Metal-ligand Cooperation Research Articles

Reactivity of a dearomatised pincer CoIIBr complex with PNCNHC donors: alkylation and Si-H bond activation via metal-ligand cooperation.

The double aminolysis reaction of [Co{N(SiMe3)2}2] by the salt 1-(6-((dicyclohexylphosphaneyl)methyl)pyridin-2-yl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bromide, which contains one phosphane, one pyridine and one imidazolium groups, of formula [o-Cy2PCH2(C5H3N)(o-C3H3N2DiPP)]Br and abbreviated as (CyPNpyrCim)Br, was previously shown to afford the Co(ii) complex [Co(CyP*NaCNHC)Br] (1) containing a dearomatised picolyl moiety in the tridentate, anionic donor ligand CyP*NaCNHC (Na = anionic amido, P* = vinylic P donor). We now report that formation of 1 is preceded by an intermediate tentatively assignable to the 5-coordinate Co(ii) complex [Co(CyPNpyrCNHC){N(SiMe3)2}Br] (2). The reaction of 1 with LiCH2SiMe3 afforded the dark purple, paramagnetic [Co(CyP*NaCNHC)CH2SiMe3] (3) with a low spin d7 CoII; the electronic configurations of 1 and 3 were corroborated by EPR spectroscopy. Addition of excess (≥4 equiv.) H2SiPh2 to a solution of 1 gave the diamagnetic [Co(CyP(SiHPh2)NCNHC)Br] (4) following Si-H activation and silylation of the ligand backbone at the α-CHP. Reduction of CoII to CoI by silanes is uncommon.

Cooperative metal-ligand influence on the formation of coordination polymers, and conducting and photophysical properties of Tl(i) β-oxodithioester complexes.

Eight novel Tl(i) β-oxodithioester complexes, [TlL]n (1-8), with ligands, L = methyl-3-hydroxy-3-(2-furyl)-2-propenedithioate (L1), methyl-3-hydroxy-3-(2-thienyl)-2-propenedithioate (L2), methyl-3-hydroxy-3-(3-pyridyl)-2-propenedithioate (L3), methyl-3-hydroxy-3-(4-pyridyl)-2-propenedithioate (L4), methyl-3-hydroxy-3-(9-anthracenyl)-2-propenedithioate (L5), methyl-3-hydroxy-3-(4-fluorophenyl)-2-propenedithioate (L6), methyl-3-hydroxy-3-(4-chlorophenyl)-2-propenedithioate (L7) and methyl-3-hydroxy-3-(4-bromophenyl)-2-propenedithioate (L8), were synthesized and thoroughly characterized by elemental analysis, and IR, UV-Vis, 1H and 13C{1H} NMR spectroscopy, and their structures were ascertained by X-ray crystallography. Complexes 1 and 2 crystallized in P21 and P212121 chiral space groups, respectively, and were studied using Circular Dichroism (CD) spectra. Solid state structural analyses revealed that the β-oxodithioester ligands are bonded to Tl(i) ions in (O, S) chelating and chelating-bridging modes, thereby forming different types of 1D and 2D coordination polymeric structures. By considering the metal-assisted bonding interactions, various coordination numbers of 5-8 and 10 are established around the metal centre. Except for 5 and 7a which have TlTl separations at 3.724(1) and 3.767(1), 3.891(1) Å respectively, the remaining complexes have no TlTl distances <4.0 Å. This indicates that the majority of structures contain only weak inter- and intramolecular thallophilic interactions. The structures of 1-8 highlight the role played by variations in substituents in the dithioester unit in the structure and properties of the complexes. The multi-dimensional assembly in these complexes rests on important non-covalent C-Hπ (TlOSC3, chelate), C-HX (X = F, Cl, O, N), C-Hπ, HH and rare TlH-C intermolecular anagostic interactions. The TlH-C anagostic interactions together with C-OTl and C-STl interactions formed 7-, 11- and 12-membered chelate rings about the metal centers. The anagostic interactions in 1, 2 and 7b were assessed by theoretical calculations. All the complexes showed bright green luminescent emissions in solution and solid phases. Time-resolved emission spectra revealed a triexponential decay curve and short mean lifetime for fluorescence behavior.

Chromogenic Detection of Fe2+ Using Schiff base–naphthalene-2-ol-modified Silver Nanoparticles

Schiff base–naphthalene-2-ol (L) was synthesized by a coupling reaction between 2-hydroxy-1-naphthaldehyde and xylylenediamine in ethanol. The product was characterized by 1H NMR, 13C NMR, and mass spectroscopy. L was utilized as reducing and surface functionalizing agents in the synthesis of silver nanoparticles (AgNPs) which were then characterized by high-resolution transmission electron microscopy (HR-TEM) and UV–Visible spectroscopy. The colorimetric sensitivity and metal ion selectivity of L-modified AgNPs with various cations were examined using UV–Vis spectrophotometry. The presence of Fe2+ induced the aggregation of AgNPs through a cooperative metal–ligand interaction, resulting in a color change from bright yellow to dark yellow with a new band at 372 nm. The interference studies confirmed the high selectivity of Fe2+ sensing in presence of cations as well as anions by L-modified AgNPs. A linear relationship between the change of absorption at 372 nm and concentration Fe2+ ranging from 1.30 × 10−5 to 1.13 × 10−3 M was observed with a correlation coefficient of 0.9949. The detection limit of the developed method was 1.06 × 10−6 M. The proposed method has been successfully applied for determination of Fe2+ content in multivitamin tablet samples. The results were in good agreement with the label claim.

Correction to "Modular Attachment of Appended Boron Lewis Acids to a Ruthenium Pincer Catalyst: Metal-Ligand Cooperativity Enables Selective Alkyne Hydrogenation".

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to “Modular Attachment of Appended Boron Lewis Acids to a Ruthenium Pincer Catalyst: Metal–Ligand Cooperativity Enables Selective Alkyne Hydrogenation”Kuei-Nin T. Tseng, Jeff W. Kampf, and Nathaniel K. Szymczak*Cite this: J. Am. Chem. Soc. 2017, 139, 49, 18122Publication Date (Web):November 27, 2017Publication History Published online27 November 2017Published inissue 13 December 2017https://doi.org/10.1021/jacs.7b09662Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views2390Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (455 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

H–H and N–H Bond Cleavages of Dihydrogen and Ammonia by a Bifunctional Imido (NH)-Bridged Diiridium Complex: A DFT Study

The mechanisms of H–H and N–H bond cleavages of dihydrogen and ammonia mediated by the diiridium μ2-imido complex [(Cp*Ir)2(μ2-H)(μ2-NH)]+ (A; Cp* = η5-C5Me5) were theoretically investigated with the density functional theory (DFT) method. Both oxidative addition and metal–ligand cooperation modes have been studied for H–H and N–H bond cleavages, respectively. The H–H bond cleavage most likely occurs through competitive oxidative addition and metal–ligand cooperation, and both cleavage modes have similar overall free energy barriers (24.3 and 25.7 kcal/mol, respectively). The ligand-assisted N–H bond heterolytic cleavage mechanism is proposed for the NH3 reaction, and the general oxidative addition pathway can be reasonably ruled out, as it is kinetically unfavorable.

Ligand Substituents Govern the Efficiency and Mechanistic Path of Hydrogen Production with [Cp*Rh] Catalysts

We demonstrate that [Cp*Rh] complexes bearing substituted 2,2'-bipyridyl ligands are effective hydrogen evolution catalysts (Cp*=η5 -pentamethylcyclopentadienyl). Disubstitution (at the 4 and 4' positions) of the bipyridyl ligand (namely -tBu, -H, and -CF3 ) modulates the catalytic overpotential, in part due to involvement of the reduced ligand character in formally rhodium(I) intermediates. These reduced species are synthesized and isolated here; protonation results in formation of complexes bearing the unusual η4 -pentamethylcyclopentadiene ligand, and the properties of these protonated intermediates further govern the catalytic performance. Electrochemical studies suggest that multiple mechanistic pathways are accessible, and that the operative pathway depends on the applied potential and solution conditions. Taken together, these results suggest synergy in metal-ligand cooperation that modulates the mechanisms of fuel-forming catalysis with organometallic compounds bearing multiple non-innocent ligands.

Facile Ruthenium(II)-Catalyzed α-Alkylation of Arylmethyl Nitriles Using Alcohols Enabled by Metal–Ligand Cooperation

A facile ruthenium(II)-catalyzed α-alkylation of arylmethyl nitriles using alcohols is reported. The ruthenium pincer catalyst serves as an efficient catalyst for this atom-economical transformation that undergoes alkylation via borrowing hydrogen pathways, producing water as the only byproduct. Arylmethyl nitriles containing different substituents can be effectively alkylated using diverse primary alcohols. Notably, using ethanol and methanol as alkylating reagents, challenging ethylation and methylation of arylmethyl nitriles were performed. Secondary alcohols do not undergo alkylation reactions. Thus, phenylacetonitrile was chemoselectively alkylated using primary alcohols in the presence of secondary alcohols. Diols provided a mixture of products. When deuterium-labeled alcohol was used, the expected deuterium transposition occurred, providing both α-alkylation and α-deuteration of arylmethyl nitriles. Consumption of nitrile was monitored by GC, which indicated the involvement of first-order kinetics....

N-Heterocyclic Phosphenium Complex of Manganese: Synthesis and Catalytic Activity in Ammonia Borane Dehydrogenation.

A neutral N-heterocyclic phosphenium complex of manganese was synthesised by a metathesis approach and characterised by IR, NMR, and XRD studies. The short P-Mn distance suggests a substantial metal-ligand double bond character. Reaction with a hydride produced an anionic phosphine complex, which was also characterised by IR and NMR spectroscopy and, after anion exchange, a single-crystal XRD study. Protonation of the anion occurs at the metal to yield a neutral phosphine metal carbonyl hydride, which releases dihydrogen upon irradiation with UV light. These reactions confirm the electrophilic nature of the phosphenium ligand and suggest that the title complex might undergo reactions displaying metal-ligand cooperativity. Surprisingly, reaction with ammonia borane (AB) did not proceed under transfer hydrogenation of the Mn=P double bond but through the catalytic dehydrogenation of AB. The phosphenium complex behaves here as a class II catalyst, which dehydrogenates AB to NH2 BH2 that was trapped with cyclohexene. Computational model studies led to the identification of two possible catalytic cycles, which differ in the regioselectivity of the initial AB activation step. In one case, the activation proceeds as cooperative transfer hydrogenation of the Mn=P bond, whereas in the other case a H+ /H- pair is transferred to the phosphorus atom and a nitrogen atom of the phosphenium unit, resulting in a ligand-centred reaction in which the metal fragment acts merely as stabilising substituent. Unexpectedly, this pathway, which constitutes a new reaction mode for phosphenium complexes, seems to be better in accord with experimental findings on the course of the catalysis.

Cooperative or Oxidative Hydrogen Addition to 2‐Hydroxypyridonate Iridium Complexes: Dependence on Oxidation State

Iridium(III)–pyridone complexes are commonly found to react in a cooperative and redox‐neutral manner with dihydrogen and alcohols. In this work, the reactivity preferences of IrI–pyridone complexes were investigated under a variety of conditions. We have found that, in contrast to IrIII–pyridones, IrI–pyridone complexes display a strong preference to react non‐cooperatively. With a new chelating 2‐hydroxy‐8‐diphenylphosphinoquinoline ligand that does not dissociate after hydrogen addition, oxidative addition is still preferred. In the preparation of mono‐ and bidentate neutral and anionic pyridone ligands, Vaska's complex was used as a point of reference. We expect these findings to have implications for catalyst development in the field of metal–ligand cooperation.

Triazolylidene-Iridium Complexes with a Pendant Pyridyl Group for Cooperative Metal-Ligand Induced Catalytic Dehydrogenation of Amines.

Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non-coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such Npyr ⋅⋅⋅H-N interactions are demonstrated by X-ray diffraction analysis, 1 H NMR, and IR spectroscopy, and suggest a pathway of substrate bond-activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligand-metal cooperative substrate activation.

Ruthenium-Catalyzed Urea Synthesis by N-H Activation of Amines.

Activation of the N-H bond of amines by a ruthenium pincer complex operating via "amine-amide" metal-ligand cooperation is demonstrated. Catalytic formyl C-H activation of N,N-dimethylformamide (DMF) is observed in situ, which resulted in the formation of CO and dimethylamine. The scope of this new mode of bond activation is extended to the synthesis of urea derivatives from amines using DMF as a carbon monoxide (CO) surrogate. This catalytic protocol allows the synthesis of simple and functionalized urea derivatives with liberation of hydrogen, devoid of any stoichiometric activating reagents, and avoids the direct use of fatal CO. The catalytic carbonylation occurred at low temperature to provide the formamide; a formamide intermediate was isolated. The consecutive addition of different amines provided unsymmetrical urea compounds. The reactions are proposed to proceed via N-H activation of amines followed by CO insertion from DMF and with liberation of dihydrogen.

Acceptorless Alcohol Dehydrogenation: OH vs NH Effect in Bifunctional NHC–Ir(III) Complexes

Bifunctional complexes bearing N-heterocyclic carbene (NHC) ligands functionalized with hydroxy or amine groups were synthesized to measure the beneficial effect of different modes of metal–ligand cooperation in the acceptorless dehydrogenation of alcohols. In comparison to complexes with an amine moiety, hydroxy-functionalized iridium catalysts showed superior activity. In contrast to alcohols, 1,4-diols underwent cyclization to give the corresponding tetrahydrofurans without involving dehydrogenation processes. Mechanistic investigations to rationalize the “OH effect” in these types of complexes have been undertaken.

Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to Alcohols

Here, we report an additive-free catalytic system for hydrogenation of carboxylic acid esters to alcohols with a well-defined cobalt pincer catalyst precursor. Various substrates, including methyl, ethyl, and benzyl esters, have been evaluated under hydrogenation conditions; however, methyl esters have low reactivity compared to those of the corresponding ethyl and benzyl esters. The biomass-derived γ-valerolactone successfully formed 1,4-pentanediol with a turnover number of 3890 with this system. Metal–ligand cooperativity is probed with the related [PN(Me)P] derivative of the cobalt catalyst, and the results suggest a nonbifunctional hydrogenation mechanism.

Hydrogenation and Hydrosilylation of Nitrous Oxide Homogeneously Catalyzed by a Metal Complex.

Due to its significant contribution to stratospheric ozone depletion and its potent greenhouse effect, nitrous oxide has stimulated much research interest regarding its reactivity modes and its transformations, which can lead to its abatement. We report the homogeneously catalyzed reaction of nitrous oxide (N2O) with H2. The reaction is catalyzed by a PNP pincer ruthenium complex, generating efficiently only dinitrogen and water, under mild conditions, thus providing a green, mild methodology for removal of nitrous oxide. The reaction proceeds through a sequence of dihydrogen activation, “O”-atom transfer, and dehydration, in which metal–ligand cooperation plays a central role. This approach was further developed to catalytic O-transfer from N2O to Si–H bonds.

Ammonia Synthesis from a Pincer Ruthenium Nitride via Metal-Ligand Cooperative Proton-Coupled Electron Transfer.

The conversion of metal nitride complexes to ammonia may be essential to dinitrogen fixation. We report a new reduction pathway that utilizes ligating acids and metal-ligand cooperation to effect this conversion without external reductants. Weak acids such as 4-methoxybenzoic acid and 2-pyridone react with nitride complex [(H-PNP)RuN]+ (H-PNP = HN(CH2CH2PtBu2)2) to generate octahedral ammine complexes that are κ2-chelated by the conjugate base. Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride. The subsequent reduction to ammonia is enabled by intramolecular 2H+/2e- proton-coupled electron transfer from the saturated pincer ligand backbone.

Valorization of CO2: Preparation of 2-Oxazolidinones by Metal–Ligand Cooperative Catalysis with SCS Indenediide Pd Complexes

The capture and utilization of CO2 to prepare high-value compounds is very attractive chemically and highly desirable socially. Indenediide-based Pd SCS pincer complexes are shown here to promote the carboxylative cyclization of propargylamines leading to 2-oxazolidinones under mild conditions (0.5–1 bar of CO2, DMSO, 40–80 °C, 1–5 mol % Pd loading). The indenediide Pd complex is competitive with known catalysts. It proved successful for a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines, and propargylanilines. Thorough experimental (NMR) and computational (DFT) investigations were undertaken to gain mechanistic insights. Accordingly, (i) the resting state of the catalytic cycle is a Pd DMSO complex; (ii) the indenediide backbone and Pd center act in concert to activate the carbamic acid intermediate and promote its cyclization; (iii) proton shuttling is essential to lower the activ...

The Ferraquinone-Ferrahydroquinone Couple: Combining Quinonic and Metal-Based Reactivity.

A ferraquinone–ferrahydroquinone organometallic redox couple was prepared and characterized. Intricate cooperativity of the metal was observed with different positions on the ligand. This allowed cooperative activation of small molecules like molecular hydrogen, oxygen, and bromine. Likewise, dehydrogenation of alcohols was achieved through 1,6 metal–ligand cooperation.

Rhenium(I) Triscarbonyl Complexes with Redox-Active Amino- and Iminopyridine Ligands: Metal–Ligand Cooperation as Trigger for the Reversible Binding of CO2 via a Dearmomatization/Rearomatization Reaction Sequence

We describe rhenium(I) triscarbonyl compounds (3 and 4) decorated with simple-to-use 2-iminomethyl-pyridine (1, impy) and 2-aminomethyl-pyridine (2, ampy) ligands, respectively, which can serve as cooperative ligand scaffolds enabling CO2 binding via a formal [1,3] addition under Re–O and C–C bond formation. fac-[Re(impy)(CO)3Br] (3) is readily prepared by stirring (1-(pyridin-2-yl)-N-(p-tolyl)methanimine (impy, 1) and [Re(CO)5Br] in refluxing THF. Alternatively, complex 3 can be readily obtained when a mixture of [Re(CO)5Br], p-toluidine, and picolinaldehyde is refluxed in ethanol. Complex 3 is reduced with excess potassium metal in THF (two-electron reduction) to give the anionic amido complex K[Re(amidopy*)(CO)3] (5b, the asterisk indicates the dearomatized ligand). Analysis of the 1H and 13C{1H} NMR spectra of 5b suggest the dearomatization of the pyridine unit. Complex 5b is highly reactive and gives rise to the facile [1,3] addition of CO2. The addition of the CO2 and thus the formation of K[Re(amidopy-COO)(CO)3] (6) is characterized by the concomitant formation of a Re–O and a C–C bond. The addition is triggered by the rearomatization of the pyridine unit in 6. Remarkably, isotopic labeling experiments involving 13CO2 suggest a reversible binding of CO2 to the complex. The related amine complex fac-[Re(ampy)(CO)3Br] (4) is similarly prepared by stirring (4-methyl-N-(pyridin-2-ylmethyl)aniline (2) and [Re(CO)5Br] in THF at 60 °C. Upon addition of excess base (LiHMDS), complex 3 is readily deprotonated twice to give likewise the anionic amido complex Li[Re(amidopy*)(CO)3] (5a). The latter reaction gives rise to a facile access to the reactive dearomatized anionic fragment [Re(amidopy*)(CO)3]− with no need for the application of strong reducing agents. The ion pair M+/[Re(amidopy*)(CO)3]− is highly reactive and combines MLC (metal–ligand cooperation) via a dearomatization/rearomatization scheme and bifunctional reactivity enabled by the nucleophilic nature of the Re complex and the Lewis acidic counter alkali cation.

Iridium versus Iridium: Nanocluster and Monometallic Catalysts Carrying the Same Ligand Behave Differently.

A specific secondary phosphine oxide (SPO) ligand (tert-butyl(phenyl)phosphine oxide) was employed to generate two iridium catalysts, an Ir-SPO complex and IrNPs (iridium nanoparticles) ligated with SPO ligands, which were compared mutually and with several supported iridium catalysts with the aim to establish the differences in their catalytic properties. The Ir-SPO-based catalysts showed totally different activities and selectivities in the hydrogenation of various substituted aldehydes, in which H2 is likely cleaved by a metal-ligand cooperation, that is, the SPO ligand and a neighboring metal centre operate in tandem to activate the hydrogen molecule. In addition, the supported IrNPs behave very differently from both Ir-SPO catalysts. This study exemplifies perfectly the advantages and disadvantages related to the use of the main types of catalysts, and thus the dissimilarities between them.

A Uranium(IV) Triamide Species with Brønsted Basic Ligand Character: Metal–Ligand Cooperativity in the f Block

Deprotonation of the tridentate triamine ligand H3N3Mes ((2,4,6-Me3C6H2N(H)CH2CH2)2NH) with 2 equiv of KCH2Ph followed by treatment with 1 equiv of UCl4 afforded the diamidoamine uranium complex (THF)2UCl2(HN3Mes) (1-THF). This species was further derivatized with either O═PPh3 or KCp* to generate (Ph3PO)UCl2(HN3Mes) (1-OPPh3) or Cp*UCl(HN3Mes) (2-Cl), respectively. Deprotonation of 2-Cl with nBuLi furnished the uranium(IV) triamido compound Cp*U(N3Mes-LiCl(THF)2) (3-LiCl), which is stabilized by the presence of LiCl. 3-LiCl reacts readily with alcohols and thiols, including HOPh, HSPh, and HOtBu, to furnish the respective products Cp*U(OPh)(HN3Mes) (2-OPh), Cp*U(SPh)(HN3Mes) (2-SPh), and Cp*U(OtBu)(HN3Mes) (2-OtBu), which show cooperative addition of the H–E (E = O, S) bond across the U–N bond, serving to regenerate the diamidoamine ligand. Similar cooperative addition was noted for 3-LiCl with benzophenone, furnishing Cp*U(N3Mes-OCPh2) (3-OCPh2), which features new U–O and N–C bonds. The Bronsted basici...