Crystal structure of bis[tetrakis(tetrahydrofuran-κO)lithium] bis[μ-2,2′,2′′-methanetriyltris(4,6-di-tert-butylphenolato)-κ4 O,O′:O′,O′′]dimagnesiate

The heterobimetallic complex comprises discrete Li–THF complex cations and centrosymmetric bimetallic Mg dianions with the tridentate phenolic ligand tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane in an ion-association mode, with each metal complex core four-coordinate with distorted tetrahedral stereochemistry.

In the crystal, a number of stabilizing intra-anion C-HÁ Á ÁO hydrogen-bonding interactions are present but no inter-species associations are found.

Chemical context
Magnesium complexes (Wang et al., 2014) and lithium complexes (Ko & Lin, 2001) display a vigorous catalytic activity in the synthesis of biodegradable polymers, through ring-opening polymerization. Heterobimetallic compounds, also called 'ate' complexes (Mulvey, 2009), have been systematically studied with a focus both on the elucidation of the solid-state structures and on the catalytic applications (Qiu et al., 2013). We have synthesized the title metal complex, 2{[Li(THF) 4 ] + }Á[Mg 2 (C 43 H 61 O 3 ) 2 ] 2À from the reaction of the tridentate phenolic ligand tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane with n-butyl-lithium and diethyl magnesium in tetrahydrofuran (THF). The structure of this novel heterobimetallic complex, in an ion-association mode, is reported herein.

Structural commentary
In the title complex ion-association compound (Fig. 1)

Database survey
A search of the Cambridge Structural Database (Groom et al., 2016) revealed 39 structures of complexes having the ligand derived from tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane. These include cage-like monometallic alkali complexes (Dinger & Scott, 2000) and an aluminum metal complex in an ion-association mode (Oishi et al., 2016). In addition, a zinc complex based on the same ligand has been found to be useful for polymerization of cyclohexene oxide and carbon dioxide (Dinger & Scott, 2001).

Synthesis and crystallization
A solution of tris(3,5-di-tert-butyl-2-hydroxyphenyl)methane (0.63 g, 1.0 mmol) and n BuLi (0.5 mL, 1.2 mmol, 2.4 M in hexane) was stirred in THF (20 mL) at 273 K under an N 2 atmosphere for 2 h. MgEt 2 (1.1 mL, 1.1 mmol, 1.0 M in hexane) was gently added to the solution. After stirring at 298 K for 6 h, the solution was filtered through celite. The filtrate was concentrated to ca 10 mL and cooled to 273 K to furnish colourless crystals, suitable for the X-ray analysis. Yield: 0.46 g (49%).

Figure 1
Molecular structure of the title compound with displacement ellipsoids given at the 40% probability level. All of the hydrogen atoms are omitted for clarity. The non-labelled atoms of one of the two cations and the binuclear anion are generated by the symmetry operation Àx + 1, Ày + 2, Àz + 1. Table 1 Selected bond lengths (Å ). Symmetry code: (i) Àx þ 1; Ày þ 2; Àz þ 1.

Figure 2
Molecular packing of the title compound in the unit cell viewed along the a axis.

Bis[tetrakis(tetrahydrofuran-κO)lithium] bis[µ-2,2′,2′′-methanetriyltris(4,6-di-tert-butylphenolato)-
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.43 e Å −3 Δρ min = −0.29 e Å −3 Special details 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.