During the last decades, sufficiently activated group 4 metallocenes and related complexes have emerged as highly active and versatile catalysts for the homogeneous Ziegler-Natta-type polymerisation of ethylene and higher α-olefins. One of the most intriguing advancements in this field was the introduction of ansa-metallocenes or metallocenophanes which contain a bridging moiety between the two η⁵-coordinated fragments. These have allowed for the synthesis of a vast range of novel stereo regular polyolefins. Another great success was the development of related Group 4 constrained geometry complexes (CGCs) where one of the η⁵-coordinated ring systems of a metallocenophane is formally replaced by an η¹-coordinated amido moiety. Due to their more accessible active site, these compounds display superior characteristics in the copolymerisation of ethylene and higher α-olefins, resulting in another class of polyolefins with exceptional features.
Both metallocenophanes and CGCs need to be activated for the polymerisation of olefins, with methylaluminoxane (MAO) and boron-based perfluorinated Lewis acids being the most common cocatalysts.

Why a boron bridge?
Having these developments in mind, our group is interested in a replacement of the most commonly applied silicon bridge in metallocenophanes and CGCs by one containing boron. This bridging element may have potential Lewis acidic character, thus favourably altering the catalytic activity of the activated species and possibly allowing for self-activation. Furthermore, the comparably short and therefore rigid boron bridge should enhance selectivity of transformations catalysed by these complexes, e.g. the stereoregular polymerisation of propylene. In [2]borametallocenophanes, the reactivity of the diborane(4)diyl bridge may potentially be used for supporting the complexes or preparation of novel polymers via ring-opening oolymerisation (ROP).

Preparation
A series of [n]borametallocenophanes (n=1, 2) of Ti, Zr and Hf with various ligand systems and related CGCs could be prepared in our group. Starting from the dihaloboranes the ligands have been synthesised by reaction with Na[C₅H₅] or Li[C₁₃H₉].

[1]Borametallocenophanes
The [1]borametallocenophanes could be prepared via salt elimination from the dilithiated ligands with group 4 metal halides, i.e. [TiCl₃(thf)₃], [ZrCl₄(thf)₂] and HfCl₄ at low temperatures.^{[1, 2, 3]}

[2]Borametallocenophanes
The [2]borametallocenophanes can be prepared in the same manner like the [1]borametallocenophanes. The ligand can be synthesized from the dihaloborane with a excess of NaCp oder LiFlu, in arbitrary order. After deprotonation of the ligand with MeLi, the metallocenophane can be prepared via salt elimination.^[4]

Constrained geometry complexes
The CGCs, where one of the η⁵-coordinated ring systems is formally replaced by a η¹-coordinated amido moiety, were synthesised via amine elimination with [M(NMe₂)₄].^{[5, 6, 7]}

Polymerisation of olefins

Our interest was driven by the expectation that introducing boron into the ligand sphere of those complexes influences and most likely enhances their reactivity. Our investigations indeed proved [n]borametallocenophanes (n = 1,2) to be highly active catalysts for the Ziegler-Natta-type polymerisation of ethene or propene.

Making the polymers
The catalysts where tested in a lab-style glas polymerisation reactor from Büchi. The standard conditions are: 60 °C, 2 bar ethene, 200 mL toluene, 10 µmol pre-catalyst, MAO:Zr ratio 4500:1, Reaction time = 5 - 60 min

Activities
The activitiy of the catalyst is defined in kg Polymer per mol catalyst, per hour, per bar ethylene. In comparison to the commercial catalyst (red), the [n]borametallocenophanes [n=1 (yellow), 2 (blue)] show comparable activities.

Outlook
Our current interests in this area focuses now on the design of boron-bridged ansa-metallocenes for the stereo selective formation of polypropylene and other “classical” polymers which make the main part of industrially relevant plastics.