Synthesis and crystallographic characterization of [2,2-bis(η5-pentamethylcyclopentadienyl)-3,4-bis(trimethylsilyl)-2-zirconafuran-5-one-κO 5]triisobutylaluminium

The crystal structure is reported of a zwitterionic zirconocene complex containing a furanone unit, namely [2,2-bis(η5-pentamethylcyclopentadienyl)-3,4-bis(trimethylsilyl)-2-zirconafuran-5-one-κO 5]triisobutylaluminium, in which the exocyclic carbonyl oxygen is coordinated to the aluminium atom of an Al(i-Bu)3 group.


Chemical context
Metallocene complexes of early transition metals can be activated by strong Lewis acids for many catalytic purposes. Reactions of group 4 metallocene complexes with Lewis acids such as HAl(i-Bu) 2 , Al(i-Bu) 3 and also B(C 6 F 5 ) 3 are therefore of great interest and have been studied intensively (Brintzinger et al., 1995). It has been reported previously that titanaand zirconacycles react readily with Al(i-Bu) 3 /HAl(i-Bu) 2 to give either heterobimetallic complexes with interesting structural features (Erker et al., 1992;Arndt et al., 2001) or zwitterionic binuclear compounds (Erker et al., 1992;Burlakov et al., 2004Burlakov et al., , 2006Burlakov et al., , 2011. The latter demonstrated remarkable catalytic activity in the ROP of "-caprolactone (Arndt et al., 1996;Arndt et al., 1997). The structure of a zwitterionic zirconocene ester enolate complex and a tantalactone, each coordinated to Al(C 6 F 5 ) 3 units, were reported recently (Tsurugi et al., 2006). The role of the zirconocene complex as an intermediate in the isospecific polymerization of methacrylates has been discussed (Zr: Bolig & Chen, 2004;Ta: Tsurugi et al., 2006). Recently, we found that the reaction of a zirconadihydrofuran with HAl(i-Bu) 2 gave a 1:1 complex where, in addition to the coordination of the aluminium atom to the oxygen of the intact furan ring, a Zr-H-Al bridge was obtained. This compound also behaves as an active catalyst in the ROP of "-caprolactone (Burlakov et al., 2017). In addition, a zwitterionic hafnocene furanone-B(C 6 F 5 ) 3 adduct has been synthesized and structurally characterized (Beweries et al., 2009). We were therefore interested in the reactivity of the zirconafuranone 1, whose crystal structure has been reported (Pellny et al., 1999), towards HAl(i-Bu) 2 . ISSN 2056-9890 In the present work, the zirconafuranone 1 reacts with two equivalents of HAl(i-Bu) 2 , and a disproportionation of the Lewis acid gives a triisobutylaluminium fragment, leading to the formation of the zwitterionic title compound 2 by coordination of Al(i-Bu) 3 to the exocyclic carbonyl oxygen of the zirconafuranone ring (see Scheme).

Supramolecular features
For the title complex 2 no significant supramolecular features are observed. The crystal packing appears to be dominated by van der Waals interactions (Fig. 3).

Synthesis and crystallisation
All operations were carried out under argon with standard Schlenk techniques or in a glovebox. The starting zircononafuranone 1 was prepared according to a method previously described in the literature (Pellny et al., 1999).
A commercial 1 M solution of i Bu 2 AlH in cyclohexane was purchased from Sigma Aldrich and used as received. Solvents were purified by conventional methods and were distilled twice over metallic sodium (toluene, n-hexane) under Ar prior Possible resonance structures of complex 2.

Figure 1
The molecular structure of the title complex 2 with the atom labelling. Displacement ellipsoids correspond to the 30% probability level. H atoms have been omitted for clarity. The minor disorder component is indicated by open bonds.

Figure 3
Packing diagram for 2 viewed along the a axis. Displacement ellipsoids correspond to the 30% probability level. H atoms and lower occupancy sites have been omitted for clarity. to use. The 1 H and 13 C NMR spectra were recorded on Bruker AMX-400 and AV-400 spectrometers. The IR spectra were recorded on a Nicolet Magna IR-750 FTIR spectrometer. The mass spectra were measured using a MAT 95-XP instrument.