An efficient one-pot synthesis of carbazole fused benzoquinolines and pyridocarbazoles

Cobalt­(II), in the presence of acetate and nitrate, quantitatively adds to the manganese–cobalt oxido cubane Mn^IVCo^III₃O₄(OAc)₅(py)₃ (<b>1</b>) to furnish the pentametallic dangler complex Mn^IVCo^III₃Co^IIO₄(OAc)₆(NO₃)­(py)₃ (<b>2</b>). Complex <b>2</b> is structurally reminiscent of photosystem II’s oxygen-evolving center, and is a rare example of a transition-metal “dangler” complex. Superconducting quantum interference device magnetometry and density functional theory calculations characterize <b>2</b> as having an <i>S</i> = 0 ground state arising from antiferromagnetic coupling between the Co^II and Mn^IV ions. At higher temperatures, an uncoupled state dominates. The voltammogram of <b>2</b> has four electrochemical events, two more than that of its parent cubane <b>1</b>, suggesting that addition of the dangler increases available redox states. Structural, electrochemical, and magnetic comparisons of complexes <b>1</b> and <b>2</b> allow a better understanding of the dangler’s influence on a cubane

Synthesis and O₂ Reactivity of a Titanium(III) Metal–Organic Framework

Metal–organic frameworks featuring pores lined with exposed metal cations have received attention for a wide range of adsorption-related applications. While many frameworks with coordinatively unsaturated M^II centers have been reported, there are relatively few examples of porous materials with coordinatively unsaturated M^III centers. Here, we report the synthesis and characterization of Ti₃O­(OEt)­(bdc)₃(solv)₂ (Ti-MIL-101; bdc^2– = 1,4-benzenedicarboxylate; solv = <i>N</i>,<i>N</i>-dimethylformamide, tetrahydrofuran), the first metal–organic framework containing exclusively Ti^III centers. Through a combination of gas adsorption, X-ray diffraction, magnetic susceptibility, and electronic and vibrational spectroscopy measurements, this high-surface-area framework is shown to contain five-coordinate Ti^III centers upon desolvation, which irreversibly bind O₂ to form titanium­(IV) superoxo and peroxo species. Electronic absorption spectra suggest that the five-coordinate Ti^III sites adopt a distorted trigonal-bipyramidal geometry that effectively shields nuclear charge and inhibits strong adsorption of nonredox-active gases

Utility of Lithium in Rare-Earth Metal Reduction Reactions to Form Nontraditional Ln²⁺ Complexes and Unusual [Li(2.2.2-cryptand)]¹⁺ Cations

The utility of lithium compared to other alkali metals in generating Ln²⁺ rare-earth metal complexes via reduction of Ln³⁺ precursors in reactions abbreviated as LnA₃/M (Ln = rare-earth metal; A = anionic ligand; M = alkali metal) is described. Lithium reduction of Cp′₃Ln (Cp′ = C₅H₄SiMe₃; Ln = Y, Tb, Dy, Ho) under Ar in the presence of 2.2.2-cryptand (crypt) forms new examples of crystallographically characterizable Ln²⁺ complexes of these metals, [Li­(crypt)]­[Cp′₃Ln]. In each complex, lithium is found in an N₂O₄ donor atom coordination geometry that is unusual for the cryptand ligand. Magnetic susceptibility data on these new examples of nontraditional divalent lanthanide complexes are consistent with 4f^<i>n</i>5d¹ electronic configurations. The Dy and Ho complexes have exceptionally high single-ion magnetic moments, 11.35 and 11.67 μ_B, respectively. Lithium reduction of Cp′₃Y under N₂ at −35 °C forms the Y²⁺ complex (Cp′₃Y)^1–, which reduces dinitrogen upon warming to room temperature to generate the (N₂)^2– complex [Cp′₂Y­(THF)]₂(μ-η²:η²-N₂). These results provide insight on the factors that lead to reduced dinitrogen complexes and/or stable divalent lanthanide complexes as a function of the specific reducing agent and conditions

Trimethylsilyl versus Bis(trimethylsilyl) Substitution in Tris(cyclopentadienyl) Complexes of La, Ce, and Pr: Comparison of Structure, Magnetic Properties, and Reactivity

To evaluate the effect of cyclopentadienyl ligand substitution in complexes of new +2 ions of the lanthanides, comparisons in reactivity and spectroscopic and magnetic properties have been made between [K­(crypt)]­[Cp′₃Ln], <b>1-Ln</b> (Cp′ = C₅H₄SiMe₃; crypt = 2.2.2-cryptand; Ln = La, Ce, Pr, and Nd), and [K­(crypt)]­[Cp′′₃Ln], <b>2-Ln</b> [Cp′′ = C₅H₃(SiMe₃)₂]. The <b>2-Ln</b> complexes (Ce, Pr, and Nd) were synthesized by reduction of Cp′′₃Ln with potassium graphite in the presence of crypt and crystallographically characterized. The structures and UV–visible spectra of <b>2-Ln</b> are similar to those of <b>1-Ln</b>, as expected, but greater thermal stability for <b>2-Ln</b>, expected from comparisons of <b>2-U</b> and <b>1-U</b>, was not observed. The magnetic susceptibilities of <b>2-Ce</b> and <b>2-Pr</b> were investigated because those of <b>1-Ce</b> and <b>1-Pr</b> did not match simple coupling models for 4f^<i>n</i>5d¹ electron configurations. The magnetic data of the <b>2-Ln</b> complexes are similar to those of <b>1-Ln</b>, which suggests that Ce²⁺ and Pr²⁺ complexes with 4f^<i>n</i>5d¹ electron configurations may have more complex electronic structures compared to nontraditional divalent complexes of the later lanthanides. Reactivity studies of isolated samples of <b>1-Ln</b> and <b>2-Ln</b> with 1,2-dimethoxyethane (DME) were conducted to determine if methoxide products, found in previous <i>in situ</i> studies of the synthesis of <b>2-Ln</b> by Lappert and co-workers, would form. Methoxide products were not observed, which shows that the chemistry of the isolated complexes differs from that of the <i>in situ</i> reduction reactions

Electronic Conductivity, Ferrimagnetic Ordering, and Reductive Insertion Mediated by Organic Mixed-Valence in a Ferric Semiquinoid Metal–Organic Framework

A three-dimensional network solid composed of Fe^III centers and para­magnetic semi­quinoid linkers, (NBu₄)₂­Fe^III₂­(dhbq)₃ (dhbq^2–/3– = 2,5-dioxido­benzo­quinone/​1,2-dioxido-4,5-semi­quinone), is shown to exhibit a conductivity of 0.16 ± 0.01 S/cm at 298 K, one of the highest values yet observed for a metal–organic framework (MOF). The origin of this electronic conductivity is determined to be ligand mixed-valency, which is characterized using a suite of spectro­scopic techniques, slow-scan cyclic voltammetry, and variable-temperature conductivity and magnetic susceptibility measurements. Importantly, UV–vis–NIR diffuse reflectance measurements reveal the first observation of Robin–Day Class II/III mixed valency in a MOF. Pursuit of stoichio­metric control over the ligand redox states resulted in synthesis of the reduced frame­work material Na_0.9­(NBu₄)_1.8Fe^III₂(dhbq)₃. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are investigated and correlated to the relative ratio of the two different ligand redox states. Overall, the transition metal–semi­quinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in MOFs

Electronic Conductivity, Ferrimagnetic Ordering, and Reductive Insertion Mediated by Organic Mixed-Valence in a Ferric Semiquinoid Metal–Organic Framework

A three-dimensional network solid composed of Fe^III centers and para­magnetic semi­quinoid linkers, (NBu₄)₂­Fe^III₂­(dhbq)₃ (dhbq^2–/3– = 2,5-dioxido­benzo­quinone/​1,2-dioxido-4,5-semi­quinone), is shown to exhibit a conductivity of 0.16 ± 0.01 S/cm at 298 K, one of the highest values yet observed for a metal–organic framework (MOF). The origin of this electronic conductivity is determined to be ligand mixed-valency, which is characterized using a suite of spectro­scopic techniques, slow-scan cyclic voltammetry, and variable-temperature conductivity and magnetic susceptibility measurements. Importantly, UV–vis–NIR diffuse reflectance measurements reveal the first observation of Robin–Day Class II/III mixed valency in a MOF. Pursuit of stoichio­metric control over the ligand redox states resulted in synthesis of the reduced frame­work material Na_0.9­(NBu₄)_1.8Fe^III₂(dhbq)₃. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are investigated and correlated to the relative ratio of the two different ligand redox states. Overall, the transition metal–semi­quinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in MOFs

Reversible CO Scavenging via Adsorbate-Dependent Spin State Transitions in an Iron(II)–Triazolate Metal–Organic Framework

A new metal–organic framework, Fe-BTTri (Fe₃[(Fe₄Cl)₃(BTTri)₈]₂·18CH₃OH, H₃BTTri =1,3,5-tris­(1<i>H</i>-1,2,3-triazol-5-yl)­benzene)), is found to be highly selective in the adsorption of CO over a variety of other gas molecules, making it extremely effective, for example, in the removal of trace CO from mixtures with H₂, N₂, and CH₄. This framework not only displays significant CO adsorption capacity at very low pressures (1.45 mmol/g at just 100 μbar), but, importantly, also exhibits readily reversible CO binding. Fe-BTTri utilizes a unique spin state change mechanism to bind CO in which the coordinatively unsaturated, high-spin Fe^II centers of the framework convert to octahedral, low-spin Fe^II centers upon CO coordination. Desorption of CO converts the Fe^II sites back to a high-spin ground state, enabling the facile regeneration and recyclability of the material. This spin state change is supported by characterization via infrared spectroscopy, single crystal X-ray analysis, Mössbauer spectroscopy, and magnetic susceptibility measurements. Importantly, the spin state change is selective for CO and is not observed in the presence of other gases, such as H₂, N₂, CO₂, CH₄, or other hydrocarbons, resulting in unprecedentedly high selectivities for CO adsorption for use in CO/H₂, CO/N₂, and CO/CH₄ separations and in preferential CO adsorption over typical strongly adsorbing gases like CO₂ and ethylene. While adsorbate-induced spin state transitions are well-known in molecular chemistry, particularly for CO, to our knowledge this is the first time such behavior has been observed in a porous material suitable for use in a gas separation process. Potentially, this effect can be extended to selective separations involving other π-acids

Selective, Tunable O₂ Binding in Cobalt(II)–Triazolate/Pyrazolate Metal–Organic Frameworks

The air-free reaction of CoCl₂ with 1,3,5-tri­(1<i>H</i>-1,2,3-triazol-5-yl)­benzene (H₃BTTri) in <i>N</i>,<i>N</i>-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co₃[(Co₄Cl)₃(BTTri)₈]₂·DMF), a sodalite-type metal–organic framework. Desolvation of this material generates coordinatively unsaturated low-spin cobalt­(II) centers that exhibit a strong preference for binding O₂ over N₂, with isosteric heats of adsorption (<i>Q</i>_st) of −34(1) and −12(1) kJ/mol, respectively. The low-spin (<i>S</i> = 1/2) electronic configuration of the metal centers in the desolvated framework is supported by structural, magnetic susceptibility, and computational studies. A single-crystal X-ray structure determination reveals that O₂ binds end-on to each framework cobalt center in a 1:1 ratio with a Co–O₂ bond distance of 1.973(6) Å. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co₃[(Co₄Cl)₃(BDTriP)₈]₂·DMF; H₃BDTriP = 5,5′-(5-(1<i>H</i>-pyrazol-4-yl)-1,3-phenylene)­bis­(1<i>H</i>-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O₂ affinities (<i>Q</i>_st = −47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O₂ adducts in Co-BTTri are best described as cobalt­(II)–dioxygen species with partial electron transfer, while the stronger binding sites in Co-BDTriP form cobalt­(III)–superoxo moieties. The stability, selectivity, and high O₂ adsorption capacity of these materials render them promising new adsorbents for air separation processes

Structural and Electronic Effects on the Properties of Fe₂(dobdc) upon Oxidation with N₂O

We report electronic, vibrational, and magnetic properties, together with their structural dependences, for the metal–organic framework Fe₂(dobdc) (dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate) and its derivatives, Fe₂(O)₂(dobdc) and Fe₂(OH)₂(dobdc)species arising in the previously proposed mechanism for the oxidation of ethane to ethanol using N₂O as an oxidant. Magnetic susceptibility measurements reported for Fe₂(dobdc) in an earlier study and reported in the current study for Fe^II_0.26[Fe^III(OH)]_1.74(dobdc)­(DMF)_0.15(THF)_0.22, which is more simply referred to as Fe₂(OH)₂(dobdc), were used to confirm the computational results. Theory was also compared to experiment for infrared spectra and powder X-ray diffraction structures. Structural and magnetic properties were computed by using Kohn–Sham density functional theory both with periodic boundary conditions and with cluster models. In addition, we studied the effects of different treatments of the exchange interactions on the magnetic coupling parameters by comparing several approaches to the exchange-correlation functional: generalized gradient approximation (GGA), GGA with empirical Coulomb and exchange integrals for 3<i>d</i> electrons (GGA+U), nonseparable gradient approximation (NGA) with empirical Coulomb and exchange integrals for 3<i>d</i> electrons (NGA+U), hybrid GGA, meta-GGA, and hybrid meta-GGA. We found the coupling between the metal centers along a chain to be ferromagnetic in the case of Fe₂(dobdc) and antiferromagnetic in the cases of Fe₂(O)₂(dobdc) and Fe₂(OH)₂(dobdc). The shift in magnetic coupling behavior correlates with the changing electronic structure of the framework, which derives from both structural and electronic changes that occur upon metal oxidation and addition of the charge-balancing oxo and hydroxo ligands

Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal–Organic Framework Fe(1,2,3-triazolate)₂(BF₄)_<i>x</i>

Metal–organic frameworks are of interest for use in a variety of electrochemical and electronic applications, although a detailed understanding of their charge transport behavior, which is of critical importance for enhancing electronic conductivities, remains limited. Herein, we report isolation of the mixed-valence framework materials, Fe­(tri)₂(BF₄)_<i>x</i> (tri^– = 1,2,3-triazolate; <i>x</i> = 0.09, 0.22, and 0.33), obtained from the stoichiometric chemical oxidation of the poorly conductive iron­(II) framework Fe­(tri)₂, and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe­(tri)₂(BF₄)_0.33, displays a room-temperature conductivity of 0.3(1) S/cm, which represents an increase of 8 orders of magnitude from that of the parent material and is one of the highest conductivity values reported among three-dimensional metal–organic frameworks. Detailed characterization of Fe­(tri)₂ and the Fe­(tri)₂(BF₄)_<i>x</i> materials via powder X-ray diffraction, Mössbauer spectroscopy, and IR and UV–vis-NIR diffuse reflectance spectroscopies reveals that the high conductivity arises from intervalence charge transfer between mixed-valence low-spin Fe^II/III centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized Fe^II/III species in Fe­(tri)₂(BF₄)_<i>x</i> at 290 K, one of the first such observations for a metal–organic framework. The electronic structure of valence-pure Fe­(tri)₂ and the charge transport mechanism and electronic structure of mixed-valence Fe­(tri)₂(BF₄)_<i>x</i> frameworks are discussed in detail