Synthesis and evaluation of dioxygen binding [Mn(depe)2X2] complexes (X=Br or NCS)

Summary Synthesis, characterization and oxygen affinity studies for [Mn(depe)₂X₂] [X=Br or NCS; depe=1,2-bis(diethylphosphino)ethane] complexes are reported. [Mn(depe)₂Br₂] crystals were obtained and characterized by single crystal x-ray and elemental analysis. Gravimetric and volumetric dioxygen uptake studies were performed at atmospheric pressure at various temperatures. These transition metal compounds show considerable dioxygen uptake at various partial pressures of dioxygen. Some isotherm and kinetic data for dioxygen coordination by these complexes at ambient pressures are presented.     

Flash photolysis studies of charge-transfer photochemistry of nickel(II) and cobalt(III) complexes

The photochemical behavior of cobalt(III) and nickel(II) complexes on excitation in the charge-transfer bands is reviewed in this article with particular reference to the study of intermediates. Investigations on the photoredox reactions of cobalt(III) and nickel(II) complexes using flash kinetic spectroscopic methods reveal details on the characteristics of the intermediates produced from the charge-transfer excited states of these metal complexes. The reactive species produced on photolysis of cobalt(III)-amine complexes activate molecular oxygen, producing mononuclear and dinuclear dioxygen species coordinated as superoxo and peroxo forms. Cobalt(III)-amino-acid complexes on photolysis lead to the formation of cobalt(III)-alkyl complexes which are identified as transients. The spectra and the decay kinetics are described with the view to elucidate mechanistic details. Nickel(II) macrocyclic complexes on excitation in the charge-transfer bands lead to oxidation of the metal centre. Scavenging experiments using dioxygen, alcohols and acids were carried out to understand the mechanistic details.     

Redox properties of NN′-X-phenylenebis(Y-salicyli-deneiminato)iron(II) complexes and reactivity of the corresponding iron(III) complexes towards catecholates

The electrochemical behaviour of a series of iron(II) complexes with the tetradentate ligand NN′-1,2-phenylenebis(salicylideneimine), [Fe(II)L], was studied in non-aqueous solvents. The redox properties of the complexes were related to the nature of the substituents in the aromatic rings. Attention was devoted to dioxygen reactivity of the complexes. The electrode activity of the catechol—[NN′-1,2-phenylenebis(salicylidene-iminato) iron(III)] system, [Fe(III)L(catH)], was also studied; the results gave evidence that both the electrochemical oxidation and the chemical oxidation by dioxygen of [Fe(II)L] in the presence of catechol lead to the complex [Fe(III)L(catH)].     

双（呋喃甲醛）缩二胺钴（II）配合物的合成及其氧合和催化氧化性能研究

合成和表征了氯化双（呋喃甲醛）缩邻苯二胺合钴（II）（1）、氯化双（呋 喃甲醛）缩乙二胺合钴（II）（2）、氯化双（呋喃甲醛）缩1,2-丙二胺合钴（ II）（3）和氯化双（呋喃甲醛）缩1,3-丙二胺合钴（II）（4）。在吡啶溶液中 和不同温度下,测定了配合物的饱和吸氧量,求出了氧加合常数和热力学参数ΔH °,ΔS°。并以这些配合物为催化剂,活化分子氧氧化环已烯得到高选择性的烯 丙位氧化产物。讨论了温度、配体结构对配合物氧合性能的影响和配体结构以及添 加NHPI（N-羟基邻苯二甲酰亚胺）对环已烯氧化反应的影响。     

四Schiff碱(二苯并-18-冠-6)及其钾(I)/钴(II)配合物的合成和 性质

由二苯并-18-冠-6出发,合成了4',4',5',5'-四(2-羟基苯亚甲基亚氨基)二苯并-18-冠-6(L^1H~4)及其5位取代衍生物(取代基R=CH~3,OH,OCH~3,Cl,NO~2)L^2H~4～L^6H~4。它们依次与硝酸钾和醋酸钴反应,制得1:1:2的钾(I)/双钴(II)配合物LCo~2.2H~2O.KNO~3(L=L^5,L^6)或其某些二氧加合物LCo~2(2O~2).2H~2O.KNO~3(L=L^1～L^4)。考察了取代基R和冠醚环配合的钾离子对二氧加合物形成及稳定性的影响。结果表明,含吸电子基(R=Cl,NO~2)的钾(I)/双钴(II)配合物不能形成二氧加合物;冠环中的钾离子会导致二氧加合物中两个Co-O~2键热稳定性的差异。     

Dioxygen affinities and catalytic oxidation performance of cobalt (ii) hydroxamates

The dioxygen affinities and catalytic oxidation performance of cobalt (II) complexes with N-phenylhydroxamic acids were investigated. The effects of substituting groups in aromatic ring of ligands on these properties were examined, and some relativity between the dioxygen affinities and catalytic oxidation activity were also discussed in this paper. This revised version was published online in June 2006 with corrections to the Cover Date.     

Schiff base complexes of vanadium(III, IV, V) as catalysts for the electroreduction of O2 to H2O in acetonitrile

Fifteen Schiff base ligands were synthesized and used to form complexes with vanadium in oxidation states III, IV, and V. Electrochemical and spectral characteristics of the complexes were evaluated and compared. In acidified solutions in acetonitrile the vanadium(IV) complexes undergo reversible disproportionation to form V(III) and V(V) complexes. With several of the ligands the V(III) complexes are much more stable in the presence of acid than is the previously studied complex with salen, an unelaborated Schiff base ligand (H(2) salen = N,N'-ethylenebis(salicylideneamine)). Equilibrium constants for the disproportionation were evaluated. The vanadium(III) complexes reduce dioxygen to form two oxo ligands. The reaction is stoichiometric in the absence of acid, and second-order rate constants were evaluated. In the presence of acid some of the complexes investigated participate in a catalytic electroreduction of dioxygen.     

Dioxygen activation and catalytic aerobic oxidation by a mononuclear nonheme iron(II) complex

We have used dioxygen, not artificial oxidants such as peracids, iodosylarenes, and hydroperoxides, in the generation of a mononuclear nonheme oxoiron(IV) complex, [Fe(IV)(TMC)(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), from its corresponding Fe(II) complex, [Fe(TMC)(CF3SO3)2]. The formation of oxoiron(IV) species by activating dioxygen was markedly dependent on iron(II) complexes and solvents, and this observation was interpreted with the electronic effect of iron(II) complexes on dioxygen activation to form oxoiron(IV) species. A catalytic aerobic oxidation of organic substrates was demonstrated in the presence of the [Fe(TMC)]2+ complex. By carrying out 18O-labeled water experiment, we were able to conclude that the oxidation of organic substrates was mediated by an oxoiron(IV) intermediate, not by a radical type of autoxidation process.     

Photoinduced ligand release in a ruthenium(II)-cobalt(III) heterodinuclear system

Two Ru(II)-Co(III) heterodinuclear complexes have been synthesised as model complexes for photoactivated cytotoxins. Photoinduced ligand release has been demonstrated and the rate shown to depend on dioxygen concentration. Emission lifetimes and quantum yields are reported. These results validate the concept and justify further work to synthesise systems containing cytotoxic ligands.     

Spectroscopic characterization of the oxo-transfer reaction from a bis(mu-oxo)dicopper(III) complex to triphenylphosphine

The oxygen-atom transfer reaction from the bis(mu-oxo)dicopper(III) complex [Cu(III)(2)(mu-O)(2)(L)(2)](2+), where L =N,N,N',N' -tetraethylethylenediamine, to PPh(3) has been studied by UV-vis, EPR, (1)H NMR and Cu K-edge X-ray absorption spectroscopy in parallel at low temperatures (193 K) and above. Under aerobic conditions (excess dioxygen), 1 reacted with PPh(3), giving O=Ph(3) and a diamagnetic species that has been assigned to an oxo-bridged dicopper(II) complex on the basis of EPR and Cu K-edge X-ray absorption spectroscopic data. Isotope-labeling experiments ((18)O(2)) established that the oxygen atom incorporated into the triphenylphosphine oxide came from both complex 1 and exogenous dioxygen. Detailed kinetic studies revealed that the process is a third-order reaction; the rate law is first order in both complex 1 and triphenylphosphine, as well as in dioxygen. At temperatures above 233 K, reaction of 1 with PPh(3) was accompanied by ligand degradation, leading to oxidative N-dealkylation of one of the ethyl groups. By contrast, when the reaction was performed in the absence of excess dioxygen, negligible substrate (PPh(3)) oxidation was observed. Instead, highly symmetrical copper complexes with a characteristic isotropic EPR signal at g= 2.11 were formed. These results are discussed in terms of parallel reaction channels that are activated under various conditions of temperature and dioxygen.     

Iron(II) complexes with amide-containing macrocycles as non-heme porphyrin analogues

Iron(II) complexes of macrocyclic pentadendate ligands 3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pydioneN5) and 16-chloro-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pyCldioneN5) were synthesized and fully characterized. Complexes with one or two deprotonated amide groups of H2pydione were both isolated. In the former case the metal ion has a distorted octahedral coordination sphere; in the latter case the complex adopts a pentagonal-bipyramidal geometry. NMR experiments show that the protonation state of the ligand is preserved in a dimethyl sulfoxide (DMSO) solution. The complexes maintain a high-spin state even at low temperatures. Detailed kinetic studies of oxygenation of the iron(II) complexes showed that the deprotonation state of the complex has a profound effect on the reactivity with dioxygen. Oxygenation of the dideprotonated complex of iron(II), Fe(pydioneN5), in aprotic solvents proceeds via a path that is analogous to that of iron(II) porphyrins: via iron(III) superoxo and diiron(III) peroxo species, as evidenced by the spectral changes during the reaction, which is second-order in the concentration of the iron(II) complex, and with an inverse dependence of the reaction rate on the concentration of dioxygen. The final products of oxygenation are crystallographically characterized iron(III) mu-oxo dimers. We have also found that the presence of 1-methylimidazole stabilizes the diiron peroxo intermediate. The reaction of Fe(pydioneN5) with dioxygen in methanol is distinctly different under the same conditions. The reaction is first-order in both iron(II) complex and dioxygen, and no intermediate is spectroscopically observed. Similar behavior was observed for the monodeprotonated complex Fe(HpydioneN5)(Cl). The presence of an accessible proton either from the solvent (reactions in methanol) or from the complex itself (in Fe(HpydioneN5)(Cl)) proves sufficient to alter the oxygenation pathway in these macrocyclic systems, which is reminiscent of the properties of iron(II) porphyrin complexes. The new amidopyridine macrocycles can be considered as new members of the "expanded porphyrin analogue" family. The expansion of the cavity provides control over the spin state and availability of protons. These macrocyclic systems also allow for easy synthetic modifications, paving the way to new, versatile metal complexes.     

EPR spectroscopy of transformations of iridium(III) and iridium(IV) hydroxo complexes in alkaline media

Processes that occur in strong alkaline solutions of iridium(III) and iridium(IV) hydroxo complexes have been studied by EPR and electronic absorption spectroscopy. It has been demonstrated that dissolution of iridium compounds in alkaline solutions should be accompanied by a series of complicated transformations involving oxygen, which lead to the formation of several binuclear iridium(III, III), (III, IV), and (IV, IV) dioxygen complexes.     

Hydroxo-bridged dinuclear cobalt(II) complexes of OBISDIEN and OBISTREN as oxygen carriers

Abstract

The hydroxo-bridged dinuclear cobalt(II) complexes of the macro-cyclic and macrobicyclic dinucleating ligands OBISDIEN and OBISTREN form thermodynamically stable dioxygen adducts. Although the values of the oxygenation constants of cobalt complexes usually increase with an increase in the number of basic amino groups coordinated to the cobalt centers, these complexes are unusual in that the oxygenation constants of the complex formed from OBISDIEN is about three orders of magnitude higher than the analogous complex formed from OBISTREN. The difference in behavior must be due to differences in steric strain. Since crystal structures are not available, steric effects were estimated by the use of molecular mechanics. By application of the molecular modelling program SYBYL it was determined that the distortions of the cryptand ligand necessary to form the hydroxo-bridged dioxygen complex are much greater and more energetic than those of the macrocyclic ligand. Both ligands were found to form excellent oxygen carriers because of facile oxygenation-deoxygenation reactions of their dinuclear cobalt complexes, and because of almost immeasurably slow metal-centered degradation. Moreover, the ligands were not changed during degradation, so that the oxygen-carrying complexes could be easily regenerated by electrolytic reduction.     

Relative dioxygenation rates of some trans-Ir(CO)X(PR3)2 complexes: The effect of phosphine and halogen changes

Relative rates of dioxygen uptake by the complexes trans-Ir(CO)X(PPh₂R)₂ (R = Ph, Me, Et; X = F, Cl, Br, I) have been measured in dichloromethane and found to follow the order R = Ph<Et<Me and X = F <Cl<Br<I. The basicity of these trans-Ir(CO)X(L)₂ complexes, as measured by their affinity for dioxygen, is not reflected in the energy of the ν(CO) absorption in the parent compounds; a previous report that complex basicity ∝1/ν(CO) does not hold for the complexes reported here.     

Aerobic catechol oxidation catalyzed by a bis(mu-oxo)dimanganese(III,III) complex via a manganese(II)-semiquinonate complex

A 3,5-di-tert-butyl-1,2-semiquinonato (DTBSQ) adduct of Mn(II) was prepared by a reaction between Mn(II)(TPA)Cl(2) (TPA = tris(pyridin-2-ylmethyl)amine) and DTBSQ anion and was isolated as a tetraphenylborate salt. The X-ray crystal structure revealed that the complex is formulated as a manganese(II)-semiquinonate complex [Mn(II)(TPA)(DTBSQ)](+) (1). The electronic spectra in solution also indicated the semiquinonate coordination to Mn. The exposure of 1 in acetonitrile to dioxygen afforded 3,5-di-tert-butyl-1,2-benzoquione and a bis(mu-oxo)dimanganese(III,III) complex [Mn(III)(2)(mu-oxo)(2)(TPA)(2)](2+) (2). The reaction of 2 with 3,5-di-tert-butylcatechol (DTBCH(2)) quantitatively afforded two equivalents of 1 under anaerobic conditions. The highly efficient catalytic oxidation of DTBCH(2) with dioxygen was achieved by combining the above two reactions, that is, by constructing a catalytic cycle involving both manganese complexes 1 and 2. It was revealed that dioxygen is reduced to water but not to hydrogen peroxide in the catalytic cycle.     

Oxo complexes of osmium(IV) formed via dioxygen activation. X-ray structures of [OsX(dcpe)2]PF6 (X = Cl, Br), [OsCl(eta 2-O2)(dcpe)2]BPh4, and [OsCl(O)(dcpe)2]BPh4 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane)

Dioxygen addition to the 16-electron complexes [OsX(P-P)2]+ (3) gives the dioxygen adducts [OsCl(eta 2-O2)(P-P)2]+ (3), which in turn react with HCl gas to give the novel osmium(IV) oxo complexes trans-[OsX(O)(P-P)2]+ (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphospholano)benzene (Me-duphos)). The complexes [OsX(dcpe)2]+ (X = Cl, Br) (3) are studied by X-ray crystallography and are shown to have a "Y-shaped" coordination geometry in the equatorial plane. The X-ray structural analysis of [OsCl(eta 2-O2)(dcpe)2]+ (4a) reveals an exceptionally short O-O bond (1.315(5) A). trans-[OsCl(O)(dcpe)2]+ (5a), the first oxo complex of osmium(IV) investigated crystallographically, exhibits a long Os-O distance of 1.834(3) A. The reactivity of 4 and 5 as oxidants is described. The dioxygen complex 4a transfers one oxygen atom to PPh3 (to give Ph3PO) or oxidizes iodide ions to triiodide ions in the presence of anhydrous HCl. In both reactions, the corresponding oxo species 5a is quantitatively formed as the only metal-containing product. Oxo complexes 5 are surprisingly stable and unreactive toward standard reducing agents such as phosphines.     

Formation and reactivity of cobalt(III)-dioxygen complexes promoted by 2-methylpropanal: Oxidation of alkenes and alcohols

2-Methylpropanal promotes the formation of cobalt(III)-dioxygen species from catalysts1 or2 and dioxygen. The cobalt(III)-dioxygen complexes efficiently catalyse the oxidation of various alcohols to carbonyl compounds in the presence of dioxygen and 2-methylpropanal. Similarly substituted alkenes are smoothly transformed to the corresponding monoepoxides under these reaction conditions. EPR study of these reactions indicates that different activated dioxygen species are formed in the presence of catalysts1 and2.     

How molecular oxygen binds to bis[trifluoroacetylacetonato(-1)]cobalt(II)--ab initio and density functional theory studies

Cobalt(II) diketonate complexes, such as bis[trifluoroacetylacetonato(-1)]cobalt(II) [Co(tfa)(2)], catalyze the aerobic oxidation of alkenols into functionalized tetrahydrofurans. To gain insight into activation of triplet dioxygen by Co(tfa)(2) in a protic solvent, as used in oxidation catalysis, the electronic structure of aquabis[trifluoroacetylacetonato(-1)]cobalt(II)--Co(tfa)(2)(H(2)O)--and the derived dioxygen adduct were characterized using ab initio (CASSCF, NEVPT2) and density functional theory (BP86, TPSSh, B3LYP) methods. The ground state of Co(tfa)(2)(H(2)O) is a high-spin, quartet state. As dioxygen approaches the cobalt atom, the quartet state couples with a triplet dioxygen molecule and forms a sextet, a quartet, and a doublet spin state with the high-spin state being the lowest in energy. At the equilibrium Co-O(2) distance of 1.9 ?, Co(tfa)(2)(H(2)O)(O(2)) has a doublet superoxo Co(III) ground state with the unpaired electron residing on the oxygen moiety, in a nearly unchanged O(2)π* orbital.     

Synthesis and properties of the binuclear vanadium(III) and oxovanadium(IV) chelates with tetradentate schiff bases

The complexes of vanadium(III) and oxovanadium(IV) with Schiff bases derived from some aliphatic diamines and 2-hydroxy-1-naphthaldehyde were prepared and characterized by elemental analysis, IR, electronic and EPR spectra. The binuclear V^III complexes have the [V₂(μ-oxo)(μ-aquo)] core in distorted octahedral environments. The VO^IV complexes form monomeric, dimeric or polymeric complexes by ligand bridging. The structures are dependent on the length of the carbon chain linking the imine groups. A complex EPR signal is observed for [VO(naphbu)]₂ in chloroform solution at room temperature, its isotropic spectrum exhibits a 15 line hyperfine signal at g_av = 2.014 and the hyperfine coupling constant is 50 G, suggesting that the centres are weakly coupled. The reactions of the binuclear V^III complexes with dioxygen and VO^IV complexes with thionylchloride were also studied.     

Oxidation of Tricarbonyl (η1 η2-but-3-en-1-yl)iron(0)and Tricarbonyl (η3-allyl)iron(0) Anion Complexes with Dioxygen

The addition of reactive carbanions to tricarbonyl(η⁴-1,3-diene)iron(0) complexes proceeded at ?78 °C to give putative tricarbonyl(η¹,η²-but-3-en-1-y1)iron(0) anion complexes and at 25 °C to produce postulated tricarbonyl(η³-allyl)iron(O) anion complexes; trapping of reactive intermediates with dioxygen produced γ,δ-unsaturated acids and allylic alcohols, respectively.