Continuous Wave and Pulsed Electron Spin Resonance Spectroscopy of Paramagnetic Framework Cupric Ions in the Zn(II) Doped Porous Coordination Polymer Cu3−xZnx(btc)2

Abstract

In the parent metal−organic framework Cu3(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 Cu3−xZnx(btc)2 coordination polymer. The formation of such paramagnetic binuclear ...