Valuing Metal-Organic Frameworks for Postcombustion Carbon Capture: A Benchmark Study for Evaluating Physical Adsorbents.

The development of practical solutions for the energy-efficient capture of carbon dioxide is of prime importance and continues to attract intensive research interest. Conceivably, the implementation of adsorption-based processes using different cycling modes, e.g., pressure-swing adsorption or temperature-swing adsorption, offers great prospects to address this challenge. Practically, the successful deployment of practical adsorption-based technologies depends on the development of made-to-order adsorbents expressing mutually two compulsory requisites: i) high selectivity/affinity for CO2 and ii) excellent chemical stability in the presence of impurities. This study presents a new comprehensive experimental protocol apposite for assessing the prospects of a given physical adsorbent for carbon capture under flue gas stream conditions. The protocol permits: i) the baseline performance of commercial adsorbents such as zeolite 13X, activated carbon versus liquid amine scrubbing to be ascertained, and ii) a standardized evaluation of the best reported metal-organic framework (MOF) materials for carbon dioxide capture from flue gas to be undertaken. This extensive study corroborates the exceptional CO2 capture performance of the recently isolated second-generation fluorinated MOF material, NbOFFIVE-1-Ni, concomitant with an impressive chemical stability and a low energy for regeneration. Essentially, the NbOFFIVE-1-Ni adsorbent presents the best compromise by satisfying all the required metrics for efficient CO2 scrubbing.

Heterobimetallic µ-oxido complexes containing discrete V(V)-O-M(III) (M = Mn, Fe) cores: targeted synthesis, structural characterization, and redox studies.

Heterobimetallic compounds [L'OV(V)(µ-O)M(III)L]n (n = 1, M = Mn, 1-5; n = 2, M = Fe, 6 and 7) containing a discrete unsupported V(V)-O-M(III) bridge have been synthesized through a targeted synthesis route. In the V-O-Mn-type complexes, the vanadium(V) centers have a square-pyramidal geometry, completed by a dithiocarbazate-based tridentate Schiff-base ligand (H2L'), while the manganese(III) centers have either a square-pyramidal (1 and 3) or an octahedral (2 and 5) geometry, made up of a Salen-type tetradentate ligand (H2L) as established by X-ray diffraction analysis. The V-O-Mn bridge angle in these compounds varies systematically from 155.3° to 128.1° in going from 1 to 5 while the corresponding dihedral angle between the basal planes around the metal centers changes from 86.82° to 20.92°, respectively. The V-O-Fe-type complexes (6 and 7) are tetranuclear, in which the two dinuclear V(µ-O)Fe units are connected together by apical iron(III)-aryl oxide interactions, forming a dimeric structure with a pair of Fe-O-Fe bridges. The X-ray data also confirm the V═O → M canonical form to contribute predominantly on the overall V-O-M bridge structure. The molecules in solution also retain their heterobinuclear composition, as established by electrospray ionization mass spectrometry and (51)V NMR spectroscopy. Electrochemically, these complexes are quite interesting; the manganese(III) complexes (1-5) display three successive reductions (processes I-III), each with a monoelectron stoichiometry. Process I is due to a Mn(III)/Mn(II) reduction (E1/2 ranges between -0.32 and -0.05 V), process II is a ligand-based reduction, and process III (E1/2 = ∼1.80 V) owes its origin to a V(V)O/V(IV)O reduction; all potentials are versus Ag/AgCl. The iron(III) compounds (6 and 7), on the other hand, show at least four irreversible processes, appearing at Epc = -0.20, -1.0, -1.58, and -1.68 V in compound 6 (processes IV-VII), together with a reversible process (process VIII) at E1/2 = -1.80 V (ΔEp = 80 mV). While the first two of these are due to Fe(III)/Fe(II) reductions at the two iron(III) centers of these tetranuclear cores, the reversible reduction at a more negative potential (ca. -1.80 V) is due to a V(V)O/V(IV)O-based electron transfer.

Tetranuclear hetero-metal [Co(II)2Ln(III)2] (Ln = Gd, Tb, Dy, Ho, La) complexes involving carboxylato bridges in a rare µ4-η(2):η(2) mode: synthesis, crystal structures, and magnetic properties.

A new family of 3d-4f heterometal 2 × 2 complexes [Co(II)2(L)2(PhCOO)2Ln(III)2(hfac)4] (1-5) (Ln = Gd (compound 1), Tb (compound 2), Dy (compound 3), Ho (compound 4), and La (compound 5)) have been synthesized in moderate yields (48-63%) following a single-pot protocol using stoichiometric amounts (1:1 mol ratio) of [Co(II)(H2L)(PhCOO)2] (H2L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine) as a metalloligand and [Ln(III)(hfac)3(H2O)2] (Hhfac = hexafluoroacetylacetone) as a lanthanide precursor compound. Also reported with this series is the Zn-Dy analog [Zn(II)2(L)2(PhCOO)2Dy(III)2(hfac)4] 6 to help us in understanding the magnetic properties of these compounds. The compounds 1-6 are isostructural. Both hexafluoroacetylacetonate and benzoate play crucial roles in these structures as coligands in generating a tetranuclear core of high thermodynamic stability through a self-assembly process. The metal centers are arranged alternately at the four corners of this rhombic core, and the carboxylato oxygen atoms of each benzoate moiety bind all of the four metal centers of this core in a rare µ4-η(2):η(2) bridging mode as confirmed by X-ray crystallography. The magnetic susceptibility and magnetization data confirm a paramagnetic behavior, and no remnant magnetization exists in any of these compounds at vanishing magnetic field. The metal centers are coupled in an antiferromagnetic manner in these compounds. The [Co(II)2Dy(III)2] compound exhibits a slow magnetic relaxation below 6 K, as proven by the AC susceptibility measurements; the activation energy reads U/kB = 8.8 K (τ0 = 2.0 × 10(-7) s) at BDC = 0, and U/kB = 7.8 K (τ0 = 3.9 × 10(-7) s) at BDC = 0.1 T. The [Zn(II)2Dy(III)2] compound also behaves as a single-molecule magnet with U/kB = 47.9 K and τ0 = 2.75 × 10(-7) s.

Homo- and heterometal complexes of oxido-metal ions with a triangular [V(V)O-MO-V(V)O] [M = V(IV) and Re(V)] core: reporting mixed-oxidation oxido-vanadium(V/IV/V) compounds with valence trapped structures.

A new family of trinuclear homo- and heterometal complexes with a triangular [V(V)O-MO-V(V)O] (M = V(IV), 1 and 2; Re(V), 3] all-oxido-metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, N,N'-bis(3-hydroxysalicylidene)-diiminoalkanes/arene (H4L(1)-H4L(3)). The upper compartment of these ligands with N2O2 donor combination (Salen-type) contains either a V(IV) or a Re(V) center, while the lower compartment with O4 donor set accommodates two V(V) centers, stabilized by a terminal and a couple of bridging methoxido ligands. The compounds have been characterized by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in 1-3. Compound 1 crystallizes in a monoclinic space group P2(1)/c, while both 2 and 3 have more symmetric structures with orthorhombic space group Pnma that renders the vanadium(V) centers in these compounds exactly identical. In DMF solution, compound 1 displays an 8-line EPR at room temperature with and values of 1.972 and 86.61 × 10(-4) cm(-1), respectively. High-resolution X-ray photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p(3/2) and 2p(1/2) electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation, confirm the trapped-valence nature of mixed-oxidation in compounds 1 and 2. Electrochemically, compound 1 undergoes two one-electron oxidations at E(1/2) = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V(IV/V) oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers. A larger cavity size in the upper compartment of the ligand H4L(3) is spacious enough to accommodate Re(V) with larger size to generate a rare type of all-oxido heterotrimetallic compound (3) as established by X-ray crystallography.

Tetranuclear homo- (Zn(II)4 and Cd(II)4) and hetero-metal (Zn(II)2Tb(III)2 and Cd(II)2Tb(III)2) complexes with a pair of carboxylate ligands in a rare η2:η2:µ4-bridging mode: syntheses, structures and emission properties.

Four tetranuclear complexes involving both homo- and hetero-metal combinations, viz. [Zn(II)(2)L(2)(µ(4)-PhCOO)(2)Zn(II)(2)(hfac)(2)] (1), [Cd(II)(2)L(2)(µ(4)-PhCOO)(2)Cd(II)(2)(hfac)(2)] (2), [Zn(II)(2)L(2)(µ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (3), and [Cd(II)(2)L(2)(µ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (4) have been prepared following a single-pot synthesis protocol using N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H(2)L) as a primary ligand. Both benzoate and hexafluoroacetylacetonate (hfac(-)), used here as ancillary ligands, play crucial roles in generating a tetranuclear core with high thermodynamic stability. Oxygen atoms of each carboxylate moiety bind all the four metal centers together in a rare η(2):η(2):µ(4)-bridging mode as confirmed by X-ray crystallography. In the homo-metallic complexes (1 and 2), the metal centers are all lying in a square plane, each occupying a corner, and remain connected together by oxygen bridges forming octagonal metallacrowns. These structures remain intact in solution as confirmed by (1)H NMR spectroscopy and photoluminescent studies. In the hetero-metal complexes (3 and 4), the metal centers are arrayed in alternate positions of the tetranuclear core. The Tb(III) centers have eight coordinate bi-capped trigonal prismatic coordination environments with different degrees of distortions. The all oxygen O(8) core surrounding each Tb(III) center is devoid of solvent molecules that make fluorescent emission from these molecules (3 and 4) quite interesting. The hfac(-)-based (1)(π-π*) emissions observed in 1 and 2 are quenched in 3 and 4. These sensitized Tb(III) emissions [(5)D(4)→(7)F(j); j = 6, 5, 4, and 3] are influenced by the local environments surrounding the 4f-metal center. The lifetime for the luminescence decay of 3 ((5)D(4)→(7)F(5) transition) is about 1.5 times longer than that of 4 in all the solvents studied at 298 K.

Syntheses, structures, and magnetic properties of a family of tetranuclear hydroxido-bridged Ni(II)2Ln(III)2 (Ln = La, Gd, Tb, and Dy) complexes: display of slow magnetic relaxation by the zinc(II)-dysprosium(III) analogue.

A new family of [2 × 2] tetranuclear 3d-4f heterometallic complexes have been synthesized. These are [Zn(2)Dy(2)L(2)(µ(3)-OH)(2)(µ(4)-OH)(dbm)(2)(MeOH)(2)](NO(3))·2H(2)O·MeOH (3), [Ni(2)Dy(2)L(2)(µ(3)-OH)(2)(µ(4)-OH)(dbm)(2)(MeOH)(2)](NO(3))·MeOH (4), [Ni(2)La(2)L(2)(µ(3)-OH)(2)(µ(4)-OH)(dbm)(2)(MeOH)(2)](ClO(4))·H(2)O·2MeOH (5), [Ni(2)Tb(2)L(2)(µ(3)-OH)(2)(µ(4)-OH)(dbm)(2) (MeOH)(2)](NO(3))·MeOH (6), and [Ni(2)Gd(2)L(2)(µ(3)-OH)(2)(µ(4)-OH)(dbm)(2)(MeOH)(2)](NO(3))·MeOH (7), [H(2)L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine and Hdbm = dibenzoylmethane] obtained through a single-pot synthesis using [Zn(HL)(dbm)] (for 3)/[Ni(HL)(dbm)]·2CH(3)OH (for 4, 5, 6, and 7) as 3d-metal ion precursors. Single-crystal X-ray diffraction analysis and electrospray ionization (ESI) mass spectroscopy have been used to establish their identities. Compounds are isostructural, in which the metal ions are all connected together by a bridging hydroxido ligand in a rare µ(4)-mode. In complexes 3-7, the metal ions are antiferromagnetically coupled. Taking a cue from the results of 3 and 5, precise estimations have been made for the antiferromagnetic Ni···Ni (J(Ni) = -50 cm(-1)), Ni···Gd (J(NiGd) = -4.65 cm(-1)), and Gd···Gd (J(Gd) = -0.02 cm(-1)) exchange interactions in 7, involving the gadolinium(III) ions. The Zn(II)(2)Dy(III)(2) compound 3 has shown the tail of an out-of-phase signal in alternating current (AC) susceptibility measurement, indicative of slow relaxation of magnetization. Interestingly, the Ni(II)(2)Dy(III)(2) compound 4 in which both the participating metal ions possess large single ion anisotropy, has failed to show up any slow magnetic relaxation.

Triple-stranded helicates of zinc(II) and cadmium(II) involving a new redox-active multiring nitrogenous heterocyclic ligand: synthesis, structure, and electrochemical and photophysical properties.

The protonated form [H(2)(L)](CF(3)SO(3))(2) (1) of a new redox-active bis-bidentate nitrogenous heterocyclic ligand, viz., 3,3'-dipyridin-2-yl[1,1']bi[imidazo[1,5-a]pyridinyl] (L), and its zinc(II) and cadmium(II) complexes (2 and 3) have been synthesized and characterized by single-crystal X-ray diffraction analysis. In the solid state, both 2 and 3 have triple-stranded helical structures involving ligands that experience twisting and bending to the extent needed by the stereoelectronic demand of the central metal ion. The metal centers in the zinc(II) complex [Zn(2)(L)(3)](ClO(4))(4) (2) are equivalent, each having a distorted octahedral geometry, flattened along the C(3) axis with a Zn1···Zn1# separation of 4.8655(13) Å. The cadmium complex [Cd(2)(L)(3)(H(2)O)](ClO(4))(4) (3), on the other hand, has a rare type of helical structure, showing coordination asymmetry around the metal centers with a drastically reduced Cd1···Cd2 separation of 4.070 Å. The coordination environment around Cd1 is a distorted pentagonal bipyramid involving a N(6)O donor set with the oxygen atom coming from a coordinated water, leaving the remaining metal center Cd2 with a distorted octahedral geometry. The structures of 2 and 3 also involve anion-π- and CH-π-type noncovalent interactions that play dominant roles in shaping the extended structures of these molecules in the solid state. In solution, these compounds exhibit strong fluxional behavior, making the individual ligand strands indistinguishable from one another, as revealed from their (1)H NMR spectra, which also provide indications about these molecules retaining their helical structures in solution. Electrochemically, these compounds are quite interesting, undergoing ligand-based oxidations in two successive one-electron steps at E(1/2) of ca. 0.65 and 0.90 V versus a Ag/AgCl (3 M NaCl) reference. These molecules are all efficient emitters in the red and blue regions because of ligand-based π*-π fluorescent emissions, tuned appropriately by the attached Lewis acid centers.

Single crystal-to-single crystal irreversible transformation from a discrete vanadium(V)-alcoholate to an aldehydic-vanadium(IV) oligomer.

An unprecedented single crystal-to-single crystal transformation occurs when a binuclear oxovanadium(V) compound [V(V)(2)O(2)(L)(2)] 1 involving 2,6-bis(hydroxymethyl)-p-cresol (H(3)L) as a bridging ligand is exposed simultaneously to white light and aerial oxygen to generate an oligomeric compound [V(IV)(2)O(2)(L*)(2)] 2 (H(2)L* is 3-hydroxymethyl-5-methylsalicylaldehyde). Each vanadium(V) center in 1 is reduced to vanadium(IV) in 2 at the expense of a two-electron alcohol-to-aldehyde oxidation in the coordinated ligand. The additional electron being released is possibly consumed by molecular oxygen to generate hydrogen peroxide.

Hetero-bimetallic complexes involving vanadium(V) and rhenium(VII) centers, connected by unsupported mu-oxido bridge: synthesis, characterization, and redox study.

Heterobimetallic complexes of a vanadium(V) and rhenium(VII) combination connected by a mu-oxido bridge [LVO(mu-O)ReO 3].H 2O [H 2L = N, N'-ethylene bis(salicylideneimine) (H 2salen) and its methoxy derivative] ( 1, 2) are reported. The compounds have been prepared by a single-pot synthesis in which the precursor [V (IV)OL] complexes are allowed to be oxidized aerially in the presence of added perrhenate. The oxidized [V (V)OL] (+) species accommodate the ReO 4 (-) anion in their vacant coordination site, trans to the terminal oxido group, providing the complexes 1 and 2. The later generates a binuclear oxovanadium(V) compound [H 2en][(TBC)VO(mu-TBC) 2OV(TBC)].5H 2O ( 3) when treated with tetrabromocatechol. Single crystal X-ray diffraction analysis and (1)H NMR spectroscopy have been used to establish their identities. In compound 2, the Re(1)-O(11)-V(1) bridge angle is barely linear [170.2(3) degrees ] with a Re...V separation of 3.9647(9) A. The redox behavior of 1 and 2 are quite interesting, each undergoing two reductions both in the positive potential range at E 1/2 = 0.59 (process I) and E 1/2 = 0.16 V (process II) versus Ag/AgCl reference (corresponding potentials are 0.59 and 0.18 V for 2). Process I has a single-electron stoichiometry involving the [VO(salen)] part of the complexes as established by combined coulometry-Electron Paramagnetic Resonance (EPR) experiments which provide an eight-line isotropic EPR pattern at room temperature ( = 1.967; = 87 x 10 (-4) cm (-1)), characteristic of an unpaired electron being coupled to a vanadium nuclear spin ( (51)V, I = 7/2). The almost linear V-O-Re bridge in 1 and 2 allows this unpaired electron to interact effectively with the neighboring Re nuclear spin, leading to familiar " two-line pattern" superhyperfine coupling ( A ( (185,187)Re) = 20.7 x 10 (-4) cm (-1)). Process II, on the other hand, is based on a Re(VII/VI) electron transfer as confirmed by differential pulse and normal pulse voltammetric experiments.