Fr Francium

The chemistry of ansa-complexes is of increasing importance in organometallic chemistry and materials science, a development that is closely related to the discovery of the first strained [1]silaferrocenophane in 1975 and their high value as efficient monomers in the ring-opening polymerization to prepare high-molecular-weight metallopolymers. By contrast, interest in boron-bridged derivatives has sparked only recently, which is rather surprising given the anticipated high degree of molecular ring strain imposed by the small covalent radius of the boron nucleus. Thus, it was not until 1997 that boron was successfully incorporated into an ansa-bridge. Since then, the chemistry of strained [n]borametalloarenophanes has been studied systematically with respectto synthesis, electronic/structural properties and reactivity patterns. As a consequence, a rather large variety of [n]borametalloarenophanes based on different sandwich complex systems such as homoleptic [Fe(η⁵-C₅H₅)₂] and [M(η⁶-C₆H₆)₂] (M = V, Cr), as well as heteroleptic [Mn(η⁵-C₅H₅)(η⁶-C₆H₆)] and [M(η⁵-C₅H₅)(η⁷-C₇H₇)] (M = Ti, V, Cr) are now available. The structural and electronic consequences of the small mono- or diatomic boron bridges have been evaluated by different spectroscopic methods and X-ray diffraction studies. Most importantly, these systems were shown to possess an enhanced reactivity. Uncommon and exciting reaction pathways could be encountered, which are evidently driven by the release of molecular ring strain. This Microreview is intended to highlight the consequences of molecular ring strain on the properties of [n]borametalloarenophanes. We begin with a comprehensive overview on the experimental approaches applied to their synthesis, and consider their unique properties in great detail. Subsequently, we will recognize that the highly strained character of the [n]bora-metalloarenophanes enables unprecedented reaction pathways.

In recent years, the chemistry of strained [n]metalloarenophanes successfully crossed the borderline between fundamental research to an application-inspired approach. In this regard, boron-bridged ansa-systems have proven highly valuable in unveiling unprecedented reaction pathways and infunctioning as suitable reagents in further derivatization reactions of simple (in)organic substrates. This Microreview provides a comprehensive overview on the chemistry of these highly strained organometallic species.

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Wiley-VCH, Weinheim 2011, 296 pp., hardcover € 129.00.—ISBN 978-3-527-32860-4

Treatment of propylperrhenate with 1 equiv. phenylzinc acetate yields dioxidodiphenylrhenium propionate. This novel complex can be synthesized with a yield of 41 % and is characterized by NMR and IR spectroscopy, as well as single-crystal X-ray diffraction. The compound is sensitive to moisture. It decomposes in sunlight and above 0 °C. The crystal structure has distorted octahedral symmetry around the rhenium center. Both rhenium–carbon bonds have a length of 2.141 Å and are significantly longer than other comparable rhenium–carbon bonds.

The reaction of (μ-propionato)trioxidorhenium with phenylzinc acetate does not lead to the expected compound phenyltrioxidorhenium, but rather to the new complex dioxidodiphenylrhenium(VII) propionate. This compound is highly sensitive to air, moisture, and light, and it is stable only below 0 °C. It has been fully characterized by means of single-crystal X-ray diffraction and NMR spectroscopy.

The synthesis of two novel organometallic π complexes of a BF₂-capped tetraoxobenzene (TOB) of the general formulae [Cp*M-η⁶-{BF₂-TOB-BF₂}] (M = Ir, 2a; M = Rh, 2b) is reported together with their molecular structures ascertained by single-crystal X-ray diffraction studies. Complexes 2a and 2b are isostructural and show the formation of 3D porous supramolecular networks. Interestingly the supramolecular networks arise from the zwitterionic nature of 2a and 2b. Indeed, the individual molecules self-assemble through the teamwork of dipolar H···F hydrogen and C···F halogen bonds, which leads to the formation of 3D networks with empty channels. These complexes hold promise for the design of a new class of porous materials with useful properties.

A unique type of Cp*M (π-arene) {M = Rh (2a); M = Ir (2b)} complexes of a bis-BF₂-capped tetraoxobenzene is described. Such organometallic backbones possess a peculiar zwitterionic nature. As a consequence, they self-assemble to give a rare example of porous 3D networks.

This paper focuses on complexes of the N-heterocyclic carbene (NHC) 1-Np, which contains a 1,1′-ferrocenediyl backbone and neopentyl substituents at the N atoms flanking the divalent C atom. 1-Np is a ring-expanded NHC with a ring size of six atoms. The square-planar group 10 metal complexes [{PdCl(μ-Cl)(1-Np)}₂], cis-[PdCl₂(1-Np)(PPh₃)], trans-[PdCl₂(1-Np)₂] and trans-[NiCl₂(1-Np)(PPh₃)] have been prepared by the reaction of 1-Np with trans-[PdCl₂(PhCN)₂], [PdCl₂(PPh₃)₂], cis-[PdCl₂(COD)] (COD = cycloocta-1,5-diene) and [NiCl₂(PPh₃)₂], respectively. In addition, the carbene–borane adduct 1-Np–BF₃ has been obtained by serendipity. All new compounds have been structurally characterised by single-crystal X-ray diffraction, which demonstrates the pronounced trans influence of 1-Np. 1-Np has been compared with the ring-expanded NHC 1,3-diisopropyltetrahydropyrimid-2-ylidene (THP-iPr) in the context of homogeneous catalysis, which utilises analogous palladium NHC complexes known to be particularly effective in the case of NHC = THP-iPr. The catalytic performance of cis-[PdCl₂(1-Np)(PPh₃)] in Suzuki–Miyaura cross-coupling reactions of PhB(OH)₂ with aryl bromides Br–p-C₆H₄–R is similar to that reported for cis-[PdCl₂(THP-iPr)(PPh₃)]. With aryl chlorides, however, the latter complex is superior to its 1-Np analogue. The catalytic performance reported for trans-[PdCl₂(THP-iPr)₂] in Mizoroki–Heck reactions of n-butyl acrylate with aryl bromides Br–p-C₆H₄–R is equal to (R = H) or moderately better than that of trans-[PdCl₂(1-Np)₂] [R = C(O)Me, OMe].

Despite its unusual reactivity in small-molecule activation, the coordination chemistry of the ferrocene-based N-heterocyclic carbene 1-Np is well-behaved, which allows easy access to catalytically competent Pd^II complexes whose performance in cross-coupling reactions rivals that of its analogues, which contain an established tetrahydropyrimidine-based ring-expanded NHC.