This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.

Core alteration, affording heteroporphyrinoids and carbaporphyrinoids, allows for the exploration of porphyrin-like or porphyrin-unlike coordination chemistry. Such a strategy has provided a fundamental change in the approach to macrocyclic organometallic chemistry. The specific reactivity of porphyrinoids has offered an insight into reactions inside particularly shaped macrocyclic architecture, including metal-mediated organic transformations. This review focuses on alterations of macrocycles, made of four carbocyclic/heterocyclic subunits, which resemble the structure of porphyrin or porphyrin isomers, providing, however, remarkable porphyrin-like (XNNN), (CNNN), (CNCN) or (C2NNN) surroundings. To facilitate the understanding of the discussed subject matter, all addressed reactions reflecting core reactivity have been formally grouped into nine categories distinguished by the principal type of transformation addressed: (1) alkylation or arylation, (2) core heteroatom replacement, (3) oxidation and oxygenation, (4) substitution, (5) macrocyclic fusion, (6) core bridging, (7) ring contraction, (8) ring expansion, and (9) C-H and C-C bond activation. The distinctive macrocyclic environment creates a long-sought opportunity to trap unique organometallic transformations which include instances of benzene ring contraction to cyclopentadiene or the formation of an unprecedented metalloporphyrinoid: 21-pallada-23-telluraporphyrin. This review offers certain challenging perspectives which target the goals of organometallic bond activation.

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The incorporation of a phenanthrene moiety into a porphyrin framework results in the formation of a hybrid macrocycle—phenanthriporphyrin—merging the structural features of polycyclic aromatic hydrocarbons and porphyrins. An antiaromatic aceneporphyrinoid, adopting the trianionic {CCNN} core, is suitable for the incorporation of a phosphorus(V) center to form a hypervalent organophosphorus(V) derivative.

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