Inorganic topochemistry. Vapor-induced solid state transformations of luminescent, three-coordinate gold(I) complexes

Abstract

The solid state interconversions of four different crystalline compounds containing the Au₂(dppe)₂I₂ unit (dppe is bis-(diphenylphosphino)ethane) have been observed and characterized through X-ray diffraction and emission spectroscopy. Treatment of AuI in acetone with solid dppe yields the crystalline polymorphs: α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) with orange emission and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) with green emission. Both polymorphs contain dimeric molecules with three-coordinate gold(I) centers that are further apart in the orange emitting (1) (Au⋯Au distance, 3.6720(2) Å) than in the green emitting (2) (Au⋯Au distance, 3.3955(2) Å). The acetone molecules do not bond to the gold centers in either polymorph. Crystals of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) undergo reversible, single-crystal to single-crystal transformations. Exposure of (2) to acetone vapor converts it into (1), while (1) is converted into (2) by exposure to air for a short time. Upon loss of acetone, α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) are converted into a microcrystalline powder (3) with a green emission. Remarkably, exposure of (3) to acetone vapor converts it into acetone-free Au₂(μ-dppe)₂(μ-I)₂ (4), which displays orange emission. The X-ray crystal structure of Au₂(μ-dppe)₂(μ-I)₂ (4) shows that each gold is in a highly distorted tetrahedral environment with bonds to two phosphorus and two iodine atoms. The transformations between these four types of crystals are notable because of the unusual role of vapors in promoting structural changes within solids that do not necessarily change composition. Thus, the reversible interconversion of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) occurs as vapor-stimulated single-crystal-to-single-crystal transformation between polymorphs. Likewise, the irreversible transformation of microcrystalline (3) into Au₂(μ-dppe)₂(μ-I)₂ (4) appears to be another case of a transformation of one polymorph into another.

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Introduction

Crystals are generally considered as fairly static, ordered arrays of atoms that provide us with some of the most detailed structural information on molecules, ionic solids and metals that can be obtained. However, it is also possible to observe structural changes and even chemical transformations within crystals. In regard to such topochemical changes, it has been postulated that reactions in crystals occur with a minimal amount of atomic movement.¹ Generally, an appropriate stimulus is needed to instigate reactions in crystals. For example, ultraviolet irradiation can induce [2 + 2] cycloadditions between olefins in molecular crystals.^2–7 Additionally, interconversions of polymorphs with attendant structural changes can be induced by thermal treatment of appropriate crystals.⁸ Application of mechanical pressure on certain crystalline metal complexes can produce marked changes in their luminescence, color and structure.^9–11

Exposure of a number of crystalline substances to vapors of water and/or volatile organic compounds can lead to alteration and sometimes destruction of the crystals. Usually, such processes are accompanied by the uptake of the vapor into the solid. For example, solid desiccants function by absorbing water. Solid anhydrous calcium sulfate (Drierite) forms the hemi-hydrate, CaSO₄·0.5H₂O, while crystalline P₄O₁₀ is converted into liquid phosphoric acid upon exposure to water vapor.¹² Extensive efforts are being made to develop crystalline solids with persistent voids from organic or metal–organic frameworks that can be used for the uptake of various gases.^13,14 Solid carboxylic acids will react with vapors of ammonia and other amines to form crystalline salts, again with uptake of the vapor.^15,16 The magnetic properties of the spin-crossover iron complex, Fe(tris(2-pyridylmethyl)amine)(NCS)₂, can be altered by the absorption of methanol vapor.¹⁷ Certain transition metal complexes, particularly those with metallophilic interactions between metal centers with d⁸ and d¹⁰ electronic configurations,¹⁸ change their luminescence or color in response to exposure to vapors of volatile organic compounds. For example, the orange form of cis-(p-EtC₆H₄CN)₂Pt(CN)₂ absorbs vapors of aromatic hydrocarbons like toluene to form yellow crystals of cis-(p-EtC₆H₄CN)₂Pt(CN)₂·0.5(toluene), a process that alters the interactions between the platinum ions in the solid.¹⁹ Similarly a platinum terpyridine complex undergoes a reversible red–orange luminescence change upon uptake and loss of methanol, which again alters the Pt⋯Pt separations.²⁰ Metal chain complexes such as {Tl[Au(C₆Cl₅)₂]}_n are vapochromic due to coordination of the volatile compounds to the metal ions in the chain that alter the interactions between the metal ions.^21–25 Such vapoluminescent and vapochromic materials have the potential to act as sensors for various volatile molecules.^26,27

Here, we report the discovery of some luminescent gold complexes whose crystals undergo changes in luminescence and structure in response to exposure to organic vapors without the uptake of the vapor itself. This work utilizes the flexible, accordion-like structure of the Au₂(μ-dppe)₂X₂ unit (dppe is bis-(diphenylphosphino)ethane).²⁸ A number of other smaller bite diphosphines such as bis-(diphenylphosphino)methane (dppm) can position gold atoms in close proximity,²⁹ but dppe shows a degree of flexibility in accommodating different Au⋯Au separations that is unusual. Previously, we documented the vapoluminescent behavior of crystals containing the dimer, Au₂(μ-dppe)₂Br₂, and described processes that required the uptake and loss of dichloromethane or acetone. That work revealed that Au₂(μ-dppe)₂Br₂ has the flexibility to accommodate Au⋯Au separations that range from 3.8479(3) to 3.0995(10) Å.²⁰ Variations in the Au⋯Au separations signal changes in the aurophilic interactions between the metal centers³⁰ and are well known to produce alterations in the luminescence of gold(I) complexes.^31,32

Results and discussion

Structures and reversible interconversions of the polymorphs α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2)

Treatment of a suspension of AuI in acetone with solid dppe resulted in the formation of a colorless solution, which was filtered and evaporated to dryness. The resulting solid was redissolved in a minimum volume of acetone. The solution was transferred to a 5 mm diameter glass tube and layered with diethyl ether. Colorless crystals of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1), which have an orange emission (emission, λ_em, 607 nm, excitation, λ_ex, 364 nm) grew along with colorless crystals of β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2), which have a green emission (λ_em, 577 nm, excitation, λ_ex, 368 nm). Thus, these crystals are concomitant polymorphs.³³

The structures of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2), are shown in Fig. 1 and 2, respectively. Crystal data are given in Table 1. In both cases, the dimeric Au₂(μ-dppe)₂I₂ molecule packs about a center of symmetry so that one half of the dimer resides in the asymmetric unit. The gold centers are three-coordinate with bonds to two phosphorus atoms of two different bridging ligands and to the terminal iodide ligand. The acetone molecules do not bond to the gold centers in either polymorph. The major differences between the two structures involve the Au⋯Au distances which are 3.6720(2) Å in the orange-glowing α-polymorph and significantly shorter, 3.3955(2) Å, in the green-glowing β-polymorph. At the shorter distance in (2), aurophilic interactions become significant and alter the luminescence of the complex. Other differences between the polymorphs include a lengthening of the Au–I distance in the β-polymorph, changes in the orientations of the phenyl rings and shifts in the positions of the acetone molecules. The structure and flexibility of Au₂(μ-dppe)₂I₂ parallels that of Au₂(μ-dppe)₂Br₂, which was reported earlier.²⁰

Fig. 1 A drawing of the structure of orange-glowing, α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1). Thermal ellipsoids are shown at the 50% probability level. The Au1⋯Au1A separation is 3.6720(2) Å. Other distances; Au1–I1, 2.9106(2); Au1–P1, 2.3157(6); Au1–P2, 2.3118(6) Å.

Fig. 2 A drawing of the structure of green-glowing β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2). Thermal ellipsoids are shown at the 50% probability level. The Au1⋯Au1A separation is 3.3955(2) Å, which is shorter than the corresponding separation (3.6720(2) Å) in the α-polymorph (I). Other distances; Au1–I1, 2.97590(18); Au1–P1, 2.3143(5); Au1–P2, 2.3177(5) Å.

Table 1 Crystal data and data collection parameters

	α-Au2(μ-dppe)2I2·2OCMe2 (1)	β-Au2(μ-dppe)2I2·2OCMe2 (2)	Au2(μ-dppe)2(μ-I)2 (4)
a For data with I > 2σ(I): R1 = ∑||Fo| − |Fc||/∑|Fo|. b For all data: wR2 = {∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]}^1/2.
Formula	C58H60Au2I2O2P4	C140H60N4NiSm2	C52H40Au2I2P4
Fw	1560.67	1560.67	1436.45
Color, habit	Colorless block	Colorless block	Colorless protopyramid
Crystal system	Monoclinic	Triclinic	Tetragonal
Space group	P21/c	P1̄	I41/a
a, Å	10.6771(3)	11.8300(3)	16.0859(4)
b, Å	22.8740(6)	11.9725(3)	16.0859(4)
c, Å	12.0196(3)	12.3591(4)	36.7500(9)
α, deg	90	65.098(2)	90
β, deg	105.824(2)	69.838(2)	97.638(2)
γ, deg	90	65.309(2)	90
V, Å^3	2824.28(13)	1411.99(7) Å	9509.3(4)
Z	2	1	8
ρ calcd, Mg m^−3	1.835	1.835	2.007
T, K	90(2)	90(2)	90(2)
Radiation (λ, Å)	Mo-Kα, 0.71073 Å	Mo-Kα, 0.71073 Å	Mo-Kα, 0.71073 Å
Unique data	8659 [R(int) = 0.035]	8630 [R(int) = 0.020]	5464 [R(int) = 0.046]
Parameters	309	309	137
Obsd (I > 2σ(I)) data	7303	7969	4428
R 1 ^a (obsd data)	0.034	0.019	0.039
wR2^b (all data)	0.047	0.041	0.095

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Remarkably, the α and β polymorphs of Au₂(μ-dppe)₂I₂·2OCMe₂ can be reversibly interconverted. Fig. 3 shows photographs of crystals of the α-polymorph of Au₂(μ-dppe)₂I₂·2OCMe₂ (1) under (a) ambient light and (b) under UV irradiation. Gentle drying of the crystals by simple exposure to air over a 30 minute period results in their transformation into the green-glowing β-polymorph as seen in (d). When these green-glowing crystals are exposed to acetone or dichloromethane vapor, they are converted back into the orange-glowing α-polymorph as seen in (c). As the photograph indicates these transformations are reproducible for an array of different crystals.

Fig. 3 Photographs of crystals of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) under (a) ambient light and (b) under UV irradiation. After the crystals are briefly allowed to stand in air, the green-emissive sample shown in (d) was obtained. Exposure of that green-glowing sample to acetone vapor restores the orange emission as shown in (c).

The interconversion of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) has been monitored by X-ray powder diffraction as shown in Fig. 4. Traces (A) and (B) show powder pattern computed from the single-crystal data and the experimental pattern for a sample of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1). After the sample stood in air until a uniform green emission was observed, the sample was exposed to acetone vapor and the powder pattern shown in (C) was obtained. This result indicates that α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) was reformed after the sample had been converted into the green-glowing β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2). Unfortunately, powder diffraction data could not be obtained for β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2), since the finely divided material rapidly lost crystallinity. However, as the next paragraph shows, larger individual crystals of β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) could be examined.

Fig. 4 X-ray powder diffraction showing the interconversion of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) and back to (1). (A) Pattern computed for α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) from single-crystal data. (B) Experimental data for α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1). (C) Experimental data for the sample that produced the data in trace B after standing in air until the sample produced green emission to form (2) followed by exposure to acetone vapor, which resulted in orange emission. Experimental data were collected with Cu Kα radiation at 90(2) K.

These transformations have also been monitored by single-crystal X-ray diffraction on an individual crystal. Initial diffraction data from a crystal of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) confirmed that the crystal was monoclinic, space group P2₁/c. After removal from the diffractometer and subsequent air exposure, the diffraction data indicated that a single-crystal-to-single-crystal transformation had occurred and that the crystal now belonged the triclinic space group P1̄, with cell parameters consistent with the transformation into β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2). Subsequently, the crystal was again removed from the diffractometer and exposed to acetone vapor as described in the Experimental section. After remounting of the crystal on the diffractometer, conversion of the crystal back into the original monoclinic P2₁/c form was observed. An entire new data set was acquired at this point, and its solution agreed with that obtained previously for the α-polymorph.

Irreversible solvate loss from α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2)

A second type of transformation occurs when crystals of the α- or β-polymorphs are allowed to stand in air for several hours. Under these conditions, the crystals crumble into a colorless powder of compound (3), which displays a green luminescence (λ_em, 570 nm, excitation, λ_ex, 395 nm). A comparison of the emission and excitation spectra of the three compounds, α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) and compound (3), is shown in Fig. 5. The process is accompanied by a loss of acetone from the crystals. The infrared spectrum of the α-polymorph shows a strong band at 1702 cm⁻¹ due to ν(C=O) from acetone. Those of the β-polymorph display a corresponding band at 1700 cm⁻¹. Upon standing in air, these prominent features disappear and are absent in the infrared spectrum of the colorless, green-luminescent powder of compound (3).

Fig. 5 Emission (solid lines) and excitation (dotted lines) spectra for: orange-glowing, α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) in orange; green-glowing β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) in light green, and the green-glowing, colorless powder (3) obtained by drying α-Au₂(μ-dppe)₂I₂·2OCMe₂ for three hours in air in dark green.

This second transformation has also been monitored by X-ray powder diffraction as shown in Fig. 6. Trace (A) shows the powder diffraction from a crushed sample of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1). This pattern agrees with the diffraction pattern computed from the single-crystal diffraction data as can be seen in Fig. 4. Upon prolonged exposure to air, the powder diffraction pattern seen in trace (B) of Fig. 6 is obtained. The sample, compound (3), shows evidence of crystallinity, but the observed diffraction data do not match those of either α- or β-Au₂(μ-dppe)₂I₂·2OCMe₂. Solvate-free compound (3) melts at 255–259 °C.

Fig. 6 X-ray powder diffraction from: (A) a sample of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1); (B) the same sample after standing in air; (C) the sample from (B) after exposure to acetone vapor. (D) The computed powder pattern obtained from the single-crystal diffraction data for solvate-free Au₂(μ-dppe)₂(μ-I)₂ (4). Experimental data were collected with Cu Kα radiation at 90(2) K.

Exposure of the green-glowing powder (3) to acetone vapor results in a third transformation into an orange-luminescent powder. Acetone is not incorporated into this powder since the infrared spectrum does not show a band at ca. 1700 cm⁻¹. The X-ray powder diffraction data for this form are shown in trace (C) of Fig. 4. The pattern is distinct from that of its immediate predecessor (trace (B)) and from those of either α- or β-Au₂(μ-dppe)₂I₂·2OCMe₂. However, the powder pattern does correspond to that of another crystalline complex, solvate-free Au₂(μ-dppe)₂(μ-I)₂ (4).

In separate experiments, uniquely shaped, bipyramidal crystals of Au₂(μ-dppe)₂(μ-I)₂ (4) shown in Fig. 7 were obtained by layering diethyl ether or n-pentane over a solution of Au(dppe)I in dichloromethane, acetone, chloroform, dimethylformamide or dimethyl sulfoxide. These crystals grew in the diethyl ether- or pentane-rich region near the upper part of the tube. Under UV irradiation, these colorless crystals produced an orange emission (λ_em, 602 nm, excitation, λ_ex, 380 nm).

Fig. 7 Photographs of crystals of solvate-free Au₂(μ-dppe)₂(μ-I)₂ (4) under (A) ambient light and (B) under UV light.

The structure of Au₂(μ-dppe)₂(μ-I)₂ (4) as determined by single-crystal X-ray diffraction is shown in Fig. 8. A crystallographic two-fold axis runs through I1 and I2. The gold ion is coordinated to two phosphorus atoms from different bridging ligands and by two iodide ions to form a highly distorted tetrahedral arrangement. The Au1⋯Au1A separation is 3.3926(5) Å. The Au–I distances (3.2116(5), 3.2030(5) Å) are nearly equivalent, but significantly longer than the corresponding distances (α, 2.9106(2); β, 2.97590(18) Å) in both polymorphs of three-coordinate Au₂(μ-dppe)₂I₂·2OCMe₂. In the related binuclear cation, [Au₂(μ-dppm)₂(μ-I)]⁺, the Au–I distances are 3.127(2) and 3.196(2) Å.³⁴ Nevertheless, the iodide ligands in Au₂(μ-dppe)₂(μ-I)₂ (4) must be considered bonded to the gold ions. The cation, [Au₂(μ-dppe)₂]²⁺, has been isolated as a salt with a non-coordinating anion and was found to have a much shorter Au⋯Au separation of 2.9220(3) Å and nearly linear P–Au–P angles.³⁵

Fig. 8 A drawing of the structure of orange-glowing Au₂(μ-dppe)₂(μ-I)₂ (4). The Au1⋯Au1A separation is 3.3926(5) Å. Other distances and angles are: Au1–I1, 3.2116(5); Au1-I2, 3.2030(5); Au1–P1, 2.3060(16); Au1–P2, 2.3042(16) Å; Au1–I1–Au1A, 63.767(14); Au1–I2–Au1, 63.958(14); P1–Au1–P2A, 163.14(6). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are not shown for clarity.

Crystals of Au₂(μ-dppe)₂(μ-I)₂ (4) do not show any changes upon standing in air and are not altered by exposure to acetone or dichloromethane vapor. Solvate-free crystals of (4) melt at 277–280 °C. The higher melting point of (4) relative to (3) suggests that (4) is more stable than (3), a situation compatible with the observation that (3) is converted into (4) through exposure to acetone vapor.

Conclusions

Scheme 1 summarizes the transformations among crystals that we have observed. The interconversions of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) polymorphs are reversible single-crystal-to-single-crystal transformations. Prolonged exposure of β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) to air results in the irreversible loss of acetone and conversion into microcrystalline (3). The detailed structure of (3) remains to be determined, but it is likely to contain the Au₂(μ-dppe)₂I₂ unit with a short Au⋯Au contact, since its luminescence resembles that of β-Au₂(μ-dppe)₂I₂ (2). Strangely, exposure of (3) to acetone or dichloromethane vapor results in its conversion into Au₂(μ-dppe)₂(μ-I)₂ (4) without uptake of the vapor. If the structure of (3) contains Au₂(μ-dppe)₂I₂ molecules with dimensions similar to those of β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2), then the iodide ligands must reposition themselves between the two gold ions to form the structure shown in Fig. 5. To accomplish this transformation, the Au1–I1 distance in (4) (2.97590(18) Å) must lengthen while the I1–Au1A distance (3.7144(2) Å) must shorten to produce Au₂(μ-dppe)₂(μ-I)₂ (4) with Au1–I1 and Au1–I2 distances of 3.2116(5) and 3.2030(5) Å, respectively. While the Au1⋯Au1A separations in β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) (3.3955(2) Å) and Au₂(μ-dppe)₂(μ-I)₂ (4) (3.3926(5) Å) are similar, the bridging diphosphine ligands must undergo some twisting to reach the arrangement found in Au₂(μ-dppe)₂(μ-I)₂ (4). Nevertheless, the key features in these solid state transformations reflect the ability of the bridging dppe ligand place the reacting centers close to one another and the flexibility of the Au₂(μ-dppe)₂ framework.

Scheme 1 Solid state transformations of crystalline gold(I) complexes.

The transformations between these four types of crystals are notable because of the unusual role of vapors in promoting structural changes within solids that do not necessarily change composition. Thus, the reversible interconversion of α-Au₂(μ-dppe)₂I₂·2OCMe₂ (1) and β-Au₂(μ-dppe)₂I₂·2OCMe₂ (2) occurs as vapor-stimulated single-crystal-to-single-crystal transformation between polymorphs. Likewise, the irreversible transformation of microcrystalline (3) into Au₂(μ-dppe)₂(μ-I)₂ (4) appears to be another case of a transformation of one polymorph into another.

At this stage, we have uncovered and documented some remarkable solid state transformations of the accordion-like dimer Au₂(μ-dppe)₂I₂. Additionally, we can offer some speculation in regard to how these transformations might occur. It appears likely that the vapor interacts with the surface of the crystals, perhaps by adsorption, to initiate the transformation. Finely ground samples of these crystals undergo these changes more rapidly than large crystals, an observation consistent with the idea that vapor may be transiently adsorbed on the surface of these crystals.

Three-coordinate gold(I) complexes, such as those encountered in this study, are frequently luminescent both in the solid state and in solution.^36–38 For such three-coordinate gold(I) compounds, the excitation process is generally attributed to a transition from the filled, in-plane d_x²⁻y²,d_xy orbitals on gold to the empty, out-of-plane p_z orbital. Emission occurs from a corresponding triplet state, which may undergo significant distortion for the ground state geometry.³⁹ Several recent experimental^40–42 and computational^33,43 studies have examined the various types of structural changes that occur in the excited states of these three-coordinate complexes. Thus, the different emission seen in (1) and (2) results from the differences in the P–Au–P bond angles, the differences in Au–I distances and the differences in proximity of the two gold ions. It is entirely likely the nature of the emissive state changes significantly depending upon whether there is or is not an aurophilic interaction between the two gold ions.

Clearly, the luminescence from these otherwise colorless crystals aided in our ability to identify the changes that are reported here. One wonders whether this sort of behavior might be more widespread and other vapor-related polymorphic phase changes might be going unnoticed in non-luminescent crystals. Control of polymorph formation and polymorph stability is frequently a challenging issue.^44–46 The phenomenon of disappearing polymorphs is one manifestation of these difficulties.⁴⁷ Vapor exposure of certain crystalline polymorphs may be an important issue in determining their apparent stability.

Experimental section

Preparation of α-Au₂(μ-dppe)₂I₂·2OCMe₂, β-Au₂(μ-dppe)₂I₂·2OCMe₂, and solvate-free Au₂(μ-dppe)₂(μ-I)₂

AuI (100 mg, 0.309 mmol) was suspended in 30 mL of acetone. While stirring, solid dppe 185 mg (0.463 mmol) was added. After additional stirring for 3 h, the solution was filtered, and then the solvent was removed in a vacuum. The white solid, Au(dppe)I, was collected and washed with diethyl ether. The yield was 180 mg (80.6%). Crystals suitable for X-ray structure determination were grown by slow diffusion of diethyl ether to a solution of the colorless solid in acetone in a 5 mm outside diameter glass tube. Colorless, orange and green emitting crystals of Au₂(μ-dppe)₂Br₂·2OCMe₂ appeared in the lower portion of the tube and were identified by their characteristic emission and external morphology. Distinctively shaped, colorless bipyramids of solvate-free Au₂(μ-dppe)₂(μ-I)₂ grew in the upper, diethyl ether-rich region of the tube. The individual types of crystals were separated manually using the differences in emission color, external morphology and spatial separation during crystal growth to identify the various phases. Crystals of solvate-free Au₂(μ-dppe)₂(μ-I)₂ could also be obtained from solutions of Au(dppe)I in dichloromethane, chloroform, dimethylformamide, and dimethyl sulfoxide that were layered with diethyl ether or pentane. In these cases the crystals of Au₂(μ-dppe)₂(μ-I)₂ always grew in the upper, diethyl ether- or pentane-rich region of the tubes.

Infrared spectra

 Orange emissive α-Au₂(μ-dppe)₂Br₂·2OCMe₂: 3050w, 3002w, 2903w, 1700vs (ν(C=O) from acetone), 1482m, 1435s, 1411m, 1224w, 1185m, 1120m, 1099m, 1070m, 1026m, 997w, 820w, 742s, 725m, 693s, 532m, 521m, 509s, 486m cm⁻¹.

Green emissive β-Au₂(μ-dppe)₂Br₂·2OCMe₂: 3051w, 3003w, 2905w, 1702vs (ν(C=O) from acetone), 1482m, 1435s, 1411m, 1307w, 1223m, 1184m, 1121m, 1099m, 1070m, 1026m, 998w, 871w, 820w, 742s, 725m, 693s, 532m, 521m, 510s, 486m cm⁻¹.

Orange emissive Au₂(μ-dppe)₂(μ-I)₂: 3069w, 3046w, 2927w, 2892w, 1478m, 1433s, 1098m, 1069w, 995w, 824m, 742m, 715w, 688s, 616m, 508s, 479m cm⁻¹.

Physical measurements

Infrared spectra were recorded on a Bruker ALPHA FT-IR spectrometer. Fluorescence excitation and emission spectra were recorded on a Perkin-Elmer LS50B luminescence spectrophotometer.

Interconversion of α-Au₂(μ-dppe)₂I₂·2OCMe₂ and β-Au₂(μ-dppe)₂I₂·2OCMe₂

Colorless, orange emitting crystals of α-Au₂(μ-dppe)₂I₂·2OCMe₂ were placed on a glass slide and allowed to stand in air for 30 min. During this period the crystals retained their clarity and shape, but were transformed into β-Au₂(μ-dppe)₂I₂·2OCMe₂ with green emission. The slide was then placed in a jar that contained a few drops of acetone that provide the vapor needed to convert α-Au₂(μ-dppe)₂I₂·2OCMe₂ into β-Au₂(μ-dppe)₂I₂·2OCMe₂.

Single-crystal X-ray crystallography and data collection

The crystals were removed from the glass tubes in which they were grown together with a small amount of mother liquor and immediately coated with a hydrocarbon oil on a microscope slide. A suitable crystal of each compound was mounted on a glass fiber with silicone grease and placed in the cold stream of a Bruker SMART Apex II CCD with graphite monochromated Mo Kα radiation at 90(2) K. Crystal data can be found in Table 1.

The structures were solved by direct methods and refined using all data (based on F²) using the software SHELX multi-scan method utilizing equivalents was employed to correct for absorptions.⁴⁸ Hydrogen atoms were added geometrically and refined with a riding model.

Crystal disorder in solvate-free Au₂(μ-dppe)₂(μ-I)₂

The structure has two kinds of disorder. There is a difference map peak that probably stems from rotation of the molecule by 90° and gives rise to an alternative Au⋯Au vector. The occupancy for the second orientation is only based on this one peak. Other atoms corresponding to this orientation are not likely to show up since the defect is only 3%. The positions of I1 and I2 are compatible with the alternative Au site. The refined occupancies for the two Au sites are: Au1/Au2 0.9688(7)/0.0312(7). The Au1⋯Au1A distance is 3.393 Å while that of Au2⋯Au2B is 3.504 Å. The second kind of disorder arises from rotational disorder of the phenyl rings bonded to P. Three of the four phenyl rings were modeled with a second orientation. Thus C1–C6 has disorder 0.707(11)/0.293(11), C15–C20 has disorder 0.571(12)/0.429(12) and C21–C26 has disorder 0.680(9)/0.320(9). In each case the six-membered ring was refined using AFIX 66 restraints. The carbon atoms were kept isotropic.

X-ray powder diffraction data collection

Each sample was finely ground with a mortar and pestle as quickly as possible and immediately mixed with minimum amount of hydrocarbon oil. This mixture was mounted on a polymer micro-loop and placed in the 90(2) K cold stream of a Bruker Apex Duo. Powder diffraction data were collected with the use of Cu-Kα radiation and the program PILOT.⁴⁹

Acknowledgements

We thank the Petroleum Research Fund (Grant 37056-AC to ALB) and the U. S. National Science Foundation [CHE-1011760 to ALB and MMO]

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files. CCDC 840562–840564. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2sc20820b

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