Bis{μ-4,4′,6,6′-tetra-tert-butyl-2,2′-[N-(2-oxidoethyl)iminodimethylene]diphenolato}dialuminium(III)

The title compound, [Al2(C32H48NO3)2], exists as a dimer with bridging ethoxide groups. It was isolated from a reaction mixture of the parent ligand and trimethylaluminium in tetrahydrofuran. The geometry around the AlIII atom is a slightly distorted trigonal-bipyramid, typical of atrane derivatives.

The title compound, [Al 2 (C 32 H 48 NO 3 ) 2 ], exists as a dimer with bridging ethoxide groups. It was isolated from a reaction mixture of the parent ligand and trimethylaluminium in tetrahydrofuran. The geometry around the Al III atom is a slightly distorted trigonal-bipyramid, typical of atrane derivatives.

Experimental
Crystal data [Al 2 (C 32 H 48  The title compound (Fig. 1), exists as a dimer with bridging ethoxides. The molecule and its core {Al 2 O 2 } ring are centered on a point of crystallographic inversion. Aluminium has a slightly distorted trigonal-bipyramidal coordination geometry around the metal with equitorial O-Al-O angles ranging from 118.60 (6)° to 124.49 (5)° and an axial N-Al-O angle of 159.06 (5)°. Other bond lengths and angles are unremarkable. The Al-Al distance of 2.9022 (10) Å in the tert-butyl substituted title complex is barely distinguishable from the 2.8991 (7) Å A l-Al distance of the related methyl substituted compound (Johnson et al., 2009). The dimer is crowded, as evidence by the proximity of the tert-butyl H atoms of one side of the dimer to the ethyloxy H atoms of the other side of the dimer (Fig. 2), which range from 2.523 (H2A-H17B) to 2.844 (H2B-H28C).
Attempts at melt polymerization of racemic lactide using the title compound proved fruitless, returning only starting materials. Indeed, variable temperature 1 H NMR (-10 to 100 °C) of the title compound in toluene-d8 showed no evidence of the dimer breaking apart, a presumed necessity for polymerization activity. At high temperatures, however, the broad singlet at 2.441 resolves into a triplet (J=5.2 Hz) and the broad singlet at 3.302 resolves into two doublets (J=13 Hz).
These dynamics are consistent with a sterically congested molecule in which rapid conformational equilibrium is achieved at elevated temperatures.

Refinement
Hydrogen atoms on carbon were added geometrically and refined using a riding model. U iso values for hydrogen atoms were assigned to be 1.20 times the U eq value of the atom to which they are attached, except for hydrogen atoms on methyl carbon atoms, which were assigned a U iso of 1.50 times the U eq of the methyl carbon atom to which they are attached. Fig. 1. The molecular structure of the title compound with 50% probability displacement ellipsoids for non-H atoms. Unlabelled atoms are related to their labelled counterparts through an inversion operation (1-x, -y, 1-z). Bis{µ-4,4',6,6'-tetra-tert-butyl-2,2'-[N-(2-oxidoethyl)iminodimethylene]diphenolato}dialuminium(III) Crystal data [Al 2 (C 32 H 48  as those based on F, and R-factors based on ALL data will be even larger.

Figures
An extinction parameter (EXTI in SHELXL-97) refined to zero and was removed from the refinement.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )