Elsevier

Inorganica Chimica Acta

Volume 361, Issue 11, 27 July 2008, Pages 3247-3253
Inorganica Chimica Acta

Lewis acidity of platinum(II)-based Baeyer–Villiger catalysts: An electrochemical approach

Dedicated to Professor Robert J. Angelici in honor of his outstanding contributions to inorganic chemistry.
https://doi.org/10.1016/j.ica.2007.10.050Get rights and content

Abstract

The electrochemical behavior of the Pt(II)-based Baeyer–Villiger catalysts of the general formulae [Pt(μ-OH)(PP)]2(BF4)2 (PP = dppe (1a), 2Fdppe (1 b), 4Fdppe (1c), dfppe (1d), dmpe (1e), depe (1f), dippe (1g), dtbpe (1h)) and [Pt(OH2)2(PP)](OTf)2 (PP = dppe (2a), 2Fdppe (2b), 4Fdppe (2c), dfppe (2d)) is reported. They exhibit irreversible reduction processes whose potentials reflect the Lewis acidity of the metal centres, showing (for the aromatic diphosphine complexes) overall relations with the number of fluorine atoms, with JPt–P, with the ν(Ctriple bondN) coordination shift of a ligand isocyanide probe and with the catalytic activity. Single-crystal X-ray diffraction analyses were carried out for [Pt(μ-OH)(4Fdppe)]2(BF4)2 (1c) and [Pt(μ-OH) (dippe)]2(BF4)2 (1g).

Graphical abstract

The electrochemical behavior of the Pt(II)-based Baeyer–Villiger catalysts of the general formulae [Pt(μ-OH)(PP)]2(BF4)2 (PP = dppe, 2Fdppe, 4Fdppe, dfppe, dmpe, depe, dippe, dtbpe) and [Pt(OH2)2(PP)](OTf)2 (PP = dppe, 2Fdppe, 4Fdppe, dfppe) is reported. They exhibit irreversible reduction processes whose potentials reflect the Lewis acidity of the metal centres. Single-crystal X-ray diffraction analyses were carried out for [Pt(μ-OH)(4Fdppe)]2(BF4)2 and [Pt(μ-OH) (dippe)]2(BF4)2.

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Introduction

Some of us have been interested in recent years in developing a class of platinum(II) complexes of general formula [Pt(μ-OH)(PP)]2(BF4)2 (with PP a chelating diphosphine ligand) as useful catalysts for the Baeyer–Villiger oxidation of ketones with hydrogen peroxide as oxidant in mild conditions.

Recently, we pointed out how the Lewis acidity of the metal centre plays an important role in the activity of such a class of catalysts by studying the effect of the electron-withdrawing power of the diphosphine ligand on the rate of oxidation of 2-methyl-cyclohexanone with H2O2 [1] in complexes of the above type. Specifically, we synthesized a series of complexes with partially fluorinated diphosphines of formula (C6H5−nFn)2P–(CH2)2–P(C6H5−nFn)2 [n = 2 (2Fdppe), 3 (3Fdppe), 4 (4Fdppe) or 5 (dfppe)] that displayed increasing catalytic activity as the number of fluorine atoms in the phosphine was increased.

It was then necessary to have a measure of the Lewis acidity of the complex to check the actual effect of the ligand on this parameter and to relate it to the activity of the catalysts.

In the past, a few different methods were developed to measure the Lewis acidity of a homogeneous catalyst, all of them concerning the coordination of a probe molecule and the change of a spectroscopic parameter which was assumed as an acidity measurement. The method developed by Childs [2] consists in the coordination of crotonaldehyde through the carbonyl oxygen at −20 °C in a chlorinated solvent; the difference in chemical shift (Δδ) of the proton bound to the C3 showed a linear dependence on the acidity of the metal centre. Beckett [3] modified the Gutmann method based on the classification of solvents by acceptor numbers [4] by measuring the change in 31P NMR of a phosphine oxide or sulphide coordinated to the metal catalyst. Both these methods need an excess of the Lewis acid in order to assure the complete coordination of the probe molecules along with anhydrous conditions, thus resulting in evident problems with our poorly soluble and hydrophilic complexes. In the meanwhile some of us have developed a method based on the observation that the shift (Δν¯) of the IR wavenumber of an isocyanide upon coordination is related to the Lewis acidity of the metal centre [5]. It was then interesting to find a different way to have a qualitative measure of the Lewis acidity that does not require the use of a probe molecule, but just the complex itself.

Electrochemical methods seemed to be a good way to solve the problem because they permit accurate and reproducible measurements and provide information on the electronic properties of the complexes without introducing any other molecule that can modify the original catalyst. In fact, redox potential–structure relationships have been recognized in coordination compounds and electrochemical parameters defined to measure the electron donor–acceptor properties of some types of metal centres and ligands [6], [7], [8], [9], [10].

In this paper, we report the preliminary results obtained in the application of cyclic voltammetry to a series of partially fluorinated aryl and alkyl diphosphine platinum(II) complexes studied previously by IR spectroscopy. The crystal structure of two of the complexes are also reported.

Section snippets

General procedures and materials

All work was carried out with the exclusion of atmospheric dioxygen under a dinitrogen atmosphere. Solvents were dried and purified according to standard methods. The complexes [Pt(μ-OH)(dppe)]2(BF4)2 (1a) [11], [Pt(μ-OH)(2Fdppe)]2(BF4)2 (1b), [Pt(μ-OH)(4Fdppe)]2(BF4)2 (1c), [Pt(μ-OH)(dfppe)]2(BF4)2 (1d) [1], [Pt(μ-OH)(dmpe)]2(BF4)2 (1e), [Pt(μ-OH)(depe)]2(BF4)2 (1f), [Pt(μ-OH)(dippe)]2(BF4)2 (1g), [Pt(μ-OH)(dcype)]2(BF4)2 (1h) [12], [Pt(OH2)2(dppe)](OTf)2 (2a), [Pt(OH2)2(2Fdppe)] (OTf)2 (2b),

Platinum(II)-based Baeyer–Villiger catalysts

The synthesis and spectroscopic characterization of the hydroxo-bridged Pt(II) complexes of the general formula [Pt(μ-OH)(PP)]2(BF4)2 (1ah), the corresponding bis-aquo derivatives [Pt(OH2)2(PP)](OTf)2 (2ad) and the isocyanide compounds [PtCl(Ctriple bondN–R)(PP)](BF4) (3ah) (R = 2,6-Me2C6H3) have been previously reported [11], [12], [13]. The structures of the compounds of the type 1, 2 and 3 are illustrated in Chart 1.

The 31P NMR data reported in Table 1 show a steady increase in the value of the

Conclusions

For the above series of related PtII diphosphine complexes, the reduction potential reflects the electron-donor character of the phosphine ligand and thus the Lewis acidity of the metal centre, as corroborated by the overall relations of that potential with the number of fluorine atoms in the diphosphine, with 1JPt–P and with the νCtriple bondN coordination shift of an isocyanide probe at related complexes. This suggests the potential application of the reduction potential to probe the reactivity of the Pt

Acknowledgements

The work has been supported by the Fundação para a Ciência e a Tecnologia (Portugal) and its POCI programme (FEDER funded) and by the University of Padova and M.I.U.R. (Italy). P.S. also thanks the AQUACHEM Network (HRTM project CMTN-CT-2003-503864) for a fellowship.

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