A bis(aluminocenophane) with a short aluminum–aluminum single bond

Wasim Haider , Diego M. Andrada *, Inga-Alexandra Bischoff , Volker Huch and André Schäfer *
Faculty of Natural Sciences and Technology, Department of Chemistry, Saarland University, Campus Saarbrücken, 66123 Saarbrücken, Saarland, Federal Republic of Germany. E-mail: andre.schaefer@uni-saarland.de

Received 27th August 2019 , Accepted 23rd September 2019

First published on 23rd September 2019


A bis(dicarba[2]aluminocenophane) was synthesized by reduction of a corresponding dichloro precursor with 1,3-β-diketiminate magnesium(I). The bis(aluminocenophane) is stable under an inert gas atmosphere and was characterized in solution and in the solid state. The crystal structure reveals an unusually short Al–Al single bond and two η5 coordinated cyclopentadienyl groups on each aluminum atom which are disposed in an orthogonal fashion. The aluminum bond can be oxidized and cleaved by element chlorides and tert-butyl isonitrile. The bonding situation was studied within the DFT framework.


Introduction

Metallocenophanes are a well-recognized class of compounds in transition metal chemistry.1,2 In contrast, only a few examples of structurally characterized metallocenophanes of p-block elements have been reported so far.3,4 In the case of the Group 13 elements, dicyclopentadienyl substituted compounds often retain at least one η1 or η2 bonded cyclopentadienyl substituent,5 with aluminocenium cations being rare examples of Group 13 compounds isostructural to ferrocene with two η5 bonded cyclopentadienyl ligands.6 In 2005, Shapiro and co-workers reported a dicarba-bridged dicyclopentadienyl-aluminum chloride dimer (Chart 1, I), which displays η15 coordination of the cyclopentadienyl ligands in the solid state while undergoing rapid sigmatropic/haptotropic shifts in solution.7 The related monomeric dicarba- and disila[2]aluminocenophane halide derivatives can be obtained by the coordination of an additional donor to the aluminum atom (Chart 1, II).3h,7
image file: c9dt03470f-c1.tif
Chart 1 Examples of bridged bis(cyclopentadienyl)aluminum compounds.

It is noteworthy that none of these compounds feature two η5 bonded cyclopentadienyl moieties at one aluminum atom. In fact, to the best of our knowledge, no neutral aluminum compound with two η5 bonded cyclopentadienyl ligands at one aluminum atom is known so far. Therefore, we were intrigued by the idea that such an unprecedented bonding motif might be realized in the case of electron-poor aluminum in lower oxidation states, as for example in a corresponding dialane. Since the pioneering work of Uhl and co-workers in 1988,8a several different dialanes have been reported, in most cases with bulky aryl or alkyl substituents and in two cases with single η5 bonded pentamethyl-cyclopentadienide ligands. In addition, donor-stabilized dialanes, including tetrahydrido dialanes, have been reported recently.8

Results and discussion

With the aim of preparing a dialane with four cyclopentadienyl moieties and to study the effects of such a ligand framework on the Al–Al bond, we targeted the dicarba-bridged dicyclopentadienylaluminum chloride dimer, 1, as a suitable precursor for the preparation of the corresponding dialane. In this regard, 1,3-β-diketiminate magnesium(I), first described and popularized by Jones and co-workers,9 has proved to be a powerful and selective reducing agent. Indeed, when the chloro bridged dimer 1 is stirred with one equivalent of β-diketiminate magnesium(I) dimer in toluene at room temperature, a uniform reduction takes place to give dialane 2 (Scheme 1), as indicated by 1H and 13C NMR spectroscopy (no signal for 2 could be detected in a range of +200 to −200 ppm in the 27Al NMR spectrum).
image file: c9dt03470f-s1.tif
Scheme 1 Synthesis of bis(dicarba[2]aluminocenophane) 2 (Mg(I) = {MesNacNacMg}2).

Notably, the structural motif of a metallocenophane dimer is very rare: to the best of our knowledge, there is only one previous example of a bis(metallocenophane) exhibiting a metal metal bond, which is a bis(dicarba[2]ruthenocenophanium) dication, reported by Manners and co-workers.10

The structure of bis(aluminocenophane) 2 as determined by X-ray crystallography reveals several interesting features (Fig. 1 and Table 1). Both the aluminum atoms exhibit two η5 bonded cyclopentadienyl substituents; dialane 2 therefore represents the first neutral aluminum species with such a bonding motif, as well as the first dialane with no formally σ bonded substituents.


image file: c9dt03470f-f1.tif
Fig. 1 Molecular structure of bis(dicarba[2]aluminocenophane), 2, in the crystal (thermal ellipsoids set at 50% probability level, H-atoms omitted for clarity, Al–Al: 250.18(5)).
Table 1 Selected experimental and computational bond lengths [pm] and angles [°] in bis(dicarba[2]aluminocenophane) 2
  Exp. Calc. (BP86+D3(BJ)/BSa) Calc. (M06-2X/BSa)
a BS = def2-TZVPP. b Dihedral angle between Cp planes. c Torsion angle between Cpcentroid–Al–Al–Cpcentroid. d Sum of interior angles around Al.
Al–Al 250.2 248.1 249.8
Al–Cpcentroid 204.3; 210.7; 227.3; 234.1 212.4; 231.3; 227.9; 224.3 201.9; 232.8; 220.0; 202.4
α 72.3; 73.9 74.4; 77.5 71.0; 69.3
τ 84.7; 81.0; 95.6; 98.7 92.7; 79.6; 105.1; 82.6 77.2; 91.3; 98.5; 93.0
∑∠Ald 357.1; 360.0 354.9; 359.1 359.7; 358.3


The two aluminocenophane moieties are twisted by ca. 90° torsion angle to each other. The dihedral angles between the cyclopentadienyl planes are 72.4° and 73.9°. The coordination spheres of the aluminum atoms are almost perfectly trigonal planar with respect to the centroids of the Cp moieties (sums of bonding angles around the aluminum atoms of 357.1° and 360.0°). Furthermore, most remarkably, bis(aluminoceno-phane) 2 exhibits an unusually short Al–Al bond of 250.2 pm. This bond length is the shortest for any acyclic dialane reported so far (Fig. 2).8,11 The Al–Cpcentroid bond lengths observed in the solid state structure of 2 are in the range of 204.3 to 234.1 pm. These relatively large differences are believed to be the result of packing effects in the crystal, because of very flexible Cp–Al bonds, which have a strongly ionic character. The 1H NMR spectrum of a solution of 2 shows two signals for Cp protons, indicating a symmetrical structure in solution.


image file: c9dt03470f-f2.tif
Fig. 2 Selected examples of acyclic dialanes (Tip = 2,4,6-iPr3-C6H2; R = tBuMe2Si; Ter = 2,6-(2,6-iPr2-C6H3)2-C6H3; Cp* = C5Me5; Bbp = 2,6-((tBu3Si)2CH)2-C6H3).8

Bis(aluminocenophane) 2 is stable under an inert gas atmosphere in the solid state at ambient temperatures for at least several days and at low temperatures for several weeks. Likewise, solutions of 2 in aromatic hydrocarbons are stable under an inert gas atmosphere at ambient temperatures for several days and at an elevated temperature of 333 K for at least two hours. Although the Al–Al bond in 2 is exceptionally short, it represents a reactive site in the molecule, as 2 is easily oxidized to give 1 when treated with element chlorides such as aluminum trichloride (AlCl3) or carbon tetrachloride(CCl4) (Scheme 2).


image file: c9dt03470f-s2.tif
Scheme 2 Reactions of 2 with element chlorides to give 1 (a) AlCl3; (b) CCl4.

Similarly, when 2 is treated with two equivalents of tert-butyl isonitrile (tBuNC), an oxidative insertion takes place along with a reductive C–C coupling to give heterocycle 3 (Scheme 3). Similar insertion reactions of unsaturated main group species into Al–Al bonds have been described before.8j,l,12


image file: c9dt03470f-s3.tif
Scheme 3 Reaction of 2 with tBuNC to give 3.

In the solid state, each of the aluminum atoms in 3 exhibits η12 coordination to the adjacent Cp moieties (Fig. 3). However, only four resonances (multiplets) are detected for Cp protons in the 1H NMR spectrum at room temperature, indicating a fluxional behaviour in solution, due to rapid sigmatropic/haptotropic rearrangements.


image file: c9dt03470f-f3.tif
Fig. 3 Molecular structure of isonitrile insertion product 3 in the crystal (thermal ellipsoids set at 50% probability level, H-atoms omitted for clarity).

To gain a deeper insight into the bonding situation in bis(aluminocenophane), 2, we carried out DFT calculations at the M06-2X/def2-TZVPP and BP86+D3(BJ)/def2-TZVPP level of theory.13 The computed Al–Al bond lengths of 249.8 pm and 248.2 pm are in good agreement with the one determined by X-ray crystallography. Omitting dispersion interactions results in a longer bond length (254.6 pm; Fig. S5). The computed Al–Cp bond lengths deviate slightly from the experimental ones, which is a common observation for Cp systems in which the distances are strongly influenced by packing effects in the solid state.14,15

Inspection of the molecular orbitals (Fig. 4(a)) reveals that the HOMO−4 corresponds to the Al–Al σ bond with small contributions of Cp orbitals. The LUMO corresponds to the antibonding combination of the 3s orbitals of the Al2 moiety (Fig. S14) and also exhibits an antibonding interaction with Cp group orbitals. Note that the natural bond orbitals suggest that the Al2 fragment of 2 is strongly charged by +2.50e. The Wiberg bond order (WBI) P = 0.93 a.u. is in line with an Al–Al single bond. The hybridization of the central atoms shows that the Al–Al bond exhibits a very large s character of 66.2% (Fig. 4(c)). For the sake of comparison, we have also performed NBO analysis on Al2H4 and Al2Cl4 (Table S1). In these compounds, the computed partial charges for the Al2 moieties are +1.52e and +2.10e, while the WBIs are 0.90 a.u. and 0.89 a.u., respectively. Interestingly, the s character of the Al–Al bond in 2 is much higher than in Al2H4 and Al2Cl4, being 36.4% and 46.3%. The NBO analysis of 2 also shows a strong ionic interaction (Scheme 4, B) with two-electron stabilizing interactions between the central Al2 moiety and the Cp ligands (Fig. S9).


image file: c9dt03470f-f4.tif
Fig. 4 (a) Kohn–Sham molecular orbitals HOMO−4 (left) and LUMO (right) of 2 (isovalue 0.05). (b) Natural bond orbital of the Al–Al bond in 2, hybridization and occupation (calculations performed at M06-2X/def2-TZVPP). (c) Laplacian distribution ∇2ρ(r) in the Al–Al–Cpcentroid plane of 2. The dashed red lines indicate areas of charge concentration (∇2ρ(r) < 0), the solid blue lines show areas of charge depletion (∇2ρ(r) > 0), the solid black lines connecting the atomic nuclei are the bond paths, the black dots are the bond critical points (BCPs) and the red dot is a non-nuclear attractor (NNA).

image file: c9dt03470f-s4.tif
Scheme 4 Resonance structures of 2 based on DFT calculations.

The Laplacian distribution ∇2ρ(r) of the electron density in the plane Al–Al–Cpcentroid of 2 (Fig. 4(b)) shows two Al–CCp bond paths to the Cp groups and one Al–Al bond path in the contour line diagram. There are two bond critical points (BCP) and one non-nuclear attractor (NNA). In a certain range of internuclear distance, the shared valence shell is maintained and eventually nonnuclear electron density maxima appear since the inner shells start to dominate the interaction, which gives two BCPs. The presence of NNAs is commonly observed in bonds with low polarity where the radial form of the valence orbitals dominates the density, for instance in Li2 and Na2 systems.16–19 The computed electron densities at the BCPs and NNA are 0.063 and 0.064e per bohr3 (Table S2), respectively. These values are comparable to the electron density computed at the BCPs and NNAs of Al2H4 (0.058 and 0.060e per bohr3) and Al2Cl4 (0.062 and 0.064e per bohr3). The calculated Al–Al bond dissociation energies for 2, Al2H4 and Al2Cl4 at M06-2X/TZ2P are 218.9 kJ mol−1, 244.8 kJ mol−1 and 240.8 kJ mol−1 (Table 2). Single point calculations performed at the local couple cluster method DF-LCCSD(T)/cc-pVTZ using the optimized geometries give 251.1 kJ mol−1, 258.3 kJ mol−1 and 261.3 kJ mol−1. Thus, the Al–Al bond in 2 is slightly weaker than in Al2H4 and Al2Cl4.

Table 2 Energy decomposition analysis (EDA-NOCV) of Al–Al bonds in 2, Al2H4 and Al2Cl4 at M06-2X/TZ2P//M06-2X/def2-TZVPP. Energies given in kJ mol−1
  2 Al2H4 Al2Cl4
a Value in parentheses gives the percentage contribution to the total attractive interactions ΔEelstat + ΔEorb. b Value in parentheses gives the percentage to the total orbital interaction.
Al–Al 249.8 pm 257.6 pm 253.2 pm
ΔEint −278.4 −261.0 −256.4
ΔEPauli 368.7 302.1 248.6
ΔEelstata −361.7 (55.8%) −311.0 (54.9%) −243.2 (47.4%)
ΔEorba −286.4 (44.2%) −263.7 (45.9%) −261.8 (52.6%)
ΔEmetaHybrid 1.0 11.6 7.9
ΔEorb ρ1b −254.1 (88.7%) −251.5 (95.4%) −256.9 (95.2%)
ΔEorb restb −32.3 (11.3%) −12.3 (4.6%) −12.8 (4.8%)
ΔEprep 59.5 16.2 15.7
De = ΔE −218.9 −244.8 −240.8


So why is the Al–Al single bond in bis(aluminocenophane) 2 so short? In order to answer this question, we performed Energy Decomposition Analysis (EDA)20 in combination with the Natural Orbital for Chemical Valence (NOCV) theory to investigate the nature of the interaction between the Al atoms and the Cp groups of the ligands (Table 2).13 This gives a more detailed understanding of the bonding situation in 2.

Although many fragmentation schemes can be imagined, we have chosen the fragmentation of the Al–Al bond. In this fragmentation, the interaction energy for 2 (−278.4 kJ mol−1) is stronger than for Al2H4 (−261.0 kJ mol−1) and Al2Cl4 (−256.4 kJ mol−1), which is in agreement with the shorter bond distance in 2. The weaker dissociation energy for 2 is a consequence of the higher preparation energy needed to deform the fragments. Further dissection of the ΔEint reveals that the total stabilizing interaction is 55.8% electrostatic and 44.2% covalent. We have also computed the dispersion interaction between aluminocenophane fragments at different levels (Table S3), suggesting a contribution of −90.0 kJ mol−1 to the overall stabilizing interaction (12.5%). What is noteworthy is that in Al2H4 and Al2Cl4, dispersion interaction accounts for only 1.2% and 2.6% of the stabilization energy.

NOCV results suggest that the orbital interaction arises mainly from the Al–Al σ bond with little contribution from the polarization of the Cp groups (Fig. S11–S13).13 Furthermore, we investigated two different bonding situations: (a) an electron sharing interaction of an Al2 fragment with (nσg+)2(nσu+)1((n + 1)σg+)1u)1(π′u)1 valence configuration with a [(Me2C)2(C5H4)2]2 fragment (Scheme 4, A) and (b) an ionic interaction of an Al24+ fragment with (nσg+)2(nσu+)0((n + 1)σg+)0u)0(π′u)0 valence configuration with a [(Me2C)2(C5H4)2]24− fragment (Scheme 4, B). The best representation can be determined by a comparison of the orbital term (ΔEorb) of each partitioning.20 The partitioning scheme that leads to the lowest absolute value is the most suitable representation since it yields less energy penalties. Notably, the orbital term in both situations has comparable strength: electron sharing −3052.4 kJ mol−1 and ionic partitioning −3310.9 kJ mol−1. In contrast, Al2H4 shows a clear preference for an electron sharing representation. In the ionic representation (Scheme 4, B), the interaction energy between the fragments is very strong, namely −8570.4 kJ mol−1. Similar results have been described for the interaction of lithium cations with amides.21 Interestingly, the splitting of the interaction energy into electrostatic (ΔEelstat), orbital (ΔEorb) and Pauli (ΔEPauli) interactions shows that the bonding for this fragmentation is 66.2% ionic and 33.8% covalent (Fig. S15, S16 and Tables S4–S6).13 Thus, the shortness of the Al–Al bond can to a small degree be attributed to attractive dispersion interactions between the ligands, but is primarily a result of the very high s character of the Al–Al bond, which is derived from the ionic nature of 2 with an Al24+ fragment (Scheme 4, B).

Conclusions

In summary, we report the synthesis, structural characterization and in-depth bonding analysis of a bis(dicarba[2]-aluminocenophane), 2. This unusual molecule represents the first dialane without formally σ bonded substituents, as well as the first neutral aluminum compound exhibiting two η5 bonded cyclopentadienyl moieties to one aluminum atom, and possesses the shortest Al–Al single bond of any acyclic dialane. DFT calculations reveal a surpassingly high s character of the Al–Al bond and a strong ionic character of the Cp–Al bonds.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Guido Kickelbick and David Scheschkewitz for their support and helpful discussions. AS thanks the Deutsche Forschungsgemeinschaft (Emmy Noether Program, SCHA1915/3-1) and the Fonds der Chemischen Industrie for funding. DMA thanks the European Research Council (ERC StG, 805113).

Notes and references

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Footnote

Electronic supplementary information (ESI) available. CCDC 1936579 and 1936580. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/C9DT03470F

This journal is © The Royal Society of Chemistry 2019