Labile Ligands Research Articles

Cyclopalladated hydrazones complexed to pyridinyl ligands

2-Benzoylpyridine and 2-acetylpyridine N-methyl- N-phenylhydrazones ( HL 1 and HL 2) in cyclopalladation reactions with Pd 3(OAc) 6 behave as N(py)N(im)C tridentate ligands and form the neutral mononuclear complexes [Pd( k 3-N,N,C–L 1,2)( k 1-OAc)] ( 1 and 2). The labile OAc ligand can be replaced by pyridine (py) and, in the presence of NH 4PF 6, a compound of the general formula [Pd( k 3-N,N,C-L 1,2)(py)]PF 6 ( 3 and 4) can be obtained. Reacting 1 or 2 with the poly(4-vinylpyridine) polymer (PVP, M ¯ w = 160 000 ) adducts of the general formula x[Pd( k 3-N,N,C-L 1,2)(PF 6)] · y[PVP] gives ( 5a– c and 6a– c, a– c: x/ y = 80/20, 50/50, 25/75 w/w) containing the laterally appended cyclopalladated “Pd( k 3-N,N,C-L 1,2)” fragments. These metallorganic polymers show high glass transition temperatures, high thermal stability and good solubility, and are optimal for obtaining transparent thin films by the spin coating method.

Well‐Defined Cationic Alkyl– and Alkoxide–Aluminum Complexes and Their Reactivity with ε‐Caprolactone and Lactides

We describe the synthesis, structure, and reactivity of low-coordinate Al-alkyl and -alkoxide cationic complexes incorporating the sterically bulky aminophenolate bidentate ligand 6-(CH(2)NMe(2))-2-CPh(3)-4-Me-C(6)H(2)O- (N,O). These complexes are derived from the ionization of neutral dialkyl Al complexes (N,O)Al2) (1 a, R=Me; 1 b, R=iBu), readily obtained by alkane elimination between AlR3 and the corresponding aminophenol ligand, with the alkyl abstracting reagents B(C(6)F(5))3 and [Ph(3)C][B(C(6)F(5))4]. The reactions of 1 a,b with B(C(6)F(5))3 yield complicated mixtures or decomposition products, however the ionization of the Al-diisobutyl derivative 1 b with [Ph(3)C][B(C(6)F(5))4] affords a stable four-coordinate Al-PhBr cationic adduct [(N,O)Al(iBu)(PhBr)]+ (3+), as deduced from elemental analysis data. Complex 3+ readily coordinates Lewis bases such as THF to form the corresponding adduct [(N,O)Al(iBu)(thf)]+ (4+), and also rapidly chain-transfers with 1-hexene to yield the three-coordinate Al-hexyl cation [(N,O)Al-hexyl]+ (5+). Both cations 3+ and 5+ slowly dimerize to form unprecedented organoaluminum dications [(N,O)AlR+]2 (3'++, R=iBu; 5'++, R=hexyl) as deduced from X-ray crystallographic analysis. Cation 3+ reacts quickly with iPrOH to form a stable Lewis acid/base adduct [(N,O)Al(iBu)(HOiPr)]+ (6+), which constitutes the first X-ray characterized adduct between an Al-alkyl complex and a simple ROH. The Al-ROH proton in 6+ is readily abstracted by NMe(2)Ph to form the neutral isopropoxide Al complex [(N,O)Al(iBu)(OiPr)] (7). Upon reaction with THF, cation 6+ undergoes an intramolecular proton transfer to yield the ammonium Al-THF complex [(eta1-HN,O)Al(iBu)(OiPr)(thf)] (8 b+), in which the aminophenolate is eta1-coordinated to the Al center. Cation 8 b+ can then be converted to the desired Al-alkoxide derivative [(N,O)Al(OiPr)(thf)](+) (10+), by an intramolecular protonolysis reaction, as confirmed by X-ray crystallography. The synthesized Al-alkyl cations form robust four-coordinate adducts in the presence of cyclic esters such as epsilon-caprolactone and (D,L)-lactide, but no insertion chemistry occurs, illustrating the poor ability of the Al-R+ moiety to ring-open. In contrast, the Al-alkoxide cation 10+ polymerizes epsilon-caprolactone in a controlled manner with excellent activity, but is inactive in the polymerization of (D,L)-lactide and L-lactide. Control experiments with L-lactide show that cation 10+ ring-opens L-lactide to yield a robust five-coordinated Al--lactate cation [(N,O)Al(eta2-L-lactate-OiPr)(thf)]+ (13+), derived from a monoinsertion of L-lactide into the Al--OiPr bond of 10+, that does not further react. Cation 13+ may be regarded as a structurally characterized close mimic of the initial intermediate in the ring opening polymerization (ROP), of lactides by [{LX}M(OR)(L)] (where LX-=bidentate monoanionic ligand and L=labile ligand) metal complex initiators.

Characteristics of Mononuclear Ruthenium–Oxo Complexes Adjusted by Axial Ligand for the Catalysis of Oxygen-Transfer Reactions

Ruthenium(II) complexes, [Ru(babp)(dmso)L], consisting of tetradentate square-planar ligand (babp = 6,6′-bis(benzoylamino)-2,2′-bipyridinato) and two monodentate axial ones, DMSO and L (L = dmso or...

Theoretical Investigation of the Stepwise Hydrolysis of the [Re3(μ-Cl)3Cl9]3- Anion

A detailed study of the stepwise substitution of the chloride ligands in the [Re3(mu-Cl)3Cl9](3-) (1) anion by water molecules is presented using theoretical methods. Ligand lability as well as the structure and relative stability of the various mono-[Re3(mu-Cl)3Cl8(H2O)](2-) (2a,b) and dihydro-[Re3(mu-Cl)3Cl7(H2O)2](-) (3a-f) conformers is examined. Clear preferences for the positions of the incoming water ligands are proposed based on calculated energy and vibrational data, which fully agree with the experimental results.

Alternating Spin Chains: Controlled Assembly from Bimetallic Building Blocks and QMC Simulation of Spin Correlation

By using the compartmental dinucleating pyrazolate ligand HL, dinickel(II) complexes [LNi2(micro-N3)(acetone)2]X2 (1: X = CIO4; 2: X = BPh4) and tetranickel(II) complex [{LNi2(micro-N3)(MeOH)2](CI04)4 (3) have been prepared and structurally characterized. Complexes 1 and 2 differ in the torsion along the bridging micro-1,3-azide moiety, while the azido ligands in 3 adopt an unusual micro-1,1,3 bridging mode to connect the two subunits. All three complexes show overall antiferromagnetic coupling and an S = 0 ground state, but the torsion along the azide moiety is a determining factor for the coupling strength. Compounds 1 and 2 serve as preorganized building blocks for the controlled synthesis of alternating 1D polymeric structures 4-6 by replacement of their labile acetone ligands by additional azido ligands. Due to the modular synthetic approach, 4-6 can be described as Heisenberg antiferromagnetic systems with inherent bond alternation (HABA), whereby the organic ligand framework ensures that the individual nickel/azido chains are well isolated in the crystal lattice. Like their precursors, 4-6 are mainly distinguished by torsion along the micro-1,3-azido bridges, both within and between the bimetallic constituents. Magnetic measurements confirm an overall 5 = 0 ground state for 4-6, and coupling parameters have been deduced from quantum Monte Carlo simulations. The two J values for the alternating 1D chains can be clearly assigned on the basis of the magnetostructural correlations established for the bimetallic building blocks. The alternation ratio gamma = J2J1(-1) places the three new systems in the HABA regime for a singlet-dimer ground state.

Catalyst stability as a factor in iridium‐mediated ortho‐deuteration

AbstractCrossover experiments show that iridium complexes such as Ir(cod)(Py)(PCy3).PF6 (1) and Ir(cod)(PPh3)2.BF4 are inactivated after a comparatively short period during deuterium exchange reactions. This effect can be limited to some extent by optimizing the concentration at which exchange is performed, but altering physical parameters and adding labile ligands in an effort to stabilize reactive intermediates does not improve matters. Complexes of bidentate arsines and PN ligands and, in some cases, bidentate phosphines, exhibit better stability. Unexpectedly, following hydrogenolysis, 1 is able to mediate deuterium exchange from deuterium oxide into amides and N‐heterocycles at room temperature. Copyright © 2007 John Wiley & Sons, Ltd.

Reaction Pathways for fac-Os(CO)3(MeCN)Me2 with Mono- and Bidentate Ligands

The reactions of fac-Os(CO)3(L)Me2 (1a, L = MeCN; 1b, L = THF) with monodentate and bidentate ligands have been studied for the first time. Treatment of 1a or 1b with PPh3 gives fac-Os(CO)3(PPh3)Me2 (2), which may be further reacted with Me3NO in acetonitrile to give the unexpected isomer cis-Os(CO)2(PPh3)(MeCN)Me2 (3). Subsequent reaction of 3 with PPh3 gives cis,trans-Os(CO)2(PPh3)2Me2 (4), with a further isomerization accompanying the ligand substitution. Oxidative decarbonylation of 4 with Me3NO was found not to be possible. Treatment of 1a or 1b with bidentate ligands gives different reaction products, depending on the choice of ligand, the reaction conditions, and the stoichiometry. Reactions with small-bite-angle bidentate ligands (L-L) gave the compounds cis-Os(CO)2(L-L)(COMe)Me (5a, L-L = dppm; 5b, L-L = dppe; 5c, L-L = 2,2‘-bipy). These mixed methyl acetyl complexes are formed by displacement of the labile ligand and subsequent chelation, accompanied by methyl migration onto carbonyl. Reaction of 1a with 1 equiv of dppp, however, resulted in a series of reactions which led to the formation of the decarbonylated product cis-Os(CO)2(dppp)Me2 (7). However, reaction of 1a with 0.5 equiv of dppp gave the bimetallic dppp-bridged complex [fac-Os(CO)3Me2]2(μ-dppp) (8). In addition to characterization of all the new compounds by analytical and spectroscopic techniques, the molecular structures of 5a and 8 have been determined by X-ray diffraction.

Bis-coordination of N-(Alkyl)-N‘-(2,6-diisopropylphenyl) Heterocyclic Carbenes to Grubbs Catalysts

N-(Alkyl)-N‘-(2,6-diisopropylphenyl) carbenes display an exceptional tendency toward bis(NHC) coordination in their reaction with the Grubbs complex [RuCl2(CHPh)(PCy3)2]. The resulting bis(NHC) complexes show substantial olefin metathesis activity at elevated temperature. One NHC ligand is expected to dissociate from the metal center for the catalyst to be activated. This NHC ligand lability is confirmed by the observation that both NHCs are exchangeable when the complexes are treated with an excess of PCy3. In addition, the isolation of a new mono(NHC) complex is described, as well as its reactivity in the ring-opening metathesis polymerization (ROMP) of cycloocta-1,5-diene and the ring-closing metathesis (RCM) of diethyl diallylmalonate.

Measuring Free Metal Ion Concentrations in Multicomponent Solutions Using the Donnan Membrane Technique

Among speciation techniques that are able to measure free metal ion concentrations, the Donnan membrane technique (DMT) has the advantage that it can measure many different free metal ion concentrations simultaneously in a multicomponent sample. Even though the DMT has been applied to several systems, like surface waters, soil solutions, and manure slurry, basic features and calibrations with model calculations of the laboratory and field DMT have not been done sufficiently yet. Therefore, we tested the application of the DMT on metal complexation with several synthetic and natural ligands and the applicability of the dynamic mode of the DMT. The results show that there is a high agreement between the calculated and measured free metal ion concentrations in solutions containing synthetic (nitriloacetic acid, diglycolic acid) and natural organic ligands (fulvic acid, humic acid) at various pH values. Both the laboratory DMT and the field DMT give very similar results. In a solution containing labile ligands, equilibrium time is smaller than in a donor solution containing inert ligands or no ligands. Moreover, when labile ligands are present in the donor solution, a dynamic procedure can be used to decrease equilibrium time. This procedure cannot be applied when no ligands or only inert ligands are present.

Lewis-Acidity of Trimethylplatinum(IV) with Labile Oxygen-Donor Ligands

Trimethylplatinum(IV) systems with labile ligands and weakly coordinating anions were investigated with respect to their Lewis-acidity and their ability to act as Lewis-acidic catalysts. The Lewis-acidity of tetrameric trimethylplatinum(IV) triflate (triflate = trifluoromethanesulfonate, OTf), [{Me3PtOTf}4], toward carbonyl compounds was quantified by NMR titration with crotonaldehyde (Childs' method) in CD2Cl2. The data show moderate but significant Lewis-acidity of the trimethyl platinum(IV) center (27% of the acidity of BBr3) and commensurate (moderate) intrinsic catalytic activity of the trimethylplatinum(IV) unit. Crystals of an aldehyde adduct were obtained. X-ray crystallography shows a binuclear metal complex, [{Me3Pt(crotonaldehyde)(μ-OTf)}2]. Despite the relatively low Lewis-acidity, in the absence of oxygen-donor ligands, the trimethylplatinum(IV) is capable of decomposing the [B(ArF)4]- anion (ArF = 3,5-(CF3)2C6H3) to yield B(ArF)3, apparently via aryl abstraction. Salt metathesis reactions of...

The insertion reaction of acetonitrile on aryl nickel complexes stabilized by bidentate N,N′-chelating ligands

Organometallic complexes to be used as single component precursors in the catalytic dimerization/polymerization of olefins usually must contain a labile ligand that can easily be displaced by the olefin. This is the first step in the activation of the precursor. One commonly used labile ligand is a nitrile. Here we report an example of incompatibility between the nickel or palladium aryl bond and acetonitrile. Neutral [MBr(Mes)NN] complexes in which Mes=2,4,6-Me3C6H2, NN=diazabutadiene (DAD), pyridinylimine (PIM), 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) gave the expected [M(Mes)(3,5-lut)(NN)][BF4] compounds and the unexpected [Ni(Mes){NH=C(Me)(2,4,6-Me3C6H2)}(NN)][BF4] complexes in the presence of TlBF4 and 3,5-lutidine or acetonitrile. The sequence of reactions that leads to the imine ligand must include an initial insertion of the nitrile on the sigma(Ni-Mes) bond. These ionic complexes remain stable under 20 bar of ethylene.

Dinuclear PCP pincer complexes from Lewis acidic [Pd(OTf)(PCP)] and basic [Pd(4-Spy)(PCP)] (OTf = triflate; 4-Spy = 4-pyridinethiolate; PCP = –CH(CH2CH2PPh2)2)

The Lewis acidic pincer with a labile triflate ligand, viz. [Pd(OTf)(PCP)] (PCP = (-)CH(CH(2)CH(2)PPh(2))(2)) was prepared from [PdCl(PCP)] with AgOTf. It reacts readily with neutral bidentate ligands [L = 4,4'-bipyridine (4,4'-bpy) and 1,1'-bis(diphenylphosphino)ferrocene (dppf)] to give dinuclear PCP pincers [{Pd(PCP)}(2)(micro-L)][OTf](2) (L = 4,4'-bpy, 2; dppf,3). [PdCl(PCP)] also reacts with 4-mercaptopyridine in the presence of KOH to give a Lewis basic pincer with a free pyridine functional group [Pd(4-Spy)(PCP)]4. Its metalloligand character is exemplified by the isolation of an asymmetric dinuclear double-pincer complex [{Pd(PCP)}(2)(micro-4-Spy)][PF(6)] 6 bridged by an ambidentate pyridinethiolato ligand. Complexes 1, 2, 3, 4 and 6 have been characterized by single-crystal X-ray diffraction analyses.

How to Compute Labile Metal–Ligand Equilibria

The properties of labile metal�ligand equilibria can be calculated readily by using the concept of a ligand function, analogous to the proton function of acid�base behavior. Use of the ligand function allows one to have a computer perform the tedious iterations, thereby bypassing the traditional trial-and-error method of solving equilibrium problems. The method is also applicable to mixed-ligand complexes.

Mechanistic Studies on the Formation of η2-Diphosphine (η6-p-cymene)ruthenium(II) Compounds

A new range of pendent diphosphine (η6-p-cymene)ruthenium(II) complexes, [RuCl(PPh3)(η1-(P-P))(η6-p-cymene)]PF6 (P-P = dppm, cis-PPh2CHCHPPh2 (dppv), dppe, dppp, dppf), have been prepared by substitution of the labile acetonitrile ligand in [RuCl(CH3CN)(PPh3)(η6-p-cymene)]PF6. The formation of chelate complexes, [RuCl(η2-(P-P))(η6-p-cymene)]+, from these pendent phosphine complexes and from the related neutral complexes, [RuCl2(η1-(P-P))(η6-p-cymene)] (P-P = dppm, dppv), has been investigated, including determination of activation enthalpies (ΔH‡) and entropies (ΔS‡). A concerted substitution mechanism is proposed for the latter complexes, in which methanol plays an important role in the ring-closing process by formation of hydrogen bonds with the chloride ligands. This proposal is supported by volumes of activation (ΔV‡) determined by variable-pressure UV-visible spectroscopy. In contrast, a dissociative mechanism is proposed for the series of cationic pendent phosphine complexes, which generally require higher temperatures to effect ring closure. Secondary reaction pathways can be observed in some cases and are discussed in terms of differences between the phosphine complexes and supplemented by investigations using electrospray ionization mass spectrometry (ESI-MS). The X-ray structures of [RuCl(PPh3)(η1-(P-P))(η6-p-cymene)]PF6 (P-P = dppm, dppv, dppp) and [RuCl(η2-dppv)(η6-p-cymene)]PF6 are also reported.

The Influence of Phosphane Coligands on the Nuclearity of Rhodium(I) 4-Thiolatobenzoic Acid Complexes

[RhCl(PR3)3] (R = Ph, Et) reacts with the potassium salt of 4-mercaptobenzoic acid to give a mixture of the monomeric and dimeric complexes, [Rh(SC6H4COOH)(PR3)3] and [{Rh(-SC6H4COOH)(PR3)2}2], respectively. With the labile PPh3 coligand, the dimer is the major product, while for the electron-richer coligand PEt3, the equilibrium is easily shifted to the monomer by the addition of excess PEt3. Phosphane dissociation and dimerization could be prevented by using the chelating coligand PPh(C2H4PPh2)2. [{Rh(-SC6H4COOH)(PPh3)2}2] (2b), [Rh(SC6H4COOH)(PEt3)3] (3a), and [Rh(SC6H4COOH){PPh(C2H4PPh2)2}] (4) were fully characterized by nuclear magnetic resonance and infrared spectroscopy, mass spectrometry, and elemental analysis. The molecular structures of 2b and 4 were determined by X-ray structure analysis. In solution, the lability of the phosphane ligands leads to the decomposition of 2b. One of the decomposition products, namely, the mixed-valent complex [{RhIRhIII(-SC6H4COO)(-SC6H4COOH)(SC6H4COOH)(PPh3)3}2] (5), was characterized by X-ray structural analysis. The dinuclear rhodium(III) complex [{Rh(-SC6H4COO)(SC6H4COOH)(PEt3)2}2] (6) was shown to be a byproduct in the synthesis of 3a, and this demonstrates the reactivity of the rhodium(I) complexes toward oxidative addition. The structurally characterized complexes 2b, 4, 5, and 6 show hydrogen bonding of the free carboxyl groups.

An Estrogen–Platinum Terpyridine Conjugate: DNA and Protein Binding and Cellular Delivery

A platinum metal complex in which terpyridine joins estradiol (via an ethynyl link) to a platinum with a labile ligand (chloride) has been designed, synthesised and its X-ray crystal structure determined. The aim of this work was to link a targeting motif (in this case estrogen) to a metal-based biomolecule recognition unit (the platinum moiety). The target molecule: 17alpha-[4'-ethynyl-2,2':6',2'-terpyridine]-17beta-estradiol platinum(II) chloride (PtEEtpy) has been shown to bind to both human and bovine serum albumin (SA) and to DNA. FTICR mass spectrometry shows that the bimolecular units are in each case linked through coordination to the platinum with displacement of the chloride ligand. Circular dichroism indicates that a termolecular entity involving PtEEtpy, SA and DNA is formed. A range of electrospray mass spectrometry experiments showed that the PtEEtpy complex breaks and forms coordination bonds relatively easily. A whole cell estrogen receptor assay in an estrogen receptor positive cell (MCF-7) confirms binding of both EEtpy and PtEEtpy to the estrogen receptor in cells. The work demonstrates the concept of linking a targeting moiety (in this case estrogen) to a DNA binding agent.

Antibiotic Resistance: Mono‐ and Dinuclear Zinc Complexes as Metallo‐β‐Lactamase Mimics

Biomimetic systems containing one or two zinc(II) ions supported by phenolate ligands were developed as functional mimics of metallo-beta-lactamase. These complexes were shown to catalytically hydrolyze beta-lactam substrates, such as oxacillin and penicillin G. The dinuclear zinc complex 1, which has a coordinated water molecule, exhibits high beta-lactamase activity, whereas the dinuclear zinc complex 2, which has no water molecules, but labile chloride ligands, shows a much lower activity. The high beta-lactamase activity of complex 1 can be ascribed to the presence of a zinc-bound water molecule that is activated by being hydrogen bonded to acetate substituents. The kinetics of the hydrolysis of oxacillin by complex 1 and the effect of pH on the reaction rates are reported in detail. In addition, the kinetic parameters obtained for the synthetic analogues are compared with those of the natural metallo-beta-lactamase from Bacillus cereus (BcII). To understand the role of the second metal ion in hydrolysis, the syntheses and catalytic activities of two mononuclear complexes (3 and 4) that include coordinated water molecules are described. Interestingly, the mononuclear zinc complexes 3 and 4 also exhibit high activity, supporting the assumption that the second zinc ion is not crucial for the beta-lactamase activity.

Application and Modification of Cyclometalated Ruthenium(II) Complex [Ru(o‐C6H4‐2‐C5H4N)‐ (MeCN)4]PF6 for Atom Transfer Radical Polymerization

AbstractPolymerizations of n‐butyl acrylate (BA), methyl methacrylate (MMA) and styrene (St) were promoted by the Ru(II) cyclometalated complex with labile MeCN ligand in the presence of Al(OiPr)3. The polymerization proceeds via radical mechanism and requires the loss of MeCN ligand. The poor control over the polymerizations can be explained in terms of the traditional ATRP scheme. However, the controllability may be significantly improved by addition of reducing SnCl2. Mechanism of the process is proposed.

Reactivity of cyclometallated platinum complexes with chiral ligands

The reaction of [Pt 2Me 4(μ-SMe 2) 2] with 3-substituted iminic thiophenes and 2-phenylpyridine gives platinum (II) [C,N] cyclometallated complexes which contain a labile ligand (SMe 2 or CH 3CN). Several platinum (II) complexes have been synthesized by substitution reactions with phosphine or sulfoxide ligands to introduce, in most cases, a second chiral center. The new complexes’ reactions with methyl iodide were subsequently studied and showed results that are dependent on the steric and electronic effects of both the cyclometallated ligand and the ancillary phosphine or sulfoxide ligand. The structure of [PtMe(( R)-C 10H 7CHMeNCHC 4H 2S)(CH 3CN)], a synthetic precursor, is also reported.

An Evaluation of DGT Performance Using a Dynamic Numerical Model

A numerical model of the transport and dynamics of metal complexes in the resin and gel layers of a DGT (diffusive gradients in thin films) device was developed and used to investigate how the chelating resin and metal-ligand complexes in solution affect metal uptake. Decreasing the stability constant or concentration of the binding resin increases the competition for free metal ions by ligands in solution, lowering the rate of mass uptake. Such effects would be rarely observed for moderately or strongly binding resins (K> 10(12)), including Chelex, which out-compete labile ligands in solution. With weakly binding resins, strongly bound solution complexes can diffuse into the resin layer before a measurable amount of dissociation occurs, such that concentrations of bound metal at the rear and front surfaces of the resin layer are equal. With more strongly binding resins, metal mainly binds to the front surface of the resin. Only complexes with the largest binding constants penetrate the gel layer containing Chelex, buttheir lack of lability means thatthe DGT sensitivity to the complex is, in any case, very low. The slow diffusion of complexes, such as those of fulvic acids, which increases the time required to establish steady state, compromises the use of the simple DGT equation. Errors are negligible for 24 h deployments, when diffusive layer thicknesses are less than 1 mm, but 3 day deployments are required to ensure accuracy with 2.4 mm thick layers. The extent to which the commonly used equation, that accounts for the concentration and diffusion of metal-complex species, overestimates DGT uptake if the rate of dissociation is slow, was estimated.