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Utilization of a Non-emissive Triphosphine Ligand to Construct a Luminescent Gold(I)-Box that Undergoes Mechanochromic Collapse into a Helical Complex Daniel T. Walters, Reza Babadi Aghakhanpour, Xian B Powers, Kamran B. Ghiassi, Marilyn M. Olmstead, and Alan L. Balch J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01666 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Journal of the American Chemical Society 1

Utilization of a Non-emissive Triphosphine Ligand to Construct a Luminescent Gold(I)-Box that Undergoes Mechanochromic Collapse into a Helical Complex Daniel T. Walters, Reza Babadi Aghakhanpour, Xian B. Powers, Kamran B. Ghiassi, Marilyn M. Olmstead* and Alan L. Balch* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA Supporting Information Placeholder ABSTRACT: Luminescent gold(I) complexes ([Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), and

[Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5) where Triphos = bis(2-diphenylphosphinoethyl)phenylphosphine)) with a box-like architecture have been prepared and crystallographically characterized. A chloride ion resides at the center of the box with two of the six gold(I) ions nearby. Mechanical grinding of blue luminescent crystals containing the cation, [Au6(Triphos)4Cl]5+, results in their conversion into amorphous solids with green emission that contain the bridged helicate cation, [µ-Cl{Au3(Triphos)2}2]5+. A mechanism of the mechanochromic transformation is proposed. The structures of the blue-emitting Helicate, [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, and the greenemitting Bridged-Helicate, [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH have been determined by single crystal X-ray diffraction.

INTRODUCTION There has been considerable interest in the construction of molecular cages through the combination of metal centers with appropriate bridging ligands.1 6 Significant advances have been made in building molecules with a variety of intricate geometric architectures including molecules with varying shapes such as tetrahedra, cubes, octahedra, icosahedra, etc. In many cases, work focused on the construction and structural characterization of the cage framework themselves. In other cases, the cages have been used to host a variety of molecules and ions.7 It is particularly interesting to see such cages used to protect and stabilize reactive entities such as white phosphorus (P4).8 In other cases, these cages have been used to house catalysts and to control and modify their catalytic activity.9,10 Molecular cages can be designed to be luminescent, particularly by incorporation of intrinsically luminescent components into the cage. 11-16 For example, Stang and co-workers have modified tetraphenylethylene through the attachment of Lewis base sites that allow construction of molecular cages through coordination of various Lewis acids such as platinum(II) complexes. The resulting assemblies show enhanced luminescence due to the restriction of rotation of the phenyl groups. Related cage structures have been formed by incorporating other emissive components (e. g. naphthalene-containing ligands, 17 dinuclear gold(I) complexes,18 etc.) into the cage. Such luminescent cages have potential in chemical sensing where guest/host interactions may alter the emission of the assembly. They may also be useful in monitoring the presence of small molecules and ions in biological samples.

In constructing these cage molecules, chelating diphosphine ligands have been used in conjunction with planar metal centers like platinum(II) to form corners where two bridging ligands can meet at right angles. Otherwise, the use of polyphosphine ligands in formation of molecular cages has been limited. In one example, Lim et. al reported the use of 1,3,5tris((diphenylphosphino)ethynyl)benzene (tppeb) to construct tetrahedral assemblies [M4(tppeb)4]4+.19 A second example was reported by Stickel et. al, who used tris(diphenylphosphine)cyclohexane to form trinuclear prismatic cages.20 Some luminescent organic or organometallic materials are sensitive to mechanical forces such as grinding, rubbing, and/or crushing.21 -25 Such materials are termed luminescence mechanochromic and have potential utility as sensors for motion or mechanical damage and as recording materials. Many mechanochromic materials have only recently been detected and the nature of the changes in absorption and emission spectra examined. In many cases, the structural changes responsible for the spectral changes are difficult to definitively identify.21,26 However, as shown by Ito and coworkers, several gold(I) complexes are known to undergo mechanochromic changes that are due to alteration in the contacts between gold(I) ions and/or changes in the π-π interactions between conjugated ligands as the complexes slide past one another in the solids.27-33 In other cases, loss of volatile ligands from metal complexes is involved with the mechanochromism.34,35 Here, we will describe the construction of a molecular box from non-luminescent components. The luminescence and structure of this molecular box change upon application of mechanical pressure.

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RESULTS AND DISCUSSION Formation and Crystal Structures of the Mechanochromic Molecular Box, [AuI6(Triphos)4Cl](EF6)5. (E = Sb, As, or a P). The reaction chemistry that leads to the formation of the luminescent Molecular Box cation, [AuI6(Triphos)4Cl]5+, and other related cations is outlined in Scheme 1. The reaction of (tht)AuICl (tht is tetrahydrothiophene) with Triphos (Triphos is bis(2-diphenylphosphinoethyl)phenylphosphine) in a 3:2 molar ratio in dichloromethane produced a colorless solution. Subsequent addition of an excess of sodium hexafluoroarsenate or sodium hexafluoroantimonate in ethanol solution or ammonium hexafluorophosphate in methanol followed by evaporation produced a white solid that was collected by filtration and vacuum dried. Each solid was dissolved in dichloromethane and filtered to remove inorganic salts. After evaporation, a pale yellow solid with a green emission (initial product) was obtained. This material was redissolved in dichloromethane. Toluene was layered over the dichloromethane solution. Colorless crystals with a blue luminescence slowly grew from the solution. Each of the three crystalline salts involved toluene solvates in differing amounts: 1B; [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 2B; and [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), [Au6(Triphos)4Cl]-(SbF6)5•7(CH3C6H5), 3B. Nevertheless, each crystal contained similar Box [Au6(Triphos)4Cl]5+ with two toluene molecules protruding into the Box.

outer surface. These interactions are illustrated in Figure 3. Parts A and B in Figure 3 show views of the cation that look down on Au2 in A and the chloride ion in B. Parts C and D in this Figure show how the anions fit into the nests provided by the phenyl groups on the [Au6(Triphos)4Cl]5+ cation. The structures of the [Au6(Triphos)4Cl]5+ cations in the hexafluorophosphate and hexafluoroarsenate salts are similar to that in [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B. Table 1 provides a comparison between some selected interionic distances and angles in the cations in the three different crystals. In addition, in each of the three crystals two toluene molecules protrude into the Box as seen in Figure 2. Thus, [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, has the minimal composition to allow this situation and has no toluene molecules in the spaces between the cations. Additionally, in the hexafluorophosphate and hexafluoroarsinate salts there are interactions between the anion and nests of phenyl groups on the surface of the cation that are analogous to those shown in Figure 3 (see Figures SI-1 and SI-2). Scheme 1. A summary of the structures and reactions described in this paper.

Figure 1 shows the core structure of one example, [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5) 3B with (SbF6)- anions, solvate molecules, and phenyl rings omitted. The chloride ion in the middle of the Box resides on a crystallographic center of symmetry. This core skeleton defines a rectangular, box-like shape. The top and bottom of the Box is a 16-atom macrocycle made up of two Triphos ligands connected to one another by two gold centers, Au1 and Au3. The non-bonded distance between these two gold ions is 8.247 Å, which defines the length of the Box. The distance between Au1A and Au3 is 6.974 Å, which determines the height of the Box. Finally, the distance between an edge gold ion, Au2 and its mate, Au2A, on the other face of the Box is 7.028 Å, which corresponds to the depth of the Box. This gives a rough dimension for the skeleton of 7.0 by 7.0 by 8.25 Å in size. Figure 2 shows a drawing of the [Au6(Triphos)4Cl]5+ cation and two of the seven toluene solvate molecules. The Box is centered about a lone chloride ion that is not directly bonded to the rest of the Box. The distance between the chloride ion and the adjacent gold ion is 3.4870(3) Å, which is too long for significant coordinate bond formation. For comparison in complexes with AuP2Cl coordination, the Au-Cl distances average 2.615 Å and range from 2.437 to 3.038 Å. Note also that the thermal ellipsoid of the chloride ion is nearly isotropic, which is consistent with an ion residing at the center of a Coulombic field produced by the centrosymmetric array of six gold ions that surround it. As shown in Figure 2, two different toluene molecules protrude into each end of the Box and are nearly symmetrically positioned between Au1 and Au3A or Au1A and Au3. The distances from Au1 to the closest two carbon atoms in the toluene molecule are 3.528(8) and 3.607(8) Å, while the corresponding distances for Au3 are 3.603(8) and 3.607(7) Å. The other five toluene solvate molecules in [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5) reside in the spaces between Boxes. The hexafluoroantimonate ions are tucked into nests in the cation that are made by the arrangements of phenyl groups on its


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Journal of the American Chemical Society 3 Table 1. Selected Bond Distances and Angles for [Au6(Triphos)4Cl]5+

Figure 1. Structure of the Box, the cation [Au6(Triphos)4Cl]5+, in [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, with the solvent molecules, phenyl rings and five hexafluoroantimonate anions omitted. Thermal contours are shown at the 30% probability level.

Compound a

[Au6(Triphos)4Cl] (SbF6)5•7(CH3C6H5) 3B

[Au6(Triphos)4Cl] (AsF6)5•8(CH3C6H5) 2B

[Au6(Triphos)4Cl] (PF6)5•2(CH3C6H5) 1B

Au2…Au3A (shortest)

6.3585(4)

6.3522(9)

6.3439(7)

Au2…Au2A (width)

6.9740(6)

6.9880(12)

7.0231(9)

Au1…Au3A (depth)

7.0282(5)

6.9907(13)

6.8686(10)

Au1…Au3 (length)

8.2472(5)

8.2803(15)

8.2683(8)

Au-Cl

3.4870(3)

3.4940(6)

3.5114(4)

Au1-P

2.3073(18) 2.3077(18)

2.3134(19) 2.3128(19)

2.3037(16) 2.3047(16)

Au2-P

2.3013(18) 2.2986(18)

2.3104(18) 2.3070(18)

2.2996(16) 2.2996(16)

Au3-P

2.3066(17) 2.3081(17)

2.3019(19) 2.3057(19)

P-Au1-P

177.75(7)

177.08(7)

176.27(6)

P-Au2-P

169.11(6)

177.75(7)

168.08(7)

P-Au3-P

177.70(7)

168.25(6)

Au-Cl-Au

180.0

180.0

180.0

a

Due to differing space groups, numbering schemes in the three salts are not identical. The numbering for [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, is used here.

Figure 2. Structure of the Box cation [Au6(Triphos)4Cl]5+ and two of the toluene solvate molecules in [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B. Hydrogen atoms, other solvate molecules, and hexafluoroantimonate anions omitted for clarity. Thermal contours are displayed at the 30% probability level.

Figure 3. Space-filling drawings of parts of the structure of [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, showing the interactions between the cation and the hexafluoroantimonate anions. Hydrogen atoms and solvate molecules are omitted for clarity. A) Side view displaying the central gold, Au2. B) Top view displaying the central chloride. C) Side view showing the anion fit. D) Top view showing anion fit.



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linear chain (Au1-Au2-Au3 angle of 177.101(3) °) with very short Au•••Au contacts (Au1-Au2, 2.9486(4); Au2-Au3, 2.9915(4) Å). Due to the helical structure of the cation, individual cations are chiral. However, the complex crystallizes in a centrosymmetric space group and the crystal is a racemate. The positioning of the phenyl groups makes the two ends of the cation distinct. At the end involving Au3, two phenyl rings extended away from the cation in a direction that is nearly parallel to the central Au1-Au2-Au3 axis. The eight other phenyl groups are oriented towards the other end of the cation, resulting in a skirtlike spiral formation with four phenyl groups protruding at the other end of the cation. As a consequence of the positioning of the phenyl substituents, Au3 is in a more exposed position than Au1.

Figure 4. The solvent accessible void spaces outlined with ochre contours in [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, as calculated by Mercury 3.9. The drawing shows two unit cells stacked on c-axis, viewed along the a-axis. Total void volume: 956.39 Å3 or 10.3% total crystal volume.

Crystals of [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, are very sensitive and fragment almost immediately upon removal from their mother liquor. The fragility of crystals of [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, can be traced to the presence of large voids within the crystal. These voids were likely to have been filled with volatile dichloromethane solvate molecules, which escaped before the X-ray diffraction data were collected.36 Figure 4 presents a drawing that shows the locations of the voids within [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B. Another view of the voids is given in Figure SI-1. For comparison crystals of [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B, and [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, have less volatile toluene molecules in somewhat similar positions between the cation and anions. Figures SI-2 and SI-3 show drawings of the voids that would be present if the toluene molecules outside the Box were removed from the structures of [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B, and [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5) ), 3B, respectively. These hypothetical voids are similar in size and positioning to the voids in [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5).

Crystallization of the green-emitting initial product used to form the Box, [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), from dichloromethane and methanol produces yellow crystals with a green cast. These contain the Bridged-Helicate, [µ-Cl {Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H. Figure 6 shows the structure of the Bridged-Helicate cation as determined by single crystal X-ray diffraction. Some relevant distances are given in Table 2. The cation consists of two of the previously seen Helicate cations, [Au3(Triphos)2]3+,30 bridged by a chloride ion. This chloride ion is coordinated to one of the terminal gold ions in each Helicate cation, but the Au-Cl distances are quite long: 2.897(5) and 2.875(6) Å. A survey of the Cambridge Structural Database indicates that most Au-Cl distances fall in the 2.45 – 2.7 Å range. 40 The Au-Cl-Au angle is nearly linear: 174.3(2) °. Note that the bridging chloride ligand in each half of the hexanuclear cation is attached to gold ions that are surrounded by only two phenyl rings, i.e. the more exposed gold ions in the individual Helicate units.

Characterization of the Helicate, [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, and the Bridged-Helicate, [µ-Cl {Au3(Triphos)2}2](PF6)5•3CH3OH Previously, Schuh, et al. used Triphos to make the Helicate salt, [Au3(Triphos)2](CF3SO3)3•3MeOH•2H2O, through the reaction of the ligand with (dimethyl sulfide)gold(I)chloride followed by the addition of thallium triflate but did not report its luminescence.37 Colorless crystals of a different solvate, [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, 4H, were obtained by recrystallization of the complex prepared by a modification of procedure of Schuh, et. al by diffusion of toluene into a dichloromethane solution of the complex. The process is outlined in Scheme 1. Since the nature of the solvate molecules has been shown to influence the structure of other gold complexes,38,39 we pursued a crystallographic study of the new solvate. However, the structure of the cation in our new salt is virtually identical to that in the methanol solvate as can be seen from the data in Table 2 and the drawing of the cation shown in Figure 5. The three gold ions form a nearly

Figure 5. The structure of the Helicate cation in [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, 4H. Thermal contours displayed at the 50% probability level. Hydrogen atoms omitted for clarity.


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Table 2. Selected Bond Distances (Å) and Angles (deg) in Helicate Crystals Compound

[Au3(Triphos)2] (CF3SO3)3 a •3MeOH•2H2O

[Au3(Triphos)2] (CF3SO3)3• 4(CH3C6H5)•H2O 4H.

[µ-Cl {Au3(Triphos)2}2] (PF6)5•3CH3OH 5µ-H

Au1…Au2

2.9486(4)

3.0075(2)

2.9366(12)

Au2…Au3

2.9915(4)

2.95657(19)

2.9170(12)

Au4-Au5

2.9194(11)

Au5-Au6

2.9415(11)

Au3-Cl

2.897(5)

Au4-Cl

2.875(6)

Au1-Au2-Au3

179.09(1)

177.101(3)

Au4-Au5-Au6

174.24(4) 171.29(3)

P1-Au1-P4

177.79(15)

167.241(19)

168.1(2)

P2-Au2-P5

170.1(2)

175.220(19)

171.94(19)

P3-Au3-P6

173.15

174.91(2)

171.98(19)

P7-Au4-PP10

169.08(18)

P8-Au5-P11

172.19(19)

P9-Au6-P12

169.11(19)

Au2-Au3-X

122.57(12)

Cl-Au4-Au5

117.55(12)

P1-Au1-Au2-P2

43.77

46.78(2)

48.54(19)

P2-Au2-Au3-P3

50.66

52.39(2)

57.88(19)

P7-Au4-Au5-P8

48.98(18)

P9-Au5-Au6-P10

49.79(18)

a

Data from Schuh, W.; Kopacka, H.; Wurst, K.; Peringer, P. Chem. Commun. 2001, 2186–2187.

Figure 6. Structure of Bridged-Helicate cation [µCl{Au3(Triphos)2}2]5+ in [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H. Hydrogen atoms and hexafluorophosphate anions omitted for clarity. Thermal contours are shown at the 50% probability level.

The individual Helicate units in Bridged-Helicate cation [µCl{Au3(Triphos)2}2]5+ are similar to one another and similar to the Helicate itself in [Au3(Triphos)2](CF3SO3)3 •4(CH3C6H5)•H2O, 4H, or [Au3(Triphos)2](CF3SO3)3•3MeOH•2H2O as seen in the data in Table 2. The individual Helicate groups are bound to the bridging chloride through the Au3 and Au4, respectively. These two gold ions are situated at the ends of the Helicates where two phenyl groups extend nearly parallel with the Au3 chain. Thus Au3 and Au4 in Figure 6 are equivalent to Au3 in Figure 5. Finally, it is important to note that the two cations, the Box ([Au6(Triphos)4Cl]5+) and the BridgedHelicate ([µ-Cl-{Au3(Triphos)2}2]5+), are structural isomers of one another. Emission Spectra and Mechanochromic Behavior of the Molecular Boxes in [AuI6(Triphos)4Cl](EF6)5•n(CH3C6H5). (E = Sb (n = 7), As (n = 8), P (n = 2)). The ligand, Triphos, is not luminescent in either crystalline form or in dichloromethane solution. However, crystals of the Helicate salt, [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, and the Bridged-Helicate salt, [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H, are luminescent. The Helicate salt, [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, 4H, produces blue luminescence while the Bridged-Helicate salt, [µCl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H, displays a green luminescence. Relevant spectra are shown in Figure 7. The gold(I) ions in both the Helicate and Bridge-Helicate Cations are involved in aurophilic interactions. Consequently, the excitation involved for each is probably due to a transition from the filled, anti-bonding molecular orbital comprised of dz2 orbitals that are directed along the Au-Au-Au axis to the corresponding empty, bonding molecular orbital comprised of pz orbitals. The two complexes show similar excitation spectra, which is consistent with this assignment. However, the emission shifts significantly in the presence of the chloride ion that bridges two gold(I) ions. The lifetimes of the emission for the two complexes (4H, 11 ms; 5µH, 14 ms) indicate that the emissions arise from phosphorescence. The change in emission appears to result from the addition of a bridging chloride ion that may alter the nature of the excited state. Previous observations on the emission spectra of three-coordinate gold(I) complexes indicate that a T-shaped excited state is favored.41,42 Thus, it has been suggested that 2-coordinate gold(I) complexes can form an exciplex with a somewhat distant halide ligand (Au…Cl distance 3.224 Å) upon photo-excitation. 43 A similar situation seems to be relevant here. Crystals of the three salts Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, [Au6(Triphos)4Cl](AsF6)5 •8(CH3C6H5), 2B, and [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, - display blue emission that is readily detected by the human eye when illuminated with a UV lamp. Figure 8 shows the emission and excitation spectra obtained from crystals of 2B, and [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5) , 3B, at room temperature. The emission lifetime for 3B is 7 µs, which indicates that the complex is phosphorescent. Why are these Boxes luminescent? The ligand Triphos is not luminescent. Two coordinate gold(I) complexes are usually not luminescent unless there is an aurophilic interaction between two or more gold centers. 44 The distances between the gold centers in the Box are all rather long, and consequently there are no aurophilic interactions. Threecoordinate gold complexes are usually luminescent,31-46 but the



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Au…Cl distances in these Boxes are quite long (3.49 – 3.51) Å. However, it has been suggested that a non-coordinated chloride that is 3.224 Å away from the gold(I) ion in the linear complex, [Au-(1,3,5-triaza-7-phosphaadamantane)2]Cl, is involved in exciplex formation and the photoluminescence of that compound.41-43 A similar formation of an exciplex with the central chloride ion in the Box cations may be responsible for the emission of the salts. Alternatively, the excitation may involve a metal-to-phosphine ligand charge transfer with the emission coming from the triplet state of the reverse process as seen in some related gold(I) complexes.47 The luminescence of these crystals changes when the crystals are ground with a mortar and pestle. Figure 9 shows photographs of crystals of [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B, and [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, taken under UV light before and after grinding. To the eye, the luminescence changes from blue to green, although the photograph does not pick up the green color well. Figure 10 shows the excitation and emission spectra for a sample of [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B before and after grinding at room temperature. After grinding, the samples do not show powder X-ray diffraction. Consequently, they appear to be amorphous. The excitation and broad emission seen after grinding corresponds to that of the Bridged-Helicate salt, [µ-Cl{Au3(Triphos)2}2](PF6)5, 5µ-H, shown in Trace B of Figure 7. The peak at 480 nm results from incomplete grinding that leaves some of the Box, 3B, intact.

Figure 8. The excitation (left) and emission (right) spectra of crystals of: A, [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, and B, [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B, and C, [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, at 22 °C.

Figure 9. Photographs of crystals of the Boxes [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B, and [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B, before (top) and after (bottom) grinding at 22 °C.

Figure 7. The excitation (left) and emission (right) spectra at 22 °C of crystals of: A, the Helicate salt, [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, 4H, and B, the Bridged-Helicate salt, [µCl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H.

Figure 10. The excitation (left) and emission (right) spectra at 22 °C of crystals of [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5). A, before grinding and B, after grinding.


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As noted previously, crystals of [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, are very sensitive to their environment. They fragment almost immediately upon removal from their mother liquor to produce a green-glowing powder. Relevant emission spectra are shown in Figure 11. Trace A shows the excitation and emission spectra taken at room temperature of the Box [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, while Trace B shows the emission and excitation spectra of these crystals after grinding. Trace C shows the emission and excitation spectra of crystals of the Bridged-Helicate [µCl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H. The similarity of Traces B and C indicate that both samples contain the BridgedHelicate cation, [µ-Cl{Au3(Triphos)2}2]5+. Finally, Trace D shows the emission and excitation spectra of crude powder from which the crystalline Box [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, or the Bridged-Helicate [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µH. can be obtained depending upon crystallization conditions. It appears the Bridged-Helicate cation, [µ-Cl{Au3(Triphos)2}2]5+, is the major species present before recrystallization.

Salts containing the Box are sensitive to mechanical grinding as seen in Figure 9. This process converts the Box ([Au6(Triphos)4Cl]5+) into the Bridged-Helicate Helicate ([mCl{Au3(Triphos)2}2]5+). We suggest that the open structure of the Box in the solid state renders it less stable than the compact structure of the Helicate with its added aurophilic interactions. At first sight, this process seems rather complex, but there is a simple and direct way to convert the two structures. If the Au2AP5 and Au2-P5A bonds are broken in the Box and reconnected so that Au2 is attached to P5 and Au2A is attached to P5A, then the Triphos/gold network of the Box is converted into two Helicates, as shown in Scheme 2. During the process as proposed here, the gold(I) ions remain two-coordinate throughout the transformation with the chloride ion replacing a phosphine ligand in the initial stage of reaction. Scheme 2. A proposed mechanism for the mechanochromic conversion of the Box into the Bridged-Helicate.

Figure 11. The excitation (left) and emission (right) spectra taken at 22 °C of: A, crystals of the Box [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B; B, crushed crystals of [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5) ), 1B; C, crystals of the Bridged-Helicate [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H; D, crude powder from which crystalline Box Bridged-Helicate [µ[Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B, or Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H, can be obtained depending upon crystallization conditions.

Conclusion The chemistry reported here is summarized in Scheme 1. The reaction of Triphos with (tht)AuCl yields the simple Helicate when thallium triflate is used to remove all chloride ion from the product. However, if chloride ion is present, the reaction of Triphos with (tht)AuCl yields a powder that can be converted into salts containing the Box ([Au6(Triphos)4Cl]5+), when crystallized from dichloromethane/toluene. Toluene seems to specifically stabilize this geometric arrangement since two toluene molecules protrude into the ends of the Box as seen in Figure 2. Alternatively, when the same powder is crystallized from dichloromethane/methanol the Bridged-Helicate ([µ-Cl {Au3(Triphos)2}2]5+) is obtained. The Box ([Au6(Triphos)4Cl]5+) and the Bridged-Helicate ([µ-Cl{Au3(Triphos)2}2]5+) are structural isomers. Salts of the Bridged-Helicate can be converted into salts of the Box by recrystallization from dichloromethane/toluene.



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The work described here demonstrates that a non-luminescent triphosphine ligand can be used to create a luminescent container cation that involves six gold(I) ions. Further utilization of polyphosphine ligands in the construction of molecular containers is anticipated. It remains to be seen whether this particular Box can contain other ions and molecules and how the encapsulated entities might alter the luminescence and stability of this molecular container. Our studies also demonstrate the subtle interplay of solvate molecules, anions, and crystal voids contributes to the mechanical properties of this sort of container assembly.

EXPERIMENTAL SECTION Materials. Chloro(tetrahydrothiophene) gold(I), (tht)AuICl), was prepared according to previous literature methods. 48 Bis(2diphenylphosphinoethyl)phenylphosphine (Triphos), sodium hexafluoroarsenate, 1,2-thiodiethanol, methanol, toluene, and dichloromethane were purchased from Sigma-Aldrich Co. LLC. Chloroauric acid, sodium hexafluoroantimonate, and thallium triflate purchased from Strem Chemicals, Inc. Ammonium hexafluorophosphate purchased from Alfa Aeser, Inc. Potassium bromide purchased from Mallinckrodt, Inc. Ethanol purchased from Decon Labs, Inc. Solids were used as received. Where noted, solvents were dried over molecular sieves, otherwise they were used as received.

Synthesis of the Gold Box [(Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B. A 50.0 mg (0.156 mmol) portion of (tht)AuCl was dissolved in 5 mL of dry dichloromethane. To this is added 55.9 mg (0.105 mmol) of Triphos, resulting in a clear solution, which was allowed to stir for approximately 30 minutes. Separately, a solution of sodium hexafluoroantimonate was prepared consisting of 218.6 mg (0.845 mmol) dissolved in 5 mL of ethanol, then added to the dichloromethane solution and allowed to stir for 1 hour. The container was left to evaporate with an air stream, leaving a powdery white solid that displayed strong green luminescence when placed under ultraviolet light. The product was collected, dissolved in dichloromethane and filtered to remove the excess salts. The supernatant liquid was evaporated to isolate the product (94.0 mg, 39.9% yield). This product was dissolved in dichloromethane and recrystallized by slow diffusion of toluene. Colorless needles with a blue luminescence formed. Vibrational Spectroscopy (cm-1): 3055(w), 1580(w), 1480(w), 1435(m), 1409(w), 1310(w), 1186(w), 1108(m), 996(w), 892(w), 729(s), 690(s), 650(vs), 514(s), 474(s). Synthesis of the Gold Box [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B. A 56.9 mg (0.177 mmol) portion of (tht)AuCl was dissolved in 10 mL of dichloromethane. Separately, 63.5 mg (0.119 mmol) of Triphos was dissolved in 5 mL of dichloromethane, then added to the (tht)AuCl solution and stirred for 30 minutes. During this time, 202.9 mg (0.958 mmol) of sodium hexafluoroarsenate was dissolved in 5 mL of ethanol. The ethanolic salt solution was added, and the mixture was allowed to stir for another 1 hour. After this, an air stream was used to aid in the evaporation of the solution before drying in vacuo. Once dry, the white solid was collected, dissolved in dichloromethane, and filtered to remove the excess salts. The solvent was evaporated to yield 108.0 mg (42.9%) of a colorless powder with a green luminescence. This product was then dissolved into dichloromethane and crystallized by slow

diffusion of toluene to form colorless needles with a blue luminescence. Vibrational Spectroscopy (cm-1): 3055(w), 2959(w), 1583(w), 1482(w), 1435(m), 1311(w), 1186(m), 1108(m), 1027(w), 998(w), 892(w), 818(w), 730(s), 685(vs), 515(s), 474(s), 388(s).

Synthesis of the Gold Box [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B. A 52.0 mg (0.162 mmol) portion of (tht)AuCl was measured and dissolved into 10 mL of dichloromethane. Separately, a 57.8 mg (0.108 mmol) portion of Triphos was measured and dissolved into 5 mL of dichloromethane, then added to the (tht)AuCl solution. This solution was allowed to stir for 30 minutes, during which time 224.3 mg (1.376 mmol) of ammonium hexafluorophosphate was dissolved into 5 mL of methanol via sonication. Then, the methanolic salt solution was added to the dichloromethane solution, resulting in an immediate cloudiness. This suspension was allowed to stir for 1 hour and then was evaporated under a stream of air. Once dry, the resulting solid was collected, washed with dichloromethane to extract the product, and filtered to remove excess salts. The dichloromethane solution was dried to isolate the product (23.1 mg, 21.0% yield) as a colorless powder with green luminescence. This powder was dissolved in dichloromethane and crystallized by slow diffusion of toluene, which resulted in the formation of colorless needles with a blue luminescence. Vibrational Spectroscopy (cm-1): 3057(w), 2931(w), 1581(w), 1480(w), 1435(m), 1310(w), 1277(w), 1187(w), 1108(m), 998(w), 829(vs), 727(s), 684(s), 553(s), 511(m), 475(s) Synthesis of the Helicate [Au3(Triphos)2](CF3SO3)3• 4(CH3C6H5)•H2O, 4H. (Synthesis modified from Schuh, et. al.30) A 51.5 mg (0.161 mmol) portion of (tht)AuCl was placed in 20 mL dried methanol. To this, 57.4 mg (0.107 mmol) of Triphos was added directly under high agitation to form a colorless solution. Separately, 59.0 mg (0.167 mmol) of thallium triflate was dissolved into 10 mL dried methanol and then added to the original solution. This addition resulted in the immediate formation of a thick white precipitate of thallium chloride as well as a change in the solution to a yellow color. This suspension was stirred for an additional 20 minutes and then filtered to remove thallium chloride, which was washed twice with 10 mL of methanol. The combined filtrate was allowed to evaporate. A yellowish solid with a blue luminescence under UV light formed after the sample evaporated to dryness (66.7 mg, 59.2 % yield). Crystallization was achieved by dissolving the solid in dichloromethane and allowing toluene to slowly diffuse into the dichloromethane solution. Faint yellow crystals of [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O with blue luminescence were obtained. Vibrational Spectroscopy (cm-1): 3055(w), 2904(w), 1695(w), 1626(w), 1585(w), 1483(w), 1436(m), 1402(m), 1274(s), 1250(s), 1221(s), 1155(vs), 1100(s), 1071(m), 1025(vs), 998(m), 919(m), 865(w), 787(m), 757(m), 740(m), 717(m), 689(vs), 634(vs), 571(m), 514(vs), 497(s) 425(w), 394(w).

Synthesis of the Bridged Helicate [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H. A 47.5 mg portion of (tht)AuCl (0.148 mmol) was dissolved into 5 mL of dry dichloromethane. Separately, 53.7 mg of Triphos (0.101 mmol) was dissolved into 5 mL of dry dichloromethane, then added slowly to the (tht)AuCl solution while stirring for 30 minutes. In 5 mL of dry ethanol, 106.6 mg of ammonium hexafluorophosphate (0.654 mmol) was dissolved and slowly added to the reaction mixture, which displayed


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temporary turbidity during the process. After one hour, agitation ceased and the solution was allowed to dry. Once dry, solid was dissolved in dichloromethane and filtered to remove excess salts. The product (76.0 mg, 75.5%) was a white solid that displayed strong green luminescence upon exposure to ultraviolet light. This white solid was re-dissolved into dichloromethane and crystallized by slow diffusion of methanol, resulting in the formation of green plates that displayed green luminescence similar to that of the crude product. Vibrational Spectroscopy (cm-1): 3055(vw), 2961(vw), 1618(w), 1580(w), 1482(w), 1435(m), 1404(w), 1186(w), 1102(m), 999(w), 833(vs), 742(s), 691(s), 555(s), 516(s). Crystal Data for the Gold Box [(Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B. [C136H132Au6P6Cl](SbF6)5(C7H8)7: M = 5179.07 g/mol, colorless needle, 0.392 × 0.098 × 0.084 mm, λ = 0.71073 Å (Mo Kα), monoclinic, space group C2/c (no. 15), a = 35.3414(19), b = 33.4913(16), c = 16.0576(8) Å, α = 90°, β = 92.3220(10)°, γ = 90°, V = 18990.6(17) Å3, Z = 4, T = 90(2) K, 17376 reflections measured, 14326 unique which were used in all calculations, Bruker Apex-II; 2θmax = 50.720°; min/max transmission = 0.277 / 0.429 (multi-scan absorption correction applied); direct and Patterson methods solution; full-matrix least squares based on F2 (SHELXT and SHELXL-2014/7).49,50 The final wR(F2) was 0.0922 (all data), conventional R1 = 0.0387 computed for 14326 reflections with I > 2σ (I) using 960 parameters with 43 restraints. Crystal Data for the Gold Box [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B. [C136H132Au6P6Cl](AsF6)5-(C7H8)8: M = 5137.01 g/mol, colorless needle, 0.350 × 0.129 × 0.078 mm, λ = 0.71073 Å (Mo Kα), monoclinic, space group C2/c (no. 15), a = 35.454(7), b = 33.300(6), c = 15.960(3) Å, α = 90°, β = 92.846(2)°, γ = 90°, V = 18819(6) Å3, Z = 4, T = 90(2) K, 17345 reflections measured, 14015 unique which were used in all calculations, Bruker Apex-II; 2θmax = 50.848°; min/max transmission = 0.003 / 0.020 (multi-scan absorption correction applied); direct and Patterson methods solution; full-matrix least squares based on F2 (SHELXT and SHELXL-2014/7).44,45 The final wR(F2) was 0.0999 (all data), conventional R1 = 0.0431 computed for 14015 reflections with I > 2σ (I) using 937 parameters with 6 restraints. Crystal Data for the Gold Box [Au6(Triphos)4Cl](PF6)5•2(CH3C6H5), 1B. [C136H132Au6P6Cl](PF6)5(C7H8)2: M = 4264.41 g/mol, colorless plate, 0.050 × 0.050 × 0.010 mm, λ = 0.77490 Å (synchotron), monoclinic, space group C2/m (no. 12), a = 15.7952(19), b = 33.133(4), c = 19.067(2) Å, α = 90°, β = 111.302(2)°, γ = 90°, V = 9296.8(19)Å3, Z = 2, T = 100(2) K, 13890 reflections measured, 11696 unique which were used in all calculations, Synchrotron; 2θmax = 60.422°; min/max transmission = 0.272 / 0.436 (multi-scan absorption correction applied); direct and Patterson methods solution; full-matrix least squares based on F2 (SHELXT and SHELXL-2014/7).44,45 The final wR(F2) was 0.1610 (all data), conventional R1 = 0.0543 computed for 11696 reflections with I > 2σ (I) using 487 parameters with 186 restraints.

Crystal Data for the Gold Helicate [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, 4H. [C68H66Au3P6](F3CSO3) •4(C7H8)•H2O: M = 2493.70 g/mol, colorless needle, 0.564 × 0.266 × 0.222 mm, λ = 0.71073 Å (Mo Kα), triclinic, space group P1 (no. 2), a = 13.9411(7), b = 16.4211(8), c = 21.984(2) Å, α = 101.193(2)°, β =

98.197(2)°, γ = 96.808(2)°, V = 4830.9(6) Å3, Z = 2, T = 90(2) K, 81545 reflections measured, 28635 unique which were used in all calculations, Bruker Apex-II; 2θmax = 62.914°; min/max transmission = 0.173 / 0.419 (multi-scan absorption correction applied); direct and Patterson methods solution; full-matrix least squares based on F2 (SHELXT and SHELXL-2014/7).44,45 The final wR(F2) was 0.0549 (all data), conventional R1 = 0.0228 computed for 28635 reflections with I > 2σ (I) using 1229 parameters with 9 restraints. Crystal Data for the Bridges Helicate [(Triphos)2Au3]2(µ5µ-H. (µ-Cl)[C68H66Au3P6]2(PF6)6Cl)(PF6)5-(CH3OH)4, •4(CH3OH): M = 4164.18 g/mol, colorless block, 0.314 × 0.230 × 0.196 mm, λ = 0.71073 Å (Mo Kα), monoclinic, space group Cc (no. 9), a = 25.058(2), b = 13.8613(13), c = 42.523(4) Å, α = 90°, β = 91.1580(13)°, γ = 90°, V = 14767(2) Å3, Z = 4, T = 90(2) K, 46672 reflections measured, 43062 unique which were used in all calculations, Bruker Apex-II; 2θmax = 61.998°; min/max transmission = 0.449 / 0.746 (multi-scan absorption correction applied); direct and Patterson methods solution; full-matrix least squares based on F2 (SHELXT and SHELXL-2014/7).44,45 The final wR(F2) was 0.1949 (all data), conventional R1 = 0.0798 computed for 43062 reflections with I > 2σ (I) using 1146 parameters with 74 restraints.

ASSOCIATED CONTENT Supporting Information. Space-filling drawings of the structures of 1B and 2B and X-ray crystallographic files in CIF format for [Au6(Triphos)4Cl](SbF6)5•7(CH3C6H5), 3B; [Au6(Triphos)4Cl](AsF6)5•8(CH3C6H5), 2B; [Au6(Triphos)4Cl](PF6)•2(CH3C6H5), 1B; [Au3(Triphos)2](CF3SO3)3•4(CH3C6H5)•H2O, 4H; and [µ-Cl{Au3(Triphos)2}2](PF6)5•3CH3OH, 5µ-H. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected], [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the National Science Foundation (Grant CHE-1305125 to ALB and MMO) for support. We thank the Advanced Light Source, supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231, for beam time. We also thank the National Science Foundation (Grant CHE-1531193) for the Dual source X-ray diffractometer.

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(50) Sheldrick, G. Acta Crystallogr., Sec. C 2015, 71, 3-8. Crystal structure refinement with SHELXL.

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