Low valent palladium benzoquinone complexes bearing different spectator ligands. The versatile coordinative capability of benzoquinone
Graphical abstract
In the palladium(0) complexes benzoquinone can coordinate either η2 or η4 and the ensuing coordinative choice was easily inferred from the 1H and 13C NMR features. The benzoquinone hapticity is not easily predictable, although the η2 coordinative mode appears to be favored in the case of rigid spectator ligands.
Introduction
The Pd(0) olefin derivatives are of paramount importance among all the palladium catalysts since they represent or are often identified as the active catalytic species in a variety of cross-coupling reactions [1]. On the basis of the original work of Dewar and Chatt and Duncanson [2] the palladium(0)–olefin bond was extensively studied from theoretical, structural [3], and synthetic points of view [4]. In particular, our group have in some cases measured the degree of stability imparted to the palladium(0) complexes by different olefins. The evaluation of the stability was based on the equilibrium constant of the direct exchange between olefins according to the following reaction [5], [5](a), [5](b):
A summary of the general results suggesting a comprehensive order of coordinative properties of the most used electron-poor olefins based on the complex [Pd(η2-nq)(Neocup)] (nq = naphthoquinone; Neocup = neocuproine) together with an indication of the dependence of such an order on the nature of the ancillary ligands, was recently reported [6]. Finally a general overview on the features of the Pd(0) olefin derivatives bearing labile or hemilabile spectator ligands was reviewed [7]. However, among the number of studied complexes the benzoquinone derivatives are not very abundant [8] and no quantitative data on its coordinative capability can be found in the literature. This fact probably depends on the nature of benzoquinone whose hapticity might be either η2 or η4 and consequently, formation of dimeric [8c] or oligomeric species via the unengaged olefinic vinylic bond cannot be excluded a priori [8e]. Moreover a further complication can arise from the fact that benzoquinone and its isofunctional naphthoquinone can give dimeric oxygen bridged species [8d]. We therefore decided to prepare some palladium(0) complexes with benzoquinone with the aim of setting its coordinative capability in the rank of the olefin stability order so far assessed [6] and understanding the structural features of the synthesized derivatives. The benzo- and naphthoquinone palladium complexes we have prepared are reported in the following Scheme 1.
Section snippets
General considerations
The palladium(0) olefin complexes were obtained according to protocol (a) of Ref. [7] by the concomitant addition of the appropriate alkene (ol) and ligand (L–L′) to Pd2DBA3 [9] in anhydrous acetone under inert atmosphere (Ar), in the dark. The formation of the complexes was unequivocally apparent by comparison of their 1H and 13C NMR spectra with those of the free olefins and from their IR spectra. As a consequence of the marked metal–olefin back donation the chemical shifts of the protons and
Conclusions
We have synthesized some Pd(0) benzoquinone derivative bearing different spectator ligands. In any case, on the basis of well characterizing 1H and 13C NMR features it was possible to assign the coordinative hapticity of the olefin which in the case of N–S ligands seems to be somehow influenced by the rigidity of the latter.
In order to rate the coordinative strength of benzoquinone within the olefin stability order proposed so far, we have measured the equilibrium constant for the displacement
Experimental
All solvents were purified by standard procedures and distilled under argon immediately before use [17]. 1D- and 2D-NMR spectra were recorded using a Bruker 300 Avance spectrometer. Chemical shifts (ppm) are given related to TMS (1H and 13C NMR). UV–Vis spectra were recorded on a Perkin–Elmer Lambda 40 spectrophotometer equipped with a Perkin–Elmer PTP 6 (Peltier temperature programmer) apparatus. IR spectra were recorded on a Perkin–Elmer Spectrum One spectrophotometer.
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