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
journal contributionposted on 2000-11-03, 00:00 authored by Russell P. Hughes, Jason S. Overby, Alex Williamson, Kin-Chung Lam, Thomas E. Concolino, Arnold L. Rheingold
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(RF) [RF = 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 RFI in the presence of light to give (TMEDA)PdI(RF) [RF = 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.