The effect of ligand basicity on the thermal stability of heterodinuclear NiII-ZnII complexes

Article abstract of DOI:10.1007/s10973-009-0302-2

Four complexes of the nuclear structure Ni^II-Zn^II were prepared with bis-N,N′-(salicylidene)-1,3-propanediamine (LH ₂), bis-N,N′-(salicylidene)-2,2′-dimethyl-1,3- propanediamine (LDMH₂) and the reduced derivativ

Full text of DOI:10.1007/s10973-009-0302-2

J Therm Anal Calorim (2009) 98:377–385
DOI 10.1007/s10973-009-0302-2
The effect of ligand basicity on the thermal stability
of heterodinuclear NiII–ZnII complexes
Bu¨lent Zeybek Æ B. Meltem Ates¸ Æ Filiz Ercan Æ
M. Levent Aksu Æ Esma Kılıc¸ Æ Orhan Atakol
Received: 10 November 2008 / Accepted: 16 January 2009 / Published online: 6 August 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract Four complexes of the nuclear structure NiII–
ZnII were prepared with bis-N,N0-(salicylidene)-1,3-pro-
panediamine (LH2), bis-N,N0-(salicylidene)-2,20-dimethyl-1,
3-propanediamine (LDMH2) and the reduced derivatives of
these Schiff bases, bis-N,N0-(2-hydroxybenzyl)-1,3-pro-
panediamine (LHH2), bis-N,N0-(2-hydroxybenzyl)-2,20-
dimethyl-1,3-propanediamine (LDMHH2). The complexes
were characterized using IR spectroscopy, elemental anal-
ysis and thermogravimetric methods. The stoichiometry of
the complex molecules were found to be NiLꢀZnCl2ꢀ
(DMF)2, NiLDMꢀZnCl2ꢀ(DMF)2, NiLHꢀZnCl2ꢀ(DMF)2 and
NiLDMHꢀZnCl2ꢀ(DMF)2. The molecular models of the
complexes prepared with the reduced Schiff bases were
determined according to the X-ray diffraction method. It is
seen that in these complexes Ni(II) is in octahedral and
Zn(II) is in tetrahedral coordination sphere. Ni(II) ion is
coordinated between two nitrogen and two oxygen donors
of the ligand and oxygen donors of the two DMF molecules.
Zn(II) ion on the other hand is coordinated between two
oxygen of the organic ligand forming two l bonds. It also
coordinates two Cl ions. The thermogravimetric analysis
showed that the complex NiLDMHꢀZnCl2ꢀ(DMF)2 con-
taining methyl groups is more stable than the other complex
NiLHꢀZnCl2ꢀ(DMF)2 containing reduced Schiff base. The
coordinative DMF molecules in NiLDMHꢀZnCl2ꢀ(DMF)2
were thermally cleaved. However, the cleavage of DMF
molecules NiLHꢀZnCl2ꢀ(DMF)2 resulted in the thermal
degradation of the complex. In order to explain the TG data
of the ligands were titrated in non-aqueous medium and
their basicity strengths were determined. It was found that
the basicity of the ligands containing two methyl groups
were stronger. It is understood that the two methyl groups
increase the negative charge density on nitrogen causing an
increase in complex stability.
Keywords Thermal decomposition ꢀ
Dinuclear complexes ꢀ Basicity order ꢀ
Potentiometric titration ꢀ Thermal stability
Introduction
Bis-N,N0-(salicylidene)-1,3-propanediamine (LH2) is
a
ligand which is highly prone to forming multi nuclear
complexes [1–4]. Ni(II) and Cu(II) complexes of this ligand
have square planar coordination sphere and the molecular
structures of these complexes determined using X-ray dif-
fraction method are given in literature [5]. In the presence of
a Lewis acid, these square planar complexes coordinate to
the Lewis acid from their phenolic oxygens and form
binuclear complexes [6–11]. Ni(II) and Cu(II) ions in these
binuclear complexes can also be coordinated with a nitrogen
containing ligand such as Py or a solvent such as DMF
[12–14]. Bis-N,N0-(salicylidene)-1,3-propanediamine (LH2)
is easily reduced with NaBH4 in amphiprotic medium and
turn into an ONNO type phenol–amine ligand [15]. In this
study, bis-N,N0-(salicylidene)-1,3-propanediamine (LH2) and
bis-N,N0-(salicylidene)-2,20-dimethyl-1,3-propanediamine
B. Zeybek (&) ꢀ E. Kılıc¸ ꢀ O. Atakol
Department of Chemistry, Faculty of Science, Ankara
˘
University, 06100 Tandogan, Ankara, Turkey
e-mail: bzeybek@science.ankara.edu.tr
B. M. Ates¸ ꢀ F. Ercan
Department of Physics Engineering, Hacettepe University,
06800 Beytepe, Ankara, Turkey
M. L. Aksu
Faculty of Gazi Education, Gazi University, 06500 Bes¸evler,
Ankara, Turkey
123

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B. Zeybek et al.
Experimental
Chemicals
1,3-diaminopropane (Merck), 2,20-dimethyl-1,3-propane-
diamine (Merck), salisylaldehyde (Fluka), ethanol (Riedel-
¨ ¨
deHaen), methanol (Riedel-deHaen), nickel(II) chloride
hexahydrate (Aldrich), zinc(II) chloride (Merck), sodium
¨
boronhydride (Fluka), acetonitrile (Riedel-deHaen), per-
chloric acid (Merck) were used without further purification.
Preparation of the ligands
Schiff bases LH2 and LDMH2 were prepared by the con-
densation reaction of 1,3-diaminopropane, 2,20-dimethyl-
1,3-propanediamine and salicylaldehyde in ethanol. They
were recrystallised in 1:1 EtOH:H2O mixture. LH2; melt-
ing point 58–59 °C, reaction yield [90%, color yellow;
LDMH2; melting point 97–98 °C, reaction yield [95%,
color yellow.
Approximately 3.0 g of these Schiff bases were dis-
solved in 50 mL MeOH by mixing and NaBH4 was added
to this solution in drop wise manner until a colorless
solution was obtained. The resulting solution was poured
into approximately 250 mL icy water and kept on the
bench for 24 h. The white colored organic precipitate
formed at the end of this period is the reduced Schiff base.
It was recrystallised from 70% EtOH. LHH2; melting point
106–108 °C, reaction yield [75%, color white; LDMHH2;
melting point 95–96 °C, reaction yield[90%, color white.
Fig. 1 The formulas of ligands a LH2, LDMH2, b LHH2 and
LDMHH2
(LDMH2) are reduced to obtain bis-N,N0-(2-hydroxyben-
zyl)-1,3-propanediamine (LHH2), bis-N,N0-(2-hydroxyben-
zyl)-2,20-dimethyl-1,3-propanediamine (LDMHH2). Hetero-
dinuclear complexes are obtained from both the Schiff bases
(LH2 and LDMH2) and the reduced Schiff bases (LHH2 and
LDMHH2) in the presence of ZnCl2 in DMF medium. The
formulas of the ligands LH2, LDMH2, LHH2 and LDMHH2
are given in Fig. 1a and b.
Preparation of the complexes
Four complexes were prepared using the same method.
0.001 mol ligand (LH2, LDMH2, LHH2, LDMHH2) was
solved in 40 mL hot DMF. Solution of 0.2 mL Et3N and
0.001 mol (0.238 g) NiCl2ꢀ6H2O in 0.20 mL hot MeOH
was added to this solution. Afterwards solution of
0.001 mol (0.136 g) ZnCl2 in 10 mL hot MeOH was added
and the mixture thus prepared was kept on the bench for 2–
3 days. The crystals which precipitated at the end of this
period were filtered and dried in air.
The molecular models of the complexes similar to I
(NiLꢀZnCl2ꢀ(DMF)2) and II (NiLDMꢀZnCl2ꢀ(DMF)2) are
reported in literature [16, 17]. The X-ray diffraction studies
of the complexes III (NiLHꢀZnCl2ꢀ(DMF)2) and IV (NiL-
DMHꢀZnCl2ꢀ(DMF)2) which gave suitable crystals were
carried out and the complexes I–IV were thermogravimet-
rically analyzed. The four complexes prepared were char-
acterized with IR and elemental analyses. They were later
studied thermogravimetrically. The two Schiff base ligands;
bis-N,N0-(salicylidene)-1,3-propanediamine, bis-N,N0-(sali-
cylidene)-2,20-dimethyl-1,3-propanediamine and the two
reduced Schiff bases; bis-N,N0-(2-hydroxybenzyl)-1,3-pro-
panediamine,bis-N,N0-(2-hydroxybenzyl)-2,20-dimethyl-1,
3-propanediamine were titrated with HClO4 in acetonitrile
solvent to determine their relative basicity strengths in order
to explain the TG data.
Measurement techniques
C, H, N element analysis of the complexes were made on
Leco 932 CHNS analyser. Ni and Zn analysis were made
on GBC Avanta PM AAS equipment. Chlorine analysis
was made gravimetrically with the help of AgNO3. IR
spectra were obtained on a Perkin Elmer Spectrum 100
FTIR spectrometer using the ATR attachment.
123

The effect of ligand basicity
379
Thermogravimetric analyses were carried out on Shi-
madzu DTG-60 H equipment in Pt pans under N2 atmo-
sphere. Heating rate was 10 °C/min. Curves were recorded
between 30 and 750 °C. The pans were cleaned with O2 at
750 °C.
All potentiometric measurements were carried out in an
80-mL jacketed titration cell thermostated at
25.0 0.1 °C and under nitrogen atmosphere. An Orion
960 automatic titrator, equipped with a combined pH
electrode (Ingold) containing a filling solution of 0.1 M
NaCl was used for measuring the cell emf values. Titration
of the ligands was carried out in MeCN and 0.034 M
HClO4 was used as titrant.
The intensity data were collected at room temperature
using an Enraf-Nonius CAD 4 diffractometer [18] with
MoKa radiation with x/2h scan mode. The cell parameters
were determined from a least-squares refinement of 25
centered reflections in the range of 2.46 B h B 23.41° in
III, 2.12 B h B 26.29° in IV. Three standard reflections
that were periodically measured for every 120 min during
data collection showed no significant intensity variations.
The ranges of h, k, l are 0 B h B 15, 0 B k B 21,
0 B l B 23 in III, 0 B h B 11, -12 B k B 12, -22 B
l B 21 in IV, respectively. There were 4,422 reflections
were collected for complex III 3572 of were utilized, the
number of reflections collected for complex IV were 4,081
of which 2,898 were utilized for structural determinations
and refinement according to I C 2r(I). Cell refinement and
data reduction were carried out using CAD4 Express [18].
The structures were solved by direct method using the
program SHELXS97 and refined using SHELXL97 [19]
in the WinGX package [20]. All non-hydrogen atoms
were refined isotropically and then anisotropically by full
matrix least squares method. All the hydrogen atoms were
placed geometrically and refined as riding with Ueq(H) =
1.2Uiso(C).
Results and discussion
Important IR data and elemental analysis results of the
ligands and complexes are given in Table 1. The data in
Table 1 are in agreement with the formulas given in
Fig. 1a and b. The band seen at 1,652 cm-1 in the IR
spectra is the stretching vibration of the carbonyl group in
DMF. mC=N stretching vibrations of complexes I and II are
seen at 1,620 cm-1. In Schiff bases which contain an ali-
phatic carbon attached to the iminic nitrogen mC=N vibra-
tion band is observed at wave numbers bigger than
1,630 cm-1. However, it is a known fact that these
vibration bands shift 10–25 cm-1 towards lower energy.
These vibrations for ligands (LH2 and LDMH2) are
observed at 1,642; 1,644 cm-1 have moved to lower
123

380
B. Zeybek et al.
Fig. 2 The curves of complexes a I, b II, c III, and d IV
energy by 20 cm-1. This shows that iminic nitrogens
participate in coordination [21].
molecules in the expected complex stoichiometry. In con-
trast, in complex III, termination of the first thermal
reaction is observed clearly after decomposition is initiated
at 203 °C. In complexes I and II ligands are Schiff bases
whereas in complexes III and IV ligands are reduced
Schiff bases. If the ligand is a Schiff base, DMF molecules
start to break off at around 135 °C and the reaction is
completed about at 200 °C. DTA peaks are observed
around 161–177 °C. This is close to the results cited in
literature [23]. If the ligand is reduced Schiff base (phenol
amine) DMF break off takes place at higher temperatures.
In complex IV, thermal reaction which starts at around
160 °C is completed at around 240 °C. In complexes I and
II second thermal reaction starts around 380 °C and con-
tinues up to 650 °C. In complex IV the complex is stabile
for by 35 °C after the completion of the DMF removal
reaction at 230 °C. Second thermal reaction starts at
around 270 °C and continues up to 700 °C. This second
decomposition reaction of the complexes I, II and IV are
similar, but their initiation temperatures are different. It is
considered that in these reactions organic ligand start to
decompose first and in parallel to the organic ligand’s
decomposition ZnCl2 leaves the structure. O2 was added to
the medium at around 750 °C and a strong exothermic peak
appeared in all four curves. This exothermic peak is the
result of the combustion of the carbon particles formed
In complexes III and IV stretching bands are observed
at mN–H 3,283–3,218 cm-1. These mN–H bands in ligands are
observed at 3,326–3,268 cm-1. Since LH2 and LDMH2
complexes do not carry N–H bonds these bands are not
observed in complexes I and II. However, when the ligand
is reduced C=N group is convert to C–NH. In this case
mN–H band becomes clearly visible and mC=N band disap-
pears for complexes III and IV.
In the IR spectrum of the ligands only the phenolic
stretching bands mO–H are observed at energies lower than
expected. The reason for this is the strong hydrogen bond
between the phenolic O–H and iminic and aminic nitrogen
atoms. This hydrogen bond may cause the proton of the
base to be transferred onto nitrogen atom and move the
m
O–H vibration to lower energy by 700–1,000 cm-1 [22]. In
this study mO–H vibrations of the ligands are observed at
2,715–2,625 cm-1
.
TG is one of the most efficient methods in determination
of complex stoichiometries [23]. The TG curves of the
complexes are given in Fig. 2a–d. Full thermoanalytical
data of the complexes are given in Table 2.
Mass loss in the first thermal reactions of complexes I,
II and IV is observed at temperatures between 135 and
240 °C. This mass loss corresponds to the two DMF
123

The effect of ligand basicity
381
during decomposition of the ligand. The mass remaining
after the formation of this peak corresponds to NiO as
shown in Table 2. Curve of complex III is different from
the other three curves. This complex remains unchanged
up to 203 °C but starting from this temperature onward it
undergoes a constant decomposition up to 700 °C. In this
complex the upper limit where the DMF molecules leave
the structure is unknown. From the curves in Fig. 2a–d we
can suggest that complex III is formed with the DMF
molecules alone. DMF molecules make coordination in
other complexes as well but these complexes remain stable
with Ni–Ligand–ZnCl2 nucleus after the DMF molecules
are thermally separated. It is considered that in complex III
Ni–ligand–ZnCl2 units also start to decompose with the
separation of DMF. However, in complexes I, II and IV
the complex remaining after the separation of the DMF
molecules maintains its stability.
D
Ni½ligandꢁ ꢀ ZnCl2 ꢀ ðDMFÞ2ꢂ2!DMFNi½ligandꢁꢀ
ꢂZnCl2
ZnCl2 ! Ni½ligandꢁ ! decomposition
I; II and IVcomplexes
D
Ni½ligandꢁ ꢀ ZnCl2 ꢀ ðDMFÞ2ꢂ2DMF
!
Ni½ligandꢁ þ ZnCl2 ! decomposition III complex
The complexes are formed by the coordination of ZnCl2
from the phenolic oxygens. Since ZnCl2 attracts the elec-
trons of the phenolic oxygens, phenolic oxygens can
transfer less electrons to Ni(II) and in turn Ni(II) ion
coordinates DMF molecules to compensate this electron
deficient. If the nitrogen atom of the organic ligand has
sufficient amount of electron density, it is considered that
the complex will maintain its existence even after the DMF
molecules leave the structure. However, in the case of
complex III the electron density on the aminic nitrogen
atoms of the organic ligand is insufficient and the complex
starts to decompose after the DMF molecules leave the
complex structure. In order to understand charge density
on the nitrogen atoms of LHH2 and LDMHH2 these ligands
were potentiometrically titrated in MeCN. Potentiometric
titration in non-aqueous solvents is one of the simplest
methods that give an idea about the acidity and basicity
strengths of organic acids and bases [24]. The basicity of
organic matter can be determined from the half-neutral-
ization potential on the potentiometric titration curves.
Half-neutralization potentials in MeCN of the phenol
amine compounds titrated in this study are given in
Table 3. LDMHH2 has a more positive half-neutralization
potential. This is in contrast to what is expected. LHH2 acts
as a stronger base. The hydrogen bond formed between
phenolic OH group and the aminic nitrogen is considered
to create this effect.
123

382
B. Zeybek et al.
Table 3 Half-neutralization potentials of ligands in MeCN
Ligands
1. half neutralization
potentials/mV
2. half neutralization
potentials/mV
LH2
204.4 3.5
252.0 1.4
-60.8 0.5
-38.2 1.3
306.7 0.6
358.0 1.5
91.7 1.9
210.0 4.5
LDMH2
LHH2
LDMHH2
Standard deviation for three results
Fig. 4 The molecular structure and atomic labeling scheme of the
complex a III and b IV. Displacement ellipsoids are drawn at the
30% probability level
Fig. 3 Titration curves of a bis-N,N0-(2-methoxybenzyl)-1,3-pro-
panediamine and b bis-N,N0-(2-methoxybenzyl)-2,20-dimethyl-1,3-
propanediamine
The titration curves LDMH2 and LDMHH2 obtained in
acetonitrile medium are given in Fig. 3a, b as an example.
It is seen that 1. half neutralization potentials for bis-N,N0-
(2-methoxybenzyl)-1,3-propanediamine -185 mV, bis-
N,N0-(2-methoxybenzyl)-2,20-dimethyl-1,3-propanedi-
amine -235 mV, respectively. These values show that the
nitrogen atoms of bis-N,N0-(2-methoxybenzyl)-2,20-dime-
thyl-1,3-propanediamine have higher electron density. Due
to this higher electron density, the bi-nuclear complex
remains somewhat more stable after thermal separation of
the DMF molecules from this ligands complex.
A strong hydrogen bond is known to exist between phe-
nolic OH group and aminic and iminic nitrogen atoms. Due
to this hydrogen bond hydrogen can be partially transferred
to the nitrogen atom [25–27]. The higher the electron density
of the aminic nitrogen atoms, the stronger the phenolic
hydrogen will be bonded to this nitrogen. In this case it is not
possible to determine the status of the charge density on the
nitrogen atoms by potentiometric titration.
Therefore, in order to determine the electron density on
the aminic nitrogen atoms, phenolic groups have been
transformed into methoxide groups and titrated in MeCN.
Since no hydrogen bonds are formed with methoxy
groups electron density of the nitrogen atoms can be
determined more clearly.
This is also seen in bond lengths. Suitable crystals of
complexes III and IV have been prepared and their
molecular models have been obtained by X-ray diffraction
method. Figure 4 shows the PLATON drawing [28] for
complexes III (a) and IV (b) and their selected
123

The effect of ligand basicity
383
Table 4 Crystallographic and
experimental data for the
complexes
III
IV
Chemical formula
Formula mass
C23H34Cl2N4NiO4Zn
625.52
C25H38Cl2N4NiO4Zn
653.57
Temperature/K
293(2)
293(2)
˚
Wavelength/A
0.71073
0.71073
Crystal system, space group
Orthorhombic, P bca
Triclinic, P 1
˚
Unit cell dimensions/A, °
a
b
c
a
b
c
13.5670(4)
8.8388(2)
10.0071(3)
17.8197(4)
101.120(2)
101.007(2)
99.514(2)
1484.32(6)
2
19.5534(5)
20.8782(3)
–
–
–
3
˚
Volume/A
5538.6(2)
Z
8
Calculated density/mg m-3
Absorption coefficient/mm-1
F (000)
1.500
1.462
1.774
1.658
2592
680
Crystal size/mm
0.12 9 0.15 9 0.60
0.15 9 0.20 9 0.60
26.29
hmax/°
Index range
23.41
0 B h B 15, 0 B k B 21,
0 B l B 23
0 B h B 11, -12 B k B 12,
-22 B l B 21
Number of reflections used
3572
2898
Number of parameters
316
334
Rint
0.0487
0.0595
0.1115
0.997
0.0448
0.589
R
Rw
0.1272
1.024
Goodness of fit
-3
˚
Dqmin, Dqmax/e A
-0.678, 0.554
-0.648, 0.667
˚
Table 5 Selected bond lengths (A) for complexes
crystallographic and experimental data are given in
Table 4, some of the important coordinative bond lengths
and angles are tabulated in Tables 5 and 6, respectively.
As it can be seen in Fig. 4a and b Ni(II) ions are in an
octahedral and Zn(II) ions in a tetrahedral coordination
sphere.
III
IV
ZnꢀꢀꢀNi
Zn–O1
Zn–O2
Zn–Cl1
Zn–Cl2
Ni–O1
Ni–O2
Ni–O3
Ni–O4
Ni–N1
Ni–N2
3.0534(19)
1.989(7)
1.986(7)
2.218(3)
2.253(3)
2.079(7)
2.084(7)
2.060(8)
2.096(7)
2.074(8)
2.073(8)
ZnꢀꢀꢀNi
Zn–O1
Zn–O2
Zn–Cl1
Zn–Cl2
Ni–O1
Ni–O2
Ni–O3
Ni–O4
Ni–N1
Ni–N2
3.1156(11)
1.977(4)
2.006(4)
2.229(2)
2.232(2)
2.048(4)
2.062(4)
2.109(4)
2.140(4)
2.055(5)
2.078(5)
In complex III distances between Ni(II) and the DMF
˚
oxygen’s are found to be 2.060 and 2.096 A. In complex
˚
IV these distances are found as 2.109 and 2.140 A. Bonds
in complex IV are clearly longer. This shows that there is
less electron requirement for the Ni(II) in complex IV.
Since the nitrogen atoms of the ligand in complex IV have
higher electron densities the bond between Ni(II) and DMF
is longer. Ni(II) ion in complex III needs more electrons,
therefore it attracts the electrons of the DMF molecules and
the respected bond length is shorter. Therefore, the tem-
perature at which the DMFs leave the structure goes up to
200 °C.
two iminic nitrogen of the organic ligand. These two
phenolic oxygen and two iminic nitrogen donors form a
basal plane. Ni(II) ions are located approximately at the
middle of this basal plane. The oxygen atoms of two DMF
molecules coordinate Ni(II) ion up and down of the basal
As seen from Fig. 4a and b, in both complexes Ni(II) ion
has O4N2 and Zn(II) ion has O2Cl2 coordination sphere.
Ni(II) ion is coordinated between two phenolic oxygen and
123

384
B. Zeybek et al.
˚
Table 6 Selected bond angles () for complexes
Acknowledgements The authors also thank the Ankara University
Scientific Research Fund (Project No: 20070705118) for their
support.
III
IV
Zn–O1–Ni
Zn–O2–Ni
O1–Zn–O2
O1–Ni–O2
O1–Ni–O3
O1–Ni–O4
N1–Ni–O2
N2–Ni–O1
Cl1–Zn–Cl2
Ni–Zn–Cl1
Ni–Zn–Cl2
97.2(3)
Zn–O1–Ni
Zn–O2–Ni
O1–Zn–O2
O1–Ni–O2
O1–Ni–O3
O1–Ni–O4
N1–Ni–O2
N2–Ni–O1
Cl1–Zn–Cl2
Ni–Zn–Cl1
Ni–Zn–Cl2
101.43(19)
99.96(18)
80.64(17)
77.66(16)
92.84(18)
94.13(17)
170.11(19)
169.96(19)
113.63(8)
121.63(6)
124.65(6)
97.2(3)
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˚
were found to be 2.060 and 2.096 A in the complex III and
they do not differ from each other very much. The bond
lengths in the basal plane and apical positions of complex
IV were found to be between 2.048–2.072 and 2.109–
˚
2.140 A. These values are also very close to each other.
¨
9. Ulku¨ D, Kaynak FB, Atakol O, Aksu M. Crystal structure of {[l-
However, the bonds in the basal plane of complex IV are
shorter and the bonds at apical positions are considerable
longer than those of complex III. This shows that the
coordinative bonds in the basal plane of complex IV are
more stabile than those in complex III. This may be
attributed to the fact that higher electron density of the
nitrogen atoms in LDMH2-. Only the coordination sphere
of Ni(II) in complex III is close to ideal. Complex IV has a
more distorted coordination sphere. The bond angles sup-
port this fact. The coordination sphere of Zn(II) ion is a
distorted tetrahedral. ZnCl2 molecule forms a l bridge with
phenolic oxygens.
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2
2
¨
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Supplementary materials
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center,
CCDC 687246 for complex III and CCDC 687245 for
complex IV. Copies of this information can be obtained free
of charge from The Director, CCDC, 12 Union Road,
Cambridge, CB2, 1EZ, UK (fax: ?44-1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
123

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