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

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-M