Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of Pd鈥揚d Distances by an Interplay of Metallophilic, Dispersive, and Coulombic Interactions
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- 作者：Manuel Weber ; Johannes E. M. N. Klein ; Burkhard Miehlich ; Wolfgang Frey ; Ren茅 Peters
- 刊名：Organometallics
- 出版年：2013
- 出版时间：October 28, 2013
- 年：2013
- 卷：32
- 期：20
- 页码：5810-5817
- 全文大小：434K
- 年卷期：v.32,no.20(October 28, 2013)
- ISSN：1520-6041
文摘
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鈥攊n general鈥攖he 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鈥揚d distances ranging from 3.15 to 5.27 脜 for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 脜 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鈥揅p 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 Br酶nsted acid such as acetic acid might facilitate different elementary steps in a catalytic cycle. By DFT calculations we have found that weak d8鈥揹8 interactions contribute to short Pd鈥揚d distances but are less important than dispersive interactions, which can even overcome the Coulombic repulsion of two cationic palladium centers.