2-Indolylphosphines, a New Class of Tunable Ligands:  Their Synthesis, Facile Derivatization, and Coordination to Palladium(II)

The generation of new metal complexes with potentially interesting properties provides the motivation for designing novel polydentate bridging ligands. Herein we report the syntheses of tertiary indolylphenylphosphines L, where L = diphenyl(3-methyl-2-indolyl)phosphine (P(C₆H₅)₂(C₉H₈N), 1), phenylbis(3-methyl-2-indolyl)phosphine (P(C₆H₅)(C₉H₈N)₂, 2), and bis(1H-3-indolyl)methane-(2,12)-phenylphosphine (P(C₆H₅)(C₁₇H₁₂N₂), 3). Ligands 1−3 were functionalized at the indolyl nitrogen with a variety of both electron-withdrawing and electron-donating groups. The solid-state structures of 1, 2, and N-functionalized indolylphosphines diphenyl(3-methyl-1-benzyl-2-indolyl)phosphine (P(C₆H₅)₂(C₉H₇N(CH₂C₆H₅)), N-Bz-1) and bis[1-(CH₂C₆F₅)-3-indolyl]methane-(2,12)-phenylphosphine (P(C₆H₅)(C₁₇H₁₀N₂[CH₂C₆F₅]₂), (N-F₅Bz)₂-3), are reported. The reaction of ligands 1−3 with 1 equiv of Pd(COD)Cl₂ led to the formation of Pd(II) complexes of the type [Pd(L)Cl(μ-Cl)]₂ (4, L = 1; 5, L = 2; 6, L = 3). The products were characterized by ¹H, ¹³C, and ³¹P NMR spectroscopy, mass spectrometry, and elemental analysis. X-ray crystallography established the dimeric structure of the products and confirmed the ability of the ligands to serve, in the absence of base, as monodentate P-donors in reaction with a transition metal. The indolyl NH groups of the complexes 4−6 demonstrate a marked propensity for hydrogen bonding in the solid state.