Palladium and platinum diphenoxide and aryl phenoxide complexes with amine donors: Effect of hydrogen bonding on structure and properties

Article abstract of DOI:10.1021/om00060a032

Several palladium and platinum phenoxides, [Pd(OPh)₂L₂] (L₂ = tmeda (1), (pyrrolidine)₂ (2), (N-methylpyrrolidine)₂ (3)) and [M(OPh)(NCN)] (NCN = C₆H₃{CH₂NMe₂}₂-2,6); M = Pd (5), Pt (6)) have been synthesized and characterized with special emphasis on the role of both N-H?O and O-H?O hydrogen bonding in the resulting structures of these metal phenoxides. The [Pd(OPh)₂L₂] (1-3) complexes were obtained from the reaction of Pd(OAc)₂ with 2 equiv of sodium phenoxide in the presence of the neutral ligand. The terdentate N-C-N-bound arylmetal complexes, [M(OPh)(NCN)], 5 and 6, were synthesized by reacting [M(C₆H₃{CH₂NMe₂} ₂-2,6)(H₂O)]BF₄ with sodium phenoxide. Both 2,5, and 6 react with excess phenol to form the corresponding phenol adducts, 4, 7, and 8, respectively. The structures of the phenoxides 2 and 5 and the phenol adducts 4 and 7 were studied by X-ray diffraction methods to establish their stereochemistry as well as the nature and structural details of the hydrogen bonding in the phenol adducts. Crystals of trans-[Pd(OPh)₂(pyrrolidine)₂] (2) are tetragonal with a = b = 18.6615 (15) ?, c = 10.7713 (6) ?, space group I4₁/a, Z = 8, and R = 0.026 for 1774 reflections with I ≥ 2.5σ(I); crystals of trans-[Pd-(OPh)₂(pyrrolidine)₂]·HOPh (4) are monoclinic with a = 6.4039 (3) ?, b = 15.497 (1) ?, c = 15.085 (1) ?, β = 99.832 (4)° space group P2₁/c, Z = 2, and R = 0.032 for 2224 reflections with I ≥ 2.5σ(I); crystals of [Pd(OPh)(C₆H₃{CH₂NMe₂} ₂-2,6)] (5) are monoclinic with a = 6.1490 (2) ?, b = 12.0595 (6) ?, c = 23.1829 (10) ?, β = 97.251 (3)° space group P2₁/c, Z = 4, and R = 0.0223 for 3387 reflections with I ≥ 2.5σ(I); and crystals of [Pd(OPh)(C₆H₃{CH₂NMe₂} ₂-2,6)]·HOPh (7) are monoclinic with a = 8.5400 (5) ?, b = 12.0445 (7) ?, c = 22.4473 (14) ?, β = 100.779 (5)°, space group P2₁/n, Z' = 4, and R = 0.0428 for 3081 reflections with 7 ≥ 2.5σ(I). Palladium phenoxide 2 has a unique dimeric structure consisting of two trans-Pd-(OPh)₂(pyrrolidine)₂ monomers being held together by four identical (pyrrolidine)N-H?O(phenoxide) hydrogen bonds (N?O = 2.866 (3) ?). The Pd atoms are in close proximity of each other, but the resulting Pd?Pd distance (3.0960 (3) ?) points to some repulsion between the metal centers. In the corresponding phenol adduct 4 the dimeric structure is broken down by interaction of two phenol molecules resulting in two (phenol)O-H?O(phenoxide) hydrogen bonds (2.638 (4) ?). The structural features of the NCN-bound palladium phenoxides, 5 and 7, reflect the effect of changing the trans ligand from phenoxide to aryl on the Pd-phenoxide bonding. In 5 a very short (phenoxide)C-O bond of 1.305 (3) ? is observed which lengthens to 1.330 (6) ? in the phenol adduct 7. The strong hydrogen bonding in 7 is indicated by the short (phenoxide)O?O (phenol) bond distance of 2.567 (6) ? (cf. 2.638 (4) ? in 4). These results have been used to synthesize new monodentate N-C-N-metal catecholate complexes, [M(C₆H₃{CH₂NMe₂} ₂-2,6)-(OC₆H₄OH-2)] (9 (M = Pd), 10 (M = Pt)), which contain an intramolecular O-H?O hydrogen bond. ¹H NMR studies show that 2 is monomeric in solution. Exchange between coordinated phenoxide and associated phenol in 4 remains slow on the NMR time scale (up to 97°C) but occurs on the laboratory time scale (experimente with pentadeuteriophenol). The arylpalladium phenoxide complex 7 undergoes fast exchange between phenoxide and associated phenol indicating the large trans effect of the aryl ligand (cf. long Pd-O bond in 5). This exchange is slow on the ¹H NMR time scale in the corresponding platinum complex 8. Similarly, intramolecular phenol/phenoxide exchange is fast on the ¹H NMR time scale in the Pd-catecholate 9 but slow in the Pt-catecholate 10. Thermodynamic parameters for the association of phenol with the palladium diphenoxide complexes 2 and 3 have been determined by means of ¹H NMR spectroscopy.