State Key Laboratory for Physical Chemistry of Solid Surfaces 厦门大学固体表面物理化学国家重点实验室 Aromaticity: From Organics to Inorganics, From 2D to 3D 吕鑫 (X. Lu) 2013. 07. 24
Outline • Overview • Practical Criteria of Aromaticity • p-aromaticity (2D) • Möbius aromaticity • Homoaromaticity • s-aromaticity • Spherical aromaticity and 3D aromaticity
Overview • Few concepts are as frequently used as AROMATICITY in the current chemical literature. • Since 1981, ca. 300,000 papers dealing with the aromatic properties of chemical systems have been published. • A thematic issue on Aromaticity: P. v. R. Schleyer, Chem. Rev. 2001, 101(5), 1115. • A recent thematic issue on aromaticity: P. v. R. Schleyer, Chem. Rev. 2005, 105(10).
The history of aromaticity can be traced back to 1825 when M. Faraday isolated for the first time benzene. Benzene (M. Faraday, 1825)
The term “aromatic” was first used by chemists in the early 19th century to designate a specific class of organic substances(e.g., benzene), which are initially distinguished from those belonging to the aliphatic class by virtue of their pleasant olfactory properties. • Aromaticity --- extra stability --- remarkableelectron delocalization /conjugation.
1.1 Types of Aromatic Systems • Before 1958, 2D planar polycyclic aromatic hydrocarbons (PAHs) reducible to molecules containing six p-electrons, e.g., -aromaticity of PAH fulfilling the Huckel 4N+2 or Clar sextet (6N) rule
After 1958 1) Monocyclic hydrocarbons containing up to 30 p-electrons, e.g., [n]annulenes Huckel & Möbius -aromaticity of annulenes
2) 3D boron and carborane cluster molecules based upon triangular face polyhedra, e.g., 3D aromaticity of clusters (ions)
3) Large carbon clusters illustrated by the famous buckminsterfullerene C60 and its homologues.
4) Analogues of PAHs containing metal atoms, such as gallium, or full metal clusters. E.g., metallabenzenes. Predicted by Hoffman in 1979. Synthesized in 1982. 1) Thorn, D. L.; Hoffman, R. Nouv. J. Chim.1979, 3, 39-45. 2) Elliott, G. P. et al. J. Chem. Soc., Chem. Commun.1982, 811-813.
5) Molecules stabilized by -electron delocalization (-aromaticity), e.g., cyclopentane. Dewar, M. J. S. Bul. Soc. Chim. Belg. 1979, 88, 957
CunHn (n=4,5,6) 6) transition-metal clusters stabilized by d-electron delocalization (-aromaticity), e.g., M4Li2 (M=Cu,Ag, Au) • Tsipis et al. J. Am. Chem. Soc. 2003, 125, 1136. • Schleyer et al. J. Am. Chem. Soc. 2005, 127, 5701.
1.2 Main developments about aromaticity 1980 Lu JX et al, quasi-aromaticity
1.3 Nature of the aromaticity concept • Like other useful and popular chemical concepts (chemical bonds, charges, electronegativities, hyperconjugations etc.), aromaticity is non-reductive, and lacks of clear physical bases. • Aromaticity is not a physical observable, having no precise experimental definition. • Aromaticity is just like to define beauty in our daily life!
Beauty (Aromaticity) is in the eye of the beholder! • Easily to recognize (but not always) • Many kinds • Hard to compare • Difficult to quantify • Various opinions, no general agreement • Interpreted differently
Aromaticity is a time-dependent concept, of which new aspects are pending for discovery. • Aromaticity is a property associated with extra stability and many other unusual manifestation!!!
1.4 Main categories of criteria characterizing aromaticity • Structural - planarity and equal bond length tendencies (simple, but unreliable!) • Energetic– enhanced stability (indirect, but impractical!) • Reactivity– lower reactivity, electrophilic aromatic substitution (neither direct nor reliable!) • Spectroscopic–UV, proton chemical shifts, magnetic susceptibility exaltation (indirect, mostly reliable, but sometimes impractical!)
1.34 1.47 1.39 Four classes of aromaticity criteria • Sructural bond length equalization • More stable than their acyclic analogues selection of reference systems, isodesmic or homodesmotic reaction! • Chemical behavior: electrophilic aromatic substitution prefered to addition butC60addition, anthracene/phenantrene Diels-Alder ! • Magnetic: ring current effects • Increased values of the magnetic susceptibility (ctot) • Large magnetic anisotropies (caniso) • Diamagnetic susceptibility exaltation ()
Drawbacks exist with these criteria: 1) Structural Criterion Bond length equalization should not be used alone as a criterion for aromaticity as some bond-equalized systems are not aromatic. e.g., B3N3H6: isoelectronic with benzene, equalized B-N bond lengths, not aromatic due to electron localization on the N atoms.
Energetic criterion. • The aromatic stabilization energy (ASE) and resonance energy (RE) have been well recognized as the cornerstone of aromaticity. • However, ASEs and REs of strained and more complicated systems are difficult to evaluate. • Such energy estimates vary significantly, strongly depending on the equations used and on the choice of reference molecules.
3) Reactivity criterion • The key characteristic reactivity feature: electrophilic aromatic substitution, not addition reaction. • However, aromaticity criteria based on chemical reactivity are not straightforward to apply!!
4) 1H NMR chemical shifts: ------- A magnetic criterion • Due to the ring current induced by an external magnetic field, the inner protons are shifted upfield, and the outer protons are downfield-shifted. • A criterion most often used experimentally.
H1: 5.78 H2: 6.26 H3: 6.36 4-membered ring is antiaromatic 4-5 antiaromatic H1: 6.10 H2: 7.71 nonaromatic H1: 8.6 H2: 8.1 H3: 8.5 Nonaromatic PW91/IGLOIII But !!!
2.1 Energetic criteria 2 Key Criteria for Aromaticity • 2.1.1 RE-Resonance Energy (VB theory). RE or Edelocalization = E(LS) – E(DS) Case study: Benzene
Ab initio MO predictions • The MO calculation on the “unrealistic” localized structure is impossible in practice. • Isodesmic reactions were proposed to evaluate RE. An isodesmic reaction is a chemical reaction in which the type of chemical bonds broken in the reactant are the same as the type of bonds formed in the reaction product
The ab initio MO-based RE depends strongly on the choice of isodesmic reactions. • It is far from trivial to balance strain, hyperconjugative effects, as well as differences in the types of bonds and atom hybridizations, using energy evaluation schemes. • Impractical for complex systems such as those with a large number of p-electrons or s-aromaticity.
VB treatment • VB/STO-6G Mo, Y et al, JPC, 1994, 98, 10048.
2.1.2 ASE (aromatic stabilization energy) Cryanski et al, Tetrahedron, 2003, 59, 1657. Homodesmic reactions for the evaluation of ASE. Homodesmic reactions are an improved form of isodesmic reactions in which all formal bonds and types of each carbon atoms are conserved in the reactants and products.
2.1.3 ISE (Isomerization stabilization energy): -------the difference between the total energies of a methyl derivative of the aromatic system and its nonaromatic exocyclic methylene isomer. Schleyer, P. v. R.; Puhlhofer, F. Org. Lett. 2002, 4 , 2873.
2.2 Magnetic Criteria magnetic criteria of aromaticity magnetic criteria of aromaticity
The ring current induces magnetic shielding within the ring, but deshielding out of the ring.
2.2.1 Diamagnetic Susceptibility exaltation (MSE, ) • Pioneering work by Pascal in 1910 • Benzene and its derivatives exhibited larger diamagnetic susceptibilities than would be expected for them from the susceptibilities of other unsaturated compounds. Pascal, P. Ann. Chim. Phys. 1910, 19, 5.
Pacault handled the discrepancy of magnetic susceptibility in the “Pascal system” by introducing a special benzene-ring parameter called “exaltation”. • Pink et al. hypothesized that the exaltation of diamagnetic susceptibility can be used to identify aromatic systems. Pacault, A. Ann. Chim., Ser. XII. 1946, 1, 567. Pink, R. C. Trans. Faraday Soc., 1948, 4, 407.
Exaltation of diamagnetic susceptibility results from the presence of cyclic delocalization of electrons, i.e. ring current. • Definition of exaltation of magnetic susceptibility: • A systematic survey of MSE of aromatic hydrocarbons was done by Dauben in 1968. Pacault, A. Ann. Chim., Ser. XII. 1946, 1, 567. Dauben, H. J. Jr. et al. J. Am. Chem. Soc.1968, 90, 811.
Magnetic susceptibility anisotropies • The tensor component perpendicular to the aromatic ring is much larger than the average of the others two components Aromatic / Antiaromatic = negative / positive canis,
Calculation of magnetic susceptibility • The magnetic susceptibility (MS) is a global property of the molecule. • Calculation of MS can be readily computed with the CSGT (Continuous Set of Gauge Transformations) method available in the Gaussian package.