π-Bonded alkene and arene complexes of silver(I): an electrospray mass spectrometric study

Electrospray mass spectra have been observed for a number of alkene and arene complexes of Ag(I) formed by the interaction of AgNO₃ and the organometallic ligand in water/methanol solution. The ES mass spectra show that almost all the alkene and arene ligands in stoichiometric excess form labile 1:2 cationic complexes with Ag(I) which are easily decomposed by collisional activation to the 1:1 species. However, with a deficiency of organic ligand polymeric species are observed. The cation [Ag(cod)₂]⁺ (cod=1,5-cyclooctadiene) was reacted with a variety of other potential ligands, such as PPh₃, AsPh₃, PhSCH₂SPh etc. In most cases, mixed complexes [Ag(cod)(ligand)]⁺ were observed, and excess ligand usually produced [Ag(ligand)₂]⁺.     

Formation of dioxygen complexes of transition metal chelates in oxygen saturated aprotic solvents. Cyclic voltammetric evidence at a micro glassy carbon disk electrode

Cyclic voltammetry at a micro electrode of Co(II) salen, Fe(II) salen, electrode generated Fe(II)(acac)₂, Fe(II) (salicylaldehyde)₂, Fe(II) (salicylaldoxime)₂, Fe(II) (bipy)₃, Fe(II) (bipy)₂, Co(II) (bipy)₃, Co(II) (benzacac)₂, and electrode generated Co(acac)₂ in oxygen saturated aprotic solvents show positive shift of the O₂ sigmoidal wave, as well as enhancement of the limiting current in the case of the first five compounds. In the case of Co(II) (bipy)₃ the slope of the sigmoidal wave due to O₂ becomes more positive, while for the other two Co(II) complexes there is no change except a small decrease in the wave height. The data are used to correlate and predict the O₂ binding properties of the chelates in solution. The data for the diketone complexes of Co(II) indicate absence of any direct association, which is in line with the interpretation offered in the literature on the mechanism of their catalytic role in the O₂ oxidation of substrates. The mechanism of the autoxidation of dimethylformamide in the presence of Fe(III) (bipy)₃ and Cu(II) (bipy)₂ is elucidated by the observation that these higher valent compounds are reduced to their next lower oxidation state by DMF.     

Enzyme inhibitor modeling with TpZn complexes of functional hydroxamates and oximates

Salicylhydroxamic acid reacts with the enzyme model Tp^Ph,MeZn-OH to form the O,O-chelating hydroxamate complex 1. The hydrogen bonding capacity of zinc enzyme bound hydroxamates is reproduced by cocrystallization of two molecules if 1 with two molecules of methanol and by cocrystallization of one molecule of Tp^Ph,MeZn-acetohydroxamate with one molecule of 3-phenyl-5-methylpyrazole. The complex formed from Tp^Ph,MeZn-OH and N-tosylproline hydroxamic acid, according to its spectra, contains the hydroxamate as an N,N-chelating ligand. In contrast, the oximate derived from pyruvic aldehyde does not act as a chelating ligand, but is monodentate via the oximate oxygen.     

The migratory insertion of carbon monoxide in pentamethylcyclopentadienyliridium (III) complexes. Structural effects on reactivity and mechanism for rhodium and iridium systems

The reactions of complex (C₅Me₅)Ir(Cl) (CO) (Me) (1a) with cyclohexylisocyanide and phosphines (L=CyNC, PHPh₂, PMePh₂, PMe₂Ph) give the products of alkyl migratory insertion (C₅Me₅Ir(Cl) (COMe) (L), in toluence or tetrahydrofuran at 323 K or higher temperature. The phenyl analogue (C₅Me₅)Ir(Cl)(CO)(Ph) or the iodide complexes (C₅Me₅)Ir(I) (CO) (R) (R=Me, Ph_are not reactive under the same conditions. The reaction of (C₅Me₅)Ir(Cl)(CO)(Me) with PMePh₂ and PMe₂Ph in acetonitrile yields the chloride substitution product [(C₅Me₅)Ir(CO)(L)(Me)]⁺Cl⁻. Kinetic measurements for the reactions of (C₅Me₅)Ir(Cl)(CO)(Me) in toluene are first order in the iridium complex and exhibit a saturation dependence on the incoming donors L. Analysis of the data suggests a two-step process involving (i) rapid formation of a molecular complex [(C₅Me₅)Ir(Cl)(CO)(Me), (L)], in which the structure of 1a is unperturbed within the limits of spectroscopic analysis, and (ii) rate determining methyl migration. The reaction parameters are K for the pre-equilibrium step (K = 1.5 (CyNC), 7.3 (PHPh₂), 7.1 (PMePh₂) dm³ mol⁻¹ at 323 K) and k₂ for the slow carbon---carbon bond formation (k₂ (10⁵) = 6.9 (CyNC), 1.2 (PHPh₂), 1.0 (PMePh₂) s⁻¹ at 323 K). The activation parameters for the methyl migration step in the reaction with PMePh₂ obtained between 308 and 338 K, are ΔH^≠ = 106±16 kJ mol⁻¹ and ΔS^≠ = − 14±5 J K⁻¹ mol⁻¹. The reaction of 1a with PMePh₂ proceeds at similar rates in tetrahydrofuran (K = 3.7 dm³ mol⁻¹, k₂ (10⁵) = 1.2 s⁻¹, 323 K). The crystal structure of (C₅Me₅)Ir(Cl)(COMe) (PMe₂Ph) has been determined by X-ray diffraction. C₂₀H₂₉ClOPIr: M_r = 544.1, monoclinic, P2₁/n, A = 8.084 (2), B = 9.030(2), C = 28.715 (3) Å, β = 91.41 (3)°, Z = 4, D_c = 1.71 g cm⁻³, V = 2095.5 ų, room temperatyre, Mo K, γ = 0.71069, μ = 65.55 cm⁻¹, F(000) = 1044, R = 0.037 for 2453 independent observed reflections. The complex shows a deformed tetrahedral coordination assuming the η⁵-C₅Me₅ molecular fragment as a single coordination site. The iridium-chlorine bond is staggered with respect to two adjacent C(ring)-methyl bonds, while the Ir---P and the Ir---COMe bonds are eclipsed with respect to C(ring)-methyl bonds.     

Electrophoretic study of cationic cobalt(III) complexes with [N2O] and [N2O2] Schiff base ligands

The electrophoretic migration behavior of acid-sensitive cationic alkylcobalt(III) complexes with tridentate Schiff bases and amines as well as that of related ‘inorganic’ cobalt(III) chelates with tridentate and tetradentate Schiff bases was studied. A correlation of the electrophoretic mobility of the organocobalt complexes in question with their structure was established. An attempt to optimize the analytical procedures for capillary electrophoretic separation of these rather labile complex cations is outlined. Their decomposition in solutions under ambient conditions was surveyed using this technique.     

Alkene rotation in [Ru(η-C5H5) (L2) (η-alkene][CF3SO3] with L2 = iPr- or pTol-diazabutadiene. X-ray crystal structure of [Ru(η-C5H5(pTol-DAB) (η-ethene][CF3SO3]

Reaction of RuCl(η⁵-C₅H₅(pTol-DAB) with AgOTf (OTf = CF₃SO₃) in CH₂Cl₂ or THF and subsequent addition of L′ (L′ = ethene (a), dimethyl fumarate (b), fumaronitrile (c) or CO (d) led to the ionic complexes [Ru(η⁵-C₅H₅)(pTol-DAB)(L′)][OTf] 2a, 2b and 2d and [Ru(η⁵-C₅H₅)(pTol-DAB)(fumarontrile-N)][OTf] 5c. With the use of resonance Raman spectroscopy, the intense absorption bands of the complexes have been assigned to MLCT transitions to the iPr-DAB ligand. The X-ray structure determination of [Ru(η⁵-C₅H₅)(pTol-DAB)(η²-ethene)][CF₃SO₃] (2a) has been carried out. Crystal data for 2a: monoclinic, space group P2₁/n with A = 10.840(1), b = 16.639(1), C = 14.463(2) Å, β = 109.6(1)°, V = 2465.6(5) ų, Z = 4. Complex 2a has a piano stool structure, with the Cp ring η⁵-bonded, the pTol-DAB ligand σN, σN′ bonded (Ru-N distances 2.052(4) and 2.055(4) Å), and the ethene η²-bonded to the ruthenium center (Ru-C distances 2.217(9) and 2.206(8) Å). The C = C bond of the ethene is almost coplanar with the plane of the Cp ring, and the angle between the plane of the Cp ring and the double of the ethene is 1.8(0.2)°. The reaction of [RuCl(η⁵-C₅H₅)(PPh)₃ with AgOTf and ligands L′ = a and d led to [Ru(η⁵-C₅H₅)(PPh₃)₂(L′)]OTf] (3a) and (3d), respectively. By variable temperature NMR spectroscopy the rottional barrier of ethene (a), dimethyl fumarate (b and fumaronitrile (c) in complexes [Ru(η⁵-C₅H₅)(L₂)(η²-alkene][OTf] with L₂ = iPr-DAB (a, 1b, 1c), pTol-DAB (2a, 2b) and L = PPh₃ (3a) was determined. For 1a, 1b and 2b the barrier is 41.5±0.5, 62±1 and 59±1 kJ mol⁻¹, respectively. The intermediate exchange could not be reached for 1c, and the ΔG^# was estimated to be at least 61 kJ mol⁻. For 2a and 3a the slow exchange could not be reached. The rotational barrier for 2a was estimated to be 40 kJ mol⁻. The rotational barier for methyl propiolate (HC≡CC(O)OCH₃) (k) in complex [Ru(η⁵-C₅H₅)(iPr-DAB) η²-HC≡CC(O)OCH₃)][OTf] (1k) is 45.3±0.2 kJ mol⁻¹. The collected data show that the barrier of rotational of the alkene in complexes 1a, 2a, 1b, 2b and 1c does not correlate with the strength of the metal-alkene interaction in the ground state.     

Synthesis, crystal structures and magnetic properties of transition metal–azide complexes coordinated with pyridyl nitronyl nitroxides

Two novel complexes Co(N₃)₂(PNN)₄ (I) and Mn(N₃)₂(PNN)₂(CH₃OH)(C₂H₅OH) (II) (PNN=2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3–oxide) were synthesized and characterized by infrared spectra, elemental analyses and UV–Vis techniques. The crystal structures of both complexes have been determined by X-ray diffraction analysis. Complex I is a neutral five-spin system and adopts a centrosymmetric tetragonally compressed octahedral coordination geometry in which Co(II) ion is coordinated to four radicals through the nitrogen atoms of the pyridine rings and two azide anions occupying the axial positions. Complex II is a neutral three-spin system in which Mn(II) ion is bound to two azide anions, two alcohol molecules and two radicals through the nitrogen atom of pyridine rings, and shows one-dimensional chain structure via hydrogen bonds (d_ON=2.78 Å). The magnetic properties for complexes I and II have been investigated in the temperature range 2–300 K. A theoretical model has been developed for complex I and the magnetic behaviors for both complexes have been discussed in detail.     

A new synthetic route towards heterotrimetallic complexes. Synthesis, crystal structure and magnetic properties of a [CuMnCr] trinuclear complex

The heterotrimetallic complex, [{LCuMn(H₂O)}{Cr(phen)(C₂O₄)₂}](ClO₄) · H₂O (1), has been obtained by assembling heterobinuclear cations, [LCuMn]²⁺, with [Cr(phen)(C₂O₄)₂]⁻ ions (H₂L is the compartmental Schiff-base resulting from the stepwise condensation of 2,6-diformyl-p-cresol with ethylenediamine and diethylenetriamine). The copper(II) and manganese(II) ions are hosted into the compartments of the macrocyclic ligand. [Cr(phen)(C₂O₄)₂]⁻ acts as a ligand, being coordinated through one oxalato oxygen atom to the apical position of the square pyramidal copper(II) ion. The cryomagnetic investigation of 1 reveals an antiferromagnetic interaction between Cu^II and Mn^II within the compartmental ligand (J = −39 cm⁻¹). The interaction between Cu^II and Cr^III across the oxalato bridge is negligible. The crystal structure of [LCuPb](ClO₄)₂ · H₂O, a useful precursor in obtaining 3d-3d′ complexes, is also reported.     

Evidence for C---C coupling between two mutually cis carbon ligands, η-crotyl and aryl in organopalladium(II) complexes

Some (η³-crotyl) (2,5-dichlorophenyl)palladium(II) complexes containing phosphine and phosphite ligands Pd(η³-CH₂CH-CHMe)(Ar)(PR₃) (Ar = C₆H₃Cl₂-2,5) were isolated as crystalline solids or generated in solution. These existed as a mixture of two geometrical isomers arising from a different way of risposition of the crotyl-methyl group and the aryl ligand. The electronic nature of the PR₃ ligand controlled the relative rates of the interconversion between the two isomers and the reductive elimination of the complexes which released MeCH=CHCH₂Ar and CH₂=CHCH(Me)(Ar). Electron-withdrawing phosphite ligands were particularly effective in enhancing the reductive elimination rate, making the contribution of the isomerization path almost negligible and allowing the formation of two coupling products to be followed separately by spectroscopic means. The observations demonstrated the occurrence of C---C coupling between mutually cis carbon ligands in (η³-allyl)(hydrocarbyl)palladium(II) complexes. The η¹-crotyl complex, (Pd(η¹-CH₂CH=CHMe)(Ar) (dppen) (dppen = cis-Ph₂PCH=CHPPh₂) was isolated and shown to exist as a sole regio-isomer in solution. Reductive elimination of this η¹-crotyl complex gave MeCH=CHCH₂Ar exclusively.     

Reconstitution of water-oxidizing complex in manganese-depleted photosystem 2 preparations with synthetic manganese complexes

Four synthetic manganese complexes in which Mn atoms have different coordination environments and valence states were used to reconstitute water-oxidizing complex (WOC) in Mn-depleted photosystem 2 preparations. Three Mn-complexes restored a significant rate of electron transfer and oxygen evolution except one complex in which Mn atom ligated to the O-atoms within the ligands by covalent linkage. The effect of coordination environment of the Mn-atom within the Mn-complexes on their efficiencies in reconstituting the electron transport and oxygen evolution was analysed.     

Silver derivatives of tris(pyrazol-1-yl)methanes. A silver(I) nitrate complex containing a tris(pyrazolyl)methane coordinated in a bridging mode

The reaction of AgX (X=ClO₄, NO₃ or SO₃CH₃) acceptors with excesses of tris(pyrazol-1-yl)methane ligands L (L=CH(pz)₃, CH(4-Mepz)₃, CH(3,5-Me₂pz)₃, CH(3,4,5-Me₃pz)₃ or CH(3-Mepz)₂(5-Mepz)) yields 1:1 [AgX(L)], 2:1 [Ag(L)₂]X or 3:2 [(AgX)₂(L)₃] complexes. The ligand to metal ratio in all complexes is dependent on the number and disposition of the Me substituents on the azole ring of the neutral ligand and on the nature of the Ag(I) acceptor. All complexes have been characterized in the solid state as well as in solution (medium- and far-IR, ¹H and ¹³C NMR and conductivity determinations) and the solid-state structures of [Ag(NO₃){(pz)₃CH}]_(∞/∞) and [Ag{(3,5-Me₂pz)₃CH}₂]NO₃ determined by single crystal X-ray studies.     

Evidence for a complex relationship between apoA-V and apoC-III in patients with severe hypertriglyceridemia

The relevance of apolipoprotein A-V (apoA-V) for human lipid homeostasis is underscored by genetic association studies and the identification of truncation-causing mutations in the APOA5 gene as a cause of type V hyperlipidemia, compatible with an LPL-activating role of apoA-V. An inverse correlation between plasma apoA-V and triglyceride (TG) levels has been surmised from animal data. Recent studies in human subjects using (semi)quantitative immunoassays, however, do not provide unambiguous support for such a relationship. Here, we used a novel, validated ELISA to measure plasma apoA-V levels in patients (n = 28) with hypertriglyceridemia (HTG; 1.8-78.7 mmol TG/l) and normolipidemic controls (n = 42). Unexpectedly, plasma apoA-V levels were markedly increased in the HTG subjects compared with controls (1,987 vs. 258 ng/ml; P < 0.001). In the HTG group, apoA-V and TG were positively correlated (r = +0.44, P = 0.02). In addition, we noted an increased level of the LPL-inhibitory protein apoC-III in the HTG group (45.8 vs. 10.6 mg/dl in controls; P < 0.001). The correlation between apoA-V and TG levels in the HTG group disappeared (partial r = +0.09, P = 0.65) when controlling for apoC-III levels. In contrast, apoC-III and TG remained positively correlated in this group when controlling for apoA-V (partial r = +0.43, P = 0.025). Our findings suggest that in HTG patients, increased TG levels are accompanied by high plasma levels of apoA-V and apoC-III, apolipoproteins with opposite modes of action. This study provides evidence for a complex interaction between apoA-V and apoC-III in patients with severe HTG.     

Monovalent copper as a potential catalyst for formation of acetaldehyde via the migration of methyl radicals to the coordinated carbonyl in the complex (CO)Cu-CH3

Cu_aq⁺ forms stable complexes with carbon monoxide in aqueous solutions. Furthermore it reacts very fast with aliphatic radicals. The reaction of Cu(CO)_maq⁺ with methyl radicals, ^•CH₃ was studied using the pulse-radiolysis technique. The results point out that methyl radicals react with Cu(CO)_aq⁺ to form an unstable intermediate with a Cu^II-C σ bond identified as (CO)Cu^II-CH₃⁺, k = (1.1±0.2) × 10⁹ M⁻¹ s⁻¹. This intermediate has a strong LMCT charge transfer band (λ_max = 385 nm, _max = 2500 M⁻¹ cm⁻¹) which is similar to the absorption bands of other transient complexes with Cu^II-alkyl σ bonds. The coordinated carbon monoxide in (CO)Cu^II-CH₃⁺ inserts into the copper—carbon bond (or rather the coordinated methyl migrates to the coordinated carbon monoxide ligand) at a rate of (3.0±0.8) × 10² s⁻¹ to form the copperacetyl complex (CO)_mCu^II-C(CH₃)=O⁺ (λ_max = 480 nm, _max = 2100 M⁻¹ cm⁻¹). The rate of formation of (CO)Cu^II-CH₃⁺ and of the insertion reaction are pH independent. The complex (CO)_mCu^II-C(CH₃)=O⁺ is also unstable and decomposes heterolytically to yield acetaldehyde and Cu_aq²⁺ as the final stable products. This reaction is slightly pH dependent. The same reactivity pattern has been observed for the Cu(CO_naq⁺ complexes (n = 2 or 3). The results clearly point out that CO remains coordinated to transient complexes of the type Cu^II-alkyl.     

Characterization and catechole oxidase activity of a family of copper complexes coordinated by tripodal pyrazole-based ligands

A family of tripodal pyrazole-based ligands has been synthesized by a condensation reaction between 1-hydroxypyrazoles and aminoalcohols. The diversity was introduced both on the substituents of the pyrazole ring and on the side chain. The corresponding copper(II) complexes have been prepared by reaction with CuCl₂ in tetrahydrofuran. They have been characterized by EPR, UV spectroscopy and cyclic voltammetry. The absence of the half-field splitting signals in EPR suggests that the complex exists in solution as mononuclear species. The influence of substituents and side chain of the tripodal ligand on the catecholase activity of the complexes was studied. The reaction rate depends on two factors. First, the presence of an oxygen atom in the third position of the side chain should be avoided to keep the effectiveness of the reaction. Second, the electronic and steric effects of substituents on the pyrazole ring strongly affect the catalytic activity of the complex. Thus, best results were obtained with complexes containing unsubstituted pyrazole based-ligands. Kinetic investigations with the best catalyst based on the Michaelis–Menten model show that the catalytic activity of the mononuclear complex is close to that of some dicopper complexes described in literature.     

Self-assembly and cytotoxicity study of waterwheel-like dinuclear metal complexes: the first metal complexes appended with multiple free hydroxamic acid groups

Two waterwheel-like dinuclear complexes [M(2)(PHA)(4)(H(2)O)(2)] (M = Cu(II) (1), Zn(II) (2); HPHA = phthal-hydroxamic acid) appended with four free hydroxamic acid groups, namely, free hydroxamic acid metal complexes (FHAMCs) have been synthesized and characterized. The crystal structure of complex 1 was determined by single crystal X-ray diffraction, which adopts the paddlewheel motif with four bidentate carboxylate ligands joining two Cu(II) ions. The relative cytotoxicities of compounds 1 and 2 against SMMC-7721 and HO-8910 cell lines are similar and more predominant than HPHA (IC(50): Cu(II)>Zn(II)>HPHA). The synergic effect of the bound water molecules, multiple free hydroxamic acid groups and dimetal active sites with bridging carboxylate may have significant impacts on their pharmacological activity. As the prototype for a new class of hydroxamic acid derivatives, the self-assembly of FHAMCs presents a promising new strategy in designing multiple hydroxamic acids with remarkable bioactivities.     

Stereochemical studies of cobalt ethylenediamine complexes and their tetrathionate salts: Interactions between complex configuration and tetrathionate conformation

A range of chiral coordination complexes of the formulae cis-[Co(en)2XY] where X, Y = hal, NH₃, NO₂, ox, SCN have been prepared as the halide or pseudohalide salts, and the novel examples have been structurally characterised and their stereochemical behaviour studied. Salt metathesis of certain of these species to the corresponding tetrathionates is described, and the tetrathionates structurally characterised and the stereochemical relationship between the conformations of the chiral complexes and the pseudochiral tetrathionates analysed. The patterns of cation-cation and cation-anion interactions are discussed in the context of the reported ability of the tetrathionate ion to induce conglomerate crystallisation in such species, and tetrathionate chirality in the literature.     

Reactivities of the coordinated organonitriles in molybdenum(0) and tungsten(0) phosphine complexes: protonation of the nitrile carbon and cleavage of the CN triple bond

The molybdenum and tungsten dinitrogen-organonitrile complexes trans-[M(N₂)(NCR)(dppe)₂] (2, M=Mo; 4, M=W; R=Ph, C₆H₄Me-p, C₆H₄OMe-p, Me; dppe=Ph₂PCH₂CH₂PPh₂) underwent double protonation at the nitrile carbon atom with loss of N₂ and a change in oxidation state to +4 on treatment with hydrochloric acid to afford the cationic imido complexes trans-[MCl(NCH₂R)(dppe)₂]⁺. The solid-state structure of trans-[WCl(NCH₂CH₃)(dppe)₂][PF₆]·CH₂Cl₂ was determined by single-crystal X-ray analysis. Protonation of complexes 2 by fluoroboric acid or hydrobromic acid also formed the similar imido complexes trans-[MoX(NCH₂R)(dppe)₂]⁺ (X=F, Br). In contrast, the dinitrogen complex trans-[Mo(N₂)₂(dppe)₂] reacted with two equiv. of benzoylacetonitrile, a nitrile with acidic CH hydrogen atoms, to give the nitrido complex trans-[Mo(N)(NKCCHCOPh)(dppe)₂] (12), which was accompanied by evolution of dinitrogen and the formation of 1-phenyl-2-propen-1-one in high yields. For complex 12, the zwitterionic structure, where the anionic enolate ligand PhC(O⁺)=CHCN coordinates to the cationic Mo(IV) center through its nitrogen atom, was confirmed by spectroscopic measurements and single-crystal X-ray analysis. A unique intermolecular aromatic C---HO hydrogen bonding was observed in that crystal structure. Complex 12 is considered to be formed via the cleavage of the CN triple bond of benzoylacetonitrile on the metal. A reaction mechanism is proposed, which includes the double protonation of the nitrile carbon atom of the ligating benzoylacetonitrile on a low-valent molybdenum center.     

Synthesis, structure and reactions of a series of 1,2-dicyanoethylenedithiolate coordinated dimeric Mo(V) complexes

(where mnt = 1,2-dicyanoethylenedithiolate) (1), reacts with HX (X = SPh, Cl, Br) to form a series of complexes, . In acidic-alcoholic medium 1 with thiophenol yields another series of compounds, . Under similar conditions tertiary-butanol does not coordinate where a complex can only be isolated in the presence of bromide as . The use of excess of methanesulfonic acid in the presence of HSPh or HSEt facilitates methanesulfonate coordination in complexes, . All these complexes are structurally characterized by single crystal X-ray study. These complexes show pH dependent hydrolytic reaction leading to quantitative reversal to the starting complex, 1. Complexes 2a-c respond to hydrolysis in CH₂Cl₂ with the intermediate formation of EPR active molybdenum(V) species.