CW and Pulsed ESR Spectroscopy of Cupric Ions in the Metal−Organic Framework Compound Cu₃(BTC)₂

The metal−organic framework (MOF) compound Cu 3 (BTC) 2 (H 2 O) 3 · x H 2 O (BTC = benzene 1,3,5-tricarboxylate) was prepared by solvothermal synthesis under ambient pressure and structurally characterized by powder X-ray diffraction and nitrogen adsorption at 77 K. X- and Q-band CW electron spin resonance and hyperfine sublevel correlation spectroscopies were used to explore the coordination state and location of the Cu(II) ions in the porous coordination polymer. Cupric ions were found to be present in two different chemical environments: (a) Cu(II) 2 clusters in the paddle-wheel building blocks of the Cu 3 (BTC) 2 network, giving rise to an antiferromagnetically coupled spin state in accordance with previous susceptibility measurements ( J. Appl. Phys. 2000, 87, 6007). However, the cross-linking of the paddle wheels by the BTC linker leads to an additional spin exchange among dimers as evidenced by the characteristics of the S = 1 ESR signal of their excited spin state. (b) In addition, paramagnetic monomer Cu(II) species are accommodated in the pore system of Cu 3 (BTC) 2 . They coordinate to adsorbed water molecules and form [Cu(H 2 O) 6 ] 2+ complexes, which are inhomogeneously distributed over the Cu 3 (BTC) 2 pore system.