Tetrapyrrole Complexes with Unusual Geometries: a Main Group Element Perspective

Area of Science:

• Coordination Chemistry
• Bioinorganic Chemistry
• Main Group Chemistry
• Porphyrin Chemistry

Background:

• The 1970s and 1980s saw significant expansion in bioinorganic chemistry, utilizing molecular coordination chemistry to model metalloenzymes and metal cofactors.
• Synthetic porphyrin ligands became crucial for studying high-valent metal-oxo/nitrido species, metal-metal multiple bonds, and organometallic chemistry.
• Historically, porphyrin chemistry focused on d-block elements (e.g., iron, cobalt), with limited exploration of s- and p-block elements.

Purpose of the Study:

• To review the concept of metalloporphyrins with unusual geometries, as introduced by Hoffmann and Tatsumi in 1981.
• To explore how main group elements coordinated within tetrapyrrole complexes have expanded upon these initial ideas.
• To highlight the unique chemistry arising from the "misfit" between main group elements and the porphyrin ligand environment.

Main Methods:

• Review of historical and contemporary research on metalloporphyrins and tetrapyrrole complexes.
• Discussion of extended Hückel calculations used to understand bonding and geometries.
• Analysis of the interplay between element properties (size, electronegativity) and the tetrapyrrole scaffold.

Main Results:

• Main group elements (Li to Bi, C, B, P, Si, Ge) form diverse porphyrin complexes, often with unusual geometries and out-of-plane coordination.
• Main group elements challenge the traditional "metalloporphyrin" definition, exhibiting rich chemistry despite their differing properties from transition metals.
• Examples include diboron porphyrinoids acting as binucleating ligands and silicon/germanium porphyrins/phthalocyanines showcasing ligand-element redox interplay.

Conclusions:

• The study of main group tetrapyrrole complexes has significantly extended the understanding of unusual geometries in coordination chemistry.
• The "misfit" between main group elements and porphyrins drives novel chemical reactivity and structural diversity.
• The concepts explored in main group tetrapyrrole chemistry offer transferable insights to broader areas of the chemical sciences.