Chemistry of Diamino-Ligated Methylpalladium(II) Alkoxide Complexes: Syntheses, x-ray Crystal Structures, and Hydrogen-Bond Formation

Abstract

The reaction of diamino-ligated dimethylpalladium(I1) complexes [Pd(Me)z(N-N)] (N-N = tmeda, bpy) with an equimolar amount of 1,1,1,3,3,3-hexafluoro-2-propanol r (substituted) phenols affords the new complexes [Pd(Me)(OR)(N-N)] (N-N = tmeda, R = CH(CF3)z (l), C6H5 (3), CsH4-4-NOz (4), CsH4-2-OH (11)); N-N = bpy, R = CH(CF3)2 (2)). These methylpalladium(I1) alkoxides and aryloxides are isolated in high yields as yelloworange solids and are remarkably thermally stable. Monoor bidentate ligands can substitute the tmeda ligand in 1 to afford a variety of methylpalladium alkoxide complexes [Pd(Me)(OCH(CF3)z)L2] (Lz = bpy (2), Ph2PCHzCH2NMez (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 P2l/c, with unit-cell dimensions a = 8.2054(5) A, b = 17.2310(9) A, c = 11.1191(12) A, P = 105.701(7), and Z = 4. Crystals of 3 are orthorhombic, space group Pbca, with unit-cell dimensions a = 10.572(2) A, b = 16.446(2) A, c = 17.029(3) A, and Z = 8. Crystals of 4 are orthorhombic, space group Pbca, with unit-cell dimensions a = 11.918(5) A, b = 12.089(3) A, c = 22.684(7) A, and Z = 8. The molecular structures of these complexes show them to be square-planar species, and in 1 the Cp-H unit of the fluorinated alkoxide is directed toward the palladium center (H-..Pd = 2.89(3) A), in what can be interpreted as the incipient stage of a P-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), C&-4-N02 (8); N-N = bpy, R = CH(CF3)z (6), C6H5 (9)). The X-ray molecular structures of [Pd(Me)(OC6H5)(tmeda)].HOCsHs (7) and [Pd(Me)(oCsH4-4-N02)(tmeda)]*HOC6H4-4-Noz (8) reveal that the aromatic alcohol is associated with the oxygen atom of the aryloxide through 0-H-0 hydrogen bonding. Crystals of 7 are monoclinic, space group P21/c, with unit-cell dimensions a = 15.5556(13) A, b = 11.0416(10) A, c = 12.0211(11) A, P = 91.343(8), and Z = 4. Crystals of 8 are monoclinic, space group P2l/c, with unit-cell dimensions a = 8.7029(5) A, b = 15.6384(11) A, c = 16.5188(9) A, P = 90.096(5), and Z = 4. Comparison of the solid state structures of [Pd(Me)(OCsH4-4-NOz)(tmeda)I (4) and its HOc6H4-4-No~ adduct (8) reveal that an electron-withdrawing substituent on the aryloxide ring leads to geometric changes in the Pd-0-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(CFs)z (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.