28538-51-4Relevant academic research and scientific papers
Multiple bonds between main-group elements and transition metals. 104. (σ-aryl)trioxorhenium(VII) complexes: Syntheses, structural characterization, and properties
De Méric De Bellefon, Claude,Herrmann, Wolfgang A.,Kiprof, Paul,Whitaker, Claire R.
, p. 1072 - 1081 (2008/10/08)
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 (Phf)2Zn (Phf = pentafluorophenyl) with Re2O7 is catalyzed by ZnCl2 and produces, after addition of quinuclidine, the five-coordinate complex (Phf)(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) A?, b = 22.924 (3) A?, c = 8.141 (1) A?, β = 102.21 (1)°, Z = 6. 1B: a = 7.343 (2) A?, b = 10.166 (2) A?, c = 13.600 (3) A?, Z = 4). Complex 4: monoclinic P21/c, a = 8.580 (1) A?, b = 11.562 (1) A?, c = 12.407 (2) A?, β = 109.34 (7)°, Z = 4. Complex 8: monoclinic P21/m, a = 8.245 (1) A?, b = 7.492 (1) A?, c = 11.991 (2) A?, 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) A?) than in 1A2 (2.063 (7) A?), 1B (2.075 (8) A?), and 8 (2.04 (1) A?). The different geometry in 4 has no influence on the Re-C bond length (2.071 (3) A?). 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.
