The interactions between arene (heteroarene) rings and metal ions, observed in solution and in the solid state, provide examples of particular geometrical flexibility. A CH moiety of the arene may participate in π-bonding but can also form a σ-bond or an agostic bond with the metal. On the other end of the scale lies the van der Waals interaction, wherein no sharing of electron density occurs between the metal and arene. Unusual cases of metal-arene bonding are encountered in various areas of inorganic chemistry. The strength of the interaction can be varied by appropriate modifications of the coordinating environment of the metal. The weaker interactions are often insufficient to stabilize the M-arene complex, which is held together by additional intramolecular restraints, such as those imposed by a pincer ligand or a cage structure. Such restraints can be readily introduced by incorporating the arene (heteroarene) moiety into a porphyrin-like macrocycle. Thus the macrocyclic brace binds the metal ion and holds it close to the arene moiety.

Carbaporphyrinoids, a recently developed class of macrocycles realize the above concept of ligand construction by fusing the structural motifs of porphyrin and arene (heteroarene). These new molecules belong to the family of carbaporphyrins, porphyrin analogues with a C-H bond in the coordination core. The goal of the present studies is to investigate metal-arene interactions in diamagnetic and paramagnetic complexes of carbaporphyrinoids including of meta-benziporphyrins 1, para-benziporphyrin 2, 2-aza-21-carbaporphyrin 3 and 3-azabenziporphyrin 4 to show how the bonding affects NMR spectroscopic parameters. In the course of this work the simplest homologues of porphyrin - pyriporphyrin 5 and subporphyrin - subpyriporphyrin 6 with a supplementary CH unit "incorporated" in a pyrrolic β-β bond were synthesized [1]. Subpyriporphyrin 6, the novel type of the contracted porphyrin, provided an environment for the shortest ever N-H···N hydrogen bond with the N···N distances equal 2.370 Å.

[1] a) R. Myśliborski, L. Latos-Grażyński, L. Eur. J. Org. Chem. 2005, 5039. b) R. Myśliborski, L. Latos-Grażyński, L. Szterenberg, T. Lis, Angew. Chem. Int. Ed. 2006, 45, 3670. c) R. Myśliborski, L. Latos-Grażyński, L. Szterenberg, Eur. J. Org. Chem. 2006, 000.

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