Spectroscopic interrogations of isostructural metalloporphyrin-based metal-organic frameworks with strongly and weakly coordinating guest molecules

<p>Two isostructural metal-organic frameworks based on cobalt(II) and nickel(II) metalloporphyrin linkers, Co-PCN222 and Ni-PCN222, are investigated using resonance Raman and X-ray absorption spectroscopy. The spectroscopic consequences of framework formation and host–guest interaction with weakly and strongly coordinating guest molecules (acetone and pyridine) are assessed. Structure sensitive vibrational modes of the resonance Raman spectra provide insights on the electronic and structural changes of the porphyrin linkers upon framework formation. XANES and EXAFS measurements reveal axial binding behavior of the metalloporphyrin units in Co-PCN222, but almost no axial interaction with guest molecules at the Ni porphyrin sites in Ni-PCN222.</p

Spectroscopic Evidence of Pore Geometry Effect on Axial Coordination of Guest Molecules in Metalloporphyrin-Based Metal Organic Frameworks

A systematic comparison of host–guest interactions in two iron porphyrin-based metal–organic frameworks (MOFs), FeCl-PCN222 and FeCl-PCN224, with drastically different pore sizes and geometries is reported in this fundamental spectroscopy study. Guest molecules (acetone, imidazole, and piperidine) of different sizes, axial binding strengths, and reactivity with the iron porphyrin centers are employed to demonstrate the range of possible interactions that occur at the porphyrin sites inside the pores of the MOF. Binding patterns of these guest species under the constraints of the pore geometries in the two frameworks are established using multiple spectroscopy methods, including UV–vis diffuse reflectance, Raman, X-ray absorption, and X-ray emission spectroscopy. Line shape analysis applied to the latter method provides quantitative information on axial ligation through its spin state sensitivity. The observed coordination behaviors derived from the spectroscopic analyses of the two MOF systems are compared to those predicted using space-filling models and relevant iron porphyrin molecular analogues. While the space-filling models show the ideal axial coordination behavior associated with these systems, the spectroscopic results provide powerful insight into the actual binding interactions that occur in practice. Evidence for potential side reactions occurring within the pores that may be responsible for the observed deviation from model coordination behavior in one of the MOF/guest molecule combinations is presented and discussed in the context of literature precedent

Spectroscopic Evidence for Room Temperature Interaction of Molecular Oxygen with Cobalt Porphyrin Linker Sites within a Metal–Organic Framework

Metalloporphyrin-based metal–organic frameworks offer a promising platform for developing solid-state porous materials with accessible, coordinatively unsaturated metal sites. Probing small-molecule interactions at the metalloporphyrin sites within these materials on a molecular level under ambient conditions is crucial for both understanding and ultimately harnessing this functionality for potential catalytic purposes. Co-PCN-222, a metal–organic framework based on cobalt­(II) porphyrin linkers. is investigated using in situ UV–vis diffuse-reflectance and X-ray absorption spectroscopy. Spectroscopic evidence for the axial interaction of diatomic oxygen with the framework’s open metalloporphyrin sites at room temperature is presented and discussed

Long-Lived Photoinduced Charge Separation in a Trinuclear Iron‑μ₃‑oxo-based Metal–Organic Framework

The presence of long-lived charge-separated excited states in metal–organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these charge-separated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100­(Fe), composed of Fe₃-μ₃-oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ₃-oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to further support this assignment and corresponding decay time. The MOF is prepared as a colloidal nanoparticle suspension for these measurements, so both its fabrication and particle size analysis are presented as well

Probing Framework-Restricted Metal Axial Ligation and Spin State Patterns in a Post-Synthetically Reduced Iron-Porphyrin-Based Metal–Organic Framework

An iron-porphyrin-based metal organic framework PCN-222­(Fe) is investigated upon postsynthetic reduction with piperidine. Fe K-edge X-ray absorption and Kβ mainline emission spectroscopy measurements reveal the local coordination geometry, oxidation, and spin state changes experienced by the Fe sites upon reaction with this axially coordinating reducing agent. Analysis and fitting of these data confirm the binding pattern predicted by a space-filling model of the structurally constrained pore environments. These results are further supported by UV–vis diffuse reflectance, IR, and resonance Raman spectroscopy data

Privileged Phosphine-Based Metal–Organic Frameworks for Broad-Scope Asymmetric Catalysis

A robust and porous Zr metal–organic framework (MOF) based on a BINAP-derived dicarb­oxyl­ate linker, BINAP-MOF, was synthesized and post-synthetically metalated with Ru and Rh complexes to afford highly enantio­selective catalysts for important organic transformations. The Rh-functionalized MOF is not only highly enantio­selective (up to >99% ee) but also 3 times as active as the homogeneous control. XAFS studies revealed that the Ru-functionalized MOF contains Ru-BINAP pre­catalysts with the same coordination environment as the homogeneous Ru complex. The post-synthetically metalated BINAP-MOFs provide a versatile family of single-site solid catalysts for catalyzing a broad scope of asymmetric organic transformations, including addition of aryl and alkyl groups to α,β-unsaturated ketones and hydrogenation of substituted alkene and carbonyl compounds

Strong Steric Hindrance Effect on Excited State Structural Dynamics of Cu(I) Diimine Complexes

The metal-to-ligand-charge-transfer (MLCT) excited state of Cu­(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral <i>D</i>_2<i>d</i> symmetry in the ground state to a flattened <i>D</i>₂ symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky <i>tert</i>-butyl groups in bis­(2,9-di-<i>tert</i>-butyl-1,10-phenanthroline)­copper­(I), [Cu^I(dtbp)₂]⁺. The two bulky <i>tert</i>-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of [Cu^I(dtbp)₂]⁺. Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that [Cu^I(dtbp)₂]⁺ exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral <i>D</i>_2<i>d</i> symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of [Cu^I(dtbp)₂]⁺ with its mono-<i>tert</i>-butyl counterpart, bis­(2-<i>tert</i>-butyl-1,10-phenanthroline)­copper­(I) [Cu^I(tbp)₂]⁺. The subpicosecond component assigned to the flattening of the <i>D</i>_2<i>d</i> coordination geometry in the MLCT excited state was again present in the latter because the absence of a <i>tert</i>-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the [Cu^I(dtbp)₂]⁺, [Cu^I(tbp)₂]⁺ exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications

Topotactic Transformations of Metal–Organic Frameworks to Highly Porous and Stable Inorganic Sorbents for Efficient Radionuclide Sequestration

Innovative solid-phase sorbent technologies are needed to extract radionuclides from harsh media for environmental remediation and in order to close the nuclear fuel cycle. Highly porous inorganic materials with remarkable sorptive properties have been prepared by topotactic transformations of metal–organic frameworks (MOFs) using both basic and acidic solutions. Treatment of Ti and Zr nanoMOFs with NaOH, Na₃PO₄, and H₃PO₄ yields Ti and Zr oxides, oxyphosphates, and phosphates via sacrificial removal of the organic ligands. This controlled ligand extraction process results in porous inorganic materials, which preserve the original MOF morphologies and impart useful surface functionalities, but are devoid of organic linkers. Structural investigation by X-ray absorption spectroscopy reveals preservation of the coordination environment of the scattering metal. Changing the MOF template introduces different metal and structural possibilities, while application of different digest solutions allows preparation of metal oxides, metal oxyphosphates, and metal phosphates. The high stability and porosity of these novel materials makes them ideally suited as nanosorbents in severe environments. Their potential for several radionuclide separations is demonstrated, including decontamination of high level nuclear waste, extraction of lanthanides, and remediation of radionuclide-contaminated seawater