Dianionic Pentadentate Ligand Research Articles

Increasing Ligand Denticity and Stability for a Water Oxidation Electrocatalyst using P(V) as Connecting Element

AbstractCarboxylate complexes have risen to prominence in the field of water oxidation catalysis. Here for the first time we use the higher valence of phosphinates [P(V)] relative to that of carboxylates [C(IV)] to increase ligand denticity. We describe the synthesis and characterization of a new dianionic pentadentate ligand, bcpq2− that contains a tridentate 2,2’‐bipyridine‐6‐carboxylato moiety, in addition to a 6’‐phosphinato substituent that acts as fourth ligand and bears a side arm containing a quinoline, the fifth ligand. The new bcpq ligand allows formation of [Ru(II)(bcpq)(L)] (2 a–b, L=picoline or isoquinoline) and in preliminary results, of a Co(II) complex. NMR spectroscopy, X‐ray diffraction, cyclic voltammetry, differential pulse and square wave voltammetry were used to characterize 2 a–b, with 2 b being characterized more extensively as a catalyst. Bulk electrolysis over 15 h at pH 7 was also used, showing that 2 b gave 100±5 % faradaic efficiency and remained completely homogeneous, whereas 1 b was no longer homogeneous; this comparison conclusively shows the advantage of the added denticity in the electrocatalytic context. Replacing carboxylate with P(V) phosphinate with an added arm may be used in other ligand systems to enhance the durability of homogeneous catalysts.

Oxidation-induced ambiphilicity triggers N-N bond formation and dinitrogen release in octahedral terminal molybdenum(v) nitrido complexes.

Coupling of octahedral, terminal d1 molybdenum(v) nitrido complexes supported by a dianionic pentadentate ligand via N-N bond formation to give μ-dinitrogen complexes was found to be thermodynamically feasible but faces significant kinetic barriers. However, upon oxidation, a kinetically favored nucleophilic/electrophilic N-N bond forming mechanism was enabled to give monocationic μ-dinitrogen dimers. Computational and experimental evidence for this "oxidation-induced ambiphilic nitrido coupling" mechanism is presented. The factors influencing release of dinitrogen from the resulting μ-dinitrogen dimers were also probed and it was found that further oxidation to a dicationic species is required to induce (very rapid) loss of dinitrogen. The mechanistic path discovered for N-N bond formation and dinitrogen release follows an ECECC sequence (E = "electrochemical step"; C = "chemical step"). Experimental evidence for the intermediacy of a highly electrophilic, cationic d0 molybdenum(vi) nitrido in the N-N bond forming mechanism via trapping with an isonitrile reagent is also discussed. Together these results are relevant to the development of molecular catalysts capable of mediating ammonia oxidation to dihydrogen and dinitrogen.

Eight-coordinate mono- and dinuclear Dy(III) complexes containing a rigid equatorial plane and an anisobidentate carboxylate ligand in the axial position: synthesis, structure and magnetism.

A rigid pentadentate chelating ligand (H2L) has been utilized to synthesize a series of octacoordinate mononuclear complexes, [Dy(L)(Ph3PO)(OOCR)] (where R = C6H5 (1), C(CH3)3 (2), CF3 (3)) and a dinuclear complex, [Dy2(L)2(Ph3PO)2{(OOC)2C6H4}] (4) based on the highly anisotropic Dy(III) ion. All the complexes were structurally characterized by single-crystal X-ray diffraction studies. The complexes were formed by the coordination action of the dianionic pentadentate ligand [L]2-, one phosphine oxide, and carboxylate ligands. DC and AC magnetic measurements were performed on 1-4. Complexes 1-4 show SMM behaviour, under zero DC field for 1 and 4, and under 500 Oe and 1000 Oe DC fields for 2 and 3 respectively, with thermally activated, Raman, and Raman and quantum tunnelling dominant relaxation mechanisms for 1 and 2, 3 and 4, respectively.

Organotitanium Complexes Supported by a Dianionic Pentadentate Ligand

Organotitanium Complexes Supported by a Dianionic Pentadentate Ligand

Spontaneous Ammonia Activation Through Coordination-Induced Bond Weakening in Molybdenum Complexes of a Dianionic Pentadentate Ligand Platform.

Ammonia oxidation catalyzed by molecular compounds is of current interest as a carbon-free source of dihydrogen. Activation of N-H bonds through coordination to transition metal centers is a key reaction in this process. We report the substantial activation of ammonia via reaction with low-valent molybdenum complexes of a diborate pentadentate ligand system. Spontaneous loss of dihydrogen from (B2 Pz4 Py)MoII -NH3 at room temperature to produce the dinuclear μ-nitrido compound (B2 Pz4 Py)Mo-N-Mo(B2 Pz4 Py) is observed due to substantial N-H bond weakening upon coordination to Mo. Mechanistic details are supported through the experimental observation/characterization of terminal amido, imido and nitrido complexes and density functional theory computations. The generally under-appreciated role of bridging nitrido intermediates is revealed and discussed, providing guidance for further catalyst development for this process.

Spontaneous Ammonia Activation Through Coordination‐Induced Bond Weakening in Molybdenum Complexes of a Dianionic Pentadentate Ligand Platform**

AbstractAmmonia oxidation catalyzed by molecular compounds is of current interest as a carbon‐free source of dihydrogen. Activation of N−H bonds through coordination to transition metal centers is a key reaction in this process. We report the substantial activation of ammonia via reaction with low‐valent molybdenum complexes of a diborate pentadentate ligand system. Spontaneous loss of dihydrogen from (B2Pz4Py)MoII−NH3 at room temperature to produce the dinuclear μ‐nitrido compound (B2Pz4Py)Mo‐N‐Mo(B2Pz4Py) is observed due to substantial N−H bond weakening upon coordination to Mo. Mechanistic details are supported through the experimental observation/characterization of terminal amido, imido and nitrido complexes and density functional theory computations. The generally under‐appreciated role of bridging nitrido intermediates is revealed and discussed, providing guidance for further catalyst development for this process.

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate.

We report the use of electron rich iron complexes supported by a dianionic diborate pentadentate ligand system, B2Pz4Py, for the coordination and activation of ammonia (NH3) and hydrazine (NH2NH2). For ammonia, coordination to neutral (B2Pz4Py)Fe(ii) or cationic [(B2Pz4Py)Fe(iii)]+ platforms leads to well characterized ammine complexes from which hydrogen atoms or protons can be removed to generate, fleetingly, a proposed (B2Pz4Py)Fe(iii)–NH2 complex (3Ar-NH2). DFT computations suggest a high degree of spin density on the amido ligand, giving it significant aminyl radical character. It rapidly traps the H atom abstracting agent 2,4,6-tri-tert-butylphenoxy radical (ArO˙) to form a C–N bond in a fully characterized product (2Ar), or scavenges hydrogen atoms to return to the ammonia complex (B2Pz4Py)Fe(ii)–NH3 (1Ar-NH3). Interestingly, when (B2Pz4Py)Fe(ii) is reacted with NH2NH2, a hydrazine bridged dimer, (B2Pz4Py)Fe(ii)–NH2NH2–Fe(ii)(B2Pz4Py) ((1Ar)2-NH2NH2), is observed at −78 °C and converts to a fully characterized bridging diazene complex, 4Ar, along with ammonia adduct 1Ar-NH3 as it is allowed to warm to room temperature. Experimental and computational evidence is presented to suggest that (B2Pz4Py)Fe(ii) induces reductive cleavage of the N–N bond in hydrazine to produce the Fe(iii)–NH2 complex 3Ar-NH2, which abstracts H˙ atoms from (1Ar)2-NH2NH2 to generate the observed products. All of these transformations are relevant to proposed steps in the ammonia oxidation reaction, an important process for the use of nitrogen-based fuels enabled by abundant first row transition metals.

Synthesis, Characterization, and Reactivity of Neutral Octahedral Alkyl-Cobalt(III) Complexes Bearing a Dianionic Pentadentate Ligand

A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal pentadentate ligand B2Pz4Py are reported. Compounds 2-R (R═CH3, CH2SiMe3, CH2SiMe2Ph, iBu, CH2(c-C5H9), and (CH2)4CH═...

Tandem deoxygenative hydrosilation of carbon dioxide with a cationic scandium hydridoborate and B(C6F5)3.

A scandium hydridoborate complex supported by the dianionic pentadentate ligand B2Pz4Py is prepared via hydride abstraction from the previously reported scandium hydride complex with tris-pentafluorophenyl borane. Exposure of [(B2Pz4Py)Sc][HB(C6F5)3] to CO2 immediately forms [(B2Pz4Py)Sc][HCOOB(C6F5)3] at room temperature. The formatoborate complex can also be synthesized directly from the starting material (B2Pz4Py)ScCl with Et3SiH and B(C6F5)3 while in the presence of an atmosphere of CO2 in 81% yield. This compound was evaluated as the transition metal component of a tandem deoxgenative CO2 hydrosilation catalyst. At 5% loadings, complete consumption of Et3SiH was observed along with CO2 reduction products, but conversion to an inactive scandium complex identified as (B2Pz4Py)ScOSiEt3 was observed.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical.

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O2 to water via Co(III)-O-O-Co(III) peroxo intermediates.

Scandium alkyl and hydride complexes supported by a pentadentate diborate ligand: reactions with CO2 and N2O.

Alkyl and hydrido scandium complexes of the dianionic pentadentate ligand B2Pz4Py are reported. The key starting material (B2Pz4Py)ScCl is readily prepared and alkylated with organolithium reagents RLi (R = CH3, CH2SiMe3, CH2SiMe2Ph, CH2CH2CH3 and CH2CHMe2) to form alkyl derivatives in 61-93% yields. These compounds are very thermally stable and do not undergo sigma bond metathesis reactions with dihydrogen. The hydrido complex was prepared from (B2Pz4Py)ScCl and NaHBEt3 in 80% yield and was found to be more stable by 28 kcal mol-1 as a dimer, rather than a monomeric hydrido complex. However, the monomer is accessible through dissociation of the dimer at 80 °C. All of the compounds (B2Pz4Py)ScR react with water to form the bridging oxo dimer (B2Pz4Py)ScOSc(B2Pz4Py). The reactivity of the hydrido and methyl complexes towards carbon dioxide was explored; heating to 80 °C results in the formation of κ2 formato and acetate complexes, respectively. The mechanisms were studied via density functional theory and distinct transition states for insertion of CO2 into the Sc-R (R = H, CH3) were found, with the insertion into Sc-CH3 being more enthalpically difficult (by 18 kcal mol-1) than insertion into Sc-H. The slow rate of reaction between [(B2Pz4Py)ScH]2 and CO2 is attributed to the barrier associated with dimer dissociation. In both insertion reactions, the kinetic products are κ1 formato or acetate complexes that are only slightly less stable than the observed κ2 derivatives. The κ1 compounds can therefore be trapped by treating the κ2 isomers with tris-pentafluorophenyl borane.

Cerium‐Complex‐Catalyzed Oxidation of Arylmethanols under Atmospheric Pressure of Dioxygen and Its Mechanism through a Side‐On μ‐Peroxo Dicerium(IV) Complex

AbstractInvited for the cover of this issue is the group of Hayato Tsurugi and Kazushi Mashima at the University of Osaka. The image depicts a cerium complex with a dianionic pentadentate ligand acting as a catalyst for aerobic oxidation of arylmethanols, in which a dinuclear cerium(IV) side‐on peroxo complex is the key intermediate. The corresponding trivalent complex was able to activate dioxygen, and the oxidation proceeded catalytically under atmospheric pressure with inexpensive and environmentally friendly dioxygen as the primary oxidant. Read the full text of the article at 10.1002/chem.201503846.

Cerium-Complex-Catalyzed Oxidation of Arylmethanols under Atmospheric Pressure of Dioxygen and Its Mechanism through a Side-On μ-Peroxo Dicerium(IV) Complex.

A new Ce(IV) complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)2] (1), bearing a dianionic pentadentate ligand with an N3O2 donor set, has been prepared by treating (NH4)2Ce(NO3)6 with the sodium salt of ligand L1. Complex 1 in the presence of TEMPO and 4 Å molecular sieves (MS4 A) has been found to serve as a catalyst for the oxidation of arylmethanols using dioxygen as an oxidant. We propose an oxidation mechanism based on the isolation and reactivity study of a trivalent cerium complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)(THF)] (2), its side-on μ-O2 adduct [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)]2(μ-η(2):η(2)-O2) (3), and the hydroxo-bridged Ce(IV) complex [Ce{NH(CH2CH2N=CHC6H2-3,5-(tBu)2-2-O)2}(NO3)]2(μ-OH)2 (4) as key intermediates during the catalytic cycle. Complex 2 was synthesized by reduction of 1 with 2,5-dimethyl-1,4-bis(trimethylsilyl)-1,4-diazacyclohexadiene. Bubbling O2 into a solution of 2 resulted in formation of the peroxo complex 3. This provides the first direct evidence for cerium-catalyzed oxidation of alcohols under an O2 atmosphere.

Facile hydrogen atom transfer to iron(iii) imido radical complexes supported by a dianionic pentadentate ligand.

A dianionic tetrapodal pentadentate diborate ligand is introduced. This ligand forms a high spin neutral iron(ii) complex that reacts with a variety of organoazides to yield transient Fe(iii) imido radicals that are extremely potent hydrogen atom abstractors. The nature of these species is supported by full characterization of the Fe(iii) amido products, kinetic studies, density functional computations and Mössbauer spectroscopy on the -C6H4-p- t Bu substituted derivative.

Synthesis and characterization of a cationic oxorhenium(V) complex containing the pentadentate N3O2-donor ligand bis ( N -methylsalicylideneiminopropyl)amine

The complex salt [ReO(bsa)]PF6 (H2bsa = bis(N-methylsalicylideneiminopropyl)amine) was prepared from the reaction of cis-[ReO2I(PPh3)2] with H2bsa in toluene. The dianionic pentadentate ligand bsa is coordinated to the ReO3+ moiety via one secondary amino and two imino nitrogens, and two anionic phenolate oxygens. The complex was characterized by spectroscopy and analytical data, and the structure has been determined by single-crystal X-ray diffraction analysis.

Synthesis, structure, and properties of N,N′-bis[2-(1-pyrazolyl)ethyl]-pyridine-2,6-dicar☐amido copper(II)

The pentadentate dipeptide ligand N,N′-bis[2-(1-pyrazolyl(ethyl]pyridine-2,6-dicar☐amide (PyPz2PH2) (1) (H denotes dissociable amide H) has been synthesized. The Cu(II) complex of this ligand namely, [Cu(PyPz2P)] (2), has been isolated and structurally characterized. Complex 2 crystallizes in the triclinic space group P1¯ with a = 7.743(3), b = 9.427(2), c = 12.809(3)Å, α = 71.01(2), β = 80.38(2), γ = 70.54(2)°, V = 831.8(4)Å and Z = 2. The structure was refined to R = 4.13% by using 3313 (I > 2σ(I)) data. The dianionic pentadentate ligand coordinates to Cu(II) in a distorted square pyramidal fashion. The Cu center is lifted out of the mean basal plane by 0.32Å. The spectral parameters of 2 have been compared with those of an analogous complex [Cu(Py3P)] that contains pyridine rings in place of pyrazoles on a similar ligand framework. Such comparison indicates that pyridine nitrogens coordinate to Cu(II) more strongly than 2-pyrazolyl nitrogens.