Ruthenium complexes with ferrocene-based P,C,P pincer ligand

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.

Steric and Electronic Effect of Cp-Substituents on the Structure of the Ruthenocene Based Pincer Palladium Borohydrides

Ruthenocene-based PCP^tBu pincer ligands were used to synthesize novel pincer palladium chloride Rc^F[PCP^tBu]PdCl (2a) and two novel palladium tetrahydroborates Rc^F[PCP^tBu]Pd(BH₄) (3a) and Rc^*[PCP^tBu]Pd(BH₄) (3b), where Rc^F[PCP^tBu] = κ³-{2,5-(tBu₂PCH₂)_2-C₅H₂}Ru(Cp^F) (Cp^F = C₅Me₄CF₃), and Rc*[PCP^tBu] = κ³-{2,5-(tBu₂PCH₂)₂C₅H₂}Ru(Cp*) (Cp* = C₅Me₅). These coordination compounds were characterized by X-ray, NMR and FTIR techniques. Analysis of the X-ray data shows that an increase of the steric bulk of non-metalated cyclopentadienyl ring in 3a and 3b relative to non-substituted Rc[PCP^tBu]Pd(BH₄) analogue (3c; where Rc[PCP^tBu] = κ³-{2,5-(tBu₂PCH₂)₂C₅H₂}Ru(Cp), Cp = C₅H₅) pushes palladium atom from the middle plane of the metalated Cp ring in the direction opposite to the ruthenium atom. This displacement increases in the order 3c < 3b < 3a following the order of the Cp-ring steric volume increase. The analysis of both X-ray and IR data suggests that BH₄ ligand in both palladium tetrahydroborates 3a and 3b has the mixed coordination mode η^1,2. The strength of the BH₄ bond with palladium atom increases in the order Rc[PCP^tBu]Pd(BH₄) < Rc*[PCP^tBu]Pd(BH₄) < Rc^F[PCP^tBu]Pd(BH₄) that appears to be affected by both steric and electronic properties of the ruthenocene moiety.