such as first row transition metals, although industrial catalysts for the same
reaction often require precious metal catalysts based on Pt, Pd, or Ru. Moreover,
using modern molecular biological techniques, enzymes can often be cheaply
produced with microbial overexpression systems. However, enzymes function
ordinarily only over the limited range of conditions in which the protein remains
correctly folded. Thus, they are not ideal for use directly in all applications. Instead,
in many cases, enzymes serve as inspiration to uncover underlying principles of fast
catalysis that can be implemented in simple, synthetic molecules.
Biological inspiration has proven particularly important in energy research.
Human-driven production of greenhouse gases and their resulting impact on global
processes including climate has increased awareness of the need to develop catalysts for carbon-neutral production of fuels [4]. Hydrogen and carbonaceous fuels
derived from carbon dioxide reduction are the most desirable targets, and many
metalloenzymes catalyzing production of these fuels are well known. In fact, the
only catalytic process for carbonaceous fuel production that has been successful on
the global scale is biological, i.e., photosynthesis. Remarkably, many fuel production enzymes feature unique heterobimetallic active sites for substrate activation.
These unusual inorganic structures serve both as templates and as aspirational goals
for development of robust catalysts.
In this chapter, we survey recent efforts to produce biologically inspired catalysts for proton and CO 2 reduction. First, we introduce the biological catalysts that
underpin this research: hydrogenases, carbon monoxide dehydrogenases, and formate dehydrogenases. Then, we describe some of the most catalytically successful
synthetic molecules that have been inspired by these biological catalysts, paying
particular attention to catalytic properties and mechanism.
2 Biological Catalysts
2.1 Hydrogenases
Hydrogen and protons have been utilized for bioenergetic processes in organisms
for billions of years. Hydrogenases are the enzymes responsible for hydrogen
oxidation and production. These proteins turnover at high rates (k cat ~ 10
4 s
À1 )
and are highly reversible, i.e., operate with minimal electrochemical overpotential
[5]. There are three types of hydrogenase, distinguished by the metal content of
their active sites: [Fe]-, [FeFe]-, and [NiFe]-hydrogenase. Figure 1 shows the active
sites of these enzymes. Unlike the other hydrogenases, the [Fe]-hydrogenase
contains a monometallic, as opposed to bimetallic, active site and no accessory
[FeS] clusters. Furthermore, it only performs the hydrogenase reaction in the
presence of an additional organic cofactor [6]. Thus, this type of hydrogenase
will not be discussed further here. The bimetallic hydrogenases, [FeFe] and
[NiFe], have been more extensively characterized. They catalyze the reaction
Biomimetic Complexes for Production of Dihydrogen and Reduction of CO 2
235
reaction often require precious metal catalysts based on Pt, Pd, or Ru. Moreover,
using modern molecular biological techniques, enzymes can often be cheaply
produced with microbial overexpression systems. However, enzymes function
ordinarily only over the limited range of conditions in which the protein remains
correctly folded. Thus, they are not ideal for use directly in all applications. Instead,
in many cases, enzymes serve as inspiration to uncover underlying principles of fast
catalysis that can be implemented in simple, synthetic molecules.
Biological inspiration has proven particularly important in energy research.
Human-driven production of greenhouse gases and their resulting impact on global
processes including climate has increased awareness of the need to develop catalysts for carbon-neutral production of fuels [4]. Hydrogen and carbonaceous fuels
derived from carbon dioxide reduction are the most desirable targets, and many
metalloenzymes catalyzing production of these fuels are well known. In fact, the
only catalytic process for carbonaceous fuel production that has been successful on
the global scale is biological, i.e., photosynthesis. Remarkably, many fuel production enzymes feature unique heterobimetallic active sites for substrate activation.
These unusual inorganic structures serve both as templates and as aspirational goals
for development of robust catalysts.
In this chapter, we survey recent efforts to produce biologically inspired catalysts for proton and CO 2 reduction. First, we introduce the biological catalysts that
underpin this research: hydrogenases, carbon monoxide dehydrogenases, and formate dehydrogenases. Then, we describe some of the most catalytically successful
synthetic molecules that have been inspired by these biological catalysts, paying
particular attention to catalytic properties and mechanism.
2 Biological Catalysts
2.1 Hydrogenases
Hydrogen and protons have been utilized for bioenergetic processes in organisms
for billions of years. Hydrogenases are the enzymes responsible for hydrogen
oxidation and production. These proteins turnover at high rates (k cat ~ 10
4 s
À1 )
and are highly reversible, i.e., operate with minimal electrochemical overpotential
[5]. There are three types of hydrogenase, distinguished by the metal content of
their active sites: [Fe]-, [FeFe]-, and [NiFe]-hydrogenase. Figure 1 shows the active
sites of these enzymes. Unlike the other hydrogenases, the [Fe]-hydrogenase
contains a monometallic, as opposed to bimetallic, active site and no accessory
[FeS] clusters. Furthermore, it only performs the hydrogenase reaction in the
presence of an additional organic cofactor [6]. Thus, this type of hydrogenase
will not be discussed further here. The bimetallic hydrogenases, [FeFe] and
[NiFe], have been more extensively characterized. They catalyze the reaction
Biomimetic Complexes for Production of Dihydrogen and Reduction of CO 2
235