Dinuclear gold(I) isocyanide complexes with luminescent properties, and displaying thermotropic liquid crystalline behavior

Article abstract of DOI:10.1021/ic060702a

Dinuclear gold(I) complexes [μ-(4,4′-CN-R-NC){Au(C ₆F₄OC₄H₉)}₂] [R = 1,4-phenylene, n = 8; R = 4,4′-biphenylene, 2,2′-dichloro-4, 4′-biphenylene, 2,2′-dimethyl-4,4′-biphenylene, n = 4,6,8,10] have been prepared and their liquid crystal behavior and optical properties studied. Although the free ligands are not mesomorphic, all the gold(I) derivatives described, except the phenylisonitrilegold(I) derivative [μ-(1,4-CN-C₆H₄-NC){Au(C₆F ₄OC₈H₁₇)}₂], display liquid crystal behavior, giving rise to a nematic mesophase. The transition temperatures decrease in the order 4-4′-biphenylene > 2,2′-dichloro-4- 4′-biphenylene > 2,2′dimethyl-4-4′-biphenylene. All compounds show photoluminescence in the solid state and in solution. The single-crystal X-ray diffraction structures of [μ-(4,4′-CN-R-NC)- {Au(C₆F₄OC_nH_2n+1)}₂] (R = 4-4′-biphenylene and 2,2′-dichloro-4-4′-biphenylene) have been determined confirming the rodlike structure of the molecule, with a linear coordination around the gold atoms. There are Au...Au interactions in the 2,2′-dichlorobiphenyl derivative but not in the 4-4′-biphenyl compound.

Full text of DOI:10.1021/ic060702a

Inorg. Chem. 2006, 45, 10180−10187
Dinuclear Gold(I) Isocyanide Complexes with Luminescent Properties,
and Displaying Thermotropic Liquid Crystalline Behavior
Silverio Coco,* Carlos Cordovilla, Pablo Espinet,* Jose´ Mart´ın-AÄ lvarez, and Paula Mun˜oz
Qu´ımica Inorga´nica, Facultad de Ciencias, UniVersidad de Valladolid, E-47005 Valladolid, Spain
Received April 25, 2006
Dinuclear gold(I) complexes [
2,2 -dichloro-4,4 -biphenylene, 2,2
crystal behavior and optical properties studied. Although the free ligands are not mesomorphic, all the gold(I)
derivatives described, except the phenylisonitrilegold(I) derivative [ -(1,4-CN-C6H4-NC) Au(C6F4OC8H17 2], display
liquid crystal behavior, giving rise to a nematic mesophase. The transition temperatures decrease in the order
4-4 -biphenylene > 2,2 -dichloro-4-4 -biphenylene > 2,2 dimethyl-4-4 -biphenylene. All compounds show photolu-
minescence in the solid state and in solution. The single-crystal X-ray diffraction structures of [ -(4,4 -CN-R-NC)-
Au(C6F4OCnH2n 2] (R 4-4 -biphenylene and 2,2 -dichloro-4-4 -biphenylene) have been determined confirming
the rodlike structure of the molecule, with a linear coordination around the gold atoms. There are Au‚‚‚Au interactions
µ-(4,4
′
-CN-R-NC){Au(C6F4OC4H9)}2] [R
)
1,4-phenylene, n
) 8; R ) 4,4′-biphenylene,
′
′
′
-dimethyl-4,4
′
-biphenylene, n
) 4,6,8,10] have been prepared and their liquid
µ
{
)}
′
′
′
′
′
µ
′
{
)}
+1
)
′
′
′
in the 2,2
′
-dichlorobiphenyl derivative but not in the 4-4
′
-biphenyl compound.
Introduction
containing molecules giving rise to mesophases (metallo-
mesogens) and displaying luminescent properties have been
described: These include a few palladium complexes,15 some
lanthanide derivatives,7,16-19 and recently a family of tetra-
fluoroarylgold(I) isocyanide complexes, which show strong
photoluminescence in the mesophase, as well as in the solid
state and in solution.20 So far, only the two latter references
report luminescence in the mesophase.
An important goal in this area of research is to establish
a clear structure-luminescence relationship, but we are still
far from predicting and controlling these properties. Numer-
ous studies have suggested that aurophilic interactions play
One reason for the interest in the preparation and study
of liquid crystals with metal-containing molecules (metallo-
mesogens) is connected to the possibility of combining the
properties of mesogenic systems (supramolecular organiza-
tion and fluidity in the mesophase) with those of transition
or post-transition metals (high birefringence, paramagnetism,
or color).1-6 In this respect, the incorporation of emission
properties into liquid crystals is currently an important area
of research, as this type of system is particularly suitable
for the development of novel liquid crystalline displays.7 A
number of luminescent liquid crystals based on organic
molecules have been reported,8-14 but just a few metal-
(10) Hoag, B. P.; Gin, D. L. AdV. Mater. 1998, 10, 1546.
(11) Boardman, F. H.; Dunmur, D. A.; Grossel, M. C.; Luckhurst, G. R.
Chem. Lett. 2002, 60.
(12) Attias, A.-J.; Cavalli, C.; Donnio, B.; Guillon, D.; Hapiot, P.; Maltheˆte,
J. Chem. Mater. 2002, 14, 375.
(13) Zhang, Y.; Zhu, W.; Wang, W.; Tian, H.; Su, J.; Wang, W. J. Mater.
Chem. 2002, 12, 1294.
(14) Pschirer, N. G.; Vaughn, M. E.; Dong, Y. B.; zur Loye, H.-C.; Bunz,
U. H. F. Chem. Commun. 2000, 85.
(15) Neve, F.; Ghedini, M.; Crispini, A. Chem. Commun. 1996, 2463. (b)
Ghedini, M.; Aiello, I.; La Deda, M.; Grisolia, A. Chem. Commun.
2003, 2198.
(16) Binnemans K.; Go¨rller-Walrand, C. Chem. ReV. 2002, 102, 2303.
(17) Galyametdinov, Y. G.; Ivanova, G. I.; Ovchinnikov, I. V. Bull. Acad.
Sci. USSR, DiV. Chem. Sci. 1991, 40, 1109.
(18) Binnemans K.; Galyametdinov, Y. G.; Collison, S. R.; Bruce, D. W.
J. Mater. Chem. 1998, 8, 1551.
* To whom correspondence should be addressed. E-mail: espinet@
qi.uva.es (P.E.).
(1) Espinet P.; Esteruelas, M. A.; Oro, L. A.; Serrano, J. L.; Sola, E. Coord.
Chem. ReV. 1992, 117, 215.
(2) Metallomesogens; Serrano, J. L., Ed.; VCH: Weinheim, 1996.
(3) Bruce, D. W. Inorganic Materials, 2nd ed.; Bruce, D. W., O’Hare,
D., Eds.; Wiley: Chichester 1996; Chapter 8. (b) Donnio, B.; Bruce,
D. W. Struct. Bonding 1999, 95 (Liquid Crystals II), 193.
(4) Espinet, P. Gold Bull. 1999, 32, 127.
(5) Collings, P. J. Liquid Crystals (Nature’s Delicate Phase of Matter);
Princeton University Press: Princeton, NJ, 1990.
(6) Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W.,
Spiess, H. W., Vill, D. V., Eds.; Wiley-VCH: Weinheim, 1998.
(7) Guillet, E.; Imbert, D.; Scopelliti, R.; Bu¨nzli, J.-C. G. Chem. Mater.
2004, 16, 4063.
(8) Yu, L. J.; Labes, M. M. Appl. Phys. Lett. 1977, 279, 835.
(9) Zhang, Y.; Zhu, W.; Wang, W.; Tian, H.; Su, J.; Wang, W. J. Mater.
Chem. 2002, 12, 1294.
(19) Sua´rez, S.; Mamula, O.; Imbert, D.; Piguet, C.; Bu¨nzli, J.-C. G. Chem.
Commun. 2003, 1226.
(20) Bayo´n, R.; Coco, S.; Espinet, P. Chem. Eur. J. 2005, 11, 1079.
10180 Inorganic Chemistry, Vol. 45, No. 25, 2006
10.1021/ic060702a CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/08/2006

Dinuclear Gold(I) Isocyanide Complexes
cyanides was directed to have a set of ligands containing a
rigid aromatic core (R ) C6H4 or biphenylenes) with different
twist angles between the two rings of the biphenyl group, to
check experimentally their influence on the properties of the
system. Here we report the synthesis and complete charac-
terization of a family of dinuclear metallomesogens display-
ing only a nematic mesophase and intense luminescence at
room temperature, including the study of two crystal
structures of [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2] (R )
4,4′-biphenylene and 2,2′-dichloro-4,4′-biphenylene; we could
not get good quality crystals for R ) 2,2′-dimethyl-4,4′-
biphenylene).
Experimental Section
Figure 1. A simple model suggesting how, in symmetric dimers, the best
multipole-multipole interactions are not those arising from layered packing
of molecules, represented in part a, but those involving some mixing of
aromatic and aliphatic regions, represented in part b (see also ref 23).
Combustion analyses were made with a Perkin-Elmer 2400
microanalyzer. IR spectra (cm-1) were recorded on a Perkin-Elmer
FT 1720X instrument and 1H and 19F NMR spectra on Bruker AC
300 or ARX 300 instruments in CDCl3. Microscopy studies were
carried out using a Leica DMRB microscope equipped with a
Mettler FP82HT hot stage and a Mettler FP90 central processor,
at a heating rate of 10 °C min-1. For differential scanning
calorimetry (DSC) a Perkin-Elmer DSC7 instrument was used,
which was calibrated with water and indium. The scanning rate
was 10 °C min-1, the samples were sealed in aluminum capsules
in the air, and the holder atmosphere was dry nitrogen. UV-vis
absorption spectra were obtained on a Shimadzu UV-1603 spec-
trophotometer in dichloromethane solutions (1 × 10-4 M). Lumi-
nescence data were recorded on a Perkin-Elmer LS-55 luminescence
spectrometer. Luminiscence quantum yields were obtained at room
temperature using the optically dilute method in degassed di-
chloromethane (quantum yields standard was quinine sulfate in 0.1
N aqueous sulfuric solution Φ ) 0.546).24
a key role in the luminescence of gold(I) complexes.21
Au‚‚‚Au intermolecular interactions have also been proposed
to be important for the mesogenic behavior of simple gold
alkylisocyanide complexes.22 However, these interactions are
not always needed. For instance, the tetrafluoroarylgold(I)
isocyanide derivatives mentioned above show two phospho-
rescent emissions in the solid state that disappear in solution.
A single-crystal X-ray diffraction study of one of these
tetrafluorophenylgold complexes demonstrated the absence
of Au‚‚‚Au contacts in the solid state. Moreover, well-defined
intermolecular Fortho‚‚‚Fmeta interactions were found, which
are responsible for the crystal packing. A detailed study
proved that the photoluminescence observed has an intra-
molecular origin and is not associated with any Au‚‚‚Au
contacts.20
We have reported previously a family of dinuclear
organometallic gold complexes [(µ-4,4′-C6F4C6F4){AuCN-
(C6H4)mOCnH2n+1}2] with unusual mesogenic behavior: Al-
though they display high transition temperatures, indicating
strong intermolecular interactions, they give rise only to a
nematic phase, and not to more ordered mesophases.23 This
apparent contradiction (strong interactions but only a less
ordered mesophase) was explained considering the electronic
and structural factors that determine the intermolecular
interactions in the system. It was suggested, but not
experimentally confirmed, that in these molecules with large
twist angles between the two rings of the octafluorobiphenyl
group the best multipole-multipole interactions should
involve a not layered packing structure (Figure 1), whether
in the solid or in the mesophase. As a consequence, only a
nematic phase might appear above the melting point.
We therefore decided to synthesize similar dinuclear
tetrafluorophenyl gold(I) complexes of type [µ-(4,4′-CN-R-
NC){Au(C6F4OCnH2n+1)}2], containing a diisocyanide ligand
bringing group two gold atoms. The selection of diiso-
Literature methods were used to prepare 1-alkyloxy-2,3,5,6-
tetrafluorobenzenes.25 1,4-diisocyanobenzene,26 4,4′-diisocyanobi-
phenyl,26 2,2′-dichloro-4,4′-dinitrobiphenyl,27 4,4′-diamino-2,2′-
dimethylbiphenyl,28 and [AuCl(tht)] (tht ) tetrahydrothiophene).29
Only typical examples are described, as the syntheses were
similar for the rest of the compounds. Yields, IR, and analytical
data are given for all the gold complexes.
CAUTION!: Aminobiphenyls are very carcinogenic materials
and must be handled with extreme care.
Preparation of 4,4′-Diamino-2,2′-dichlorobiphenyl. Graphite
(2.0 g) was added to a solution of 2,2′-dichloro-4,4′-dinitrobiphenyl
(0.63 g, 2.01 mmol) and hydrazine monohydrate (1.36 mL, 28.16
mmol) in 50 mL of dried ethanol. The flask was purged with N2,
and the suspension was refluxed for 6 h. The reaction mixture was
allowed to cool to 40 °C and filtered. The solvent was removed on
a rotary evaporator to obtain the product as white solid (80.5 g,
98% yield).
Preparation of 2,2′-Dichloro-4,4′-diisocyanobiphenyl. The
procedure described by Ugi was followed, but with triphosgene as
(24) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229.
(25) Coco, S.; Ferna´ndez- Mayordomo, C.; Falaga´n, S. Espinet, P. Inorg.
Chim. Acta 2003, 350, 366.
(26) Efraty, A.; Feinstein, I.; Wackerle, L.; Goldman, A. J. Org. Chem.
1980, 45, 4059.
(27) Ln˜itunenko, L. M.; Grekov, A. P.; Verkhovod, N. N.; Dzyuba, U. P.
Chem. Abstr. 1957, 51, 5007.
(28) Calin, R. B.; Odioso, R. C. J. Am. Chem. Soc. 1954, 76, 2345.
(29) Uso´n, R.; Laguna, A.; Vicente, J. J. Organomet. Chem. 1977, 131,
471.
(21) White-Morris, R. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem.
Soc. 2003, 125, 1033.
(22) Bachman, R. E.; Fioritto, M. S.; Fetics, S. K.; Cocker, T. M. J. Am.
Chem. Soc. 2001, 123, 5376.
(23) Bayo´n, R.; Coco, S.; Espinet, P.; Ferna´ndez-Mayordomo, C.; Mart´ın-
AÄ lvarez, J. M. Inorg. Chem. 1997, 36, 2329.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10181

Coco et al.
dehydrating agent.30 To a solution of 4,4′-di-N-formamido-2,2′-
dichlorobiphenyl (0.6 g, 1.94 mmol), prepared by reaction of the
diamine with formic acid, and triethylamine (1.08 mL, 7.76 mmol)
in 50 mL of CH2Cl2 was added dropwise a solution of trichloro-
methyl carbonate (triphosgene) (0.69 g, 2.32 mmol) in 25 mL of
CH2Cl2. The mixture was refluxed for 2 h, and then, the solvent
was removed on a rotary evaporator. The resulting residue was
chromatographed (silica gel, CH2Cl2/hexane, 3:1 as eluent), and
the solvent was evaporated to obtain the product as a white solid
R ) 2,2′-Dimethyl-4,4′-biphenylene. n ) 4. Yield: 42%. IR
1
ν(CtN)/cm-1: (CH2Cl2) 2218, (Nujol) 2214. H NMR (CDCl3):
δA 7.51, δM ) 7.48, δX ) 7.23, AMX spin system (JAM ) 1.5 Hz,
JMX ) 8.0 Hz), 4.16 (t, 4H, J ) 6.6, -O-CH2-), 2.12 (s, 3H, CH3),
1.79-0.90 (m, 9H, alkoxy chains). 19F NMR (CDCl3): δ1 -118.25,
δ2 -157.36, AA′XX′ spin system (3JA,B + 5JA,B′ ) 17.9 Hz, 4JA,A′
4
+ JB,B′ ) 5.9 Hz). Anal. Calcd for C36H30Au2F8N2O2: C, 40.46;
H, 2.83; N, 2.62. Found: C, 40.80; H, 2.87; N, 2.80.
n ) 6. Yield: 46%. IR ν(CtN)/cm-1: (CH2Cl2) 2217, (Nujol)
2208. Anal. Calcd for C40H38Au2F8N2O2: C, 42.72; H, 3.40; N,
2.49. Found: C, 42.60; H, 3.39; N, 2.56.
n ) 8. Yield: 55%. IR ν(CtN)/cm-1: (CH2Cl2) 2217, (Nujol)
2211. Anal. Calcd for C44H46Au2F8N2O2: C, 44.76; H, 3.93; N,
2.37. Found: C, 44.63; H, 3.95; N, 2.32.
n ) 10. Yield: 55%. IR ν(CtN)/cm-1: (CH2Cl2) 2218, (Nujol)
2214. Anal. Calcd for C48H54Au2F8N2O2: C, 46.61; H, 4.41; N,
2.26. Found: C, 46.30; H, 4.21; N, 2.33.
1
(0.34 g, 64% yield). IR ν(CtN)/(CH2Cl2): 2130 cm-1. H NMR
(CDCl3): δA 7.55 (d, 2H, JAM ) 1.8 Hz), δM 7.38 (dd, 2H, JMX
8.2 Hz), JAM ) 1.8 Hz), δX 7.29 (d, 2H, JMX ) 8.2 Hz).
)
Preparation of 2,2′-Dimethyl-4,4′-diisocyanobiphenyl. This
isonitrile was prepared as described for the analogous 2,2′-dichloro-
4,4′-diisocyanobiphenyl, starting from 4,4′-diamino-2,2′-dimethyl-
1
biphenyl. IR ν(CtN)/(CH2Cl2): 2126 cm-1. H NMR (CDCl3):
δA 7.31 (d, 2H, JAM ) 1.7 Hz), δM 7.27 (dd, 2H, JMX ) 8.0 Hz,
JAM ) 1.7 Hz), δX 7.07 (d, 2H, JMX ) 8.0 Hz), 2.03 (s, 6H, CH3).
Preparation of [µ-(4,4′-CN-R-NC){Au(C6F4OCnH2n+1)}2]. To
a solution of HC6F4OCnH2n+1 (0.09 mmol) in 25 mL of dry diethyl
ether was added a solution of LiBun in hexane (0.56 mL, 0.09
mmol) at -78 °C under nitrogen. After stirring for 1 h at -50 °C,
solid [AuCl(tht)] (0.29 g, 0.09 mmol) was added at -78 °C, and
the reaction mixture was slowly brought to room temperature (3
h). Then, a few drops of water were added and the solution was
filtered in air through anhydrous MgSO4. CN-R-NC (0.045 mmol)
was added to the solution obtained. After stirring for 15 min, the
solvent was removed on a rotary evaporator, and the brown solid
obtained was recrystallized from dichloromethane/hexane at -15
°C to give a microcrystalline white solid.
R ) 4,4′-Biphenylene. n ) 4. Yield: 45%. IR ν(CtN)/cm-1
:
1
(CH2Cl2) 2214, (Nujol) 2212. H NMR (CDCl3): δ 7.73 (m, 8H,
aromatics), AA′BB′ spin system (δA ≈ δB), 4.17 (t, 4H, J ) 6.4,
-O-CH2-), 1.77-0.90 (m, 9H, alkoxy chains). 19F NMR
(CDCl3): δA -118.29, δB -157.34, AA′XX′ spin system (3JA,B
+
4
5JA,B′ ) 17.9 Hz, JA,A′
+
4JB,B′ ) 6.3 Hz). Anal. Calcd for
C34H26Au2F8N2O2: C, 39.25; H2.52; N, 2.69. Found: C, 39.14;
H, 2.56; N, 2.55.
n ) 6: Yield: 50%. IR ν(CtN)/ cm-1: (CH2Cl2) 2214, (Nujol)
2209. Anal. Calcd for C38H34Au2F8N2O2: C, 41.62; H, 3.13; N,
2.55. Found: C, 42.10; H, 3.22; N2.53.
n ) 8: Yield: 85%. IR ν(CtN)/ cm-1: (CH2Cl2) 2214, (Nujol)
2211. Anal. Calcd for C42H42Au2F8N2O2: C, 43.76; H, 3.67; N,
2.43. Found: C, 43.69; H, 3.65; N, 2.01.
n ) 10: Yield: 45%. IR ν(CtN)/cm-1: (CH2Cl2) 2214, (Nujol)
2216. Anal. Calcd for C46H50Au2F8N2O2: C, 45.71; H, 4.17; N,
2.32. Found: C, 45.60; H, 4.13; N, 2.07.
Experimental Procedure for X-ray Crystallography. Crystals
of [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2] (R ) 4,4′-biphenylene
and 2,2′dichloro-4-4′-biphenylene) were obtained by direct diffusion
of hexane into a solution of the complex in dichloromethane.
Suitable single crystals were mounted in glass fibers, and diffraction
measurements were made using a Bruker SMART CCD area-
detector diffractometer with Mo KR radiation (λ ) 0.71073 Å).31
Intensities were integrated from several series of exposures, each
exposure covering 0.3° in ω, the total data set being a hemisphere.32
Absorption corrections were applied based on multiple and sym-
metry-equivalent measurements.33 The structure was solved by
Patterson synthesis or direct methods and refined by least squares
on weighted F2 values for all reflections (see Table 1).34 All non-
hydrogen atoms were assigned anisotropic displacement parameters
and refined without positional constraints. Hydrogen atoms were
taken into account at calculated positions, and their positional
parameters were refined. Complex neutral-atom scattering factors
were used.35 Crystallographic data (excluding structure factors) for
the structures reported in this paper have been
Yields, IR, analytical data, and representative 1H and 19F NMR
follow below. When NMR data are not given, these are practically
1
identical to those provided for similar complexes (except the H
NMR spectra which differ in the intensity of the multiplet
comprising the undefined hydrogen atoms of the alkoxy chains,
which is in each case proportional to their number).
R ) 1,4-Phenylene; n ) 8. Yield: 70%. IR ν(CtN)/cm-1
:
1
(CH2Cl2) 2219, (Nujol) 2215. H NMR (CDCl3): δ 7.73 (s, 4H,
aromatics), 4.13 (t, 4H, J ) 6.6, -O-CH2-), 1.77-0.85 (m, 30H,
alkoxy chains). 19F NMR (CDCl3): δ1 -118.19, δ2 -157.15,
5
4
4
AA′XX′ spin system (3J1,2 + J1,2′ ) 18.1 Hz, J1,1′ + J2,2′ ) 6.2
Hz,). Anal. Calcd for C36H38Au2F8N2O2: C, 40.16; H, 3.56; N, 2.60.
Found: C, 40.09; H, 3.60; N, 2.75.
R ) 2,2′-Dichloro-4,4′-biphenylene. n ) 4. Yield: 77%. IR
1
ν(CtN)/cm-1: (CH2Cl2) 2215, (Nujol) 2214. H NMR (CDCl3):
δA 7.77, δM ) 7.60, δX ) 7.45, AMX spin system (JAM ) 2.0 Hz,
JMX ) 8.4 Hz), 4.16 (t, 4H, J ) 6.6, -O-CH2-), 1.79-0.90 (m,
9H, alkoxy chains). 19F NMR (CDCl3): δ1 -118.25, δ2 -157.36,
5
4
4
AA′XX′ spin system (3JA,B + JA,B′ ) 18.0 Hz, JA,A′ + JB,B′
)
6.5 Hz). Anal. Calcd for C34H24Au2Cl2F8N2O2: C, 36.81; H, 2.18;
N, 2.52. Found: C, 36.84; H, 2.15; N, 2.24.
n ) 6. Yield: 57%. IR ν(CtN)/cm-1: (CH2Cl2) 2216, (Nujol)
2218. Anal. Calcd for C38H32Au2Cl2F8N2O2: C, 39.16; H, 2.72;
N, 2.52. Found: C, 36.84; H, 2.15; N, 2.24.
n ) 8. Yield: 65%. IR ν(CtN)/cm-1: (CH2Cl2) 2217, (Nujol)
2215. Anal. Calcd for C42H40Au2Cl2F8N2O2: C, 41.30; H, 3.30;
N, 2.29. Found: C, 41.45; H, 3.08; N, 3.88.
n ) 10. Yield: 40%. IR ν(CtN)/cm-1: (CH2Cl2) 2216, (Nujol)
2218. Anal. Calcd for C46H48Au2Cl2F8N2O2: C, 43.24; H, 3.78;
N, 2.19. Found: C, 43.21; H, 3.78; N, 2.08.
(31) SMART V5.051 Diffractometer Control Software; Bruker Analytical
X-ray Instruments Inc.: Madison, WI, 1998.
(32) SAINT V6.02 Integration Software; Bruker Analytical X-ray Instru-
ments Inc.: Madison, WI, 1999.
(33) Sheldrick, G. M. SADABS: A program for absorption correction with
the Siemens SMART system; University of Go¨ttingen: Go¨ttingen,
Germany, 1996.
(34) SHELXTL program system Version 5.1; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(30) Ugi, I.; Fetzer, U.; Knupfer, H.; Offermann, K. Angew. Chem., Int.
Ed. Engl. 1965, 4, 472.
(35) International Tables for Crystallography; Kluwer: Dordrecht, 1992;
Vol. C.
10182 Inorganic Chemistry, Vol. 45, No. 25, 2006

Dinuclear Gold(I) Isocyanide Complexes
Table 1. X-ray Data and Data Collection and Refinement Parameters for [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2] (R ) 4,4′-Biphenylene and
2,2′-Dichloro-4,4′-biphenylene)
empirical formula
fw
C34H26Au2F8N2O2
1040.50
C34H24Au2Cl2F8N2O2
1109.39
cryst syst
space group
a (Å)
monoclinic
P21/c
12.052(3)
monoclinic
P21/n
14.007(5)
b (Å)
16.387(4)
10.607(4)
c (Å)
16.981(4)
24.343(9)
â (deg)
91.881(5)
3351(13)
103.275(8)
3520(2)
V (Å3)
Z
D
4
4
calcd (g/cm-3
)
2.062
8.822
1960
2.093
8.555
2088
abs coeff (mm-1
F(000)
)
cryst size (mm3)
temp (K)
0.23 × 0.08 × 0.03
0.12 × 0.07 × 0.02
297(2)
298(2)
θ range for data collcn (deg)
wavelength (Å)
1.69-23.28
0.71073
1.54-23.26
0.71073
index ranges
reflns collected
-13 e h e 13, 0 e k e 18, 0 e l e 18
15627
-15 e h e 15, 0 e k e 11, 0 e l e 26
16411
indep reflns
completeness to θ
abs corr
4812 [Rint ) 0.0403]
23.28 (99.6%)
SADABS
5037 [Rint ) 0.0460]
23.26 (99.5%)
SADABS
max and min transm
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff. peak and hole (e Å-3
1.000000 and 0.506552
4812/0/435
0.936
R1 ) 0.0292, wR2 ) 0.0597
R1 ) 0.0504, wR2 ) 0.0645
0.963 and -0.484
1.000000 and 0.544350
5037/0/453
1.000
R1 ) 0.0346, wR2 ) 0.0800
R1 ) 0.0688, wR2 ) 0.0963
1.331 and -0.623
)
Scheme 1a
deposited as Supporting Information, and with the Cambridge
Crystallographic Data Centre as supplementary publication with
the following deposition numbers: CCDC-605387 for [µ-(4,4′-CN-
C6H4-C6H4-NC){Au(C6F4OC4H9)}2] and CCDC-605388 for [µ-(4,4′-
CN-C6H3Cl-C6H3Cl-NC){Au(C6F4OC4H9)}2]. Copies of the data
can be obtained free of charge on application to the CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. [fax (international) +44-
1223/336-033; e-mail deposit@ccdc.cam.ac.uk].
Results and Discussion
Synthesis and Characterization. The isocyanides, 4,4′-
diisocyano-2,2′-dichlorobiphenyl and 4,4′-diisocyano-2,2′-
dimethylbiphenyl, have not been reported before and were
prepared starting from the corresponding dinitrobiphenyl
compounds in three steps, involving reduction to the diamines
H2NRNH2 by hydrazine/graphite, formylation to their N-
diformamides, and finally dehydration with bis(trichloro-
methyl)carbonate (“triphosgene”) and triethylamine to the
diisocyanides CN-R-NC, as summarized in Scheme 1. None
of the free isocyanide ligands prepared and used in this work
show liquid crystal behavior. At room temperature they are
white solids, not particularly smelling, that can be stored for
long periods in the freezer.
The gold(I) isocyanide complexes were prepared as
described in the literature for mononuclear fluorophenyl-
gold(I) isocyanide derivatives,23,25 as depicted in Scheme 1
(tht ) tetrahydrothiophene). The compounds [(µ-4,4′-CN-
R-NC){Au(C6F4OCnH2n+1)}2] [R ) 2,2′-dichloro-4,4′-bi-
phenylene, 2,2′-dimethyl-4,4′-biphenylene, 4,4′-biphenylene;
n ) 4,6,8,10; R ) 1,4-phenylene, n ) 8] were obtained by
arylation of [AuCl(tht)] followed by ligand exchange of tht
with the appropriate isocyanide ligand. They were isolated
as white solids. The biphenyl moiety is not planar, and the
complexes with 2,2′-dichlorobiphenylene and 2,2′-di-
methylbiphenylene contain an asymmetric biphenyl core. The
a Reagents: i, N2H4-graphite; ii, HCO2H; iii, (Cl3CO)2CO, Et3N.
rotational barriers for these biphenyl moieties are reasonably
high.36 This means that the compounds should be racemic
mixtures, at least in the solid state.
The C, H, N analyses for the complexes, yields, and
relevant IR data are given in the Experimental Section. The
IR spectra are all similar and show one ν(CtN) absorption
for the isocyanide group at higher wavenumbers (ca. 90
cm-1) than for the free ligands, as reported for other gold(I)
(36) Leroux, F. ChemBioChem 2004, 5, 644.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10183

Coco et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) within
Molecules [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2]
R ) C6H4-C6H4
R ) C6H3Cl-C6H3Cl
Au(1)-C(1)
1.958(8)
2.031(6)
1.131(8)
1.987(9)
2.026(7)
1.137(8)
177.5(3)
177.6(7)
178.1(8)
179.3(3)
1.984(11)
2.046(9)
1.137(11)
1.969(10)
2.023(9)
1.155(11)
175.7(4)
176.6(9)
177.5(9)
175.7(4)
Au(1)-C(21)
C(1)-(N(1)
Au(2)-C(2)
Au(2)-C(31)
N(2)-C(2)
C(1)-Au(1)-C(21)
N(1)-C(1)-Au(1)
N(2)-C(2)-Au(2)
C(2)-Au(2)-C(31)
isocyanide compounds.23,25 The 1H NMR spectra of the metal
complexes are all very similar for each family. Thus, at 300
MHz the 4,4′-diisocyano-2,2′-dichlorobiphenyl and 4,4′-
diisocyano-2,2′-dimethylbiphenyl complexes show similar
spectra, displaying three resonances from aromatic protons
(AMX spin systems), as expected. The 4,4′-diisocyano-2,2′-
dimethylbyphenyl complexes display also a singlet from the
2- and 2′-methyl groups, at 2.03 ppm. In addition, one triplet
is observed at ca. 4.1 ppm corresponding to the first
methylene group of the C6F4OCnH2n+1 moiety. The remaining
chain hydrogen atoms appear overlapped in the range 0.8-
1.8 ppm. The 1H NMR spectrum of 4,4′-diisocyanobiphenyl
shows two somewhat distorted “pseudodoublets” (a decep-
tively simple pattern for the AA′XX′ spin system with δA
≈ δB) for the biphenyl group present in the molecule. The
1H NMR spectra of 1,4-isocyanobenzene complexes show
one signal for aromatic protons, as expected.
Figure 2. (Top) Molecular ORTEP for [µ-(4,4′-CN-C6H4-C6H4--NC)-
{Au(C6F4OC4H9)}2] and zigzag molecular arrangement. (Middle) View of
one sequence of molecules showing the Fmeta‚‚‚Au short distances. (Bottom)
View of the arrangement of these zigzag sequences.
not the contribution of an electrostatic interaction at the
exclusion distance of the aryl rings. This might be the reason
for the zigzag arrangement of the stacking (Figure 2).
[µ-(4,4′-CN-C6H3Cl-C6H3Cl-NC){Au(C6F4OC4H9)}2]. The
compound crystallizes in the monoclinic space group P21/n,
with 4 formula units per unit cell. The molecule takes a
rodlike shape, as in the biphenylisocyanide derivative
discussed above, but the dihedral angle between the two rings
of the 2,2′-dichlorobiphenylisocyanide group is greater
(104.9° considering the position of the substituents; effective
angle between ring planes 75.1°), revealing in this complex
a lower electronic conjugation between the phenyl rings. The
crystal packing of the complex shows a molecular arrange-
ment in planes (Figure 3, middle). Within the plane the
intermolecular distances Au‚‚‚Fmeta (3.60 Å) and Au‚‚‚Cl
(3.67 Å) are higher than the sum of the van der Waals radii
of gold and the corresponding halogen. As discussed above,
this would exclude the existence of intraplanar Au‚‚‚halogen
covalent interactions, but not of electrostatic interactions.
These layers produce a stacking with each plane shifted with
respect to the previous (Figure 3, down). The Au‚‚‚Au
distances between molecules contained in consecutive planes
(3.61 Å) are suggestive of some aurophilic interactions.
It is important to note that neither [µ-(4,4′-CN-C6H4-C6H4-
NC){Au(C6F4OC4H9)}2] nor [µ-(4,4′-CN-C6H3Cl-C6H3Cl-
NC){Au(C6F4OC4H9)}2] adopt a layered packing of mol-
ecules in the solid state. This reluctance to adopt layered
packing is in coincidence with the absence of smectic phases
in their mesophases.
The 19F NMR spectra of these complexes show two
somewhat distorted “doublets” flanked by two pseudotriplets
corresponding to an AA′XX′ spin system with JAA′ ≈ JXX′
.
The signals appear at ca. -118 ppm and ca. -157 ppm (ref
CFCl3), assigned to F(ortho) and F(meta), respectively, with
apparent coupling constants (N ) JAX + JAX′) of 18.0 Hz,
as reported for mononuclear tetrafluorophenylgold(I) iso-
cyanide complexes.
X-ray Crystallography. Compounds [µ-(4,4′-CN-R-NC)-
{Au(C6F4OC4H9)}2] (R ) 4,4′-biphenylene and 2,2′-dichloro-
4,4′-biphenylene) have been characterized by X-ray diffrac-
tion. The crystal data and experimental details are given in
the Experimental Section and in Table 1.
[µ-(4,4′-CN-C6H4-C6H4-NC){Au(C6F4OC4H9)}2]. The
compound crystallizes in the monoclinic space group P21/c,
with 4 formula units per unit cell. The gold atoms are linearly
coordinated by the diisocyanide ligand and tetrafluorophenyl
groups. Bond angles and bond lengths fall within normal
ranges (Table 2). The two rings of the biphenylisocyanide
system make a dihedral angle of 28.3°. The butoxy chains
are extended, and the molecule takes a rodlike shape. The
crystal packing of the complex shows stacking of molecules
in a parallel disposition, but each molecule is slightly shifted
from the previous, so that the shortest intermolecular distance
observed is 3.56 Å, between one Fmeta and a gold atom of
neighboring molecules (Figure 1). Considering that the sum
of the van der Waals radius of gold and fluorine is 3.05 Å,37
the intermolecular Au‚‚‚Fmeta distance observed would ex-
clude the existence of any covalent Au‚‚‚F interaction, but
Mesogenic Behavior. Although none of the free ligands
used in this work behave as a liquid crystal, all the
(37) Emsley, J. The Elements, 3rd ed.; Oxford University Press: New York,
2000.
10184 Inorganic Chemistry, Vol. 45, No. 25, 2006

Dinuclear Gold(I) Isocyanide Complexes
Figure 3. (Top) Molecular ORTEP for [µ-(4,4′-CN-C6H3Cl-C6H3Cl-NC)-
{Au(C6F4OC4H9)}2] and molecular arrangement. (Middle) View of one
sequence of molecules showing the halide‚‚‚Au short distances. (Bottom)
View of the arrangement of these molecular sequences.
Figure 4. Polarized optical microscopic texture (×100) observed for
[µ-(4,4′-CN-C6H3Cl-C6H3Cl-NC){Au(C6F4OC6H13)}2]. The picture shows
the marbled texture of the N phase, at 152 °C on heating from the crystal.
Table 3. Optical, Thermal, and Thermodynamic Data for
[µ-(4,4′-CN-R-NC){Au(C6F4OCnH2n+1)}2] with R ) 4,4′-Biphenylene
(R1), 2,2′-Dichloro-4,4′-biphenylene (R2), 2,2′-Dimethyl-4,4′-biphenylene
(R3), and 1,4-Phenylene (R4)
The variation in thermal properties can be summarized as
follows: The clearing temperatures decrease with increasing
length of the alkoxy chain, and in the order phenyl >
biphenyl > 2,2′-dichlorobiphenyl > 2,2′-dimethylbiphenyl.
The same trend is observed for the melting temperatures with
one exception, the octyloxy-2,2′-dichlorobiphenylene com-
pound, which shows the highest melting point of the
octiloxybiphenyl derivatives.
R
n
transitiona
tempb (°C)
∆Hb (kJ/mol)
R1
4
Cr f N
N f I
Cr fCr
Cr f N
Ν f I
Cr f Cr’
Cr’ f N
N f I
Cr f Cr
Cr f N
N f I
Cr f Cr’
C’ f N f Id
N f Cr
Cr f Cr’
Cr’ f N
N f I
N f Cr
Cr f N
N f I
256.0
290c (dec)
112.7
194.5
275c (dec)
114.1
135.2
257c (dec)
98.1
139.4
222c (dec)
110.5
194.6
72.2
19.4
R1
R1
R1
6
8
3.4
24.8
5.8
27.5
The simplest way to look at the rodlike molecules
containing a biphenyl moiety, [µ-(4,4′-CN-R-NC){Au-
(C6F4OCnH2n+1)}2] [R ) 2,2′-dichloro-4,4′-biphenylene,
1 0
2.1
29.6
2,2′-dimethyl-4,4′-biphenylene, 4,4′-biphenylene;
n )
4,6,8,10; R ) 1,4-phenylene, n ) 8], is to consider them as
formal derivatives of [µ-(4,4′-CN-C6H4C6H4-NC){Au-
(C6F4OCnH2n+1)}2] by substitution of a H atom by a Cl or a
CH3 group in the 2 and 2′ positions of the biphenyl system,
which will determine the magnitude of intermolecular
electrostatic interactions. In general, more planar molecules
should lead to more compact packing and stronger interac-
tions than nonplanar molecules. Theoretical studies have
shown that, for biphenyl systems, the substituents in 2 and
2′ positions determine the twist angle of the biphenyl in the
gas phase, as well as its rotational barrier. Both increase in
the order H , Cl < Me.36 This trend is supported by the
two X-ray structures determined here (H/H twist angle )
28.3°; Cl/Cl twist angle ) 104.9°, equivalent to 75.1°
between planes). Surprisingly this trend is the same as that
observed for the variation in melting and clearing temper-
atures experimentally observed for the biphenyl compounds
(H/ H-biphenyl > Cl/Cl-biphenyl > Me/Me-biphenyl),
despite their different solid-state structures and bond dipoles
(C-H, C-Me, C-Cl) in the biphenyls, suggesting that the
molecular packing in the melt (and hence the properties of
the bulk material) is still strongly influenced by the higher
R2
R2
4
6
5.03
36.0e
-18.6
-4.3
41.1
0.8
-31.8
38.0
0.3
99.7
142.3
170.3
65.8
150.8
162.9
127.8
159.2
202.2
79.0
118.4
143.9
105.0
145.2
101.4
130.1
212.0
232.6
R2
R2
8
1 0
Cr f N
N f I
36.7
1.3
R3
R3
4
6
Cr f N f Id
I f Cr
36.9e
-20.6
21.9
0.9
Cr f N
N f I
Cr f N
N f I
Cr f N
N f I
Cr f Cr
Cr f I
R3
R3
R4
8
1 0
8
22.2e
1.2
24.6
0.6
0.5
44.8
a Cr, crystal; N, nematic; I, isotropic liquid. b Data referred to the second
DSC. Temperatures referred to the onset temperatures. c Microscopy data.
d Data referred to the first DSC cycle starting from the crystal. e Combined
enthalpies.
tetrafluorophenylgold compounds prepared are mesomorphic,
except the derivative of 1,4-diisocyanobenzene. Their optical,
thermal, and thermodynamic data are collected in Table 3.
All the mesogenic gold complexes display only an N
mesophase, identified in optical microscopy by the typical
marbled and schlieren textures (Figure 4).38-40 Several
compounds show crystal-to-crystal transitions before melting.
(38) Demus, D.; Richter, L. Textures of liquid crystals, 2nd ed.; VEB:
Leipzig, 1980; p 790.
(39) Gray, G. W.; Goodby, J. W. Smectic liquid crystals. Textures and
Structures; Hill: London, 1984.
(40) Dierking, I. Textures of liquid Crystals; Wiley-VCH: Weinheim, 2003.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10185

Coco et al.
Table 4. UV-Vis Data in Dichloromethane Solution of the Free
Isocyanides and the Complexes [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2]
with R ) 4,4′-Biphenylene (R1), 2,2′-Dichloro-4,4′-biphenylene (R2),
and 2,2′-Dimethyl-4,4′-biphenylene (R3)
Table 5. Emission and Excitation Maxima (in nm) for Free Isocyanide
Ligands and Complexes [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2] with R
) 4,4′-Biphenylene (R1), 2,2′-Dichloro-4,4′-biphenylene (R2), and
2,2′-Dimethyl-4,4′-biphenylene (R3) at 298 K
R or compd
λ/nm (ꢀ)/dm3 mol-1 cm-1
KBr
CH2Cl2
R1a
228 (6000), 272 (24000)
241 (47000)
243 (27500)
230 (1000), 262 (500)
228 (29000), 297 (70500)
229 sh (46000),b 260 (49000), 283 sh (44500)b
259 (50000), 280 (52500)
R
λex
λem
λex
λem
R2a
R1a
R2a
R3a
R1
247
340
259
328
312
301
314, 325
448
R3a
H9C4OC6H4H
323
R1
R2
R3
340
348
334
496, 532
519
480, 510
491,b 524
454
R2
R3
452,b 480
a Free isocyanide ligands. b Apparent molar absorptivity.
a Free isocyanide ligands. b Emission band exhibiting vibronic progres-
sion. For the compound with R1, a third peak seems to merge as a shoulder
at about 560 nm.
or lower nonplanarity of the individual molecules. Yet, the
rather high transition temperatures found suggest the exist-
ence of fairly good intermolecular interactions, as expected
from large multipolar molecules even if the efficiency of
packing has been relaxed. However, only the phase of lower
disordered, N, is observed. The crystal packing of the 4,4′-
biphenylene and the 2,2′-dichloro-4,4′-biphenylene deriva-
tives shows, in both cases, a nonlayered structure confirming
that, in this type of symmetric dimers, the best multipole-
multipole interactions is achieved in packings with some
mixing of aromatic and aliphatic regions, hence giving a
nonlayered structure. For this reason, only the nematic
mesophase is obtained.
It is worth mentioning that only [µ-(4,4′-CN-C6H4-
NC){Au(C6F4OCnH2n+1)}2], with a flat rigid core and show-
ing the highest melting point, is not mesogenic. This supports
the idea that, in the case of strong intermolecular multipolar
interactions, the molecules have to be designed so as to
preclude much too efficient molecular packing in order to
obtain mesogenic behavior. While this conclusion seems
clear, the good coincidence of variation of transition tem-
peratures with the expected rotational angles is somewhat
surprising, considering that the molecules involve substituents
as different as Cl and H or Me.
Photophysical Studies. The electronic spectra of free
isocyanides and some representative gold(I) complexes are
summarized in Table 4. The free isocyanides show a strong
UV absorption in the range 241-272 nm which is assigned
to a π-π* transition in the biphenyl system (K-band).41 This
absorption in the biphenyl system undergoes a noticeable
bathochromic shift upon complexation of the isocyanide
ligand to give the gold complexes, as reported for analogous
complexes [AuX(CNC6H4C6H4OC10H21)] (X ) Cl, Br, I),41
[AuX(CN(o-xylyl)],42 and [Au(C6F4OCnH2n+1)(CNC6H4C6H4-
OC10H21)].20 Moreover, it is interesting to note that the largest
molar absorptivity for free isocyanide ligands is that of the
2,2-dichloro derivative in accordance with the presence of a
strong auxochromic group (Cl) in the aromatic rings.43 In
contrast, in the gold complexes, which have a higher
electronic density available for interaction with the π-electron
Figure 5. Luminescence excitation and emission spectra of 2,2′-dimethyl-
4,4′-biphenyldiisocyanide in dichloromethane solution at different concen-
trations.
system of the aromatic ring than the free isocyanide ligands,
the effect of the auxochromic group would be lower, and
the largest molar absorptivity is that of biphenyl derivative
according to the smaller twist angle between the two aromatic
rings (28.3°) and consequently greater delocalization of
electrons over the biphenyl systems in this complex.
The luminescence spectroscopic data of three representa-
tive gold complexes and the free isocyanide ligands are listed
in Table 5. The free isocyanides are luminescent at room
temperature in dichloromethane solution and show similar
emission spectra consisting of a broad emission band in the
region 314-448 nm. In the solid state (KBr pellets), the
emission is lost. The luminescent behavior of 4,4′-diisocyano-
2,2′-dimethylbiphenyl in dichloromethane solution as a
function of concentration is shown in Figure 5. As the
concentration is gradually increased from 10-4 to 10-2 M,
the intensity of the emission band at 323 nm decreases and
shifts to lower energy. The emission is lost at higher
concentration. This behavior suggests the formation of an
excimer or excimerlike adduct, as observed frequently in
aromatic compounds.44
It is important to note that with time and manipulation at
room temperature these isocyanides develop a cream color,
probably due to partial polymerization. Despite still showing
(41) Benouazzane, M.; Coco, S.; Espinet, P.; Mart´ın-AÄ lvarez, J. M. J. Mater.
Chem. 1995, 5, 441.
(42) Ecken, H.; Olmstead, M. M.; Noll, B. C.; Attar, S.; Schlyer, B.; Balch,
A. L. Dalton Trans. 1998, 3715.
(43) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 5th ed.; John Wiley & Sons:
Singapore,1991; Chapter 7.
(44) Birks, J. B. Photophysics of Aromatic Molecules, John-Wiley: London,
1970.
10186 Inorganic Chemistry, Vol. 45, No. 25, 2006

Dinuclear Gold(I) Isocyanide Complexes
respectively). The rather long value supports a phosphores-
cent nature for the emission. A similar behavior is obtained
for the peaks at 480 nm (120 µs) and 510 nm (128 µs) for
the 4,4′-diisocyano-2,2′-dimethylbiphenyl gold derivative and
for the 2,2′-dichlorobiphenylene gold derivative (20 µs). In
degassed dichloromethane solution at room temperature, the
gold complexes show similar luminescence spectra with
quantum yields of 0.001 for both 2,2′-dimethylbiphenyl and
biphenyl gold complexes, and 0.002 for the 2,2′-dichloro-
biphenyl gold derivative. These low quantum yields can be
explained considering that, due to the long radiative lifetimes
of phosphorescent emissions, their quenching is especially
important in fluid media (solution), where collisions are
frequent. For [µ-(4,4′-CN-C6H3Cl-C6H3Cl-NC){Au(C6F4-
OC4H9)}2], there is a clear red shift on going from solution
to the solid state. This effect is usually observed in the
emission spectra of gold(I) complexes and is attributed to
the presence of intermolecular Au‚‚‚Au or π-stacking
interactions.47 Considering that our chloro complex [µ-(4,4′-
CN-C6H3Cl-C6H3Cl-NC){Au(C6F4OC4H9)}2] shows Au‚‚‚Au
intermolecular interactions in the solid state, and its emission
band is structureless, we conclude that its luminescence
corresponds to excimerlike phosphorescence in the solid
state, and to ligand-localized monomer phosphorescence in
solution. In contrast, in [µ-(4,4′-CN-C6H4-C6H4-NC){Au(C6F4-
OC4H9)}2], the emission band is structured, the red shift on
going from solution to the solid state is insignificant, and
there are no Au‚‚‚Au or π-stacking interactions. Thus, the
emission band is assigned as ligand-localized monomer
phosphorescence both in the solid state and in solution. For
[µ-(4,4′-CN-C6H3CH3-C6H4CH3-NC){Au(C6F4OC4H9)}2], we
do not have an X-ray structure, and consequently, we cannot
structurally ascertain that Au‚‚‚Au or π-stacking interactions
exist. However, considering that its structured luminescence
profile is similar to that of [µ-(4,4′-CN-C6H4-C6H4-NC)-
{Au(C6F4OC4H9)}2], it is reasonable to assign its emission
behavior as ligand-localized monomer phosphorescence both
in the solid state and solution.
Figure 6. Luminescence excitation and emission spectra in the solid state
(KBr pellets) of [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2] with R ) 4,4′-
biphenylene (R1) (black), 2,2′-dichloro-4,4′-biphenylene (R2) (red), and 2,2′-
dimethyl-4,4′-biphenylene (R3) (blue) at 298 K.
Figure 7. Luminescence excitation and emission spectra in dichloro-
methane solution of [µ-(4,4′-CN-R-NC){Au(C6F4OC4H9)}2] with R )
4,4′biphenylene (R1) (black), 2,2′-dichloro-4,4′biphenylene (R2) (red), and
2,2′-dimethyl-4,4′-biphenylene (R3) (blue) at 298 K.
1
no extra signals in the H NMR spectrum, these cream
samples display in the emission spectra an additional weak
luminescence at 383 nm (λexc ) 324 nm), for which the
intensity does not change significantly with concentration.
The dinuclear gold compounds are luminescent at room
temperature, both in solution and in the solid state (Table 5
and Figures 6 and 7). In the solid state they exhibit a yellow-
green luminescence visually observed under UV irradiation
at 365 nm. The emission spectra of the 4,4′-diisocyano-2,2′-
biphenyl and 4,4′-diisocyano-2,2′-dimethylbiphenyl gold
complexes show a structured emission band in the region
452-532 nm. The energy difference between neighboring
vibronic peaks is close to 1300 cm-1 suggesting a vibronic
progression typical of C-C aromatic stretchings, which is
consistent with π-π* ligand-centered transitions. In contrast,
the 2-2′-dichlorobiphenyl gold complex displays a broad
structureless emission, even at 77 K.
In summary, we have found a new system, based on
dinuclear gold complexes with biphenyl isocyanides, which
shows luminescence properties and also liquid crystal
behavior. This kind of system is promising to obtain a
delicate tuning of properties by changing the dihedral angle
between the two aromatic rings of the biphenyl group.
In general, biphenyl systems with low twist angles between
the two aromatic rings, and consequently high conjugation
in the aromatic biphenyl core, show stabilization of π*
states.45 Thus, it is not unexpected that the 4,4′-diisocyano-
biphenyl gold complex, that shows the lower twist angle
between the two aromatic ring (28.3°) in the series of gold
complexes studied, displays lower energy emission bands,
in accordance to the absorption UV spectrum discussed
above. The lifetimes in the solid state have been measured
for the 4,4′-diisocyano-2,2′-biphenyl gold complex.46 Both
vibronic peaks at 496 and 532 nm show, as expected, the
same (within experimental error) lifetimes (187 and 175 µs,
Acknowledgment. This work was sponsored by the
Direccio´n General de Investigacio´n (Project CTQ2005-
08729/BQU) and the Junta de Castilla y Leo´n (Project
VA099A05).
Supporting Information Available: Crystallographic data in
CIF format for [µ-(4,4′-CN-C6H3Cl-C6H3Cl-NC){Au(C6F4OC4H9)}2]
and [µ-(4,4′-CN-C6H4-C6H4-NC){Au(C6F4OC4H9)}2]. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC060702A
(45) Leslie, W.; Batsanov, A. S.; Howard, J. A. K.; Williams, J. A. G.
Dalton Trans. 2004, 623.
(46) Wayne, C. E.; Wayne, R. P. Photochemistry, Oxford University
Press: New York, 1996.
(47) Brandys M.-C.; Jenning, M. C.; Puddephatt, R. J. Dalton Trans. 2000,
4601.
Inorganic Chemistry, Vol. 45, No. 25, 2006 10187