Polynuclear Magnesium and Magnesium−Titanium Species. Syntheses and Crystal Structures of [Mg<sub>4</sub>(μ<sub>3</sub>,η<sup>2</sup>-ddbfo)<sub>2</sub>(μ,η<sup>2</sup>-ddbfo)<sub>2</sub>(μ,η<sup>1</sup>-ddbfo)<sub>2</sub>(η<sup>1</sup>-ddbfo)<sub>2</sub>], [Mg<sub>4</sub>(μ<sub>3</sub>-OMe)<sub>2</sub>(μ,η<sup>2</sup>-ddbfo)<sub>2</sub>(μ,η<sup>1</sup>-ddbfo)<sub>2</sub>(η<sup>1</sup>-ddbfo)<sub>2</sub>(CH<sub>3</sub>OH)<sub>5</sub>], and [Mg<sub>4</sub>(μ<sub>3</sub>,η<sup>2</sup>-thffo)<sub>2</sub>(μ,η<sup>2</sup>-thffo)<sub>2</sub>(μ,η<sup>1</sup>-thffo)<sub>2</sub>{μ-OTi(DIPP)<sub>3</sub>}<sub>2</sub>] Aggregates
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Tetranuclear magnesium complexes with chelating alkoxo ligands have been synthesized with the aim of investigating coordinatively unsaturated magnesium sites able to bind TiX4 (X = Cl, OR), of the type necessary for the formation of the active centers in polymerization catalysts. The magnesium compound [Mg4(μ3,η2-ddbfo)2(μ,η2-ddbfo)2(μ,η1-ddbfo)2(η1-ddbfo)2]·2CH2Cl2 (1) (ddbfo = 2,3-dihydro-2,2-dimethyl-7-benzofuranoxide) was prepared by the reaction of MgBu2 with ddbfoH in dichloromethane. Complex 1 exists as a centrosymmetric tetranuclear species with two different types of magnesium centers corresponding to octahedral MgO6 and trigonal bipyramidal MgO5 geometry. Compound 1 is monoclinic, space group P21/c, with a = 12.053(2) Å, b = 13.323(3) Å, c = 17.069(3) Å, β = 98.50(3)o, and Z = 4. The reaction of 1 with methanol in tetrahydrofuran (THF) gave compound [Mg4(μ3-OMe)2(μ,η2-ddbfo)2(μ,η1-ddbfo)2(η1-ddbfo)2(CH3OH)5]·CH3OH·THF (2). During this reaction one of the two five-coordinate MgO5 centers in 1 is completed by a methanol molecule and becomes octahedral in 2. Species 2 belongs to the P21/n monoclinic space group, with a = 13.323(3) Å, b = 20.768(4) Å, c = 27.584(6) Å, β = 104.26(3)°, and Z = 4. Compound [Mg4(μ3,η2-thffo)2(μ,η2-thffo)2(μ,η1-thffo)2{μ-OTi(DIPP)3}2]·2CH2Cl2 (3) is formed as a result of substitution of two thffo (thffo = 2-tetrahydrofurfuroxide) ligands bonded to the five-coordinate magnesium atom in [Mg4(thffo)8] by bulky OTi(DIPP)3 (DIPP = diisopropylphenolate) groups. Crystals of 3 are monoclinic, space group P21/n, with a = 17.069(3) Å, b = 18.421(4) Å, 17.815(4) Å, β = 90.77(3)°, and Z = 4. The X-ray crystal structures of complexes 1−3 are discussed in terms of explaining the role of the coordinatively unsaturated magnesium site in chiral catalyst active center formation.
