Axial Ligand Substitution Research Articles

Kinetics of Reactions of Dirhodium and Diruthenium Paddlewheel Tetraacetate Complexes with Nucleophilic Protein Sites: Computational Insights.

Recently, dirhodium and diruthenium paddlewheel complexes have drawn attention as perspective anticancer drugs. In this study, the kinetics of reaction of typical paddlewheel scaffolds Rh2(μ-O2CCH3)4(H2O)2, Ru2(μ-O2CCH3)4(H2O)Cl, and [Ru2(μ-O2CCH3)4(HO)Cl]- with protein nucleophiles were investigated by means of the density functional theory. The substitution of axial ligands─water and chloride─by the models of protein residue side chains was analyzed, revealing the binding selectivity displayed by these paddlewheel metal scaffolds. The substitution of water is under a thermodynamic control, in which, although the Arg, Cys-, and Sec- residues are the most favorable, their binding is expected to be scarcely selective in a biological context. On the other hand, the replacement of the axial water with a more stable hydroxo ligand induces the chloride substitution in diRu complexes, which also targets Arg, Cys-, and Sec-, although with a moderately higher activation barrier for any examined protein residue. Additionally, the carried out characterization of the geometrical parameters of the transition states permitted determination of the impact of an increased steric hindrance of diRh and diRu complexes on their protein site selectivity. This study corroborates the idea of the substitution of the acetate ligands with biologically active, but more hindering, carboxylate ligands, in order to yield dual acting metallodrugs. This study allows us to assume that the delivery of diRu paddlewheel complexes in their monoanionic form [Ru2(μ-O2CR)4(OH)Cl]- decorated by the bulky substituents R may constitute an approach to augment the selectivity toward anticancer targets, such as TrxR in tumor cells, although under the condition that such a selectivity is operative only in high pH conditions.

Reaction of dirhodium and diruthenium paddlewheel tetraacetate complexes with nucleophilic protein sites: A computational study

The biomolecular affinity of three paddlewheel dinuclear complexes Rh2(µ-O2CCH3)4(H2O)2, Ru2(µ-O2CCH3)4(H2O)Cl, [Ru2(µ-O2CCH3)4(H2O)2]+ for the binding at cancer-specific protein targets was determined by means of DFT approaches. Thermodynamics of axial ligand substitution reaction by models of suitable protein sites was investigated in order to assess the binding selectivity displayed by the paddlewheel complexes. Both Rh2(µ-O2CCH3)4(H2O)2 and Ru2(µ-O2CCH3)4(H2O)Cl resulted to react favorably with most of the examined models of the protein side chains, with higher exergodicity demonstrated in the reaction of Arg, Cys, His, Lys, Sec, thus showing a limited selectivity. On the other hand, we identified Sec and His as the most suitable targets of the examined paddlewheel complexes, while Arg and Lys are expected to be also targetable at higher pH. Therefore, our calculations showed how both dirhodium and, majorly, diruthenium examined complexes undergo to a certain degree of destabilization upon the binding at nucleophilic protein sites. The presented computational results allowed us to shed light onto the protein sites’ affinity and on the therapeutic potential of both Rh2(µ-O2CCH3)4(H2O)2 and Ru2(µ-O2CCH3)4(H2O)Cl, and provided for a structural basis for the development of newly designed paddlewheel complexes.

How Does Replacement of the Axial Histidine Ligand in Cytochrome c Peroxidase by Nδ-Methyl Histidine Affect Its Properties and Functions? A Computational Study.

Heme peroxidases have important functions in nature related to the detoxification of H2O2. They generally undergo a catalytic cycle where, in the first stage, the iron(III)–heme–H2O2 complex is converted into an iron(IV)–oxo–heme cation radical species called Compound I. Cytochrome c peroxidase Compound I has a unique electronic configuration among heme enzymes where a metal-based biradical is coupled to a protein radical on a nearby Trp residue. Recent work using the engineered Nδ-methyl histidine-ligated cytochrome c peroxidase highlighted changes in spectroscopic and catalytic properties upon axial ligand substitution. To understand the axial ligand effect on structure and reactivity of peroxidases and their axially Nδ-methyl histidine engineered forms, we did a computational study. We created active site cluster models of various sizes as mimics of horseradish peroxidase and cytochrome c peroxidase Compound I. Subsequently, we performed density functional theory studies on the structure and reactivity of these complexes with a model substrate (styrene). Thus, the work shows that the Nδ-methyl histidine group has little effect on the electronic configuration and structure of Compound I and little changes in bond lengths and the same orbital occupation is obtained. However, the Nδ-methyl histidine modification impacts electron transfer processes due to a change in the reduction potential and thereby influences reactivity patterns for oxygen atom transfer. As such, the substitution of the axial histidine by Nδ-methyl histidine in peroxidases slows down oxygen atom transfer to substrates and makes Compound I a weaker oxidant. These studies are in line with experimental work on Nδ-methyl histidine-ligated cytochrome c peroxidases and highlight how the hydrogen bonding network in the second coordination sphere has a major impact on the function and properties of the enzyme.

Communication—Semi-Rigid Ru(II)Porphyrinate-Based Macrocyclic Receptor as Endotopic Catalyst for Carbene Transfer Reactions

A 5,15-bis(1,1′-biphenyl)porphyrin-based macrocyclic receptor with a well-defined cavity is suitable for coordination of Ru(II) ions with carbonyl axial ligands. Axial ligand substitution reaction using diphenyldiazomethane as reactant affords a macrocyclic Ru(II)porphyrinate with a diphenylcarbene moiety that functions as an excellent endotopic catalysts for the dimerization reaction of ethyldiazoacetate. The extraordinary stability of the diphenylcarbene axial ligand in conjunction with the high reactivity of the Ru(II)porphyrinate moiety towards diazoderivatives render the macrocyclic complex a promising candidate for the active metal template synthesis of interlocked molecules.

Ligand-induced symmetry breaking and concomitant blueshift in the emission wavelength of an octahedral chromium complex.

The resulting distortion of the octahedral symmetry of the complex [CrIII(NH3)6]3+ upon replacing the axial ligands with halides (i.e., weaker ligands) affects the stability of the doublet state with respect to that of the quartet ground state. This substitution affects the doublet-to-quartet transition responsible for phosphorescence. The position of the halide with respect to ammonia in the spectrochemical series is a major influence on the emission wavelength of the complex. The close proximity of fluorine and ammonia in the spectrochemical series leads to a blueshift in the emission wavelength when fluoride ions are introduced into the complex, thus providing a rational approach to the design of blue-phosphorescent materials, which are desirable for OLEDs used in full-color displays. Graphical abstract Shifts in the phosphorescence emission wavelength of an octahedral Cr(III) complex caused by axial ligand substitution. Replacing the axial ligands leads to a change in the relative positions of the axial and equatorial ligands in the spectrochemical series, which in turn induces a redshift or a blueshift in the emission wavelength.

Facile Axial Ligand Substitution in Linear Mo≣Mo-Ni Complexes.

Clean axial ligand substitution reactions of heterometallic extended metal atom chains (HEMACs) supported by the dpa ligand (dpa = 2,2'-dipyridylamine) have been synthetically challenging due to side reactions that alter the trimetallic core. Following the hypothesis that a heterometallic core containing second-row transition metals would be more robust toward ligand substitution, we report the synthesis of three new heterotrimetallic compounds, Mo2Ni(dpa)4(OTf)2 (1), Mo2Ni(dpa)4(NCS)2 (2), and Mo2Ni(dpa)4(NCSe)2 (3) that are obtained cleanly and in good yield. Compound 1 may be synthesized either directly by reaction of Ni(OTf)2 with Mo2(dpa)4 (4) or indirectly, by reaction of Mo2Ni(dpa)4Cl2 (5) with 2 equiv of TlOTf. Axial ligand substitution on 1 via solutions containing NaNCS or KNCSe afford 2 or 3, respectively. X-ray crystal structures of 1, 2, and 3 present short Mo-Ni distances of 2.458(8)Å /2.47(1) Å, 2.548(1), and 2.546(1), respectively. Density functional theory (DFT) calculations indicate a 3-center 3-electron σ bonding interaction between the Mo2 quadruply bonded core and the Ni in both 1 and 2. These complexes were analyzed by SQUID magnetometry, supporting the presence of a high spin Ni2+ center with S = 1.

Axial ligand substitution reactions of head-to-head α-pyridonato-bridged Pt(III) binuclear complexes bearing various equatorial ligands with chloride ion and olefin

In this study, axial ligand-substitution reactions of head-to-head (HH) α-pyridonato-bridged platinum(III) binuclear complexes bearing equatorial amine ligands (L = MeNH2, EtNH2, and L–L = ethylenediamine (en), and N,N′-dimethylethylenediamine (Me2en)) [(Cl)(NH3)2Pt(μ-α-pyridonato)2Pt(L)2(Cl)]2+ are synthesized, and the axial ligand substitution on these complexes with chloride ions are kinetically investigated in acidic aqueous solution. For these complexes, the formation reactions of the dichloro complexes from the monochloro complexes proceeded through three parallel reaction pathways, in contrast to those for the HH tetraammine amidato-bridged platinum (III) binuclear complexes. The effects of equatorial amine ligands as well as the bridging ligands on the axial ligand substitution are discussed. The reaction of [(H2O)(NH3)2Pt(μ-α-pyridonato)2Pt(en)(OH2)]2+ with p-styrenesulfonate is also kinetically investigated.

Axial-ligand substitution reactions of a head-to-head pivalamidato-bridged Pt(III) binuclear complex bearing equatorial bromide ligands: A mechanistic study

Axial ligand-substitution reactions of head-to-head (HH) a binuclear pivalamidato-bridged platinum(III) complex bearing equatorial bromide ligands, [(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(Br)2(OH2)]2+ (1), with chloride and bromide ions were thermodynamically and kinetically investigated in acidic aqueous solutions. Reactions of 1 with p-styrenesulfonate and 4-penten-1-ol were also kinetically investigated in order to clarify the reaction mechanisms involving olefins. In contrast to the reactions of the HH tetraammine pivalamidato-bridged platinum(III) binuclear complex, [(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(NH3)2(OH2)]4+ (2′), all substitution processes on 1 involve single-step reactions. The effect of the equatorial halide ligands on the axial ligand substitution reactions of 1 is discussed in relation to 2′; the rate-determining first substitution of H2O by X− at the Pt(Br2O2) site in 1 is followed by a fast second substitution of H2O by X− at Pt(N4). In contrast, axial ligand substitution on 2′ by X− proceeds in two consecutive steps. Reactions of 1 with p-styrenesulfonate or 4-penten-1-ol proceed in one step to form only mono-π complexes that are in rapid equilibrium with σ complexes, whereas reactions of 2′ with these olefins proceed in three consecutive steps via di-π complexes. These mechanistic differences are interpreted in terms of enhanced charge localization in the mono-π complexes of 1 ([PtII(NH3)2(μ-pivalamidato)2PtIV(Br)2(p-styrenesulfonate or 4-penten-1-ol)]2+).

Synthesis, Characterization And Electrolytic Behavior Of Cadmium(II) Complexes Of 5,7,7,12,14,14- Hexamethyl-1,4,8,11-Tetraazacyclotetradeca-4,11- Diene And Isomers Of Its Saturated Analogue

Condensation of ethylendiamine with acetone in the presence of quantitative amount of perchloric acid, yielded the ligand 5,7,7,12,14,14- hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene dihydroperchlorate (denoted by L.2HClO4). The ligand L.2HClO4 on reduction with NaBH4, yielded an isomeric mixture of saturated macrocycles, the Me6[14]anes, which were resolved into two distinct C-chiral isomers (denoted by ‘tet-a’ and ‘tet-b’). Interaction of ligands L.2HClO4, ‘tet-a’ and ‘tet-b’ with salts CdI2, Cd(NO3)2.4H2O, CdCl2.2H2O and Cd(ClO4)2.6H2O produced different five coordinated square pyramidal and six coordinated octahedral cdmium(II) complexes. Among them the complexes, cis-[Cd(teta)( NO3)](NO3) and cis-[Cd(tet-b)(NO3)](NO3) underwent axial ligand substitution reaction with KCNS; whereas complex [Cd(tet-a)I2] underwent axial ligand substitution reaction and complex [CdLI](ClO4) underwent simultaneous ligand substitution and addition reaction with NaNO2. Characterization of all the complexes were carried out on the basis of elemental analysis; FT-IR, UV-Vis. and 1H-NMR spectroscopy; melting point determination as well as by magnetic moment and conductivity measurements. Molar conductivity measurment of the complexes reavealed that they show different electrolytic behavior in different solvents.

Kinetics of the chemical oxidation of (5,10,15,20-tetraphenyl-21H,23H-porphinato)(chloro)(aqua)iridium(III)

The oxidation of (Cl)(H2O)IrTPP with atmospheric oxygen in the presence of concentrated H2SO4 accompanied by coordination of molecular O2 and substitution of axial ligands was studied spectrophotometrically. In 16.785–18.09 М H2SO4 at 298–318 K, (Cl)(H2O)IrTPP experienced two single-electron oxidations in sequence: with an increase in the oxidation state of the iridium cation and with formation of the π-radical cation form (HSO4)IrIVTPP•+ oxidized at the aromatic ligand (k 298 = 7.2 × 10–6 mol–1 L s–1). Referring to the literature data on the oxidation of (Cl)(H2O)IrTPP in AcOH and CF3COOH, it was shown that the medium acidity and the nature of the axial ligands affect the electron removal site in the chemical oxidation of (Cl)(H2O)IrTPP with atmospheric oxygen in proton-donor solvents.

Axial ligand effect on the reaction mechanism of head-to-head pivalamidato-bridged Pt(III) binuclear complex containing an equatorial bromide ligand with acetone

The axial ligand substitution reactions of head-to-head (HH) pivalamidato-bridged platinum(III) binuclear complexes containing equatorial Br− ligands, ([(H2O)(NH3)2Pt(μ-pivalamidato)2Pt(Br)2(OH2)]2+ (complex 1) and [(Br)(NH3)2Pt(μ-pivalamidato)2Pt(Br)2(Br)] (complex 2)) with acetone were kinetically investigated in acidic aqueous solutions. Both the complexes reacted with acetone slowly and acetonyl PtIII binuclear complexes were formed in two steps (1 and 2). The observed pseudo first-order rate constants (kobs1) for step 1 of the reactions of complexes 1 and 2 were linearly and quadratically dependent on the excess ligand concentration (CL) at a given [H+], respectively. The rate constants for step 2 in both the reactions were independent of both CL and [H+]. A reaction mechanism involving an intermediate with one π-coordinated acetone molecule was proposed for complex 1, which is similar to those proposed previously for the reactions of HH tetraammine amidato-bridged diplatinum(III) complexes with acetone. In contrast, a reaction mechanism involving an intermediate with two π-coordinated acetone molecules on the axial sites was proposed for complex 2.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

Synthesis and spectroscopic characterization of super-stable rhenium(V)porphyrins

The preparation of rhenium(V) porphyrin complexes {μ-oxo-bis[(oxo)(5,10,15,20-tetraphenyl-21H,23H-porphinato)rhenium(V)] [OReTPP]2O (1), (oxo)(phenoxo)(2,3,7,8,12,13,17,18-octaethyl-5-monophenyl-21H,23H-porphinato)rhenium(V) ORe(PhO)MPOEP (2), (cloro)(oxo)(2,3,7,8,12,13,17,18-octaethyl-5,15-diphenyl-21H,23H-porphinato)rhenium(V) ORe(Cl)5,15DPOEP (4), and (oxo)(phenoxo)(2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphinato)rhenium(V) ORe(PhO)OEP (5)} by the interaction of H2ReCl6 with corresponding porphyrin in boiling phenol is described. (Cloro)(oxo)(2,3,7,8,12,13,17,18-octaethyl-5-monophenyl-21H,23H-porphinato)rhenium(V) ORe(Cl)MPOEP (3) and (oxo)(chloro)(2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphinato)rhenium(V) ORe(Cl)OEP (6) have been prepared by the reaction of axial-ligand substitution from (2) and (5), respectively. Compounds (2–4) were newly synthesized. Characterization of the compounds (1–6) reported herein was made mainly by UV–Visible, IR, 1Н NMR, 1H1H 2D COSY, 1H1H 2D DOSY, 1H1H 2D ROESY, 1H1H 2D TOCSY spectroscopic techniques and elemental analysis. The stability of the complexes in solutions when exposed to strong acids at the presence of atmospheric oxygen has been estimated. Compounds (2–4) and (6) show them super-stable since they do not undergo dissociation along MN bonds in concentrated H2SO4 under heating up to 363K. Compounds (3) and (4) undergo one-electron oxidation to form stable π-cation radicals ORe(HSO4)P+ under these conditions. The products of the reaction between all studied porphyrins and concentrated H2SO4 were isolated in CHCl3 by reprecipitation onto ice and proved to be rhenium(V) complexes ORe(HSO4)P.

Axial Ligand Exchange of N-heterocyclic Cobalt(III) Schiff Base Complexes: Molecular Structure and NMR Solution Dynamics

The kinetic and thermodynamic ligand exchange dynamics are important considerations in the rational design of metal-based therapeutics and therefore, require detailed investigation. Co(III) Schiff base complex derivatives of bis(acetylacetone)ethylenediimine [acacen] have been found to be potent enzyme and transcription factor inhibitors. These complexes undergo solution exchange of labile axial ligands. Upon dissociation, Co(III) irreversibly interacts with specific histidine residues of a protein, and consequently alters structure and causes inhibition. To guide the rational design of next generation agents, understanding the mechanism and dynamics of the ligand exchange process is essential. To investigate the lability, pH stability, and axial ligand exchange of these complexes in the absence of proteins, the pD- and temperature-dependent axial ligand substitution dynamics of a series of N-heterocyclic [Co(acacen)(X)(2)](+) complexes [where X = 2-methylimidazole (2MeIm), 4-methylimidazole (4MeIm), ammine (NH(3)), N-methylimidazole (NMeIm), and pyridine (Py)] were characterized by NMR spectroscopy. The pD stability was shown to be closely related to the nature of the axial ligand with the following trend toward aquation: 2MeIm > NH(3) ≫ 4MeIm > Py > Im > NMeIm. Reaction of each [Co(III)(acacen)(X)(2)](+) derivative with 4MeIm showed formation of a mixed ligand Co(III) intermediate via a dissociative ligand exchange mechanism. The stability of the mixed ligand adduct was directly correlated to the pD-dependent stability of the starting Co(III) Schiff base with respect to [Co(acacen)(4MeIm)(2)](+). Crystal structure analysis of the [Co(acacen)(X)(2)](+) derivatives confirmed the trends in stability observed by NMR spectroscopy. Bond distances between the Co(III) and the axial nitrogen atoms were longest in the 2MeIm derivative as a result of distortion in the planar tetradentate ligand, and this was directly correlated to axial ligand lability and propensity toward exchange.

Cobalt (III) Complexes as Novel Matrix Metalloproteinase-9 Inhibitors

Matrix metalloproteinases (MMPs) are a family of zinc binding endopeptidases that play crucial roles in various physiological processes and diseases such as embryogenic growth, angiogenesis, arthritis, skin ulceration, liver fibrosis and tumor metastasis. Because of their implications in a wide range of diseases, MMPs are considered as intriguing drug targets. The majority of MMP inhibitors are organic small molecules containing a hydroxamate functionality for the zinc binding group. This hydroxamate group binds to a zinc(II) center in a bidentate fashion and creates a distorted trigonal bipyramidal geometry. Although the hydroxamate group is the most effective zinc binding moiety reported, it has shown two major limitations in clinical trials: low bioavailability and lack of specificity. Because the hydroxamate group is prone to rapid metabolism and has a high binding affinity towards various transition metals, many efforts have been made to develop novel zinc binding groups, including reverse hydroxamate, phosphate, and pyridinone. However, the in vitro activity of these newly developed inhibitors are not as high as the hydroxamate derivatives. It was previously reported that cobalt(III) acacen complexes could interact with histidine residues of proteins and model peptides. The cobalt(III) complexes bind histidine residues in active sites and on enzyme surfaces in a random fashion. Spectroscopic and chromatographic data suggested that the complexes bind to a histidine residue by axial ligand substitution. When an appropriate targeting group is attached, the cobalt(III) complexes can selectively inhibit the histidinecontaining enzymes such as thermolysin, human α-thrombin, and carbonic anhydrase. Since histidine is the most commonly found residue in the active sites of zinc enzymes and zinc-binding proteins, it has been demonstrated that the cobalt(III) complexes can interact with zinc finger proteins, such as HIV-1 nucleocapsid protein NCp7 and human zinc finger transcription factor Sp1. It was also reported that a Co(III) complex with a oligonucleotide targeting Snail family zinc finger transcription factor resulted in a selective inactivation of a transcriptional activity implicated in embryonic development and breast cancer. In this work, a series of the cobalt(III) complexes are prepared as novel MMP inhibitors. Since cobalt(III) complexes have high affinity towards histidine residues and zinc binding proteins, they can be engineered to disrupt the zinc binding active site of MMP. Furthermore, the cobalt(III) complexes may be able to provide better solubility and higher binding affinity than the known organic inhibitors. To test this hypothesis, a series of cobalt(III) complexes were designed and synthesized. For active site directing groups, biphenyl sulfonate and biphenyl amide group were chosen based on the structures of well known MMP inhibitors. In vitro activity of these complexes was evaluated for MMP-9 (gelatinase B), because MMP-9 is one of the most highly expressed MMPs in tumors and has been implicated in tumor aggressiveness. The syntheses of cobalt(III) complexes are shown in Schemes 1-2. The synthesis of 1 and 2 begins with the methylation of commercially available Cbz-Lys(Boc)-OH as

Thermodynamics of Axial Substitution and Kinetics of Reactions with Amino Acids for the Paddlewheel Complex Tetrakis(acetato)chloridodiruthenium(II,III)

The known paddlewheel, tetrakis(acetato)chloridodiruthenium(II,III), offers a versatile synthetic route to a novel class of antitumor diruthenium(II,III) metallo drugs, where the equatorial ligands are nonsteroidal anti-inflammatory carboxylates. This complex was studied here as a soluble starting prototype model for antitumor analogues to elucidate the reactivity of the [Ru(2)(CH(3)COO)(4)](+) framework. Thermodynamic studies on equilibration reactions for axial substitution of water by chloride and kinetic studies on reactions of the diaqua complexes with the amino acids glycine, cysteine, histidine, and tryptophan were performed. The standard thermodynamic reaction parameters ΔH°, ΔS°, and ΔV° were determined and showed that both of the sequential axial substitution reactions are enthalpy driven. Kinetic rate laws and rate constants were determined for the axial substitution reactions of coordinated water by the amino acids that gave the corresponding aqua(amino acid)-Ru(2) substituted species. The results revealed that the [Ru(2)(CH(3)COO)(4)](+) paddlewheel framework remained stable during the axial ligand substitution reactions and was also mostly preserved in the presence of the amino acids.

Effect of the axial halogen ligand on the substitution reactions of chromium(III) porphyrin complex

The thermodynamics and kinetics of the substitution reaction of the chromium(III) complex of 5,10,15,20-tetraphenylporphyrin, [Cr(TPP)(X)(L)] (X=F, Cl, Br), where L represents a non-charged ligand such as H2O, pyridine, or 1-methylimidazole, was investigated. The present study aimed to elucidate the effect of the axial halogen ligand, X on the substitution reaction of the ligand trans to X. The substitution reaction of the axial pyridine ligand of [Cr(TPP)(X)(Py)] by 1-methylimidazole was studied spectrophotometrically in dichloromethane, and it was found that the reaction proceeds via a limiting dissociative mechanism and the activation enthalpy of the dissociation of pyridine is much smaller for the fluorine complex, [Cr(TPP)(F)(Py)], than for the chlorine complex, [Cr(TPP)(Cl)(Py)], i.e., ΔH‡=57.5±1.1 and 97.2±1.4kJmol−1, respectively. These results are discussed in terms of the trans effect of the halogen ligand on the axial ligand substitution reaction of [Cr(TPP)(X)(L)], and it is concluded that the strength of the bond shows the tendency of Cr–F>Cr–Cl>Cr–Br, leading to the strong trans effect of the F− ligand.

Remote Effects of Axial Ligand Substitution in Heterometallic Cr≡Cr···M Chains

The heterometallic complexes CrCrM(dpa)(4)Cl(2) (dpa = 2,2'-dipyridylamide) featuring linear Cl-Cr≡Cr···M-Cl chains can regiospecifically be modified via axial ligand substitution to yield OTf-Cr≡Cr···M-Cl chains (OTf = triflate) with M being Fe, Mn, or Co. The effect of OTf substitution on the Cr side of the molecule has an unusual and profound structural impact on the square-pyramidal transition metal M. Specifically, elongation of the four equatorial M-N(py) bonds and the axial M-Cl bonds by 0.03 and 0.09 Å for Fe and 0.07 and 0.11 Å for Mn is observed. The longer M-Cl and M-N(py) bonds result from subtle interactions between the equatorial dpa ligand and the three metal ions. The equatorial dpa ligand responds to the introduction of the more labile OTf ligand at Cr by binding more strongly to this Cr ion which in turn weakens bonding to M. The ligand field experienced by M can be tuned by changing the Cr axial ligand, and this effect is observed in electrochemical measurements of the iron compounds.

ChemInform Abstract: Synthesis of Axial Dialkoxy Phosphorus(V)porphyrin Derivatives - Novel Hypervalent Phosphorus Compounds.

Water-stable, axial dialkoxy hypervalent phosphorus(V)tetraphenylporphyrins have been synthesised in a novel fashion by efficient substitution of axial chloride ligands of dichlorophosphorus(V)-tetraphenylporphyrin.

ChemInform Abstract: Synthesis, Spectroscopic and Electrochemical Properties, and Electronic Structures of Octahedral Hexatechnetium(III) Clusters [Tc6Q8(CN)6]4‐ (Q: S, Se).

AbstractThe discrete molecular sulfide‐ and selenide‐capped hexatechnetium(III) title complexes with axial cyano ligands are prepared from an octahedral hexatechnetium complex by depolymerization and axial ligand substitution and characterized by IR spectroscopy, UV/VIS spectroscopy, cyclic voltammetry, and DFT calculations.