JP6154409B2 - Method for producing heterometallic multinuclear complex containing Ni and Au - Google Patents

Description

  The present invention relates to a method for producing a dissimilar metal polynuclear complex containing Ni and Au.

The exhaust gas discharged from automobile internal combustion engines such as gasoline engines and diesel engines contains harmful components such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). For this reason, in general, an exhaust gas purification device for decomposing and removing these harmful components is provided in the vehicle, and these harmful components are rendered harmless by the exhaust gas purification catalyst provided in the exhaust gas purification device. Has been. Conventionally, as such an exhaust gas purifying catalyst, a catalyst carrier made of a porous metal oxide such as alumina (Al ₂ O ₃ ), a noble metal, particularly platinum (Pt), rhodium (Rh), palladium (Pd) A material carrying a platinum group element such as is widely known.

  However, these platinum group elements are very expensive rare metals because they are produced in few regions and their production is biased to specific regions such as South Africa and Russia. In addition, these platinum group elements are used in an increased amount with the tightening of exhaust gas regulations for automobiles, and therefore there is a concern about the depletion of resources. For this reason, while reducing the usage-amount of a platinum group element, it is required to substitute the role of the said platinum group element with another metal etc. in the future. Thus, much research has been conducted on catalyst components for reducing or replacing the amount of platinum group elements used.

  On the other hand, by adding the second and third other metal components to the active point of a supported catalyst such as an exhaust gas purifying catalyst, the activity and selectivity of the supported catalyst often show dramatic improvements. .

  For example, Patent Document 1 describes a method of purifying NOx with an alloy catalyst in which iron and gold as base metals are combined.

  Here, as a method for preparing an exhaust gas purifying catalyst in which another metal component is added to the catalyst metal, for example, a compound containing a catalyst metal and a compound containing another metal component are used. A method of impregnating and supporting the components on the catalyst support simultaneously or sequentially is generally known. However, in such a catalyst preparation method, it is generally difficult to form metal particles or active sites in which the catalyst metal and other metal components are combined at the atomic level. And when such compounding is not enough, the effect by having added another metal component with respect to a catalyst metal may not fully be exhibited.

  On the other hand, a method using a multinuclear complex having a plurality of types of metal atoms has been proposed as a method for forming metal particles or active sites in which different metal components are compounded at the atomic level.

For example, in Patent Document 2, by treating a mononuclear complex such as Pd or Pt coordinated with a nitrogen-containing group such as an NH ₃ group with a base, a plurality of kinds of metals are used in a multimerization reaction involving deprotonation. It is described that a catalyst containing a heterogeneous metal cluster can be prepared by forming a heterometallic multinuclear complex having atoms and impregnating a porous carrier into an aqueous solution containing the heterometallic multinuclear complex thus formed. .

JP 2011-098265 A JP 2011-101862 A

  As a catalyst component that can be substituted for the platinum group element, in addition to iron (Fe) described in Patent Document 1, several metals have been proposed in other documents, and one of them is nickel ( Ni). Therefore, for example, for the purpose of improving the activity and selectivity of the catalyst, Ni is combined with other metal components in the same manner as in Patent Document 1, and further, the Ni and other metal components are combined at the atomic level. Therefore, even if it is going to apply the method as described in patent document 2, it is very difficult to manufacture the metal particle which these metal components compounded at the atomic level. Because Ni is not a diamagnetic metal, it is difficult to identify its molecular structure using nuclear magnetic resonance (NMR) during the synthesis of the complex, and therefore the synthesis of a multinuclear complex containing Ni itself This is because it is very difficult.

  Therefore, an object of the present invention is to provide a method for producing a dissimilar metal multinuclear complex containing Ni, which can be used in the preparation of an exhaust gas purifying catalyst.

In order to solve the above problems, the present invention provides a method for producing a heterometallic multinuclear complex containing Ni and Au, and includes a Ni2 nucleus complex containing a bridged hydroxyl group and Au (PPh ₃ ) [N (SiMe ₃ ) _2. And a reaction step of reacting.

  According to the method of the present invention, a heterometallic multinuclear complex containing Ni and Au can be produced. Further, by using a heterogeneous metal multinuclear complex containing Ni and Au produced by the method of the present invention, bimetallic particles in which Ni and Au coexist at the nano level, particularly at the atomic level, Compared with the method, it is possible to produce an exhaust gas purifying catalyst in which finer binary metal particles are supported on a catalyst carrier.

₂ is a diagram showing the results of X-ray crystal structure analysis of a Ni ₂ Au ₂ tetranuclear complex produced in Example 1. FIG. The analysis result by STEM of the catalyst for exhaust gas purification of Example 2 is shown. The analysis result by STEM of the catalyst for exhaust gas purification of the comparative example 1 is shown.

<Method for Producing Dissimilar Metal Multinuclear Complex Containing Ni and Au>
The method for producing a heterometallic multinuclear complex containing Ni and Au according to the present invention includes a reaction step of reacting a Ni2 nucleus complex containing a crosslinked hydroxyl group and Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ]. It is said.

  As described above, as a method for preparing an exhaust gas purification catalyst in which other metal components are added to a catalyst metal, a method of impregnating and supporting these components simultaneously or sequentially on a catalyst carrier is generally known. ing. As another method, a method is known in which metal particles containing a plurality of metal elements are produced, and then the metal particles are supported on a catalyst carrier by a conventional impregnation method. And as one of the methods for producing such metal particles containing a plurality of metal elements, a reducing agent such as alcohol or sodium borohydride is added to a mixed solution containing a salt of each metal element constituting the metal particles. And the method of reducing simultaneously the ion of each metal element contained in a mixed solution, performing a heating etc. as needed is known.

  However, since the method for producing metal particles using the reducing agent as described above includes a step of reducing the salt or ion of each metal element dissolved in the solution, the reduction of the salt or ion of each metal element. If there is a difference in the ease with which it is performed, it is very difficult to form fine metal particles in which each metal element coexists at the nano level, particularly at the atomic level. And if each metal element does not coexist on a nano level, the special effect by combining these metal elements may not fully be exhibited.

This time, the present inventors used a Ni2 nucleus complex containing a bridged hydroxyl group and a Au mononuclear complex Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] to produce a heterometallic multinuclear complex containing Ni and Au. A new manufacturing method has been found. In addition, the inventors of the present invention compared with a conventional method using a reducing agent such as sodium borohydride by impregnating and supporting Ni and Au on a catalyst support using a heterogeneous metal multinuclear complex obtained by this method. The present inventors have further found that it is possible to produce an exhaust gas purification catalyst in which finer binary metal particles are supported on a catalyst carrier.

[Reaction process]
According to the present invention, in the reaction step described above, the Ni2 nucleus complex containing a crosslinked hydroxyl group is contacted with Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ].

Although it does not specifically limit as Ni2 nucleus complex containing a bridge | crosslinking hydroxyl group, For example, the following general formula (1), ie,
Wherein R ¹ and R ² are each independently a hydrogen atom, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, a phenyl group, an alkoxycarbonyl group, or an acyl group, and X is a counter ion. And a complex containing an N-heterocyclic carbene ligand. Moreover, as a specific example of the complex containing such N-heterocyclic carbene ligand, the compound which has the following Structural formula (2) is mentioned, for example.

  By including an N-heterocyclic carbene ligand having a strong electron donating property, the Ni2 nucleus complex can be made a diamagnetic substance. In this case, nuclear magnetic resonance (NMR) can be used to identify the molecular structure during synthesis of the complex, so that it is possible to more easily synthesize heterogeneous metal multinuclear complexes containing Ni and Au. It becomes.

The Ni2 nucleus complex containing the above-mentioned bridged hydroxyl group can be prepared by any method known to those skilled in the art. For example, a Ni2 nucleus complex containing an N-heterocyclic carbene ligand having the above structural formula (2) has the following structural formula (3):
It can be synthesized by adding a base such as sodium hydroxide (NaOH) and extracting an iodide ion as shown in the following reaction formula (4). is there. The base is not particularly limited, and potassium hydroxide (KOH) or the like can be used in addition to NaOH.

In this specific example, the Ni2 nucleus complex contains I ⁻ as a counter ion. In this case, if necessary, the counter ion may be replaced with a counter ion having hydrophobic characteristics, for example, [B (C ₆ F ₅ ) ₄ ] ⁻ as shown in the following formula (5). Good. By using a Ni2 nucleus complex containing a counter ion having hydrophobic properties, for example, in the organic solvent used in the reaction of the Ni2 nucleus complex with Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] It is possible to reliably dissolve the Ni2 nucleus complex.

In the reaction step, the reaction of the Ni2 nucleus complex and Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] proceeds as specifically represented by the following formula (6).

Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] more specifically has the following structural formula (7), which can be prepared by any method known to those skilled in the art.

By reacting a Ni2 nucleus complex containing a bridged hydroxyl group with an Au mononuclear complex Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ], an OH group that crosslinks two Ni in the Ni2 nucleus complex and Au A heterometallic multinuclear complex containing Ni and Au can be obtained by bonding with Au in (PPh ₃ ) [N (SiMe ₃ ) ₂ ].

Since Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] has the characteristics as a base, the Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] removes protons from the bridging OH group in the Ni2 nucleus complex. Is done. By such deprotonation, a condensation reaction involving elimination of silylamine proceeds between the Ni2 nucleus complex and Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ]. As a result, dissimilar metal multinuclear complex containing Ni and Au in which the bridging OH group in Ni2 nucleus complex and Au in Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] are bonded, more specifically, Ni ₂ Au ₂ tetranuclear complex can be obtained.

The Au mononuclear complex, that is, Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] used in the reaction process of the present invention is triphenylphosphine (PPh) in addition to a silylamine group, more specifically, a bis (trimethylsilyl) amine group. ₃ ) Including group. However, as is clear from the above formula (6), the Au mononuclear complex may be any one as long as condensation with elimination of silylamine occurs with the Ni2 nucleus complex, and therefore, a silylamine group, particularly Any material containing a bis (trimethylsilyl) amine group as an essential component may be used. Therefore, the triphenylphosphine (PPh ₃ ) group in the Au mononuclear complex that does not directly participate in the reaction of the above formula (6) is substituted with any other appropriate group or ligand as necessary. It is possible. For example, as such an Au mononuclear complex, the following general structural formula (8)
In which R ³ is a hydrogen atom, a substituted or unsubstituted alkyl group having 1 to 20 carbon atoms, a phenyl group, an alkoxycarbonyl group, or an acyl group.

The amount of Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] introduced in the above reaction step depends on the desired molar ratio with Ni in the Ni2 nucleus complex or is finally different. What is necessary is just to determine suitably according to desired Au content in a metal polynuclear complex. Although not particularly limited, in general, Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] has a molar ratio of Au to Ni (Au: Ni) with respect to Ni in the Ni ₂ nucleus complex of 1: 0. It can be introduced in the range of 1 to 10.

In the above reaction step, for example, a solution in which Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] is dissolved in a solvent is introduced into a solution in which a Ni ₂ nucleus complex is dissolved in a solvent, and then a predetermined temperature and time are set. For example, by stirring at a temperature not lower than the freezing point of the solvent and not higher than about 25 ° C., particularly at a temperature of about −95 ° C. to about 25 ° C. (room temperature) for about 1 hour to about 20 hours. As the above solvent, Ni @ 2 binuclear complex and _{Au (PPh 3) [N (} SiMe 3) 2] can be used any solvent which can dissolve the is not particularly limited, preferably an organic solvent Can be used.

  The organic solvent in the reaction step is not particularly limited, and examples thereof include ethers such as tetrahydrofuran (THF) and diethyl ether, halogenated hydrocarbons such as dichloromethane, dichloroethane, chloroform, chlorobenzene and dichlorobenzene, N-methyl 2 Examples thereof include amides such as pyrrolidone and N, N-dimethylacetamide, and mixed solvents of at least one of these and hydrocarbons such as n-hexane (may be simply abbreviated as “hexane”).

After reacting the Ni2 nucleus complex with Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ] (ie after the reaction step), an additional purification step may optionally be added. Such a purification step can be performed by any method known to those skilled in the art. Although not particularly limited, for example, by purifying the crude product after the reaction step by an operation such as recrystallization, it is possible to obtain a heterometallic multinuclear complex containing Ni and Au with higher purity in the above formula (6). It becomes possible.

<Manufacture of exhaust gas purification catalyst>
For example, according to another aspect of the present invention, the heterogeneous metal multinuclear complex containing Ni and Au obtained by the method of the present invention can be used in the preparation of exhaust gas purification catalysts for automobiles and the like. Specifically, the dissimilar metal multinuclear complex containing Ni and Au produced as described above through the reaction step and further the optional purification step is then supported on the catalyst carrier in the supporting step and used for exhaust gas purification. A catalyst is prepared.

[Catalyst support]
The catalyst carrier to be introduced in this supporting step is not particularly limited, and any metal oxide generally used as a catalyst carrier in the technical field of exhaust gas purification catalyst can be used. Examples of such a catalyst carrier include alumina (Al ₂ O ₃ ), silica (SiO ₂ ), ceria (CeO ₂ ), zirconia (ZrO ₂ ), titania (TiO ₂ ), and combinations thereof. Can do.

[Supporting process]
In the supporting step, a predetermined amount of a metal oxide (catalyst support) powder is added to a solution containing a heterogeneous metal multinuclear complex containing Ni and Au produced by the method of the present invention, and the amount of Ni and / or Au (metal). It is introduced in such an amount that the equivalent supported amount is generally in the range of 0.01 to 10 wt% with respect to the catalyst carrier. This is then dried and / or at a predetermined temperature and time, in particular at a temperature and time sufficient to decompose and remove the complex salt portion of the heterometallic multinuclear complex and to support the metal particles comprising Ni and Au on the catalyst support. By calcination, an exhaust gas purifying catalyst in which metal particles made of Ni and Au are supported on a catalyst carrier can be obtained.

  In general, the drying can be performed at a temperature of about 80 ° C. to about 250 ° C. for about 1 hour to about 24 hours under reduced or normal pressure, while the calcination is oxidative. It can be carried out in an atmosphere, for example in air, at a temperature of about 200 ° C. to about 800 ° C. for about 30 minutes to about 30 hours.

  Finally, the exhaust gas-purifying catalyst obtained as described above is formed into a pellet by pressing under high pressure, for example, or formed into a slurry by adding a predetermined binder or the like. It can be used by coating on a catalyst substrate such as a cordierite honeycomb substrate.

  By using the dissimilar metal polynuclear complex containing Ni and Au produced by the method of the present invention, Ni and Au are binary metal particles coexisting at the nano level, particularly at the atomic level, and more uniform and fine An exhaust gas purifying catalyst in which such binary metal particles are supported on a catalyst carrier can be produced.

  In contrast, even if Ni and Au are supported on the catalyst carrier simply by a conventionally known co-impregnation method or sequential impregnation method, it is not possible to form bimetallic particles in which they coexist at the nano level. On the other hand, according to the method using the reducing agent such as sodium borohydride described above, it is possible to produce bimetallic particles in which Ni and Au coexist at the nano level. However, the Ni—Au binary metal particles obtained by such a method have a larger average particle diameter than the Ni—Au binary metal particles obtained using the heterometallic multinuclear complex according to the present invention, and The uniformity of composition may also be inferior. As a result, there are cases where the exhaust gas purification performance of Ni as a catalyst component cannot be fully exhibited, or the effects of combining Ni and Au cannot be fully exhibited.

  By using a heterometallic multinuclear complex containing Ni and Au produced by the method of the present invention, it is generally more than 0 nm to 20 nm, particularly more than 0 nm to 15 nm, more than 0 nm to 10 nm, more than 0 nm to 5 nm, It is possible to produce an exhaust gas purifying catalyst in which binary metal particles composed of Ni and Au of 0 nm to 3 nm or less, 0 nm to 2 nm or less, or 0 nm to 1 nm or less are supported on a catalyst carrier.

  In general, it is known that base metals such as Ni are more easily oxidized than platinum group elements. When Ni is oxidized by an oxidizing component such as oxygen contained in the exhaust gas, the metallization becomes insufficient, and as a result, the catalyst is sufficient for purification of exhaust gas, particularly NOx reduction purification. Unable to achieve activity. However, according to the present invention, Ni and Au coexist at the nano level, particularly at the atomic level, so that the oxidation of Ni existing in close proximity to Au due to relatively weak affinity for oxygen is remarkable. It is considered to be suppressed. As a result, Ni can be maintained in a highly active metal state, and the exhaust gas purification activity of the obtained exhaust gas purification catalyst can be remarkably improved.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples at all.

  In the following examples, dissimilar metal polynuclear complexes containing Ni and Au were produced by the method of the present invention, and the structures and the like were examined. Further, an exhaust gas purification catalyst was prepared using the heterogeneous metal polynuclear complex, and the characteristics thereof were investigated.

[Example 1]
[Production of dissimilar metal polynuclear complex containing Ni and Au]
Unless otherwise noted, all the following experimental operations were performed using ordinary Schlenk under a nitrogen atmosphere. Tetrahydrofuran (THF), hexane, and diethyl ether were purchased from Kanto Chemical and degassed before use. Acetonitrile was dehydrated from Wako Pure Chemical Industries and used. The analysis was performed using the following measuring equipment and conditions.
IR measurement: JASCO FT-IR4100
NMR measurement: measured at room temperature using JEOL ECP500 X-ray structure analysis: using Rigaku R-AXIS Rapid, processing with Rigaku PROCESS-AUTO and ABSCOR programs, then performing structural analysis using the direct method (SHELXS 97), The structure is illustrated by the ORTEP-3 program.
Elemental analysis: Perkin Elmer 2400 CHNS / O analyzer

Production of a heterometallic multinuclear complex containing Ni and Au was carried out as shown in the following formula (9).

First, Compound 1 as a starting material was treated with an aqueous solution of sodium hydroxide (NaOH) to obtain Ni2 nucleus complex 2 containing a crosslinked hydroxyl group in a yield of 89%. Next, counter ions were exchanged from I ⁻ to [B (C ₆ F ₅ ) ₄ ] ⁻ to obtain Ni 2 nucleus complex 3. Next, 365.4 mg (0.183 mmol) of the Ni2 nucleus complex was dissolved in 5 mL of tetrahydrofuran (THF), cooled to −90 ° C., and 3 equivalents of Au (PPh ₃ ) [N (SiMe ₃ ) ₂ prepared separately. The mixture was stirred for 12 hours at room temperature. The reaction solution did not change from yellow. Then, dryness, and washed with hexane (10mL + 5mL + 4mL), the THF / hexane bilayer recrystallization, the Ni ₂ Au ₂ 4 nucleus complex containing a crosslinking hydroxyl group as a yellow blocky crystals, yield 83 against complex 3 % (437.9 mg, 0.152 mmol). The analysis results of the obtained complex were as follows.

¹ H NMR (500 MHz, CD ₃ CN): d 3.79 (s, NMe, 24H), 6.77 (s, HC═CH, 8H), 7.45 to 7.65 (m, PPh ₃ , 30H) )
³¹ P { ¹ H} NMR (202 MHz, CD ₃ CN): δ 27.8 (s, PPh ₃ ).

FIG. 1 is a diagram showing the results of X-ray crystal structure analysis of the Ni ₂ Au ₂ tetranuclear complex produced in Example 1.

[Example 2]
[Preparation of exhaust gas purification catalyst (Ni—Au / SiO ₂ ) using Ni ₂ Au ₂ tetranuclear complex]
First, 20 mL of acetonitrile was placed in a 50 mL beaker, and 0.28 g (0.10 mmol) of Ni ₂ Au ₂ tetranuclear complex produced in Example 1 was dissolved therein. Next, while stirring at room temperature, 2.0 g of silica support (Nanotek S _BET = 94 m ^{2 /} g) was added and stirred for 10 minutes, and then the powder was separated by filtration. Next, this was washed with acetonitrile and then dried at 120 ° C. overnight to obtain an exhaust gas purifying catalyst (Ni—Au / SiO ₂ ) powder having a Ni loading of 1 wt% in terms of the carrier weight ratio.

[Comparative Example 1]
[Synthesis of Ni-Au particles by sodium borohydride reduction]
First, 2.71 g (10.9 mmol) of nickel acetate tetrahydrate (Ni (CH ₃ COO) ₂ .4H ₂ O) was weighed in a 200 mL beaker, and 80 mL of tetraethylene glycol was added thereto and heated at about 80 ° C. Stir to dissolve and then cool in an ice bath. Next, 21.6 g (194 mmol) of polyvinylpyrrolidone (PVP K-25) as a protective agent was weighed in a 500 mL beaker, 160 mL of tetraethylene glycol was added thereto, and the mixture was dissolved by heating and stirring at about 80 ° C. Cooled with. Next, 5.3 g (10.9 mmol) of 30% chloroauric acid (HAuCl ₄ ) aqueous solution was weighed in a 100 mL beaker, dissolved in 40 mL of tetraethylene glycol, and mixed with the above nickel acetate solution and PVP solution. Next, the resulting solution was transferred to a flask, and 4.4 g (117 mmol) of sodium borohydride dissolved in 40 mL of tetraethylene glycol was added thereto while bubbling with nitrogen, and heated to reflux at a bath temperature of 220 ° C. for 30 minutes. . The reaction solution was cooled to room temperature, diluted with 3.6 L of acetone, centrifuged (3000 rpm × 10 minutes), and the supernatant was removed. 100 mL of ethanol was added to the precipitate to obtain a dispersion containing Ni—Au particles.

[Preparation of exhaust gas purifying catalyst (Ni—Au / SiO ₂ ) using Ni—Au particles by sodium borohydride reduction]
First, the dispersion liquid (about 50 mL) containing the Ni—Au particles obtained above was placed in a 300 mL beaker, 50 mL of water was added, and the mixture was stirred with a magnetic stirrer. Next, the dispersion medium was removed by placing the silica support (Nanotek SiO ₂ ) in a beaker in such an amount that the Ni loading was 1 wt% in terms of the weight ratio of the support and stirring with heating. Next, after drying at 120 ° C. for 12 hours in a drier, the powder was pulverized in a mortar and finally calcined at 300 ° C. for 30 hours in air to obtain exhaust gas purifying catalyst (Ni—Au / SiO ₂ ) powder.

[Analysis of catalyst by STEM]
The exhaust gas purifying catalysts of Example 2 and Comparative Example 1 were measured using a scanning transmission electron microscope (STEM) (JEOL JEM-1000, acceleration voltage: 200 kV). Each measurement sample was dispersed in ethanol, dropped onto a molybdenum grid, and then dried and measured. The results are shown in FIGS.

2 and 3 show the analysis results by STEM of the exhaust gas purifying catalysts of Example 2 and Comparative Example 1, respectively. Referring to FIG. 3, it can be confirmed that the catalyst prepared by using sodium borohydride reduction has particles of about 5 nm on the silica support. In contrast, referring to FIG. 2, the catalyst prepared using the Ni ₂ Au ₂ tetranuclear complex has smaller primary particles, eg, several nm, compared to the catalyst prepared using sodium borohydride reduction. In particular, it can be confirmed that many fine particles having a particle diameter of about 1 nm or less are formed. From the composition analysis by EDX, both Ni and Au elements were detected in these particles for both the catalyst of Example 2 and Comparative Example 1. These results indicate that in both the catalyst prepared using the Ni ₂ Au ₂ tetranuclear complex and the catalyst prepared using sodium borohydride reduction, nanoparticles in which both Ni and Au elements coexist are formed. It is to support that there is.

Claims (1)

A method for producing a heterometallic multinuclear complex containing Ni and Au, comprising a reaction step of reacting a Ni2 nucleus complex containing a crosslinked hydroxyl group and Au (PPh ₃ ) [N (SiMe ₃ ) ₂ ].