Abstract
The behaviour of platinum(II) and palladium(0) complexes coordinated by various hydrosoluble monodentate phosphane ligands has been investigated by 31P{1H} NMR spectroscopy in the presence of randomly methylated β-cyclodextrin (RAME-β-CD). This molecular receptor can have no impact on the organometallic complexes, induce the formation of phosphane low-coordinated complexes or form coordination second sphere species. These three behaviours are under thermodynamic control and are governed not only by the affinity of RAME-β-CD for the phosphane but also by the phosphane stereoelectronic properties. When observed, the low-coordinated complexes may be formed either via a preliminary decoordination of the phosphane followed by a complexation of the free ligand by the CD or via the generation of organometallic species complexed by CD which then lead to expulsion of ligands to decrease their internal steric hindrance.
Highlights
Biphasic aqueous organometallic catalysis is one of the greenest solutions to produce organic chemicals [1,2]
As only one third of the platinum (195 Pt) has a spin of 1/2, the 31 P{1 H} NMR spectrum of this complex is characterized by a 1:4:1 triplet of doublets centred at δ = 24.5 ppm (1 JPcis –Pt = 2491 Hz; 2 JPcis –Ptrans = 19 Hz) corresponding to the two phosphorus atoms cis to the chloride and a 1:4:1 triplet of triplets centred at δ = 13.3 ppm (1 JPtrans –Pt = 3682 Hz; 2 JPcis –Ptrans = 19 Hz) corresponding to the phosphorus atom trans to the chloride (Figure 1)
Comparison of the Results Obtained with Palladium and Platinum Complexes
Summary
Biphasic aqueous organometallic catalysis is one of the greenest solutions to produce organic chemicals [1,2]. Water is cheap and non-toxic, and permits one to immobilize the catalyst in an aqueous phase by the use of water-soluble ligands, leading to ease of recycling by simple decantation at the end of the reaction [3]. The most widespread ligand used in such systems is TPPTS (tris(3-sulfonatophenyl)phosphane sodium salt, Table 1, entry 1) which is responsible for the industrial success of the aqueous biphasic propene hydroformylation (Ruhrchemie-Rhône Poulenc process, 1984) [4]. The alternative approaches to overcome this drawback have been recently reviewed [5,6]. These include the use of cyclodextrins (CDs) which are widely recognized as outstanding water-soluble receptors in aqueous biphasic organometallic processes [7]
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