Steric effects and aurophilic interactions in crystals of Au2(μ-1,2-bis(diphenylphosphino)ethane)X2and Au2(μ-1,2-bis(dicyclohexylphosphino)ethane)X2(X?=?Cl, Br, I)

Article abstract of DOI:10.1016/j.poly.2016.06.031

Crystals in the isostructural series Au₂(μ-dppe)X₂(dppe is 1,2-bis(diphenylphosphino)ethane, X?=?Cl, Br, I) form centrosymmetric, dimeric structures with two short Au?Au contacts between the component molecules. We found no evidence for a second polymorph for any member of the Au₂(μ-dppe)X₂series, although two polymorphs were observed previously for the related Au₂(Ph₂As(CH₂)₂AsPh₂)X₂series. Crystals of Au₂(μ-dcpe)X₂(where dcpe is 1,2-bis(dicyclohexylphosphino)ethane) show structures that differ appreciably from those of Au₂(μ-dppe)X₂. The bulky cyclohexyl substituents appear to be playing a major role in eliminating the formation of polymeric or dimeric structures. Au₂(μ-dcpe)Cl₂has a monomeric structure without aurophilic interactions. In contrast, Au₂(μ-dcpe)Br₂and Au₂(μ-dcpe)I₂have similar structures with intramolecular aurophilic interactions. Au₂(μ-dcpe)I₂displays a form of aggregation-induced emission. It displays a red-pink luminescence in crystalline form but is non-emissive in solution.

Full text of DOI:10.1016/j.poly.2016.06.031

Accepted Manuscript
Steric Effects and Aurophilic Interactions in Crystals of Au2(μ-1,2-bis(diphe-
nylphosphino)ethane)X2 and Au2(μ-1,2-bis(dicyclohexylphosphino)ethane)X2
(X = Cl, Br, I)
Daniel T. Walters, Kellie R. England, Kamran B. Ghiassi, Fikerete Z. Semma,
Marilyn M. Olmstead, Alan L. Balch
PII:
S0277-5387(16)30265-0
DOI:
http://dx.doi.org/10.1016/j.poly.2016.06.031
POLY 12072
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
7 May 2016
20 June 2016
Please cite this article as: D.T. Walters, K.R. England, K.B. Ghiassi, F.Z. Semma, M.M. Olmstead, A.L. Balch,
Steric Effects and Aurophilic Interactions in Crystals of Au2(μ-1,2-bis(diphenylphosphino)ethane)X2 and
Au2(μ-1,2-bis(dicyclohexylphosphino)ethane)X2 (X = Cl, Br, I), Polyhedron (2016), doi: http://dx.doi.org/10.1016/
j.poly.2016.06.031
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Revised version
Steric Effects and Aurophilic Interactions in Crystals of Au2(µ-1,2-
bis(diphenylphosphino)ethane)X2 and Au2(µ-1,2-bis(dicyclohexylphosphino)ethane)X2 (X =
Cl, Br, I).
Daniel T. Walters, Kellie R. England, Kamran B. Ghiassi, Fikerete Z. Semma, Marilyn M.
Olmstead, and Alan L. Balch*
Department of Chemistry, University of California, Davis, California 95616, U. S. A.
E-mail, albalch@ucdavis.edu
1

Abstract
Crystals
in
the
isostructural
series
Au2(µ-dppe)X2
(dppe
is
1,2-
bis(diphenylphosphino)ethane, X = Cl, Br, I) form centrosymmetric, dimeric structures with two
short Au…Au contacts between the component molecules. We found no evidence for a second
polymorph for any member of the Au2(µ-dppe)X2 series, although two polymorphs were
observed previously for the related Au2(Ph2As(CH2)2AsPh2)X2 series. Crystals of Au2(µ-
dcpe)X2 (where dcpe is 1,2-bis(dicyclohexylphosphino)ethane) show structures that differ
appreciably from those of Au2(µ-dppe)X2. The bulky cyclohexyl substituents appear to be
playing a major role eliminating the formation of polymeric or dimeric structures. Au2(µ-
dcpe)Cl2 has a monomeric structure without aurophilic interactions. In contrast, Au2(µ-dcpe)Br2
and Au2(µ-dcpe)I2 have similar structures with intramolecular aurophilic interactions. Au2(µ-
dcpe)I2 displays a form of aggregation-induced emission. It displays a red-pink luminescence in
crystalline form but is non-emissive in solution.
2

1. Introduction
The solid-state organization of molecules of gold(I) complexes frequently reflects the
presence of aurophilic attractions between neighboring gold(I) centers. 1 These aurophilic
interactions arise from correlation and relativistic effects2,3 and result in attractive forces that are
similar in energy to hydrogen bonds.4,5 Such aurophilic bonding results in close contacts between
individual gold(I) centers with typical Au•••Au distances that are less than 3.6 Å.6,7,8,9
Diphosphine ligands of the type R2P(CH2)nPR2 can function effectively to connect two
gold centers in a somewhat flexible fashion. For example, the short methylene connector in
Ph2P(CH2)PPh2 (dppm) facilitates the close approach of two metals and has been widely used to
support metal-metal bonding and reactions that involve breaking or making metal-metal
bonds.10 , 11 , 12 In contrast, when diphosphines with connectors consisting of three or more
methylene units bridge two gold(I) centers, the gold ions are generally rather far apart.13,14,15,16,17
With Ph2P(CH2)2PPh2 (dppe) and related ligands that utilize two methylene connectors, there is
considerable flexibility in positioning two metal centers. For example, the dimers Au2(µ-
dppe)2Br2 and Au2(µ-dppe)2I2 consist of two, three-coordinate P2AuX units.18,19 Remarkably,
the spacing between these three-coordinate, planar gold centers is variable and can range from
~3.2 Å to ~3.8 Å.18,19 Thus, these molecules resemble molecular accordions. They are
luminescent with their emission depending upon the Au•••Au separation. When that spacing is
less than 3.5 Å, the dimers have a green emission, but when the spacing is greater than 3.5 Å, a
red emission is observed.18,19
Our attempts to make the hypothetical molecular accordion
“Au2(µ-dcpe)2X2” (where dcpe is 1,2-bis(dicyclohexylphosphino)ethane and X can be Cl, Br, or
I), in which the phenyl groups were replaced with cyclohexyl substituents that could not
3

contribute to the luminescence, resulted exclusively in the formation of the two-coordinate
complexes, Au2(µ-dcpe)X2.
Here, we present results on the related series of two-coordinate gold(I) complexes -
Au2(µ-dppe)X2 and Au2(µ-dcpe)X2. Molecules similar to Au2(µ-dppe)X2 and Au2(µ-dcpe)X2
can produce a variety of different structural types as shown in Scheme 1. The formation of
polymeric structures (A) that involve aurophilic connecting these complexes is particularly
common.20 For example, the molecules Au2(µ-Ph2P(CH2)nPPh2)I2 with n = 4 through 8 form
polymers of this sort. 21 The case of Au2(µ-Ph2P(CH2)6PPh2)I2 is particularly interesting since
the individual polymer strands in the crystal are interwoven to form discrete layers.22 As a result
of this layered structure, preferential cleavage of crystals of Au2(µ-Ph2P(CH2)6PPh2)I2 occurs
along the crystallographic ab plane, which is the plane containing the individual polymeric
interwoven layers. As an alternative to polymer formation, molecules of the type Au2(µ-
R2P(CH2)nPR2)X2 can dimerize through the formation of two aurophilic interactions as shown in
(B) in Scheme 1. This laboratory has recently shown that each of the related arsine analogs
Au2(µ-dpae)X2 (dpae is 1,2-bis(diphenylarsino)ethane) forms two polymorphs, one with the
polymeric structure (A) and the other with the dimeric structure (B).23 Molecules of the type
Au2(µ-R2P(CH2)nPR2)X2 can also from intramolecular aurophilic bonds as shown in (C) of
Scheme 1. Thus, Au2(µ-cis-dppee)X2 (dppee is cis-bis(diphenylphosphine)ethylene) adopts
structure of type (C) because the rigid backbone positions the two gold centers in close
proximity.24,25 Finally, molecules of the type Au2(µ-R2P(CH2)nPR2)X2 may have no aurophilic
interactions at all as shown in (D) in Scheme 1. With four known structural motifs possible for
the Au2(µ-dppe)X2 and Au2(µ-dcpe)X2 series, we identify here which motifs are actually found
for these two series and explore their luminescent properties.
4

Scheme 1. Binuclear Gold Complexes with Ligand Bridges and Varying
Aurophilic Interactions.
5

2. Experimental Section
2.1 Materials. Chloroauric acid, bromoauric acid, and gold(I) iodide were purchased from
Strem Chemicals, Inc. The phosphine ligands were purchased from Sigma-Aldrich Co. LLC. 2
2'-thiodiethanol was purchased from Sigma-Aldrich. Solvents were obtained commercially. All
materials were used as received unless otherwise specified. Au2(µ-dppe)Cl2 was prepared
according to literature methods.26,27
2.2 Preparation of Au2(µ-dppe)Br2. A 200 mg (0.386 mmol) portion of bromoauric acid was
added to 10 mL methanol at room temperature. While stirring, 0.50 mL of 2,2’-thiodiethanol (4.8
mmol) was added dropwise. During 10 minutes of stirring, the solution faded from red-orange to
gold. A 150 mg (0.376 mmol) portion of dppe was dissolved in methanol and the solutions
combined. The resulting solution was stirred until the solution color faded to pale gold. The
solution was heated to 50 °C for 30 minutes. Upon addition of 50 mg (0.62 mmol) of sodium
thiocyanate, the color of the solution faded and a white suspension formed. Evaporation of the
solvent yielded a white powder. The crude solid (168 mg 83%) was collected by filtration and
washed with three 10 mL aliquots of diethyl ether. Colorless crystals formed via slow diffusion
of hexane or diethyl ether into a dichloromethane solution of the complex. IR spectrum: 3328w,
3050w, 2937w, 2921w, 2871w, 2172w, 2128w, 2072w, 1959w, 1738m, 1578w, 1476m, 1429s,
1407m, 1376m, 1333w, 1284w, 1221w, 1164m, 1100s, 1042s, 999s, 944w, 883w, 848w, 792w,
726s, 684s, 518s, 470s, 398w. 31P{1H} NMR (chloroform-d, 298 K): singlet at 35.1 ppm.
Melting point: 245-250°C (decomposition)
6

2.3 Preparation of Au2(µ-dppe)I2. AuI (100 mg, 0.309 mmol) was suspended in 30 mL of
acetone. Solid dppe (185 mg, 0.463 mmol) was added while stirring. After stirring for three
hours, the solution was filtered and the solvent removed in a vacuum to yield “Au(dppe)I”19 (180
mg). A 100 mg portion of “Au(dppe)I” (0.0692 mmol) was dissolved in 25 mL dichloromethane.
While stirring, 22 mg of I2 (0.087 mmol) dissolved in a minimal amount of dichloromethane was
added. Upon addition of I2, the suspension turned gold, which faded to colorless after an hour.
The suspension was stirred for 3 hours and the solvent removed via evaporation. The crude
white powder was washed with three aliquots of diethyl ether. The crude yield was 112 mg
(77.2 %). Crystals were formed by slow diffusion of diethyl ether into a solution of the product
in dichloromethane. IR spectrum: 3053w, 1588w, 1482w, 1433m, 1406w, 1316w, 1310w,
1261w, 1164w, 1140w, 1121m, 1095w, 1067w, 989w, 995w, 920w, 876w, 846w, 765w, 726s,
1
689s, 616w, 530m, 504s, 421w. 31P{1H} NMR (chloroform-d, 298 K): singlet at 32.5 ppm. H-
NMR (chloroform-d, 298 K): 2.01 (br) methylenes, 7.45 (t) m-H, 7.52 (t) p-H, 7.70 (q) o-H.
Melting point: 154-156°C. Mass spectrum positive mode: m/z 918.795 [dppe(AuI)Au]+.
2.4 Preparation of Au2(µ-dcpe)Cl2. A 217 mg (0.639 mmol) portion of chloroauric acid was
dissolved in a mixture of 5 mL deionized water and 7.5 mL of methanol and cooled to 0° C in a
bath of acetone and ice. Separately, 163 µL (1.63 mmol) of 2,2’-thiodiethanol was dissolved in
2.5 mL of methanol and then slowly added to the chloroauric acid solution. After 15 minutes of
stirring, the solution became colorless. A 139 mg (0.330 mmol) portion of dcpe was dissolved in
a mixture of 5 mL methanol and 5 mL of acetonitrile. This mixture was added slowly to the
chloroauric acid solution. Upon addition, a precipitate formed rapidly and redissolved while
stirring. After 30 minutes of stirring, persistent precipitation was observed and 50 mL of dry
7

methanol was added to aid in completion of precipitation. The crude solid was collected by
filtration and washed with three 10 mL aliquots of methanol. Additional precipitate was
observed in the supernatant upon warming to room temperature. This precipitate was also
collected by filtration, washed with methanol, and combined with the other precipitate. The
yield of crude product was 182 mg (94.5%). The product was dissolved in 8 mL of
dichloromethane and recrystallized by slow diffusion of diethyl ether, methanol, and toluene in
separate containers. Each combination produced identical crystals suitable for X-ray diffraction.
Infrared spectrum: 2922s, 2848s, 1440s, 1334w, 1296w, 1267w, 1188m, 1114m, 1047w, 1002m,
889m, 852m, 820m, 750s, 704m, 674m, 524s, 484s, 441w, 402m. 31P{1H} NMR (chloroform-d,
298 K): singlet at 50.0 ppm. 1H-NMR (chloroform-d, 298 K): 1.20-1.45 (m), 1.72-2.05 (m).
Melting Point: 225° (decomposition into red-pink solid). Mass spectrum positive mode: m/z
851.084 [dcpe(AuCl)Au)]+.
2.5 Preparation of Au2(µ-dcpe)Br2. A 288 mg (0.557 mmol) portion of bromoauric acid was
dissolved in a mixture of 7.5 mL methanol and 5 mL deionized water cooled to 0° C in a bath of
acetone and ice, resulting in a red solution. Separately, 173 µL of (1.67 mmol) 2,2’-
thiodiethanol was dissolved in 2.5 mL of methanol and added drop wise over 10 minutes to the
chilled bromoauric acid solution. The color of the solution gradually faded to produce a
colorless mixture. A 118 mg (0.281 mmol) portion of dcpe was dissolved in a mixture of 5 mL
methanol and 5 mL acetonitrile. The two solutions were mixed and produced immediate
precipitation of the crude product. The solution was warmed to room temperature, then 50 mL
of methanol was added to complete the precipitation. The colorless solid was collected by
filtration and washed with three 10 mL portions of methanol. The total yield of crude product
8

was 149 mg (55 %). The product was dissolved in 8 mL of dichloromethane and recrystallized
by slow diffusion of diethyl ether, ethanol, and toluene in separate containers. Each combination
produced identical crystals suitable for X-ray diffraction. Infrared spectrum: 2921s, 2849s,
1442s, 1413w, 1327w, 1296w, 1270m, 1201w, 1187s, 1171m, 1110m, 1051w, 1004m, 913w,
891m, 852s, 822m, 752s, 706m, 676m, 514s, 513s, 484s, 440m, 402m. 31P{1H} NMR
(chloroform-d, 298 K): singlet at 52.1 ppm. 1H-NMR (chloroform-d, 298 K): 1.20-1.48 (m),
1.70-2.06 (m). Melting Point: 235° (decomposition). Mass spectrum positive mode: m/z
895.060 [dcpe(AuBr)Au)]+.
2.6 Preparation of Au2(µ-dcpe)I2. A 58 mg (0.18 mmol) portion of gold(I) iodide was placed
in 6 mL of dichloromethane and sonicated for 10 minutes to produce a yellow suspension.
Excess dcpe (114 mg, 0.270 mmol) was added directly, which resulted in the formation of a clear,
colorless solution. This solution was covered and stirred vigorously for two hours, then
uncovered and allowed to evaporate overnight. The crude solid product was obtained in
quantitative yield. Upon drying, the solid product was dissolved in dichloromethane and
recrystallized by slow diffusion of diethyl ether, methanol, ethanol, propanol, and toluene in
separate containers. Each combination produced identical crystals suitable for X-ray diffraction.
Infrared spectrum: 2922s, 2848s, 1443s, 1329w, 1267w, 1187m, 1111w, 1050w, 1004m, 888m,
852m, 818m, 753s, 707m, 676m, 525s, 485s, 440w, 407m. 31P{1H} NMR (chloroform-d, 298
1
K): singlet at 54.9 ppm. H-NMR (chloroform-d, 298 K): 1.20-2.14 (m). Melting Point: 251-
258° C (decomposition). Mass spectrum positive mode: m/z 943.3719 [dcpe(AuI)Au)]+.
9

2.7 X-ray Crystallography and Data Collection. Crystals of Au2(µ-dcpe)Br2, Au2(µ-dcpe)Cl2,
and Au2(µ-dppe)Cl2 were mounted in the 100 K nitrogen cold stream provided by an Oxford
Cryostream low-temperature apparatus on the goniometer head of a Bruker D8 diffractometer
equipped with a Photon 100 CMOS detector. Data were collected using synchrotron radiation (λ
= 0.6888 Å) at Beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National
Laboratory. Crystals of Au2(µ-dppe)Br2, and Au2(µ-dppe)I2, Au2(µ-dcpe)I2 were mounted in a
90 K cold stream provided by a Cryo Industries low-temperature apparatus on the goniometer
head of a Bruker D8 Apex II diffractometer employing Mo Kα (λ = 0.71073 Å) radiation and a
CCD detector. All data sets were reduced using SAINT ,28 and a multiscan absorption correction
was applied using SADABS.29 Structure solution and refinement were executed using direct or
dual methods30 and refined using all data (based on F2) using SHELXL-2014.31 Additional
refinement details can be found in the crystallographic information files (CIFs).
2.8 Physical Measurements. Infrared spectra were recorded on neat powders on a Bruker
ALPHA FT-IR spectrometer. Fluorescence excitation and emission spectra were recorded on a
Perkin-Elmer LS50B luminescence spectrophotometer. NMR data were collected on a Bruker
Avance DRX 500 MHz spectrometer. 31P{1H} chemical shifts are referenced to external
phosphoric acid.
3. Results
3.1 Synthesis and Crystal Growth
Colorless Au2(µ-dppe)Cl2 was prepared in a similar fashion as reported earlier.26,27
Crystals of Au2(µ-dppe)Br2 were obtained by reduction of bromoauric acid with 2,2’-
thiodiethanol followed by the addition of dppe. This produced a golden solution that was treated
10

with sodium thiocyanate, which appears to act as a second reducing agent, since its addition
caused the solution to fade to colorless. Evaporation of the solvent yielded a white powder in
83% yield. Colorless crystals of Au2(µ-dppe)Br2 were formed via slow diffusion of hexane or
diethyl ether into a dichloromethane solution of the complex. Treatment of solid gold(I) iodide
with an acetone solution of dppe produced a colorless solution that upon evaporation yielded
“Au(dppe)I” as a white powder.19
As previously reported, recrystallization of this white
powder produced Au2(µ-dppe)2I2 with two, three-coordinate gold centers and a variable spacing
between these gold centers and Au2(µ-dppe)2(µ−I)2 with two, four-coordinate gold centers.19
Attempts to oxidize “Au(dppe)I” with diiodine produced only colorless Au2(µ-dppe)I2 in 77 %
yield.
Efforts to obtain the target molecule “Au2(µ-dcpe)2X2”, were unsuccessful although we
did examine a variety of reaction conditions and reaction stoichiometries, but we were able to
prepare and characterize the two-coordinate complexes, Au2(µ-dcpe)X2. Colorless crystals of
Au2(µ-dcpe)Cl2 and Au2(µ-dcpe)Br2 were obtained in 94 or 55 % yield respectively by reduction
of either HAuCl4 or HAuBr4 with 2,2’-thiodiethanol followed by the addition of dcpe.
Treatment of a yellow suspension of AuI in dichloromethane with dcpe yielded colorless Au2(µ-
dcpe)I2 in essentially quantitative yield.
Infrared and 31P{1H} NMR spectral data for all of these complexes are given in the
experimental section.
3.2 The Molecular and Supramolecular Structures of the Isostructural Series: Au2(µ-
dppe)Cl2, Au2(µ-dppe)Br2, and Au2(µ-dppe)I2
Crystallographic data for this series of compounds are presented in Table1. Selected
bond distances and angles are given in Table 2. The molecular structure of Au2(µ-dppe)I2, which
11

is representative of the series, is shown in Figure 1 and is akin to D of Scheme 1. The Newman
projection angles, X-Au•••Au-X, and part (B) of Figure 1 show that the monomer is twisted into a
non-planar shape. Within this molecule, the Au1•••Au2 distance is rather long, 6.3686(4) Å. As
shown in Figure 2, in the solid state, two molecules of Au2(µ-dppe)I2 self associate to form a
centrosymmetric dimer through intermolecular aurophilic interactions, as in B of Scheme 1. The
Au1•••Au2A distance in the dimer is 3.1552(2) Å. Similar molecular structures and self-
association are seen in Au(µ-dppe)Br2 and for Au(µ-dppe)Cl2. The structure of Au2(µ-dppe)Cl2
is entirely consistent with previous reports,27,28 but we include our low temperature structure for
direct comparison with the other complexes reported here. Additionally, Au2(µ-dppe)Cl2,
Au2(µ-dppe)Br2, and Au2(µ-dppe)I2 are isostructural with the α-polymorphs of Au2(µ-dpae)X2
(X = Cl, Br, I). However, despite repeated efforts, we were not able to detect any other
polymorphic forms of Au2(µ-dppe)Cl2, Au2(µ-dppe)Br2, or Au2(µ-dppe)I2.
3.3 The Structure of Au2(µ-dcpe)Cl2.
Crystallographic data for the Au2(µ-dcpe)Cl2 (X = Cl, Br, and I) series of compounds are
given in Table 3, while selected bond distances and angles are given in Table 4. Figure 3 shows
the molecular structure of Au2(µ-dcpe)Cl2. Within the molecule, the gold ions are separated by
6.8375(9) Å. Moreover, there are no close Au•••Au contacts between molecules. Hence, the
monomeric structure of Au2(µ-dcpe)Cl2 contrasts with that of the series Au2(µ-dppe)X2 which
self-associate through two intermolecular Au•••Au interactions. In the absence of any aurophilic
interactions, the P-Au-Cl angle, 177.07(3) °, in Au2(µ-dcpe)Cl2 is more nearly linear than the
corresponding angles seen in any of the other complexes reported here.
12

3.4 The Structures of Au2(µ-dcpe)Br2 and Au2(µ-dcpe)I2.
As the crystallographic data in Table 3 show, these two compounds are isostructural.
Drawings of the two similar molecules are shown in Figure 4. These two complexes have a
twisted shape in which the two gold centers are drawn together by aurophilic interactions as in C
of Scheme 1. The Au1•••Au2 distances in these monomers (Au2(µ-dcpe)Br2, 3.1511(7) Å; Au2(µ-
dcpe)I2, 3.1087(4) Å) are significantly shorter than the corresponding intermolecular distances
seen in the Au2(µ-dppe)Cl2 (X = Cl, Br, and I) series. The P-Au-X angles in these two
monomers are slightly bent inward in a fashion that reflects the neighboring attractive aurophilic
interactions.
3.5 Luminescence of Crystalline Au2(µ-dcpe)I2.
Samples of Au2(µ-dppe)X2 and Au2(µ-dcpe)X2 (X = Cl, Br, and I) both in solution and in
the solid state have been examined for luminescence. As seen in Figures 5 and 6, crystalline
Au2(µ-dcpe)I2 displays a pink luminescence. None of the other five compounds discussed here
show any luminescence in the crystalline state or in solution. As Figure 5 shows, a solution of
Au2(µ-dcpe)I2 is not luminescent, but the material becomes luminescent upon crystallization.
The emission spectrum shows two peaks, which may come from vibrational fine structure or the
fact that there are two non-equivalent gold centers in the crystal structure. We have ruled out the
possibility that two polymorphs are present by examining a number of crystals by both X-ray
diffraction and emission spectroscopy. All crystals show the same emission features with
identical intensity ratios.”
4. Discussion
13

Crystals in the series Au2(µ-dppe)X2 (X = Cl, Br, I) are isostructural and isostructural
with the α-polymorphs of Au2(µ-dpae)X2. They form centrosymmetric, dimeric structures of
type (B) in Scheme 1. Although a second, unique polymorph was identified for each member of
the Au2(µ-dpae)X2 series that had the polymeric structure (A) in Scheme 1, we found no
evidence for a second polymorph for any member of the Au2(µ-dppe)X2 (X = Cl, Br, I) series.
Crystals of Au2(µ-dcpe)X2 show structures that differ appreciably from those of Au2(µ-
dppe)X2. The bulky cyclohexyl substituents appear to be playing a major role eliminating the
formation of polymeric or dimeric structures. Au2(µ-dcpe)Cl2 has a rather uncommon,
monomeric structure with no aurophilic interactions. In this situation, the cyclohexyl groups are
position to avoid any intramolecular contact. In contrast, Au2(µ-dcpe)Br2 and Au2(µ-dcpe)I2
have similar structures with intramolecular aurophilic interactions. As seen in Figure 4, the
bulky cyclohexyl substituents in these two complexes again are situated so that contact between
any of them is minimized. The intramolecular Au•••Au distances in Au2(µ-dcpe)Br2 and Au2(µ-
dcpe)I2 (3.1511(7) and 3.1087(4) Å, respectively) are similar to the intermolecular Au…Au
distances in Au2(µ-dppe)Br2 and Au2(µ-dppe)I2 (3.1703(4) and 3.1552(2)Å, respectively). In
Au2(µ-dppm)Cl2, which also has an intramolecular aurophilic interaction, the Au•••Au distance is
considerably longer (3.341 Å).32 However, Au2(µ-cis-dppee)I2, where dppee is cis-di(phenyl-
phosphine)ethylene, has a structure of type (C) with the two gold centers positioned closer
together with a Au•••Au distance of 2.9526(5) Å.25 The differences between the structure of
Au2(µ-dcpe)X2 and those of Au2(µ-dcpe)Br2 and Au2(µ-dcpe)I2 are likely to result from subtle
differences in packing efficiency that are tied to the differences in the lengths of the Au-X bonds,
the sizes of the halides, and the strength of the aurophilic interactions.
14

Of the compounds reported here, only crystalline Au2(µ-dcpe)I2 is luminescent at room
temperature. Given the large Stokes’ shift involved, the emission appears to be phosphorescence.
Solution of Au2(µ-dcpe)I2 are non-luminescent. Consequently, Au2(µ-dcpe)I2 displays
aggregation induced emission.33 Many luminescent molecules show diminished luminescence
intensity, if any, when condensed into solid form, a phenomenon known as aggregation caused
quenching. Here, we have the opposite situation: only upon crystallization does Au2(µ-dcpe)I2
emit. A number of gold complexes undergo self-association upon crystallization and become
emissive with the emission linked to the formation of aurophilic interactions between individual
gold(I) complexes.34,35,36 However, in the present case no self-association occurs. Rather, we
suggest that the Au2(µ-dcpe)I2 molecule exists in solution in a freely rotating form that is free
from aurophilic interactions, but upon crystallization the molecule becomes locked in an
arrangement that favors interaction between the two gold centers. In this context, the non-
emissive nature of crystalline Au2(µ-dcpe)Br2 is some what surprising, until one realizes that
both polymorphs of Au2(µ-dpae)Cl2 and Au2(µ-dpae)I2 are luminescent, but neither polymorph
of Au2(µ-dpae)Br2 is emissive.23
Acknowledgements. We thank the Petroleum Research Fund (Grant 37056-AC) and the
National Science Foundation for financial support (Grant CHE-1305125 to A.L.B. and M.M.O.
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 and a fellowship to K.B.G.
15

Appendix. Supplementary data
CCDC 1478417, 1478418, 1478419, 1478420, 1478421, 1478422 contain the
supplementary crystallographic data. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223- 336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
References
[1]
[2]
[3]
[4]
[5]
A.L. Balch, Struct. Bond. 123 (2007) 1.
P. Pyykkö, Chem. Soc. Rev. 37 (2008) 1967.
P. Pyykkö, Inorg. Chim. Acta 358 (2005) 4113.
H. Schmidbaur, W. Graf, G. Müller, Angew. Chem., Int. Ed. 27 (1988) 417.
D.E. Harwell, M.D. Mortimer, C.B. Knobler, F.A.L. Anet, M.F. Hawthorne, J. Am.
Chem. Soc. 118 (1996) 2679.
[6]
[7]
[8]
[9]
S.S. Pathaneni, G.R. Desiraju, J. Chem. Soc. Dalton Trans. (1993) 319.
K.A. Anderson, A.E. Goeta, J.W. Steed, Inorg. Chem. 46 (2007) 6444.
H. Schmidbaur, A. Schier, Chem. Soc. Rev. 37 (2008) 1931.
H. Schmidbaur, A. Schier, Chem. Soc. Rev. 41 (2012) 370.
[10] B. Chaudret, B. Delavaux, R. Poilblanc, Coord. Chem. Rev. 86 (1988) 191.
[11] R.J. Puddephatt, Chem. Soc. Rev. 12 (1983) 99.
[12] M.M. Olmstead, H. Hope, L.S. Benner, A.L. Balch, J. Am. Chem. Soc. 99 (1977) 5502.
[13] A.L. Balch, B. Tulyathan. Inorg. Chem. 16 (1977) 2840.
16

[14] P.M. Van Calcar, M.M. Olmstead, A.L. Balch. Inorg. Chem. 36 (1997) 5231.
[15] S. Onaka, M. Yaguchi, R. Yamauchi, T. Ozeki, M. Ito, T. Sunahara, Y. Sugiura, M.
Shiotsuka, K. Nunokawa, M.Horibe, K. Okazaki, A. Iida, H. Chiba, K. Inoue, H. Imai, K.
Sako, J. Organometal. Chem. 690 (2005) 57.
[16] S.H. Lim, J.C. Schmitt, J. Shearer, J. Jia, M.M. Olmstead, J.C. Fettinger, A.L. Balch,
Inorg. Chem. 52 (2013) 823.
[17] M. Streitberger, A. Schmied, E. Hey-Hawkins, Inorg. Chem. 53 (2014) 6794.
[18] S.H. Lim, M.M. Olmstead, A.L. Balch, J. Am. Chem. Soc. 133 (2011) 10229.
[19] S. H. Lim, M.M. Olmstead, A.L. Balch, Chem. Science 4 (2013) 311.
[20] R.J. Puddephatt, Coord. Chem. Rev. 216-217 (2001) 313.
[21] P.M. Van Calcar, M.M. Olmstead, A.L. Balch, Inorg. Chem. 36 (1997) 5231.
[22] P.M. Van Calcar, M.M. Olmstead, A.L. Balch, J. Chem. Soc., Chem. Commun. (1995)
1773.
[23] S.H. Lim, M.M. Olmstead, J.C. Fettinger, A.L. Balch, Inorg. Chem. 51 (2012) 1925.
[24] P.G. Jones, Acta Crystallogr., Sect. B. 36 (1980) 2775.
[25] J.B. Foley, S.E. Gay, M.J. Vela, B.M. Foxman, A.E. Bruce, M.R.M. Bruce, Eu. J. Inorg.
Chem. 31 (2007) 4946.
[26] P.A. Bates, J.M. Waters, Inorg. Chim. Acta. 98 (1985) 125.
[27] D.S. Eggleston, D.F. Chodosh, G.R. Girard, D.T. Hill, Inorg. Chim. Acta. 108 (1985) 221.
[28] Bruker AXS Inc., Madison,Wisconsin, USA (2015).
[29] G. M. Sheldrick, University of Gőttingen, Germany (2015).
[30] G. M. Sheldrick, Acta Crystallogr., Sect A. 64 (2008) 112; G. M. Sheldrick, Acta
Crystallogr. Sect. A. 71 (2015) 3.
17

[31] G. M. Sheldrick, Acta Crystallogr. Sect. C. 71 (2015) 3.
[32] H. Schmidbaur, A. Wohlleben, F. Wagner, O. Orama, G. Huttner, Chem. Ber. 110, (1977)
1748.
[33] J. Mei, N. L. C. Leung, R. T.K Kwok, J.W.Y. Lam, BZ Tang, Chem. Rev. 115(2015)
11718.
[34] R.L. White-Morris, M.M. Olmstead, F. Jiang, D.S. Tinti, A.L. Balch, J. Am. Chem. Soc.
124 (2002) 2327.
[35] R.L. White-Morris, M.M. Olmstead, A.L. Balch, J. Am. Chem. Soc. 125 (2003) 1033.
[36] M.A. Malwitz, S.H. Lim, R.L. White-Morris, D.M. Pham, M.M. Olmstead, A.L. Balch, J.
Am. Chem. Soc. 134 (2012) 10885.
18

Table 1.
Crystal Data and Structure Refinement for Au2(µ-dppe)X2
Au2(µ-dppe)Cl2
C26H24Au2Cl2P2
863.22
(Au2(µ-dppe)Br2
C26H24Au2Br2P2
952.14
Au2(µ-dppe)I2
C26H24Au2I2P2
1046.12
90(2)
Formula
Formula weight
T, K
100(2)
90(2)
λ, Å
0.6888
0.71073
colorless prism
monoclinic
P21/n
0.71073
colorless block
monoclinic
P21/n
Color and Habit
Crystal system
Space group
a, Å
colorless block
monoclinic
P21/n
12.6499(4)
11.0650(4)
18.8155(6)
90
12.8214(7)
11.2555(6)
18.9160(11)
90
13.0117(7)
11.5017(6)
18.997(2)
90
b, Å
c, Å
α, deg
β, deg
109.094(2)
90
108.2320(10)
90
107.268(2)
90
γ, deg
V, Å3
2488.73(15)
4
2592.7(2)
4
2714.9(4)
4
Z
d
, g•cm-3
calc
2.304
2.439
2.559
µ, mm-1
11.128
14.52
13.20
unique data
restraints
parameters
9485
5379
7926
0
0
0
289
289
289
a
R1
0.0269
0.0333
0.0226
b
wR2
0.0424
0.0752
0.0499
aR1= ∑Fo-Fc/ ∑Fo.
2
2 2
2
2
bwR2 = [∑[w(Fo -Fc2)2]/ ∑[w(Fo ) ]]1/2, w-1 = [ σ2(Fo ) + ( aP)2 + bP ], where P = (Fo + 2Fc2)/3.
19

Table 2.
Selected Bond Distances (Å) and Angles (deg) for Au2(µ-dppe)X2
Au2(µ-dppe)Br2
Au2(µ-dppe)I2
Au2(µ-dppe)Cl2
Distance, Å
Au1---Au2
intramolecular
6.2299(2)
6.3040(5)
3.1703(4)
6.3686(4)
3.1552(2)
Au1•••Au2A
3.16188(17)
intermolecular
Au1-P1
Au2-P2
Au1-X1
Au2-X2
2.2309(8)
2.2333(8)
2.2991(8)
2.3055(7)
61.1
2.2454(19)
2.240(2)
2.4242(9)
2.4263(9)
60.9
2.2624(9)
2.2552(10)
2.5822(3)
2.5738(4)
58.2
X1-Au1---Au2-X
Newman
projection, deg
Bond Angles, deg
P1-Au1-X1
172.86(3)
175.27(3)
171.74(6)
175.10(5)
169.56(2)
174.17(3)
P2-Au2-X2
20

Table 3
Crystal Data and Structure Refinement for Au2(µ-dcpe)X2
Au2(µ-dcpe)Cl2
C26H48Au2Cl2P2
887.41
100(2)
0.6888
colorless lath
orthorhombic
Pbca
Au2(µ-dcpe)Br2
C26H48Au2Br2P2
976.33
Au2(µ-dcpe)I2
C26H48Au2I2P2
1070.31
90(2)
Formula
Formula weight
T, K
100(2)
λ, Å
0.6888
0.71073
colorless lath
monoclinic
P21/n
Color and Habit
Crystal system
Space group
a, Å
colorless needle
monoclinic
P21/n
11.408(2)
15.602(3)
16.397(3)
90
9.7007(19)
15.171(3)
20.785(4)
90
9.7969(13)
15.829(2)
20.548(3)
90
b, Å
c, Å
α, deg
β, deg
90
102.48(3)
90
102.285(2)
90
γ, deg
90
V, Å3
2918.5(10)
4
2986.6(11)
4
3113.5(7)
4
Z
d
, g•cm-3
calc
2.020
2.171
2.283
µ, mm-1
9.491
11.586
11.512
9920
unique data
restraints
parameters.
3768
7396
0
0
0
145
289
289
a
R1
0.0202
0.0490
0.0289
0.0152
0.0304
b
wR2
0.0632
a
R1= ∑Fo-Fc/ ∑Fo.
b
2
2 2
2
wR2 = [∑[w(Fo -Fc2)2]/ ∑[w(Fo ) ]]1/2, w-1 = [ σ2(Fo ) + ( aP)2 + bP ], where
P = (Fo + 2Fc2)/3.
2
21

Table 4.
Selected Bond Distances (Å) and Angles (deg) for Au2(µ-dcpe)X2
Au2(µ-dcpe)Cl2
Au2(µ-dcpe)Br2
Au2(µ-dcpe)I2
Distance, Å
Au1••• Au2(Au1A)
intramolecular
6.8374(9)
2.2448(9)
3.1511(7)
2.2436(13)
2.2384(13)
2.4060(7)
2.4061(6)
68.9
3.1087(4)
2.2607(5)
2.2582(5)
2.5672(3)
2.5708(3)
67.4
Au1-P1
Au2-P2
Au1-X1
Au2-X2
2.3158(8)
180
X-Au•••Au-X
Newman projection,
deg
Bond Angles, deg
P1-Au1-X1
P2-Au2-X2
177.07(3)
171.74(4)
174.42(4)
169.75(2)
172.95(2)
22

(A)
(B)
Figure 1.
Two views of the structure of an individual Au2(µ-dppe)I2.molecule. Thermal
ellipsoids are shown at the 50 % probability level in this and all subsequent
figures. Hydrogen atoms are omitted for clarity in all figures.
23

Figure 2.
The interactions between two molecules of Au2(µ-dppe)I2..
24

Figure 3. The structure of Au2(µ-dcpe)Cl2.
25

(A)
(B)
Figure 4. The structures of (A) Au2(µ-dcpe)I2 and (B) Au2(µ-dcpe)Br2.
26

Figure 5. Luminescence from crystalline Au2(µ-dcpe)I2. The photograph of a cuvette
containing a dichloromethane solution of Au2(µ-dcpe)I2 was taken under irradiation with
a UV lamp. Notice that the solution is not luminescent, but that crystals of the compound,
which formed by capillary action and solvent evaporation, have a pink emission.
27

Figure 6. Emission and excitation spectra for crystalline Au2(µ-dcpe)I2.
28

Graphical Abstract.
Synopsis
Aurophilic interactions in the crystal structures of the series Au2(µ-1,2-bis(diphenylphosphino)-
ethane)X2 and Au2(µ-1,2-bis(dicyclohexylphosphino)ethane)X2 (X = Cl, Br, I) trade off with
steric effects as a function of halogen atom and phenyl vs. cyclohexyl substituent. The shortest
Au…Au distance of these is observed in Au2(µ-dcpe)I2
29