The use of recyclable catalysts in chemical reactions seems
to be a hot research area! I have looked the a recent issue of Chem. Commun.
and have read another nice paper about the immobilization of a ferrocene-containing
phosphine onto iron nanoparticles, through the use of dopamine as a linker [1]. The
corresponding rhodium and palladium complexes of the magnetic
ferrocenyl-phosphine ligands were prepared, and they have been shown to be
great and recyclable (hence re-usable) catalysts for hydroformylation (Rh) and
Heck coupling reactions (Pd) respectively.
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| Figure 1. Synthesis of the ligand and preparation of the metal complexes. |
Let me show you the design of this modular ligand (Figure 1). First,
the ligand was a ferrocenyl phosphine known as BPPFA. This phosphine ligand was
first conjugated to dopamine. When the dopamine-linked phosphine was sonicated
with Fe3O4, the phosphine was thus functionalized with
magnetic nanoparticles, and the structure was designated as Fe3O4@dop-BPPF.
These magnetite nanoparticles were formed as a black precipitate, and it was
notable that they could be collected and separated from the reaction mixture
through the use of a piece of magnet.
When Fe3O4@dop-BPPF
was allowed to react with a slight excess of [Rh(nbd)Cl]2 or [Pd(C3H5)Cl]2,
the resulting rhodium or palladium complexes were afforded respectively.
Why did the researchers use dopamine as a linker? They
rationalized that the 2 hydroxyl groups on dopamine, which were arranged in a
1,2 relationship, was great as a bidentate ligand and thus could provide
tighter binding to the iron oxide, leading to higher stability for the
resulting catalyst.
Before submitting to rounds of catalysis, the chemical and
magnetic properties of these compounds were first investigated with a number of
techniques. Besides NMR and FTIR spectroscopy, transmission electron microscopy,
selected area electron diffraction, and X-ray diffraction studies have also
been employed to access the appearances of these ligands and metal complexes.
One aspect I find particularly fascinating is the
investigations of the magnetic properties of the nanoparticles. It is related
to a phenomenon known as ‘magnetic hysteresis’, and the concept of hysteresis
is actually important in the field of Nonlinear Dynamics, something I am also highly
fascinated in.
The researchers have found that, the dop-BFFP phosphine’s
magnetic properties, namely its coercivity (Hc) and remanence (Mr), were not
affected by the functionalization with iron nor complexation to rhodium or
palladium. While the saturation magnetization value was decreased slightly upon
the iron nanoparticle functionalization and complexation with metals, the bulk
magnetization has not been affected, which meant that the magnetic properties
were retained even if the iron particles were functionalized with the phosphine
and complexed to the metal center, and this would benefit the separation
process at the end of a catalytic trial through the use of a magnet. Indeed, if
we look at the magnetic hysteresis loops of
Fe3O4@dop-BPPF, Fe3O4@dop-BPPF-Pd
and Fe3O4@dop-BPPF-Rh at room temperature were almost
overlapping with each other, confirming the above arguments.
 |
| Figure 2. Hydroformylation catalysis. |
Next, the researchers used their newly-prepared metal
complexes to do 2 catalytic reactions – Rh-catalyzed hydroformylation and
Pd-catalyzed Heck coupling reaction. Not only they wanted to investigate the conversion
and substrate scope, a prime aim was to see whether these nanoparticle catalysts
could be recycled and recused efficiently.
For the hydroformylation (Figure 2), they have focused much of their
studies on the substrate styrene. They have been able to optimize the reaction
conditions and the branch to linear selectivity was reasonably high, with the
branched product as the predominant. They have also found that styrene
substituted with electron withdrawing groups (such as NO2 and Br)
gave a very high branch to linear selectivity of 99:1. Linear alkenes, such as
1-octene, gave the opposite selectivity, with the normal isomer as the
predominant product (b : l = 0 : 100).
The hydroformylation catalyst could be recycled and re-used
for 3 further times, and then a gradual loss of reactivity was observed, and
the researchers attributed to a higher pressure in the reaction system, leading
to catalyst leaching.
 |
| Figure 3. Heck coupling catalysis. |
The Pd catalyst was subject to Heck coupling reaction, with
styrene and iodobenzene as the 2 coupling partners. Reaction temperature,
choice of base, and solvent system were optimized. When the less reactive
bromobenzene was employed instead of iodobenzene, full conversion could be
achieved through a longer reaction time, and this signified the stability of
the new catalyst.
With the styrene / iodobenzene system, the Pd catalyst could
also be recycled and re-used, by simply using an external magnet to separate
the catalyst from the reaction mixture. The clean catalyst could be re-used for
at least 9 more times, which was very impressive.
 |
| Figure 4. Cobalt-phosphine catalyzed hydroarylation - a potential reaction to explore? |
This is great research, and the authors are working on other
catalytic reactions to explore the scope of their novel ligands. Since they
have been using styrene as a substrate, that fact rings a bell on me as I
remember reading an article about a cobalt-catalyzed hydroarylation of styrene
from J. Am. Chem. Soc. [2]. That could also be an interesting and potentially useful
reaction to try, too!
by Ed Law
19/7/2016
Reference:
1. Magnetic nanoparticle-supported ferrocenylphosphine: a reusable catalyst for hydroformylation of alkene and Mizoroki–Heck olefination
M. Nasiruzzaman
Shaikh , Md.
RSC Adv., 2016, 6, 41687–41695
2. Regioselectivity-Switchable Hydroarylation of Styrenes
Ke Gao and Naohiko Yoshikai
J. Am. Chem.
Soc., 2011, 133, 400–402
