Rhenium(V) oxo complexes with acetylacetone derived Schiff bases: structure and catalytic epoxidation

Substitution reactions of rhenium(V) oxo precursors [ReOCl3(PPh3)2] or [NBu4][ReOCl4] with the bidentate acetylacetone-derived ketoamine ligands APOH = 4-anilino-3-penten-2-one, DPOH = 4-[2,6-dimethylanilino]-3-penten-2-one, and MTPOH = 4-[2-(methylthio)anilino]-3-penten-2-one gave the complexes [ReO(APO)Cl2(PPh3)] (1), [ReO(DPO)Cl2(PPh3)] (2), and [NBu4][ReOLCl3] (3, L = APO; 4, L = DPO; 5, L = MTPO), respectively. All complexes exhibit only one ketoamino chelate, independent of the amount of ligand added to the rhenium precursors. The complexes were characterized by 1H and 13C NMR spectroscopy. X-ray crystal structures of the complexes 1, 2, 4, and 5, including that of MTPOH, were determined, revealing the trans position of the two oxygen atoms and the trans-Cl,Cl conformation in 1 and 2, in contrast to most other rhenium complexes of this type where the cis-Cl,Cl conformation is observed. Coordination of the potentially tridentate ligand MTPOH in 5 is bidentate with a dangling thioether substituent. Compound 2 shows catalytic activity in the oxidation of cis-cyclooctene with tert-butylhydroperoxide.     

Rhodium and iridium complexes of a new ferrocene-derived chelating bis(NHC) ligand

Rhodium and iridium complexes of a new ferrocene-derived bis(N-heterocyclic carbene) ligand, [M(cod){1,2-(MeNCHCHNCCH(2))(2)C(5)H(3)}Fe(C(5)H(5))]BF(4) (M = Rh, 8a; M = Ir, 8b; cod = 1,5-cyclooctadiene), were synthesized from the corresponding bis(imidazolium) salt 6. The molecular structure of 8a was determined by single-crystal X-ray diffraction. Complexes 8a and 8b smoothly react with CO with displacement of the chelating cod ligand to give the corresponding dicarbonyl derivatives 9a and 9b.     

Synthesis and structural characterization of new multiferrocenyl diyne ligands and their cobalt carbonyl complexes

New multiferrocenyl diyne ligands FcC(CH₃)₂Fc′–C≡C–C≡C–Fc [L ₁ ; Fc?=?C₅H₅FeC₅H₄; Fc′?=?C₅H₅Fe(1,3-disubstituted)C₅H₃], FcC(CH₃)₂Fc′–C≡C–C≡C–Fc′C(CH₃)₂Fc (L ₂ ) and their complexes [FcC(CH₃)₂Fc′–C≡C–C≡C–Fc][Co₂(CO)₆] _n [n?=?1, (1); n?=?2, (2)], [FcC(CH₃)₂Fc′–C≡C–C≡C–Fc′C(CH₃)₂Fc][Co₂(CO)₆] _n [n?=?1, (3); n?=?2, (4)] have been synthesized by the coupling reaction of terminal ferrocenylacetylene and the reaction of ligands L₁ and L₂ with Co₂(CO)₈. The composition and molecular structure of the ligands L₁ , L₂ and their cobalt complexes were characterized by element analysis, IR, ¹H(¹³C)NMR and MS. The electrochemical properties of compounds L₁ , L₂ , 1, 2, 3, 4 were studied by cyclic voltammetry(CV). The results of the electrochemical research reveal that all three ferrocenyl groups in L₁ become redox active centers, but there are only two (not four) ferrocenyl redox active centers in L₂ .     

Ruthenium and chromium complexes bearing pH-indicators as the η6-arene ligand: Synthesis,characterization, and protonation behavior

A series of ruthenium and chromium complexes bearing pH indicators as the η⁶-arene ligand, (η⁶-X)(ML_n)_y [X = methyl yellow, crystal violet lactone, phenolphthalein; ML_n = RuCp¹⁺, RuCl₂(L), Cr(CO)₃; y = 1, 2] is prepared and characterized by spectroscopic and crystallographic methods. Of the plural arene rings in the indicators, a specific arene ring can be successfully coordinated to the metal center in a selective manner under appropriate conditions (i.e. use of the precursors of different oxidation states and reaction with the non-protonated and protonated pH indicator). The obtained indicator complexes show halochromic behavior depending on pH as observed for the parent molecules but the transition pH ranges are shifted to the more acidic side because of the attachment of the electron-withdrawing metal fragments, which decrease the basicity of the attached pH indicators.     

Rhenium(IV)-copper(II) heterobimetallic complexes with a bridge malonato ligand. Synthesis, crystal structure, and magnetic properties

The Re(IV) complex [ReCl4(mal)]2-, in the form of two slightly different salts, (AsPh4)1.5(HNEt3)0.5[ReCl4(mal)] (1a) and (AsPh4)(HNEt3)[ReCl4(mal)] (1b), and the Re(IV)-Cu(II) bimetallic complexes [ReCl4(mu-mal)Cu(phen)2].CH3CN (2), [ReCl4(mu-mal)Cu(bpy)2] (3), and [ReCl4(mu-mal)Cu(terpy)] (4) (mal=malonate dianion, AsPh4=tetraphenylarsonium cation, HNEt3=triethylammonium cation, phen=1,10-phenanthroline, bpy=2,2'-bipyridine and terpy=2,2':6',2' '-terpyridine) have been synthesized and the structures of 1a, 1b, 2, and 3 determined by single-crystal X-ray diffraction. The structures of 1a and 1b are made up of discrete [ReCl4(mal)]2- anions and AsPh4+ and HNEt3+ cations, held together by electrostatic forces and hydrogen bonds. The Re(IV) atom is surrounded by four chloride anions and a bidentate malonate group, in a distorted octahedral environment. The structure of 2 consist of neutral dinuclear units [ReCl4(mu-mal)Cu(phen)2], with the metal ions united through a bridge carboxilato. The environment of Re(IV) is nearly identical to that in the mononuclear complex, and Cu(II) is five coordinate, being surrounded by four nitrogen atoms of two bidentate phen ligands and one oxygen atom of the malonato ligand. In 3, there are also dinuclear units, [ReCl4(mu-mal)Cu(bpy)2], but the Cu(II) ions complete a distorted octahedral coordination by binding with the free malonato oxygen atom of a neighbor unit, resulting in an infinite chain. The magnetic properties of 1-4 were also investigated in the temperature range 2.0-300 K. The magnetic behavior of 1a and 1b is as expected for a Re(IV) complex with a large value of the zero-field splitting (2D ca. 110 cm(-1)). For the bimetallic complexes, the magnetic coupling between Re(IV) and Cu(II) is antiferromagnetic in 2 (J=-0.39 cm(-1)), ferromagnetic in 4 (J=+1.51 cm(-1)), and nearly negligible in 3 (J=-0.09 cm(-1)).     

Oxidation-state and metal-ion dependent stereoisomerization in oxo molybdenum and tungsten complexes of a bulky alkoxy heteroscorpionate ligand

Monooxo Mo(V) complexes of a N2O heteroscorpionate ligand designated (L10O) are found to exist as isolable cis and trans isomers. We have been able to trap the kinetically labile cis isomer and follow its isomerization to the thermodynamically more stable trans form. We have also followed the kinetics of isomerization between the cis and trans isomers of the corresponding dioxo Mo(VI) and W(VI) species. Here the trans is the labile isomer that spontaneously converts to the thermodynamically more stable cis. It is observed that at 60 degrees C in DMSO the Mo(VI) complex isomerizes approximately 6.5 times faster than the Mo(V) and nearly 5 times faster than the corresponding W(VI) analogs. The temperature dependence to the kinetics of the Mo(V) and Mo(VI) isomerizations give activation parameters that are similar for both oxidation states and consistent with those previously observed in [(L1O)MoOCl2] suggesting a similar twist mechanism is operating in all cases. Thus there are oxidation state, metal ion and donor atom dependent differences in isomeric stability that could have significant implications for understanding the mechanisms of both enzymatic and nonenzymatic oxo atom transfer reactions catalyzed by complexes of Mo, W and Re.     

Impact of ligand protonation on eigen-type metal complexation kinetics in aqueous systems

The impact of ligand protonation on metal speciation dynamics is quantitatively described. Starting from the usual situation for metal complex formation reactions in aqueous systems, i.e., exchange of water for the ligand in the inner coordination sphere as the rate-determining step (Eigen mechanism), expressions are derived for the lability of metal complexes with protonated and unprotonated ligand species being involved in formation of the precursor outer-sphere complex. A differentiated approach is developed whereby the contributions from all outer-sphere complexes are included in the rate of complex formation, to an extent weighted by their respective stabilities. The stability of the ion pair type outer-sphere complex is given particular attention, especially for the case of multidentate ligands containing several charged sites. It turns out that in such cases, the effective ligand charge can be considerably different from the formal charge. The lability of Cd(II) complexes with 1,2-diaminoethane-N,N'-diethanoic acid at a microelectrode is reasonably well predicted by the new approach.     

Synthesis and properties of a temperature-sensitive chelating hydrogel and its metal complexes

A temperature-sensitive chelating hydrogel was synthesized by the copolymerization of 1-(β-acrylamidoethyl)-3-hydroxy-2-methyl-4(1H)-pyridinone (AHMP) and N-isopropylacrylamide (NIPAA) in the presence of N,N' -ethylene-bis-acrylamide as a crosslinking agent. The AHMP-NIPAA hydrogel formed a red complex with iron(III) and a pale green complex with Cu(II), respectively. It was observed that the hydrogel and its metal complexes had a high swelling ratio below the temperature of 35°C, while above that temperature the swelling ratios were dramatically decreased. Furthermore, the swelling ratio of the metal complexes was much lower at the swelling temperature (below 35°C) than that of the hydrogel itself, which might be due to the lower flexibility of the complexes. The iron(III) chelating study showed that the hydrogel had a high chelating efficiency at its swelling temperature, while the chelating efficiency of the hydrogel was very low at its deswelling temperature (>35°C). It was found that the chelating efficiency depended on the swelling ratio of the hydrogel in water, which could be explained by the difference in contactable internal surfaces at different temperatures. The hydrogel and its metal complexes could be easily separated at their deswelling temperature. It was also convenient to regenerate the hydrogel with 1 M HCl for reuse.     

Impact of ligand protonation on higher-order metal complexation kinetics in aqueous systems

The impact of ligand protonation on the complexation kinetics of higher-order complexes is quantitatively described. The theory is formulated on the basis of the usual situation for metal complex formation in aqueous systems in which the exchange of water for the ligand in the inner coordination sphere is rate-determining (Eigen mechanism). We derive expressions for the general case of lability of ML(n) species that account for the contributions from all outer-sphere complexes to the rate of complex formation. For dynamic complexes, dissociation of ML is usually the rate-determining step in the overall process ML(n) --> M. Under such conditions, it is the role of ligand protonation in the step ML --> M that is relevant for the kinetic flux. 1:2 complexes of Cd(II) with pyridine-2,6-dicarboxylic acid fall into this category, and their lability at a microelectrode is reasonably well predicted by the differentiated approach. For non-dynamic systems, the kinetic flux arising from dissociation of higher-order complexes contributes to the rate-determining step. In this case, the weighted contribution of protonated and unprotonated outer-sphere complexes in all contributing dissociation reactions must be taken into account. The kinetic flux arising from the dissociation of 1:2 complexes of Ni(II) with bicine at a conventional electrode was quite well described by this combined approach. The results establish the generic role of ligand protonation within the overall framework of metal complexation kinetics in which complexes may be dynamic to an extent that depends on the operational time scale of the measurement technique.     

Synthesis of naturally occurring polyacetylenes via a bis-silylated diyne

A straightforward synthesis of a series of naturally occurring polyacetylenes has been developed, including the montiporynes A and C, possessing cytotoxic activity against several human solid tumor cells, the atractylodin, with antibiotic activity against Escherichia coli, and triynes, which display insecticidal activities, starting from the readily available 1,4-bis(trimethylsilyl)-1,3-butadiyne.     

Synthesis of chelating resins and its application in ligand exchange chromatography

Summary Three kinds of new chelating resin containing -hydroxydithiocinnamic acid, ethyl -hydroxydithiocinnamate and N-(hydroxymethyl)-thioamide functional groups were synthesized. Cu(II), Cd(II) and Hg(II) were used as complex forming metal ions. By varying the combination of metal ion and chelating resin, a great variety of separation systems could be realized. For the on-line trace enrichment of 2-mercaptobenzimidazole, the copper-loaded N-(hydroxymethyl)-thioamide concentration column was found most suitable. UV detection at 300 nm was used. The detection limit was 0.2 ng/g. A 10 ml sample was found to be sufficient to attain the described sensitivity, and linear relationship was obtained in the range of 0.75 ng/g to 12 ng/g. The best separation of the thiol compound mixture thioglycolic acid, methionine, cysteine and 2-mercaptoethanol was obtained with copper-loaded ethyl -hydroxydithiocinnamate resin; UV absorbance detection was at 215 nm.     

Synthesis,chemical and electrochemical studies of complexes of a tridentate ONS chelating ligand built around the elusive [MoVIOS]2+ core

Synthesis and characterization of four Mo(VI) complexes of a diprotic tridentate ONS chelating ligand (H₂L) containing the rather elusive [Mo^VIOS]²⁺ core is reported. These [Mo^VIOSL] complexes are obtained from their corresponding [Mo^VIO₂L] precursors using a combination of PPh₃ and PPh₃S. This process of oxo-abstraction and sulfido-inclusion affected by PPh₃–PPh₃S is reported for the first time and may be considered as a general method of converting [Mo^VIO₂L] complexes to the corresponding [Mo^VIOSL] complexes. Direct structural characterization of these complexes could not be done due to the ease of solvolysis of these oxosulfidomolybdenum(VI) complexes to the corresponding dioxomolybdenum(VI) analogues. However, the structure of these [Mo^VIOSL] complexes could be reasonably surmised from the corresponding structurally characterized [Mo^VIO₂L] complexes. Points of attachment of the potentially pentadentate but functionally tridentate ONS chelating ligands to [Mo^VIOS]²⁺ are located mainly through analysis of IR and UV-Vis spectral data and comparison with corresponding [Mo^VIO₂L] complexes of known structure. Conditions under which solvolysis of [Mo^VIOS]²⁺ to the [Mo^VIO₂]²⁺ core is significantly retarded have been identified and make us hopeful of obtaining single crystals of [Mo^VIOSL].     

Tetravalent titanium, zirconium, and cerium oxo and peroxo complexes containing an imidodiphosphinate ligand

Dinuclear Ti(IV), Zr(IV), and Ce(IV) oxo and peroxo complexes containing the imidodiphosphinate ligand [N(i-Pr(2)PO)(2)](-) have been synthesized and structurally characterized. Treatment of Ti(O-i-Pr)(2)Cl(2) with KN(i-Pr(2)PO)(2) afforded the Ti(IV) di-μ-oxo complex [Ti{N(i-Pr(2)PO)(2)}(2)](2)(μ-O)(2) (1) that reacted with 35% H(2)O(2) to give the peroxo complex Ti[N(i-Pr(2)PO)(2)](2)(η(2)-O(2)) (2). Treatment of HN(i-Pr(2)PO)(2) with Zr(O-t-Bu)(4) and Ce(2)(O-i-Pr)(8)(i-PrOH)(2) afforded the di-μ-peroxo-bridged dimers [M{N(i-Pr(2)PO)(2)}(2)](2)(μ-O(2))(2) [M = Zr (3), Ce (4)]. 4 was also obtained from the reaction of Ce[N(i-Pr(2)PO)(2)](3) with 35% H(2)O(2). Treatment of (Et(4)N)(2)[CeCl(6)] with 3 equiv of KN(i-Pr(2)PO)(2) afforded Ce[N(i-Pr(2)PO)(2)](3)Cl (5). Reaction of (Et(4)N)(2)[CeCl(6)] with 2 equiv of KN(i-Pr(2)PO)(2) in acetonitrile, followed by treatment with Ag(2)O, afforded the μ-oxo-bridged complex [Ce{N(i-Pr(2)PO)(2)}Cl](2)[μ-N(i-Pr(2)PO)(2)](2)(μ-O) (6). 6 undergoes ligand redistribution in CH(2)Cl(2) in air to give 5. The solid-state structures of [K(2){N(i-Pr(2)PO)(2)}(2)(H(2)O)(8)](n) and complexes 1-6 have been determined.     

Synthesis of aluminium complexes bearing a piperazine-based ligand system

Aluminium complexes bearing the N,N-chelating ligand 1,4-bis(2-hydroxy-3,5-di-tert-butyl)piperazine (1) have been synthesised. Both monometallic and bimetallic aluminium methyl complexes (2 and 3, respectively) were prepared by treatment of 1 with the appropriate amount of AlMe₃. Complex 2 can be converted to 3 by addition of excess AlMe₃. Bimetallic aluminium-ethyl complex 4 was also prepared. Treatment of 1 with AlEt₂Cl afforded the monometallic chloride complex 5. Treatment of this latter complex with potassium alkoxides (KOR, R = Me, Et, ⁱPr, ^tBu) or AgOTf afforded the corresponding aluminium alkoxide complexes (6, R = Et; 7, R = Me; 8, R = ⁱPr; 9, R = ^tBu; 10, R = OTf) in good yields. Aluminium ethoxide complex 6 was also synthesised by treatment of 1 with AlEt₂OEt. All of these complexes were tested as potential catalysts in the ring-opening polymerisation of rac-lactide and caprolactone with limited success.     

Coordination chemistry of a chelating amidoximato ligand

A potentially general strategy for accessing the rarely encountered coordination of amidoximates through both oximato O and amido N atoms was developed. The new amidoxime (Z)-R(1)C(=NOH)NR(2)H (R(1) = mesityl, R(2) = 2-[[dimethylamino]methyl]phenyl), H(2)L, was prepared by reaction of equimolar amounts of the appropriate nitrile oxide and primary amine. Treatment of H(2)L with 1 equiv of AlMe(3) produced the dimeric species (MeAlL)(2) (1) possessing the formally dianionic ligand L(2)(-) bound to the aluminum via the oximato oxygen and the amido and amino nitrogens. The oximato oxygen atoms serve to link the two monomeric organoaluminum units together. Reaction of (ON)Cr(N(i)Pr(2))(3) with H(2)L provided the diamagnetic complex (ON)Cr(N(i)Pr(2))(eta(3)-L) (2). Interaction of 2 with excess methanol led, in particular, to the oxidative loss of NO and yielded the paramagnetic dimer [Cr(LH)(OMe)(mu-OMe)](2) (3) containing the eta(3)-bound amidoximato ligand protonated at its oximato nitrogen. In addition to spectroscopic characterizations of the new substances, single-crystal X-ray structures of H(2)L, 1.(4)/(3)C(6)H(6), and 3.MeOH were obtained.     

Studies on chelating resins-I General equation for the calculation of the protonation constants of chelating resins

A general equation has been derived which describes the protonation equilibria of basic groups attached to an insoluble polymer matrix. The derivation is based on the application of the charge and mass-balance equations. When the relationship obtained is applied to water-soluble ligands, it reduces to the well-known Schwarzenbach equation for titration of polybasic acids.