Pavel M. Usov

Cooperative electrochemical water oxidation by Zr nodes and Ni–porphyrin linkers of a PCN-224 MOF thin film

Here, we demonstrate a new strategy for cooperative catalysis and proton abstraction via the incorporation of independent species competent in the desired reactivity into a metal–organic framework (MOF) thin film. The highly porous MOF, designated as PCN-224-Ni, is constructed by Zr–oxo nodes and nickel(II) porphyrin linkers. Films of PCN-224-Ni were grown in situ on FTO and were found to electrochemically facilitate the water oxidation reaction at near neutral pH.

A New Class of Metal-Cyclam-Based Zirconium Metal–Organic Frameworks for CO2 Adsorption and Chemical Fixation

Metal-organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr6 clusters and metallocyclams as organic linkers. Importantly, the cyclam core has accessible axial coordination sites for guest interactions and maintains the electronic properties exhibited by the parent cyclam ring. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO2 uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zr-based MOFs for the chemical fixation of CO2 with epoxides, including sterically hindered epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance.

Study of Electrocatalytic Properties of Metal–Organic Framework PCN-223 for the Oxygen Reduction Reaction

A highly robust metal-organic framework (MOF) constructed from Zr6 oxo clusters and Fe(III) porphyrin linkers, PCN-223-Fe was investigated as a heterogeneous catalyst for oxygen reduction reaction (ORR). Films of the framework were grown on a conductive FTO substrate and showed a high catalytic current upon application of cathodic potentials and achieved high H2O/H2O2 selectivity. In addition, the effect of the proton source on the catalytic performance was also investigated.

Magnetic, electrochemical and optical properties of a sulfate-bridged Co(II) imidazole dimer

The synthesis, as well as the electrochemical, optical and magnetic properties of a sulfate-bridged Co(II) dimer, [Co2(DMIM)4(μ2-O2,O,O′-SO4)2]·2MeCN (DMIM = 1,2-dimethylimidazole) (2), are reported. The crystal structure consists of two distorted pseudo-octahedral Co(II) centres, each ligated by the μ2-(O-O′-sulfato) and μ2-(O2-sulfato) bridging modes of the SO42− anions. Magnetic studies have identified significant antiferromagnetic coupling between the high spin Co(II) centres, with J = −28 cm−1. Cyclic voltammetry indicated the presence of a quasi-reversible ligand centred oxidation, while the vis-NIR spectrum of 2 revealed complex splitting of the absorption bands due to the tetragonal distortion of the octahedral geometry of the Co(II) centres.

Structural and optical investigations of charge transfer complexes involving the radical anions of TCNQ and F4TCNQ

The structures and optical band gaps of twelve radical anionic 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 7,7,8,8-tetracyano-2,3,4,5-tetrafluoroquinodimethane (F4TCNQ) based charge-transfer complexes are reported. The compounds described have been categorised into three general types based upon solid-state arrangements of the donor and acceptor molecules. Crystallographic, EPR and IR spectroscopic investigations indicated that both TCNQ and F4TCNQ in each of the compounds described exist in the radical monoanion form. Visible-NIR absorption measurements indicate optical band gaps in the range of 0.79 to 1.08 eV. Whilst the packing arrangements in CT complexes are known to affect the band gap, in the cases considered here no clear relationship between the packing arrangement and the optical band gap is apparent. The results suggest that in the absence of mixed valency the packing arrangement does not impact significantly upon the magnitude of the optical band gap.

A Comparative Study of the Structural, Optical, and Electrochemical Properties of Squarate-Based Coordination Frameworks

Systematic studies of the thermal expansion, optical, and redox properties of a series of six squarate-based frameworks, [MII(C4O4)(H2O)2] (MII = MnII, FeII, CoII, NiII, ZnII, CdII) have revealed that five members of the series exhibit cubic structures in which the squarate ligands are configured in an ‘eclipsed’ phase, while the CdII analogue exhibits a trigonal structure with a ‘staggered’ orientation of the ligands. The ‘eclipsed’ structures are characterised by a positive coefficient of thermal expansion, while the CdII analogue exhibits zero thermal expansion. Ultraviolet-visible-near infrared (UV-Vis-NIR) spectra and electrochemical measurements indicate that electron delocalisation across the dianionic squarate bridge is absent.

Structural and optical investigations of charge transfer complexes involving the F4TCNQ dianion

7,7,8,8-Tetracyano-2,3,4,5-tetrafluoroquinodimethane (F4TCNQ) in its dianionic form, F4TCNQ2−, is shown to form charge transfer complexes with a wide variety of organic cations. The structures and spectroscopic properties of fourteen F4TCNQ2− salts are described, thirteen of which have colours consistent with the formation of charge transfer complexes. Unlike neutral F4TCNQ charge transfer complexes, the dianion, F4TCNQ2− is able to act as a donor in its interaction with suitable cations that serve as acceptors in solid-state complexes. The F4TCNQ2− salts described in this work have been categorised into five different structural types according to the relative arrangements of cations and anions. In each case, structural and IR spectroscopic data indicate that the anions retain a formal −2 charge upon formation of the salt. The optical band gaps, determined from Vis-NIR spectra, are found to have the lowest values when the cation is a viologen, either methyl viologen or diphenylmethyl viologen.

Chapter 7:Conducting Framework Materials

Electronic conductivity in framework materials is a highly-sought after functional property, with the capacity to revolutionise a wide range of technologically- and industrially-useful fields. Examples of conducting metal-organic frameworks remain relatively limited at the present time; however enormous advances have been made over the past five years. This chapter details the latest developments in the exciting research frontier of electronically-conducting metal-organic frameworks (MOFs). Central to the discussion is a series of parameters which have now emerged for materials’ design, including redox matching, donor–acceptor, mixed-valence, and π-interactions. At both the fundamental and applied levels, enormous opportunities exist if long-range electronic conductivity can be harnessed, including intimate understandings of charge transport in 3D coordination space, and potential applications across catalysis, solid-state sensing, energy storage and conversion devices, amongst numerous others.

Untangling Complex Redox Chemistry in Zeolitic Imidazolate Frameworks Using Fourier Transformed Alternating Current Voltammetry

Two zeolitic imidazolate frameworks, ZIF-67 and ZIF-8, were interrogated for their redox properties using Fourier transformed alternating current voltammetry, which revealed that the 2-methylimidazolate ligand is responsible for multiple redox transformations. Further insight was gained by employing discrete tetrahedral complexes, [M(DMIM)4]2+ (DMIM = 1,2-dimethylimidazole, M = CoII or ZnII) which have similar structural motifs to ZIFs. In this work we demonstrate a multidirectional approach that enables the complex electrochemical behavior of ZIFs to be unraveled.

Insights into CO2 adsorption and chemical fixation properties of VPI-100 metal–organic frameworks

Metal–organic frameworks (MOFs) have shown great promise as efficient CO2 adsorbents, as well as an emerging class of heterogeneous catalysts for conversion of CO2 to other useful chemicals. In this work, we synthesized and characterized two isostructural hafnium-based MOFs, denoted as Hf-VPI-100 (Cu) and Hf-VPI-100 (Ni). Both frameworks demonstrated high catalytic efficiency for cycloaddition of CO2 to epoxides under ambient pressure. In situ PXRD, QCM and DRIFTs have been used to probe the interaction between the guest molecules (CO2/epoxide) and Hf-VPI-100. The crystal structures of these frameworks were preserved during the exposure to CO2 atmosphere up to 20 bar. The epoxide uptake per unit cell of VPI-100 and diffusion coefficients have been calculated by QCM analysis. Comparison of catalytic efficiency between Hf-VPI-100 and the previously reported Zr-based VPI-100, aided by electronic structure calculations revealed that the open metal centers in the metallocyclam act as the primary Lewis acid sites to facilitate the catalytic conversion of CO2.