2.2 Nitrogen or Sulfur Modification of Metal NP Surface
Although heterogeneous metal NP catalysts have many advantages over homogeneous catalysts, such as high stability and easy separation and reuse, fine-tuning the
active metal center remains difficult. Beller et al. reported the heterogenization of
well-defined organometallic complex catalysts via immobilization and pyrolysis on
activated carbon [47]. The synthetic scheme for novel cobalt oxide NPs is shown in
Fig. 12. The deposition of cobalt(II) acetate–phenanthroline complex onto commercially available carbon followed by pyrolysis under inert conditions provided cobalt
oxide NPs coated with N-doped graphene (Co oxide–N/C).
The synthesized novel N-ligated Co oxide NPs showed high chemoselectivity for
nitroarenes bearing easily reducible moieties including alkenes, alkynes, aldehydes,
and ketones (Fig. 13). Furthermore, this heterogeneous Co catalyst was reusable
without a significant decrease in activity during the recycling experiments, although
an initial loss of activity was observed.
Beller et al. also showed that the convenient pyrolysis heterogenization method of
well-defined organometallic complex catalysts could be applied to the most earthabundant metal, iron [48], because the development of efficient and cost-effective
catalyst systems is greatly needed in current chemical research. The pyrolysis of
iron–phenanthroline complexes adsorbed on carbon at 800
C provided stable Fe 2 O 3
NPs surrounded by a nitrogen-doped carbon layer (Fe-phen/C-800). The Fe-phen/C800 catalyst promoted more than 80 examples of structurally diverse nitroarenes,
giving the corresponding functionalized anilines in good to excellent yields under
industrially acceptable conditions (Fig. 14).
One rational design approach toward metal NP catalysts involves metal modification with self-assembled monolayers (SAMs) to create a favorable surface environment for selective formation of the desired products [49, 50]. SAMs are organic
assemblies of molecules containing a headgroup that noncovalently bind to the
metal, producing ordered and stable monolayer films on the metal surface. A unique
property of SAM modification is the formation of highly ordered assembly structures
on metal surfaces, where modifiers with different tail structures can finely tune the
metal surface environment to facilitate specific interactions between reactants and
metal NPs.
Medlin et al. showed that SAMs on metal NP surfaces can act as “molecular
recognition monolayers” to give the reactant a desirable orientation, which greatly
improves the catalytic performance for hydrogenation reactions [51]. For example,
in the hydrogenation of cinnamaldehyde, modification of a Pt NP catalyst with
3-phenylpropanethiol significantly improved the selectivity for cinnamyl alcohol
to over 95% compared with 25% selectivity using uncoated Pt catalysts [52]. This
high chemoselectivity was attributed to the favorable orientation of cinnamaldehyde
in the vertical configuration through aromatic stacking interactions between the
modifier and reactant. The proposed adsorption mechanism on SAMs is shown in
Fig. 15. This represents the first successful demonstration of using specific
58
K. Jitsukawa and T. Mitsudome
Although heterogeneous metal NP catalysts have many advantages over homogeneous catalysts, such as high stability and easy separation and reuse, fine-tuning the
active metal center remains difficult. Beller et al. reported the heterogenization of
well-defined organometallic complex catalysts via immobilization and pyrolysis on
activated carbon [47]. The synthetic scheme for novel cobalt oxide NPs is shown in
Fig. 12. The deposition of cobalt(II) acetate–phenanthroline complex onto commercially available carbon followed by pyrolysis under inert conditions provided cobalt
oxide NPs coated with N-doped graphene (Co oxide–N/C).
The synthesized novel N-ligated Co oxide NPs showed high chemoselectivity for
nitroarenes bearing easily reducible moieties including alkenes, alkynes, aldehydes,
and ketones (Fig. 13). Furthermore, this heterogeneous Co catalyst was reusable
without a significant decrease in activity during the recycling experiments, although
an initial loss of activity was observed.
Beller et al. also showed that the convenient pyrolysis heterogenization method of
well-defined organometallic complex catalysts could be applied to the most earthabundant metal, iron [48], because the development of efficient and cost-effective
catalyst systems is greatly needed in current chemical research. The pyrolysis of
iron–phenanthroline complexes adsorbed on carbon at 800
C provided stable Fe 2 O 3
NPs surrounded by a nitrogen-doped carbon layer (Fe-phen/C-800). The Fe-phen/C800 catalyst promoted more than 80 examples of structurally diverse nitroarenes,
giving the corresponding functionalized anilines in good to excellent yields under
industrially acceptable conditions (Fig. 14).
One rational design approach toward metal NP catalysts involves metal modification with self-assembled monolayers (SAMs) to create a favorable surface environment for selective formation of the desired products [49, 50]. SAMs are organic
assemblies of molecules containing a headgroup that noncovalently bind to the
metal, producing ordered and stable monolayer films on the metal surface. A unique
property of SAM modification is the formation of highly ordered assembly structures
on metal surfaces, where modifiers with different tail structures can finely tune the
metal surface environment to facilitate specific interactions between reactants and
metal NPs.
Medlin et al. showed that SAMs on metal NP surfaces can act as “molecular
recognition monolayers” to give the reactant a desirable orientation, which greatly
improves the catalytic performance for hydrogenation reactions [51]. For example,
in the hydrogenation of cinnamaldehyde, modification of a Pt NP catalyst with
3-phenylpropanethiol significantly improved the selectivity for cinnamyl alcohol
to over 95% compared with 25% selectivity using uncoated Pt catalysts [52]. This
high chemoselectivity was attributed to the favorable orientation of cinnamaldehyde
in the vertical configuration through aromatic stacking interactions between the
modifier and reactant. The proposed adsorption mechanism on SAMs is shown in
Fig. 15. This represents the first successful demonstration of using specific
58
K. Jitsukawa and T. Mitsudome