Isomeric separation in donor-acceptor systems of Pd(ii) and Pt(ii) and a combined structural, electrochemical and spectroelectrochemical study

Article abstract of DOI:10.1039/c1dt10856e

Compounds of the form [(pap)M(Q^2-)] (pap = phenylazopyridine; Q = 3,5-di-tert-butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl-benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one-pot reaction.

Full text of DOI:10.1039/c1dt10856e

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 9925
www.rsc.org/dalton
PAPER
Isomeric separation in donor–acceptor systems of Pd(II) and Pt(II) and a
combined structural, electrochemical and spectroelectrochemical study†
Naina Deibel,a David Schweinfurth,a Jan Fiedler,b Stanislav Za´lisˇb and Biprajit Sarkar*a
Received 6th May 2011, Accepted 18th July 2011
DOI: 10.1039/c1dt10856e
Compounds of the form [(pap)M(Q2-)] (pap = phenylazopyridine; Q = 3,5-di-tert-butyl-benzoquinone,
M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl-benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a
and 4b) were synthesized in a one-pot reaction. The geometrical isomers, which are possible because of
the built in asymmetry of these ligands, have been separated by using different temperatures and
variable solubility. Structural characterization of 1b shows that the metal centers are in a square planar
environment, the pap ligand is in the unreduced neutral state and the quinones are in the doubly
reduced, Q2- catecholate form. Cyclic voltammetric measurements on the complexes display two
one-electron oxidations and two one-electron reductions. EPR and vis-NIR spectra of the one-electron
oxidized forms of the complexes indicate that the first oxidation takes place on the Q2- ligands to
produce a metal bound semiquinone (Q∑-) radical. Reduction takes place on the pap ligand, generating
metal bound pap∑- as seen from the 14N (I = 1) coupling in their EPR spectrum. All the complexes in
their [(pap)M(Q2-)] neutral forms show strong absorptions in the NIR region which are largely LLCT
(ligand to ligand charge transfer) in origin. These NIR bands can be tuned over a wide energy range by
varying the metal center as well as the Q ligand. In addition, the intensity of NIR bands can be
switched on and off by a simple electron transfer at relatively low potentials. DFT studies were used to
corroborate these findings.
systems based on them,38 we looked into systems [(pap)Pt(Q2-)]
Introduction
which resulted in our preliminary report on isomer separation.18
Donor–acceptor systems based on Pt(II) and containing redox
Encouraged by our initial results we decided to systematically
non-innocent ligands have been studied for a while because of
probe the phenomenon of isomer formation in such complexes
their exciting photochemical properties and their possible use in
and also extended this work to include an additional Q ligand
harnessing solar energy.1–7 After initial studies, which were based
mainly on dithiolates as donors and diimines as acceptors, recent
focus has also been on other related redox-active donors.8–17 In
as well as to Pd(II) complexes. Herein we report on the synthesis
and isomer separation of [(pap)M(Q2-)] (pap = phenylazopyridine;
Q = 3,5-di-tert-butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt,
2a and 2b; Q = 4-tert-butyl-benzoquinone, M = Pd, 3a and 3b;
this regard we recently reported on the isomeric separation in
complexes of the form [(pap)Pt(Q2-)] (pap = phenylazopyridine,
M = Pt, 4a and 4b). The separation of the isomers was achieved
Q = 3,5-di-tert-butyl-benzoquinone).18 The quinones, Q can exist
by either varying the reaction temperature or by preferentially
in the three different redox forms of Q0, Q∑- and Q2- which are
precipitating one isomer. Structural characterization of some of
connected by one-electron transfer processes. The ligand pap
the complexes helped in isomer identification. Compounds of
which contains an azo group19 can also exist as pap0, pap∑- or
the form mentioned here have been reported in the literature.
pap2-. Non-innocent behavior of such ligands has been studied
However, no mention of isomer formation or separation can be
in various metal complexes.20–30 In view of our interest in the
chemistry of quinone ligands31–37 and building up donor–acceptor
found in those reports.13,39 We have also investigated the complexes
electrochemically to probe their redox properties. A combined
vis-NIR and EPR spectroelectrochemical study is presented to
elucidate the spectroscopic properties of these complexes. Control
of absorptions in the NIR region is another aspect of this work
and we show here that such absorptions, which can be useful
for future opto-electronic systems40,41 can be tuned by varying the
metal centre as well as the quinone ligand. In addition, controlling
of the intensity of such NIR absorptions via electron transfer is
also presented.
aInstitut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring
55, D-70550, Stuttgart, Germany. E-mail: sarkar@iac.uni-stuttgart.de
bJ. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of
the Czech Republic, Dolejsˇkova 3, CZ-18223, Prague, Czech Republic
† Electronic supplementary information (ESI) available. CCDC reference
numbers 805489. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10856e
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9925–9934 | 9925
©

View Article Online
Table 1 Crystallographic details
Results and discussion
Chemical formula
Mr
Crystal system, space group
C25H29N3O2Pd
509.91
Triclinic, P ¯ı
173
6.9087(4), 10.8143(7), 15.7231(9)
101.526(3), 97.418(3), 91.407(3)
1139.86(12)
2
1.486
524
Mo-Ka
0.84
0.15 ¥ 0.05 ¥ 0.01
9621
4081
3484
0.117
Synthesis and crystal structure
The complexes (Scheme 1) were synthesized in a one-pot reaction
by using Pd(pap)Cl2 or Pt(pap)Cl2 and the relevant QH2 ligand
with NEt3 as a base in acetonitrile (Experimental section).
Encouraged by our finding that isomer separation is possible in
such complexes,18 we systematically varied the reaction conditions
in order to be able to force the formation of one isomer or the other
based on reaction conditions. After screening a set of conditions,
we were delighted to find that temperature, time and solubility
are the factors that can result in the favorable formation of one
isomer or the other. Thus carrying out the reaction for 3 h at
room temperature results in the preferential formation of 1a or
3a (kinetic control). On refluxing the solution at 70 ◦C, the other
isomer, 1b or 3b is formed preferentially (thermodynamic control,
Scheme 1 and Experimental section). For the platinum complexes,
solubility differences were used for separation of 4a from 4b. This
is in contrast to our earlier finding where we had achieved isomer
separation exclusively through column chromatography.18 The
T/K
˚
a, b, c/A
a, b, g (◦)
3
˚
V/A
Z
Density/g cm-3
F000
Radiation type
m/mm-1
Crystal size/mm
meas. refl.
indep. refl.
obsvd. [I > 2s(I)] refl.
Rint
R[F2 > 2s(F2)], wR(F2), S
0.081, 0.221, 1.034
4.32, -1.50
-3
˚
Drmax, Drmin/e A
diffraction studies. This result was used for the unambiguous
identification of the configuration of the isomers. The same was
previously done by us for 2a and 2b. For 3a/3b and 4a/4b,
isomer identification was done by comparing their 1H-NMR
spectra with those of 1a/1b and 2a/2b respectively (Figure S1†).
Crystallographic details for 1b are presented in Table 1. 1b
crystallizes in the triclinic P¯ı space group (Fig. 1).
1
metal complexes were characterized by H NMR spectroscopy,
elemental analyses and mass spectrometry. 1H NMR spectroscopy
can be put to good use to check the formation of one isomer or
the other (Experimental section, Figure S1†).
Fig. 1 ORTEP plot of 1b. Ellipsoids are drawn at 30% probability.
Hydrogen atoms are omitted for clarity.
The Pd(II) center in 1b adopts a distorted square planar
environment, the distortion being imposed by the chelating nature
of the pap and Q ligands. This is evident from the O1–Pd1–O2
angle of 83.6(2)◦ and N1–Pd1–N2 of 78.3(2)◦ respectively. The
Pd–N and Pd–O distances are in the expected range. Bond length
analyses within the Q ring clearly establish its completely reduced
Q2- catecholate form. Thus the C1–O1 and C2–O2 distances of
˚
1.331(9) and 1.351(9) A respectively point to C–O single bond
distances. The virtually identical intra-ring C–C bond distances
˚
and their average values of about 1.4 A suggests the aromatic
character of the ring as would be expected for a catecholate
form.21,22 The distances within the pap ring are typical for the
unreduced and neutral pap0 form (Fig. 1). The slightly elongated
˚
N–N distance of 1.286(9) A compared to an authentic N
N
Scheme 1 Isomers of the complexes reported in this work.
double bond has to do with back-donation from the Pd(II)
center which is bonded to the electron-rich Q2- ligand.19,42,43 Bond
length analyses thus show that the best formulation for 1b is
[(pap0)Pd(Q2-)]. The data presented here match well with our
1b could be crystallized by slow evaporation of a CH2Cl2
solution of it layered with n-hexane for single crystal X-ray
9926 | Dalton Trans., 2011, 40, 9925–9934
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
previous report on the structural parameters of the platinum
complexes 2a and 2b.18 The uncoordinated phenyl ring of pap is
twisted with respected to the rest of the molecule. This is apparent
from the dihedral angle of 31.2◦ between the planes defined by the
phenyl ring and the Pd(II) center together with its coordinating
atoms. The twisting of the phenyl ring is most likely a result of
either steric repulsion between the C–H bond of the phenyl and
the adjacent coordinating O atom of the Q2- ring or packing effects
in the solid state.
Various attempts at crystallizing the isomer 1a invariably
led to the isolation of the crystals of 1b. The conversion was
1
independently verified by performing H NMR experiments on
1
1a before the crystallization process and then measuring a H
NMR spectrum of the resulting crystals (Figure S2†). From
the synthetic protocols of the isomers (vide supra) it is known
that 1b is the thermodynamically stable form. Hence over the
period of crystallization in solution, 1a is converted to the
thermodynamically stable product 1b. This phenomenon was not
observed for the platinum complexes 2a/2b, for which we had
reported the crystal structures of both the isomers previously. With
comparable ligands, platinum is known to form more robust bonds
than palladium. This fact is possibly responsible for the conversion
of 1a to 1b over a period of time but not of 2a to 2b.
Fig. 2 Cyclic voltammogram of 3a and 4a in CH2Cl2/0.1 M Bu4NPF6
at 298 K. * corresponds to a re-reduction peak that appears after the 2nd
oxidation. The red curves show the reversibility of the first oxidation step
when the scan direction is reversed without scanning the second oxidation
step.
Cyclic voltammetry
potentials of 1a and 2a is about 500 mV. This result is a first
indication of some amount of metal influence in the first oxidation
process. The first oxidation potential of the reported complexes
[(Q)Pt(bpy*)]8 and [(Q)Pt(dpphen)]9 (bpy* = 4,4/-(di-tert-butyl)-
2,2/-bipyridine, dpphen = 4,7-diphenyl-1,10-phenanthroline) are
cathodically shifted compared to 2a (Table 2). This can be
rationalized by considering the better p-accepting ability of pap
compared to bpy* or dpphen. The second oxidation is irreversible
for ◦all the reported complexes at all tested temperatures (-50 to
25 C) and scan rates (50 to 1000 mV s-1). This phenomenon
was also observed for [(Q)Pt(bpy*)] and [(Q)Pt(dpphen)]. A likely
explanation is the extremely weak basicity of the neutral o-quinone
form, Q0 that is generated on second oxidation. The weak basicity
probably leads to complex dissociation and hence makes the
second oxidation irreversible. The irreversibility of the second
The presence of two redox-active ligands makes the complexes
presented here ideal candidates for cyclic voltammetric measure-
ments. Each of the complexes described in this work show two
oxidation and two reduction processes within the dichloromethane
solvent window. Both isomers of each complex were investi-
gated electrochemically in CH2Cl2/0.1 M Bu4NPF6. However,
as expected, and as can be seen from Table 2, their behavior
is virtually identical and hence in the following, the discussion
will be restricted to one set of isomers. All the complexes show
a completely reversible first one-electron oxidation at relatively
low potentials (Fig. 2 and, Table 2). The Pd(II) complexes 1a
ox1
ox1
(E1/2 = -0.37 V) and 3a (E1/2 = -0.30 V) are respectively easier
ox1
to oxidize compared to their Pt(II) analogues 2a (E1/2 = 0.14 V)
ox1
and 4a (E1/2 = 0.22 V). The difference in the first oxidation
Table 2 Electrochemical data from cyclic voltammetrya
neu,c
red,d
Compounds
Eox2,b
Eox1
Ered1
Ered2
Kc
Kc
1a
0.45
0.55
0.98
0.93
0.48
0.51
1.08
1.06
~1.0
0.56
n.o.
n.o.
-0.37
-0.30
0.14
-1.12
-1.04
-0.90
-0.94
-1.07
-1.03
-0.82
-0.83
-1.82
-1.66
-0.56
-0.79
-1.99
-1.90
-1.74
-1.75
-1.95
-1.90
-1.70
-1.75
n.o.
5 ¥ 1012
5 ¥ 1012
6 ¥ 1017
9 ¥ 1017
1 ¥ 1013
1 ¥ 1013
4 ¥ 1017
8 ¥ 1017
5 ¥ 1027
9 ¥ 1026
—
6 ¥ 1014
6 ¥ 1014
1 ¥ 1014
5 ¥ 1013
8 ¥ 1014
5 ¥ 1014
8 ¥ 1014
4 ¥ 1015
—
1b
2a18
2b18
0.12
3a
-0.30
-0.25
0.22
3b
4a
4b
0.23
[Pt(Q)(bpy*)]e, 9
[Pt(Q)(dpphen)]f, 8
[Pd(pap)Cl2]g, 44
[Pt(pap)Cl2]44
-0.18
-0.07
n.o.
-2.28
-1.39h
-1.74
3 ¥ 1010
—
n.o.
—
1 ¥ 1016
a Half-wave potentials from cyclic voltammetric measurements in CH2Cl2/0.1 M Bu4NPF6 for reversible processes at 298 K, scan rate 100 mV s-1,
neu
red
ferrocene/ferrocenium was used as an internal standard. b
Ered1 - Ered2
at 253 K. h Epc for irreversible reduction. n.o. = not observed.
E
pa for irreversible process. c Kc = 10DEneu/59 mV, DEneu = Eox1 - Ered1
.
d Kc = 10DEred/59 mV, DEred
=
.
e Q = 3,5-di-tert-butyl-catecholate, bpy* = 4,4/-(di-tert-butyl)-2,2/-bipyridine. f dpphen = 4,7-diphenyl-1,10-phenanthroline. g measurements
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9925–9934 | 9927
©

View Article Online
oxidation process precluded the quantitative determination of the
thermodynamic stability of the one-electron oxidized form.
Both the reduction processes are completely reversible for the
complexes reported here. The large differences between the first
oxidation and first reduction potentials lead to comproportiona-
tion constant (Kc) values of the order of 1012 for the palladium
complexes 1a and 3a, and 1017 for the platinum complexes 2a
and 4a, showing their high thermodynamic stability. Just like the
oxidation processes, the reduction potentials for the palladium
The reduction potentials of [Pt(Q)(bpy*)]9 and [Pt(Q)(dpphen)]8
are shifted to higher negative values compared to 2a (Table 2). This
can be rationalized by the energetically lower lying p*-LUMO of
pap compared to bpy* or dpphen. The large differences between
the 1st and 2nd reduction potentials for all the complexes leads to
a very high thermodynamic stability of the one-electron reduced
forms, as seen from the comproportionation constant values (Kc)
of the order of 1014 (Table 2). The effect of the tert-butyl groups
(one or two) of Q, on the redox potentials of the complexes is
negligible. In order to gain further insight into the redox steps, vis-
NIR as well as EPR spectroelectrochemical studies were carried
out on the complexes. Since positional isomers such as the ones
described here are known to have virtually identical spectroscopic
properties, only one isomer was investigated with these methods.
red1
red1
complexes 1a (E1/2
= -1.12 V) and 3a (E1/2
= -1.07 V)
are cathodically shifted compared to their platinum analogues
red1
red1
2a (E1/2
= -0.90 V) and 4a (E1/2
= -0.82 V); the shift,
however, is less compared to that for the oxidation potentials
(Table 2). The trend in the reduction potentials observed here
is the opposite of what was observed for the precursor complexes
[Pd(pap)Cl2] and [Pt(pap)Cl2],44 where the Pd(II) complex (E1/2
=
-0.56 V) has a lower negative reduction potential compared to
that of the Pt(II) analogue (E1/2 = -0.79 V). Pd(II) is normally
capable of a better s-polarization effect compared to Pt(II) and
this phenomenon is responsible for the lower negative reduction
potential of [Pd(pap)Cl2] compared to [Pt(pap)Cl2]. In the present
case, this trend is likely reversed because of the superior donor
ability of the [Pd(Q)2-] unit containing the Q2- ligand in 1a and
3a, thus leading to shift of reduction potentials to higher negative
values. The same phenomenon is also responsible for the cathodic
shift of all reduction potentials of 1a–4a compared to the precursor
complexes [Pd(pap)Cl2] and [Pt(pap)Cl2].44 It should be noted here
that for a series of related Pd(II) and Pt(II) complexes, reported with
2,2/-bipyridine (bpy) and catecholate, amidophenolate, amidoth-
iolate or dithiolate ligands, the reduction potentials were shown to
be independent of the metal centers as well as the donor ligands.15
The p*-LUMO of the bpy ligands are energetically much higher
compared to the p*-LUMO of pap. Thus, in the complexes with
bpy, the effect of the [M(Q)2-] fragment on the reduction potentials
for the bpy centered process is negligible.
Vis-NIR spectroelectrochemistry
All the investigated complexes show intense absorption bands in
the NIR region and this band dominates their absorption spectrum
(Fig. 3 and 4, Table 3). For the Pd(II) complexes, the NIR band is
shifted to lower energies compared to their Pt(II) counterparts
[1378 nm (7256 cm-1) for 1b versus 970 nm (10309 cm-1) for
2b]. The origin of this band lies in the spin and dipole allowed
LLCT (with some MLCT contribution) transition from a highest
occupied molecular orbital (HOMO) predominantly based on the
Q2- donor to the lowest unoccupied molecular orbital (LUMO)
located primarily on the pap ligand as has been confirmed by
TD-DFT calculations (vide infra). The position of this band
correlates well with the difference between the first oxidation
and first reduction potentials of the complexes as obtained from
cyclic voltammetry experiments (Table 2 and 3). All the complexes
display further bands in the visible region which can be assigned
to metal to ligand charge transfer (MLCT, M(dp) → pap(p*)) and
intra ligand (IL) transitions. Some of these bands are reminiscent
Table 3 vis-NIR data of the complexes in various redox formsa
Compound
l/nm (e/M-1cm-1)
1b∑+
1b
304(13700), 417(28200), 779(1600)
297(15300), 365(20500), 1378(9200)
397(29500), 490 (2100)
306(17600), 392(18500), 500(4000), 582(5600), 635(5000), 805(1900)
309(17100), 392sh
298(7000), 402(12800), 420(13700), 478(8800), 537 (3800), 567(4500), 790(1800), 1017sh
319(8400), 390(9300), 477 (2100), 650 (1700), 970(9500)
392(11400), 415 sh, 512(2300)
313(14000), 395(12500), 450(3800), 505(3600), 601(3200), 649(3400), 786(3300)
302(13800), 345sh, 367(15600), 380sh, 425sh, 480(4100), 592(1600)
310(12000), 395(5800), 504(1800)
355(10900), 490sh
304(11300), 420(20800), 568sh, 740(1700)
300(13100), 371(16300), 1208(6000)
309(14900), 390(18200), 580(6800), 635sh, 775(2100)
308(15100), 395sh
298(10900), 419(20600), 468(12900), 562(7300), 763(2800)
304(13700), 385(14600), 440sh, 635sh, 960(13600)
311(15800), 393(17600), 496(5600), 585sh, 646(6500), 675sh, 750(5500)
307(13700), 385sh, 491(2200)
44
Pd(pap)Cl2
1b∑-
1b2-
2b∑+
2b
44
Pt(pap)Cl2
2b∑-
[Pt(pap)Cl2]∑-, 44
2b2-
[Pt(pap)Cl2]2-, 44
3b∑+
3b
3b∑-
3b2-
4a∑+
4a
4a∑-
4a2-
a From OTTLE spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.
9928 | Dalton Trans., 2011, 40, 9925–9934
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 4 EPR data of the paramagnetic complexesa
Compound
giso
aiso (1H)b
aiso (14N)b
aiso (M)b
1b∑+
2.002
2.002
2.004
2.004
1.994
1.992
2.005
2.006
2.001
2.003
1.995
2.005
3.3
3.4
n.o.
0.7
3.6
3.5
n.o.
2.6
3.7
n.o.
3.6
n.o.
n.o.
2.2e
1b∑+,c
1b∑-
0.3, 0.2, 0.1
9.0, 3.1, 2.6d
3.6, 1.0, 0.8
n.o.
1.1, 0.6, 0.8
n.o.
1.7
3.0e
1b∑-,c
2a∑+, 18
2a∑+,c
2a∑-, 18
2a∑-,c
3b∑+
6.0
24.5f
15.8f
104.0f
90.0f
2.3e
3.4, 0.9, 0.6
n.o.
3b∑-
8.6, 3.4, 2.8d
n.o.
3.5e
4a∑+
22.2f
108.0f
4a∑-
n.o.
a X-band EPR data obtained from in situ generated species in CH2Cl2/0.1
M Bu4NPF6 at 298 K. b Isotropic hyperfine coupling constants in Gauss
obtained from simulation. c Calculated using ADF/BP at optimized
geometry. d Hyperfine coupling constants of nitrogen atoms belonging to
the azo group and pyridine ring of pap. e Hyperfine coupling to 105Pd,
Fig. 3 Changes in the vis-NIR spectrum of 1b during the various redox
processes in CH2Cl2/0.1 M Bu4NPF6 at 298 K.
1
I = 5/2, natural abundance = 22.2%. f Hyperfine coupling to 195Pt, I =
natural abundance = 33.3%.
,
2
and [PtII(pap∑-)Cl2]∑- which shows very similar transitions in the
visible region.39 Thus the reduction takes place primarily on the
pap center leading to complexes of the form [(Q2-)MII(pap∑-)]∑-. On
further one-electron reduction, almost all the bands in the visible
and NIR region lose their intensity as would be expected for the
formation of [(Q2-)MII(pap2-)]2-.
The NIR bands observed for the neutral complexes 1b–3b and
4a dominate their spectra. The position of these bands can be
tuned by changing the metal centers as well as the Q ligand
(Table 3) and the band positions correlate well with the difference
between the 1st oxidation and 1st reduction potentials of these
complexes. Reversible one-electron oxidation as well as reduction
leads to almost complete depletion of these bands as seen above.
Thus, a simple electron transfer can lead to the change in intensity
of these NIR bands from extinction coefficient values of about
10000 M-1 cm-1 to almost 0. The redox processes, particularly the
first oxidation occur at extremely low and accessible potentials.
Compounds with such characteristics have been postulated as
useful candidates for future opto-electronic systems.40
Fig. 4 Changes in the vis-NIR spectrum of 4a during the various redox
processes in CH2Cl2/0.1 M Bu4NPF6 at 298 K.
of the precursor complexes [Pd(pap)Cl2] and [Pt(pap)Cl2] thus
helping in their assignment.44
One-electron oxidation of the complexes in CH2Cl2/0.1 M
Bu4NPF6 in an optically transparent thin layer electrochemical
(OTTLE)45 cell leads to depletion and diminishing of intensity of
the NIR bands. Additionally, new intense bands appear between
400 and 500 nm (for example at 468 nm for 4a∑+). These bands
are typically observed as IL transitions for the semiquinone Q∑-
radical,8,46,47 thus indicating that the oxidation process occurs
primarily at the Q2- center, resulting in [(Q∑-)MII(pap0)]∑+. Other
bands of the MLCT and LLCT nature are also seen for the
one-electron oxidized forms of the complexes. Even though the
second oxidation processes could not be investigated owing to their
irreversible nature, attempts at generating these species showed a
complete depletion of bands in the vis-NIR region as would be
expected for the formation of a [(Q0)MII(pap0)]2+ complex.
One-electron reduction also leads to diminishing of intensity of
the NIR absorptions. Additionally, multiple bands appear in the
visible region that can be assigned to MLCT and IL transitions
of a metal bound azo radical (Fig. 3 and 4, Table 3). Confidence
in this assignment comes from the spectra of [PdII(pap∑-)Cl2]∑-
EPR spectroelectrochemistry
In order to further consolidate the assignment of redox pro-
cesses made by vis-NIR spectroelectrochemistry, EPR spec-
troscopy was carried out on the odd-electron forms of the
complexes. The substances for carrying out the EPR mea-
surements were generated by in situ electrolysis of the neu-
tral complexes in CH2Cl2/0.1 M Bu4NPF6. The one-electron
oxidized form 1b∑+ shows a well resolved signal at 298 K
centered at g = 2.002 (Fig. 5 and Table 4). This spectrum
could be simulated by considering hyperfine coupling to one
1H nucleus (I = 21 ) of 3.3 G and one 105Pd nucleus (I = 5/2,
natural abundance = 22.2%) of 2.2 G. The 105Pd satellites are
visible on the extremities of the experimental signal. The g-value
and the hyperfine coupling constants are typical for a metal bound
semiquinone radical thus pointing to a form [(Q)∑-PdII(pap0)]∑+ for
the one-electron oxidized species.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9925–9934 | 9929
©

View Article Online
as compared to 105Pd (-2.683 mT).48 Thus 4a∑+ can be formulated
as [(Q)∑-PtII(pap0)]∑+.18
The one-electron reduced form 1b∑- shows a signal centered
at g = 2.004 at 298 K. This spectrum shows multiple lines which
could be reasonably simulated by using hyperfine coupling to three
different 14N (I = 1) nuclei of 9.0, 3.1 and 2.6 G respectively as well
as a 105Pd coupling of 3 G (Fig. 5). An unambiguous assignment
of the nitrogen coupling to specific nitrogen atoms is not possible.
However, it can be said with a reasonable amount of confidence
that the largest coupling of 9.0 G is probably to the nitrogen
atom of the azo part which is coordinated to the palladium center.
The compound 4a∑- shows a signal at 298 K which is centered at
g = 2.005. This spectrum could be simulated by considering a 195Pt
coupling of 108 G (Fig. 6). The large line-width of this signal which
is associated with the large 195Pt hyperfine coupling precludes the
resolution of the 14N coupling for the platinum complex. The much
larger 195Pt coupling for the reduced form as compared to the
oxidized form has to do with a large amount of back donation
that is possible from the Pt(II) center to the pap ligand. The EPR
data thus unambiguously establish the one-electron reduced forms
as [(Q)2-MII(pap∑-)]∑-.
Fig. 5 X-band EPR spectrum of in situ generated 1b∑+ and 1b∑- at 298 K
in CH2Cl2/0.1 M Bu4NPF6 together with simulation.
DFT calculations
The one-electron oxidized platinum complex 4a∑+ shows a
spectrum centered at g = 1.995, that is similar in features to that
of 1b∑+. The only difference being the observance of 195Pt (I =
1/2, natural abundance = 33.3%) satellites instead of palladium
satellites (Fig. 6). This spectrum could be simulated by using a
The DFT optimized bond lengths and angles of 1b and 2b well
describe the experimental crystal structures (Table S1†), bond
˚
lengths are reproduced within 0.02 A with an exception of the
N3–C15 bond. Positional isomers b have slightly lower energy
than a, free energy differences are 0.013 and 0.020 eV for 1b and
2b, respectively.
1
hyperfine coupling of 3.6 G to the H nucleus and of 22.2 G to
the 195Pt nucleus. The much larger hyperfine coupling constant
to the 195Pt nucleus in the case of 4a∑+ as compared to the
hyperfine coupling to 105Pd in the case of 1b∑+ is due to the larger
isotropic hyperfine coupling constant value for 195Pt (1227.8 mT)
Frontier molecular orbitals (FMO) of 2b are depicted in Figure
S3†, shapes of FMOs of 1b differ only slightly. HOMOs of both
complexes are formed by the p system of the Q ligand with some
M contribution. LUMOs are composed of a p pap orbital with
a contributing metal dxz orbital (12% and 8.5% for Pt and Pd
complex, respectively). In the course of oxidation an electron
is withdrawn from the Q based MO, during the reduction an
electron is accepted in preferably the pap localized orbital with
partly contributing metal orbital. Fig. 7 depicts spin densities for
oxidized and reduced forms of 2b; analogous shapes are obtained
in the case of 1b.
ADF/BP calculated spin densities on Pt are 0.014 and 0.147
for oxidized and reduced species, respectively. Spin densities on
Pd are 0.009 and 0.080 in the case of oxidized and reduced
species, respectively. Spin density distribution is reflected by the
calculated EPR parameters. Calculated g values and hyperfine
couplings for radical cations and anions of both complexes are
listed in Table 4. The calculated EPR parameters correlate well
with the experimental ones. The ratio between aiso(Pd) and aiso(Pt)
is reasonably well interpreted; calculations underestimate aiso(Pt)
and overestimate the aiso(Pd) parameter of 1b∑-.
TD-DFT calculations well interpret the experimental spectral
features of 2b (Table 5 and S3†). The intense allowed feature
calculated at 841 nm can be characterized as a LLCT (Q2- to
pap0) transition. A second intense transition calculated at 373 nm
has mixed MLCT and IL character and also well reproduces the
experimental feature. Shifts of lowest lying allowed transitions
to shorter wavelengths and the intensity variations are well
reproduced by calculations for oxidized and reduced species.
Fig. 6 X-band EPR spectrum of in situ generated 4a∑+ and 4a∑- at 298 K
in CH2Cl2/0.1 M Bu4NPF6 together with simulation.
9930 | Dalton Trans., 2011, 40, 9925–9934
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Table 5 Main TD-DFT (PRE0/PCM-CH2Cl2) calculated transitions of 1bn and 2bn compared with experimental data
Main contributing
excitations (%)
Transition energy
eV(nm)
Transition
Oscillator
strength
Molar absorption
Compound
energy (nm)a
coefficient, M-1 cm-1,a
1b
HOMO → LUMO (97)
Mixed
1.11 (1118)
3.25 (381)
1.64 (754)
2.96 (419)
1.40 (833)
2.35 (528)
2.66 (466)
3.42 (362)
1.47 (841)
2.89 (427)
3.32 (373)
1378
365
779
417
805
582
500
392
970
477
390
0.211
0.264
0.031
0.222
0.056
0.062
0.096
0.190
0.299
0.068
0.386
9200
20500
1600
28200
1900
5600
5000
18500
9500
2100
9300
1b+
1b-
aHOMO → aLUMO (90)
Mixed
bHOMO → bLUMO (98)
bHOMO-1 → bLUMO (70)
bHOMO-2 → bLUMO (80)
Mixed
HOMO → LUMO (97)
HOMO-2 → LUMO (55)
HOMO-2 → LUMO (40)
HOMO-4 → LUMO (41)
aHOMO → aLUMO (80)
Mixed
2b
2b+
2b-
1.67 (741)
2.80 (441)
2.93 (423)
3.17 (391)
1.56 (794)
2.14 (578)
2.74 (453)
3.51 (353)
790
478
420
402
786
601
450
395
0.043
0.150
0.082
0.090
0.106
0.036
0.077
0.195
1800
8800
13700
12800
3300
3200
3800
12500
Mixed
Mixed
bHOMO → bLUMO (98)
bHOMO-1 → bLUMO (84)
bHOMO-2 → bLUMO (80)
Mixed
a From OTTLE spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.
agreement with experimental finding lowest lying excitations for
1b are calculated at longer wavelengths than in 2b.
Conclusion
By combining two different non-innocent ligands we were able to
characterize donor acceptor systems of the form [(Qn)MII(papm)],
M = Pd or Pt; n = -1, -2; m = 0, -1, -2. The in-built asymmetry of
the ligands makes isomer formation possible and these could be
isolated by using thermodynamic versus kinetic reaction control
as well as solubility differences. Structural characterization of 1b
was used for isomer identification. Bond length analyses in 1b
lead to the formulation [(Q2-)PdII(pap0)] for the neutral complexes.
All the complexes show two one-electron oxidation and two one-
electron reduction processes. Additionally, the neutral forms of all
the complexes show strong absorptions in the NIR region which
can be tuned by changing the metal center and the Q ligand. The
intensity of the NIR bands can be influenced by electron transfer
processes. The oxidation of the complexes occur predominantly
on the Q2- ligand generating successively metal bound Q∑- and Q0.
The identity of these forms was verified through the typical metal-
bound semiquinone absorption bands in the visible region of the
absorption spectrum as well as through hyperfine coupling to the
1H center of the semiquinone ring in the EPR spectrum. The ligand
in the Q0 form does not bind well to the metal center, thus making
the second oxidation step irreversible. Reduction takes place on the
pap part of the complex generating successively metal-bound pap∑-
and pap2-. The identity of these species was also verified through
vis-NIR and EPR spectroscopy. DFT studies corroborate the
experimental findings. The presence of two redox-active ligands
in the same metal complex as well as a four-coordinated metal
center provides opportunities in such complexes for studies of
ligand redox-induced reactivity at the metal centers. Our current
studies are focussed in these directions.
Fig. 7 Spin density plots of 1b∑+ and 1b∑- from DFT calculations.
FMOs contributing to lowest lying intense transitions in 2b+ and
2b- are depicted in Figures S4 and S5†.
TD-DFT calculated transitions for 1b, listed in Table 5 and
S2†, also well interpret the experimental spectral features. In
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9925–9934 | 9931
©

View Article Online
3a. Similar to 1a by using Pd(pap)Cl2 (72.1 mg, 0.20 mmol)
and 4-tert-butyl catechol (33.2 mg, 0.20 mmol). Yield: 67.1 mg
(74%). 1H-NMR (250 MHz, CDCl3): d = 1.28 (s, 9H, tert-butyl);
Experimental section
General considerations
6.59 (m, 2H, catecholate); 6.74 (dd, 1H, 3JH–H = 7.38 Hz, 4JH–H
=
Pd(pap)Cl2, Pt(pap)Cl2,44 2a18 and 2b18 were prepared according to
reported procedures. All other reagents are commercially available
and were used as received. All solvents were dried and distilled
using common techniques unless otherwise mentioned.
2.05 Hz, catecholate); 7.54 (m, 4H); 8.08 (m, 2H); 8.51 (m, 2H);
8.95 (m, 1H). Anal. Calc. for C21H21N3O2Pd: C 55.58; H 4.66; N
9.26%; found: C 55.27; H 4.61; N 9.11%. HRMS (ESI): Calc. for
C21H21N3O2Pd ([M]+): m/z 453.0671; found 453.0698.
Instrumentation
3b. Similar to 1b by using Pd(pap)Cl2 (72.1 mg, 0.2 mmol)
and 4-tert-butyl catechol (33.2 mg, 0.2 mmol). Yield: 19.2 mg
(21%). 1H NMR (250 MHz, CDCl3): d = 1.29 (s, 9H, tert-butyl);
1H NMR spectra were recorded at 250.13 MHz on a Brucker
AC250 instrument. EPR spectra in the X band were recorded with
a Bruker System EMX. Simulations of EPR spectra were done
using the Simfonia program. UV-vis-NIR absorption spectra were
recorded on a J&M TIDAS spectrometer. Cyclic voltammetry
was carried out in 0.1 M Bu4NPF6 solution using a three-
electrode configuration (glassy carbon working electrode, Pt
counter electrode, Ag wire as pseudoreference) and PAR 273
potentiostat and function generator. The ferrocene/ferrocenium
(Fc/Fc+) couple served as internal reference. Spectroelectrochem-
ical measurements were carried out using an optically transparent
thin layer electrochemical (OTTLE) cell. Elemental analysis was
performed on a Perkin Elmer Analyser 240. Mass spectrometry
experiments were carried out on a Bruker Daltronics Mictrotof-Q
mass spectrometer.
6.60 (m, 2H, catecholate); 6.78 (dd, 1H, 3JH–H = 4.65 Hz, 4JH–H
=
2.10 Hz, catecholate); 7.58 (m, 4H); 8.12 (m, 2H); 8.52 (m, 2H);
8.99 (m, 1H). Anal. Calc. for C21H21N3O2Pd: C 55.58; H 4.66; N
9.26%; found: C 55.31; H 4.51; N 9.06%. HRMS (ESI): Calc. for
C21H21N3O2Pd ([M]+): m/z 453.0671; found 453.0684.
4a and 4b. Pt(pap)Cl2 (44.9 mg, 0.1 mmol) and 4-tert-butyl
catechol (16.6 mg, 0.1 mmol) were taken together in 15 mL of
acetonitrile. Triethylamine (0.1 ml) was added to the solution. The
reaction mixture was heated to reflux for 3 h. The colour of the
solution changed to deep green. The precipitate that formed was
filtered and washed with diethylether. This compound corresponds
to 4a. The filtrate was collected and the solvent evaporated. The
crude product was purified by column chromatography on silica
gel (CH2Cl2/CH3CN: 10/1). First green band corresponds to the
compound 4b.
Syntheses
1a. Pd(pap)Cl2 (72.1 mg, 0.20 mmol) and 3,5-di-tert-butyl
catechol (44.4 mg, 0.20 mmol) were taken together in 20 mL of
acetonitrile. Triethylamine (0.1 ml) was added to the solution. The
reaction mixture was stirred at room temperature for 3 h. The green
precipitate was filtered, washed with diethylether and purified by
column chromatography on aluminiumoxide (CH2Cl2/CH3CN:
1
4a. Yield: 24.2 mg (44%). H-NMR (250 MHz, CDCl3): d =
1.32 (s, 9H, tert-butyl); 6.64 (m, 1H, catecholate); 6.99 (m, 1H,
3
4
catecholate); 7.12 (dd, 1H, JH–H = 6.59 Hz, JH–H = 2.19 Hz,
catecholate); 7.58 (m, 4H); 8.04 (m, 1H); 8.24 (m, 1H); 8.56 (m,
2H); 9.65 (m, 1H). Anal. Calc. for C21H21N3O2Pt: C 46.49; H 3.90;
N 7.75%; found: C 45.94; H 3.88; N 7.59%. HRMS (ESI): Calc.
for C21H21N3O2Pt ([M]+): m/z 542.1278; found 542.1273.
1
10/1). Yield: 50.0 mg (49%). H-NMR (250 MHz, CDCl3): d =
1.27 (s, 9H, tert-butyl); 1.46 (s, 9H, tert-butyl); 6.58 (m, 1H,
4
catecholate); 6.72 (d, 1H, JH–H = 2.17 Hz, catecholate); 7.52 (m,
1
4b. Yield: 12.3 mg (22%). H-NMR (250 MHz, CDCl3): d =
4H); 7.96 (m, 1H); 8.06 (m, 1H); 8.59 (m, 2H); 8.97 (m, 1H).
Anal. Calc. for C25H29N3O2Pd: C 58.88; H 5.73; N 8.24%; found:
C 58.75; H 5.36; N 8.16%. HRMS (ESI): Calc. for C25H29N3O2Pd
([M]+): m/z 509.1299; found 509.1284.
1.32 (s, 9H, tert-butyl); 6.64 (m, 1H, catecholate); 6.98 (m, 1H,
3
4
catecholate); 7.11 (dd, 1H, JH–H = 6.73 Hz, JH–H = 2.20 Hz,
catecholate); 7.57 (m, 4H); 8.01 (m, 1H); 8.23 (m, 1H); 8.54 (m,
2H); 9.63 (m, 1H). Anal. Calc. for C21H21N3O2Pt: C 46.49; H 3.90;
N 7.75%; found: C 46.01; H 3.81; N 7.59%. HRMS (ESI): Calc.
for C21H22N3O2Pt ([M + H]+): m/z 543.1356; found 543.1373.
1b. Pd(pap)Cl2 (72.1 mg, 0.20 mmol) and 3,5-di-tert-butyl
catechol (44.4 mg, 0.20 mmol) were taken together in 20 ml of
acetonitrile. Triethylamine (0.1 ml) was added to the solution. The
reaction mixture was heated to 70 ◦C for 1 h. After cooling to
room temperature a green precipitate was formed. The precipitate
was filtered, washed with diethylether and purified by column
chromatography on aluminium oxide (CH2Cl2/CH3CN: 10/1).
X-ray crystallography
Single crystals of 1b were grown by slow evaporation of a
CH2Cl2 solution of it layered with n-hexane. The intensity data
were collected at 173(2) K on a Kappa CCD diffractometer
1
Yield: 39.3 mg (38%). H-NMR (250 MHz, CDCl3): d = 1.30 (s,
9H, tert-butyl); 1.49 (s, 9H, tert-butyl); 6.61 (d, 1H, 4JH–H = 2.20 Hz,
(graphite monochromated MoKa radiation, l = 0.71073 A). The
˚
4
catecholate); 6.76 (d, 1H, JH–H = 2.20 Hz, catecholate); 7.53 (m,
structures were solved by direct methods (SHELXS-97) and
refined by full-matrix least-square procedures (based on F2,
SHELXL-97) with anisotropic thermal parameters for all the
non-hydrogen atoms.49 CCDC 805489 contains the supplementary
crystallographic data for this paper that can be obtained free
of charge from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
4H); 8.10 (m, 1H); 8.20 (m, 1H); 8.62 (m, 2H); 8.99 (m, 1H). Anal.
Calc. for C25H29N3O2Pd: C 58.88; H 5.73; N 8.24%; found: C 58.81;
H 5.67; N 8.17%. HRMS (ESI): Calc. for C25H29N3O2Pd ([M]+):
m/z 509.1299; found 509.1292. Recrystallization by evaporation
of a dichloromethane/n-hexane (1/3) solution afforded dark green
crystals of 1b.
9932 | Dalton Trans., 2011, 40, 9925–9934
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Quantum chemical calculations
13 G. K. Rauth, S. Pal, D. Das, C. Sinha, A. M. Slawin and J. D. Woolins,
Polyhedron, 2001, 20, 363.
14 Z. Sun, H. Chun, K. Hildebrand, E. Bothe, T. Weyhermu¨ller, F. Neese
and K. Wieghardt, Inorg. Chem., 2002, 41, 4295.
15 P. Ghosh, A. Begum, D. Herebian, E. Boethe, K. Hildenbrand, T.
Weyhermu¨ller and K. Wieghardt, Angew. Chem., Int. Ed., 2003, 42,
563.
The electronic structures of 1bn and 2bn (n = -1, 0, 1) were
calculated by density functional theory (DFT) methods using the
Gaussian 0950
and Amsterdam Density Functional
(ADF2009.01) 51,52 program packages.
G09 calculations employed Perdew, Burke, Ernzerhof53,54 PBE0
hybrid functional (G09/PBE0). The geometry of the open shell
systems was calculated by UKS approach. Geometry optimization
was followed by vibrational analysis. For H, C, N and O
atoms polarized triple-z basis sets 6-311G(d),55 together with
quasirelativistic effective core pseudopotentials and correspond-
ing optimized set of basis functions for Pd and Pt were used.56,57
Low-lying excitation energies were calculated by time-dependent
DFT (TD-DFT) at the optimized geometry of the corresponding
oxidation state. The solvent was described by the polarizable
continuum model (PCM)58 in TD DFT calculations.
16 S. Mondal, N. Paul, P. Banerjee, T. K. Mondal and S. Goswami, Dalton
Trans., 2010, 39, 2717.
17 S. Mandal, A. Castineiras, T. K. Mondal, A. Mondal, D. Chattopad-
hyay and S. Goswami, Dalton Trans., 2010, 39, 9514.
18 B. Sarkar, R. Huebner, R. Pattacini and I. Hartenbach, Dalton Trans.,
2009, 4653.
19 W. Kaim, Coord. Chem. Rev., 2001, 219–221, 463.
20 M. D. Ward and J. A. McCleverty, J. Chem. Soc., Dalton Trans., 2002,
275.
21 S. Bhattacharya, P. Gupta, F. Basuli and C. G. Pierpont, Inorg. Chem.,
2002, 41, 5810.
22 D. Herebian, E. Bothe, F. Neese, T. Weyhermu¨ller and K. Wieghardt,
J. Am. Chem. Soc., 2003, 125, 9116.
23 G. K. Lahiri, S. Bhattacharya, S. Goswami and A. Chakravorty, J.
Chem. Soc., Dalton Trans., 1990, 561.
Within the ADF program Slater type orbital (STO) basis sets
of triple-z quality with two polarisation functions for C, N, O and
H atoms and quadruple-z with four polarisation functions for
the Pd and Pt atom were employed. Core electrons were included
in the calculations. Within ADF the functional including Becke’s
gradient correctiontothe localexchangeexpressioninconjunction
with Perdew’s gradient correction to local density approximation
(LDA) with VWN parametrisation of electron gas data was used
(ADF/BP86).59,60 A and g tensors were obtained by first-order
perturbation theory from a ZORA Hamiltonian in the presence of
a time-independent magnetic field.61,62 The g tensor was obtained
from a spin-nonpolarised wave function after incorporating the
spin–orbit (SO) coupling.
24 B. K. Santra, G. A. Thakur, P. Ghosh, A. Pramanik and G. K. Lahiri,
Inorg. Chem., 1996, 35, 3050.
25 S. Patra, B. Sarkar, S. Maji, J. Fiedler, F. A. Urbanos, R. J.-. Aparicio,
W. Kaim and G. K. Lahiri, Chem.–Eur. J., 2006, 12, 489.
26 A. I. Poddel’sky, V. K. Cherkasov and G. A. Abakomov, Coord. Chem.
Rev., 2009, 253, 291.
27 C. G. Pierpont, Coord. Chem. Rev., 2001, 216–217, 95.
28 P. Zanello, Coord. Chem. Rev., 2006, 250, 2000.
29 M. Nomura, T. Cauchy and M. Fourmigue´, Coord. Chem. Rev., 2010,
254, 1406.
30 B. G. d. Bonneval, K. I. M.-C. Ching, F. Alary, T.-T. Bui and L. Valade,
Coord. Chem. Rev., 2010, 254, 1457.
31 P. Braunstein, D. Bubrin and B. Sarkar, Inorg. Chem., 2009, 48, 2534.
32 A. Paretzki, R. Pattacini, R. Huebner, P. Braunstein and B. Sarkar,
Chem. Commun., 2010, 46, 1497.
33 H. S. Das, F. Weisser, D. Schweinfurth, C. Y.-. Su, L. Bogani, J. Fiedler
and B. Sarkar, Chem.–Eur. J., 2010, 16, 2977.
34 D. Schweinfurth, H. S. Das, F. Weisser, D. Bubrin and B. Sarkar, Inorg.
Chem., 2011, 50, 1150.
35 F. Weisser, R. Huebner, D. Schweinfurth and B. Sarkar, Chem.–Eur. J.,
2011, 17, 5727.
36 H. S. Das, A. K. Das, R. Pattacini, R. Huebner, B. Sarkar and P.
Braunstein, Chem Commun., 2009, 4387.
37 A. Paretzki, H. S. Das, F. Weisser, T. Scherer, D. Bubrin, J. Fiedler, J.
E. Nycx and B. Sarkar, Eur. J. Inorg. Chem., 2011, 2413.
38 N. Deibel, D. Schweinfurth, R. Huebner, P. Braunstein and B. Sarkar,
Dalton Trans., 2011, 40, 431.
Acknowledgements
We are thankful to the Baden-Wu¨rttemberg Stiftung, Deutsche
Forschungsgemeinschaft (DFG) and the Fonds der Chemischen
Industrie (FCI) for financial support. Dr R. Pattacini is kindly
acknowledged for crystal structure solving. S. Z. thanks the
Ministry of Education of the Czech Republic (Grant COST
LD11086) and the European collaboration program COST D35.
39 R. Roy, P. Chattopadhyay, C. Sinha and S. Chattapodhyay, Polyhedron,
1996, 15, 3361.
40 M. D. Ward, J. Solid State Electrochem., 2005, 9, 778.
41 J. Garcia-Canadas, A. P. Meacham, L. M. Peter and M. D. Ward,
Angew. Chem., Int. Ed., 2003, 42, 3011.
42 B. Sarkar, S. Patra, J. Fiedler, R. B. Sunoj, D. Janardanan, G. K. Lahiri
and W. Kaim, J. Am. Chem. Soc., 2008, 130, 3532.
43 B. Sarkar, S. Patra, J. Fiedler, R. B. Sunoj, D. Janardanan, S. M. Mobin,
M. Niemeyer, G. K. Lahiri and W. Kaim, Angew. Chem., Int. Ed., 2005,
44, 5655.
44 S. Roy, I. Hartenbach and B. Sarkar, Eur. J. Inorg. Chem., 2009, 2553.
45 M. Krejcik, M. Danek and F. Hartl, J. Electroanal. Chem., 1991, 317,
179.
46 J. A. Zuleta, J. M. Bevilacqua, D. M. Proserpio, P. D. Harvey and R.
Eisenberg, Inorg. Chem., 1992, 31, 2396.
References
1 S. D. Cummings and R. Eisenberg, J. Am. Chem. Soc., 1996, 118, 1949.
2 J. A. Zuleta, C. A. Chesta and R. Eisenberg, J. Am. Chem. Soc., 1989,
111, 8916.
3 W. B. Connick, D. Geiger and R. Eisenberg, Inorg. Chem., 1999, 38,
3264.
4 N. M. Shavaleev, G. Accorsi, D. Virgili, Z. R. Bell, T. Lazarides, G.
Calogero, N. Armaroli and M. D. Ward, Inorg. Chem., 2005, 44, 61.
5 A. Vogler and H. Kunkely, J. Am. Chem. Soc., 1981, 103, 1559.
6 K. Base and M. W. Grinstaff, Inorg. Chem., 1998, 37, 1432.
7 J. A. Weinstein, N. N. Zheligovskaya, M. Y. Mel’nikov and F. Hartl, J.
Chem. Soc., Dalton Trans., 1998, 2459.
8 J. A. Weinstein, M. T. Tierney, E. S. Davies, K. Base, A. A. Robeiro
and M. W. Grinstaff, Inorg. Chem., 2006, 45, 4544.
9 N. M. Shavaleev, E. S. Davies, H. Adams, J. Best and J. A. Weinstein,
Inorg. Chem., 2008, 47, 1532.
47 A. Vogler, H. Kunkely and J. Hlavatsch, Inorg. Chem., 1984, 23, 506.
48 J. A. Weil and J. R. Bolton, ‘Electron Paramagnetic Resonance’, John
Wiley & Sons, Inc., Hoboken, New Jersey, 2007.
49 G. M. Sheldrick, SHELXL-97, Program for crystal structure refinement,
University of Go¨ttingen, 1997.
10 J. Best, I. V. Sazanovich, H. Adams, R. D. Bennett, E. S. Davies, A.
J. H. M. Meijer, M. Towrie, S. A. Tikhomirov, O. V. Bouganov, M. D.
Ward and J. A. Weinstein, Inorg. Chem., 2010, 49, 10041.
11 E. A. M. Geary, K. L. McCall, A. Turner, P. R. Murray, E. J. L.
McInnes, L. A. Jack, L. J. Yellowlees and N. Robertson, Dalton Trans.,
2008, 3701.
50 R. A. Gaussian 09, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery,
12 K. Heinze and S. Reinhardt, Chem.–Eur. J., 2008, 14, 9482.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9925–9934 | 9933
©

View Article Online
Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N.
Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.
M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V.
G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,
A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and
D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
54 C. Adamo and V. Barone, J. Chem. Phys., 1999, 110, 6158.
55 L. A. Curtiss, M. P. McGrath, J.-P. Blaudeau, N. E. Davis, R. C. B. Jr.
and L. Radom, J. Chem. Phys., 1995, 103, 6104.
56 D. Andrae, U. Ha¨ussermann, M. S. Dolg, H. and H. Preuss, Theor.
Chim. Acta, 1990, 77, 123.
57 J. M. L. Martin and A. Sundermann, J. Chem. Phys., 2001, 114,
3408.
58 M. Cossi, N. Rega, G. Scalmani and V. Barone, J. Comput. Chem.,
2003, 24, 669.
51 G. t. Velde, F. M. Bickelhaupt, S. J. A. v. Gisbergen, C. F. Guerra, E.
J. Baerends, J. G. Snijders and T. Ziegler, J. Comput. Chem., 2001, 22,
931.
59 A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38,
3098.
60 J. P. Perdew, Phys. Rev. B, 1986, 33, 8822.
61 E. v. Lenthe, A. v. d. Avoird and P. E. S. Wormer, J. Chem. Phys., 1997,
107, 2488.
52 S. ADF2009.01, Theoretical Chemistry, Vrije Universiteit, Amsterdam,
The Netherlands, http://www.scm.com.
53 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77,
3865.
62 E. v. Lenthe, A. v. d. Avoird and P. E. S. Wormer, J. Chem. Phys., 1998,
108, 4783.
9934 | Dalton Trans., 2011, 40, 9925–9934
This journal is
The Royal Society of Chemistry 2011
©