Bis(1,1,1-trifluoro-5,5-dimethyl-5-methoxy-acetylacetonato)copper(II) was prepared in two polymorphic modifications. The orthorhombic R-form is stable and densely packed, with four trans and four cis square bischelate building blocks per unit cell. These are connected through additional coordination bonds to form a dense polymer network. For the trigonal β-form, the square bischelate complex units are present exclusively as the trans isomers. The distinctive assembly of these units results in a lattice with an open pore volume of about 17% that is accessible to a wide range of guests. The compound has a remarkably strong affinity for the porous β-form as evident from the efficient R-to-β conversion on contact not only with liquid guests but also with organic vapors at pressures well below the saturation pressure. Although the open β-form is metastable, it has a remarkable kinetic stability, most likely because of the trans-to-cis isomerization that must accompany the β-to-R transformation. Many sorbents play a dual role as stabilizing guest and as catalyst promoting the R-to-β or β-to-R conversion. Because of its versatile sorption properties and relative robustness, the β-form of the complex can be classified as a novel organic zeolite mimic.

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