Platinum Metals Rev., 1975, 19, (1), 22
Trifluorophosphine Complexes of the Platinum Metals
A Review of Recent Developments
The chemistry of the trifluorophosphine complexes of the transition metals, and of the platinum metals in particular, is of increasing importance. This article describes some of the more recent developments in trifluorophosphine complexes of the platinum metals, including their syntheses, structures, reactions, and potential as homogeneous catalysts.
The rapid development of transition metal carbonyl chemistry originated with the discovery of Ni(CO)4 in 1890. About the same time the first trifluorophosphine coordination complex [PtF2(PF3)]2 was synthesised by Moissan, but its true identity was not recognised at the time. It was only following the synthesis of the complexes cis -PtCl2 (PF3)2 and [PtCl2(PF3)]2 by Chatt and Williams in 1950 that attention was drawn to the similarity of trifluorophosphine and carbon monoxide as ligands towards transition metals. It was proposed that the dative sigma bond formed by donation of the lone-pair of electrons on phosphorus to the metal was supplemented by a symmetry allowed π-bond involving back donation of transition metal d -electrons into empty phosphorus 3d -orbitals. The σ- and π-components were considered to operate in the same “synergic” fashion as that already suggested for the analogous metal carbonyls. The π-component was expected to be favoured in trifluorophosphine-metal complexes because of the presence of the highly electronegative fluorine atoms attached to phosphorus (1). A large number of trifluorophosphine-transition metal complexes are now known and their chemistry has been extensively studied (2, 3, 4).
Platinum metal complexes in which trifluorophosphine is the only ligand present are listed in Table I. It is interesting to note that the corresponding carbonyl complexes M(CO)4, (M=Pd, Pt) and M′2(CO)8, (M′=Rh, Ir) are unstable. As yet there is no extensive chemistry of polynuclear metal trifluorophosphine complexes.
Methods of Preparation
The main synthetic route developed by Th. Kruck and co-workers in Germany involves “reductive fluorophosphination”. In this method the platinum metal halide is heated in an autoclave with copper or zinc in the presence of high pressures (50–500 atm) of trifluorophosphine (2, 5, 6).
Interestingly, when IrCl3 is heated with PF3 and copper only small amounts of the orange red crystalline μ3-phosphido-nonakis-(trifluorophosphine)tri-iridium Ir3P(PF3)9 are formed rather than Ir2(PF3)8 (7). The latter can be obtained in good yield from the reaction between K[Ir(PF3)4] and IrCl(PF3)4 (8) or by u.v. irradiation of IrH(PF3)4 (9). It is thermally stable up to 200°C. The rhodium analogue, on the other hand, evolves two moles of PF3 per dimer at 170°C to afford a red volatile solid, [Rh(PF3)3]x, whose identity is not yet fully established (8).
The platinum and palladium complexes M(PF3)4, which are readily available by reductive fluorophosphination of the appropriate metal halide (10) can also be obtained in the absence of any reducing metal since PF3 can act as its own reducing agent (11, 12), viz.
Pd(PF3)4 can also be obtained by the direct reaction between palladium powder and trifluorophosphine at high temperature and pressure (15).
Vibrational spectroscopic studies suggested that the M(PF3)4 complexes, (M = Pd, Pt), have perfectly tetrahedral structures and this has been subsequently confirmed in the case of the platinum complex (16). In contrast to the very short metal-phosphorus bond length found for Ni(PF3)4 (2.116 Å), the platinum-phosphorus distance in Pt(PF3)4 (2.230 Å) is not substantially different from that in other platinum-pbosphine complexes. This may reflect the decreased importance of metal-phosphorus π-bonding for the heavier platinum metals.
The pentakis(trifluorophosphine)ruthenium and osmium complexes, M″(PF3)5, almost certainly have trigonal bipyramid structures. Detailed 19F and 31P nuclear magnetic resonance studies show that these molecules are “fluxional” even at temperatures as low as −160°C, the barrier to intramolecular ligand exchange between equatorial and axial positions being less than 5 kcal/mole (17) (Fig. 1).
The structures of the M′2(PF3)8 complexes (M′=Rh, Ir), are not yet known, but presumably consist of two staggered trigonal bipyramid M′(PF3)4 units held together by a metal-metal bond. Ir3P(PF3)9 is thought to have the structure shown below (7).
The hydrides M′H(PF3)4 (M′ = Rh, Ir) and M″H2(PF3)4 (M″ = Ru, Os) listed in Table II are obtained in high yields by heating a mixture of the metal chloride and copper under high pressures of trifluorophosphine and hydrogen (18, 19).
|Trifluorophosphine Platinum Metal Hydrides|
|RuH2(PF3)4 (colourless, m.p. −76°C)||RhH(PF3)4 (colourless, m.p. −40°C, b.p. 89°C/725)|
|OsH2(PF3)4 (colourless, m.p. −72°C)||IrH(PF3)4 (colourless, m.p. −39°C, b.p. 95°C/730)|
The rhodium and iridium hydrides can also be obtained by acidification of the metallates [M′(PF3)4]− which are available from alternative low pressure synthetic routes. All the hydrides are very volatile liquids and, except for RhH(PF3)4, exhibit considerable thermal stability.
The binuclear complex Rh2(PF3)8 is readily cleaved by hydrogen gas to yield RhH(PF3)4 quantitatively, while Ir2(PF3)8 forms IrH(PF3)4 in the presence of moisture (2). When a mixture of HIr(PF3)4 and HCo(PF3)4 is subjected to u.v. irradiation, the interesting bridged complex shown below is obtained as a red oil (7).
All the trifluorophosphine metal hydrides exhibit acidic properties, readily forming the metallates [M′(PF3)4]−, (M′−Rh, Ir), [M″H(PF3)4]− and [M″(PF3)4]2−, (M″ = Ru, Os), with bases or potassium amalgam (18, 19).
The M′H(PF3)4 complexes react with triphenylphosphine to produce the salts [Ph3PH]+[M′(PF3)4]− while the substituted products RhH(PF3)(PPh3)3 and RhH(PF3)2 (PPh3)2 have recently been obtained by reaction of the stoichiometric quantities of PF3 and RhH(PPh3)4 (20). The complexes RhH(PF3)(PPh3)3 and RhH(PF3)2(PPh3)2 exhibit high activity as homogeneous catalysts. For example, RhH(PF3)(PPh3)3 catalyses the hydrogenation of terminal olefins (at 25°C, 1 atm H2) while RhH(PF3)2(PPh3)2 has been found to bring about rapid isomerisation of terminal olefins (20). The ruthenium complexes RuH2(PPh3)3 (PF3) and RuH2(PPh3)2(PF3)2 have been obtained very recently from RuH4(PPh3)3, but are not as catalytically active as their rhodium counterparts (21).
The structure of RhH(PF3)4 (Fig. 2), which has recently been determined by electron diffraction (22), may be considered to be pseudo-tetrahedral, i.e. based on a distorted trigonal bipyramid in which the hydrogen atom occupies an axial position. The rhodium-phosphorus distance is shorter than that in RhH(PPh3)4 and of the same order of magnitude as that found for Rh(NO) (PF3)3 (Rh−P = 2.245 Å) (23). It is not, however, the shortest rhodium-phosphorus bond length known. In the analogous cobalt complex, CoH(PF3)4, the cobalt-phosphorus bond is exceptionally short (24).
Variable temperature n.m.r. studies on M′H(PF3)4 (M′=Co, Rh, Ir) establish that all these complexes are stereochemically non-rigid (fluxional) and it has been proposed that the phosphorus environments are interchanged by a “tetrahedral tunnelling” rearrangement mechanism (17).
A recent X-ray crystallographic study (25) of the catalytically active complex RhH (PF3)(PPh3)3 has confirmed the basic trigonal bipyramidal structure proposed earlier on the basis of its 1H, 19F, and 31P n.m.r. spectra (20) (Fig. 3). The rhodium-trifluorophosphine distance (2.15 Å) is very much shorter than the rhodium-triphenylphosphine distances (average 2.33 Å), which is consistent with the stronger π-bonding ability of PF3.
The colourless complex cis -PtCl2(PF3)2 and the orange dimeric compound [PtCl2 (PF3)]2 mentioned above are both obtained by passing PF3 through a plug of platinous chloride at 200−220°C.
Passage of PF3 over heated rhodium trichloride in the presence of copper powder gives very low yields of the red crystalline, volatile, chlorine bridged dimer, [RhCl(PF3)2]2 (2), but this complex has subsequently been conveniently synthesised in high yield by reaction of PF3 with [RhCl(L2)]2 (L=ethylene, cyclo-octene, CO, etc.) under mild conditions (8, 26, 27).
The complexes [RhX (PF3)2]2 (X=Br, I) have been prepared similarly and are also obtained from halogen exchange reactions of [RhCl(PF3)2]2 with LiBr or NaI (27). Treatment of [IrCl(C8H14)2]2 (C8H14=cyclo-octene) with trifluorophosphine yields the rather unstable IrCl(PF3)4 which readily decomposes to the dimeric [IrCl(PF3)2]2 on warming (8). It is interesting that the corresponding carbonyl complex has the composition IrCl(CO)3. The very volatile IrI(PF3)4 has been made by treating the metallate salt KIr(PF3)4 with iodine at −80°C (28).
The yellow crystalline complexes RhX(PF3)4 (X = Cl, Br, I) are stable at room temperature only under a pressure of PF3, rapidly reverting to the red [RhX(PF3)2]2 compounds on removal of the trifluorophosphine pressure (27).
The synthetic utility of the dimeric [RhCl(PF3)2]2 complex is summarised in Fig. 4. The complex trans -RhCl(PPh3)2(PF3) formed by treating the dimer with an excess of triphenylphosphine has been shown to undergo fast ligand exchange reactions with traces of trifluorophosphine or triphenylphosphine (26).
No structural data are available at present on any of the above halide complexes, but [RhCl(PF3)2]2 probably has a similar structure to that found for [RhCl(CO)2]2 with two chlorine bridges between the metal atoms and possibly some degree of metal-metal interaction in the solid state.
Some typical complexes are listed in Table III. Rh2(PF3)8 reacts with a variety of acetylenes to give volatile compounds of the type Rh2(PF3)6(acetylene). The structural analogy between these compounds and the better known, related Co2(CO)6(acetylene) complexes has been confirmed by an X-ray diffraction study of Rh2(PF3)4(PPh3)2(PhC ≡CPh) (29) (Fig. 5). The rhodium-trifluoro-phosphine distances (average 2.216 Å) are again significantly shorter than the rhodium triphenylphosphine distances (average 2.391 Å).
A series of volatile, π-allylic complexes Rh(π-allylic)(PF3)3 have been obtained by insertion of various dienes into the metal-hydride bond of RhH(PF3)4 (30, 31). These complexes react with hydrogen chloride to produce ultimately [RhCl(PF3)2]2 and an olefin, e.g.
The intermediate π-allyl rhodium(III) hydride complex has been detected by a low temperature n.m.r. study and this observation is important in providing support for the involvement of this type of intermediate during the isomerisation of olefins catalysed by transition metal complexes (32). A novel thermal isomerisation of Rh(π-1,1-dimethylallyl)(PF3)3 to the 1,2-isomer has been observed (33) and a diene metal-hydride intermediate postulated (Fig. 6).
The π-allyl group in Rh(π-C3H5)(PF3)3 is readily displaced by hydrogen and trifluoro-phosphine to yield RhH(PF3)4 quantitatively (34), and treatment with nitric oxide affords the volatile liquid nitrosyl Rh(NO)(PF3)3, (2, 34), whose structure has been determined by electron diffraction (23).
The structure of the unusual bis-π-allylic ruthenium complex dichloro-2,7-dimethylocta-2,6-diene-1,8-diyl(trifluorophosphine) ruthenium, RuCl2(C10H16)(PF3), (35) is shown in Fig. 7. This represents the first structural study of a trifluorophosphine complex of a metal in an oxidation state greater than one (25). It is interesting to note that the ruthenium-phosphorus bond length (2.237 Å) is still considerably shorter than in other ruthenium phosphine complexes (typically 2.30–2.37 Å).
A new synthetic method which promises to have considerable general application involves transfer of PF3 initially coordinated to nickel to other transition metals. Thus treatment of the pentamethylcyclopentadienyl complexes (Me5C5MCl2)2 (M = Rh, Ir) with an excess of the commercially available complex Ni(PF3)4 in boiling toluene gives good yields of the complexes π-(Me5C5) M(PF3)2 (36).
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