1 Introduction
Studies on metal–metal bonded compounds have largely been focused on their
structures, bonding, and electronic spectra [1–5]. This class of compounds is
increasingly being applied in different fields of research [6–9]. Metal–metal bonded
bimetallic complexes are of particular interest due to their applications in catalysis.
The bimetal platform provides scope for cooperative action between two metal ions
[10–14] and offers prospect for multi-electron redox chemistry [15–19]. Reactivity
studies on bimetallic systems provide valuable mechanistic information on chemical reactions catalyzed by metal cluster [20–22], metal nanoparticle [23–27], and
reactions occurring on metal surfaces [28–30]. Majority of the reactions on bimetallic complexes occur at equatorial sites [31–34]. Metal–metal singly bonded
compounds provide suitable molecular platform for axial site chemistry. The
metal–metal single bond allows close approach of the substrate to axial site whereas
strong trans effect of the metal–metal multiple bonds does not permit strong axial
binding.
Dirhodium(II,II) tetraacetate and related complexes are most prominent among
bimetallic complexes for their extensive applications as catalysts for a range of
organic reactions. Several reviews [35–38] and book chapters [39–42] have
appeared recently covering dirhodium(II,II) compounds. Other bimetallic compounds featuring metal–metal single bond include complexes containing [Ir
II –Ir
II ]
[43–46] and [Pd
I –Pd
I ] [47–52] bonds. Lack of synthetic procedures for their large
scale synthesis, however, has impeded their wider applications.
The isoelectronic [Ru
I –Ru
I ] complexes are relatively more accessible, and such
complexes are proving to be useful for a range of chemical reactions. This article
collects the chemistry at sites trans to the [Ru–Ru] single bond. The C–H activation, C–C bond formation, acceptorless alcohol dehydrogenation, cyclopropanation, carbenoid C–H insertion, and C–H amination reactions are covered.
Reactivity studies at axial sites on [Ru–Ru] multiply bonded systems are also
included because of their direct relevance in catalytic chemistry. The purpose of
this article is to highlight the recent progress on the axial-site chemistry on [Ru
I –
Ru
I ] platforms with intent to infuse interests for further development.
2 Diruthenium(I,I) Complexes Bearing Paddlewheel
[Ru 2 (CO) 4 ]
2+ Core
Compounds
of
general
formula
[Ru 2 (O 2 CR) 2 (CO) 4 ] n
and
[Ru 2 (O 2 CR) 2 (CO) 4 (AL) 2 ] (Scheme 1) are the commonly used diruthenium(I,I)
precursors containing metal–metal single bond [53]. Two acetates bridge between
two metals, and four carbonyls bind at equatorial positions to complete the
paddlewheel geometry for [Ru 2 (O 2 CR) 2 (CO) 4 ]. Two ligands additionally occupy
axial sites for [Ru 2 (O 2 CR) 2 (CO) 4 (AL) 2 ]. Polymeric structure is observed for
Reactivity and Catalysis at Sites Trans to the [Ru–Ru] Bond
61
Studies on metal–metal bonded compounds have largely been focused on their
structures, bonding, and electronic spectra [1–5]. This class of compounds is
increasingly being applied in different fields of research [6–9]. Metal–metal bonded
bimetallic complexes are of particular interest due to their applications in catalysis.
The bimetal platform provides scope for cooperative action between two metal ions
[10–14] and offers prospect for multi-electron redox chemistry [15–19]. Reactivity
studies on bimetallic systems provide valuable mechanistic information on chemical reactions catalyzed by metal cluster [20–22], metal nanoparticle [23–27], and
reactions occurring on metal surfaces [28–30]. Majority of the reactions on bimetallic complexes occur at equatorial sites [31–34]. Metal–metal singly bonded
compounds provide suitable molecular platform for axial site chemistry. The
metal–metal single bond allows close approach of the substrate to axial site whereas
strong trans effect of the metal–metal multiple bonds does not permit strong axial
binding.
Dirhodium(II,II) tetraacetate and related complexes are most prominent among
bimetallic complexes for their extensive applications as catalysts for a range of
organic reactions. Several reviews [35–38] and book chapters [39–42] have
appeared recently covering dirhodium(II,II) compounds. Other bimetallic compounds featuring metal–metal single bond include complexes containing [Ir
II –Ir
II ]
[43–46] and [Pd
I –Pd
I ] [47–52] bonds. Lack of synthetic procedures for their large
scale synthesis, however, has impeded their wider applications.
The isoelectronic [Ru
I –Ru
I ] complexes are relatively more accessible, and such
complexes are proving to be useful for a range of chemical reactions. This article
collects the chemistry at sites trans to the [Ru–Ru] single bond. The C–H activation, C–C bond formation, acceptorless alcohol dehydrogenation, cyclopropanation, carbenoid C–H insertion, and C–H amination reactions are covered.
Reactivity studies at axial sites on [Ru–Ru] multiply bonded systems are also
included because of their direct relevance in catalytic chemistry. The purpose of
this article is to highlight the recent progress on the axial-site chemistry on [Ru
I –
Ru
I ] platforms with intent to infuse interests for further development.
2 Diruthenium(I,I) Complexes Bearing Paddlewheel
[Ru 2 (CO) 4 ]
2+ Core
Compounds
of
general
formula
[Ru 2 (O 2 CR) 2 (CO) 4 ] n
and
[Ru 2 (O 2 CR) 2 (CO) 4 (AL) 2 ] (Scheme 1) are the commonly used diruthenium(I,I)
precursors containing metal–metal single bond [53]. Two acetates bridge between
two metals, and four carbonyls bind at equatorial positions to complete the
paddlewheel geometry for [Ru 2 (O 2 CR) 2 (CO) 4 ]. Two ligands additionally occupy
axial sites for [Ru 2 (O 2 CR) 2 (CO) 4 (AL) 2 ]. Polymeric structure is observed for
Reactivity and Catalysis at Sites Trans to the [Ru–Ru] Bond
61