Gas-Phase Structural and Optical Properties of Homo-and

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Gas-Phase Structural and Optical Properties of Homo and Heterobimetallic Rhombic Dodecahedral Nanoclusters [Ag Cu(C#CtBu) X] Clusters (X = Cl and Br): Ion Mobility, VUV and UV Spectroscopy, and DFT Calculations 14-n

n

12

+

Steven Daly, Chang Min Choi, Athanasios Zavras, Marjan Krstic, Fabien Chirot, Timothy U Connell, Spencer J. Williams, Paul S. Donnelly, Rodolphe Antoine, Alexandre Giuliani, Vlasta Bonacic-Koutecky, Philippe Dugourd, and Richard A. J. O'Hair J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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Gas-Phase Structural and Optical Properties of Homo and Heterobimetallic Rhombic Dodecahedral Nanoclusters [Ag14-nCun(C≡CtBu)12X]+ Clusters (X = Cl and Br): Ion Mobility, VUV and UV Spectroscopy, and DFT Calculations

Steven Daly,a Chang Min Choi,a Athanasios Zavras,b Marjan Krstić,c Fabien Chirot,d Timothy U. Connell,b Spencer J. Williams,b Paul S. Donnelly,b Rodolphe Antoine,a Alexandre Giuliani,e Vlasta Bonačić-Koutecký,c* Philippe Dugourd,a* Richard A. J. O'Hairb*

a

Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F69622, LYON, France. E-mail: [email protected] b School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, 30 Flemington Rd, Parkville, Victoria 3010, Australia. Email: [email protected] c Interdisciplinary Center for Advanced Science and Technology (ICAST) at University of Split, Split, Croatia. E-mail: [email protected] d Université Claude Bernard Lyon 1, Ens de Lyon, CNRS, Institut des Sciences Analytiques, UMR 5280, 5 rue de la Doua, F-69100, Villeurbanne, France e SOLEIL, l’Orme des Merisiers, St Aubin, BP48, F-91192 Gif sur Yvette Cedex, France

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Abstract: The rhombic dodecahedral nanocluster [Ag14(C≡CtBu)12Cl]+, which has been structurally characterized via X-ray crystallography, was transferred to the gas-phase using electrospray ionization, where it was characterized by ion mobility (IM), vacuum ultraviolet (VUV) and ultraviolet (UV) spectroscopies in conjunction with DFT calculations. IM reveals a single peak and modelling of the collision cross section with the X-ray structure suggests that the cluster maintains its condensed phase structure upon transfer to the gas-phase. The VUV spectra exhibit rich fragmentation, including: photoionization to give [Ag14(C≡CtBu)12Cl]2+• with an onset of 8.84 ± 0.08 eV; cluster fission fragmentation via losses of (AgC≡CtBu)n and (AgC≡CtBu)n-1(AgCl); and via reductive elimination of (tBuC≡C)2. Apart from channels associated with photoionization, similar fragment ions are observed in the UVPD spectra, although their relative intensities differ. The TDDFT absorption spectra are symmetry allowed transitions belonging Au → Ag, Eu → Ag and Eu → Eg irreducible representations. Comparing the collision cross sections with the X-ray structures for the related clusters [Ag8Cu6(C≡CtBu)12Cl]+, [Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ suggests that they maintain their condensed phase structures in the gas-phase. The VUV spectra of [Ag8Cu6(C≡CtBu)12Cl]+ and [Ag14(C≡CtBu)12Br]+ exhibit similar fragmentation channels and ionization onsets (8.86 ± 0.03 and 8.86 ± 0.05 respectively) compared to [Ag14(C≡CtBu)12Cl]+.


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Introduction: Clusters provide important conceptual bridges between physics and chemistry.1,2 In the gas-phase isolated clusters can be characterized by a range of spectroscopic and other techniques,3-7 while in the condensed-phase clusters can be synthesized on the bulk scale 8 and their structures determined by X-ray crystallography.9 Yet studies that bridge the gas and condensed-phase using the same cluster are rare. Mass spectrometry based methods provide exciting opportunities to bridge these realms utilizing the concepts of mass spectrometry inspired 10 or directed 11-18 synthesis of clusters. Arguably the most famous example of the former concept is buckminsterfullerene (C60),19 which was first discovered as its cation as formed in a cluster beam source.20 Subsequent experiments by Kroto et al. led to the proposal of the buckminsterfullerene structure.21 This prompted a search for a synthesis of isolated samples of C60; 5 years later bulk samples became available.22 As a consequence of this breakthrough, C60 is one of the most widely studied species in both the gas-phase and condensed phase, with a recent highlight the gas-phase spectroscopic investigations 23 that have led to the suggestion that it is one of the elusive interstellar bands.24

Anion-templated coinage metal-ligand cage-type nanoclusters are of interest due to their structural and photophysical properties and their potential to offer multisite reactivity and activation of small molecules that resembles metallic heterogeneous catalysts.25 We recently used electrospray ionization mass spectrometry (ESI-MS) to monitor the stepwise exchange of silver cations for copper cations in the rhombic dodecahedral nanoclusters [Ag14(C≡CtBu)12X]+ (X = Cl and Br).26 We found that only a maximum of six silver cations could be exchanged for copper.14 These experiments informed a synthetic campaign that led to the isolation of [Ag8Cu6(C≡CtBu)12Cl]+ and [Ag8Cu6(C≡CtBu)12Br]+ in the condensed phase, with subsequent characterization by X-ray crystallography identifying that the exchange retains the overall rhombic dodecahedron and places the six copper atoms as caps on each of the faces of the central silver cube. As examples of studies that bridge the gas and condensed phases in metal nanocluster chemistry are rare, in this work we exploit the availability of bulk samples of these homo- and heterobimetallic nanoclusters to explore their gas-phase: (i) structure using a combination of ion-mobility




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measurements 27-29 and DFT calculations; (ii) ionization energies by vacuum ultraviolet (VUV) radiation 30 using a synchrotron source (SOLEIL DESIRS beamline) 31,32 and DFT calculations; and optical spectra using UVPD action spectroscopy.33,34 In all of these experiments ESI-MS 35 is the enabling experimental bridge linking the condensed and gas phases by transferring solutions of clusters to vacuum.

Experimental and Theoretical Methods Materials: Chloroform and methanol were HPLC grade sourced from Sigma Aldrich and

were

used

without

further

purification.

[Ag14(C≡CtBu)12Cl]+,

[Ag8Cu6(C≡CtBu)12Cl]+, [Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ were the tetrafluoroborate salts available from a previous study.14

Mass spectrometry and ion-mobility experiments.

Ion mobility measurements were performed using a custom-built tandem ion mobility spectrometer (IMS; Lyon) described in detail elsewhere 27 and only a brief description is given. Measurement used a a freshly prepared solution of [Ag14(C≡CtBu)12Cl]+ and [Ag8Cu6(C≡CtBu)12Cl]+ tetrafluoroborates, in methanol at a concentration of approx. 75 µM. This solution was electrosprayed using a syringe pump (flow rate 100 µL.hr-1). The ESI conditions were: spray voltage 4.5 kV, capillary temperature 150°C, nebulizer gas pressure 0.8 bar, and desolvation gas flow 2.5 L.min-1. Mobilities were measured by injecting short (200 µs) ion bunches in a 79 cm-long drift tube filled with 4 Torr helium, in which a constant drift field was maintained through the controlled voltage drop across the tube (in the 200-500 V range). The temperature of the entire instrument was kept at 297 K. After their drift, ions were conveyed to a reflectron time-of-flight (TOF) mass spectrometer. Mass spectra were finally recorded as a function of the IMS drift time, allowing extraction of arrival time distributions (ATDs) for ions with any desired mass-to-charge ratio. ATDs were recorded for different drift voltages in order to determine absolute values for the collision crosssections (CCS) based on the linear relation between the drift time and the inverse drift field.36,37


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Mass spectrometry and VUVPD experiments. Mass spectrometry experiments were conducted on a Thermo Fisher Scientific LTQ XL linear ion trap mass spectrometer that has been coupled to the DESIRS beamline of the SOLEIL synchrotron.31,32 This undulator beamline produces a high flux of photon (typically in the 1012-1013 ph/sec/0.1% bandwidth), tuneable over the whole VUV range (5-40 eV) with a high spectral purity, ie with no high harmonics of the undulator that can be transmitted by the higher orders of the grating, and which are very efficiently cut-off by a gas filter. This is a crucial issue in the context of the mass spectrometry based action spectroscopy experiments and especially when onsets are to be measured, where excitation by higher order harmonics must be avoided. All photon-energy spectra were normalised to the incident photon flux measured using a calibrated photodiode (IRD AXUV100). The condensed phase cluster samples solutions of [Ag14(C≡CtBu)12Cl]+, [Ag8Cu6(C≡CtBu)12Cl]+, [Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ tetrafluoroborates were diluted in methanol to concentrations of 50 µM and injected at a flow rate 5 µL.min-1 into the Finnigan ESI source. ESI source conditions typically involved needle potentials of 3.2 – 4.8 kV to give a stable source current of ca. 0.5 µA and a nitrogen sheath gas pressure of 5 arbitrary units. The ion transfer capillary temperature was set to 250 ˚C. The tube lens voltage and capillary voltage were both set to ca. 10.0 V. The onset for ionization for [Ag14(C≡CtBu)12Cl]+, [Ag8Cu6(C≡CtBu)12Cl]+ and [Ag14(C≡CtBu)12Br]+ were determined from a Wannier-type fit (Fig. S3).

Mass spectrometry and UVPD experiments. Solutions of [Ag14(C≡CtBu)12Cl]+, [Ag8Cu6(C≡CtBu)12Cl]+, [Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ tetrafluoroborates were prepared in methanol to a concentration of approx. 75 µM and introduced into a modified quadrupole linear ion trap mass spectrometer (LTQ, Thermo Fisher Scientific, San Jose, CA, USA) by electrospray ionization (ESI) using a syringe pump set to a flow rate of 5 µL.min-1. The ESI conditions were: spray voltage, 4.2 – 5.0 kV, capillary temperature, 300°C, nitrogen sheath gas pressure, 5 (arbitrary units), capillary voltage 15 V. Modification to the mass spectrometer consisted of installation of a quartz window fitted on the rear of the MS chamber to allow coupling of a laser with the linear ion trap.33,34 The laser used was a nanosecond frequency-doubled tuneable Horizon OPO (Optical




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Parametric Oscillator) laser pumped by a SureliteTM II Nd:YAG laser (both from Continuum, Santa Clara, CA, USA). The repetition rate of the laser was 10 Hz with pulse widths of 5 ns. The laser beam passes through a mechanical shutter electronically synchronized with the mass spectrometer, after which it is focused with a 1000 mm long focal lens into the linear trap on axis. The laser power was monitored with a power meter located just before the injection point in the ion trap. The mechanical shutter used synchronizes the laser irradiation with the trapping of the ions. To perform laser irradiation for a given number of laser pulses, the ion trap radio frequency (RF) sequence was altered to include an MSn step with an activation amplitude of 0% and a reaction time of 100 ms, during which the shutter located on the laser beam is opened. The activation q value was set to 0.20.

For action spectroscopy, mass spectra were recorded after laser irradiation as a function of the laser wavelength as described in detail elsewhere.38 At each laser wavelength from 235 nm to 291 nm (with a 0.1 nm step and 1 second dwell), a lasernormalized yield of photo-fragmentation is deduced from the mass spectrum through eq. 1:

σ = log((parent + daughter)/parent)/ Φ

(1)

Where Φ is the laser fluence or the synchrotron radiation photon flux, parent is the intensity of the precursor ion, and daughter represents the intensity of the product ion peaks. Optical action spectra were obtained by plotting the normalized yield of photofragmentation as a function of the laser wavelength.

Theory The Gaussian09

39

program was used with the PBE RI functional 40,41 and the RECP

basis set for silver atoms

42

and the def2-SVP basis set for all other atoms

43

to fully

optimise the [Ag14(C≡CtBu)12Cl]+, [Ag8Cu6(C≡CtBu)12Cl]+, [Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ clusters using their X-ray structures as the starting geometries and to determine their vertical and adiabatic ionization energies and their optical spectra.

DFT absorption spectra for

[Ag14(C≡CtBu)12Cl]+ and

Ag8Cu6(C≡CtBu)12Cl]+ are calculated with cam-B3LYP functional basis set

43

with Stuttgart RECP for silver atoms.

42


44

and def2-SVP

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Results and Discussions: Ion-mobility

experiments

on


[Ag8Cu6(C≡CtBu)12Cl]+,

[Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ Ion-mobility has been widely used to probe the structures of clusters since the collision cross section is sensitive to the “shape” of the ion and thus it is a valuable method to probe whether a population of ions is homogenous or whether it is a mixture of isomers.28,29 We were interested in using this method to establish if the [Ag14(C≡CtBu)12Cl]+,

[Ag8Cu6(C≡CtBu)12Cl]+,

[Ag14(C≡CtBu)12Br]+

and

+

[Ag8Cu6(C≡CtBu)12Br] cluster ions formed using ESI-MS exhibited arrival time profiles consistent with the compact structures found via X-ray crystallography.

DFT calculations were carried out on the structures of the isolated cations taken from the single crystal X-ray structures in order to allow them to relax to gas-phase structures in the absence of counter anions. These fully optimized DFT structures, illustrated for [Ag14(C≡CtBu)12Cl]+ and [Ag8Cu6(C≡CtBu)12Cl]+ (Fig. 1a and 1b) are almost identical to the reported solid state structures determined using X-ray crystallography, highlighting that the counter anion has little effect on the intrinsic structure of the cluster.14,26 For complexes with homogeneous and heterogeneous cores deviations from crystal structures are negligible. They are: 0.03 Å for Ag-Ag bonds, 0.08 Å for C-C bonds and 0.05 Å for C-H bonds. In the case of the homogenous Ag14 core, the Ag-Ag bond length is slightly shorter for complexes with Cl than for Br (2.97 Å vs. 3.00 Å). For the heterogeneous Ag8Cu6 complexes the AgAg bonds are slightly longer (3.45 Å vs. 3.53 Å) with respect to complexes with homogeneous cores, in agreement with X-ray crystal structures.

The arrival time distributions of these clusters, as determined via ion-mobility measurements, highlight that only one type of structure is present and that the heteronuclear clusters have a shorter arrival time than that of the homonuclear clusters as illustrated for [Ag14(C≡CtBu)12Cl]+ and [Ag8Cu6(C≡CtBu)12Cl]+ (Fig. 1c). This is consistent with their slightly more compact structures as determined both using X-ray crystallography and DFT calculations.




a)

b)

c)

1.0

CCS(Å2 ) + [ Ag8 Cu 6 L1 2 Cl] : 311. 634 +

[ Ag14 L12Cl]

Arb. Intensity

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: 321.187

0.5

0.0 40

42

44

46

48

50

52

54

56

58

60

Arrival time (ms)

Figure 1. DFT optimized structures (chlorine = light green; silver = light grey; copper = orange; carbon = dark green; hydrogen = white) for: a) [Ag14(C≡CtBu)12Cl]+ and b) [Ag8Cu6(C≡CtBu)12Cl]+. c) ion-mobility arrival times [Ag8Cu6(C≡CtBu)12Cl]+ (black peak) and [Ag14(C≡CtBu)12Cl]+ (red peak).

Modelling

the

collision

cross

sections

of


[Ag8Cu6(C≡CtBu)12Cl]+, [Ag14(C≡CtBu)12Br]+ and [Ag8Cu6(C≡CtBu)12Br]+ using their X-ray crystal structures and their gas-phase structures as determined using DFT calculations.


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Table 1 lists the experimentally determined collision cross sections (CCS) of all of the homo- and heteronuclear clusters. The CCS was found to decrease for both the heteronuclear clusters [Ag8Cu6(C≡CtBu)12Cl]+ and [Ag8Cu6(C≡CtBu)12Br]+, relative to the equivalent Ag14 homonuclear clusters, consistent with the smaller atomic radius of copper giving rise to more compact structures. CCS calculations were carried out using two different models: (1) the trajectory method (TM); and (2) the exact hardsphere scattering model (HS).37 CCS calculations used both the solid-state structures determined by X-ray crystallography as well as the fully optimized DFT structures. The calculated CCS for the copper-containing clusters is smaller in all cases, highlighting their more compact structures. The CCS values calculated from the crystal structures most closely match the experimental data, with the trajectory method generally providing a better match than the exact hard-sphere scattering model. The difference in CCS for the bromine- and chlorine-containing clusters is at the limit of experimental uncertainty, whereas from the crystal structure, no difference is expected. This small difference between experiment and theory may originate from the limitations of the CCS calculation models and from the absolute error on the experimental CCS values taken from distinct datasets. The CCS calculated using the DFT optimized structures are slightly larger than the ones determined from the crystal structures. The main conclusions from the IMS data is that the solution structure obtained from X-ray measurements is maintained in the gas phase and that the heteronuclear clusters have a smaller CCS than the homonuclear clusters.




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Table 1. Comparison of the collision cross sections of [Ag14-nCun(C≡CtBu)12X]+ (n = 0 and 6; X = Cl and Br) determined experimentally using ion-mobility with those determined from theoretical approaches. Species

CCS 2

CCS

CCS

2

(Å )

(Å )

expt

HS

CCS

2

2

(Å ) X- TM

(Å )

CCS (Å2)

X- HS DFT TM DFT

ray (a,b)

ray (b,c)

(b,d)

(b,e)

[Ag14(C≡CtBu)12Cl]+

313 ± 6

323 ± 2

311 ± 5

337 ± 6

325 ± 6

[Ag8Cu6(C≡CtBu)12Cl]+

302 ± 6

306 ± 2

299 ± 5

328 ± 6

315 ± 6

[Ag14(C≡CtBu)12Br]+

319 ± 6

325 ± 2

310 ± 5

336 ± 6

325 ± 6

[Ag8Cu6(C≡CtBu)12Br]+

313 ± 6

306 ± 2

299 ± 5

328 ± 6

310 ± 6

(a) CCS was calculated using the exact hard-sphere scattering (HS) model with the coordinates of the X-ray structure used as input. (b) The given error corresponds to a standard deviation on 15 different Monte-Carlo integrations. (c) CCS was calculated using the trajectory method (TM) with the coordinates of the X-ray structure used as input. (d) CCS was calculated using the exact hard-sphere scattering (HS) model with the coordinates from the DFT calculations used as input. (e) CCS was calculated using the trajectory method (TM) with the coordinates from the DFT calculations used as input.

Products formed upon irradiation of [Ag14(C≡CtBu)12Cl]+, [Ag8Cu6(C≡CtBu)12Cl]+ and [Ag14(C≡CtBu)12Br]+ by VUV radiation. Mass selection of [Ag14(C≡CtBu)12Cl]+ (m/z 2525) in an ion trap followed by irradiation with 11.0 eV of VUV radiation resulted in the mass spectrum shown in Fig. 2a. Key product channels observed include: photoionization to give [Ag14(C≡CtBu)12Cl]2+• (eq. 2); cluster fragmentation via losses of silver acetylides to give [Ag14-n(C≡CtBu)12-nCl]+ (eq. 3) and via losses of both silver acetylides and AgCl to give [Ag14-n(C≡CtBu)12-n+1]+ (eq. 4); and formation of [Ag14(C≡CtBu)10Cl]+, which might occur by “Glaser-type homocoupling” via reductive elimination of (tBuC≡C)2 (eq. 5).45 Other minor losses involving C–C bond activation reactions of the acetylide ligands also occur.45,46 The cluster fragmentation reactions are likely to involve complex rearrangement of the cluster via processes including acetylide ligand


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migration and Ag induced extrusion of the central halide. For example formation of [Ag12(C≡CtBu)11]+ from [Ag14(C≡CtBu)12Cl]+ (Fig. 2c) requires opening up of the cluster to allow loss of the combined elements of (AgC≡CtBu) and AgCl. [Ag14(C≡CtBu)12X]+ + hν →

[Ag14(C≡CtBu)12X]2+• + e+

(2)

→

[Ag14-n(C≡CtBu)12-nX]

→

[Ag14-n(C≡CtBu)12-(n-1)]+ + (AgC≡CtBu)n-1(AgCl) (4)

→

[Ag14(C≡CtBu)10X]+ + (tBuC≡C)2

(5)

→

[Ag14(C≡CtBu)10X]2+• + e- + (tBuC≡C)2

(6)

+ (AgC≡CtBu)n

(3)

The templating anion does not seem to exert a major effect since irradiation of [Ag14(C≡CtBu)12Br]+ (m/z 2568) results in the formation of product ions through related reaction channels, although the yields are slightly higher (Fig. 2b). [Ag14(C≡CtBu)12Br]+ also undergoes a minor radical loss channel (eq. 6) as well as photoionization coupled with “Glaser-type homocoupling” via reductive elimination of (tBuC≡C)2 to give [Ag14(C≡CtBu)10Br]2+• (eq. 7). The spectrum for [Ag8Cu6(C≡CtBu)12Cl]+ in which six silver atoms have been replaced by copper is more complex (Fig. 2c), but this is due to the fact that while the same types of fragment channels operate, cluster fragmentation can proceed by losses of various combinations of (AgC≡CtBu) and (CuC≡CtBu) (cf eq. 3) and (AgC≡CtBu), (CuC≡CtBu), AgCl and CuCl (cf eq. 4). [Ag14(C≡CtBu)12Br]+ + hν

→

[Ag14(C≡CtBu)11Br]+• + tBuC≡C•


(7)



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800

1000

1200

1400

1600

1800

2000

2200

2400

2600

14,0,12,1

(a) 14,0,12,12+• 7,0,6,0 14,0,10,12+•

14,0,12-tBu,1

10,0,8,1

12,0,11,0

8,0,6,1

11,0,10,0

10,0,9,0

6,0,5,0 *

11,0,9,1

*

*

*

14,0,10,1

* 14,0,12,1

(b)

10,0,8,1 7,0,6,0 14,0,10,12+• 6,0,5,0

14,0,12,12+•

*

11,0,9,1 12,0,11,0 14,0,10,1

8,0,7,0 *

(c) 4,2,5,0

6,5,9,1 6,4,8,1 8,2,8,1 7,4,9,1

4,3,6,0

5,1,5,0

8,3,9,1

5,5,8,1

*

5,2,6,0

3,3,5,0 800

8,6,12,1

7,3,8,1

8,6,10,12+•

1000

1200

1400

1600

1800

* 2000

** * * 2200

* 2400

2600

m/z 2

Figure 2. LTQ ESI-MS of the mass-selected metal clusters irradiated at 11 eV for a period of 200 ms: (a) [Ag14(C≡CtBu)12Cl]+; (b) [Ag14(C≡CtBu)12Br]+; and (c) [Ag8Cu6(C≡CtBu)12Cl]+.

Peaks are labeled as a,b,c,d to reflect the product ion

stoichiometries [AgaCub(C≡CtBu)cXd]+ (X = Cl or Br). The peaks highlighted with an asterisk are related to metastable fragmentation of the radical dication (see Fig. S1 and S2).

It is also possible to determine whether fragments originate from a dissociative ionization event, or if they are related to a delayed Coulomb explosion of the radical cation. To determine the presence of the latter process, the radical dication [Ag14(C≡CtBu)12Cl]2+• was formed by irradiation with photons at 13 eV, massselected, and trapped without further activation. The resultant mass spectrum (Fig. S1) yields a range of fragment ions that are associated with a delayed decay of the radical dications, labelled such in Tables S1-3. Related delayed fragmentation is observed for [Ag8Cu6(C≡CtBu)12Cl]+ (Fig. S2), and whilst the same experiments were not performed for [Ag14(C≡CtBu)12Br]+, it can be assumed that the equivalent fragmentation channels in this species originate from delayed fragmentation of the


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radical dication. Indeed, it is possible to identify the corresponding channels in [Ag14(C≡CtBu)12Br]+ since they display breakdown curves with identical profiles.

Determination

of

the

[Ag8Cu6(C≡CtBu)12Cl]+

onset and

for

ionization

of

[Ag14(C≡CtBu)12Br]+ by

[Ag14(C≡CtBu)12Cl]+, VUV

radiation

and

comparison with the DFT calculated vertical and adiabatic ionization energies.

While there are several experimental studies on the ionization of bare metal coinage metal cluster cations,47,48 examples of the ionization of ligated coinage metal cluster cations are rare.30 Thus we examined the onset for ionization of [Ag14(C≡CtBu)12Cl]+, [Ag8Cu6(C≡CtBu)12Cl]+ and [Ag14(C≡CtBu)12Br]+ as determined from a Wanniertype

fit (Fig. S3) of the yields of all product ions associated with

[Ag14(C≡CtBu)12Cl]2+•, [Ag8Cu6(C≡CtBu)12Cl]2+• and [Ag14(C≡CtBu)12Br]2+• (see above), as a function of the VUV energy used. The experimental ionization onset together with the DFT calculated vertical ionization energies (VIE) and adiabatic ionization energies (AIE) are given in Table 2. For completeness, we have also tabulated the DFT calculated VIE and AIE of [Ag8Cu6(C≡CtBu)12Br]+.




6

14,0,12,Cl2+• 14,0,12,Br2+• 8,6,12,Cl2+•

5

Ionization Yield (x10-14)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4

3

2

1

0 8

9

10

11

12

13

14

15

Photon Energy / eV Figure 3. Determination of the onset of ionization of [Ag14(C≡CtBu)12Cl]+ (black curve), [Ag8Cu6(C≡CtBu)12Cl]+ (blue curve) and [Ag14(C≡CtBu)12Br]+ (red curve) as a function of the VUV energy used, which was varied from 8 to 15.5 eV. Note that the missing points for [Ag8Cu6(C≡CtBu)12Cl]+ is due to loss of the synchrotron source for those photon energies.

Table 2. Experimentally determined ionization onsets compared with the DFT calculated vertical ionization energies (VIE) and adiabatic ionization energies (AIE). All energies are in eV. Obtained for DFT optimized structures. species

ionization

onset VIE

AIE

(a)


8.84 ± 0.08(a)

8.23

8.16

+

8.86 ± 0.05

(a)

8.22

8.12

8.86 ± 0.03

(a)

8.37

8.29

8.12

8.05

[Ag14(C≡CtBu)12Br]

[Ag8Cu6(C≡CtBu)12Cl]

+ +

[Ag8Cu6(C≡CtBu)12Br]

(b)

ND

(a) Obtained from a Wannier type fit (Fig. S3). (b) ND = not determined.


Page 15 of 25


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Within experimental uncertainty, the ionization energies are independent of the templating halide (Cl versus Br) or replacement of six silver atoms with six copper atoms. The DFT calculated ionization energies slightly underestimate experimental ones.

Analysis

of

the

molecular

orbitals

of


and

[Ag14(C≡CtBu)12Cl]2+• that are involved in the ionization process (Fig. S4) shows that the removal of one electron from the HOMO takes place from the region of the square pyramids of the rhombic dodecahedron.

Product ion yields as a function of the irradiation energy. We next examined the yields of these product ions as a function of the photon energy (Fig. 4). The dissociative processes observed when changing either the templating halide or exchanging six silver atoms for copper are much more pronounced than is the case for the ionization curves presented above. In all cases, the fragmentation close to threshold is dominated by the loss of four metal acetylides (note that in Fig. 4c, the curves represent the sum of the equivalent loss channels with either silver or copper acetylide losses), with loss of three metal acetylides also being important. In both cases, the breakdown curves are strongly peaked at ~11 eV in [Ag14(C≡CtBu)12Cl]+, ~10.2 eV in [Ag14(C≡CtBu)12Br]+ and ~11.4 eV in [Ag8Cu6(C≡CtBu)12Cl]+. This indicates that the process leading to this fragmentation channel depends strongly on the templating halide, but only weakly on the exchange of silver for copper. The strong resonances may be related to resonant electronic excitations within the continuum (Feshbach resonances). The metal losses suggest electronic excitation within the metal cores. Calculation of electronic excited states at these energies is beyond the scope of this paper. The exchange of the central halide ion induces a shift of one eV further indicating a transition located in the core of the cluster.

It is also interesting to note that in all cases (in particular for

[Ag14(C≡CtBu)12Br]+) the onset of cluster fragmentation in which the cation maintains the halide (eq 3) appeared below the ionization onset determined above.

Above 10 eV cluster fragmentation where the cation has lost the halide (eq 4) becomes accessible, with [M6(C≡CtBu)5]+ the most important fragment ion observed. Here, the onset in [Ag8Cu6(C≡CtBu)12Cl]+ shows a shift of 0.5 eV compared to




[Ag14(C≡CtBu)12Cl]+. [Ag14(C≡CtBu)12Br]+ possesses the lowest appearance energy, and has a shallower onset compared to the other two species.

Above 13 eV, reductive elimination of (tBuC≡C)2 from the radical dication (eq 6) becomes the dominant fragmentation channel in both [Ag14(C≡CtBu)12Cl]+ and [Ag14(C≡CtBu)12Br]+, but not in [Ag8Cu6(C≡CtBu)12Cl]+, where this fragmentation channel is unavailable. Instead, [Ag8Cu6(C≡CtBu)12Cl]+ shows an increase in the photoionization yield without any apparent alternative fragmentation channel occurring.

Comparison +

[Ag14(C≡CtBu)12Cl]

of

the

breakdown

and [Ag14(C≡CtBu)12Br]

+

curves

for

this

channel

in

showed no shift in the onset,

indicating that the loss of (tBuC≡C)2 from the radical dication required equal activation energy in both systems; i.e. that the templating halide had no influence. It should be noted that the same channel from the cation (eq 5) shows no dependence on the photon energy, and is open below the ionization threshold.

6.5

5.5

5.0

(c)

(b) 14,0,12,12+• 11,0,10,0+ 11,0,9,1+ 10,0,9,0+ 10,0,8,1+ 7,0,6,0+ 14,0,10,12+• 6,0,5,0+ Fragmentation Yield

6.0

14,0,12,12+• 12,0,11,0+ 11,0,9,1+ 10,0,8,1+ 14,0,10,12+• 6,0,5,0+ Fragmentation Yield

6.0

5.5

5.0

6.0

5.5

4.5

4.0

4.0

4.0

3.5

3.5

3.5

3.0

3.0

3.0

2.5

2.5

2.5

2.0

2.0

2.0

1.5

1.5

1.5

1.0

1.0

1.0

0.5

0.5

0.5

0.0

0.0

0.0 8

9

10

11

12

13

Photon Energy / eV

14

15

8,6,12,12+• x,y,9,1+ (x+y = 11) x,y,8,1+ (x+y = 10) x,y,6,0+ (x+y = 7) x,y,5,0+ (x+y = 6) Fragmentation Yield

5.0

4.5

4.5

-14

6.5

6.5

(a)

Branching Ratio (x10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8

9

10

11

12

13

14

15

Photon Energy / eV

8

10

12

14

Photon Energy / eV

Figure 4. LTQ ESI-MS/MS yields of product ions formed from irradiation of (a) [Ag14(C≡CtBu)12Cl]+, (b) [Ag14(C≡CtBu)12Br]+ and (c) [Ag8Cu6(C≡CtBu)12Cl]+ for a period of 200 ms as a function of the VUV irradiation energy, which was varied from


Page 17 of 25


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8 to 15.5 eV. Note that the missing points for [Ag8Cu6(C≡CtBu)12Cl]+ is due to loss of the synchrotron source for those photon energies.

The VUV branching ratios show a strong dependence upon photon energy for some fragmentation channels (Fig. 4), with very well defined bands. This suggests that there are orbital specific photophysical mechanisms in operation.

UVPD action spectroscopy of [Ag14(C≡CtBu)12Cl]+ and [Ag8Cu6(C≡CtBu)12Cl]+. The

experimental

optical

action

spectra

of


and

+

[Ag8Cu6(C≡CtBu)12Cl] , as well as calculated TDDFT absorption spectra for the lowest energy structures with S6 symmetry, are presented in Fig. 5. Apart from channels associated with photoionization (eq. 2), similar fragment ions are observed in the UVPD (Fig. S5b) and VUVPD spectra (Fig. S5c) although their relative intensities are different. For example, loss of (AgC≡CtBu)4 and (AgC≡CtBu)3, which dominate the VUV spectrum (Fig. 5b), are minor in the UV spectrum (Fig. 5a), while losses of (AgC≡CtBu)7(AgCl) and (AgC≡CtBu)6(AgCl) dominate the UV spectrum. The UV spectra do not show changes in the branching ratios as a function of wavelength. Indeed, the types and relative abundances of the fragment ions formed via UVPD (Fig. S5b) and collision-induced dissociation (Fig. S5a) are similar, suggesting that fragmentation after heating and UV photoexcitation proceeds via similar mechanisms.

The TDDFT absorption spectra are characterized by dominant transitions located at 245 nm and the shoulder at 250 nm for homogenous system, while for heterogeneous cluster the same features with two groups of transitions are red shifted as shown in Fig. 5. The main spectral features are due to symmetry allowed transitions belonging Au → Ag, Eu → Ag and Eu → Eg irreducible representations. Leading excitations within these two groups of transitions involve peripheral metal (Ag or Cu) as well as carbon atoms, as shown in Supporting Information Fig. S4. Calculated absorption spectra are in qualitative agreement with the experimental findings, thus allowing for the structural assignment.




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure

5.

Optical

action

spectra

of:

a)

[Ag14(C≡CtBu)12Cl]+;

b)

[Ag8Cu6(C≡CtBu)12Cl]+ and TDDFT calculated absorption spectra for: c) [Ag14(C≡CtBu)12Cl]+; d) [Ag8Cu6(C≡CtBu)12Cl]+ using the cam-B3LYP functional and def2-SVP basis set and Stuttgart RECP for silver atoms obtained for the lowest energy structures also shown.

Conclusions: Just as the high symmetry C60 cluster is stable and maintains its structure in the gas and condensed-phases, so too do the homo and heterobimetallic rhombic dodecahedral nanoclusters of the type [Ag14-nCun(C≡CtBu)12X]+ (where n = 0, 6; X = Cl, Br). Thus IM reveals a single peak for each cluster and their collision cross section are well modelled by their X-ray structures. A series of nanoclusters can be obtained by titration of the [Ag14(C≡CtBu)12Cl]+ nanocluster with copper(I) or the [Ag8Cu6(C≡CtBu)12Cl]+ nanocluster with silver(I) and can be detected in the gas


Page 19 of 25


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phase following transfer by electrospray;14 this work suggests that this family of nanoclusters also maintain discrete stable structures in the gas and solution phases. Measurements of the VUV ionization onsets and fragmentation threshold and UV electronic excitation of [Ag14(C≡CtBu)12Cl]+ provide experimental benchmarks for theory. Moreover, the observation of some strong resonances in the fragmentation yields, which could be related to population of resonant electronic excitations within the continuum (Feshbach resonances), is a particular feature of these nanoclusters. Given the importance of metallic nanoparticles in a range of catalytic processes, and the development of metathesis approaches for the synthesis of higher-order metallic nanoclusters,49,50 the present work provides new insights into the intrinsic stability and lability of well-defined homo and heterometallic species that complements their rich solution phase reactivity.14,51

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (VBK); [email protected] (PD); [email protected] (RAJO)

ACKNOWLEDGEMENTS R.A.J.O., S.J.W. and P.D. thank the Australian Research Council for financial support (DP150101388). SOLEIL support is acknowledged under project no. 20150734. We also thank the general technical staff of SOLEIL for running the facility. AZ acknowledges the award of an Australian Postgraduate PhD Scholarship. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013 Grant agreement N°320659). VBK and MK acknowledge Prof. Miroslav Radman at MedILS and Split-Dalmatia County for kind support. We thank George Khairallah for discussions.

ASSOCIATED CONTENT




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Supporting Information Description: Mass spectra, fit used to extract ionization onset from VUV data, DFT calculated HOMOs and leading excitations of the TDDFT calculated absorption spectra.

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Page 25 of 25


Table of content graphic: )

1.0

ESI-MS!

6.5

C CS(Å2 )

(a)

+

[ Ag8 Cu 6 L1 2 Cl] : 311. 634

IM! Ar b. Intensity

+

[ Ag1 4 L1 2Cl]

: 321.187

5.5 0.5

5.0

4.5

-14

0.0 40

42

44

46

48

50

52

54

56

Ar rival time (ms)

UV!

58

60

VUV!

14,0,12,1 2+• 11,0,10,0 + 11,0,9,1+ 10,0,9,0+ 10,0,8,1+ 7,0,6,0 + 14,0,10,1 2+• 6,0,5,0 + Fragmentation Yield

6.0

Branching Ratio (x10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5


0.0 8

9

10

11

12

13

Photon Energy / eV


14

15

Gas-Phase Structural and Optical Properties of Homo-and

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