Strong Rigidochromism Induced by Jahn–Teller Disto

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Tetranuclear Au2Cu2 Clusters with Butterfly- and Planar-Shaped Metal Cores: Strong Rigidochromism Induced by Jahn−Teller Distortion in Two-Coordinated Gold(I) Centers Sara Nayeri,† Sirous Jamali,*,† Ali Jamjah,† and Hamidreza Samouei‡ †

Department of Chemistry, Sharif University of Technology, P.O. Box 11155-3516, Tehran, Iran Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States

‡

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S Supporting Information *

ABSTRACT: Two tetranuclear Au 2 Cu 2 cluster complexes [Au2Cu2(μ-(PPh2)2py)2(μ-OH)](PF6)3, 2, and [Au2Cu2Cl2(μ(PPh2)2 py)2](OTf)2, 4, have been prepared by the reactions of precursor complexes [Au2(μ-(PPh2)2py)2](OTf)2, 1, and [Cu2(μ(PPh2)2py)2(μ-SMe2)(OTf)2], 3, with [Cu(NCCH3)4]PF6 and AuCl(SMe2), respectively. The crystal structures of complexes 2 and 4 were determined by X-ray crystallography, indicating a butterfly-shaped Au2Cu2 metal core for 2 and a planar-shaped Au2Cu2 metal core for 4. In complex 2, the Cu atoms occupy the edge-sharing bond, while in complex 4, alternating Au and Cu atoms occupy the tetragon vertices. The optical properties of the complexes were investigated by experimental and computational methods. Although complex 2 displayed a luminescence vapochromic behavior in the presence several volatile organic compounds, complex 4 indicated only an distinguishable change in its emission color when it was exposed to vapor of hydrogen-bond donor solvents. The calculations showed that 2 undergoes an unsymmetrical distortion in its two-coordinated gold(I) centers upon excitation to the first triplet excited state. This distortion induces a large Stokes shift and a strong rigidochromism behavior that is unprecedented for two-coordinated gold(I) complexes.

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INTRODUCTION Chromism is an emerging topic in chemistry. By definition, chromism is a process in which an alteration in the color of compounds takes place upon a change in their electronic states. The change in the energy of the electronic states can be induced by various external stimuli such as the temperature, pH, solvent, rigidity of the medium, and mechanical force. Among various types of compounds undergoing chromism, stimulus responsive luminescent materials have attracted a great deal of attention from the point of view of fundamental understanding and designing new materials with improved properties. For example, Cu4I4py4 shows an intense emission band that originates from a cluster center excited state (CC*). The emission wavelength of this band is quite responsive to temperature and medium rigidity; the cluster displays a strong rigidochromism in frozen solutions. Both experimental and computational investigations suggest that the luminescence rigidochromism in copper(I) iodide clusters is due to their highly distorted excited states.1 Meanwhile, the rigidochromic effect in other types of transition metal complexes such as rhenium carbonyl complexes,2 cyclometalated iridium complexes,3 and platinum complexes4 that have been previously reported is attributed to the difference between the emission of the fully relaxed excited state in nonrigid medium and the emission of the excited state before solvent-induced relaxation © XXXX American Chemical Society

(which occurs in frozen solutions or at solid states). However, the range of the blue shift in such complexes is lower in comparison to those of copper(I) iodide clusters in which rigidochromism is induced by distorted excited states. It is well-known that in the absence of Au−Au interactions, twocoordinated gold(I) complexes (AuL2) are nonluminescent while three-coordinated gold(I) complexes (AuL3) are emissive5 and display a large Stokes shift and a rigidochromic effect that is induced by highly distorted excited states.6 Omary and co-workers showed that the large observed Stokes shift in the phosphorescence emission of AuL3 complexes of type [Au(PR3)3]+ can be attributed to the Jahn−Teller distortion from a trigonal geometry to a T-shaped structure.7 Laguna and co-workers prepared a dinuclear three-coordinated gold(I) complex, [Au2((Ph2Sb)2O)3](ClO4)2, that had a trigonal geometry around the Au(I) centers with an aurophilic interaction between them.8 While this three-coordinated complex displayed a large Stokes shift and a Jahn−Teller distortion occurred at its lowest triplet excited state, calculations showed that the distortion took place for only one of the gold(I) centers that involved an out-of-plane Received: May 14, 2019

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DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

HSQC NMR spectrum did not show any correlation for these C atoms, while the 1H−13C HMBC NMR spectrum of 1 showed correlations for this signal with meta and para protons of the pyridyl rings (Figure S5a,b). Two other pyridyl C atoms (139.2 and 130.3 ppm) and the C atoms of the phenyl rings (134.5, 133.2, 130.1, and 125.3 ppm) were assigned as shown in the Supporting Information. As shown in Figure 1, the

bending of the Au atom and a moderate decrease in the Au− Au distance. Luminescence vapochromism is an emission or a color switch in emission triggered by vapor stimulation, which is useful in the detection of volatile organic compounds (VOCs). Among coinage metal complexes, heteronuclear AuI−CuI clusters present themselves as an attractive class of phosphors that are known to display luminescence vapochromic behavior. For an earlier example, Catalano and co-workers have demonstrated that the displacement of acetonitrile by a methanol ligand in the trinuclear complex [Au(im(CH2py)2)2(Cu(NCMe)2)2](PF6)3 induces a large change in the Au−Cu bonding distance and shows a reversible luminescence vapochromic behavior through ligand exchange reactions.9 Also, recently a family of interesting AuI−CuI clusters containing hydroxyl alkynyl ligands have been prepared that are examples of luminescence polymorphism in which the emission color of the polymorphs depends on the metal−metal distances in the core of the clusters. Furthermore, it has been shown that metal−metal distances in these clusters and their subsequent luminescence behavior can be influenced by vapors of certain organic solvents via noncovalent interactions with the hydroxyaliphatic (R-OH) and hydroxyaromatic (Ar-OH) substituents of alkynyl ligands in the solid state.10,11 However, it seems that although the alkynyl functional group −CC− in these systems is coordinated to gold(I) and Cu(I) metal centers in a combination of σ and π bonding modes, they do not directly affect the vapoluminescence properties of the clusters. On the basis of these findings and aiming to investigate the optical behavior of Au2Cu2 clusters in absence of alkynyl ligands, herein we report the preparation of alkynyl-free new tetranuclear cluster complexes, [Au2Cu2(μ-(PPh2)2py)2(μ-OH)](PF6)3, 2, and [Au2Cu2Cl2(μ(PPh2)2py)2](OTf)2, 4, with butterfly- and planar-shaped Au2Cu2 metal cores, respectively. Cluster 2 has a bridging hydroxide ligand and displayed a luminescence vapochromic behavior upon exposure to the vapor of several hydrogen-bond donor and acceptor organic solvents, while cluster 4 indicated only a distinguishable vapochromic behavior in the presence of the hydrogen-bond donor organic solvents. Although cluster 2 comprised two-coordinated gold(I) centers with a linear P− Au−P geometry, both experimental and calculated data for this cluster demonstrated a large Stokes shift and strong rigidochromic behavior that was found to be due to the unsymmetrical Jahn−Teller distortion in two-coordinated gold(I) metal centers from linear geometry to bent structure.

Figure 1. Molecular crystal structure of 1. The anions (OTf), solvents of crystallization, and H atoms have been omitted for the sake of clarity.

crystal structure of 1 reveals a binuclear two-coordinated gold(I) complex with a linear geometry around the Au(I) centers that are separated by 5.229 Å and do not show any intra- or intermolecular Au−Au interactions. Complex 1 contains two uncoordinated pyridyl groups that are largely tilted relative to the plane defined by four phosphorus atoms by 56.3° and 58.1°, providing the possibility of coordination of the pyridyl nitrogen atoms to other metal centers above and below the plane of the phosphorus atoms. Recently, we have shown that the copper(I) complex [Cu(NCCH3)4]PF6 can activate a water molecule in a cooperative manner with the help of a bis-phosphine platinum(0) complex to generate Cu(I) hydroxide and platinum(II) hydride species.12 Thus, aiming to prepare a new AuICuI cluster containing a hydroxyl group, we examined the reaction of the dinuclear gold(I) complex 1 with [Cu(NCCH3)4]PF6 in wet dichloromethane in an inert atmosphere (Scheme S1). We have found that the reaction of 1 with an excess amount of [Cu(NCCH3)4]PF6 formed a separable mixture of the tetranuclear Au2Cu2 cluster complex [Au2Cu2(μ-(PPh2)2py)2(μ-OH)](PF6)3, 2, and an unknown product. Analysis of 2 via ESI-MS spectrometry in positive ion mode revealed an intense signal at m/z 644 due to binuclear gold species [Au2(μ-(PPh2)2py)2]2+, indicating that 2 is labile in the gas phase and undergoes dissociation via rupturing the Cu−N bonds. However, the experimental diffraction pattern of 2 shows the substantial changes in pattern compared to that observed for 1 and the simulated PXRD spectra of 1 and 2 are in good agreement with corresponding experimental diffraction patterns (Figure S6). Yellow crystals of 2 were grown by vapor diffusion of ether into a CH2Cl2 solution of 2 and were structurally characterized using X-ray crystallography. The complex crystallizes in the orthorhombic system, in space group Pnma. The crystal structure of 2 (Figure 2) indicates a square-pyramidal geometry, in which the basal plane crosses from four phosphorus atoms belonging to two (PPh2)2py ligands, while the axial vertex is occupied by the oxygen atom

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RESULTS AND DISCUSSION Synthesis and Characterization. The dinuclear gold(I) complex, [Au2(μ-(PPh2)2py)2](OTf)2, 1, was prepared by simple reaction of 2,6-bis(diphenylphosphino)pyridine, (PPh2)2py, with 1 equiv of AuCl(SMe2) and 1 equiv of Ag(OTf) in a dry dichloromethane solvent (Scheme S1). After workup, 1 was isolated in 71% yield as a white powder. The dicationic molecular peak (m/z 644) in electrospray ionization mass spectrometry (ESI-MS) (Figure S1) confirms the chemical formula of the cationic fragment [Au 2 (μ(PPh2)2py)2]2+. The 31P{1H} NMR spectrum of 1 in DMSO-d6 (Figure S2) showed a singlet signal at 43.5 ppm. In the 13C{1H} NMR spectrum of 1 (Figure S3), the C atoms of the pyridyl rings connected to phosphorus atoms appeared as a triplet of triplet signal at 156 ppm due to coupling with the P atoms of two (PPh2)2py ligands. Accordingly, the 1H−13C B

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

resonances, the 1H NMR spectrum of 2 in a CD2Cl2 solvent (Figure S8) showed the resonance of the hydroxide ligand proton at 1.5 ppm. This signal disappeared upon the addition of excess D2O. In the 13C{1H} NMR spectrum of 2, the resonances of the C atoms of the pyridyl rings showed a downfield shift compared to those observed for 1 (Figure S9). The 31P{1H} NMR spectrum of cluster 2 in DMSO-d6 at 298 K (Figure S10) showed a sharp singlet signal at 44.7 ppm due to the P atoms of the two (PPh2)2py ligands. However, on the basis of the observed distortion in the crystal structure of 2, the two P atoms that were coordinated to one Au(I) center are equivalent with each other and are different from the two P atoms connected to the other Au(I) center. These observations revealed that 2 undergoes a dynamic process in solution. Therefore, the variable-temperature (VT) 31P{1H} NMR spectrum of 2 was monitored from 298 to 193 K to further explore the dynamic process in a CD2Cl2 solution (Figure 3). The results revealed that the phosphorus signal of 2 was broadened as the temperature decreased, resulting in a spectrum with four singlet signals; two resonances appeared at 45.0 and 44.0 ppm, and two other singlet resonances appeared at 44.3 and 42.7 ppm. When the temperature was decreased to 193 K, the intensity of the latter singlet signals increased and became sharper while the intensity of the former singlet signals decreased. It is important to note that the chemical shift differences between each pair of these singlet signals showed a large change during a temperature decrease (a change from 173 Hz at 253 K to 99 Hz at 193 K for the former singlets). On the basis of these NMR data, we tentatively assume that at low temperatures complex 2 adopts a distorted structure with two distinct phosphorus atom environments and two different wing sizes for the butterfly metal core. As the temperature is increased, an equilibrium can be established between the two distorted structures with different amounts of distortion by the motion of the copper(I) atoms toward the gold(I) centers (Scheme 1). It is important to note that the copper(I) ellipsoids in the crystal structure of 2 are unusually large, indicating a fluxional process involving copper(I) centers. When the temperature was increased to room temperature, the two sets of singlet signals re-emerged as a

Figure 2. Molecular crystal structure of 2. The anions (PF6), solvents of crystallization, and H atoms have been omitted for the sake of clarity.

from the hydroxo ligand. The two gold atoms are located on opposite edges of the square basal plane, while the two copper atoms are positioned on two nonadjacent trigonal faces, in front of each other, forming a tetranuclear butterfly Au2Cu2 metal core of alternating Au and Cu atoms. It is important to note that 2 is not completely symmetrical and has a butterfly metal core with different wing sizes. The distance for either of the Au(1)−Cu(1) and Au(1)−Cu(2) interactions amounts to 2.943 Å, which is shorter than the Au(2)−Cu(1) and Au(2)− Cu(2) distances (3.053 Å), and the P(1)−Au(1)−P(1) angle is 173.55°, which is smaller than the P(2)−Au(2)−P(2) angle (178.34°). The crystal structure did not show any inter- or intramolecular Au−Au interactions (intramolecular Au−Au distance of 5.110 Å). In contrast to the previously reported AuI−CuI clusters containing alkynyl ligands,10,11 complex 2 showed a strong CuI−CuI bonding interaction [d(Cu−Cu) = 2.423 Å] that is supported by the bridging hydroxide ligand (with the Cu−O bond distances being 2.046 Å). The IR spectrum of 2 (Figure S7) showed a broad band at 3500 cm−1, confirming the presence of the hydroxide ligand in the cluster. In addition to pyridyl and phenyl group proton

Figure 3. Variable-temperature 31P{1H} NMR spectra of 2 in CD2Cl2. C

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Equilibrium between Two Distorted Structures of 2 with Different Amounts of Distortion in the Solution Phase

sharp singlet resonance. The density functional theory (DFT) calculations showed the presence of a second ground state that is close in energy (1.2 kJ/mol energy difference) and differs in core bond distances with respect to the distorted structure of 2. However, the presence a dynamic process involving cluster 2 with different phosphorus atoms connected to the same gold(I) center cannot be possible on the basis of these NMR data, although there may be alternative mechanisms that are also consistent with the observed data. However, VT 1H NMR spectra of cluster 2 in a dichloromethane-d2 solution (Figure S8c) show a second dynamic process involving the exchange hydrogen atom of the hydroxide group by water. Decreasing the temperature to 193 K broadens the hydroxyl and water signals, merges them into one single broad peak, and moves toward the downfield region. This indicates that a second dynamic process (exchange with water) predominates at lower temperatures and the first dynamic process closed to stop exchange at 193 K as confirmed by VT 31P NMR spectra. The reaction of the bis(diphenylphosphino)pyridine ligand, (PPh2)2py, with 1 equiv of Cu(SMe2)Br and 1 equiv of Ag(OTf) in a dichloromethane solution at room temperature gave the binuclear Cu(I) complex [Cu2(μ-(PPh2)2py)2(μSMe2)(OTf)2], 3, in 50% yield (Scheme S1). The dicationic molecular peak (m/z 510) in ESI-MS (Figure S11) confirms the chemical formula of the cationic fragment [Cu2(μ(PPh2)2py)2]2+, indicating that dimethyl sulfide and the OTF ligands are dissociated in the gas phase. In the 31P{1H} NMR spectrum of 3 (Figure S12), the four equivalent phosphorus atoms of the phosphine ligands appeared as a singlet signal at −1.6 ppm and the 1H NMR spectrum of an acetone-d6 solution of complex 3 (Figure S13) showed a singlet signal due to the dimethyl sulfide ligand at 1.99 ppm. As shown in Figure 4, the crystal structure of 3 revealed a binuclear copper(I) complex with a tetrahedral geometry around copper(I) centers that are separated by 5.229 Å and does not show any Cu−Cu bonding interactions. While copper(I) centers are held together by the bridging (PPh2)2py and SMe2 ligands, the OTf and dimethyl sulfide ligands are labile and can be replaced by other ligands. Similar to 1, complex 3 contains two uncoordinated pyridyl groups that provide the possibility of coordination of the pyridyl nitrogen atoms to other metal centers. Binuclear complex 3 undergoes a transmetalation reaction with 2 equiv of AuCl(SMe2) and affords the tetranuclear cluster [Au2Cu2Cl2(μ-(PPh2)2py)2](OTf)2, 4, in good yield. Suitable crystals of 4 for single-crystal X-ray analysis were obtained from vapor diffusion of ether into a dichloromethane solution of 4. The complex crystallized in the monoclinic system, in space group C2/c. The asymmetric unit of complex 4 comprised half of the cluster cation, one OTf counterion, and one dichloromethane molecule as the solvent

Figure 4. Molecular crystal structure of 3. The solvents of crystallization and H atoms have been omitted for the sake of clarity.

of crystallization. A view of the molecular structure of 4 is depicted in Figure 5, and the selected bond lengths and angles

Figure 5. Molecular crystal structure of 4. The solvents of crystallization, counterions, and H atoms have been omitted for the sake of clarity.

are listed in Table S5. In comparison to the tetranuclear Au2Cu2 cluster 2 in which both of the Cu(I) centers are coordinated by the pyridine nitrogen atoms on the same side of the plane defined by four phosphorus atoms, the Cu(I) centers in 4 are coordinated by the pyridine nitrogen atoms on the opposite side of the phosphorus plane. The metal centers D

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry in 4 form a tetranuclear planar-shaped metal core of alternating Au and Cu atoms that are positioned in the vertices of a tetragon without Au−Au (5.213 Å) and Cu−Cu (3.165 Å) interactions. While Cu(I) centers in 2 are held together by a bridging hydroxide ligand, the Cu(I) centers in 4 are coordinated by terminal chloride ligands. The cluster contains two different types of Au−Cu bonds, short (Au−Cu distances of 3.034 Å) and long (Au−Cu distances of 3.065 Å). The ESIMS spectrum of cluster 4 is particularly informative as it showed a dicationic peak at m/z 744 (Figure S14), which proves that the cationic cluster [Au2Cu2Cl2(μ-(PPh2)2py)2]2+, observed in the solid state, is retained in the gas phase. The 31 1 P{ H} NMR spectrum of 4 in CD2Cl2 (Figure S15) showed a singlet signal at 45.0 ppm, and the resonances of the aromatic protons of the (PPh2)2py ligands appeared in the range of 7.36−8.4 ppm in the 1H NMR spectrum of 4 (Figure S16). Photophysical Properties. Ultraviolet−visible (UV−vis) absorption spectra of 1−4 were measured in CH2Cl2 (Figures S17 and S18). The UV−vis absorption spectrum of 1 shows high-energy intense absorption bands (λ < 300 nm) that can tentatively be assigned to intraligand transitions. In addition to the UV−vis absorption bands observed for 1, tetranuclear cluster 2 showed a low-intensity forbidden absorption band around 380 nm that can be assigned as a triplet clustercentered transition (further explanation given below). The absorption spectrum of 4 is similar to that of the precursor complex 3. The absorption spectra of 3 and 4 exhibit intense high-energy absorptions (λ < 350 nm) and a broad weak absorption at λ values ranging from 350 to 400 nm that can be typically ascribed to charge transfer transitions (further explanation given below). Photophysical data for 2−4 are summarized in Table 1, and their emission spectra are shown

Figure 6. (a) Emission spectra of 2 (light green), 3 (yellow), and 4 (blue-green) in the solid state. (b) Emission spectra of 2 in a CH2Cl2 solution at 298 K (red), in frozen CH2Cl2 at 77 K (yellow), and in a solvent-free solid state at 298 K (green).

Table 1. Photophysical Data for Complexes 2−4 in the Solid State complex

δem (nm)

δex (nm)

τ (μs)

δ (%)

2 4 3

517 490 536

370 334, 394 316, 371

1.9 7.71 10.66

1.2 11.8 5.9

ochromism that arises upon changes in the rigidity of the medium. In light of the optical properties mentioned above, DFT calculations were performed at the PBE0-TDDFT level of theory on tetranuclear clusters 2 and 4, binuclear complex 3, and the model complex [Au2Cu2(μ-(PH2)2py)2(μ-OH)](PF6)3, 5. Initially, the S0 → S1 singlet excitation transitions of 2−4 at the experimental geometries were calculated, which were in good agreement with the experimental absorption data (Figure 7 and Table S6). Next, for exploring the origin of the emission and the rigidochromism behavior, the geometries of 2−5 were fully optimized in the ground and first triplet excited electronic states. The optimized geometries are shown in Figure 8a and Figures S22−S25, and the selected bond lengths and bond angles are listed in Tables S7−S10. For the S0 → S1 transition in 2, the transition density was mostly centered on the Cu atoms and the hydroxide ligand while the (PPh2)2py ligands and the gold atoms played a minor role. For the same electronic transition in 3, the contribution of one copper(I) center and its coordinated pyridyl group becomes more significant with a transfer of electron density from the pyridyl ring to the Cu(I) center (LMCT). Meanwhile, for cluster 4, the transition densities were localized on the Cu(I) centers, pyridyl groups, and chloride ligands with transfer of electron density from one Py−CuI−Cl unit to another. The T1 → S0 transition for binuclear complex 3 was centered on one CuI center and its coordinated pyridyl group (MLCT), while in cluster 4, a transfer of electron density took place between the Py−CuI−Cl units. The transition densities for the lowest-

in Figure 6a. In contrast to the previously reported binuclear three-coordinated gold(I) complex, [Au2(μ-(PPh2)2py)3](ClO4)2, which displayed photoluminescence both in the solid state and in the degassed solution,13 1 showed only a weak emission that was assigned to the intraligand excited state, because the free ligand (PPh2)2py also shows a similar weak emission at the same energy.13 Although complexes 3 and 4 are not emissive in solution, they show intense yellow and blue-green emissions in the solid state at λmax values of 536 and 490 nm, respectively. At room temperature and using 370 nm excitation, a solution of complex 2 in dichloromethane showed a red emission at a λmax of 686 nm with an excited state lifetime on the microsecond scale (1.8 μs) and a quantum yield of 1% (Figure 6b). The emission band of 2 in dichloromethane displayed a large blue shift (116 nm) as the temperature was decreased from 298 to 77 K. Although the emission band of a solvent-free powder of 2 at 298 K showed a further blue shift [λmax = 517 nm (Figure 6b)], the powder of 2 that was exposed to the vapor of CH2Cl2 at 298 K showed a similar blue shift compared to that observed for the frozen solution. These results are consistent with the strong luminescence rigidE

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. PBE0-DFT transition densities for S0 → S1 and T1 → S0 transitions of 2−5. During the electronic transition, the electron density increases in the violet areas and decreases in the brown areas. Transition densities for the T1 → S0 transition of 2 and the S0 → S1 transition of 5 are shown in Figures S20 and S21.

energy triplet emission, T1 → S0, of 2 and 5 were mostly centered on one wing of the butterfly metal core defined by the Cu(1)−Au(1)−Cu(2) angle, and the hydroxide ligand and other parts of the cluster had minor contributions. The calculated data (Table S10) indicate that the model complex 5 undergoes a distortion upon excitation to the triplet excited state, including a decrease in the angle between the wings of the butterfly metal core and a change in the geometry of the P−Au−P moieties from linear to bent. Compared to the corresponding singlet ground state structure, the Au−Au distance in the triplet excited state was shortened significantly from 5.159 to 4.604 Å and the Au(1)−Cu(1)−Au(2) and Au(1)−Cu(2)−Au(2) angles were reduced from 111.18° and 111.89° to 106.23° and 105.53°, respectively. It is important to note that the distortion was largely localized on the wing defined by three metal centers [Cu(1), Au(1), and Cu(2)] and on the P(1)−Au(1)−P(3) angle. The P(1)−Au(1)−P(3) angle showed a significant difference from 173.6° to 152.3°, while the P(2)−Au(2)−P(4) angle showed a slight change from 171.3° to 167.0°. Consistent with this observation, the

Cu(1)−Au(1) and Cu(2)−Au(1) bond distances were reduced notably from 3.103 and 3.172 Å to 2.738 and 2.760 Å, respectively, while the Cu(1)−Au(2) and Cu(2)−Au(2) bond distances decreased slightly from 3.150 and 3.054 Å to 3.013 and 3.018 Å, respectively. On the basis of Jahn−Teller considerations, it is expected that the unsymetrical distortion of two gold(I) metal centers prevails because compared to the symmetrical distortion of two gold(I) metal centers, a further decrease in symmetry has occurred. Calculated data also indicated that the Jahn−Teller splitting and the stability of the lowest triplet excited state with unsymetraical distortion of two gold centers [Au(1) and Au(2) by 21° and 4°] is larger than that observed for the symmetrical distortion of both gold(I) centers [Au(1) and Au(2) by 12.5°] (Figure 8b). To probe the sensitivity of the singlet−triplet energy gap to geometry changes, we scanned the surface potential energy of 5 by altering the P(1)−Au(1)−P(3) and P(2)−Au(2)−P(4) angles from 180° to the optimized geometry (Table S11), which resulted in drastic changes in the calculated energies of T1 → S0 transitions. In contrast, via application of variations in F

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. (a) Optimized molecular structure of 5 in the singlet ground state and lowest triplet excited state. (b) HOMO and LUMO molecular orbital energies of 5 in the singlet ground state (left), the lowest triplet excited state with symmetrical distortion of gold(I) centers (middle), and the lowest triplet excited state with unsymmetrical distortion of the gold(I) metal centers (right). The numbers show the P−Au−P angles in the singlet and triplet states.

the solvent polarity, the energies of T1 → S0 transitions changed very slightly (Table S12). The optimized geometry for complex 2 in the lowest triplet excited state showed similar structural features. However, the distortion of 2 (Figure S22 and Table S7) was smaller than that observed in the model complex 5. Therefore, the results of both theoretical and experimental investigations (the range of blue shift) verified that the distortion of the lowest triplet excited state was the most plausible cause of the rigidochromic effect in 2. Luminescence vapochromism was seen for clusters 2 and 4 upon their exposure to a series of VOCs such as methanol, tetrahydrofuran (THF), acetonitrile, pyridine, and dichloromethane at room temperature. The samples in which 2 and 4 were exposed to the VOCs underwent thermal decomposition upon heating and did not show a reversible luminescence vapochromic behavior. As shown in Figure 9a, the emission color of 2 was orange, yellow, green, and white in response to CH2Cl2, MeOH, pyridine, and THF vapors, respectively. Moreover, the luminescence spectra showed that the λmax of the emissions shifted from 600 to 480 nm. With regard to cluster 4, the changes in the emission wavelengths were small and showed a large shift only upon exposure to the vapor of a hydrogen-bond donor solvent such as methanol (Figure S26). These experimental results demonstrated that the presence of a hydroxyl group or the presence of hydrogen-bond interactions plays an important role in the luminescence vapochromic

behavior of AuICuI clusters in the absence or presence11 of alkynyl ligands. The vapochromic behavior of clusters 2 and 4 was investigated via comparison of the powder X-ray diffraction (PXRD) patterns before and after their exposure to VOCs. Analysis of the PXRD patterns of cluster 2 exposed to VOCs showed that changes are minor in the case of hydrogen-bond donor solvents such as methanol but substantial for hydrogen-bond acceptor solvents, including THF, dichloromethane, and pyridine (Figure 9b). Similar changes were observed in the PXRD for cluster 4 (Figure S27). The different behavior of 2 against hydrogen-bond donor and acceptor solvents was also observed in a dichloromethane solution. Quenching occurred in the emission of cluster 2 in a dichloromethane solution upon addition of a hydrogen-bond solvent. As shown in Figure S28 and the Stern−Volmer plots (Figure S29), the quenching in the emission of 2 together with changes in its emission wavelength occurs with a higher rate upon the addition of hydrogen-bond acceptor solvents, in comparison to those observed for hydrogen-bond donor solvents.

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EXPERIMENTAL SECTION

General Comments. All manipulations were performed under a dry and oxygen-free atmosphere (Ar). All solvents were dried over activated molecular sieves after distillation. 1H, 31P, and 13C NMR spectra were recorded on Bruker 400 and 500 MHz spectrometers. G

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Ph ring C atoms), (139.2, 130.3, and 156 pyridine C atoms). Anal. Calcd for C60H46N2P4F6O6S2Au2: C, 45.4; H, 2.9; N, 1.8. Found: C, 45.8; H, 2.7; N, 1.9. [Au2Cu2(μ-(PPh2)2py)2(μ-OH)](PF6)3 (2). 1 (300 mg, 0.19 mmol) was added to an excess of [Cu(NCMe)4]PF6 in wet dichloromethane (30 mL). The mixture was stirred for 2 h at room temperature and then filtered. The volatile solvents were removed under reduced pressure. The residue was triturated with Et2O and dried under vacuum. Single crystals suitable for X-ray analysis were grown from a slow diffusion of hexane into a concentrated solution of 2 in CH2Cl2 to afford pale yellow crystals of complex 2: yield 76%; 1H NMR (CD2Cl2, 500 MHz) δ 7.2−7.26 (d, JHH = 7.5 Hz, 4H, py), 7.6−7.8 (m, 40H, Ph ring protons), 8.1−8.2 (broad t, JHH = 7.5 Hz, 2H, py), 1.5 (broad, 1H, hydroxide proton); 31P{1H} NMR (DMSO-d6, 203 MHz) δ 44.7 (P atom of PPh2), −145 (PF6−); 13C{1H} NMR (CD2Cl2, 126 MHz) δ (125, 132, 134, and 135 C atoms of phenyl rings) (131, 140, and 157 C atoms of pyridine rings). Anal. Calcd for C58H47N2P7F18OCu2Au2: C, 37.3; H, 2.5; N, 1.5. Found: C, 37.5; H, 2.4; N, 1.6. [Cu2(μ-(PPh2)2py)2(μ-SMe2)(OTf)2] (3). AgOTf (125 mg, 48 mmol) was added to a solution of [(PPh2)2py] (217 mg, 48 mmol) in freshly distilled dry dichloromethane (30 mL). The mixture was stirred for 5 min at room temperature, and then CuBr(SMe2) (100 mg, 48 mmol) was added and the mixture stirred for an additional 4 h at room temperature. The mixture was filtered, and all volatile solvents were removed under reduced pressure. The residue was triturated with Et2O and dried under vacuum. Slow diffusion of ether into a concentrated solution of 3 afforded yellowish crystals of complex 3: yield 50%; 1H NMR (acetone-d6, 500 MHz) δ 7.2−7.28 (m, 44H, ArH), 8 (m, 2H, py), 1.99 (S, 6H, SMe2); 31P{1H} NMR (acetone-d6, 203 MHz) δ −1.6 (s). Anal. Calcd for C62H52N2P4F6O6S3Cu2: C, 53.8; H, 3.8; N, 2.0. Found: C, 53.7; H, 3.9; N, 1.9. [Au2Cu2Cl2(μ-(PPh2)2py)2](OTf)2 (4). AuCl(SMe2) (42 mg, 144 mmmol) was added to 3 (100 mg, 72 mmol) in dichloromethane (30 mL). The mixture was stirred for 2 h at room temperature and then filtered. The volatile solvents were removed under reduced pressure. The residue was triturated with Et2O and dried under vacuum. Single crystals suitable for X-ray analysis were grown from slow diffusion of ether into a concentrated solution of 4 resulting in yellowish crystals of complex 4: yield 77%; 1H NMR (DMSO-d6, 500 MHz) δ 7.35− 7.85 (m, 44H, ArH), 8.15 (broad, 1H, Py), 8.4 (broad, 1H, py); 31 1 P{ H} NMR (DMSO-d6, 203 MHz) δ 44.6 (PPh2). Anal. Calcd for C60H46N2Cl2P4F6O6S2Cu2Au2: C, 40.3; H, 2.6; N, 1.5. Found: C, 40.4; H, 2.6; N, 1.7. X-ray Structure Determination. X-ray intensity data were collected using the full sphere routine by an ω scan strategy on the Agilent SuperNova dual-wavelength EoS S2 diffractometer with mirror monochromated Cu Kα radiation (λ = 1.54184 Å) for 1 and 4 and the Bruker SMART Apex2CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) for 2 and 3. For all data collection, the crystals were cooled to 110−150 K using an Oxford diffraction Cryojet low-temperature attachment. Data reduction, including an empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm,14 was performed using the CrysAlisPro software package and SAINT15 for 1−4. The crystal structures of 1−4 were determined by direct methods using the online version of AutoChem 2.0 in conjunction with the OLEX216 suite of programs implemented in the CrysAlis software and then refined by full-matrix least squares (SHELXL-2018)17 on F2. The non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were positioned geometrically in idealized positions and refined with the riding model approximation, with Uiso(H) = 1.2 or 1.5Ueq(C). All geometric calculations were carried out using the PLATON software.18 The solvent accessible voids of both structures were squeezed by the SQUEEZE19 routine in PLATON by back Fourier transform. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, No. CCDC 1895349, 1895350, 1915162, and 1915161.

Figure 9. (a) Powder samples of 2 in glass tubes that were exposed to selected organic vapors under UV light and their emission spectra. (b) Comparison of PXRD patterns of 2 after exposure to selected organic vapors and the corresponding PXRD pattern obtained from a solventfree powder of 2. (PPh2)2py, Ag(OTf), and [Cu(NCCH3)4]PF6 were purchased from commercial sources. Electrospray ionization mass spectra (ESI-MS) were recorded on a Hewlett-Packard Series 1100 spectrometer. PXRD measurements (GBCMMA, Instrument) were taken in the 2θ range from 0° to 80° using Cu Kα radiation. [Au2(μ-(PPh2)2py)2](OTf)2 (1). AgOTf (60 mg, 0.23 mmol) was added to a solution of [(PPh2)2py] (105 mg, 0.23 mmol) in freshly distilled dry dichloromethane (30 mL). The mixture was stirred for 5 min at room temperature, and then AuCl(SMe2) (70 mg, 0.23 mmol) was added and the mixture stirred for an additional 4 h at room temperature. The mixture was filtered, and all volatile solvents were removed under reduced pressure. The residue was triturated with Et2O and dried under vacuum. Slow diffusion of hexane into a concentrated solution of 1 provided colorless crystals of complex 1: yield 71%; 1H NMR (CD2Cl2, 500 MHz) δ 7.2−7.28 (d, 4H, py, JHH = 7.93 Hz), 7.5−7.7 (m, 40H, Ph ring protons), 8−8.05 (t, 2H, py, JHH = 7.93 Hz); 31P{1H} NMR (DMSO-d6, 203 MHz) δ 43.5 (s); 13 C{1H} NMR (CD2Cl2, 126 MHz) δ (134.5, 133.2, 130.1, and 125.3 H

DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Photophysical Measurements. Emission and excitation spectra for complexes 2 in solution upon exposure to the air were recorded on a FluoroMax 4 (JY Horiba Inc.) spectrofluorometer in a 1 cm quartz cuvette (freshly distilled dichloromethane, concentration of ∼10−6 M, 298 K). The emission quantum yields in solution were determined by a comparative method using a 370 nm light-emitting diode as the excitation source and ruthenium [Ru(bpy)3]Cl2, in water (Φp = 0.04 ± 0.002 in air) as the reference dye.20 Excitation and emission spectra of complexes 2−4 in the solid state at 298 and 77 K were recorded with a HORIBA FluoroMax-4 spectrofluorometer. Lifetime and emission quantum yields in the solid phase were performed on a HORIBA Scientific FluoroLog-3 spectrofluorometer. The samples were placed in a model optCRYO 105 cryostat for measurements at 77 K. The absolute emission quantum yields of solid samples, which were loaded in Teflon cuvettes and covered by a quartz glass ring, were measured using a FluoroLog 3 (JY Horiba Inc.) spectrofluorometer and a Quanta-phi integration sphere (Horiba). Computational Details. Compounds 2−5 were studied using the hybrid PBE0 density functional method. The gold and copper atoms were described by a triple-valence ζ-quality basis set with polarization functions (def2-TZVP). Scalar relativistic effects were taken into account by employing a 60-electron relativistic effective core potential for gold. A split-valence basis set with polarization functions was used for the other atoms (6-31G*). The electronic transitions of the complexes were investigated by means of time-dependent DFT calculations (PBE0-TDDFT). For 2−5, the first triplet excited state (T1) geometry was calculated from the crystal structure and the optimized ground state (S0) geometry, respectively. The geometries of the lowest-energy triplet excited and ground states of each complex were used to investigate their absorption and emission features. All electronic structure calculations and geometry optimizations were performed with ORCA version 4.1.0[1].

Experimental details, NMR spectra of 1−4, ESI mass spectra of 1, 3, and 4 for absorption and emission, details of theoretical studies, and X-ray crystallographic data (PDF) Accession Codes

CCDC 1895349−1895350 and 1915161−1915162 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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*E-mail: [email protected]. ORCID

Sirous Jamali: 0000-0002-5997-2619 Hamidreza Samouei: 0000-0003-4125-9556 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Sharif University of Technology Research Council (Grant G930615) and the Iran national science foundation (Grant 96017102) for financial support. We would like to acknowledge Prof. Sergey P. Tunik and Dr. Julia R. Shakirova for the photophysical measurement facilities.

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CONCLUSION In summary, we present two new tetranuclear AuI2CuI2 cluster complexes 2 and 4 with butterfly- and planar-shaped metal cores, respectively. Although cluster 2 comprises twocoordinated gold(I) centers with two metalophilic interactions, it shows a large Stokes shift and a strong rigidochromism behavior. The amount of blue shift along with the computational data for 2 and the model complex 5 shows that the rigidochromic behavior of 2 can be attributed to the Jahn− Teller distortion in the lowest triplet excited state from a linear to bent geometry around the gold(I) centers. Calculations showed that the unsymmetrical distortion of the two gold(I) metal centers gave a further reduction in the symmetry and an increase in the stability of the lowest triplet excited state, compared to the symmetrical distortion of two gold(I) centers. The investigations herein showed that this behavior is not a typical behavior of three-coordinated gold(I) centers7,8,21 and can be observed in two-coordinated gold(I) centers and opened new perspectives for the design of new gold(I)-based sensing systems that are sensitive to the rigidity of the medium. Also, both clusters 2 and 4 showed luminescence vapochromic behavior when they were exposed to a series of VOCs. Experimental results show that the hydroxide ligand and hydrogen-bond interactions play a key role in the vapochromic behavior of these types of clusters.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01414 Inorg. Chem. XXXX, XXX, XXX−XXX