Showing papers on "Denticity published in 2000"
TL;DR: In this paper, two families of molecular frameworks which grow as homochiral single crystals are described, which consist of multiple interpenetration of the three-connected chiral (10,3)-a (Y*) network and result from the tridentate coordination of the 1,3,5-benzenetricarboxylate (btc) ligand to octahedral metal centers.
Abstract: Two families of molecular frameworks which grow as homochiral single crystals are described. Both consist of multiple interpenetration of the three-connected chiral (10,3)-a (Y*) network and result from the tridentate coordination of the 1,3,5-benzenetricarboxylate (btc) ligand to octahedral metal centers which act as linear connectors. The nature of the interpenetration is controlled by the auxiliary ligands bound in the equatorial plane of the metal center. Ethylene glycol (eg) binds in a unidentate fashion to form phase A which has 28% accessible solvent volume and contains four interpenetrating (10,3)-a networks. 1,2-Propanediol (1,2-pd) coordinates as a bidentate ligand to yield a phase B with a greatly enhanced 51% of solvent accessible volume, because only two (distorted) (10,3)-a‘ networks interpenetrate. Ligands in the void space and bound to the metal center can both be liberated thermally: the kinetics of this process allow isolation of microporous desolvated crystalline A and B. The porous ph...
564 citations
TL;DR: Two triazine ligands have been synthesized by derivatisation of cyanuric chloride: 6-(diethylamino)-2,4-disulfide group present on one ring and a co-ordinated thiolate on the second.
Abstract: Two ligands have been synthesized by derivatisation of cyanuric chloride: 6-(diethylamino)-2,4-disulfanyl-1,3,5-triazine (H2SSta) 1 and 6-(diethylamino)-2-hydroxo-4-sulfanyl-1,3,5-triazine (H2OSta) 2 have been characterised by X-ray crystallography, which shows intermolecular hydrogen bonding in the solid state, leading to dimers of 1 and ribbons of 2. On reaction with metal salts both ligands undergo oligomerisation reactions. Compound 1 reacts with nickel chloride to form a mononuclear complex, [Ni{(Sta)S(S2ta)}] 3. In 3 two triazine ligands have reacted, to form a tetradentate ligand in which two triazine rings are bridged by a sulfur group, with a co-ordinated disulfide group present on one ring and a co-ordinated thiolate on the second. Compound 2 reacts with cobalt(II) chloride to form a cage complex, [Co6NaO(OStaH)7{S(Ota)2}2(O2CPh)2(H2O)2] 4. This complicated structure contains two polydentate ligands formed by linking triazine groups through a bridging sulfur. The cage contains four cobalt(II) and two cobalt(III) sites which are assigned by bond length considerations. The compound [Co(OSta)3] 5 co-crystallises with 4, and its structure has also been determined.
495 citations
TL;DR: In this article, Heck et al. synthesize PdII complexes of donor-functionalized heterocyclic carbene ligands through a route involving carbene transfer from AgI carbene precursors.
471 citations
TL;DR: In this article, the stereochemical properties of over 120 pyrazole-based chelating ligands, studied by single-crystal X-ray crystallography were analyzed.
426 citations
TL;DR: Chiral monophosphines have been prepared from enantiomerically pure 2, 2'-dihydroxy-1,1'-binaphthyl and demonstrated to be highly efficient chiral ligands for transition-metal-catalyzed organic transformations, especially for reactions where chelating bisphosphine ligands cannot be used.
Abstract: Chiral monophosphines, whose chirality is due to biaryl axial chirality, have been prepared from enantiomerically pure 2, 2'-dihydroxy-1,1'-binaphthyl and demonstrated to be highly efficient chiral ligands for transition-metal-catalyzed organic transformations, especially for reactions where chelating bisphosphine ligands cannot be used. The high efficiency is observed in palladium-catalyzed asymmetric hydrosilylation of a wide variety of olefins such as alkyl-substituted terminal olefins and in asymmetric reactions via pi-allylpalladium intermediates represented by asymmetric reduction of allylic esters with formic acid.
416 citations
TL;DR: Lanthanide atom sizes (the lanthanide contraction) directly control the type of structure formed by the coordination of a single multidentate ligand, 3,5-pyrazoledicarboxylic acid (H(3)pdc).
Abstract: Lanthanide atom sizes (the lanthanide contraction) directly control the type of structure formed by the coordination of a single multidentate ligand, 3,5-pyrazoledicarboxylic acid (H3pdc). Single-layer, double-layer ([Eu2(Hpdc)3(H2O)6], see picture), and three-dimensional networks were found. Control of the reaction pH plays a key role in the structure formation in this system.
411 citations
TL;DR: A review of the literature on the metal ion complexes of carbohydrate type ligands with the emphasis on the past 20 years, discussing the equilibria and structure in aqueous solution together with the structures determined for the solid state complexes are collected for comparison as discussed by the authors.
390 citations
TL;DR: The homogeneous enantioselective hydrogenation of functionalized prochiral olefins is one of the most frequently studied and most efficient transition metal-catalyzed reactions as mentioned in this paper.
Abstract: The homogeneous enantioselective hydrogenation of functionalized prochiral olefins is one of the most frequently studied and most efficient transition metal-catalyzed reactions. In the first reports using chiral Wilkinson type catalysts, low enantiomeric excesses (ee’s) were obtained using monodentate phosphines as ligands. All attempts to develop monodentate ligands which would afford high ee’s in this reaction met with limited success, the best result being reached with CAMP, already published in 1972, giving ee’s up to 90% in the hydrogenation of dehydroamino acids. Although new monodentate phosphorus ligands play a significant role in other transition metal-catalyzed reactions, highly enantioselective hydrogenations are exclusively based on bidentate phosphorus ligands. Starting with Kagan’s diop a large number of bidentate ligands with excellent selectivities was designed. Among the most successful are DIPAMP, which gives superior results compared to its monodentate analogue PAMP, the frequently used BINAP ligand, the ferrocenyl-based ligands and the DuPHOS, BPE, and FerroTANE ligands, the latter showing extremely high enantioselectivities and broad scope. To date, not only phosphines (phosphanes) are used but also bidentate aminophosphines, phosphites, phosphinites, phosphonites, and hybrid ligands such as phosphine-phosphite, aminophosphine-phosphinite, and phosphine-phosphonite,
366 citations
TL;DR: It appears that functionalization of biomolecules with tridentate-chelating ligand systems is preferable for the labeling with fac-[(99m)Tc(OH(2)(3)(CO)(3)](+), since this will presumably result in radioactive bioconjugates with better pharmacokinetic profiles.
362 citations
TL;DR: In this paper, the structure of disilver(I) complexes of o-phenylenedimethylenebis(imidazol-2-ylidene) has been determined by X-ray crystallography.
Abstract: N-Functionalised imidazol-2-ylidene carbene complexes of AgI have been prepared by interaction of the corresponding imidazolium salt with Ag2O in dichloromethane or 1,2-dichloroethane in the presence of molecular sieves or with Ag2CO3 in 1,2-dichloroethane. In an analogous way disilver(I) complexes of o-phenylenedimethylenebis(imidazol-2-ylidene) have been obtained. Their structures, as determined by X-ray crystallography, indicate that the carbene ligand acts as monodentate from the carbene end or as a bridge between two different metal atoms. In one case the silver is coordinated by two carbene ligands. In the majority of the structurally characterised complexes the metal centres adopt linear geometries with M–C(carbene) bond lengths in the range of 207–210 pm characteristic of a single bond.
238 citations
TL;DR: A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility.
Abstract: The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn4(O2CMe)2(pdmH)6](ClO4)2 (1). Complex 1·2MeCN·Et2O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) A, b = 15.347(4) A, c = 9.660(3) A, α = 104.58(1)°, β = 93.42(1)°, γ = 106.06(1)°, and Z = 1. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1·2.5H2O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1·2.5H2O and indicate that the desolvated complex has a S = 8 ground state...
TL;DR: In this article, the first crystal structure of the hydrogen-bonded metal complex of 2,2′-biimidazolate monoanion ligand Hbim − with Cu(II) as a neutral dimer metal complex was obtained.
TL;DR: In this article, the photoreactivities of methoxy and ethoxy groups on TiO2 have been studied by Fourier transformed infrared spectroscopy and a Russell-like mechanism was invoked to explain the formation of the reaction products.
Abstract: Thermo- and photoreactivities of methoxy (CH3O(a)) and ethoxy groups (C2H5O(a)) bonded, via the oxygen atom, to one Ti ion (monodentate adsorption form) or to two Ti ions (bidentate adsorption form) on TiO2 have been studied by Fourier transformed infrared spectroscopy. Regardless of the similar thermal stability for the two adsorption geometries of the adsorbed alkoxy groups, difference in photoreactivity is observed by monitoring the change of their integrated IR absorptions as a function of UV irradiation time. The monodentate photooxidation rate is ∼1.5 times that of bidentate for both methoxy and ethoxy groups. CH3O(a) on TiO2 is photooxidized to H2O(a) and HCOO(a) in the presence of O2. On the other hand, C2H5O(a) is photooxidized to H2O(a), HCOO(a), and CH3COO(a). A Russell-like mechanism is invoked to explain the formation of the reaction products. Possible reaction steps that control the photoreactivity of the monodentate and bidentate adsorption forms are discussed in terms of this mechanism.
TL;DR: The first functional mimic of the tetramanganese−oxo core was synthesized in this article, containing a cubical [Mn4O4]n+ core surrounded by six facially bridging bidentate chelates to the manganese ions.
Abstract: Photosynthesis produces molecular oxygen from water catalyzed by an enzyme whose active site contains a tetramanganese−oxo core of incompletely established structure. The first functional mimic of this core has been synthesized containing a cubical [Mn4O4]n+ core, surrounded by six facially bridging bidentate chelates to the manganese ions ((dpp)6Mn4O4 (1); dpp- = diphenylphosphinate anion). Bond enthalpy data predict that the Mn4O46+ core is thermodynamically capable of releasing molecular O2, but is kinetically prevented from doing so by an activation barrier. UV light absorption into a Mn−O charge-transfer excited state (but not excitation of a Mn ligand-field excited state) efficiently releases an O2 molecule if performed in the gas phase and concomitantly releases a bridging dpp- anion and the cationic species (dpp)5Mn4O2+ (presumed Mn4O2-butterfly core type). All species were identified by high resolution mass spectrometry. This reaction proceeds with high quantum efficiency (>50%) and is the only o...
TL;DR: Early transition metal complexes containing polydentate amido-donor ligands not only display unprecedented patterns of reactivity but are the focus of recent developments in olefin polymerization catalysis as discussed by the authors.
TL;DR: The nature of the ground state is confirmed by the good simulation of the magnetization curves at 2 and 5 K (diagonalization of the full matrix taking into account all energy levels obtained with the parameter set resulting from the fit of the susceptibility curve).
Abstract: The tetranuclear [Ni4(pypentO)(pym)(μ3-OH)2(μ-Oac)2(NCS)2(OH2)] cubane-like complex has been prepared, and its structure and magnetic properties have been studied (pypentO and pym are the deprotonated forms of 1,5-bis[(2-pyridylmethyl)amino]pentane-3-ol and 2-pyridylmethanol, respectively). The X-ray diffraction analysis of this novel nickel complex (C61H74N14O25.5S4Ni8, monoclinic, P21, a = 13.9375(14) A, b = 20.6604(18) A, c = 16.6684(19) A, β = 110.619(12)°, Z = 2) showed a Ni4O4 cubane arrangement of four nickel atoms, four μ3-O bridging ligands (one pypentO, one pym, and two OH-), two syn−syn bridging acetates, and three terminal monodentate ligands (two NCS- and one OH2). In this low-symmetry elongated cubane, the four Ni−Ni long distances (3.18 A) correspond to the faces of the cube including two μ3-OR bridges, and the two Ni−Ni short distances (2.94 A) correspond to the faces including two μ3-OR and one acetate bridges. The temperature dependence of the magnetic susceptibility was fitted with J1 =...
TL;DR: In this article, the bidentate nitrogen-donor ligand 1,4-bis(3-pyridyl)-2,3-diaza-1,3butadiene (4) has been synthesized.
Abstract: The new bidentate nitrogen-donor ligand 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (4) has been synthesized (monoclinic, P21/c, a = 11.074(2) A, b = 4.737(1) A, c = 11.475(2) A, β = 115.861(9)°, V = 541.7(1) A3, Z = 4) and its coordination chemistry with cadmium and cobalt nitrate hydrates investigated. Two new noninterpenetrating coordination polymers based on “T-shaped” building blocks have been obtained: [Cd(NO3)2(C12H10N4)1.5·CH2Cl2]n (5) (monoclinic, P21/n, a = 7.891(1) A, b = 17.172(3) A, c = 18.576(4) A, β = 97.72(1)°, V = 2494.5(7) A3, Z = 4) and [Co(NO3)2(C12H10N4)1.5·CH2Cl2]n (6) (monoclinic, P21/n, a = 7.7954(8) A, b = 17.026(3) A, c = 18.412(3) A, β = 97.89(1)°, V = 2420.6(6) A3, Z = 4). Both 5 and 6 adopt a novel “polycyclohexane” structural motif, consisting of “all-equatorially” condensed cyclohexane-like M6(ligand)6 units. They are open-framework materials with unusually large cavities (ca. 28 × 15 A), and reversibly absorb and desorb guest molecules between room temperature and 213 °C wi...
TL;DR: Four new coordination polymers are prepared by solution reactions and fully characterized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction, generating two new types of building blocks that are distinctly different from the common "T-shaped" building block generated from the same (MN3O4) coordination environment reported previously.
Abstract: The coordination chemistry of the long conjugated bidentate Schiff-base ligands 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L1) and 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (L2) with cadmium and cobalt nitrate hydrates is investigated. Four new coordination polymers are prepared by solution reactions and fully characterized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction. [Cd(NO3)2(L1)1.5·0.5(L1)]n (1; monoclinic, P21/c; a = 7.7729(16) A, b = 19.049(4) A, c = 17.865(4) A, β = 93.13(3)°, Z = 4) is obtained by combination of L1 with Cd(NO3)2·4H2O in a benzene/methanol or THF/methanol mixed-solvent system. The structure features two-dimensional brick wall sheets that are cross-linked by weak noncovalent π−π interactions (alternating face-to-face stacking of coordinated and uncoordinated L1 molecules) to generate a novel three-dimensional network. [Co(NO3)2(L1)1.5·H2O]n (2; orthorhombic, Ccca; a = 19.031(4) A, b = 33.627(7) A, c = 14.299(3) A, Z...
TL;DR: A series of palladium complexes with a hemilabile terdentate carbene ligand, 1,3-bis(2-pyridyl)imidazol-2-ylidene (pyimypy), has been synthesized as discussed by the authors.
TL;DR: In this paper, a series of metal-L1 complexes which cover the lanthanides have been prepared, and five different structural types have been established for the ligands in the asymmetric unit.
Abstract: Phenyl-substituted derivatives of 2,2′∶6′,2′′-terpyridine and a corresponding bipyridine–pyrazine derivative have been shown to have metal extraction properties and separation factors for americium(III) over europium(III) which are comparable to those previously obtained for 2,2′∶6′,2′′-terpyridine (L1). The extracting agents in either tert-butylbenzene (TBB) or hydrogenated tetrapropene (TPH) gave DAm/DEu separation factors (SFs) between 7 and 9 when used to extract the metal ions from 0.01–0.1 M nitric acid solution in synergistic combination with 2-bromodecanoic acid. In contrast to L1, the new hydrophobic ligands have little or no solubility in the aqueous phase. In an effort to better understand the nature of the species which may be involved in the extraction process, a series of metal–L1 complexes which cover the lanthanides have been prepared. Five different structural types have been established for the lanthanide coordination complexes. In type 1, [M(NO3)3(L1)(H2O)] (M = Nd), the metal is 10-coordinate being bonded to one terdentate L1 ligand, three bidentate nitrates and a water molecule. In type 2, [M(NO3)2(L1)2]+[M(NO3)4(L1)]− (M = Nd, Sm, Tb, Dy and Ho), the metal atom in the cation is 10-coordinate, being bonded to two terdentate L1 ligands and two bidentate nitrates; in the anion the metal is also 10-coordinate, being bonded to one terdentate L1 ligand and four nitrates, of which three are bidentate and one unidentate. In type 3, [M(NO3)3(L1)(H2O)]·L1 (M = Ho, Er, Tm and Yb), the metal is 10-coordinate, being bonded to three bidentate nitrates, one terdentate L1 and a water molecule. In addition, an L1 ligand is found in the asymmetric unit which is hydrogen-bonded to the coordinated water molecule. In type 4, [M(NO3)3(L1)(H2O)] (M = Tm), the metal is 9-coordinate, being bonded to two bidentate nitrates, one unidentate nitrate, one terdentate L1 ligand and a water molecule. In type 5, [M(NO3)3(L1)] (M = Yb), the metal is 9-coordinate, being bonded to three bidentate nitrates and one terdentate L1 ligand. A sixth structural type was observed for M = La in the crystal structure [(H2L1)(NO3)]+[(H2L1)]2+[La(NO3)6]3−. The metal is not bound to L1 but instead forms the well-known hexanitrate anion. This complex may give some indication of the type of species which could be formed at higher acid concentrations in the aqueous phase, where protonation of L1 can occur.
TL;DR: The new versatile multidentate nonchelating ligand 1,2-bis[(2-pyr-imidinyl)-sulfanylmethyl]benzene (bpsb) was designed and prepared for supramolecular syntheses and resulted in the polymer with single-stranded helical chain structure.
Abstract: The new versatile multidentate nonchelating ligand 1,2-bis[(2-pyr-imidinyl)-sulfanylmethyl]benzene (bpsb) was designed and prepared for supramolecular syntheses. Self-assembly between silver nitrate and the bpsb ligand resulted in the polymer [Ag4(bpsb)2-(NO3)4]n (1) with a single-stranded helical chain structure. Each bpsb ligand in 1 acts as a tetradentate ligand, in which two sulfur atoms and two nitrogen atoms from different pyrimidine groups coordinate to four Ag atoms in four different directions. The nitrate anions serve as a template for the formation of the helix and are either embedded in the interior of the helix or located in the flank of the helix. Self-assembly between silver perchlorate and the bpsb ligand under the same conditions gave rise to the polymer [Ag2(bpsb)3(ClO4)2]n (2) comprising a two-dimensional lamellar network containing crownlike cavities. The silver atoms in two adjacent layers are arranged staggered in 2. The two-dimensional lamellar network comprising isolated cavities of [Ag6(bpsb)6] is very different from that of usual honeycomb structures.
TL;DR: The ferromagnetic exchange coupling is discussed using DFT calculations on model compounds that have shown a countercomplementary effect of the hydroxo and acetate bridges.
Abstract: The structural and magnetic data of the trinuclear compound [Cu3(L)2(CH3COO)2(OH)2(dmf)2] (HL = N-(2-methylpyridyl)toluensulfonylamide) are reported. The compound crystallizes in the monoclinic system, space group P2(1)/n (no. 14), with a = 11.6482(6) A, b = 13.5772(6) A, c = 13.5306(7) A, alpha = 90 degrees, beta = 92.859(5) degrees, gamma = 90 degrees, and Z = 2. The three copper atoms form an exact linear arrangement. Neighboring coppers are connected by a hydroxo bridge and a bidentate syn-syn carboxylato group. The coordination spheres of the terminal copper atoms are square pyramidal with a dmf molecule as the apical ligand. The central copper has a regular square planar geometry. The mixed bridging by a hydroxide oxygen atom and a bidentate carboxylato group leads to a noncoplanarity of the adjacent basal coordination planes with a dihedral angle of 51.96(9) degrees. Susceptibility measurements (2-300 K) reveal a strong ferromagnetic coupling, J = 93(6) cm-1, in the mixed-bridged moiety leading to a quartet ground state that is confirmed by the EPR spectra. The ferromagnetic exchange coupling is discussed using DFT calculations on model compounds that have shown a countercomplementary effect of the hydroxo and acetate bridges.
TL;DR: The first Cu-II coordination compound, catena-[Cu(N(CN)2)2(ampym)2] (in which ampym=2-aminopyrimidine) has been synthesized and investigated by X-ray analysis, ligand field, EPR spectroscopy and magnetic susceptibility.
TL;DR: The bis(ortho-)chelated platinum complex (PtCl(NCN)), 1 (NCN ) (C6H3(CH2NMe2)2-2,6) - ), has been used as a novel metal precursor for C-H bond activation and subsequent cyclometalation of the potentially terdentate coordinating pincer ligand PCHP (PCP) (C 6H3 (CH2PPh2)
Abstract: The bis(ortho-)chelated platinum complex (PtCl(NCN)), 1 (NCN ) (C6H3(CH2NMe2)2-2,6) - ), has been used as a novel metal precursor for C-H bond activation and subsequent cyclometalation of the potentially terdentate coordinating pincer ligand PCHP (PCP ) (C6H3(CH2PPh2)2-2,6) - ). Depending on the reaction temperature various intermediates, each with unique structural features, could be isolated and identified during the formation of the final, bis(ortho-)cyclometalated product (PtCl(PCP)), 2. These results provide valuable insight into the intimate steps of this reaction for which we like to introduce the term transcyclometalation process (see note). At room temperature, a sparingly soluble dimetallic complex is formed, which comprises a 1 -monodentate C-bound NCN ligand and a bridging, P,P 2 - 2 -coordinating PCHP ligand. Solution spectroscopy and X-ray analysis of this platinum dimer established a C-H bond which is interacting by intramolecular noncovalent H‚‚‚Cl hydrogen bonding with the Pt-Cl motif. As a further intermediate, the formation of a trans-bis(phenyl) complex (Pt( 3 -PCP)( 1 -HNCN))Cl, 5, has been identified, which is characterized by a bis(ortho-)cyclometalated PCP ligand which is 3 -P,C,Pcoordinated to the platinum center as well as an 1 -monodentate C-bound NCN ligand. The equivalent HCl, which is formally released during the formation of 5, is intramolecularly trapped by one of the basic amine groups, as is apparent from the identification of precursors of 5, i.e., compounds in which either one ((3)Cl) or two ((4)X2) of the NMe2 groups have become protonated (see Scheme 5). Remarkably, the proton in (4)X2 is not only bound to the nitrogen lone pair but also interacts with the filled d z2 orbital of the nucleophilic platinum(II) center, thus disclosing complex 5 as an organometallic proton sponge that is able to sequester protons due to Pt,N-bidentate chelating properties. Prolonged exposure of the reaction mixture at 80 °C or performing the reaction at 110 °C afforded the transcyclometalated complex 2 and 1 equiv of the diaminoarene NCHN as the two ultimate products.
TL;DR: In this paper, solvent substituted MoO2X2(Solv)2 complexes (Solv =THF, CH3CN) with mono-and bidentate nitrogen and oxygen donor ligands were examined by single crystal X-ray analysis.
Abstract: Reaction of solvent substituted MoO2X2(Solv)2 complexes ((Solv)=THF, CH3CN) with mono- and bidentate nitrogen and oxygen donor ligands leads to complexes of the type MoO2X2L2 in nearly quantitative yields at room temperature within a few minutes. The 95 Mo and 17 O NMR data of selected complexes as well as the MoO IR vibrations were used to probe the influence of the ligands on the electronic properties of the metal and the MoO bond. Two complexes have additionally been examined by single crystal X-ray analysis. The activity of the MoO2X2L2 complexes as catalysts in olefin epoxidation with t-butylhydroperoxide as oxidizing agent depends on both the nature of the organic ligand L and the halogeno ligand X. The difference in activity observed between Cl and Br substituted complexes is not very pronounced. In general, the Cl derivatives are more active than their Br analogues. The organic ligands L display a significant influence on the catalytic performance. Complexes with ligands bearing aromatic substituents at N are in all cases, much more active than those bearing aliphatic substituents. The less active complexes can be activated by raising the temperature and extending the reaction time. In all observed cases, these changes produce a significant increase of the product yield.
TL;DR: The structure of the 4-NC complex supports the view that resonance delocalization of the positive character of the nitrogen prevents substrate activation and confirms previous work that protocatechuate 3,4-dioxygenases have three coordination sites available for binding by exogenous substrates.
Abstract: The crystal structures of protocatechuate 3,4-dioxygenase from the soil bacteria Acinetobacterstrain ADP1 (Ac 3,4-PCD) have been determined in space group I23 at pH 8.5 and 5.75. In addition, the structures of Ac 3,4-PCD complexed with its substrate 3, 4-dihydroxybenzoic acid (PCA), the inhibitor 4-nitrocatechol (4-NC), or cyanide (CN(-)) have been solved using native phases. The overall tertiary and quaternary structures of Ac 3,4-PCD are similar to those of the same enzyme from Pseudomonas putida[Ohlendorf et al. (1994) J. Mol. Biol. 244, 586-608]. At pH 8.5, the catalytic non-heme Fe(3+) is coordinated by two axial ligands, Tyr447(OH) (147beta) and His460(N)(epsilon)(2) (160beta), and three equatorial ligands, Tyr408(OH) (108beta), His462(N)(epsilon)(2) (162beta), and a hydroxide ion (d(Fe-OH) = 1.91 A) in a distorted bipyramidal geometry. At pH 5.75, difference maps suggest a sulfate binds to the Fe(3+) in an equatorial position and the hydroxide is shifted [d(Fe-OH) = 2.3 A] yielding octahedral geometry for the active site Fe(3+). This change in ligation geometry is concomitant with a shift in the optical absorbance spectrum of the enzyme from lambda(max) = 450 nm to lambda(max) = 520 nm. Binding of substrate or 4-NC to the Fe(3+) is bidentate with the axial ligand Tyr447(OH) (147beta) dissociating. The structure of the 4-NC complex supports the view that resonance delocalization of the positive character of the nitrogen prevents substrate activation. The cyanide complex confirms previous work that protocatechuate 3,4-dioxygenases have three coordination sites available for binding by exogenous substrates. A significant conformational change extending away from the active site is seen in all structures when compared to the native enzyme at pH 8.5. This conformational change is discussed in its relevance to enhancing catalysis in protocatechuate 3,4-dioxygenases.
TL;DR: The first homoleptic nitrile dipalladium (I) complex was constructed in this article, where the CH3CN ligand undergoes facile displacement by phosphine or bidentate nitrogen ligands while the Pd-Pd bond remains intact.
TL;DR: A tridentate Schiff base ligand (LH) has been synthesized from 2-hydroxyacetophenone and 2-dimethylaminoethylamine as discussed by the authors, which forms the neutral complexes [Co(L)(N3){o-(CH3CO)C6H4O}] (1) and [Co[L)(SCN]·1/2H2O (2) in presence of equivalent amount of Co(II) acetate, and sodium azide for 1 and sodium thiocyanate for 2).
TL;DR: The cationic Rh(I) dicarbonyl complex [{Rh((mim)2CH2)(CO)2}+BPh4-] (1), containing a bidentate bisimidazolylmethane ligand [mim = N-methylimidazol-2-yl], acts as an efficient catalyst for the intra...
TL;DR: The presence of the σ-bonded 2-(dimethylaminomethyl)phenyl ligand is mandatory to prevent reduction of the gold(III) centre.
Abstract: Dichloro[2-(N,N-dimethylaminomethyl)phenyl-C1,N]gold(III), [Au(damp-C1,N)Cl2] (1), reacts with salicylaldehyde thiosemicarbazone (H2saltsc), vanilline thiosemicarbazone (Hvantsc), N-methylpyrrole aldehyde thiosemicarbazone (Hmepyrtsc), pyridoxal methylthiosemicarbazone (H2pydoxmetsc), 2-diphenylphosphinobenzaldehyde thiosemicarbazone (HPtsc) or variously substituted acetylpyridine thiosemicarbazones (HapRtsc; R = H, Me, Ph) with cleavage of the Au–N bond and protonation of the dimethylamino group. Compounds of general formulae [Au(Hdamp-C1)Cl(L)]+ (L = Hsaltsc−, vantsc−, mepyrtsc−), [Au(Hdamp-C1)Cl(L)]2+ (L = H2pydoxmetsc) or [Au(Hdamp-C1)(L)]2+ (L = Ptsc−, apRtsc−, R = H, Me, Ph) have been isolated and characterized. The presence of the σ-bonded 2-(dimethylaminomethyl)phenyl ligand is mandatory to prevent reduction of the gold(III) centre. The crystal structures of [Au(Hdamp-C1)Cl(Hsaltsc)](PF6) (3a), [Au(Hdamp-C1)Cl(mepyrtsc)]Cl (3c), [Au(Hdamp-C1)Cl(H2pydoxmetsc)]Cl2·MeOH (4), [Au(Hdamp-C1)(apPhtsc)]Cl2·2 MeOH (5c) and [Au(Hdamp-C1)(Ptsc)]Cl2· 1.5MeOH (6) have been elucidated, showing the gold atoms in distorted square-planar co-ordination environments.
The potentially O,N,S-tridentate ligands H2saltsc and H2pydoxmetsc co-ordinate in a bidentate fashion and do not incorporate the OH groups in the chelating framework, whereas HapRtsc or HPtsc co-ordinate in a tridentate manner. Generally, one or more hydrogen atoms of the heterocyclic ligands and/or the NMe2H+ group form hydrogen bridges in the solid state structures. The preliminary results of antiproliferation tests on tumor cells demonstrate the considerable cytotoxicity of these new gold complexes.
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