A Robust and Efficient Pd₃ Cluster Catalyst for the Suzuki Reaction and Its Odd Mechanism

Abstract

The palladium-catalyzed Suzuki−Miyaura coupling reaction is one of the most versatile and powerful tools for constructing synthetically useful unsymmetrical aryl−aryl bonds. In designing a Pd cluster as a candidate for efficient catalysis and mechanistic investigations, it was envisaged to study a case intermediate between, although very different from, the "classic" Pd(0)L n and Pd nanoparticle families of catalysts. In this work, the cluster [Pd 3 Cl(PPh 2 ) 2 (PPh 3 ) 3 ] + [SbF 6 ] − (abbreviated Pd 3 Cl) was synthesized and fully characterized as a remarkably robust framework that is stable up to 170 °C and fully air-stable. Pd 3 Cl was found to catalyze the Suzuki−Miyaura C−C crosscoupling of a variety of aryl bromides and arylboronic acids under ambient aerobic conditions. The reaction proceeds while keeping the integrity of the cluster framework all along the catalytic cycle via the intermediate Pd 3 Ar, as evidenced by mass spectrometry and quick X-ray absorption fine structure. In the absence of the substrate under the reaction conditions, the Pd 3 OH species was detected by mass spectrometry, which strongly favors the "oxo-Pd" pathway for the transmetalation step involving substitution of the Cl ligand by OH followed by binding of the OH ligand with the arylboronic acid. The kinetics of the Suzuki− Miyaura reaction shows a lack of an induction period, consistent with the lack of cluster dissociation. This study may provide new perspectives for the catalytic mechanisms of C−C cross-coupling reactions catalyzed by metal clusters.

Highly Robust but Surface‐Active: An N‐Heterocyclic Carbene‐Stabilized Au25 Nanocluster

“…[3] Forthis purpose,the structurally welldefined NCs should be soluble and stable enough in reaction media while having open surface metal sites accessible by reactants. [4] Although al arge number of ligated atomically precise Au NCs have been successfully attained in recent decades by employing organic ligands,i ncluding phosphines, [5] thiols, [6] and alkynyls, [7] quite limited success has been achieved to demonstrate the direct use of the ligated atomically precise Au NCs as atruly homogeneous catalyst in liquid-phase reactions without being loaded on supports. [8] Recently,N -heterocyclic carbenes (NHCs) have shown excellent attributes in stabilizing surfaces,n anoparticles,o r clusters of coinage metals.…”

Highly Robust but Surface-Active: N-Heterocyclic Carbene-Stabilized Au25 Nanocluster as a Homogeneous Catalyst

“…[26][27][28] Therefore, limited success has been achieved to demonstrate the direct use of the ligated atomically precise metal NCs as a truly homogeneous catalyst in liquid-phase reactions without being loaded on supports. [4][5]29 Recently, N-heterocyclic carbenes (NHCs) have been shown excellent attributes in stabilizing surfaces, nanoparticles or clusters of coinage metals. 30 Several groups have demonstrated that NHCs readily form robust self-assembled monolayer on coinage metal surface and function as a performance-enhancing modifier.…”

Usefulness of the σ‐Aromaticity and σ‐Antiaromaticity Concepts for Clusters and Solid‐State Compounds

“…In recent years s-aromaticity was extended into the solid state. [47][48][49][50][51][52] Robilotto et al [47] reportedt he synthesis of the [(LAu) 3 ] + OTf À compound with an Au 3 + core. The X-ray diffraction revealed that this compound is of approximate D 3 symme- (Figure 9).…”

“…The crystal structureo f[ Pd 3 Cl(PPh) 2 (PPh 3 ) 3 ] + [SbF 6 ] À .R eprinted with permission from ref [49]…”