Using the flytrap approach, paramagnetic ansa-metallocenes of the late transition metals cobalt and nickel containing a tetra-tertbutyldistannane bridge have been prepared. The complexes were identified using a combination of analytical methods (NMR, EPR, cyclic voltammetry, and X-ray crystallography) and further converted to their corresponding cations by one-electron oxidation with ferrocenium hexafluorophosphate. Spectral and structural analyses of the ionic products are consistent with metal-based oxidations. ■ INTRODUCTIONMetallocenophanes, or ansa-metallocenes, are a popular class of organometallic compounds, which have found use in catalytic processes and materials science. While the success of earlytransition-metal metallocenes as Ziegler−Natta catalysts is documented in the industrial production of polyolefins, 1 strained derivatives of the late transition metals are gaining increasing importance, due to their ability to undergo ringopening polymerization (ROP) and to produce metallopolymers with functional properties. 2 As the central metal atom, the nature and size of the ansa bridge, and the η 5 -coordinating ligands are all factors influencing their properties and chemical reactivity, many variations within this group of compounds have been reported. 3 In comparison to the many different bridging units known for [n]ferrocenophanes (n = number of bridging atoms), the structural variety of ansa-metallocenes involving late transition metals of the higher groups (i.e., above group 8) is rather limited (see Chart 1 for an overview). 3 This phenomenon can be explained in part by the failure to 1,1′-dimetalate these metallocenes, which constitutes a key step in the usual synthesis of metallocenophanes. Hence, several other strategies have been explored. The first examples of ansa-cobaltocenes (I) were based on the reductive coupling of fulvene ligand precursors using metal atoms. 4 While it is versatile and offers access to chiral ansa-metallocenes (II), 5 the method is restricted to the use of rather large substituents in the 6-position of the fulvene, which results in sterically demanding bridging units. Intramolecular ring-closing olefin metathesis was used to construct the first structurally characterized ansa-nickelocenes (IV), 6 in which an unsaturated carbon bridge connects the two cyclopentadienyl fragments. 7 Interestingly, the bridge is readily converted into its saturated analogue by hydrogenation, without affecting the π system of the Cp ligands. 8 In another approach, the so-called flytrap method, 3 which involves the reaction of dianionic bis(cyclopentadienyl) ligand sets with appropriate metal halides, the Manners group synthesized two hydrocarbon-bridged [n]cobaltocenophanes (III; n = 2, 3) with varying lengths of the bridge. 9 It is important to note that the more strained system was shown to produce a high-molecularweight polymetallocenium by thermal ring opening and subsequent oxidation. 10 In a similar fashion, our group introduced the tetramethyldisilane bridge into both cobalt 11
The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.
We report the molecular structures of 1,1'-dilithiometallocenes of ruthenium and osmium. These compounds served as precursors for the synthesis and subsequent structural characterization of the first [2]osmocenophanes with disilane and distannane bridges, as well as of a distanna[2]ruthenocenophane. In addition, the insertion of sulfur and selenium into the Sn-Sn bridges was studied and it was observed that the presence of the Lewis base pmdta (N,N,N',N″,N″-pentamethyldiethylenetriamine) dramatically accelerates the reaction.
The synthesis and characterization of new [3]silametallocenophanes of the group 8 metals via salt elimination is presented. Thereby, new [3]silaferrocenophanes as well as the first [3]silametallocenophanes of the heavier metals ruthenium and osmium could be synthesized and characterized. Also, the first solid-state structure of a [3]silaferrocenophane was determined by X-ray crystallographic analysis.
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