Bulky PNP Ligands Blocking Metal-Ligand Cooperation Allow for Isolation of Ru(0), and Lead to Catalytically Active Ru Complexes in Acceptorless Alcohol Dehydrogenation

We synthesized two 4Me−PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me−PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono-CO Ru(0) complexes. Reactivity with CO and H₂ allows for stabilization and extensive characterization of bis-CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp²) −H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between ⁱPr and ^tBu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.     

High‐Oxidation‐State Palladium Catalysis: New Reactivity for Organic Synthesis

Recent years have seen the rapid development of a new field of palladium catalysis in organic synthesis. This chemistry takes place outside the usually encountered Pd⁰/Pd^II cycles. It is characterized by the presence of strong oxidants, which prevent further palladium(II)‐promoted reactions at a given point of the catalytic cycle by selective metal oxidation. The resulting higher‐oxidation‐state palladium complexes have been used to develop a series of new synthetic transformations that cannnot be realized within conventional palladium catalysis. This type of catalysis by palladium in a higher oxidation state is of significant synthetic potential.     

Supported palladium catalysis using a heteroleptic 2-methylthiomethylpyridine-N,S-donor motif for Mizoroki-Heck and Suzuki-Miyaura coupling, including continuous organic monolith in capillary microscale flow-through mode

Flow-through catalysis utilising (2-methylthiomethylpyridine)palladium(II) chloride species covalently attached to a macroporous continuous organic polymer monolith synthesised within fused silica capillaries of internal diameter 250 μm is described, together with related studies of ground bulk monolith compared with supported catalysis on Merrifield and Wang beads and homogeneous catalysis under identical conditions to bulk supported catalysis. The monolith substrate, poly(chloromethylstyrene-co-divinylbenzene), has a backbone directly related to Merrifield and Wang resins. The homogeneous precatalyst PdCl₂(L²) (L²=4-(4-benzyloxyphenyl)-2-methylthiomethylpyridine) contains the benzyloxyphenyl group on its periphery as a model for the spacer between the ‘PdCl₂(N∼S)’ centre and the polymer substituent of the resins and monolith. Suzuki-Miyaura and Mizoroki-Heck catalysis exhibit anticipated trends in reactivity with variation of aryl halide reagents for each system, and show that supported catalysis on beads and monolith gives higher yields than for homogeneous catalysis. The synthesis of 2-methylthiomethylpyridines is presented, together with crystal structures of 4-bromo-2-bromomethylpyridine hydrobromide, 4-(4-hydroxyphenyl)-2-methylthiomethylpyridine (L¹), PdCl₂(L¹) and PdCl₂(L²). Hydrogen bonding occurs in 4-bromo-2-bromomethylpyridine hydrobromide as N-H?Br interactions, in 4-(4-hydroxyphenyl)-2-methylthiomethylpyridine as O-H?N to form chains, and in PdCl₂(L¹) as O-H?Cl interactions leading to adjacent π-stacked chains oriented in an antiparallel fashion.     

Kinetic studies of a catalyzed metalloporphyrin formation

Kinetics of the incorporation of mercury(II) ion in tetra (p-trimethylammoniumphenyl)porphine have been investigated in aqueous solution at 30.0°C and 0.2 M (NaNO₃) ionic strength. The reaction was found to be first order each in mercury(II) and the porphyrin. The forward (formation) and the reverse (dissociation) rate constants were found to be 1.9 ± 0.2 × 10³ M^?1 s^?1 and 7 ± 2 × 10⁶ M^?1 s^?1, respectively. Kinetics of zinc(II) incorporation in tetra(p-trimethylammoniumphenyl)porphine catalyzed by mercury(II) were also investigated. This catalysis is explained in terms of steady-state formation of mono mercury(II) porphyrin followed by zinc(II) displacement of mercury(II) ion from the porphyrin. Such a mechanism also illustrates the importance of porphyrin core deformation to metal incorporation.     

Cationic Gold(II) Complexes: Experimental and Theoretical Study**

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [Au^II(L)(X)]⁺ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [Au^II(L)(X)]⁺ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes.     

Polymeric amines and their copper(II) complexes: Catalysts for the hydrolysis of organophosphate esters

Catalysis of the solvolysis of organophosphorus esters by polymers of aliphatic amines, imidazole, pyridine, 2,2′-bipyridine, and their copper(II) complexes was studied using diisopropyl fluorophosphate (DEP) as a model substrate. The polymeric catalysts were synthesized either by (1) derivation of available polymers, including polyethylenimine, polyvinyl amine, polyvinyl alcohol, polystyrene, and poly-4-vinylpyridine or (2) by polymerization of functionalized monomers such as 4(5)-vinylimidazole and 4-vinyl-4′-methyl-2,2′-bipyridine. Polymer hydrophilicity was controlled by partial quaternization of amine groups with different alkyl halides. The greatest catalytic activity was exhibited by copper(II) complexes of polymers containing the 2,2′-bipyridine group. At pH 7.6 and 3.7 × 10^?3M, the most active of these catalysts reduced the half-life of DFP from 800 to 9 min. The rate was largely independent of the pH in the range 6.5–8.5 but was limited by the aqueous solubility of the catalyst. Heterogeneous catalysis by some polymers was observed but was less effective. A Lineweaver-Burk plot of V₀^?1 versus [DFP]^?1 for a soluble polymeric 2,2′-bipyridine-copper(II) catalyst was linear. There was no correlation between catalysis of solvolysis of DFP and the carboxylic ester, p-nitrophenyl acetate.     

Electrocatalytic effect of iron sulfates on oxygen reduction. Influence of the solvation sphere of the mediator

The redox catalysis of oxygen reduction was performed on a platinum rotating disk electrode. The Fe(III)/Fe(II)/H₂SO₄ system at different pH's was used as a MEDIATOR. The catalytic effect of mediator was directly related to the solvation sphere of Fe(III) and Fe(II). Only the redox couple FeHSO ₄ ²⁺ /FeHSO ₄ ⁺ (pH<0) showed a catalytic effect on oxygen reduction.     

ApA Cleavage Promoted by Oxa-aza Macrocycles and Their Zn(II) Complexes. The Role of pH and Metal Coordination in the Hydrolytic Mechanism

Abstract

The thermodynamic parameters for protonation and Zn(II) complex formation with ligand 1,4,7,16,19,22-hexaza-10,13,25,28-tetraoxacyclotriacontane (L1) have been determined. L1 forms stable dizinc complexes from neutral to alkaline pH. The hydrolytic ability toward adenylyl(3′-5′)adenosine (ApA) of L1 and its dizinc(II) complexes have been analyzed by means of HPLC chromatography. Only partially protonated species of L promote ApA hydrolysis suggesting that the cleavage process is cooperatively promoted by a general base catalysis by neutral amine groups and a general acid catalysis by protonated ammonium functions. Concerning the Zn(II) complexes, the hydrolysis rates increase in the presence of the hydroxo complexes [Zn₂L1(OH)]³⁺ and [Zn₂L1(OH)²]²⁺. This indicates that Zn-OH functions play a crucial role in the hydrolytic process, assisting the deprotonation of the 2′-OH group of ApA, which may act as nucleophile in the cleavage process. Both binuclear L1 complexes are better catalysts than the mononuclear [ZnL₂(OH)]⁺ complex (L₂ = 1,4-Dioxa-7,10,13-triazacyclopentadecane), indicating a cooperative role of the two Zn(II) ions in ApA cleavage by [Zn₂L1(OH)]³⁺ and [Zn₂L1(OH)₂]²⁺, probably due to a bridging coordination of the phosphate moiety of ApA to the two metal centers.     

Equilibrium investigation of complex formation reactions involving copper(II), nitrilo-tris(methyl phosphonic acid) and amino acids,peptides or DNA constitutents. The kinetics,mechanism and correlation of rates with complex stability for metal ion promoted hydrolysis of glycine methyl ester

The complex formation reactions of [Cu(NTP)(OH₂)]^4? (NTP?=?nitrilo-tris(methyl phosphonic acid)) with some selected bio-relevant ligands containing different functional groups, are investigated. Stoichiometry and stability constants for the complexes formed are reported. The results show that the ternary complexes are formed in a stepwise mechanism whereby NTP binds to copper(II), followed by coordination of amino acid, peptide or DNA. Copper(II) is found to form Cu(NTP)H _n species with n?=?0, 1, 2 or 3. The concentration distribution of the various complex species has been evaluated. The kinetics of base hydrolysis of glycine methyl ester in the presence of copper(II)-NTP complex is studied in aqueous solution at different temperatures. It is proposed that the catalysis of GlyOMe ester occurs by attack of OH^? ion on the uncoordinated carbonyl carbon atom of the ester group. Activation parameters for the base hydrolysis of the complex [Cu(NTP)NH₂CH₂CO₂Me]^4? are, ΔH^±?=?9.5?±?0.3?kJ?mol^?1 and ΔS^±?=??179.3?±?0.9?J?K^?1?mol^?1. These show that catalysis is due to a substantial lowering of ΔH^±.     

Ligand Substitution of RuII–Alkylidenes to Ru(bpy)32+: Sequential Olefin Metathesis/Photoredox Catalysis

Ruthenium(II) alkylidene complexes such as the Grubbs’ 1st and 2nd generation catalysts undergo a ligand substitution with 2,2′-bipyridine, which readily leads to the common photoredox catalyst Ru(bpy)₃²⁺. The application of this catalyst transformation in sequential olefin metathesis/photoredox catalysis is demonstrated by way of ring-closing metathesis (RCM)/photoredox ATRA reactions.     

Equilibrium and hydrolysis of α-amino acid esters in mixed-ligand complexes withN-(acetamido)-iminodiacetatecopper(II)

Summary The formation equilibria of the binary and ternary complexes of Cu^II withN-(acetamido)-iminodiacetic acid (ADA) and amino acids or their esters were investigated potentiometrically. The chelation mode was ascertained by conductivity measurements. The kinetics of the base hydrolysis of amino acid esters in the presence of the copper(II)-ADA complex were studied. The rate and catalysis constants were estimated.     

Chemoselective Amination of Propargylic C(sp3)?H Bonds by Cobalt(II)‐Based Metalloradical Catalysis

Highly chemoselective intramolecular amination of propargylic C(sp³) H bonds has been demonstrated for N‐bishomopropargylic sulfamoyl azides through cobalt(II)‐based metalloradical catalysis. Supported by D_2h‐symmetric amidoporphyrin ligand 3,5‐Di^tBu‐IbuPhyrin, the cobalt(II)‐catalyzed C H amination proceeds effectively under neutral and nonoxidative conditions without the need of any additives, and generates N₂ as the only byproduct. The metalloradical amination is suitable for both secondary and tertiary propargylic C H substrates with an unusually high degree of functional‐group tolerance, thus providing a direct method for high‐yielding synthesis of functionalized propargylamine derivatives.     

Chemoselective Amination of Propargylic C(sp3)H Bonds by Cobalt(II)‐Based Metalloradical Catalysis

Highly chemoselective intramolecular amination of propargylic C(sp³)? H bonds has been demonstrated for N‐bishomopropargylic sulfamoyl azides through cobalt(II)‐based metalloradical catalysis. Supported by D_2h‐symmetric amidoporphyrin ligand 3,5‐Di^tBu‐IbuPhyrin, the cobalt(II)‐catalyzed C? H amination proceeds effectively under neutral and nonoxidative conditions without the need of any additives, and generates N₂ as the only byproduct. The metalloradical amination is suitable for both secondary and tertiary propargylic C? H substrates with an unusually high degree of functional‐group tolerance, thus providing a direct method for high‐yielding synthesis of functionalized propargylamine derivatives.     

Ligand-Enabled Palladium(II)-Catalyzed Enantioselective β-C(sp3)−H Arylation of Aliphatic Tertiary Amides**

Amide is one of the most widespread functional groups in organic and bioorganic chemistry, and it would be valuable to achieve stereoselective C(sp³)−H functionalization in amide molecules. Palladium(II) catalysis has been prevalently used in the C−H activation chemistry in the past decades, however, due to the weakly-coordinating feature of simple amides, it is challenging to achieve their direct C(sp³)−H functionalization with enantiocontrol by Pd^II catalysis. Our group has developed sulfoxide-2-hydroxypridine (SOHP) ligands, which exhibited remarkable activity in Pd-catalyzed C(sp²)−H activation. In this work, we demonstrate that chiral SOHP ligands served as an ideal solution to enantioselective C(sp³)−H activation in simple amides. Herein, we report an efficient asymmetric Pd^II/SOHP-catalyzed β-C(sp³)−H arylation of aliphatic tertiary amides, in which the SOHP ligand plays a key role in the stereoselective C−H deprotonation-metalation step.     

Structural studies on Helicobacter pylori 3‐deoxy‐D‐manno‐2‐octulosonate‐8‐phosphate synthase using electrospray ionization mass spectrometry: a tetrameric complex composed of dimeric dimers

Helicobacter pylori 3‐deoxy‐D ‐manno‐2‐octulosonate‐8‐phosphate (KDO8P) synthase catalyzes the conversion of D ‐arabinose‐5‐phosphate (A5P) and phosphoenolpyruvate (PEP) to produce KDO8P and inorganic phosphate. Since this protein is absent in mammals, it might therefore be an attractive target for the development of new antibiotics. Unlike E. coli KDO8P synthase (class I), the H. pylori counterpart is a class II enzyme, where it requires a divalent transition metal ion for catalysis. Although the metal ions have been shown to be important for catalysis, their role in the structure is not understood. Using electrospray ionization mass spectrometry (ESI‐MS), the role of the metal ions in H. pylori KDO8P synthase has been investigated. This protein is found to be a tetramer in the gas phase but dissociates into the dimer with increasing declustering potential (DP2) suggesting an existence of a ‘structurally specific’ tetramer. An examination of mass spectra revealed that the tetrameric state of the Cd²⁺‐reconstituted enzyme is less stable than those of the Zn²⁺‐, Co²⁺‐ and Cu²⁺‐enzymes. The stoichiometry of metal binding to the protein depends on the nature of the metal ion. Taken together, our data suggest that divalent metal ions play an important role in the quaternary structure of the protein and the tetrameric state may be primarily responsible for catalysis. This study demonstrates the first structural characterization and stoichiometry of metal binding in class II KDO8P synthase using electrospray ionization quadrupole time‐of‐flight mass spectrometry under nondenaturing conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Decomposition of di-tert-butyl nitroxide in aqueous solutions

Di-tert-butyl nitroxide (DTBN) decomposes in aqueous solutions producing 2-methyl-2-nitroso propane (MNP) and tert-butanol. The process is acid catalyzed, it is of second order in DTBN, and takes place with a rate constant of (1.0 ± 0.1) M^?2 s^?1. The reaction is also catalyzed by the anionic surfactant sodium dodecyl sulfate and by Fe(II) and Fe(III) ions. The catalysis by Fe(III) involves a very fast reduction of Fe(III) ions with concomitant formation of 2-methyl-2-nitroso propane. The reaction catalyzed by Fe(II) also produces 2-methyl-2-nitroso propane with a formation rate given by: d[MNP]/dt = (0.25 ± 0.10) [Fe(II)] [DTBN]. This reaction rate is nearly pH independent.     

Novel benzoferrocenyl chiral ligands: Synthesis and evaluation of their suitability for asymmetric catalysis

Four benzoferrocenyl phosphorus chiral ligands were conveniently prepared in good overall yields. These ligands were found to be stable in solid form and in solution. Two of the four ligands were resolved by chiral HPLC. Unlike a reported bis(phosphino-η⁵-indenyl)iron(II) complex, in which the indenyl ligands undergo ring flipping through an η¹-intermediate, these two ligands were found to be configurationally stable in solution and in solid state. The suitability of these ligands for enantioselective catalysis was assessed in studies on allylic alkylation reactions. When the two less sterically hindered ligands were used, excellent chemical yields were obtained, but the other two more sterically hindered ones gave lower yields. When the two enantiopure ligands were used, enantioselectivity of up to 51% ee was observed. These findings suggest that benzoferrocene derivatives may be used as chiral ligands for asymmetric catalysis.     

A novel iron-enhanced pathway for base-catalyzed catechol oxidation by dioxygen

Summary The kinetics and mechanisms of the catecholase-type biomimetic activation of O₂ by the new dioximatoiron(II) complexes [Fe(Hdmed)]⁺, [Fe(Hdmpd)]⁺ and [Fe(H₂dmdt)]²⁺ have been studied in methanol. Kinetic measurements reveal first order behavior with respect to catalyst and O₂ and a saturation type dependence on the 3,5-di-tert-butylcatechol (H₂dtbc) substrate. Added triethylamine increases the rate by changing the reaction mechanism. An important, hitherto unknown feature is iron(II)-enhanced base catalysis of H₂dtbc oxidation, via coordination of HdtbcO₂^- to the Fe(II) complex present, resulting in a significant acceleration of oxidation. A mechanism involving free radicals is suggested on grounds of ESR evidence. The activity pattern of the catalyst complexes correlates with coordination number and symmetry as revealed by M?ssbauer spectra.     

Experimental and Theoretical Investigations of the Existence of CuII,CuIII, and CuIV in Copper Corrolato Complexes

The most common oxidation states of copper in stable complexes are +I and +II. Cu^III complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu^IV species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu^IV complexes being non‐existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu^IV species, and describe a redox series where Cu^II, Cu^III, and Cu^IV are discussed within the same molecular platform.     

Flow-injection catalytic kinetic determination of manganese using stopped-flow and gradient calibration

Based on the principle of Mn(II) catalysis of the Tiron-hydrogen peroxide reaction, a catalytic kinetic spectrophotometric determination of traces of manganese (ca. 10^?7 M) by flow injection was established. In combination with a microcomputer, by using gradient dilution and the stopped-flow method, onlya single standard solution was needed for calibration. The method has a high selectivity and a sampling rate of 40 h^?1. Traces of manganese in natural water were determined with a maximum relative standard deviation of 5.5% (n = 6).