Strained Metallocenophanes


Bis(ansa) sandwich complexes
Ansa half-sandwich complexes
Organometallic polymers
Transition-metal catalyzed diboration

In contrast to the unstrained metallocenophanes of early transition metals (e.g. group 4), the introduction of a bridge into sandwich complexes of late transition metals leads to strained molecules. The extent of strain can be expressed by the tilt angle α, i.e. the angle between the aromatic ring planes (Fig. 1). The strain allows the molecules to react in unexpected ways (vide infra).



Fig. 1: Deformation parameters for metallocenophanes

The most commonly applied approach to this kind of molecule involves a salt elimination reaction between the dilithiated sandwich compound and an organoelement dihalide (Fig. 2).


Fig. 2: Formation of strained metalloarenophanes by salt elimination

By applying this strategy, several compounds of this kind have been synthesized in our group, among them the [1]boraferrocenophane 1, the ferrocenophane with the largest tilt angle observed so far (Fig. 3).[1, 2]



Fig. 3: Molecular structure of 1

Besides the ferrocene-based systems several compounds derived from other sandwich complexes like bis(benzene)vanadium [V(η6-C6H6)2], “benzmancene” [Mn(η5-C5H5)(η6-C6H5)][3], "trochrocene” [Cr(η5-C5H5)(η7-C7H7)][4, 5, 6] or "troticene" [Ti(η5-C5H5)(η7-C7H7)][7] could be obtained (Fig. 4).




Fig. 4: Molecular structures of selected strained metalloarenophanes

[2]Metallocenophanes are useful reagents for the metal mediated functionalization of unsaturated organic substrates, which is initiated by oxidative addition of the ansa bridges to group 10 metals (see above). Further reactivity studies concerning the activation of the bridging elements of [2]metallocenophanes were performed in our laboratories. For instance, the activation of a Si–Si bridge is shown by the reaction of the nickel derivative [{(Me2Si)25-C5H4)2}Ni] with the Ni(0) species [(tBuNC)4Ni], yielding in the dinuclear complex [(Me2Si)(η5-C5H4)Ni(CNtBu)]2 with cleavage of the Si–Si bond (Fig. 5).[8]



Fig. 5: Reaction of [(Me2Si)25-C5H4)2Ni] with [(tBuNC)4Ni].



Bis(ansa) sandwich complexes

The first intramolecular activation of a E–E bridge of a [2]metallocenophane was accomplished by photolysis of the ansa complex [(Me2Si)25-C5H4)2MoH2]. The molybdenum species reacts by reductive elimination of hydrogen and oxidative addition of the Si–Si bond to the metal atom, thus forming an unprecedented bis(ansa) complex 2 (Fig. 6). The unusual structural motif of the isolated complex depicts two silicon nuclei, each bridging the metal atom and a cyclopentadienyl ring.



Fig. 6: Synthesis of [1],[1]disilamolybdenocenophane 2.

DFT studies confirm the η5Si-coordination mode of the formal dianionic silylcyclopentadienyl ligands exhibiting Wiberg bond indices of the Mo–Si bonds of 0.61 (Fig. 7). Further calculations as well as the structural parameters obtained from X ray diffraction studies indicate molecular strain of the bis(ansa) complex and a high reactivity of this derivative is presumed.[9]



Fig. 7: Left: QTAIM-based molecular graph showing the Mo–Si–Cipso unit and the slightly bent Mo–Si bond (red dots: bond critical points). Right: Contour plot of the Laplacian in a Mo–Si–Cipso plane of 2

Based on the facile metal mediated functionalization of alkynes with [2]metallocenophanes the reactivity of the bis(ansa) complex towards unsaturated organic substrates was investigated. The molybdenocenophane reacts with the nonpolar compounds 2-butyne and trans-azobenzene by reformation of the Si–Si bridge and side-on coordination of the substrates to the metal center (Fig. 8).[10]



Fig. 8: Reactivity of 2 towards 2-butyne.

Within the scope of this work, an ansa carbene complex, which exhibit an unusual structural motif, was isolated by treating the molybdenum complex with tert-butyl isonitrile (Fig. 9).



Fig. 9: Molecular structure of an ansa carbene complex.

Further reactivity studies were performed using platinum(0) complexes. Treatment of the molybdenum compound with two equivalents of [Pt(PEt3)3] yields in the product of twofold oxidative addition of the Si–C bonds to platinum fragments (Fig. 10), whereas reactions of the bis(ansa) species with [Pt(PCy3)2] in different stoichiometries lead to products of more complex structures.[11]



Fig. 10: Molecular structure of the trinuclear molybdenum platinum complex.



Ansa half-sandwich complexes

Ansa half-sandwich complexes have proven to be an interesting subclass of metallocenes. Their distinct structural feature is a σ-bonded bridge, which links the η5-coordinated cyclopentadienyl ring to the metal center. This imposes different reactivity patterns and therefore new properties and additional synthetic possibilities compared to metallocene counterparts.


Our synthetic route to element-bridged ansa half-sandwich complexes comprehends the use of the dilithiated complexes Li[(η5-C5H4Li)M(CO)3] (3a,b: M = Mo, W) as highly reactive precursors.[12] These were treated with distanna dichlorides and trisila dichlorides, respectively, to obtain the desired compounds 4a,b and 5a,b in good yields.[13, 14]



Fig. 11: Synthesis of ansa half-sandwich complexes.

The biatomar bridge of the distanna compounds 5a,b causes a high ring stain within the system, which is the driving force for the insertion of elemental chalcogens and [Pd(CNtBu)2] into the tin–tin bond.[15]



Fig. 12: Insertion into the tin–tin bond.

A different approach to the 1,3-Distanna-2-thia tungsten complex is the reaction of the bis(stannyl) compound 6 with sodium sulfide by double salt elimination.[16]



Fig. 13: Alternative synthesis for ansa half-sandwich complexes

In contrast to that, the diborane [(Me2N)BBr]2 does not react in an analogous manner but forms the oxycarbyne complex [M{η1,µ-CO-BNMe2-BNMe2-(η5-C5H4)}(CO)2] (7a,b: M = Mo, W), which can be stabilized by reaction with [(Ph3P)2Pt(C2H4)].[17]



Fig. 14: Different reactivity upon treatment with [(Me2N)BBr]2.



Organometallic polymers

The ring strain can be used to create metal-containing polymers by ring-opening polymerisation (ROP). In collaboration with I. Manners and his group at Bristol, we undertook a differential scanning calorimetry (DSC) study to explore the thermal behaviour of the monomer 1 (Fig. 15).[1, 2] At approximately 115 °C the DSC thermogram showed a melt endothermic, followed by thermally-induced ROP at about 190 °C (onset). The enthalpy for the ROP of 1 was determined to be 95 kJ mol-1 and is greater than the values for silicon-bridged [1]ferrocenophanes (70-80 kJ mol-1).


Fig. 15: DSC-spectrum of 1

In contrast to [n]ferrocenophanes, polymerization studies of other bridged sandwich compounds are rare. We achieved a transition metal catalyzed ROP by treatment of [Cr(η5-C5H4)(η7-C7H6)SiMe2] with Karstedt’s catalyst (0.5% wt) (Fig. 16).



Fig. 16: Transition-metal catalyzed ROP of a [1]trochrocenophane

The obtained polymer was characterized by gel permeation chromatography (GPC) and revealed a moderate molecular weight (Mw = 6.4x103, Mn = 4.0x103, PDI = 1.6).[6]

Furthermore we investigated the polymerization behaviour of the highly strained compound [V(η6-C6H5)2SiMe2]. The transition metal catalyzed ROP afforded a well-defined paramagnetic polymer (Fig. 17). Characterisation by small angle X-ray scattering (SAXS) revealed a molecular weight (Mw) of at least 28000 g mol-1.



Fig. 17: Transition-metal catalyzed ROP of a [1]vanadoarenophane



Transition-metal catalyzed diboration

Similar to the well-established hydroboration reaction, the transition metal mediated diboration has developed into a highly useful method for the functionalization of unsaturated organic substrates. The borylated species thus obtained have become of major importance for various applications e.g. in organic synthesis or materials science. Due to their pronounced reactivity towards low-valent Pt and Pd species, [2]borametalloarenophanes were studied as facile diborane(4) precursors for the 1,2-diboration of alkynes. Interestingly, the addition of the B–B-bond towards the substrate was not only observed under stoichiometric and homogeneously catalyzed conditions, but also in the presence of Pt-metal or Pd/C, i.e for the first time under heterogeneous conditions (Fig. 18).[18, 19]



Fig. 18: Stoichiometric and catalytic diboration of alkynes.

By expanding this strategy toward dialkynes, three different structural motifs can be observed, including single (Fig. 19, top) and double diboration (Fig. 19, bottom). The latter products are obtained as diastereomeric pairs and, interestingly, the single isomers can be interconverted by heating in solution.[20, 21]



Fig. 19: Stoichiometric and catalytic diboration of alkynes

Beyond the carbon-based systems described above, other multiple bond systems were used in diboration reactions. Thus we were able to demonstrate the first diboration of a N=N bond. Starting from azobenzene, the [4]diboradiazachromarenophane 8 (Fig. 19) was obtained.[22]



Fig. 20: Molecular structure of the [4]diboradiazachromarenophane 8.

Unlike alkynes and azobenzene, isocyanides are diborated in a 1,1-fashion. Surprisingly,the reactions of [2]borametalloarenophanes with isocyanides occur instantaneous already at room temperature and without the necessity of catalysis.[23]



Fig. 21: Diboration of isocyanides.

Selected references:


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