Continuous Wave and Pulsed Electron Spin Resonance Spectroscopy of Paramagnetic Framework Cupric Ions in the Zn(II) Doped Porous Coordination Polymer Cu_3−xZn_x(btc)₂

Abstract

In the parent metal-organic framework Cu 3 (btc) 2 material the Cu(II) pairs in the paddle wheel building blocks of the framework give rise to an antiferromagnetic spin state with an electron spin resonance (ESR) silent S ) 0 ground state. The thermally excited S ) 1 state of the Cu(II) pairs can be observed for temperatures above 80 K by ESR spectroscopy but give rise to an exchanged narrowed resonance line preventing the exploration of any structural details in the environment of the paddle wheel units. However, magnetically diluted paramagnetic binuclear Cu-Zn clusters can be formed by substitution of Cu(II) ions by Zn(II) at low doping levels, as already known for zinc-doped copper acetate monohydrate. Indeed, ESR, hyperfine sublevel correlation spectroscopy (HYSCORE) and pulsed electron nuclear double resonance (ENDOR) verify the successful incorporation of zinc ions at cupric ion sites into the framework of the resulting Cu 3-x Zn x (btc) 2 coordination polymer. The formation of such paramagnetic binuclear Cu-Zn paddle wheel building blocks allows the investigation of the interaction between the Cu(II) ions and various adsorbates by advanced pulsed ESR methods with high accuracy. As a first example we present the adsorption of methanol over Cu 3-x Zn x (btc) 2 , which was found to coordinate directly to the Cu(II) ions via their open axial binding site.

Graphene inclusion controlling conductivity and gas sorption of metal–organic framework

“…One explanation for this behaviour could be that graphene protects the EPR active copper centres towards further hydration and possible degradation in excess water. 59,62 This would be in good agreement with the gas sorption analysis, in which we propose that graphene akes partly wrap around HKUST-1 crystals and partly block the access to the pores. Thus, the access of water to the copper sites may be hindered by hydrophobic graphene causing slower kinetics of the hydration.…”

“…The EPR spectrum of HKUST-1 narrows aer only 2 minutes exposure to water, and features which must be due to two different types of copper EPR signals are observed, in accordance with literature. 62 With prolonged exposure to water, the features of the narrowest signal disappear and the other line shis slowly to lower elds. For HKUST-1/graphene the development to a narrow spectrum with features belonging to two different Cu species is much slower (15 minutes) and the change in spectrum stops at this stage; no further development or loss of intensity are observed even aer more than an hour.…”

Chemical principles underpinning the performance of the metal–organic framework HKUST-1

“…In the case of hydrated HKUST-1, exposure to a strong base – ammonia is one example – weakens the aqueous O–H bond, resulting in labile protons 56,57. This phenomenon was further demonstrated by Hupp and co-workers, who showed an increase in proton conductivity (Grotthus mechanism58) upon loading of methanol in the hydrated structure 59…”

Investigation of the spin-lattice relaxation of 13CO and 13CO2 adsorbed in the metal-organic frameworks Cu3(btc)2 and Cu3−xZnx(btc)2

“…In case of Cu 2.97 Zn 0.03 (btc) 2 where a very small fraction of Cu is substituted by Zn, a small amount of electron spins with a spin quantum number of S = 1 2 is present and should also be taken into account. 28 Moreover, also in case of MOFs without Zn we cannot exclude the presence of S = 1 2 spin states in the parent material due to a low concentration of "extraframework" Cu 2+ species. 28,31 The contribution from the CSA mechanism can be calculated by the following Eq.…”

“…28 Moreover, also in case of MOFs without Zn we cannot exclude the presence of S = 1 2 spin states in the parent material due to a low concentration of "extraframework" Cu 2+ species. 28,31 The contribution from the CSA mechanism can be calculated by the following Eq. (1) 32…”