Photoinduced isomerisation of cis-[M(L-S,O)2] (M = Pt II and PdII) complexes of N,N-diethyl-N′-3,4,5- trimethoxybenzoylthiourea: Key to preparation of the trans isomer

Article abstract of DOI:10.1039/b415306e

In acetonitrile solutions at room temperature, cis-[M(L-S,O)₂] Pt^II and Pd^II complexes of N,N-diethyl-N′-3,4,5- trimethoxybenzoylthiourea undergo reversible photoinduced isomerisation to the corresponding trans isomer upon irradiation with visible light in the 320-570 nm range, the rate and extent of isomerisation being significantly higher for the cis-[Pd(L-S,O)₂] complex compared to the Pt^II analogue; in the dark trans-[M(L-S,O)₂] cleanly reverts back to the cis complex at a rate dependent on the solution temperature, indicating a thermally controlled reverse process. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415306e

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Photoinduced isomerisation of cis-[M(L-S,O)2] (M 5 PtII and PdII)
complexes of N,N-diethyl-N9-3,4,5-trimethoxybenzoylthiourea: key to
preparation of the trans isomer
Dirk Hanekom, Jean M. McKenzie, Nocky M. Derix and Klaus R. Koch*
Received (in Cambridge, UK) 4th October 2004, Accepted 11th November 2004
First published as an Advance Article on the web 21st December 2004
DOI: 10.1039/b415306e
steady state within 25 h. As shown for cis-[Pt(L1-S,O)2] in Fig. 1,
In acetonitrile solutions at room temperature, cis-[M(L-S,O)2]
PtII and PdII complexes of N,N-diethyl-N9-3,4,5-trimethoxyben-
zoylthiourea undergo reversible photoinduced isomerisation to
the corresponding trans isomer upon irradiation with visible
light in the 320–570 nm range, the rate and extent of
isomerisation being significantly higher for the cis-[Pd(L-
S,O)2] complex compared to the PtII analogue; in the dark
trans-[M(L-S,O)2] cleanly reverts back to the cis complex at a
rate dependent on the solution temperature, indicating a
thermally controlled reverse process.
the area of the second peak at tR y 8.6 min grows with length of
exposure to light, while the major peak area correspondingly
decreases, eventually reaching a steady state. The absorbance
profiles of the two eluted species as obtained by a diode array
photometric detector are virtually identical (lmax 5 307 nm),
suggesting that the two peaks are due to cis–trans isomers.
Moreover rp-HPLC coupled to electrospray mass spectrometry
(ESMS) shows that the two peaks in the chromatogram of cis-
[Pt(L1-S,O)2] solutions exposed to light have the same values (m/z
846.83 and 846.57, calc. for [C30H42N4O8PtS2?H]+ # 846.22),
confirming that the smaller peak corresponds to the trans-[Pt(L1-
S,O)2] complex. Fig. 1 shows the peak area ratio Ke 5 [trans]/[cis]
as a function of the time exposed to ambient light at room
temperature. Similar observation can be made for cis-[Pd(L1-
S,O)2], although the appearance of the second peak occurs much
sooner, and steady state is reached within ca. 1 h.
We have in the last decade extensively studied N,N-dialkyl-
N9-aroylthioureas (RRNC(S)NHC(O)R9) for their potential
analytical and process chemistry applications in the platinum
group metals refining industry.1 These ligands have long been
known to readily form stable complexes with softer 1st row
transition metal ions as shown from the studies of Hoyer and
Beyer2 and later Ko¨nig and Schuster.3 Generally these molecules
show an overwhelming tendency to coordinate particularly to d8
metal ions resulting, upon loss of a proton, in a cis-S,O mode of
coordination. We have exploited the favorable physiochemical
properties of N,N-dialkyl-N9-acylthioureas (HL) for the convenient
reversed-phase high performance liquid chromatographic (rp-
HPLC) determination of PtII, PdII and RhIII in acid chloride
media.4 Several years ago we serendipidously isolated a first
example of a trans-bis(N,N-di(n-butyl)-N9-naphthoylthioureato)-
platinum(II) complex in ca. 15% yield.5 This is one of only two
examples of trans complexes with these ligands of the more than
25 related crystal structures reported in the Cambridge Structural
Database.6 Despite considerable effort we have not been able to
predictably prepare substantial quantities of trans-[Pt(L-S,O)2] or
trans-[Pd(L-S,O)2] complexes with N,N-dialkyl-N9-aroylthioureas
by any standard synthetic route.
Experiments show that the rate of photoisomerisation observed
for cis-[Pt(Ln-S,O)2] or cis-[Pd(Ln-S,O)2] at room temperature in
dilute acetonitrile solutions is significantly influenced by the
relative intensity as well as the wavelength range of the light used
for irradiation. Irradiation of solutions of cis-[Pt(L1-S,O)2] or cis-
[Pd(L1-S,O)2] in a water jacketed, 15 cm glass cell using intense
white light§ with relatively constant light flux (y320 mmol s21 m22),
results in isomerisation of cis-[Pt(L1-S,O)2] to a steady state within
ca. 70 min (Ke 5 0.14 ¡ 0.005), while cis-[Pd(L1-S,O)2] reaches a
steady state within ca. 21 min (Ke 5 0.43 ¡ 0.02). The relatively
higher rate of isomerisation for cis-[Pd(L1-S,O)2] complexes at
constant photon flux is consistent with the fact that PdII complexes
We here report that the key to obtaining trans-[M(L-S,O)2]
complexes is a photoinduced isomerisation of the cis-[M(L1-S,O)2]
complexes in acetonitrile solution7 as monitored by rp-HPLC
(M
5
PtII, PdII and N,N-diethyl-N9-3,4,5-trimethoxyben-
zoylthiourea (HL1)){. Repeated injection of freshly prepared
solutions (200 mg cm23) of authentic cis-[Pt(L1-S,O)2] in MeCN
at room temperature, which are kept in the dark over a period
of several weeks, show the elution of only a single peak
(tR y 10.5 min){. Identical solutions of cis-[Pt(L1-S,O)2] exposed
to ambient daylight show the development of a second peak in the
chromatogram within ca. 30–60 min of exposure, reaching a
Fig. 1 Ke (ratio of trans/cis peak areas) for pure cis-[Pt(L1-S,O)2] in
MeCN (ca. 100 mg cm23), as a function of the time exposed to ambient
daylight at 20 uC, as monitored by rp-HPLC. The inset shows typical
chromatograms obtained. Control dark experiments show only one peak.
*krk@sun.ac.za
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are generally more kinetically labile compared to those of PtII.
Experiments using optical filters to yield blue, yellow and red light
respectively, show that light in the wavelength range of 320–570 nm
is responsible for the observed photoisomerisation. Fig. 2 shows
the influence of the wavelength range of visible light on the relative
Ke values of cis-[Pd(L1-S,O)2] being 0.43 ¡ 0.02, 0.40 ¡ 0.05,
0.20 ¡ 0.01 and 0.01 for white, blue, yellow and red light
respectively.
When the (yellow) light intensity is increased from y 320 to
2280 mmol s21 m22, the Ke value increases from 0.20 to 0.40, at
20 uC for cis-[Pd(L1-S,O)2]. Similar trends are obtained for cis-
[Pt(L1-S,O)2] in MeCN, showing relative Ke values of 0.14 ¡ 0.01,
0.10 ¡ 0.01, and 0 for white (and blue), yellow and red light
respectively. These experiments confirm that cis-[M(L1-S,O)2]
(M 5 PtII, PdII) in solution undergo wavelength dependent
photoinduced cis–trans isomerisation in MeCN. In the absence of
light the isomerisation is reversed resulting in pure cis-[M(L1-S,O)2]
again, suggesting a thermally controlled reverse reaction.
Fig. 3 Ke for ca. 200 mg cm23 cis-[Pd(L1-S,O)2] in MeCN kept in the
dark after irradiation at relatively constant intensity with white light as a
function of temperature, as monitored by rp-HPLC.
Fig. 4 Expanded 1H NMR spectra of cis-[Pd(L1-S,O)2] in MeCN at
20 uC in the dark, showing a trans to cis isomerisation after irradiation
with white light to steady state; H(2,6) trimethoxyphenyl resonances.
Fig. 3 clearly shows that Ke is temperature dependent, and that
after an appropriate time in the dark, only the cis complex is again
found in solution as monitored by rp-HPLC. Monitoring the
isomerisation by rp-HPLC has the disadvantage of a time delay
corresponding to the retention-time of the complexes on column.
Thus monitoring the reverse trans to cis thermal reaction of inter
alia cis-[Pd(L1-S,O)2] after irradiation with white light at various
temperatures by means of 1H NMR, confirms the results obtained
with rp-HPLC. Fig. 4 shows a typical series of 1H NMR spectra as
a function of time after irradiation, confirming that the resonance
at d y 7.33 ppm (assigned to H2 and H6 of the trimethoxybenzoyl
moiety) due to the trans complex decreases with time and the d y
7.53 ppm resonance of the cis isomer grows to eventually dominate
the spectrum again.
1
are known,8 such geometrical isomerisations are rare for chelating
ligands, and there is to our knowledge only one well-studied case
of a photoinduced cis–trans isomerisation observed for the
[Pt(glycinato)2] complex in the literature,8 with no examples of
comparable PdII complexes. The well studied facile cis–trans
isomerism of bis(glycinato)copper(II) complexes,9 is apparently not
photoinduced, occurring spontaneously at ambient temperatures,
presumably via an energetically favourable ring-twisting mechan-
ism.10 We are currently investigating in detail the possible
mechanisms of the photoisomerisation of cis-[M(L-S,O)2] reported
here.
In conclusion, although photoinduced geometrical isomeriza-
tions of metal complexes particularly with monodentate ligands
Financial support from the University of Stellenbosch, the NRF
(GUN 2046827), THRIP (project 2921) and Angloplatinum is
gratefully acknowledged.
Dirk Hanekom, Jean M. McKenzie, Nocky M. Derix and
Klaus R. Koch*
Department of Chemistry, University of Stellenbosch, Private Bag X1,
Matieland, 7602, Stellenbosch, South Africa. E-mail: krk@sun.ac.za;
Fax: +27 21 808 3342; Tel: +27 21 808 3020
Notes and references
{ Ligands HL1, and the corresponding cis-[M(L1-S,O)2] complexes
(M 5 PtII , PdII) were prepared as previously described,1,4 fully
characterised by elemental analysis, mp and 1H and 13C NMR in
CDCl3; cis-[Pt(L1-S,O)2] was confirmed by single crystal X-ray diffraction
(unpublished results; K. R. Koch, J. Miller and L. Barbour, 2003).
{ rp-HPLC conditions: 150 mm 6 4.6 mm, Luna (end-capped) 5 mm C18
column, isocratic flow at 1 cm3 min21, mobile phase: 90% CH3CN, 10%
0.1 M sodium acetate buffer pH 6, 20 mL injections, photometric detection.
Fig. 2 Ke for pure cis-[Pd(L1-S,O)2] in MeCN (20 uC) as a function of
wavelength of irradiation at relatively constant intensity. % white; r blue
(cutoff 310 nm); e yellow (465 nm); m red (580 nm) light.
768 | Chem. Commun., 2005, 767–769
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§ Light source 150 W quartz-halogen lamp from a conventional slide
projector; intensity measured with an LI-250 quantum meter (Lincoln, LI-
COR, USA). Photographic optical filters blue (80B), yellow (Y2) and red
(25A) provided coloured light.
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