4.2.3 Manganese as an Alternative to Rhenium
Manganese is approximately one million times more abundant than rhenium in the
earth’s crust [168], making it a much better candidate for catalyst design with future
large-scale applications. The complexes Mn 2 (CO) 10 and Mn(CO) 5 I were first
reported in the 1950s [169], and fac-Mn(bpy-R)(CO) 3 X, where X is a halogen,
complexes have been known since 1959 [170]. However, their catalytic properties
were not described until much later. In 1996, Hartl and coworkers concluded from an
IR spectroelectrochemical study that [Mn(bpy)(CO) 3 ]
1À does not react with CO 2
[171]. Further, IR studies showed that air stable fac-Mn(bpy)(CO) 3 Br quickly forms a
dimer under reducing conditions [172] and further reduction formed the [Mn(bpy)
(CO) 3 ]
1À anion. The X-ray crystal structure of the two-electron-reduced [Mn(bpy)
(CO) 3 ]
1À was reported in 2007 by the same group [173]. The breakthrough did not
come until 2011 when Deronzier et al. [174] reported that a proton source is the
missing ingredient. Both fac-Mn(bpy-R)(CO) 3 Br and [Mn(bpy-R)(CO) 3 ] 2 (R¼H,
Me) electrocatalytically reduce CO 2 to CO in acetonitrile with water as a proton
source. The catalysis occurs at the potential at which [Mn(bpy)(CO) 3 ]
1À is formed,
suggesting that the two-electron-reduced anion promotes the catalysis. Inspired by
Deronzier’s work, related Mn(bpy-R)(CO) 3 X complexes with different substitution
of the bipyridyl ligand were also shown to electrocatalytically reduce CO 2 in solution
with a proton source. In particular, Kubiak et al. [175] observed similar behavior with
tert-butyl-substituted manganese complex. Upon reduction, dimerization occurs
followed by formation of the catalytically active anion. Remarkably, the activity of
the tert-butyl derivative approaches that of the rhenium analogs and exceeds them in
the presence of water. It is also important to note that the activity increases with
increasing strength of the Br€ onsted acid. Several groups have explored the idea of
replacing the bipyridyl ligand. Hartl and coworkers [176] reported a related manganese catalyst with a diimine ligand, Mn(CO) 3 (R-DAB)X (R-DAB¼N,N
0 -Di-R-1,4diazabuta-1,3-diene). Although formation of dimer was not observed, as for the
bipyridyl complexes, the catalyst has been identified as the reduced anion, Mn
(CO) 3 (R-DAB)
1À . Catalysis is thought to follow an ECE mechanism, and an extra
100 mV of overpotential is required relative to the bipyridyl analogue.
4.2.4 Metal Complexes with Macrocyclic Ligands: Role of the Outer
Coordination Sphere
A number of different metallocomplexes supported by macrocyclic ligands have
been shown to reduce CO 2 to CO. A common theme seen in many of these
complexes is hydrogen bonding between ligand protons and bound CO 2 or transfer
Biomimetic Complexes for Production of Dihydrogen and Reduction of CO 2
259
Manganese is approximately one million times more abundant than rhenium in the
earth’s crust [168], making it a much better candidate for catalyst design with future
large-scale applications. The complexes Mn 2 (CO) 10 and Mn(CO) 5 I were first
reported in the 1950s [169], and fac-Mn(bpy-R)(CO) 3 X, where X is a halogen,
complexes have been known since 1959 [170]. However, their catalytic properties
were not described until much later. In 1996, Hartl and coworkers concluded from an
IR spectroelectrochemical study that [Mn(bpy)(CO) 3 ]
1À does not react with CO 2
[171]. Further, IR studies showed that air stable fac-Mn(bpy)(CO) 3 Br quickly forms a
dimer under reducing conditions [172] and further reduction formed the [Mn(bpy)
(CO) 3 ]
1À anion. The X-ray crystal structure of the two-electron-reduced [Mn(bpy)
(CO) 3 ]
1À was reported in 2007 by the same group [173]. The breakthrough did not
come until 2011 when Deronzier et al. [174] reported that a proton source is the
missing ingredient. Both fac-Mn(bpy-R)(CO) 3 Br and [Mn(bpy-R)(CO) 3 ] 2 (R¼H,
Me) electrocatalytically reduce CO 2 to CO in acetonitrile with water as a proton
source. The catalysis occurs at the potential at which [Mn(bpy)(CO) 3 ]
1À is formed,
suggesting that the two-electron-reduced anion promotes the catalysis. Inspired by
Deronzier’s work, related Mn(bpy-R)(CO) 3 X complexes with different substitution
of the bipyridyl ligand were also shown to electrocatalytically reduce CO 2 in solution
with a proton source. In particular, Kubiak et al. [175] observed similar behavior with
tert-butyl-substituted manganese complex. Upon reduction, dimerization occurs
followed by formation of the catalytically active anion. Remarkably, the activity of
the tert-butyl derivative approaches that of the rhenium analogs and exceeds them in
the presence of water. It is also important to note that the activity increases with
increasing strength of the Br€ onsted acid. Several groups have explored the idea of
replacing the bipyridyl ligand. Hartl and coworkers [176] reported a related manganese catalyst with a diimine ligand, Mn(CO) 3 (R-DAB)X (R-DAB¼N,N
0 -Di-R-1,4diazabuta-1,3-diene). Although formation of dimer was not observed, as for the
bipyridyl complexes, the catalyst has been identified as the reduced anion, Mn
(CO) 3 (R-DAB)
1À . Catalysis is thought to follow an ECE mechanism, and an extra
100 mV of overpotential is required relative to the bipyridyl analogue.
4.2.4 Metal Complexes with Macrocyclic Ligands: Role of the Outer
Coordination Sphere
A number of different metallocomplexes supported by macrocyclic ligands have
been shown to reduce CO 2 to CO. A common theme seen in many of these
complexes is hydrogen bonding between ligand protons and bound CO 2 or transfer
Biomimetic Complexes for Production of Dihydrogen and Reduction of CO 2
259