New molecule sets stage for nickel as a 'greener' photocatalyst, reveals
key steps in reaction process
Researchers have developed a new ligand that promotes a direct nickel- photocatalyzed cross-coupling reaction.

Date:
April 26, 2022
Source:
University of Illinois College of Liberal Arts & Sciences
Summary:
Novel system could lead to catalysts based on cheaper, more abundant
nickel rather than more expensive precious metals.



FULL STORY ==========================================================================
In recent years, the golden word in precious metals is palladium.


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A crucial component in automobile catalytic converters and in emerging
hydrogen fuel cell technology, the demand for this rare silvery white transition metal continues to outstrip supply, driving its price per
ounce far above gold and silver.

Palladium and other rare, costly precious metals like platinum, iridium,
and ruthenium, are also crucial in chemical transformations, specifically transition metal catalysis, which has become an indispensable tool for
putting together complex molecules in the development of pharmaceutical
drugs, polymers, and other useful chemicals.

The scarcity and expense of these precious metals has created a need
to develop catalysts from transition metals that are more abundant and generally cheaper, like nickel, a cousin of palladium.

As a result, the last decade has seen a dramatic expansion of new
catalytic bond-forming transformations involving nickel.

"We know from the literature that nickel complexes are extremely useful in performing some transformations, maybe better than other transition metals
out there," said Liviu Mirica, William H. and Janet G. Lycan Professor of Chemistry at the University of Illinois at Urbana-Champaign. "People have gotten very good at optimizing conditions for specific transformations,
so we are slowly getting to where nickel could rival palladium in
these transformations." More recently, scientists have been focusing on developing nickel catalysts that can be directly photoinitiated by light,
which Mirica said has proven to be a very successful area of research
producing reactions that have not been previously possible.



========================================================================== However, they still require the use of an additional photocatalyst --
typically based on precious metals such as iridium or ruthenium that
are even more expensive than palladium.

In a paper recently published in Nature Communications, Mirica and
postdoctoral researcher Hanah Na report their work on the development of a completely novel tridentate ligand that coordinates with nickel to create
a catalyst that can be directly activated by light to form a carbon-oxygen
bond without the use of an additional photocatalyst. C-O bonds are
prevalent in many natural products, pharmaceuticals, and agrochemicals.

Mirica and Na believe their new class of tridentate pyridinophane
ligands (RN3) can lead to the development of new nickel catalysts
and are a practical platform for detailed mechanical studies of other nickel-catalyzed chemical reactions.

"It is a competent catalyst and on top of it, it can actually do this photocatalysis by itself, it doesn't require these other photocatalysts," Mirica said. "It opens up many avenues of research that we think
could be used for many additional applications." These tridentate pyridinophane ligands (RN3) build on previous work by Mirica, who had
already developed a novel four-pronged molecule known as a tetradentate
ligand, whose structure resembles the pocket of a baseball glove. This
ligand structure promoted rapid C-C bond forming reactivity while also stabilizing the higher oxidation states of nickel.



========================================================================== "It's very stable. But all of those intermediates over the past
decade have been way too stable. They're not competent in catalytic applications," Mirica said.

Then there is the bidentate ligand framework bipyridyl that most chemists
are using in nickel photocatalytic processes, which provides enhanced reactivity and the ability to adjust optimization to get the desired
reaction.

"It's great for catalytic chemistry but you can't isolate or see these
special nickel species," Mirica said.

Typically, Mirica explained, classic organic chemists have a particular chemical transformation in mind and try whatever catalysts they think
will be good, and whatever conditions or additives would be useful and
optimize it, focusing on a very specific transformation.

"We have a slightly different approach: a metallo-centric approach and
in this case nickel is the metal of interest," he said. "I am interested
in being able to design, isolate, characterize nickel complexes with
different coordination numbers, different ligand environments, and
in different oxidation states, which ultimately will dictate their
reactivity." This latest ligand structure is somewhere between the
other two.

"We open up a coordination site, we open up that nickel center, by
removing one of the four nitrogens, to allow other things to bind to it
and eventually it allows you to do catalytic activity, but still be able
to isolate and characterize intermediates," he said.

Their novel tridentate ligand enabled them to reveal for the first
time the key reaction steps and intermediate species in this catalytic
cycle. An in-depth mechanistic understanding of Ni-mediated photocatalysis
is essential for rational reaction design and optimization of the nickel-mediated chemical process, the researchers explain in the report.

Their mechanistic study employed techniques including Nuclear Magnetic Resonance (NMR), electron paramagnetic resonance (EPR), in situ infrared
(IR) spectroscopy and electrochemical and photophysical measurements,
and computational studies.

From a mechanical perspective, the photocatalytic cycle is
well-understood, but the Ni-mediated redox cycle has remained a
mystery. Paramagnetic Ni(I) and Ni (III) species are assumed to be part
of the process, but have not been thoroughly investigated, and the key catalytic steps of oxidative addition, trans-metalation, and reductive elimination at the nickel centers have never been directly observed.

In the past several decades, Na explained, visible light-mediated
photoredox catalysis has made vital contributions in the field of
synthetic organic chemistry. Traditionally, developing new methodologies
and reaction condition optimization are often achieved by trial and error rather than being based on a thorough understanding of the underlying
reaction mechanism.

Na said this might be because understanding of the underlying chemistry requires a major contribution from the inorganic and organometallic
chemistry fields (beyond the scope of the research interests in synthetic organic chemistry), including the synthesis and characterization
of related metal complexes and study of their photochemistry and
photophysics.

"As inorganic and organometallic chemists, we want to contribute to
this emerging research field, mostly focusing on unraveling clues to
understand underlying reaction mechanisms -- which is not much done by
organic chemists," Na said. "We believe that our work would provide
crucial insight into the reaction design and search for new chemical transformations in the burgeoning field of photoredox catalysis, and
thus can impact both the organic and inorganic chemistry community."
The goal, Mirica explained, is to unleash new reactivity that could
ultimately be helpful to organic chemists, who could then employ this
system and use it for very particular synthetic targets.

"They may not work now as well as the finely optimized or finely tuned
systems that people use on a daily basis in an organic lab, but we hope
that our new Ni catalysts will be commonly used several years down the
line," Mirica said.


========================================================================== Story Source: Materials provided by University_of_Illinois_College_of_Liberal_Arts_& Sciences. Original
written by Tracy Crane. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Hanah Na, Liviu M. Mirica. Deciphering the mechanism of the Ni-
photocatalyzed C‒O cross-coupling reaction using a tridentate
pyridinophane ligand. Nature Communications, 2022; 13 (1) DOI:
10.1038/ s41467-022-28948-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220426162558.htm

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