Synthesis and coordination chemistry of meta-perfluoroalkyl-derivatised triarylphosphines
Introduction
Following the introduction of the fluorous biphase approach to the heterogenisation of homogeneous catalysis [1], [2], a range of ligand systems and metal complexes have been derivatised with long perfluoroalkyl ‘ponytails’ and their solubilities and reactivities under fluorous biphasic conditions have been described [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Throughout this work, phosphorus(III) ligands, in particular phosphines, have been pre-eminent [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. We have described a general route to the synthesis of para-derivatised triaryl phosphine ligands [8] and have recently established the criteria for preferential perfluorocarbon solubility as well as the electronic influence of the perfluoroalkyl substituents on the coordination chemistry of these ligands [11]. Here, we extend this work to an analogous series of meta-substituted ligands to establish the steric and electronic implications of introducing the perfluoroalkyl substituents in this position.
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Experimental
Proton, 19F and 31P NMR spectroscopies were carried out on a Bruker ARX250 spectrometer at 250.13, 235.34 and 101.26 MHz or on a Bruker DRX 400 spectrometer at 400.13, 376.50 and 161.98 MHz. All chemical shifts are quoted in ppm using the high-frequency positive convention; 1H NMR spectra were referenced to external SiMe4, 19F NMR spectra to external CFCl3 and 31P NMR spectra to external 85% H3PO4. Assignments of the 19F NMR resonances were made using correlation experiments and follow the
Results and discussion
The syntheses of the meta-derivatised triaryl phosphines 2–4 with one, two and three perfluoroalkyl substituents respectively, via 3-bromo-perfluorohexylbenzene, (Scheme 1) is essentially the same as the route used to prepare the analogous para-derivatised compounds. The ligands were formed in 54–70% yields as air-stable, white solids. The spectroscopic data (Experimental) are entirely consistent with the formulations, in particular, each gave a single resonance in the 31P{1H} NMR spectrum (at
Conclusions
The introduction of perfluoroalkyl ponytails into the meta-positions of triarylphosphine ligands is straightforward. These ligands readily coordinate to a series of platinum group metals wherein the meta-substitution induces little or no greater electronic influence than the related para-substitution, although there is some evidence for a greater steric influence. Only the free tris-derivatised ligand and the complexes trans-[RhCl(CO)L2] and [RhClL3] {L=P(C6H4-4-C6F13)3} are preferentially
Acknowledgements
We thank the EPSRC (D.R.P. and A.M.S.), BP Chemicals Ltd. (D.R.P.) and the Royal Society (E.G.H.) for financial support.
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