Multiple Bonds between Main-Group Elements and Transition Metals. 104. (σ-Aryl)trioxorhenium(VII) Complexes: Syntheses, Structural Characterization, and Properties

Reaction of arylzinc reagents with 2 equiv of Re₂O₇ in THF affords in good yields the (σ-aryl)trioxorhenium(VII) complexes MesReO₃ (1) (Mes = 2,4,6-trimethylphenyl), XyReO₃ (2) (Xy = 2,6-dimethylphenyl), Ph(THF)ReO₃ (4) (Ph = phenyl), and TolReO₃ (5) (Tol = 4-methylphenyl). The weakly coordinated THF in 4 is easily removed to afford PhReO₃ (3). Complexes 3 and 4 are in a fast equilibrium, which is shifted toward 3, even in the presence of excess THF. The reaction of (Ph_f)₂Zn (Ph_f = pentafluorophenyl) with Re₂O₇ is catalyzed by ZnCl₂ and produces, after addition of quinuclidine, the five-coordinate complex (Ph_f)(quin)ReO₃ (6) (quin = N(CH₂CH₂)₃CH). The new organozinc reagent (Me₃SiO-Xy)ZnCl(THF) (7) (Me₃SiO-Xy = 2,6-dimethyl-4-(trimethylsiloxy)phenyl) reacts with Re₂O₇ to yield the deep yellow complex (Me₃SiO-Xy)ReO₃ (8) in high yields. Reaction of 8 with HCl gives the first, pale yellow hydroxyphenyl complex (Ho-Xy)ReO₃ (9) (HO-Xy = 2,6-dimethyl-4-hydroxyphenyl). The Lewis acidity of the Re center in the RReO₃ complexes depends on both electronic and steric factors of the aryl group R. Thus, 1 does not form adducts with THF and quinuclidine, whereas 6 is isolable only as a quinuclidine adduct. Compound 3 forms the labile THF adduct 4 and the strong quinuclidine adduct Ph(quin)Reo₃ (10). All compounds have been characterized by IR and ¹H, ¹³C, and ¹⁷O NMR spectroscopy. The ¹³C NMR data for the series 1–3 and 5 indicate that the ReO₃ group is a strong electron-withdrawing substituent. Complexes 1,4, and 8 have been characterized by single-crystal X-ray diffraction. Complex 1 crystallizes in two different space groups: monoclinic P2₁/m (type 1A) and orthorhombic Pnma (type 1B), from diethyl ether and n-pentane, respectively. Structure 1A has two different molecules, 1A1 and 1A2, in the unit cell, differing by the relative orientation of the phenyl plane to the ReO₃ fragment (1A: a = 8.210 (1) Å, b = 22.924 (3) Å, c = 8.141 (1) Å, β - 102.21 (1)°, Z = 6. 1B: a = 7.343 (2) Å, b = 10.166 (2) Å, c = 13.600 (3) Å, Z = 4). Complex 4: monoclinic P2₁/m, a = 8.580 (1) Å, b = 11.562 (1) Å, c = 12.407 (2) Å, β = 109.34 (7)°, Z = 4. Complex 8: monoclinic P2₁/m, a = 8.245 (1) Å, b = 7.492 (1) Å, c = 11.991 (2) Å, β = 100.83 (<1)°, Z = 2. Molecules 1A, 1B, and 8 are pseudotetrahedral. In comparison, the rhenium center in 4 has a distorted trigonal-bipyramidal coordination geometry, with the three oxo ligands defining the equatorial plane. The Re-C bond length is noticeably shorter in molecule 1A1 (2.006 (9) Å) than in 1A2 (2.063 (7) Å), 1B (2.075 (8) Å), and 8 (2.04 (1) Å). The different geometry in 4 has no influence on the Re-C bond length (2.071 (3) Å). Thermal degradation of 3 at 60–80 °C gives only biphenyl, indicating an intermolecular decomposition pathway. In comparison, 1 and 2 are stable up to 200 °C. This remarkable difference in thermal stability can be attributed to increased steric hindrance, thus avoiding intermolecular coupling decomposition.