MetalâLigand Complexation through Redox ... - ACS Publications

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Metal−Ligand Complexation through Redox Assembly at Surfaces Characterized by Vibrational Spectroscopy Christopher G. Williams,† Miao Wang,‡ Daniel Skomski,†,§ Christopher D. Tempas,† Larry L. Kesmodel,‡ and Steven L. Tait*,†,‡ †

Department of Chemistry and ‡Department of Physics, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: The formation of metal−organic complexes on metal surfaces is a research topic of high interest to develop tunable functional surfaces. One such focus of this research is the formation of single site metal centers that have uniform ligand environments and thus uniform chemistry. We report the complexation of Pt and Ag with the ligand dipyridyl-tetrazine (DPTZ) on Ag(111) and of Pt with DPTZ on the reconstructed Au(100) surface. Each metal atom binds two DPTZ molecules resulting in one-dimensional supramolecular chains across the surface. Pt complexation occurs immediately after Pt deposition at room temperature on either surface. This complexation is improved with annealing to 170 °C on Au(100). DPTZ forms complexes with Ag atoms from the Ag(111) substrate when annealed to 110 °C. No similar complexation with substrate atoms is seen on Au(100). This metal−organic complexation with substrate atoms on Ag(111) takes place even when the DPTZ is already complexed to Pt, demonstrating a Pt replacement reaction by Ag at 80 °C, which has not been reported previously. The metal−organic complexes are characterized by highresolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS). This research is among the few to use HREELS to characterize the formation of extended metal organic networks on a surface formed through the redox of the organic species into an anionic state. XMCD).22−24 Those results have demonstrated formation of tailored MOF structures at surfaces. In order to achieve successful redox assembly of metal−ligand coordination structures at surfaces, the ligand design must include both a favorable ligating geometry and a strong electron accepting character. In some cases XPS has been used to determine the oxidation state of the metal centers.9,10,25−27 Vibrational spectroscopies can provide new insight into metal−organic systems at surfaces, but have not been extensively utilized in the analysis of these systems. High-resolution electron energy loss spectroscopy (HREELS) is particularly useful because both structural and chemical information can be determined from the spectra and because it is sensitive to low energy modes (below 900 cm−1) that are difficult to detect with IR spectroscopy, including many significant modes in organic adsorbates, such as out-of-plane aromatic modes. Vibrational spectroscopy characterization of such systems can allow new insight into the nature of metal centers that are both stabilized by a planar ligand field and are interacting with support surface atoms. Vibrational spectroscopies (Infrared methods and HREELS) have been used to characterize the formation of MOFs in

1. INTRODUCTION Typical metal nanoparticle catalysts have a variety of surface sites with different local coordination environments,1−5 which leads to the lower selectivity of the catalyst.6,7 On the other hand, metal−organic complexes commonly have single-site metal atom centers with specific coordination environments due to a well-defined ligand field,8−10 resulting in metal centers that have a uniform oxidation state and chemical reactivity. It has been suggested that metal−organic complexes at surfaces can stabilize single-site transition metal centers in a heterogeneous catalyst environment, which may allow significant improvements in selectivity of next-generation catalysts. Significant progress has been made in developing thin film growth of metal organic frameworks (MOFs),11−14 as well as more fundamental studies of metal−organic coordination in 1D and 2D geometries in the first monolayer at surfaces.15−17 There is growing interest in this class of materials, including exploration of on-surface metal−ligand redox reaction to form metal centers that are in intimate chemical contact with the underlying surface to take advantage of reverse spillover and other surface-assisted reaction pathways. Surface-supported metal−organic complexes and networks at surfaces have been analyzed by scanning tunneling microscopy (STM),9,10,18−20 low-energy electron diffraction (LEED),21 Xray photoelectron spectroscopy (XPS),9,10,15,18 and synchrotron based techniques (XSW, XPS, NEXAFS, XAS, and © XXXX American Chemical Society

Received: March 24, 2017 Revised: May 30, 2017 Published: June 1, 2017 A

DOI: 10.1021/acs.jpcc.7b02809 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C solution28 as well as metal−organic complexation at surfaces,29−31 including an example of an extended metal−organic assembly.29 It has been reported that the metal−organic complexation will alter the vibrational modes of the organic species due to the increased electron density in the ligand upon bond formation.29,32,33 The changes in the distribution of electron density also impact the relative intensities of features in vibrational spectra.34,35 In this study, we looked at the formation of Pt complexes on Ag(111) and Au(100) using the organic ligand dipyridyltetrazine (DPTZ, Scheme 1). Previous STM and XPS studies

further chemical analysis of these systems, including potential catalytic and gas adsorption studies.

2. EXPERIMENTAL SECTION Ag and Au single crystals, cut to expose (111) and (100) surfaces, respectively, were purchased from Princeton Scientific. All experiments were conducted in ultrahigh vacuum (UHV). In each experiment, the Ag(111) surface was cleaned by cycles of 1.5 keV Ar+ sputtering followed by thermal annealing at 460 °C. Au(100) was cleaned by cycles of 500 eV Ar+ sputtering followed by annealing at 460 °C. Sample surface temperature was monitored by a thermocouple attached to the back side of the sample stage, which was calibrated to the surface temperature. Surface cleanliness was verified by Auger electron spectroscopy (AES) metal to carbon signal ratio and by sharp LEED patterns. The surfaces were cooled gradually to room temperature before deposition in a separate but connected UHV chamber. DPTZ was purchased from Sigma-Aldrich (96% purity). The DPTZ was degassed in UHV at 50 °C for 1 h. DPTZ was vapor deposited from a Knudsen-type evaporator held at 85 °C to achieve a deposition rate of about 0.1 monolayers per minute, as monitored by quartz crystal microbalance. The surface coverage could not be verified using AES as the electron beam induced molecular desorption of the molecule. Submonolayer coverages were used to allow mobility of the molecules and metal atoms for efficient self-assembly. Pt was vapor deposited by resistive heating of a W filament that was wrapped with fine Pt wire (99.99%, Goodfellow). Postdeposition annealing treatments were done in most experiments, as described below. All HREELS experiments were performed with a double-pass 127° angle cylindrical deflection electron spectrometer (LK Technologies, model LK 2000) operated at an initial beam energy of 5−8 eV and a peak width of 55 ± 10 cm−1. Each spectrum was normalized by the average intensity of a featureless background region to account for differences in initial beam current (see the Supporting Information (SI) for more detail).

Scheme 1

have characterized the structure and metal oxidation state for the Pt-DPTZ and V-DPTZ systems on Au(100).9,10 Further insight into the impact of the surface and added electron density in DPTZ will be presented here using vibrational spectroscopy analysis. In addition to Pt-DPTZ complexation on Au(100), the HREELS experiments presented here show the on-surface redox formation of Pt-DPTZ metal−organic coordination complexes on Ag(111) for the first time. We observe DPTZ complexation with Ag adatoms on Ag(111). AgDPTZ complexation can also occur as a replacement reaction with Pt-DPTZ on the Ag(111) surface, replacing Pt with Ag atoms. To the best of our knowledge, this is the first observation of the replacement of Pt by Ag within a metal− organic coordination complex in solution or at surfaces. The relative stability of different metals in coordination complexes has been studied previously in solution-based systems,36 including ion exchange processes,37,38 transmetalation MOF studies,39 and metal-pyridine complexes.36 However, there are few prior studies of metal replacement reactions at surfaces.25 Here, we measure differences in HREEL spectra of DPTZ before and after complexation by Ag and Pt to gain insight into the metal−organic complexation process on a metal surface. This work provides an example of vibrational spectroscopy of metal−organic systems at surfaces, which will be important for

3. RESULTS AND DISCUSSION DPTZ on Ag(111) and Au(100). Submonolayer coverages of DPTZ in the range of 0.5−0.75 ML were vapor deposited onto Ag(111) or Au(100) surfaces and studied by HREELS at

Figure 1. HREELS of the bending mode and in-plane mode regions for (a) DPTZ and (b) DPTZ+Pt on the Ag(111) and Au(100) surfaces. Peak assignments are listed in Table 1 and discussed in the text. (a) Several differences for in-plane mode positions of noncomplexed DPTZ can be observed on the two surfaces, such as the pyridine in-plane mode at a higher energy on Ag(111) (618 cm−1) than on Au(100) (583 cm−1). (b) Several spectral changes after codeposition of Pt and DPTZ indicate Pt-DPTZ complexation on each surface. The spectrum shown here for Au(100) was recorded after annealing to drive the complexation to completion, but initial complexation is evident before annealing. The Pt-pyridine mode on Ag(111) (660 cm−1) is positioned at a higher energy than on Au(100) (618 cm−1). Multiple differences to the in-plane modes are also observed. The traces have been scaled by the factors indicated in the figure for clarity. B


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Table 1. DPTZ and Coordinated DPTZ Peak Assignments Compared with Prior IR Analysis of DPTZ and of Metal−DPTZ complexes prior IR studiesa

HREELS experiments in this work DPTZ on Ag(111) (cm−1) out-of-plane ring def.

C−H outof-plane bending

DPTZ on Ag(111) annealed (cm−1)b

DPTZ+Pt on Ag(111) (cm−1)

DPTZ+Pt on Ag(111) annealed (cm−1)b

DPTZ+Pt on Au(100) annealed (cm−1)c

DPTZ (cm−1)40

98

84

179

194

179

128

400

400

618 743

583 730

414 494 640 743

407 487 660 736

414 487 640 750

618 758

745

772

774 890

868

in-plane ring vib.

772 890

DPTZ + Au (cm−1)40

DPTZ + Ni (cm−1)41

615−625 700

440 598 750

799 920

791

796

956

956 1095

1000 1095

993 1088

1015 1095

985 1051

993 1092

1005 1100

947

1147 1242

1139 1257

1161 1257

1161 1264

1154 1264

1130

1142

1293

1138 1256

1433 1579

1396 1462 1572

1389 1462 1579

1396 1455 1579

1440 1587

1391 1443 1582 1600

1371 1430 1560 1618

1390 1440 1578 1590

2916

2916

3055

3055

2953

3060−3075

3060−3075

3045

C−H inplane bending in-plane ring vib.

DPTZ on Au(100) (cm−1)

1367 1575 C−H stretching 3054

3041

3055

DPTZ, [AuCl2(DPTZ)]Cl·1/2H2O, or Ni(DPTZ)2Cl2·H2O samples in these studies were suspended in KBr pellets. Several of the minor peaks reported are not included here for clarity of presentation. bSamples annealed to 140 °C for 90 min, then cooled to room temperature for HREELS. c Samples annealed to 170 °C for 90 min, then cooled to room temperature for HREELS. a

Figure 2. HREELS C−H stretch region of 0.75 ML DPTZ on Ag(111) (a) without and (b) with codeposited Pt. Following deposition, the sample was annealed to the temperature indicated in the figure label, then cooled to room temperate for HREELS measurement. (a) Without coadsorbed Pt, there is a significant intensity increase after the 110 °C anneal (green trace). (b) With Pt deposition (red trace), there is a significant change due to Pt complexation, then a more gradual change and intensity shift at higher annealing temperatures as the Pt metal is replaced by Ag in the complexes. The vertical dashed line in each panel at 3055 cm−1 is drawn for the purpose of comparing the two panels.

from out-of-plane ring deformation, the latter is usually assigned to a pyridine out-of-plane mode. The peaks at 743 and 772 cm−1 are C−H bending out-of-plane modes. The inplane modes can be broken down into the peak at 956 cm−1 resulting from the aromatic ring system undergoing in-plane deformations, the peak at 1147 cm−1 resulting from in-plane C−H bending, and the peaks at 1242, 1367, and 1575 cm−1 from other in-plane ring deformations. The C−H stretching feature is the peak at 3054 cm−1. We note that these modes of DPTZ on Ag or Au are generally softened relative to studies of

room temperature (Figure 1). The HREELS features can be organized into three main groups: out-of-plane modes (300− 900 cm−1), in-plane modes (900−2000 cm−1), and C−H stretching modes (2800−3300 cm−1). These are based on assignments made in previous vibrational spectroscopy studies of DPTZ40,41 (Table 1) and on DFT-calculated vibrational modes (see the SI). General assignment regions are expected to be similar on Ag(111) and Au(100) surfaces; we discuss first the modes on Ag(111) and then will discuss the differences (Table 1). Within the out-of-plane region, the peaks at 400 and 618 cm−1 result C


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Figure 3. HREELS of submonolayer DPTZ on (a) Ag(111) or (b) Au(100) surfaces with increasing amounts of Pt deposited onto the surface. (a) On Ag(111), no annealing was done between depositions to avoid conversion from Pt-DPTZ to Ag-DPTZ. The pyridine peak is seen to shift from 618 to 660 cm−1 with increasing amounts of Pt. (b) The Au(100) surface was annealed to 110 °C for 90 min after each deposition to aid complexation. Complexation is observable via the decrease and shift of the out-of-plane pyridine mode. Excess Pt is not seen to cause any significant additional changes compared to 1:1 Pt:DPTZ ratio. (0.5 ML DPTZ on Au(100) and 0.75 ML DPTZ on Ag(111).)

Figure 4. HREELS of 0.75 ML DPTZ on Ag(111) after various annealing temperatures (each step was a 90 min anneal) (a) without and (b) with codeposited Pt. The sample was cooled to room temperature before each HREEL spectrum was acquired. (a) Spectral changes after annealing at 110 °C in the absence of Pt indicate the start of metal−organic coordination between DPTZ and Ag atoms. This is observable by the increase in the inplane modes (1090 cm−1), decrease/shift to the pyridine out-of-plane mode (618 to 640 cm−1), and the increase to the C−H stretching region (see Figure 2a). Further complexation is seen at 140 °C. (b) When Pt and DPTZ are codeposited on Ag(111), complexation is observed at room temperature (red trace, pyridine out-of-plane mode at 660 cm−1). After annealing at 80 °C, the pyridine out-of-plane mode shifts to 640 cm−1, indicating replacement of the Pt with Ag.

DPTZ suspended in KBr40,41 due to the interaction of DPTZ with the metal surface (Table 1). Several interesting observations can be made by comparing the HREELS spectra of DPTZ on Ag(111) with DPTZ on Au(100). The pyridine out-of-plane mode at 583 cm−1 on Au(100) or 618 cm−1 on Ag(111) (Figure 1a) is the feature that is most impacted by the difference in support surface. The energy loss of this mode on Ag(111) falls in the range of prior studies of DPTZ pyridine (615−625 cm−1) when complexed to metals,40 molecular pyridine complexed to metals,42,43 or molecular pyridine bound to metal surfaces,44,45 indicating that DPTZ adsorption on Ag(111) may tend toward a strong complexation-like interaction to the surface. The positions of in-plane deformation modes also differ between the surfaces; the 890 cm−1 peak on Au(100) has shifted into the C−H bending feature on Ag(111) creating a shoulder around 845 cm−1, and the other prominent in-plane features at 1147 and 1367 cm−1 on Ag(111) could be shifted from 1095 and 1433 cm−1 on Au(100). Differences in relative peak intensities between the Au(100) and Ag(111) surfaces are expected in HREELS measurements.46 Complexation of Pt-DPTZ on Ag(111) and Au(100). On-surface redox complexation of DPTZ with Pt on Au(100) has been reported previously using STM and XPS.9 Upon codeposition of Pt with DPTZ on Au(100) or on Ag(111), we observe significant HREEL spectra changes consistent with this complexation reaction (Figure 1b). The in-plane pyridine mode

and several other in-plane modes shift to higher energy on either Ag(111) or Au(100) (Table 1). Upon Pt deposition, the out-of-plane deformation mode intensity decreases with respect to the in-plane mode intensity. The ratio of the DPTZ pyridine peak (583 cm−1 out-of-plane mode) to in-plane peak height just below 1000 cm−1 changes from 3.5 to 2.5 on Ag(111) and from 2.7 to 2.0 on Au(100). A C−H stretching mode at 2916 cm−1 appears with Pt deposition at room temperature (Figure 2b, red trace). This peak appearance suggests that some of the aryl C−H are made more aliphatic by the presence of Pt. These relative intensity changes may be partially due to the charge transfer into DPTZ (decrease to out-of-plane modes, increase to in-plane modes, and increase to C−H modes), consistent with previous observations for neutral vs anionic polyarene species.34,35 As far as we can tell, this is one of the first instances of surfaceconfined redox of a metal and organic species observed through HREELS. Incremental Pt deposition experiments show stepwise complexation of Pt-DPTZ (Figure 3). Depositing more than an equimolar amount of Pt did not alter the spectra, likely due to Pt nanoparticle formation separate from the metal−ligand complexes. This confirms a 1:1 Pt:DPTZ stoichiometric ratio which is also corroborated by molecular level structure elucidated by STM.9 The coordination structures are different on these surfaces in that Pt-DPTZ on Au(100) readily forms long 1D chains,9,10 D


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Figure 5. Comparison of HREEL spectra of DPTZ and DPTZ+Pt, before and after annealing at 140−170 °C, on (a) Ag(111) and (b) Au(100). (a) DPTZ on Ag(111) shows a pyridine peak at 618 cm−1 (red), but, with Pt codeposition, that peak shifts to 660 cm−1 and in-plane modes also change (orange), indicating Pt complexation with DPTZ. When either DPTZ (green) or DPTZ+Pt (blue) are annealed to 140 °C for 90 min, the pyridine peak moves to 640 cm−1 and other in-plane peak position align, indicating that both form the same Ag-DPTZ complex, i.e., Ag replaces Pt. (b) On the Au(100) surface, the pyridine mode for DPTZ appears at 583 cm−1 (blue) and the molecule begins to desorb with annealing at 140 °C. With Pt codeposition and annealing to 170 °C, the pyridine mode shifts to 618 cm−1. Note that Pt-DPTZ shows indications of partial complexation after deposition at room temperature before annealing and that complexation with Pt stabilizes DPTZ against thermal desorption.

Figure 6. HREELS of DPTZ codeposited with Pt on Ag(111) in a series of increasing Pt:DPTZ ratios (a) before and (b) after annealing at 170 °C. (a) Relative peak intensities and peak positions differ depending on the amount of Pt on the surface. (b) After annealing, HREELS peak positions converge regardless of Pt coverage showing the preference for Ag-DPTZ over Pt-DPTZ. (DPTZ coverage is 0.5−0.75 ML.).

within the 1400−1600 cm−1 region. These peaks are found to significantly increase in intensity in addition to their shift to higher energy (Table 1, Figure 4a). There is also a significant increase in intensity to the C−H stretching modes at 3055 cm−1 (Figure 2a). For HREELS, intensity changes can be due to changes in molecular orientation, but off-specular analysis of the peaks shows that the orientation of the molecule rings remain predominantly parallel with the surface even after the spectral change (Figure S3). It is thus concluded that this annealing caused a redox reaction of the DPTZ with surface Ag atoms creating Ag-DPTZ complexes. DPTZ oxidation of substrate atoms to form Ag-DPTZ complexes on Ag(111) is consistent with the observation above that DPTZ on Ag(111) has a strong pyridine-Ag surface interaction. Conversion of Pt-DPTZ to Ag-DPTZ by Replacement Reaction on Ag(111). Annealing Pt-DPTZ complexes on Ag(111) at 80 °C (for 90 min) led to HREEL spectral changes similar to Ag-DPTZ on Ag(111), indicating a change in the complex from Pt-DPTZ to Ag-DPTZ (Figure 4b). The most notable change upon annealing is a shift of the pyridine out-ofplane deformation from 660 to 640 cm−1. The direction of this shift is consistent with the difference between Pt and Ag pyridine complexes reported in previous calculations.47 This shift can also be found in a metal-N bond observed at 194 cm−1 for Pt-DPTZ and at 179 cm−1 for Ag-DPTZ, albeit this peak is harder to characterize due to the elastic peak background (see Figure S4). The vibrational spectrum for Pt-DPTZ on Ag(111) after annealing shows a better match to the spectrum for DPTZ annealed on Ag(111) (forming Ag-DPTZ complexes) than it

while on Ag(111) the M-DPTZ chain direction is less uniform and the chains are generally more disordered (Figure S2). Annealing Pt-DPTZ on Au(100) to 140−170 °C for 90 min does not change the HREELS observations, consistent with the prior STM study.9 The same annealing process for Pt and DPTZ on Ag(111) leads to significant spectral changes (vide infra). Ag-DPTZ Complexation on Ag(111). HREEL spectra of DPTZ on Ag(111) at room temperature or after annealing at temperatures up to 80 °C are similar to spectra of DPTZ on Au(100), except for the minor spectral differences discussed above. However, when DPTZ on Ag(111) is annealed at 110− 170 °C (for 90 min), significant spectral changes are observed (Figure 4a) that resemble the spectra for Pt-DPTZ on either Ag(111) or Au(100) more than the spectra for uncomplexed DPTZ on Ag(111) at room temperature (Figure 5), indicating that the DPTZ has complexed with Ag atoms during the annealing. Annealing DPTZ on Au(100) at these temperatures does not produce any notable spectral changes compared to room temperature until DPTZ desorbs from Au(100) at 140 °C, consistent with previous work reporting DPTZ desorption from Au(100) at the same temperature.9 After annealing DPTZ on Ag(111) at 110−170 °C (Figure 4a), we see an intensity decrease of the pyridine out-of-plane ring deformation in addition to a 22 cm−1 shift from 618 to 640 cm−1. The out-of-plane C−H bending modes also decrease in intensity and shift to appear as a single peak around 740 cm−1. The peaks within the 900−1600 cm−1 region shift to higher energy with the most significant changes occurring to the peaks E


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in these chemistries and that there is a need for further development of theoretical models for these systems.

does to the spectrum for Pt-DPTZ before annealing (Figure 5a). A strong feature in the C−H stretch region at 2916 cm−1 is observed for Pt-DPTZ at room temperature, but becomes less significant than the feature at 3055 cm−1 after annealing (Figure 2b). The 2916 cm−1 mode is not observed for DPTZ annealed on Ag(111) without Pt (Figure 2a). This conversion of Pt-DPTZ to Ag-DPTZ was observed with various initial Pt coverages (Figure 6). While there are variations in the Pt-DPTZ features due to the relative concentration of Pt (Figure 6a), the resulting spectra of AgDPTZ after annealing are highly uniform (Figure 6b); the DPTZ complexation with Ag seems to proceed regardless of the initial amount of Pt complexed. This conversion is also indicated by a shift in the XPS Pt 4d 5/2 peak from 316.6 to 315 eV with annealing (Figure 7), indicating a reduction of the Pt. The reduced Pt appears in STM images to aggregate into nanoparticles on the surface after the replacement reaction (Figure S2).

4. CONCLUSION HREEL spectroscopy was employed to characterize the adsorption and transition metal complexation of DPTZ on Ag(111) and Au(100) surfaces. The surface impacted the complexation behavior after annealing the ligand on each surface; when DPTZ is deposited on Ag(111) and annealed, the ligand is able to capture Ag adatoms to form Ag-DPTZ complexes, but no such reaction occurs on Au(100). Pt codeposition with DPTZ on Ag(111) or on Au(100) produces Pt-DPTZ immediately after deposition at room temperature. We also observed a replacement reaction by which Pt-DPTZ complexes on Ag(111) converted to Ag-DPTZ complexes upon annealing. The replacement of Pt by Ag has not been reported in any prior metal−organic coordination study at surfaces or in solution. This replacement reaction occurred at a lower temperature than the complexation of neutral DPTZ with Ag adatoms, indicating that the prior redox state facilitated the replacement reaction. Development and understanding of the formation of metal organic complexes is important when trying to design functional surface structures. Here we showed that the surface plays a critical role in the formation of metal organic surface structures and demonstrated the utility of HREELS for studying on-surface redox processes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02809. HREELS normalization procedure; STM images of DPTZ and DPTZ+Pt on Ag(111); supplemental HREELS data; discussion of electron-stimulated desorption of DPTZ; LEED of DPTZ on Ag(111); standard electrode potentials for relevant redox steps; DFT calculations of vibrational modes (PDF)

Figure 7. X-ray photoelectron spectroscopy of DPTZ with Pt on Ag(111), measured at room temperature after annealing the sample at the temperature indicated. At room temperature (red trace), the Pt 4 d5/2 binding energy is 316.6 eV, corresponding to a +2 oxidation state and this shifts to lower binding energy with annealing, indicating reduction of the Pt metal. This corresponds to the HREELS observation that Ag is replacing Pt in the metal−organic complexes.

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AUTHOR INFORMATION

Corresponding Author

When starting from Pt-DPTZ, the replacement reaction occurs at annealing temperatures of 80 °C or above (Figure 4b). We note that the metalation of DPTZ by Ag adatoms on Ag(111) (without coadsorbed Pt) was not observed at 80 °C, but did occur at annealing temperatures of 110 °C or higher (Figure 4a). This temperature difference indicates that the PtDPTZ redox state facilitates the subsequent reaction with Ag. We note that the replacement reaction by which Pt centers in DTPZ complexes are reduced and replaced by Ag cation centers is in agreement with electrochemical redox potentials, which indicate that the reaction would be energetically favorable (see the SI).48,49 Thus, redox exchange of Pt and Ag (galvanic replacement) is used in the growth of porous Pt nanoparticles of Pt/Ag bimetallic nanostructures.50−53 Note that redox potentials are also consistent with our observation that Au does not replace Pt in a similar reaction. However, the replacement of Pt by Ag in metal−organic complexes does not agree with the slightly stronger M−N bond strength for Pt (indicated by our HREELS study, Figure S4, and by prior studies of M-pyridine bonds at surfaces47,54−56) and the general observation that a quasi-square planar coordination environment is more favorable for Pt than for Ag (usually tetrahedral). All of this points to the idea that the surface plays a critical role

*E-mail: [email protected]. Tel: 812-855-1302. ORCID

Steven L. Tait: 0000-0001-8251-5232 Present Address §

D.S.: Formulation Sciences, Merck Research Laboratories, Merck & Co., Inc., West Point, PA 19486, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support for this work from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant DE-FG02-12ER16351. The authors thank Benjamin W. Noffke for helpful discussions and assistance with the DFT calculations. The authors also thank Mike Hosek and Dave Sprinkle of the IU Physics Department and the Mechanical Instrument Services group of the IU Chemistry Department for technical development and support of the HREELS instrument and organic evaporation source. F


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