The Cu(II) pairs in the paddle-wheel building blocks of the metal−organic framework compound Cu3(btc)2 give rise to an antiferromagnetic spin state with an electron spin resonance (ESR)-silent S = 0 ground state. However, the adsorption of di-tert-butyl nitroxide (DTBN) radicals leads to the formation of an unusual nitroxide ESR spectrum and later, upon thermal treatment of the samples, to distinct paramagnetic Cu(II) centers, whose ESR signals can be observed at temperatures below 70 K. Various scenarios for the suppression of the antiferromagnetic coupling of the Cu(II) ions in the paddle-wheel units by interaction with the nitroxide and the subsequent formation of these S = 1/2 copper centers are discussed.

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