Shaping Antiaromatic π‐Systems by Metalation: Synthesis of a Bowl‐Shaped Antiaromatic Palladium Norcorrole

Abstract

The synthesis of a bowl‐shaped antiaromatic molecule was achieved through the deformation of a planar antiaromatic porphyrinic π‐conjugation system by insertion of palladium into the small cavity of a metal‐free norcorrole. The bowl‐to‐bowl inversion dynamics of the antiaromatic Pd‐coordinated norcorrole was determined by variable‐temperature 1H NMR spectroscopy. The metal‐free norcorrole was prepared from acid‐induced demetalation of a copper norcorrole, which was obtained from the intramolecular coupling of a bis(diiododipyrrin) copper complex with copper thiophenecarboxylate.

Norcorrole as a Delocalized, Antiaromatic System

“…Figure 2 depicts highlights of the B3LYP 7 – 9 -D3 10 /STO-TZ2P optimized geometries. The energy minima turned out to be a C i -symmetric wave conformation for H 2 Nc 11 and a C 2v -symmetric dome conformation for NiNc. The planar D 2h form of NiNc, just 0.03 eV (0.7 kcal/mol) higher in energy relative to the C 2v minimum, was found to correspond to the transition state for the bowl inversion process.…”

“…The X-ray structures of certain other NiNc derivatives (YEQKUC 6 ; CALRAL 12 ; MUJTIW, MUJTOC 15 , on the other hand, exhibit larger differences (>0.04 Å) between adjacent C meso -C α bonds. To this list may be added a slightly saddled CuNc (YEHTOX) 11 and a strongly domed PdNc (YEHTEN) 11 structure, which exhibit an intermediate difference (~0.03 Å) between adjacent C meso -C α bonds. Significant bond localization has been observed for several NiNc derivatives with strongly conjugating β -substituents such as cyano 15 , nitro 16 or amino 17 ; these systems are not within the purview of the present study.…”

Norcorrole: Aromaticity and Antiaromaticity in Contest

“…The flatter Ci geometry is preferred in the solid state and in solution. 14 A better estimate of the experimentally observed Ni-C distance in NiNc (1.771 Å to 1.789 Å 15 ) was obtained at the TPSSh-D3BJ/def2-TZVP level, 1.796 Å against 1.810 Å at the B3LYP-D3BJ/def2-TZVP level. Before discussing the NiNc and H2Nc σiso(r) isosurfaces (Figure 1), it is instructive to examine shielding around benzene and cyclobutadiene (C6H6 and C4H4, Figure 2, at D6h and D2h optimized geometries, same levels of theory as for for NiNc and H2Nc).…”

“…Experimental evidence of antiaromatic behavior comes from the NiNc 1 H NMR and MCD spectra; 11 on the theory side, antiaromaticity is suggested by DFT-level NICS, HOMA values, ACID plots, out-of-plane bond magnetizabilities and magnetically induced current densities. 11,[13][14][15][16] NiNc units have been used in the construction of "an antiaromaticwalled nanospace". 17 In contrast to other antiaromatic species, such as the classical example of antiaromaticity, cyclobutadiene, C4H4, NiNc does not undergo a 2 nd -order Jahn-Teller transition lowering the energy, reducing the symmetry and introducing bond length alternation (in the case of C4H4, from square D4h to rectangular D2h).…”

From Antiaromatic Norcorrolatonickel(II) to Aromatic and Nonaromatic Zwitterions: Innocent Ligands with Unbalanced Charge of the Core

“…Norcorrrole is a tetrapyrrolic macrocycle of [16]porphyrin(1.0.1.0) structure and antiaromatic character due to a 16π-electron delocalization pathway. The pioneering work of Bröring 1 followed by a gram-scale synthesis of norcorrole nickel(II) ( Chart 1 , 1 , R = mesityl) and copper(II) complexes introduced by Shinokubo et al 2 opened an avenue for application of this porphyrinoid as an emblematic example of a stable and easy to handle antiaromatic system. 3 Complex 1 undergoes various substitution, 4 addition, 4e , 4g , 5 or insertion 4g , 5a , 5b , 5d reactions.…”