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 d<sup>8</sup>–d<sup>8</sup> 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

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