From all-metal aromatic clusters to open-shell spherical aromaticity

Jordi PoaterInstitut de Química Computacional

and Departament de Química, Universitat de Girona

• The concept of aromaticity is no longer confined to "carbon chemistry" but can be applied to the entire periodic table.

1985

1991

2001

Fullerenes

2007

All-metal Aromaticity

2005d-orbital Aromaticity

Nanotubes

2008

-Aromaticity

f-orbital Aromaticity

Open-shell spherical aromaticity

• Hückel’s 4n+2 rule

Dnh

n

Aromaticity of (4n+2)p-annulenes with Dnh symmetry comes from the fulfillment of a closed-shell that provides extra stability.

coronene pyrene

Open-shell spherical aromaticity

• Baird’s 4n rule

Dnh

n

Aromaticity of lowest-lying triplet state of(4n)p-annulenes with Dnh symmetry comes from the same-spin half-filled degenerate highest-occupied MOs that provides extra stability.

Open-shell spherical aromaticity

• Hirsch’s 2(n+1)2 rule for spherical aromaticity

2, 8, 18, 32, 50, 72, 98… = 2(n+1)2

2

8

18

32

n = 0

n = 1

n = 2

n = 3

Rotor rigid solution

A. Hirsch, Z. Chen and H. Jiao, Angew. Chem. Int. Ed. 2000, 39, 3915

Open-shell spherical aromaticity

• Hirsch’s 2(n+1)2 rule for spherical aromaticity

Z. Cheng and R. B. Bruce Chem. Rev., 2005, 105, 3613

Open-shell spherical aromaticity

2(n+1)2 Hirsch’s rule

4n lowest-lying triplet excited state Baird’s rule

4n+2 Hückel’s rule

?OPEN-SHELL

CLOSED-SHELL

PLANAR SPHERICAL

Open-shell spherical aromaticity

• 2n2+2n+1 (S=n+1/2) rule for open-shell spherical aromaticity

2

8

18

32

1, 5, 13, 25, 41, 61, 75… = 2n2+2n+1; (S=n+1/2)

n = 0

n = 1

n = 2

n = 3

J. Poater and M. Solà Chem. Commun. 2011, 47, 11647

Open-shell spherical aromaticity• Aromaticity indices

avzzyyxxNICS 3

1

zzzzNICS Magnetic shielding tensor zzNICS

Multicenter delocalization indices

For monodeterminantal WFs:

M. Giambiagi, M. S. de Giambiagi, C. D. dos Santos Silva and A. P. de Figuereido, Phys. Chem. Chem. Phys. 2000, 2, 3381

P. Bultinck, R. Ponec and S. van Damme, J. Phys. Org. Chem. 2005, 18, 706

A = {A1, A2, …, AN}

Niiiii

iiiiiiring ASASASnnIN

N

N 1

21

32211,,,

21)( A

)(2

1)(

)(

AAAP

ringIN

MCI

Open-shell spherical aromaticity

J. Poater and M. Solà Chem. Commun. 2011, 47, 11647

Systems symm. NICS(1)zz MCI r(C,C) Spin

C202+ Ih -7.4 0.020 1.447 S = 0

C207+ Ih -4.0 0.035 1.494 S = 3/2

C205- Ih -18.0 0.024 1.508 S = 7/2

Open-shell spherical aromaticity

Systems symm Ring NICS(1)zz MCI BLA Spin

C60 Ih 6-MR 0.8 0.018 0.058 S = 0

5-MR 21.5 0.011C60

1- Ih 6-MR -1.4 0.017 0.002 S = 11/2

5-MR -19.9 0.049C60

19+ Ih 6-MR -14.9 0.019 0.013 S = 9/2

5-MR -25.3 0.041C60

10+ Ih 6-MR -18.6 0.011 0.030 S = 0

5-MR -29.5 0.017

J. Poater and M. Solà Chem. Commun. 2011, 47, 11647

Open-shell spherical aromaticitySystems symm. Ring NICS(1)zz MCI BLA Spin

C80 S6 5-MR 10.7 0.019 S = 0

6-MR -5.2 0.012 0.025

5-MR 26.3 0.018

6-MR 11.3 0.014 0.001

6-MR -5.1 0.012 0.025C80

8+ Ih 6-MR -7.2 0.011 0.015 S = 0

5-MR -4.0 0.017C80

5- Ih 6-MR -20.8 0.019 0.012 S = 13/2

5-MR -5.5 0.034J. Poater and M. Solà Chem. Commun. 2011, 47, 11647

Open-shell spherical aromaticity

Systems symm. NICS(1)zz MCI Spin

Ge122- Ih -5.5 0.049 S = 0

Ge121- Ih -405.9 0.113 S = 5/2

Ge124+ Ih -69.0 0.088 S = 0

All-metal aromatic clusters

• All-metal and semimetal aromatic clusters are among the most exciting molecules synthesized since the beginning of the present century.

• These compounds can have not only the conventional -(anti)aromaticity of classical organic compounds, but also - or even - and -(anti)aromaticity, thus giving rise to the so-called multifold aromaticity.

All-metal aromatic clusters• The tendency of p-electrons in classical organic aromatic

molecules is always to localize the double bonds (distortive character), against the delocalizing force of the s-electrons.

S.C.A.H. Pierrefixe, F.M. Bickelhaupt, Chem. Eur. J. 2007, 13, 6321

• Thus, while many of the properties attributed to aromaticity derive from the p-electrons, the s-electrons are the ones responsible for the symmetric framework of monocyclic aromatic compounds.

All-metal aromatic clusters• Al4

2- was the first all-metal compounds synthesized, containing a pair of delocalized -electrons (orbital 1a2u) and two pairs of -electrons (orbitals 1b2g and 2a1g) that contribute to the overall aromaticity of this species.X. Li, A.E. Kuznetsov, H.F. Zhang, A.I. Boldyrev, L.S. Wang, Science 2001, 291, 859.

• Thus, Al42- can be considered as a “3-fold”

aromatic system ( + s p aromatic)

All-metal aromatic clusters

• Are the -electrons of the Al42- cluster

distortive, i.e., do they work against the regular "aromatic" structure with delocalized, equivalent bonds? Or is this propensity a characteristic of only classical aromatic organic molecules?

J. Poater, F. Feixas, F.M. Bickelhaupt and M. Solà Chem. Commun. 2011, 47, 11647

All-metal aromatic clusters

• Amsterdam Density Functional• BP86/TZ2P, QUILD• Energy Decomposition Analysis

∆E : M2- (aaa) + M2

- (bbb) M42-

∆E = ∆Eprep + ∆Eint

∆Eint = ∆Velstat + ∆EPauli + ∆Eoi

∆Eoi = SG ∆EG

∆Eint = ∆Velstat + “total s” + “total p”J. Poater, F. Feixas, F.M. Bickelhaupt and M. Solà Chem. Commun. 2011, 47, 11647

All-metal aromatic clusters

J. Poater, F. Feixas, F.M. Bickelhaupt and M. Solà Chem. Commun. 2011, 47, 11647

90 91 92 93 94 95 96 97 98 99 100

-30

-20

-10

0

10

20

30

40

50

total s

total p

DVelstat

DEint

J. Poater, F. Feixas, F.M. Bickelhaupt and M. Solà Chem. Commun. 2011, 47, 11647

All-metal aromatic clusters

• Equivalent trends to those found in benzene.

NICS(1b2g)= +10.8ppmNICS(2a1g)=-3.9ppm

ACKNOWLEDGMENTS

Prof. Dr. Miquel Solà (UdG)Dr. Ferran Feixas (San Diego, USA)

Prof. Dr. F. Matthias Bickelhaupt

19/4/2012 Toulouse RECENT ADVANCES IN AROMATICITY 27

http://iqc.udg.edu