Triflate Complexes Research Articles

Al(III) and Ga(III) triflate complexes as solvate ionic liquids: speciation and application as soluble and recyclable Lewis acidic catalysts.

This work reports on the first solvate ionic liquids (SILs) based on aluminium(III) and gallium(III) triflates, M(OTf)3, and triglyme (G3). Liquid-phase speciation of these new SILs was studied by multinuclear NMR spectroscopy. Across the compositional range of G3 : M(OTf)3 mixtures, both metals were found to be in a hexacoordinate environment, with both G3 and [OTf]- ligands present in the first coordination sphere, and apparently exchanging through a dynamic equilibrium. The Lewis acidity was quantified by the Gutmann acceptor number (AN) and compared to the performance of SILs as Lewis acidic catalysts in model [3 + 3] cycloadditions. Despite saturated coordination, AN values were relatively high, reaching AN = ca. 71-83 for Al-SILs and ca. 80-93 for Ga-SILs, corroborating the labile nature of the metal-ligand bonding. In a model catalytic reaction, SILs were fully soluble in the reaction mixtures, in contrast to the corresponding triflate salts. The catalytic performance of SILs exceeded that of the corresponding triflate salts, and Ga-SILs were more active than Al-SILs, in agreement with AN measurements. In conclusion, the new Group 13 SILs can be considered as soluble and catalytically active forms of their corresponding metal triflates, with potential uses in catalysis.

Oxygenolytic cleavage of 1,2-diols with dioxygen by a mononuclear nonheme iron complex: Mimicking the reaction of myo-inositol oxygenase

A mononuclear iron(II) complex, [(TpPh2)FeII(OTf)(CH3CN)] (1) (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, OTf = triflate) has been isolated and its efficiency toward the aliphatic CC bond cleavage reaction of 1,2-diols with dioxygen has been investigated. Separate reactions between 1 and different 1,2-diolates form the corresponding iron(II)-diolate complexes in solution. While the iron(II) complex of the tetradentate TPA (tris(2-pyridylmethyl)amine) ligand is not efficient in affecting the CC cleavage of 1,2-diol with dioxygen, complex 1 displays catalytic activity to afford carboxylic acid and aldehyde. Isotope labeling studies with 18O2 reveal that one oxygen atom from dioxygen is incorporated into the carboxylic acid product. The oxygenative CC cleavage reactions occur on the 1,2-diols containing at least one α-H atom. The kinetic isotope effect value of 5.7 supports the abstraction of an α-H by an iron(III)-superoxo species to propagate the CC cleavage reactions. The oxidative cleavage of 1,2-diolates by the iron(II) complex mimics the reaction catalyzed by the nonheme diiron enzyme, myo-inositol oxygenase.

Y(OTf)3-Salazin-Catalyzed Asymmetric Aldol Condensation.

The chiral aziridine-containing vicinal iminophenol tridentate ligands (named salazins) are a class of readily prepared chiral ligands from enantiopure aziridines and salicylaldehydes. Their scandium and yttrium triflate complexes show excellent reactivity and enantioselectivities in the catalytic asymmetric aldol condensation of electron-deficient aromatic aldehydes and ketones, including acetone and cycloalkanones. The stereoselectivity is rationalized to the strong π-stacking interaction between aromatic aldehydes and the vicinal iminophenol group in the chiral ligands.

Synthesis and Derivatives of Beryllium Triflate.

Various pathways for the synthesis of beryllium triflate were investigated. The reaction of triflic acid or trimethylsilyl triflate with beryllium metal in liquid ammonia led to the formation of mono-, di-, and tetra-nuclear beryllium ammine complexes. Utilization of SMe2 as a solvent gave homoleptic Be(OTf)2. Various beryllium triflate complexes with N- and O-donor ligands as well as the complex anions [Be(OTf)4]2- and [Be2(OTf)6]2- were synthesized to evaluate the reactivity and solution properties of beryllium triflate. This showed that OTf- is not a weakly coordinating anion for Be2+ cations and that it exhibits good bridging properties.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction.

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

Bis(phenanthrolinyl)amine manganese complexes with pseudo-trigonal prismatic geometry

Two manganese(II) complexes supported by an isoamyl-substituted bis(phenanthrolinyl)amine ligand have been prepared and characterized. Determination of the molecular structures through X-ray crystallography demonstrated that the triflate compound crystallizes as a monomeric species with an outer-sphere anion, while the chloride complex exists as a dimer containing bridging chloride ligands in the solid state. Importantly, these structures revealed a coordination environment about the metal centre that is best described as pseudo-trigonal prismatic in both cases. Computational analysis found an optimized geometry for the triflate complex in a high-spin state that is consistent with the experimental observations, while the low-spin calculation showed a significant shift toward octahedral geometry. These results and their contrast with previously reported data provide evidence for the influence of electronic structure and metal ion size on molecular geometry in this system.

Structural Diversity in Divalent Group 14 Triflate Complexes Involving Endocyclic Thia-Macrocyclic Coordination.

A highly unusual series of M(II) (M = Ge, Sn, Pb) complexes with endocyclic thioether macrocyclic coordination and with coordination numbers ranging from three to nine have been prepared by the reaction of [9]aneS3 (1,4,7-trithiacyclononane), [12]aneS4 (1,4,7,10-tetrathiacyclododecane), or [24]aneS8 (1,4,7,10,13,16,19,22-octathiacyclotetracosane) with M(OTf)2 (M = Sn and Pb; OTf = CF3SO3-) or with GeCl2·dioxane and 2 mol equiv of TMSOTf (Me3SiO3SCF3) in a mixture of anhydrous CH2Cl2 and MeCN. The isolated bulk products are characterized by 1H, 13C{1H}, 19F{1H}, and 119Sn{1H} NMR and IR spectroscopy, high-resolution ESI+ MS, and microanalytical data. Crystal structures are also reported for [M(L)][OTf]2 (M = Ge, Sn, Pb; L = [9]aneS3, [12]aneS4) and for [M([24]aneS8)][OTf]2 (M = Sn, Pb). In all cases, the ligand is bound in an endocyclic fashion, but the coordination environment and number are highly dependent on the group 14 ion, the macrocyclic ring size, and the number of S-donor atoms it presents. Solution NMR spectroscopic data suggest that the metal-macrocycle coordination is retained in solution but that the triflate anions are extensively dissociated on the NMR timescale. Density functional theory calculations on the [M([9]aneS3)]2+ and [M([12]aneS4)]2+ (M = Ge, Sn, Pb) dications reveal that the HOMO is centered on the group 14 atom as a directional "lone pair"; it also retains a significant amount of positive charge.

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate.

The catalytic transformation of N2 to NH3 by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N2 fixation, well-defined vanadium complexes that can catalyze the conversion of N2 to NH3 are scarce. In particular, a V(NxHy) intermediate derived from proton/electron transfer reactions of coordinated N2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(tBu2PO)2-C6H3) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N2 to NH3 and N2H4. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH2), which mediated 15N2 conversion to 15NH3, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH2 possessed a N-H bond dissociation free energy (BDFEN-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH2) with [Ph2NH2][OTf] resulted in the release of NH3 and the formation of a vanadium triflate complex, which upon reduction under N2 afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N2 reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH3. These findings provide unprecedented insights into the mechanism of N2 reduction related to FeV nitrogenase.

Synthesis, spectroscopic and structural properties of Sn(II) and Pb(II) triflate complexes with soft phosphine and arsine coordination.

Reaction of the divalent M(OTf)2 (M = Sn, Pb; OTf = CF3SO3) with soft phosphine and arsine ligands, L, where L = o-C6H4(ER2)2 (E = P, R = Me or Ph; E = As, R = Me), MeC(CH2ER2)3 (E = P, R = Ph; E = As, R = Me), PhP(CH2CH2PPh2)2 or P(CH2CH2PPh2)3, affords complexes of stoichiometry M(L)(OTf)2 as white powders, which have been characterised via elemental analysis, 1H, 19F{1H}, 31P{1H} and 119Sn NMR spectroscopy, with the expected 31P-119Sn and 31P-207Pb couplings clearly evident. The crystal structures of nine of these pnictine complexes are reported, in each case revealing retention of one or both OTf anions, which gives rise to a diverse range of coordination environments including monomers, as well as varying degrees of oligomerisation to form weakly associated (OTf-bridged) dimers, trimers and polymers. 19F{1H} NMR spectra indicate that the OTf is essentially anionic (dissociated) in solution. Anion metathesis of [M(OTf)2{MeC(CH2PPh2)3}] with Na[BArF] (BArF = B{3,5-(CF3)2C6H3}4) yields the corresponding [M{MeC(CH2PPh2)3}][BArF]2 salts, the crystal structures of all three (M = Ge, Sn, Pb) reveal pyramidal dications with discrete [BArF]- anions providing charge balance. Density functional theory (DFT) calculations on these [M{MeC(CH2PPh2)3}]2+ (M = Ge, Sn, Pb) dications using the B3LYP-D3 functional show the presence of a directional lone pair, which is a mixture of valence s and pz character, with the valence p-orbital character decreasing down group 14. Natural bond orbital (NBO) analysis also shows that the natural charge at the metal centre increases and the charge on the P centre decreases upon going down group 14.

FeII complexes supported by an iminophosphorane ligand: synthesis and reactivity.

The synthesis of iron complexes supported by a mixed phosphine-lutidine-iminophosphorane (PPyNP) ligand was carried out. While bidentate κ2-N,N coordination was observed for FeCl2, pincer coordination modes were adopted at cationic iron centers, either through dechlorination of [LFe(PPyNP)Cl2] (1) or direct coordination of PPyNP to Fe(OTf)2. Reaction with tert-butylisocyanide gave access to the diamagnetic octahedral complex [Fe(PPyNP)(CNtBu)3]X2 (X = OTf (4), Cl (4')). Both 1 and 4 were shown to undergo deprotonation of the phosphinomethyl group, but the resulting complexes were not active for the dehydrogenative coupling of hexan-1-ol. The hydrosilylation of acetophenones was catalyzed at room temperature with 1 mol% of a catalyst generated in situ from cationic PPyNP-supported iron triflate complexes and KHBEt3.

Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes.

The synthesis of P,N-phosphaalkene ligands, py-CH═PMes* (1, py = 2-pyridyl, Mes* = 2,4,6-tBu-C6H2) and the novel quin-CH═PMes* (2, quin = 2-quinolinyl) is described. The reaction with [Rh(μ-Cl)cod]2 produces Rh(I) bis(phosphaalkene) chlorido complexes 3 and 4 with distorted trigonal bipyramidal coordination environments. Complexes 3 and 4 show a pronounced metal-to-ligand charge transfer (MLCT) from Rh into the ligand P═C π* orbitals. Upon heating, quinoline-based complex 4 undergoes twofold C-H bond activation at the o-tBu groups of the Mes* substituents to yield the cationic bis(phosphaindane) Rh(I) complex 5, which could not be observed for the pyridine-based analogue 3. Using sub- or superstoichiometric amounts of AgOTf the C-H bond activation at an o-tBu group of one or at both Mes* was detected, respectively. Density functional theory (DFT) studies suggest an oxidative proton shift pathway as an alternative to a previously reported high-barrier oxidative addition at Rh(I). The Rh(I) mono- and bis(phosphaindane) triflate complexes 6 and 7, respectively, undergo deprotonation at the benzylic CH2 group of the phosphaindane unit in the presence of KOtBu to furnish neutral, distorted square-planar Rh(I) complexes 8 and 9, respectively, with one of the P,N ligands being dearomatized. All complexes were fully characterized, including multinuclear NMR, vibrational, and ultraviolet-visible (UV-vis) spectroscopy, as well as single-crystal X-ray and elemental analysis.

Biphenyl appended non-noble metal complexes as electrocatalysts for the electrochemical oxygen reduction reaction

The oxygen reduction reaction (ORR) is one of the most important reactions in many electrochemical processes. There has been a growing interest in the replacement of noble metal electrocatalysts for oxygen reduction reaction (ORR). In continuation of these efforts herein we report the design and synthesis of biphenyl appended non-noble transition metal and Zn(II) complexes as electrocatalysts in the aforementioned reaction. In this regard nitrogen-based bidentate ligand, 6-(quinolin-8-yl)-6,7-dihydro-5H-dibenzo[c,e]azepine as Ligand, L was synthesized and used for the preparation of a series of mononuclear Ni(II) and Zn(II) complexes, [ML(X)2] which differ only by the halide ion attached to the respective metal center. All complexes of this series showed tetrahedral geometry regardless of the halide ion and metal center (Ni/Zn). The complexes were characterized by different spectroscopic techniques. The triflate (OTf) complexes of Fe(II) and Cu(II) were also synthesized to evaluate their ORR activity. All the complexes were investigated for their electrocatalytic activity in oxygen reduction reaction (ORR) in which complex 1, [NiL(Cl)2] showed the highest activity with an onset potential of 0.75 V. The remaining complexes also showed significant ORR performance with onset potentials close to that of complex 1.

Selective Reduction of CO2 to a Tantalum Formate Complex and Release of Methyl Formate from the Tantalum(V) Center.

A cyclometalated NHC-coordinated hydrido tantalum alkoxide was found to selectively react with CO2 to afford the genuine tantalum formate (NHC)(HCOO)Ta(ORF)3 with ORF = OC(CF3)2CH3. In the solid state, the presence of a κ2-O,O-formate moiety was established by single-crystal X-ray diffraction and ATR-IR spectroscopy, while NMR experiments and DFT modeling studies suggest that the κ1-O-coordination mode is preferred in solution. Despite the accessibility of the latter κ1-O-formate in solution, no over-reduction to a dinuclear methylene diolate was observed. Upon treatment with MeOTf, the κ1-O-formate was methylated selectively, which led to the formation of a tantalum triflate complex along with methyl formate. This is a rare example in which a value-added oxygen-containing organic product (here HCOOMe) is released from an oxophilic early transition metal (here TaV).

Communication—Voltammetry of Lanthanide (III) Triflates Accessible in Acetonitrile at Nafion Modified Electrodes

Lanthanides are common in catalysis and advanced technologies, but difficult to assess voltammetrically. Voltammetry for seven lanthanide(III) trifluoromethanesulfonate complexes Ln(OTf)3 is enabled at Nafion® modified platinum electrodes in acetonitrile. Voltammetric morphologies for La3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, and Yb3+ trifluoromethanesulfonate (triflate) complexes exhibit reduction waves between 0.2 to 0.1 V and −0.7 to −0.8 V vs NHE with coupled chemical processes.

Spin State Tunes Oxygen Atom Transfer towards FeIV O Formation in FeII Complexes.

Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non-heme iron-dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes [FeII (CH3 CN)2 L](SbF6 )2 ([1](SbF6 )2 and [2](SbF6 )2 ) and [FeII (CF3 SO3 )2 L] ([1](OTf)2 and [2](OTf)2 (1, L=Me,H Pytacn; 2, L=nP,H Pytacn; R,R' Pytacn=1-[(6-R'-2-pyridyl)methyl]-4,7- di-R-1,4,7-triazacyclononane) with Bu4 NIO4 to form the corresponding [FeIV (O)(CH3 CN)L]2+ (3, L=Me,H Pytacn; 4, L=nP,H Pytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl- as entering ligand, which indicates that formation of [FeIV (O)(CH3 CN)L]2+ is kinetically controlled by substitution in the starting complex to form [FeII (IO4 )(CH3 CN)L]+ intermediates that are converted rapidly to oxo complexes 3 and 4. The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin-crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high-valent iron oxo species.

Cationic Group VI Metal Imido Alkylidene N‐Heterocyclic Carbene Nitrile Complexes: Bench‐Stable, Functional‐Group‐Tolerant Olefin Metathesis Catalysts

AbstractDespite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes.

Cationic Group VI Metal Imido Alkylidene N-Heterocyclic Carbene Nitrile Complexes: Bench-Stable, Functional-Group-Tolerant Olefin Metathesis Catalysts.

Despite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes.

Asymmetric hetero‐Diels‐Alder Reaction of trans‐1‐Methoxy‐3‐trimethylsilyloxy‐buta‐1,3‐diene Catalyzed by Zinc Complexes

Asymmetric hetero‐Diels‐Alder reactions of Danishefsky's diene and glyoxylate/pyruvate esters catalyzed by readily available zinc triflate complex with BOX ligand afforded the corresponding adduct which upon treatment with trifluoroacetic acid furnished the hetero‐Diels‐Alder product in 91 % enantiomeric excess and 87 % isolated yield. This reaction was applied to the concise synthesis of six‐membered ulosonic acid C‐glycoside.

Synthesis of LnII‐in‐Cryptand Complexes by Chemical Reduction of LnIII‐in‐Cryptand Precursors: Isolation of a NdII‐in‐Cryptand Complex

AbstractLanthanide triflates have been used to incorporate NdIII and SmIII ions into the 2.2.2‐cryptand ligand (crypt) to explore their reductive chemistry. The Ln(OTf)3 complexes (Ln=Nd, Sm; OTf=SO3CF3) react with crypt in THF to form the THF‐soluble complexes [LnIII(crypt)(OTf)2][OTf] with two triflates bound to the metal encapsulated in the crypt. Reduction of these LnIII‐in‐crypt complexes using KC8 in THF forms the neutral LnII‐in‐crypt triflate complexes [LnII(crypt)(OTf)2]. DFT calculations on [NdII(crypt)]2+], the first NdII cryptand complex, assign a 4f4 electron configuration to this ion.

Synthesis of LnII -in-Cryptand Complexes by Chemical Reduction of LnIII -in-Cryptand Precursors: Isolation of a NdII -in-Cryptand Complex.

Lanthanide triflates have been used to incorporate NdIII and SmIII ions into the 2.2.2-cryptand ligand (crypt) to explore their reductive chemistry. The Ln(OTf)3 complexes (Ln=Nd, Sm; OTf=SO3 CF3 ) react with crypt in THF to form the THF-soluble complexes [LnIII (crypt)(OTf)2 ][OTf] with two triflates bound to the metal encapsulated in the crypt. Reduction of these LnIII -in-crypt complexes using KC8 in THF forms the neutral LnII -in-crypt triflate complexes [LnII (crypt)(OTf)2 ]. DFT calculations on [NdII (crypt)]2+ ], the first NdII cryptand complex, assign a 4f4 electron configuration to this ion.