Catalytic reduction of dioxygen to water with a monomeric manganese complex at room temperature

There have been numerous efforts to incorporate dioxygen into chemical processes because of its economic and environmental benefits. The conversion of dioxygen to water is one such example, having importance in both biology and fuel cell technology. Metals or metal complexes are usually necessary to promote this type of reaction and several systems have been reported. However, mechanistic insights into this conversion are still lacking, especially the detection of intermediates. Reported herein is the first example of a monomeric manganese(II) complex that can catalytically convert dioxygen to water. The complex contains a tripodal ligand with two urea groups and one carboxyamidopyridyl unit; this ligand creates an intramolecular hydrogen-bonding network within the secondary coordination sphere that aids in the observed chemistry. The manganese(II) complex is five-coordinate with an N(4)O primary coordination sphere; the oxygen donor comes from the deprotonated carboxyamido moiety. Two key intermediates were detected and characterized: a peroxo-manganese(III) species and a hybrid oxo/hydroxo-manganese(III) species (1). The formulation of 1 was based on spectroscopic and analytical data, including an X-ray diffraction analysis. Reactivity studies showed dioxygen was catalytically converted to water in the presence of reductants, such as diphenylhydrazine and hydrazine. Water was confirmed as a product in greater than 90% yield. A mechanism was proposed that is consistent with the spectroscopy and product distribution, in which the carboxyamido group switches between a coordinated ligand and a basic site to scavenge protons produced during the catalytic cycle. These results highlight the importance of incorporating intramolecular functional groups within the secondary coordination sphere of metal-containing catalysts.     

DFT investigation of the molybdenum cofactor in periplasmic nitrate reductases: structure of the Mo(V) EPR-active species

The periplasmic nitrate reductase NAP belongs to the DMSO reductase family that regroups molybdoenzymes housing a bis-molybdopterin cofactor as the active site. Several forms of the Mo(V) state, an intermediate redox state in the catalytic cycle of the enzyme, have been evidenced by EPR spectroscopy under various conditions, but their structure and catalytic relevance are not fully understood. On the basis of structural data available from the literature, we built several models that reproduce the first coordination sphere of the molybdenum cofactor and used DFT methods to make magneto-structural correlations on EPR-detected species. "High-g" states, which are the most abundant Mo(V) species, are characterized by a low-anisotropy g tensor and a high g(min) value. We assign this signature to a six-sulfur coordination sphere in a pseudotrigonal prismatic geometry with a partial disulfide bond. The "very high-g" species is well described with a sulfido ion as the sixth ligand. The "low-g" signal can be successfully associated to a Mo(V) sulfite-oxidase-type active site with only one pterin moiety coordinated to the molybdenum ion with an oxo or sulfido axial ligand. For all these species we investigate their catalytic activity using a thermodynamic point of view on the molybdenum coordination sphere. Beyond the periplasmic nitrate reductase case, this work provides useful magneto-structural correlations to characterize EPR-detected species in mononuclear molybdoenzymes.     

Asymmetric Hydroformylation Using a Rhodium Catalyst Encapsulated in a Chiral Capsule

Supramolecular capsules can be used to change the activity and selectivity of a catalyst through the influence of the second coordination sphere, reminiscent of how enzymes control the selectivity of their processes. In enzymes, this approach is used to also control the enantioselectivity of reactions in which the active catalytic site is often not chiral but the second coordination sphere is. We are interested in the possibility to generate a chiral second coordination sphere around an otherwise achiral transition metal complex for asymmetric catalysis. In this paper we show that the ligand template approach can be used to generate a chiral second coordination sphere around a rhodium complex, which is used in asymmetric hydroformylation.     

A nickel(II) coordination polymer derived from a tridentate Schiff base ligand with N,O-donor groups: synthesis,crystal structure,spectroscopy, electrochemical behavior and electrocatalytic activity for H<Subscript>2</Subscript>O<Subscript>2</Subscript> electroreduction in alkaline medium

A Ni(II) coordination polymer [C₄₂H₄₂K₂N₄Ni₄O₂₇] has been synthesized under open-air mild reaction conditions and characterized by physicochemical and spectroscopic methods. The X-ray crystal structure of the complex has been obtained. The crystallographic data revealed that each metal center is in a distorted octahedral geometry where the ligand coordinates to the metal centers by a nitrogen from the imine group, an oxygen from the carboxylic acid and a phenoxide group as an endogenous bridge to the metal centers. The coordination sphere is completed by an acetate, coordinated as an exogenous bridging ligand to both nickel centers, plus one terminal water ligand on each nickel. The polymeric structure is an infinite chain involving the binuclear nickel structure and K⁺ ions. Carbon paste electrodes modified with the Ni(II) coordination polymer were prepared, and the electrochemical behavior and electrocatalytic activity toward H₂O₂ reduction were investigated. The electrochemical results suggest that this Ni(II) coordination polymer has good catalytic activity with respect to H₂O₂ reduction.     

Hexacoordinate Nickel(II)/(III) Complexes that Mimic the Catalytic Cycle of Nickel Superoxide Dismutase

A functional model complex of nickel superoxide dismutase (NiSOD) with a non‐peptide ligand which mimics the full catalytic cycle of NiSOD is unknown. Similarly, it has not been fully elucidated whether NiSOD activity is a result of an outer‐ or inner‐sphere electron‐transfer mechanism. With this in mind, two octahedral nickel(II)/(III) complexes of a bis‐tridentate N₂S donor carboxamide ligand, N‐2‐phenylthiophenyl‐2′‐pyridinecarboxamide (HL^Ph), have been synthesized, structurally characterized, and their SOD activities examined. These complexes mimic the full catalytic cycle of NiSOD. Electrochemical experiments support an outer‐sphere electron‐transfer mechanism for their SOD activity.     

Development, mechanism, and scope of the palladium-catalyzed enantioselective allene diboration

In the presence of a chiral phosphoramidite ligand, the palladium-catalyzed diboration of allenes can be executed with high enantioselectivity. This reaction provides high levels of selectivity with a range of aromatic and aliphatic allene substrates. Isotopic-labeling experiments, stereodifferentiating reactions, kinetic analysis, and computational experiments suggest that the catalytic cycle proceeds by a mechanism involving rate-determining oxidative addition of the diboron to Pd followed by transfer of both boron groups to the unsaturated substrate. This transfer reaction most likely occurs by coordination and insertion of the more accessible terminal alkene of the allene substrate, by a mechanism that directly provides the eta3 pi-allyl complex in a stereospecific, concerted fashion.     

Steric, electronic, and secondary effects on the coordination chemistry of ionic phosphine ligands and the catalytic behavior of their metal complexes

The effects of introducing ionic functionalities in phosphine ligands on the coordination chemistry of these ligands and the catalytic behavior of the corresponding metal complexes are reviewed. The steric and electronic consequences of such functionalizations are discussed. Apart from these steric and electronic effects, the presence of charged groups often leads to additional, supramolecular interactions that occur in the second coordination sphere of the metal complex, such as intramolecular, interligand hydrogen bonding and Coulombic repulsion. These interactions can significantly alter the behavior of the phosphine ligand in question. Such effects have been observed in phosphine-metal association/dissociation equilibria, ligand substitution reactions, and stereoisomerism in phosphine-metal complexes. By drawing general conclusions, this review offers an insight into the coordination and catalytic behavior of phosphine ligands containing ionic functionalities and their corresponding metal complexes.     

Nickel(II) Complexes with a Hydroxyl Pendant‐Armed Macrocyclic Ligand: Synthesis,Characterization, and Crystal Structures

The reactions of different nickel(II) salts with a mixed‐donor macrocyclic ligand L (6,7,8,9,10,11,18,19‐octahydro‐5H, 17H‐dibenzo[f,o][1,5,9,13] dioxadiazacyclohexadecin‐18‐ol), potentially pentadentate N₂O₃ donor sets containing one pendant alcohol function have been investigated. The physical properties and the chemical structures of 1:1 (metal:ligand) NiLX₂ (X = Cl^?, Br^?, NO₃^?, ClO₄^?) complexes have been characterized by using IR, UV‐Vis spectroscopy and conductance measurements. The X‐ray determination have been employed to probe the nature of the respective complexes in solid state. The nickel atom in [NiL(NO₃)]NO₃·0.5H₂O complex is six‐coordinate with a distorted octahedral coordination in which the all N₂O₃ donor atoms are coordinated to the nickel atom. The coordination sphere is completed by a nitrate anion. In contrast to the above nickel complex, in [NiLCl₂] complex the pendant hydroxyl arm of macrocycle remains uncoordinated and ligand acts as tetradentate N₂O₂ donor atoms. The coordination sphere is completed by two chloride anions and the nickel atom is six‐coordinate with a distorted octahedral coordination.     

Nickel‐Catalyzed Asymmetric Hydrogenation of N‐Sulfonyl Imines

An efficient nickel‐catalyzed asymmetric hydrogenation of N‐tBu ‐ sulfonyl imines was developed with excellent yields and enantioselectivities using (R,R)‐QuinoxP* as a chiral ligand. The use of a much lower catalyst loading (0.0095 mol %, S/C=10500) represents the highest catalytic activity for the Ni‐catalyzed asymmetric hydrogenations reported so far. Mechanistic studies suggest that a coordination equilibrium exists between the nickel salt and its complex, and that excess nickel salt promotes the formation of the active Ni‐complex, and therefore improved the efficiency of the hydrogenation. The catalytic cycle was also investigated by calculations to determine the origin of the enantioselectivity. An extensive network of numerous weak attractive interactions was found to exist between the catalyst and substrate in the transition state and may also contribute to the high catalytic activity.     

Site-isolation effects in a dendritic nickel catalyst for the oligomerization of ethylene

Dendrimers, specifically suited to construct site-isolated groups due to their well-defined hyperbranched structure, have been used as a ligand design element for the construction of nickel catalysts for ethylene oligomerization. The dendritic P,O ligand indeed suppresses the formation of inactive bis(P,O)Ni complexes in toluene, as is evident from NMR studies, and, as a consequence, outperforms the parent ligand in catalysis in this solvent. The dendritic effect observed in methanol is more subtle because both the dendritic ligand 1 and the parent 2 form bis(P,O)nickel complexes in solution according to NMR spectroscopy. Unlike the parent complex 8, the dendritic bis(P,O)Ni complex 7 derived from dendrimer ligand 1 is able to dissociate to a mono-ligated species under catalytic conditions, that is, 40 bar ethylene and 80 degrees C, which can enter the catalytic cycle. Indeed, dendritic ligand 1 gives much more active nickel catalysts for the oligomerization in methanol than does 2.     

N-allyl-bis(diphenylphosphino)amide nickel dibromide as a catalyst of the polymerization of methyl methacrylate

The polymerization of methyl methacrylate initiated by a catalytic system based on the complex CH₂=CHCH₂N(PPh₂)₂NiBr₂ containing a bidentate phosphine ligand in the coordination sphere of nickel is studied. The activities of this system and of the catalytic system based on NiBr₂(PPh₃)₂ with a monodentate ligand at the nickel atom in the synthesis of PMMA are compared. A scheme of polymerization catalyzed by the CH₂=CHCH₂N(PPh₂)₂)NiBr₂ system is advanced.     

Reevaluation of the mechanism of the amination of aryl halides catalyzed by BINAP-ligated palladium complexes

Two previous mechanistic studies of the amination of aryl halides catalyzed by palladium complexes of 1,1'-binaphthalene-2,2'-diylbis(diphenylphosphine) (BINAP) are reexamined by the authors of both studies. This current work includes a detailed study of the identity of the BINAP-ligated palladium complexes present in reactions of amines with aryl halides and rate measurements of these catalytic reactions initiated with pure precatalysts and precatalysts generated in situ from [Pd2(dba)3] and BINAP. This work reveals errors in both previous studies, and we describe our current state of understanding of the mechanism of this synthetically important transformation. 31P NMR spectroscopy shows that several palladium(0) species are present in the catalytic system when the catalyst is generated in situ from [Pd2(dba)3] and BINAP, and that at least two of these complexes generate catalytic intermediates. Further, these spectroscopic studies and accompanying kinetic data demonstrate that an apparent positive order in the concentration of amine during reactions of secondary amines is best attributed to catalyst decomposition. Kinetic studies with isolated precatalysts show that the rates of the catalytic reactions are independent of the identity and the concentration of amine, and studies with catalysts generated in situ show that the rates of these reactions are independent of the concentration of amine. Further, reactions catalyzed by [Pd(BINAP)2] with added BINAP are found to be first-order in bromoarene and inverse first-order in ligand, in contrast to previous work indicating zero-order kinetics in both. These data, as well as a correlation between the decay of bromobenzene in the catalytic reaction and the predicted decay of bromobenzene from rate constants of studies on stoichiometric oxidative addition, are consistent with a catalytic process in which oxidative addition of the bromoarene occurs to [Pd(BINAP)] prior to coordination of amine and in which [Pd(BINAP)2], which generates [Pd(BINAP)] by dissociation of BINAP, lies off the cycle. By this mechanism, the amine and base react with [Pd(BINAP)(Ar)(Br)] to form an arylpalladium amido complex, and reductive elimination from this amido complex forms the arylamine.     

Ammonia Activation by a Nickel NCN‐Pincer Complex featuring a Non‐Innocent N‐Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

A Ni⁰‐NCN pincer complex featuring a six‐membered N‐heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its Ni^II precursor. It retained chloride in the square‐planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni⁰ complex rapidly activated ammonia at room temperature, in a ligand‐assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent‐dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non‐innocent behavior of the NHC ligand.     

Complexes of N-hydroxyphthalimide and cobalt(II) acetate in reactions of alkylarene oxidation by molecular oxygen

The synergetic effect of the catalytic system cobalt(II) acetate-N-hydroxyphthalimide (NHPI) was studied in acetic acid during the oxidation of alkylbenzenes. The formation of complex between the components of catalytic system NHPI and Co(II) was demonstrated with the use of 1H NMR spectroscopy. It was established that N-hydroxyphthalimide is included in the complex as a ligand substituting for acetic acid in the internal coordination sphere of the metal; the reaction of NHPI with cobalt ions in the internal sphere leads to the formation of phthalimide-N-oxyl radicals that than participate in the catalytic cycle of the oxidation of organic substrates. The stability constants of complexes of NHPI with cobalt acetate were determined, as were those for the outer-sphere complexes of alkylbenzenes with metallocomplex.     

Realistic Energy Surfaces for Real‐World Systems: An IMOMO CCSD(T):DFT Scheme for Rhodium‐Catalyzed Hydroformylation with the 6‐DPPon Ligand

The hydroformylation of terminal alkenes is one of the most important homogeneously catalyzed processes in industry, and the atomistic understanding of this reaction has attracted enormous interest in the past. Herein, the whole catalytic cycle for rhodium‐catalyzed hydroformylation with the 6‐diphenylphosphinopyridine‐(2H)‐1‐one (6‐DPPon) ligand 1 was studied. This catalytic transformation is challenging to describe computationally, since two requirements must be met: 1) changes in the hydrogen‐bond network must be modeled accurately and 2) bond‐formation/bond‐breaking processes in the coordination sphere of the rhodium center must be calculated accurately. Depending on the functionals used (BP86, B3LYP), the results were found to differ strongly. Therefore, the complete cycle was calculated by using highly accurate CCSD(T) computations for a PH₃ model ligand. By applying an integrated molecular orbital plus molecular orbital (IMOMO) method consisting of CCSD(T) as high level and DFT as low‐level method, excellent agreement between the two functionals was achieved. To further test the reliability of the calculations, the energetic‐span model was used to compare experimentally derived and computed activation barriers. The accuracy of the new IMOMO method apparently makes it possible to predict the catalytic potential of real‐world systems.     

Catalysis by Transition Metal Derivatives Bound to Structurally Ordered Polymers

It is well known that metalloenzymes are able to catalyze chemical reactions with a very high activity and selectivity|1–3|. Therefore the synthesis of complexes between transition metal derivatives and structurally ordered macromolecular ligands to give catalytic systems having high activity and stereoselectivity is of large interest from both applicative and speculative point of view. In this connection the main aim of this article is to emphasize the role of the constitutional and configurational order of the macromolecular ligand in determining the properties of the corresponding complexes with transition metal derivatives. This is of particular importance for designing catalytic systems displaying peculiar features as far as activity, selectivity and stereochemistry are concerned. Moreover a better insight into environment and coordination sphere of the polymer attached metal derivative can be of great help for mechanistic studies of the reactions involved.     

Mechanistic elucidation of intramolecular aminoalkene hydroamination catalyzed by a tethered bis(ureate) complex: evidence for proton-assisted C-N bond formation at zirconium

A broad mechanistic investigation regarding hydroamination reactions catalyzed by a tethered bis(ureate) zirconium species, [ureate(2-)]Zr(NMe(2))(2)(HNMe(2)), is described. The cyclization of both primary and secondary aminoalkene substrates gives similar kinetic profiles, with zero-order dependence on substrate concentration up to ～60-75% conversion, followed by first-order dependence for the remainder of the reaction. The addition of 2-methylpiperidine changes the observed substrate dependence to first order throughout the reaction, but does not act as a competitive inhibitor. The reactions are first order in precatalyst up to loadings of ～0.15 M, indicating that a well-defined, mononuclear catalytic species is operative. Several model complexes have been structurally characterized, including dimeric imido and amido species, and evaluated for catalytic performance. These results indicate that imido species need not be invoked as catalytically relevant intermediates, and that the mono(amido) complex [ureate(2-)]Zr(NMe(2))(Cl)(HNMe(2)) is much less active than its bis(amido) counterpart. Structural evidence suggests that this is due to differences in coordination geometry between the mono- and bis(amido) complexes, and that an equatorial amido ligand is required for efficient catalytic turnover. On the basis of the determination of kinetic isotope effects and stoichiometric reactivity, the catalytic turnover-limiting step is proposed to be a concerted C-H, C-N bond-forming process with a highly ordered, unimolecular transition state (ΔS(?) = -21 ± 1 eu). In addition to this key bond-forming step, the catalytic cycle involves an on-cycle pre-equilibrium between six- and seven-coordinate intermediates, leading to the observed switch from zero- to first-order kinetics.     

Coordination properties of cyclam (1,4,8,11-tetraazacyclotetradecane) endowed with two methylphosphonic acid pendant arms in the 1,4-positions

The title ligand, 1,4,8,11-tetraazacyclotetradecane-1,4-diyl-bis(methylphosphonic acid) (H4te2p1,4, H4L), was prepared by an optimized synthetic approach and its complexing properties towards selected metal ions were studied by means of potentiometry. The ligand forms a very stable complex with copper(II) (log beta(CuL) = 27.21), with a high selectivity over binding of other metal ions (e.g. log beta(ZnL) = 20.16, log beta(NiL) = 21.92). The crystal structures of two intermediates in the ligand synthesis and two forms of the nickel(II) complex (obtained by crystallization at different pH) were determined. From acid solution, the crystals of trans-O,O-[Ni(H3L)]Cl.H2O were isolated. In such complex species, one phosphonate pendant arm is double- and the second arm is monoprotonated. The isolation of such species demonstrates a high kinetic inertness of the complex. The central metal ion is surrounded by four in-plane nitrogen atoms (in the ring configuration III) and two oxygen atoms of pendant moieties in the apical positions of octahedral coordination sphere. From neutral solution, the crystals of (trans-O,O-[Ni(H2L)])3.5H2O were isolated. The molecular structures of the complex units found in this structure are analogous to that found in trans-O,O-[Ni(H3L)]Cl.H2O.     

Nickel(I) complex as the final product of the sequence of spontaneous transformations in the system Ni(allyl)2-(2,6-diisopropylphenyl)diazabutadiene

A reaction of Ni(Allyl)₂ with bis(2,6-diisopropylphenyl)diazabutadiene gave an imino amide allyl nickel(II) complex (I). Complicated rearrangements of the imino amide ligand in the coordination sphere of complex I spontaneously yielded a paramagnetic Ni(I) ??-allyl complex as a final reaction product. The nickel complexes produced in this system were studied by EPR, IR, and 2D NMR spectroscopy and mass spectrometry. Structure I was examined by X-ray diffraction.     

Energetics of the first and second coordination spheres of transition metal ions

For complexes of transition metals (manganese, iron, cobalt, nickel) with monodentate ligands, equilibrium metal-ligand distances and ligand bond energies in the first and second coordination spheres have been calculated by the CNDO method. Some effects of ligand bond energies in different coordination spheres are analyzed. These effects significantly differ between the first and second coordination spheres. In the first sphere, the ligand bond energy is mainly determined by the nature of the central ion and the type of donor atom of the ligand, but weakly depends on the structure of the ligand. Conversely, in the second coordination sphere, the ligand bond energy weakly depends on the nature of the central ion and the type of donor atom, but considerably depends on the structure of the ligands in the first coordination sphere. In the second coordination sphere, ligand binding is determined by ligand interactions with both the central ion and the ligands of the first sphere. In the general case, when strong specific interactions between ligands are absent, the energetics of the second sphere is determined by the size of the inner-spheric ligands, which may be considered to be a specific steric effect. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 370–374, March–April, 1995. Translated from L. Smolina