Non-iron [n]metalloarenophanes.
The past 15 years have witnessed the creation of a wealth of fascinating applications for strained [n]ferrocenophanes, in which the aromatic sandwich rings of ferrocene are covalently tethered (n denotes the number of bridging elements). By contrast, the related [n]metalloarenophanes, that is, ansa-complexes not derived from ferrocene, have been neglected for a long time. In this Account, we present the tremendous progress that has been achieved in this field, mostly over the last five years. We focus on systems that have been developed in our laboratories, namely, those based on [M(eta(6)-C(6)H(6))(2)] (where M is V, Cr, or Mo), [Mn(eta(5)-C(5)H(5))(eta(6)-C(6)H(6))], and [M(eta(5)-C(5)H(5))(eta(7)-C(7)H(7))] (where M is V or Cr). We begin by examining the synthetic precursors to [n]metalloarenophanes, the selectively 1,1'-dilithiated sandwich complexes. These species can be isolated and characterized both in the solid state and in solution, and an appreciation of their structural properties is essential for the controlled synthesis of strained [n]metalloarenophanes. About 25 different [n]metalloarenophanes (n = 1, 2) have been obtained from 1,1'-dilithiated sandwich complexes by their stoichiometric reaction with appropriate element dihalides, and most have been fully characterized. X-ray diffraction data confirm the presence of tilted structures, with the extent of the tilt depending on the number of bridging elements, their covalent radii, and the nature of the metal center. The tilt angle, alpha, between the planes of the two carbocyclic ligands represents a good measure of the amount of ring strain present in these species, ranging from 31.23 degrees for a highly strained [1]boravanadoarenophane to 2.60 degrees for the almost unstrained [2]silatrochrocenophane. The strained character of [n]metalloarenophanes is reflected in their rich and unusual reactivity. The thermodynamic driving force for most of the observed transformations is a significant reduction of molecular ring strain, as evidenced by smaller tilt angles in the products. For example, the [1]sila derivatives undergo facile oxidative addition of the strained Si-(arene)C bond to low-valent transition metal complexes. Catalytic reaction with Karstedt's catalyst results in the transition-metal-catalyzed ring-opening polymerization (ROP) of a [1]silatrochrocenophane, yielding a polymeric species (M(W) = 6.4 x 10(3) g mol(-1)). The transition-metal-catalyzed ROP of a paramagnetic [1]silavandoarenophane provides a rare example of a well-characterized macromolecule (M(W) > or = 2.8 x 10(4) g mol(-1)) containing spin-active metal centers in the main chain. The B-B bond of the [2]borametalloarenophanes was found to be particularly susceptible to further functionalizations. Facile oxidative addition was observed, and the resulting [3]diboraplatina derivatives could be successfully employed in the diboration of alkynes to form ansa-bis(boryl)alkenes. This transformation was also accomplished directly from the [2]borametalloarenophanes, that is, a diboration of alkynes under both homogeneous and heterogeneous catalysis conditions. Similarly, the stoichiometric diboration of the N=N double bond of azobenzene by a [3]diboraplatinachromoarenophane is possible. The Si-Si bond in [2]silachromoarenophanes can also be used in further derivatizations, although higher temperatures are required. The bis-silylation of propyne proceeded via palladium mediation under homogeneous conditions to yield ansa-bis(silyl)alkenes. Finally, we discuss the electronic properties of [n]metalloarenophanes developed in our laboratories, revealing correlations between selected NMR, EPR, and UV-visible parameters and molecular distortion.
Braunschweig
Holger
H
Institut für Anorganische Chemie, Julius-Maximilians Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany. h.braunschweig@mail.uni-wuerzburg.de
Kupfer
Thomas
T