Synthesis, Molecular Structures, and Dynamics of Primary and Secondary Fluoroalkyl Complexes of Palladium(II) with Tetramethylethylenediamine (TMEDA) Ligands. Evaluation of the Structural trans-Influences of Methyl and Fluoroalkyl Groups as Ligands within the Same Coordination Sphere

Reaction of (TMEDA)Pd(CH₃)₂ (TMEDA = tetramethylethylenediamine) with primary perfluoroalkyl iodides gives a mixture of products that depend markedly on the choice of solvent. In hexanes, (TMEDA)PdMe(R_F) [R_F = n-C₃F₇ (1); n-C₄F₉ (2)] is afforded in about 50% yield, with (TMEDA)Pd(CH₃)I (3) formed concurrently. In hexanes 3 precipitates from solution and does not react further, but with THF as the solvent 3 remains in solution and reacts with R_FI in the presence of light to give (TMEDA)PdI(R_F) [R_F = n-C₃F₇ (4); n-C₄F₉ (5)] and (TMEDA)PdI₂, as well as 1 and 2, respectively. Treatment of either 1 or 2 with I₂ yields the corresponding iodide compounds 4 and 5, with elimination of CH₃I, while an analogous reaction of 2 with concentrated HCl produces the chloride compound (TMEDA)PdCl(C₄F₉) (8). In contrast, reaction of (TMEDA)Pd(CH₃)₂ with the secondary iodide ICF(CF₃)₂ gives only one fluoroalkyl product, (TMEDA)Pd(CH₃)(i-C₃F₇) (6), in either hexanes or THF. Reaction of 6 with I₂ occurs with selective cleavage of the Pd−CH₃ bond to give the iodide product (TMEDA)PdI(i-C₃F₇) (7). The solid-state structures of 1−5, 7, and 8 have been confirmed by X-ray crystallography. Examination of bond distances illustrates that fluoroalkyl ligands have smaller trans-influences than methyl ligands, but larger trans-influences than iodide or chloride ligands toward the N-donor TMEDA ligand. This results in a shorter, presumably stronger Pd−fluoroalkyl bond being trans to a shorter, presumably stronger Pd−N bond. The principal steric effect of the perfluoroisopropyl group in the solid state is to generate an almost planar TMEDA ring. The solution NMR data for 1, 2, 4, 5, and 8 show resonances due to diastereotopic CF₂ groups below room temperature, with the CF₂ group acting as a remote sensor for formation of a stereocenter at the metal due to slowing of the inversion of the puckered five-membered metallacycle formed by the TMEDA ligand and Pd. The other TMEDA complexes show the same behavior below room temperature, with the exception of (TMEDA)PdI(i-C₃F₇) (7), in which the TMEDA ring is presumably almost planar in the ground state.