Stable mixed-valent radicals from platinum(II) complexes of a bis(dioxolene) ligand

Article abstract of DOI:10.1002/chem.201304848

Three diplatinum(II) complexes [{PtL}₂(μ-thea)] (H ₄thea=2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9, 10-ethanoanthracene) have been prepared, with diphosphine or bipyridyl L co-ligands. One-electron oxidation of these complexes gave radical cations containing a mixed-valent [thea .]^3- ligand with discrete catecholate and semiquinonate centers separated by quaternary methylene spacers. The electronic character of these radicals is near the Robin-Day class II/III border determined by UV/Vis/NIR and EPR spectroscopies. Crystal-structure determinations and a DFT calculation imply that oxidation of the thea ^4- ligand may lead to an increased through-space interaction between the dioxolene π systems. Electron hopping: Diplatinum complexes, such as the one shown here, are readily oxidized to mixed-valent catechol/semiquinone radical species, some of which can be crystallized at room temperature. The dioxolene rings in the radical products are moderately strongly coupled by EPR and UV/Vis/NIR spectroscopies, despite being separated by saturated methylene spacers.

Full text of DOI:10.1002/chem.201304848

DOI: 10.1002/chem.201304848
Communication
&
Mixed-Valent Compounds
Stable Mixed-Valent Radicals from Platinum(II) Complexes of
a Bis(dioxolene) Ligand
Jonathan J. Loughrey,[a, b] Stephen Sproules,[c, d] Eric J. L. McInnes,[c] Michaele J. Hardie,[a] and
Malcolm A. Halcrow*[a]
Abstract: Three diplatinum(II) complexes [{PtL}2(m-thea)]
(H4thea=2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-
9,10-ethanoanthracene) have been prepared, with diphos-
phine or bipyridyl “L” co-ligands. One-electron oxidation
of these complexes gave radical cations containing
3ꢀ
C
a mixed-valent [thea] ligand with discrete catecholate
and semiquinonate centers separated by quaternary
methylene spacers. The electronic character of these radi-
cals is near the Robin–Day class II/III border determined
by UV/Vis/NIR and EPR spectroscopies. Crystal-structure
determinations and a DFT calculation imply that oxidation
of the thea4ꢀ ligand may lead to an increased through-
space interaction between the dioxolene p systems.
Scheme 1. Compounds reported herein and other complexes referred to in
the discussion. Co-ligand abbreviations: dppb=1,2-bis(diphenylphosphino)-
benzene; dppe=1,2-bis(diphenyl-phosphino)ethane; and tBu2bipy=4,4’-
Metal complexes of dioxolenes can exhibit a fascinating
bis(tert-butyl)-2,2’-bipyridyl.
ligand-based redox chemistry, involving conversion between
catecholate (“cat”), semiquinone (“sq”), and quinone (“q”)
ligand oxidation levels.[1–3] Complexes of dinucleating dioxo-
lenes add another layer of complexity to this behavior with
multiple metal and ligand redox sites,[2–4] which may give rise
to ligand-based mixed valency.[5] One example is 4,4’-bis-
(catechol) (H4bis(cat), Scheme 1), whose derivatives form delo-
calized radicals at the sq/cat oxidation state, but are spin cou-
pled at the sq/sq level reflecting formal oxidation of the cen-
tral CꢀC bond.[6–8] Conversely, cat/sq radicals generated from
spiro4ꢀ complexes (H4spiro=3,3,3’,3’-tetramethyl-1,1’-spiro-bis-
{5,6-dihydroxyindane}, Scheme 1) are localized on individual di-
oxolene rings,[2] with electron hopping between the dioxolene
groups occurring near the EPR timescale.[9] Transition-metal di-
oxolene complexes can also exhibit valence tautomerism and
spin-transition equilibrium involving metalÐligand charge
transfer,[10] but few examples of these phenomena in bis(dioxo-
lene) ligand systems have been reported to date.[2,9,11]
Herein, we report the redox chemistry of 2,3,6,7-tetrahy-
droxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene
(H4thea) when complexed to platinum(II) (1–3, Scheme 1).
Others have used thea4ꢀ as a component in metallacycle and
cage complexes, but the redox chemistry of those products
was not reported.[12,13] The dimethyl-bicyclo[2.2.2]octyl spacer
prevents formal conjugation of the theanꢀ dioxolene groups
but places them close in space. Therefore, we predicted
[a] Dr. J. J. Loughrey, Prof. M. J. Hardie, Prof. M. A. Halcrow
School of Chemistry, University of Leeds
Woodhouse Lane, Leeds, LS2 9JT (UK)
Fax: (+44)113-343-6565
3ꢀ
C
that the cat/sq species [thea] should exhibit mixed-valence
3ꢀ
3ꢀ
C
C
behavior intermediate between [bis(cat)]
and [spiro]
(Scheme 1).
E-mail: m.a.halcrow@leeds.ac.uk
The synthesis of compounds 1–3 was achieved by reacting
H4thea with two equivalents of preformed [PtCl2L] (L=dppb,
dppe, or tBu2bipy) in the presence of base. The complexes can
be handled in air in the solid state and in solution, but must
be stored under an inert atmosphere for extended periods.
X-ray structure determinations of 1 and 2 were achieved
from solvate crystals grown from dichloromethane/pentane
(Figure 1). Although both structures are crystallographically
non-routine,[14] the metric parameters about the thea4ꢀ ligands
confirm that the dioxolene rings are at the catecholate oxida-
tion level (ꢀ1.7(3)ꢁDꢁꢀ2.1(2),[15] Table 1). The dihedral angle
[b] Dr. J. J. Loughrey
Current address: Department of Chemistry and Biochemistry
University of Arizona, P.O. Box 210041
1306 East University Blvd., Tucson, AZ 85721-0041 (USA)
[c] Dr. S. Sproules, Prof. E. J. L. McInnes
School of Chemistry and Photon Science Institute
University of Manchester, Oxford Road, Manchester, M13 9PL (UK)
[d] Dr. S. Sproules
Current address: School of Chemistry, University of Glasgow
Joseph Black Building. University Avenue, Glasgow, G12 8QQ (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304848.
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The green oxidized products [1]+ and [2]+, and purple [3]+,
can be generated by treatment of the neutral precursors with
one equivalent of [FeCp2]PF6 in CH2Cl2. Solutions of [1]+ and
[2]+ are stable for hours at 298 K under an inert atmosphere,
which allowed [1]PF6 and [2]PF6 to be isolated and crystallized
(see below), but [3]PF6 decomposes slowly under those condi-
tions. The oxidations were monitored by UV/Vis/NIR titrations,
which proceeded isosbestically for 1 and 2 (Figure 2). In both
cases, ingrowth of a new intervalence charge transfer (IVCT)
transition at lmax =1810 nm with at least one low-wavelength
shoulder was observed, along with a smaller increase in inten-
Figure 1. “A” disorder site of the [{Pt(dppb)}2(thea)] molecule in
[1·xC5H12 ꢁ(4ꢀx)CH2Cl2] (top),[12] and the [{Pt(dppe)}2(thea)]+ cation in
[2]PF6·3CH2Cl2 (bottom). Displacement ellipsoids are at the 50% probability
level, and all hydrogen atoms have been omitted for clarity. Symmetry code:
(i) 3/2ꢀx, 1/2ꢀy, z. Additional crystallographic Figures and Tables are given
in the Supporting Information.[16]
between the thea4ꢀ dioxolene groups (q, Table 1) is approxi-
mately 108 larger in 2 than in 1, showing that there is some
conformational flexibility in the thea4ꢀ framework.
Cyclic and differential pulse voltammetry of 1–3 in CH2Cl2/
0.5m nBu4NPF6 at 298 K revealed two chemically reversible
low-potential oxidations at ꢀ0.37ꢂ0.02 and ꢀ0.11ꢂ0.02 V
versus [FeCp2]/[FeCp2]+.[16] These were assigned to the
3ꢀ
2ꢀ
[thea]4ꢀÐ[thea] Ð[thea ]
(cat/catÐcat/sqÐsq/sq) redox
C
CC
series. The separation of these processes (DE) is 250ꢂ20 mV,
between complexes of bis(cat)4ꢀ (DE=320–500 mV)[2,6,7] and of
spiro4ꢀ (140–170 mV).[2,9] The subsequent [thea ] Ð[thea] Ð-
[thea]0 (sq/sqÐsq/qÐq/q) oxidations occurred near +0.75 V,
were more closely separated (DEꢃ110 mV) and were only
partly reversible at room temperature.
2ꢀ
ꢀ
C
CC
Figure 2. UV/Vis/NIR titrations for the chemical oxidation of 1 (top) and 2
(bottom) by up to one equivalent of [FeCp2]PF6 (CH2Cl2, 296 K). The spectra
of pure 1, [1]+, 2, and [2]+ are highlighted as black lines, whereas the inter-
mediate stoichiometries are in grey. Isosbestic points are shown as insets.[16]
sity of the dioxolene!L (L=
Table 1. Selected metric parameters from the crystal structures in this work. D is a bond-valence sum parame-
ter giving the oxidation state of dioxolene groups, which takes the values of 0, ꢀ1, and ꢀ2 for the q, sq, and
cat levels, respectively.[15] q is the dihedral angle between the least squares planes of the thean- dioxolene
rings. More detailed information about the structures is given in the Supporting Information.
dppb or dppe) ligand-to-ligand
charge transfer (LLCT) band near
690 nm.[7] The intensity of the
IVCT band is around three times
greater in [2]+ than in [1]+.
PtꢀO [ꢂ]
1.98(2)–2.05(2)
2.026(8)–2.048(8)
2.006(13)–2.049(9)
2.034(9)–2.071(15)
2.042(4), 2.060(4)
PtꢀP [ꢂ]
D
q [8]
1
2.201(3)–2.207(3)
2.202(4)–2.227(3)
2.205(4)–2.236(4)
2.195(6)–2.222(4)
2.2101(14), 2.2197(14)
ꢀ1.7(3)ꢁDꢁꢀ1.9(2)[a]
ꢀ1.77(15), ꢀ2.1(2)
ꢀ1.86(14), ꢀ2.1(2)
ꢀ1.90(15)[b]
140.8(5)–141.1(5)[a]
130.8(5)
An IVCT band with lmax
=
2 molecule A
molecule B
molecule C
1917 nm is also formed during
the oxidation of 3, reaching
emax =6.3ꢁ103 mꢀ1 cmꢀ1 for [3]+,
which is twice as intense as that
exhibited by [2]+ (Figure 2). That
titration was not isosbestic, how-
131.1(6)
129.8(9)–131.5(6)[a]
117.14(11)
[c]
[2]PF6
ꢀ1.57(15)
[a] Range of values given for disorder sites in this residue.[14] [b] D for the second Pt/dioxolene center in this
molecule was not determined because of restraints applied in the crystallographic refinement. [c] There is only
one unique Pt/dioxolene center in this crystal structure.[14]
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ever, which indicates slow decomposition of [3]+ under these
conditions.
+
1
The width at half height, Dn = , of the IVCT bands in [1] and
2
[2]+ is ꢃ2300 cmꢀ1, taking account of the low-wavelength
shoulder. That is smaller than predicted by Equation (1) for
[1]+ and [2] , which gives Dn = ꢄ3550 cm
for a class II
+
ꢀ1
1
2
mixed-valent system with an IVCT maximum of 1810 nm (E=
5525 cmꢀ1).[17]
1
=
2
ð1Þ
Dn=
1
¼ ð2310EÞ
2
Although this criterion should be applied with care, the
3ꢀ
C
[thea] framework is clearly approaching the class III formal-
ism. In the class III limit, the electron-coupling energy HAB for
[1]+ and [2]+ is approximately 1150 cmꢀ1 according to Equa-
tion (2):[5,17]
HAB¼1= Dn=
1
ð2Þ
2
2
That is comparable to cyclophane radical ions and related
species, which show through-space coupling between stacked
aromatic rings.[18] Solutions of [2]+ exhibit a correlation be-
tween the IVCT maximum and the donor number of the sol-
vent,[19] in the order DMF (lmax =1883 nm)>THF (1847)>ace-
tone (1829)>CH2Cl2 (1810). The maximum variation in IVCT
energy between these solvents (215 cmꢀ1) is smaller than ex-
Figure 3. Fluid solution S band and frozen solution X band EPR spectra of
[1]+ and [3]+ in CH2Cl2/THF 10:1. Simulation parameters are given in
Table 2.[16]
line widths of [1]+ and [2]+ are almost invariant between 200–
300 K, but the spectrum of [3]+ broadens considerably below
230 K, which could indicate the slowing of this electron hop-
ping[9] and/or aggregation of the complex in solution (see
below).[16] In contrast, frozen solution X-band spectra of [1]+
–[3]+ are near-axial and show coupling to just one 195Pt nu-
cleus, and for [1]+ and [2]+, two 31P nuclei (Figure 3). There-
fore, electron hopping between their dioxolene groups is
frozen out, apparently coinciding with freezing of the solvent
medium.[9] Although the hyperfine coupling for [3]+ is poorly
resolved in the frozen solution spectrum, the g anisotropy
and195Pt couplings in [3]+ are over double those in [1]+
and [2]+. This indicates a greater Pt contribution to the frontier
orbital in the presence of the more strongly p-accepting
tBu2bipy ligand.[21]
pected for a class II organic radical,[20] and again implies
3ꢀ
C
a degree of delocalization between the [thea] dioxolene
groups.[5] The IVCT linewidth Dn = does not vary significantly in
1
2
these spectra, but the relative intensities of the IVCT band and
the LLCT absorption near 700 nm show a much stronger sol-
vent dependence.[16]
The S band and X band EPR spectra of [1]+ and [2]+ in
CH2Cl2/THF 10:1 fluid solution are very similar, with g values
close to that of the free electron (Table 2 and Figure 3). Al-
Table 2. Simulated EPR spectroscopic parameters for [1]+–[3]+ in CH2Cl2/
THF 10:1 solution. Hyperfine couplings are related to 195Pt, and are re-
ported in 10ꢀ4 cmꢀ1
.
Single-crystal X-ray structures were obtained of
[2]PF6·3CH2Cl2 and a solvate of [1]PF6, although the latter
structure was of too low resolution for a detailed analysis of its
metric parameters.[14] The complex cation in [2]PF6·3CH2Cl2 has
crystallographic C2 symmetry, meaning that the oxidized and
unoxidized dioxolene groups are crystallographically equiva-
lent (Figure 1). Although the bond lengths to the Pt atom are
indistinguishable from the neutral complexes, the metric pa-
rameters in the unique dioxolene center are consistent with
[b]
[b]
[b]
210 Kg (A[a])
100 Kg1 (A1
)
g2 (A2
)
g3 (A3 )
[1]+[c]
[2]+[c]
[3]+
2.0011 (4.9)
2.0012 (4.7)
2.0019 (13.5)
2.0045 (20)
2.0031 (19)
2.0032 (19)
1.9872 (48)
1.9844 (ꢀ10)
1.9833 (ꢀ10)
1.9796 (40)
2.0055 (19.5)
2.0292 (48)
[a] Coupling to two 195Pt nuclei. [b] Coupling to one 195Pt nucleus. [c] Ad-
ditional superhyperfine coupling of 1–3ꢁ10ꢀ4 cmꢀ1 to 31P nuclei can also
be extracted from the line shapes of these spectra.
a singly oxidized [thea]3ꢀ ligand (D=ꢀ1.57(15), Table 1; the ex-
C
though hyperfine coupling was not clearly resolved, features
on the S-band line shape could be modelled by considering
pected value is ꢀ1.5[15]). The dihedral angle between the diox-
olene groups (q) in [2]+ is contracted to 117.14(11)8, which is
approximately 148 lower than in 2 (Table 1; the corresponding
values for the two unique complex cations in [1]PF6 are
108.7(5) and 112.8(7)8). Although they are not isomorphous, in
both structures the radical cations associate into nested dimers
(Figure 4). The assignment of these dimers as charge-transfer
assemblies is uncertain, because there are no interatomic con-
hyperfine coupling to two 195Pt (I=1= , 34% abundant) and
2
four 31P (I=1= , 100% abundant) nuclei. The spectrum of [3]+
2
contains five resolved lines that more obviously arise from hy-
perfine coupling to two 195Pt nuclei. This demonstrates elec-
3ꢀ
C
tron hopping between the [thea] dioxolene rings that is
rapid on the EPR timescale at these temperatures. The X-band
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Acknowledgements
This work was supported by the EPSRC. We thank Diamond
Light Source for access to beamline I19 (MT8517) that contrib-
uted to the results presented herein.
Keywords: charge transfer
·
dioxolenes
·
mixed-valent
compounds · platinum · radical ions
Figure 4. View of the association of the radical cations in [2]PF6·3CH2Cl2 into
nested dimers, generated from the unique half-molecule by crystallographic
S4 symmetry. The carbon atoms of the two molecules have grey and black
colors, and hydrogen atoms have been omitted for clarity. Pt(1ii) is at the
front of the figure, whereas Pt(1iii) is directly behind it. Symmetry codes: (i)
3/2ꢀx, 1/2ꢀy, z; (ii) 1/2+y, 1ꢀx, 1ꢀz; (iii) 1ꢀy, ꢀ1/2+x, 1ꢀz.
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their van der Waals radii. However, a dimerization equilibrium
of this type could explain the EPR line broadening observed
for [3]+ below 230 K.[16] Notably, unoxidized 1 and 2 do not di-
merize in this manner in the crystals of those compounds. The
ꢀ
PF6 ions in both radical structures only associate with the cat-
ions through peripheral van der Waals contacts.[14,16]
A DFT calculation of the model complex [{Pt(bipy)}2(thea)]
showed that the HOMO lies predominantly on the thea4ꢀ
ligand and has p-antibonding character between the two diox-
olene rings.[16] The reduced q values in [1]PF6 and [2]PF6 com-
pared to 1 and 2 (Table 1) are consistent with depopulation of
this HOMO upon oxidation, which would strengthen any bond-
ing interaction between the dioxolene groups. The HOMOꢀ1
is the corresponding in-phase combination between the
thea4ꢀ dioxolene rings. The calculated energy gap between
the HOMO and HOMOꢀ1, 0.24 V, is a good match for the elec-
trochemical separation between the cat/sq oxidations in 1–3
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(DEꢄ250 mV).
3ꢀ
C
In conclusion, oxidation of 1–3 gave [thea] radical deriva-
tives. Although their dioxolene centers are not directly conju-
gated, [1]+–[3]+ show electron hopping between the dioxo-
lene rings in fluid solution by EPR, and a degree of electron de-
localization that is comparable to cyclophane-derived radi-
cals.[18] The strength of this electron coupling may reflect the
proximity of the dioxolene rings, which are only 2.4 ꢂ apart at
their closest approach in [2]PF6. More detailed spectroscopic
and theoretical studies are in progress to characterize the
other redox states of 1–3 and to clarify the electronic struc-
tures of radical species based on theanꢀ and related bis- and
tris(dioxolenes).
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[14] The complex molecule in 1·xC5H12·(4ꢀx)CH2Cl2 is disordered across
a noncrystallographic mirror plane. The crystal also contains channels
of disordered solvent. The asymmetric unit of 2·0.67H2O·2.07CH2Cl2
contains three unique complex molecules, which also contain some dis-
order. Solvated [1]PF6 contains two unique complex half-molecules
ꢀ
with crystallographic C2 symmetry. The PF6 ions occupy defined lattice
sites, but the unit cell contains 26% disordered void space making the
crystals poor diffractors of X-rays. The complex cation in [2]PF6·3CH2Cl2
also has crystallographic C2 symmetry. The anions and solvent lie in dis-
ordered sheets between layers of the cations, although the anion sites
were clearly resolved in the Fourier map.[16]
Experimental Section
´
[15] O. Carugo, C. B. Castellani, K. Djinovic, M. Rizzi, J. Chem. Soc. Dalton
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Synthetic procedures and characterization data for 1–3, as well as
details of the instrumentation and computational procedures used
for the spectroscopic and electrochemical measurements, crystal-
structure determinations, and DFT calculations, are given in the
Supporting Information.[16]
[16] Additional Tables and Figure of the voltammetry, EPR, and UV/Vis/NIR
data, the crystal structures, and the DFT calculation are given in the
Supporting Information.
[17] K. D. Demadis, C. M. Hartshorn, T. J. Meyer, Chem. Rev. 2001, 101, 2655–
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Received: December 11, 2013
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Published online on && &&, 0000
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Electron hopping: Diplatinum com-
plexes, such as the one shown herein,
are readily oxidized to mixed-valent cat-
echol/semiquinone radical species,
some of which can be crystallized at
room temperature. The dioxolene rings
in the radical products are moderately
strongly coupled by EPR and UV/Vis/NIR
spectroscopies, despite being separated
by saturated methylene spacers.
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Mixed-Valent Compounds
J. J. Loughrey, S. Sproules, E. J. L. McInnes,
M. J. Hardie, M. A. Halcrow*
&& – &&
Stable Mixed-Valent Radicals from
Platinum(II) Complexes of
a Bis(dioxolene) Ligand
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Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!