Group VII Transition Metal Carbene Cluster Complexes

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Historical development of organometallic chemistry

The first recorded organometallic compound, “Cadet’s fuming liquid”, was prepared as early as 1760 in a Parisian military pharmacy.1 In an effort to make cobalt-based inks, cobalt minerals containing arsenic was used, and in situ formation of [Me2As]2O resulted. However, as arsenic is classified as a metalloid or ‘semi-metal’, the title of the first organometallic compound may well belong to the first olefin complex, the complex Na[PtCl3C2H4] known as Zeise’s salt,2 prepared by the Danish pharmacist Zeise by boiling a solution of chloroplatinic acid in ethanol, and then adding KCl. 3 The term ‘organometallic’ was only introduced by Frankland in 1849, following his preparation of important alkylmercury compounds such as Me2Hg. 4 After this, a wealth of main-group organometallic compounds were isolated and characterized in the late nineteenth century, 5 and industrial applications for these complexes grew exponentially. However, little was known about the true nature and structure of these complexes. For example, the first binary metal carbonyl Ni(CO)4 was discovered by Mond during the commercial process for refining nickel.6 Iron pentacarbonyl was reported almost simultaneously,7 but the product was initially misformulated as Fe(CO)4, and the nature of the compound was not elucidated. Organometallic chemistry came to be recognized in its full right with the discovery of the first sandwich complex, ferrocene.8 In 1951 and 1952, Pauson reported in the journals Nature9 and Journal of the Chemical Society10 the serendipitous preparation of “a new type of organo-iron compound”. Pauson proposed a linear structural arrangement where two planar cyclopentadienyl rings linked to the iron metal as shown for resonance structure I in Figure 1.1. He attributed the remarkable stability of FeC10H10 to the prevalence of resonance structure II, where the cyclopentadienyl groups attain aromatic character with the acquisition of a negative charge.

Recent developments of polymetallic carbene complexes

The activation of simple organic molecules by more than one transition metal constitutes an area of research that has grown in importance.23 The applications of carbenes as active or auxiliary ligands in organic synthesis and catalysis, however, are mostly focused on monocarbene systems. No assemblies of carbene units (carbene supramolecular chemistry) have been reported and only a few carbene complexes are known to be part of small metal organic frameworks (MOF).29 In addition, no carbene complexes have featured in materials of note, no dendrimers with carbene functionalities have been prepared and no carbene complexes showing liquid crystalline properties have been reported. In fact very few studies on multimetal carbene complexes or cluster carbene complexes have been recorded.30 In the field of non-linear optics, conjugated unsaturated systems with a transition metal moiety have been employed for their electron delocalisation and so-called ‘push-pull’ characteristics.31 However, the incorporation of different transition metal fragments in complexes has been widely investigated to study the role of different metal fragments on the reactivity of ligands and the chemistry of the complexes32 . When applied in the area of Fischer carbene complexes of the type [M(CO)5{C(OR’)R}], the carbene properties have either been modified by introducing metal-containing substituents to further activate the carbene carbon33 or the carbene ligand is used as a connector to bridge the other transition metals.34 The introduction of a metal fragment to the carbene oxygen offers the possibility to modulate the carbene reactivity by the electronic and steric properties of this second metal fragment.3

eteroatom-bonded carbene ligand substituent

The incorporation of a second metal-containing fragment, joined to the heteroatom bonded to the carbene carbon has first been explored by Fischer et al.8 This involved the O-alkylation of an acyl chromate with titanocene dichloride to yield the corresponding metaloxycarbene complex [Cr(CO)5{C(Me)OTiCp2Cl}] as well as the trimetallic bismetaloxycarbene complex [{-O2TiCp2- O,O’}{C(Me)Cr(CO)5}2]. However, the above Fischer method is limited by the reactivity of the acyl metalate intermediate (A in Figure 2.1 below). The intermediate A requires stabilization; but if too stable, O-alkylation is resisted in the following step and metal alkylation can result in an acyl complex rather than the desired Fisher-type metaloxycarbene complex B.

Carbon-bonded carbene ligand substituent

In a study performed prior to this investigation in our laboratories, mono- and dimetal substituted carbene complexes (listed as complexes 14 – 21 in Scheme 2.5) were derived from a lithiated benzothienyl substituent (2-BT) or a benzo[b]thienyl ring -coordinated to Cr(CO)3, (Cr(CO)3( 6 -2-BT)), followed by alkylation of the acyl metalate by triethyloxonium tetrafluoroborate or metalation with titanocene dichloride.17 The [Cr(CO)3( 6 -2-BT)]-substituent has the -bonded chromium tricarbonyl fragment coordinated to the ring furthest away from the carbene carbon atom, leaving an unoccupied space directly beneath the thiophene bonded to the carbene carbon atom. This prompted the investigation into the possibility of replacing the benzo[b]thiophene fragment by a more compact, redox-active ferrocenyl substituent in order to increase the electronic communication between the -bonded transition metal and the carbene carbon atom, as well as heteroaryl rings with the -Cr(CO)3-moiety in closer proximity to the carbene carbon atom, using the Group VI transition metals Cr, Mo and W (Figure 2.10).

Ferrocenyl carbene ligand substituent

Ferrocene was chosen as carbene substituent due to its wide application and the rapidly expanding field of ferrocenyl-containing ligands, where they can be used in catalytic transformations of organic compounds, especially when in contact with a second transition metal.18 The first examples of ferrocenyl Fischer carbene complexes were synthesized by Connor et al.5 as part of an investigation into the electron withdrawing nature of metal carbonyl carbene groups and comprehensively reviewed as dimetallic heteroatom stabilized Fischer carbenes.19

pi-aryl carbene ligand substituent

Besides ferrocene, both thiophene and benzene with a chromium tricarbonyl fragment -bonded to it were chosen as carbene substituents. The effect of – coordination of metal fragments to different types of ring systems has been extensively studied, especially for arenes containing no heteroatoms like cyclopentadienyl and benzene. In particular, arene chromium tricarbonyls have attracted attention (Figure 2.4) as the activating substituent.

Table of Contents :

• Summary
• List of Abbreviations
• List of Compounds
• Chapter 1 Introduction
  • 1.1 Background
    • 1.1.1 Historical development of organometallic chemistry
    • 1.1.2 Early development of carbene chemistry
    • 1.1.3 Recent developments of polymetallic carbene complexes
  • 1.2 Aim of the study
• Chapter 2 Group VI Transition Metal Carbene Cluster Complexes
  • 2.1 Introduction
    • 2.1.1 Background
    • 2.1.2 Hetero-atom bonded carbene ligand substituent
    • 2.1.3 Carbon-bonded carbene ligand substituent
      • 2.1.3.1 Ferrocenyl carbene ligand substituent
      • 2.1.3.2 -aryl carbene ligand substituent
    • 2.1.4 Homo- and heteronuclear polymetallic biscarbene complexes
      • 2.1.4.1 Conjugated bridging biscarbene complexes
      • 2.1.4.2 Biscarbene complexes by connecting heteroatom
• substituents
• 2.2 Results and discussion
  • 2.2.1 Focus of this study
• 2.3 Synthesis
  • 2.3.1 Synthesis of ferrocenyl mono- and biscarbene Cr and Mo
• complexes
  • 2.3.2 Synthesis of tungsten carbene complexes
  • 2.3.3 Synthesis of mixed heteronuclear bridging biscarbene complex
  • 2.3.4 Synthesis of-aryl-Cr(CO)3 titanoxycarbene complexes of
• chromium
• 2.4 Spectroscopic characterization
  • 2.4.1 1H NMR spectroscopy
  • 2.4.2 13C NMR spectroscopy
  • 2.4.3 IR spectroscopy
  • 2.4.4 Mass spectrometry
  • 2.4.5 Single crystal X-ray crystallography
    • 2.4.5.1 Molecular structures
    • 2.4.5.2 Crystal packing
• 2.5 Concluding remarks
• Chapter 3 Group VII Transition Metal Carbene Cluster Complexes
  • 3.1 Introduction
    • 3.1.1 Background
    • 3.1.2 Monomanganese carbene complexes
    • 3.1.3-arene substituted carbene complexes
    • 3.1.4 Dirhenium carbene complexes
    • 3.1.5 Axial or equatorial carbene ligands of nonacarbonyl dimetal
  • complexes
    • 3.1.6 Different reactivities of manganese and rhenium complexes
    • 3.1.7 Hydrido-acyl and hydroxycarbene transition metal complexes
  • 3.2 Results and discussion
    • 3.2.1 Focus of this study
  • 3.3 Synthesis
    • 3.3.1 Synthesis of cyclopentadienyl manganese carbene complexes
    • 3.3.2 Synthesis of dirhenium ethoxycarbene complexes
    • 3.3.3 Synthesis of dirhenium cluster carbene cluster complexes
  • 3.4 Spectroscopic investigation
    • 3.4.1 1H NMR spectroscopy
    • 3.4.2 13C NMR spectroscopy
    • 3.4.3 IR spectroscopy
    • 3.4.4 Mass spectrometry
    • 3.4.5 Single crystal X-ray crystallography
      • 3.4.5.1 Molecular structures
      • 3.4.5.2 Crystal packing
• 3.5 Concluding remarks
• Chapter 4 Investigation of substituent effect on carbene ligands
  • 4.1 Introduction
    • 4.1.1 Background
    • 4.1.2 Theoretical bonding model of carbene ligands
  • 4.2 Molecular modelling
    • 4.2.1 The theoretical method
    • 4.2.2 Molecular modelling of transition metal complexes
    • 4.2.3 Modelling of Fischer carbene complexes
    • 4.2.4 Substituent effect
  • 4.3 Electrochemical approach
    • 4.3.1 Anodic electrochemical behaviour of Fischer carbene complexes
    • 4.3.2 Cathodic behaviour of Fischer carbene complexes
  • 4.4 Results and discussion
  • 4.4.1 Focus of this study
  • 4.5 Theoretical investigation of substituent effect
    • 4.5.1 Computational details
    • 4.5.2 Theoretical results
    • 4.5.3 Vibrational spectroscopy results
    • 4.5.4 Molecular orbital analysis
    • 4.5.5 Correlation between UV/Vis spectroscopy and MO analysis
    • 4.5.6 Natural bond orbital analysis
  • 4.6 Electrochemical investigation of substituent effect
    • 4.6.1 Cyclic voltammetric studies
  • 4.7 Concluding remarks
    • 4.7.1 Summary
    • 4.7.2 Future work
• Chapter 5 Experimental
  • 5.1 Standard operating procedure
  • 5.2 Characterization techniques
    • 5.2.1 Nuclear magnetic resonance spectroscopy
    • 5.2.2 Infrared spectroscopy
    • 5.2.3 Raman spectroscopy
    • 5.2.4 Fast atom bombardment mass spectrometry
    • 5.2.5 X-ray crystallography
    • 5.2.6 UV/Visible spectroscopy
  • 5.3 Electrochemistry
  • 5.4 Preparation of compounds
    • 5.4.1 Preparation of starting material compounds
      • 5.4.1.1 Triethyl oxonium tetrafluoroborate
      • 5.4.1.2 Chloromercury ferrocene
      • 5.4.1.3 Bromoferrocene
      • 5.4.1.4 Iodoferrocene
      • 5.4.1.5 Trisammine tricarbonyl chromium
    • 5.4.1.-thiophene chromium tricarbonyl
  • 5.4.1.-benzene chromium tricarbonyl
  • 5.4.2 Preparation of organometallic complexes
    • 5.4.2.1 General carbene preparation with direct lithiation of
• ferrocene in the presence of TMEDA
• 5.4.2.2 General carbene complex preparation with aryl lithiation
• at low temperatures
• 5.4.2.3 Preparation of mixed heteronuclear carbene complex
• 5.5 Analytical data
• Appendices
• Attached compact disk
• Appendix 1 Crystallographic data of Complex
• Appendix 2 Crystallographic data of Complex
• Appendix 3 Crystallographic data of Complex
• Appendix 4 Crystallographic data of Complex
• Appendix 5 Crystallographic data of Complex
• Appendix 6 Crystallographic data of Complex
• Appendix 7 Crystallographic data of Complex
• Appendix 8 Crystallographic data of Complex
• Appendix 9 Crystallographic data of Complex
• Appendix 10 Crystallographic data of Complex

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Multimetal complexes of Fischer carbenes