Dual stimuli-responsive interconvertible heteroleptic platinum coordination modes

Article abstract of DOI:10.1039/c0cc01668c

The exchange between different equilibrium and out-of-equilibrium platinum coordination modes can be orthogonally controlled thermodynamically with a proton input and kinetically using light.

Full text of DOI:10.1039/c0cc01668c

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Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 44 | 28 November 2010 | Pages 8293–8488
ISSN 1359-7345
COMMUNICATION
Sarah J. Pike and Paul J. Lusby
Dual stimuli-responsive interconvertible heteroleptic platinum coordination modes

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Dual stimuli-responsive interconvertible heteroleptic platinum
coordination modesw
Sarah J. Pike and Paul J. Lusby*
Received 31st May 2010, Accepted 5th July 2010
DOI: 10.1039/c0cc01668c
The exchange between different equilibrium and out-of-equilibrium
platinum coordination modes can be orthogonally controlled
thermodynamically with
using light.
a proton input and kinetically
The operational basis for devices and smart materials, such as
molecular machines1 and switchable polymers,2 is often a
non-covalent (supramolecular) or transition metal–ligand
motif which undergoes reversible change in response to an
external stimulus. In almost all cases,3 the stimulus functions
by adjusting the thermodynamic position of the equilibrium;
in systems that rely upon exclusively weak interactions such as
hydrogen-bonding or p–p interactions,1a–c re-equilibration
often occurs very quickly such that out-of-equilibrium states
cannot be easily detected. In contrast, systems which utilise
transition metal–ligand interactions often occur on a much
slower timescale, minutes, hours or even days.4,5 As such,
metastable states, which have been shown to be important in
molecular electronics6 and machine ratcheting mechanisms,7
can be observed or even isolated and these systems often
require the input of heat to overcome the kinetic barrier to
re-equilibration.5,8,9 However, heat does not alter the magnitude
of this barrier and so cannot be thought of as a ‘‘kinetic
stimulus’’. Here we report the switchable behaviour of
heteroleptic, acceptor–donor-type platinum coordination
complexes in which we orthogonally use a proton input to
adjust the equilibrium position and exploit light to selectively
lower the energetic barrier to equilibration.
Scheme 1 Dual stimuli-responsive exchange between Pt coordinated
PyNMe2 and PyR (R = H or CF3) as a function of light and protons.
All reactions carried out at 0.01 M in 1 : 1 [D7]DMF : CD2Cl2 at
313 K, either in the presence or absence of light (broad band
275–375 nm). The ratios correspond to the equilibrium ratios obtained
The system under study aimed to combine the preference of
group 10 transition metals to selectively bind heterocycles as a
function of basicity,2b,5 a thermodynamic bias that can be
reversed by the addition of an equivalent of acid,5 with the
known photochemical liability of 2nd and 3rd row transition
metals. While light-promoted ligand exchange is well documented
for Ru(II),10 in particular, it has recently been shown that
the rates of Pt(II) substitution can also be increased upon
photoirradiation.11 In order to control the selective exchange
of just a single ligand, we chose to investigate the neutral
platinum motif12 [LPt(PyR)] (Scheme 1) as it was envisaged
that the tridentate, doubly charged L2ꢀ would not undergo
dissociation from the platinum centre on exposure to light.
Initially, the thermodynamically downhill exchange of N,N0-
dimethylaminopyridine (PyNMe2) for the pyridine ligand of
after
x h starting from (a) [LPt(PyR)]; (b) [cis-HLPt(PyR)-
(PyNMe2)]OTs and (c) [cis-HLPt(PyNMe2)(S)]OTs.15 aRatio taken
prior to equilibration due to increasing decomposition caused by
prolonged heating. b264 h at 313 K and then 264 h at 333 K.15
c312 h at 313 K and then 264 h at 333 K.
[LPt(PyH)] was investigated (Scheme 1a). Upon broad band
275–375 nm irradiation (with a lmax of 312 nm, see ESIw)
of a 0.01 M solution of [LPt(PyH)] and PyNMe2 in 1 : 1
[D7]DMF : CD2Cl2, a steady-state equilibrium of 17 : 83
[LPt(PyH)] : [LPt(PyNMe2)] was obtained after 10.5 h. To
investigate the effect an increase in the pKa difference of
the two exchangeable heterocycles would have on both the
thermodynamic bias and the rate of equilibration, the reaction
of PyNMe2 and [LPt(PyCF3)] was also investigated
(Scheme 1a). Under the same conditions (0.01 M, 1 : 1
[D7]DMF : CD2Cl2) 4.5 h irradiation gave a steady-state ratio
of 11 : 89 [LPt(PyCF3)] : [LPt(PyNMe2)]. In both cases, the
exchange experiment was followed directly by 1H NMR
spectroscopy (Fig. 1). To assess the effect of light, both
School of Chemistry, Joseph Black Building, West Mains Road,
Edinburgh, EH9 3JJ, Scotland, UK. E-mail: Paul.Lusby@ed.ac.uk;
Fax: +44 (0)131 6506453; Tel: +44 (0)131 6504832
w Electronic supplementary information (ESI) available: Synthesis
and characterisation of all new compounds, details of all irradiative
and non-irradiative ligand exchange experiments. See DOI: 10.1039/
c0cc01668c
c
8338 Chem. Commun., 2010, 46, 8338–8340
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after 11 days at 313 K and a further 11 days at 333 K. In both
cases, the thermal reactions were halted prior to equilibrium
due to small amounts of decomposition.
To reverse the coordinative bias of the heterocycles, it was
anticipated that a second equivalent of acid could be used.
Promisingly, the addition of 1 equiv. of TsOH to [trans-
HLPt(PyNMe2)(PyR)]OTs (R = CF3 or H) in 1 : 1 [D7]DMF
: CD2Cl2 resulted in liberation of PyRꢁTsOH along with
formation of a complex we presume to be the solvato complex
[cis-HLPt(PyNMe2)(S)]OTs, where S = [D7]DMF. Irradia-
tion of these solutions resulted in the emergence of PyNMe2ꢁ
TsOH and the concomitant reduction of PyRꢁTsOH
(R = CF3 or H) (Scheme 1c). However, the complexity of
these NMR spectra, again, prohibited a direct measure of the
equilibrium position, so the samples were treated with P1–tBu
after 5.2 h (R = H) and 5.5 h (R = CF3) irradiation, from
which ratios of [LPt(PyR)] : [LPt(PyNMe2)] of 70 : 30
(R = H) and 85 : 15 (R = CF3,) were obtained. Again, for
comparison, the thermal reactions produced ratios of 41 : 59
(R = H) and 63 : 37 (R = CF3) after 13 days at 313 K and a
further 11 days at 333 K. Both thermal reactions were
again stopped prior to equilibrium due to small amounts of
decomposition.
Fig. 1 1H NMR spectra (400 MHz, 1 : 1 [D7]DMF : CD2Cl2, 300 K)
of the photochemical reaction between [LPt(PyCF3)] and PyNMe2
over time. The o-pyridyl protons of [LPt(PyNMe2)] and free PyNMe2
are highlighted in blue and the o-pyridyl protons of [LPt(PyCF3)] and
free PyCF3 in red.
reactions were also carried out under identical conditions
(0.01 M in 1 : 1 [D7]DMF : CD2Cl2, 313 K) but in the dark.
The reaction of PyNMe2 with [LPt(PyCF3)] took 44 h to reach
an equilibrium ratio of 15 : 85, within error of the ratio
obtained under irradiative conditions,13 indicating that light
significantly lowers the activation barrier to exchange without
perturbing the relative ground state energies. In the absence of
light, equilibration of [LPt(PyH)] with PyNMe2 at the same
temperature (313 K) gave a ratio of 28 : 72 after 11 days.
However, the sluggishness of the thermal exchange process
(even at 333 K), coupled with the slow but steady decomposition
of the complexes that occurred after further, prolonged
heating, ensured that this reaction could not be followed to
completion.
A likely mechanism for the pronounced rate enhancements
upon irradiation is through excitation into a metal centred
(MC) state with s antibonding dx2–y2 character.11a,b Due to the
heavy atom effect, it is probable that this proceeds via a
3MLCT (metal–ligand charge transfer) state, from which the
3MC state is thermally accessible.12 However, strong field
cyclometallated ligands are known to raise the energy of the
MC state, which is why many similar Pt complexes are
luminescent, rather than photochemically active, at room
temperature.16 In this current investigation, it could be
anticipated that the MC states of [LPt(PyR)] would be
less accessible than the corresponding states for either
[cis-HLPt(PyR)(PyNMe2)]OTs or [cis-HLPt(PyNMe2)(S)]OTs.
This appears to be supported, experimentally, as the C4N
complexes exhibit greater irradiative rate enhancements in
comparison to the thermal reactions (in effect, protonation
photochemically activates the C4N4C complexes).17
Interestingly, it is known that the related C4N4C complex
[LPt(CNPh)] is not luminescent at room temperature and a
recent theoretical analysis has shown that a likely reason for
this is due to significant distortion of the triplet ground state,
which reduces the ligand field and brings the MC state into
thermal range.18
The platinum-binding preference of the heterocycles cannot
be reversed with just one equivalent of acid as addition of a
stoichiometric amount of p-toluenesulfonic acid (TsOH) to a
mixture of [LPt(PyNMe2)] and PyR (R = H or CF3) results
in protonation of the C4N4C complexes to give [trans-
HLPt(PyNMe2)(PyR)]OTs (Scheme 1b).14 Furthermore, we
reasoned that the stereochemical relationship of the PyNMe2
and PyR ligands with respect to the strong trans influence of
the phenylato group of HLꢀ would lead to isomers of
noticeably different energy. To investigate this, [LPt(PyR)]
(R = H or CF3) were initially treated with TsOH and
then with PyNMe2 to give the presumed higher energy
[cis-HLPt(PyR)(PyNMe2)]OTs isomers. Upon light irradiation
of these samples, it could be roughly observed that an increase
in [trans-HLPt(PyNMe2)(PyR)]OTs was accompanied by a
decrease in [cis-HLPt(PyR)(PyNMe2)]OTs (Scheme 1b).
However, the complexity of these NMR spectra, caused in
part by the unsymmetrical nature of the C^N complexes,
prohibited a direct measure of the cis : trans complex ratio.
Instead, an indirect measure was obtained by treating the
samples, after irradiative equilibration was complete (6.5 h
for R = H, 5 h for R = CF3), with the base P1–tBu to affect
cyclometallation, from which ratios of 26 : 74 (R = H) and
20 : 80 (R = CF3) [LPt(PyR)] : [LPt(PyNMe2)] could be
obtained.15 For comparison, the non-irradiative reactions
produced ratios of 49 : 51 (R = H) and 30 : 70 (R = CF3)
In conclusion, we have described the orthogonal, dual
stimuli-responsive behaviour of heteroleptic cyclometalled
platinum complexes, in which protons are used to switch the
thermodynamic bias, and in which light functions as a true
kinetic stimulus by lowering the activation barrier to ligand
exchange. We envisage that switchable complexes such as
these, where both the thermodynamic and kinetic components
of switching can be independently manipulated, will play a
pivotal role in the development of future devices and smart
materials.
This work was supported by the EPSRC and The Royal
Society. P.J.L. is a Royal Society University Research Fellow.
We thank Dr Jonathan E. Beves (Edinburgh) for useful
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This journal is The Royal Society of Chemistry 2010
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discussions. We gratefully acknowledge Johnson Matthey for
the loan of the platinum salts.
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8340 Chem. Commun., 2010, 46, 8338–8340
This journal is The Royal Society of Chemistry 2010