Dinuclear Cobalt Research Articles

New cobalt-mediated radical polymerization (CMRP) of methyl methacrylate initiated by two single-component dinuclear β-diketone cobalt (II) catalysts.

Two dinuclear cobalt complexes based on bis-diketonate ligands (ligand 1: 3,3′-(1,3-phenylene)bis(1-phenylpropane-1,3-dione); ligand 2: 3,3′-(1,4-phenylene)bis(1-phenylpropane-1,3-dione)) were successfully synthesized. The two neutral catalysts all showed satisfactory activities in the cobalt-mediated radical polymerization (CMRP) of methyl methacrylate (MMA) with the common initiator of azodiisobutyronitrile (AIBN). The resulting polymerizations have all of the characteristics of a living polymerization and displayed linear semilogarithmic kinetic plots, a linear correlation between the number-average molecular weight and the monomer conversion, and low polydispersities. Mono- or dicomponent low polydispersity polymers could be obtained by using the two dinuclear catalysts under proper reaction conditions. All these improvements facilitate the implementation of the acrylate CMRP and open the door to the scale-up of the syntheses and applications of the multicomponent low polydispersity polymers.

Unexpected Magnetic Behavior of Dinuclear Co(II) Complex in Dithiocarbamates: A Possibility of Intramolecular Exchange Interactions

This study mainly interested in the magnetism of tetrahedrally coordinated dinuclear cobalt (II) complexes. Using the observed room temperature magnetic measurements suggested that the predicted magnetic behavior has to be tested with additional experimental measurements in order to testify the validity of the UV measurements. Using the theoretical background for magnetic behavior of Co(II) ion in a tetrahedral coordination, it is strongly recommended that the proposed structure of the complex has to be tested with additional characterization.

Structure and magnetism of dinuclear cobalt complexes containing furopyridine derivatives

Structure and magnetism of dinuclear cobalt complexes containing furopyridine derivatives

Synthesis, structures and properties of cobalt(II) and copper(I) complexes of 2,6-diamino-3-[(2-carboxymethyl)phenylazo]-pyridine

The reaction of methyl anthranilate, sodium nitrite, and 2,6-diamino-pyridine affords a new compound, 2,6-diamino-3-[(2-carboxymethyl) phenylazo]-pyridine (L), which has been characterized with X-ray crystallography and NMR spectrum. The reaction of L and CoCl2⋅ 6H2O or Cu(ClO4)2⋅ 6H2O gives a dinuclear cobalt(II) complex [Co2L2Cl3]Cl (1), and a copper(I) complex [CuL2]ClO4 (2)，respectively. They are characterized by X-ray crystallography, X-ray photoelectron spectroscopy, NMR spectra, magnetic measurement and emission spectra. Magnetic behavior of 1 denotes the occurrence of intramolecular antiferromagnetic interactions (J = − 141.8 cm− 1). Complex 2 exhibits photoluminescence at 557 nm.

Synthesis, characterization, and electrocatalysis of a dinuclear cobalt(III) thioxanthate complex

This study reports on the synthesis, characterization, and performance of a new dinuclear cobalt(III) thioxanthate complex of [Co2(μ-SC2H4OH)2(HOC2H4SCS2)4] as an electrocatalyst for trichloroacetic acid (TCA) and bromate reduction. Its structure was characterized by X-ray crystallography and elemental analysis. The structure contains two different anions of 2-sulfanylethanol thioxanthate and 2-sulfanylethanol. The electrochemical behavior and the electrocatalysis of the cobalt complex bulk-modified carbon paste electrode have been studied by cyclic voltammetry. It shows good electrocatalytic activities toward the reduction of TCA and bromate. The values for the detection limit and the sensitivity are 0.06 µmol L−1 and 19.40 µA µmol L−1 for TCA detection and 0.01 µmol L−1 and 177.6 µA µmol L−1 for bromate detection, respectively. This modified electrode exhibits good reproducibility, high stability, low detection limit and technical simplicity, and allows a possibility for rapid preparation, which is important for practical applications.

Hydrothermal Syntheses and Crystal Structures of Two New Three‐dimensional Coordination Polymers Based on 4‐Pyrid‐3‐ylbenzoic Acid

AbstractTwo new coordination polymers [Co2(pbc)4(H2O)]n (1) and [Mn(pbc)2] (2) (Hpbc = 4‐pyrid‐3‐ylbenzoic acid) were obtained by hydrothermal reaction and characterized by elemental analysis, IR spectroscopy, and single‐crystal X‐ray diffraction. Compound 1 features a 3D network with a four‐connected 66 net constructed from secondary building units of dinuclear cobalt. Compound 2 exhibits a six‐connected 412·63 topology based on dinuclear manganese.

Stabilization of the cobalt coordination site in transmetalation processes on dinuclear salen derivatives

Abstract Two dinuclear cobalt/copper compounds have been isolated from the reaction between N,N′-ethylenebis(salicylideniminato)cobalt(II), [Co(salen)], and copper(II) chloride in different conditions. The first one is a dinuclear cobalt(III)/copper(II) derivative, [Co(salen)Cl2Cu(EtOH)2Cl], 1, that have the cobalt atom six-coordinated to the four donor atoms of the salen ligand and to two chlorine atoms in a slightly distorted octahedral environment and the copper atom five-coordinated to the two bridging oxygen atoms of the salen ligand, two ethanol molecules and one extra chlorine atom. This compound is the only reported example of a cobalt/copper derivative with the cobalt maintaining the salen coordinative site, since the usual reaction takes place by a transmetalation process. This reaction is observed in the second derivative, [Cu(salen)CoCl2], 2, where the copper atom displaces the cobalt from the salen cavity. The copper atom adopts a square-planar coordinative environment, while the cobalt is tetrahedrically coordinated to the two bridging oxygen and two chlorine atoms. Both compounds present several intermolecular contacts that increase the dimensionality in the crystal and some of which can transmit magnetic interactions. The magnetic properties confirm the structural picture, with isolated copper(II) centres in 1, where the cobalt(III) is in the low spin form, and with antiferromagnetically coupled S = 1/2 and S = 3/2 centres in 2.

In vitro bacteriostatic and DNA interaction studies of drug-based mixed-ligand complexes of cobalt(II)

The dinuclear cobalt(II) complexes with ciprofloxacin and bidentate ligands were synthesized and characterized using infrared spectra, electronic spectra, magnetic measurements, elemental analyses, thermal investigation, and mass spectroscopy. Here in we tried to increase an antibacterial activity of ciprofloxacin drug due to formation of mixed-ligand complexes. Synthesized compounds were found to be more potent compared to drugs, ligands, and metal salt against selective gram(+ve) and gram(−ve) organisms. Interaction of the complexes with nucleic acid (DNA) was investigated using spectroscopic technique, viscosity measurement, and gel electrophoresis and it was found that the complexes bind to DNA via intercalative mode.

Structural, MALDI-TOF-MS, Magnetic and Spectroscopic Studies of New Dinuclear Copper(ii), Cobalt(ii) and Zinc(ii) Complexes Containing a Biomimicking μ-OH bridge

The Py(2)N(4)S(2) octadentate coordinating ligand afforded dinuclear cobalt, copper and zinc complexes and the corresponding mixed metal compounds. The overall geometry and bonding modes have been deduced on the basis of elemental analysis data, MALDI-TOF-MS, IR, UV-vis and EPR spectroscopies, single-crystal X-Ray diffraction, conductivity and magnetic susceptibility measurements. In the copper and zinc complexes, a μ-hydroxo bridge links the two metal ions. In both cases, the coordination geometry is distorted octahedral. Magnetic and EPR data reveal weakly antiferromagnetic high spin Co(II) ions, compatible with a dinuclear structure. The magnetic characterization of the dinuclear Cu(II) compound indicates a ferromagnetically coupled dimer with weak antiferromagnetic intermolecular interactions. The intra-dimer ferromagnetic behaviour was unexpected for a Cu(II) dimer with such μ-hydroxo bridging topology. We discuss the influence on the magnetic properties of non-covalent interactions between the bridging moiety and the lattice free water molecules.

Synthesis and structural and magnetic characterisation of cobalt(ii) complexes of mixed phosphonate-antimonate ligands

The polynucleating oxygen donor ligands, [(SbAr)(4)O(2)(PhPO(3)H)(4)(PhPO(3))(4)] 1 and [(SbAr)(2)O(HO(3)P(t)Bu)(6)] 2, based on condensation of p-chlorophenylstibonic acid (ArSbO(3)H(2)) and phosphonic acids, were used to prepare polymetallic cobalt(ii) complexes. Reaction of 1 with cobalt acetate under solvothermal conditions produces three different types of polymetallic cobalt complexes. With LiOMe/pyridine as base in MeOH a dinuclear cobalt cage, [Co(2)(SbAr)(4)O(4)(O(3)PPh)(4)(OMe)(4)py(2)] 3, is formed, with four Sb(ii) and two Co(ii) centres bridged by μ(3)-oxides and phosphonates. The pyridine in the structure can be replaced by 3-picoline 4, 4-picoline 5, quinoline 6, 1,2-diazole 7, 4-phenylpyridine 8, 4-ethyl pyridine 9 and methanol 10. Using pyrazine (pyz) instead of pyridine under the same conditions a polymer, [Co(2)(SbAr)(4)O(4)(O(3)PPh)(4)(OMe)(4)(C(4)H(4)N(2))](n)11, is formed. A similar 1D-polymer forms with 4,4'-bipyridyl-ethylene 12 in place of pyrazine. With Et(3)N/pyridine as base in MeCN a tetranuclear cobalt cage, [Co(4)(SbAr)(5)O(9)(O(3)PPh)(6)(py)(4)] 13, with five Sb(ii) centres forming a "bowtie" and bridging to four Co(ii) centres by phosphonates and μ(3)-oxides, is formed. The reaction of 2 with cobalt acetate using LiOMe/pyridine as base in methanol, under solvothermal conditions, produces a dinuclear Co(ii) complex, [Co(2)(SbAr)(2)(O(3)P(t)Bu)(3)O(2)(OMe)(2)(py)(2)] 14, with two Co(II) and two Sb centres at the vertices of a distorted tetrahedron. Magnetic measurements on selected number of these cobalt cages are reported.

PH-Sensitive mesoporous zirconium diphosphonates for controllable colon-targeted delivery

pH-Sensitive mesoporous zirconium diphosphonates have been successfully synthesized through the surfactant-assisted procedure using the organophosphonate derivative of piperazine, 1,4-bis(phosphomethyl)piperazine (BPMP), and ZrCl4 as the metal precursor. The pH-sensitivity was derived from the reversible protonation–deprotonation of piperazine groups integrated in the mesoporous walls under different pH values, which endows mesoporous zirconium diphosphonates with reversible cationic–neutral surface properties. These pH-sensitive materials can effectively adsorb PDS (dinuclear cobalt phthalocyanine ammonium sulfonate, a photosensitizer of sulfonated phthalocyanine for photodynamic therapy of tumors) through strong electrostatic interaction. The PDS-loaded samples exhibit pH-dependent release behaviors for PDS in physiological buffer solutions as evidenced by the fact that almost no PDS release can be observed in simulated gastric fluid (pH 1.2) and a sustained release of PDS with a slower initial release rate was triggered with the electrostatic interaction disappearing due to the deprotonation of piperazine groups in the mesoporous walls under the weakly basic medium of simulated intestinal fluid (pH 7.5). The pH-sensitive mesoporous zirconium diphosphonates as smart carriers for pH-controllable release of anionic photosensitizer can be used to develop an oral colon-targeted drug delivery system in which the drug molecules will be trapped without release when passing through the stomach, and be released completely in a sustained-release method in the colon followed by a minimal drug release in the small intestine.

Solvothermal in situ synthesis of (Schiff-base)-containing dinuclear cobalt compound

Abstract A new dinuclear cobalt compound, namely Co2(L)(H2O)Cl2 (1, H2L = N,N′-o-phenylenebis(salicylide-neimine) was obtained by one-pot solvothermal self-assembly of CoCl2, 1,2-phenylenediamine, and salicylaldehyde in C2H5OH. The magnetic studies suggest weak antiferromagnetic behavior and the magnetic data were interpreted by means of a dinuclear cobalt model with the parameters of g = 2.12, J = −1.25 cm−1, θ = −3.12 K.

Dinuclear Manganese and Cobalt Complexes with Cyclic Polyoxovanadate Ligands: Synthesis and Characterization of [Mn2V10O30]6– and [Co2(H2O)2V10O30]6–

AbstractAn all‐inorganic complex, [Mn2{(VO3)5}2]6– (1), was synthesized, and the structure determination reveals a dinuclear manganese complex coordinated by two cyclic pentavanadate ligands. The cyclic pentavanadate units sandwich the edge‐sharing octahedral dimanganese core through coordination of the oxido group of the pentavanadate. A dinuclear cobalt complex with a cyclic decavanadate, [Co2(OH2)2(VO3)10]6– (2), was also synthesized. The structure analysis reveals a dinuclear cobalt complex with a macrocyclic decavanadate, which is composed of 10 VO4 units joined by the vertex sharings. The CoO6 octahedrons are edge‐shared, with each cobalt octahedron coordinated to five oxido groups from the decavanadate. The remaining site is occupied by water. The coordinated water molecules are supported with hydrogen bonds in two directions. Complex 2 in acetonitrile shows no reactivity with dioxygen even at low temperature, and the cyclic voltammogram of 2 shows no redox chemistry in acetonitrile. Complex 2 exhibits chromism by water exposure both in the solid state and in acetonitrile. Complex 2 is green–yellow in color, and the addition of water causes the complex to turn brown. After heating the sample, it returns to its original color in a reversible manner. The EXAFS data in acetonitrile is also reported and is consistent with the solid‐state structure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Mechanism of the cobalt-catalyzed carbonylation of ethyl diazoacetate

Carbonyl cobalt complexes serve as catalysts or catalyst precursors for the facile and selective transformation of primary diazoalkanes into the corresponding ketene. The mechanism of this carbonylation reaction has been elucidated in the case of ethyl diazoacetate as model diazoalkane using octacarbonyl dicobalt as the catalyst precursor. Dinuclear cobalt complexes having ethoxycarbonylcarbene ligand(s) in bridging position(s) have been identified as active intermediary of the catalytic cycles and their relevant chemical properties have been explored. Key step of the carbonylation is the formation of the highly reactive ethoxycarbonylketene by intramolecular coupling of a carbonyl ligand with the ethoxycarbonylcarbene ligand. DFT calculations reveal that the ketene formation takes place via a rapid coupling of the carbene ligand with one terminal CO followed by coordination of an external carbon monoxide and by a facile intramolecular rearrangement and ketene elimination. The ethoxycarbonylketene can be in situ trapped by OH, NH, or CH acid compounds or by N-substituted imines. In the presence of ethanol diethyl malonate is the only product of the catalytic carbonylation of ethyl diazoacetate. On the bases of the kinetics of the composing steps of the catalytic cycles, localization of the rate-determining step(s) under various reaction conditions has been made.

Synthesis, structures and electrochemical properties of two novel metal–organic coordination complexes based on trimesic acid (H 3BTC) and 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (BPO)

Two novel metal–organic coordination complexes [Cu(HBTC)(BPO)]·H 2O ( 1) and [Co 3(BTC) 2(BPO) 3(H 2O) 2]·5.25H 2O ( 2), have been synthesized from hydrothermal reaction of metal chloride with the mixed ligands 1,3,5-benzenetricarboxylate (H 3BTC) and bent dipyridyl based ligand 2,5-bis(3-pyridyl)-1,3,4-oxadiazole (BPO), and structurally characterized by elemental analyses, IR, TG and single-crystal X-ray diffraction analysis. The results reveal that each dinuclear Cu II unit is bridged by two kinds of different ligands (H 3BTC and BPO) to form one-dimensional (1-D) chain structure in complex 1. The adjacent chains for 1 are further linked by π–π stacking interactions and hydrogen bonding interactions to form a three-dimensional (3-D) supramolecular framework. Complex 2 possesses a 3-D network composed of three different cobalt(II) centers [carboxylate-bridged dinuclear cobalt units and mononuclear cobalt ion] and bridging ligands BTC and BPO, which presents the first example of 3-D coordination polymer constructed from the BPO ligands simultaneously showing three different coordination modes. Moreover, the electrochemical behaviors of the two complexes bulk-modified carbon paste electrodes ( 1-CPE and 2-CPE) have been reported.

Mechanism and Transition State Structure of Aryl Methylphosphonate Esters Doubly Coordinated to a Dinuclear Cobalt(III) Center

Reactivities of five phosphonate esters each coordinated to a dinuclear Co(III) complex were investigated ([Co(2)(tacn)(2)(OH)(2){O(2)P(Me)OAr}](3+); tacn = 1,4,7-triazacyclononane; substituent = m-F, p-NO(2) (1a); p-NO(2) (1b); m-NO(2) (1c); p-Cl (1d); unsubstituted (1e)). Hydrolysis of the phosphonate esters in 1a to 1e is specific base catalyzed and takes place by intramolecular oxide attack on the bridging phosphonate. These data define a Brønsted beta(lg) of -1.12, considerably more negative than that of the hydrolysis of the uncomplexed phosphonates (-0.69). For 1b, the kinetic isotope effects in the leaving group are (18)k(lg) = 1.0228 and (15)k = 1.0014, at the nonbridging phosphoryl oxygens (18)k(nonbridge) = 0.9954, and at the nucleophilic oxygen(18)k(nuc) = 1.0105. The KIEs and the beta(lg) data point to a transition state for the alkaline hydrolysis of 1b that is similar to that of a phosphate monoester complex with the same leaving group, rather than the isoelectronic diester complex. The data from these model systems parallel the observation that in protein phosphatase-1, which has an active site that resembles the structures of these complexes, the catalyzed hydrolysis of aryl methylphosphonates and aryl phosphates are much more similar to one another than the uncomplexed hydrolysis reactions of the two substrates.

Synthesis and structural characterization of a novel peroxo bridged dinuclear cobalt(III) complex of succinimide showing three varieties of hydrogen bonding interactions

The reaction of aqueous cobaltous nitrate hexahydrate with the anion of succinimide (sucH) in the presence of excess ethylenediamine (en) in air results in the formation of a dinuclear complex µ-peroxo-bis[bis(ethylenediamine)succinimidato-cobalt(III)] dinitrate dihydrate, 1, in good yield. Compound 1 was characterized by elemental analysis, IR, visible spectra and magnetic susceptibility studies. The explosive nature of [Co(en)2(suc)(µ-O2)Co(en)2(suc)](NO3)2·2H2O, 1, precluded its thermal characterization. Compound 1 crystallises in the monoclinic space group P21/c and a half of the molecule, constitutes its asymmetric unit. In the centrosymmetric dinuclear complex 1, two Co(III) centres are linked by a planar peroxide bridge. Each cobalt atom is surrounded by four nitrogen atoms of ethylenediamine ligands, a nitrogen atom of succinimidato anion and an oxygen atom of peroxo bridge resulting in a slightly distorted {CoN5O} octahedron. Due to steric hindrance between the two Co(III) centres, the peroxide bridge is planar with a Co-O-O-Co torsion angle of 180°. The dinuclear complex cation, the nitrate anion and the lattice water are involved in three varieties of H-bonding interactions namely N-H⋯O, O-H⋯O and C-H⋯O.

Observation of Redox-Induced Electron Transfer and Spin Crossover for Dinuclear Cobalt and Iron Complexes with the 2,5-Di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate Bridging Ligand

Dinuclear [(TPyA)M(II)(DBQ(2-))M(II)(TPyA)](BF(4))(2) [TPyA = tris(2-pyridylmethyl)amine; DBQ(2-) = 2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate; M = Co (1(2+)), Fe (2(2+)), Ni (3(2+))] complexes have been prepared by the reaction of M(2+), TPyA, H(2)DBQ, and triethylamine in MeOH solution. Their monooxidized form [(TPyA)M(III)(DBQ(*3-))M(III)(TPyA)](3+) [Co = (1(3+)), Fe (2(3+))] has been synthesized by using ferrocenium tetrafluoroborate, and the dioxidized form of 1(2+), [(TPyA)Co(III)(DBQ(2-))Co(III)(TPyA)](4+) (1(4+)), has been obtained by using thianthrinium tetrafluoroborate. These dinuclear compounds were characterized by X-ray crystallography, electrochemistry, magnetism, and EPR spectroscopy. Valence ambiguous 1(3+) forms via redox-induced electron transfer, whereby the one-electron oxidation of the [Co(II)(DBQ(2-))Co(II)](2+) core forms [Co(III)(DBQ(*3-))Co(III)](3+), and it also exhibits spin crossover behavior to the core [Co(III)(DBQ(2-))Co(II)](3+) above room temperature. The M ions in 1 and 2 have a distorted octahedral geometry by coordination with four nitrogens of a TPyA, two oxygens of a DBQ(2-/*3-). Due to the interdimer offset face-to-face pi-pi and/or herringbone interactions, 1(2+), 1(3+), and 2(2+) show extended 1-D and/or 2-D supramolecular structures. The existence of DBQ(*3-) in 1(3+) is confirmed from both solid-state magnetic and solution EPR data. Co- and Ni-based 1(2+) and 3(2+) show weak antiferromagnetic interactions [1(2+): g = 2.44, J/k(B) = -3.20 K (-2.22 cm(-1)); 3(2+): g = 2.13, J/k(B) = -3.22 K (-2.24 cm(-1)), H = -2JS(1)*S(2) for 1(2+) and 3(2+)], while Fe-based 2(2+) exhibits strong spin crossover behavior above room temperature. 1(2+) has three reversible one-electron transfer waves at E(1/2) (vs SCE in MeCN) = -1.121, 0.007, and 0.329 V, and a fourth wave at -1.741 V that exhibits a slight chemical irreversibility. The first three correspond to [Co(II)DBQ(2-)Co(II)](2+) reduction to [Co(II)DBQ(*3-)Co(II)](+), and oxidation to [Co(III)DBQ(*3-)Co(III)](3+) and [Co(III)DBQ(2-)Co(III)](4+), respectively. The mechanism of the multielectron transfer oxidation from [Co(II)DBQ(2-)Co(II)](2+) to [Co(III)DBQ(*3-)Co(III)](3+) is unknown; the energy of stabilization for oxidizing the Co(II) centers in the presence of DBQ(*3-), relative to oxidizing the Co(II) centers in the presence of DBQ(2-) is computed to be 1.45 eV. 2(2+) also has three reversible one-electron transfer waves at 0.802, 0.281, and -1.007 V that correspond to two successive one-electron oxidations (2(2+)/2(3+) and 2(3+)/2(4+)), and a one-electron reduction (2(2+)/2(+)). 2(2+) has the [Fe(hs)(II)(DBQ(2-))Fe(hs)(II)](2+) electronic structure that becomes [Fe(hs)(III)(DBQ(*3-))Fe(hs)(III)](3+) upon oxidation. The latter undergoes spin crossover above room temperature to populate the [Fe(hs)(III)(DBQ(2-))Fe(hs)(II)](3+) excited state.

ChemInform Abstract: Self‐Assembled Dinuclear Cobalt(II)‐Salen Catalyst Through Hydrogen‐Bonding and Its Application to Enantioselective Nitro‐Aldol (Henry) Reaction.

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X-ray structure and spectroscopic characterization of divalent dinuclear cobalt complexes containing carboxylate- and phosphodiester- auxiliary bridges

Two dinuclear spin-coupled divalent cobalt complexes, [Co 2(P1-O)(μ 2-OAc)](ClO 4) 2, ( 1) and [Co 2(P1-O)(μ 2-BNPP)](ClO 4) 2, ( 2) containing μ-1,3 acetate (OAc) and bis(4-nitrophenyl)phosphate (BNPP) auxiliary bridges, respectively, were synthesized by the reaction of a classic dinucleating ligand, P1–OH with cobalt(II) perchlorate in presence of acetic acid/bis(4-nitrophenyl)phosphate. They were characterized by single crystal X-ray diffraction, to show a trigonal bipyramidal geometry around each cobalt center and the intervening bridging atoms that are responsible for spin-transfer between the two divalent cobalt centers; the alkoxo oxygen donor occupies an equatorial position, and the auxiliary ligand oxygens (OAc/BNPP) occupy the axial positions. Solution state magnetic moment measurement together with UV–Vis/NIR spectra revealed a high-spin ground state ( S = 3/2) for Co(II) in these compounds. Complexes 1 and 2 show interesting 1H NMR spectral features of resonances with relatively narrower linewidths in conjunction with a sizable chemical shift dispersion of −5 and 265 ppm. Complex 2 containing the bis(4-nitrophenyl)phosphate auxiliary bridge showed narrower spectral window than complex 1 that has the acetate auxiliary bridge.