Hexacoordinate Complexes Research Articles

Reactivity Comparison of Primary Aromatic Amines and Thiols in E–H Insertion Reactions with Diazoacetates Catalyzed by Iridium(III) Tetratolylporphyrin

A detailed kinetic study is described for the insertion of carbenes from methyl diazoacetate into the N–H bond of aniline, using Ir(TTP)CH3 (TTP = tetratolylporphyrinato) as a catalyst. Aniline strongly coordinates to the Ir center with a binding constant of K = (2.5 ± 0.5) × 104 at 296 K, forming an inactive hexacoordinate complex, (aniline)Ir(TTP)CH3. The rate of N–H insertion is first order in both diazo ester and catalyst. When the true amount of active, five-coordinate Ir(TTP)CH3 is taken into account, the insertion rate is found to be independent of the aniline concentration. This indicates that the rate-limiting step in the catalytic cycle occurs prior to the nucleophilic attack of aniline on an Ir carbene complex to generate a coordinated ylide. Thus, with N–H insertion catalyzed by Ir(TTP)CH3, aniline is both a strong ligand and a potent nucleophile. This is in contrast to the analogous catalytic insertion of carbenes into S–H bonds. p-Toluenethiol is a more weakly binding ligand toward Ir(TTP)CH...

Synthesis, structural characterization and crystal structure of some dimethyltin complexes containing substituted 1,10-phenanthroline

The reaction of dimethyltin dichloride with four substituted 1, 10- phenanthroline has been studied. The reactions of dimethyltin dichloride with 5-methyl-1,10-phenanthroline (Mephen); 5,6-dimethyl-1,10-phenanthroline (Me2phen); 5-nitro-1,10-phenanthroline (NO2phen); 5-chloro-1,10-phenanthroline (Clphen) resulted in the formation of the hexa-coordinated complexes of [SnMe2Cl2(NN)] {Mephen (1), Me2phen (2), NO2phen (3), Clphen (4)}. The resulting products have been fully characterized by elemental analysis, multinuclear (1H, 13C, 119Sn) NMR, DEPT-135, HHCOSY and HSQC NMR spectroscopy. The solid state X-ray determination of complexes [SnMe2Cl2(Mephen)] (1) and [SnMe2Cl2(Me2phen)] (2) revealed that the complexes 1 and 2 contain the hexa-coordinated tin(IV) atom in an octahedral geometry with the trans-[SnMe2] configuration. The SnN bond distances in 1–2 are 2.47–2.48 Å which are almost among the largest values.

Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (tBuPCP)IrH(Cl) (1; tBuPCP = κ3-2,6-(CH2PtBu2)2C6H3) appeared to be moderately active in NHMe2·BH3 (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (tBuPCP)IrHCl(η1-BH3·NHMe2) (3) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)IrH2(η1-BH3·NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and tert-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of 1) the h...

Manganese extraction with mixtures of bis(2,4,4-trimethylpentyl)dithiophosphinic acid and trioctyl phosphine oxide

ABSTRACTIt was shown that a strong synergistic effect (S ≈ 4000) takes place when extracting manganese (II) from sulfate solutions with mixtures of bis(2,4,4-trimethylpentyl)dithiophosphinic acid (CYANEX 301) and trioctyl phosphine oxide (TOPO. The synergistic effect is caused by the formation of the mixed complex Mn-HR-TOPO. Analysis of the manganese partition between organic and aqueous phase as well as of the IR and electron paramagnetic resonance spectra of the extracts showed that manganese forms a hexacoordinated complex with the dithiophosphinate ions and TOPO with an octahedral geometry. The composition of the complex can be presented as MnR2∙2TOPO.Examples of the possible use of the CYANEX 301 and trialkyl phosphine oxide (TAPO, CYANEX 923) mixtures when processing manganese-containing liquors are given.

Studies on the reactions of 3,2′-polymethylene-2-phenylbenzo[b]-1,10-phenanthrolines with Ru(tpy)Cl3 and properties of the products

A series of 3,2-polymethylene-2-phenylbenzo[b]-1,10-phenanthrolines was reacted with Ru(tpy)Cl3 to afford two ruthenium (Ru) complexes, a pentaaza-coordinated (N5Cl) complex [Ru(tpy)(L)Cl]+ and a hexa-coordinated (N5C) complex [Ru(tpy)(L)]+. The ratio of these two complexes was found to be highly dependent on the length of the polymethylene bridge between terminal phenyl and central pyridine rings. The reaction between the dimethylene-bridged ligand and Ru(tpy)Cl3 afforded a hexa-coordinated (N5C) complex as an only isolatable product in 83% yield, while the others afforded pentaaza-coordinated products and hexa-coordinated products in ratios of 1:1.6-3.5 with 80-90% overall yields. The UV spectra of pentaaza-coordinated complexes (N5Cl) and hexa-coordinated (N5C) cycloruthenated complexes were similar to show absorbances at 253-255, 276-286, 312-324, 360-373, and 490-532nm. All of these complexes showed greenish blue light emissions in the range 450-460nm upon excitation in the range 368-382nm, while excitation at 500-532nm resulted in green light emissions at 570nm for pentaaza-coordinated complexes and 577-579nm for hexaaza-coordinated species. Irradiation of the plasmid (100μM) in the presence of 8c (λirr>395nm, 10min) in air resulted in single-strand cleavage leading to the production of nicked plasmid (Form II), probably via intercalation.

Generation, spectroscopic characterization and antioxidant activity of tetracoordinate and hexacoordinate complexes of diethylgermanium(IV) of relatively sterically undemanding N-protected amino acids and sterically demanding heterocyclic β-diketones containing radical sensitive Ge-O bond

ABSTRACTTwo new sets of diethylgermanium(IV) complexes of the types Et2GeA2 and Et2GeB2 have been generated by the reactions of diethylgermanium(IV) dichloride with sodium salts of N-protected amino acids (AH) [where AH = , R = -CH(CH3)C2H5: A1H, R = -CH2CH(CH3)2: A2H, R = -CH(CH3)2: A3H] and with sodium salts of sterically demanding heterocyclic β-diketones (BH) [where BH = , R’ = -C6H5: B1H, R’ = -p-(Cl)C6H4: B2H, R’ = -CH3: B3H, R = -C2H5: B4H], respectively in 1:2 molar ratios in refluxing dry benzene. Plausible structures of these complexes were suggested on the basis of physico-chemical and spectroscopic studies. It is suggested that the complexes of the types Et2GeA2 and Et2GeB2 contain four coordinated and six coordinated germanium centers, respectively. The ligands and organogermanium(IV) complexes were evaluated for their free radical scavenging activity by DPPH method. These compounds/complexes demonstrate potential antioxidant activity.

Synthesis and Characterisation of Organosilicon and Organotin Complexes derived from DHA Ligand

Organosilicon(IV) and organotin(IV) complexes with dehydroacetic acid and amino ether Schiff base ligand have been described with the help of elemental analyses, electronic, infrared, multinuclear magnetic resonance spectroscopy. Analysis of data suggested that the Schiff base provides two bidentate NO domains and was coordinated to silicon and tin moiety through the imine nitrogen and hydroxyl oxygen atom forming penta coordinated and hexa coordinate complexes in 1: 2 and 1: 1 molar ratio. Thermal studies of the synthesized organotin compounds shows that they are stable and decomposition of the complexes occur in two steps with the formation of metal oxides as the end product.

Fixation of atmospheric CO2 as novel carbonate-(water)2-carbonate cluster and entrapment of double sulfate within a linear tetrameric barrel of a neutral bis-urea scaffold.

A meta-phenylenediamine-based disubstituted bis-urea receptor L1 with electron-withdrawing 3-chloro and electron-donating 4-methylphenyl terminals has been established as a potential system to fix and efficiently capture atmospheric CO2 as air-stable entrapment of an unprecedented {CO32--(H2O)2-CO32-} cluster (complex 1a) within its tetrameric long straight pillar-like assembly entirely sealed by n-TBA cations via formation of a barrel-type architecture. L1 and its isomeric 4-bromo-3-methyl disubstituted bis-urea receptor L2 have been found to entrap similar kinds of water-free naked sulfate-sulfate double anion (complexes 1b and 2a) by cooperative binding of urea moieties inside the two pairs of the inversion-symmetric linear tetrameric barrel of L1 and L2, respectively. On the other hand, in the presence of excess halides, L1 self-assembles to form hexa-coordinated fluoride complex 1c and tetra-coordinated bromide complex 1d, while L2 self-assembles to form penta-coordinated fluoride complex 2b in the solid state via semicircular receptor architectures and non-cooperative H-bonding interactions of urea moieties.

A mononuclear iron carbonyl complex [Fe(μ-bdt)(CO)2(PTA)2] with bulky phosphine ligands: a model for the [FeFe] hydrogenase enzyme active site with an inverted redox potential.

A mononuclear hexa-coordinated iron carbonyl complex [Fe(μ-bdt)(CO)2(PTA)2] 1 (bdt = 1,2-benzenedithiolate; PTA = 1,3,5-triaza-7-phosphaadamantane) with two bulky phosphine ligands in the trans position was synthesized and characterized by X-ray structural analysis coulometry data, FTIR, electrochemistry and electronic structure calculations. The complex undergoes a facilitated two-electron reduction 1/12- and shows an inverted one-electron reduction for 1/1- at higher potentials. Electrochemical investigations of 1 are compared to the closely related [Fe(bdt)(CO)2(PMe3)2] compound. A mechanistic suggestion for the hydrogen evolution reaction upon proton reduction from acid media is derived. The stability of 1 in both weak and strong acids is monitored by cyclic voltammetry.

A mononuclear Co(ii) complex formed from pyridinedimethanol with manifold slow relaxation channels.

A mononuclear hexacoordinate complex [Co(pydm)2](dnbz)2 formed from 2,6-pyridinedimethanol in the coordination sphere of Co(ii) and dinitrobenzoato anions exhibits magnetic anisotropy of an easy axis type and a field induced slow magnetic relaxation with manifold relaxation channels. The low-frequency relaxation time is as slow as τLF = 0.13 s at BDC = 0.4 T and T = 1.9 K.

Cadmium(II) and indium(III) complexes derived from 2-benzoylpyridine N(4)-cyclohexylthiosemicarbazone: Synthesis, crystal structures, spectroscopic characterization and cytotoxicity

Two metal complexes [Cd(L)2]·0.275H2O(1) and [In(L)2]NO3 (2) (HL=2-benzoylpyridineN(4)-cyclohexylthiosemicarbazone) have been synthesized and structurally characterized by elemental analysis, a number of spectroscopic methods (IR, UV-vis, NMR), mass spectrometry, and X-ray crystallography. The Schiff’s base ligand forms hexacoordinated complexes having octahedral geometry for Cd(II) and In(III) complexes, respectively. The synthesized compounds were tested for antiproliferative activity and showed the ability to kill HepG2 cells (human hepato cellular carcinoma) significantly, especially 2 with IC50=2.02±0.14μM. Of particular note is the fact that complex 1 has ca 12-fold lower toxicity in the normal hepatocyte QSG7701 cells than in the hepatocellular carcinoma HepG2 cells. In addition, complex 2 also exhibited excellent luminescent property. Upon the addition of 1 equiv of In3+ ion, 200-fold fluorescence enhancement of HL at λem=504nm has been observed. Moreover, the fluorescent color change (from transparent to light-green) could be observed by naked eyes under the light of 365nm. These findings can expand the applications of thiosemicarbazone derivatives in the fields of colorimetric and fluorescent probes.

Dioxomolybdenum(VI) and ‐tungsten(VI) Complexes with Multidentate Aminobisphenol Ligands as Catalysts for Olefin Epoxidation

AbstractThe synthesis of four molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene (1a) or hydroxyglycolene (1b) substituents is reported. The molybdenum dioxo complexes [MoO2L] (L = 2a, 2b) and tungsten complexes [WO2L] (3a, 3b) were synthesized using [MoO2(acac)2] and [W(eg)3] (eg = 1,2‐ethanediolato, ethylene glycolate), respectively, as precursors. All complexes were characterized by spectroscopic means as well as by single‐crystal X‐ray diffraction analyses. The latter reveal, in all cases, hexacoordinate complexes in which the hydrogen atom of the hydroxy group is involved in hydrogen bonding with one of the metal oxo groups. In the case of the glycol substituent, the ether oxygen atom is coordinated to the metal whereas the hydroxy group remains uncoordinated. The complexes were tested as catalysts in the epoxidation of cyclooctene under eco‐friendly conditions, using an aqueous solution of H2O2 as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions, as substitutes for chlorinated solvents. Molybdenum complexes 2a and 2b showed good catalytic activity using H2O2 without added solvent, and tungsten complexes 3a and 3b showed very high activity in the epoxidation of cyclooctene using H2O2 and DMC as solvents.

ChemInform Abstract: Investigation of Catalytic Activity and Catalytic Mechanism of Chiral Amino Diol Tridentate Ligands in the Asymmetric Addition of Aldehydes in the Present of Methyllithium Reagent.

Abstract Several highly modular chiral amino diol tridentate ligands were found to be effective for the asymmetric alkylation reaction of aromatic aldehydes in the presence of methyllithium reagent, providing up to 96% ee values and up to 94% yields under relatively mild conditions. The investigation of the catalytic activity of these ligands shows a correlation of enantioselectivities of ligands with steric properties of substituent N-alkyl and nucleophilic Li-alkyl. With the addition of Ti(O-i-Pr)4 to the ligand 3c, the 1H NMR spectrum reveals a hexa-coordinate Ti transition state complex with only one Ti metal center generated in solution.

Zirconium complexes stabilized by amine-bridged bis(phenolato) ligands as precatalysts for intermolecular hydroamination reactions

A series of zirconium complexes bearing amine-bridged bis(phenolato) ligands of different steric and electronic properties have been synthesized, and their activities in catalyzing intermolecular hydroamination reactions have been studied and compared. In general, hexacoordinate zirconium dibenzyl complexes 1-4 stabilized by [ONNO]- or [ONOO]-type ligands were found to be less active than pentacoordinate complexes 5 and 7 that carry [ONO]-type ligands, which clearly imply that amine-bridged bis(phenolato) ligands play crucial roles in influencing catalytic activities. Complex 5 showed good activities and regioselectivities in catalysing reactions of various primary amines and alkynes. Moreover, reactions of challenging substrates, including secondary amines, internal alkynes, and hydrazines, were achieved with in situ generated cationic species from complex 5 and [Ph3C][B(C6F5)4].

Heme Coordination Versatility in a Truncated Hemoglobin

GlbN, the “truncated” hemoglobin from Synechocystis sp. PCC 6803, coordinates the heme iron with His46 (E10, distal) and His70 (F8, proximal). Displacement of His E10 by an exogenous ligand (e.g., cyanide in the ferric state) drives a significant conformational change allowing Tyr22 (B10), Gln43 (E7) and Gln47 (E11) to establish a hydrogen bond network stabilizing the distal ligand.THB1, a closely related hemoglobin from Chlamydomonas reinhardtii, also binds cyanide and forms the same network of interactions. However, in the absence of an exogenous ligand, the neutral amino group of Lys53 (E10) coordinates the heme iron on the distal side [1]. Because of the importance of the coordination scheme in controlling the reactivity of heme proteins, the flexibility of the truncated hemoglobin fold, and the rarity of lysine coordination, we explored the ability of GlbN to use lysine at position E10 as an axial ligand. The His46Lys replacement yielded a protein with UV-Vis and NMR spectra similar to those of His46Leu and comparable pH response, suggesting that GlbN does not accommodate lysine coordination at position E10. However, the spectra were incompatible with water coordination and suggested that in the ferric state these variants were low-spin endogenously hexacoordinate complexes. We performed amino acid replacements within the distal H-bond network to characterize the perturbed heme environment and ligand sets. A combination of pH titrations and NMR experiments illustrates the delicate balance of interactions governing the heme pocket conformation of GlbN.[1] Johnson et al. (2014) Biochemistry 53:4573Supported by NSF grant MCB-1330488

Computational Insights into the Isomerism of Hexacoordinate Metal–Sarcophagine Complexes: The Relationship between Structure and Stability

The hexacoordinate complexes that the macrobicyclic ligands {(NH3)(2)sar)(2+) and {NMe3)(2)sar}(2+) (sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) form with transition metals such as Co-III, Co-II and Cu-II can adopt several isomeric structures. In this article, we have firstly employed DFT methods lo compute the relative stability of their Delta-ob(3), Delta-ob(2)lel, Delta-lel(2)ob and Delta-lel(3) isomers, as well as the activation barriers for their interconversion. In agreement with the experimental data, the results show that, in general, the different isomers of the Co-III and Co-II complexes present similar free energies, whereas the Cu-II complexes show a strong tendency towards the lel(3) form. In addition, the interplay between the structure and stability of these species has been studied by combining shape maps with a distortion/interaction energy analysis. In contrast to the geometries close to the ideal octahedron that all the studied Co complexes present, the le)3 structures of [Cu{(NH3)(2)sar}](4+) and [Cu{(NMe3)(2)sar](4+) are better described. as trigonal prisms. In such structures the ligand adopts a conformation significantly more stable than in the other isomers, and this drives the formation of lel(3)-[Cu{(NH3)(2)sar}](4+) and lel(3)-[Cu{(NNe3)(2)sar}](4+). Overall, the results show a clear relationship between the stability of a given isomer and its degree of distortion with respect to the ideal octahedron (or trigonal prism), with the latter being ultimately dependent on the transition metal and its radius.

Molybdenum complex with bulky chelates as a functional model for molybdenum oxidases.

The novel bulky Schiff base chelate ligand [(4,5-diisopropyl-1H-pyrrole-2-yl)methylene]-4-(tert-butyl)aniline ((iPr2)HL) bearing two isopropyl groups close to the pyrrole nitrogen atom reacts with MoCl2(dme)O2 (dme = 1,2-dimethoxyethane) to give the sterically congested complex Mo(VI)((iPr2)L)2O2 ((iPr2)1; OC-6-4-4 configuration). In spite of the increased steric shielding of the [MoO2] unit (iPr2)1 is active in oxygen-atom transfer to PMe3 and PPh3 to give OPMe3 and OPPh3, respectively. Because of the increased steric bulk of the chelate ligand, formation of dinuclear complexes [Mo(V)((iPr2)L)2O]2(μ-O) ((iPr2)3) by comportionation is effectively prevented in contrast to the highly favored formation of [Mo(V)((H2)L)2O]2(μ-O) ((H2)3) with the less bulky ligand (H2)HL. Instead, the smaller PMe3 ligand coordinates to the resulting pentacoordinate intermediate Mo(IV)((iPr2)L)2O ((iPr2)5), giving the hexacoordinate complex Mo(IV)((iPr2)L)2O(PMe3) ((iPr2)2) with OC-6-3-3 configuration. The larger potential ligands PPh3 and OPPh3 are only able to weakly coordinate to (iPr2)5, giving labile and sensitive Mo(IV)((iPr2)L)2O(L) complexes ((iPr2)6, L = PPh3; (iPr2)7, L = OPPh3). Traces of water and dioxygen in solutions of (iPr2)6/(iPr2)7 yield the di(μ-oxido) complex [Mo(V)((iPr2)L)O]2(μ-O)2 ((iPr2)4) with reduced steric congestion due to dissociation of the bulky chelate ligands. According to electron paramagnetic resonance studies, the much more strongly bound small PMe3 ligand in (iPr2)2 can be slowly liberated by one-electron oxidation to Mo(V), with Ag(+) leaving a free coordination site at Mo(V). Hence, essentially pentacoordinate Mo(IV) and Mo(V) complexes are accessible as a result of the increased steric bulk.

Donor-stabilized hypercoordinated silicon(IV) chelates with cyclohexylideneimino-N ligand group: Role of substituents on ionization

N-Cyclohexylideneimino-O-(trimethylsilyl)acylimidates react smoothly with XSiCl3 (X=Me, Ph, vinyl and Cl) in a transsilylation reaction producing ionic pentacoordinate bischelates and neutral hexacoordinated complexes depending on the electronic and steric behavior of the ligands. These hypercoordinated silicon complexes are characterized by UV–Vis, FT-IR and NMR spectroscopic techniques. A neutral hexacoordinate silicon complex with dichloro ligands was characterized by single crystal X-ray analysis. The UV–Vis absorption data showed a significant bathochromic shift of λmax in hypercoordinated silicon complexes on comparison with their corresponding hydrazide ligands. The 29Si NMR chemical shifts strongly depend on nature of X group directly attached to the central silicon atom toward ionization.

Investigation of catalytic activity and catalytic mechanism of chiral amino diol tridentate ligands in the asymmetric addition of aldehydes in the present of methyllithium reagent

Several highly modular chiral amino diol tridentate ligands were found to be effective for the asymmetric alkylation reaction of aromatic aldehydes in the presence of methyllithium reagent, providing up to 96% ee values and up to 94% yields under relatively mild conditions. The investigation of the catalytic activity of these ligands shows a correlation of enantioselectivities of ligands with steric properties of substituent N-alkyl and nucleophilic Li-alkyl. With the addition of Ti(O-i-Pr)4 to the ligand 3c, the 1H NMR spectrum reveals a hexa-coordinate Ti transition state complex with only one Ti metal center generated in solution.

Isomorph supramolecular coordination compounds of cobalt(II) and nickel(II) based on pyridine-2,4-dicarboxylic acid and creatinine adduct

Pyridine-2,4-dicarboxylic acid (py-2,4-dcH2) and creatinine (creat) with MCl2·6H2O (M = CoII and NiII) formed two isomorph supramolecular coordination compounds. Their three-dimensional supramolecular architecture is formed not only by simple proton transfer from pyridine-2,4-dicarboxylic acid to creatinine, but also by other very important non-covalent interactions, especially the ion pairing, hydrogen bonds, C–H···π, N–H···π and C–O···π interactions. In these complexes, pyridine-2,4-dicarboxylate acts as a bidentate ligand and the lattice is composed of anionic hexacoordinated complex, [M(py-2,4-dc)2(H2O)2]2− (M = CoII and NiII), creatinium counter ion, (creatH)+, and two uncoordinated water molecules. The complexation equilibrium and stabilities of proton transfer, binary and ternary complexes of pyridine-2,4-dicarboxylic acid as a primary ligand and creatinine as a secondary ligand with Co2+ and Ni2+ transition metal ions in comparison with X-ray crystal structures in solid state have studied.