Molecular accordion: Vapoluminescence and molecular flexibility in the orange and green luminescent crystals of the dimer, Au2(μ-bis- (diphenylphosphino)ethane)2Br2

Article abstract of DOI:10.1021/ja2026807

Solutions containing the components Au⁺, dppe (dppe is bis(diphenylphosphino)ethane), and Br^- in a 1:1:1 ratio can produce three different types of crystals: type A, orange luminescent solvates of the dimer Au₂(dppe)₂Br₂ (Au₂(μ-dppe) ₂Br₂·2(OSMe₂), Au₂(μ-dppe) ₂Br₂·2(OCMe₂), Au₂(μ-dppe) ₂Br₂·2(CH₂Cl₂), Au ₂(μ-dppe)₂Br₂·2(HC(O)NMe ₂)); type B, green luminescent solvates of the same dimer (Au ₂(μ-dppe)₂Br₂·(NCMe) and Au ₂(μ-dppe)₂Br₂·0.5(C₄H ₁₀O)); and type C, orange luminescent solvates of a polymer ({Au(μ-dppe)Br}_n·0.5(C₄H₁₀O) and {Au(μ-dppe)Br}_n·(CH₂Cl₂)). Some crystals of types A are solvoluminescent. Exposure of type A crystals of Au ₂(μ-dppe)₂Br₂·2(OCMe₂) or Au₂(μ-dppe)₂Br₂·2(CH ₂Cl₂) to air or vacuum results in the loss of the orange luminescence and the formation of new green luminescent crystals. Subsequent exposure of these crystals to acetone or dichloromethane vapor results in the reformation of crystals of type A. The dimeric complexes in crystals of types A and B are all centrosymmetric and share a common ring conformation. Within these dimers, the coordination geometry of each gold center is planar with a P ₂Br donor set. In other respects, the Au₂(μ-dppe) ₂Br₂ molecule is remarkably flexible and behaves as a molecular accordion, whose dimensions depend upon the solvate content of a particular crystalline phase. In particular, the dimer Au₂(μ-dppe) ₂Br₂ is able to accommodate Au···Au separations that range from 3.8479(3) to 3.0943(2) A, and these variations along with alterations in the Au - Br distances and in the P - Au - P angles are the likely causes of the differences in the luminescence properties of these crystals.

Full text of DOI:10.1021/ja2026807

ARTICLE
pubs.acs.org/JACS
Molecular Accordion: Vapoluminescence and Molecular Flexibility
in the Orange and Green Luminescent Crystals of the Dimer,
Au2(μ-bis-(diphenylphosphino)ethane)2Br2
Sang Ho Lim, Marilyn M. Olmstead, and Alan L. Balch*
Department of Chemistry, University of California, Davis, California 95616, United States
S Supporting Information
b
ABSTRACT: Solutions containing the components Auþ, dppe
(dppe is bis(diphenylphosphino)ethane), and Brꢀ in a 1:1:1
ratio can produce three different types of crystals: type A, orange
luminescent solvates of the dimer Au2(dppe)2Br2 (Au2(μ-
dppe)2Br2 2(OSMe2), Au2(μ-dppe)2Br2 2(OCMe2), Au2(μ-
3
3
dppe)2Br2 2(CH2Cl2), Au2(μ-dppe)2Br2 2(HC(O)NMe2));
3
3
type B, green luminescent solvates of the same dimer (Au2(μ-dppe)2Br2 (NCMe) and Au2(μ-dppe)2Br2 0.5(C4H10O)); and
type C, orange luminescent solvates of a polymer ({Au(μ-dppe)Br}n 0.5(C4H10O) and {Au(μ-dppe)Br}n (CH2Cl2)). Some
crystals of types A are solvoluminescent. Exposure of type A crystals of Au2(μ-dppe)2Br2 2(OCMe2) or Au2(μ-dppe)2Br2 2
(CH2Cl2) to air or vacuum results in the loss of the orange luminescence and the formation of new green luminescent crystals.
Subsequent exposure of these crystals to acetone or dichloromethane vapor results in the reformation of crystals of type A. The
dimeric complexes in crystals of types A and B are all centrosymmetric and share a common ring conformation. Within these dimers,
the coordination geometry of each gold center is planar with a P2Br donor set. In other respects, the Au2(μ-dppe)2Br2 molecule is
remarkably flexible and behaves as a molecular accordion, whose dimensions depend upon the solvate content of a particular
3
3
3
3
3
3
crystalline phase. In particular, the dimer Au2(μ-dppe)2Br2 is able to accommodate Au Au separations that range from 3.8479(3)
3 3 3
to 3.0943(2) Å, and these variations along with alterations in the AuꢀBr distances and in the PꢀAuꢀP angles are the likely causes of
the differences in the luminescence properties of these crystals.
’ INTRODUCTION
The closed-shell, diamagnetic d10 gold(I) ion produces an
interesting array of luminescent complexes with varied
properties.14,15 Mononuclear, two-coordinate gold(I) complexes
are generally colorless and are usually nonluminescent when the
individual gold(I) centers are isolated from one another. How-
ever, two coordinate gold(I) complexes can become luminescent
when two or more gold(I) centers are placed in close proximity.
Aurophilic interactions, attractive interactions between gold
centers caused by a combination of relativistic effects and
correlation effects,16 facilitate the formation of aggregates of
two-coordinate gold(I) complexes, and these aurophilic interac-
tions can be promoted through the use of suitable bridging
ligands. The distance between gold atoms in metallic gold is
Vapoluminescent and vapochromic transition metal complexes
change their luminescence or color in response to exposure to
vapors of volatile organic compounds. Consequently, they can be
developedintosensors forvarious volatile molecules.1ꢀ3 For metal
complexes with low coordination numbers, the formation of
metalꢀmetal interactions (metallophillic attractions)4,5 pro-
duces suitable chromophores that can exhibit vapochromic and/
or vapoluminescent behavior. These vapoluminescent and vapo-
chromic compounds generally function by absorbing the volatile
molecule, which can interact with the chromophor or lumophor
in a variety of ways. In many cases, the volatile molecule becomes
coordinated to a metal ion as happens in the linear polymeric
chain complex, {Tl[Au(C6Cl5)2]}n, where various volatile Lewis
bases bind to the thallium ions and alter the thalliumꢀgold
interactions.6 Other interesting examples that also entail metal ion
coordination of the volatile substance include a trinuclear Cu2Au
complex,7 a polymeric AuꢀAg compound,8 and simple copper-
(I) cyanide.9 Platinum chain complexes of the type [Pt(CNR)4]-
[Pt(CN)4] show particularly dramatic vapochromic and vapo-
2.89 Å, while the Au Au separation expected from the van der
3 3 3
Waals radius is about 3.6 Å.17 When the Au Au separation is
3 3 3
less than 3.6 Å, some degree of aurophilic interaction may be
expected, and numerous gold(I) complexes display structural
evidence of aurophilic attractions.2,3,18
Despite the abundance of aurophilic interactions that connect
two-coordinate gold(I) complexes, aurophilic interactions between
three-coordinate gold(I) complexes are rare.19 The cases where
such an interaction is found involve the complexes shown in
luminescent effects, which result from alterations in the Pt Pt
3 3 3
separations that occur when volatile molecules are incorporated
into these solids.10ꢀ12 Hydrogen bonding between the cyanide
ligands and the adsorbed molecules also can play an important
role in causing the vapochromic and vapoluminescent changes.13
Received: March 24, 2011
Published: May 26, 2011
r
2011 American Chemical Society
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Journal of the American Chemical Society
ARTICLE
Scheme 1 where two or three small-bite ligands bridge the
two gold(I) centers and enforce their proximity. Thus, in [Au2-
(μ-dmpm)3](BF4)2 (where dmpm is bis(dimethylphosphino)-
generally attributed to a transition from the filled, in-plane
dx ꢀy ,dxy orbitals on gold to its empty pz orbital. Recent
2
co2mputational and experimental studies have analyzed the
structural changes that are found in the excited states of these
three-coordinate complexes. In particular, distortions from nom-
inally trigonal ground states to and beyond a T-shape have been
proposed based upon computational studies of complexes of the
types [Au(PR3)3]þ and Au(PR3)2X (X = halide).29,30 In con-
methane, E = PMe2, B = CH2) the Au Au distances are
3 3 3
3.040(1) and 3.050(1) Å for the two different molecules in the
asymmetric unit.20 Similarly, in [Au2{(Ph2Sb)2O}3](ClO4)2
(E = SbPh2, B = O) the Au Au distance is 3.0320(4) Å,21 and
3 3 3
in Au2(μ-dppm)2Br2 (E = PPh2, B = CH2, X = Br) the Au Au
3 3 3
distance is 3.015(1) Å.22 Other cases of aurophilic interac-
tions between possibly three-coordinate gold centers involve
the accordion-like structure shown in Scheme 1 with two
bridging small-bite phosphine ligands and terminal chloride
ligands.23ꢀ25 However, in these cases the AuꢀCl distances
are quite long (greater than 2.7 Å).
trast, photocrystallographic studies of Au(PR3)2Cl CHCl3 and
3
Au(PR3)3Cl showed bond shortening in the excited state, while a
similar study of the second polymorph of Au(PR3)2Cl revealed
little change between the structures of the crystals with and
without photoexcitation.31ꢀ33
Here we report the remarkable structural flexibility and
vapoluminescent properties of three-coordinate gold(I),
accordion-like complexes (see II in Scheme 1) formed with dppe
(dppe is bis(diphenylphosphino)ethane) and bromide ion as
ligands.
Unlike monomeric, two-coordinate gold(I) complexes,
mononuclear three-coordinate gold(I) complexes are frequently
luminescent both in the solid state and in solution.26ꢀ28 For
these three-coordinate complexes, the excitation process is
Scheme 1. Binuclear Gold(I) Complexes
’ RESULTS AND DISCUSSION
Preparation and Crystal Growth. Simple mixing of the labile
complex (tht)AuBr with dppe in either dichloromethane or
acetone results in the formation of a colorless product with a
green luminescence that can be crystallized from a variety
of solvents to produce three distinct types of colorless crystals,
each of which is a solvate. Colorless crystals that produce an
orange emission when irradiated with a UV lamp (type A) are
obtained from dimethyl sulfoxide, acetone, dichloromethane, or
N,N-dimethylformamide and contain molecules of the dimer,
Au2(μ-dppe)2Br2, along with those of the respective solvent.
However, colorless crystals that produce green emission (type B)
are obtained as acetonitrile or diethyl ether solvates. These
Figure 1. Photographs of a type A crystal of Au2(μ-dppe)2Br2 2(HC(O)NMe2) under (a) ambient and (b) UV light and of a type B crystal of Au2(μ-
3
dppe)2Br2 (NCMe) under (c) ambient and (d) UV light.
3
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ARTICLE
Table 1. Crystal Data and Data Collection Parameters
Au2(μ-dppe)2Br2 2(OSMe2)
Au2(μ-dppe)2Br2 2(OCMe2)
Au2(μ-dppe)2Br2 2CH2Cl2
Au2(μ-dppe)2Br2 2(HC(O)NMe2)
3
3
3
3
formula
C
52H48Au2Br2P4 2(C2H6OS)
C52H48Au2Br2P4 2(C3H6O)
C52H48Au2Br2P4 2(CH2Cl2)
C52H48Au2Br2P4 2(C3H7NO)
3
3
3
3
type A
type A
type A
type A
formula weight
T, K
1506.79
1466.69
1520.38
1496.73
90(2)
90(2)
90(2)
90(2)
color and habit
crystal system
space group
a, Å
colorless block
monoclinic
P21/c
colorless block
monoclinic
P21/c
colorless block
monoclinic
P21/c
colorless parallelepiped
triclinic
P1
10.6725(4)
23.1389(9)
11.7127(5)
90
10.776(2)
22.739(4)
11.738(2)
90
10.7883(4)
22.4579(8)
11.6966(4)
90
11.0206(10)
11.6968(11)
12.2211(11)
103.357(2)
101.085(2)
109.337(2)
1382.9(2)
1
b, Å
c, Å
R, deg
β, deg
106.416(4)
90
106.848(3)
90
108.544(4)
90
γ, deg
V, Å3
2774.54(19)
2
2752.8(8)
2
2686.75(17)
2
Z
dcalcd, g cmꢀ3
1.804
1.769
1.879
1.797
3
μ, mmꢀ1
unique data
restraints
params.
R1a
6.953
6.932
7.295
6.902
6391
8241
6165
8277
7
0
4
0
312
309
297
318
0.0264
0.0555
0.0206
0.0444
0.022
0.0210
wR2b
0.055
0.0488
Au2(μ-dppe)2Br2 (NCMe)
Au2(μ-dppe)2Br2 0.5(C4H10O)
{Au(μ-dppe)Br}n 0.5(C4H10O)
{Au(μ-dppe)Br}n (CH2Cl2)
3
3
3
3
formula
C52H48Au2Br2P4 (C2H3N)
C52H48Au2Br2P4 0.5(C4H10O)
C26H24AuBrP 0.5(C4H10O)
C26H24AuBrP (CH2Cl2)
3
3
3
3
type B
1391.59
type B
1424.64
type C
713.32
type C
760.19
formula weight
T, K
90(2)
90(2)
90(2)
90(2)
color and habit
crystal system
space group
a, Å
colorless plate
triclinic
P1
colorless irregular block
triclinic
P1
colorless irregular block
colorless block
triclinic
P1
monoclinic
C2/c
11.8336(4)
14.4702(5)
15.2751(5)
77.090(2)
78.172(2)
80.294(3)
2474.62(15)
2
11.7530(3)
14.4241(3)
15.4879(3)
76.550(3)
77.600(3)
79.600(3)
2470.35(9)
2
26.0742(13)
9.0076(5)
24.0854(12)
90
9.1275(4)
12.0795(5)
13.7259(6)
67.432(3)
71.848(3)
83.965(4)
1327.75(10)
2
b, Å
c, Å
R, deg
β, deg
111.493(5)
90
γ, deg
V, Å3
5263.5(5)
4
Z
dcalcd, g cmꢀ3
1.868
1.865
1.798
1.901
3
μ, mmꢀ1
unique data
restraints
params.
R1a
7.703
7.716
7.246
7.381
11403
14398
6079
8089
0
6
0
0
565
577
312
298
0.017
0.023
0.031
0.032
wR2b
0.041
0.046
0.0713
0.0832
a For data with I > 2σI, R1 = ∑||Fo| ꢀ Fc||/∑|Fo|. b For all data. wR2 = (∑[w(Fo2 ꢀ Fc2)2])/(∑[w(Fo2)2])1/2
.
crystals also contain the dimeric Au2(μ-dppe)2Br2. Figure 1
pseudopolymorphs incorporating either dichloromethane or
diethyl ether as described below.
shows photographs of examples of type A (Au2(μ-dppe)2Br2 2
3
(HC(O)NMe2)) and type B (Au2(μ-dppe)2Br2 (NCMe)) crys-
The type A crystals of Au2(μ-dppe)2Br2 2(OSMe2) and
3
3
tals under ambient light and under UV irradiation. Finally, colorless,
orange luminescent crystals of the polymer {Au(μ-dppe)Br}n
(type C) have been obtained as concomitant polymorphs or
Au2(μ-dppe)2Br2 2(HC(O)NMe2) are readily obtained as the
3
only type of crystal produced when the initial colorless product is
dissolved in the appropriate solvent and the solution is allowed to
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Table 2. Selected Bond Distances (Å) and Angles (deg) for Gold Complexes
bond distances (Å)
Au2(μ-dppe)2Br2 2(OSMe2)
Au2(μ-dppe)2Br2 2(OCMe2)
Au2(μ-dppe)2Br2 2(CH2Cl2)
Au2(μ-dppe)2Br2 2(HC(O)NMe2)
3
3
3
3
type A
type A
type A
type A
Au1 Au1A
3.8479(3)
2.8292(5)
2.3097(10)
2.3050(10)
3.7339(9)
2.8226(4)
2.3051(6)
2.3109(6)
3.5676(3)
2.8034(4)
2.3060(9)
2.3096(8)
3.5142(3)
2.8848(3)
2.3041(6)
2.3032(6)
3 3 3
Au1ꢀBr1
Au1ꢀP1
Au1ꢀP2
bond angles (deg)
P1 Au1 P2
156.21(4)
103.04(3)
100.60(3)
359.85
155.797(19)
101.824(18)
102.356(19)
359.977
154.19(3)
102.11(2)
103.63(2)
359.93
157.80(2)
96.582(16)
105.099(16)
359.48
3 3 3
3 3 3
P1 Au1 Br1
3 3 3 3 3 3
P2 Au1 Br1
3 3 3
3 3 3
Σ angles about Au1
bond distances (Å)
Au2(μ-dppe)2Br2 (NCMe)
Au2(μ-dppe)2Br2 (NCMe)
Au2(μ-dppe)2Br2 0.5(C4H10O)
Au2(μ-dppe)2Br2 0.5(C4H10O)
3
3
3
3
site A
site B
site A
site B
type B
type B
type B
type B
Au1 Au1A
3.31740(19)
2.7247(3)
2.3033(6)
2.3126(6)
3.09841(18)
2.7115(11)
2.2995(6)
2.3205(6)
3.3249(2)
2.7374(3)
2.3085(7)
2.3154(7)
3.0943(2)
2.7127(3)
2.3019(7)
2.3152(7)
3 3 3
Au1ꢀBr1
Au1ꢀP1
Au1ꢀP2
bond angles (deg)
P1 Au1 P2
145.10(2)
110.442(15)
104.266(15)
359.81
139.87(2)
114.328(15)
104.749(15)
358.95
146.69(2)
108.721(17)
104.443(18)
359.85
141.48(2)
112.073(18)
105.706(18)
359.25
3 3 3
3 3 3
P1 Au1 Br1
3 3 3 3 3 3
P2 Au1 Br1
3 3 3
3 3 3
Σ angles about Au1
bond distances (Å)
{Au(μ-dppe)Br}n 0.5(C4H10O)
{Au(μ-dppe)Br}n (CH2Cl2)
3
3
type C
type C
Au1 Au1A
7.1529(7) intra
9.0082(7) inter
2.7894(5)
7.3645(4) intra
8.3827(7) inter
2.8166(4)
3 3 3
Au1ꢀBr1
Au1ꢀP1
Au1ꢀP2
2.3118(13)
2.3123(10)
2.3095(13)
2.3104(10)
bond angles (deg)
P1 Au1 P2
150.67(4)
108.41(3)
100.86(3)
359.94
153.36(3)
98.74(2)
107.78(2)
359.88
3 3 3
3 3 3
P1 Au1 Br1
3 3 3 3 3 3
P2 Au1 Br1
3 3 3 3 3 3
Σ angles about Au1
evaporate. Likewise, crystals of Au2(μ-dppe)2Br2 (NCMe) of type
the diethyl ether-rich region in the upper part of the tube.
Additionally, in some tubes, but not in all, there is a region in
the upper portion in which type B, green glowing crystals of
3
B are formed when an acetonitrile solution of the initial product
is allowed to evaporate. When crystals are grown by layering diethyl
ether over a dichloromethane solution of the initial colorless
Au2(μ-dppe)2Br2 0.5(C4H10O) grow.
3
product, crystals of Au2(μ-dppe)2Br2 2(CH2Cl2) of type A develop
Although we have developed procedures to crystallize dimeric
Au2(μ-dppe)2Br2 and polymeric {Au(μ-dppe)Br}n in several
different forms, we have not attempted to directly study the species
present in solution. The ability to form both Au2(μ-dppe)2Br2
and {Au(μ-dppe)Br}n from a single mixture of solvents certainly
indicates that these complexes are quite labile in solution. An
earlier study of related complexes by 31P NMR spectroscopy has
revealed the presence of several unidentified compounds along
with [Au(dppe)2]2þ in a solution containing a 1:1:1 ratio of
Auþ, dppe, and Clꢀ.34
3
in the lower region of the tube where the concentration of dichloro-
methane is high, while crystals of {Au(μ-dppe)Br}n (CH2Cl2) of
3
type C grow in the upper portion of the tube in the diethyl ether-rich
region. The situation for crystal growth involving layering diethyl
ether over an acetone solution of the initial product is even more
complex. Type A crystals of the orange glowing acetone solvate,
Au2(μ-dppe)2Br2 2(OCMe2), grow in the lower, acetone-rich
3
portion of the tube, while type C crystals of the orange glowing
diethyl ether solvate, {Au(μ-dppe)Br}n 0.5(C4H10O), grow in
3
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Journal of the American Chemical Society
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Figure 2. Drawing of the structure of Au2(μ-dppe)2Br2 2OSMe2, a
3
representative crystal of type A. Thermal ellipsoids are shown at the 50%
probability level.
Figure 4. Drawing of the structure of Au2(μ-dppe)2Br2 (CH3CN), a
3
representative crystal of type B. Thermal ellipsoids are shown at the 50%
probability level. Hydrogen atoms are not shown for clarity.
the ten-membered ring at the core of the dimer. Despite the fact
that dimethyl sulfoxide is a good ligand, there is no bonding
between it and the gold center. Indeed, it appears that the
dimethyl sulfoxide molecule is merely filling space in the crystal.
The other crystals of type A have similar structural features.
Indeed Au2(μ-dppe)2Br2 2(OSMe2), Au2(μ-dppe)2Br2 2(OCMe2),
3
3
and Au2(μ-dppe)2Br2 2(CH2Cl2) are isostructural, while Au2-
3
(μ-dppe)2Br2 2(HC(O)NMe2) crystallizes differently. In each
3
case, the basic structures of the dimeric gold complex are similar,
and there are no interactions of the solvate molecules with the
three-coordinate gold centers. In all four structures, the ten-
membered ring within the dimer has the conformation shown
in Figure 3. The most noticeable difference in the structures
is found in the separation between the gold centers, which
range from 3.8479(3)Å in Au2(μ-dppe)2Br2 2(OSMe2) to
3
3.5142(3) Å in Au2(μ-dppe)2Br2 2(HC(O)NMe2).
Structures of the Green Luminescent Type B Crystals:
3
Au2(μ-dppe)2Br2 (NCMe) and Au2(μ-dppe)2Br2 0.5(C4H10O).
3
3
Crystals of Au2(μ-dppe)2Br2 (NCMe) and Au2(μ-dppe)2Br2 0.5
3
3
(C4H10O) are isostructural as seen from the data in Table 1. A
drawing of the molecular components in Au2(μ-dppe)2Br2
3
(NCMe) is shown in Figure 4. There are two separate half
molecules of the dimeric gold complex and an acetonitrile molecule
in the asymmetric unit. As is the case in crystals of type A, the
dimeric molecules in the type 2 crystals pack about crystallographic
centers of symmetry. In each of the two solvates, the two gold
dimers have structures remarkably similar to those found in the
type A crystals. Each dimer has the same conformation for the ten-
membered ring at its core as shown in Figure 3. However, in each
Figure 3. View of the ten-membered ring in Au2(μ-dppe)2Br2
(OSMe2). Thermal ellipsoids are shown at the 50% probability level.
For clarity, the phenyl rings and the hydrogen atoms are omitted.
2
3
Structures of the Orange Luminescent Type A Crystals of
Au2(μ-dppe)2Br2. Crystal data for all complexes considered here
are given in Table 1. Selected bond distances and angles are
presented in Table 2. The structure of a representative example
type B crystal the Au Au separations in the two different dimers
3 3 3
of a type A crystal, Au2(μ-dppe)2Br2 2(OSMe2), is shown in
differ significantly. In the acetonitrile solvate the two Au Au
3
3 3 3
Figure 2. The dimeric Au2(μ-dppe)2Br2 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 bromide ligand. Figure 3 shows a drawing of
distances are 3.31740(19) and 3.09841(18) Å, while in the diethyl
ether solvate those distances are 3.3249(2) and 3.0943(2) Å.
Additionally, these Au Au distances are all considerably shorter
3 3 3
than the corresponding Au Au distances in the type A crystals,
3 3 3
which range from 3.8479(3) to 3.5142(3) Å.
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Figure 5. Drawing of a portion of the polymeric chain in {Au(μ-dppe)Br}n (CH2Cl2), a representative crystal of type C. Thermal ellipsoids are shown
3
at the 50% probability level. Hydrogen atoms are not shown for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) for Three-Coordinate, Mononuclear Gold Complexes
Au(PCy3)2Bra
Au(PCy3)2Bra
Au(PCy3)2Bra
Au(PCy3)2Bra
R
β molecule 1
β molecule 2
γ
Au(PPh3)2Brb
bond distances (Å)
Au1ꢀBr1
3.764(4)
2.328(3)
2.305(3)
2.894(1)
2.317(3)
2.311(3)
2.842(1)
2.292(3)
2.302(3)
2.777(2)
2.323(2)
2.323(2)
2.625(2)
2.323(2)
2.323(2)
Au1ꢀP1
Au1ꢀP2
bond angles (deg)
P1 Au1 P2
178.4(1)
72.52(9)
109.0(1)
359.92
162.06(9)
99.56(7)
97.69(7)
359.31
157.7(1)
100.51(8)
106.27(6)
364.48
147.5(1)
106.27(6)
106.27(6)
357.04
132.45(8)
113.78(5)
113.78(5)
360.34
3 3 3
3 3 3
P1 Au1 Br1
3 3 3 3 3 3
P2 Au1 Br1
3 3 3 3 3 3
Σ angles about Au1
a Data from: Bowmaker, G. A.; Brown, C. L.; Hart, R. D.; Healy, P. C.; Englehardt, L. M.; Rickard, C. E. F.; White, A. H. J. Chem. Soc., Dalton Trans. 1999,
881ꢀ889. b Data from: Bowmaker, G. A.; Dyason, J. C.; Healy, P. C.; Englehardt, L. M.; Pakawatchai, C.; White, A. H. J. Chem. Soc., Dalton Trans. 1987,
1089ꢀ1097.
Structures of the Orange Luminescent Polymers {Au-
(μ-dppe)Br}n 0.5(C4H10O) and {Au(μ-dppe)Br}n (CH2Cl2).
Type C crystals contain polymeric chains as shown in Figure 5
However, in other respects the Au2(μ-dppe)2Br2 molecule is
remarkably flexible and able to function as a molecular accordion,
whose structure depends upon the local crystal environ-
3
3
for {Au(μ-dppe)Br}n (CH2Cl2). The structure of {Au(μ-dppe)-
ment. Most notably, it is able to accommodate Au1 Au1A
3
3 3 3
Br}n 0.5(C4H10O) is similar. Within these polymers each gold
separations that range from 3.8479(3) to 3.0943(2) Å. Addi-
tionally, the AuꢀBr distances span a broad range (2.8848(3) to
2.7115(11) Å). The AuꢀBr distances for the type A crystals
are longer (>2.8 Å) than those for the type B crystals, which are
less than 2.8 Å. Long AuꢀBr distances are associated with
3
center is planar and three coordinate with ligation similar to
that in crystals of type A and B as the data in Table 2 show.
The gold centers are widely separated in these polymers. The
closest contact between two gold centers is 7.3645(4) Å
for {Au(μ-dppe)Br}n (CH2Cl2) and 7.1529(7) Å for {Au(μ-dppe)-
long Au1 Au1A separations and also with wide P1ꢀAuꢀP2
3
3 3 3
angles, which are found to range from 157.89(2) to 139.98(4)° in
these dimers. Finally, there are considerable variations in
the BrꢀAuꢀP angles, which can range from 114.06(4) to
96.582(14)°. Moreover, within a single molecule, the two
BrꢀAuꢀP angles can differ by as much as 9.1° or as little as
0.5°. While these structural differences are associated with the
presence of various solvate molecules in the individual crystals,
there do not appear to be readily identifiable interactions such as
coordination to gold or hydrogen bonding between the dimer
molecules and the solvate molecules. Rather, the solvate mole-
cules occupy space within the crystals, and the dimer seems to
alter its geometry to accommodate the space available within a
particular crystalline environment.
Br}n 0.5(C4H10O). These distances are similar because they
3
are the distances between gold centers within individual chains.
The Au Au separations between the closest gold centers in
3 3 3
two adjacent chains are even longer, and clearly there are no
aurophilic interactions in these polymers.
Comparisons of Structures within the Three Types of
Crystals. The dimeric complexes in crystals of type A and B
are all centrosymmetric and share a common ring conformation
that is shown in Figure 3. Within these dimers, the coordina-
tion geometry of each gold center is planar as shown by the
sum of the PꢀAuꢀP and PꢀAuꢀBr angles given in Table 2.
Additionally, the AuꢀP distances fall in a narrow range
(2.3032(6) to 2.3238(14) Å).
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Figure 7. Emission and excitation spectra for representative crystals of
types A, B, and C at room temperature.
Figure 6. Emission spectra for crystals of type A and B and dried crystals
at room temperature.
morphology, the change in the color of the emission is readily
apparent. Exposure of the solvate-free, green glowing crystals to
acetone vapor results in their conversion into orange-glowing
crystals as shown in parts c and d of Figure 8. The loss of the
solvate molecules can be monitored by infrared spectroscopy.
The bond angles and distances about gold in the polymeric
crystals of type C, where the separations between gold centers are
quite long, are most similar to those in Au2(μ-dppe)2Br2 2
(OSMe2), which is the type A crystal with the longest Au Au
separation.
Variations in the AuꢀBr distances and PꢀAuꢀP angles are
common in complexes of the general type, (R3P)2AuBr, as the
data in Table 3 show. The case of (Cy3P)2AuBr, where Cy =
cyclohexyl is particularly interesting.35 This compound crystal-
lizes in three polymorphic forms (R, β, γ) with the β form
containing two molecules in the asymmetric unit. Within the four
different molecular structures, the AuꢀBr distances range from
3.764(4) to 2.777(2) Å, while the PꢀAuꢀP angles vary from
178.4(1) to 147.5(1)°. In (Ph3P)2AuBr, the AuꢀBr distance,
2.625(2) Å, is even shorter, and the PꢀAuꢀP angle is narrower,
132.45(8)°, than any of the corresponding parameters for
(Cy3P)2AuBr.36
3
For example, after drying in air, crystals of Au2(μ-dppe)2Br2 2
3 3 3
3
(OCMe2) lose the band at 1703vs cmꢀ1, which is ν(CdO) for
acetone.
The processes shown in Figure 8 have also been monitored by
X-ray powder diffraction as shown in Figure 9. Traces A and B
of Figure 9 compare the powder diffraction pattern computed
from the single crystal data for Au2(μ-dppe)2Br2 2(OCMe2)
3
with the experimental data obtained from a polycrystalline
sample of Au2(μ-dppe)2Br2 2(OCMe2). As shown in trace C
3
of Figure 9, air drying of Au2(μ-dppe)2Br2 2(OCMe2) results in
3
the loss of the characteristic features seen in traces A and B and
the development of a new pattern of a second crystalline phase,
one that is green glowing when irradiated with near UV light.
However, upon exposure to acetone vapor, the powder pattern
shown in trace C is replaced by the powder pattern shown in
trace D. The data in traces A, B, and D are similar and consistent
It is also clear that, despite the variations in the AuꢀBr
distances and in the BrꢀAuꢀP and PꢀAuꢀP angles, the
bromide ions are coordinated in all of these dimeric solvates.
The cation, [Au2(μ-dppe)2]2þ, with two-coordinate gold, has
been isolated in crystalline form with poorly coordinating anions.
with the reformation of Au2(μ-dppe)2Br2 2(OCMe2) after
3
exposure to acetone vapor. The new crystalline phase responsible
for the green luminescence shown in Figure 8 produces the
powder pattern shown in trace C of Figure 9. Unfortunately,
efforts to obtain single crystals of this phase have not succeeded.
However, based on the data for the green emitting solvates of
type B (Au2(μ-dppe)2Br2 (NCMe) and Au2(μ-dppe)2Br2 0.5
For [Au2(μ-dppe)2](CF3SO3)2 2(CH3CN),37 the PꢀAuꢀP
3
angles are nearly linear (177.10 (5), 171.77 (5)°) and the
Au Au separation (2.9220 (3) Å) is much shorter than any
3 3 3
of the Au Au separations reported here for solvates of Au2-
3 3 3
3
3
(μ-dppe)2Br2. Similarly, in [Au2(μ-dppe)2](PF6)2,38 the Au
(C4H10O)) it appears that the new phase responsible for
the powder pattern seen in trace C of Figure 9 retains the
3 3 3
Au separation (2.935 (1) Å) is short, and the PꢀAuꢀP angle
(160.6(1)°) is bent inward in a fashion that accommodates the
dimeric structure but possesses a shorter Au Au separation
3 3 3
short Au Au distance.
than that seen in the original, orange luminescent Au2(μ-dppe)2-
3 3 3
Luminescence and Vapoluminescence. Emission spectra
for the various crystals of types A and B are shown in Figure 6. As
the figure shows, various crystals of type A produce similar room-
temperature emissions with λmax in the range 640ꢀ620 nm,
while those of type B produce emissions centered at 550 nm.
Figure 7 shows emission and excitation spectra for representative
crystals of each type. Type A crystals of the dimer and those of the
Br2 2(OCMe2).
3
The green luminescent crystals obtained after drying the type
A crystals of Au2(μ-dppe)2Br2 2(CH2Cl2) and Au2(μ-dppe)2-
3
Br2 2(OCMe2) respond to dichloromethane vapor in the
3
same fashion as they do with acetone vapor. However, they do
not change their luminescence when exposed to vapors of carbon
disulfide, diethyl ether, benzene, acetonitrile, pyridine, nitromethane,
dimethyl formamide, or dimethyl sulfoxide. With methanol, the
green emitting phase slowly transforms into an orange emitting
form. After a two to three hour period partial conversion to the
orange emitting form can be visually detected, but about 15 h is
required for the complete conversion.
polymer {Au(μ-dppe)Br}n (CH2Cl2) exhibit similar emission
maxima at ca. 620 nm.
Some of the crystals of types A and B exhibit vapoluminescent
properties. Thus, upon exposure to air or drying in vacuum,
crystals of Au2(μ-dppe)2Br2 2(CH2Cl2) and Au2(μ-dppe)2-
3
3
Br2 2(OCMe2) of type A lose their orange emission and acquire
Not all crystals of type A exhibit this sort of behavior. Thus,
3
a green luminescence. Figure 8 shows photographs of crystals
crystals of Au2(μ-dppe)2Br2 2(OSMe2) and Au2(μ-dppe)2-
3
of Au2(μ-dppe)2Br2 2(CH2Cl2) before and after solvate removal
by drying in air. While the crystals retain their external
Br2 2(HC(O)NMe2) retain their orange luminescence when
3
3
exposed to air or dried under vacuum. The volatility of the solvate
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Journal of the American Chemical Society
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Figure 8. Photographs of crystals of Au2(μ-dppe)2Br2 2(CH2Cl2): A, under ambient light; B, under UV irradiation; C, under UV irradiation after
3
drying in air; and D, under UV irradiation after exposing the dried crystals to acetone vapor.
molecules appears to determine which crystals of type A undergo
the change from orange to green emission. Only those crystals
with the most volatile solvate molecules (acetone and dichloro-
methane) show this transformation.
However, the orange emitting crystals of type C, which involve
polymeric strands rather than discrete dimers, do not show any
change in emission upon drying. Thus, while crystals of the dimeric
The orange-luminescent type A crystals of Au2(μ-dppe)2-
Br2 2(CH2Cl2) and Au2(μ-dppe)2Br2 2(OCMe2) acquire a
3
3
green luminescence upon solvate loss but regain the orange
luminescence upon exposure to vapors of acetone or dichloro-
methane. This is a selective process. Vapors of carbon disulfide,
diethyl ether, benzene, acetonitrile, pyridine, nitromethane,
dimethyl formamide, or dimethyl sulfoxide do not restore the
orange luminescence. The marked flexibility of the Au2(μ-dppe)2-
Br2 molecule is intimately associated with the variations in
luminescence and the vapoluminescent behavior shown in
Figures 1 and 6ꢀ8. The approach of the two gold centers
seen in the crystals of type B results in greater overlap of the
out-of-plane pz orbitals and a corresponding diminution in the
HOMOꢀLUMO gap. As a result, the excitation profile for
crystals of type B shows a low-energy shoulder at ca. 400 nm
that is not present in the excitation spectrum of the crystals of
type A. The difference in luminescence between the crystals of
type A and B can be ascribed largely to a difference in the Stokes’
shifts for the two types with the green emitting crystals having a
lesser Stokes’ shift. The fact that the orange emitting crystalline
polymers of type C polymers do not display variations in
luminescence is consistent with the fact that these polymers do
not contain the flexible Au2(μ-dppe)2Br2 molecule. To obtain
that molecule from the polymer, considerable bond breaking
and remaking would have to occur, a situation that is unlikely to
occur in the solid state.
complexes, Au2(μ-dppe)2Br2 2(OCMe2) and Au2(μ-dppe)2-
3
Br2 2(CH2Cl2), change their emission from orange to green
3
after standing in air for 10 min, crystals of {Au(μ-dppe)Br}n 0.5
(C4H10O) and Au(μ-dppe)Br}n (CH2Cl2) retain their orange
luminescence after standing in air for a day. Likewise, crystals
of type B do not change their luminescence when exposed to air
or vacuum, and they do not show vapoluminescent behavior.
3
3
’ CONCLUSIONS
Our results indicate that crystallization of materials containing
the components Auþ, dppe, and Brꢀ in a 1:1:1 ratio can produce
three different types of crystals suitable for analysis by single
crystal X-ray diffraction. In one case, all three phases crystallize
in the same tube. Orange emitting crystals of type A and green
glowing crystals of type B contain the dimer, Au2(μ-dppe)2Br2,
while crystals of type C involve a linear polymer as shown
in Figure 5. A number of other linear polymers of this type with
other bridging diphosphines have been reported previously.39ꢀ41
In all three types of crystals, dppe functions as a bridging ligand.
Interestingly, we have not encountered dppe acting as a chelat-
ing ligand in any of the complexes we have crystallized. Dppe
is, of course, well designed to function as a chelating ligand
and does form chelate rings in the four-coordinate cation,
[Au(dppe)2]þ.34
Finally, it is important to note that the present compounds
present the first evidence for the significance of aurophilic
interactions between three-coordinate gold(I) centers where
close contacts between the gold centers are not enforced by
small-bite bridging ligands, as is the case with [Au2(μ-dmpm)3]-
(BF4)2 and related compounds mentioned in the Introduction.20
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Journal of the American Chemical Society
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997w, 917w, 879w, 819w, 744m, 724s, 707m, 689vs, 521m, 508s,
475m cmꢀ1. Infrared spectrum polymeric {Au(μ-dppe)Br}n (CH2Cl2):
3
3047w, 2973w, 2895w, 1479s, 1433s, 1412m, 1311m, 1098s, 1073m,
950m, 750m, 726s, 688s, 522m, 508s, 487m, 478m cmꢀ1
.
Au2(μ-dppe)2Br2 2(HC(O)NMe2). The white solid was dissolved
3
in a minimum volume of N,N-dimethylformamide, and the colorless
solution was filtered. After evaporation for a week, colorless blocks grew
in the solution. These crystals, which produced an orange luminescence,
were used for the spectroscopy and crystallography. Infrared spectrum:
3049w, 3008w, 2895w, 1658s (νCdO from HC(O)NMe2), 1484m,
1432s, 1414m, 1379m, 1098s, 822m, 740w, 715s, 689s, 659m, 523m,
510s, 490m, 475m cmꢀ1
.
Au2(μ-dppe)2Br2 (NCMe). The white solid was dissolved in a
3
minimum volume of acetonitrile, and the solution was filtered. After an
hour, the colorless plates with a green luminescence grew in the solution.
These crystals were utilized in the spectroscopic and crystallographic
studies. Infrared spectrum: 3049w, 3008w, 2893w, 2251w (νCꢁN from
acetonitrile), 1481m, 1432s, 1412m, 1100s, 825m, 745m, 688s, 523m,
508s, 483m, 475m cmꢀ1
.
Au2(μ-dppe)2Br2 2(OCMe2), Au2(μ-dppe)2Br2 0.5(C4H10O),
3
3
4
and {Au(μ-dppe)Br}n 0.5(C H10O). Colorless (tht)AuBr (100 mg,
3
0.274 mmol) was dissolved in 30 mL of acetone. While stirring, solid
dppe (164 mg, 0.412 mmol) was added. After additional stirring for 2 h,
the solution was filtered, and then the solvent was removed in a vacuum.
The white solid was collected and washed with diethyl ether. The yield
was 155 mg (84.3%). Crystals suitable for X-ray structure determination
were grown by slow diffusion of diethyl ether to a solution of the colorless
solid in acetone. Colorless, orange emitting crystals of Au2(μ-
dppe)2Br2 2(OCMe2) appeared near the bottom of the tube in the
3
acetone-rich section. Colorless, orange luminescent crystals of {Au(μ-
Figure 9. Powder X-ray diffraction data for orange luminescent Au2-
dppe)Br}n 0.5(C4H10O) appeared in the upper, diethyl ether-rich
3
(μ-dppe)2Br2 2(OCMe2): A, calculated pattern from the single crystal
3
portion of the tube. Additionally, but not in every case, colorless, green
data; B, experimental powder data; C, dried sample from B with green
luminescence; D, orange luminescent sample obtained after exposing
sample shown in C to acetone vapor.
luminescent crystals of Au2(μ-dppe)2Br2 0.5(C4H10O) also appeared
3
in a separate region in the upper, diethyl ether-rich portion of the tube.
Infrared spectrum for Au2(μ-dppe)2Br2 2(OCMe2): 3047w, 3003w,
3
2905w, 1703vs (ν(CdO) from acetone), 1584m, 1481m, 1432vs,
1410m, 1358m, 1309m, 1275m, 1221m, 1095m, 1070m, 1025m, 997m,
87m1, 833m, 809m, 738s, 689vs, 661m, 642w, 616w, 556w, 519m, 508s,
’ EXPERIMENTAL SECTION
Materials. A previously reported procedure was used for the
preparation of (tht)AuBr (tht = tetrahydrothiophene).42
475m cmꢀ1. Infrared spectrum for Au2(μ-dppe)2Br2 0.5(C4H10O):
3
3045w, 3007w, 2892w, 1480m, 1433s, 1411m, 1309w, 1124m, 1100s,
Preparation of Au(dppe)Br. A 100 mg (0.274 mmol) portion of
(tht)AuBr was dissolved in 30 mL of dichloromethane. While stirring,
164 mg (0.412 mmol) of dppe was added. After stirring for 2 h, the
solution was filtered, and then the solvent was removed in a vacuum. The
white solid with green luminescence was collected and washed with
diethyl ether: yield, 160 mg (86.5%). This material was used to obtain
crystalline samples of the following compounds.
1069m, 998w, 826m, 748s, 689s, 524m, 508s, 485m, 470m cmꢀ1. Infrared
spectrum for {Au(μ-dppe)Br}n 0.5(C4H10O): 3048w, 2975w, 2896w,
3
1481s, 1432s, 1412m, 1315m, 1100s, 1073m, 950m, 750m, 726s, 690s,
521m, 511s, 487m, 475m cmꢀ1
.
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 hydro-
carbon 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 CCD with graphite monochromated Mo KR
radiation at 90(2) K.
The structures were solved by direct methods and refined using all data
(based on F2) using the software SHELXTL 5.1. A semiempirical method
utilizing equivalents was employed to correct for absorptions. Hydrogen
atoms were added geometrically and refined with a riding model.43
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 lumine-
scence spectrophotometer.
Au2(μ-dppe)2Br2 2(OSMe2). The original white solid was dissolved
3
in a minimum volume of dimethyl sulfoxide, and the colorless solution
was filtered. After evaporation for a week, colorless blocks with an orange
luminescence grew from the solution. These were used for the X-ray
diffraction and spectral studies. Infrared spectrum: 3047w, 3005w,
2892w, 1484s, 1432s, 1406m, 1315m, 1100w, 1013s, 950m, 746m, 705m,
690s, 522m, 509m, 485m cmꢀ1
.
Au2(μ-dppe)2Br2 2(CH2Cl2) and {Au(μ-dppe)Br}n (CH2Cl2).
3
3
The suitable crystals for X-ray structure determination were grown by
slow diffusion of diethyl ether to a saturated solution of the initial white
solid in dichloromethane. The colorless blocks of Au2(μ-dppe)2Br2
2
3
(CH2Cl2) with an orange luminescence formed over a one day period in
the dichloromethane-rich layer at the bottom of the tube. Simulta-
neously, another set of colorless blocks appeared in the diethyl ether-rich
layer at the upper part of the tube. These orange luminescent crystals
’ ASSOCIATED CONTENT
were identified as the polymer, {Au(μ-dppe)Br}n (CH2Cl2). Infrared
3
S
Supporting Information. Drawings of the molecular struc-
spectrum Au2(μ-dppe)2Br2 2(CH2Cl2): 3048w, 3006w, 2891w, 1480m,
b
3
1433s, 1413m, 1307w, 1265w, 1179w, 1157w, 1100s, 1069m, 1026w,
tures from all of the crystals. X-ray crystallographic files in CIF
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Journal of the American Chemical Society
ARTICLE
format for Au2(μ-dppe)2Br2 2(OSMe2), Au2(μ-dppe)2Br2 2-
(24) Schmidbaur, H.; Herr, R.; Muller, G.; Riedet, J. Organometallics
1985, 4, 1208–1213.
3
3
(OCMe2), Au2(μ-dppe)2Br2 2(CH2Cl2), Au2(μ-dppe)2Br2 2-
3
3
(25) Schmidbaur, H.; Pollok, Th.; Herr, R.; Wagner, F. E.; Bau, R.;
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(27) McCleskey, T. M.; Gray, H. B. Inorg. Chem. 1992, 31,
1773–1734.
(HC(O)NMe2), Au2(μ-dppe)2Br2 (NCMe), Au2(μ-dppe)2Br2 0.5-
3
3
(C4H10O), {Au(μ-dppe)Br}n 0.5(C4H10O), and {Au(μ-dppe)-
3
Br}n (CH2Cl2). This material is available free of charge via the
3
Internet at http://pubs.acs.org.
(28) King, C.; Khan, M. N. I.; Staples, R. J.; Fackler, J. P., Jr. Inorg.
Chem. 1992, 31, 3236–3238.
’ AUTHOR INFORMATION
(29) Barakat, K. A.; Cundari, T. R.; Omary, M. A. J. Am. Chem. Soc.
2003, 125, 14228–14229.
(30) Sinha, P.; Wilson, A. K.; Omary, M. A. J. Am. Chem. Soc. 2005,
127, 12488–12489.
Corresponding Author
albalch@ucdavis.edu
(31) Hoshino, M; Uekusa, H; Ohashi, Y. Bull. Chem. Soc. Jpn. 2006,
79, 1362–1366.
’ ACKNOWLEDGMENT
(32) Hoshino, M.; Uekusa, H.; Sonoda, S.; Takuhiro Otsuka, T.;
Kaizu, Y. Dalton Trans. 2009, 3085–3091.
We thank the Petroleum Research Fund (Grant 37056-AC to
ALB) and U.S. National Science Foundation [CHE-0716843
and CHE-1011760 to ALB and MMO] for support and Dr.
D. Rios for experimental assistance.
(33) Hoshino, M.; Uekusa, H.; Ishii, S.; Otsuka, T.; Kaizu, Y.; Ozawa,
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