Donor-acceptor systems of Pt(ii) and redox-induced reactivity towards small molecules

Article abstract of DOI:10.1039/c2cc15245b

Oxidation at a redox-active ligand is shown to enhance reactivity at the metal center and makes it susceptible to chemical reactions which also include H₂ activation. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc15245b

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Donor–acceptor systems of Pt(II) and redox-induced reactivity towards
small moleculesw
Naina Deibel,a David Schweinfurth,a Stephan Hohloch,a Jan Fiedlerb and Biprajit Sarkar*a
Received 23rd August 2011, Accepted 28th November 2011
DOI: 10.1039/c2cc15245b
Oxidation at a redox-active ligand is shown to enhance reactivity
at the metal center and makes it susceptible to chemical reactions
which also include H2 activation.
Donor–acceptor systems based on Pt(II) and non-innocent
ligands have been long studied for their intriguing electronic
properties and their possible use in solar energy harnessing.1
Metal complexes of non-innocent ligands,2 once considered
spectroscopic curiosities, are making a strong comeback due
to their unusual chemical reactivities and possible use in
catalysis.3 We have recently reported on donor–acceptor
systems of the type [(pap)0MII(Q)2À] (M = Pd or Pt, pap =
phenylazopyridine, Q = 3,5-di-tert-butyl-o-benzoquinone)
and investigated their redox and spectroscopic properties in
detail.4 Even though the ligand Q can exist in three different
Scheme 1 Influence of the ligand redox step on the reactivity at the
Pt(II) center.
1a and 1b were synthesized from [Pt(pap)Cl2] and H2Q1 in
the presence of a base (experimental section). 1b crystallizes in
the monoclinic P21/c space group. The platinum center in 1b is
in a distorted square planar environment, being coordinated
by the two N atoms from pap and the N and O atoms from Q1
(Fig. 1). The Pt–O and Pt–N distances are in the expected
range (Table S2, ESIw).4a,7 Analysis of bond lengths within Q1
shows that with distances of C1–O1 of 1.329(4) A, C2–N1 of
1.386(5) A and the sensitive meta C5–C6 distance of 1.388(5) A,
the ligand Q1 is present in the fully reduced (Q1)2À amidophenolate
form. For comparison, statistical values of the above mentioned
bond lengths from the literature are 1.34, 1.38 and 1.39 A
respectively.8 The N2–N3 distance of 1.321(4) A is longer than
the typical limit of 1.30 A for an unreduced NQN azo bond.
The elongation in the present case is because of the strongly
À
redox forms o-quinone, semiquinone and catecholate (Q0, Qꢀ
and Q2À), the use of redox equivalents from the Q ligand for
inducing chemical reactions is difficult because of the known
problems of chemical instabilityÀof the metal complexes of
such ligands in the oxidised Qꢀ and Q0 forms, and their
tendency to polymerise.3e,5 We thus decided to use the ligand
4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone, Q1, 6 in donor–
acceptor systems [(pap)PtII(Q1)], 1a and 1b (isomers). Just like Q,
Q1 can also exist as (Q1)0, (Q1)ꢀÀ or (Q1)2À and steric protection
from the substituents on the N-atom is known to stabilize the
metal bound oxidized forms of this ligand and also prevent
polymerisation.3e In the following, we present the synthesis,
structural, electrochemical and UV-vis-NIR and EPR spectro-
electrochemical investigations of [(pap)mPtII(Q1)n]x. Subsequently,
we demonstrate how redox processes at the Q1 ligand can
induce reactivity at the Pt(II) center towards molecules such as
PPh3 and H2 (Scheme 1). Similar observations have been made
recently by Rauchfuss et al.3e
a Institut fur Anorganische Chemie, Universitat Stuttgart,
¨
¨
Pfaffenwaldring 55, D-70550, Stuttgart, Germany.
E-mail: sarkar@iac.uni-stuttgart.de; Fax: +49 71168564165;
Tel: +49 71168564235
b J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of
Sciences of the Czech Republic, Dolejsˇkova 3, CZ-18223 Prague,
Czech Republic
w Electronic supplementary information (ESI) available: Synthetic
and spectroscopic characterization of the complex. CCDC 837298.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2cc15245b
Fig. 1 ORTEP plot of 1b. Ellipsoids are drawn at 50% probability
and hydrogen atoms are omitted for clarity.
c
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Fig. 3 X-Band EPR spectrum of 1aꢀ+ in dichloromethane at 295 K.
multiple new bands in the visible region (Fig. SÀ2 and Table S3,
ESIw). These bands are reminiscent of a papꢀ attached to a
Pt(II) center.9 The EPR spectrum of 1aꢀÀ at 295 K is centered
at g = 2.008 and could be simulated with a(195Pt) = 127 G
(Fig. S3, ESIw). Such a speÀctrum has been prevÀiously observed
for a Pt(II) bound papꢀ radical and 1aꢀ is thus best
Fig. 2 Cyclic voltammograms of 1.6 mM 1a in CH2Cl2/0.1 M
Bu4NPF6 in the absence (top) and presence of PPh3 (PPh3/1a molar
ratio: 0, 1/4, 1/2, 3/4, 1).
À 4a
ꢀ
] .
formulated as [(pap)ꢀÀPtII(Q1)2À
À
The much larger 195Pt coupling for 1aꢀ as compared to
+
p-donating (Q1)2À ligand that is bound to the Pt(II) center
which results in back-bonding into the p* orbital of pap and
the concomitant elongation of the NQN bond. Such an
elongation was also observed in the case of the corresponding
complex with the Q2À ligand.4a However, the extent of the
elongation is more prominent in the present case possibly due
to the better p-donor ability of the (Q1)2À ligand in comparison
to (Q)2À. The complex 1b is thus best described as the diamagnetic
[(pap)0PtII(Q1)2À] form.
1aꢀ results from a back-bonding from Pt(II) to pap in the
reduced form.7b The expected hyperfine couplings to the
N atoms of pap are not resolved, possibly due to the Àlarge
195
line-widths arising out of the large Pt coupling in 1aꢀ
.
Oxidation induced reactivity was then tested by cyclic
voltammetry as shown in Fig. 2. In a typical experiment,
increasing amounts (up to 1 equivalent) of a dichloromethane
solution of PPh3 was added to a solution of 1a in dichloro-
methane. Addition of increasing amounts of PPh3 led to
progressive depletion of the oxidation waves of 1a at 0.22 and
À0.47 V and to the appearance of new waves at Epa = À0.25
and E1/2 = 0.20 V. Furthermore, a new peak in the cathodic
side appeared at Epc = À0.79 V. The changes continued till
the addition of 1 equivalent of PPh3, whereupon both the
original oxidation waves of 1a had completely disappeared.
During all these changes, the reduction peak of 1a at E1/2 = À1.57
remained intact, the second reduction wave at À2.12 V was
changed partially (Fig. 2); its shape is presumably influenced
by a species remaining adsorbed on the electrode surface after
the scan in the positive potential region (Fig. S4, ESIw). We
propose a tentative mechanism to explain these observations
(Scheme 2). Oxidation of 1a to 1aꢀ+ leads to the oxidation of
Since positional isomers are known to show identical electro-
chemical and spectroscopic properties, all studies were carried
out only with 1a.4a Cyclic voltammetry of 1a shows two
oxidation processes at 0.22 and À0.47 V and two reduction
processes at À1.57 and À2.12 V (Fig. 2, top). The large
potential differences between the 1st oxidation and 1st
reduction steps as well as those between the two oxidation
and two reduction steps result in comproportionation constant
(Kc) values of the order of 1018, 1011, and 109 for the neutral, one-
electron oxidized and one-electron reduced forms respectively.
These values show the high thermodynamic stability of all the
three forms and possible quantitative access to these redox
forms. Accordingly, 1aꢀ+ could be isolated by the oxidation of
1a with ferrocenium hexafluorophosphate.
À
(Q1)2À to (Q1)ꢀ and to an indirect increase in the Lewis
The UV-vis-NIR spectrum of 1a shows a spin and dipole
allowed intense LLCT [p(Q1)2À to p*(pap)0] transition at 897 nm
acidity of the Pt(II) center. The Lewis acidic Pt(II) center now
binds PPh3 and the five coordinated complex is then oxidized
+
(e = 20 900 MÀ1 cmÀ1).7b One-electron oxidation to 1aꢀ
at Epa = À0.25 V. The oxidation wave at E1/2 = 0.20 ÀV is
assigned to the reversible oxidation of [PPh3(pap)0PtII(Q1)ꢀ
]
ꢀ
+
leads to a decrease in the intensity of the LLCT band and the
appearance of a typical iminosemiquinone based transition at
415 nm (Fig. S2, ESIw). The EPR spectrum of 1aꢀ+ at 295 K is
centered at g = 1.988 and could be simulated with a(195Pt) =
25.3 G (I = 1/2, nat. abundance 33.3%); a(1H) = 4.4 G;
a(14N) = 6.1 G (Fig. 3). These values are typical for a Pt(II)
to the dicationic form. The Epc wave at À0.79 V is tentatively
assigned to an EC step, which involves the re-reduction of
À
+
[PPh3(pap)0PtII(Q1)ꢀ
now less Lewis acidic metal center to regenerate 1a.
]
and the release of PPh3 from the
ꢀ
Partial support for the mechanism proposed in Scheme 2 comes
from mass spectrometric measurements, where in an in situ experi-
+
bound iminosemiquinone raÀdical4a,7b and 1aꢀ is thus best
described as [(pap)0PtII(Q1)ꢀ
]
+. Further oxidation to 1a2+
ment, the species [PPh3(pap)0PtII(Q1)ꢀ
]
was observed when
À
+
ꢀ
ꢀ
leads to depletion of most bands in the visible and NIR regions
as would be expected for a [(pap)0PtII(Q1)0]2+ form.
One-electron reduction to 1aꢀ leads to a decrease in the
intensity of the LLCT band at 897 nm and the appearance of
1aꢀ+ and PPh3 were injected inside the mass spectrometer (Fig. S5,
ESIw). The neutral complex 1a showed no reactivity whatsoever
towards PPh3, supporting our hypothesis that oxidation at the
(Q1)2À site is necessary in order to make the Pt(II) center reactive.
À
c
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Chem. Commun., 2012, 48, 2388–2390 2389

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+
reduce two equivalents of 1aꢀ to 1a with an additional
proton abstraction by KOtBu. The redox equivalents for H2
activation are thus concentrated on the non-innocent Q1
ligand. Such a mechanism has precedence in the literature
for iridium complexes with non-innocent ligands.3e,10 We were
able to reduce ferrocenium hexafluorophosphate to ferrocene
with H2 in the presence of 1aꢀ+ as catalyst and repeat the cycle 15
times with about 60% catalyst recovery (Fig. S8 and S9, ESIw).
In summary, we have presented here novel, isomeric Pt(II)
based donor–acceptor systems and have shown that redox
steps at a non-innocent ligand can increase Lewis acidity at the
Pt(II) center and make it reactive towards PPh3 and for
activating H2. The detailed mechanism of H2 activation, as
well as the potential of the system described here for activating
other small molecules is the focus of our current research
activities. In view of the fact that it is ligand-centered redox
that is primarily responsible for reactivity in the present case, it
will be intriguing to see if such reactivity can also be realised
for other inexpensive and earth abundant metals.
+
Scheme 2 Mechanism for the reaction between 1aꢀ and PPh3.
Encouraged by these initial findings, we tried to figure out if
such a redox induced increase in reactivity can also be used in
activating small molecules like H2. UV-vis-NIR spectroscopy
turned out to be the method of choice for unravelling such
reactivities, because from our spectroelectrochemical experi-
ments, as well as from the UV-vis-NIR spectrum of the isolated
Fonds der Chemischen Industrie (FCI) and Grant Agency of
the Czech Republic (grant 203/09/1607) are kindly acknowledged
for the financial support of this project.
Notes and references
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+
species 1a and 1aꢀ we had standard quantitative spectra for
these two different redox states in hand (Fig. S2, ESIw). As
would be expected, bubbling H2 gas through a dichloromethane
solution of 1a resulted in no reaction. In contrast, on reacting
+
1aꢀ with H2 in the presence of a stoichiometric amount of
KOtBu as base, activation of H2 was observed together with a
+
concomitant reduction of 1aꢀ to 1a. This process can be
followed with the naked eye because of a change in color from red
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+
1aꢀ to 1a (Fig. S6, ESIw).
+
We suggest that the more Lewis acidic metal center in 1aꢀ
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bound H2. The resulting hydride complex (detected by mass
spectrometry, Fig. S7, ESIw) then has the required electrons to
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Fig. 4 Change of color of the complexes accompanied by H2
activation.
c
2390 Chem. Commun., 2012, 48, 2388–2390
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