Solvent effects on the dimensionality of oxamato-bridged manganese(II) compounds

Abstract

Two new oxamate-containing manganese(II) complexes, [{Mn(H₂edpba)(H₂O)₂}₂]_n (1) and [Mn(H₂edpba)(dmso)₂]?dmso?CH₃COCH₃?H₂O (2) (H₄edpba = N,N′-ethylenediphenylenebis(oxamic acid) and dmso = dimethylsulfoxide), have been synthesized and the structures of 1 and 2 were characterized by single crystal X-ray diffraction. The structure of 1 consists of neutral honeycomb networks in which each manganese(II) is six-coordinate by one H₂edpba^2? ligand and two carboxylate–oxygens from two other H₂edpba^2? ligands building the equatorial plane. Each manganese is connected to its nearest neighbor through two carboxylate(monoprotonated oxamate) bridges in an anti-syn conformation. A dmso solution of single crystals of 1 was placed under acetone atmosphere affording 2, whereas putting 2 in equimolar water:ethanol mixture results in 1. The molecular structure of 2 is made up of mononuclear manganese(II) units which are interlinked by weak C–H?π and edge-to-face π-stacking interactions leading to supramolecular chains along the crystallographic b axis. Magnetic measurements reveal the occurrence of an antiferromagnetic coupling between two manganese(II) ions through anti-syn carboxylate bridges for 1 [J = ?1.18 cm^?1, the Hamiltonian being defined as H = ?J S₁^.S₂] and very weak intrachain ferromagnetic interactions in 2 [J = + 0.046 cm^?1, H = ?J ∑_iS_i^.S_i + 1].     

Inorganic-organic hybrid structures: open-framework iron phosphite-oxalates of varying dimensionality

Inorganic-organic hybrid structures belonging to the family of iron phosphite-oxalates have been prepared by employing hydrothermal methods. Their structures, determined by single-crystal X-ray diffraction, show a hierarchy within the family. While compounds I and II are low dimensional, III-VI have three-dimensional structures. Compound I has edge-shared ladders of iron phosphite with oxalate units hanging from the iron centers. Compound II has a layer structure with a honeycomb-like arrangement. The three-dimensional hybrid structures have the oxalate units connected in both in-plane and out-of-plane modes. A newly identified secondary building unit (SBU-7) and the oxalate units satisfying the valence and coordination requirements in the structure of V are novel and noteworthy structural features. Magnetic studies show that the dominant interactions between the iron centers are antiferromagnetic. Similar to other known hybrid structures, the phosphite-oxalate structures appear to show wide compositional and structural diversity.     

Direct in situ observation of increasing structural dimensionality during the hydrothermal formation of open-framework zinc phosphates

The first time-resolved in situ X-ray diffraction studies of the hydrothermal crystallisation of open-framework zinc phosphates reveal a pathway of sequential crystallisation involving formation of a metastable low dimensional chain phase before the growth of three-dimensional zeolitic architectures.     

Ligand effects on the stereochemical outcome of Suzuki-Miyaura couplings

The ligands associated with various Pd catalysts play a crucial role in determining the stereochemistry of cross-couplings between boronic acids and Z-alkenyl halides. A ligand on palladium has been found that leads to the desired products under mild conditions and in high yields that, in most cases, retain their Z-olefin geometry.     

Ligand effects on migratory insertion by the Heck reaction

Ligand effects on ethene insertion into a series of cationic phenylpalladium complexes with diverse bidentate phosphine ligands were studied by using the density functional methods. For the complexes with n-membered ring ligands (n = 4–6), a correlation was found between the ring size and the insertion barrier. This behavior was explained by considering the P—Pd—P bond angle. In the case of complexes with ligands of different rigidity, almost no difference was found for the insertion barriers. Furthermore, the bidentate phosphine ligand was systematically substituted by Me, t-Bu, F and Ph groups. It was found that the electron-donating substituents increased the insertion barrier, whereas the electron-withdrawing groups decreased it. The substantial increase of insertion barrier by the t-Bu group indicated that steric effect also had great effect on the migratory insertion.     

Ligand macrocycle structural effects on copper-dioxygen reactivity

With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the relative stability of isomeric mu-eta 2:eta 2-peroxo- and bis(mu-oxo)dicopper complexes, a comparative study was undertaken of the O2 reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R3-1,4,7-triazacyclodecane (R3TACD; R = Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr3TACDD). While the 3-coordinate complex [(iPr3TACDD)Cu]SbF6 was unreactive with O2, oxygenation of [(R3TACD)Cu(CH3CN)]X (R = Me or Bn; X = ClO4- or SbF6-) at -80 degrees C yielded bis(mu-oxo) species [(R3TACD)2Cu2(mu O)2]X2 as revealed by UV-vis and resonance Raman spectroscopy. Interestingly, unlike the previously reported system supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN), which yielded interconverting mixtures of peroxo and bis(mu-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161), low-temperature oxygenation of [(iPr3TACD)Cu(CH3CN)]SbF6 in a variety of solvents cleanly yielded a mu-eta 2:eta 2-peroxo product, with no trace of the bis(mu-oxo) isomer. The peroxo complex was characterized by UV-vis and resonance Raman spectroscopy, as well as an X-ray crystal structure (albeit of marginal quality due to disorder problems). Intramolecular attack at the alpha C-H bonds of the substituents was indicated as the primary decomposition pathway of the oxygenated compounds through examination of the decay kinetics and the reaction products, which included bis(mu-hydroxo)- and mu-carbonato-dicopper complexes that were characterized by X-ray diffraction. A rationale for the varying results of the oxygenation reactions was provided by analysis of (a) the X-ray crystal structures and electrochemical behavior of the Cu(I) precursors and (b) the results of theoretical calculations of the complete oxygenated complexes, including all ligand atoms, using combined quantum chemical/molecular mechanics (integrated molecular orbital molecular mechanics, IMOMM) methods. The size of the ligand substituents was shown to be a key factor in controlling the relative stabilities of the peroxo and bis(mu-oxo) forms, and the nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.     

Ligand effects on the size and purity of Pd nanoparticles

Palladium nanoparticles (Pd-NPs) were prepared by a single-phase reduction of palladium acetate in the presence of different organic thiol ligands. Sizes, size distributions and crystallinity of the Pd-NPs were determined by high resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction (XRD) while thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) was employed to measure their organic ligand to palladium ratios and to quantify contaminants. No systematic effect of the different ligands on the size and purity of the Pd-NPs was observed but 1^st-generation Frechet dendron thiols had an about 4 times larger foot-print at the surface of the NPs than the other thiol ligands.     

Ligand effects on Negishi couplings of alkenyl halides

Negishi couplings at olefinic centers do not always occur with the anticipated maintenance of stereochemistry. The source of erosion has been traced to the ligand, and a modified method has been developed that solves the stereochemical issue and significantly improves yields of Negishi couplings in general.     

Ligand effects on optical properties of CdSe nanocrystals

We report effects of various organic and inorganic ligands on optical properties of CdSe nanocrystals (NCs) by changes in their photoluminescence and absorbance spectra. Surface ligand loss occurring during dilution and purification of solutions of CdSe NCs leads to a decrease of photoluminescence intensity. The complex of trioctylphosphine with Se atoms on the surface of CdSe NCs is found responsible for the trap emission band that is red-shifted relative to the photoluminescence band edge.     

Ligand and substrate effects on the mechanism of rhodium-catalyzed hydrogenation of enamides

The rhodium-catalyzed hydrogenation reaction of enamides is studied computationally using the B3LYP/LACVP** level of theory for a range of ligands and substrates. Two model bidentate phosphine ligands, 1,2-bis(dimethylphosphino)ethane (DMPE) and (Z)-1,2-bis(dimethylphosphino) ethene (ZDMP), and two chiral bidentate phosphine ligands, (R,R)-MeDuPHOS and (R,R)-tetramethylbisoxaphospinane (TMBOP), are investigated in the hydrogenation of alpha-formamidoacrylonitrile as a model substrate. The ZDMP ligand is then studied for three additional substrates: N-(2-propenyl)formamide, (Z)-3-formamido-2-butenenitrile, and (E)-3-formamido-2-butenenitrile. The potential-energy surfaces calculated for the four ligands and alpha-formamidoacrylonitrile are in general agreement with previous computational studies using QM/MM (ONIOM) methods but show consistently higher relative barriers rather than lower. The calculated potential-energy surfaces of hydrogenations of various substrates with a common ligand indicate a mechanistic change based on substrate. The sequence of hydrogen transfer to the two olefinic carbons is calculated to change based on substrate electronics. This has a significant impact on the origins of enantioselectivity for such varied substrates as the first hydride transfer to the substrate is calculated to be irreversible for all substrates, independent of whether it occurs at the alpha or beta carbon of the olefin.     

Ligand effects on the structures of extended networks of dicyanamide-containing transition-metal ions

The structural characterization of a series of complexes of formula [M(dca)2L]n, where dca = dicyanamide, L = 1,10-phenanthroline (phen) [1-4] and 2,9-dimethylphenanthroline (2,9-dmphen) [9-12], and M = Mn (1 and 9), Fe (2 and 10), Co (3 and 11), and Ni (4 and 12), has revealed the effect of the presence of the methyl substituents of L on the resulting network. The structure of [Mn(dca)2(phen)]n (1), which is identical to those of 2-4, together with the investigation of its magnetic properties in the temperature range of 77-300 K were reported elsewhere. The use of the 4,7-dimethylphenanthroline (4,7-dmphen) as the co-ligand yielded a series of compounds of formula [M(dca)2(4,7-dmphen)]n [M = Mn (5), Fe (6), Co (7), and Ni (8)], which are isostructural with 1-4. Compounds containing phen (1-4) and 4,7-dmphen (5-8) are made of two-dimensional grids of metal atoms, each metal atom being linked to three other metal centers through single (three metal atoms involved) and double (two metal atoms involved) dca bridges exhibiting the mu-1,5 coordination mode. The isostructural complexes [M(dca)2(2,9-dmphen)]n (9-12) also have a sheetlike structure, the metal atoms in each layer being linked by two single and one double mu-1,5-dca units, as in 1-8. However, the topology of the network in 9-12 is different from that in 1-8 because of the different arrangement of the two single mu-1,5 dca bridges: cis in 1-8 versus trans in 9-12. The magnetic study of compounds 1-12 in the temperature range of 1.9-290 K has revealed the occurrence of weak ferromagnetic (M = Ni) and antiferromagnetic interactions (M = Mn, Fe, and Co). The different magnetic behavior in 1-12 was analyzed in the light of their structures, and the values of the magnetic interactions were compared to those of related systems.     

Insight into the construction of open-framework aluminophosphates

Over the past twenty five years, a class of open-framework aluminophosphates, denoted AlPOs, has been prepared with neutral zeolitic frameworks and anionic frameworks showing wonderfully complex structural and compositional diversity. An insight into the construction of open-framework AlPOs revealing their general structural features and topological chemistry is provided in this tutorial review, and the role of templating and the designed construction and synthesis of AlPOs are discussed.     

Photochemical properties of mixed-metal supramolecular complexes

We have prepared a series of mixed-metal trimetallic complexes of the form {[(bpy)₂Ru(BL)]₂MCl₂}ⁿ⁺(bpy 2,2′-bipyridine; BL 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq) or 2,3-bis(2-pyridyl)-benzoquinozaline (dpb); M Ir(III), Rh(III) or Os(II). This new class of trimetallic complexes can be prepared with a good yield, often as high as 95%, using our building block strategy. The central rhodium and iridium fragments of these trimetallic, namely [M(BL)₂Cl₂]⁺, have been shown in our laboratory to be capable of delivering multiple electrons, “stored” on the bridging ligand π* orbitals, to a substrate as they functioned as electrocatalysts for the reduction of carbon dioxide to formate. The two terminal ruthenium metals are good light absorbers designed to give rise to photochemical activity. These bichromophoric systems should be capable of absorbing two photons of light, each giving rise to a desired photochemical reaction, namely excited-state electron transfer. Thus these systems form the basis of a molecular device for photoinitiated electron collection. The properties of these supramolecular complexes have been tuned by variation in the central metal and bridging ligand. Comparison of this array of nine complexes is described herein.     

Ligand effects in the rhodium acetate catalyzed hydrogen transfer

In the presence of some coordinating ligands, rhodium(II) acetate dimer Rh₂(OCOCH₃)₄, shows a good catalytic activity towards the hydrogen transfer from 2-propanol to cyclohexanone and some other unsaturated compounds. The catalytic activity is the function of the nature of ligands and their ratio to Rh₂(OCOCH₃)₄. The most active system is obtained using Rh₂(OCOCH₃)₄ and 2,2-bipyridine in molar ratio 1:6.

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, (II), Rh₂(OCOCH₃)₄, 2- . Rh₂(OCOCH₃)₄. , Rh₂(OCOCH₃)₄ 2,2- 16.

     

Ligand and electrolyte effects in the electroreduction of copper-substituted heteropolyanions

Focus is put on the electrochemical behaviours of the Cu²⁺ centre in several copper-substituted heteropolyanions. These behaviours are found to depend clearly on the structure and composition of the precursor lacunary oxometalates. In sulfate medium, complete demetallation is easily observed upon reduction. However, quantitative recovery of the complex is obtained by reoxidation. Therefore, after the deposition process, the lacunary heteropolyanions retain their original properties and can be used again to capture metal ions in solution. Various behaviours of the copper deposition process are observed depending on the presence of chloride in the supporting electrolyte.     

Ligand effects in the hydroformylation of acrylonitrile by cobalt carbonyl

The hydroformylation of acrylonitrile (VCN) using Co₂(CO)₈/L (L  HN(CH₂CN)₂, H₂C(CH₂)₃NMe, Me₂N(CH₂)₂NMeH, PPh₃, and PCy₃) has been examined in methanol solvent. Four reaction pathways are observed which are dependent on L. With no L or with L  HN(CH₂CN)₂, the reaction produces the desired acetal (MeO)₂CHCH₂CH₂CN. For the more basic amines the reaction produces ～ 50% yields of hydrodimerization products NCCHMe(CH₂)₂CN/NC(CH₂)₄CN in a 10/1 ratio and an ～ 30% yield of the hydrogenation product CH₃CH₂CN. These reactions are shown to be metal catalyzed. The main reaction for Co₂(CO)₈/PR₃ catalyzed systems appears to be a classical Michael addition reaction of the solvent, methanol, with acrylonitrile to give MeOCH₂CH₂CN. Evidence is given to show that this reaction is catalyzed by phosphine which has dissociated under reaction conditions and not by a ligated cobalt complex.     

Ligand influence on connectivity and processing: magnetic crystals based on metalloceniums and films of TCNE-based magnets (TCNE=tetracyanoethylene)

Molecular materials built from coordination complexes exhibit properties that can be explained through intermolecular electronic interactions driven by the ligand moieties. The nature of the ligand in the precursor molecules governs the connectivity of the magnetic phases and the possibility of producing them by using a gas-phase process. Metallocenium, metal bisdithiolate materials, and solvated and solvent-free [M(tcne)2] (tcne=tetracyanoethylene) magnets illustrate such features.     

Multitechnique investigation of the physisorption and thermal treatment of mixed-metal clusters on carbon

The three clusters [Ru6PtC(CO)16(COD)] (1), [Ru5PtC(CO)14(COD)] (2) (COD = 1,5-cyclooctadiene), and (NEt4)2[Pd6Ru6(CO)24] (3) were physisorbed onto an active carbon support and used as molecular precursors for supported particles. The samples before and after thermal treatment at 300 degrees C were characterized by a combination of SIMS, XPS, SEM and XRD. It was found that the clusters are molecularly intact, but present in the form of agglomerates on the support at the end of the physisorption step. After thermal treatment, the ligands were lost in all cases, leaving on the surface bimetallic particles containing both starting metals (Ru-Pt in the case of 1 and 2, and Ru-Pd in the case of 3). The presence of the counterion was evidenced for 3, before, but not after, heating. Molecular clusters are thus interesting precursors for the preparation of supported bimetallic phases, and SIMS is the ideal technique to characterize them at each step of their preparation.     

Nonlinear magneto-optical effects and photomagnetism of electrochemically synthesized molecule-based magnets

This article describes novel optical functionalities such as photomagnetic effects and magnetization-induced second harmonic generation (MSHG) in several cyano-bridged metal assemblies. Single crystal- and film-types of a cyano-bridged Cu–Mo bimetallic assembly, , were electrochemically prepared. When this compound was irradiated with light, spontaneous magnetization with a Curie temperature (T _C) of 23 K was observed. Electrochemically prepared Fe^II[Cr^III(CN)₆]_2/3·5H₂O thin film, which was a ferromagnet with T _C=21 K, showed photoreduced magnetization. This photomagnetism is due to the change of ferromagnetic coupling between Fe^II and Cr^III. MSHG was observed in Cs^ICo^II[Cr^III(CN)₆]·0.5H₂O. This -type Prussian blue analog-based magnet is proven to be a piezoelectric ferromagnet, i.e., condensed matter with both piezoelectric and ferromagnetism. This MSHG is due to the coupling between a piezoelectric structure of and ferromagnetism with a T _C of 46 K.