Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2 Elementary Steps in Binuclear Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.1 Oxidative Addition/Reductive Elimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.2 Migratory Insertion, β-Hydrogen Elimination, and Ligand Migration . . . . . . . . . . . . . . . 35
3 Trans Effect in Bimetallic Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.1 Position of Bridging Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 Intermetallic Trans Effect/Influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4 Homobimetallic Iridium Complexes: Reactivity and Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1 Bond Activation by Homobimetallic Iridium Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2 Catalysis by Homobimetallic Iridium Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
1 Introduction
Metal cooperation is a widely used concept in enzyme catalysis, often invoked in
order to rationalize activities and selectivity patterns inaccessible by action of a
single metal center [1–13]. An illustrative example is the enzyme tyrosinase, which
catalyzes the hydroxylation of tyrosine to DOPA (3,4-dihydroxyphenylalanine) and
its subsequent oxidation to the corresponding quinone. Tyrosinase makes use of an
active site based on a bimetallic copper complex that activates molecular oxygen.
The deoxy form consists of a [Cu(I) Cu(I)] core, while the oxygenated site presents
a μ-η
2 :η
2 -peroxide (O 2
2À ) bridge: [Cu(I)O 2 Cu(I)]. According to the generally
accepted catalytic cycle (Scheme 1), the binuclear nature of the catalyst is required,
not only for the activation of molecular oxygen but also for the transfer of the
oxygen atom in the hydroxylation of tyrosinase, as well as for the two-electron
oxidation of DOPA [14–21].
Despite the vast number of outstanding examples of enzymatic catalysis that rely
on the collaboration of two or more vicinal metal centers hitherto disclosed, the
design and development of efficient binuclear organometallic complexes able to
enhance the performance of mononuclear catalyst by means of an intermetallic
cooperative process remains widely unexplored [22–24]. In fact, the formation of
bi- or polynuclear complexes has been often described as a catalyst deactivation
pathway [25–30]. However, the availability of more electron density at the active
site, extra coordination positions, and the possibility to develop more preorganized
systems that allow for (enantio)selective reactions shows great promise for an
improved catalytic performance [9].
Binuclear rhodium complexes in particular have met with great success as
catalysts for various transformations, especially remarkable are the Rh
(II) examples reported by Doyle et al. [31] (and references therein) and Stanley’s
system for hydroformylation [32–38]. The latter showed an excellent
regioselectivity for the hydroformylation of 1-hexene with remarkable turnover
numbers (ca. 12,000 cycles) and frequencies (73 min
À1 ). The enhanced activity and
32
M. Iglesias et al.
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2 Elementary Steps in Binuclear Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.1 Oxidative Addition/Reductive Elimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.2 Migratory Insertion, β-Hydrogen Elimination, and Ligand Migration . . . . . . . . . . . . . . . 35
3 Trans Effect in Bimetallic Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.1 Position of Bridging Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 Intermetallic Trans Effect/Influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4 Homobimetallic Iridium Complexes: Reactivity and Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1 Bond Activation by Homobimetallic Iridium Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2 Catalysis by Homobimetallic Iridium Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
1 Introduction
Metal cooperation is a widely used concept in enzyme catalysis, often invoked in
order to rationalize activities and selectivity patterns inaccessible by action of a
single metal center [1–13]. An illustrative example is the enzyme tyrosinase, which
catalyzes the hydroxylation of tyrosine to DOPA (3,4-dihydroxyphenylalanine) and
its subsequent oxidation to the corresponding quinone. Tyrosinase makes use of an
active site based on a bimetallic copper complex that activates molecular oxygen.
The deoxy form consists of a [Cu(I) Cu(I)] core, while the oxygenated site presents
a μ-η
2 :η
2 -peroxide (O 2
2À ) bridge: [Cu(I)O 2 Cu(I)]. According to the generally
accepted catalytic cycle (Scheme 1), the binuclear nature of the catalyst is required,
not only for the activation of molecular oxygen but also for the transfer of the
oxygen atom in the hydroxylation of tyrosinase, as well as for the two-electron
oxidation of DOPA [14–21].
Despite the vast number of outstanding examples of enzymatic catalysis that rely
on the collaboration of two or more vicinal metal centers hitherto disclosed, the
design and development of efficient binuclear organometallic complexes able to
enhance the performance of mononuclear catalyst by means of an intermetallic
cooperative process remains widely unexplored [22–24]. In fact, the formation of
bi- or polynuclear complexes has been often described as a catalyst deactivation
pathway [25–30]. However, the availability of more electron density at the active
site, extra coordination positions, and the possibility to develop more preorganized
systems that allow for (enantio)selective reactions shows great promise for an
improved catalytic performance [9].
Binuclear rhodium complexes in particular have met with great success as
catalysts for various transformations, especially remarkable are the Rh
(II) examples reported by Doyle et al. [31] (and references therein) and Stanley’s
system for hydroformylation [32–38]. The latter showed an excellent
regioselectivity for the hydroformylation of 1-hexene with remarkable turnover
numbers (ca. 12,000 cycles) and frequencies (73 min
À1 ). The enhanced activity and
32
M. Iglesias et al.