Reticular Chemistry in All Dimensions

Chemistry, first and foremost, is concerned with the geometry that atoms and molecules adopt in space. Whether we are pursuing chemical reactions or studying the properties of materials, inevitably we seek to know and understand the geometrical aspects of chemical structures. Indeed, it has been historically the case that no matter how far chemists veer away from this practice, especially when racing toward making materials that “benefit” society, they come back to questions regarding how atoms are linked into molecules and how molecules interact with each other to account for their observations. It has also been our quest, once we acquire this knowledge, to use it for controlling chemical structures and in many ways “bending them to our will”. Chemists have done so for molecules (0D) and to some extent polymers (1D), but beyond these our ability to express control in infinite 2D and 3D remained undeveloped throughout the twentieth century. Over the last 25 years, reticular chemistry has emerged from this thinking and in its current practice serves to fill the gulf between what we know and can do on the molecular level in 0D and 1D, and what could be possible in 2D and 3D. It started first with linking inorganic clusters into extended porous frameworks, and then linking organic molecules and metal ions into metal−organic frameworks (MOFs), and organic molecules together into covalent organic frameworks (COFs). In other words, it opened the way for developing chemistry beyond the molecule. It is now widely accepted that reticular chemistry is the chemistry of linking molecular building blocks by strong bonds to make extended crystalline structures as exemplified by MOFs and COFs. What is in this definition? (i) Molecular building blocks provide control in the construction of frameworks because of their well-defined structure and geometry, (ii) strong bonds impart architectural, thermal, and chemical stability to the resulting frameworks, and (iii) crystallinity, which was the challenge impeding the progress toward realizing such frameworks, ensures that their structures can be definitively characterized by X-ray or electron diffraction techniques. Each of these aspects has been established through the synthesis and study of over eighty thousand MOFs and hundreds of COFs, where it is no longer a major challenge to control matter in 2D and 3D. As we take stock of the progress made in reticular chemistry, we are struck by how quickly it has developed in employing libraries of organic molecules in the chemistry of reticulating them with metal ions and other organic molecules into a seemingly infinite variety of ways, leading to what can only be imagined to be an infinite structure space. When we add the vast possibilities of functionalizing these structures, the infiniteness of reticular chemistry supported by the exquisite control with which such structures can be made and modified is exceptional. Although I have emphasized the structural aspects of these synthetic creations because they form the foundation onto which further levels of control can be achieved, the vastness of this field requires simplifying concepts with the hope to achieve better understanding of the chemistry.