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Abstract
Nickel norcorrole provides an unusual example of a molecule that is strongly antiaromatic according to the magnetic criterion, but which exhibits, according to high-quality DFT calculations, a symmetric, delocalized structure with no difference in bond length between adjacent Cmeso-Cα bonds. A fragment molecular orbital analysis suggests that these discordant observations are a manifestation of the high stability of the dipyrrin fragments, which retain their electronic and structural integrity even as part of the norcorrole ring system.
Highlights
Over a dozen years ago, one of us conceptualized norcorrole (H2Nc) as the smallest, realistic, fully conjugated tetrapyrrole ring system as part of a theoretical exercise (Fig. 1)[1]
The experimental 1H NMR spectra and nucleus independent chemical shift (NICS) calculations clearly implicated nickel norcorrole (NiNc) as an antiaromatic system[6]. These findings pose an interesting conundrum: NiNc is antiaromatic according to the magnetic criterion, density functional theory (DFT) geometry optimizations indicate a symmetric structure with little or no bond length alternation
The optimized geometry parameters for NiNc are in excellent agreement with the X-ray structures of several Ni meso-diarylnorcorrole derivatives
Summary
Over a dozen years ago, one of us conceptualized norcorrole (H2Nc) as the smallest, realistic, fully conjugated tetrapyrrole ring system as part of a theoretical exercise (Fig. 1)[1]. Among the more notable conclusions of the study, which employed standard density functional theory (DFT) methods, was that nickel norcorrole (NiNc) should exhibit a slightly domed, but otherwise fully symmetric structure. The X-ray structure reported by these authors (CCDC: YEQKUC) revealed a planar macrocycle geometry with significant bond length alternation[6]. The experimental 1H NMR spectra and nucleus independent chemical shift (NICS) calculations clearly implicated NiNc as an antiaromatic system[6]. Together, these findings pose an interesting conundrum: NiNc is antiaromatic according to the magnetic criterion, DFT geometry optimizations indicate a symmetric structure with little or no bond length alternation. Presented is a detailed DFT investigation aimed at establishing the true equilibrium geometry of NiNc
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