TEtraQuinolines: A Missing Link in the Family of Porphyrinoid Macrocycles

Porphyrin contains four inwardly oriented nitrogen atoms. It is arguably the most ubiquitous multifunctional naturally occurring macrocycle that has inspired the design of novel nitrogen-containing heterocycles for decades. While cyclic tetramers of pyrrole, indole, and pyridine have been exploited as macrocycles in this category, quinoline has been largely neglected as a synthon. Herein, we report the synthesis of TEtraQuinoline (TEQ) as a 'missing link' in this N4 macrocycle family. In TEQs, four quinoline units are concatenated to produce an S₄-symmetric architecture. TEQs are characterized by a highly rigid saddle shape, wherein the lone-pair orbitals of the four nitrogen atoms are not aligned in a planar fashion. Nevertheless, TEQs can coordinate a series of transition-metal cations (Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Pd²⁺). TEQs are inherently fluorescence-silent but become strongly emissive upon protonation or complexation of Zn(II) cations (ϕ = 0.71). TEQ/Fe(II) complexes can catalyze dehydrogenation and oxygenation reactions with catalyst loadings as low as 0.1 mol %.