Benzylic Complexes of Palladium(II): Bonding Modes and Pentacoordination for Steric Relief

Abstract

A large variety of α-(pentafluorophenylmethyl)benzylic palladium complexes with different ligands have been synthesized and characterized. Multinuclear NMR spectroscopic data allow to determine the σ-or η 3-benzylic nature of the complexes in solution. The adoption of either coordination mode is a function of the number of ligands coordinated to palladium and, remarkably, the presence of bulky phosphines favors the adoption of a bidentate η 3-benzylic mode and palladium pentacoordinated complexes. Experimental data and DFT calculations indicate that this fivecoordination could alleviate the steric hindrance of two cis bulky phosphines. The benzylic complexes show a rich fluxional behavior that involves both ligand exchange and σ-to η 3-benzylic interconversion.

Highly efficient vinylic addition polymerization of 5-vinyl-2-norbornene using benzylic palladium complexes as precatalysts

“…We have been working for some time with η 3 -benzylic palladium complexes and studying their structure and reactivity. 22,23 We rationalized that in the context of the vinylic addition (VA) polymerization of norbornenes these complexes might be ideal precursors since the η 3 -benzyl moiety can easily interconvert into the σ-benzyl form, leaving a coordination site available and facilitating the subsequent initiation by coordination and insertion of NB. Because of the easy transformation into the σ-benzyl, driven by the recovery of the aromaticity of the ring, these complexes might be superior precatalysts than the more commonly used η 3 -allyl palladium complexes (Scheme 2).…”

“…We used commercial VNB which is a mixture of endo:exo isomers in a 80:20 ratio. The isolated complexes [Pd(η 3 -C 6 H 5 CHCH 2 C 6 F 5 )(PPh 3 ) 2 ]X (X = BF 4 , BAr F 4 ), 22 do not polymerize VNB. However, analogous cationic species with a lower amount of phosphine generated in situ from a mixture of 1/PPh 3 /NaBAr F 4 (ratio Pd:PPh 3 :BAr F 4 -= 1:1:1) led to a polymer in 65% yield (entry 1, Table 1).…”

“…The formation of the monomeric complex 2 occurs when one equivalent of PCy 3 per palladium reacts with 1 (Scheme 3). Complex 2 is a η 3 -benzylic derivative, as shown by the characteristic chemical shift of C α at 53 ppm, 22 and it is stable only at very low temperature. As it is observed for most benzylic phosphine complexes described here, 2 undergoes a fast syn-anti interconversion in solution even at 195 K probably via a suprafacial sigmatropic rearrangement (see ESI for details).…”

α-Substituted Benzylic Complexes of Palladium(II) as Precursors of Palladium Hydrides

“…We have previously prepared a variety of benzylic palladium complexes with different ligands and an α-methylpentafluorophenyl substituent (Scheme 2). 5 These derivatives eventually decompose by β-H elimination at very different rates, and they are suitable model complexes to analyze this competing reaction.…”

“…A rich fluxional behavior is also observed in some cases that involve ligand exchange as well as η 3 -σ benzylic exchange. 5 As a result of these manifold situations, the stability of the complexes is quite different in solution, but all of them decompose cleanly by β-H elimination to give pentafluorostilbene 9 (the trans isomer is always the major one) and a palladium hydride that has been detected in some cases. This hydride either decomposes or is transferred to another complex molecule to give, by reductive elimination, the reduction product 10 (eq 1).…”