Kielmann M, Senge MO. Molecular Engineering of Free-Base Porphyrins as Ligands-The N-H⋅⋅⋅X Binding Motif in Tetrapyrroles.
Angew Chem Int Ed Engl 2019;
58:418-441. [PMID:
30067890 PMCID:
PMC6391963 DOI:
10.1002/anie.201806281]
[Citation(s) in RCA: 57] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2018] [Indexed: 12/15/2022]
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.
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