Impact of steric bulk on photoinduced ligand exchange reactions in Mn(I) photoCORMs

© 2020 Elsevier B.V. The investigation of ligand exchange reactions in Mn(I)-based photoactivated CO-releasing molecules, or “photoCORMs,” has largely focused on the electronic effects of the ligand set. In this work, we report the effects of sterically bulky bidentate (NN) ligands on the efficiency for CO release and the formation of photochemical intermediates in fac-[Mn(NN)(CO)3(L)]n+ photoCORMs. The vibrational and electronic absorption spectroscopy and photochemistry of two new Mn(I) photoCORMs with a sterically bulky 6,6′-dimethyl-2,2′-bipyridine ligand, fac-[Mn(6,6′-Me2bpy)(CO)3Br] (6,6′-Me2bpy-Br) and fac-[Mn(6,6′-Me2bpy)(CO)3(py)]+ (6,6′-Me2bpy-py), are reported in comparison to two previously reported analogues, fac-[Mn(4,4′-Me2bpy)(CO)3Br] (4,4′-Me2bpy-Br) and fac-[Mn(4,4′-Me2bpy)(CO)3(py)]+ (4,4′-Me2bpy-py) with the 4,4′-dimethyl-2,2′-bipyridine ligand. The steric demands of the methyl substituents in the 6,6′ positions on bpy significantly distort the structure, as the crystal structure shows contraction between the equatorial CO ligands and tilting of the bidentate ligand relative to the 4,4′-Me2bpy complexes. The movement of the methyl substituents from the 4,4′ to the 6,6′ positions on bpy has little impact on the electronic properties of the complexes, as observed by FTIR and electronic absorption spectroscopy, while the steric bulk of 6,6′-Me2bpy increases the quantum yield of CO release (ΦCO) and increases the lability of the Br− and py ligands compared to the 4,4′-Me2bpy complexes with less steric bulk

Ferrocene Bis(Sulfonate) Salt as Redoxmer for Fast and Steady Redox Flow Desalination

Desalination is considered a promising solution to alleviate water shortages, yet current methods are often restricted, due to challenges like high energy consumption, significant cost, or limited desalination capacity. In this study, we present a novel approach of redox flow desalination (RFD) utilizing the highly aqueous-soluble and reversible redox-active compound, potassium 1,1′-bis(sulfonate) ferrocene (1,1′-FcDS). This water-soluble organic compound yielded stable and rapid desalination, sustaining extended operation without notable decay and achieving an impressive desalination rate of up to 457.5 mmol·h−1·m−2 and energy consumption as low as 40.2 kJ·molNaCl−1. Specifically, the RFD device effectively desalinated a 50 mM NaCl solution to potable standards within 6000 s using 1,1′-FcDS. It maintained an average energy consumption of 178.16 kJ·molNaCl−1 and exhibited negligible deterioration in desalination rate, energy efficiency, and charge efficiency throughout a rigorous 12,000 s cycling test. Furthermore, the versatility of this method was demonstrated by effectively treating saline water with varying initial concentrations from 10 mM to 50 mM, showcasing its potential across a broad spectrum of applications

Fluorine labeling of ortho-phenylenes to facilitate conformational analysis

¹H NMR spectroscopy is a powerful tool for the conformational analysis of ortho-phenylene foldamers in solution. However, as o-phenylenes are integrated into ever-more-complex systems, we are reaching the limits of what can be analyzed by ¹H- and ¹³C-based NMR techniques. Here, we explore fluorine labeling of o-phenylene oligomers for analysis by ¹⁹F NMR spectroscopy. Two series of fluorinated oligomers have been synthesized. Optimization of monomers for Suzuki coupling enables an efficient stepwise oligomer synthesis. The oligomers all adopt well-folded geometries in solution, as determined by ¹H NMR spectroscopy and X-ray crystallography. ¹⁹F NMR experiments complement these methods well. The resolved singlets of one-dimensional ¹⁹F{¹H} spectra are very useful for determining relative conformer populations. The additional information from two-dimensional ¹⁹F NMR spectra is also clearly valuable when making ¹H assignments. Comparison of ¹⁹F isotropic shielding predictions to experimental chemical shifts is not, however, currently sufficient by itself to establish o-phenylene geometries

Visible-Light-Driven Photosystems Using Heteroleptic Cu(I) Photosensitizers and Rh(III) Catalysts To Produce H₂

The synthesis of two new heteroleptic Cu­(I) photosensitizers (PS), [Cu­(Xantphos)­(NN)]­PF₆ (NN = biq = 2,2′-biquinoline, dmebiq = 2,2′-biquinoline-4,4′-dimethyl ester; Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene), along with the associated structural, photophysical, and electrochemical properties, are described. The biquinoline diimine ligand extends the PS light absorbing properties into the visible with a maximum absorption at 455 and 505 nm for NN = biq and dmebiq, respectively, in CH₂Cl₂ solvent. Following photoexcitation, both Cu­(I) PS are emissive at low energy, albeit displaying stark differences in their excited state lifetimes (τ_MLCT = 410 ± 5 (biq) and 44 ± 4 ns (dmebiq)). Cyclic voltammetry indicates a Cu-based HOMO and NN-based LUMO for both complexes, whereby the methyl ester substituents stabilize the LUMO within [Cu­(Xantphos)­(dmebiq)]⁺ by ∼0.37 V compared to the unsubstituted analogue. When combined with H₂O, <i>N,N</i>-dimethylaniline (DMA) electron donor, and <i>cis</i>-[Rh­(NN)₂Cl₂]­PF₆ (NN = Me₂bpy = 4,4′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, dmebpy = 2,2′-bipyridine-4,4′-dimethyl ester) water reduction catalysts (WRC), photocatalytic H₂ evolution is only observed using the [Cu­(Xantphos)­(biq)]⁺ PS. Furthermore, the choice of <i>cis</i>-[Rh­(NN)₂Cl₂]⁺ WRC strongly affects the catalytic activity with turnover numbers (TON_Rh = mol H₂ per mol Rh catalyst) of 25 ± 3, 22 ± 1, and 43 ± 3 for NN = Me₂bpy, bpy, and dmebpy, respectively. This work illustrates how ligand modification to carefully tune the PS light absorbing, excited state, and redox-active properties, along with the WRC redox potentials, can have a profound impact on the photoinduced intermolecular electron transfer between components and the subsequent catalytic activity

Resolving a Half-Century-Long Controversy between (Magneto)optical and EPR Spectra of Single-Electron-Reduced [PcFe]⁻, [PcFeL]⁻, and [PcFeX]^2– Complexes: Story of a Double Flip

The reduction of iron(II) phthalocyanine (Pc(2−)FeII) or its bisaxially coordinated complexes results in the formation of the purple/red [PcFe]−, [PcFeL]−, and [PcFeX]2– (L is neutral and X is anionic ligand) species. The X-ray structure of the [K(DME)4][PcFe] complex exhibits a square-planar [PcFe]− anion. 1H NMR spectra of the reduced species have one or two phthalocyanine broad peaks between 15 and 17 ppm. Solution magnetic moments are consistent with the presence of a single unpaired electron. A solid-state Mössbauer spectrum of [K(DME)4][PcFe] is consistent with an early report [Taube, R. Pure Appl. Chem.1974, 38, 427−438]. The solid-state EPR spectrum of the [PcFe]− anion is close to that recorded by Konarev et al. [Dalton Trans.2012, 41, 13841−13847]. Solution EPR spectra of reduced species have axial symmetry (g⊥ ∼ 2.08–2.17 and g|| ∼ 1.95–1.96) and correlate well with spectra reported by Lever and Wilshire in 1978 [Inorg. Chem.1978, 17, 1145−1151]. The UV–vis spectra of pentacoordinated [PcFeL]− and [PcFeX]2– anions consist of the characteristic bands around 810, 690, and 515 nm. These bands correlate well with the set of MCD pseudo A-terms and resemble transitions in the [Pc(3−)M]− and [Pc(3−)ML]− compounds. The UV–vis and MCD spectra of [PcFeL]− and [PcFeX]2– complexes are in stark contrast to the crystallographically characterized reference [Pc(2−)CoI]− anion, which is EPR silent, has a regular diamagnetic 1H NMR spectrum, and has an intense Q-band at 699 nm, which correlates well with the strong MCD A-term. The DFT and TDDFT calculations are suggestive of the iron(II) center in a (dxy)2(dxz,yz)3(dz2)1 (s = 1) electronic configuration that is antiferromagnetically coupled with the one-electron-reduced Pc(3−) ligand (i.e., [Pc(3−)FeII]−, [Pc(3−)FeIIL]−, and [Pc(3−)FeIIX]2–). The calculated EPR, Mössbauer, and UV–vis spectra of [PcFe]−, [PcFeL]−, and [PcFeX]2– complexes are in excellent agreement with the experimental data, thus resolving the controversy between axial s = 1/2 like EPR and Pc(3−)-like UV–vis spectra of these compounds

Lone-Pair-Induced Topicity Observed in Macrobicyclic Tetra-thia Lactams and Cryptands: Synthesis, Spectral Identification, and Computational Assessment

The synthesis of a rigid macrobicyclic N,S lactam <b>L1</b> and a topologically favored in/in N,S cryptand <b>L2</b> are reported with X-ray structure analysis, dynamic correlation NMR spectroscopy, and computational analysis. Lactam <b>L1</b> exhibits two distinct rotameric conformations (plus their enantiomeric counterparts) at 25 °C, as confirmed via NMR spectroscopy and computational analysis. Coalescence of the resonances of <b>L1</b> was observed at 115 °C, allowing for complete nuclei to frequency correlation. Combining computational investigations with experimental data, topological equilibria and relative energies/strain relating to the perturbation of the pore were determined. Due to the increased conformational strain of the N₂S₂ template, the nitrogen lone pairs in <b>L2</b> elicit a unique transannular interaction, resulting in a thermodynamically favored in/in nephroidal racemate. The combination of preferred topology, steric relief, and electronic localization of <b>L2</b> induces a chiral environment imparted through the amine with a computed inversion barrier of 10.3 kcal mol^–1

Lone-Pair-Induced Topicity Observed in Macrobicyclic Tetra-thia Lactams and Cryptands: Synthesis, Spectral Identification, and Computational Assessment

The synthesis of a rigid macrobicyclic N,S lactam <b>L1</b> and a topologically favored in/in N,S cryptand <b>L2</b> are reported with X-ray structure analysis, dynamic correlation NMR spectroscopy, and computational analysis. Lactam <b>L1</b> exhibits two distinct rotameric conformations (plus their enantiomeric counterparts) at 25 °C, as confirmed via NMR spectroscopy and computational analysis. Coalescence of the resonances of <b>L1</b> was observed at 115 °C, allowing for complete nuclei to frequency correlation. Combining computational investigations with experimental data, topological equilibria and relative energies/strain relating to the perturbation of the pore were determined. Due to the increased conformational strain of the N₂S₂ template, the nitrogen lone pairs in <b>L2</b> elicit a unique transannular interaction, resulting in a thermodynamically favored in/in nephroidal racemate. The combination of preferred topology, steric relief, and electronic localization of <b>L2</b> induces a chiral environment imparted through the amine with a computed inversion barrier of 10.3 kcal mol^–1

Lone-Pair-Induced Topicity Observed in Macrobicyclic Tetra-thia Lactams and Cryptands: Synthesis, Spectral Identification, and Computational Assessment

The synthesis of a rigid macrobicyclic N,S lactam <b>L1</b> and a topologically favored in/in N,S cryptand <b>L2</b> are reported with X-ray structure analysis, dynamic correlation NMR spectroscopy, and computational analysis. Lactam <b>L1</b> exhibits two distinct rotameric conformations (plus their enantiomeric counterparts) at 25 °C, as confirmed via NMR spectroscopy and computational analysis. Coalescence of the resonances of <b>L1</b> was observed at 115 °C, allowing for complete nuclei to frequency correlation. Combining computational investigations with experimental data, topological equilibria and relative energies/strain relating to the perturbation of the pore were determined. Due to the increased conformational strain of the N₂S₂ template, the nitrogen lone pairs in <b>L2</b> elicit a unique transannular interaction, resulting in a thermodynamically favored in/in nephroidal racemate. The combination of preferred topology, steric relief, and electronic localization of <b>L2</b> induces a chiral environment imparted through the amine with a computed inversion barrier of 10.3 kcal mol^–1

CCDC 2065533: Experimental Crystal Structure Determination

Related Article: Ebube E. Oyeka, Ilknur Babahan, Bernard Eboma, Kenechukwu J. Ifeanyieze, Obinna C. Okpareke, Esin P. Coban, Ali Özmen, Burak Coban, Mehran Aksel, Namık Özdemir, Tatiana.V. Groutso, Jude I. Ayogu, Ufuk Yildiz, Mehmet Dinçer Bilgin, H. Halil Biyik, Briana R. Schrage, Christopher J. Ziegler, Jonnie N. Asegbeloyin|2021|Inorg.Chim.Acta|528|120590|doi:10.1016/j.ica.2021.12059

CCDC 2009344: Experimental Crystal Structure Determination

Related Article: Ebube E. Oyeka, Ilknur Babahan, Bernard Eboma, Kenechukwu J. Ifeanyieze, Obinna C. Okpareke, Esin P. Coban, Ali Özmen, Burak Coban, Mehran Aksel, Namık Özdemir, Tatiana.V. Groutso, Jude I. Ayogu, Ufuk Yildiz, Mehmet Dinçer Bilgin, H. Halil Biyik, Briana R. Schrage, Christopher J. Ziegler, Jonnie N. Asegbeloyin|2021|Inorg.Chim.Acta|528|120590|doi:10.1016/j.ica.2021.12059