Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of Pd–Pd Distances by an Interplay of Metallophilic, Dispersive, and Coulombic Interactions

Abstract

Monomeric ferrocene bis-imidazoline bis-palladacycles (FBIP) have recently been reported to be efficient bimetallic catalysts in different sorts of asymmetric reactions by the cooperation of two Pd(II) centers. A crucial parameter regarding the efficiency of reactions catalyzed in a bimetallic mode isin generalthe intermetallic distance of both catalytically relevant metal centers. In this article we describe the structural elucidation of the monomeric FBIP catalyst type (usually generated in situ from a catalytically inactive dimeric chloride bridged precatalyst) by X-ray crystal structure analysis. Two dicationic monomeric complexes are compared to a neutral complex. The solid-state structures reveal varying Pd–Pd distances ranging from 3.15 to 5.27 Å for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 Å has been found. This variability is supposed to be one of the key advantages of a ferrocene backbone in a bimetallic catalyst system, since the Fe–Cp bonds allow the bimetallic complex to readily open and close like a pair of scissors, employing just a few degrees of rotational freedom. In addition, on the basis of the nature of the reported catalyst species, we suggest that a permanent switch among neutral and mono- and dicationic catalyst species by a Brønsted acid such as acetic acid might facilitate different elementary steps in a catalytic cycle. By DFT calculations we have found that weak d8–d8 interactions contribute to short Pd–Pd distances but are less important than dispersive interactions, which can even overcome the Coulombic repulsion of two cationic palladium centers.

Recent Advances in the Synthesis of Imidazolines (2009–2020)

“…[27] These imidazoline derivatives had been commenced as ligands in transition metal-catalyzed processes. [28] The reaction was initiated by the formation of an intermediate amide (10) from the ferrocene containing carboxylic acid (8) and the diamine (9). Next, the reaction utilized a bis(triphenyl) oxodiphosphonium trifluoromethanesulfonate mediated cyclodehydration of the amide to generate imidazolines in excellent yields.…”

“…reported the synthesis of ferrocenyl imidazolines ( 11 ) from ferrocenyl carboxylic acids ( 8 ) and enantiopure diamines ( 9 ) in high yields (Scheme ) . These imidazoline derivatives had been commenced as ligands in transition metal‐catalyzed processes . The reaction was initiated by the formation of an intermediate amide ( 10 ) from the ferrocene containing carboxylic acid ( 8 ) and the diamine ( 9 ).…”

Cooperative Effects in Multimetallic Complexes Applied in Catalysis

“…The bimetallic cooperative mechanism operating in this reaction has also been confirmed from the absolute configuration of the reaction product. [100,101] Similar heterotrimetallic complexes Ru(II)/Pd 2 (II) and Fe(II)/ Pd(II)/Pt(II) have also been successfully synthesized, and used for the different cooperative organic transformations. [102] Peters and co-workers have also synthesized heterobimetallic Ni(II)/ Pd(II) complexes using salen bis-NHC hybrid type ligands using a stepwise synthetic protocol.…”

Regioselective Asymmetric Allylic Alkylation Reaction of α‐Cyanoacet­ates Catalyzed by a Heterobimetallic Platina‐/Palladacycle

“…Because the heterobimetallic FBIPP‐catalyst is clearly superior to the mono‐palladacycle and ‐platinacycle catalysts (Table 6), and because the control experiment shown in Scheme indicates that the cyanoacetate is also activated by the catalyst, an intramolecular bimetallic mechanism appears reasonable 24,25. In addition, it seems likely that the corresponding enolate of the cyanoacetate undergoes C–C bond formation, because no reaction was observed in the absence of base.…”