Charge-transfer bonding in metal–arene coordination

https://doi.org/10.1016/S0010-8545(00)00322-2Get rights and content

Abstract

X-ray crystallographic structures of donor–acceptor complexes of aromatic hydrocarbons with transition metals are re-examined with the focus on the arene ligands. Thus, significant structural and electronic changes are revealed in the arene moiety due to coordination to the metal center including: (i) expansion of the aromatic six-carbon ring; (ii) endocyclic π-bond localization; (iii) distortion of the planarity (folding) of the arene ring; and (iv) shortening of the metalarene bond distances. All structural features are characteristic of metal–arene (π- or σ-) complexes that exhibit various degrees of (metal-to-ligand) charge transfer. The concept of charge-transfer bonding not only explains the structural details but also the various facets of chemical reactivity of metal-coordinated arenes including efficient carbonhydrogen bond activation and nucleophilic–electrophilic umpolung, both of which are critical factors in homogeneous metal catalysis.

Introduction

Arenes represent one of the most important classes of π-ligands in organometallic chemistry. Owing to their ability to provide up to six electrons for coordination, numerous donor–acceptor complexes of various hapticities (η1 through η6) are known between arene ligands and metal centers, many of which have been identified as crucial intermediates in homogeneous metal catalysis [1], [2], [3], [4], [5] of various aromatic reactions. It is well known that arenes undergo substantial changes in reactivity upon coordination to a metal center  the two most important chemical effects include efficient carbonhydrogen bond activation and nucleophilic–electrophilic umpolung of the arene reactivity. The favorable energetics of CH bond activation of metal-coordinated arenes is exploited in organometallic catalysis, and the facile attack of metal–arene complexes by nucleophiles is extensively utilized to functionalize arenes (vide infra). Thus, the question arises as to the structural and electronic origin of such substantial changes in the reactivity of arenes in organometallic complexes.

X-ray crystallographic studies of metal–arene complexes generally focus on the metal center and the symmetry of its orbitals in the ligand environment. However, the structural details of the arene ligand, which are relevant for understanding its reactivity, are rarely discussed and often completely ignored. In this review, we take an alternative approach and examine metal–arene bonding from the viewpoint of the aromatic ligand. Thus, we provide a careful re-evaluation of X-ray crystallographic data of metal–arene complexes with the focus on the frequently subtle and often unnoticed structural changes in the arene ligands due to coordination to a metal center. The structural changes include ring expansion, endocyclic π-bond localization, and distortion of the planarity of the arene ring which (separately or all together) ultimately lead to a complete loss of the aromaticity of the π-system. These structural features all point to strong electronic interactions between arene and metal center, which result in the formation of π- or σ-complexes with various degrees of charge transfer. On the basis of the presented X-ray crystallographic evidence, we develop a donor–acceptor or charge-transfer concept that satisfactorily explains the structural details as well as the various facets of chemical reactivity of metal-coordinated arenes.

The donor–acceptor terminology for the description of organometallic coordination compounds is not new, since  in its conventional form  it considers a formal electron distribution merely based on Lewis acid–base interactions and electron count to determine how many electrons are donated by the arene ‘donor’ to provide a stable (18- or 16-electron, closed-shell) environment for the otherwise open-shell metal ‘acceptor’ [6]. In other words, η1- through η6-coordination [7] can be readily explained by this ‘donor–acceptor’ concept combined with the 18-electron rule. Although hapticity and electron count are extremely useful descriptors to classify organometallic complexes [7], they certainly do not satisfactorily explain the above mentioned structural features and the related chemical reactivity. To understand structural details such as the ring expansion of metal-coordinated arenes or the unusual bond strength in metal carbonyl complexes, additional electronic effects are commonly invoked  the most important being back-donation [6]. For example, in metal carbonyl complexes the vacant (π*) orbitals of the CO ligands are considered to interact with the (formally) occupied metal orbitals which leads to a redistribution of electrons and a strong metalcarbonyl bond. A closer look reveals that the back-donation concept is merely a special case of the general charge-transfer concept put forward in this review. In general, the description of organometallic substrates as ‘donor–acceptor’ complexes (with the ligand formally donating electrons to the metal center in accord with the 18-electron rule) does not provide any information on the real distribution of charges between metal and ligand. On the other hand, the charge-transfer concept considers the effective donor or acceptor strength1 (see Section 2.1) of the ligand and the metal center and thus allows us to determine the actual location of charges and the degree of (metal-to-ligand or ligand-to-metal) charge transfer.2

We will first discuss a variety of structural (Section 2) and chemical (Section 3) effects in metal–arene complexes that arise from charge-transfer from and to the arene ligand. We will demonstrate with a few striking examples that the observed charge-transfer phenomena are independent of hapticity and/or electron count and, most importantly, have direct analogies in organic and inorganic donor–acceptor complexes. Detailed analyses of various η1- and η2-coordinated metal–arene complexes will then follow in Section 4 since structural and chemical effects of charge transfer are most pronounced in these low-hapticity complexes. To limit the scope of this review, we focus mostly on arene/transition-metal complexes, and donor–acceptor complexes with main-group metals are only discussed for comparison purposes.

Section snippets

Effects of charge transfer on the structure of metal–arene complexes

Several significant structural changes can be observed when aromatic hydrocarbons are coordinated to a metal center. Such effects include:

  • 1.

    expansion of the aromatic ring;

  • 2.

    π-bond localization in the arene ring;

  • 3.

    distortion of planarity of the arene ring (either by folding or by deviation of the position of substituents out of the plane of the ring);

  • 4.

    shortening of the arenemetal bond distance (from van der Waals contact to covalent (σ) bonds).

Moreover, there are striking examples of secondary

Charge-transfer activation of metal-coordinated arene ligands

The structural changes in arenes upon coordination with a metal center as illustrated in the previous section, would be of little interest if there were not closely related to the reactivity of organometallic complexes. In fact, arene–metal complexes are crucial intermediates in homogeneous metal catalysis [1], [2], [3], [4], [5], which means that the coordination of an aromatic substrate to a metal center changes its chemical properties to such an extent that reactions readily occur which are

η1- and η2-Coordinated arene ligands

The following section will describe structural and electronic effects in η1- and η2-coordinated metal–arene complexes in more detail. First, we need to emphasize that the topological classification of metal–arene complexes in terms of hapticity which was originally proposed by Cotton in 1968 [122] as the basis to distinguish organometallic complexes and their chemistry, was never intended to characterize the chemical bonding between metal and ligands. In fact, the hapticity concept was

Conclusions

In general, crystal structures of organometallic coordination compounds are reported from the metal center's point of view in order to discuss the symmetry of the valence orbitals and the ligand environment. In this review, we examine the X-ray structures of numerous transition metal–arene complexes with the focus on the arene ligand. This alternative approach reveals a series of significant structural details that clearly point to the importance of charge transfer in metal–arene coordination.

Acknowledgements

We thank the National Science Foundation and the Robert A. Welch Foundation for financial support.

References (145)

  • S.G. Davies et al.
  • F. Calderazzo et al.

    J. Organomet. Chem.

    (1996)
  • J.K. Kochi et al.

    J. Organomet. Chem.

    (1993)
  • F. Scott et al.

    J. Organomet. Chem.

    (1990)
  • A.S. Batsanov et al.

    J. Organomet. Chem.

    (1998)
  • I.L. Fedushkin et al.

    J. Organomet. Chem.

    (1995)
  • D.L. Clark et al.

    Inorg. Chim. Acta

    (1996)
  • F. Jellinek

    J. Organomet. Chem.

    (1963)
  • J. Müller et al.

    J. Organomet. Chem.

    (1997)
  • A. Ceccon et al.

    J. Organomet. Chem.

    (1984)
  • R.D. Pike et al.

    Coord. Chem. Rev.

    (1999)
  • M.F. Semmelhack
  • J.K. Kochi

    Organometallic Mechanisms and Catalysis

    (1978)
  • K.H. Dötz, R.W. Hoffmann (Eds.), Organic Synthesis via Organometallics, Vieweg, Braunschweig,...
  • D. Astruc

    Electron Transfer and Radical Processes in Transition-Metal Chemistry

    (1995)
  • D. Astruc

    Top. Curr. Chem.

    (1992)
  • L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1960. See also Ref. [8], p....
  • F.A. Cotton

    J. Am. Chem. Soc.

    (1968)
  • R.S. Mulliken et al.

    Molecular Complexes

    (1969)
  • G. Briegleb

    Elektronen-Donator-Acceptor-Komplexe

    (1961)
  • M. Tamres et al.

    J. Am. Chem. Soc.

    (1960)
  • L. Pauling
  • F.H. Allen et al.

    J. Chem. Soc. Perkin Trans.

    (1987)
  • G.A. Jeffrey et al.

    Proc. R. Soc. Lond. Ser. A

    (1987)
  • J. Hecht, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138870,...
  • E.K. Kim et al.

    J. Am. Chem. Soc.

    (1991)
  • J. Hecht, R. Rathore, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138871,...
  • S.V. Lindeman, E. Bosch, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138872,...
  • J. Hecht, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138873,...
  • J. Hecht, R. Rathore and J.K. Kochi, Electronic submission to the Cambridge Structural Database 138874,...
  • R.J. Bernhardt et al.

    Organometallics

    (1986)
  • B.P. Byers et al.

    Inorg. Chem.

    (1987)
  • A.J. Steedman et al.

    Acta Crystallogr. Sect. C

    (1997)
  • J. Hecht, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138875,...
  • R.K. McMullan et al.

    Acta Crystallogr. Sect. B

    (1997)
  • P. LeMagueres, R. Rathore, J.K. Kochi, submitted to J. Org. Chem. Electronic submission to the Cambridge Structural...
  • K.-B. Shiu et al.

    Organometallics

    (1990)
  • J.M. Casas et al.

    Organometallics

    (1993)
  • C.D. Tagge et al.

    J. Am. Chem. Soc.

    (1996)
  • I. Bach et al.

    Organometallics

    (1996)
  • S.V. Lindeman, T. Mori, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138879,...
  • H. Chen et al.

    J. Am. Chem. Soc.

    (1990)
  • J. Terheijden et al.

    J. Am. Chem. Soc.

    (1985)
  • M. Mascal, J. Hansen, A.J. Blake, W.-S. Li, Chem. Commun. (1998)...
  • J.B. Lambert et al.

    Organometallics

    (1994)
  • G.S. Hair et al.

    J. Am. Chem. Soc.

    (1999)
  • S.V. Lindeman, T. Dhanasekaran, J.K. Kochi, Electronic submission to the Cambridge Structural Database 138880,...
  • R. Rathore et al.

    J. Am. Chem. Soc.

    (1998)
  • W. Lau et al.

    J. Am. Chem. Soc.

    (1982)
  • C.L. Higgitt, A.H. Klahn, M.H. Moore, B. Oelckers, M.G. Partridge, R.N. Perutz, J. Chem. Soc. Dalton Trans. (1997)...
  • Cited by (156)

    • Indium and thallium

      2021, Comprehensive Coordination Chemistry III
    • Gaseous mercury removal by graphene-like carbon nitride impregnated with ammonium bromide

      2020, Fuel
      Citation Excerpt :

      And the delocalization of charges through the graphene-like triazine structure may provide enough electron deficiency on the armchair carbons to accept electrons from mercury atoms. As a consequence, the Hg0 is likely captured on the armchair edge carbons (carbyne type) of the g-C3N4 by producing an σ-bonded (η1) or π-bonded (η2) mercury-triazine complex via Lewis acid-base conjugation [61,62]. As for NH4Br-modified g-C3N4 (Fig. 18), the Hg0 capture mechanism can be elucidated via a carbene structure-activity model.

    • Interaction between Cu and Ag free ions and central metals in complexes with XH<inf>n</inf> units (X = B, Si, N, O, C, Al, Zn, Mg; n = 1, 2)

      2020, Coordination Chemistry Reviews
      Citation Excerpt :

      weak MI⋯(C–H)arene interactions using group 11 metal ions are mainly of an electrostatic nature (vide infra); η1 (referring to such M⋯CH interaction) and η2 (involving two π-bonded carbon atoms) complexation to metals are energetically very similar [189–191]; the final mode of coordination is influenced by steric and packing effects.

    View all citing articles on Scopus
    View full text