A platinum(II)–palladium(II)–nickel(II) heterotrimetallic coordination polymer showing a cooperative effect on catalytic hydrogen evolution

Abstract

Stepwise construction of a 1D heterotrimetallic coordination polymer containing all three group 10 metal ions via an ammineplatinum(II) metalloligand with D-penicillamine is reported. This system showed a significant enhancement in heterogeneous catalytic activity for electrochemical hydrogen evolution by the stepwise introduction of Pd^II and Ni^II into the Pt^II metalloligand.

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In recent years, heterometallic coordination compounds have received increased attention because the incorporation of different kinds of metal ions into one molecule often leads to an intriguing cooperative effect on physical and chemical properties, such as magnetism, emission, and catalysis.¹ To date, many examples of ‘heterobimetallic’ compounds that show some cooperative effect on their properties have been reported. However, reports on a cooperative effect for ‘heterotrimetallic’ species are still relatively uncommon.² This is owing to a lack of rational synthetic methods to incorporate three different kinds of metal ions into a single molecule; most of the heterotrimetallic coordination compounds have been synthesized via one-pot reactions in which three kinds of metal ions are self-assembled with organic and/or inorganic ligands under specific conditions.³ To overcome this problem, we have developed a metalloligand approach, in which a metal complex having several donor sites coordinates in a stepwise fashion to second and third metal ions to form the desired heterotrimetallic coordination compounds.^4–6 Previously, we showed that a linear-type Au^I complex, [Au(D-pen)₂]³⁻ (D-H₂pen = D-penicillamine), in which an Au^I centre is linearly coordinated by two thiolato groups from two D-pen ligands, can serve as a multifunctional metalloligand by using coordinated thiolato and free amine and carboxylate groups, forming a variety of heterobimetallic and heterotrimetallic species.⁶ For example, a cage-type metallosupramolecular compound that contains all group 11 metal ions (Au^I, Ag^I, and Cu^II) has been synthesized by the stepwise reaction of [Au(D-pen)₂]³⁻ with Ag⁺ and Cu²⁺.^6a This was the first example of a heterotrimetallic coordination compound consisting of all group 11 metal ions. Other heterotrimetallic compounds that consist of all metal ions belonging to the same group have rarely been synthesized.⁷ Recently, we found that a newly prepared Pt^II metalloligand with D-pen, trans-[Pt(NH₃)₂(D-pen)₂]²⁻ ([1]²⁻), in which the Au^I linker in [Au(D-pen)₂]³⁻ is replaced by a trans-{Pt^II(NH₃)₂}²⁺ moiety, also acts as a multifunctional metalloligand.⁸ This finding prompted us to investigate the reactivity of [1]²⁻ toward Pd²⁺ and Ni²⁺ to create a heterotrimetallic compound containing all group 10 metal ions (Pt^II, Pd^II, and Ni^II) in one molecule. Here, we report that [1]²⁻ reacts with Pd(OAc)₂ to afford a Pt^II₂Pd^II₂ tetranuclear complex ([2]), which further reacts with NiCl₂ to produce a Pt^II₂Pd^II₂Ni^II coordination polymer ([3]Cl₂) (Scheme 1). Remarkably, [3]Cl₂ showed excellent catalytic activity for electrochemical hydrogen evolution compared with [1]²⁻ and [2], indicating a significant enhancement of the catalytic activity due to the incorporation of all three group 10 metal ions in one molecule. To our knowledge, such a cooperative effect on catalytic activity has not been reported.

Scheme 1 Synthetic route of [2] and [3]²⁺via [1]²⁻.

The reaction of [Pt(NH₃)₂(D-Hpen)₂] ([H₂1]) with Pd(OAc)₂ in H₂O gave an orange solution, from which yellow crystals ([2]) were isolated. The ¹H NMR spectrum of [2] displayed only a single set of signals (δ 1.43, 1.56, and 3.35 ppm) due to D-pen ligands in the complex, indicative of its high symmetrical structure (Fig. S1, ESI†). The presence of a deprotonated carboxyl group in [2] was implied by its IR spectrum that shows a strong band at 1600 cm⁻¹ (Fig. S2, ESI†).⁹ The elemental analytical data of [2] were in agreement with a neutral formula containing [1]²⁻ and Pd^II in a 1 : 1 ratio. Single crystals of [2] suitable for X-ray crystallography were obtained as the protonated form, [H₄2]Cl₄. The asymmetric unit of [H₄2]Cl₄ contains one complex cation and four chloride anions, besides solvated water molecules. As shown in Fig. 1, the complex cation has an S-bridged Pt^II₂Pd^II₂ tetranuclear structure in [Pd₂Pt₂(NH₃)₄(D-Hpen)₄]⁴⁺, consisting of two metalloligands with protonated carboxyl groups ([H₂1]) and two Pd^II atoms. Each of the two metalloligands span two Pd^II atoms in a bis(bidentate-N,S) mode, such that each Pd^II centre has a square planar geometry with a cis-N₂S₂ donor set (av. Pd–N: 2.084 Å, Pd–S: 2.272 Å). The Pt⋯Pt distance in [H₄2]⁴⁺ is 3.300 Å, indicating the lack of an intramolecular Pt⋯Pt interaction.¹⁰ Each NH₃ ligand is hydrogen-bonded with a D-pen thiolato group (av. N_NH3⋯S = 3.46 Å), which appears to stabilize the Pt^II₂Pd^II₂ structure in [2]. Here, it should be noted that the ¹H NMR spectral features of the reaction solution are the same as those of isolated [2] (Fig. S1, ESI†), indicating that only a single isomer with two cis-[PdN₂S₂] planes is selectively formed by the reaction of [H₂1] with Pd²⁺. This result is different from the corresponding reaction of [Au(D-pen)₂]³⁻ with Pd²⁺, which produced a mixture of cis and trans isomers of [Au₂Pd₂(D-pen)₄]²⁻.^6c The overall structure of [H₄2]⁴⁺ resembles that of the cis isomer of [Au₂Pd₂(D-pen)₄]²⁻. However, all of the bridging S atoms in [H₄2]⁴⁺ have the R configuration and form a twisted Pt^II₂Pd^II₂S₄ framework with a D₂ molecular symmetry (Fig. 1). In contrast, cis-[Au₂Pd₂(D-pen)₄]²⁻ has two S and two R configurational bridging S atoms, forming a boat-like framework with a C₂ symmetry.^6c While a number of S-bridged M₂M₂′ tetranuclear complexes with two MN₂S₂ square-planes have been prepared and structurally characterized,¹¹ the formation of the twisted form is rare. Molecular model examinations revealed that, in [H₄2]⁴⁺, there exist steric repulsions between the NH₃ ligand and the methyl group of a D-pen ligand in the trans isomer and the boat and chair forms of the cis isomer (Fig. S3, ESI†).

Fig. 1 (a) Top and (b) side views of the molecular structure of [H₄2]⁴⁺.

To create a heterotrimetallic coordination compound containing all group 10 metal ions (Pt^II, Pd^II, and Ni^II), [2] was reacted with NiCl₂ in water. This reaction gave a clear green solution, from which yellow crystals of [3]Cl₂ were obtained. X-ray fluorescence analyses indicated the presence of Pt, Pd, and Ni atoms in [3]Cl₂, and its elemental analytical data were consistent with a formula containing [2] and NiCl₂ in a 1 : 1 ratio. The IR spectrum of [3]Cl₂ showed a strong absorption band at 1640 cm⁻¹, indicating the presence of deprotonated carboxylate groups (Fig. S2, ESI†).⁹ The structure of [3]Cl₂ was established by a single-crystal X-ray diffraction study. The asymmetric unit of [3]Cl₂ contains one [Pd₂Pt₂(NH₃)₄(D-pen)₄] molecule, one Ni^II ion, and two chloride anions, besides solvated water molecules. As shown in Fig. 2, the overall structure of the Pt^II₂Pd^II₂ tetranuclear molecule in [3]Cl₂ is essentially the same as that of [H₄2]Cl₄. However, the four carboxyl groups of the Pt^II₂Pd^II₂ molecule in [3]Cl₂ are deprotonated, consistent with the IR spectral feature. In addition, two of these groups each coordinate to a Ni^II ion to form a cationic 1D chain structure in [{Ni(H₂O)₄}{Pd₂Pt₂(NH₃)₄(D-pen)₄}]²⁺ (Fig. 2). Note that [3]Cl₂ is the first example of a structurally characterized coordination compound with all three group 10 metal ions. In [3]²⁺, each Ni^II ion is bound by four aqua ligands and two carboxylate groups in a trans octahedral geometry (av. Ni–O_COO⁻: 2.064 Å, Ni–O_water: 2.093 Å). The chloride ions are not involved in the coordination and are located between the Pt^II₂Pd^II₂Ni^II chains, forming hydrogen bonds with ammine groups and water molecules (av. Cl⋯N_NH3: 3.29 Å, Cl⋯O_water: 3.09 Å, Fig. S3, ESI†).

Fig. 2 1D chain heterotrimetallic structure in [3]²⁺.

The catalytic property of [3]Cl₂ in the solid state was electrochemically investigated, and the results were compared with those of [H₂1] and [2]. The linear sweep voltammetry of a solid sample of [3]Cl₂, attached to the surface of a glassy carbon electrode (0.07 cm²), in H₂O/CH₃CN (v/v = 1/9) containing 0.1 M LiClO₄ displayed a sharp current increase at an onset potential of −0.98 V (vs. Ag/AgCl). A similar but lower current increase was also observed for a solid sample of [2] at −1.10 V. (Fig. 3).¹² However, a solid sample of [H₂1] showed only a slight current increase at −1.15 V. Since the control experiment using a bare glassy carbon electrode under the same conditions did not show any current increase until reaching −1.5 V, this observation is indicative of a catalytic reduction,¹³ the activity of which drastically increases in the following order: [H₂1] < [2] < [3]Cl₂. During the voltammetric scans, the evolution of bubbles from the electrode surface was observed for [2] and [3]Cl₂. The bubbles were analyzed using gas chromatography (GC) and found to be H₂ gas (Fig. S5, ESI†). Such an evolution of bubbles, as well as a current increase, was not detected when absolute CH₃CN was used as the solvent instead of H₂O/CH₃CN (Fig. S6, ESI†). Thus, [2] and [3]Cl₂ catalyze the electrochemical reduction of water to produce H₂ gas. These are relatively few examples of coordination compounds that exhibit a heterogeneous catalytic activity for H₂ gas evolution on an electrode surface.¹⁴ Considering that [H₂1] is significantly less active for catalytic reduction of water than [2] and [3]Cl₂, the catalytic reaction mainly occurs at the Pd^II centre of the [Pd(D-pen)₂]²⁻ square-planes. It has been proposed that Pd is the best alternative to Pt in catalytic systems for H₂ evolution.¹⁵ However, examples reporting catalytic hydrogen evolution are limited to several homogeneous catalytic systems,¹⁶ and this is the first Pd^II coordination system that evidences heterogeneous catalytic H₂ evolution.

Fig. 3 Linear sweep voltammograms of (a) [3]Cl₂, (b) [2], and (c) [H₂1] attached to a glassy carbon electrode (0.07 cm²), and (d) a bare glassy carbon electrode, in H₂O–CH₃CN (v/v = 1/9) containing 0.1 M LiClO₄. The scan rate is 0.01 V s⁻¹. Inset: charge build-up versus time in extended potential-controlled electrolysis experiments at an applied potential of −1.20 V.

To evaluate the H₂ evolution rate, potential-controlled electrolyses were carried out under the same conditions as those of the voltammetric experiments. At an applied potential of −1.20 V, the rate of Coulomb charge increased in the order [H₂1] < [2] < [3]Cl₂, which is in parallel with the voltammetric results (Fig. 3). The turnover frequency (TOF) of H₂ (mol H₂/mol cat) was calculated to be 3.73, 18.2, and 34.4 mol min⁻¹ cm⁻² for [H₂1], [2], and [3]Cl₂, respectively. The GC analysis indicated that the amount of H₂ gas generated during the electrolysis using [2] and [3]Cl₂ was 1.56 μmol and 2.88 μmol, respectively. From these values, the Faraday efficiencies for [2] and [3]Cl₂ were calculated to be 81% and 100%, respectively, indicating that no side redox reaction occurs during the electrolysis for [3]Cl₂. It has been shown that the increase of Lewis acidity around the reaction centre promotes the efficiency of an electrocatalytic hydrogen evolution.¹⁷ In [3]Cl₂, the D-pen carboxylate group coordinates to a Ni^II centre, which might lead to an increase of the Lewis acidity around a Pd^II catalytic centre.¹⁸

In summary, we showed that the metalloligand trans-[Pt(NH₃)₂(D-pen)₂]²⁻ ([1]²⁻) reacts with Pd^II to afford an S-bridged Pt^II₂Pd^II₂ complex, [Pd₂Pt₂(NH₃)₄(D-pen)₄] ([2]), which exclusively forms a twisted isomer consisting of cis-[Pd(D-pen)₂]²⁻ planes. This coordination behaviour is due to the presence of two NH₃ ligands bound to each Pt^II centre, exerting non-bonding steric and NH⋯S hydrogen bonding interactions. Complex [2] acts as an O-donating metalloligand to Ni^II to create a Pt^II₂Pd^II₂Ni^II coordination polymer, [{Ni(H₂O)₄}{Pd₂Pt₂(NH₃)₄(D-pen)₄}]Cl₂ ([3]Cl₂), which is the first example of a structurally characterized heterotrimetallic coordination compound with all three group 10 metal ions in one molecule. Due to the stepwise combination of Pd^II and Ni^II centres with [1]²⁻, the electrocatalytic water reduction was largely enhanced by heterogeneous activity, demonstrating a significant cooperative effect due to metal ions belonging to the same group, for the first time.

This work was supported by CREST, JST and JSPS KAKENHI Grant Numbers 16K13609 and 16K17945. One of the authors (NK) acknowledges the support from the Kurita Water and Environment Foundation and the Kansai Research Foundation for Technology Promotion.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, NMR, IR, LSV, GC, and crystal packing. CCDC 1514220 and 1514221. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc08789b

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