Wakatsuki and co-workers [128]. This complex is able to reversibly deprotonate
acidic organic compounds, which makes possible a novel catalytic reaction for the
cleavage of the C–C bond in aromatic 1,2-diols (Scheme 30). Besides, this complex
is able to reversibly deprotonate acidic methylenic protons and act as a catalyst for
Michael addition reactions under neutral conditions.
A new example of a binuclear Ir(II) catalyst is complex [{Ir(μ-κC NHC ,η
6
Dipp -
IDipp)(H)} 2 ][BF 4 ] 2 , which efficiently promotes the hydroalkynylation of imines
[129]. The Ir 2
II,II compound is remarkably stable in the presence of air or moisture,
even at high temperatures in the presence of imines. Addition of one equivalent of
alkyne to [{Ir(μ-κC NHC ,η
6
Dipp -IDipp)(H)} 2 ][BF 4 ] 2 results in the formation of a
non-identified hydride complex in low yields. However, when excess alkyne is
added, the C–C coupling reaction takes place to give a mixture of organic compounds (dimerization, trimerization, and cyclotrimerization) and the initial
binuclear complex. The proposed reaction mechanism, substantiated by theoretical
calculations at the DFT level, entails (i) single-site oxidative addition of the
alkyne’s C–H bond to give the alkynyl–trihydride complex, in agreement with
the reactivity described above for terminal alkynes (vide supra); (ii) deprotonation
of the binuclear complex by the imine to give the protonated imine and the
corresponding dihydride Ir 2
I,III compound with the end-on coordinated alkynyl
ligand; (iii) coordination of the protonated imine by the C¼N bond and subsequent
migratory insertion into the Ir–C(alkynyl) bond; and to end with, (iv) the release of
the propargyl amine with the concomitant formation of the Ir 2
II,II active species,
which restarts the catalytic cycle (Scheme 31).
Noteworthy, the arene ligands undergo multiple hapticity changes during the
course of the proposed catalytic cycle, which regulates the activity and stability of
the catalyst by providing the vacant coordination sites required for substrate
coordination while, at the same time, maintains the integrity of the bimetallic entity.
As already mentioned in Sect. 4.1, the use of binuclear Ir 2
I,I complexes for
hydrogenation reactions is not favored. In particular, Ir 2
I,I complexes, containing
binucleating N-donor ligands presenting open-book structures, are usually inactive
toward molecular hydrogen activation. However, a binuclear Ir 2
I,I complex
containing bridging amido ligands of formula [{Ir(cod)(μ-NH 2 )} 2 ] is an active
homogeneous catalyst for the transfer hydrogenation of acetophenone to
1-phenylethanol, using
i
PrOH as hydrogen donor. The unusual activity of this Ir 2
I,
I system has been attributed to the cooperation between the two Ir centers and the
non-innocent amido ligand [130]. The proposed reaction mechanism, substantiated
by experimental observations and theoretical calculations at the DFT level, is
shown in Scheme 32.
The process is initiated by an unusual metalacycle opening to yield [(cod)Ir
(μ-NH 2 )Ir(NH 2 )(cod)] species that promotes the concerted dehydrogenation of
isopropanol and simultaneous transference of two hydrogen atoms to form a
diiridium hydride intermediate [(cod)(H)Ir(μ-NH 2 )Ir(NH 3 )(cod)] containing a
bridging amido ligand. This is followed by the concerted hydrogenation of
acetophenone, by simultaneous transference of two hydrogen atoms to the C¼O
bond, to yield 1-phenylethanol. The concerted dehydrogenation of isopropanol, and
52
M. Iglesias et al.
acidic organic compounds, which makes possible a novel catalytic reaction for the
cleavage of the C–C bond in aromatic 1,2-diols (Scheme 30). Besides, this complex
is able to reversibly deprotonate acidic methylenic protons and act as a catalyst for
Michael addition reactions under neutral conditions.
A new example of a binuclear Ir(II) catalyst is complex [{Ir(μ-κC NHC ,η
6
Dipp -
IDipp)(H)} 2 ][BF 4 ] 2 , which efficiently promotes the hydroalkynylation of imines
[129]. The Ir 2
II,II compound is remarkably stable in the presence of air or moisture,
even at high temperatures in the presence of imines. Addition of one equivalent of
alkyne to [{Ir(μ-κC NHC ,η
6
Dipp -IDipp)(H)} 2 ][BF 4 ] 2 results in the formation of a
non-identified hydride complex in low yields. However, when excess alkyne is
added, the C–C coupling reaction takes place to give a mixture of organic compounds (dimerization, trimerization, and cyclotrimerization) and the initial
binuclear complex. The proposed reaction mechanism, substantiated by theoretical
calculations at the DFT level, entails (i) single-site oxidative addition of the
alkyne’s C–H bond to give the alkynyl–trihydride complex, in agreement with
the reactivity described above for terminal alkynes (vide supra); (ii) deprotonation
of the binuclear complex by the imine to give the protonated imine and the
corresponding dihydride Ir 2
I,III compound with the end-on coordinated alkynyl
ligand; (iii) coordination of the protonated imine by the C¼N bond and subsequent
migratory insertion into the Ir–C(alkynyl) bond; and to end with, (iv) the release of
the propargyl amine with the concomitant formation of the Ir 2
II,II active species,
which restarts the catalytic cycle (Scheme 31).
Noteworthy, the arene ligands undergo multiple hapticity changes during the
course of the proposed catalytic cycle, which regulates the activity and stability of
the catalyst by providing the vacant coordination sites required for substrate
coordination while, at the same time, maintains the integrity of the bimetallic entity.
As already mentioned in Sect. 4.1, the use of binuclear Ir 2
I,I complexes for
hydrogenation reactions is not favored. In particular, Ir 2
I,I complexes, containing
binucleating N-donor ligands presenting open-book structures, are usually inactive
toward molecular hydrogen activation. However, a binuclear Ir 2
I,I complex
containing bridging amido ligands of formula [{Ir(cod)(μ-NH 2 )} 2 ] is an active
homogeneous catalyst for the transfer hydrogenation of acetophenone to
1-phenylethanol, using
i
PrOH as hydrogen donor. The unusual activity of this Ir 2
I,
I system has been attributed to the cooperation between the two Ir centers and the
non-innocent amido ligand [130]. The proposed reaction mechanism, substantiated
by experimental observations and theoretical calculations at the DFT level, is
shown in Scheme 32.
The process is initiated by an unusual metalacycle opening to yield [(cod)Ir
(μ-NH 2 )Ir(NH 2 )(cod)] species that promotes the concerted dehydrogenation of
isopropanol and simultaneous transference of two hydrogen atoms to form a
diiridium hydride intermediate [(cod)(H)Ir(μ-NH 2 )Ir(NH 3 )(cod)] containing a
bridging amido ligand. This is followed by the concerted hydrogenation of
acetophenone, by simultaneous transference of two hydrogen atoms to the C¼O
bond, to yield 1-phenylethanol. The concerted dehydrogenation of isopropanol, and
52
M. Iglesias et al.