Labile Coordination Sites Research Articles

Preparation and structures of 6- and 7-coordinate salen-type zirconium complexes and their catalytic properties for oligomerization of ethylene

A series of salen-type zirconium complexes of the general formula LZrCl 2 (L = N, N′-ethylenebis(salicylideneiminate), 3a; N, N′-ethylenebis(3,5-di- tert-butylsalicylideneiminate), 3b; N, N′-ethylenebis(5-methoxysalicylideneiminate), 3c; N, N′-ethylenebis(5-chlorosalicylideneiminate), 3d; N, N′-ethylenebis(5-nitrosalicylideneiminate), 3e; N, N′- o-phenylenebis(salicylideneiminate), 4a; N, N′- o-phenylenebis(3,5-di- tert-butylsalicylideneiminate), 4b; N, N′- o-phenylenebis(5-methoxysalicylideneiminate), 4c; N, N′- o-phenylenebis(5-chloro-salicylideneiminate), 4d) were prepared. The crystal structures of 6- and 7-coordinate zirconium complexes 4b and [ 4b · OCMe 2] were determined by X-ray crystallography, which reveals that a salen-type zirconium complex possesses a labile coordination site on the Zr center with a relatively stable framework and that the coordination and the dissociation of O-donor molecules occur readily at this site. The catalytic properties of 3( a– e) and 4( a– d) were studied for ethylene oligomerization in combination with Et 2AlCl as co-catalyst. Complex 3c featuring a methoxy-substituted salen ligand displayed higher activity than its analogous precursors having chloro and nitro groups as substituents. The catalytic reactions by 3( a– e) and 4( a– d) gave C 4–C 10 olefins and low-carbon linear α-olefins in good selectivity.

Efficient catalysis of Nazarov cyclization using a cationic iridium complex possessing adjacent labile coordination sites.

The dicationic Ir(III) complex [IrMe(CO)(dppe)(DIB)](BARF)2 having adjacent labile sites has been found to be a very effective catalyst for promoting the Nazarov cyclization of aryl vinyl and divinyl ketones. Spectroscopic evidence for a substate-catalyst complex before cyclization is presented. The efficiency of the cyclization is attributed to the electrophilicity of the Ir(III) complex and substrate activation via chelation.

A Reaction Intermediate Involved in Oxygenation of Catecholatoiron(III) Complexes with Molecular Oxygen — Relevance to Catechol Dioxygenases

Abstract Formation of an intermediate in the step of O2 binding to a catecholatoiron(III) complex was first clearly shown by construction of a labile coordination site on the ferric center. In addition, spectral changes indicated reversible interconversion between the catecholatoiron(III) complex and the intermediate.

Structural studies on manganese(III) and manganese(IV) complexes of tetrachlorocatechol and the catalytic reduction of dioxygen to hydrogen peroxide

The mononuclear complexes (Bu 4N)[Mn(Cl 4Cat) 2(H 2O)(EtOH)] and (Bu 4N) 2[Mn(Cl 4Cat) 3] (H 2Cat=1,2-dihydroxybenzene) have been synthesised and characterised by X-ray diffraction. This work provides a direct, independent, synthesis of these complexes and an interesting example of how solvent effects can promote the formation of either a manganese(III) or manganese(IV) complex of the same ligand. The characterisation of (Bu 4N)[Mn(Cl 4Cat) 2(H 2O)(EtOH)] supports previous work that manganese(III) is extremely reluctant to form tris (catecholato) complexes due to the short `bite distance' of catecholate oxygen atoms (2.79 Å) which are unable to span the elongated coordination axes of the Jahn-Teller distorted Mn(III) ion and explains the 2:1 and 3:1 tetrachlorocatechol:manganese ratios in the Mn(III) and Mn(IV) complexes, respectively. Hydrogen peroxide production using dioxygen and hydroxylamine as substrates in acetonitrile/water mixtures, under ambient conditions, can be demonstrated with both complexes, suggesting that neither labile coordination sites nor the oxidation state of the manganese are important to the catalytic system. Turn over frequencies (TOF, moles of H 2O 2 per moles of manganese per hour) of ∼10 000 h −1 are obtained and this compares very favourably with the commercial production of hydrogen peroxide by the autoxidation of 2-ethylanthrahydroquinone (AO process).

Carrier-facilitated bulk liquid membrane transport of iron(III)-siderophore complexes utilizing first coordination sphere recognition.

Carrier-facilitated bulk liquid membrane (BLM) transport from an aqueous source phase through a chloroform membrane phase to an aqueous receiving phase was studied for various hydrophilic synthetic and naturally occurring Fe(III)-siderophore complexes using first coordination sphere recognition. Iron transport systems were designed such that two cis coordination sites on a hydrophilic Fe(III) complex are occupied by labile aquo ligands, while the other four coordination sites are blocked by strong tetradentate ligands (siderophores). The labile aquo coordination sites can be "recognized" by a liquid membrane-bound hydrophobic bidentate ligand, which carries the hydrophilic Fe(III)-siderophore complex across the hydrophobic membrane to an aqueous receiving phase. The system is further designed for uphill transport of Fe(III) against a concentration gradient, driven by anti-port H(+) transport. Three tetradentate siderophore and siderophore mimic ligands were investigated: rhodotorulic acid (H(2)L(RA)), alcaligin (H(2)L(AG)), and N,N'-dihydroxy-N,N'-dimethyldecanediamide (H(2)L(8)). Flux values for the transport of Fe(L(x))(OH(2))(2)(+) (x = RA, AG, 8) facilitated by the hydrophobic lauroyl hydroxamic acid (HLHA) membrane carrier were the highest when x = 8, which is attributed to substrate lipophilicity. Ferrioxamine B (FeHDFB(+)) was also selectively transported through a BLM by HLHA. The process involves partial dechelation of ferrioxamine B to produce the tetradentate form of the complex (Fe(H(2)DFB)(OH(2))(2)(2+)), followed by ternary complex formation with HLHA (Fe(H(2)DFB)(LHA)(+)) and transport across the membrane into the receiving phase. Uphill transport of ferrioxamine B was confirmed by increased flux as [H(+)](source phase) < [H(+)](receiving phase). The membrane flux of ferrioxamine B occurs near neutral pH, as evidence that ternary complex formation and ligand exchange are viable processes at the membrane/receptor surface of microbial cells.

Conversion of core oxos to water molecules by 4e-/4H+ reductive dehydration of the Mn4O2(6+) core in the manganese-oxo cubane complex Mn4O4(Ph2PO2)6: a partial model for photosynthetic water binding and activation.

Reaction of the Mn4O4(6+) "cubane" core complex, Mn4O4L6 (1) (L = diphenylphosphinate, Ph2PO2-), with a hydrogen atom donor, phenothiazine (pzH), forms the dehydrated cluster Mn4O2L6 (2), which has lost two mu-oxo bridges by reduction to water (H2O). The formation of 2 was established by electrospray mass spectrometry, whereas FTIR spectroscopy confirmed the release of water molecules into solution during the reduction of 1. UV-vis and EPR spectroscopies established the stoichiometry and chemical form of the pzH product by showing the production of 4 equiv of the neutral pz radical. By contrast, the irreversible decomposition of 1 to individual Mn(II) ions occurs if the reduction is performed using electrons provided by various proton-lacking reductants, such as cobaltocene or electrochemical reduction. Thus, cubane 1 undergoes coupled four-electron/four-proton reduction with the release of two water molecules, a reaction formally analogous to the reverse sequence of the steps that occur during photosynthetic water oxidation leading to O2 evolution. 1H NMR of solutions of 2 reveal that all six of the phosphinate ligands exhibit paramagnetic broadening, due to coordination to Mn ions, and are magnetically equivalent. A symmetrical core structure is thus indicated. We hypothesize that this structure is produced by the dynamic averaging of phosphinato ligand coordination or exchange of mu-oxos between vacant mu-oxo sites. The paramagnetic 1H NMR of water molecules in solution shows that they are able to freely exchange with water molecules that are bound to the Mn ion(s) in 2, and this exchange can be inhibited by the addition of coordinating anions, such as chloride. Thus, 2 possesses open or labile coordination sites for water and anions, in contrast to solutions of 1, which reveal no evidence for water coordination. Complex 2 exhibits greater paramagnetism than that of 1, as seen by 1H NMR, and it possesses a broad (440 G wide) EPR absorption, centered at g = 2, that follows a Curie-Weiss temperature dependence (10-40 K) and is visible only at low temperatures, compared to EPR-silent 1. Its comparison to a spin-integration standard reveals that 2 contains 2 equiv of Mn(II), which is in agreement with the formal oxidation state of 2Mn(II)2Mn(III) that was derived from the titration. The EPR and NMR data for 2 are consistent with a loss of two of the intermanganese spin-exchange coupling pathways, versus 1, which results in two "wingtip" Mn(II) S = 5/2 spins that are essentially magnetically uncoupled from the diamagnetic Mn2O2 base. Bond-enthalpy data, which show that O2 evolution via the reaction 1-->2 + O2, is strongly favored thermodynamically but is not observed in the ground state due to an activation barrier, are included. This activation barrier is hypothesized to arise, in part, from the constraining effect of the facially bridging phosphinate ligands.

Synthesis and characterization of magnesium and zinc complexes of 1,4,7-triazacyclononane- N-acetate. Potential models for the active site of RuBisCo

Reaction of the tetradentate pendant-arm macrocycle 1,4,7-triazacyclononane- N-acetate (L) with Zn(O 3SCF 3) 2 or Mg(O 3SCF 3) 2 in alcohol produces the six-coordinate complexes [ZnL(H 2O)]O 3SCF 3·C 2H 5OH ( 1) and [MgL(H 2O) 2]O 3SCF 3 ( 2). Compounds 1 and 2 were characterized by single-crystal X-ray crystallography. The complex cation of 1 forms a polymeric chain with zinc centers bridged in a syn, anti fashion by acetate pendant arms. The complex cation of 2 is mononuclear, with two adjacent coordination sites on magnesium occupied by water molecules. These labile coordination sites may provide a binding site for activation of substrates in functional modeling of magnesium enzymes such as RuBisCo. 1H and 13C NMR spectroscopy demonstrates that L remains bound to magnesium in methanol solution. Crystal data: 1, orthorhombic, space group P2 12 12 1, a=9.0536(7), b=14.3750(11), c=14.4741(11) Å, V=1883.7(2) Å 3, Z=4, R=0.041; 2, orthorhombic, space group P2 12 12 1, a=8.5949(3), b=13.9694(4), c=14.0076(5) Å, V=1681.83(10) Å 3, Z=4, R=0.0248.

Structural similarity and functional diversity in diiron-oxo proteins

Diiron-oxo proteins currently represent one of the most rapidly developing areas of bioinorganic chemistry. All of these proteins contain a four-helix bundle protein fold surrounding a (μ-carboxylato)diiron core, and most, if not all, of the diiron(II) sites appear to react with O2 as part of their functional processes. Despite these common characteristics, an emerging functional diversity is one of the most striking aspects of this class of proteins. X-ray crystal structures of diiron(II) sites are now available for four of these proteins: hemerythrin (Hr), the hydroxylase protein of methane monooxygenase (MMOH), the R2 protein of Escherichia coli ribonucleotide reductase (RNR-R2), and a plant acyl-carrier protein Δ9-desaturase. The structure of the diiron(II) site in Hr, the sole O2 carrier in the group, is clearly distinct from the other three, whose function is oxygen activation. The Hr diiron site is more histidine rich, and the oxygen-activating diiron sites contain a pair of (D/E)X30–37EX2H ligand sequence motifs, which is clearly not found in Hr. The Hr diiron site apparently permits only terminal O2 coordination to a single iron, whereas the oxygen-activating diiron(II) centers present open or labile coordination sites on both irons of the center, and show a much greater coordinative flexibility upon oxidation to the diiron(III) state. Intermediates at the formal FeIIIFeIII and FeIVFeIV oxidation levels for MMOH and formal FeIIIFeIV oxidation level for RNR-R2 have been identified during reactions of the diiron(II) sites with O2. An [Fe2(μ-O)2]4+, 3+ "diamond core" structure has been proposed for the latter two oxidation levels. The intermediate at the FeIIIFeIV oxidation level in RNR-R2 is kinetically competent to generate a stable, functionally essential tyrosyl radical. The FeIVFeIV oxidation level is presumed to effect hydroxylation of hydrocarbons in MMOH, but the mechanism of this hydroxylation, particularly the involvement of discrete radicals, is currently controversial. The biological function of diiron sites in three members of this class, rubrerythrin, ferritin and bacterioferritin, remains enigmatic.

Hypothesis: iron chelation plays a vital role in neutrophilic inflammation.

Neutrophil influx into tissues occurs in many diverse diseases and can be associated with both beneficial and injurious effects. We hypothesize that the stimulus for certain neutrophilic inflammatory responses can be reduced to a series of competing reactions for iron, with either a labile or reactive coordination site available, between host chelators and chelators not indigenous to that specific living system. The iron focuses the transport of host phagocytic cells through a metal catalyzed generation of oxidant sensitive mediators including cytokines and eicosanoids. Many of these products are chemotactic for neutrophils. We also postulate that the iron increases the activity of the phagocyte associated NADPH oxidoreductase in the neutrophil. The function of this enzyme is likely to be the generation of superoxide in the host's attempt to chemically reduce and dislodge the iron from its chelate complex. After the reoxidation of Fe2+ in an aerobic environment, Fe3+ will be coordinated by host lactoferrin released by the neutrophil. When complexed by this glycoprotein, the metal does not readily undergo oxidation/reduction and is safely transported to the macrophages of the reticuloendothelial system where it is stored in ferritin. Finally, we propose that the neutrophil will attempt to destroy the chelator not indigenous to the host by releasing granular contents other than lactoferrin. Inability to eliminate the chelator allows this sequence to repeat itself, which can lead to tissue injury. Such persistence of a metal chelate in the host may be associated with biomineralization, fibrosis, and cancer.

Synthesis and Properties of Dinuciear μ-Oxo-μ-carboxylato Iron(III) Complexes with a Labile Coordination Site. Models of Non-haem Iron Proteins

The synthesis by self-assembly and the spectroscopic characterization of novel complexes containing an Fe 2 O core with one or two bridge monocarboxylate (AcO - , BzO - , StO - ), bidentate (bpy, phen) and labile monodentate (H 2 O, MeOH) terminal ligands are reported.

A general method for assembling (.mu.-oxo)bis(.mu.-carboxylato)diiron(III) complexes with labile terminal sites using a bridging dicarboxylate ligand

Structural and spectroscopic models of the (μ-oxo)bis(μ-carboxylato)diiron(III) center found in the marine invertebrate respiratory protein hemerythrin have been synthesized with a variety of facially coordinating tridentate ligands. In order to mimic the functions and the related diiron proteins ribonucleotide reductase and methane monooxygenase new model complexes are required having vacant or labile terminal coordination sites

Industrial applications of homogeneous catalysis. A review

Two sets of bio-inspired non-heme iron complexes, each comprising of an FeII and FeIII species, bearing 16-membered macrocyclic tetradentate N-heterocyclic carbene ligands are reported. The complexes exhibit trans labile coordination sites, are characterized by means of NMR, ESI-MS, elemental analysis, SC-XRD and cyclic voltammetry and assessed as olefin epoxidation (pre-)catalysts applying H2O2 as oxidant. Sc(OTf)3 and AcOH are evaluated as Lewis and Brønsted acidic additives, respectively, resulting in partially noticeable improvement in catalytic performance. Hereby, complex 2b shows high stability (TON = 1000 at 20 °C), high temperature tolerance and advances in the more challenging epoxidation of terminal and functionalized olefins. Furthermore, in-depth DFT calculations are conducted to put the catalysts’ structural and electronic features into relation with the catalytic results.

A pulse radiolysis and flash photolysis study of the formation and characterization of platinum (III) amine complex ions

An investigation into the formation and characterization of the transients arising from the reactions of hydroxyl radical with the complexes bis(ethylenediamine)platinum(II) perchlorate and chloride, chloro(diethylenetriamine)platinum(II) chloride and chloro(1,1,7,7-tetraethyldiethylenetriamine)platinum(II) chloride in aqueous media saturated with nitrous oxide has been carried out using the technique of pulse radiolysis. In general, the transients exhibit absorption bands with either a single, intense peak below 300 nm (ϵ ⪞ 400 m 2 mol −1) or for the longer-lived species, two peaks of lesser intensity at about 350 and 500 nm. For the bis(ethylenediamine)-platinum(II) system, the transients are interrelated by acid-base reactions and in the presence of free chloride ion by substitution process. The disappearance of the longer-lived intermediate, which obeys second-order kinetics, does not lead to the formation of detectable amounts of ammonia (or free ethylenediamine). This feature in conjunction with other results indicates that the transients are not those arising from the degradation or loss of the co-ordinated amine ligand. Some of the intermediates exhibit reactivity towards either copper(II) perchlorate, an oxidant, or the reducing agent potassium ferrocyanide. The flash photolysis of trans-dichlorobis(ethylenediamine)platinum(IV) perchlorate has been used to generate and further characterize some of the transients encountered in the pulse radiolysis study. It is concluded that the intermediates are platinum(III) amine complex ions. For the Pt(III) species with absorption peaks below 300 nm, their structures are proposed to be distorted octahedra with substitutionally labile co-ordination sites. It is suggested that the products absorbing above 300 nm may be structurally different from those absorbing below 300 nm.