n-C₄H₉, i-C₄H₉, t-C₄H₉, n-C₅H₁₁, n-C₆H₁₃, C₆H₁₂(C₂H₅)] to form well-defined solid adducts. Compositional analysis of adducts indicates that the stoichiometric ratio of magnesium dichloride to alcohol depends on length of alkyl group and nature of isomeric alcohol. Magnesium dichloride-2-ethyl-l-hexanol adduct was treated with diphenyldichlorosilane in the presence of dibutylphthalate to obtain active magnesium dichloride support. The titanation process of active magnesium dichloride gives supported magnesium–titanium catalyst (Mg–Ti). The catalyst was characterized by compositional analysis and specific surface area measurements. Performance of the catalyst for polymerization of propene was evaluated with triethylaluminum (TEAL) and phenyltriethoxysilane (PES) as cocatalyst. The yield and isotacticity of the polymer is governed by polymerization parameters such as Si/A1 ratio and polymerization time.     

Temperature effect on ethylene polymerization with a catalyst prepared by mixing Mg(C2H5)(n-C4H9), Al(C2H5)1.5Cl1.5 and iron(II)bis (imino)pyridyl complex

Summary Ethylene polymerization was conducted with a catalyst prepared by mixing 2,6-bis{1-[2,6-(diisopropylphenyl)imino]ethyl}pyridine iron dichloride, Mg(C₂H₅)(n-C₄H₉) and Al(C₂H₅)_1.5Cl_1.5 in the presence of common alkylaluminium as cocatalyst. Both the activity and the molecular weight of polymers produced were markedly dependent upon the polymerization temperature. The end-group analysis of polymers showed that the molecular weight of polymers produced at higher temperature was reduced by chain transfer with Al(C₂H₅)₃ in addition to β-hydrogen elimination.     

Comparison of the Composition and Structural Parameters of W/O Microemulsions Containing Gemini Imidazoliums with Those Containing Monomeric Analogues

The composition and structural parameters of W/O microemulsions containing the gemini surfactant 1,4‐bis(3‐alkylimidazolium‐1‐yl) butane bromide [(C_n‐4‐C_n)Br₂, n = 12, 14, 16] + pentan‐1‐ol + octane + water and W/O microemulsions containing the ionic liquid surfactant 1‐alkyl‐3‐methylimidazolium (C_nmimBr, n = 12, 14, 16) + pentan‐1‐ol + octane + water were studied and compared. The mole fractions of the n‐alkyl alcohol at the interfacial layer in (C_n‐4‐C_n)Br₂ based microemulsion systems are always larger than those in C_nmimBr based microemulsion systems. However, the mole fractions of the n‐alkyl alcohol in the oil phase are nearly the same for both the microemulsion systems. The (C_n‐4‐C_n)Br₂ based microemulsion systems have greater absolute values of the free enthalpy values than that for C_nmimBr based systems. In the (C_n‐4‐C_n)Br₂ based microemulsion systems, a large number of cosurfactants at the interfacial layer is conducive to the formation of a smaller droplet W/O microemulsion. The effects of n‐alkyl alcohols, alkanes, salinity and temperature on the composition and structural parameters of the (C_n‐4‐C_n)Br₂ based and C_nmimBr based microemulsion systems were also investigated and discussed.     

Syntheses and Characterization of 1D, 2D and 3D Organotin Polymers with Benzimidazole-5,6-dicarboxylic Acid and Trimethyltin Chloride

Three types of di- and trimethyltin(IV) polymers [Me₂Sn(C₉H₄N₂O₄)]_n · 4H₂O 1, [(Me₃Sn)₂(C₉H₄N₂O₄)]_n · H₂O 2 and [(Me₃Sn)₂(C₉H₄N₂O₄)]_n · CH₃OH 3 have been synthesized by the reaction of trimethyltin chloride with benzimidazole-5,6-dicarboxylic acid under three different experimental conditions. All the complexes were characterized by elemental analysis, IR, NMR (¹H, ¹³C, ¹¹⁹Sn) spectroscopy and X-ray crystallography diffraction analysis. The structure analyses reveal that complex 1 has a 1D helical chain in which benzimidazole-5,6-dicarboxylic acid act as a tetradentate (O,O-chelation) ligand coordinating to dimethyltin (IV) ions, two water molecules take part in the coordination giving seven-coordinated tin centers in the component. Complex 2 and 3 are 2D and 3D corrugated polymers in which the deprotoned acid as tetradentate ligand affords by three oxygen atoms and a nitrogen atom.