Tripodal Hexadentate Ligand Research Articles

Self-Photosensitizing Dinuclear Ruthenium Catalyst for CO2 Reduction to CO.

The promise of artificial photosynthesis to solve environmental and energy issues such as global warming and the depletion of fossil fuels has inspired intensive research into photocatalytic systems for CO2 reduction to produce value-added chemicals such as CO and CH3OH. Among the photocatalytic systems for CO2 reduction, self-photosensitizing catalysts, bearing the functions of both photosensitization and catalysis, have attracted considerable attention recently, as such catalysts do not depend on the efficiency of electron transfer from the photosensitizer to the catalyst. Here, we have synthesized and characterized a dinuclear RuII complex bearing two molecules of a tripodal hexadentate ligand as chelating and linking ligands by X-ray crystallography to establish the structure explicitly and have used various spectroscopic and electrochemical methods to elucidate the photoredox characteristics. The dinuclear complex has been revealed to act as a self-photosensitizing catalyst, which acts not only as a photosensitizer but also as a catalyst for CO2 reduction. The dinuclear RuII complex is highly durable and performs efficient and selective CO2 reduction to produce CO with a turnover number of 2400 for 26 h. The quantum yield of the CO formation is also very high─19.7%─and the catalysis is efficient, even at a low concentration (∼1.5%) of CO2.

Interconversion of tetrahedral [(M)4∩{Mg4(L1)6}]/[In4(L2)4] and cyclic [In6Cl6(L3)6]. Enantiotopization of diastereotopic protons monitored by means of VT 1H NMR spectroscopy

AbstractIn this review we report on the enantiomerization of tetrahedral racemic‐[(NH4)4∩{Mg4(L1)6}] monitored by means of variable temperature (VT) 1H NMR spectroscopy. The enantiotopization of the diastereotopic protons is due to simultaneous Bailar twists at the four octahedrally coordinated magnesium centres, synchronized with atropisomerization of the six edge‐bridging ligands (L1)2‐.The VT 1H NMR spectrum of tetrahedral meso‐[In4(L2)4] is coined through the helicity of the four face‐centred tripodal hexadentate ligands (L2)3‐ and can, alike the 1H NMR of cyclic meso‐[In6Cl6(L3)6], be explained by the enantiotopization of the diastereotopic protons via mesomerization.

Trigonal Prismatic Cobalt(II) Single-Ion Magnets: Manipulating the Magnetic Relaxation Through Symmetry Control.

Two mononuclear trigonal prismatic Co(II) complexes [Co(tppm*)][BPh4]2 (1) and [Co(hpy)][BPh4]2·3CH2Cl2 (2) (tppm* = 6,6',6″-(methoxymethanetriyl)tris(2-(1H-pyrazol-1-yl)pyridine; hpy = tris(2,2'-bipyrid-6-yl)methanol) were synthesized by incorporating the Co(II) ions in two pocketing tripodal hexadentate ligands. Magnetic studies indicate similar uniaxial magnetic anisotropy while having distinct dynamic magnetic properties for two complexes, of which 1 exhibits clear hysteresis loops and Orbach process governed magnetic relaxation with an effective energy barrier (Ueff) of 192 cm-1, among the best examples in transition metallic SIMs, about 10 times larger than that of 2 (Ueff = 20 cm-1, extracted by fitting the data to an Orbach relaxation process but there is no real state at this energy). Such pronounced difference is ascribed to the dominant Raman process and quantum tunneling of magnetization (QTM) in 2 owing to the structural distortion and symmetry breaking, indicated by a nearly perfect trigonal prismatic geometry (D3 local symmetry) for 1 and a more distorted configuration for 2 (C3 local symmetry). Ab initio calculations predict strong axial anisotropy for 1 with minimal QTM probability, with the transverse component of anisotropy being estimated to be much higher for 2 than 1, leading to a 10-fold lower Ueff value than 1.

Construction and Magnetic Study of a Trigonal-Prismatic Cobalt(II) Single-Ion Magnet.

A new tripodal hexadentate ligand of tris[6-(1 H-pyrazol-1-yl)pyridin-2-yl]methanol (tppm) was synthesized and explored for constructing the trigonal-prismatic cobalt(II) complex [Co(tppm)][ClO4]2·2CH3CN·H2O (1). Magnetic study showed that 1 exhibited large uniaxial magnetic anisotropy with a zero-field-splitting parameter of -80.7 cm-1 and typical single-molecule-magnet behavior.

A novel tripodal tris-hydroxypyrimidinone sequestering agent for trivalent hard metal ions: synthesis, complexation and in vivo studies

A new tripodal hexadentate ligand, NTP(PrHPM)(3), having three hydroxypyrimidinone (HPM) chelating units attached to a nitrilotripropionic acid (NTP) has been prepared and studied in terms of thermodynamic stability of the complexes with iron, aluminium and gallium and it has been subsequently in vivo assayed for its capacity to remove hard metal ions from an animal model overloaded with (67)Ga. The anchoring of the HPM units to the NTP scaffold revealed to be an interesting alternative to the reported hexadentate tris(3-hydroxy-4-pyridinone) analogue, NTP(PrHP)(3), because the new tris-HPM ligand still keeps high chelating capacity for hard metal ions and presents better water-solubility (log P = -1.51). The in vivo studies show that NTP(PrHPM)(3) induces a faster clearance from main organs and an enhancement of overall excretion, as compared with the commercial drug, DFP, or the bidentate HPM compound (HOPY-PrN), albeit slightly lower than the tris-hydroxypyridinone analogue, NTP(PrHP)(3). The solution and in vivo results herein presented encourage further studies envisaging the potential clinical applications of hexadentate HPM derivatives as metal sequestering agents.

三脚型配位子を用いた新規金属錯体の結晶構造とさまざまな性質

Metal complexes with tripodal hexadentate ligands containing three bidentate arms have a pair of optical isomers, that is Δ form and Λ form. We have prepared the metal complexes with two types of tripodal ligands. The complexes are not only mononuclear ones but also multinuclear and self-assembled supramolecular ones. Here, we describe the structures and properties with the emphasis on chirality-related features of them.

Synthesis and Characterization of a Six-Coordinate Silicon (IV) Complex with a Tripodal Phenalenyl-Based Hexadentate Ligand

We report the preparation and solid state characterization of a silicon (IV) salt based on a new redox active tripodal hexadentate ligand [H 3L = l,l,l-Tris(l′-oxophenalenyl-9′-N-methyl) ethane]. X-ray analysis of the complex reveals a short Si-N bond distance (av 1.878(16)). The compound is shown to undergo three reversible one-electron reduction steps (without decomposition) due to the formation of radical, anion and dianion of the phenalenyl moiety.

Allosteric Binding of Alkali Metal Ions to a Pseudocryptand Formed by a C-Pivot Tripodal Ligand Containing 3-Hydroxy-2(1H)-pyridinone and Ga(III)

A novel C-pivot tripodal hexadentate ligand (3,2-HOPOHL) composed of 3-hydroxy-2(1H)-pyridinone as a bidentate ligand, the ethyleneoxy chain as a spacer, and tris(carboxyric acid) as an anchor was synthesized. 3,2-HOPOHL recognized only Na + ion, suggesting that it pre-organized a cavity due to the electrostatic interaction among the 2(1H)-pyridinone rings. UV-VIS spectroscopic analysis indicated that 3,2-HOPOHL formed a stable intramolecular 1:1 Fe(III) complex in aqueous solution. The stability constant (log K) of 3,2-HOPOHL-Fe(III) complex was estimated to be 27.6 from the competitive reaction with EDTA. 1 H-NMR titration of 3,2-HOPOHL-Ga(III) complex with Na + and K + ions in CDCl 3 -CD 3 CN indicated the formation of 1:1 complexes. The binding constants of Na + - and K + -3,2-HOPOHL-Ga(III) complexes were estimated to be 3.3×10 3 and 7.8×10 3 M - 1 , respectively, the ion selectivity of K + toward Na + being more than two-fold.

Mechanism of the Stepwise Dissociation of FeIII Complexes with Tripodal Ligands as Siderophore Models

AbstractThe acid hydrolysis and formation kinetics of 1:1 ferric complexes with several tripodal hexadentate ligands were investigated over the [H+] range of 0.02−1.0 M at 25 °C and at an ionic strength of 2.0 M (NaClO4/HClO4). These ligands are based on a tris(2‐aminoethyl)amine (TREN) spacer connected through a tris(amide) moiety to identical pendant bidentate chelating group arms such as catecholate (TRENCAMS, L3), 8‐hydroxyquinolinate (O‐TRENSOX, L0) or pyridinophenolate (TRENPYPOLS, L5). Mixed ligands with chelating arms such as catecholate and 8‐hydroxyquinolinate (TRENSOX2CAMS, L1 and TRENSOXCAMS2, L2) were also investigated. An analogue of L0 (C‐SOX, L4) with a C‐pivot scaffold instead of TREN was studied for comparison. The acid hydrolysis reaction was found to proceed in four kinetically distinguishable stages. A common mechanism in the series was followed when the ligand structure was varied. The first stage is very fast and can be attributed to the dissociation of one arm of the tripodal ligand. The second and third stages involve protonation of the complex which induces a change of coordination giving the corresponding tetracoordinated bis(salicylate) complex (coordination with carbonyl and o‐hydroxy oxygen atoms). The final dissociation of the ligand proceeds in the fourth stage by one or two kinetically detectable steps. For the last step an important structural effect may be observed from 1.2 × 10−5 to 2.2 × 10−2 s−1 for the proton‐independent step and from 1.2 × 10−6 to 0.18 M−1s−1 for the proton‐dependent step. Differences in the rates and mechanism between the different tripodal ligands are discussed in terms of the global electrostatic charge of the ligand and the donor ability of the chelating subunit. The C−N bond rotation of the amide moiety and the importance, depending on the charge, of the H‐bond network in the partially uncoordinated complexes have also been considered. The role of electrostatics in complex formation is discussed in relation to the Eigen‐Wilkins mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

A Tripodal Ligand Containing Three Imidazole Groups Inducing Spin Crossover in Both Fe(II) and Fe(III) Complexes; Structures and Spin Crossover Behaviors of the Complexes

Abstract A tripodal hexadentate ligand containing three imidazole groups gives spin-crossover iron complexes in both the +II and the +III oxidation states. The tetrafluoroborate salt of the Fe(II) complex shows a gradual spin transition in the temperature range 110–270 K. The corresponding Fe(III) complex shows a spin transition above 150 K, and at 300 K ca. 87% of the Fe(III) ion is in the high-spin state.

タンデムクライゼン転位 新規機能性フェノール誘導体の構築

The reaction of 3-chloro-2-chloromethyl-l-propene with 2 equimolar phenolic compounds gave isobutenyl bis (aryl) ether compounds. The thermal or Lewis acid-assisted Claisen rearrangement of the ether compounds results in generating phenolic compounds having plural hydroxyl groups at one step reaction. We named this reaction as “tandem Claisen rearrangement, ” where the Claisen rearrangement successively occurs two times at each isobutenyl ether moiety. This reaction can be applied not only to acyclic, but also to macrocyclic compounds having isobutenyl ether moiety. Tetradentate ligands with two quinolinol moieties, tripodal hexadentate ligands with either three quinolinol or catechol moieties, etc. have been synthesized as acyclic rearranged products. As macrocyclic rearranged products, various kinds of novel crownophane and cyclophane derivatives having plural hydroxyl groups have been synthesized. These compounds prepared via tandem Claisen rearrangement have been found to be able to work as ionophore, fluorophore, host molecules for neutral organic molecules, chiral ones, ligands for transition metal ions and so on.

Allosteric Binding of Alkali Metal Ions to a Pseudocryptand Formed by a Tripodal Heterocyclic Hexadentate Ligand and Ga(III) †

A new tripodal heterocyclic hexadentate ligand, in which three sets of 3-hydroxy-4(1 H)-pyridinone and ethyleneoxy chain are linked to tris(carboxyethoxymethyl)ethane, was synthesized. UV-Vis spectroscopic analysis in aqueous solution indicated 1:1 stoichiometric complexation of the hexadentate ligand with iron(III) ( λ max 460 nm and ε 5100 M −1 cm −1; the relative stability constant log K=32.9). Allosteric binding of alkali metal ions to a pseudocryptand formed by the tripodal hexadentate ligand and Ga(III) was examined in CDCl 3–CD 3CN (1:1) mixture by means of 1H NMR spectroscopy. The pseudocryptand formed 1:1 complexes with Na + and K + ions, and the binding constant was estimated to be almost the same ( K=5.9×10 2 for Na + and 5.8×10 2 M −1 for K +).

Di- and Trinuclear Nickel(II) Complexes Containing Tripodal Hexadentate Ligands

Abstract The reaction of Ni2+ with a tripodal hexadentate ligand (H3L = 1,1,1-tris(N-salicylideneaminomethyl)ethane) afforded the O–H···O bridged dinuclear [Ni2(HL)2] complex. When Ni2+ was allowed to react with H3L in the presence of triethylamine (3 : 2 : 6), the linear trinuclear [Ni3(L)2] complex formed, where the central nickel(II) ion is bridged by six phenolate oxygen atoms to the terminal nickel(II) ions.

Optical Resolution of a Six-Coordinate Silicon(IV) Complex with a Tripodal Hexadentate Ligand

Abstract The reaction of silicon tetraacetate with a tripodal hexadentate ligand (H3L = 1,1,1-tris(N-salicylideneaminomethyl)ethane) afforded the colorless [Si(L)]+ complex. The X-ray analysis of the complex revealed that the Si–N bond distance, av. 1.926(2) Å, is much longer than the Si–O bond distance, av. 1.723(2) Å. The complex is stable toward hydrolysis and could be optically resolved by fractional crystallization of the diastereomeric salt with (−)-dibenzoyl-l-tartaric acid.

Structural and Photophysical Properties of Mononuclear and Dinuclear Lanthanide(III) Complexes of Multidentate Podand Ligands Based on Poly(pyrazolyl)borates

Lanthanide(III) complexes have been prepared with [L1]- [the tetradentate chelating ligand bis{3-(2-pyridyl)pyrazolyl}dihydroborate], [L2]- [the tetradentate chelating ligand bis{3-(2-pyrazinyl)pyrazolyl}dihydroborate], [L3]- [the hexadentate chelating ligand bis[3-{6‘-(2,2‘-bipyridyl)}pyrazol-1-yl]dihydroborate], and [L4]2- [the 12-dentate compartmental ligand hexakis{3-(2-pyridyl)pyrazol-1-yl}diboran(IV)ate, which has two hexadentate tris(pyrazolyl)borate-based cavities linked “back-to-back” by a B−B bond]. [Ln(L1)2(NO)3] are 10-coordinate with two tetradentate N-donor ligands and one bidentate nitrate. [Ln(L2)2(NO)3] have 10-coordinate structures similar to those of the [L1]- complexes except that the coordinated N1 of the pyrazine rings is not such a good donor as the pyridine rings in the [L1]- complexes, leading to marked lengthening of these Ln−N bonds. [Ln(L3)(NO3)2] are also 10-coordinate from one hexadentate chelating ligand which has a pseudoequatorial coordination mode and two pseudoaxial bidentate nitrate ligands; the hexadentate ligand has a shallow helical twist to prevent steric interference between its ends. Finally [{Ln(NO3)2(L4)] are dinuclear, with each metal center being 10-coordinate from a tripodal hexadentate ligand cavity and two bidentate nitrates. Five complexes were structurally characterized: [Tb(L2)2(NO3)]·dmf is monoclinic (P21/c) with a = 14.881(3) Å, b = 15.5199(12) Å, c = 15.845(2) Å, β = 92.387(12)°, and Z = 4. [Gd(L2)2(NO3)]·dmf is monoclinic (P21/c) with a = 14.926(2) Å, b = 15.465(2) Å, c = 15.878(2) Å, β = 92.698(11)°, and Z = 4. [Eu(L3)(NO3)2]·dmf·0.5Et2O is triclinic (P1̄) with a = 10.020(3) Å, b = 13.036(3) Å, c = 14.740(3) Å, α = 70.114(14)°, β = 71.55(2)°, γ = 79.66(2)°, and Z = 2. [{La(NO3)(dmf)2}2(L4)](NO3)2·dmf is orthorhombic (Pbca) with a = 18.813(2) Å, b = 15.241(2) Å, c = 27.322(2), and Z = 4. [{Gd(NO3)2}2(L4)]·2.4dmf is tetragonal (P42/n) with a = 16.622(6), c = 24.19(5) Å, and Z = 4. Detailed photophysical studies have been performed on the free ligands and their complexes with Gd(III), Eu(III), and Tb(III) in several solvents. The results show a wide range in the emission properties of the complexes which can be rationalized in terms of subtle variations in the steric and electronic properties of the ligands. In particular the dinuclear Tb(III) complex of [L4]2- has an emission quantum yield of ca. 0.5 in D2O and MeOD.

ChemInform Abstract: Synthesis and Binding Properties of New Tripodal Hexadentate Ligands Having Three Quinolinol Moieties for Trivalent Metal Cations.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Synthesis and Binding Property of a Novel Tripodal Hexadentate Ligand Having Catechol Moieties.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis and binding property of a novel tripodal hexadentate ligand having catechol moieties

A novel tripodal hexadentate ligand, consisting of three catechol units, three isobutenyl ether arms, and one aromatic core, was synthesized through four steps, in which the Claisen rearrangement was included as the key step. The hexadentate ligand formed a 1:1 complex with iron(III) trichloride hexahydrate with an equilibrium constant (conditional) of 6.3 × 104 M−1 in acetonitrile in the presence of 2,4,6-trimethylpyridine.

Synthesis and binding properties of new tripodal hexadentate ligands having three quinolinol moieties for trivalent metal cations

Tripodal hexadentate ligands, having three 8-hydroxyquinolyl or 2-methyl-8-hydroxyquinolyl groups as binding sites, were successfully prepared from the corresponding tris(isobutenyl 8-quinolyl ether) via the Claisen rearrangement. The 2-methyl-8-hydroxyquinolyl derivative exhibited binding ability for several kinds of trivalent metal cations to form 1:1 complexes, and showed selective extractability for Ga3+ among trivalent cations of Group 13 elements.

機能性Ｎ‐ヒドロキシアミド含有複素環化合物‐合成・反応・金属錯体形成能

Synthesis, reaction, and metal-chelating property of N-hydroxyamide-containing heterocycles such as 1-hydroxy-2 (1H) -pyrimidinones and -pyrazinones are described. The diazines showed remarkable high water solubility and acidity, which enable them to act as an effective acylating agent for amines, amino acids, and alcohols. The diazines were also found to be an bidentate ligand to a ferric iron in aqueous solution, in particular, under acidic to neutral conditions. The five types of tripodal hexadentate ligands bearing 1-hydroxy-2 (1H) -pyrimidinone or -pyrazinone as bidentate ligands were synthesized, and effects of their structural feature and the chirality on their metal-chelating behavior were also discussed. Among them, a hexadenate ligand bearing 1-hydroxy-2 (1H) -pyrazinone, L-alanine, and tris (2-aminoethyl)amine showed high iron removal ability from human transferrin, and its pseudo-first-order-rate constant was more than five times greater than naturally-occurring siderophore, desferrioxamine B, even in small excess of the ligand concentration to transferrin.