Molecular Engineering of Free‐Base Porphyrins as Ligands—The N−H⋅⋅⋅X Binding Motif in Tetrapyrroles

Abstract

The core N-H units of planar porphyrins are often inaccessible to forming hydrogen-bonding complexes with acceptor molecules. This is due to the fact that the amine moieties are "shielded" by the macrocyclic system, impeding the formation of intermolecular H-bonds. However, methods exist to modulate the tetrapyrrole conformations and to reshape the vector of N-H orientation outwards, thus increasing their availability and reactivity. Strategies include the use of porpho(di)methenes and phlorins (calixphyrins), as well as saddle-distorted porphyrins. The former form cavities due to interruption of the aromatic system. The latter are highly basic systems and capable of binding anions and neutral molecules via N-H⋅⋅⋅X-type H-bonds. This Review discusses the role of porphyrin(oid) ligands in various coordination-type complexes, means to access the core for hydrogen bonding, the concept of conformational control, and emerging applications, such as organocatalysis and sensors.

Conformational Re‐engineering of Porphyrins as Receptors with Switchable N−H⋅⋅⋅X‐Type Binding Modes

“…This finding-while preliminary-suggests that the geometrical and electronic prop- erties of phosphoric and sulfonic moieties influence the porphyrin-analyte complex formation, similar to the proposed "lock-and-key" concept. [4] In operational terms,t he affinities for anions were found to decrease according to the following sequence: PP i > BS > MP as observed from UV/Vis displacement studies ( Figures S51 and S54). Thea ffinities of MSA and BSA were not investigated due to their poor solubility in CHCl 3 .S toichiometry for a 4 -P1 was determined using Jobsp lots (Figure S52).…”

“…Such porphyrins recently found their first application as organocatalysts and promoters for dioxygen/hydrogen peroxide interconversion . This indicates that specific core N−H⋅⋅⋅substrate interactions can be achieved via macrocycle engineering . Since the beginning of porphyrin structural chemistry, weak interactions have been observed in crystals .…”

“…[3] This indicates that specific core NÀH···substrate interactions can be achieved via macrocycle engineering. [4] Since the beginning of porphyrin structural chemistry, [5a] weak interactions have been observed in crystals. [5] Densely packed free base porphyrin systems that encapsulate substrates in their lattices are well known.…”

Konformativer Umbau von Porphyrinen als Rezeptoren mit schaltbaren N‐H⋅⋅⋅X‐Bindungsmodi

“…Da sterisch überladene Porphyrine stark sattelförmig deformierte 3D‐Konformationen einnehmen, kann eine Kombination mit zusätzlichen Koordinationsstellen an der Porphyrinperipherie Rezeptoren mit mehrfachen Bindungsstellen liefern. Dies ergibt auf der molekularen Ebene eine dem Schlüssel‐Schloss‐Prinzip der Enzyme vergleichbare Situation . Hierbei resultiert das Zusammenspiel aller peripheren Substituenten (d. h., peri ‐Wechselwirkungen und periphere Kontrolle durch funktionelle Gruppen) zu einem wohldefinierten aktiven Zentrum, das Substrat‐/Analytbindung und ‐erkennung in den Porphyrinkern dirigiert (Schema ).…”

Reactive Dissolution of Organic Nanocrystals at Controlled pH

“…Among many organic materials, porphyrin molecules represent relevant building-blocks for functional material [1] whose optical/ electronic properties can be modulated electrochemically [2] or by the proper control of pH and solvent. [3] Nowadays, controlling the morphology of advanced organic materials is a key task for their applications in different fields, ranging from the fabrication of organic electronic devices, [4] sensors, [5][6][7] as well as applications to the biomedical field. [8][9][10][11] In this framework, studying the formation of porphyrin thin films, their phase stability and dissolution mechanisms is beneficial.…”