Facile Synthesis and Structure of Heavy Alkali Metal Thiocarboxylates: Structural Comparison with the Selenium and Tellurium Isologues

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Abstract
Potassium, rubidium, and cesium thiocarboxylates were found to be synthesized by the reaction of thiocarboxylic acid or its O-trimethylsilyl esters with KF, RbF, and CsF. Tetramethylammonium thiocarboxylates were prepared in good yields by the reaction of sodium thiocarboxylates with tetramethylammonium chloride. The structures of potassium benzene-, 2-methoxybenzene-, and 4-methoxybenzenecarbothioates, rubidium 2-methoxybenzenecarbothioate, tetramethylammonium 2-methoxy- and 2-trifluoromethylbenzenecarbothioates, potassium 2-methoxybenzenecarboselenoate, and rubidium 2-methoxybenzenecarbotelluroate were characterized by X-ray structural analysis. All of these alkali metal salts exhibit a dimeric structure in which the oxygen and/or sulfur atom is associated with the metal of the opposite molecule, while the tetramethylammonium thiocarboxylates are monomeric. In potassium 2-methoxybenzenecarbothioate, the two thiocarboxylate groups chelate to the potassium atoms above and below the plane which involves the thiocarboxylate groups. In potassium 4-methoxybenzenecarbothioate, one thiocarboxylate group chelates to potassium. Without exception, the o-methoxy oxygen intermolecularly coordinates to the metal of the opposite molecule. The C−O bond lengths of the thiocarboxylate group are in the range 1.22−1.24 Å, which is slightly longer than those of common thioesters. The C(sp2)−S distances are in the range 1.70−1.72 Å, which is close to that of a typical C−S single bond and suggests that the negative charge may be somewhat localized on the sulfur atom. The interaction number of the metals with oxygen and sulfur atoms was 7 for K and 8 for both Rb and Cs. The dihedral angle between the thiocarboxyl group and the phenyl ring is substantially increased by the introduction of the o-methoxy group. In these alkali metal thio-, seleno-, and tellurocarboxylates, only cesium 2-methoxybenzenecarbotelluroate showed an intermolecular nonbonding interaction between the cesium and the phenyl ring carbons.

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