Ligand Transfer Research Articles

Nano-NiFe2O4 catalyzed microwave assisted one-pot regioselective synthesis of novel 2-alkoxyimidazo[1,2-a]pyridines under aerobic conditions

Here, we have demonstrated regioselective synthesis of novel 2-alkoxy-3-arylimidazo[1,2-a]pyridines using nano-NiFe2O4 as reusable catalyst via one-pot sequential aza-Michael addition, Fe-promoted ligand transfer and C–H imination reactions.

Geminal Dichlorination of Phenyliodonium Ylides of β-Dicarbonyl Compounds through Double Ligand Transfer from (Dichloroiodo)benzene

Pre-formed phenyliodonium ylides of cyclic and acyclic β-diketones, β-keto esters and β-diesters were reacted with (dichloroiodo)benzene, resulting in transfer of both chloride ligands onto the ylidic carbon. These two hypervalent iodine(III) compounds exhibit high reactivity towards each other under mild reaction conditions and typically afford the <i>gem</i>-dichloride products in good yield. Upon comparison of these chlorination reactions with those of the analogous diazocarbonyl compounds, reactions of iodonium ylides were unilaterally faster, and often gave the products in higher yield.

Luminescent Properties of a Polymer Photoluminescent Composite Containing the Binuclear (EU, TB) Complex as an Emitter

abstract Uninuclear europium (Eu), as well as binuclear Eu and terbium (Tb), complexes were synthesized using acrylic acid (AA) as the first ligand and 1,10-phenanthroline (Phen) as the second ligand. The relative weight ratio of the europium (III) (Eu3+) to terbium (III) (Tb3+) ions of the binuclear complex was 1:1 as determined via energy dispersive X-ray analysis. The structures of the Eu(AA)3Phen and Eu0.5Tb0.5(AA)3Phen complexes were characterized by Fourier transform infrared spectroscopy. A series of tri-cellulose acetate (TCA)/ the Eu(AA)3Phen and TCA/Eu0.5Tb0.5(AA)3Phen composites were prepared by solution blending, and their luminescent properties were investigated by fluorescence spectrophotometry. The excitation spectra of all composites indicated that the TCA matrix probably affected the energy absorption and transfer of organic ligands. In TCA/Eu0.5Tb0.5(AA)3Phen composites the introduced Tb3+ ions had some influence on energy absorption and transfer of organic ligands; the energy transfer process of the complex is suggested to be as follows: Phen→AA→Tb3+ion→Eu3+ion. The emission spectra indicated that the luminescent intensity of the TCA/Eu0.5Tb0.5(AA)3Phen composites was noticeably stronger than that of the TCA/Eu(AA)3Phen composites, suggesting that the comparatively stable and high-efficiency energy transfer process was only slightly influenced by the TCA matrix. In summary, the TCA/Eu0.5Tb0.5(AA)3Phen (90/10) composite possesses fine luminescent properties for potential usage as red fluorescent materials.

Mechanistic Study of Photocatalytic Hydrogen Generation with Simple Iron Carbonyls as Water Reduction Catalysts

AbstractThis study provides new insights into light‐driven hydrogen generation using an iridium photosensitizer (IrPS) and simple iron carbonyls as water reduction catalysts (WRCs). Stopped‐flow rapid‐scan FTIR and operando continuous‐flow FTIR spectroscopy as well as time‐dependent density functional theory (TD‐DFT) has been applied to study the reaction. The conversion of the WRC precursor [Fe3(CO)12] into the radicals [Fe3(CO)11].− and [Fe2(CO)8].− as well as [Fe(CO)5] in the absence of light in a solvent mixture of tetrahydrofuran, triethylamine, and water has been studied quantitatively. During light‐induced hydrogen production in the presence of the IrPS, the trimeric [HFe3(CO)11]− and the monomeric [HFe(CO)4]− anion could be identified as major WRC species. The equilibrium between both species can be shifted completely towards [HFe(CO)4]− by increasing the water content of the solvent mixture. Application of other iron(0) carbonyl compounds as WRC precursors also results in the exclusive formation of [HFe(CO)4]−. Kinetic experiments show that the stability of the system is primarily influenced by the applied amount of WRC precursor, whereas the reaction rate is mainly determined by the concentration of the IrPS. At least two loss channels could be identified: light‐induced CO dissociation from the WRC and decomposition of the IrPS at high IrPS/WRC ratios, accompanied by a ligand transfer from the iridium towards the iron center of the WRC. To reveal the nature of the catalytically active complex, binding energies and charge‐transfer probabilities of all coordination geometries of various IrPS⋅⋅⋅WRC complexes have been calculated. These computations indicate an increased probability of charge transfer for dimeric and trimeric iron carbonyl species.

Effect of water—ethanol solvent on the stability of copper(ii) coordination compound with the nicotinate ion

The stability constants of copper(II) complexes with nicotinate ion in water—ethanol solvent were determined by the potentiometric method at 25.0±0.1 °C and ionic strength of 0.25 (NaClO4) in the range XEtOH = 0—0.7 mole fractions. The stability constant of copper(II) nicotinate complex considerably increases with increasing ethanol concentration in the solvent. The contributions of reactants to the Gibbs energy of the complex formation reaction on going from water to aqueous ethanol were analyzed. The results of thermodynamic analysis of solvation effects were used to evaluate the ratio of the ion and ligand transfer Gibbs energy contributions to the change in the reaction Gibbs energy in the water—ethanol system.

Hydrogenation of biomass-derived levulinic acid to γ-valerolactone catalyzed by PNP-Ir pincer complexes: A computational study

Density functional theory (DFT) calculations were used to study the mechanisms for hydrogenation of biomass-derived levulinic acid (LA) to γ-valerolactone (GVL) catalyzed by PNP-ligated (PNP = 2,6-bis(di-ter-butylphosphinomethyl) pyridine) iridium complexes, (tBuPNP)IrH3, 1. The transformation proceeds dearomatization/rearomatization including four stages: hydride transfer to give LA-H− and dihydride iridium specie (BtuPNP)IrH2+, methylene proton in the PNP ligand transfer to substrate to form γ-hydroxyvaleric acid (HVA), catalyst regeneration via H2 splitting, cyclization of HVA to GVL. Compared with the hydrogenation of CO2 catalyzed by the same complex, herein the rate-limiting step is the hydride transfer step rather than the methylene proton transfer. The calculations show that the hydride transfer step has a significantly higher barrier (21.1 vs 5.3 kcal mol−1 for the hydrogenation of CO2). Besides, the cyclization is very easily achieved due to the low barrier. As an alternative pathway, different from the above dearomatization/rearomatization process, a base abstracts a proton from the dihydrogen ligand to form the trihydride iridium with a free energy barrier of 24.6 kcal mol−1. Our results will help in the understanding of the mechanisms of hydrogenation of LA to GVL catalyzed by metal complexes.

Role of N-donor groups on the stability of hydrazide based hypercoordinate silicon(IV) complexes: Theoretical and experimental perceptions

A triphenylphosphinimino donor group is illustrated as a ligand in pentacoordinate siliconium halide dichelates, [YSiL2]+X−, where L is the bidentate ligand –OC(R)NNPPh3)– (R=t-Bu-Ph or Ph), Y=Me, Ph, CH2Cl, CHCl2, Cl or Br, and X=Cl or Br. All the new complexes were characterized by NMR spectroscopy and elemental analysis. The remote substituent, the t-Bu-phenyl or phenyl group, imparts more pentacoordinate character, i.e. more ionization to the complexes, compared to the PhCH2 group. DFT calculations indicate that the central silicon atom, due to the more positive charge, demands greater electron density. As a result of this, shorter Si–O, Si–N and Si–Cl bonds were observed. Both theoretical and experimental analysis indicate that the phosphinimino ligand is a stronger donor than the previously studied dimethylamino and isopropylidenimino ligands, causing all of the complexes to be pentacoordinate siliconium-halide salts in solution. The hypercoordinate silicon dichelates undergo unique intermolecular chelate exchange reactions: (i) complete ligand transfer from the dichelates to PhSiCl3 by a ligand priority order and (ii) bidentate ligand interchange between the dichelates and a trimethylsilyl-hydrazide precursor. Thermolysis of some selected hypercoordinated silicon(IV) complexes containing a silicon–carbon σ-bond significantly undergo a two step decomposition, while other complexes with silicon–halogen σ-bonds follow three steps. The thermal decomposition strongly depends on the nature of the substituents directly attached to the central silicon atom.

N–H Activation of Ammonia by [{M(μ-OMe)(cod)}2] (M = Ir, Rh) Complexes: A DFT Study

In this work, a computational study at the DFT level is carried out to determine the reaction mechanism for the N–H bond activation of ammonia by dinuclear [{M(μ-OMe)(cod)}2] complexes (M = Ir, Rh) to yield amido species [{M(μ-NH2)(cod)}2] reported experimentally by Mena et al. (Angew. Chem., Int. Ed. 2011, 50, 11735–11738). A stepwise mechanism is proposed for the replacement of μ-OMe bridging ligands considering associative or dissociative approaches for NH3 coordination to the metal. Reaction pathways for the homolytic and heterolytic N–H σ-bond cleavage of ammonia, such as oxidative addition through MIII species or hydrogen transfer to the ligand, are investigated. The energetically preferred mechanism involves the participation of both metallic centers through the formation of and intermediate bearing M1-NH3 and M2-OMe moieties followed by heterolytic hydrogen transfer of the amino ligand to the methoxo ligand. A bonding analysis on the metallacycle [M2X2] core (M = Ir, Rh; X = μ-OMe, μ-NH2) is perfo...

Calcium Hydride Catalyzed Highly 1,2-Selective Pyridine Hydrosilylation.

Reaction of the calcium hydride complex (DIPPnacnac-CaH⋅THF)2 with pyridine is much faster and selective than that of the corresponding magnesium hydride complex (DIPPnacnac = [(2,6-iPr2 C6 H3 )NC(Me)]2 CH). With a range of pyridine, picoline and quinoline substrates, exclusive transfer of the hydride ligand to the 2-position is observed and also at higher temperatures no 1,2→1,4 isomerization is found. The heteroleptic product DIPPnacnac-Ca(1,2-dihydropyridide)⋅(pyridine) shows fast ligand exchange into homoleptic calcium complexes and therefore could not be isolated. Calcium hydride reduction of isoquinoline gave well-defined homoleptic products which could be characterized by X-ray diffraction: Ca(1,2-dihydroisoquinolide)2 ⋅(isoquinoline)4 and Ca3 (1,2-dihydroisoquinolide)6 ⋅(isoquinoline)6 . The striking selectivity difference in the dearomatization of pyridines by Mg or Ca complexes could be explained by DFT theory and was utilized in catalysis. Whereas hydroboration of pyridine with pinacol borane with a calcium hydride catalyst gave only minor conversion, the hydrosilylation of pyridine and quinolines with PhSiH3 yields exclusively 1,2-dihydropyridine and 1,2-dihydroquinoline silanes with 80-90 % conversion. Similar results can be achieved with the catalyst Ca[N(SiMe3 )2 ]2 ⋅(THF)2 . These calcium complexes represent the first catalysts for the 1,2-selective hydrosilylation of pyridines.

Transfer of the human NKG2D ligands UL16 binding proteins (ULBP) 1-3 is related to lytic granule release and leads to ligand retransfer and killing of ULBP-recipient natural killer cells.

After immune interactions, membrane fragments can be transferred between cells. This fast transfer of molecules is transient and shows selectivity for certain proteins; however, the constraints underlying acquisition of a protein are unknown. To characterize the mechanism and functional consequences of this process in natural killer (NK) cells, we have compared the transfer of different NKG2D ligands. We show that human NKG2D ligands can be acquired by NK cells with different efficiencies. The main findings are that NKG2D ligand transfer is related to immune activation and receptor-ligand interaction and that NK cells acquire these proteins during interactions with target cells that lead to degranulation. Our results further demonstrate that NK cells that have acquired NKG2D ligands can stimulate activation of autologous NK cells. Surprisingly, NK cells can also re-transfer the acquired molecule to autologous effector cells during this immune recognition that leads to their death. These data demonstrate that transfer of molecules occurs as a consequence of immune recognition and imply that this process might play a role in homeostatic tuning-down of the immune response or be used as marker of interaction.

Ligand Binding to Distinct Sites on Nanocrystals Affecting Energy and Charge Transfer.

Hybrid optoelectronic devices are attractive because they offer the promise of low-cost, roll-to-roll fabrication. Despite this, energy transfer between organic and inorganic interfaces is not well understood. Device engineering on this class of solution-processed materials generally focuses on replacing the long insulating ligands with short ones. Here, we show that energy and charge transfer between an inorganic nanocrystal (NC) donor and organic molecular acceptor is acutely sensitive to the chemical moiety linking the two species. Our results reveal that the CdS NCs have distinct binding sites for different chemical species because only resonance energy transfer (RET) is observed for the carboxylic-acid-functionalized ligand, while both RET and charge transfer are observed for the amine-functionalized ligand. We observe that the equilibrium constant for this static quenching term increases with decreasing particle size. This finding offers a new approach in the design of hybrid thin films for devices and NC probes based on RET used for imaging, sensing, signal transduction, and photon management.

Bis(pyridylimino)isoindolato (BPI) Osmium Complexes: Structural Chemistry and Reactivity

A series of osmium complexes with monoanionic, meridionally coordinating 1,3-bis(2-pyridylimino)isoindolates (BPI) as spectator ligands has been synthesized. Reaction of the dichlorido metal precursor [OsCl2(PPh3)3] with the lithiated BPI ligand transfer reagent gave the chlorido complex [(tBu-BPIMe)Os(PPh3)2Cl] (2) which, in turn, was reacted with lithium triethylborohydride to yield the hydrido complex [(tBu-BPIMe)Os(PPh3)2H] (3). Treatment of complex 2 with thallium hexafluorophosphate under a nitrogen pressure afforded the cationic dinitrogen complex [(tBu-BPIMe)Os(PPh3)2(N2)]PF6 (4), which contains an end-on coordinated dinitrogen molecule trans to the isoindolato nitrogen atom. To synthesize the alkynyl complex [(tBu-BPIMe)Os(PPh3)2(C≡CPh)] (5), the chlorido complex 2 was treated with lithium phenylacetylide. Complex 5 was subsequently converted quantitatively by addition of one or two equivalents of HBF4 to the vinylidene complex [(tBu-BPIMe)Os(PPh3)2(═C═CHPh)]BF4 (6), and the cationic species [(H-tBu-BPIMe)Os(PPh3)2(═C═CHPh)](BF4)2 (7), respectively; the latter being formed via protonation of one imine nitrogen atom of the BPI ligand. Upon stirring, a toluene solution of the chlorido complex 2 with benzyl potassium, the four-membered metallacycle [(tBu-BPIMe)Os(PPh3)(o-C6H4PPh2)] (8) was obtained, which reacted with molecular hydrogen and phenyl acetylene to give the hydrido complex 3 and the acetylide complex 5, respectively. Stirring of 8 with methyl acetylenedicarboxylate (DMAD) yielded [(tBu-BPIMe)Os(DMAD)(o-C6H4PPh2)] (9), while treatment with carbon monoxide gave the acyl complex [(tBu-BPIMe)Os(o–C(O)-C6H4PPh2)(CO)] (10) by an insertion of CO into the osmium carbon bond.

Reactivity of transition-metal borylene complexes: recent advances in B-C and B-B bond formation via borylene ligand coupling.

Terminal borylene complexes of group 6-10 metals have been prepared through a number of synthetic strategies, displaying a wide structural diversity with numerous coordination modes of the borylene ligand [:BR]. Earlier, amino-substituted borylenes were found to serve as borylene ligand transfer reagents to a range of organic and organometallic substrates. In contrast, aryl- and alkyl-substituted borylenes display metathesis behavior when combined with ketones. Recent studies on these complexes revealed new reactivity patterns involving borylene-borylene and carbonyl-borylene ligand coupling. Herein a brief account on the recent progress of these borylene ligand coupling reactions made in our laboratories is provided. These reactions offer unprecedented alternative methods for electron-precise B-B and B-C bond formation.

Group 1 Complexes of an (Amido-amino)silane and Their Use in the Synthesis of the Bi(III) Amide, Bi(Me2Si{NAr}{N(H)Ar})Cl2 (Ar = 2,6-i-Pr2C6H3)

The synthesis of sodium and potassium salts of the monoanionic, (amido-amino)silane, [Me2Si{NAr}{N(H)Ar}]− (Ar = 2,6-i-Pr2C6H3), and their use as ligand transfer reagents to bismuth are presented. Initial attempts to form Li(Me2Si{NAr}{N(H)Ar) (2) using equimolar amounts of n-BuLi and the neutral compound Me2Si{N(H)Ar}2 (1) gave mixtures of the unreacted di(amino)silane and the known dianion, Li2(Me2Si{NAr}2), on workup. Reaction of 1 with NaH and KH, however, afforded the desired anion in the soluble, crystalline compounds Na(Me2Si{NAr}{N(H)Ar})(THF)3 (3(THF)3) and K(Me2Si{NAr}{N(H)Ar}) (4). The crystal structure of 4 shows a cyclic hexamer [4]6, containing intra- and intermolecular K···π-aryl interactions. Reaction of 3 or 4 with BiCl3 demonstrated that both compounds are effective ligand transfer reagents, affording the bismuth compound Bi(Me2Si{NAr}{N(H)Ar})Cl2 (5). The crystal structure of 5 showed a primary monodentate κ1-N-bonding mode for the ligand with a close Bi···N contact to the neutral amino nitrogen atom. Long range Bi···Cl interactions associate these molecules into dimers, [5]2.

Dimeric platinum-stannylene complexes by twofold ligand transfer from an NHC adduct to an organotin(II) hydride.

Upon ligand transfer of hydrides and NHC a dimeric formal Pt(I)–Sn(I) (2) complex was obtained from a 1:1 reaction of Pt(cod)2 (cod = 1,5-cyclooctadiene) and the NHC adduct to a bulky terphenyltin(II) hydride Ar*SnH(NHC) (1) [NHC = 1,3,4,5-tetramethylimidazol-2-ylidene, Ar* = 2,6-(2,4,6-triisopropylphenyl)C6H3]. The bridging stannylene Sn atoms each reveal an individual coordinative interaction with the platinum atoms and the hydrides with ‘non-classic’ Pt–H–Sn bonds.

Synthesis and physico-chemical studies of a novel organo-metallic compound CdCl2(C6H4FNH2)2

A new Cd(II) complex with the monodentate ligand 3-fluoroaniline, CdCl2(C6H4FNH2)2, has been prepared and characterized by single crystal X-ray diffraction, solid state NMR, IR, Raman and UV–visible spectroscopies. In the complex, the Cd(II) ion is surrounded by four chlorine atoms and two nitrogen atoms, forming a chain of edge-charing octahedra spreading along the a-axis direction. These chains are interconnected via NH…Cl hydrogen bonds to form inorganic layers parallel to the (a, b) plane. The organic 3-fluorophenyl groups are inserted between these layers and are interconnected via CH…F hydrogen bonds to perform an infinite three-dimensional network. Intermolecular π–π stacking interactions between neighboring aromatic rings are also observed with a face-to-face distance of 3.6816 (4)Å. The 13C and 19F CP-MAS NMR spectra are in agreement with the X-ray structure. DFT calculations allow the attribution of the carbon peaks to the different atoms. The absorption bands of the Cd2Cl4N2 units were identified by Raman spectroscopy. The solid-state UV–vis spectrum of the complex has been assigned to ligand and charge transfer transitions.

Heterometallic europium disiloxanediolates: synthesis, structural diversity, and photoluminescence properties.

This contribution presents a full account of a structurally diverse class of heterometallic europium disiloxanediolates. The synthetic protocol involves in situ metalation of (HO)SiPh2OSiPh2(OH) (1) with either (n)BuLi or KN(SiMe3)2 followed by treatment with EuCl3 in suitable solvents such as 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF). Reaction of EuCl3 with 2 equiv of (LiO)SiPh2OSiPh2(OLi) in DME afforded the Eu(III) bis(disiloxanediolate) "ate" complex [{(Ph2SiO)2O}2{Li(DME)}3]EuCl2 (2), which upon attempted reduction with Zn gave the tris(disiloxanediolate) [{(Ph2SiO)2O}3{Li(DME)}3]Eu (3). Treatment of EuCl3 with (LiO)SiPh2OSiPh2(OLi) in a molar ratio of 1:2 yielded both the ate complex [{(Ph2SiO)2O}3Li{Li(THF)2}{Li(THF)}]EuCl·Li(THF)3 (4) and the LiCl-free europium(III) complex [{(Ph2SiO)2O}2{Li(THF)2}2]EuCl (5). Compound 5 was found to exhibit a brilliant red triboluminescence. When (KO)SiPh2OSiPh2(OK) was used as starting material in a 3:1 reaction with EuCl3, the Eu(III) tris(disiloxanediolate) [{(Ph2SiO)2O}3{K(DME)}3]Eu (6) was isolated. Attempted ligand transfer between 5 and (DAD(Dipp))2Ba(DME) (DAD(Dipp) = N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene) afforded the unique mixed-valent Eu(III)/Eu(II) disiloxanediolate cluster [(Ph2SiO)2O]6Eu(II)4Eu(III)2Li4O2Cl2 (7). All new complexes were structurally characterized by X-ray diffraction. Photoluminescence studies were carried out for complex 5 showing an excellent color quality, due to the strong (5)D0→(7)F2 transition, but a weak antenna effect.

Controlled Living Polymerization of Carbodiimides Using Versatile, Air-Stable Nickel(II) Initiators: Facile Incorporation of Helical, Rod-like Materials

The new polymerization of carbodiimides using two, simple [bis(triphenylphosphino)aryl]nickel(II) bromide complexes has been discovered to occur in a controlled, living fashion. These initiators are substantially more air and moisture stable compared to their titanium(IV) counterparts making them significantly easier to synthesize, purify, and utilize. The polymerization is initiated via aryl ligand transfer to the electrophilic center carbon of the carbodiimide. Sequential insertions of the carbodiimide π-bond into the nickel–nitrogen amidinate coordination bond propagates the polymer chain in a living chain growth manner as evident by the linear relationship in the plots of percent conversion vs Mn, ln([M]o/[M]) vs time, and monomer: initiator ratio vs Mn. The transferred aryl ligand was confirmed to be appended to the terminus of the polymer chain by MALDI–TOF and 19F NMR. This added control element offers new opportunities to end functionalize rigid-rod, helical polycarbodiimides. This new technique a...

Anionic access to silylated and germylated binuclear heterocycles.

A simple access to silylated and germylated binuclear heterocycles, based on an original anionic rearrangement, is described. A set of electron-rich and electron-poor silylated aromatic and heteroaromatic substrates were tested to understand the mechanism and the factors controlling this rearrangement, in particular its regioselectivity. This parameter was shown to follow the rules proposed before from a few examples. Then, the effect of the substituents borne by the silicon itself, in particular the selectivity of the ligand transfer, was studied. Additionally, this chemistry was extended to germylated substrates. A hypervalent germanium species, comparable to the putative intermediate proposed with silicon, seems to be involved. However, a pathway implicating the elimination of LiCH2Cl was observed for the first time with this element, leading to unexpected products of the benzo-oxa (or benzo-aza) germol-type.

Transfer of the chelating ligands in the systems [LAuCl2]+-[PdCl4]2−: Synthesis and structure of the salt (NH4)0.20[(Bipy)AuCl2]1.04[(Bipy)PdCl2]0.96[AuCl4]0.76[PdCl4]0.24

The formation of binary complex salts containing gold(III) in the cation and palladium(II) in the anion in the systems [(Bipy)AuCl2]+-[PdCl4]2− occurs by transfer of the N,N-electron-donating chelating ligand bipyridine and the chloride ligands between the gold-containing cation and the palladium-containing anion. The resulting neutral salt [(Bipy)PdCl2] crystallizes together with the anion [AuCl4]− from acetonitrile-water (1 : 1-1 : 2, v/v) to give the complex salt (NH4+)0.20[(Bipy)AuCl2+]1.04[(Bipy)PdCl2]0.96[AuCl4−]0.76PdCl42−]0.24 with a total Au : Pd ratio of 3 : 2. The ammonium cation is formed from acetonitrile upon its hydrolysis most likely catalyzed by Pd complexes. Quantum-chemical calculations were performed to study the transfer of the chelating ligand theoretically.