Dibora[2]ferrocenophane: A Carbene‐Stabilized Diborene in a Strained cis‐Configuration

Abstract

Metallocenes with bridged cyclopentadienyl ligands, commonly named ansa metallocenes or metallocenophanes, have emerged as a class of organometallic compounds with an exceptionally wide and diverse range of applications.[1] Among other applications, ansa metallocenes are employed as catalyst precursors in the industrial production of polyolefins [2] and as monomers for ring-opening polymerization to form functional metallopolymers.[3] Their versatility and usefulness stems largely from the fact that their physical properties, and hence reactivity, can be tuned through structural modifications of the ligand framework. For instance, unsaturated two-atom bridges have been developed to increase the configurational rigidity [4] as well as the molecular strain of metallocenophanes, but also to add additional functionality to the metallocene fragment. In part due to difficulties encountered in their synthesis, these types of bridges are relatively rare and only a handful of these have been successfully incorporated into the ferrocene structure, as shown in Figure 1 (I-V). The first examples involved bridging aromatic rings, such as an ortho-phenylene (I), [5,6] a cyclobutadiene cobalt (II) [6] and a ruthenacyclopentadiene fragment (III). [7] Whereas the focus of the initial studies was on synthesis and structural features, the vinylene-bridged dicarba[2]ferrocenophanes IV and V have been developed as candidates for ring-opening metathesis polymerization (ROMP) to produce conjugated metal-containing polymers. [8][9][10] As shown by the groups of Tilley and Manners, such strained ferrocenophanes can indeed undergo ROMP with Schrock-and Grubbs-type catalysts to form conjugated metallopolymers. [9,10] Although homo-and heteronuclear multiple bonding is common for other p-block elements, especially the second-period elements, [11] unsaturated ansa bridges in metallocenophanes are to date restricted to carbon. By capitalizing on the isoelectronic relationship between Lewis base-stabilized diborenes, [(L)RB=BR(L)] (L = Lewis base), [12,13] and olefins, R2C=CR2, we sought to prepare the first ansa metallocene with a heteroatomcontaining multiple bond in the bridge. Herein, we describe the successful synthesis of a strained dibora[2]ferrocenophane (VI), in which the bridging diborene moiety is forced to adopt a cis rather than the prevailing trans configuration. The effects of changing the regiochemistry, as well as the interrelationship between the strain and properties of the diborene are addressed.[a]Prof.

Enantiopure Phospha[1]ferrocenophanes: Textbook Examples of Through‐Space Nuclear Spin–Spin Coupling

“…[2b, 8] The chemistry of strained sandwich compounds still yieldss urprising results as it can be exemplified with two recent highlights in this area, the FCP D and the nickelocenophane E (Figure 1). While the carbene-stabilized compound D exhibits an unusual bonding pattern, [9] species E is the first strained sandwichcompound revealing reversible ROP. [10] In addition to their application as monomers, phospha[1]ferrocenophanes are interestingp hosphines and can coordinate as monodentate ligandst ot ransition metals.…”

“…[4] However,t he exploration into FCPs already began a few years after the discoveryo ff errocene, [5] with the first published derivative being ac arbon-bridged [3]FCP. While the carbene-stabilized compound D exhibits an unusual bonding pattern, [9] species E is the first strained sandwichcompound revealing reversible ROP. [2b, 8] The chemistry of strained sandwich compounds still yieldss urprising results as it can be exemplified with two recent highlights in this area, the FCP D and the nickelocenophane E (Figure 1).…”

Near‐Infrared Quadrupolar Chromophores Combining Three‐Coordinate Boron‐Based Superdonor and Superacceptor Units

“…Cyclic voltammetry studies on THF solutions of compounds 5a and 5b (see Supporting Information) revealed oxidation waves;t he oxidation of 5a occurs at À1.19 V( all values vs.F c/Fc + ,F c = [(h-C 5 H 5 ) 2 Fe]), indicating that it is significantly more difficult to oxidise than previously studied NHC-stabilised diborenes (E 1/2 = À1.53 to À1.56 V), [16,17] whereas the oxidation wave of 5b is found at À0.80 V, av alue surpassed only by ad iborene stabilised by the relatively poor donor bis(diphenylphosphino)methane (E 1/2 = À0.51 V) TheHOMO energy is thus markedly affected by the thienyl substituent. Thep resence of the BMes 2 and B F Mes 2 groups also allows the species to be reduced; 5a displays ar eduction wave at À2.60 V, whereas for 5b this occurs at À1.73 V, showing the profound effect of fluorination of the aryl substituents.Indeed, the similarity of these values to the reduction potentials of related B F Mes 2 -a nd BMes 2 -substituted compounds,4 -tBuC 6 H 4 B F Mes 2 (E 1/2 = À1.63 V), 4-Ph 2 NC 6 H 4 B F Mes 2 (E 1/2 = À1.66 V) and 4-Ph 2 NC 6 H 4 BMes 2 (E 1/2 = À2.60 V), [10b] indicates that the effect of the donor on the reduction potential is relatively small.…”

Dreifach koordiniertes Bor als Superdonor und ‐akzeptor für quadrupolare Nahinfrarot‐Chromophore

“…Die Oxidation von 5a erfolgt bei E 1/2 = À1.19 V (alle Werte vs.Fc/Fc + ), was darauf hindeutet, dass es sich im Vergleich mit bereits untersuchten NHC-stabilisierten Diborenen (E 1/2 = À1.53 bis À1.56 V) [16,17] wesentlich schwieriger oxidieren lässt. Die Oxidation von 5a erfolgt bei E 1/2 = À1.19 V (alle Werte vs.Fc/Fc + ), was darauf hindeutet, dass es sich im Vergleich mit bereits untersuchten NHC-stabilisierten Diborenen (E 1/2 = À1.53 bis À1.56 V) [16,17] wesentlich schwieriger oxidieren lässt.…”