Tin-Bridgedansa-Metallocenes of the Late Transition Metals Cobalt and Nickel: Preparation, Molecular and Electronic Structures, and Redox Chemistry

Abstract

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

Redox Chemistry of Nickelocene-Based Monomers and Polymers

“…In a comparison of the hydrocarbonbridged species, neutral 3 is significantly more tilted than 12 (α = 16.6°and 1.0°, respectively), and for the silicon-containing bridges, 10 is more tilted than 11 (α = 9.4°versus 3.5°). This increased tilt/stabilization relationship has also been observed in work by Braunschweig et al, 21 and although the effect of tilting on the frontier orbitals in ferrocene is well-understood, 32 further theoretical investigation is required to justify this observation in ansa-nickelocenes. To provide further insight into the effect of Cp ring tilt on the ease of oxidation in these monomers, computational studies (see the Supporting Information for details) were performed to compare the first oxidation potential of hydrocarbon-bridged species, where the electronic influence of the bridge is expected to be similar across the three species probed.…”

“…A very similar effect is observed in distanna [2]nickelocenophane 9 and its oxidation product [9] + , where τ increases from 12.92(4)°to 24.56(4)°and staggering of the Cp rings (ϕ) also increases significantly from 11.00(12)°to 22.3(3)°. 21 Compound [11][B(C 6 F 5 ) 4 ] crystallizes in the monoclinic space group P2 1 /c and contains three formula units per asymmetric unit. One of these is disordered over the metallocene unit, the ansa bridge, and one of the C 6 F 5 rings of the tetrakis(pentafluorophenyl)borate anion.…”

“…). 21 The 20 VE nickelocene unit in neutral 9 features two electrons occupying formally antibonding e″ 1 orbitals (Figure 2). Oxidation affords the 19 VE nickelocenium species, where the Ni−Cp cent distance is decreased (9: α = 4.60 (8)°, Ni−Cp cent = 1.8141(8) Å, [9] + : α = 3.67 (13)°, Ni−Cp cent = 1.7186(14) Å).…”

“…This structural distortion can be related to the removal of a valence electron from the e′ 2 bonding orbital in the ferrocene unit (Figure ). In contrast, upon oxidation of the 19 VE cobaltocene unit and removal of an electron from the e″ 1 antibonding orbital (Figure ), the opposite effect is observed. ,,, [ n ]­Cobaltocenophanium ions (18 VE diamagnetic species) feature shorter Co–Cp cent distances and smaller tilt angles than their neutral precursors (e.g., dicarba[2]­cobaltocenophanium [6] + : α = 21.4(2)° as compared to the neutral dicarba[2]­cobaltocenophane 6 : α = 27.1(4)°) …”

“…A similar behavior was noted for the unsaturated tetracarba-bridged 8 (Figure ). The only isolated [ n ]­nickelocenophanium ion is the distanna[2]­nickelocenophanium hexafluorophosphate salt ( [9] [PF 6 ]) . The 20 VE nickelocene unit in neutral 9 features two electrons occupying formally antibonding e″ 1 orbitals (Figure ).…”