Tripalladium clusters of the type [Pd3(PPh3)4]2+, wherein three linearly connected Pd atoms are stabilized by phosphine and arsine ligands, have been detected and isolated as intermediates during the reduction of well-defined mononuclear [Pd(OTf)2(XPh3)2] (X = P and X = As, respectively) to Pd nanoparticles (PdNPs). The isolated [Pd3(PPh3)4]2+ cluster isomerizes on broad-band UV irradiation to form an unexpected photoisomer, produced by a remarkable change in conformation at one of the bridging PPh3 ligands. A catalytic role for these [Pd3(XPh3)4]2+ species is exemplified in Suzuki-Miyaura cross-coupling (SMCC) reactions, with high activity seen in the arylation of a brominated heterocyclic 2-pyrone. Use of the [Pd3(PPh3)4]2+ cluster enables a switch in site selectivity for SMCC reactions involving 2,4-dibromopyridine from the typical C2-bromide site (seen previously for mononuclear Pd catalysts) to the atypical C4-bromide site, thereby mirroring recently reported cyclic Pd3 clusters and PdNPs. We have further determined that the thermal isomer and photoisomer of [Pd3(PPh3)4]2+ are similarly catalytically active in the Pd-catalyzed hydrogenation of phenylacetylene to give styrene. Our findings link the evolution of mononuclear Pd(II) salts to PdNPs via the intermediacy of linear [Pd3(XPh3)4]2+ clusters.
Bibliographical noteFunding Information:
We are grateful for financial support from the EPSRC (grant no. EP/G009546/1) and Bruker BioSpin (CASE studentship for K.M.A. and equipment). N.W.J.S. was funded by Bayer AG (Ph.D. studentship). S.B.D. and I.J.S.F. thank the Wellcome Trust for funding (Reference Grants 092506 and 098335).
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