Tetranuclear macrocyclic gold(I) alkynyl phosphine complex containing azobenzene functionalities: A dual-input molecular logic with photoswitching behavior controllable via silver(I) coordination/decoordination

Article abstract of DOI:10.1021/om0609719

Photoisomerization studies on a tetranuclear macrocyclic gold(I) alkynyl phosphine complex demonstrate the generation of a dual-input molecular logic with photoswitching behavior that can be controlled by addition or removal of silver-(I) ions.

Full text of DOI:10.1021/om0609719

22
Organometallics 2007, 26, 22-25
Tetranuclear Macrocyclic Gold(I) Alkynyl Phosphine Complex
Containing Azobenzene Functionalities: A Dual-Input Molecular
Logic with Photoswitching Behavior Controllable via Silver(I)
Coordination/Decoordination
Hau-San Tang, Nianyong Zhu, and Vivian Wing-Wah Yam*
Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials Research and Department
of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
ReceiVed October 23, 2006
Summary: Photoisomerization studies on a tetranuclear mac-
rocyclic gold(I) alkynyl phosphine complex demonstrate the
generation of a dual-input molecular logic with photoswitching
behaVior that can be controlled by addition or remoVal of silVer-
(I) ions.
compounds.4-6 With our recent interest in the study of confor-
mational changes induced by photo- and ion-responsive pro-
cesses7 and our longstanding interest in luminescent polynuclear
gold(I) systems,6,8 we became interested in the utilization of
the photoresponsive azobenzene moiety as a linker in the
synthesis of gold(I) macrocycles with photoswitchable functions.
Although a related azobenzene-containing mononuclear gold-
(I) alkynyl complex has been reported previously and its NLO
properties studied,9 relatively little is known of their photoi-
somerization behavior. Herein we report the synthesis, X-ray
crystal structure, and photoisomerization behavior of a tetra-
nuclear macrocyclic gold(I) alkynyl phosphine complex, the
photoswitching behavior of which could be locked or unlocked
with a second input brought about by the addition or removal
of silver(I) ions. A related dinuclear gold(I) molecular rod with
photoswitchable capability has also been synthesized and
studied.
Azobenzene and its derivatives are well-known to undergo
reversible trans-cis isomerization upon irradiation of UV light.1
Because of the difference in the chemical and physical properties
of the trans and cis isomers, the reversible optically induced
isomerizable unit has been widely utilized as the photorespon-
sive component in optical switching, molecular logics, and
devices.2 Azo-containing transition-metal complexes present an
intriguing system because of the combination of optical, redox,
and magnetic properties of metal complexes with the photoi-
somerization properties of the azo moieties.3 Recent works have
shown that a number of gold(I) phosphine complexes were
capable of forming macrocyclic rings, catenanes, or polymers
depending on the flexibility and geometry of the bridging ligands
used, some of which underwent supramolecular self-assembly
driven by the aurophilic attraction commonly found in gold(I)
Dinuclear and tetranuclear macrocyclic gold(I) alkynyl
phosphine complexes containing azobenzene functionalities,
[{Au(PPh3)}2(CtC-L-CtC)] (1) and [Au4(dppm)2(CtC-
L-CtC)2] (2), were synthesized by the reaction of 4,4′-
(4) (a) Irwin, M. J.; Rendina, L. M.; Vittal, J. J.; Puddephatt, R. J. Chem.
Commun. 1996, 1281. (b) Irwin, M. J.; Vittal, J. J.; Yap, G. P. A.;
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* To whom correspondence should be addressed. E-mail: wwyam@hku.hk.
Fax: +(852)-2857-1586. Tel: +(852)-2859-2153.
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10.1021/om0609719 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/07/2006

Communications
Organometallics, Vol. 26, No. 1, 2007 23
diethynylazobenzene (HCtC-L-CtCH) with 2 equiv of
[Au(PPh3)Cl] and with 1 equiv of [Au2(dppm)Cl2] (dppm )
bis(diphenylphosphine)methane) in dichloromethane, respec-
tively (L ) C6H4NdNC6H4).10 Subsequent recrystallization from
layering of n-hexane onto concentrated dichloromethane solu-
tions of 1 and 2 yielded the desired complexes as orange crystals.
Reaction of 2 with 2 equiv of [Ag(MeCN)4]PF6 in acetone at
room temperature, followed by recrystallization from diffusion
of diethyl ether into an acetone solution, gave [{Au4(dppm)2-
(CtC-L-CtC)2}Ag2](PF6)2 (3) as a deep orange solid.11 All
1
of the complexes have been characterized by H and 31P{1H}
NMR spectroscopy, FT-Raman spectroscopy, and positive FAB
mass spectrometry and gave satisfactory elemental analyses. A
shift of the ν(CtC) stretch from 2113 cm-1 in 2 to 2043 cm-1
in 3 was observed, supporting the π coordination of the alkynyl
units to the Ag(I) ions in 3.
An X-ray crystal structure determination of 2 confirmed the
trans conformation of the two azobenzene moieties in the
tetranuclear macrocyclic gold(I) alkynyl complex (Figure 1a).
The Au‚‚‚Au distances were found to be 3.221(18) and 3.131-
(2) Å, indicative of the presence of intramolecular Au‚‚‚Au
interaction. The two Au‚‚‚Au units in each molecule of 2 were
twisted against each other with a dihedral angle of 26.9° (see
Figure 1b). The dihedral angles of C(7)-C(6)-N(1)-N(2),
C(6)-N(1)-N(2)-C(14), N(1)-N(2)-C(14)-C(13), C(23)-
C(22)-N(3)-N(4), C(22)-N(3)-N(4)-C(30), and N(3)-
N(4)-C(30)-C(29) were close to 180°, where the benzene rings
in each of the two azobenzenes were roughly parallel to each
other. Intermolecular weak π-π stacking interactions exist
between the azobenzene moieties of adjacent molecules to give
discrete dimeric structures of 2 in the crystal-packing diagram
(Figure 1b). The lack of an extended π-stacked oligomeric
structure is probably due to the steric hindrance exerted by the
neighboring dppm ligands, which prevents the close approach
of the molecules.
Figure 1. (a) Perspective drawing of [Au4(dppm)2(CtC-L-Ct
C)2] (2) with the atomic numbering scheme. Thermal ellipsoids
are shown at the 30% probability level. (b) Crystal packing of
[Au4(dppm)2(CtC-L-CtC)2] (2) showing the dimeric structure.
Hydrogen atoms and solvent molecules have been omitted for
clarity.
chains is possible upon dissolution of the complexes, the 31P
NMR spectra of 2 showed only a sharp singlet, suggestive of
effective equivalence of all phosphorus atoms and probably the
presence of one chemical species, though one cannot completely
exclude the possibility of a dynamic process that occurs at a
rate faster than that of the NMR time scale. However, it is
believed that the macrocyclic structure is favored in the present
system, given the syn conformation of the diphosphine ligand
(dppm) and the possibility of the presence of intramolecular
Au‚‚‚Au interactions in the metallamacrocycle, although zigzag
oligomers are also known to be formed with a syn conformation
of the dppm ligand and Au‚‚‚Au interactions. Similar sugges-
The X-ray crystal structure suggested that 2 attained a
macrocyclic structure. Although the formation of oligomers or
(10) A mixture of HCtC-L-CtCH (18 mg, 0.077 mmol) and
triethylamine (2 mL) in THF (5 mL) was added dropwise to a solution of
[Au(PPh3)Cl] (80 mg, 0.162 mmol) in THF (5 mL). An orange suspension
appeared rapidly, and the mixture was stirred for 20 h at room temperature,
after which the orange precipitate was filtered and washed with deionized
water, methanol, and diethyl ether. Subsequent recrystallization by vapor
diffusion of diethyl ether into a concentrated dichloromethane solution of
the product gave 1 as orange crystals (yield 68 %). Complex 2 was
synthesized in a way similar to that for 1, except [Au2(dppm)Cl2] (84 mg,
0.10 mmol) was used in place of [Au(PPh3)Cl] (yield 70 %). 1: 1H NMR
(400 MHz, CDCl3, 298 K, relative to Me4Si) δ 7.54-7.65 (m, 34H, PPh3
and aryl H ortho to N), 7.85 (d, 4H, J ) 8.5 Hz, aryl H meta to N); 31P-
{1H} NMR (202 MHz, CDCl3, 298 K, relative to 85 % H3PO4) δ 43.4, s;
12
tions have also been made for Au2(dppm)Cl2 and for other
related systems.4 Given the size, the conformation, and the
flexibility of the linkers, it is believed that the 2:4:2 (ligand:
metal:ligand) metallamacrocycle remains intact in solution, as
thermodynamic control is also an important factor in governing
the size of the macrocycles. Since the 1:2:1 metallamacrocycle
is not possible in our system, the 2:4:2 metallamacrocycle is
the smallest possible macrocycle, which is thought to be favored
over larger macrocycles for entropic reasons.13
positive FAB-MS m/z 1148 {M + H}+; FT-Raman (solid with KBr, cm-1
)
2111 (m) ν(CtC). Anal. Found: C, 54.34; H, 3.50; N, 2.51. Calcd for
Au2C68H46P2N2: C, 54.50; H, 3.32; N, 2.45. 2: 1H NMR (400 MHz, CDCl3,
298 K, relative to Me4Si) δ 3.59 (t, 4H, J ) 11 Hz, CH2), 7.30-7.44 (m,
40H, PPh2), 7.61 (d, 8H, J ) 7.0 Hz, aryl H ortho to N), 7.65 (d, 8H, J )
7.0 Hz, aryl H meta to N); 31P{1H} NMR (202 MHz, CDCl3, 298 K, relative
to 85 % H3PO4) δ 32.7, s; positive FAB-MS m/z 2014 {M + H}+; FT-
Raman (solid with KBr, cm-1) 2113 (m) ν(CtC). Anal. Found: C, 47.80;
H, 3.15, N, 2.56. Calcd for Au4C82H60P4N4‚CH2Cl2: C, 47.50; H, 3.08; N,
2.67.
The electronic absorption spectra of complexes 1-3 in
dichloromethane show moderately intense absorption shoulders
at ca. 250-280 nm and an intense band at ca. 380-390 nm,
with a tail extending to ca. 500 nm. The absence of the
moderately intense absorption at the high-energy region in
HCtC-L-CtCH is suggestive of its assignment as an
(11) To an orange suspension of 2 (20 mg, 0.01 mmol) in acetone (5
mL) was added dropwise [Ag(MeCN)4]PF6 (8.3 mg, 0.02 mmol) in acetone
(5 mL). The reaction mixture immediately turned clear reddish orange and
was stirred for 30 min at room temperature. Recrystallization by vapor
diffusion of diethyl ether into an acetone solution of the product yielded 3
as a reddish orange solid (yield 63%): 1H NMR (400 MHz, CDCl3, 298
K, relative to Me4Si) δ 3.60 (m, 4H, CH2), 7.31-7.48 (m, 40H, PPh2),
7.60 (d, 8H, J ) 7.0 Hz, aryl H ortho to N), 7.69 (d, 8H, J ) 7.0 Hz, aryl
H meta to N); 31P{1H} NMR (202 MHz, CDCl3, 298 K, relative to 85%
H3PO4) δ 32.7, s; positive ESI-MS m/z 1114 {M}2+; FT-Raman (solid with
KBr, cm-1): 2043 (w) ν(CtC). Anal. Found: C, 39.46; H, 2.51; N, 2.28.
Calcd for Au4C82H60P6N4Ag2F12‚CH3COCH3: C, 39.60; H, 2.56; N, 2.17.
(12) Schmidbaur, H.; Wohlleben, A.; Wagner, F.; Orama, O.; Huttner,
G. Chem. Ber. 1977, 110, 1748.
(13) (a) Sweigers, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483.
(b) Radhakrishnan, U.; Schweiger, M.; Stang, P. J. Org. Lett. 2001, 3, 3141.

24 Organometallics, Vol. 26, No. 1, 2007
Communications
Figure 2. UV-vis spectral changes of [{Au(PPh3)}2(CtC-L-
CtC)] (1) in dichloromethane upon irradiation at 360 nm. The
inset shows the absorbance change at 300 nm with irradiation time
and its theoretical fit.
intraligand transition of the phosphine ligands. The intense
absorption band at 380-390 nm, which also occurs in the
azobenzene ligand, is assigned as a π f π* intraligand transition
of the azo moiety, while the tail to 500 nm is assigned as an n
f π* intraligand transition of the azo moiety.
Upon irradiation of a dichloromethane solution of 1 at 360
nm into the IL azo (π f π*) transition, trans-cis photoisomer-
ization occurred, while irradiation of visible light into the IL (n
f π*) transition at 486 nm caused a cis-to-trans photoisomer-
ization (Figure 2). In order to obtain a quantitative measure of
the photoinduced isomerization processes of 1, the photochemi-
cal quantum yield for the trans-cis isomerization process of 1
was determined using chemical actinometry. The photoreactions
Figure 3. (a) UV-vis spectral changes of [Au4(dppm)2(CtC-
L-CtC)2] (2; 1.34 × 10-5 M) in dichloromethane upon irradiation
at 360 nm for 240 s. (b) UV-vis spectral traces of [{Au4(dppm)2-
(CtC-L-CtC)2}Ag2](PF6)2 (3; 1.53 × 10-5 M) in dichlo-
romethane upon irradiation at 360 nm for 360 s.
1
were monitored by both UV-vis spectrophotometry and H
NMR spectroscopy. Upon irradiation at 360 nm, two new
Scheme 1. Schematic Diagram Demonstrating the
“Locking” and “Unlocking” Mechanism Brought About by
the Addition and Removal of Ag+ Ions in Preventing and
Facilitating Trans-Cis Isomerization in
1
doublets appeared in the H NMR spectrum, corresponding to
the aryl protons meta and ortho to the NdN moiety of the cis
isomer formed, from which determination of the trans/cis
integral ratio would give rise to the extinction coefficients of
the trans and cis isomers. Subsequent monitoring of the
photoreactions by UV-vis spectrophotometry gave a quantum
yield of 0.1 for the trans-to-cis photoisomerization of 1. A
control experiment to correct for the presence of any thermal
trans-cis isomerization reaction was performed, in which an
equal amount of 1 was dissolved in the same amount of
dichloromethane and placed in the dark. The thermal reaction
was similarly monitored by UV-vis spectrophotometry, in
which no observable cis isomer was detected in the absence of
light.
[Au4(P∧P)2(CtC-L-CtC)2]
The photoisomerization behavior of the tetranuclear macro-
cyclic gold(I) complex 2 was studied, in which similar UV-
vis absorption changes were observed (Figure 3a). Interestingly,
introduction of Ag+ ions to 2 inhibited the trans-cis photoi-
somerization process (Figure 3b), probably due to the locking
up of the photoresponsive moiety via π coordination of adjacent
pairs of alkynyls to the Ag+ ions in a sandwich binding fashion
to give [{Au4(dppm)2(CtC-L-CtC)2}Ag2]2+, as revealed
from the FT-Raman studies,14 which resulted in the suppression
n
of the isomerization process. In contrast, addition of Bu4NCl
to [{Au4(dppm)2(CtC-L-CtC)2}Ag2]2+ resulted in the ab-
straction of the Ag+ ions, restoring the conformational flexibility
(14) It should be noted that there was a report on the silver(I) perturbation
of trans-cis isomerization of azobenzene in the literature, in which the
π-π* absorption band of the azo moiety for the azobenzene-silver adduct
was shifted by ca. 1100 cm-1 to lower energy.15 However, in the present
study, we believed that the silver(I) ions were bound to the alkynyl units
because of the absence of a red-shifted π-π* absorption band, and more
importantly, a lowering of the frequency of the ν(CtC) stretch from 2113
cm-1 in [Au4(dppm)2(CtC-L-CtC)2] to 2043 cm-1 in [{Au4(dppm)2-
(CtC-L-CtC)2}Ag2]2+ was observed, which was indicative of the
π coordination mode of the alkynyl groups to the d10 Ag(I) centers.16
(15) Bortolus, P.; Flamigni, L.; Monti, S.; Bolte, M.; Guyot, G. J. Chem.
Soc., Faraday Trans. 1991, 87, 1303.
(16) See recent examples on π coordination of silver(I) ions to alkynyl
ligands of gold(I) alkynyl complexes: (a) Abu-Salah, O. M. J. Organomet.
Chem. 1998, 565, 211. (b) Yam, V. W. W.; Cheung, K. L.; Cheng, E. C.
C.; Zhu, N.; Cheung, K. K. Dalton Trans. 2003, 1830. (c) Schuster, O.;
Monkowius, U.; Schmidbaur, H.; Ray, R. S.; Kru¨ger, S.; Ro¨sch, N.
Organometallics 2006, 25, 1004.

Communications
Organometallics, Vol. 26, No. 1, 2007 25
within the gold(I) macrocycle. Scheme 1 shows the hypothesized
schematic diagram of the “locking” and “unlocking” of the trans
conformation on addition and elimination of Ag+ to prevent
and facilitate trans-cis photoisomerization in 2. Since the UV-
vis spectral change in 2 is similar to that observed in 1 and the
conversion is relatively low and comparable to that of 1, it is
believed that photoisomerization of 2 would give rise to a trans,-
cis product. Assuming only the trans,cis product is formed, a
photoisomerization quantum yield of 0.07 is determined on the
basis of the photoreactions monitored by both UV-vis spec-
trophotometry and 1H NMR spectroscopy. Interestingly, addition
of Ag+ salts to the trans,cis product of 2 did not have an effect
on its reverse photoisomerization back to the trans,trans isomer
of 2, further confirming the sandwich binding mode of Ag+ to
the trans,trans metallamacrocycle, which becomes impossible
in the trans,cis form.
addition of nBu4NCl restored the ability of the azobenzene
moiety to undergo photoisomerization. The present work serves
as a proof of principle in demonstrating the versatility of
molecular design in generating a dual-input lockable molecular
logic photoswitch.
Acknowledgment. V.W.-W.Y. acknowledges support from
the University Development Fund, The University of Hong
Kong Foundation for Educational Development and Research
Limited, the URC Seed Funding for Strategic Research Theme
on Organic Optoelectronics, and the Faculty Development Fund
of The University of Hong Kong. This work has been supported
by the Research Grants Council of the Hong Kong SAR,
People’s Republic of China (Project No. HKU2/05, HKU7050/
04P). H.-S.T. acknowledges the receipt of a Postgraduate
Studentship administered by The University of Hong Kong.
In conclusion, photoisomerization studies showed that upon
UV irradiation into the IL (π f π*) transition at 360 nm, trans-
to-cis photoisomerization occurred, while irradiation with visible
light at 486 nm promoted the reverse isomerization process.
Interestingly, addition of Ag+ ions to the macrocyclic gold(I)
alkynyl complex affected the flexibility within the gold(I)
macrocycle, inhibiting the photoisomerization reactions, while
Supporting Information Available: Text giving the details of
experimental procedures for the synthesis and photoisomerization
study of 1-3 and crystal data and refinement details for 2 and a
CIF file giving crystal data for 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM0609719