Evidence for Suzuki-Miyaura cross-couplings catalyzed by ligated Pd-clusters: from cradle to grave

• Published in: Chemical science (Cambridge) Vol. 15; no. 8; pp. 2763 - 2777
• Main Authors: Jeddi, Neda, Scott, Neil W. J, Tanner, Theo, Beaumont, Simon K, Fairlamb, Ian J. S
• Format: Journal Article
• Published: 22-02-2024
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d3sc06447f

Pd
n
clusters offer unique selectivity and exploitable reactivity in catalysis. Understanding the behavior of Pd
n
clusters is thus critical for catalysis, applied synthetic organic chemistry and greener outcomes for precious Pd. The Pd
3
cluster, [Pd
3
(μ-Cl)(μ-PPh
2
)
2
(PPh
3
)
3
][Cl] (denoted as Pd
3
Cl
2
), which exhibits distinctive reactivity, was synthesized and immobilized on a phosphine-functionalized polystyrene resin (denoted as immob-Pd
3
Cl
2
). The resultant material served as a tool to study closely the role of Pd
3
clusters in a prototypical Suzuki-Miyaura cross-coupling of 4-fluoro-1-bromobenzene and 4-methoxyphenyl boronic acid at varying low Pd ppm concentrations (24, 45, and 68 ppm). Advanced heterogeneity tests such as Hg poisoning and the three-phase test showed that leached mononuclear or nanoparticulate Pd are unlikely to be the major active catalyst species under the reaction conditions tested. EXAFS/XANES analysis from (pre)catalyst and filtered catalysts during and after catalysis has shown the intactness of the triangular structure of the Pd
3
X
2
cluster, with exchange of chloride (X) by bromide during catalytic turnover of bromoarene substrate. This finding is further corroborated by treatment of immob-Pd
3
Cl
2
after catalyzing the Suzuki-Miyaura reaction with excess PPh
3
, which releases the cluster from the polymer support and so permits direct observation of [Pd
3
(μ-Br)(μ-PPh
2
)
2
(PPh
3
)
3
]
+
ions by ESI-MS. No evidence is seen for a proposed intermediate in which the bridging halogen on the Pd
3
motif is replaced by an aryl group from the organoboronic acid,
i.e.
formed by a transmetallation-first process. Our findings taken together indicate that the 'Pd
3
X
2
' motif is an active catalyst species, which is stabilized by being immobilized, providing a more robust Pd
3
cluster catalyst system. Non-immobilized Pd
3
Cl
2
is less stable, as is followed by stepwise XAFS of the non-immobilized Pd
3
Cl
2
, which gradually changes to a species consistent with 'Pd
x
(PPh
3
)
y
' type material. Our findings have far-reaching future implications for Pd
3
cluster involvement in catalysis, showing that immobilization of Pd
3
cluster species offers advantages for rigorous mechanistic examination and applied chemistries.
Pd
n
clusters offer unique selectivity and exploitable reactivity in catalysis. Tethering Pd
3
clusters to a resin unlocks compelling new insight into the speciation of Pd during catalytic turnover for Suzuki-Miyaura cross-couplings.