Defining the refractory bond: exploring the nature and possibility of titanophilic, zirconophilic, and hafnophilic non-covalent interactions

Abstract

We propose a new subclass of non-covalent interactions, which we call refractory bonds. These bonds are characterized by the attractive interaction between a group 4 element (Ti, Zr, or Hf) and a nucleophilic site, either within the same molecule or with a neighboring entity. This interaction can be seen as a sister to the other σ-/π-hole interactions. In σ-hole (π-hole) bonding, the σ-hole (or a π-hole) is an electron-density deficient region on the surface of an atom (or an array of atoms) (R), opposite (or orthogonal) to the outermost extension of a covalent or coordinate bond formed by a substituent (S) in an S–R molecule. The σ-/π-hole acts as an electrophilic center, capable of interacting with electron-rich species. We provide both crystallographic and computational gas-phase evidence to support the existence of the refractory bond in chemical systems. We show that these σ-hole bonds, as well as refractory π-hole interactions, exhibit significant similarities with other non-covalent interactions, including triel, tetrel, pnictogen, chalcogen, halogen, aerogen, coinage, alkali, alkaline earth, as well as erythronium, wolfium, osme, spodium, and regium bonds. Our findings open new avenues for the study of non-covalent interactions and are expected to offer valuable insights to the broader non-covalent chemistry community, particularly in molecular recognition, crystallography, self-assembly, supramolecular chemisty and catalysis.