Polymorphism and Pseudopolymorphism of the [Ni(4-Methylpyridine)4(NCS)2] Werner Complex, the Compound that Led to the Concept of “Organic Zeolites”

Abstract

The inclusion chemistry of the title complex was revisited in the context of three topical problems of present-day crystal engineering: conventional and pseudo-polymorphism, the design of organic zeolite mimics, and the creation of “third generation” porous metal−organic frameworks. The crystal structures of dense (α) and microporous (β) polymorphs of the complex were redetermined and two new crystal structures were studied: β- and γ-inclusion phases of the complex (1:1 and 1:2 host-to-guest ratio, respectively) with benzene as the simplest aromatic guest. Although both the β- and γ-crystal architectures display a remarkable adaptability toward inclusion of various guests, the mechanisms of this adaptability are essentially different. The microporous β-phase is capable of expanding by 14.5% in response to the size of the included guest or variations in temperature. This flexibility is the highest ever observed for a metal−organic microporous framework and is responsible for the zeolitic behavior of the material. In contrast, the γ-phase shows an ability to transform to a number of architectures that are topologically similar but crystallographically different.