DOI: 10.1002/chem.201404763
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& Copper Nanoclusters
Phosphinothiolates as Ligands for Polyhydrido Copper Nanoclusters Miguel A. Huertos,[a] Israel Cano,[a] Nuno A. G. Bandeira,[a] Jordi Benet-Buchholz,[a] Carles Bo,[a, b] and Piet W. N. M. van Leeuwen*[a]
and 10 S atoms). Seven hydrogen atoms, in hydride form, are needed for charge balance and were located by density functional theory methods. H2 was released from the copper hydride nanoparticles by thermolysis and visible light irradiation.
Abstract: The reaction of [CuI(HSC6H4PPh2)]2 with NaBH4 in CH2Cl2/EtOH led to air- and moisture-stable copper hydride nanoparticles (CuNPs) containing phosphinothiolates as new ligands, one of which was isolated by crystallization. The Xray crystal structure of [Cu18H7L10I] (L = S(C6H4)PPh2) shows unprecedented features in its 28-atom framework (18 Cu
Introduction
properties. Herein we describe the synthesis and characterization of novel copper nanoparticles with phosphinothiolatekP,kS ligands. Copper nanoparticles with 2-(diphenylphosphino)benzenethiol were prepared following the same procedure as that used for phosphine-capped gold nanoparticles.[3b, 10]
In recent decades, special attention has been paid to Au and Ag nanoclusters because of their chemical and physical characteristics, which are different from those of the molecular compounds.[1] The change of the properties has been correlated with the size of the clusters. Core size can be controlled by surface bound ligands, such as thiolates[2] and phosphines, which each lead to specific properties.[3] Phosphine-ligated Au and Ag nanoclusters are less stable than the thiolate ones and in addition they need anions to compensate the charge of the surface atoms. Studies on copper nanoclusters are still in their infancy. In the last two decades, Fenske et al. have reported several chalcogenide copper nanoclusters.[4] Metal hydrides were extensively pursued as they are key intermediates in catalysis[5] and promising materials for hydrogen storage.[6] Copper hydrides are well-known catalysts for the hydrogenation of a variety of unsaturated C C and C O bonds.[7] Since Wrtz prepared the first copper(I) hydride in 1844,[8] several copper hydrides have been synthesized (mono-, di-, tri-, hexa-, or octanuclear) and stabilized by phosphines, pyridines, N-heterocyclic carbenes or thiolates.[7c, 9] In 2013, Liu et al. published the first Cu20 nanocluster ligated by dichalcogen donor ligands.[9a] Our strategy to obtain Cu nanoclusters involves the use of bidentate (S,P) ligands that contain thiolate as a stabilizer and a phosphine that can be used for tuning the electronic
Results and Discussion Synthesis and characterization of the precursor complex The reaction of CuI with one equivalent of [SH(C6H4)PPh2] (LH) in CH2Cl2 solution led to the formation of a new complex as a white crystalline solid in good yield (Scheme 1). The solid
Figure 1. Molecular structure of 1: Displacement ellipsoids depicted at 50 % probability level. Minor disordered components and most H atoms omitted for clarity. Selected bond lengths () and angles (8): Cu1 P1 2.2411(12), Cu1 I1 2.5483(8), Cu1 S1 2.649(8); P1-Cu1-I1 128.06(4), P1-Cu1-S1 77.20(18), I1-Cu1-S1 113.4(3), Cu1-I1-Cu1’ 61.33(2).
[a] Dr. M. A. Huertos, Dr. I. Cano, Dr. N. A. G. Bandeira, Dr. J. Benet-Buchholz, Prof. Dr. C. Bo, Prof. Dr. P. W. N. M. van Leeuwen Institute of Chemical Research of Catalonia (ICIQ) Avinguda Paı¨sos Catalans 16, 43007 Tarragona (Spain) E-mail: [email protected]
state structure, as determined by single-crystal X-ray diffraction (Figure 1), identifies this new product as [{CuI(LH)}2] (1). This complex is characterized by a {Cu2(m-I)2} rhombus as a structural unit with a bidentate LH ligand coordinated to each copper ion. The {Cu2(m-I)2} unit is planar with a crystallographic inversion center. The Cu ion adopts a pseudotetrahedral coordina-
[b] Prof. Dr. C. Bo Departament de Qumica Fsica i Inorganica, Universitat Rovira i Virgili Carrer de Marcel·l Domingo s/n, 43007 Tarragona (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404763. Chem. Eur. J. 2014, 20, 1 – 8
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Full Paper ing 17 % of copper. EDX also displays 6 % of iodine (approximate formula Cu7L5I1). We explain the ligand content calculated (Cu/L = 1.2:1) by the formation of small clusters [CunLn] with n < 10 not visible by TEM. The UV/Vis absorption band (300 nm; see Figure S.3 in the Supporting Information) indicates the formation of small copper nanoparticles (CuNPs).[12] In addition, the absence of a plasmon resonance band in the region around 560 nm indicates that there is not a detectable number of Cu nanoparticles with a size larger than 4.5 nm. The transmission IR spectrum of the CuNPs shows the absence of the S–H stretching band, in contrast to the observed band at 2550 cm 1 for the free ligand and complex 1 (Figure 3). These experiments are in agreement with coordination of the ligand as a thiolate on the copper nanoparticles.
Scheme 1. Synthesis of 1.
tion geometry, comprising two halide ions and one P and one S atom, both of the same LH ligand. The small Cu-I-Cu angle (61.33(2)8) is probably due to the large ionic radius of iodide.[11] The 1H NMR spectrum in solution for compound 1 shows a relative integral 2H broad signal at d = 4.23 ppm, corresponding to two S–H units of the thiolphosphine ligand.
Synthesis and characterization of phosphinothiolatestabilized Cu nanoparticles In a typical nanoparticle synthesis, NaBH4 (15 equiv) suspended in EtOH was added to a solution of 1 in CH2Cl2 previously cooled at 0 8C (Scheme 2). A color change, from colorless to
Scheme 2. Synthesis of CuNPs. Figure 3. Overlay of the ATR FT-IR spectra of ligand, complex 1, and CuNPs.
bright yellow, was immediately observed and the solution was stirred overnight (16 h) at room temperature. The resultant product was purified (see Experimental Section) and a bright yellow powder was obtained in 63 % yield. The product shows a broad size distribution by TEM (Figure 2) but most of the particles visible by TEM exhibit a size around 1.2–1.6 (0.87) nm. Elemental analysis (EA) and EDX are in good agreement, show-
The 31P{1H} NMR spectrum in solution of CuNPs shows a very-broad signal centered at 0.5 ppm. This signal shifts 9.5 ppm from precursor 1 (d = 9.0 ppm; see Experimental Section) and shows that also in CuNPs the phosphine is coordinated to Cu. The oxidation state of the CuNPs was then investigated by XPS. Figure 4 shows the binding energies of 2p3/2 and 2p1/2 electrons of copper. The XPS spectrum of 1 (Figure 4, black) with copper atoms in oxidation state + 1, fits perfectly with the binding energies of the electrons of copper in CuNPs (Figure 4, grey). It is known that the 2p3/2 binding energy of
Figure 2. Representative TEM image of CuNPs and size distribution (inset) determined from TEM images by counting > 300 non-touching particles obtained from images captured from distinct quadrants of the grid.
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Figure 4. X-ray photoelectron spectra of the CuNPs (gray) and 1 (black).
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Full Paper Cu0 is only 0.1 eV away from that of CuI species. Although one isolated species (see below) may contain only CuI, for the CuNPs mixture we cannot exclude the presence of Cu0. The 1H NMR spectrum in solution of CuNPs shows two very broad signals d 4.50 and 2.20 ppm that correspond to Cu hydrides. These data suggest the existence of two types of hydrides. The higher-field signal (2.20 ppm) is in good agreement with {Cu(m3-H)}.[13] Jin et al. reported different shifts for the bridging hydrides in Cu4H4X (X = B, C, N, O) depending on the electronegativity of X.[14] In this context, we propose that the lower-field signal (4.50 ppm) could belong to a {Cu(m2-H)}, species where the copper is ligated by a sulfur atom.
H2 evolution from CuNPs Three different experiments were performed with the aim to support the existence of Cu-hydride clusters. A solution of CuNPs in THF (4 mg, 2 mL) was irradiated with visible light using a Xenon Arc Lamp with a UV filter keeping a constant temperature of 25 8C. After 30 min, H2 evolution (27 mL of H2, Figure 5) was observed by gas chromatography (GC). A solution of CuNPs in THF (4 mg, 2 mL) was heated for 30 min at 70 8C and H2 was detected by GC. As expected, addition of acid also produced H2 (for more details, see the Supporting Information). Figure 6. a) Central core of 8 Cu atoms; b) skeleton of nanocluster 2 formed by 18 Cu and 10 S atoms; c) molecular structure of 2. Displacement ellipsoids depicted at 50 % probably level. Minor disordered components and all H atoms omitted for clarity.
central core, the distances between the 8 Cu atoms (2.5261(11)–2.9157(10) ) are similar to those in Cu clusters previously described.[9a] The remaining 10 Cu atoms are surrounding the internal Cu core always connected to S and P atoms (Figure 6 b). Only Cu3/Cu3’ atoms are connected directly to the inner copper core atoms (distance Cu3–Cu4: 2.6411(9) ). At each side of the structure (as drawn in Figure 6) there are 4 Cu atoms (Cu1, Cu6, Cu8 and Cu9, and symmetry equivalents), which are connected between them and to the central core by four Cu m3-S bonds. The skeleton of the cluster is formed by 18 Cu atoms and 10 S atoms (Figure 6 b, Cu m3-S distances: 2.2205(15)–2.4402(14) ). In the external part of the skeleton, the 10 outer copper atoms are capped by a phosphine (Figure 6 c). Additionally, the atoms Cu1, Cu8, and Cu9 are linked to an iodide anion. This iodide anion has an occupancy of 50 %, since it is disordered in the two symmetry-equivalent positions (Figure 6 c). Alternating its location with the iodide anion, we speculate that a hydride is present also having an occupancy of 50 % in each symmetryequivalent position. The hydrogen atoms (probably 7 hydrides, based on DFT calculations; see below) could not be localized from single-crystal X-ray diffraction experiments. Efforts to grow a larger crystal (1 mm3) suitable for neutron diffraction were unsuccessful.
Figure 5. Calibration plot for hydrogen measurement. White circles: Addition of known amounts of hydrogen into the cell. Black circles: Experimental value for the photolysis experiment. 27 mL of H2 (0.0001 mmol) are released for 4 mg of CuNCs (0.02 mmol Cu).
X-ray crystal structure Crystals of an air- and moisture-stable Cu–H nanocluster [Cu18H7L10I] (2),[15] which are a small portion of the mixture of nanoparticles, were grown from a solution of CuNPs in CH2Cl2 layered with hexane. The size of this species coincides with the number-wise most abundant size in Figure 2. Herein we report the unprecedented crystal structure of 2 (Figure 6 c). The compound crystallizes in the space group P1¯, the cluster having a crystallographic Ci symmetry. The framework of 18 copper atoms can be described as a central core of 8 Cu atoms (Figure 6 a, atoms Cu2, Cu4, Cu5, Cu7 and symmetry equivalents) formed by two butterfly structures (4 Cu atoms each). In this Chem. Eur. J. 2014, 20, 1 – 8
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atoms with distances between 1.658 and 1.840 . Two other hydrides (H220 and H223) are coordinated in Cu(m2-H) fashion. Cu m2-H distances are in the range 1.587–1.696 . Several examples for this type of coordination were reported previously.[9a–f] The other 4 hydrides are placed on 4 faces of the tetrahedrons of the 8 Cu atoms of the central core. The coordination of the hydrogen atom on a tetrahedral copper face has been reported for [Cu20H11(S2P(OiPr)2)9].[9a] X-ray and DFT structures are in good agreement, as shown by the distances summarized in Table 1. An alternative structure built by replacing the iodine atom with a hydride provided a more symmetrical structure, but did not affect significantly the nature of the Cu atoms core (Figure 7).
Density functional theory (DFT)-based full geometry optimizations were carried out for 2 in order to locate the hypothesized 7 hydrogen atoms (see the Supporting Information). It is worth mentioning that without any additional hydrogen atom, the nanocluster’s electronic structure is not stable, as it displayed a null HOMO–LUMO gap. By analyzing the spatial shape of the lowest empty energy levels, seven hydrogen atoms and seven additional electrons were added to the system, and the geometry of a model cluster, that is, [Cu18H7L10I], wherein the ligand L = [SC6H4PPh2] was truncated to [SC6H4PMe2] for computational expediency, was subsequently optimized. The iodine atom in only one apical position and the hydride in the opposite apical site entail the loss of symmetry. Six hydrogen atoms were placed in the {Cu10} core and optimized in several different arrangements. Initially, the optimization was constrained to maintain the coordinates of the crystal structure optimizing only the six core hydrides plus the one in the apical position. In the subsequent step, the structures were fully relaxed. Two stationary points were located in the procedure, the least stable one being 98.7 kJ mol 1 higher in energy. In the least stable structure the {Cu10} core hydrides were each coordinated in tetrahaptic (h4-H) fashion within the Cu tetrahedra, in a structural pattern akin to Wrtz’s original CuH structure, in which each hydride ion is enclosed by a copper(I) tetrahedron and vice versa. The lowest energy structure exhibits four hydrides bridging the outward faces of each Cu tetrahedron in m3-H coordination (Figure 7 a) and on this we base our proposal for the hydride positions. Hydride H226 is depicted at the bottom of Figure 7 a and is coordinated as a bridge between 3 copper
Table 1. Selected bond lengths. Bond 2
Cu m -H Cu -m3-H Cu Cu (central core) Cu3 Cu4 Cu m3-S
X-Ray
DFT
— — 2.5261(11)–2.9157(10) 2.6411(9) 2.2205(15)–2.4402(14)
1.587–1.696 1.644–1.757 2.485–2.890 2.515–2.543 2.264–2.608
Figure 7 also contains a schematic MO diagram of the frontier orbitals of the minimum energy model (b) and 3D plots displaying the composition of the highest occupied (HOMO, c) and lowest unoccupied molecular orbital (LUMO, d). The HOMO ( 4.08 eV; Figure 7 c) of 2 is principally located in the copper atoms with 52 % atomic contribution from copper and 26 % from sulfur. The highest filled levels correspond to a corelike 3d10 region of the copper fragments whereas the CuI 4s and 4p orbitals only come up with any significant degree of prominence in the virtual MO space at 0.97 eV, higher in energy than the phenyl rings’ antibonding p* orbital set, which begins at the LUMO ( 1.99 eV). Natural population analysis (NPA)[16] of 2 clearly indicates that the oxidation state of all copper atoms is + 1, and that all seven added hydrogen atoms must be regarded as hydrides. Indeed, the atomic population of copper atoms falls into a 0.42–0.65 range, with nearly 10 e in the d orbitals (average 9.87) plus roughly 0.5 e in the s orbital. For the seven additional hydrogen atoms, the NPA atomic charge average is 0.5.
Conclusion In summary, we have prepared and characterized new copper hydride nanoclusters ligated by a thiolate-phosphine chelate ligand. This unsymmetric bidentate ligand gives CuNPs with the characteristic stability supplied by the thiolate and a tunable phosphine, which might lead to catalytic activity. The CuNPs release hydrogen gas under heating and upon irradiation with visible light. The latter is key to the synthesis of new materials for hydrogen storage, although in this material the hydrogen content by weight is low. We were able to isolate one of the clusters with the stoichiometry [Cu18H7L10I]. The unprecedented geometry of this Cu18 structure has been
Figure 7. a) Optimized molecular structure of 2; b) schematic molecular orbital diagram diagram; c) HOMO; d) LUMO.
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Full Paper characterized by X-ray diffraction and the hydride positions were calculated by DFT methods.
X-ray crystal structures determination Crystals of 1 and 2 were obtained by slow diffusion of a 10/30 dichloromethane/hexane solution at room temperature. The measured crystals were prepared under inert conditions immersed in perfluoropolyether as protecting oil for manipulation.
Experimental Section
Data collection: Crystal structures determinations for 1 and 2 were carried out using an Apex DUO Kappa 4-axis goniometer equipped with an APPEX 2 4 K CCD area detector, a Microfocus Source E025 IuS using MoKa radiation, Quazar MX multilayer Optics as monochromator and an Oxford Cryosystems low temperature device Cryostream 700 plus (T = 173 8C). Full-sphere data collection was used with w and f scans. Programs used: Data collection APEX2,[18] data reduction Bruker Saint[19] and absorption correction SADABS.[20]
General All oxygen and moisture sensitive operations were carried out under an argon atmosphere using standard vacuum-line and Schlenk techniques. Solvents were purchased from Sigma–Aldrich as HPLC grade and dried by means of an MBraun MB SPS800 purification system. 2-(Diphenylphosphino)benzenethiol (LH) was prepared according to literature procedure reported by Block et al.[17] Chemical shifts of 1H, 13C, and 31P NMR are reported in ppm, with the solvent used as internal standard. Signals are quoted as s (singlet), d (doublet), dd (double doublet), m (multiplet), b (broad). Elemental analyses were performed by Kolbe (Mlheim, Germany).
Structure solution and refinement: Crystal structures were solved and refined by Dr. Jordi Benet-Buchholz. Crystal structures solution were achieved using direct methods as implemented in SHELXTL[21] and visualized using the program XP. Missing atoms were subsequently located from difference Fourier synthesis and added to the atom list. Least-squares refinement on F2 using all measured intensities was carried out using the program SHELXTL. All non-hydrogen atoms were refined including anisotropic displacement parameters.
Gas chromatography (GC) Gas chromatography measurements were done using an Agilent Technologies 7890B GC System with thermal conductivity detector and capillary molecular sieves column.
Comments on the structures: The asymmetric unit of the crystal structure of 1 contains half a molecule of the organometallic complex and half a molecule of cyclohexane. The complex molecule shows Ci symmetry. The aromatic ring with the sulfur atom is disordered in two positions through the center of inversion with a ratio of 55:45. The cyclohexane ring is disordered around a twofold rotation axis (ratio 50:50).
Transmission electron microscopy (TEM) TEM analyses were performed on a Zeiss 10 CA electron microscope at 100 kV with a resolution of 3 . Samples were prepared by drop casting (from various THF solutions) onto a holey Formvar/carbon-coated copper grid.
The metal cluster 2 crystallizes having Ci symmetry in the triclinic space group P1¯. The asymmetric unit contains half a molecule of the metal cluster, two molecules of dichloromethane and half a molecule of n-hexane. The metal cluster contains an iodine atom which is disordered in two positions related by the center of inversion (one iodine atom for each complete metal cluster, ratio 50:50). One of the dichloromethane molecules is also disordered in two positions (ratio 50:50). Expected hydrides could not be localized from the single-crystal X-ray structure determination and are missing in the empirical formula. Probably the disordered iodine anion is alternating its position with a hydride.
X-ray photoelectron spectroscopy (XPS) These experiments were carried out on our behalf by Dr. L. Calvo Barrio at the Centres Cientfics i Tecnolgics, Universitat de Barcelona, using a PHI 5500 Multitechnique System (from Physical Electronics) with a monochromatic X-ray source (aluminum K-alfa line of 1486.6 eV energy and 350 W), placed perpendicular to the analyzer axis and calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 0.8 eV. The analyzed area was a circle of 0.8 mm diameter, and the selected resolution for the spectra was 187.5 eV of Pass Energy and 0.8 eV per step for the general spectra and 23.5 eV of pass energy and 0.1 eV per step for the spectra of the different elements in the depth profile spectra. A low-energy electron gun (less than 10 eV) was used to discharge the surface when necessary. All measurements were made in an ultra-high vacuum (UHV) chamber with pressure between 5 10 9 and 2 10 8 torr.
Computational details The Amsterdam Density Functional (ADF) program package[22] version 2013.01 was used throughout to optimize the geometries of the species mentioned in the paper. The Becke[23] and Perdew[24] gradient-corrected exchange and correlation functionals (BP86) respectively were used in the calculations, wherein the local part was set up from the Slater exchange and the Vosko, Wilk, and Nusair correlation functionals.[25] The ZORA[26] scalar relativistic Hamiltonian was employed with a triple zeta Slater-type orbital[27] (STO) augmented with one polarization function (TZP) for copper and iodine, and double zeta STO type functions augmented with one additional polarization function on the remaining elements. A frozen core approximation was used for all the elements ([Ar] for Cu, [Pd] for iodine, [He] for C and [Ne] for phosphorus and sulfur) except hydrogen. The geometry optimizations were performed via the default numerical integration Scheme of Becke[28] without symmetry constraints and with default integration accuracy. A natural population analysis[29] was performed on the lowest energy structure using an all electron basis set.
Fourier transform infrared spectroscopy (FT-IR) FT-IR measurements were carried out on a Bruker Optics FT-IR Alpha spectrometer equipped with a DTGS detector, KBr beamsplitter at 4 cm 1 resolution in the range 4000–400 cm 1.
Energy dispersive X-ray spectroscopy (EDX) EDX analyses were performed on a FEI Quanta 600 ESEM equipped with an Oxford Instruments EDS Detector (20 kV accelerating voltage, 10 mm working distance) employing cobalt as internal standard. Samples were prepared by drop casting (from various THF solutions) onto an aluminum holder. Chem. Eur. J. 2014, 20, 1 – 8
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[{2-(Diphenylphosphino)benzenethiol)CuI}2] (1): A solution of 2-(diphenylphosphino)benzenethiol (294 mg, 1.0 mmol) and CuI (190 mg, 1.0 mmol) in CH2Cl2 (20 mL) was stirred for 16 h at room temperature. The solvent was removed by evaporation under high vacuum to a volume of 5 mL. Next, hexane (5 mL) was added and the resulting solution was stored at 30 8C, precipitating a white solid that was isolated by decantation of the solvent and dried under high vacuum., affording the prodduct as a white crystalline solid (397 mg, 0.41 mmol, 82 %). Slow diffusion of hexane into a concentrated solution of 1 in CH2Cl2 at 30 8C afforded crystals, one of which was employed for an X-ray structure determination. 1 H NMR (500 MHz, CD2Cl2, ): d = 7.61–7.57 (m, 1 H, HAr), 7.66–7.10 (m, 12 H, HAr), 7.07 (t, J = 6.8 Hz, 1 H, HAr), 4.23 ppm (s, br, SH); 31 1 P{ H} NMR (202 MHz, CD2Cl2): d = 9.33 ppm (s, br). Copper nanoparticles: The appropriate copper complex 1 (0.04 mmol) was dissolved in degassed CH2Cl2 (30 mL) and cooled to 0 8C in an ice bath. A solution of NaBH4 (15 mg, 0.4 mmol, 10 equiv) in ethanol (3 mL) was prepared immediately prior to use and then added in one portion via syringe to the rapidly stirred CH2Cl2 solution resulting in the immediate formation of a yellow suspension, which was allowed to warm to room temperature slowly overnight. The solvents were removed under high vacuum and the dry residue was extracted with toluene (30 mL). The yellow solution was concentrated under reduced pressure and the residue was extracted with CH2Cl2 (30 mL). The solvent was removed by evaporation to a volume of approximately 5 mL and a bright yellow solid was precipitated with hexane, which was washed with ethanol (2 20 mL, 0 8C) and diethyl ether (2 20 mL, 0 8C), affording the product as a yellow powder (24 mg, 63 % yield, based on Cu). 1H NMR (500 MHz, C6D6, 128 scans, 25 8C): d = 8.15– 5.8 ppm (br, HAr); 31P{1H} NMR (202 MHz, C6D6, 3000 scans, 25 8C): d = 3.1 ppm (very br); elemental analysis (%) found: C 52.76, H 4.47, Cu 16.41, P 6.83, S 6.67; EDX (%): C 61.14, Cu 17.95, P 7.74, S 7.85, I 5.31.
Acknowledgements This work was supported by an ERC Advanced Grant (NANOSONWINGS 2009–246763) and Spanish Ministerio de Economia y Competitividad (MINECO, grant CTQ2011–29054-C02–02). The authors acknowledge the ICIQ support units and Dr. Rita Marimon and Dr. Mariana Stefanova from Universitat Rovira i Virgili for TEM and EDX measurements. Keywords: copper · density functional calculations · hydrides · nanoclusters · P,S ligands [1] a) X. Yuan, Z. Luo, Y. Yu, Q. Yao, J. Xie, Chem. Asian J. 2013, 8, 858 – 871; b) C. M. Cobley, J. Chen, E. C. Cho, L. V. Wang, Y. Xia, Chem. Soc. Rev. 2011, 40, 44 – 56; c) M.-C. Daniel, D. Astruc, Chem. Rev. 2003, 103, 293 – 346. [2] a) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. 1994, 801 – 802; b) G. Yang, Z. Zhang, S. Zhang, L. Yu, P. Zhang, Y. Hou, Surf. Interface Anal. 2013, 45, 1695 – 1701; c) X. Jia, X. Yang, J. Li, D. Li, E. Wang, Chem. Commun. 2014, 50, 237 – 239; d) T.-Y. Dong, H.-H. Wu, M.-C. Lin, Langmuir 2006, 22, 6754 – 6756; e) W. Wei, Y. Lu, W. Chen, S. Chen, J. Am. Chem. Soc. 2011, 133, 2060 – 2063; f) T. P. Ang, T. S. A. Wee, W. S. Chin, J. Phys. Chem. B 2004, 108, 11001 – 11010; g) A. Desireddy, B. E. Conn, J. Guo, B. Yoon, R. N. Barnett, B. M. Monahan, K. Kirschbaum, W. P. Griffith, R. L. Whetten, U. Landman, T. P. Bigioni, Nature 2013, 501, 399 – 402.
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Full Paper [19] Data reduction with Bruker SAINT version V8.30c. Bruker (2007). Bruker AXS Inc., Madison, Wisconsin, USA. [20] a) SADABS: V2012/1 Bruker (2001). Bruker AXS Inc., Madison, Wisconsin, USA. Blessing; b) R. H. Blessing, Acta Crystallogr. 1995, A51, 33 – 38. [21] G. M. Sheldrick, Acta Crystallogr. 2008, A64, 112 – 122. SHELXTL version V6.14. [22] E. J. Baerends, J. Autschbach, A. B rces, C. Bo, P. M. Boerrigter, L. Cavallo, D. P. Chong, L. Deng, R. M. Dickson, D. E. Ellis, M. van Faassen, L. Fan, T. H. Fischer, C. F. Guerra, S. J. A. van Gisbergen, J. A. Groeneveld, O. V. Gritsenko, M. Grning, F. E. Harris, P. v. d. Hoek, H. Jacobsen, L. Jensen, G. van Kessel, F. Kootstra, E. van Lenthe, D. A. McCormack, A. Michalak, V. P. Osinga, S. Patchkovskii, P. H. T. Philipsen, D. Post, C. C. Pye, W. Ravenek, P. Ros, P. R. T. Schipper, G. Schreckenbach, J. G. Snijders, M. Sola, M. Swart, D. Swerhone, G. te Velde, P. Vernooijs, L. Versluis, O. Visser, F. Wang, E. van Wezenbeek, G. Wiesenekker, S. K. Wolff, T. K. Woo, A. L. Yakovlev, T. Ziegler, http://www.scm.com, Scientific Computing and Modelling ADF-2013.01.
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[23] A. D. Becke, Phys. Rev. A 1988, 38, 3098 – 3100. [24] a) J. P. Perdew, Phys. Rev. B 1986, 33, 8822 – 8824; b) J. P. Perdew, Phys. Rev. B 1986, 34, 7406. [25] a) S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980, 58, 1200 – 1211; b) J. C. Slater, Phys. Rev. 1951, 81, 385 – 390. [26] a) E. van Lenthe, E. J. Baerends, J. G. Snijders, J. Chem. Phys. 1993, 99, 4597 – 4610; b) E. van Lenthe, A. E. Ehlers, E. J. Baerends, J. Chem. Phys. 1999, 110, 8943 – 8953. [27] E. Van Lenthe, E. J. Baerends, J. Comput. Chem. 2003, 24, 1142 – 1156. [28] A. D. Becke, J. Chem. Phys. 1988, 88, 2547 – 2553. [29] E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M. Morales, F. Weinhold, NBO 6.0 ed., University of Wisconsin, Madison, US, 2013.
Received: August 7, 2014 Published online on && &&, 0000
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FULL PAPER & Copper Nanoclusters
Post scriptum: Copper hydride nanoclusters have been prepared by reduction of a copper(I) phosphinothiol complex; they release H2 by visible light irradiation. A Cu18 cluster was isolated by crystallization, showing an unprecedented geometry that was evaluated by X-ray diffraction.
M. A. Huertos, I. Cano, N. A. G. Bandeira, J. Benet-Buchholz, C. Bo, P. W. N. M. van Leeuwen* && – && Phosphinothiolates as Ligands for Polyhydrido Copper Nanoclusters
The peculiar structure… …of the nanocluster, reported by P. W. N. M. van Leeuwen et al. in their Full Paper on page && ff., consists of eighteen copper atoms and ten phosphinothiolate anions with a core of eight copper atoms, four additional atoms sticking out via bridging ligands, and on top and below the central core three atoms also glued to the cluster via bridging ligand interactions. The structure of the cluster is unusually “unorganized” compared to most other clusters, although it does have an inversion center.
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& Copper Nanoclusters
Phosphinothiolates as Ligands for Polyhydrido Copper Nanoclusters Miguel A. Huertos,[a] Israel Cano,[a] Nuno A. G. Bandeira,[a] Jordi Benet-Buchholz,[a] Carles Bo,[a, b] and Piet W. N. M. van Leeuwen*[a]
and 10 S atoms). Seven hydrogen atoms, in hydride form, are needed for charge balance and were located by density functional theory methods. H2 was released from the copper hydride nanoparticles by thermolysis and visible light irradiation.
Abstract: The reaction of [CuI(HSC6H4PPh2)]2 with NaBH4 in CH2Cl2/EtOH led to air- and moisture-stable copper hydride nanoparticles (CuNPs) containing phosphinothiolates as new ligands, one of which was isolated by crystallization. The Xray crystal structure of [Cu18H7L10I] (L = S(C6H4)PPh2) shows unprecedented features in its 28-atom framework (18 Cu
Introduction
properties. Herein we describe the synthesis and characterization of novel copper nanoparticles with phosphinothiolatekP,kS ligands. Copper nanoparticles with 2-(diphenylphosphino)benzenethiol were prepared following the same procedure as that used for phosphine-capped gold nanoparticles.[3b, 10]
In recent decades, special attention has been paid to Au and Ag nanoclusters because of their chemical and physical characteristics, which are different from those of the molecular compounds.[1] The change of the properties has been correlated with the size of the clusters. Core size can be controlled by surface bound ligands, such as thiolates[2] and phosphines, which each lead to specific properties.[3] Phosphine-ligated Au and Ag nanoclusters are less stable than the thiolate ones and in addition they need anions to compensate the charge of the surface atoms. Studies on copper nanoclusters are still in their infancy. In the last two decades, Fenske et al. have reported several chalcogenide copper nanoclusters.[4] Metal hydrides were extensively pursued as they are key intermediates in catalysis[5] and promising materials for hydrogen storage.[6] Copper hydrides are well-known catalysts for the hydrogenation of a variety of unsaturated C C and C O bonds.[7] Since Wrtz prepared the first copper(I) hydride in 1844,[8] several copper hydrides have been synthesized (mono-, di-, tri-, hexa-, or octanuclear) and stabilized by phosphines, pyridines, N-heterocyclic carbenes or thiolates.[7c, 9] In 2013, Liu et al. published the first Cu20 nanocluster ligated by dichalcogen donor ligands.[9a] Our strategy to obtain Cu nanoclusters involves the use of bidentate (S,P) ligands that contain thiolate as a stabilizer and a phosphine that can be used for tuning the electronic
Results and Discussion Synthesis and characterization of the precursor complex The reaction of CuI with one equivalent of [SH(C6H4)PPh2] (LH) in CH2Cl2 solution led to the formation of a new complex as a white crystalline solid in good yield (Scheme 1). The solid
Figure 1. Molecular structure of 1: Displacement ellipsoids depicted at 50 % probability level. Minor disordered components and most H atoms omitted for clarity. Selected bond lengths () and angles (8): Cu1 P1 2.2411(12), Cu1 I1 2.5483(8), Cu1 S1 2.649(8); P1-Cu1-I1 128.06(4), P1-Cu1-S1 77.20(18), I1-Cu1-S1 113.4(3), Cu1-I1-Cu1’ 61.33(2).
[a] Dr. M. A. Huertos, Dr. I. Cano, Dr. N. A. G. Bandeira, Dr. J. Benet-Buchholz, Prof. Dr. C. Bo, Prof. Dr. P. W. N. M. van Leeuwen Institute of Chemical Research of Catalonia (ICIQ) Avinguda Paı¨sos Catalans 16, 43007 Tarragona (Spain) E-mail: [email protected]
state structure, as determined by single-crystal X-ray diffraction (Figure 1), identifies this new product as [{CuI(LH)}2] (1). This complex is characterized by a {Cu2(m-I)2} rhombus as a structural unit with a bidentate LH ligand coordinated to each copper ion. The {Cu2(m-I)2} unit is planar with a crystallographic inversion center. The Cu ion adopts a pseudotetrahedral coordina-
[b] Prof. Dr. C. Bo Departament de Qumica Fsica i Inorganica, Universitat Rovira i Virgili Carrer de Marcel·l Domingo s/n, 43007 Tarragona (Spain) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404763. Chem. Eur. J. 2014, 20, 1 – 8
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Full Paper ing 17 % of copper. EDX also displays 6 % of iodine (approximate formula Cu7L5I1). We explain the ligand content calculated (Cu/L = 1.2:1) by the formation of small clusters [CunLn] with n < 10 not visible by TEM. The UV/Vis absorption band (300 nm; see Figure S.3 in the Supporting Information) indicates the formation of small copper nanoparticles (CuNPs).[12] In addition, the absence of a plasmon resonance band in the region around 560 nm indicates that there is not a detectable number of Cu nanoparticles with a size larger than 4.5 nm. The transmission IR spectrum of the CuNPs shows the absence of the S–H stretching band, in contrast to the observed band at 2550 cm 1 for the free ligand and complex 1 (Figure 3). These experiments are in agreement with coordination of the ligand as a thiolate on the copper nanoparticles.
Scheme 1. Synthesis of 1.
tion geometry, comprising two halide ions and one P and one S atom, both of the same LH ligand. The small Cu-I-Cu angle (61.33(2)8) is probably due to the large ionic radius of iodide.[11] The 1H NMR spectrum in solution for compound 1 shows a relative integral 2H broad signal at d = 4.23 ppm, corresponding to two S–H units of the thiolphosphine ligand.
Synthesis and characterization of phosphinothiolatestabilized Cu nanoparticles In a typical nanoparticle synthesis, NaBH4 (15 equiv) suspended in EtOH was added to a solution of 1 in CH2Cl2 previously cooled at 0 8C (Scheme 2). A color change, from colorless to
Scheme 2. Synthesis of CuNPs. Figure 3. Overlay of the ATR FT-IR spectra of ligand, complex 1, and CuNPs.
bright yellow, was immediately observed and the solution was stirred overnight (16 h) at room temperature. The resultant product was purified (see Experimental Section) and a bright yellow powder was obtained in 63 % yield. The product shows a broad size distribution by TEM (Figure 2) but most of the particles visible by TEM exhibit a size around 1.2–1.6 (0.87) nm. Elemental analysis (EA) and EDX are in good agreement, show-
The 31P{1H} NMR spectrum in solution of CuNPs shows a very-broad signal centered at 0.5 ppm. This signal shifts 9.5 ppm from precursor 1 (d = 9.0 ppm; see Experimental Section) and shows that also in CuNPs the phosphine is coordinated to Cu. The oxidation state of the CuNPs was then investigated by XPS. Figure 4 shows the binding energies of 2p3/2 and 2p1/2 electrons of copper. The XPS spectrum of 1 (Figure 4, black) with copper atoms in oxidation state + 1, fits perfectly with the binding energies of the electrons of copper in CuNPs (Figure 4, grey). It is known that the 2p3/2 binding energy of
Figure 2. Representative TEM image of CuNPs and size distribution (inset) determined from TEM images by counting > 300 non-touching particles obtained from images captured from distinct quadrants of the grid.
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Figure 4. X-ray photoelectron spectra of the CuNPs (gray) and 1 (black).
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Full Paper Cu0 is only 0.1 eV away from that of CuI species. Although one isolated species (see below) may contain only CuI, for the CuNPs mixture we cannot exclude the presence of Cu0. The 1H NMR spectrum in solution of CuNPs shows two very broad signals d 4.50 and 2.20 ppm that correspond to Cu hydrides. These data suggest the existence of two types of hydrides. The higher-field signal (2.20 ppm) is in good agreement with {Cu(m3-H)}.[13] Jin et al. reported different shifts for the bridging hydrides in Cu4H4X (X = B, C, N, O) depending on the electronegativity of X.[14] In this context, we propose that the lower-field signal (4.50 ppm) could belong to a {Cu(m2-H)}, species where the copper is ligated by a sulfur atom.
H2 evolution from CuNPs Three different experiments were performed with the aim to support the existence of Cu-hydride clusters. A solution of CuNPs in THF (4 mg, 2 mL) was irradiated with visible light using a Xenon Arc Lamp with a UV filter keeping a constant temperature of 25 8C. After 30 min, H2 evolution (27 mL of H2, Figure 5) was observed by gas chromatography (GC). A solution of CuNPs in THF (4 mg, 2 mL) was heated for 30 min at 70 8C and H2 was detected by GC. As expected, addition of acid also produced H2 (for more details, see the Supporting Information). Figure 6. a) Central core of 8 Cu atoms; b) skeleton of nanocluster 2 formed by 18 Cu and 10 S atoms; c) molecular structure of 2. Displacement ellipsoids depicted at 50 % probably level. Minor disordered components and all H atoms omitted for clarity.
central core, the distances between the 8 Cu atoms (2.5261(11)–2.9157(10) ) are similar to those in Cu clusters previously described.[9a] The remaining 10 Cu atoms are surrounding the internal Cu core always connected to S and P atoms (Figure 6 b). Only Cu3/Cu3’ atoms are connected directly to the inner copper core atoms (distance Cu3–Cu4: 2.6411(9) ). At each side of the structure (as drawn in Figure 6) there are 4 Cu atoms (Cu1, Cu6, Cu8 and Cu9, and symmetry equivalents), which are connected between them and to the central core by four Cu m3-S bonds. The skeleton of the cluster is formed by 18 Cu atoms and 10 S atoms (Figure 6 b, Cu m3-S distances: 2.2205(15)–2.4402(14) ). In the external part of the skeleton, the 10 outer copper atoms are capped by a phosphine (Figure 6 c). Additionally, the atoms Cu1, Cu8, and Cu9 are linked to an iodide anion. This iodide anion has an occupancy of 50 %, since it is disordered in the two symmetry-equivalent positions (Figure 6 c). Alternating its location with the iodide anion, we speculate that a hydride is present also having an occupancy of 50 % in each symmetryequivalent position. The hydrogen atoms (probably 7 hydrides, based on DFT calculations; see below) could not be localized from single-crystal X-ray diffraction experiments. Efforts to grow a larger crystal (1 mm3) suitable for neutron diffraction were unsuccessful.
Figure 5. Calibration plot for hydrogen measurement. White circles: Addition of known amounts of hydrogen into the cell. Black circles: Experimental value for the photolysis experiment. 27 mL of H2 (0.0001 mmol) are released for 4 mg of CuNCs (0.02 mmol Cu).
X-ray crystal structure Crystals of an air- and moisture-stable Cu–H nanocluster [Cu18H7L10I] (2),[15] which are a small portion of the mixture of nanoparticles, were grown from a solution of CuNPs in CH2Cl2 layered with hexane. The size of this species coincides with the number-wise most abundant size in Figure 2. Herein we report the unprecedented crystal structure of 2 (Figure 6 c). The compound crystallizes in the space group P1¯, the cluster having a crystallographic Ci symmetry. The framework of 18 copper atoms can be described as a central core of 8 Cu atoms (Figure 6 a, atoms Cu2, Cu4, Cu5, Cu7 and symmetry equivalents) formed by two butterfly structures (4 Cu atoms each). In this Chem. Eur. J. 2014, 20, 1 – 8
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atoms with distances between 1.658 and 1.840 . Two other hydrides (H220 and H223) are coordinated in Cu(m2-H) fashion. Cu m2-H distances are in the range 1.587–1.696 . Several examples for this type of coordination were reported previously.[9a–f] The other 4 hydrides are placed on 4 faces of the tetrahedrons of the 8 Cu atoms of the central core. The coordination of the hydrogen atom on a tetrahedral copper face has been reported for [Cu20H11(S2P(OiPr)2)9].[9a] X-ray and DFT structures are in good agreement, as shown by the distances summarized in Table 1. An alternative structure built by replacing the iodine atom with a hydride provided a more symmetrical structure, but did not affect significantly the nature of the Cu atoms core (Figure 7).
Density functional theory (DFT)-based full geometry optimizations were carried out for 2 in order to locate the hypothesized 7 hydrogen atoms (see the Supporting Information). It is worth mentioning that without any additional hydrogen atom, the nanocluster’s electronic structure is not stable, as it displayed a null HOMO–LUMO gap. By analyzing the spatial shape of the lowest empty energy levels, seven hydrogen atoms and seven additional electrons were added to the system, and the geometry of a model cluster, that is, [Cu18H7L10I], wherein the ligand L = [SC6H4PPh2] was truncated to [SC6H4PMe2] for computational expediency, was subsequently optimized. The iodine atom in only one apical position and the hydride in the opposite apical site entail the loss of symmetry. Six hydrogen atoms were placed in the {Cu10} core and optimized in several different arrangements. Initially, the optimization was constrained to maintain the coordinates of the crystal structure optimizing only the six core hydrides plus the one in the apical position. In the subsequent step, the structures were fully relaxed. Two stationary points were located in the procedure, the least stable one being 98.7 kJ mol 1 higher in energy. In the least stable structure the {Cu10} core hydrides were each coordinated in tetrahaptic (h4-H) fashion within the Cu tetrahedra, in a structural pattern akin to Wrtz’s original CuH structure, in which each hydride ion is enclosed by a copper(I) tetrahedron and vice versa. The lowest energy structure exhibits four hydrides bridging the outward faces of each Cu tetrahedron in m3-H coordination (Figure 7 a) and on this we base our proposal for the hydride positions. Hydride H226 is depicted at the bottom of Figure 7 a and is coordinated as a bridge between 3 copper
Table 1. Selected bond lengths. Bond 2
Cu m -H Cu -m3-H Cu Cu (central core) Cu3 Cu4 Cu m3-S
X-Ray
DFT
— — 2.5261(11)–2.9157(10) 2.6411(9) 2.2205(15)–2.4402(14)
1.587–1.696 1.644–1.757 2.485–2.890 2.515–2.543 2.264–2.608
Figure 7 also contains a schematic MO diagram of the frontier orbitals of the minimum energy model (b) and 3D plots displaying the composition of the highest occupied (HOMO, c) and lowest unoccupied molecular orbital (LUMO, d). The HOMO ( 4.08 eV; Figure 7 c) of 2 is principally located in the copper atoms with 52 % atomic contribution from copper and 26 % from sulfur. The highest filled levels correspond to a corelike 3d10 region of the copper fragments whereas the CuI 4s and 4p orbitals only come up with any significant degree of prominence in the virtual MO space at 0.97 eV, higher in energy than the phenyl rings’ antibonding p* orbital set, which begins at the LUMO ( 1.99 eV). Natural population analysis (NPA)[16] of 2 clearly indicates that the oxidation state of all copper atoms is + 1, and that all seven added hydrogen atoms must be regarded as hydrides. Indeed, the atomic population of copper atoms falls into a 0.42–0.65 range, with nearly 10 e in the d orbitals (average 9.87) plus roughly 0.5 e in the s orbital. For the seven additional hydrogen atoms, the NPA atomic charge average is 0.5.
Conclusion In summary, we have prepared and characterized new copper hydride nanoclusters ligated by a thiolate-phosphine chelate ligand. This unsymmetric bidentate ligand gives CuNPs with the characteristic stability supplied by the thiolate and a tunable phosphine, which might lead to catalytic activity. The CuNPs release hydrogen gas under heating and upon irradiation with visible light. The latter is key to the synthesis of new materials for hydrogen storage, although in this material the hydrogen content by weight is low. We were able to isolate one of the clusters with the stoichiometry [Cu18H7L10I]. The unprecedented geometry of this Cu18 structure has been
Figure 7. a) Optimized molecular structure of 2; b) schematic molecular orbital diagram diagram; c) HOMO; d) LUMO.
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Full Paper characterized by X-ray diffraction and the hydride positions were calculated by DFT methods.
X-ray crystal structures determination Crystals of 1 and 2 were obtained by slow diffusion of a 10/30 dichloromethane/hexane solution at room temperature. The measured crystals were prepared under inert conditions immersed in perfluoropolyether as protecting oil for manipulation.
Experimental Section
Data collection: Crystal structures determinations for 1 and 2 were carried out using an Apex DUO Kappa 4-axis goniometer equipped with an APPEX 2 4 K CCD area detector, a Microfocus Source E025 IuS using MoKa radiation, Quazar MX multilayer Optics as monochromator and an Oxford Cryosystems low temperature device Cryostream 700 plus (T = 173 8C). Full-sphere data collection was used with w and f scans. Programs used: Data collection APEX2,[18] data reduction Bruker Saint[19] and absorption correction SADABS.[20]
General All oxygen and moisture sensitive operations were carried out under an argon atmosphere using standard vacuum-line and Schlenk techniques. Solvents were purchased from Sigma–Aldrich as HPLC grade and dried by means of an MBraun MB SPS800 purification system. 2-(Diphenylphosphino)benzenethiol (LH) was prepared according to literature procedure reported by Block et al.[17] Chemical shifts of 1H, 13C, and 31P NMR are reported in ppm, with the solvent used as internal standard. Signals are quoted as s (singlet), d (doublet), dd (double doublet), m (multiplet), b (broad). Elemental analyses were performed by Kolbe (Mlheim, Germany).
Structure solution and refinement: Crystal structures were solved and refined by Dr. Jordi Benet-Buchholz. Crystal structures solution were achieved using direct methods as implemented in SHELXTL[21] and visualized using the program XP. Missing atoms were subsequently located from difference Fourier synthesis and added to the atom list. Least-squares refinement on F2 using all measured intensities was carried out using the program SHELXTL. All non-hydrogen atoms were refined including anisotropic displacement parameters.
Gas chromatography (GC) Gas chromatography measurements were done using an Agilent Technologies 7890B GC System with thermal conductivity detector and capillary molecular sieves column.
Comments on the structures: The asymmetric unit of the crystal structure of 1 contains half a molecule of the organometallic complex and half a molecule of cyclohexane. The complex molecule shows Ci symmetry. The aromatic ring with the sulfur atom is disordered in two positions through the center of inversion with a ratio of 55:45. The cyclohexane ring is disordered around a twofold rotation axis (ratio 50:50).
Transmission electron microscopy (TEM) TEM analyses were performed on a Zeiss 10 CA electron microscope at 100 kV with a resolution of 3 . Samples were prepared by drop casting (from various THF solutions) onto a holey Formvar/carbon-coated copper grid.
The metal cluster 2 crystallizes having Ci symmetry in the triclinic space group P1¯. The asymmetric unit contains half a molecule of the metal cluster, two molecules of dichloromethane and half a molecule of n-hexane. The metal cluster contains an iodine atom which is disordered in two positions related by the center of inversion (one iodine atom for each complete metal cluster, ratio 50:50). One of the dichloromethane molecules is also disordered in two positions (ratio 50:50). Expected hydrides could not be localized from the single-crystal X-ray structure determination and are missing in the empirical formula. Probably the disordered iodine anion is alternating its position with a hydride.
X-ray photoelectron spectroscopy (XPS) These experiments were carried out on our behalf by Dr. L. Calvo Barrio at the Centres Cientfics i Tecnolgics, Universitat de Barcelona, using a PHI 5500 Multitechnique System (from Physical Electronics) with a monochromatic X-ray source (aluminum K-alfa line of 1486.6 eV energy and 350 W), placed perpendicular to the analyzer axis and calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 0.8 eV. The analyzed area was a circle of 0.8 mm diameter, and the selected resolution for the spectra was 187.5 eV of Pass Energy and 0.8 eV per step for the general spectra and 23.5 eV of pass energy and 0.1 eV per step for the spectra of the different elements in the depth profile spectra. A low-energy electron gun (less than 10 eV) was used to discharge the surface when necessary. All measurements were made in an ultra-high vacuum (UHV) chamber with pressure between 5 10 9 and 2 10 8 torr.
Computational details The Amsterdam Density Functional (ADF) program package[22] version 2013.01 was used throughout to optimize the geometries of the species mentioned in the paper. The Becke[23] and Perdew[24] gradient-corrected exchange and correlation functionals (BP86) respectively were used in the calculations, wherein the local part was set up from the Slater exchange and the Vosko, Wilk, and Nusair correlation functionals.[25] The ZORA[26] scalar relativistic Hamiltonian was employed with a triple zeta Slater-type orbital[27] (STO) augmented with one polarization function (TZP) for copper and iodine, and double zeta STO type functions augmented with one additional polarization function on the remaining elements. A frozen core approximation was used for all the elements ([Ar] for Cu, [Pd] for iodine, [He] for C and [Ne] for phosphorus and sulfur) except hydrogen. The geometry optimizations were performed via the default numerical integration Scheme of Becke[28] without symmetry constraints and with default integration accuracy. A natural population analysis[29] was performed on the lowest energy structure using an all electron basis set.
Fourier transform infrared spectroscopy (FT-IR) FT-IR measurements were carried out on a Bruker Optics FT-IR Alpha spectrometer equipped with a DTGS detector, KBr beamsplitter at 4 cm 1 resolution in the range 4000–400 cm 1.
Energy dispersive X-ray spectroscopy (EDX) EDX analyses were performed on a FEI Quanta 600 ESEM equipped with an Oxford Instruments EDS Detector (20 kV accelerating voltage, 10 mm working distance) employing cobalt as internal standard. Samples were prepared by drop casting (from various THF solutions) onto an aluminum holder. Chem. Eur. J. 2014, 20, 1 – 8
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[{2-(Diphenylphosphino)benzenethiol)CuI}2] (1): A solution of 2-(diphenylphosphino)benzenethiol (294 mg, 1.0 mmol) and CuI (190 mg, 1.0 mmol) in CH2Cl2 (20 mL) was stirred for 16 h at room temperature. The solvent was removed by evaporation under high vacuum to a volume of 5 mL. Next, hexane (5 mL) was added and the resulting solution was stored at 30 8C, precipitating a white solid that was isolated by decantation of the solvent and dried under high vacuum., affording the prodduct as a white crystalline solid (397 mg, 0.41 mmol, 82 %). Slow diffusion of hexane into a concentrated solution of 1 in CH2Cl2 at 30 8C afforded crystals, one of which was employed for an X-ray structure determination. 1 H NMR (500 MHz, CD2Cl2, ): d = 7.61–7.57 (m, 1 H, HAr), 7.66–7.10 (m, 12 H, HAr), 7.07 (t, J = 6.8 Hz, 1 H, HAr), 4.23 ppm (s, br, SH); 31 1 P{ H} NMR (202 MHz, CD2Cl2): d = 9.33 ppm (s, br). Copper nanoparticles: The appropriate copper complex 1 (0.04 mmol) was dissolved in degassed CH2Cl2 (30 mL) and cooled to 0 8C in an ice bath. A solution of NaBH4 (15 mg, 0.4 mmol, 10 equiv) in ethanol (3 mL) was prepared immediately prior to use and then added in one portion via syringe to the rapidly stirred CH2Cl2 solution resulting in the immediate formation of a yellow suspension, which was allowed to warm to room temperature slowly overnight. The solvents were removed under high vacuum and the dry residue was extracted with toluene (30 mL). The yellow solution was concentrated under reduced pressure and the residue was extracted with CH2Cl2 (30 mL). The solvent was removed by evaporation to a volume of approximately 5 mL and a bright yellow solid was precipitated with hexane, which was washed with ethanol (2 20 mL, 0 8C) and diethyl ether (2 20 mL, 0 8C), affording the product as a yellow powder (24 mg, 63 % yield, based on Cu). 1H NMR (500 MHz, C6D6, 128 scans, 25 8C): d = 8.15– 5.8 ppm (br, HAr); 31P{1H} NMR (202 MHz, C6D6, 3000 scans, 25 8C): d = 3.1 ppm (very br); elemental analysis (%) found: C 52.76, H 4.47, Cu 16.41, P 6.83, S 6.67; EDX (%): C 61.14, Cu 17.95, P 7.74, S 7.85, I 5.31.
Acknowledgements This work was supported by an ERC Advanced Grant (NANOSONWINGS 2009–246763) and Spanish Ministerio de Economia y Competitividad (MINECO, grant CTQ2011–29054-C02–02). The authors acknowledge the ICIQ support units and Dr. Rita Marimon and Dr. Mariana Stefanova from Universitat Rovira i Virgili for TEM and EDX measurements. Keywords: copper · density functional calculations · hydrides · nanoclusters · P,S ligands [1] a) X. Yuan, Z. Luo, Y. Yu, Q. Yao, J. Xie, Chem. Asian J. 2013, 8, 858 – 871; b) C. M. Cobley, J. Chen, E. C. Cho, L. V. Wang, Y. Xia, Chem. Soc. Rev. 2011, 40, 44 – 56; c) M.-C. Daniel, D. Astruc, Chem. Rev. 2003, 103, 293 – 346. [2] a) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. 1994, 801 – 802; b) G. Yang, Z. Zhang, S. Zhang, L. Yu, P. Zhang, Y. Hou, Surf. Interface Anal. 2013, 45, 1695 – 1701; c) X. Jia, X. Yang, J. Li, D. Li, E. Wang, Chem. Commun. 2014, 50, 237 – 239; d) T.-Y. Dong, H.-H. Wu, M.-C. Lin, Langmuir 2006, 22, 6754 – 6756; e) W. Wei, Y. Lu, W. Chen, S. Chen, J. Am. Chem. Soc. 2011, 133, 2060 – 2063; f) T. P. Ang, T. S. A. Wee, W. S. Chin, J. Phys. Chem. B 2004, 108, 11001 – 11010; g) A. Desireddy, B. E. Conn, J. Guo, B. Yoon, R. N. Barnett, B. M. Monahan, K. Kirschbaum, W. P. Griffith, R. L. Whetten, U. Landman, T. P. Bigioni, Nature 2013, 501, 399 – 402.
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[23] A. D. Becke, Phys. Rev. A 1988, 38, 3098 – 3100. [24] a) J. P. Perdew, Phys. Rev. B 1986, 33, 8822 – 8824; b) J. P. Perdew, Phys. Rev. B 1986, 34, 7406. [25] a) S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980, 58, 1200 – 1211; b) J. C. Slater, Phys. Rev. 1951, 81, 385 – 390. [26] a) E. van Lenthe, E. J. Baerends, J. G. Snijders, J. Chem. Phys. 1993, 99, 4597 – 4610; b) E. van Lenthe, A. E. Ehlers, E. J. Baerends, J. Chem. Phys. 1999, 110, 8943 – 8953. [27] E. Van Lenthe, E. J. Baerends, J. Comput. Chem. 2003, 24, 1142 – 1156. [28] A. D. Becke, J. Chem. Phys. 1988, 88, 2547 – 2553. [29] E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M. Morales, F. Weinhold, NBO 6.0 ed., University of Wisconsin, Madison, US, 2013.
Received: August 7, 2014 Published online on && &&, 0000
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FULL PAPER & Copper Nanoclusters
Post scriptum: Copper hydride nanoclusters have been prepared by reduction of a copper(I) phosphinothiol complex; they release H2 by visible light irradiation. A Cu18 cluster was isolated by crystallization, showing an unprecedented geometry that was evaluated by X-ray diffraction.
M. A. Huertos, I. Cano, N. A. G. Bandeira, J. Benet-Buchholz, C. Bo, P. W. N. M. van Leeuwen* && – && Phosphinothiolates as Ligands for Polyhydrido Copper Nanoclusters
The peculiar structure… …of the nanocluster, reported by P. W. N. M. van Leeuwen et al. in their Full Paper on page && ff., consists of eighteen copper atoms and ten phosphinothiolate anions with a core of eight copper atoms, four additional atoms sticking out via bridging ligands, and on top and below the central core three atoms also glued to the cluster via bridging ligand interactions. The structure of the cluster is unusually “unorganized” compared to most other clusters, although it does have an inversion center.
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