Proton and Electron Transfer in the Formation of a Copper Dithiolene-Based Coordination Polymer.

Metal bis(dithiolene) complexes are promising building blocks for electrically conductive coordination polymers. N-Heterocyclic dithiolene complexes allow their cross-linking via the coordination of N-donor atoms to additional transition metal ions. In this study, we present the formal copper(II) and copper(III) 6,7-quinoxalinedithiolene complexes [Cu(qdt)2]- and [Cu(qdt)2]2- (qdt2-: 6,7-quinoxalinedithiolate), as well as the 2D coordination polymer Cu[Cu(Hqdt)(qdt)] (3). The dithiolene complexes were isolated as (Bu4N)2[Cu(qdt)2] (1), Na[Cu(qdt)2]·4H2O (2a), [Na(acetone)4][Cu(qdt)2] (2b), and [Ni(MeOH)6][Cu(qdt)2]2·2H2O (2c). Their crystal structures reveal nearly planar complexes with a high tendency of π-stacking. For a better understanding of their coordination behavior, the electronic properties are investigated by UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT simulations. The synthesis of the 2D coordination polymer 3 involves the reduction and protonation of the monoanionic copper(III) complex. A combination of powder X-ray diffraction, magnetic susceptibility measurements, as well as IR and EPR spectroscopy confirm that formal [CuII(Hqdt)(qdt)]- units link trigonal planar copper(I) atoms to a dense 2D coordination polymer. The electrical conductivity of 3 at room temperature is 2 × 10-7 S/cm. Temperature dependent conductivity measurements confirm the semiconducting behavior of 3 with an Arrhenius derived activation energy of 0.33 eV. The strong absorption of 3 in the visible and NIR regions of the spectrum is caused by the small optical band gap of Eg,opt = 0.65 eV, determined by diffuse reflectance spectroscopy. This study sheds light on the coordination chemistry of N-heterocyclic dithiolene complexes and may serve as a reference for the future design and synthesis of dithiolene-based coordination polymers with interesting electrical and magnetic properties.

Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal-Organic Frameworks.

Metal-organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal-organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe-Cu paddlewheels as well as monometallic Cu-Cu and Fe-Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal-organic framework materials and the thorough characterization thereof is particularly important to derive structure-property or structure-activity correlations.

A combined continuous wave electron paramagnetic resonance and DFT calculations of copper-doped [Cd0.98Cu0.02(prz-trz-ia)] metal-organic framework.

Continuous wave X-band electron paramagnetic resonance (EPR) and density functional theory (DFT) were successfully applied to explore the incorporation and coordination state of the Cu2+ ions in the [Cd0.98Cu0.02(prz-trz-ia)] porous metal-organic frameworks. EPR measurements on powder samples and single crystals provided the full electron Zeeman g and copper hyperfine ACu interaction tensors including the orientation of their principal axes frames. DFT computations allowed for a detailed interpretation of the experimental results in terms of coordination symmetry and binding properties of the paramagnetic Cu2+ ions. Cupric ions were found to substitute Cd2+ ions in the dinuclear Cd-Cd units where they experience a noticeably distorted elongated pyramidal coordination environment formed by three nitrogen and two oxygen atoms from three linker molecules.

Atomic-Scale Quantum Sensing of Ensembles of Guest Molecules in a Metal-Organic Framework with Intrinsic Electron Spin Centers.

One of the exciting applications of electron-spin-based quantum sensing is the detection of distant nuclear spins of external molecular species. Here, we explore the application of a metal-organic framework (MOF) material as a host matrix for sensing spin centers. As a sensor, we employ inherent Cu2+ ions in the structure of a Zn-doped HKUST-1 framework. As a target molecular species, we use butane gas that exhibits no specific chemical reactivity toward the inner surface of HKUST-1 and is thus randomly distributed inside the MOF pore network. By employing a conventional double-resonance pulse sequence, we can effectively detect the coupling of the distant 1H nuclear spins of butane to the electron spin of the sensor and gain atomic-scale insight into their spatial distribution. Thus, our proof-of-the-concept experiment demonstrates that MOFs, the materials featuring extremely large surface area and great tunability, are perfectly suited as a key element for emerging magnetic quantum sensing solutions.

Adsorption of Olefins at Cupric Ions in Metal-Organic Framework HKUST-1 Explored by Hyperfine Spectroscopy.

Metal-organic frameworks (MOFs) represent a promising platform for gas storage and separation. In this work, adsorption of olefins in Zn-doped HKUST-1 metal-organic framework is explored with hyperfine spectroscopy. By means of electron-nuclear double resonance and hyperfine sublevel correlation spectroscopy, we detect the interaction between the electron spins of the Cu2+ sites of the MOF and the 1H nuclear spins of adsorbed C2H4 and C4H8. Further analysis of the measured spectra allows us to precisely locate the protons in the vicinity of the Cu2+ ions and thereby establish ensemble-averaged structural models of the olefin molecules adsorbed at the open metal sites of HKUST-1.