Showcasing results from a combined NMR and FTIR

spectroscopic study with isotopically labelled alkyne ligands

carried out by the Chen Group at the University of California,

Santa Cruz.

Identification of the formation of metal–vinylidene interfacial

bonds of alkyne-capped platinum nanoparticles by isotopic

labeling

When n-alkynes self-assemble onto platinum nanoparticle

surface, spectroscopic measurements confirm the formation

of Pt–vinylidene (Pt==C==CH–) interfacial linkages rather than

Pt–acetylide (Pt–C≡≡C–) and platinum–hydride (Pt–H) bonds,

based on a comparative study with 1-dodecyne and

dodec-1-deuteroyne as the capping ligands.

As featured in:

See Shaowei Chen et al., Chem. Commun., 2016, 52, 11631.

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Cite this:Chem. Commun., 2016,

52, 11631

Identification of the formation of metal–vinylidene interfacial bonds of alkyne-cappedplatinum nanoparticles by isotopic labeling†

Peiguang Hu, Limei Chen, Christopher P. Deming, Lewis W. Bonny, Hsiau-Wei Leeand Shaowei Chen*

Stable platinum nanoparticles were prepared by the self-assembly

of 1-dodecyne and dodec-1-deuteroyne onto bare platinum colloid

surfaces. The nanoparticles exhibited consistent core size and optical

properties. FTIR and NMR measurements confirmed the formation of

Pt–vinylidene (PtQQQCQQQCH–) interfacial linkages rather than Pt–acetylide

(Pt–CRRRC–) and platinum–hydride (Pt–H) bonds.

Recently it has been observed that metal–ligand interfacial bondinginteractions may be exploited as a new, effective variable in themanipulation of the chemical and physical properties of metalnanoparticles.1–3 Of these, acetylene derivatives represent aunique capping ligand, as they may be readily self-assembledonto transition metal surfaces, forming conjugated metal–carbonbonds. This leads to apparent intraparticle charge delocalizationbetween the particle-bound functional moieties and hence newoptical and electronic properties. For instance, ruthenium nano-particles have been functionalized with n-octynyl fragments(deprotonated n-octyne) through the formation of ruthenium–acetylide (Ru–CR) bonds.4,5 The resulting nanoparticles exhibitan apparent red-shift of the CRC vibrational energy and photo-luminescence emission that is analogous to those of diacetylenederivatives (–CRC–CRC–); and with ethynylferrocenyl fragmentsincorporated into the nanoparticle capping layer, intervalencecharge transfer occurs between the ferrocenyl metal centers atmixed valence, a behavior analogous to ferrocene oligomers,which suggests nanoparticle-mediated electronic communicationthanks to the conjugated metal–ligand interfacial linkages.4,5 Morerecently, it has been found that metal nanoparticles can alsobe stabilized by the direct self-assembly of n-alkynes onto‘‘bare’’ metal colloids,6–8 which is ascribed to the formationof metal–vinylidene (MQCQCH–) interfacial bonding linkagesthat are in a dynamic equilibrium with metal–acetylide (M–CR)and metal hydride (M–H) bonds by a tautomeric rearrangement

process (Scheme 1). Such a hypothesis is supported by the specificreactivity of the nanoparticles with imine derivatives. Note thatin alkyne-transition metal complexes, the metal–vinylidene(MQCQCH–) structure is thermodynamically favorable.9,10

Thus, one immediate question arises. Is metal–vinylidene(MQCQCH–) bond also the dominant interfacial bondinglinkages on alkyne-capped metal nanoparticles? This has remainedan outstanding question and is the primary motivation of thepresent study.

Herein, we compare the spectroscopic characteristics of platinumnanoparticles functionalized with n-dodecyne (HC12) and dodec-1-deuteroyne (DC12). FTIR and 1H and 2H NMR spectroscopicmeasurements showed clear signatures of platinum–vinylidene(PtQCQCH–) linkages whereas none for platinum–hydrideand platinum–acetylide bonds.

Experimentally, DC12 was synthesized by following a pre-viously reported procedure,11 where HC12 was deuterated byK2CO3 in D2O. 1H NMR measurements showed that about 95%of the original 1-dodecyne was deuterated (Fig. S1, ESI†). HC12or DC12-functionalized platinum nanoparticles (PtHC12 orPtDC12) were then prepared by carbon monoxide reduction

Scheme 1

Department of Chemistry and Biochemistry, University of California,

1156 High Street, Santa Cruz, California, 95064, USA. E-mail: shaowei@ucsc.edu

† Electronic supplementary information (ESI) available: NMR, TEM, UV-vis andphotoluminescence spectra, and reaction scheme. See DOI: 10.1039/c6cc05626a

Received 7th July 2016,Accepted 8th August 2016

DOI: 10.1039/c6cc05626a

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of PtCl4 in the presence of HC12 or DC12.12,13 The syntheticdetails were included in the ESI.†

As manifested in TEM measurements (Fig. S2, ESI†), bothPtHC12 and PtDC12 nanoparticles were dispersed rather wellwithout apparent agglomeration, suggesting sufficient protectionof the nanoparticles by the self-assembly of the alkyne ligandson platinum colloid surfaces, as observed previously.14 Statisticalanalysis based on more than 150 nanoparticles showed that theaverage core sizes of the samples were very close, at 1.26� 0.18 nmfor PtHC12 and 1.32 � 0.22 nm for PtDC12, with the majorityof the nanoparticles in the range of 1.0 to 1.5 nm in bothsamples. Moreover, clearly defined lattice fringes can be seenin high-resolution TEM studies where the interplanar distance of0.232 nm is consistent with that of the Pt(111) crystalline planes(PDF Card 4-802). The optical properties of both nanoparticleswere also consistent with prior results.14 For instance, whereasonly featureless UV-vis absorption profiles were observed, bothnanoparticles displayed apparent photoluminescence emissions(Fig. S3, ESI†), with an excitation maximum around 350 nm andthe corresponding emission maximum around 440 nm, that wereanalogous to those of diacetylene derivatives (–CRC–CRC–).This was ascribed to intraparticle charge delocalization between theparticle-bound acetylene moieties, as observed previously for noblemetal nanoparticles functionalized with acetylene derivatives.4,5,15

As mentioned above, in prior studies, the self-assembly ofn-alkynes on metal nanoparticles was thought to involve adynamic equilibrium between metal–vinylidene (MQCQCH–)and metal–acetylide (M–CR) + metal–hydride (M–H) bonds(Scheme 1). These were first examined by FTIR measurements.From Fig. 1, several characteristic bands can be identified forthe HC12 free ligands (green curve), 3315 cm�1 for the terminalRC–H vibration, 2120 cm�1 for the CRC vibrations, andmultiple vibration bands at 2956, 2927, and 2853 cm�1 thatarose from the –CH2– and –CH3 vibrations of the hydrocarbonchains. DC12 free ligands (blue curve) displayed similar charac-teristics, except that the terminal RC–D vibration appeared at2600 cm�1 (the fact that the 3315 cm�1 band remained visible

was because not all alkynyl protons were replaced by the deuteriumatoms,11 as shown in Fig. S1, ESI†). When the ligands were self-assembled onto Pt nanoparticle surfaces, for both PtHC12 andPtDC12 nanoparticles the –CH2– and –CH3 vibrations remainedvirtually unchanged, and the RC–H and RC–D vibrationalbands vanished completely. Furthermore, PtHC12 exhibited avibrational band at 2047 cm�1, a red-shift of 73 cm�1 from thatof the CRC stretch of HC12 free ligands. Previously this wasaccounted for by the formation of conjugated Pt–CR interfacialbonds that led to intraparticle charge delocalization and hence adiminishing bonding order of the CRC moiety, with additionalcontributions from Pt–H bonds on the nanoparticle surface(Scheme 1).14 These two contributions may be differentiatedwith the PtDC12 nanoparticles which exhibited a vibrationalband at an almost identical position of 2051 cm�1; yet no vibra-tional feature was observed around 1450 cm�1 that is anticipatedfor the Pt–D bonds.16,17 Instead, a new band emerged at around2475 cm�1 for PtDC12, which is in good agreement with the vinylcarbon deuterium (QC–D) stretch.18,19 Taken together, theseresults strongly suggest the formation of the metal–vinylideneinterfacial structure, rather than the metal–acetylide + metal–hydride linkages, at the metal–ligand interface (Scheme 1).

Consistent results were obtained in NMR measurements, asshown in Fig. 2. From the 1H NMR spectra in panel (A), one cansee that both PtHC12 (black curve) and PtDC12 (red curve)nanoparticles exhibited two prominent broad peaks at 0.89 and1.26 ppm, which can be assigned to the terminal methyl (CH3)and methylene (CH2) protons of the HC12 and DC12 ligands,respectively. Notably, the absence of sharp features at thesetwo peaks indicates the successful attachment of the ligandsonto the nanoparticle surface, and both Pt nanoparticles werespectroscopically clean and free of excess ligands20 (the sharppeaks at 1.56, 2.01, and 7.26 ppm are due to protons from residualsolvents of water, acetonitrile, and chloroform, respectively).In addition, two broad peaks can be identified at 2.10 and6.01 ppm for both nanoparticles. Of these, whereas the peak at6.01 ppm (figure inset) is consistent with the vinylidene protons(PtQCQCH–), the peak at 2.10 ppm may be ascribed to theprotons of the a-carbon of both Pt–vinylidene (PtQCQCH–CH2–)and Pt–acetylide (Pt–CRC–CH2–). However, for the PtHC12nanoparticles, the ratio of the integrated peak areas betweenthe vinylidene and a-methylene protons was calculated to be0.93 : 2 (Fig. S4, ESI†), close to the theoretical value of 1 : 2expected for PtQCQCH–CH2–, indicating that indeed thealkyne ligands all formed Pt–vinylidene (PtQCQCH–) ratherthan Pt–acetylide (Pt–CRC–) interfacial bonds on the Pt nano-particle surface, in agreement with the FTIR results (Fig. 1).Such a ratio was significantly lower for the PtDC12 nano-particles at only 0.14 : 2 (Fig. S4, ESI†), due to the replacementof the vinylidene proton with deuterium (Fig. S1, ESI†).

Further structural insights were obtained in 2H NMR mea-surements. As shown in panel (B), in sharp contrast to thePtHC12 nanoparticles which only showed two sharp peaks at0.89 and 1.28 ppm from the methyl and methylene deuteriumsof the solvent n-hexane-d14, PtDC12 nanoparticles exhibited anadditional broad peak within the range of 5 to 8 ppm, which is

Fig. 1 FTIR spectra of PtHC12 (black) and PtDC12 (red) nanoparticles,along with those of the free ligands of HC12 (green) and DC12 (blue).

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consistent with vinylidene deuterium (QCQCD–). This, again,shows good agreement with results from the FTIR measurements(Fig. 1), and further confirmed the formation of Pt–vinylidene(PtQCQCH–) interfacial linkages. One may also notice that thereappears another broad peak within the range of 1.5 to 3.5 ppm.While it is plausible that this peak arose from methylene deuterium(PtQCQCH–CHD–) produced by hydrogen/deuterium migrationdue to the sigmatropic reaction (Scheme S1, ESI†),21–24 the exactorigin is not clear at this point. Further studies are desired.

Furthermore, in the above 1H and 2H NMR measurements,no spectral feature was observed in the negative chemical shiftregime, which suggested the absence of metal–hydride linkagesin both nanoparticles (Scheme 1).

In summary, using isotopically labelled alkyne ligands, FTIRand NMR spectroscopic measurements confirmed that whenn-alkynes self-assembled on metal nanoparticle surfaces, thedominant interfacial bonding structures entailed metal–vinylidenelinkages.

This work was supported in part by the National ScienceFoundation (CHE-1265635 and DMR-1409396). TEM work wascarried out at the National Center for Electron Microscopy atthe Lawrence Berkeley National Laboratory, which is supportedby the US Department of Energy, as part of a user project.

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221–245.2 W. Chen, J. R. Davies, D. Ghosh, M. C. Tong, J. P. Konopelski and

S. W. Chen, Chem. Mater., 2006, 18, 5253–5259.3 C. M. Crudden, J. H. Horton, I. I. Ebralidze, O. V. Zenkina, A. B.

McLean, B. Drevniok, Z. She, H. B. Kraatz, N. J. Mosey, T. Seki,E. C. Keske, J. D. Leake, A. Rousina-Webb and G. Wu, Nat. Chem.,2014, 6, 409–414.

4 W. Chen, N. B. Zuckerman, X. W. Kang, D. Ghosh, J. P. Konopelskiand S. W. Chen, J. Phys. Chem. C, 2010, 114, 18146–18152.

5 X. Kang, N. B. Zuckerman, J. P. Konopelski and S. Chen, Angew.Chem., Int. Ed., 2010, 122, 9686–9689.

6 X. Kang, N. B. Zuckerman, J. P. Konopelski and S. Chen, J. Am. Chem.Soc., 2012, 134, 1412–1415.

7 P. Maity, S. Takano, S. Yamazoe, T. Wakabayashi and T. Tsukuda,J. Am. Chem. Soc., 2013, 135, 9450–9457.

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9 C. Bianchini, M. Peruzzini, A. Vacca and F. Zanobini, Organometallics,1991, 10, 3697–3707.

10 J. Silvestre and R. Hoffmann, Helv. Chim. Acta, 1985, 68, 1461–1506.11 S. P. Bew, G. D. Hiatt-Gipson, J. A. Lovell and C. Poullain, Org. Lett.,

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and J. P. Xie, J. Am. Chem. Soc., 2014, 136, 1246–1249.13 P. G. Hu, L. M. Chen, C. P. Deming, X. W. Kang and S. W. Chen,

Angew. Chem., Int. Ed., 2016, 55, 1455–1459.14 K. Liu, X. W. Kang, Z. Y. Zhou, Y. Song, L. J. Lee, D. Tian and

S. W. Chen, J. Electroanal. Chem., 2013, 688, 143–150.15 P. G. Hu, Y. Song, M. D. Rojas-Andrade and S. W. Chen, Langmuir,

2014, 30, 5224–5229.16 W. A. Pliskin and R. P. Eischens, Z. Phys. Chem., 1960, 24, 11–23.17 T. Toya, J. Res. Inst. Catal., Hokkaido Univ., 1962, 10, 236–260.18 M. Nikow, M. J. Wilhelm and H. L. Dai, J. Phys. Chem. A, 2009, 113,

8857–8870.19 Y. J. Wu, M. Y. Lin, B. M. Cheng, H. F. Chen and Y. P. Lee, J. Chem.

Phys., 2008, 128, 204509.20 M. J. Hostetler, J. E. Wingate, C. J. Zhong, J. E. Harris, R. W. Vachet,

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Fig. 2 (A) 1H and (B) 2H NMR spectra of PtHC12 (black) and PtDC12 (red)nanoparticles in (A) CDCl3 and (B) n-hexane-d14. Inset in panel (A) is thezoom-in of the region between 4 and 8 ppm.

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