Synthesis and coordination chemistry of a photoswitchable bis(phosphine) ligand

Article abstract of DOI:10.1021/ic050429x

A 1,2-dithienylethene compound bearing bis(phosphine) groups (1o) represents a new class of photoresponsive ligands where there are steric and electronic differences between two photogenerated isomers. The coordination chemistry of this ligand class is demonstrated by preparing a gold(I) complex (2o) and a phosphine selenide (3o).

Full text of DOI:10.1021/ic050429x

Inorg. Chem. 2005, 44, 5960−5962
Synthesis and Coordination Chemistry of a Photoswitchable
Bis(phosphine) Ligand
David Sud, Robert McDonald,† and Neil R. Branda*
Department of Chemistry, Simon Fraser UniVersity, 8888 UniVersity DriVe,
Burnaby, British Columbia, Canada V5A 1S6
Received March 22, 2005
A 1,2-dithienylethene compound bearing bis(phosphine) groups
(1o) represents a new class of photoresponsive ligands where
there are steric and electronic differences between two photoge-
nerated isomers. The coordination chemistry of this ligand class
is demonstrated by preparing a gold(I) complex (2o) and a
phosphine selenide (3o).
throline) are used to construct closed-shell structures and
coordination polymers.3,4 Other ligand choices have been less
fruitful. For example, the coordination complexes formed
using a cyano-functionalized DTE and ruthenium proved to
be photochemically unstable.4 Similar complexes using
rhenium could not be isolated, and only the starting material
was recovered.
It is surprising that there are no examples of photorespon-
sive DTE derivatives decorated with the ligand family most
ubiquitous in coordination chemistry, the tertiary phosphorus
family of ligands. This is particularly surprising because these
are the ligands that boast the most dramatic and versatile
changes in properties based on the fine-tuning of the
electronic and steric framework of the groups attached to
the phosphorus atom.5 In this Communication, we describe
the first example of a photoresponsive bis(triarylphosphine)
based on the DTE backbone (1o) and demonstrate how it
binds and affects metal centers. Specifically, gold(I) and
selenium compounds are used as illustrative examples.
The photoresponsive bis(phosphine) ligand is prepared as
its ring-open form (1o) in one step from the known 1,2-bis-
(5′-chloro-2′-methylthien-3′-yl)perfluorocyclopentene6 as shown
in Scheme 1. The bis(phosphine) is isolated as air-stable
colorless crystals and is freely soluble in a wide range of
common organic solvents such as CH2Cl2, THF, CH3CN,
and alcohols. Irradiation of a colorless CH2Cl2 or CH3CN
solution of 1o with 313-nm light6 results in the immediate
appearance of a deep purple color (λmax ) 570 nm) because
of the generation of the ring-closed isomer 1c. In CH2Cl2,
the photostationary state (PSS) contains 80% of 1c as
Compounds that interconvert between two thermally stable
forms when irradiated with appropriate wavelengths of light
provide a practical means to regulate the chemical and
physical properties of molecular devices.1 This is due to the
fact that the two isomers possess unique physical properties
such as the way they absorb and emit light, their refractive
indices, and their redox potentials. Photoresponsive 1,2-
dithienylethenes (DTEs) are especially useful and undergo
ring-closing and ring-opening reactions, often with a high
degree of photostability.2 Because metal-coordination com-
plexes provide their own diverse assortment of photophysical
and electrochemical characteristics and because the metal
centers’ properties are highly sensitive to the steric and
electronic nature of their associated ligands, developing
tunable coordination complexes by combining a switching
ligand with a metal ion will advance the use of coordination
compounds in optoelectronics as well as in chemical reactiv-
ity and metal catalysis. In this regard, the DTE backbone is
particularly attractive because it exhibits dramatic steric and
electronic differences between its two interconverting isomers
and metals bound to ligands made from this architecture will
experience significant variations.
Several reports have described the use of DTE derivatives
where nitrogen ligands (pyridine, bipyridine, and phenan-
(3) Yam, V. W.-W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 2004, 126,
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J.-P. Eur. J. Inorg. Chem. 2000, 1581.
* To whom correspondence should be addressed. E-mail:
nbranda@sfu.ca.
† Present address: X-ray Crystallography Laboratory, Department of
Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2.
(1) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, Germany,
2001.
(2) Irie, M. In Organic Photochromic and Thermochromic Compounds;
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(6) See the Supporting Information for details.
5960 Inorganic Chemistry, Vol. 44, No. 17, 2005
10.1021/ic050429x CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/22/2005

COMMUNICATION
Scheme 1
Scheme 2
Figure 2. Molecular structure of 2o in the crystal. Ellipsoids are shown
at the 20% probability level.
by X-ray crystallography. Single crystals suitable for X-ray
analysis can be grown by slowly evaporating a solution of
the complex in a mixture of CH2Cl2 and hexanes.
Figure 1. UV-vis absorption spectra of CH3CN solutions of ring-open
isomers 1o and 2o (solid lines) and the PSSs generated when the solutions
are irradiated with 313-nm light (broken lines).
The structure of the complex in the single crystal is shown
in Figure 2.6,7 In the crystal, the gold complex is locked into
an unproductive conformation. It exists in a distorted parallel
conformation, and the distance between the reactive carbons
is too large for the photocyclization to occur in the single
crystal (4.78 Å). This is verified by the fact that irradiating
a single crystal of 2o with 313-nm light for 30 min results
in no observable change in the color of the crystal. The
coordination geometry around the gold atom is linear with
bond angles of 178°. The inter- and intramolecular distance
between gold atoms indicates that there is no gold-gold
bond, as shown for other (diphenylphosphino)thiophenegold-
(I) complexes.8
The photochemistry of complex 2o is well behaved in
solution. Irradiation of CH2Cl2 or CH3CN solutions of 2o
with 313-nm light induces an immediate color change
(colorless to purple) and the appearance of an absorption
peak at 565 nm in the UV-vis spectrum (Figure 1). In the
case of CH2Cl2, a PSS consisting of 60% of the ring-closed
isomer 2c is reached using our light sources. Cycling can
be performed with minimal changes in the spectra (313 and
>434 nm). A solvent that can coordinate to the gold (CH3-
CN), should Au-P bond rupture be a part of the photo-
chemical process, causes a slight decrease in the PSS to 50%,
the same value that was observed for the free ligand in this
solvent. However, other factors may lead to this observation.
measured by 1H NMR spectroscopy, where changes in
chemical shifts typical for the ring-closing reaction are
observed. The remaining 20% is the ring-open isomer. The
UV-vis absorption spectra of both states are illustrated in
Figure 1. The photocyclization reaction is highly solvent-
dependent, and the photoconversion drops to 50% when CH3-
CN is used. The original absorption spectrum corresponding
to 1o can be regenerated by irradiating the solution of the
PSS with light of wavelengths greater than 434 nm, which
triggers the reverse reaction and produces the ring-open
isomer. This ring-closing/ring-opening cycle can be per-
formed numerous times with no significant degradation. The
signals in the 31P NMR spectrum appear at -18.7 and -8.3
ppm for the ring-open and ring-closed isomers, respectively,
illustrating the electronic differences between the two DTE
isomers.
The coordination chemistry of ligand 1o is typified by the
preparation and isolation of the air-stable gold complex 2o
(Scheme 1). This is best done by treating the ring-open form
of the ligand with AuCl(tht) in CH2Cl2 at 25 °C.6,7 The
coordination complex can be purified by column chroma-
1
tography (silica, hexanes/CH2Cl2, 3:1). The H NMR spec-
trum is consistent with the structure shown in Scheme 2, as
is the 31P NMR spectrum, which has a single signal appearing
at 19.8 ppm.6 Confirmation of the structure of 2o is provided
The electron-withdrawing ability of the phosphine ligands
in both forms of the photoresponsive compound is compared
(7) X-ray data were collected at 193 K on a Bruker PLATFORM/SMART
1000 CCD with Mo KR radiation. The crystal structure was solved
using direct methods (SHELXS-86) and refined by full-matrix least
squares on F 2 (SHELXL-93). Crystal data for C39H28Au2Cl2F6P2S2:
fw ) 1201.51, triclinic P1h (No. 2), a ) 9.7100(6) Å, b ) 10.2317(6)
Å, c ) 20.4590(12) Å, R ) 83.1693(9)°, â ) 76.9049(8)°, γ )
84.1473(9)°, V ) 1959.8(2) Å3, Fc ) 2.036 g cm-3, Z ) 2, µ ) 7.860
1
by direct measurement of the J(77Se-31P) spin-spin cou-
pling constants of the derived phosphine selenides.9 The bis-
(selenide) compound 3o is prepared quantitatively by heating
2
2
mm-1, R1 [F02 g 2σ(F0 )] ) 0.0214, wR2 [F02 g -3σ(F0 )] ) 0.0542,
(8) Clot, O.; Akahori, Y.; Moorlag, C.; Leznoff, D. B.; Wolf, M. O.;
and 4.32° < 2θ < 52.76°.
Batchelor, R. J.; Patrick, B. O.; Ishii, M. Inorg. Chem. 2003, 42, 2704.
Inorganic Chemistry, Vol. 44, No. 17, 2005 5961

COMMUNICATION
a mixture of the bis(phosphine) 1o, selenium powder, and
CHCl3 at reflux.6 The excess selenium is removed by filtering
through a short silica plug. The photoactivity of this
compound is demonstrated by the change in color of CHCl3
solutions of 3o from colorless to purple when irradiated with
313-nm light and by the changes in the 31P NMR spectra. In
this case, the PSS contains 55% of the ring-closed isomer
3c. The signals in the 31P NMR spectra are different for the
two isomers and appear at 22.4 and 27.0 ppm for 3o and 3c,
respectively. What is more indicative of the electronic
differences affecting the nucleophilicity of the lone pair on
3o and 3c is comparable in magnitude to that obtained when
a phenyl group in triphenylphosphine is replaced by an alkyl
group, which is a change that is well accepted to lead to
significant differences in reactivity.
Further research efforts will focus on illustrating that the
steric and electronic differences between the two intercon-
verting isomers (1o and 1c) can be harnessed in practical
applications such as chemical reactivity and catalysis. These
results will be reported in due course.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada, the Canada
Research Chairs Program, and Simon Fraser University
(SFU) for financial support of this research. We thank
Nippon Zeon Corp. for supplying the octafluorocyclopentene
needed to prepare the photochromic compounds and Prof.
Daniel Leznoff (SFU) for AuCl(tht) and helpful advice.
1
the phosphorus atoms are the coupling constants J(77Se-
31P), which are 744 and 756 Hz for 3o and 3c respectively.
This indicates that the ring-closed isomer of the parent bis-
(phosphine), 1c, is a weaker nucleophile than the ring-open
isomer 1o. This difference can be attributed to the increased
electron-withdrawing properties of the molecular backbone
in 1c. The difference in the 77Se-31P coupling constants for
Supporting Information Available: Synthetic details and
photochemical reactions for compounds 1-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
(9) Chevykalova, M. N.; Manzhukova, L. F.; Artemova, N. V.; Luzikov,
Y. N.; Nifant’ev, I. E.; Nifant’ev, E. E. Russ. Chem. Bull. 2003, 52,
78. Allen, D. W.; Taylor, B. F. J. Chem. Soc., Dalton Trans. 1982,
51.
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