Imidazolyl Donors Research Articles

Tris(imidazolyl) dicopper(I) complex and its reactivity to exert the catalytic oxidation of sterically hindered phenol substrates via a [Cu2O]2+ core.

The Cu ion ligated with histidine residues is a common active site motif of various Cu-containing metalloenzymes exerting versatile catalytic oxidation reactions. Due to the scarcity of structurally characterized biomimetic binuclear Cu(I)-imidazolyl complexes, the bonding interactions between the Cu(I) center and imidazolyl donor ligands as well as their stoichiometric/catalytic oxidation reactivities remain relatively unexplored. In this study, we successfully synthesized a tris(imidazolyl) dicopper(I) complex [CuI(μ-bimeta)3CuI][PF6]2 (1) characterized by single-crystal X-ray diffraction, cyclic voltammetry, and UV-vis absorption spectroscopy. The coordination environment around each Cu(I) center of complex 1 is best described as a regular trigonal geometry and the distance between two Cu(I) centers is ∼3.0521(18) Å. From the O2-/PhIO-titration reactions of complex 1, only 0.5 equiv. of O2(g) and one equiv. of PhIO are required to produce the corresponding oxygenated product complex 1ox, respectively, exhibiting a distinct UV-vis absorption band at ∼640 nm. From the characterization of ESI mass spectrometry, IR, UV-vis spectroscopy, and O2-/PhIO-titration reactions, the molecular identity of complex 1ox is tentatively assigned as [Cu(μ-O)(μ-bimeta)3Cu]2+, which is further corroborated by the spectroscopically calibrated DFT calculations. The oxidation reactivities of complex 1ox were investigated using the well-understood sterically hindered phenol substrates DTBP and TBBP in both stoichiometric and catalytic fashions. In stoichiometric reactions, one equiv. of 1ox is capable of fully converting TBBP into TBOBF in the CH3CN solution at room temperature within 1 h. The optimized selectivity of catalytically oxidizing DTBP into TBOBF (>95% selectivity, ∼100% conversion rate) could be achieved with a 10 mol% loading of complex 1ox using air as the oxidant. From the time-course product distributions of this catalyst system, ∼40% of DTBP was converted into the corresponding TBBP product in the first 5 min. From 10 to 120 min, the oxidation of DTBP to TBBP and the oxidation of TBBP to TBOBF simultaneously proceeded, which was evidenced by the decrease of TBBP and the increase TBOBF.

New fac-[ReI(CO)3(Ltri)]Y complexes of linear N,N′,N′′-tridentate ligands (Ltri) with terminal imidazolyl or pyridyl ring (N and N′′) and central tertiary sulfonamide (N′) N-donors. Effects of size and substituents of the terminal N-donor rings and the rarely found direct sulfonamide N to metal bond

fac-[Re(CO)3Ltri]Y (Y = ClO4- or PF6-) complexes were prepared with N,N',N''-donor tridentate ligands (Ltri), N(SO2R)1,1′-Me2dma and N(SO2R)Mendpa (R = Me or Tol). The two new N(SO2R)1,1′-Me2dma ligands are derivatives of the parent ligand, N(H)1,1′-Me2dma (bis(1-methyl-2-methylimidazolyl)amine), by having the central N within a tertiary sulfonamide group instead of a secondary amine group. In addition to having a tertiary sulfonamide group, the N(SO2R)Mendpa ligands differ from the parent ligand, N(H)dpa (2-picolylamine), by having Me groups at the 6,6′- (n = 2) or 3,3′,5,5′- (n = 4) positions of the pyridyl rings. Ltri is facially coordinated with a Re—N' (sulfonamide N) bond of normal length. Only the Ltri = N(SO2R)6,6′-Me2dpa complexes exhibit ligand dissociation over time in DMSO‑d6, a finding attributable to Re—N and Re—N'' bond weakening caused by the steric effect of the pyridyl 6,6′-Me groups projecting toward the coordination sphere, while the Ltri = N(SO2R)1,1′-Me2dma and N(SO2R)3,3′,5,5′-Me4dpa complexes are indefinitely robust in solution. In contrast to the similar behavior of fac-[Re(CO)3Ltri]Y complexes for Ltri = N(SO2R)1,1′-Me2dma and N(SO2R)3,3′,5,5′-Me4dpa, only the latter dissociates over time in DMSO‑d6 for [Pt(“Ltri”)Cl2] complexes (unpublished), where the sulfonamide N in “Ltri” is not bound, forming a rare 8-membered chelate ring. On the basis of NMR and X-ray crystallographic data, we attribute the high robustness of [Pt(N(SO2R)1,1'-Me2dma)Cl2] mainly to the presence of the terminal N imidazolyl donors. The smaller imidazolyl moieties for Ltri = N(SO2R)1,1'-Me2dma (relative to the pyridyl rings in the dpa ligands) make dma ligands a better platform for developing fac-[99mTc(CO)3Ltri]Y diagnostic agents and also most probably for developing Pt(II) anticancer drugs.

Iron and Silver Complexes of 4‐(Imidazol‐1‐yl)‐2,6‐di(pyrazol‐1‐yl)‐pyridine (L), Including a [Fe3(µ‐F)2F6L8]+ Assembly

Treatment of 2,4,6‐trifluoropyridine with sodium imidazolate, then 2 equiv. sodium pyrazolate, in two reaction steps yields the title ligand L in 40 % overall yield. Crystalline [FeL2][BF4]2 and [FeL2][ClO4]2 are isostructural high‐spin complexes, with highly twisted six‐coordinate geometries and pendant imidazolyl groups. Conversely, [Ag(µ‐L)]BF4·0.5MeCN is a linear coordination polymer of distorted square planar silver ions linked by κ1:κ3,µ‐L ligands. Some samples of [FeL2][BF4]2 were contaminated by [FeIII3(µ‐F)2F6L8]BF4, which was prepared in pure form following reaction of Fe[BF4]2·6H2O, Na3[FeF6] and L in appropriate ratios. The linear [{trans‐FeFL4}2(µ‐F)2FeF4]+ assembly is supported by monodentate L ligands coordinated through their imidazolyl donors, and donating C–H····F interactions to the central [FeF6]3– moiety. The cluster exhibits moderate antiferromagnetic coupling between its iron(III) centres.

Ruthenium(II) Metalla[2]catenanes and Macrocycles via Donor-Dependent Self-Assembly.

Donor-selective coordination-driven self-assembly of a bis(dipyrrin)-bridged new diruthenium acceptor (RuA) with dipyridyl and diimidazolyl donors has been reported. The self-assembly of the ruthenium(II) acceptor with imidazolyl donors resulted in the formation of either [2 + 2] self-assembled monomeric macrocycles (MMs) or a mixture of metalla[2]catenanes (MCs) and MMs depending on the solvents used. On the contrary, similar self-assembly with the pyridyl donors resulted in simple [2 + 2] macrocycles (MMs) exclusively, irrespective of the solvents used. The new ruthenium acceptor and self-assembled macrocycles were systematically characterized by multinuclear NMR and electrospray ionization mass spectrometry study. The structure of one of the metalla[2]catenanes (MC1) was further confirmed by single-crystal X-ray diffraction studies. Density functional theory calculations inferred that the interlocked structures with imidazolyl donors are stabilized by π-π interactions between the benzene rings, while such interactions cease to exist with the pyridyl linkers, leading to the formation of noninterlocked macrocycles.

Imidazole for Pyridine Substitution Leads to Enhanced Activity Under Milder Conditions in Cobalt Water Oxidation Electrocatalysis.

A previously reported cobalt complex featuring a tetraimidazolyl-substituted pyridine chelate is an active water oxidation electrocatalyst with moderate overpotential at pH 7. While this complex decomposes rapidly to a less-active species under electrocatalytic conditions, detailed electrochemical studies support the agency of an initial molecular catalyst. Cyclic voltammetry measurements confirm that the imidazolyl donors result in a more electron-rich Co center when compared with previous pyridine-based systems. The primary changes in electrocatalytic behavior of the present case are enhanced activity at lower pH and a marked dependence of catalytic activity on pH.

Supramolecular gels based on boronate esters and imidazolyl donors.

Dative boron-nitrogen bonds can be used for the controlled assembly of boronate esters and N-donor ligands. So far, most investigations have focused on crystalline assemblies. In contrast, there are very few reports about soft materials based on dative B-N bonds. Here, we describe the formation of gels by aggregation of diboronate esters and ditopic bisimidazolyl donor ligands. Analyses of low molecular weight model compounds by NMR spectroscopy, isothermal titration calorimetry, and X-ray crystallography reveal that imidazolyl ligands form particularly strong dative bonds with boronate esters. The strong interaction allows gel formation in competitive polar solvents such as THF and acetone. In less polar solvents, very low critical gel concentrations of down to 0.02 wt% are observed. By using a diboronate ester linked by an azobenzene group, it is possible to form a photoresponsive gel.

Bioinspired copper(I) complexes that exhibit monooxygenase and catechol dioxygenase activity.

New tripodal ligand L2 featuring three different pyridyl/imidazolyl-based N-donor units at a bridgehead C atom, from which one of the imidazolyl units is separated by a phenylene linker, was synthesized and investigated with regards to copper(I) complexation. The resulting complex [(L2)Cu]OTf (2(OTf)), the known complex [(L1)Cu]OTf (1(OTf); L1 differs from L2 in that it lacks the phenylene spacer) and [(L3)Cu]OTf (3(OTf)), prepared from a known chiral, tripodal, N-donor ligand featuring pyridyl, pyrazolyl, and imidazolyl donors, were tested as catalysts for the oxidation of sodium 2,4-di-tert-butylphenolate (NaDTBP) with O2. Indeed, they mediated NaDTBP oxidation to give mainly the corresponding catecholate and quinone (Q). None of the complexes 1(OTf), 2(OTf), and 3(OTf) is superior to the others, as yields were comparable and, if the presence of protons is guaranteed by concomitant addition of the phenol DTBP, the oxidation can also be performed catalytically. For all complexes stoichiometric oxidations under certain conditions (concentrated solutions, high NaDTBP content) were found to also generate products typical for metal-mediated intradiol cleavage of the catecholate with O2. As shown representatively for 1(OTf) this dioxygenation sets in at a later stage of the reaction. Initially a copper species responsible for the monooxygenation must form from 1(OTf)/NaDTBP/O2, and only thereafter is the copper species responsible for dioxygenation formed and consumes Q as substrate. Hence, under these circumstances complexes 1(OTf)-3(OTf) show both monooxygenase and catechol dioxygenase activity.

Chiral Tripodal Ligand Containing Three N-Heterocyclic Donor Functions and Its Copper Complexes: Crystallization of [LCuI]22+/[L2CuII]2+ Stereoisomers and Tyrosinase Activity

A novel chiral ligand system L containing one pyridyl and two imidazolyl donor functions has been synthesized and investigated with respect to its Cu(I) and Cu(II) coordination chemistry. Reaction with [Cu(MeCN)4]PF6 and [Cu(MeCN)4]OTf led to the dimeric complexes [LCu]2X2 (1, X = PF6; 2, X = OTf) with the ligands L in different configurations (R,S). The ligand matrix formed in these complexes can also host a Cu(II) ion instead of two Cu(I) ions so that mixed crystals of [L2Cu]X2 and [LCu]2X2 can be produced. The pure compounds [L2Cu]X2 (3, X = PF6; 4, X = OTf) can be obtained by treatment of 1 and 2 with O2 in acetonitrile, respectively. From the corresponding solution 3 crystallizes with the two L molecules in different configurations, while 4 crystallizes with the ligands in (S,S) or (R,R) configurations, respectively. Crystals containing the analogous diastereomers of 3 were obtained, besides those isolated previously, when this compound was synthesized by reaction of 1 with AgPF6. On treating 2 with O2 as the oxidant in acetonitrile, besides formation of 4, additional evidence for oxygenation of L to L(ox), where one of the original phenyl units corresponds to a phenolate function, was found: The dinuclear complex [L(ox)Cu(OH)(OTf)CuL](OTf) (5) was isolated as the final product of O2 activation and conversion, which resembles the one of tyrosinase. In acetonitrile 5 reacts further to give 4 and [L(ox)2Cu2](OTf)2 (6), and hence, product mixtures are obtained. In CH2Cl2 decomposition can be avoided, and hence, changing the solvent from acetonitrile to CH2Cl2 leads to selective formation of 5.

The synthesis and photophysical studies of cyclometalated Pt(ii) complexes with C,N,N-ligands containing imidazolyl donors

Two new C,N,N-type ligands (HL(2) and HL(3)), containing a C(phenyl), a N(pyridyl), and a N(imidazolyl) donor, and their cycloplatinated complexes, [Pt(L(2))Cl] (1), [Pt(L(3))Cl] (2), [Pt(L(2))(PPh(3))](+) (3) and [Pt(L(3))(PPh(3))](+) (4), have been successfully synthesized and characterized. Spectroscopic and (3)MLCT luminescent properties of these Pt(II) cyclometalated complexes were found to be pH dependent. This was attributed to the protonation/deprotonation of the acidic 1-imidazolyl-NH moieties on the ligands. All the cycloplatinated complexes (both protonated and deprotonated forms) possessed two-photon excitability with two-photon absorption cross-sections ranging from 6.0 to 30.0 GM (protonated forms) and from 16.2 to 24.9 GM (deprotonated forms).

Copper(I)/O2 Chemistry with Imidazole Containing Tripodal Tetradentate Ligands Leading to μ-1,2-Peroxo−Dicopper(II) Species

Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {L(MIm), (1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine, and L(EIm), 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)ethanamine}, have been investigated to probe differences in their chemistry, especially in copper(I)-dioxygen chemistry, compared to that already known for the pyridyl analogue TMPA, tris(2-pyridyl)methyl)amine. Infrared (IR) stretching frequencies obtained from carbon monoxide adducts of [(L(MIm))Cu(I)](+) (1a) and [(L(EIm))Cu(I)](+) (2a) show that the imidazolyl donor is stronger than its pyridyl analogue. Electrochemical data suggest differences in the binding constant of Cu(II) to L(EIm) compared to TMPA and L(MIm), reflecting geometric changes. Oxygenation of [(L(MIm))Cu(I)](+) (1a) in 2-methyltetrahydrofuran (MeTHF) solvent at -128 degrees C leads to an intensely purple colored species with a UV-vis spectrum characteristic of an end-on bound peroxodicopper(II) complex [{(L(MIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (1b(P)) {lambda(max) = 528 nm}, very similar to the previously well characterized complex [{(TMPA)Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) {lambda(max) = 520 nm (epsilon = 12 000 M(-1) cm(-1)), in MeTHF; resonance Raman (rR) spectroscopy: nu(O-O) = 832 (Delta((18)O(2)) = -44) cm(-1)}. In the low-temperature oxygenation of 2a, benchtop (-128 degrees C) and stopped-flow (-90 degrees C) experiments reveal the formation of an initial superoxo-Cu(II) species [(L(EIm))Cu(II)(O(2)(*-))](+) (2b(S)), lambda(max) = 431 nm in THF) . This converts to the low-temperature stable peroxo complex [{(L(EIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (2b(P)) {rR spectroscopy: nu(O-O) = 822 (Delta((18)O(2)) = -46) cm(-1)}. Complex 2b(P) possess distinctly reduced Cu-O and O-O stretching frequencies and a red-shifted UV-vis feature {to lambda(max) = 535 nm (epsilon = 11 000 M(-1) cm(-1))} compared to the TMPA analogue due to a distortion from trigonal bipyramidal (TBP) to a square pyramidal ligand field. This distortion is supported by the structural characterization of related ligand-copper(II) complexes: A stable tetramer cluster complex [(mu(2)-L(EIm-))(4)(Cu(II))(4)](4+), obtained from thermal decomposition of 2b(P) (with formation of H(2)O(2)), also exhibits a distorted square pyramidal Cu(II) ion geometry as does the copper(II) complex [(L(EIm))Cu(II)(CH(3)CN)](2+) (2c), characterized by X-ray crystallography and solution electron paramagnetic resonance (EPR) spectroscopy.

Multiple bonding modes exhibited by heteroscorpionate N 2S(alkylthiolate) ligands with Zn(II) and Fe(II)

Two related heteroscorpionate N 2S(thiolate) ligands L 1 and L 2 have been employed in the synthesis of new zinc(II) and iron(II) complexes. These ligands contain two imidazolyl donors and one alkylthiolate donor connected by a central carbon atom. The ligand L 1 was reacted with ZnBr 2 to give the trinuclear zinc complex [(L 1) 3Zn 3Br 2]Br ( 1). The X-ray structure of 1 reveals one of the zinc ions to be coordinated by two L 1 ligands in a tripodal arrangement resulting in a distorted octahedral geometry, while the other two zinc ions are both held in a distorted tetrahedral geometry and are coordinated to another molecule of L 1 in an unusual tridentate, multibridging coordination mode. Modification of L 1 to include phenyl substituents on the imidazole groups leads to L 2, and these substituents cause quite a different result to be obtained in the reaction with ZnBr 2. The combination of L 2 and ZnBr 2 leads to the dimeric zinc complex (L 2) 2Zn 2Br 2, ( 2). Characterization of 2 by X-ray diffraction shows this complex to be a dimer with a central Zn 2(μ-SR) 2 rhomb. Each zinc ion is found in a distorted tetrahedral geometry coordinated by L 2 in a bidentate, N,S fashion. The 1H NMR spectrum of 2 provides strong evidence this structure is retained in solution. In comparison, reaction of L 2 with FeCl 2 leads to the monomeric complex (L 2) 2Fe II ( 3), which has each ligand coordinated in the same bidentate, N,S bonding mode. Interestingly, the methoxy groups of the two ligands in 3 are close enough to the iron center for a possible bonding interaction, suggesting an unexpected N 2O coordination mode.

Calorimetric investigation of the complexation of metal ions by pentadentate ligands having pyridyl and imidazolyl donor groups and the oxygenation of the cobaltous complexes

Calorimetric investigation of the complexation of metal ions by pentadentate ligands having pyridyl and imidazolyl donor groups and the oxygenation of the cobaltous complexes

Coordination chemistry of methylmercury(II). Synthesis, hydrogen-1 NMR, and crystallographic studies of cationic complexes of MeHg(II) with ambidentate and polydentate ligands containing pyridyl and N-substituted imidazolyl donors and involving unusual coordination geometries

Complexes [MeHgL]N03 (L = 4,4’,4’’-triethyL2,2':6’,2”-terpyridyl (Et3terpy), terpy, bis(2-pyridy1)methanes ((py)CH2's), (py)2CMe2, (py)2CEt2, tris(2-pyridyl)methane, several N-alkylimidazoles, N-(2-pyridyl)imidazole, and N-methyl-2-(2-pyridy1)imidazoles) are obtained from addition reactions in acetone. Proton magnetic resonance spectroscopy is used to deduce coordination behavior of potential uni- or polydentate ligands in methanol, indicating that Et3terpy, terpy, (py)2CH2's, and N-methyl-2-(2-pyridyl)imidazoles act as bidentates to give three-coordinate mercury, but (py)2CR2 (R = Me, Et) are present as unidentates. A linear relation is obtained between 2J(1H-199Hg) for [MeHgL]+ and protonation constants for L, where L = N-alkylimidazoles, and the relation is similar to that obtained previously for L = pyridines. The ambidentate ligand N-(2-pyridyl)imidazole binds to mercury via the imidazole ring. Crystalline [MeHg((py)2CH2)]N03 has (py)CH2 present as a bidentate ligand with “T-shape” coordination geometry based on a dominant C-Hg-N moiety [Hg-N = 2.16(1) A, C-Hg-N = 172 (1) and a weaker Hg-N’ bond [2.75(2) A]. Crystalline [MeHg(Et3terpy)]N03 has Et3terpy present as a tridentate ligand with MeHg(II) bonded strongly to the central nitrogen [2.26(2) A, C-Hg-N = 171(1) and weakly to the terminal nitrogens [2.51(2), 2.61(2) A]. Syntheses of new ligands containing pyridyl and N-methylimidazolyl donor groups are described.

Synthesis of potential platinum(II) anti-tumor complexes: Complexes containing bidentate pyridyl and imidazolyl donors

The synthesis of complexes PtCl2(L2) of some bidentate ligands (L2) containing pyridyl rings connected by a carbon atom, di-2-pyridylmethane (py2CH2) and py2SEt2, and containing both pyridyl and imidazolyl rings, (2,2’-pyridyl)imidazole (pyIm) and py-NMeIm. Preliminary screening of the complexes, including the 2,2’-bipyridyl complex and cis-PtL2Cl2 (L = NH3, pyridine), on cultures of L1210 mouse leukemia cells has been carried out. Results were obtained over a range of concentrations required to give lo-90% inhibition of growth, and the expected sigmoidal curves (percent growth vs. concentration) were obtained. The complex cis-Pt(pyridine)2Cl2 gave an I.D.50 value higher than that of cis-Pt(NH3)2C12, consistent with earlier studies showing that this complex has lower activity than cis-Pt(NH3)2Cl2 against Ehrlich ascites and Sarcoma 180 tumors in mice. The complex Pt(pyIm)C12. H20 gave an I.D.50 value similar to that of cis- Pt(NH3)2Cl2, and below values for the other complexes. These results suggest that further testing with tumor bearing animals is warranted for Pt(pyIm)C12. H20.