The role of ancillary ligands and of electron poor alkenes and alkynes in stabilizing Pd(0) derivatives: A comparative study

Abstract

a b s t r a c tThe peculiar characteristics of the ligand neocuproine (2,9-dimethylphenanthroline) allow a number of exchange equilibrium studies between the low valence complex [Pd(g 2 -nq)(Neocup)] (nq = naphthoquinone; Neocup = neocuproine) and several alkenes and alkynes. A new order of stability which compares differently unsaturated molecules was established. An overview of the factors governing the stability of palladium(0) alkene and alkyne derivatives as a function of the steric and electronic characteristics of both the unsaturated molecule and the ligand becomes accessible and a comparison with the previously determined order was therefore feasible. Such a comparison enlightens the importance of the substituent methyl groups in ortho position of the hetero-aromatic ring which represents the molecular fragment common to all the ligands considered. Taking advantage of the steric requirements of the alkene tmetc (tmetc = tetramethylethylenetetracarboxylate) a kinetic investigation of the reaction between the olefin itself and the complexes [ -deta)(Neocup)] (deta = but-2-ynedioc acid diethyl ester) were also reported in the present paper. The structure of the latter represents the first example of a palladium(0) complex in which the N 2 C 2 donor set around the metal centre is supported by a chelating g 2 -alkyne.

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…A comparative study quite recently appeared [26] which, while confirming the stabilization capability of the olefins hitherto studied [30][31][32], also analyzes the influence of the nature of the ancillary ligands in determining the stability constants. The olefin stability order can be summarized as follows, while the comparison that takes into account the nature of some ancillary ligands is reported in Table 1 [26].…”

“…Protocol (a): This protocol, consisting in reacting the complex Pd 2 (DBA) 3 ÁCHCl 3 [23] (DBA = dibenzyliden-acetone) with the appropriate ligand and olefin in anhydrous acetone under inert atmosphere, represents the first synthetic method [24,25] which is still widely employed [26]. It is noteworthy that the complex Pd 3 (TBAA) 3 ÁCHCl 3 [27] (TBAA = tribenzylidenacetyl-acetone) can be used as an alternative starting substrate [24].…”

“…As will be discussed later, the coordinative capability of the electron-poor olefins can be very different. Therefore, different derivatives can be synthesized simply by adding an appropriate and better coordinating olefin to a solution of a complex containing a labile alkene [26,30,31].…”

“…In particular, ancillary bidentate ligands with one trivalent sulfur atom [49,50] coordinating a low-valent palladium bearing Z-type olefins may display apparent olefin rotation when the residual metallophilic lone pair on sulfur attacks the metal centre [4,26,32,39]. Notably, it has been Scheme 2.…”

“…Taking advantage from the peculiar steric characteristics of the tetramethyl-ethene-tetracarboxylate (tmetc) the olefin exchange equilibria were also studied from the kinetic point of view [26,31,32,34,39,43,[63][64][65]. It has been shown that the rates of exchange are strongly dependent on the steric requirement of the ancillary ligand and the mechanism is associative in nature.…”