Why Examine Ligands of Formula C_nP-F_2–n Instead of PF₃?

Organofluorophosphines are compounds involving a carbon-phosphorus-fluorine (C-P-F) moiety. In contrast to other fluorophosphines, for example Si-P-F, N-P-F, P-P-F, O-P-F, S-P-F, and X-P-F (where X = halogen), they show a stable element-phosphorus bond (here C-P). The physical and chemical properties of organofluorophosphines also resemble those of PF₃ very closely (8–10), see Table II. This has been confirmed by X-ray diffraction studies on two organofluorophosphines: the dimeric anthracene difluorophosphine, shown in Figure 2 (9), and for comparison the dimethylamino difluorophosphine (10).

One can clearly see the similarities in the bond angles of PF₃ and RPF₂ (where R = carbon bonded groups), despite strong steric hindrance, in contrast to N-PF₂ without this steric factor. In the example, Figure 2, the PF₂ moiety is unable to rotate at the C-P bond because of strong F-H “through-space” interactions.

This property serves to combine the special electronic effects of PF₃ with other characteristics of co-ordination compounds that are important in catalytic reactions, for example: stability, solubility, volatility, chirality, the bulk, the possibility of heterogenisation, and so on.

Surprisingly, the highly sterically hindered and air-stable compound shown in Figure 2 was obtained in a single step synthesis, see Equation (ii):

(ii)

the photochemical dimerisation of difluorophosphino anthracene (9)

Methods of Preparation

The known types of organofluorophosphine complexes of platinum group metals are given in Table III. For most of these compounds an appropriate PF₃ complex is also known.

Most of these compounds have been prepared by an exchange reaction between a labile ligand and an excess of the organofluorophosphine (7, 14, 15), that is:

(iii)

(iv)

where COD = 1,8-cyclooctadiene, L = RPF₂, R₂PF, and R = carbon bonded organic moiety.

In the case of platinum(O) complexes a very large excess of organodifluorophosphine may be used to get complexes of the type PtL₄ (reduction of platinum(II) under formation of some organofluorophosphorane (16, 17), see Equation (v)). These complexes are not as stable against oxygen and moisture as their Cl₂PtL₂ analogs (17) and they may be transformed to mixed phosphine substituted complexes using tertiary phosphines (16), for example triphenylphosphine, see Equation (vi):

(v)

(vi)

Bulky organic substituents involved in the fluorophosphine ligand have only a minor effect in the given preparation methods. For example two ‘BuPF₂ or (o, p -(CF₃)C₆H₃)₂PF can coordinate to one platinum(II) atom (18). In the case of tungsten a hexaco-ordinated species is known, that is WCBuPF₂)₂ (13). On the other hand it is impossible to co-ordinate two phosphines such as ‘Bu₂PF or (Me₃Si)₃PF₂ onto one platinum(II) atom (7), but each of these ligands can co-ordinate to one metal, for example osmium or ruthenium (14). In a cluster compound, for example Os₃(CO)₁₀L₂, two ligands are always bonded to two different metal atoms (14, 15). Only if there are more RPF₂ ligands than metal atoms in a cluster, as in Os₃ (CO)₆ L₆, will two organodifluorophosphines be connected with one metal centre (14).

Reactions of Platinum(O) and Platinum(II) Complexes

The reactions of complexes of the type PtL₄ with water and oxygen lead to the formation of partly hydrolysed and oxidised organofluorophosphines co-ordinated to platinum(II), see Equation (vii)(17). These complexes may be reacted with boron trifluoride etherate to yield BF -bridged, chelating organofluorophosphine complexes, as shown in Figure 3 on the previous page (17).

(vii)

(viii)

where R = a carbon bonded organic moiety.

Organofluorophosphine platinum(II) complexes are very stable compounds which do not decompose under normal conditions. These complexes are less stable only if the organic moiety is chosen from the group of multiple halogenated substituents, for example C₆F₅ orCCl₂H. This is in strong contrast to the stability of the corresponding organofluorophosphines themselves, which become more stable if they are either halogenated in the organic part or if the organic moiety shows a reasonable steric bulk.

As a typical example of a platinum(II) complex involving a bulky substituent, the X-ray structure of PtCl₂(Cp^★PF₂)₂ is shown in Figure 4 (19).

On the other hand bulky substituents involved in the phosphine ligand, such as 2, 4, 6-trimethylphenyl difhiorophosphine, display a most interesting chemistry if they are co-ordinated to platinum(II). Pta₂(PF₂C₆H₂Me₃)₂, for example, splits off HQ if heated in vacuum (20) and gives a compound involving a Pt-C bond, see Figure 5.

The remaining chlorine atom in the compound shown in Figure 5 may be replaced by organic moieties using, for example, SnMe₄ or HgPh₂, and yielding the methylated or phenylated complexes, respectively.

Another most interesting aspect of organofluorophosphines bonded to platinum(II) has been found in the compound shown in Figure 6. In the solid state the ortfo-substitutent of the aromatic ring system (-CH₂NMe₂) is found to co-ordinate into the PF₂ moiety, representing the first example of an hypervalent phosphorus in this type of compound (21).

Reactions of Ruthenium and Osmium Complexes

Complexes of type M₃(CO)_12–nL_n (M = Os, Ru, n = 1 –4; M = Os, n = 6) are stable compounds in the solid state, and in the case of osmium also in solution (14, 15), see Figure 7. Treatment of these complexes with trimethylamine oxide in the presence of free ligand leads to the formation of mixtures of compounds involving different numbers of ligands. Therefore one may start with M₃(CO)₁₂ (M = Ru, Os) and end up with all the possible complexes given above (n = 1-0 see Table (III) of which n = 3 represents the main product if an excess of amine oxide and ligand is used (14) Numbers n > 3 are difficult to obtain by this method, indeed n = 5 has never been fully characterised. Using ultraviol et light, Os₃ (CO)₁₂ and excess ligand n = 6 has been found to be the major product (14).

Rhodium, Palladium and Iridium Complexes

Even though rhodium is one of the few metals showing a strong nuclear magnetic spin of which suggests that any conceivable rhodium organofluorophosphine complexes would have an interesting NMR spectra, little is known about this class of compounds (7). In the opinion of the author, organofluorophosphine complexes of rhodium as well as of iridium represent a challenge for every preparative chemist, especially with respect to possible compounds analogous to Wilkinson’s and Vaska’s complexes.

Palladium organofluorophosphine complexes are known to undergo some of the reactions given for platinum; however, the appropriate platinum complexes are more stable and, for this reason, have been studied in greater detail. The identity of some palladium compounds involving organofluorophosphine ligands are claimed by Kruck (13) but have not yet been published.

Properties of Complexes of Organofluorophosphines

In contrast to the more or less volatile PF₃-complexes of transition metals, organofluorophosphine complexes are much less volatile, or non-volatile, solids; and due to the orgamc group they are more resistant to moisture and to air. Almost all of the known organofluorophosphine complexes of the platinum group metals may be stored for months in the solid state at room temperature, without using any protective methods, and without any changes occurring. Indeed, even in solution most of them are stable against moisture and oxygen.

The solubility of organofluorophosphine complexes of these metals is influenced by the organic group bonded to the phosphorus and by the general bonding state on the metal. Dichloromethane or other polar solvents are perfectly capable of dissolving reasonable amounts of most of these complexes. It is possible, however, to use hydrophobic or hydrophilic organic moieties to increase or decrease the solubility of a given co-ordination state in a given solvent. For example, using dichloromethane as the solvent the solubility of complexes of the type Cl₂Pt(PF₂[o, o -R₂C₆H₃])₂ increases in the following order: R = MeO < Me < PhO < H <C₁₆H₃₃O (7).

In some dozens of X-ray determinations of organofluorophosphine complexes of transition metals a somewhat shorter metal-phosphorus bond length has always been observed, compared with that of the appropriate triorganophosphine co-ordination compound (7). In the case of platinum(II) complexes, the Pt-P bond length correlates with the ³¹P-^l,5Pt coupling constant in the NMR spectra of these compounds (7, 21). Both indicate a strong metal-phosphorus interaction, which is reflected in strong chemical and physical bonding of the organofluorophosphine to the metal.

Heterogenisation of Complexes of Organofluorophosphines

One main synthetic route to organofluorophosphines first developed by Lines and Centofanti (22) and then optimised by the author (7, 23) involves lithiation of an aromatic ring system, and treatment of this product with PF₂C1, which is a gas. (PF₃ may also be used but gives insufficient yields). It has been shown that this method is useful for transforming poly sty rol to PF₂-functionalised polystyrol, which readily yields platinum(II) complexes on treatment with PtCl₂(COD) (7).

PF₂-functionalised polystyrol should also be a good starting material for other transition metal complexes, especially for the platinum metals.

Outlook

The investigations on fluorophosphine complexes started with the hope in finding some interesting new compounds which would become as relevant for catalytic processes as some particular CO complexes. The high sensitivity of compounds involving PF, to moisture and oxygen has destroyed the initial optimism. As organofluorophosphines are readily available and are much less sensitive to wet air this optimism may return. In addition, the possibility of heterogenisation of active complexes by standard organic reactions may increase the interest of industrial chemists.

Heterogenisation may not only find application in catalytic reactions, one should also keep in mind the strong bonding of the platinum group metals to organofluorophosphines, a property which may be useful in the recovery of these expensive metals from solutions and mixtures in organic solvents, by absorption using polymeric organofluorophosphines.

Lastly, Pt(PF₃)₄ has been used for the preparation of special platinum metal catalysts by thermal decomposition (24). Because of the high toxicity and volatility of PF₃, which is split off during this reaction, it may be better to use organofluorophosphine ligands instead.