Light switches the ligand! photochromic azobenzene-phosphanes

Article abstract of DOI:10.1002/ejic.201000063

Three phosphanes containing azobenzene groups were synthesized, and the photochemical behaviour of their platinum complexes was studied by using the light-induced (E)/(Z) isomerization as a switch to change ligand properties.

Full text of DOI:10.1002/ejic.201000063

SHORT COMMUNICATION
DOI: 10.1002/ejic.201000063
Light Switches the Ligand! Photochromic Azobenzene–Phosphanes
Maria Dolores Segarra-Maset,[a] Piet W. N. M. van Leeuwen,[a] and Zoraida Freixa*[a]
Keywords: P ligands / Molecular switches / Azo compounds / Ligand effects / Homogeneous catalysis
Three phosphanes containing azobenzene groups were syn-
thesized, and the photochemical behaviour of their platinum
complexes was studied by using the light-induced (E)/(Z)
isomerization as a switch to change ligand properties.
here, because either the possible coordination of the N
atom to the metal atom or its equilibrium with the inner
phosphonium salt could interfere with the catalytic pro-
cess.[20]
Aiming at light-induced steric effects in catalysis, phos-
phane 3 was synthesized (Scheme 1). As shown by molecu-
lar modelling,[21] one might expect that (E)/(Z) isomeriza-
tion of the three azo bonds of phosphane 3 generates a
structure resembling the reported bowl-shaped phosphanes,
which found several applications in catalysis (Fig-
ure 1).[22–27] Even if only two of its azo groups adopt a (Z)
conformation, a ligand with steric properties different from
those of the tri-(E) isomer could be generated.
The synthesis of the ligands involves Pd-mediated P–C
coupling of diphenylphosphane and 4- or 3-iodoazoben-
zene for 1 and 2, respectively, and lithiation of 3-iodoazo-
benzene and reaction with PCl3 for tris(azobenzene)–phos-
phane 3 (Scheme 1).[28,29]
The switchability of the free ligands was initially studied
by 31P{1H} NMR spectroscopy.[30] A CDCl3 solution of
phosphane 1 shows a singlet at δ = –1.97 ppm (Figure 2a)
of the pure (E) isomer. After irradiation at λ = 350 nm dur-
ing 1 h, a new singlet appeared at δ = –2.7 ppm [attributed
to the (Z) isomer], whilst the initial signal at δ = –1.97 ppm
[(E) isomer] decreased in intensity. When the sample was
left in the dark, the reversibility of the process was con-
firmed; the original singlet due to the (E) form of the azo
bond was progressively recovered.
Introduction
Molecular switches toggle reversibly between different
(meta)stable states. This change (or switch) can be triggered
by external stimuli such as light, heat, pressure, electric or
magnetic fields, etc. Nature relies on this phenomenon to
regulate many biological functions using switchable mole-
cules. Numerous examples of synthetic counterparts based
on spiropyrans, azobenzenes, diarylethenes, etc. have been
used to switch properties of materials, and more recently to
develop nanoscale functional devices.[1–5]
We are interested in the application of such molecular
switches for the construction of light-tunable ligands for
homogeneous catalysis. There are several examples in the
literature of ligands containing photoreversible, bistable
systems.[6–16] In these cases, the photosensitive group in the
ligand undergoes a conformational change when irradiated
at the appropriate wavelength. Surprisingly, ligand proper-
ties, crucial parameters for a potential catalytic application,
have been scarcely studied while undergoing changes due to
irradiation.[7]
Herein, we present the synthesis, coordination and
switchability of phototunable, azobenzene–phosphanes.
Early examples of azo-containing phosphanes appeared in
1996,[11–15,17–19] but only two examples of catalytic applica-
tions are known, and their photochromic properties have
never been investigated in depth.[9,10]
The isomerization process and its reversibility were also
studied by UV/Vis spectroscopy. A solution of 1 in toluene
was irradiated at λ = 350 nm during 1 h; then the sample
was placed in the UV/Vis quartz cell, and spectra were re-
corded at fixed time intervals to study the (Z)/(E) isomer-
ization process. In the spectra two bands were observed; a
very intense one at 325 nm due to the transition π1Ǟπ1*,
characteristic of the (E)-azo isomer, and a less intense one
at 440 nm, attributed to the partly forbidden transition
nsǞπ1*, typical of a (Z)-azo moiety. The former one pro-
gressively increases when the irradiated sample is left in the
dark. Accordingly, the weak band decreases when the (Z)
Results and Discussion
Triarylphosphanes containing only one azo group in para
and meta positions (1 and 2) have been chosen as simple
model compounds to study the isomerization process. The
previously reported ortho derivative has not been studied
[a] Institute of Chemical Research of Catalonia (ICIQ),
Av. Paisos Catalans 16, 43007 Tarragona, Spain
Fax: +34-977-920-221
E-mail: zfreixa@iciq.es
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000063.
Eur. J. Inorg. Chem. 2010, 2075–2078
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. D. Segarra-Maset, P. W. N. M. van Leeuwen, Z. Freixa
SHORT COMMUNICATION
Scheme 1. Synthesis of the ligands.
form returns to the (E) form (Figure 2b). The same behav-
iour was observed for phosphane 2 (see Supporting Infor-
mation).
As regards tris(azobenzene)–phosphane 3, the three azo
groups in the molecule may isomerize independently ren-
dering up to four different compounds [(EEE), (EEZ),
(EZZ), (ZZZ)]. Ligand 3 before irradiation shows one
31P{1H} NMR signal, indicating that all azo groups have
an (E) conformation. After irradiation at λ = 350 nm during
1 h, only two additional signals at δ = –2.48 and –2.84 ppm
appeared next to the initial singlet at δ = –1.94 ppm (Fig-
ure 3). As was observed for the monoazo ligands 1 and 2,
the initial singlet was recovered after several days in the
dark.
Figure 1. Molecular modelling of tris(azobenzene)–phosphane 3 in
tri-(E)- and tri-(Z)-azo conformations showing maximized steric
difference.
Figure 3. Isomerization of phosphane 3 monitored by 31P{1H}
NMR spectroscopy in CDCl3.
Because of our interest in catalysis, we subsequently
studied if the presence of a metal complex or coordination
of the phosphane ligand to a metal atom affect the
(E)/(Z)/(E) isomerization processes of the azo group. First
the photochemical (E)/(Z) isomerization was studied. The
coordination properties of the ligands were studied by in
situ 31P{1H} NMR analysis of the platinum dichloride
complexes [Cl2PtL2] of ligand 1. The 31P{1H} NMR spectra
show the formation of cis-[Cl2PtPEPE] as major compound
(Figure 4a, Table 1). At δ = 23 ppm, traces of the trans com-
plex are observed.[31] When the sample is irradiated at λ =
350 nm for 1 h, the original signal at δ = 17.33 ppm evolves
to a pair of doublets at δ = 17.91 and 16.70 ppm, and a
Figure 2. Isomerization of phosphane 1: (a) 31P{1H} NMR spec-
troscopy in CDCl3; (b) UV/Vis absorption spectroscopic change
after irradiation of the sample at λ = 350 nm in toluene during 1 h. new singlet at δ = 17.25 ppm (with their corresponding sat-
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Photochromic Azobenzene–Phosphanes
ellites). The former signals are attributed to a cis- Table 1. 31P{1H} NMR spectroscopic data of cis-[Cl2Pt(1)2] com-
plexes in CDCl3.
[Cl2PtPEPZ] species and the latter to a cis complex in which
both ligands were converted into the (Z) form cis-
[Cl2PtPZPZ]. As for the free phosphanes, the original signal
is recovered after five days, demonstrating that metal coor-
dination neither inhibits nor catalyses the (Z)/(E) isomeriza-
tion. Secondly, the metal complexes do not catalyze (Z)/
(E) conversion either. The latter is somewhat surprising as
Nakamura et al. reported rapid catalytic (Z)/(E) isomeriza-
tion of azobenzene for several metal complexes. Pt(PPh3)4
catalyzed (Z)/(E) conversion of azobenzene very efficiently
(t1/2 = 1 min at –20 °C and 0.03 m Pt). Divalent group 10
metal complexes were found to be inactive in their studies.
They proposed that back-donation from a zero-valent metal
atom to the azo functionality in a transient complex causes
the catalytic (Z)/(E) transformation; the present complexes
are not sufficiently electron-rich to do so.[31] The switch-
ability of cis-[Cl2PtP2] complexes was also studied by
UV/Vis spectroscopy (Figure 4b) corroborating that (E) Ǟ
(Z) Ǟ (E) isomerization is not influenced by coordination
to Pt or intermolecular interactions. Similar behaviour was
observed when the platinum dichloride complexes of ligand
3 were studied, but the complexity of the signals is much
δ
1J(PE,Pt)
[Hz]
1J(PZ,Pt)
[Hz]
2J(PE,PZ)
[Hz]
[ppm]
[Cl2PtPEPE]
[Cl2PtPEPZ]
17.33
17.91
16.70
17.25
3662
3683
–
–
–
3640
3660
–
15.85
[Cl2PtPZPZ]
–
–
Conclusions
New light-triggered switchable azobenzene–phosphanes
have been synthesized. Herein, we have demonstrated that
the molecular switch is compatible with the phosphane
functionality, and Pt coordination does not influence the
photochemical and thermal isomerization processes. This
result is essential for potential applications in catalysis. Pre-
liminary experiments with these model ligands showed, as
expected, only a slight influence of the isomerization pro-
cesses in catalysis (Rh hydroformylation, Pd allylic alkyl-
ation, Rh hydrosylilation). This result can be taken as a
proof of concept that encourages us to design more elabo-
rated systems.
higher due to the many isomers possible (see Supporting Supporting Information (see footnote on the first page of this arti-
cle): Synthesis and characterization of compounds, NMR details,
complete UV/Vis experiments and experimental conditions for irra-
diation.
Information).
Acknowledgments
The Spanish Ministerio de Educación y Ciencia (MEC) is kindly
acknowledged for
a “Ramon y Cajal” contract (Z. F.), the
Formación de Personal Universitario (FPU) for a grant (AP2005-
3339, M. D. S.-M.), the Project INNOCAT (CTQ2008-00683), and
the Consolider Ingenio 2010 (CSD2006_0003).
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Eur. J. Inorg. Chem. 2010, 2075–2078
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Received: January 21, 2010
Published Online: April 8, 2010
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