Clusters: a Joint Theoretical and Experimental Work

with the program PLATON.39 The crystallographic data for compound 1a are summarized in. Table 1. Optical characterization .... Figure 1. Investigated ...
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A: Spectroscopy, Photochemistry, and Excited States 4

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Structural and Spectroscopic Investigations of Two [CuX]- (X= Cl, Br) Clusters: A Joint Theoretical and Experimental Work -

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Camille Latouche, Romain Gautier, Romain Génois, and Florian Massuyeau J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02663 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Structural

and

Spectroscopic

Investigations of Two [Cu4X6]2- (X= Cl-, Br-) Clusters:

a

Joint

Theoretical

and

Experimental Work Camille Latouche,* Romain Gautier,* Romain Génois, Florian Massuyeau Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 03, France

Abstract Herein we report a joint experimental and theoretical investigation on two tetranuclear Cu(I) clusters stabilized by halide ligands. These clusters are of high interest due to their spectroscopic and optical properties, more precisely both clusters exhibit thermochromism. The compounds synthesized by hydrothermal method have been characterized by singlecrystal X-ray diffraction, UV-Visible spectroscopy and quantum calculations. Modelled structures have been investigated by means of DFT and TD-DFT methods. Anharmonic computations have been performed to better achieve the vibrational investigation. Computations of the triplet excited states permit to get more insights into the structure and electronic structure of the excited states responsible for the luminescence properties. Calculations are in agreement with the observed phosphorescence wavelengths.

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Introduction Nowadays materials exhibiting photoluminescence properties as a response to stimuli such as temperature, electricity or grinding have demonstrated their promises for several types of applications as sensors, memories or display devices.1–11 As a matter of fact, the past years have been effective to target new compounds devoted for these applications and chemists have provided much effort to synthetize and design new functional materials. On these grounds, computationally and experimentally oriented chemists jointed their forces to obtain new compounds and especially new clusters exhibiting strong luminescence properties, based on Cu+/Ag+ cluster types (formally d10).12–25 Among all these clusters, it should be mentioned that some of them are able to trap anion species (ion, molecule) allowing to shape new architectures together with promising optical properties.26–34 Analyzing these assemblies becomes mandatory and it allows a rationalization of the phenomena occurring when spectroscopic properties (optical, vibrational) are observed. Recently, some of us have reported the synthesis, the characterization and a DFT rationalization of the observed properties of the [Cu4Br6]2- cluster.35 In the latter investigation, two phosphorescence wavelengths were observed and the computations retrieved with an acceptable accuracy both of them allowing a good description of the triplet excited states. In this paper, we present a combined theoretical and experimental investigation on two copper halide clusters, i. e. [Cu4Cl6]2- (1) in [C6H16N2]3[Cu4Cl6][Cu2Cl6] (1a), H2O and [Cu4Br6]2- (2) in [C6H16N2]3[Cu4Br6][Cu2Br6] (2a). For compound 1, the electronic structure is described, a band assignment of the observed absorption band using TD-DFT method is proposed, IR and RAMAN spectra are simulated using anharmonic computations and the optical properties issued from the triplet excited states are rationalized. As stated before, the optical properties of 2 have already been reported, therefore we mainly focus our attention on the vibrational properties for this compound. Finally, the possibility to entrap an hydride into 1 has been investigated in order to evaluate the capability of the cluster to be a potential candidate for hydrogen storage. All the results from DFT computations are compared with the available experimental ones.

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Experimental Section Synthesis The compound [C6H16N2]3[Cu4Cl6][Cu2Cl6], H2O 1a was synthesized by hydrothermal method from a mixture of 7.87 mmol of Cu metal, 4.43 mmol of N,N'-dimethylpiperazine in 3ml of HCl 37%. The mixture was heated at 180°C during 24h and slowly cooled down to room temperature at the rate of 10°C/h using a 23mL Teflon-lined stainless steel autoclave. Single crystals suitable for single crystal X-ray diffraction were recovered by filtration. Compound 2a was synthesized as previously described in the literature.35 Structure determination The structure determination of compound 1a was carried out from Single-crystal X-ray diffraction with a Bruker-Nonius Kappa CCD diffractometer (monochromated Mo Kα radiation). Absorption corrections were carried out using SADABS.36 The crystal structure was determined by direct methods and was completed by Fourier difference syntheses with SIR200437. SHELXL-2013 was used to refine the crystal structures and anisotropic displacement parameters were considered.38Additional symmetry elements were checked with the program PLATON.39 The crystallographic data for compound 1a are summarized in Table 1. Optical characterization Diffuse reflectance spectra were collected from 250 nm to 2500 nm using a Varian Cary 5G spectrophotometer with a 60 mm integrating sphere. Absorbance spectra were calculated from reflectance measurements using the Kubelka Munk function (a/S = (1-R)2/2R where a is the absorption coefficient, S the scattering coefficient and R the reflectance). The photoluminescence spectra at low and room temperature were collected using a Spex Fluorolog-3 spectrofluorometer from Instruments Jobin Yvon. The excitation source is a 450W Xe lamp.

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Table 1. Crystallographic data for compound 1a. [C6H16N2]3[Cu4Cl6][Cu2Cl6], H2O Space group P212121 a/Å 13.3441(13) b/Å 14.6101(9) c/Å 20.1415(19) α/° 90 β/° 90 γ/° 90 Radiation Mo Kα 2Θ range for data collection/° 12.84 to 51.58 Reflections collected 31711 Data/restraints/parameters 7409/0/387 Goodness-of-fit on F2 1.043 R1 = 0.0569 Final R indexes [I>=2σ (I)] wR2 = 0.1213 Largest diff. peak/hole / e Å-3 1.11/-0.87

Computational details Ground state (GS) and triplet excited state (ES) computations have been performed on the models using the PBE040,41 functional together with the Def2TZVPD42 basis set. The Gaussian package has been used for all the calculations.43 All computations have been performed in vacuum and the geometries have been checked to be at the minimum of the potential energy surface by diagonalizing their Hessian for both ground and excited states. Simulated spectra have been modeled using the VMS package.44 Optical properties were investigated using TD-DFT. Calculations of IR and RAMAN have been performed with the GVPT245–50 model as implemented in the used version of Gaussian. Structures and orbital plots have been drawn with the GaussView software.51

Results and Discussion Structure Description The new compound 1a is isostructural to the previously reported d10 copper chloride compound 2a.35 The crystal structure of 1a is described in the noncentrosymmetric spacegroup P212121. The compound is built from two clusters ([Cu2Cl6]4- and [Cu4Cl6]2- units). The Cu-Cl bond lengths are in the range 2.223(3) Å < d < 2.691(3) Å. In [Cu2Cl6]4- units, Cu-

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Cu distances are 2.6724(17) Å and 2.6431(16) Å < d < 3.066(2) Å for [Cu4Cl6]2- units. The cations, namely 1,4-dimethylpiperazine-1,4-diium, together with water molecules are located between the different anionic species. Ground State Investigation As the geometrical parameters of 2 have already been reported and discussed in Ref.35, this section is mainly focused on the crystal and electronic structures of 1. The data from the simulated optimized geometries are compared to the X-ray diffraction ones. Similarly to cluster 2, the optimized simulated structure of the cluster with X= Cl is in reasonable agreement with respect to the experimental one. In this geometry, the four metal atoms are forming a tetrahedron in which the six edges are capped by a µ2-halide ligand. Consequently, each metal atom is linked to three halides in a trigonal-planar MX3 configuration (Figure 1). This statement seems of high interest since such fragments characterized up to now generally have a fourth ligand attach to each Cu, sometimes with an enchlatrated oxo ion.52–56

2 1 (Td) (Td) Figure 1. Investigated compounds: left corresponds to 1(Cl) and right to 2(Br). When the Td symmetry is enforced, the computed Cu-Cu and Cu-X distances match nicely the averaged observed ones (Cu-Cu (Å): 2.717 [2.843], 2.749 [2.869] and Cu-X(Å): 2.329 [2.300], 2.459 [2.410] Å for 1 and 2, respectively) (Table 2). According to the X-ray structures of each compound, the tetranuclear copper species is connected to a binuclear copper one through a long Cu-X contact. For compound 1, the latter distance is ca 2.70 Å, which is by far larger than the other Cu-Cl distances (ca 2.30) and is also widely larger than the sum of covalent and atomic radii of Cu and Cl.57 This distance is also higher than the sum of the ionic radii of Cl and Cu in tetravalent coordination mode (2.40 Å).58

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Furthermore, one should mention that according to our level of computations, it was impossible to get the optimized structures of the bi- and hexa-nuclear species. On these grounds, the electronic structures of 1 and 2 have been investigated and the case of 1 is herein discussed. The frontier orbitals are depicted in the qualitative Kohn-Sham molecular diagram in Figure 2.

7 6

t2

e a1

t2 a1

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4.67 eV

0 -1

t2 e t1

t1 t2

1

Figure 2. Kohn-Sham orbital diagram and plots of the HOMO, LUMO and LUMO+1 orbitals of 1. First, as one can see, the LUMO and LUMO+1 orbitals are strongly localized on the metallic cluster cage whereas the HOMO is mixed between Cl and Cu atoms. Second, it turned out that metallic contribution for the triply degenerated HOMO (t2) is mainly constituted on the “d” orbitals whereas the LUMO and LUMO +1, both in a1 symmetry, have a strong “s” and “p” character. Third, the quite large HOMO-LUMO gap (4.67 eV) indicates that 1 is thermodynamically stable. For compound 2, the HOMO-LUMO gap slightly decreases to 4.51 eV. According to the Kohn-Sham orbital diagram, the LUMO+1 and the LUMO are quite out from the unoccupied block. Furthermore, their symmetries (a1) make them highly interesting for a potential anion trap inside the metallic cluster cage, such as H-. This point is discussed later.

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Table 2. Relevant computed and averaged experimental (between square brackets) parametric data of clusters 1 and 2. Cu-Cu (Å) Cu-X (Å)

1

2.717 [2.843]

2.329 [2.300]

2

2.749 [2.869]

2.459 [2.410]

Anharmonic vibrational IR and RAMAN spectroscopies The experimental vibrational spectra are given in SI. However, it should be mentioned that in the crystal there are water, counter-ion and the binuclear cluster entities. All these entities together with a Z= 4 factor for this crystal makes very complicated the identification of the origin of the experimental signal. At this stage, the following results should be taken as fully predictive. To be the most accurate in the vibrational characterization of compounds 1 and 2, IR (Figure 3a) and RAMAN (Figure 3b) spectra have been simulated using the anharmonic approximation keeping the same functional and basis set (Figure 3). Furthermore, the used methodology is the same as reported by some of us in previous reports where the agreements between the available experimental data and the computations were good.59,60 1

1

1

1

Normalized Intensity

2

Normalized Intensity

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0.5

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0.5

0 50

100

150

200 250 Wavenumbers (cm-1)

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350

0

50

100

150 200 250 Wavenumbers (cm-1)

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a) b) Figure 3. Simulated Anharmonic IR (left, a)) and RAMAN (right, b)) spectra of 1 and 2.

It should be pointed out that the anharmonic corrections to the fundamental bands are weak in this region (< 5 cm-1), but the use of such a high level of accuracy permits to take into account overtones and combination bands in the simulations. First, as one can see, the general signature of the simulated IR spectra is similar for 1 and 2. It is composed of an intense and large peak around 240 and 180 cm-1 for 1 and 2. This intense band is computed at 183 cm-1 and 235 cm-1 for 1 and 2, respectively, and can mainly be assigned as a double

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stretching along an axis, together with a slight breathing of the metallic tetrahedron. Lower in wavenumbers, three hills are present for both species where the first is always the most intense one. For 1, another signature is present in the vicinity of the large peak. This signature is computed at 231 cm-1 with a rather small intensity and it corresponds to the opposite movement of the previous one. The latter signature is not present for 2. The simulated peaks of 1 falling ca 205, 172 and 77 cm-1 are now discussed. The 77 cm-1 and 172 cm-1 hills correspond to the bending (scissoring) of the Cl atoms. The weak intensity of the hill falling around 205 cm-1 is fully explained by the fact that this hill is constituted by many combination bands, involving mods computed at 158 and 47 cm-1. Both vibrations correspond to a global breathing, stretching and bending of atoms in the molecule. All these vibrations are depicted in Figure 4. Other peaks with moderate intensities do not correspond to any fundamental band. According to our simulations, they are predicted to be a set of overtones and combination bands. In addition to the IR spectra, the simulated RAMAN ones of 1 and 2 are also rather similar. There is a double signature around 75 cm-1 (62 and 77 cm-1, respectively) for 1, whereas for 2 it is around 50 cm-1 (47 and 54 cm-1). For compound 1, the computed vibration at 62 cm-1 corresponds to a whole breathing of the metallic tetrahedron and the 77 cm-1 is the same one already discussed for IR spectra. The vibration motions are identic for 2. A vibration for compound 2 shows up at 114 cm-1 with a moderate intensity and is tentatively assigned as Cu-Br-Cu rocking centered on the bridging halogen. The very intense peak computed at 155 cm-1 (2), corresponds to the Cu-Br in phase stretching. As for the IR spectrum, the RAMAN spectrum of compound 1 exhibits a weak peak at 172 cm-1.

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235 cm-1

231 cm-1

77 cm-1

47 and 165 cm-1 (combination bands)

Figure 4. Relevant vibration motions from anharmonic computation performed on 1. Two RAMAN peaks centered at 188 and 183 cm-1 with a very moderate intensity appear for 2. Around 205 cm-1, the small peak of 1 is assigned to a set of overtones and combination bands. The final signature of 1 is constituted by an intense peak ca 231 cm-1 and a shoulder around 238 cm-1. The first peak is mainly driven by a single vibration and corresponds to

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the in phase Cu-Cl stretching, whereas the second peak is more likely constituted by several combination bands and overtones. Optical properties The discussion is now focused on the optical properties. In 2017 some of us reported the optical properties of 2, including a band assignment of the observed absorption bands.35 In the present manuscript the discussion is emphasized on 1. Experimentally, the absorption maxima are observed at 307 and 334.5 nm. The absorption band related to these transitions is depicted in Figure 5. The later results are consistent with the ones reported previously for 2.35 In our computations for 1, it turned out that the first excitation falls around 350 nm (t2, f= 0.0565) involving the triply degenerated HOMO to the LUMO and the LUMO +1. As for the cluster 2, two excitations show up around 300 nm with a smaller intensity (i) t2, f= 0.0044, λ= 303 nm; ii) t2, f= 0.0060, λ= 290 nm). These computed results are in good agreement with the experimental ones, giving us confidence to get into the electronic structure of the triplet excited states. However, it should be pointed out that recorded band above 700 nm is not present in our computations. This behavior was already noticed in the previous communication on cluster 2 and did not interfere with the investigation of the excited state.

Figure 5. Absorption spectrum for compound 1a.

On these grounds, the luminescence properties have been investigated and computations of the triplet excited states have been performed in order to elucidate the states responsible

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for the phosphorescence signature. The observed emission signals are about 520 and 700 nm (Figure 6). In our simulations, two minima (check by frequency calculations) were found with 521 and 930 nm (Table 3) simulated electronic wavelengths. The simulated shortest wavelength is in very good agreement whereas the highest one is only in reasonable agreement. Indeed, the energy deviation for this lowest energy emission band is around 0.40 eV which is larger the usual standard for transition metal complexes.61–63 However, when one deals clusters the deviation seems to be larger.64 As a matter of fact, the optimized cluster possesses the same geometry as the cluster 2.35 Finally, as demonstrated in Figure 6, the second transition is redshifted when the temperature decreases. As all the computations have been performed at 0K, the lower experimental temperature the better the agreement is with computations.

Figure 6. Solid-state photoluminescence (excitation vs. emission) for compound 1a in function of temperature.

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Table 3. Relevant experimental and electronic theoretical phosphorescence wavelengths. Experimental Theoretical λ1 λ2 λ1 λ2 520 nm ≈ 700 nm 519 nm ≈ 930 nm 1 (2.38) (1.77) (2.39) (1.33) 490 nm ≈ 680 nm 488 nm ≈ 850 nm 2 (2.53 eV) (1.83 eV) (2.54 eV) (1.46 eV) Prediction and stability of anion encapsulation Based on these statements, the capability to entrap negatively charged species inside the metallic cluster cage has been evaluated. In this study, the global negative charge of the isolated modelled cluster (-II) and the relative small host size prevent the ability of the whole cluster to entrap large anion. Therefore, solely the hypothetical encapsulation of Hhas been modelled. First, it should be pointed out that the energy minimum is not in Td symmetry (67i, t1) but in an approximated D2d, with a stabilization energy of 1.96 eV. In this configuration H- is at the center of the Cu4 tetrahedron and linked to each metal atom with a 1.723 Å bond length. This strong stabilization energy is mainly issued from the breaking of four Cu-Cl bonds. In this new architecture, only two Cl atoms are bonded to two Cu (µ2), the four other halogens become terminal atoms. As a result the Cu-Cu distances are not equivalent anymore. In comparison to 1, two are shortened (2.562 Å) and four are extended (2.932 Å)(Figure 7 and Table 4).

Figure 7. Optimized D2d structure of 1 with an encapsulated hydride. Table 4. Relevant computed parametric data of 1 with an encapsulated hydride. Cu-Cu (Å) Cu-Cl (Å) Cu-H (Å) 4*2.932 4* 2.424 (µ2) [Cu4(µ4-H)Cl6]34*1.723 2*2.562 4* 2.244 (µ1) Interestingly, a related hydride containing cluster has been very recently reported by Teo and coworkers, namely [Cu4(µ4-H)(µ2-Cl)2(PPh2Py)4]+.65 Its Cu-Cu and (µ-H)-Cu bond-

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lengths are in good agreement with that of our predicted model. Furthermore, two (µ2-Cl)Cu are capping edges of the tetrahedron like the model.

Conclusion In this paper, a thorough quantum investigation has been performed on two tetranuclear copper clusters. Results issuing from DFT have been compared to the available experimental ones. A band assignment of the observed optical transitions has been performed on cluster 1. Simulations of the IR and RAMAN spectra of 1 and 2 have been done employing the anharmonic level of accuracy. The simulated peaks with highest intensities have been assigned unambiguously and it has also been shown that many overtones and combination bands are present. The luminescence wavelengths of 1 have been computed and fit nicely the experimental ones. As for compound 2a,35 it has been shown that two different triplet excited states are responsible for the observed luminescence signatures. Finally, the encapsulation of H- into this tiny cluster cage has been evaluated. Even if, according to the calculations, it is possible to entrap such anion into the Cu4 cage it remains unlikely due to the cage modification. Despite its small size, H- forces the whole cluster to strongly distort and thus induces multiple bonds breaking.

Acknowledgement The work was supported by the National Agency for Research (ANR Young Researchers, ANR-16-CE08-0003-01, Combi-SSL project). We thank the “Centre de Calcul Intensif des Pays de la Loire” (CCIPL) for computational supplies. Dr. Samia Kahlal from Université de Rennes 1 for fruitful discussions is greatly acknowledged. CL thanks Prof. Bernard Humbert for fruitful discussions.

Supplementary Information Experimental IR and RAMAN spectra of 1a and 2a. X-ray data (cif) for compound 1a.

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