Method of Manufacturing a Platinum Complex for Plating

The present invention relates to a method of manufacturing platinum (II) complexes and their use in plating.

It is well known to use platinum (IV) and (II) salts for plating. A common class of platinum salts contain one or more nitro ligands, and these complexes can be written by the general formula (I)

An example of a complex of Formula I is dihydrogen dinitrosulfatoplatinum (II) H[Pt(NO)(SO)] which is also referred to in the literature as dinitrosulfatoplatinous acid, PtDNS or simply DNS. This complex has been described for use in platinum electroplating, see GB2059440A (Johnson Matthey) and is commercially available from Johnson Matthey and others (CAS 12033-81-7).

Dinitrosulfatoplatinous acid (hereafter “H.DNS”) and related complexes such as potassium dinitrosulfatoplatinate (hereafter “K.DNS”) and potassium tetranitroplatinate K[Pt(NO)] are used to produce thin platinum films on a variety of substrates. For example, the article “The Electrodeposition of Platinum and Platinum Alloys” (1988, 32, (4), 188-197) describes that these electrolytes can be used to coat platinum onto a wide range of materials including copper, brass, silver, nickel, lead and titanium. CN105132964A (Wuxi Qingyang) describes a platinum plating solution comprising K[Pt(NO)], a water-soluble phosphate, dialkyltrimethylammonium bromide and sulfuric acid. The article “Hot corrosion behaviour of single-phase platinum-modified aluminide coatings: Effect of Pt content and pre-oxidation” (2017, 127, 82-90) describes the use of K.DNS (K[Pt(NO)(SO)] to electroplate Ni-based superalloys.

The manufacture of H.DNS and K.DNS has been reported in the literature. GB897690 (Johnson Matthey) describes that H.DNS can be prepared by the reaction between tetranitroplatinous acid H[Pt(NO)] and sulfuric acid. The complex H[Pt(NO)] is prepared in this reference by passing a solution of K[Pt(NO)] through a cation exchange column with sulfuric acid. The complex K[Pt(NO)] itself is usually prepared from a platinum-chloride salt. For instance the articles “The Stereoisomerism of Complex Inorganic Compounds. X/V. Studies upon the Stereochemistry of Saturated Tervalent Nitrogen Compounds” (1952, 74, 14, 3535-3538) and “Molecular Photocrystallography: A Study of Metastable and Transient Species by Non-Ambient Crystallographic Techniques” (Hatcher, L., Doctoral Thesis 2014, University of Bath) describe the preparation of K[Pt(NO)] by the reaction between KPtCland KNO.

The article1988, 32, (4), 188-197 describes that H.DNS can be prepared from a platinum nitro salt such as K[Pt(NO)Cl], K[Pt(NO)Cl] or K[Pt(NO)SO](K. DNS). These in turn have to be prepared from a precursor platinum salt which is normally a platinum chloride salt.

The article “Hot corrosion behaviour of single-phase platinum-modified aluminide coatings: Effect of Pt content and pre-oxidation” (2017, 127, 82-90) describes the use of K.DNS (K[Pt(NO)(SO)] to electroplate Ni-based superalloys. The K.DNS is prepared by aqueous reaction between KPtCland KNOat 80° C. to obtain K[Pt(NO)] crystals, followed by the aqueous reaction between K[Pt(NO)] and sulfuric acid.

Existing routes to H.DNS, K.DNS and related complexes involve several steps and generally require the use of platinum halide complexes, which are sensitizing. There is a need for a simpler, safer and scalable method to produce platinum (II) complexes such as H.DNS, K.DNS and related complexes. The present invention addresses this need.

The present inventors have found that platinum (II) complexes can be prepared in a simple process by the aqueous acidic reaction between a chloride-free platinum (IV) compound and a source of nitrous acid (HNO). The nitrite ions reduce the Pt(IV) to Pt(II) and are oxidized to nitrate and/or nitrogen oxides, especially nitrogen dioxide. Other nitrite ions may complex with the Pt(II) centre to produce a nitro complex. A heating step is carried out to promote the reduction of platinum (IV) to platinum (II) and to promote decomposition of residual nitrous acid. An exemplary reaction is:

This process offers several advantages over known methods for producing Pt(II) complexes, especially H.DNS and K.DNS.

Firstly, the Pt(II) complex is prepared in a single step from the Pt(IV) compound. This provides a higher yielding route to the desired complex compared with the complex multistep routes described above. The single step method also allows simplification of the manufacturing equipment and requires less space in the plant.

Secondly, nitrites are relatively easy to handle materials compared to reducing agents like H, N/Hor hydrazine which have been used previously for the reduction of Pt(IV) to Pt(II). In addition, nitrites do not reduce the Pt(II) further to Pt(0).

Thirdly, the reduction and subsequent processing are quick. This allows increased throughput compared to existing alternative routes.

Fourthly, preferred sources of platinum (IV) used in this route are hexahydroxyplatinic acid and hexahydroxyplatinate salts, which are non-sensitizing unlike the platinum chloride salts which are used in existing processes to make H.DNS/K.DNS and related complexes.

It is known to use Pt(IV) compounds as precursors to Pt(II) salts. US2015/0315224 (Umicore) describes a process in which H[Pt(OH)] is reacted with an uncharged donor ligand L in the presence of a reducing agent and at least one of the hydroxo ligands is replaced. Preferred reducing agents are H, N/Hmixtures, hydrazine, formaldehyde, oxalic acid and formic acid. Typically the ligand L is a monodentate or bidentate amine or phosphine. However, this reference does not describe the use of HNOas a reducing agent.

The combination of a platinum (IV) compound and a nitrite salt is described in U.S. Pat. No. 4,200,626 (Toyo Seiyaku Kasei Co Ltd) which describes a dental composition comprising a water-soluble haloplatinate and a pharmaceutical carrier or diluent. Examples 2 and 3 of this reference describe the combination of sodium hexachloroplatinate hexahydrate and sodium nitrite (Example 2) or sodium hexachloroplatinate hexahydrate, sodium nitrite and sodium chloride (Example 3) at pH 5.5 or 3.5. However, sodium nitrite is not present in sufficient quantity to complete the reduction of Pt(IV) to Pt(II) and, even if some reduction does take place, a heating step is not carried out.

The book “Handbook of Preparative Inorganic Chemistry” (Volume 1, Second Edition, Academic Press Inc, 1963) describes on pages 1579-1580 the preparation of cis-dinitrodiammineplatinum(II), based on a method described in U.S. Pat. No. 1,779,436. In a first step potassium hexachloroplatinate (IV) (KPtCl) and sodium nitrite are dissolved in water and heated with stirring. NOis evolved as fine bubbles. When no further gas evolves the solution now containing K[Pt(NO)] is cooled and filtered. This solution is then reacted with a stoichiometric quantity of 20% aqueous ammonia to produce the desired complex [Pt(NO)(NH)], which can be recrystallized from hot water. This route uses KPtClwhich contains chloride and is sensitizing.

In a first aspect the invention relates to a method of manufacturing a plating solution comprising a platinum (II) complex, comprising the steps of:

In the present invention the Pt(IV) compound can be converted into a diverse range of Pt(II) nitro complexes depending on the choice of acid and source of nitrous acid.

In a second aspect the invention relates to a plating solution produced by a process described herein. Without wishing to be bound by theory, it is thought that the new method may produce solutions with different speciation compared to those described previously in the literature as H[Pt(NO)(SO)] or K[Pt(NO)(SO)].

Any sub-headings are for convenience only and are not intended to limit the invention.

The process of the invention uses a Pt(IV) compound as a starting material. The Pt(IV) centre is generally surrounded by ligands and the Pt(IV) compound may also be referred to herein as a complex. The Pt(IV) compound is typically a salt in which the anion contains Pt (IV).

Platinum chloride salts and complexes such as KPtCland KPtClwhich have been used previously to prepare platinum (II) complexes are often sensitizing. The Pt(IV) compound used in step (i) is therefore chloride-free. By “chloride free” we mean that the compound does not include chloride complexed to platinum such as KPtClor HPtCletc. The Pt(IV) compound is preferably halide-free, i.e. the compound does not include any halide complexed to platinum. A particularly preferred Pt(IV) compound is hexahydroxyplatinic (IV) acid, HPt(OH). This compound is available commercially and is considered to be a non-sensitizing Pt(IV) compound. Salts of hexahydroxyplatinic (IV) acid, containing the hexahydroxyplatinate ion, are also preferred Pt(IV) compounds. The counterion to hexahydroxyplatinate may be a metal ion or a non-metal ion, such as an ammonium or alkylammonium ion.

The role of the source of nitrous acid is to form nitrous acid in situ under the acidic conditions and thereby reduce the Pt(IV) to Pt(II). It is preferred that the source of nitrous acid is added in an amount sufficient to reduce all of the Pt(IV) to Pt(II). While an excess of reducing agent is desirable for rapid reaction conversion, too much is undesirable for cost reasons and to avoid excess NOx release.

The reducing agent is referred to herein for convenience as being “nitrous acid” but it will be appreciated that the active reducing agent may be more complicated, and is probably a mixture of nitrous acid (HNO) and nitrous anhydride (NO) which are known to exist in equilibrium in aqueous solution according to the equation:

Under the acidic conditions in steps (i) and (ii) nitrite will exist as a mixture of NO, HNOand NO, and possibly other species, depending on the pH of the solution.

In one embodiment the source of nitrous acid is provided as a nitrite salt e.g. of formula [M(NO))] where Mis a metal or non-metal cation and a is an integer. Nitrous acid is formed from the nitrite salt under acidic conditions. One type of preferred salts are Group I nitrites, preferably lithium nitrite, sodium nitrite or potassium nitrite. Another type of preferred salts are Group II nitrites, preferably magnesium nitrite, calcium nitrite, strontium nitrite or barium nitrite. Another preferred salt is silver nitrite. In a preferred embodiment the source of nitrous acid is a metal nitrite selected from magnesium nitrite, calcium nitrite, strontium nitrite, barium nitrite and silver nitrite. These salts are preferred because the metal ions (Mg, Ca, Sr, Ba, Ag) form insoluble salts with many mineral and some organic acids, meaning that they can be precipitated from solution in step (i) or in optional step (iii). For example, additional of phosphoric acid in step (i) or in optional step (iii) will precipitate the residual metal ions as magnesium phosphate, calcium phosphate, strontium phosphate, barium phosphate or silver phosphate. Magnesium nitrite, calcium nitrite and strontium nitrite are preferred over barium nitrite because the latter is toxic, and are preferred over silver nitrite because the latter is expensive. Therefore, a preferred source of nitrous acid is a metal nitrite selected from magnesium nitrite, calcium nitrite and strontium nitrite. Ammonium nitrite is a preferred source of nitrous acid with a non-metal cation.

It is also possible to use alternative sources of nitrous acid. In one embodiment the source of nitrous acid is a M[Pt(NO)] salt, where x=2 when M has a charge of +1 and x=1 when M has a charge of +2, for example potassium tetranitroplatinate (II) (K[Pt(NO)]).

If excess nitrite is present then this is released as brown nitrogen dioxide gas as the reaction warms up. If a metal nitrite salt is used as the source of nitrous acid it may be undesirable to have excessive amounts of countercation remaining in solution after steps (i)/(ii). The molar ratio of Pt(IV):source of nitrous acid at the beginning of step (i) will depend on the choice of source of nitrous acid. For example one equivalent of KNO, Ca(NO)or K[Pt(NO)] contains 1, 2 and 4 equivalents of nitrite respectively. Where the source of nitrous acid is a nitrite salt it is preferred that the molar ratio of Pt(IV):nitrite at the beginning of step (i) is between 1:2 to 1:10, preferably between 1:2 to 1:8.

In one embodiment the source of nitrous acid is pure HNO. By “pure HNO” we mean that the solution is substantially free of metal ions. Pure nitrous acid can be prepared by treatment of a solution containing a nitrite salt in an organic polar solvent aqueous solution with an ion exchange resin. Examples include the procedures reported in U.S. Pat. No. 3,113,837 and the article “Preparation of Solutions of Pure Nitrous Acid” (1963, 85, 23, 3888). Pure nitrous acid may also be prepared by electrolysis of nitric acid solutions using a platinum electrode, as described in the article “Alternative Electrode Reactions. Part I. Reactions at a platinum cathode in nitric acid solutions” (1932, 1565-1579). Where pure HNOis used the solution in step (i) is substantially free of metal ions, other than those from the Pt(IV) compound, optional acid (described later), and optional salt which dissolves to give an acidic solution (described later).

The source of nitrous acid may also exchange with the ligands on the Pt(IV) or Pt(II) centre to form a nitro complex. As used herein “nitro complex” should be understood in its broadest sense and includes bonding via nitrogen (Pt—NO) or via oxygen (Pt—ONO).

Some Pt(IV) compounds are not appreciably soluble in water at room temperature, such as hexahydroxyplatinic (IV) acid which is insoluble in water at room temperature. Therefore it is preferred that the mixture is agitated in order to promote dissolution during steps (i) and (ii). Agitation is preferably achieved by moderate stirring. Rapid stirring if needed should be limited to short bursts as it promotes nitrogen dioxide loss. The present inventors have found that an aqueous suspension of hexahydroxyplatinic (IV) acid solubilised rapidly in the presence of nitrous acid, presumably due to the reduction of Pt(IV) to Pt(II) by nitrite.

The concentration of Pt in step (i) should be chosen with consideration of process economy (dilute solutions require more energy to heat). Typically the Pt concentration is 5 to 100 g/L, such as 10 to 100 g/L. The upper limit is determined by the solubility of the Pt(IV) compound. When manufacturing at commercial scale a Pt concentration of 30 to 80 g/L may be appropriate.

The reaction is carried out under acidic conditions, i.e. pH<7, preferably pH<6, more preferably pH<5, more preferably pH<4. The minimum pH is not especially limited but is preferably not less than −1, preferably not less than 0. Typically the reaction is carried out at pH<4, such as pH 0-4. The acidic conditions promote formation of nitrous acid (HNO) which is believed to be a major species responsible for reducing the Pt(IV).

Depending on the Pt(IV) compound and source of nitrous acid used it may not be necessary to add any additional acid. However, in some embodiments an acid is added in step (i) to promote the formation of nitrous acid. The acid is distinct from the Pt(IV) compound or source of nitrous acid. The skilled person will appreciate that if the conjugate base of the acid is strongly coordinating towards platinum (II) then it is possible for it to be incorporated into the platinum (II) complex. For example, if sulfuric acid is added as the acid then it is possible that a platinum (II) sulfato complex will be formed. The choice of acid will therefore be guided by the desired platinum (II) complex. The acid may be a mineral acid or an organic acid. Preferred mineral acids include phosphoric acid, sulfuric acid and nitric acid. Preferred organic acids include carboxylic acids, and aldehydes that often contain carboxylic acid impurities on ageing, such as glyoxal.

The addition of phosphoric acid or sulfuric acid as the acid in step (i) has the additional advantage that they form insoluble salts with a variety of metal ions. Therefore, if the source of nitrous acid is a metal nitrite salt in which the metal forms an insoluble salt with either phosphate or sulfate then it may be possible to acidify the solution and precipitate a metal phosphate or sulfate at the same time in step (i).

In one embodiment the source of nitrous acid is a metal nitrite, the acid is phosphoric acid and a metal phosphate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is phosphoric acid and magnesium phosphate, calcium phosphate or strontium phosphate is precipitated in step (i).

In one embodiment the source of nitrous acid is a metal nitrite, the acid is sulfuric acid and a metal sulfate is precipitated. In a preferred embodiment the source of nitrous acid is selected from magnesium nitrite, calcium nitrite or strontium nitrite, the acid is sulfuric acid and magnesium sulfate, calcium sulfate or strontium sulfate is precipitated in step (i). The use of calcium nitrite or strontium nitrite with sulfuric acid is preferred over the use of magnesium nitrite because of the much greater insolubility of calcium sulfate and strontium sulfate compared to magnesium sulfate.

Preferred organic acids include acetic acid, oxamic acid and oxalic acid. These acids dissolve in aqueous solution to provide the acidic conditions necessary to form HNOand reduce the Pt(IV) to Pt(II). These acids, particularly oxalic acid, also form precipitates with some metal ions and are particularly advantageous where a metal nitrite is used as the source of nitrous acid. Oxalic acid is a particularly preferred organic acid. In one embodiment the source of nitrous acid is a metal nitrite, the acid is oxalic acid and a metal oxalate is precipitated. In a particularly preferred process the source of nitrous acid is magnesium nitrite, calcium nitrite, or strontium nitrite, the organic acid is oxalic acid and magnesium oxalate, calcium oxalate or strontium oxalate is precipitated.

Using an excess of oxalic acid tends to lead to a blackening and matt finish when the product from step (ii) is used for electroplating. While there are some instances where a dark and matt finish is desired, avoiding excess oxalic acid and/or adding a mixture of oxalic acid and sulfuric acid mitigates this and gives bright platinum plates.

In some embodiments the acid is provided in step (i) by a salt which dissolves in aqueous solution to give an acidic solution. This may be in place of or in addition to a mineral acid or organic acid. It is preferred that the salt dissolves in solution to produce a pH below 6. This helps to ensure that the NO/HNO/NOequilibrium is in favour of NO/HNOwhich are believed to be the active species for reducing Pt(IV) to Pt(II). Examples of suitable salts include Group I metal sulfates, Group I metal hydrogensulfates, Group I metal dihydrogenphosphates and Group I metal hydrogenoxalates, in each case the sodium or potassium salts are preferred for their commercial availability. Preferred salts are those which dissolve in solution to produce a pH below 4.

The addition of the Pt(IV) compound, source of nitrous acid, and any acid, should be controlled so as to avoid exotherms. If necessary the solution may be cooled to a temperature of 0-10° C. such as 0-5° C.

The order of addition of Pt(IV) compound, source of nitrous acid and any additional acid in step (i) is not particularly important, unless the acid is capable of reducing Pt(IV) to Pt(0), in which case the acid should be added after combining the Pt(IV) compound and source of nitrous acid or at the same time as combining the Pt(IV) compound and source of nitrous acid. This is particularly the case for oxalic acid which is capable of fully reducing Pt(IV) to Pt(0).

In step (ii) the reaction mixture is heated. The role of step (ii) is to promote the reduction of Pt(IV) to Pt(II) and to promote decomposition of any residual nitrous acid to nitrogen oxides.

In some embodiments heating is only commenced once the chloride-free platinum (IV) compound and source of nitrous acid are both present in solution.

In alternative embodiments a solution of the chloride-free platinum (IV) compound is pre-heated and the source of nitrous acid is added to the pre-heated solution. In this embodiment steps (i) and (ii) take place simultaneously. The source of nitrous acid may be added as a solid or as a solution.

Excess nitrous acid was seen to escape as brown nitrogen dioxide gas in the 50-80° C. range in the present invention. Therefore it is preferred that the reaction is heated to a temperature of at least 50° C. during step (ii), preferably at least 80° C. Typically the reaction is heated to a temperature of at least 90° C. to ensure any conversion based on final solution color change, e.g. reddening. However, in many cases solutions held for an extended duration at lesser temperatures e.g. 60° C. will plate successfully. If the source of nitrous acid is added to a pre-heated solution of the chloride-free platinum (IV) compound then it is preferred that the pre-heated solution is at a temperature of at least 50° C.