Hydrogen storage properties and neutron scattering studies of Mg2(dobdc)--a metal-organic framework with open Mg2+ adsorption sites

The hydrogen storage properties of Mg(2)(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate), a metal-organic framework possessing hexagonal one-dimensional channels decorated with unsaturated Mg(2+) coordination sites, have been examined through low- and high-pressure adsorption experiments, infrared spectroscopy, and neutron scattering studies.     

Selective binding of O2 over N2 in a redox-active metal-organic framework with open iron(II) coordination sites

The air-free reaction between FeCl(2) and H(4)dobdc (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe(2)(dobdc)·4DMF, a metal-organic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a Brunauer-Emmett-Teller (BET) surface area of 1360 m(2)/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated Fe(II) centers. Gas adsorption isotherms at 298 K indicate that Fe(2)(dobdc) binds O(2) preferentially over N(2), with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O(2) molecule per two iron centers. Remarkably, at 211 K, O(2) uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O(2) molecule per iron center. Mo?ssbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O(2) at low temperature and complete charge transfer to form iron(III) and O(2)(2-) at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O(2) bound to iron in a symmetric side-on mode with d(O-O) = 1.25(1) ? at low temperature and in a slipped side-on mode with d(O-O) = 1.6(1) ? when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe(2)(dobdc) to be a promising material for the separation of O(2) from air at temperatures well above those currently employed in industrial settings.     

Carbon dioxide adsorption by physisorption and chemisorption interactions in piperazine-grafted Ni(2)(dobdc) (dobdc = 1,4-dioxido-2,5-benzenedicarboxylate)

The metal-organic framework Ni(2)(dobdc) (CPO-27-Ni, where dobdc = 1,4-dioxido-2,5-benzenedicarboxylate) has been post-synthetically modified with piperazine (pip) - a known 'accelerator' to improve the kinetics of CO(2) uptake in alkanolamine solvents for chemical absorption - and the impact of the modification on the CO(2) uptake and selectivity over N(2) has been probed. While the modified framework, Ni(2)(dobdc)(pip)(0.5) (pip-CPO-27-Ni), exhibits a lower uptake of CO(2) compared with the non-grafted material, the selectivity for CO(2) over N(2) at 25 °C and at pressures pertinent to post-combustion flue gas capture (0.1-0.15 bar) is enhanced. Mechanistically, the interaction between the CO(2) molecules and the free amine sites in pip-CPO-27-Ni occurs via physisorption and chemisorption interactions, in which CO(2) binds to the framework with an isosteric heat of adsorption (-Q(st)) of 40.5 kJ mol(-1) at very low coverage (P = 0.033 mbar), followed by binding at a higher heat of adsorption (-Q(st) = 46.2 kJ mol(-1) at P = 3.55 mbar). Pure water adsorption isotherms revealed a two-step mechanism for uptake in CPO-27-Ni, consistent with adsorption into the first and second hydration spheres of Ni(2+) followed by subsequent uptake via physisorption into the pores. Additional steric hindrance in pip-CPO-27-Ni results in a single step only. The working capacity over multiple cycles was also investigated using a temperature swing adsorption process which revealed reversible CO(2) adsorption and desorption of 10 wt% over 10 cycles.     

Spin-crossover and liquid crystal properties in 2D cyanide-bridged Fe(II)-M(I/II) metalorganic frameworks

Novel two-dimensional heterometallic Fe(II)-M(Ni(II), Pd(II), Pt(II), Ag(I), and Au(I)) cyanide-bridged metalorganic frameworks exhibiting spin-crossover and liquid crystal properties, formulated as {FeL(2)[M(I/II)(CN)(x)](y)}·sH(2)O, where L are the ligands 4-(4-alkoxyphenyl)pyridine, 4-(3,4-dialkoxyphenyl)pyridine, and 4-(3,4,5-trisalkoxyphenyl)pyridine, have been synthesized and characterized. The physical characterization has been carried out by means of EXAFS, X-ray powder diffraction, magnetic susceptibility, differential scanning measurements, and Mo?ssbauer spectroscopy. The 2D Fe(II) metallomesogens undergo incomplete and continuous thermally induced spin transition at T(1/2) ≈ 170 K and crystal-to-smectic transition above 370 K.     

Metal-organic frameworks as adsorbents for hydrogen purification and precombustion carbon dioxide capture

Selected metal-organic frameworks exhibiting representative properties--high surface area, structural flexibility, or the presence of open metal cation sites--were tested for utility in the separation of CO(2) from H(2) via pressure swing adsorption. Single-component CO(2) and H(2) adsorption isotherms were measured at 313 K and pressures up to 40 bar for Zn(4)O(BTB)(2) (MOF-177, BTB(3-) = 1,3,5-benzenetribenzoate), Be(12)(OH)(12)(BTB)(4) (Be-BTB), Co(BDP) (BDP(2-) = 1,4-benzenedipyrazolate), H(3)[(Cu(4)Cl)(3)(BTTri)(8)] (Cu-BTTri, BTTri(3-) = 1,3,5-benzenetristriazolate), and Mg(2)(dobdc) (dobdc(4-) = 1,4-dioxido-2,5-benzenedicarboxylate). Ideal adsorbed solution theory was used to estimate realistic isotherms for the 80:20 and 60:40 H(2)/CO(2) gas mixtures relevant to H(2) purification and precombustion CO(2) capture, respectively. In the former case, the results afford CO(2)/H(2) selectivities between 2 and 860 and mixed-gas working capacities, assuming a 1 bar purge pressure, as high as 8.6 mol/kg and 7.4 mol/L. In particular, metal-organic frameworks with a high concentration of exposed metal cation sites, Mg(2)(dobdc) and Cu-BTTri, offer significant improvements over commonly used adsorbents, indicating the promise of such materials for applications in CO(2)/H(2) separations.     

2:2 Fe(III):ligand and "adamantane core" 4:2 Fe(III):ligand (hydr)oxo complexes of an acyclic ditopic ligand

A bis-hydroxo-bridged diiron(III) complex and a bis-mu-oxo-bis-mu-hydroxo-bridged tetrairon(III) complex are isolated from the reaction of 2,6-bis((N,N'-bis-(2-picolyl)amino)methyl)-4-tert-butylphenol (Hbpbp) with iron perchlorate in acidic and neutral solutions respectively. The X-ray structure of the dinuclear complex [{(Hbpbp)Fe([mu-OH)}(2)](ClO(4))(4).2C(3)H(6)O (1.2C3H6O) shows that only one of the metal-binding cavities of each ligand is occupied by an iron(III) atom and two [Fe(Hbpbp)]3+ units are linked together by two hydroxo bridging groups to form a [Fe(III)-(mu-OH)](2) rhomb structure with Fe...Fe = 3.109(1)A. The non-coordinated tertiary amine of Hbpbp is protonated. Magnetic susceptibility measurements show a well-behaved weak antiferromagnetic coupling between the two Fe(III) atoms, J= -8 cm(-1). The tetranuclear complex [(bpbp)(2)Fe(4)(mu-O)(2)(mu-OH)(2)](ClO(4))(4)(2) was isolated as two different solvates .4CH(3)OH and .6H(2)O with markedly different crystal morphologies at pH ca. 6. Complex .4CH(3)OH forms red cubic crystals and .6H(2)O forms green crystalline platelets. The Fe(4)O(6) core of shows an adamantane-like structure: The six bridging oxygen atoms are provided by the two phenolato groups of the two bpbp(-) ligands, two bridging oxo groups and two bridging hydroxo groups. The hydroxo and oxo ligands could be distinguished on the basis of Fe-O bond lengths of the two different octahedral iron sites: Fe-mu-OH, 1.953(5), 2.013(5)A and Fe-mu-O, 1.803(5), 1.802(5)A. The difference in ligand environment is too small for allowing Mossbauer spectroscopy to distinguish between the two crystallographically independent Fe sites. The best fit to the magnetic susceptibility of .4CH(3)OH was achieved by using three coupling constants J(Fe-OPh-Fe)= 2.6 cm(-1), J(Fe-OH-Fe)=-0.9 cm(-1), J(Fe-O-Fe)=-101 cm(-1) and iron(III) single ion ZFS (|D|= 0.15 cm(-1)).     

Detailed investigations of phase transitions and magnetic structure in Fe(III), Mn(II), Co(II) and Ni(II) 3,4,5-trihydroxybenzoate (gallate) dihydrates by neutron and X-ray diffraction

The effect of cation valency on the complex structures of divalent and trivalent transition metal gallates has been examined using a combination of neutron and synchrotron X-ray powder diffraction, single-crystal X-ray diffraction and XANES spectroscopy. In the divalent frameworks, M(C(7)H(4)O(5))·2H(2)O (M = Mn, Co and Ni), it was found that charge balance was achieved via the presence of protons on the meta-hydroxyl groups. It was also established that these compounds undergo a discontinuous phase transition at lower temperatures, which is driven by the position of the extra-framework water molecules in these materials. By contrast, in the trivalent Fe gallate, Fe(C(7)H(3)O(5))·2H(2)O, it was found that the stronger bonding between the meta-hydroxy oxygen and the cations leads to a weakening of the bond between this oxygen and its proton. This is turn is thought to lead to stronger hydrogen bonding with the extra-framework water. The lattice water is disordered in the Fe(III) case, which prevents the phase transition found in the M(II) gallates. Refinement against the neutron diffraction patterns also revealed that the relatively mild microwave synthesis of gallate frameworks in D(2)O led to an extensive deuteration of the ortho-hydrogen sites on the aromatic ring, which may suggest a more versatile method of deuterating aromatic organics. The antiferromagnetic structure of Co gallate has also been determined.     

Temperature and pressure evolution of the crystal structure of A(x)(Fe(1-y)Se)2 (A = Cs, Rb, K) studied by synchrotron powder diffraction

Temperature-dependent synchrotron powder diffraction on Cs(0.83)(Fe(0.86)Se)(2) revealed first-order I4/m to I4/mmm structural transformation around 216 °C associated with a disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of the intercalated alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs(0.83)(Fe(1-y)Se)(2), Rb(0.85)(Fe(1-y)Se)(2), and K(0.8)(Fe(1-y)Se)(2) (y ~ 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and nonsuperconductive states.     

Synthesis, magnetic properties, and magnetic structure of a natrochalcite structural variant, KM(II)2D3O2(MoO4)2 (M = Mn, Fe, or Co)

We report the syntheses, crystal structures, and magnetic properties of KMn(2)(H(3)O(2))(MoO(4))(2) (MnH), KMn(2)(D(3)O(2))(MoO(4))(2) (MnD), KFe(2)(H(3)O(2))(MoO(4))(2) (FeH), KFe(2)(D(3)O(2))(MoO(4))(2) (FeD), KCo(2)(H(3)O(2))(MoO(4))(2) (CoH), and KCo(2)(D(3)O(2))(MoO(4))(2) (CoD), and the magnetic structures of MnD and FeD. They belong to the structural variant (space group I2/m) of the mineral natrochalcite NaCu(2)(H(3)O(2))(SO(4))(2) (space group C2/m) where the diagonal within the ac-plane of the latter become one axis of the former. The structure of MnD, obtained from Rietveld refinement of a high-resolution neutron pattern taken at 300 K, consists of chains of edge-sharing octahedra bridged by MoO(4) and D(3)O(2) to form layers, which are connected to K through the oxygen atoms to form the three-dimensional (3D)-network. The X-ray powder diffraction patterns of the other two compounds were found to belong to the same space group with similar parameters. The magnetic susceptibilities of MnH and FeH exhibit long-range ordering of the moments at a Ne?el temperature of 8 and 11 K, respectively, which are accompanied by additional strong Bragg reflections in the neutron diffraction in the ordered state, consistent with antiferromagnetism. Analyses of the neutron data for MnD and FeD reveal the presence of both long- and short-range orderings and commensurate magnetic structures with a propagation vector of (?, 0, ?). The moments are antiferromagnetically ordered within the chains with alternation between chains to generate four nonequivalent nuclear unit cells. For MnD the moments are perpendicular to the chain axis (b-axis) while for FeD they are parallel to the b-axis. The overall total is a fully compensated magnetic structure with zero moment in each case. Surprisingly, for KCo(2)(D(3)O(2))(MoO(4))(2) neither additional peaks nor increase of the nuclear peaks' intensities were observed in the neutron diffraction patterns below the magnetic anomaly at 12 K which was identified to originate from a small quantity of a ferromagnetic compound, Co(2)(OH)(2)MoO(4).     

Hydrothermal synthesis in the system Ni(OH)(2)-NiSO(4): nuclear and magnetic structures and magnetic properties of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2)

We present the synthesis, characterization by DT-TGA and IR, single crystal X-ray nuclear structure at 300 K, nuclear and magnetic structure from neutron powder diffraction on a deuterated sample at 1.4 K, and magnetic properties as a function of temperature and magnetic field of Ni(3)(OH)(2)(SO(4))(2)(H(2)O)(2). The structure is formed of chains, parallel to the c-axis, of edge-sharing Ni(1)O(6) octahedra, connected by the corners of Ni(2)O(6) octahedra to form corrugated sheets along the bc-plane. The sheets are connected to one another by the sulfate groups to form the 3D network. The magnetic properties measured by ac and dc magnetization, isothermal magnetization at 2 K, and heat capacity are characterized by a transition from a paramagnet (C = 3.954 emu K/mol and theta = -31 K) to a canted antiferromagnet at T(N) = 29 K with an estimated canting angle of 0.2-0.3 degrees. Deduced from powder neutron diffraction data, the magnetic structure is modeled by alternate pairs of Ni(1) within a chain having their moments pointing along [010] and [010], respectively. The moments of Ni(2) atoms are oppositely oriented with respect to their adjacent pairs. The resulting structure is that of a compensated arrangement of moments within one layer, comprising one ferromagnetic and three antiferromagnetic superexchange pathways between the nickel atoms.     

Self-assembled 3D heterometallic Cu(II)/Fe(II) coordination polymers with octahedral net skeletons: structural features, molecular magnetism, thermal and oxidation catalytic properties

The new three-dimensional (3D) heterometallic Cu(II)/Fe(II) coordination polymers [Cu(6)(H(2)tea)(6)Fe(CN)(6)](n)(NO(3))(2n)·6nH(2)O (1) and [Cu(6)(Hmdea)(6)Fe(CN)(6)](n)(NO(3))(2n)·7nH(2)O (2) have been easily generated by aqueous-medium self-assembly reactions of copper(II) nitrate with triethanolamine or N-methyldiethanolamine (H(3)tea or H(2)mdea, respectively), in the presence of potassium ferricyanide and sodium hydroxide. They have been isolated as air-stable crystalline solids and fully characterized including by single-crystal X-ray diffraction analyses. The latter reveal the formation of 3D metal-organic frameworks that are constructed from the [Cu(2)(μ-H(2)tea)(2)](2+) or [Cu(2)(μ-Hmdea)(2)](2+) nodes and the octahedral [Fe(CN)(6)](4-) linkers, featuring regular (1) or distorted (2) octahedral net skeletons. Upon dehydration, both compounds show reversible escape and binding processes toward water or methanol molecules. Magnetic susceptibility measurements of 1 and 2 reveal strong antiferromagnetic [J = -199(1) cm(-1)] or strong ferromagnetic [J = +153(1) cm(-1)] couplings between the copper(II) ions through the μ-O-alkoxo atoms in 1 or 2, respectively. The differences in magnetic behavior are explained in terms of the dependence of the magnetic coupling constant on the Cu-O-Cu bridging angle. Compounds 1 and 2 also act as efficient catalyst precursors for the mild oxidation of cyclohexane by aqueous hydrogen peroxide to cyclohexanol and cyclohexanone (homogeneous catalytic system), leading to maximum total yields (based on cyclohexane) and turnover numbers (TONs) up to about 22% and 470, respectively.     

Polymorphism and spin crossover in mononuclear Fe(II) species containing new dipyridylamino-substituted s-triazine ligands

Four new dipyridylamino-substituted s-triazine ligands DBB (N(2),N(2),N(4),N(4)-tetrabenzyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DDB (N(2),N(2),N(4),N(4)-tetrabutyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DCCl (6-chloro-N(2),N(2)-dicyclohexyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine) and DDT (N(2),N(2),N(4),N(4)-tetraphenyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), have been incorporated into eight new, 0D Fe(II) compounds of type [Fe(II)(NCX)(2)(L)(2)]·Solvent (where NCX = NCS(-), NCSe(-) or N(CN)(2)(-)). The polymorphic compounds α-trans-[Fe(II)(NCS)(2)(DBB)(2)] (1) and β-trans-[Fe(II)(NCS)(2)(DBB)(2)] (2) display, respectively, a relatively abrupt, complete, one-step spin transition with T(?) ~ 170 K, and a more gradual, complete, one-step spin transition with T(?) ~ 300 K. Gradual, one-step spin transitions are observed for trans-[Fe(II)(N(CN)(2))(2)(DBB)(2)]·2CH(3)CH(2)OH (3) and trans-[Fe(II)(NCSe)(2)(DCCl)(2)]·2CH(3)OH (6) with T(?) ~ 280 K for both, while the one-step spin transition observed for a desolvated sample of trans-[Fe(II)(NCSe)(2)(DDB)(2)]·2CH(3)OH (4) is relatively abrupt, showing hysteresis with T(?↑) = 285 K and T(?↓) = 275 K. The compounds cis-[Fe(II)(NCS)(2)(DDB)(2)] (5) and trans-[Fe(II)(NCS)(2)(DDT)(2)]·4CH(2)Cl(2) (7) remain high spin, while structural data on trans-[Fe(II)(NCSe)(2)(DDT)(2)]·4CH(2)Cl(2) (8) suggests a spin transition at low temperatures. It is likely that distortion of the Fe(II)N(6) octahedron, intermolecular interactions and molecular conformation are crucial in deciding both the T(?) and abruptness of the spin transition for these species, although the nature of their influence varies. Variable temperature powder X-ray diffraction measurements on the polymorphs 1 and 2 reveal anisotropy in the unit cell parameters as the spin transition occurs.     

Spectroscopic and structural characterization of the [Fe(imidazole)(6)](2+) cation

Spectroscopic, structural, and magnetic data are presented for Fe(C(3)H(4)N(2))(6)(NO(3))(2), which facilitate a precise definition of the electronic and molecular structure of the [Fe(Im)(6)](2+) cation. The structure was determined at 120(1) K by X-ray diffraction methods. The salt crystallizes in the trigonal space group R3 with unit-cell parameters a = 12.4380(14) A, c = 14.5511(18) A, and Z = 3. All the imidazole ligands are equivalent with an Fe-N bond distance of 2.204(1) A. Variable-temperature inelastic neutron scattering (INS) measurements identify a cold magnetic transition at 19.4(2) cm(-1) and a hot transition at 75.7(6) cm(-1). The data are interpreted using a ligand field Hamiltonian acting in the weak-field (5)D basis, from which the diagonal trigonal field splitting of the (5)T(2g) (O(h)) term is estimated as approximately 485 cm(-1), with the (5)A(g) (S(6)) component lower lying. High-field multifrequency (HFMF) EPR data and measurements of the magnetic susceptibility are also reported and can be satisfactorily modeled using the energies and wave functions derived from analysis of the INS data. The electronic and molecular structures are related through angular overlap model calculations, treating the imidazole ligand as a weak pi-donor.     

A series of chiral coordination polymers containing helicals assembled from a new chiral (R)-2-(4'-(4'-carboxybenzyloxy)phenoxy)propanoic acid: syntheses, structures and photoluminescent properties

Ten new chiral coordination polymers, namely, [Ni(L)(H(2)O)(2)] (1), [Co(L)(H(2)O)(2)] (2), [Cd(L)(H(2)O)] (3), [Cd(L)(phen)] (4), [Mn(2)(L)(2) (phen)(2)]·H(2)O (5), [Cd(2)(L)(2)(biim-4)(2)] (6), [Zn(2)(L)(2)(biim-4)(2)] (7), [Cd(L)(pbib)] (8), [Cd(L)(bbtz)] (9) and [Cd(L)(biim-6)] (10), where phen = 1,10-phenathroline, biim-4 = 1,1'-(1,4-butanediyl)bis(imidazole), pbib = 1,4-bis(imidazole-1-ylmethyl)benzene, bbtz = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene, biim-6 = 1,1'-(1,6-hexanedidyl)bis(imidazole), and H(2)L = (R)-2-(4'-(4'-carboxybenzyloxy)phenoxy)propanoic acid, have been synthesized under hydrothermal conditions. Their structures have been determined by single-crystal X-ray diffraction analyses and further characterized by infrared spectra (IR), powder X-ray diffraction (PXRD), elemental analyses and thermogravimetric (TG) analyses. Compounds 1 and 2 exhibit similar 1D left-handed helical chains, which are further extended into 3D supramolecular structures through O-H···O hydrogen-bonding interactions, respectively. Compound 3 shows a 2D double-layer architecture containing helical chains. Compound 4 features two types of 2D undulated sheets with helical chains, which are stacked in an ABAB fashion along the c direction. Compound 5 possesses a 1D double chain ribbon structure containing unusual meso-helical chains, which is linked by π-π interactions into a 2D supramolecular layer. These layers are further extended by hydrogen-bonding interactions to form a 3D supramolecular assembly. Compounds 6 and 7 are isostructural and exhibit 2D (4(4))-sql networks with helical chains. Neighboring sheets are further linked by C-H···O hydrogen-bonding interactions to generate 3D supramolecular architectures. Compounds 8-10 are isostructural and display 3D 3-fold interpenetrating diamond frameworks with helical chains. The effects of coordination modes of L anions, metal ions and N-donor ligands on the structures of the coordination polymers have been discussed. The luminescent properties of 3, 4 and 6-10 have also been investigated in detail.     

Porous Cu-Cd mixed-metal-organic frameworks constructed from Cu(Pyac)2 [Bis[3-(4-pyridyl)pentane-2,4-dionato]copper(II)

Porous Cu-Cd mixed-metal-organic frameworks [[Cd(NO(3))(2)](2)[(Cu(Pyac)(2)](3)] (M'MOF 1) and [[CdCl(2)][Cu(Pyac)(2)](2)] (M'MOF2) [Cu(Pyac)(2) = bis[3-(4-pyridyl)pentane-2,4-dionato]copper(II)] have been synthesized by the reaction of Cu(Pyac)(2) with Cd(NO(3))(2) and CdCl(2). They are noninterpenetrating 1D ladder and 2D square-grid frameworks, constructed from Cu(Pyac)(2) building blocks with T-shaped Cd(NO(3))(2) nodes and square-planar CdCl(2) nodes, respectively. The 1D ladders and 2D square grids are stacked in ABCABC... and ABCDEF... packings, leading to 2D interconnected channels of ca. 5.7 x 10.2 and 4.1 x 9.8 A in 1 and 1D channels of ca. 8.0 x 8.2 A in 2, respectively. The copper sites in these two M'MOFs are coordinated by solvent molecules and exposed to the pores.     

CO(2) Dynamics in a Metal-Organic Framework with Open Metal Sites

Metal-organic frameworks (MOFs) with open metal sites are promising candidates for CO(2) capture from dry flue gas. We applied in situ(13)C NMR spectroscopy to investigate CO(2) adsorbed in Mg(2)(dobdc) (H(4)dobdc = 2,5-dihydroxyterephthalic acid; Mg-MOF-74, CPO-27-Mg), a key MOF in which exposed Mg(2+) cation sites give rise to exceptional CO(2) capture properties. Analysis of the resulting spectra reveals details of the binding and CO(2) rotational motion within the material. The dynamics of the motional processes are evaluated via analysis of the NMR line shapes and relaxation times observed between 12 and 400 K. These results form stringent and quantifiable metrics for computer simulations that seek to screen and improve the design of new MOFs for CO(2) capture.     

Fluxionality of hydrogen ligands in Fe(H)2(H2)(PEtPh2)3

Extensive computational investigations along with additional quasielastic neutron scattering data were used to obtain a consistent picture of the extensive fluxionality of hydride and dihydrogen ligands in Fe(H)(2)(H(2))(PEtPh(2))(3) over a wide range of temperatures from 1.5 to 320 K. We were able to identify three different regimes in the dynamical processes based on activation energies obtained from line spectral broadening. The rotational tunneling lines (coherent exchange of the two hydrogens of the H(2) ligand) are broadened with increasing temperature by incoherent exchange up to about 80 K at which point they merge into a quasielastic spectrum from 100 K to about 225 K. The effective activation energies for the two regions are 0.14 and 0.1 kcal mol(-1), respectively. A third dynamical process with a higher activation energy of 0.44 kcal mol(-1) dominates above 225 K, which we attribute to a quantum dynamical exchange of dihydrogen and hydride ligands. Our detailed density functional theory (DFT) structural calculations involving the three functionals (B3LYP, TPSS, and wB97XD) provide a good account of the experimental structure and rotational barriers when only the hydrogen ligands are relaxed. Full relaxation of the "gas-phase" molecule, however, appears to occur to a greater degree than what is possible in the crystal structure. The classical dihydrogen-hydride exchange path involves a cis-dihydrogen and tetrahydride structure with energies of 6.49 and 7.38 kcal mol(-1), respectively. Experimental observation of this process with much lower energies would seem to suggest involvement of translational tunneling in addition to the rotational tunneling. Dynamics of this type may be presumed to be important in hydrogen spillover from metal particles, and therefore need to be elucidated in an effort to utilize this phenomenon.     

The effect of ligand charge on the coordination geometry of an Fe(III)ion: five- and six-coordinate Fe(III) complexes of tris(2-benzimidazolylmethyl)amine

By using the tripodal tetradentate ligand tris(2-benzimidazolylmethyl)amine (H(3)ntb), which can have several charge states depending on the number of secondary amine protons, mononuclear octahedral and dinuclear trigonal bipyramidal Fe(III) complexes were prepared. The reaction of mononuclear octahedral [Fe(III)(H(3)ntb)Cl(2)]ClO(4), 1, with 3 equiv of sec-butylamine in methanol led to the formation of mononuclear cis-dimethoxo octahedral Fe(III)(H(2)ntb)(OMe)(2), 2. One equivalent of the sec-butylamine was used to generate the monoanionic H(2)ntb(-) ligand where one of the three amines in the benzimidazolyl groups was deprotonated. The remaining 2 equiv were used to generate two methoxides that were coordinated to the octahedral Fe(III) ion in a cis fashion as demonstrated by the chlorides in 1. Reaction of 1 with excess (7 equiv) sec-butylamine generated the doubly deprotonated dianionic Hntb(2-) that stabilized the dinuclear mu-oxo Fe(III)(2)O(Hntb)(2), 3, adopting a five-coordinate trigonal bipyramidal geometry. The magnetic data for 3 are consistent with the antiferromagnetically coupled Fe(III) (S = 5/2) sites with the coupling constant J = -127 cm(-1).     

Dinitrosyl iron complexes (DNICs) [L(2)Fe(NO)2]- (L = thiolate): interconversion among {Fe(NO)2}9 DNICs, {Fe(NO)2}10 DNICs, and [2Fe-2S] clusters, and the critical role of the thiolate ligands in regulating NO release of DNICs

Dinitrosyl iron complex [(-SC(7)H(4)SN)(2)Fe(NO)(2)](-) (1) was prepared by reaction of [S(5)Fe(NO)(2)](-) and bis(2-benzothiozolyl) disulfide. In synthesis of the analogous dinitrosyl iron compounds (DNICs), the stronger electron-donating thiolates [RS](-) (R = C(6)H(4)-o-NHCOCH(3), C(4)H(3)S, C(6)H(4)NH(2), Ph), compared to [-SC(7)H(4)SN](-) of complex 1, trigger thiolate-ligand substitution to yield [(-SC(6)H(4)-o-NHCOCH(3))(2)Fe(NO)(2)](-) (2), [(-SC(4)H(3)S)(2)Fe(NO)(2)](-) (3), and [(SPh)(2)Fe(NO)(2)](-) (4), respectively. At 298 K, complexes 2 and 3 exhibit a well-resolved five-line EPR signal at g = 2.038 and 2.027, respectively, the characteristic g value of DNICs. The magnetic susceptibility fit indicates that the resonance hybrid of {Fe(+)((*)NO)(2)}(9) and {Fe(-)((+)NO)(2)}(9) in 2 is dynamic by temperature. The IR nu(NO) stretching frequencies (ranging from (1766, 1716) to (1737, 1693) cm(-)(1) (THF)) of complexes 1-4 signal the entire window of possible electronic configurations for such stable and isolable {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-). The NO-releasing ability of {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) is finely tuned by the coordinated thiolate ligands. The less electron-donating thiolate ligands coordinated to {Fe(NO)(2)}(9) motif act as better NO-donor DNICs in the presence of NO-trapping agent [Fe(S,S-C(6)H(4))(2)](2)(2-). Interconversion between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(10) [(Ph(3)P)(2)Fe(NO)(2)] was verified in the reaction of (a) [(RS)(2)Fe(NO)(2)](-), 10 equiv of PPh(3) and sodium-biphenyl, and (b) 2 equiv of thiol, [RS](-), and [(Ph(3)P)(2)Fe(NO)(2)], respectively. The biomimetic reaction cycle, transformation between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(9) [(R'S)(2)Fe(NO)(2)](-), reversible interconversion of {Fe(NO)(2)}(9) and {Fe(NO)(2)}(10) DNICs, and degradation/reassembly of [2Fe-2S] clusters may decipher and predict the biological cycle of interconversion of {Fe(NO)(2)}(9) DNICs, {Fe(NO)(2)}(10) DNICs, and the [Fe-S] clusters in proteins.     

Synthesis, structure, and magnetic properties of tetranuclear cubane-like and chain-like iron(II) complexes based on the N(4)O pentadentate dinucleating ligand 1,5-bis[(2-pyridylmethyl)amino]pentan-3-ol

The tetranuclear complexes [Fe(4)(pypentO)(pym)(3)(Oac)(NCS)(3)] x 1.5EtOH (1), [Fe(4)(pypentO)(pym)(Oac)(2)(NCS)(2)(MeO)(2)(H(2)O)] x H(2)O (2), [Fe(2)(pypentO)(NCO)(3)](2) (3), and [Fe(2)(pypentO)(N(3))(3)](2) (4) have been prepared, and their structure and magnetic properties have been studied (pypentOH = 1,5-bis[(2-pyridylmethyl)amino]pentan-3-ol, pymH = 2-pyridylmethanol). The X-ray diffraction analysis of 1 (C(43)H(53)N(10)O(7.5)S(3)Fe(4), monoclinic, P2(1)/n, a = 11.6153(17) A, b = 34.391(17) A, c = 14.2150(18) A, beta = 110.88(5) degrees, V = 5305(3) A(3), Z = 4) and 2 (C(31)H(45)N(7)O(10)S(2)Fe(4), monoclinic, C2/c, a = 19.9165(17) A, b = 21.1001(12) A, c = 21.2617(19) A, beta = 104.441(10) degrees, V = 8652.7(12) A(3), Z = 8) showed a Fe(4)O(4) cubane-like arrangement of four iron(II) atoms, four mu(3)-O bridging ligands, one (1) or two (2) syn-syn bridging acetates. The X-ray diffraction analysis of 3 (C(40)H(46)N(14)O(8)Fe(4), monoclinic, P2(1)/c, a = 11.7633(18) A, b = 18.234(3) A, c = 10.4792(16) A, beta = 99.359(18) degrees, V = 2217.7(6) A(3), Z = 2) and 4 (C(34)H(46)N(26)O(2)Fe(4), monoclinic, P2(1)/c, V = 4412.4(10) A(3), a = 23.534(3) A, b = 18.046(2) A, c = 10.4865(16) A, beta = 97.80(2) degrees, Z = 4) showed a zigzag bis-dinuclear arrangement of four iron(II) cations, two mu(2)-O bridging pypentO ligands, four mu(2)-N-cyanato bridging ligands (3) or four end-on azido bridging ligands (4): they are the first examples of cyanato and azido bridged discrete polynuclear ferrous compounds, respectively. The M?ssbauer spectra of 1 are consistent with four different high-spin iron(II) sites in the Fe(4)O(4) cubane-type structure. The M?ssbauer spectra of 3 are consistent with two high-spin iron(II) sites (N(5)O and N(4)O). Below 190 K, the M?ssbauer spectra of 4 are consistent with one N(5)O and two N(4)O high-spin iron(II) sites. The temperature dependence of the magnetic susceptibility was fitted with J(1) approximately 0 cm(-1), J(2) = -1.3 cm(-1), J(3) = 4.6 cm(-1), D = 6.4 cm(-1), and g = 2.21 for 1; J(1) = 2.6 cm(-1), J(2) = 2.5 cm(-1), J(3) = - 5.6 cm(-1), D = 4.5 cm(-1), and g = 2.09 for 2; J(1) = 1.5 cm(-1), J(2) = 0.2 cm(-1), D = - 5.6 cm(-1), D' = 4.5 cm(-1), and g = 2.14 for 3; and J(1) = - 2.6 cm(-1), J(2) = 0.8 cm(-1), D= 6.3 cm(-1), D' = 1.6 cm(-1), and g = 2.18 for 4. The differences in sign among the J(1), J(2), and J(3) super-exchange interactions indicate that the faces including only mu(3)-OR bridges exhibit ferromagnetic interactions. The nature of the ground state in 1-3 is confirmed by simulation of the magnetization curves at 2 and 5 K. In the bis-dinuclear iron(II) compounds 3 and 4, the J(2) interaction resulting from the bridging of two Fe(2)(pypentO)X(3) units through two pseudo-halide anions is ferromagnetic in 3 (X = mu(2)-N-cyanato) and may be either ferro- or antiferromagnetic in 4 (X = end-on azido). The J(1) interaction through the central O(alkoxo) and pseudo-halide bridges inside the dinuclear units is ferromagnetic in 3 (X = mu(2)-N-cyanato) and antiferromagnetic in 4 (X = end-on azido). In agreement with the symmetry of the two Fe(II) sites in complexes 3 and 4, D (pentacoordinated sites) is larger than D' (octahedral sites).