Trigonal-Planar Zinc(II) and Cadmium(II) Tris(phenoxide) Complexes.
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Biomedical subjects
Sharon A. Niezgoda
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Structural Diversity in Monomeric Cadmium Phenoxides.
Three monomeric Cd(II) phenoxide complexes have been prepared by reacting Cd[N(SiMe(3))(2)](2) with 2 equiv of 2,6-disubstituted phenols bearing sterically bulky tert-butyl or phenyl groups. The strongly coordinating solvents THF, tetrahydrothiophene (THT), and pyridine used for these reactions were incorporated into the metal's coordination sphere, leading to complexes with a general formulation of Cd(O-2,6-R(2)C(6)H(3))(2)(solv)(2)(-)(3). The Cd complexes obtained have been characterized crystallographically and have been found to adopt differing solid-state geometries. The X-ray crystal structure of Cd(O-2,6-(t)BuC(6)H(3))(2)(THF)(2), 1, previously reported by Buhro, displayed unusual square-planar coordination of the metal center. Complex 2, Cd(O(t)BuC(6)H(3))(2)(THT)(2), has been found to take on the same square-planar geometry, even with the more strongly donating thioether ligand. Alternatively, complex 3, Cd(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), has been found to adopt distorted-tetrahedral geometry, quite similar to its zinc congener. The O(1)-Cd-O(2) angle between the phenoxide ligands in 3 is 150.1(2) degrees, and the angle between the ether ligands is 83.1(3) degrees. When the strongly basic solvent pyridine was used, a five-coordinate complex 4, Cd(O-2,6-(t)BuC(6)H(3))(2)(py)(3), was isolated. This complex 4 is best described as having trigonal bipyramidal geometry with the phenoxide ligands and one pyridine defining the equatorial plane and two axial pyridine ligands having an angle of 169.7(2) degrees. The angle between the phenoxide ligands in 4 is 156.1(2) degrees. These complexes 1-4 have also been characterized in noncoordinating solvent solutions by (1)H, (13)C, and (113)Cd NMR spectroscopy and have been found to contain labile donor ligands. Preliminary studies indicate that, in a noncoordinating solvent, a rapid equilibrium exists between species with and without coordinated donor solvent ligands.
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Syntheses, Structures, and Binding Constants of Cyclic Ether and Thioether Adducts of Soluble Cadmium(II) Carboxylates. Intermediates in the Homopolymerization of Oxiranes and Thiiranes and in Carbon Dioxide Coupling Processes.
A synthetic methodology for the preparation of a large variety of eta(3)-HB(3-Phpz)(3)Cd(acetate) adducts is presented which involves replacement of toluene in the eta(3)-HB(3-Phpz)(3)Cd(acetate) solvate complex by the appropriate cyclic ether or cyclic thioether. In this manner, adducts of THF, dioxane, propylene oxide, cyclohexene oxide, and propylene sulfide were isolated. The solid-state structures of several of these complexes were determined by X-ray crystallography, revealing a six-coordinate complex where the acetate ligand is shown to be fairly symmetrically bonded to the cadmium center. In methylene chloride solution, the cyclic ether or thioether readily dissociates to afford the five-coordinate complex, as demonstrated by (113)Cd NMR. A quantitative assessment of the binding of these base adducts of eta(3)-HB(3-Phpz)(3)Cd(acetate) was determined by measuring the temperature dependence of the equilibrium constants for the five- and six-coordinate derivatives. The presence of one sharp (113)Cd resonance in this equilibrium mixture is indicative of rapid intermolecular exchange between the five- and six-coordinate complexes when compared to the chemical shift differences in these two species ( approximately 6600 Hz at 89 MHz). The order established for ether binding is THF > dioxane > propylene sulfide > cyclohexene oxide >/= propylene oxide, with DeltaH degrees and DeltaS degrees spanning the ranges -27.7 to 24.3 kJ/mol and -89.7 to -94.1 J/(mol K). The epoxide and thioepoxide adducts were shown to serve as models for the initiation step in the copolymerization of epoxides with carbon dioxide catalyzed by metal carboxylates. That is, the carboxylate ligand was shown to ring-open the epoxide or thioepoxide, subsequently affording polyethers or polythioethers with ester end groups. By way of contrast, in the presence of CO(2) and epoxides, this system led to cyclic carbonate production.
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