  HKUST-1 is a metal-organic framework (MOF) which plays a significant role
both in applicative and basic fields of research, thanks to its outstanding
properties of adsorption and catalysis but also because it is a reference
material for the study of many general properties of MOFs. Its metallic group
comprises a pair of Cu2+ ions chelated by four carboxylate bridges, forming a
structure known as paddle-wheel unit, which is the heart of the material.
However, previous studies have well established that the paddle-wheel is
incline to hydrolysis. In fact, the prolonged exposure of the material to
moisture promotes the hydrolysis of Cu-O bonds in the paddle-wheels, so
breaking the crystalline network. The main objective of the present
experimental investigation is the determination of the details of the
structural defects induced by this process in the crystal and it has been
successfully pursued by coupling the electron paramagnetic resonance
spectroscopy with other more commonly considered techniques, as X-ray
diffraction, surface area estimation and scanning electron microscopy. Thanks
to this original approach we have recognized three stages of the process of
decomposition of HKUST-1 and we have unveiled the details of the corresponding
equilibrium structures of the paddle-wheels at the atomic scale level.
