Crystal structure of bis(μ2-methanolato-κO:κO)hexamethylbis(μ2-triphenylacetato-κO:κO′)bis(μ2-triphenylacetato-κ2 O,O′:κO)dialuminiumdilanthanum toluene tetrasolvate

The complex [{La(Ph3CCOO)2(Me3AlOMe)}2] has an La2(μ-OCO)4 core, contains the carboxylate ligands in μ2-κ1 O:κ1 O′ bridging and μ2-κ2 O,O′:κ1 O semi-bridging coordination modes, and displays La—C interactions with the π-system of a phenyl ring.

The title compound, [Al 2 La 2 (C 20 H 15 O 2 ) 4 (CH 3 ) 6 (CH 3 O) 2 ]Á4CH 3 C 6 H 5 or [{La(Ph 3 CCOO) 2 (Me 3 AlOMe)} 2 ]Á4CH 3 C 6 H 5 , was formed in a reaction between lanthanum tris(tetramethylaluminate) and triphenylacetic acid (1:1) with unintended partial oxidation. The triphenylacetate ligand exhibits 2 -1 O: 1 O 0 bridging and 2 -2 O,O 0 : 1 O semi-bridging coordination modes, forming a dimeric La 2 (-OCO) 4 core. The semi-bridging triphenylacetate group provides additional bonding with an La 3+ cation via the -system of one of its phenyl rings. The trimethylmethoxyaluminate anion, which is coordinated to the La 3+ cation by its O atom, displays a rather long La-C Me bond. Two toluene molecules are each disordered over two orientations about centres of symmetry with site occupancy factors of 0.5. The title compound represents the first example of an Ln III complex containing both alkyl alkoxide aluminate andbounded arene fragments.

Chemical context
Heteroleptic tetraalkylaluminate complexes of rare-earth metals attract significant attention because of their intriguing role in the stereospecific polymerization of conjugated dienes (Anwander, 2002). Stereoregular elastomers obtained in the polymerization process of isoprene and butadiene are fundamentally important for the production of modern wear-resistant rubbers (Friebe et al., 2006). It is assumed that this type of complex plays the key role in the formation of catalytically active species. Meanwhile, little is known about the structure of such complexes (Fischbach et al., 2006a, and reference therein). The exceptionally high oxidative instability of aluminate complexes is one of the reasons for the lack of information on the structures of catalytically active heteroleptic bimetallic Ln-Al complexes.
This report describes the product of unintentional oxidation of a carboxylate-aluminate La complex while reacting lanthanum tris(tetramethylaluminiumate) with the corresponding acid (Fig. 1). This reaction should have led initially to the heteroleptic triphenylacetate-tetramethylaluminate complex that is supposed to be a model of the active species in the catalyst system. The accidental partial oxidation resulted ISSN 2056-9890 in the formation of the triphenylacetate-trimethylmethoxyaluminate lanthanum complex [{La(Ph 3 CCOO) 2 Me 3 Al-OMe} 2 ].
The La 3+ cation is also coordinated by the -system of a phenyl ring of the semi-bridging carboxylate ligand (Fig. 3, atoms C7 i -C12 i ; Table 1). The interaction with the phenyl (Ph) group is close to symmetrical: the LaÁ Á ÁPh centroid distance is 2.938 (2) Å , the normal to the Ph-ring plane is 2.9353 (16)  The molecular structure of the {La(Ph 3 CCOO) 2 (Me 3 AlOMe)} 2 unit in the title compound with displacement ellipsoids drawn at the 30% probability level. Hydrogen atoms and toluene solvent molecules are omitted for clarity. The La-O bonds are shown with thinner solid lines.

Figure 3
Metal-ligand interactions within the {La(Ph 3 CCOO) 2 (Me 3 AlOMe)} 2 unit. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted, only C ipso atoms (labeled as Ph) are shown for non-coordinating phenyl groups for clarity. The Ln-C contacts are shown with thin dashed lines. Symmetry code: (i) Àx, Ày + 1, Àz + 1.  Synthesis of [{La(Ph 3 CCOO) 2 Me 3 AlOMe} 2 ]Á4(CH 3 C 6 H 5 ). and the LaÁ Á ÁC Ph bond lengths lie in the range 3.201 (4) to 3.318 (4) Å . Ten crystal structures exhibiting the interaction of La 3+ with the -system of an uncharged C 6 aromatic ring have been found in the Cambridge Structural Database (CSD, Version 5.39, February 2018 update; Groom et al., 2016). The corresponding distances in these compounds vary from 2.93 to 3.27 Å for LaÁ Á ÁC Aryl and from 2.61 to 2.87 Å for LaÁ Á ÁAryl centroid . The LaÁ Á ÁPh centroid and LaÁ Á ÁC Ph distances in the title compound are therefore the longest, which is likely caused by steric hindrance induced by the presence of many phenyl groups within the inner coordination sphere. The trimethylmetoxyaluminate anions are coordinated to the La 3+ cations via oxygen atoms (La1-O1, La1 i -O1 i ), and exhibit a slightly distorted tetrahedral environment about the Al atoms, with an O1-Al1-C2 angle of 100.03 (17) and with other O-Al-C and C-Al-C bond angles ranging from 108.32 (18) to 113.2 (2) . The small value for the O1-Al1-C2 angle is due to the additional coordination of the [Al(CH 3 ) 3 (OCH 3 )] anion with La 3+ by the C2 atom (Fig. 3). However, the La1-C2 bond length [3.042 (4) Å ] is rather long compared to those of previously characterized compounds possessing the La-[(-Me) 2 AlMe 2 ] fragment, which have La-C Me distances lying in the range 2.66 to 2.98 Å with the average value of 2.76 Å (32 compounds with 128 crystallographically independent La-C Me-Al distances retrieved from the CSD). The La1Á Á ÁAl1 distance  (Table 1) is considerably longer (by 0.24-0.28 Å ) than the corresponding La-C distances in MIMPED [2.800 (5), 2.759 (5) Å ], presumably due to steric reasons.
In the studied compound, the La-O Me (La1-O1) bond is the shortest, compared to the other La-O bonds, which may be due to delocalization of negative charge on the carboxy oxygen atoms and/or steric repulsion of the bulky carboxylate anion.

Supramolecular features
Weak intra-and intermolecular interactions among complex molecules and non-coordinating toluene molecules are mainly represented by the C Ph -HÁÁ type (Table 2). An interesting feature of the crystal packing is that the centres of all non-coordinating toluene molecules are located nearly in one plane parallel to the ab plane, separating 2D molecular layers of the complex (Fig. 4).

Database survey
The number of crystal structures for rare-earth compounds containing the Ln-C-Al fragment (CSD, Version 5.39, February 2018 update; Groom et al., 2016) Table 2 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
Synthetic operations were carried out under a purified argon atmosphere. Toluene was distilled from sodium/benzophenone ketyl, hexane was distilled from Na/K alloy. Triphenylacetic acid was purified by azeotrope removal of water from its toluene solution with a Dean-Stark trap, followed by crystallization from a cold saturated solution and then by vacuum drying. The complex La(AlMe 4 ) 3 was prepared according to the literature procedure (Zimmermann et al., 2007).
A solution of Ph 3 CCOOH (0.144 g, 0.50 mmol) in toluene (20 ml) was added to a stirred solution of La(AlMe 4 ) 3 (0.196 g, 0.49 mmol) in toluene (10 ml), producing a suspension, which was stirred overnight at room temperature. The precipitate was removed by decantation and the solution was concentrated to a volume of 10 ml. Slow and careful layering of hexane (40 ml) on the top of the residual solution resulted in the formation of an inseparable compound mixture and a few colourless crystals suitable for X-ray single crystal diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atom were positioned geometrically (C-H = 0.95 Å for aromatic, 0.98 Å for methyl H atoms) and refined as riding atoms with U iso (H) =  1.5U eq (C) for methyl or 1.2U eq (C) for aromatic H atoms. A rotating group model was applied for methyl groups. Three reflections (100, 010, 001) were affected by the beam stop, and were therefore omitted from the refinement. Two non-coordinating toluene molecules disordered over inversion centres with occupancy factors of 0.5 were modelled by fitting the phenyl rings to regular hexagons, by constraining the C ipso -C Me bond distances to 1.52 (1) Å , and by using equal anisotropic displacement parameters for atoms C52, C53, C54, C55, C60, C62 and C65. Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication:

Special details
Experimental. moisture and air sensitive Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.