Chemistry of diamino-ligated methylpalladium(II) alkoxide complexes: Syntheses, X-ray crystal structures, and hydrogen-bond formation

Article abstract of DOI:10.1021/ja00149a017

The reaction of diamino-ligated dimethylpalladium(II) complexes [Pd(Me)₂(N～N)] (N～N = tmeda, bpy) with an equimolar amount of 1,1,1,3,3,3-hexafluoro-2-propanol or (substituted) phenols affords the new complexes [Pd(Me)(OR)(N～N)] (N～N = tmeda, R = CH(CF₃)₂ (1), C₆H₅ (3), C₆H₄-4-NO₂ (4), C₆H₄-2-OH (11)); N～N = bpy, R = CH(CF₃)₂ (2)). These methylpalladium(II) alkoxides and aryloxides are isolated in high yields as yellow-orange solids and are remarkably thermally stable. Mono- or bidentate ligands can substitute the tmeda ligand in 1 to afford a variety of methylpalladium alkoxide complexes [Pd(Me)(OCH(CF₃)₂)L₂] (L₂ = bpy (2), Ph₂PCH₂CH₂-NMe₂ (12), dppe (13), 2PMe₃ (14)). Crystals of 1, 3, and 4 have been subjected to X-ray diffraction studies. Crystals of 1 are monoclinic, space group P2₁/c, with unit-cell dimensions a = 8.2054(5) ?, b = 17.2310(9) ?, c = 11.1191-(12) ?, β = 105.701(7)°, and Z = 4. Crystals of 3 are orthorhombic, space group Pbca, with unit-cell dimensions a = 10.572(2) ?, b = 16.446(2) ?, c = 17.029(3) ?, and Z = 8. Crystals of 4 are orthorhombic, space group Pbca, with unit-cell dimensions a = 11.918(5) ?, b = 12.089(3) ?, c = 22.684(7) ?, and Z = 8. The molecular structures of these complexes show them to be square-planar species, and in 1 the C_β-H unit of the fluorinated alkoxide is directed toward the palladium center (H?Pd = 2.89(3) ?), in what can be interpreted as the incipient stage of a β-hydrogen elimination. The palladium alkoxide or aryloxide complexes 1-4 when treated with 1 equiv of the corresponding alcohol or (substituted) phenol afford O-H?O hydrogen-bonded adducts [Pd(Me)(OR)(N～N)]·-HOR (N～N = tmeda, R = CH(CF₃)₂ (5), C₆H₅ (7), C₆,H₄-4-NO₂ (8); N～N = bpy, R = CH(CF₃)₂ (6), C₆H₅ (9)). The X-ray molecular structures of [Pd(Me)(OC₆H₅)(tmeda)]·HOC₆H₅ (7) and [Pd(Me)(OC₆H4-4-NO₂)(tmeda)]·-HOC ₆H4-4-NO₂ (8) reveal that the aromatic alcohol is associated with the oxygen atom of the aryloxide through O-H?O hydrogen bonding. Crystals of 7 are monoclinic, space group P2₁/c, with unit-cell dimensions a = 15.5556-(13) ?, b = 11.0416(10) ?, c = 12.0211(11) ?, β = 91.343(8)°, and Z = 4. Crystals of 8 are monoclinic, space group P2₁/c, with unit-cell dimensions a = 8.7029(5) ?, b= 15.6384(11) ?, c = 16.5188(9) ?, β = 90.096(5)°, and Z = 4. Comparison of the solid state structures of [Pd(Me)(OC₆H₄-4-NO₂)(tmeda)] (4) and its HOC₆H₄-NO₂ adduct (8) reveal that an electron-withdrawing substituent on the aryloxide ring leads to geometric changes in the Pd-O-C unit. Proton NMR for 5-9 and the X-ray structural data of 7 and 8 indicate that the O-H?O hydrogen bonding in these adducts is strong both in solution and in the solid state. The thermodynamic parameters for the alkoxide-alcohol exchange in the adduct [Pd(Me)(OCH(CF₃)₂)(tmeda)]·HOCH(CF ₃)₂ (5) have been determined with NMR spin saturation transfer techniques and provide evidence for a five-coordinate species being the key intermediate in this intramolecular exchange reaction. Furthermore, the thermodynamic parameters for adduct formation, obtained by the Scatchard method, quantify the O-H?O hydrogen bond as being strong.