Role of exposed metal sites in hydrogen storage in MOFs

The role of exposed metal sites in increasing the H2 storage performances in metal-organic frameworks (MOFs) has been investigated by means of IR spectrometry. Three MOFs have been considered: MOF-5, with unexposed metal sites, and HKUST-1 and CPO-27-Ni, with exposed Cu(2+) and Ni(2+), respectively. The onset temperature of spectroscopic features associated with adsorbed H2 correlates with the adsorption enthalpy obtained by the VTIR method and with the shift experienced by the H-H stretching frequency. This relationship can be ascribed to the different nature and accessibility of the metal sites. On the basis of a pure energetic evaluation, it was observed that the best performance was shown by CPO-27-Ni that exhibits also an initial adsorption enthalpy of -13.5 kJ mol(-1), the highest yet observed for a MOF. Unfortunately, upon comparison of the hydrogen amounts stored at high pressure, the hydrogen capacities in these conditions are mostly dependent on the surface area and total pore volume of the material. This means that if control of MOF surface area can benefit the total stored amounts, only the presence of a great number of strong adsorption sites can make the (P, T) storage conditions more economically favorable. These observations lead to the prediction that efficient H2 storage by physisorption can be obtained by increasing the surface density of strong adsorption sites.     

Synthesis,structures and properties of three copper complexes created via in situ ligand cross-coupling reactions

Three copper complexes {[Cu₂(L¹)₂]·I₃} _n (1), [Cu(L²)₂] (2), and [Cu₂I₂(L³)₂(MBI)₂] (3) (MBI = 2-mercaptobenzimidazole, L¹ = N-(benzothiazol-2-yl)acetamidine anion, L² = N-(thiazol-2-yl) acetamidine anion, L³ = 3-methyl-[1,2,4]thiadiazolo[4,5-a]benzimidazole) have been synthesized solvothermally by the reactions of CuI with 2-benzothiazolamine, 2-aminothiazole and 2-mercaptobenzimidazole (MBI), respectively, in acetonitrile. In situ C–N (or C–S) cross-coupling ligand reactions were observed in all three complexes, and hypothetical reaction mechanisms are proposed for the formation of the ligands and their complexes. The single-crystal X-ray structural analysis reveals that both the Cu(II) and Cu(I) atoms are located in pseudo-tetrahedral environments in complex 1, and L¹ acts as a double bidentate ligand which coordinates with the Cu(I) and Cu(II) atoms to form a 1D coordination polymer. Unlike complex 1, the Cu(II) atom in complex 2 is in a square planar geometry, coordinated by two L² ligands with relatively small steric hindrance. In complex 3, the Cu(I) atoms have a distorted tetrahedral geometry, being coordinated by one nitrogen atom from L³, two sulfur atoms of MBI ligands, and one iodide. The sulfur atoms from MBI ligands bridge two Cu(I) atoms to form a binuclear complex. All three complexes exhibit relatively high thermal stabilities. Complex 1 displays intense fluorescence emission at 382 nm and complex 3 displays two intense fluorescence emissions at 401 and 555 nm.     

Rare earth metal benzyl complexes bearing bridged-indenyl ligand for highly active polymerization of methyl methacrylate

The novel anionic bridged-indenyl rare earth metal benzyl complexes [{C₉H₆SiMe₂(CH₂)₂SiMe₂C₉H₆}Ln(CH₂C₆H₄-p-^tBu)₂][Li(THF)₄] (Ln = Y (1), Lu (2)) were synthesized by an acid-base reaction of C₉H₇SiMe₂(CH₂)₂SiMe₂C₉H₇ with one equiv. of rare earth metal trisbenzyl complexes, which were formed in situ from the reaction of anhydrous LnCl₃ with LiCH₂C₆H₄-p-^tBu in 1:3 molar ratio in THF. The complexes were characterized by elemental analysis, NMR spectroscopy, FT-IR spectroscopy, and X-ray structural analysis in the case of 2. Both complexes are active for the polymerization of methyl methacrylate (MMA) to afford high molecular weight and narrow molecular weight distribution PMMA. The molecular weights of PMMA could be controlled using 1 as a polymerization initiator in chlorobenzene at −40 °C.     

Novel metal complexes containing a chiral trinitrogen isoindoline-based pincer ligand: in situ synthesis and structural characterization

The first synthesis and characterization of metal coordinated complexes containing in situ prepared chiral trinitrogen 1,3-bis(4,5-dihydrooxazol-2-ylimino)isoindoline-based pincer ligands are reported. Two zinc complexes, isolated as Zn(L)(2), where L = 1,3-bis(4,5-dihydro-4-(R)-phenyloxazol-2-ylimino)isoindoline ((R,R)-5) or 1,3-bis(4,5-dihydro-4-(S)-iso-propyloxazol-2-ylimino)isoindoline ((S,S)-6), respectively, are reported. Complexes Zn((R,R)-5)(2) and Zn((S,S)-6)(2) were prepared in situ through the condensation of phthalonitrile with enantiopure 2-amino-4-(R)-phenyloxazoline ((R)-3) or 2-amino-4-(S)-iso-propyloxazoline ((S)-4) in the presence of ZnCl(2) at 80 °C in dry toluene over 3-4 days. The characterizations of Zn((R,R)-5)(2) and Zn((S,S)-6)(2) in both the solid (X-ray crystallography) and solution (multinuclear NMR spectroscopy) states are reported.     

Dynamics of the molecular hydrogen ligand in metal complexes

The recent discovery that certain metal complexes can bind hydrogen in molecular form as they do other small molecules has presented a unique opportunity for neutron scattering to assist in the determination of electronic details of the novel chemical bond formed between this dihydrogen ligand and the metal. This can be accomplished by comparative studies of the barriers to rotation for the dihydrogen ligand in various complexes where either the metal center or the other metal ligands are changed together with appropriate theoretical analyses. This information can be extracted from vibrational and rotational inelastic neutron scattering spectra. Results from a wide variety of molecular hydrogen complexes are reviewed and their implications for the metal—dihydrogen chemical bond are discussed along with reference to various theortical approaches to this problem.     

In situ generation of active sites in olefin metathesis

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Br?nsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.     

Highly active double metal cyanide complexes： Effect of central metal and ligand on reaction of epoxide/CO2

Various novel double metal cyanide (DMC) catalysts were successfully prepared by modifying the central metal (M) and one of cyanide ion (CN-) in Zna[M(CN)b]c complex. Such modifications have significant impact on the catalytic efficiency as well as the polymer selectivity for the reaction of PO/CO2. Zn–Ni(Ⅱ) DMC is a potential catalyst for alternating copolymerization of PO/CO2, and DMC catalysts based on Zn3[Co(CN)5X]2 (X = Br-and N3-) exhibit moderate efficiency for the production of polycarbonates. This research presents the preliminary exploration of novel DMC complex via chemical modification of its central metal and ligand.     

New optically active amido-phosphinite ligand and ruthenium complexes

The hydrogenation of acetoacetanilide in the presence of optically active ruthenium catalysts leads to 3-hydroxy-N-phenylbutanamide in good yield and high enantioselectivity (ee>95%). Its direct phosphinylation with chlorodiphenylphosphine affords a new bifunctional amido-phosphinite ligand that can easily be coordinated to [(arene)RuCl₂] and to the CpRuCl(PPh₃) moiety with very good stereoselectivity from CpRuCl(PPh₃)₂.     

Mixed ligand complexes of coumarilic acid/nicotinamide with transition metal complexes

Coumarilate–nicotinamide complexes of Co^II and Zn^II were synthesized and investigated by elemental analysis, magnetic susceptibility, solid state UV–Vis, direct injection probe mass spectra, FTIR spectra, thermoanalytic TG-DTG/DTA, and crystal X-ray diffraction methods. It was obtained that both complex structures contain 2 mol aqua ligands, 2 mol coumarilate (CCA^?) and 2 mol nicotinamide (NA) ligands per formula unit. The CCA^? and NA ligands were bonded to metal cations as monodentate through acidic oxygen and nitrogen of pyridine ring, respectively. Thermal decomposition of each complex starts with dehydration and continue removing of 1 mol NA ligand. The thermal dehydration of the complexes takes place in one or two steps. The decomposition mechanism and thermal stability of the investigated complexes are interpreted in terms of their structures. The final decomposition products are found to be metal oxides.     

Coinage metal complexes of a boron-substituted soft scorpionate ligand

An improved synthesis of lithium phenyltris(methimazolyl)borate, Li[PhTm(Me)], (methimazole = 1-methylimidazole-2-thione) is described, and the structure of the methanol-solvated [Li(OHMe)4][PhTm(Me)] has been determined. The syntheses and characterization of complexes [M(PhTm(Me))(PR3)] (M = Cu, Ag, Au; R = Et, Ph;) are reported, and the complexes [Cu(PhTm(Me))(PPh3)], [Ag(PhTm(Me))(PEt3)] and [Au(PhTm(Me))(PEt3)] are crystallographically characterized, showing a progression from pseudo-tetrahedral geometry (copper, S3P coordination) to trigonal planar geometry (silver, S2P coordination) to linear geometry (gold, SP coordination). In addition, the copper(I) and silver(I) triphenylphosphine complexes of the adventitiously formed phenylhydrobis(methimazolyl)borate ligand, [M(PhBm(Me))(PPh3)], have been crystallographically characterized, showing both species to have a trigonal planar primary coordination sphere, with a secondary M...H-B interaction. Finally, reaction of copper(II) chloride with Li[PhTm(Me)] results in formation of a compound analyzing as [Cu(II)(PhTm(Me))Cl], although its extreme insolubility and marked instability have precluded its complete characterization. Attempts to prepare this by ultra-slow diffusion of the reactants through solvent blanks has led to isolation of a mixed-valence copper(I/II) methimazolate cluster, [Cu(I)10Cu(II)2(mt)12Cl2] and a copper(I) dimeric complex [Cu2(PhTm(Me))2], indicating that copper(II) ions oxidatively decompose the phenyltris(methimazolyl)borate anion.     

Transition metal complexes of a potential anticancer quinazoline ligand

A new heterocycle, namely 2-(furyl)-3-(furfuralimino)-1,2-dihydroquinazolin-4(3H)-one (ffdq) was formed by the condensation of 2-aminobenzoylhydrazide with furfural and characterized by physico-chemical, spectroscopic, and single crystal X-ray diffraction studies. A series of complexes of ffdq have been synthesized and characterized by physico-chemical, spectroscopic, and thermal studies. According to the i.r. and ¹H-n.m.r. spectra ffdq behaves as a bidentate ligand coordinating through quinazoline oxygen and azomethine nitrogen. The FAB-mass spectrum of the Cd(II) complex indicates the monomeric nature of this complex. The X-band e.p.r. spectrum of the Cu(II) complex and thermal stabilities of the Co(II) and Ni(II) complexes are discussed.     

Syntheses,structures and properties of four second-sphere coordination complexes via metal halide anion and naphthalene-based ligand

Second-sphere coordination refers to any intermolecular interactions with the ligands directly bound to the primary coordination sphere of a metal ion. Four supramolecular complexes, 0.5[L·2H]²⁺·0.5[MCl₄]^2?·[CH₃OH]·0.5[CH₂Cl₂] (M = Co, crystal 1; M = Mn, crystal 2), 0.5[L·2H]²⁺·0.5[ZnBr₄]^2?·[CH₃OH]·0.5[CH₂Cl₂] (crystal 3), and 0.5[L·2H]²⁺·0.5[Cu₂Br₄]^2?·H₂O (crystal 4), based on naphthalene-based ligand N,N,N′,N′-tetra-p-methylnaphthyl-ethanediamine (L), have been synthesized. X-ray analysis reveals that 1–3 are isostructural, in which the methanol molecules are bridges, connecting the protonated L and metal chloride anions via N–H?O and O–H?Cl (Br) interactions to construct the host framework, and forming X-shaped cavity accessible for the inclusion of weakly polar guest molecules of dichloromethane. Dichloromethane is connected with the host framework through van der Waals forces. In 4, a dinuclear anion [Cu₂Br₄]^2? is connected with the ligand through N–H?Br interactions, in which the water molecules are accommodated between chains formed by the ligand and [Cu₂Br₄]^2?. Structure stability, thermal analysis, and photoluminescent properties were studied for 1–4.     

Insertion of unnatural metal ligand in the heme pocket of nitrophorin through protein semi-synthesis: Toward biomimicking binuclear active sites

Herein, we report a semi-synthetic strategy affording a nitrophorin 2(NP2) variant with a N,N-bis(2-pyridylmethyl)amine(Dpa) ligand as sidechain selectively installed at position 27, which was assembled from a synthetic peptide thioester bearing the Dpa ligand and an expressed protein segment via native chemical ligation. The semi-synthetic NP2 was able to accept the natural heme b cofactor and the Dpa ligand was able to bind Cu(II)/Fe(III) ions, leading to heteronuclear active site.     

Formation of stable iron/cobalt NHC complexes via unexpected ring opening and in situ generation of a tridentate ligand

The reactions of FeCl(2) and CoCl(2) with a bistriazolium salt yields the NHC complexes [Fe(III)()(2)]I·H(2)O, [Fe(III)()(2)]PF(6)·CH(3)CN and [Co(III)()(2)]I·0.5CH(3)CN, through an unusual ring opening of one of the triazoyl rings, which leads to the formation of C,N,O tridentate ligands or . Furthermore, a Fe(ii) species [Fe(II)()(2)]·CH(2)Cl(2) was also obtained.     

Structural studies of metal ligand complexes by ion mobility-mass spectrometry

Collision cross sections (CCS) have been measured for three salen ligands, and their complexes with copper and zinc using travelling-wave ion mobility-mass spectrometry (TWIMS) and drift tube ion mobility-mass spectrometry (DTIMS), allowing a comparative size evaluation of the ligands and complexes. CCS measurements using TWIMS were determined using peptide and TAAH calibration standards. TWIMS measurements gave significantly larger CCS than DTIMS in helium, by 9 % for TAAH standards and 3 % for peptide standards, indicating that the choice of calibration standards is important in ensuring the accuracy of TWIMS-derived CCS measurements. Repeatability data for TWIMS was obtained for inter- and intra-day studies with mean RSDs of 1.1 % and 0.7 %, respectively. The CCS data obtained from IM-MS measurements are compared to CCS values obtained via the projection approximation, the exact hard spheres method and the trajectory method from X-ray coordinates and modelled structures using density functional theory (DFT) based methods.     

s-Block metal complexes of a bulky, donor-functionalized allyl ligand

Crystallographic and NMR spectroscopic studies on allyl complexes of lithium, potassium and magnesium containing an O-donor functionality are described.     

Diarylethene-containing cyclometalated platinum(II) complexes: tunable photochromism via metal coordination and rational ligand design

The synthesis, characterization, electrochemistry, photophysics and photochromic behavior of a new class of cyclometalated platinum(II) complexes [Pt(C(∧)N)(O(∧)O)] (1a-5a and 1b-5b), where C(∧)N is a cyclometalating 2-(2'-thienyl)pyridyl (thpy) or 2-(2'-thienothienyl)pyridyl (tthpy) ligand containing the photochromic dithienylethene (DTE) unit and O(∧)O is a β-diketonato ligand of acetylacetonato (acac) or hexafluoroacetylacetonato (hfac), have been reported. The X-ray crystal structures of five of the complexes have also been determined. The electrochemical studies reveal that the first quasi-reversible reduction couple, and hence the nature of lowest unoccupied molecular orbital (LUMO) of the complexes, is sensitive to the nature of the ancillary O(∧)O ligands. Upon photoexcitation, complexes 1a-3a and 1b-3b exhibit drastic color changes, ascribed to the reversible photochromic behavior, which is found to be sensitive to the substituents on the pyridyl ring and the extent of π-conjugation of the C(∧)N ligand as well as the nature of the ancillary ligand. The thermal bleaching kinetics of complex 1a has been studied in toluene at various temperatures, and the activation barrier for the thermal cycloreversion of the complex has been determined. Density functional theory (DFT) calculations have been performed to provide an insight into the electrochemical, photophysical and photochromic properties.     

Catalysis of the Suzuki reaction by acyclic diaminocarbene palladium complexes generated in situ

Acyclic diaminocarbene palladium complexes generated in situ by nucleophilic attack of morpholine, 4-nitrophenylhydrazine, or benzhydrazide on bis(isocyanide) palladium(II) complexes catalyzed the Suzuki reaction of 4-iodo- or 4-bromoanisole with phenylboronic acid. Morpholine turned out to be the best catalyst modifier. The cross coupling reaction under fairly mild conditions (reflux in ethanol in the presence of potassium carbonate) in 2 h afforded 4-methoxybiphenyl whose yield was insignificantly lower than in the presence of preliminarily prepared catalyst. Neither preliminary degassing nor protection from atmospheric moisture and oxygen was necessary.     

Solvothermal synthesis of new open-framework metal phosphites with structure-directing agents generated in situ

By using amide molecules and a urea derivative as the source of structure-directing agents (SDAs), three open-framework metal phosphites, (CH₃NH₃)₂·[Be₃(HPO₃)₄] (1), (CH₃CH₂NH₃)₂·[Be₃(HPO₃)₄] (2), and [(CH₃)₂NH₂]_1.5·(H₃O)_0.5·[Al₄(HPO₃)₇(H₂O)₃]·(H₂O)_0.5 (3), have been synthesized under solvothermal conditions. Compound 1 has left- and right-handed helical channels running along the [001] directions. Compound 2 possesses zigzag 12-ring channels running along the [001] direction. Compound 3 contains multidirectional 12-ring channels running along the [100], [010], and [110] directions. It is noteworthy that the direct use of methylamine, ethylamine and dimethylamine as SDAs can't produce corresponding single crystals of compounds 1–3, demonstrating the slow release of amines from amide molecules and urea derivative is a key process for the crystal growth.