Synthesis and Resolution of Planar-Chiral Ruthenium-Palladium Complexes with ECE′ Pincer Ligands

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.

Synthesis and application in asymmetric catalysis of P-stereogenic pincer–metal complexes

P-stereogenic pincer: synthesis and application in asymmetric catalysis.

Synthesis, structural properties and reactivity of ruthenocene-based pincer Pd(ii) tetrahydroborate

Non-covalent interactions determine the structure, crystal packing and reactivity of isolated ruthenocene-based pincer Pd(ii) complexes. Bifurcate dihydrogen-bonded complexes are active intermediates of tetrahydroborate alcoholysis.

Non-symmetric pincer ligands: complexes and applications in catalysis

Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

Heterodinuclear complexes of 4,5-diazafluorene derivatives displaying η(5),κ(2)-[N,N] and η(5),κ(1)-N coordination modes

The syntheses and structures for a series of heterodinuclear complexes of 4,5-diazafluorenyl (L(-)) and 3,6-dimesityl-4,5-diazafluorenyl (LMes(-)) ligands are reported herein. In all these heterodinuclear complexes, the Ru(II) centre is sandwiched between a pentamethylcyclopentadienyl (Cp*) ligand and the C5 ring of L(-) or LMes(-) in a double η(5) fashion, while the other metal (Fe(II), Co(II), Pt(II), or Cu(I)) is bound to the N-donors.

Complexation of DNA with ruthenium organometallic compounds: the high complexation ratio limit

Organometallic compounds possess two modes of interaction with DNA: intercalation at low complexation ratios and electrostatic adsorption at high ratios.

Symmetrical and unsymmetrical pincer complexes with group 10 metals: synthesis via aryl C-H activation and some catalytic applications

Aryl-based pincer metal complexes with anionic terdentate ligands have been widely applied in organic synthesis, organometallic catalysis and other related areas. Synthetically, the most simple and convenient method for the construction of these complexes is the direct metal-induced C(aryl)-H bond activation, which can be fulfilled by choosing the appropriate functional donor groups in the two side arms of the aryl-based pincer preligands. In this perspective, we wish to summarize some results achieved by our group in this context. Successful examples include symmetrical chiral bis(imidazoline) NCN pincer complexes with Ni(II), Pd(II) and Pt(II), bis(phosphinite) and bis(phosphoramidite) PCP pincer Pd(II) complexes, unsymmetrical (pyrazolyl)phosphinite, (amino)phosphinite and (imino)phosphinite PCN pincer Pd(II) complexes, chiral (imidazolinyl)phosphinite and (imidazolinyl)phosphoramidite PCN pincer complexes with Ni(II) and Pd(II) as well as unsymmetrical (oxazolinyl)amine and (oxazolinyl)pyrazole NCN' pincer Pd(II) complexes. Among them, the P-donor containing complexes are efficiently synthesized by the "one-pot phosphorylation/metalation" method. The obtained symmetrical and unsymmetrical pincer complexes have been used as catalysts in Suzuki-Miyaura reaction (Pd), asymmetric Friedel-Crafts alkylation of indole with trans-β-nitrostyrene (Pt) as well as in asymmetric allylation of aldehyde and sulfonimine (Pd). In the Suzuki couplings conducted at 40-50 °C, some unsymmetrical Pd complexes exhibit much higher activity than the related symmetrical ones which can be attributed to their faster release of active Pd(0) species resulting from the hemilabile coordination of the ligands. Literature results on the synthesis of some related pincer complexes as well as their activities in the above catalytic reactions are also presented.