A magnetic look into the protecting layer of Au25 clusters
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This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. A magnetic look into the protecting layer of Au25 clusters Agrachev, Mikhail; Antonello, Sabrina; Dainese, Tiziano; Gasrcón, José A.; Pan, Fangfang; Rissanen, Kari; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Maran, Flavio Agrachev, M., Antonello, S., Dainese, T., Gasrcón, J. A., Pan, F., Rissanen, K., Ruzzi, M., Venzo, A., Zoleo, A., & Maran, F. (2016). A magnetic look into the protecting layer of Au25 clusters. Chemical Science, 7(12), Article 6910. https://doi.org/10.1039/c6sc03691k 2016
Registered charity number: 207890 Showcasing research from international collaboration between the laboratories of Flavio Maran (Department of Chemistry, University of Padova, Italy), José Gascón (Department of Chemistry, University of Connecticut, USA), and Kari Rissanen (Department of Chemistry, University of Jyväskylä, Finland). A magnetic look into the protecting layer of Au25 clusters In ligand-protected gold clusters, the capping monolayer has the special role of interfacing the metal core to the surrounding medium. Pulse electron nuclear double resonance, NMR, molecular-dynamics calculations, and single crystal X-ray crystallography are used to study the fi ne interactions between the unpaired electron in Au25(SR)18 0 and protons of the ligands. Comparative analysis, carried out on four clusters, allows distinguishing between ligand types and precisely assessing distance eff ects. It is shown that magnetism can be used as a very precise tool to probe important features at the ligand-core interface of ultrasmall clusters. As featured in: See Alfonso Zoleo, Flavio Maran et al., Chem. Sci., 2016, 7, 6910. www.rsc.org/chemicalscience
A magnetic look into the protecting layer of Au 25 clusters† Mikhail Agrachev, a Sabrina Antonello, a Tiziano Dainese, a Jos´ e A. Gasc´ on, b Fangfang Pan, c Kari Rissanen, c Marco Ruzzi, a Alfonso Venzo, d Alfonso Zoleo* a and Flavio Maran* ab The field of molecular metal clusters protected by organothiolates is experiencing a very rapid growth. So far, however, a clear understanding of the fine interactions between the cluster core and the capping monolayer has remained elusive, despite the importance of the latter in interfacing the former to the surrounding medium. Here, we describe a very sensitive methodology that enables comprehensive assessment of these interactions. Pulse electron nuclear double resonance (ENDOR) was employed to study the interaction of the unpaired electron with the protons of the alkanethiolate ligands in four structurally related paramagnetic Au 25 (SR) 0 18 clusters (R ¼ethyl, propyl, butyl, 2-methylpropyl). Whereas some of these structures were known, we present the first structural description of the highly symmetric Au 25 (SPr) 0 18 cluster. Through knowledge of the structural data, the ENDOR signals could be successfully related to the types of ligand and the distance of the relevant protons from the central gold core. We found that orbital distribution affects atoms that can be as far as 6 ˚ A from the icosahedral core. Simulations of the spectra provided the values of the hyperfine coupling constants. The resulting information was compared with that provided by 1 H NMR spectroscopy, and molecular dynamics calculations provided useful hints to understanding differences between the ENDOR and NMR results. It is shown that the unpaired electron can be used as a very precise probe of the main structural features of the interface between the metal core and the capping ligands. Introduction In monolayer-protected gold clusters (MPCs) with gold cores of diameter <1.6 nm the number of Au atoms is sufficiently small to make them display molecular features. This makes the study of their fundamental properties particularly fascinating and oen intriguing. 1–5 Instrumental to these studies has been the possibility of preparing molecule-like gold MPCs in an atomically precise form, as assessed by mass spectrometry and single-crystal X-ray crystallography. 6 The structure of Au 25 (SR) 18 clusters, by far the most well known among molecular clusters, is based on a 13 gold-atom icosahedral core surrounded by 6 Au 2 (SR) 3 units, with minor differences induced by the charge state (1, 0, and +1) and the ligands, 7–11 even when the linear polymer (Au 25 ) n forms. 12 Several molecular features of Au 25 (SR) 18 clusters have been studied in detail, such as the characteristic electrochemical behavior 13–15 and charge-dependent optical 16 and nuclear magnetic resonance (NMR) patterns. 17 Studies of their photophysical behavior, 18–21 chirality, 22 electron-transfer and redox-catalysis properties, 23–27 have been described. Several theoretical studies have been carried out and reviewed. 28–30 Whereas the as prepared anionic cluster Au 25 (SC 2 H 4 Ph) 18 is a diamagnetic species, the corresponding, indenitely stable neutral form Au 25 (SC 2 H 4 Ph) 0 18 is paramagnetic. The effect of the unpaired electron was detected by 1 Hand 13 C NMR at room temperature 17 or electron paramagnetic resonance (EPR) at temperatures typically lower than 100 K. 16,31 The Jin's group showed that the magnetic state can be switched offby reduction to diamagnetic Au 25 (SC 2 H 4 Ph) 18 . 31 Similarly, we demonstrated by both NMR 17 and EPR 16 that oxidation to cation Au 25 (SC 2 H 4 Ph) 18+ generates a diamagnetic species. DFT calculations indicated that the magnetic behavior is controlled by signicant splitting of the relevant orbital energy levels. 17 The NMR spectrum of the three charge states evidenced very profound charge-induced variations in the position and shape a Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy. E-mail: alfonso.zole[email protected]; [email protected]t b Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, 06269 Connecticut, USA c Department of Chemistry, Nanoscience Center, University of Jyv¨ askyl¨ a, P.O. Box 35, 40014 JYU, Finland d National Research Council, ICMATE c/o Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy †Electronic supplementary information (ESI) available: Details on the synthesis and characterization of Au 25 (SPr) 0 18 and Au 25 (SMePr) 0 18 , full NMR spectroscopy data, electrochemistry, simulation of the ENDOR spectra, DFT calculations, and X-ray crystallography, including check CIF le. CCDC 1453036. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc03691k Cite this: Chem. Sci.,2016,7, 6910 Received 17th August 2016 Accepted 18th September 2016 DOI: 10.1039/c6sc03691k www.rsc.org/chemicalscience 6910 |Chem. Sci.,2016,7, 6910–6918 This journal is © The Royal Society of Chemistry 2016 Chemical Science EDGE ARTICLE Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 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of the peaks. 17 Most notably, some of the ligands' resonances undergo a signicant downeld shiupon formation of paramagnet Au 25 (SR) 0 18 . This was rst observed by the Murray's group 32 and then perfected by us through identication of all resonances as a function of temperature or ligand type. 10,12,17,26 In this context, it is important to stress that the 18 thiolated ligands present in the 6 Au 2 (SR) 3 half-crowns (or staples) capping the central Au 13 core split into a group of 12 inner and a group of 6 outer ligands. Here, inner refers to the fact that the two terminal SR groups of –(SR)–Au–(SR)–Au–(SR)–also bind to Au 13 , whereas outer refers to the outmost, remaining thiolate of the half-crown. 1D and 2D NMR analysis allowed distinguishing between the two ligands' families, also on a quantitative basis. There is a general consensus that the properties of molecular MPCs mostly depend on the number and relative position of the gold atoms. 5,6 On the other hand, NMR evidence and corresponding DFT calculations indicated that the singly occupied molecular orbital (SOMO) spreads onto the rst groups of the thiolated ligands. Very recent studies also concluded that the ligand structure/composition can be a factor affecting the optical behavior of molecular and larger MPCs. 33,34 These results thus indicate that the highest occupied and the lowest unoccupied MOs (HOMOs and LUMOs) are not just limited to the Au 13 core, as oen implicitly assumed, but rather involve to some extent the ligands. Another example is provided by the optical spectrum of Au 25 capped by thiophenolate-type ligands, which shows band shis 35 and a small decrease of the HOMO– LUMO gap compared to that of Au 25 capped by alkanethiolates, for which the spectrum does not depend on the ligand length: 26 this effect shows that changing the carbon type at the aposition to sulfur affects the electronic properties of the cluster. The way by which the capping ligands interact with the core, the shape and spreading of the chemically relevant orbitals, and the actual environment experienced by molecules or ions penetrating the monolayer 15 are expected to be crucial factors also for understanding the catalytic effects of ultrasmall clusters 27 on a truly molecular basis. Very recently, we illustrated the remarkable potentialities of pulse electron nuclear double resonance (ENDOR). 10 This technique is a very sensitive way of performing ENDOR 36 and is meant to characterize hyperne coupling (A) between an unpaired electron and nuclei nearby. This interaction consists of isotropic and anisotropic parts: the former is a through-bond contribution that depends on the number and type of bonds involved, whereas the latter depends on both through-bond and through-space (electron-dipole/nuclear-dipole) interactions. In the ENDOR spectrum, a doublet of lines is associated with a magnetic nucleus with nuclear spin I¼1/2. When A<2n, where nis the nucleus Larmor frequency, the doublet is centered at nand the separation between the two lines is A.On the other hand, for A>2nthe doublet is centered at A/2 and the separation between the two lines is 2n. The Larmor frequency ndepends only on the magnetic eld Bat which the ENDOR spectrum is recorded, according to the relation n¼g I B/(2p), where g I is the nucleus gyromagnetic ratio. For larger nuclear spins, such as for 197 Au whose I¼3/2, the quadrupolar and hyperne interactions split the ENDOR lines further. This was experimentally veried in the ENDOR analysis of Au 25 (SEt) 0 18 . 10 The hyperne interaction between the unpaired electron and the gold atoms could be assessed quantitatively, and the ENDOR results could be nicely reproduced by density functional theory (DFT) calculations, which could be particularly precise due to the very small thiolate used. Here, we describe a methodology and results that accurately enabled assessing the spin density and, therefore, the distribution of the SOMO in Au 25 (SR) 0 18 clusters. We employed pulse ENDOR to study the interaction of the unpaired electron with the protons of the alkanethiolate ligands, an approach that was never described before. The resulting information was compared with that obtained by 1 H NMR spectroscopy of how and how much the chemical shis change when the charge state of the cluster is varied from 1 to 0. Molecular dynamics (MD) calculations provided useful hints in understanding differences between the ENDOR and NMR results. We noted that by reducing the temperature to 5 K, a pronounced increase of spin-polarization occurs. It is thus shown that the unpaired electron can be used as a particularly sensitive probe of the main structural features of the interface between the metal core and the capping ligands, leading to establish a very precise and consistent picture of these complex systems. Experimental Au 25 (SEt) 18 and Au 25 (SBu) 18 were prepared as described previously. 10,12 The two new clusters, Au 25 (SPr) 18 and Au 25 (SMePr) 18 , were synthesized and oxidized along similar lines. 1 H and 13 C NMR spectroscopy measurements were carried out on 3 mM [n-Oct 4 N + ][Au 25 (SR) 18 ]orAu 25 (SR) 0 18 in benzened 6 (100%, 99.96% d 6 , Aldrich). We used a Bruker Avance DMX600 MHz spectrometer equipped with a 5 mm TX-1 x,y,zgradient powered, triple resonance inverse probe operating at 599.90 ( 1 H NMR) or 150.07 MHz ( 13 C NMR). The temperature was controlled at 298 K with a Bruker BVT-300 automatic temperature controller. Chemical shis are in ppm units (d) with reference to tetramethylsilane used as an internal standard for both 1 H and 13 C NMR. The proton assignments were either already known or performed by 2D correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), and nuclear Overhauser enhancement spectroscopy (NOESY) experiments. 13 C chemical shis were obtained and assigned through heteronuclear multiple quantum coherence (HMQC) correlation experiments. For ENDOR measurements, the samples consisted in 100 ml of 0.5 mM Au 25 (SR) 0 18 in toluene. Each solution was introduced into the EPR sample holder, a 3 (o.d.) 2 mm (i.d.) quartz tube, and degassed through freeze–pump–thaw cycles in a vacuum line and sealed at low pressure (5 10 5 torr). The samples were frozen at 80 K and then introduced in the probehead. The experiments were carried out at 5 K. 1 H Pulse ENDOR measurements were carried out with a Bruker Elexsys E580 instrument equipped with a pulse ENDOR dieletric probehead and an Oxford CF935 cryostat. We used the Davies pulse sequence, with 160 ns microwave inversion pulse and 80–160 ns pulse-sequence for electron-spin echo detection. The This journal is © The Royal Society of Chemistry 2016 Chem. Sci.,2016,7, 6910–6918 | 6911 Edge Article Chemical Science Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
radiofrequency pulse was 8–10 ms long and amplied with a 150 W RF Bruker amplier. The spectra were recorded at the top of the echo detected EPR spectra (approximately 290 mT), with a RF scan ranging from 1 to 20 MHz, where the proton peaks were expected. The narrow spectral range and the long microwave pulses were chosen to spot and enhance ENDOR lines from 1 H with respect to 197 Au. 10 ENDOR simulations were performed with a homebuilt program running on the opensource Scilab-5.5.1 calculation package [http://www.scilab.org]. Results and discussion We used a series of related Au 25 (SR) 0/1 18 clusters. The ligands were chosen to provide a progressive variation of the chain length from two to four carbon atoms, as shown in Chart 1. The single crystal structures of the SEt and SBu protected clusters were available from previous work, whereas that of Au 25 (SPr) 0 18 is described here for the rst time. The effect of branching was checked by changing a hydrogen atom with a methyl group at the bposition: for this ligand, 2-methyl-1propanethiolate, we will use the notation SMePr to stress both methyl branching and that the fully extended chain length is the same as that of SPr. Starting from sulfur, the carbon atoms and associated hydrogen atoms are dened as a,b,g, and d; the second CH 3 group of HSMePr is denoted as g0. Monodisperse samples of the four Au 25 (SR) 18 clusters were prepared, and the clusters were oxidized according to a method already described. 10 Full characterization of the puried neutral clusters was carried out by a combination of matrix-assisted laser desorption ionization time-of-ight mass spectrometry (e.g., Fig. S1 in ESI†), UV-vis absorption spectroscopy, differential-pulse voltammetry, and 1 H NMR spectroscopy techniques. Au 25 (SPr) 18 and Au 25 (SMePr) 18 are new clusters. X-ray crystallography Au 25 (SPr) 0 18 crystallizes (Fig. 1c) in trigonal space group P 3, with three cluster molecules in the unit cell. This MPC is highly symmetric with a 3-fold rotoinversion axis running through the central Au atom. As for the other known Au 25 (SR) 18 structures, 7–12 the 25 gold atoms can be regarded as being formed by two shells composed by an Au 13 icosahedral core, consisting of a central Au atom with 12 Au atoms directly interacting with it, and an outer shell of 12 Au atoms bound to thiolate groups to form –(SR)–Au–(SR)–Au–(SR)–motifs (Fig. 1a): The Au–Au bondstrength order is Au central –Au ico >Au ico –Au ico >Au ico –Au staple . These bonds correspond to average Au–Au bond lengths of 2.784, 2.927 and 3.163 ˚ A, respectively. It is worth noting that there is asignicantly shorter Au ico –Au ico bond (2.7746 ˚ A) and a relatively longer Au ico –Au staple bond (3.3206 ˚ A). We found this feature also in the closely related SEt and SBu analogues. 10,12 Concerning the orientation of the carbon chains with respect to the plane of the same half-crown, Au 25 (SPr) 0 18 features the rst case of a Au 25 cluster where only alternate orientations are observed. The space-lling model (Fig. 1b) illustrates that the ligands are quite folded around the gold core, thereby forming a relatively thin monolayer, at least in the solid state. Evidence for the formation of quite thin capping monolayers was previously gathered also in solution, through electron-transfer measurements 26 of a series of monodisperse Au 25 (SC n H 2n+1 ) 18 clusters with nvarying from 2 to 18. Of particular importance for the current investigation, from the structures of Au 25 (SEt) 0 18 ,Au 25 (SPr) 0 18 , and Au 25 (SBu) 0 18 we Chart 1 Thiols. Fig. 1 (a) Projection showing the X-ray crystal structure of Au 25 (SPr) 0 18 .Au ¼yellow, S ¼red, C ¼gray, H ¼white. For clarity, the icosahedral core (yellow), one of the staples (blue), and the corresponding inner- (red) and outer-ligands (green) are highlighted. The positions of the chain carbons with respect to sulfur are also indicated. (b) Space-filling model; the dashed line and the arrow highlight the approximately spherical shape and the average diameter (1.72 nm). (c) Single crystal bricks (ca. 1 mm) from which the structure was solved. 6912 |Chem. Sci.,2016,7, 6910–6918 This journal is © The Royal Society of Chemistry 2016 Chemical Science Edge Article Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
could calculate the average distances (mediated over the six staples) of the corresponding hydrogen atoms for both the inner and outer ligands. In this connection, it is worth noting that the average radii (r MPC ) of these three clusters nicely match those calculated from the Stokes–Einstein–Sutherland equation, D¼ k B T/6phr MPC , where Dis the electrochemically determined diffusion coefficient, 15 k B is the Boltzmann constant, and his the solvent viscosity: for Au 25 (SEt) 0 18 ,Au 25 (SPr) 0 18 ,andAu 25 (SBu) 0 18 we nd the couple of values 8.3 and 7.8, 8.6 and 8.6, 10.2 and 9.4 ˚ A, respectively. NMR spectroscopy The 12 inner and the 6 outer ligands experience a different chemical environment and thus show distinct NMR spectroscopy signals. When the cluster is in its paramagnetic state, differences are enhanced, especially for those resonances related to the proton and carbon atoms closer to the gold core. We studied the spectra of the selected clusters in either charge state, using benzene-d 6 as the solvent. Fig. 2 illustrates for the case of Au 25 (SPr) 18 the most salient differences in chemical shi as one goes from the paramagnetic to the diamagnetic states. Further spectra (Fig. S2–S4†) and data are in ESI.† The most signicant effect of the one-electron oxidation of the native cluster is to shidowneld the NMR peaks pertaining to the protons in positions a,band g(except for Au 25 (SEt) 18 , which has no ggroups) of the inner ligands, and in positions aand (to a small extent) gof the outer ligands; instead, the bprotons of the outer ligands undergo an upeld shi. For the three clusters of known crystallographic structure, the chemical shidifferences (Dd¼d radical d anion ) are displayed in Fig. 3 as a function of the average distance between the central Au atom and the two or three hydrogen atoms of the specic resonance, averaged for the six half-crowns. The positive differences roughly obey an exponential dependence on distance (taking into account the error on the latter), as already commented upon for Au 25 (SBu) 18 . 12 As to Au 25 (SMePr) 18 , we observed the same charge-dependent effect (Fig. S3 and S4†). The NMR shis observed upon changing the charge state from 1 to 0 are related to the contact interaction of the nuclear magnetic moments with the unpaired electron, and can thus be taken as a measure of how far the spin density spreads outside the Au 13 core. 17 The Ddvalues can be used to estimate the isotropic hyperne coupling constant Aaccording to the relationship (eqn (1)): 37 Dd¼Agb e S(S+ 1)/(3ħg I k B T)(1) where ħis the reduced Planck constant, b e is the Bohr magneton, g I is the proton nuclear gyromagnetic ratio, and gis the isotropic gvalue for the unpaired electron. The latter can be exactly calculated as g¼(g xx +g yy +g zz )/3, where g xx ,g yy , and g zz are the main values of the g-tensor. For Au 25 (SBu) 0 18 , they are 1.78, 2.40, and 2.56, respectively, 12 and the same values are found for Au 25 (SEt) 0 18 : 10 therefore, we used these values also for the other clusters. By using these values and 298 K, Ddcan be expressed as 29.74 (ppm MHz 1 )A(MHz). We note that eqn (1) holds true provided the pseudo-contact contribution to the chemical shis of the ligand protons is negligible; this is indeed supported by the remarkable agreement observed between NMR chemical shis and DFT calculations of the electron spindensity. 17 Table 1 shows the so-calculated Avalues. ENDOR ENDOR experiments were carried out at 5 K in frozen solutions of 0.5 mM Au 25 (SR) 0 18 in toluene. The spectra of the four clusters (Fig. 4) show a large background between 2 and 20 MHz due to an ENDOR line of gold atoms. 10 By focusing on the region between 8 and 16 MHz, as shown in Fig. 5 for Au 25 (SBu) 0 18 , one can notice the presence of three symmetrical line doublets Fig. 2 (a) 1 H NMR spectrum of [n-Oct 4 N + ] [Au 25 (SPr) 18 ]at25C. The peaks marked with a star refer to n-Oct 4 N + . (b) 1 H NMR spectrum of Au 25 (SPr) 0 18 at 25 C; the portion of the spectrum showing the (a-CH) in protons (at 70 C) is offset and enlarged. Fig. 3 Plot of the Ddvalues for Au 25 (SR) 18 , obtained at 298 K, against the average crystallographic distance of the specific proton type from the central Au atom. The color codes for the R groups are: Et, green; Pr, red; Bu, blue. The resonances are indicated as: (a-CH) in ,;(b-CH) in , ;(a-CH) out ,;(g-CH) in ,;(b-CH) out , . For clarity, a scale break has been inserted into the Ddscale and vertical dashed lines group the protons at similar distances. This journal is © The Royal Society of Chemistry 2016 Chem. Sci.,2016,7, 6910–6918 | 6913 Edge Article Chemical Science Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
centered at the Larmor frequency n. At a magnetic eld of 0.29 T, which is the eld at which ENDOR spectra were acquired, the Larmor frequency is 12.34 MHz for the protons. The three symmetric doublets are marked by brown (outer), green (middle), or black (inner) lines. Similar ENDOR spectra are observed for the other clusters capped by linear-chain thiolates, but the relative intensity ratios are different. The simplest case to analyze is Au 25 (SEt) 0 18 . The two proton types and the two ligand families generate four groups of equivalent protons: 24 (a-CH) in ,36(b-CH) in ,12(a-CH) out , and 18 (b-CH) out . These groups should give rise to four doublets of lines. Three doublets of lines are clearly observed and positioned symmetrically around the Larmor frequency, whereas a broad line is observed at ca. 3 MHz (Fig. 4, trace a). The latter Table 1 1 H NMR and ENDOR parameters Ligand and position A a (kHz) A b,c (MHz) T xxc,d (MHz) T yyc,d (MHz) T zzc,d (MHz) n e SEt (a-CH) in 723 n.d. f (b-CH) in 81.1 4.2 0.8 2.7 3.5 36 (a-CH) out 59.2 1.6 0.9 0.9 1.8 12 (b-CH) out 5.78 0.24 0.01 0.01 0.02 14 g SPr (a-CH) in 713 n.d. f (b-CH) in 39.3 5.5 1.2 1.2 2.4 24 (a-CH) out 59.7 2.0 0.6 0.6 1.2 12 (b-CH) out 6.89 0.24 0.01 0.01 0.02 14 g SBu (a-CH) in 708 n.d. f (b-CH) in 42.9 5.8 0.7 0.7 1.4 24 (a-CH) out 61.9 2.2 0.6 0.6 1.2 12 (b-CH) out 5.82 0.24 0.01 0.01 0.02 16 g SMePr (a-CH) in 355 n.d. f (b-CH) in 4.94 5.5 1.2 1.2 2.4 12 (a-CH) out 63.5 2.8 0.6 0.6 1.2 12 (b-CH) out 10.1 0.22 0.01 0.01 0.02 28 g (g-CH) in 31.9 2.0 0.5 0.5 1.0 60 a From NMR measurements at 298 K, using eqn (1). b From ENDOR measurements at 5 K. c The error associated with the simulations is ca. 0.1 MHz. d T xx ,T yy , and T zz are the main values of the anisotropic hyperne tensor. e nis the number of equivalent nuclei corresponding to the best simulation. f Not determined: see text. g As discussed in the text, this number is affected by further contributions. Fig. 4 1 HENDORspectraof(a)Au 25 (SEt) 0 18 ,(b)Au 25 (SPr) 0 18 ,(c) Au 25 (SBu) 0 18 , and (d) Au 25 (SMePr) 0 18 in toluene solution at 5 K. For clarity, the spectra have been offset. The asterisks mark background signals due to the probe head. Fig. 5 Baseline-corrected 1 H-ENDOR spectrum (blue) and simulation (red) for Au 25 (SBu) 0 18 in toluene at 5 K. The lines mark the outer (brown), middle (green), and inner (black) proton ENDOR doublets. 6914 |Chem. Sci.,2016,7, 6910–6918 This journal is © The Royal Society of Chemistry 2016 Chemical Science Edge Article Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
could be attributed to the low-frequency part of a doublet of lines pertaining to the most strongly coupled protons. However, the corresponding high-frequency component, which should occur at ca. 21 MHz, is almost undetectable in the spectra (not shown). Furthermore, in the region around 3 MHz hyperne couplings from 13 C nuclei could also contribute. This makes uncertain the attribution of this feature to a proton line. For Au 25 (SEt) 0 18 , the average crystallographic distance between the alkanthiolate protons and the central gold atom increases in the order 6.27 (a-CH) in , 6.92 (b-CH) in , 7.23 (a-CH) out , and 8.26 ˚ A(b-CH) out . The (a-CH) in protons are closer to the gold cluster than the other protons and, therefore, they should give rise to the most strongly coupled doublet. In the hypothesis the 3 MHz line is a proton line, this could be related to the (a-CH) in , in keeping with the aforementioned NMR results. However, the ENDOR lines of the (a-CH) in protons could be simply undetectable, which is indeed not unusual for strongly coupled nuclei. The (b-CH) out protons are located at the largest distance from the center of the Au core and the inner lines are thus attributed to them. The distances characterizing the (b-CH) in and (a-CH) out proton groups are quite similar, in the solid state at least, and thus a direct assignment is difficult. This problem can be addressed by simulation of the ENDOR outer, middle and inner doublets. The spectrum of Au 25 (SEt) 0 18 is well simulated (ESI, red line in Fig. S6†) by using the parameters shown in Table 1, in which a ratio of 36 : 12 for the intensity of the outer and middle doublets is considered. This ratio corresponds to the ratio between the nuclei (b-CH) in and (a-CH) out . The position of the doublets is also in agreement with the crystallographic relative distances from the central Au atom, which for this cluster are 6.92 vs. 7.23 ˚ A. This allows assigning the outer, middle and inner doublets to (b-CH) in , (a-CH) out , and (b-CH) out , respectively. Regarding the (b-CH) out protons, the simulation provides a number, 14, that does not fully agree with that expected, 18. For weak couplings, however, some differences are not unusual because the ENDOR selectivity effect reduces the ENDOR line intensity, whereas the presence of the proton-free Larmor line (a single line associated with the solvent protons) could contribute to the inner doublet by increasing the line intensity. 38 Depending on the prevailing effect, either a decrease or an increase in intensity may occur. For the other clusters, we assign the (b-CH) in and (a-CH) out doublets as for Au 25 (SEt) 0 18 ; for (a-CH) in , the above considerations about the broad peak at 3 MHz are also valid. The crystallographic distances of the (b-CH) in and (a-CH) out groups from the cluster's center are similar, i.e., 7.23 and 7.16 ˚ A (Au 25 (SPr) 0 18 ), and 7.16 and 7.28 ˚ A (Au 25 (SBu) 0 18 ), respectively. In addition to the signals already discussed, the ENDOR spectrum of Au 25 (SPr) 0 18 is liable to show a contribution also from the 36 (g-CH) in , whose average crystallographic distance is 8.35 ˚ A, and thus shorter than that of the (b-CH) out protons, 8.64 ˚ A; a similar outcome is observed for Au 25 (SBu) 0 18 whose values are 8.12 and 8.20 ˚ A, respectively. However, we could still simulate the spectrum of Au 25 (SPr) 0 18 well (ESI, Fig. S7†) by using for the outer and middle doublets an ENDOR intensity ratio of 24 : 12, which corresponds to the number of equivalent (b-CH) in and (a-CH) out protons. Apparently, no (g-CH) in protons need to be taken into account. In fact, according to the NMR results, the (g-CH) in protons could have a hyperne coupling smaller than that of the (a-CH) out protons but still detectable. However, the roomtemperature NMR data cannot be directly compared with the low-temperature ENDOR data. At room temperature, the random motion of the alkyl chain is fast, with the (g-CH) in atoms moving oen closer to the metal core than the (b-CH) in atoms, but NMR spectroscopy only probes the average contact shi. Electron-transfer 26 and diffusion-coefficient 15 measurements provided evidence for the ligand chains being quite mobile in solution; for example, the Dvalues yield r MPC values smaller than the average radius of the same clusters as calculated from the crystallographic structure. At 5 K, however, whereas the ligands' motion is very limited by the frozen glassy solution, (g-CH) in can still be present in different conformations. It is thus conceivable that the ENDOR doublet of the (g-CH) in protons is associated with a wide conformational distribution (larger than that experienced by the band, even more, the a-groups) and this would cause signicant line broadening and thus spreading of the signal under the outer and middle doublets. To shed further light onto this issue, it is useful to compare the ENDOR spectra of Au 25 (SPr) 0 18 and Au 25 (SMePr) 0 18 (Fig. 6). Whereas in the latter there are 72 (g-CH) in protons that can contribute to the spectrum, the (b-CH) in and (b-CH) out protons are only 12 and 6, respectively, i.e., one half than those in Au 25 (SPr) 0 18 . Fig. 6 shows that in Au 25 (SMePr) 0 18 the ENDOR lines of the outer doublet are indeed signicantly smaller than those of Au 25 (SPr) 0 18 , and this conrms that for all clusters the outer doublet is consistently associated with the (b-CH) in protons. The increase in the middle doublet is particularly worth noting. This increase is attributed to a strong contribution from the (g-CH) in protons: compared to those in Au 25 (SPr) 0 18 ,in Au 25 (SMePr) 0 18 these protons have a narrower conformational distribution due to the steric hindrance introduced by the second methyl group. Hindrance not only makes these methyl Fig. 6 Comparison between the normalized ENDOR spectrum of Au 25 (SPr) 0 18 (black line) and Au 25 (SMePr) 0 18 (red line). For comparison, the spectra were normalized for the height. This journal is © The Royal Society of Chemistry 2016 Chem. Sci.,2016,7, 6910–6918 | 6915 Edge Article Chemical Science Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
groups less mobile but also the whole monolayer stiffer and thus the MPC radius larger. Support to this view comes from the electrochemical determination of the Dvalue in dicholoromethane (see ESI and Fig. S5†)and,thus,r MPC value of Au 25 (SMePr) 0 18 : they are 5.16 10 6 cm 2 s 1 and 10.3 ˚ A, respectively, whereas for Au 25 (SPr) 0 18 (which has the same fully extended length but more uid chains in the monolayer) they corresponding values are 6.15 10 6 cm 2 s 1 and 8.6 ˚ A. 15 The simulation of the ENDOR spectrum of Au 25 (SMePr) 0 18 (ESI, Fig. S7†) was carried out as summarized in Table 1. The number of protons required to obtain the best t to the inner signals is indeed signicantly larger than 6, which in this specic cluster corresponds to the particularly small number of (b-CH) out protons. We note, however, that the average crystallographic distance for the (g-CH) out protons in Au 25 (SPr) 0 18 and Au 25 (SBu) 0 18 is 8.9 ˚ Aandthus only slightly larger than for the (b-CH) out protons. A weak coupling is thus expected also for the (g-CH) out protons: for Au 25 (SMePr) 0 18 these protons are particularly numerous, 36, and this could make their contribution to the inner ENDOR lines quite signicant. This hypothesis is reasonable but not quanti- able also because of the aforementioned problems associated with weak couplings. Regarding Au 25 (SBu) 0 18 (Fig. 5), the most evident new feature is that the outer lines are higher than the middle lines. However, the number of protons causing an ENDOR line is related to the area, not to the line height. The simulation (ESI, Fig. S8†) results in a ratio of 24 : 12 between the outer and middle lines, in agreement with the attribution of the outer line to (b-CH) in and the middle line to (a-CH) out ; the outer lines are just narrower than in Au 25 (SEt) 0 18 and Au 25 (SPr) 0 18 . As for Au 25 (SPr) 0 18 , conformational distribution would make the contribution of the (g-CH) in protons spread in the region pertaining to the outer and middle lines. As a matter of fact, the Stokes radius of this cluster, 9.4 ˚ A, is smaller than that of the stiffer Au 25 (SMePr) 0 18 cluster. Regarding the (b-CH) out protons, the best t to the inner lines is obtained by using a number of protons, 16, larger than 12. Interestingly, also for Au 25 (SPr) 0 18 the number is larger, 14. We believe that the reason is as already described for Au 25 (SMePr) 0 18 ,i.e., a non vanishingly small contribution from the (g-CH) out protons. Electron–nucleus interaction The isotropic coupling is proportional to the spin density on the nucleus. The simulations of the ENDOR spectra provide both the isotropic and the anisotropic hyperne values. The latter are mainly related to the magnetic dipole–dipole interaction between electron and nucleus, therefore providing geometrical information. This interaction can be described by three main values, T xx ,T yy and T zz , called hyperne tensor main values, which are specic for each nucleus. 39 Here, the axes x,y,z represent a main reference system whose origin is at the center of the electron spin-distribution (in our case, the gold-cluster center). The values of T xx ,T yy and T zz can be obtained by averaging spatially the dipole–dipole interaction over the spin distribution, according to the following equation (eqn (2)), exemplied for x: Txx ¼m0 4pgebegNbNR23x2 R5(2) where m 0 is the vacuum permeability, Ris the electron-nucleus distance, g e is the electron g-factor, g N is the nucleus g-factor, and b N is the nuclear magneton. Analogous equations hold for y and z. From these equations, it results that T xx +T yy +T zz ¼0. If the spin-distribution is axially symmetric with respect the z direction, then T xx ¼T yy and the hyperne tensor main values are T xx ¼T yy ¼Tand T zz ¼2T. Deviation from an axially symmetric distribution leads to a hyperne tensor whose main values are [T xx ,T yy ,T zz ] with T xx sT yy . Table 1 shows that the various proton types are consistently in the form [T,T,+2T], except for (b-CH) in in Au 25 (SEt) 0 18 in which a [T xx ,T yy ,T zz ] form is observed, with a marked difference between T xx and T yy . With all cautions already discussed, if we assume that the feature at 3 MHz is due to (a-CH) in protons, its A can be estimated to be around 15–20 MHz. This would be about one order of magnitude larger than the value pertaining to the (a-CH) out protons. In this connection, it is worth recalling that DFT calculations carried out for Au 25 (SCH 2 CH 2 Ph) 0 18 showed that the spin density at (a-CH) in is one order of magnitude larger than at (a-CH) out . 17 If we now compare the (b-CH) in and the (a-CH) out isotropic hyperne couplings for alkanethiolates of increasing length (Table 1, third column), we note an increase in the absolute value of A, particularly in the passage from Et to Pr. This suggest that the ligands are not completely indifferent to the spin distribution in Au 25 (SR) 0 18 , and thus to the SOMO structure. Another aspect regards the sign of A. According to the theory, the isotropic hyperne coupling with a nucleus is given by (eqn (3)): A¼2m0 3gebegNbNrarb(3) where r a is the direct spin-density and r b is the spin-polarized density. 39 r a is mainly contributed by the unpaired electron in the SOMO, whereas r b is due to spin-polarization. The latter results from the tendency of the unpaired electron to withdraw electrons with the same spin, because of the favorable exchange interaction, and vice versa.Ifg N is positive, which is true for protons, then a positive Ais found for a dominant direct contribution from the SOMO, whereas a negative value is found if spin-polarization prevails. From the simulations, we obtain the sign with respect to [T xx , T yy ,T zz ], i.e., the pattern of the doublet does not change if we were to revert both the sign of Aand [T xx ,T yy ,T zz ]. However, if we assume that the anisotropic interaction is mainly dipolar, then the form of the anisotropic tensor main values is [T,T,+2T] with T> 0, and thus the sign of Ais determined as shown in Table 1. Consequently, a negative value in Ais an indication that Ais mainly determined by spin-polarization through bonds, with a small contribution from direct spin density on the proton nuclei from the SOMO orbital, according to eqn (3). We can thus speculate that the increase of |A| as one goes from Et to Bu is the result of a decreasing, positive contribution to the hyperne coupling from the direct spin-density of the SOMO, suggesting again that in the Et cluster spin-density is more diffuse toward the ligands than in the Pr or Bu cases. 6916 |Chem. Sci.,2016,7, 6910–6918 This journal is © The Royal Society of Chemistry 2016 Chemical Science Edge Article Open Access Article. Published on 19 September 2016. Downloaded on 14/11/2016 15:09:56. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online