Weak metal-arene interactions have been investigated in Zn, Cd, Hg, and Ni complexes of meso-tetraaryl m- and p-benziporphyrin (1 and 2) and of the new compound, m-benziporphodimethene (3). Compounds 1-3 incorporate the phenylene moiety into a macrocyclic structure so as to facilitate the interaction between the arene and coordinated metal ion. X-ray studies performed on Cd(II) and Ni(II) complexes show that the arene fragment approaches the ion at a distance much shorter than the sum of van der Waals radii. In chloronickel(II) m-benziporphyrin, a weak agostic bond is actually formed. In the NMR spectra of the Cd(II) and Hg(II) species, unusual (1)H-M and (13)C-M scalar couplings have been observed that are transmitted directly between the metal and the arene. DFT calculations performed for two Cd(II) species and subsequent AIM analysis show that the accumulation of electron density between the metal and arene necessary to induce these couplings is fairly small and the interaction is steric in nature. In the paramagnetic Ni(II) complexes of 1 and 3, the agostic proton of the m-phenylene exhibits large downfield (1)H NMR shifts (386 and 208 ppm at 298 K, respectively). An agostic mechanism of spin density transfer is proposed to explain these shifts as resulting from electron donation from the CH bond to the metal. In chloronickel(II) p-benziporphyrin, the inner protons of the p-phenylene have a contrastingly small shift (0.0 ppm at 298 K), indicating that in this case the agostic interaction is inefficient, in agreement with the X-ray data.

Dual identity: It takes a single phenylene twist to reveal the dichotomous nature of a di‐para‐benzihexaphyrin (see picture; phenylene rings highlighted in red). This expanded porphyrinoid switches between Hückel and Möbius topologies in an unusual solvent‐ and temperature‐dependent equilibrium. Each of the two incarnations of the macrocycle has its own unmistakable spectral signature.

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5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid) which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The metal ion of square-planar coordination geometry is firmly held via three pyrrolic nitrogen atoms where the fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin. The macrocycle reveals the unique structural flexibility triggered by coordination of palladium. The structural rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between two pyrrolic rings of vacataporphyrin albeit the significant ruffling of the tripyrrolic block is also of importance. Two fundamental modes of interactions between the palladium ion and annulene moiety have been recognized. The first one resembles an eta(2)-type interaction and involves the C(2)C(3) unit of the butadiene part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires Hückel or extremely rare Möbius topologies readily reflected by spectroscopic properties. The palladium vacataporphyrin complexes reveal Hückel aromaticity or Möbius antiaromaticity of [18]annulene applying the butadiene fragment of vacataporphyrin as a topology selector. The properties of specific conformers were determined using (1)H NMR and density functional theory calculations.

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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8,19-Dimethyl-9,13,14,18-tetraethyloxybenziporphyrin coordinates palladium(II) to form the four-coordinate anionic complex [(OBP)Pd(II)](-). The NMR data provide evidence for the retention of macrocyclic aromaticity and coordination via a carbon sigma-donor. Protonation of the external oxygen atom to give [(HOBP)Pd(II)] switches the molecule to a less aromatic phenol-like state, which is manifested by a significant reduction of the macrocyclic ring current. [(AcOBP)Pd(II)] and [(TsOBP)Pd(II)], two ester derivatives of [(OBP)Pd(II)](-), are similar to the protonated species, and their benzenoid character is more pronounced. However, reaction of [(OBP)Pd(II)](-) with methyl iodide leads to selective methylation of the coordinating C(22) atom to form a novel organopalladium complex (OBPMe)Pd(II). The strong shielding of the inner Me(22) (delta((1)H) -2.00 ppm in CDCl(3)) indicates that the aromaticity of the macrocycle has been retained. At the same time the (13)C chemical shift of C(22) (44 ppm) shows that the palladium-bound carbon has undergone a drastic hybridization change. Alkylation with n-BuI yields a mixture of the O-substituted [(n-BuOBP)Pd(II)] and the C-substituted [(OBP-n-Bu)Pd(II)], which confirms the ambident nucleophilicity of [(OBP)Pd(II)](-). DFT calculations carried out for six tautomers of oxybenziporphyrin and the 22-methylated palladium species provide further insight into the electronic structure of the ligand and its complexes. Relative energies of the tautomers, increasing in the order [CH,NH,N,NH,O] < [CH,N,NH,N,OH] < [CH(2),N,NH,N,O] < [CH(2),N,N,N,OH], have been used to estimate the accessibility of four limiting delocalization modes postulated for oxybenziporphyrin and its derivatives. The state of macrocyclic aromaticity observed experimentally in the free base and the phenolic aromaticity of the O-protonated tautomer are the most favorable, and the latter has its energy higher by only 13 kcal/mol. The peculiar bonding situation in (OBPMe)Pd(II), which can be inferred from the NMR data, is also predicted by the DFT methods, which show a strongly distorted tetrahedral environment of C(22).