Tridentate Bis Research Articles

Carbon-carbon bond formation and cleavage at redox active bis(pyridylimino)isoindole (BPI) germylene compounds.

Redox-active ligands provide alternative reaction pathways by facilitating redox events. Among these, tridentate bis(piridylimino)isoindole (BPI) fragments offer great potential, though their redox-active behaviour remains largely underdeveloped. We describe herein a family of BPI germanium(II) complexes and the study of their redox properties. Amido complexes (RBPI)Ge[N(SiMe3)2] 1-R (R = H, Me, Et) showing a κ2-Npy,Niso coordination to the germanium(II) centre were prepared. In contrast, chloride derivatives (RBPI)GeCl, 2-R, display dynamic κ3-Npy,Niso,Npy coordination to the metal centre. The addition of silver bis(trifluoromethane)sulfonimide to compound 1-H generates a dinuclear complex, 3, where the silver atoms are bound to the germanium and one of the imine nitrogen atoms of another BPI fragment. The reduction of 2-R with KC8 generates dinuclear complexes (4-R) characterized by the formation of new C-C bonds between the isoindoline five-membered rings of two different BPI ligands via a radical mechanism, a transformation that does not take place in the absence of germanium. Interestingly, computational and spectroscopic studies support that the reduction takes place exclusively over the RBPI ligand. Strikingly, the newly formed C-C bond is also readily cleaved. Thus, subsequent reduction of 4-R (R = H, Me) using additional KC8 affords dinuclear species 5-R, with polymeric structures between potassium atoms and the corresponding dinuclear Ge2(RBPI2) fragments.

Synthesis and characterization of C2-symmetric bis(carboxamide) pincer ligands

Structural, spectroscopic, and computational studies on C2-symmetric bis(carboxamide) pincer ligands indicate excess populations of one atropisomer (enantiomer) are favoured both in solution and the solid state for sufficiently bulky derivatives.

Dinitrogen Functionalization with Carbon Dioxide and Carbon Disulfide Giving Symmetric and Unsymmetric Hydrazido Complexes

AbstractHere we show that a tridentate bis(aryloxide)anilide‐ligated titanium/potassium scaffold promotes functionalization of coordinated N2 with CO2 and CS2 through formation of N−C bonds. Treatment of a naphthalene complex with N2 gave an end‐on bridging dinitrogen complex featuring a [Ti2K2N2] core. The dinitrogen complex underwent insertion of CO2 into each Ti−NN bond to afford an N,N′‐dicarboxylated hydrazido complex. Stepwise nitrogen‐carbon bond formation at coordinated N2 proceeded to afford an unsymmetric hydrazido complex upon sequentially treating the dinitrogen complex with CS2 and CO2. Addition of Me3SiCl to the dicarboxylated hydrazido complex resulted in partial silylation of the carboxylate groups but did not lead to removal of the functionalized N2 unit from the metal centers. However, reduction of the dicarboxylated hydrazido complex with potassium naphthalenide afforded an oxo‐bridged dinuclear complex along with release of free potassium cyanate.

Dinitrogen Functionalization with Carbon Dioxide and Carbon Disulfide Giving Symmetric and Unsymmetric Hydrazido Complexes.

Here we show that a tridentate bis(aryloxide)anilide-ligated titanium/potassium scaffold promotes functionalization of coordinated N2 with CO2 and CS2 through formation of N-C bonds. Treatment of a naphthalene complex with N2 gave an end-on bridging dinitrogen complex featuring a [Ti2 K2 N2 ] core. The dinitrogen complex underwent insertion of CO2 into each Ti-NN bond to afford an N,N'-dicarboxylated hydrazido complex. Stepwise nitrogen-carbon bond formation at coordinated N2 proceeded to afford an unsymmetric hydrazido complex upon sequentially treating the dinitrogen complex with CS2 and CO2 . Addition of Me3 SiCl to the dicarboxylated hydrazido complex resulted in partial silylation of the carboxylate groups but did not lead to removal of the functionalized N2 unit from the metal centers. However, reduction of the dicarboxylated hydrazido complex with potassium naphthalenide afforded an oxo-bridged dinuclear complex along with release of free potassium cyanate.

Electronic Effect on Phenoxide Migration at a Nickel(II) Center Supported by a Tridentate Bis(phosphinophenyl)phosphido Ligand.

A phosphide nickel(II) phenoxide pincer complex (2) reacts with CO(g) to give a pseudo-tetrahedral nickel(0) monocarbonyl complex (3) possessing a phosphinite moiety. This metal-ligand cooperative (MLC) transformation occurs with a (PPP)Ni scaffold (PPP- = P[2-PiPr2-C6H4]2-), which can accommodate both square planar and tetrahedral geometries. The 2-electron reduction of a nickel(II) species induced by CO coordination involves group transfer to generate a P-O bond. For better mechanistic understanding, a series of nickel(II) phenolate complexes (2a-2e, XC6H4O- (X = OMe, Me, H, and CF3) and pentafluorophenolate) were prepared. Kinetic experimental data reveal that a phenolate species with an electron-withdrawing group reacts faster than those with electron-donating groups. The reaction kinetic experiments were conducted in pseudo-first order conditions at room temperature monitored by UV-vis spectroscopy. A pentafluorophenolate nickel(II) complex (2e) reveals instantaneous reactions even at -40 °C to give a nickel(0) monocarbonyl species (3e) and the reverse reaction is also possible. According to kinetic experiments, the rate determining step (RDS) would be the formation of a 5-coordinate intermediate 4 with a negative entropy value (ΔS‡ < 0), and a positive ρ value based on the Hammett plot indicates that the electron-deficient phenolate leads to a faster CO association. Furthermore, scramble experiments suggest that phenolate de-coordinates from the intermediate 4, which gives a (PPP)Ni-CO species 6. The cationic nickel monocarbonyl intermediate can possess a P--Ni(II), P•-Ni(I), or even a P+-Ni(0) character. Such an inner-sphere electron transfer is suggested when a π-acidic ligand such as CO coordinates to a metal ion. Another possible reaction is homolysis of a Ni-O bond to give P--Ni(I) or P•-Ni(0), when a phenoxyl radical is liberated. Considering the P-O bond formation, closed-shell nucleophilic and open-shell radical pathways are suggested. A phenolate pathway reveals a lower energy state for 2e relative to other complexes (2c and 2d), while its radical pathway undergoes via a higher energy state. Therefore, the formation of a P-O bond may occur with the binding of a closed-shell phenolate to the electron-deficient P center.

Construction of spin crossover-fluorescence bifunctional iron(ii) complexes with modified bis(pyrazole)pyridine ligands

Two Feii-based spin crossover complexes were synthesized by modifying tridentate bis(pyrazole)pyridine ligand with naphthalene and pyrene groups. The naphthalene-decorated FeII-complex showed the synergistic effect of spin-crossover and fluorescence.

Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3 and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3 (1) with one or two equivalents of AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imide) or AgSbF6 gives stibaalkene mono- and dications of the form [(CDC)SbBr3-n][A]n (2-4; n = 1,2; A = NTf2 or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3 (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3 (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2 (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

New NNN pincer copper complexes as potential anti-prostate cancer agents

Eleven novel NNN Cu(II) complexes supported by a tridentate bis(imidazo[1,2-α]pyridin-2-yl)pyridine ligand were synthesized and characterized by elemental analysis, HRMS, and X-ray determination. Target prediction and docking studies indicated that these pincer complexes formed hydrogen bonds with Asp33 and Gly35 of Cathepsin D protein, which is highly associated with prognosis of advanced prostate cancer. Furthermore, they exhibited anti-proliferation activity in both androgen-sensitive and androgen-insensitive prostate cancer cells according to WST-1 assay results. Mechanistic study showed that pincer complexes arrested cell cycle progression at G0/G1 phase and inhibited Cathepsin D regulated signaling pathways. Most importantly, new pincer copper complexes significantly inhibited xenograft prostate cancer growth along with a promising in vivo safety profile. In summary, these results suggest the applicability of the developed novel pincer copper complexes as promising anticancer agents for prostate cancer treatment.

Spin state of two mononuclear iron(II) complexes of a tridentate bis(imino)pyridine N-donor ligand: Experimental and theoretical investigations

Investigations on the spin states of octahedral Fe(II) complexes have received special attention due to their clear discrimination in the spin states of the d-orbitals. As a means to further understand the factors that influence the spin-crossover (SCO) phenomenon in Fe(II) systems, we herein report two mononuclear Fe(II) complexes, [FeL2](ClO4)2·2CH3OH (1) and [FeL2](BF4)2·CH3CN·CH3OH (2), derived from a novel N3-donor Schiff base ligand, 2,6-bis[(3-methylbenzylimino)methyl]pyridine (L) with varying counteranion and the diamagnetic [ZnL2](BF4)2 congener for a comparative investigation. The complexes have been synthesized and characterized by electrospray-ionization mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction (XRD) and magnetic susceptibility studies. Structural and magnetic investigations reveal that both 1 and 2 show Fe__N6 distorted octahedral geometry and are locked in the diamagnetic LS state throughout the entire explored temperature range from 1.8 to 400 K. The LS state of [FeL2]2+ is also confirmed by comparing the experimentally found structural parameters, NMR chemical shifts and excitation energies in the visible region with density functional theory (DFT) calculations.

Isoselective mechanism for asymmetric kinetic resolution polymerization of rac-lactide catalyzed by chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes

Chiral tridentate bis(oxazolinylphenyl)amido ligand supported zinc complexes were synthesized and utilized for the asymmetric kinetic resolution polymerization of rac-lactide. These polymerizations catalyzed by chiral catalysts were performed efficiently, resulting in isotactic polylactide with stereogradient microstructure and narrow dispersity. The asymmetric kinetic resolution polymerization and block polymerization experiments indicated that the combination of enantiomorphic site control and chain-end control should be responsible for the stereocontrol mechanism during the polymerization process.

Bis(pyridyl)carbodicarbene supported ruthenium complexes and their catalytic application in hydrogen‐transfer reaction

AbstractWe report the preparation and structural characterization of the first known pincer carbodicarbene (CDC) supported ruthenium complexes. The single‐crystal X‐ray diffraction experiment indicated that tridentate bis(pyridyl)‐CDC free ligand is coordinated to the six‐coordinated ruthenium (II) in meridional conformation with each of two chlorides trans to the carbone carbon and phosphine. The pincer effect of new ligand bis(pyridyl)‐CDC has shortened the Ru‐carbodicarbene bond length in comparison to monodentate CDC. The catalytic activity of the new CDC complexes was tested in hydrogen transfer reaction, revealing the efficient activity of CDC‐supported Ru complexes in catalytic transfer hydrogenation of ketone using 2‐propanol as hydrogen source.

Rational Design of Chiral Tridentate Ligands: Bifunctional Cobalt(II) Complex/Hydrogen Bond for Enantioselective Michael Reactions.

Bifunctional chiral tridentate bis(pyrroloimidazolone)pyridine (PyBPI) ligands have been designed, synthesized, and applied in an asymmetric Michael addition. With a 0.05 mol % PyBPI-Co(II) complex, β,γ-unsaturated α-keto esters reacted with 4-hydroxycoumarin to give the adducts in 93-99% yields and 90-97% ee. Experiments and DFT calculations supported the dual activation manner, in which the tridentate ligand coordinated with Co(II) to activate the keto ester, and the hydroxyl and carbonyl groups in PyBPI interacted with 4-hydroxycoumarin via two different H bonds.

Synthesis, electronic structures, and reactivity of mononuclear and dinuclear low-valent molybdenum complexes in iminopyridine and bis(imino)pyridine ligand environments

Molybdenum in redox non-innocent ligand environments features prominently in biological inorganic systems. While Holm and coworkers, along with many other researchers, have thoroughly investigated formally high-oxidation-state molybdenum (Mo(IV)-Mo(VI)) ligated by dithiolenes, less is known about molybdenum in other formal oxidation states and/or different redox-active ligand environments. This work focuses on the investigation of low-valent molybdenum in four different redox non-innocent nitrogen ligand type environments (mononucleating and dinucleating iminopyridine, mononucleating and dinucleating bis(imino)pyridine). The reaction of iminopyridine N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine (L1) with Mo(CO)3(NCMe)3 produced Mo(L1)(CO)3(NCMe). Mo(L1)(CO)3(NCMe) undergoes transformation to Mo(L1)(CO)4 upon treatment with CS2 or prolonged stirring in dichloromethane. The reaction of the open-chain dinucleating bis(iminopyridine) ligand N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine) (L2) similarly produced an hexacarbonyl complex Mo2(L2)(CO)6(NCMe)2 which also underwent transformation to the octacarbonyl Mo2(L2)(CO)8. Both complexes featured anti-parallel geometry of the chelating units. The oxidation of Mo(L1)(CO)3(NCMe) with I2 resulted in Mo(L1)(CO)3I2. The reaction of mononucleating potentially tridentate bis(imino)pyridine ligand (L3) (N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine) with both Mo(CO)3(NCMe)3 and Mo(CO)4(NCMe)2 produced complexes Mo(L3)(CO)3(NCMe) and Mo(L3)(CO)4 in which L3 was coordinated in a bidentate fashion, with one imino sidearm unbound. The reaction of dinucleating macrocyclic di(bis(imino)pyridine) analogue (L4) led to the similar chemistry of Mo2(L4)(CO)6(NCMe)2 and Mo2(L4)(CO)8 complexes. Treatment of Mo(L3)(CO)3(NCMe) with I2 formed a mono(carbonyl) complex Mo(L3)(CO)I2 in which molybdenum was formally oxidized and L3 underwent coordination mode change to tridentate. The electronic structures of formally Mo(0) complexes in iminopyridine and bis(imino)pyridine ligand environments were investigated by density functional theory calculations.

Synthesis, structure, and properties of the Sc chloride complex coordinated by the tridentate bis(phenolate)-tethered NHC ligand

Synthesis, structure, and properties of the Sc chloride complex coordinated by the tridentate bis(phenolate)-tethered NHC ligand

Using N-Heterocyclic Carbenes as Weak Equatorial Ligands to Design Single-Molecule Magnets: Zero-Field Slow Relaxation in Two Octahedral Dysprosium(III) Complexes.

We report the synthesis, structures, and magnetic investigations of two new octahedral dysprosium complexes, based on the original N-heterocyclic carbene (NHC) tridentate bis(phenoxide) ligand, of the respective formulas mer-[DyL(THF)2Cl] (1) and mer-[DyL(THF)3][BPh4] (2), where L = 1,3-bis(3,5-di-tert-butyl-2-oxidophenyl)-5,5-dimethyl-3,4,5,6-tetrahydropyrimidin-1-ium chloride and THF = tetrahydrofuran. The short Dy-O distances in the axial direction in association with the weak donor ability of the NHC moiety provide a suitable environment for slow relaxation of magnetization, overcoming the previous single-molecule magnets based on NHC ligands.

Multi analyte sensing of amphiphilic tridentate bis(benzimidazolyl)pyridine incorporated in liposomes and potential application in enzyme assay.

A liposome based nanosensor Lipo-1 for efficient detection of copper, cyanide (CN-) and ATP in a pure aqueous medium has been described. Lipo-1 shows a fluorescence ON-OFF response with copper. However, Lipo-1.Cu (Lipo-1 and copper ensemble) was used for the OFF-ON detection of ATP with nM and CN- with μM detection levels, lower than the WHO permissible level for safe drinking. Lipo-1 has better and enhanced binding properties over the counter organic amphiphilic compound Bzimpy-LC, which is not soluble in water. The significant changes in the emission spectra in the presence of Cu2+, CN- and ATP ions, as variable inputs, are used to construct INHIBIT and OR logic operations in a nano-scale environment. The fluorescent detection of CN- ions with Lipo-1.Cu was used to develop an enzyme assay for β-glucosidase using amygdalin as the substrate. β-Glucosidase enzymatic activity was monitored by the emission OFF-ON signal of the probe Lipo-1.Cu by CN- detection. This approach could be an efficient method for developing a fluorescence-based β-glucosidase enzyme assay. A switch ON luminescence response, low detection limit, fast response, 100% aqueous solution, biocompatibility, multi-analyte detection, and improved sensitivity and selectivity of Bzimpy-LC in lipid bilayer membranes are the main features of the nanoprobe Lipo-1. These properties give it a clear advantage for analytical applications.

Three-phase electrochemistry of a highly lipophilic neutral ru-complex having tridentate bis(benzimidazolate)pyridine ligands

ABSTRACT Here we describe the synthesis and electrochemical testing of a heteroleptic bis(tridentate) ruthenium(II) complex [RuII(LR)(L)] (LR =2,6-bis(1-(2-octyldodecan)benzimidazol-2-yl)pyridine, L = 2,6-bis(benzimidazolate)pyridine). It is a neutral complex which undergoes a quasireversible oxidation and reduction at relatively low potential. The newly synthesised compound was used for studies of ion-transfer at the three-phase junction because of the sensitivity of this method to cation expulsion. The [RuII(LR)(L)] shows exceptional stability during cycling and is sufficiently lipophilic even after oxidation to persist in the organic phase also using very hydrophilic anions such as Cl−. We believe that this, in combination with a comparably low redox potential, will make [RuII(LR)(L)] useful as electron-donor in phase-separated catalysis and voltammetric ion-selective membranes.

Synthesis and Crystal and Molecular Structure of the Solvated Complex [MoO2(L) · C5H5N], Where L2– Is the 2-[N-(2-Hydroxynaphthalidene)amino]propane-1,2,3-Triol Anion

A new dioxomolybdenum(VI) compound [MoO2(L) · C5H5N], where L2– is 2-[N-(2-hydroxynaphthalidene)amino]propane-1,2,3-triol, was synthesized and its structure was determined by IR spectroscopy and X-ray diffraction. The Mo atom is in an octahedral coordination environment formed by two oxo ligands in cis positions to each other, two O atoms and the N(2) atom of the tridentate bis(chelate) ligand L, and the N(1) atom of the pyridine (Py) molecule. The N(2)(L) and N(1)(Py) atoms are in trans positions to the O(oxo) ligands at distances that are elongated due to the structural trans effect of the multiply bonded oxo ligands (Mo–N(1), 2.476 A; Mo–N(2), 2.272 A).

Palladium(II) complexes of tridentate bis(benzazole) ligands: Structural, substitution kinetics, DNA interactions and cytotoxicity studies

Reactions of 2,6-bis(benzimidazol-2-yl)pyridine (L1), 2,6-bis(benzoxazol-2-yl)pyridine (L2), and 2,6-bis(benzothiazol-2-yl)pyridine (L3) with [Pd(NCMe)2Cl2] in the presence of NaBF4 afforded the corresponding Pd(II) complexes, [Pd(L1)Cl]BF4, PdL1; [Pd(L2)Cl]BF4, PdL2; [Pd(L3)Cl]BF4, PdL3; respectively, while reaction of bis[(1H-benzimidazol-2-yl)methyl]amine (L4) with [Pd(NCMe)2Cl2] afforded complex [Pd(L4)Cl]Cl, PdL4. Characterisation of the complexes was accomplished using NMR, IR, MS, elemental analyses and single crystal X-ray crystallography. Ligand substitution kinetics of these complexes by biological nucleophiles thiourea (Tu), L-methionine (L-Met) and guanosine 5′-diphosphate disodium salt (5-GMP) were examined under pseudo-first order conditions. The reactivity of the complexes decreased in the order: PdL1 > PdL2 > PdL3 > PdL4, ascribed to electronic effects. Density functional theory (DFT) supported this trend. Studies of interaction of the Pd(II) complexes with calf thymus DNA (CT-DNA) revealed strong binding affinities via intercalative binding mode. Molecular docking studies established associative non-covalent interactions between the Pd complexes and DNA. The in vitro cytotoxic activities of PdL1–PdL4 were assessed in cancer cell lines HeLa and MRC5-SV2 and a normal cell line MRC-5, using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. PdL1 exhibited cytotoxic potency and selectivity against HeLa cell that was comparable to cisplatin's. Complex PdL1, unlike cisplatin, did not significantly induce caspase-dependent apoptosis.

Ligand-Based Reactivity of Oxygenation and Alkylation in Cobalt Complexes Binding with (Thiolato)phosphine Derivatives.

In our efforts to understand the nature of metal thiolates, we have explored the chemistry of cobalt ion supported by (thiolato)phosphine ligand derivatives. Herein, we synthesized and characterized a square-planar CoII complex binding with a bidentate (thiolato)phosphine ligand, Co(PS1″)2 (1) ([PS1″]- = [P(Ph)2(C6H3-3-SiMe3-2-S)]-). The complex activates O2 to form a ligand-based oxygenation product, Co(OPS1″)2 (2) ([OPS1″]- = [PO(Ph)2(C6H3-3-SiMe3-2-S)]-). In addition, an octahedral CoIII complex with a tridentate bis(thiolato)phosphine ligand, [NEt4][Co(PS2*)2] (3) ([PS2*]2- = [P(Ph)(C6H3-3-Ph-2-S)2]2-), was obtained. Compound 3 cleaves the C-Cl bond in dichloromethane via an S-based nucleophilic attack to generate a chloromethyl thioether group. Two isomeric products, [Co(PS2*)(PSSCH2Cl*)] (4 and 4') ([PSSCH2Cl*]- = [P(Ph)(C6H3-3-Ph-2-S)(C6H3-3-Ph-2-SCH2Cl)]-), were isolated and fully characterized. Both transformations, oxygenation of a CoII-bound phosphine donor in 1 and alkylation of a CoIII-bound thiolate in 3, were monitored by spectroscopic methods. These reaction products were isolated and fully characterized. Density functional theory (DFT, the B3LYP functional) calculations were performed to understand the electronic structure of 1 as well as the pathway of its transformation to 2.