Metalla-aromatics: Planar, Nonplanar, and Spiro

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS:The concept of aromaticity is one of the most fundamental principles in chemistry. It is generally accepted that planarity is a prerequisite for aromaticity, and typically the more planar the geometry of an aromatic compound is, the stronger aromatic it is. However, it is not always the case, particularly when transition metals are involved in conjugation and electron delocalization of aromatic systems, i.e., metalla-aromatics. Because of the intrinsic nature of transition-metal orbitals, besides planar geometries, the most stable molecular structures of metallaaromatic compounds could take nonplanar and even spiro geometries. In this Account, we outline several unprecedented types of metalla-aromatics developed recently in our research group.Around seven years ago, we found that 1,4-dilithio-1,3-butadienes, dilithio reagents with π-conjugation, could function as noninnocent ligands and react with low-valent transition-metal complexes, generating monocyclic metalla-aromatic compounds. Later on, by taking advantage of the unique behavior of dilithio reagents and the intrinsic nature of different transition metals, we have synthesized a series of metalla-aromatic compounds, of which four types are discussed here, and each of them represents the first of its kind. First, nearly planar aromatic dicupra[10]annulenes, a 10 π-electron aromatic system with two bridging Cu atoms participating in the orbital conjugation and electron delocalization, are synthesized by annulating two dilithio reagents with two Cu(I) complexes.Second, four kinds of spiro metalla-aromatics, featuring planar (with Pd, Pt, or Rh as the spiro atom) geometry with a whole 10π aromatic system, octahedral (tris-spiro metalla-aromatics with V as the spiro atom) geometry with an entire 40π Craig−Mobius aromatic system, tetrahedral (with Mn as the spiro atom) geometry having two independent and perpendicular 6π planar aromatic rings, and tetrahedral (with Mn as the spiro atom) geometry with one planar and one nonplanar 6π aromatic rings, respectively, are generated. In sharp contrast to spiroaromaticity with carbon acting as the spiro atom described in Organic Chemistry, the metal spiro atom herein takes part in orbital conjugation and electron delocalization. Third, nonplanar aromatic butadienyl diiron complexes are realized. Different from planar aromatic systems featuring delocalized πtype overlap, this nonplanar metalla-aromaticity is achieved by the novel σ-type overlap between the two Fe 3d xz orbitals and the butadienyl π orbital, forming a 6π aromatic system. Fourth, dinickelaferrocene, a ferrocene analogue with two aromatic nickeloles, is synthesized from our monocyclic aromatic dilithionickelole and FeBr 2 . The aromaticity of dinickelaferrocene and its nickelole ligands is realized by electron back-donation from the Fe 3d orbital to the π* orbital of nickeloles, which also deepens our understanding of the origin of aromaticity.The search for unprecedented and exciting aromatic systems, particul...

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

“… 8 Most of the effort has been centered on the metal counterparts of hydrocarbons: i.e., metallabenzenes, 9 metallabenzynes, 10 metallanaphthalenes, 11 metallaanthracenes, 12 metalloles, 13 and some condensed species bearing the metal bonded to four carbons such as carbolongs 14 and spiro metalloles. 15 In contrast, organometallic metallaheteroaromatic compounds have received little attention. 16 Although the number of known heteroaromatic, organically pure molecules is extremely large, 17 only the existence of α- 18 and β-metallafurans, 19 α-metallathiophenes, 20 α- 21 and β-metallapyrroles, 22 metallapyryliums, 23 metallathiobenzenes, 24 and metallapyridines 25 has been demonstrated ( Chart 2 ).…”

“…Since the prediction of the metallabenzenes by Thorn and Hoffmann in 1979 and the preparation of the first osmabenzene by Roper and co-workers in 1982, the chemistry of these types of compounds has experienced a tremendous development, mainly from a conceptual point of view . Most of the effort has been centered on the metal counterparts of hydrocarbons: i.e., metallabenzenes, metallabenzynes, metallanaphthalenes, metallaanthracenes, metalloles, and some condensed species bearing the metal bonded to four carbons such as carbolongs and spiro metalloles . In contrast, organometallic metallaheteroaromatic compounds have received little attention .…”

A One‐Pot Strategy for the Synthesis of β‐Substituted Rhoda‐ and Irida‐Carbolong Complexes

“…Metallaaromatics are aromatic complexes with at least one metal fragment in the aromatic ring. [ 1‐13 ] Metallaaromatics were realized by Thorn and Hoffman in 1979 for the first time, who predicted theoretically that metallabenzenes are aromatic. [ 14 ] Three years later, the prediction was confirmed experimentally by the Roper group.…”

Helical versus linear Jahn-Teller distortions in allene and spiropentadiene radical cations

“…76,77 However, the 6 π-electron [2,4]spiroheptatriene is perpendicular around the spiro-atom, and both neutral [3,3]spirosystems and the spiro[2,3]hexadienyl anion are unstable. 78,79 Spirometallic 4n+2 π-systems do assume planarity, [80][81][82][83][84] and the spiropentadiene dication is also planar. 42,[85][86][87][88] The torsion energy profiles of neutral and dication spiropentadiene are very similar to the equivalent allene species (Figure 3b and 4b), and we shall explore this connection for their cations.…”