Thiocarboxylic acids, such as selenocarboxylic acids, exist predominantly in the thioxo form (RCSOH, thion acid) in polar solvents such as tetrahydrofuran (THF) at temperatures below −50 °C. Tellurocarboxylic acids (5) were observed for the first time by acidolysis of the corresponding cesium tellurocarboxylates with hydrogen chloride. The telluroic acids (6) exist predominantly in the telluroxo form (RCTeOH, telluron acid) in THF at temperatures below −70 °C. Telluron acids were reddish to blue violet for the aliphatics (R = alkyl) and dark green for the aromatics (R = aryl) and reacted with aryl isocyanates at −70 °C to give crystalline acyl carbamoyl tellurides in good yields.

A series of rubidium and cesium tellurocarboxylates (2 and 3) were synthesized from the reaction of O-trimethylsilyl tellurocarboxylates (1) with rubidium and cesium fluorides in moderate isolated yields and characterized by 1H, 13C, and 125Te NMR spectroscopy and X-ray diffraction. The carbon–oxygen distance [1.235(6) Å] of the tellurocarboxyl group of crystalline cesium 2-methoxybenzenecarbotelluroate (3f) showed a typical value of the carbon–oxygen double bond. The crystal packing of 3f shows the possibility of an interaction between the cesium cation and the aromatic ring carbon. The thio- and selenocarboxyl groups of the corresponding sulfur and selenium isologues (4 and 5) also indicate a double bond [1.231(4) and 1.226(5) Å] between the carbon and oxygen, respectively. The salts (2 and 3) readily react with iodomethane to afford the corresponding Te-methyl tellurocarboxylates in good yields. The reaction between 3 and tetramethylammonium chloride was found to give a novel quaternary ammonium salt (8) of tellurocarboxylic acid, where the dihedral angle between benzene ring and the tellurocarboxyl group of 8 is ca. 90°. Crystallographic data for 3f, 4, 5, and 8 are as follows. 3f: P21/n, a = 7.182(2), b = 12.161(5), c = 12.151(3) Å, β = 92.57(2)°, V = 1060.2(6) Å3, dcalcd = 2.479 g cm−3, Z = 4, R = 0.035, Rw = 0.043. 4: Pbca, a = 8.995(2), b = 29.936(2), c = 7.291(1) Å, V = 1963.3(5) Å3, dcalcd = 2.030 g cm−3, Z = 8, R = 0.024, Rw = 0.021. 5: Pbca, a = 9.290(1), b = 30.207(2), c = 7.193(2) Å, V = 2018.5(3) Å3, dcalcd = 2.284 g cm−3, Z = 8, R = 0.029, Rw = 0.020. 8: P212121, a = 11.844(2), b = 14.014(2), c = 8.650(1) Å, V = 1435.8(3) Å3, dcalcd = 1.558 g cm−3, Z = 4, R = 0.044, Rw = 0.047.

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The first synthesis and characterization of sodium and potassium tellurocarboxylates were achieved by the reaction of acyl chlorides with the corresponding alkali metal tellurides. The salts are yellow to red solids or oils. The aliphatic derivatives are very sensitive toward oxygen and very thermally sensitive. However, the aromatic derivatives are relatively stable and can be stored for 1 week below -5 degrees C and under oxygen-free conditions. The most practical method for the synthesis of Te-alkyl telluroesters was established by the reaction of the sodium and potassium tellurocarboxylates with alkyl iodides at 0 degrees C. The first X-ray structural analysis of telluroesters (RCOTeR') was carried out for Te-methyl 4-chlorobenzenecarbotelluroate (4-ClC(6)H(4)COTeCH(3)), whose crystals are monoclinic (P2(1)/a) with a = 5.975(3) Å, b = 14.517(2) Å, c = 10.617(3) Å, beta = 92.74(3) degrees, V = 919.8(4) Å(3), and Z = 4. The molecule was nearly planar. The C=O and C-Te bond lengths are 1.204(3) and 2.153(3) Å, respectively, indicating C=O double and C-Te single bonds. The C-Te-C angle of the C(O)TeCH(3) moiety is close to a right angle (92.3 degrees ), much more narrow compared with those [C-E-C, E = O (>105 degrees ), S (>102 degrees ), Se (>95)] of common esters and thio- and selenoesters (ArC(O)ER', E = O, S, Se; R' = alkyl). The nu(C=O) bands and the (13)C=O and (125)Te NMR spectra of the sodium and potassium tellurocarboxylates are discussed in comparison with those of other alkali metal or oxygen, sulfur, and selenium isologues.

Rubidium and cesium selenocarboxylates (2 and 3) were found to be synthesized in good to high yields from the reaction of O-trimethylsilyl selenocarboxylates with rubidium and cesium fluorides. The spectroscopic properties of 2 and 3 are also given.