STRUCTURAL RELATIONSHIPS BETWEEN MONOBIS(ALKOXIDES) AND DIMERIC BIS(ALKOXIDES) AND BIS(ARYLOXIDES) OF CALCIUM AND BARIUM - X-RAY CRYSTAL-STRUCTURES OF M(OC6H2-T-BU2-2,6-ME-4)2(THF)3THF (M=CA, BA) AND [CA(MU-OR)(OR)(THF)]2.2(TOLUENE) [R=C(C6H5)2CH2C6H4CL-4]

Soluble alkoxides and aryloxides of calcium and barium have been prepared from the reaction of potassium salts of the alcohols and phenols with alkaline-earth diiodides. Reaction of the bulky aryloxide ligand [OC6H2-t-Bu2-2,6-Me-4]- with CaI2 in THF generates a hydrocarbon-soluble compound of the formula Ca(OAr)2(THF)3.THF. Single-crystal X-ray diffraction at -172-degrees-C reveals it as a 5-coordinate monomeric species, in which the ligating oxygen atoms form a distorted trigonal bipyramidal arrangement about the calcium. The axis is formed by two THF ligands, with a nearly linear O-Ca-O angle of 177-degrees; the third THF and the two aryloxide ligands lie in the equatorial plane, with an RO-Ca-OR angle of 152-degrees. Use of [OC6H2-t-BU2-2,6-Me-4]- with BaI2 generates the Ba(OAr)2(THF)3.THF complex that X-ray crystallography reveals is isostructural with the calcium analog. In the barium aryloxide, the axis is again formed by two THF ligands, with an essentially linear O-Ba-O angle of 179-degrees. The third THF and the two aryloxide ligands lie in the equatorial plane, with an RO-Ba-OR angle of 150-degrees. Reaction of the bulky alkoxide [OC(C6H5)2CH2C6H4Cl-41- with CaI2 and recrystallization of the product from THF also produces a monomeric 5-coordinate species, analogous to the aryloxides. In the Ca(OR)2(THF)3.THF complex, the axis is formed by two THF ligands, with an O-Ca-O angle of 163-degrees. The third THF and the two alkoxide ligands lie in the equatorial plane; the RO-Ca-OR angle of 126-degrees is markedly reduced from that for the analogous aryloxide. Recrystallization of the monomer from toluene partially desolvates the complex and generates a calcium species of the formula Ca(OR)2(THF). In the solid state it exists as a dimer, [Ca(mu-OR)(OR)(THF)]2, with each calcium coordinated by a single terminal and two bridging alkoxide ligands and a single THF. A systematic comparison of structural relationships among these complexes emphasizes the contribution of ligand packing to their structures.