Tiara-like Hexanuclear Nickel–Platinum Alloy Nanocluster

Tiara-like metal nanoclusters (TNCs) have attracted a great deal of attention because of their high stability and easy synthesis under atmospheric conditions as well as their high activity in various catalytic reactions. Alloying is one of the methods that can be used to control the physicochemical properties of nanoclusters, but few studies have reported on alloy TNCs. In this study, we synthesized alloy TNCs [NixPt6–x(PET)12, where x = 1–5 and PET = 2-phenylethanethiolate] consisting of thiolate, nickel (Ni), and platinum (Pt). We further evaluated the stability, geometric structure, and electronic structure by high-performance liquid chromatography and density functional theory calculations. The results revealed that NixPt6–x(PET)12 has a distorted structure and is therefore less stable than single-metal TNCs.


Characterization
Matrix-assisted laser desorption/ionization (MALDI) mass spectra were recorded with a JMS-S3000 spiral time-of-flight mass spectrometer (JEOL, Tokyo, Japan) equipped with a semiconductor laser (λ = 349 nm).DCTB was used as the MALDI matrix.To minimize nanocluster (NC) dissociation induced by laser irradiation, the sample-to-matrix ratio was fixed at 1:1000.
The transmission electron microscope (TEM) images were recorded with a H-9500 electron microscope (HITACHI, Tokyo, Japan) or JEM-2100 electron microscope (JEOL, Tokyo, Japan) operating at 200 kV, typically using magnification of 600,000.
Optical absorbance spectra of products were acquired in DCM at 25 °C with a V-630 spectrometer (JASCO, Tokyo, Japan).
Ni K-edge and Pt L3-edge X-ray absorption fine structure (XAFS) measurements were performed at beamline BL01B1 of the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (proposal numbers 2020A0695, 2021A1102, 2021B1163, 2022A1075, and 2022B1823).The incident X-ray beam was monochromatized with a Si(111) double-crystal monochromator.XAFS spectra of all samples (as well as Ni foil, solid NiO, Pt foil, and solid PtO2 as a reference) were recorded in transmission mode with ionization chambers.The X-ray energies for the Ni K-edges and Pt L3-edges were calibrated with Ni foil and Pt foil, respectively.X-ray absorption near-edge structure (XANES) and extended XAFS (EXAFS) spectra were analyzed with xTunes [1] as follows.The χ spectra were extracted by subtracting the atomic absorption background by cubic spline interpolation and normalized to the edge height.The normalized data were used as the XANES spectra.The k 3 -weighted χ spectra in the k range 3.0-13.0Å −1 for the Ni K-edge and Pt L3-edge were Fourier-transformed into r space for structural analysis.
Separation and isolation of products were performed with a Shimadzu Prominence high-performance liquid chromatography (HPLC) system consisting of LC-20AD (2 pumps), an SPD-M20A [photodiode array (PDA) detector], a CTO-20AC (column oven), and a DGU-20A3R (on-line degasser).A YMC core-shell ODS column (Meteoric Core C18, 150 mm × 4.6 mm, I.D., 2.7 µm) was used as the stationary phase.The column and detector were aged (stabilized) for sufficient time prior to analysis.Reversed phase (RP) mode was used for separation.The mobile phase was continuously changed from pure acetonitrile to pure acetone/toluene by the gradient program.The flow rate of the mobile phase was set to 1.0 mL/min, and the optical absorption spectra of the separated products were obtained with a PDA detector in the range of 190-800 nm.The chromatogram was obtained from the absorption intensity at 420 nm.
Inductively coupled plasma-mass spectrometry was performed with an Agilent 7850c spectrometer (Agilent Technologies, Tokyo, Japan).Yttrium was used as the internal standard for Ni and bismuth as the internal standard for Pt.
The Pt 4f and Ni 2p X-ray photoelectron spectroscopy (XPS) spectra were collected by using a JPS-9030 electron spectrometer (JEOL, Tokyo, Japan) at a base pressure of ∼2 × 10 −8 Torr.X-rays from the Mg-Kα line (1253.6eV) were used for excitation.Each TNCs was deposited on a molybdenum (Mo) plate and the spectra were calibrated with the peak energies of Mo 3d5/2 (227.7 eV).
General density functional theory (DFT) and time-dependent (TD)-DFT calculations were performed with Gaussian 16 (ES64L-G16, RevB.01) [2] .To reduce the CPU cost, all 2-phenylethanethiolate (PET) ligands in the cluster were replaced with SCH3 (MT) ligands.The optimized geometric structure of NixPt6-x(MT)12 (x = 0-6) in the ground state was determined by using the BP86 functional [3] with the basis sets of def2-SV(P) [4] for Ni and Pt atoms; and 6-31G(d,p) [5] for H, C, and S atoms.SDD pseudopotentials with scalar relativistic effects were used. [6]Note that no symmetry constraints were added during the calculations.The obtained structure was confirmed to be an optimized structure by harmonic vibrational frequency analysis.[9] In this calculation, the basis set def2-SV(P) was used for the Ni and Pt atoms; 6-31G(d,p) for the H, C, and S atoms; and the SDD pseudopotential was also used.The molecular geometries and MOs were drawn with Avogadro 1.2.0.The irreducible representation of each molecular orbital was confirmed by recalculating with symmetry constraints.

Experiment
Synthesis of 1 [NixPt6-x(PET)12 (x = 0-6)] The overall reaction process was carried out in a 50-mL vial at room temperature and under air.First, Ni(NO3)2⋅6H2O, (100 mg, 0.34 mmol) and 0.2 M H2PtCl6•6H2O (1.7 mL, 0.34 mmol) were added to 1-propanol (12 mL) and stirred for 20 min until complete dissolution.The solution was then stirred vigorously and 2-phenylethanethiol (0.186 mL, 1.38 mmol) was slowly added to the solution.This caused the color of the solution to gradually change from light green to brown.The solution was stirred for 15 min to react with divalent Ni ions (Ni 2+ ), tetravalent platinum ions (Pt 4+ ), and 2-phenylethanethiol.NEt3 (0.5 mL) was then added to this solution straight away.The color of the solution immediately changed to dark brown.After stirring for 3 h, the crude product was washed with MeOH to remove, for example, unreacted Ni 2+ , Pt 4+ , 2-phenylethanethiol, and NEt3.The crude product was then extracted with DCM (Scheme S1).

Synthesis of 5 [NixPt6-x(PET)12 (x = 0-6) via metal exchange reaction]
The overall reaction process was carried out in a 50-mL vial at room temperature and under air.First, Ni6(PET)12 (2.0 mg, 1 µmol), synthesized in accordance with a previous report, [10] was dissolved in 10 mL of THF.H2PtCl6 (1 µmol), dissolved in 2 mL of THF, was added to this Ni6(PET)12 solution and stirred for 30 min.Then, the THF solution was removed with a rotary evaporator, and the target material was extracted with DCM.Finally, the insoluble components were removed by filtration to obtain 5.
Preparation of Ni6(PET)12/CB and Product 1/CB (1 μmol of metal per 4 mg of CB) Ni6(PET)12/CB and Product 1/CB (1 μmol of metal per 4 mg of CB) was prepared by the impregnation method.Each cluster dissolved in THF solution was dropped on the CB and then the catalysts were dried.

Preparation of catalyst slurry
To conduct the electrochemical measurements on Ni6(PET)12/CB and Product 1/CB (1 μmol of metal per 4 mg of CB), a catalyst slurry was prepared.First, the catalyst powder (10.4 mg; Ni6(PET)12/CB and Product 1/CB (1 μmol of metal per 4 mg of CB)), was dispersed in a solution containing H2O (19.1 mL) and 2-propanol (6.0 mL).Then, Nafion TM solution (100 μL) was added to this solution.The vial containing this mixture was sealed and ultrasonicated for 60 min in an ice bath.

Electrochemical measurements
All electrochemical measurements for the hydrogen evolution reaction were performed with an ECstat-302 (EC FRONTIER, Kyoto, Japan) with a RRDE-3A rotating ring disk electrode apparatus (BAS, Tokyo, Japan).A rotating disk electrode (RDE, φ = 5 mm) was polished with alumina paste and then sonicated in water before use.A Pt ring electrode was used as the counter electrode.A silver/silver chloride (Ag/AgCl) electrode was used as the reference electrode.In the setup, first the catalyst slurry was sonicated in an ice bath for 30 min, and then 10 μL of the catalyst slurry was carefully dropped onto the RDE by the dropcast method.After the catalyst slurry was sufficiently dried, each electrode was set in an electrochemical measurement system containing 0.10 mol L −1 KOH (pH = 13) as the electrolyte.
In the measurements, N2 gas was bubbled for 30 min and then cyclic voltammetry (CV) was conducted 100 times in the region from 0.0 to 1.0 V (vs.reversible hydrogen electrode; RHE) at a scanning rate of 200 mV s −1 for cleaning the electrodes.After CV, linear sweep voltammetry (LSV) was performed under N2 in the region from 0.1 to −0.7 V (vs.RHE) at a rate of 20 mV s −1 .

Figure S2 .
Figure S2.(A) The TEM image and (B) resulting histograms of the particle-size distribution of synthesized 1.

Figure S3 .
Figure S3.Results of (A) Ni K-edge and (B) Pt L3-edge XANES spectra for synthesized 1 together with Ni foil and NiO powder in (A) and Pt foil and PtO2 powder in (B) as a standard sample.

Figure S7 .
Figure S7.(A) Optical absorbance spectra of 1 and pure Ni6(PET)12, and (B) RP-HPLC chromatogram of 1 and (C) UV-vis optical absorption spectra of the peak (i-xi) obtained by PDA detector attached to RP-HPLC apparatus.In (B), * indicates unknown compounds.

Figure S9 .
Figure S9.RP-HPLC chromatogram of synthesized 5 by metal exchange from Ni6(PET)12.In the metal exchange of adding Pt ions to Ni6(PET)12, almost no alloy TNC was obtained because of the high stability of Ni6(PET)12, and it is assumed that unstable species generated during the reaction produced Ni5(PET)10 (peak ii).

Figure S10 .
Figure S10.Positive-ion MALDI mass spectra of separated products for peak v in Figure S7.

Figure S11 .
Figure S11.RP-HPLC chromatogram of separated products for peak v from 1 in Figure S7.

Figure S14 .
Figure S14.(A) Bond length [(a) M-M distances and (b) M-S bonds] and (B) bond angles [(a) M-S-M, (b) M-M-M and (c) S-M-S angles] from an analysis of geometric structures for [NixPt6-x(MT)12] 0 determined by DFT calculations from Figure S10.

Table S1 . Calculated corresponding energies for NixPt6-x(MT)12 (x = 0-6) Composition Isomer a ΔE (meV) b PG c h d g e P f
a Numbers in brackets indicate the position of the substituted Pt atom in the cyclic Ni6.b Energy difference compared with most stable isomer.c Point group.d Order of point group.e Degeneracy of molecule, which was calculated by dividing the order of the point group D3d in Ni6(MT)12 (h = 12) by the order of the point group in the molecule of interest.f Boltzmann distribution at 300 K, which was obtained with the following equation:   =   exp(− ∆   B  ⁄ )  ⁄ ,  = ∑   exp�− ∆   B  ⁄ � ; where ΔEi is the energy difference between the stable isomer and the target molecule i, kB is Boltzmann's constant, and T is temperature.Note that the Boltzmann distribution