Tetrapyrrole Complexes with Unusual Geometries: a Main Group Element Perspective

ConspectusIn 1981, the year he won the Nobel Prize, Roald Hoffmann together with Kazuyuki Tatsumi published two papers entitled "Metalloporphyrins with Unusual Geometries" that strongly influenced the state of the art in porphyrin and tetrapyrrole research at that time. The 1970s and 1980s saw the dramatic expansion of bioinorganic chemistry, using the tools of molecular coordination chemistry to model complex processes of metalloenzymes and metal cofactors. Synthetic porphyrin ligands emerged as key platforms for high-valent metal-oxo and -nitrido species, metal-metal multiple bonds and the emergence of organometallic chemistry with porphyrins as the supporting ligands. These developments were the drivers for the Hoffmann papers, which featured extended Hückel calculations to expand our then understanding of types of metalloporphyrins for which experimental evidence was just emerging, and which challenged the notion of porphyrin functioning simply as a tetradentate macrocycle with a coordinated metal ion sitting squarely in the middle.Another unquestioned assumption from that time was that porphyrin and tetrapyrrole coordination chemistry was anchored firmly in the <i>d</i> block of the periodic table, a not unreasonable stance given the origins of this field in heme and vitamin B12 featuring iron and cobalt. Surprisingly, the presence of group 2 element magnesium in chlorophyll had not tempted chemists to interrogate more deeply the role of main group elements in tetrapyrrole chemistry, and at the time of the Hoffmann papers, examples of <i>s</i> and <i>p</i> block elements as porphyrin complexes could be almost counted on one hand. In the nearly half century since then, as the chemistry of tetrapyrrole main group complexes has unfolded, major new examples of complexes with unusual geometries have emerged. The chemistry of the <i>d</i> block elements is largely governed by oxidation states and <i>d</i>-electron configurations while in the <i>s</i> and <i>p</i> blocks the fundamental properties of size and electronegativity dominate. Porphyrins and tetrapyrroles offer four nitrogen donors in a square-planar arrangement of fairly fixed radii, not an obvious fit for main group elements with their widely ranging sizes, electronegativities, coordination geometries and bonding types. Main group tetrapyrrole complexes are "misfits" in which the poor match between the ligand environment and the requirements of the coordinated elements stimulates unusual chemistry for both partners.In this <i>Account</i>, I will use the concept of metalloporphyrins with unusual geometries addressed in the Hoffmann papers to look at how tetrapyrroles bearing coordinated main group elements have extended these ideas well beyond those originally envisaged. Main group metals range from lightweight lithium to the <i>p</i> block heavies thallium, lead and bismuth; all are known to form porphyrin complexes, some with dramatic out-of-plane metal coordination. The classic <i>p</i> block elements carbon, boron, and phosphorus challenge the "metalloporphyrin" paradigm; these small, light nonmetals nevertheless exhibit a rich chemistry in a tetrapyrrole setting. The extensive range of diboron porphyrinoids feature tetrapyrroles acting as binucleating ligands, incorporating not one but two elements within the N₄ coordination site. Silicon and germanium porphyrins and phthalocynanines demonstrate the interplay between redox properties of the ligand and central element. The underlying theme in this discussion will be the new concepts that can be translated into other areas of the chemical sciences.