Computational Study of Intramolecular Coordination Enhanced Oxidative Addition to form PdIV-Pincer Complexes, and Selectivity in Aryloxide Attack at PdIVCH2CRR′ Motifs in Palladium-Mediated Organic Synthesis

Bimetallic complexes that merge metallocene and pincer-metal building blocks: synthesis, stereochemistry and catalytic reactivity

This perspective is to illustrate the synthesis and applications of bimetallic complexes by merging a metallocene and a (cyclopentadienyl/aryl) pincer metal complex. Four possible ways to merge metallocene and pincer-metal motifs are reported and representative examples are discussed in more detail. These bimetallic complexes have been employed in some important catalytic reactions such as cross-coupling, transfer hydrogenation or synthesis of ammonia. The metallocene fragment may tune the electronic properties of the pincer ligand, due to its redox reversible properties. Also, the presence of two metals in a single complex allows their electronic communication, which proved beneficial for, e.g., the catalytic activity of some species. The presence of the metallocene fragment provides an excellent opportunity to develop chiral catalysts, because the metallocene merger generally renders the two faces of the pincer-metal catalytic site diastereotopic. Besides, an extra chiral functionality may be added to the bimetallic species by using pincer motifs that are planar chiral, e.g. by using the different substituents of pincer ligand "arms" or non-symmetrical arene groupings. Post-functionalization of pre-formed pincer-metal complexes, via η⁶-coordination with an areneophile such as [CpRu]⁺ and [Cp*Ru]⁺ presents a striking strategy to obtain diastereomeric metallocene-pincer type derivatives, that actually involve half-sandwich metallocenes. This approach offers the possibility to create diastereomerically pure derivatives by using the chiral TRISPHAT anion. The authors hope that this report of the synthetic, physico-chemical properties and remarkable catalytic activities of metallocene-based pincer-metal complexes will inspire other researchers to continue exploring this realm.

Computational Study of Bridge Splitting, Aryl Halide Oxidative Addition to PtII , and Reductive Elimination from PtIV : Route to Pincer-PtII Reagents with Chemical and Biological Applications

Density functional theory computation indicates that bridge splitting of [Pt^II R₂ (μ-SEt₂ )]₂ proceeds by partial dissociation to form R₂ Pt^a (μ-SEt₂ )Pt^b R₂ (SEt₂ ), followed by coordination of N-donor bromoarenes (L-Br) at Pt^a leading to release of Pt^b R₂ (SEt₂ ), which reacts with a second molecule of L-Br, providing two molecules of PtR₂ (SEt₂ )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH₂ )₂ C₆ H₃ Br (pz=pyrazol-1-yl) and 2,6-(Me₂ NCH₂ )₂ C₆ H₃ Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via Pt^II Tol₂ (L-N,Br) and reductive elimination from Pt^IV intermediates gives mer-Pt^II (L-N,C,N)Br and Tol₂ . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH₂ )₂ C₆ H₃ Br, a stable Pt^IV product, fac-Pt^IV Me₂ {2,6-(pzCH₂ )₂ C₆ H₃ -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable Pt^IV Tol₂ {L-N,C,N}Br undergoing facile Tol₂ reductive elimination. The mechanisms reported herein enable the synthesis of Pt^II pincer reagents with applications in materials and bio-organometallic chemistry.