Synthesis and structure of an asymmetrical sila[1]magnesocenophane

1 Introduction

Metallocenes with interlinked cyclopentadienyl moieties with a one-atomic bridging motif are commonly referred to as [1]metallocenophanes. Examples of such compounds are known for various transition metals as well as group 2 s-block elements [1], [2], [3], [4]. These compounds have seen applications as monomers in ring-opening polymerization and in homogenous catalysis [5], [6], [7], [8], [9], [10], [11], [12]. A limited number of [1]magnesocenophanes is known – only some of them being structurally characterized – but so far, most examples exhibited a symmetrical ansa-ligand system [4, 11, 13], [14], [15], [16], [17], [18]. Herein we present the first example of an asymmetrical sila[1]magnesocenophane featuring two different ring substitution patterns that has been structurally characterized.

2 Experimental

3 Results

The synthesis started from the neutral ansa-ligand system, (cyclopentadienyl)(tetramethylcyclopentadienyl)dimethylsilane. Treatment of the ligand with a hexane solution of n-butyl-sec-butylmagnesium at 273 K yielded the title compound, 1, as a colorless solid. As it is common with magnesocenophanes, 1 is essentially insoluble in apolar solvents, such as hexane or benzene. However, it readily dissolves in donor solvents such as tetrahydrofuran with the formation of the corresponding solvent complex, due to the high Lewis-acidity of the magnesium center (Scheme 1). Single crystals of 1, suitable for X-ray diffraction analysis, could be obtained by storing a tetrahydrofuran solution at 248 K.

Scheme 1:

Synthesis of sila[1]magnesocenophane bis(tetrahydrofuran) 1‧(thf)₂.

All structurally characterized examples of [1]magnesocenophanes are donor-solvent complexes, mostly bis(tetrahydrofuran) complexes. The solid state structures of these complexes feature slipped sandwich structures with one Cp ligand bound <η⁵ to the magnesium center [4]. A similar situation is found in 1 (Figure 1 and Table 1).

Figure 1:

Molecular structure of sila[1]magnesocenophane bis(tetrahydrofuran) 1‧(thf)₂ in the crystal. a) Showing only occupation A (64%), and b) showing both occupations (displacement ellipsoids drawn at 50% probability level; H-atoms, except for H5 in b), omitted for clarity).

It is worth noting that a disorder was found in the crystal structure of 1‧(thf)₂. The entire structure was split over two positions, with the occupancy factors refined to 64% for the major component. Since all C-atoms of 1‧(thf)₂ are placed on two different split positions, this requires the introduction of a relatively large set of restraint parameters for the refinement of the disorder. The restraints were applied to the anisotropic displacement parameters of the disordered atoms. All non H-atoms were located on the electron density maps and refined anisotropically. C-Bound H atoms were placed in positions of optimized geometry and treated as riding atoms. Their isotropic displacement parameters were coupled to the corresponding carrier atoms by a factor of 1.2 (CH, CH₂) or 1.5 (CH₃). The hydrogen atoms H5A and H5B were found on the electron density maps. Their positional parameters were refined using isotropic displacement parameters, which were set at 1.2 times the U_eq value of their parent atoms. Restraints (DFIX) of 0.95 (0.01) Å were used for the C–H bond lengths.

Within the major component (64% occupation) of 1‧(thf)₂, Mg–C distances of the η⁵ coordinated tetramethylcyclopentadienyl group are 232.8(8)–259.7(7) pm and the Mg–Cp^centroid distance is 215.2(5) pm. The C–C distances found within this Cp ring are 140.1(7)–144.1(8) pm, indicating a high degree of π conjugation. On the other hand, the unsubstituted cyclopentadienyl group exhibits a distorted η¹ coordination to the magnesium atom, with significantly more diverging Mg–C distances of 238.0(8)–399.7(7) pm and C–C distance of 138.7(6)–144.1(8) pm, indicating more localized single and double bond character. The Mg–O distances of 201.0(9) and 204(1) pm are comparable to those in other sila[1]magnesocenophane tetrahydrofuran complexes [15].

The solution NMR spectra indicate that the structure is fluxional in solution at room temperature, thus that the slipped-sandwich structure is not retained, as only two signals for Cp protons are observed in the ¹H NMR spectrum, along with just six resonances in the ¹³C{¹H} NMR spectrum for Cp ring carbon atoms (Figures 2 and 3). Furthermore, a resonance at δ²⁹Si = −22.6 is detected in the ²⁹Si{¹H} INEPT NMR spectrum, typical for a tetracoordinated silicon atom (Figure 4).

Figure 2:

¹H NMR spectrum (400.13 MHz, THF-D8, 293 K, TMS) of sila[1]magnesocenophane bis(tetrahydrofuran) 1‧(thf)₂.

Figure 3:

¹³C{¹H} NMR spectrum (100.61 MHz, THF-D8, 294 K, TMS) of sila[1]magnesocenophane bis(tetrahydrofuran) 1‧(thf)₂.

Figure 4:

²⁹Si{¹H} INEPT NMR spectrum (79.49 MHz, THF-D8, 294 K, TMS) of sila[1]magnesocenophane bis(tetrahydrofuran) 1‧(thf)₂.

In summary, the first crystal structure of a sila[1]magnesocenophane, 1, with different substitution patterns on the Cp rings is reported. The compound was crystallized as a bis(tetrahydrofuran) complex and shows a slipped sandwich structure in the solid state, with the unsubstituted C₅H₄ moiety exhibiting an η¹ and the permethylated C₅Me₄ moiety exhibiting an η⁵ coordination mode. It holds potential applicability as a precursor in the synthesis of corresponding metallocenophanes of transition metals by transmetalation.