Stereoselective formation of dinuclear complexes with anomalous CD spectra

A novel chiral tetradentate Schiff base ligand forms dinuclear lambda,lambda-[M2L2Cl2]2+ complexes with high diastereoselectivity; the complexes exhibit anomalous CD spectra due to exciton coupling of chromophores located on different metal centres.     

Thermal Elimination of Ethylene from Cyclobutyl Groups Characterized by X-ray Crystallography in a Metal–Organic Framework Matrix

Methods to synthesize and characterize aromatic molecules with vinyl substituents are sought after yet limited in the literature. Here, we introduce cyclobutyl groups into a metal–organic framework (MOF) matrix that are poised to produce ethylene upon heating. The expulsion of ethylene produces vinyl groups on an aromatic core, which are isolated by the crystalline matrix of the framework. This enables full characterization of the thermolysis by single-crystal X-ray diffraction. Further, we modify the vinyl groups by a bromine addition reaction. Importantly, the two transformations happen in a single-crystal-to-single-crystal manner without changing the overall network structure of the parent framework. New insights into the structural and synthetic chemistry of this important class of compound are generated. Installing reactive vinyl tags in materials by the high temperature thermolysis of cyclobutyl groups is a powerful strategy for altering their physicochemical characteristics.     

Enhancing Multicomponent Metal–Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal–organic frameworks (MOFs) that contain the three-dimensional cubane-1,4-dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB-30), comprising cdc, 4,4′-biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π–π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.     

Enhancing Multicomponent Metal–Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal–organic frameworks (MOFs) that contain the three‐dimensional cubane‐1,4‐dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB‐30), comprising cdc, 4,4′‐biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π–π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.     

CD spectra of d-f heterobimetallic helicates with segmental di-imine ligands

We have investigated the CD spectra of a series of enantiomerically pure heterobimetallic helicates, Lambda,Lambda-[LnCr(1)(3)](6+) (Ln = Eu, Gd, Tb), which contain segmental di-imine ligands. For the mononuclear precursor of these helicates, Lambda-[Cr(1)(3)](3+), a positive exciton couplet was observed around 330 nm, as expected for a tris(di-imine) complex with this absolute configuration. The titration of Ln(III) ions into a solution of this complex leads to the formation of Lambda,Lambda-[LnCr(1)(3)](6+). During this process, the CD signal was observed to invert to give a signal which was negative at lower energies. We investigated the observed changes in the CD spectra using a ZINDO-based computational method which we have previously developed. We were able to show that the exciton coupling of the chromophores coordinated to the Cr and Ln ions give rise to CD signals of opposite phase, despite having the same nominal absolute configuration. Exciton coupling between chromophores located on different metal centers ("internuclear" exciton coupling) is also predicted to have a significant impact on the observed spectrum. We were able to "deconstruct" the observed CD spectra into a set of competing exciton coupling effects and show that the sign of these spectra does not correlate with the absolute configuration of the individual metal centers.     

Complexes of 5,5'-aminoacido-substituted 2,2'-bipyridyl ligands: control of diastereoselectivity with a pH switch and a chloride-responsive combinatorial library

The synthesis and coordination chemistry of a new chiral ligand, 2,2'-bipyridine substituted at the 5 and 5' positions by N-methyl-L-valine methyl ester (5), is presented. The ligand readily forms complexes [M(5)3]2+ where M = Co(II) and Fe(II) in CH3CN, and the complexation reaction is slightly diastereoselective (d.e. =ca. 20%) in favour of the Delta diastereomer. The addition of six equivalents of HCl to these complexes [M(II)(5)3]2+ leads to formation of Delta-[M(II)(5H2)3]8+ with a d.e. of 100%. This high diastereoselectivity can be reversed by the addition of base i.e. the diastereoselectivity can be controlled by the pH. Delta-[Fe(5H2)3]8+ was found to bind chloride ions in CD3OD-CD3CN (6:1) with a binding constant of 260 M(-1). [Co(II)(5)3]2+ can be oxidised to Delta-[Co(III)(5H2)3]9+. Formation constants for both [Co(II)(5)3]2+ and [Co(II)(5H2)3]8+ in acetonitrile were obtained by spectrophotometric titrations. In the former case, the stability constant, log beta3 = 19.5(8), is very similar to that measured for [Co(II)(bipy)3]2+ (log beta3 = 19.3(7)) but this drops significantly when the amine groups of are protonated (log beta3 = 16.5(2)). A dynamic combinatorial library was prepared by mixing three equivalents of, three equivalents of bipy, and two equivalents of Co(II) in CD3CN. The presence of all possible Delta- and Lambda-[Co(II)(5)x(bipy)(3-x)]2+ complexes was inferred from 1H NMR and ES-MS spectra. Addition of protons to this library reduced the number of components by inducing diastereoselectivity, and presence of chloride further simplified the 1H NMR spectrum, indicating that [Cl2 ligand Delta-[Co(II)(5H2)3]]6+ and [Co(II)(bipy)3]2+ were the dominant products.     

The first example of a mixed alkoxide hydride of boron: sodium boron isopropoxide trihydride

The title compound, poly[[μ‐trihydro(isopropoxy)borato]sodium(I)], [Na(C₃H₁₀BO)]_n, forms unique polymeric layers normal to the c axis via Na⁺...O [2.3405 (15) Å] and Na⁺...H(borane) [2.22 (3) and 2.28 (3) Å] interactions. This arrangement builds on distorted tetrahedral Na⁺, oxygen and boron environments, with one of the borane hydride units uncoordinated, and highlights potential H₃B—O‐based chemistry.