Redox and Acid–Base Properties of Binuclear 4-Terphenyldithiophenolate Complexes of Nickel

Article abstract of DOI:10.1002/chem.201603060

This work reports on the redox and acid–base properties of binuclear complexes of nickel from 1,4-terphenyldithiophenol ligands. The results provide insight into the cooperative electronic interaction between a dinickel core and its ligand. Donor/acceptor contributions flexibly adjust to stabilize different redox states at the metals, which is relevant for redox reactions like proton reduction. Proton transfer to the [S₂Ni₂] core and Ni?H bond formation are kinetically favored over the thermodynamically favored yet unproductive proton transfer to ligand.

Full text of DOI:10.1002/chem.201603060

DOI: 10.1002/chem.201603060
Full Paper
&
Binuclear Complexes
Redox and Acid–Base Properties of Binuclear 4-
Terphenyldithiophenolate Complexes of Nickel
Felix Koch, Andreas Berkefeld,* Hartmut Schubert, and Claudius Grauer[a]
Abstract: This work reports on the redox and acid–base
properties of binuclear complexes of nickel from 1,4-terphe-
nyldithiophenol ligands. The results provide insight into the
cooperative electronic interaction between a dinickel core
and its ligand. Donor/acceptor contributions flexibly adjust
to stabilize different redox states at the metals, which is rele-
vant for redox reactions like proton reduction. Proton trans-
fer to the [S2Ni2] core and NiÀH bond formation are kinetical-
ly favored over the thermodynamically favored yet unpro-
ductive proton transfer to ligand.
Introduction
The search for complexes of earth-abundant 3rd-row transi-
tion-metals that can mediate HÀH bond formation from pro-
tons and electrons is a vibrant field of research.[1] The efficacy
of sequential electron- and proton-transfer steps can be fine-
tuned through ligand design. For example, it has been recog-
nized that proximal basic substituents act as intramolecular
relays for fast proton transfer to a metal or metal-H site.[2]
Thiolates are a special class of ligands in this regard. The
comparatively high covalent character of metal–thiolate bonds
and thiolate basicity are the elements that support intramolec-
ular electron and proton transfer,[3] and examples have been
reported for thiolate complexes of Mo,[4] Fe,[5] Ru,[6] Co,[7] Rh,[8]
and Ni.[9] In fact, proton- and (m-S)2-binding by thiolate residues
are among the key factors that contribute to the function of
[NiFe] hydrogenase.[10] Reactivity studies of model complexes
Scheme 1. Electrochemical generation of putative di-NiI or di-NiIII species
from a (m-S)2NiII complex for electrocatalytic reduction and evolution of pro-
2
tons.[14e]
add to the general notion that low- and high-valent binuclear
complexes of Ni play a role in a variety of reactions.[15]
The structural and electronic properties of the ligands are of
profound importance in this context since the metals cycle be-
tween at least two redox states during substrate turnover. A
flexible adjustment of donor and acceptor contributions in
metal–ligand bonding may accommodate changes of the ste-
reoelectronic preferences of the metals[16] and provide means
to manipulate their reactivity. Conjugated p-systems have
been recognized to act as electronically flexible donor/accept-
or ligands to single and multiple metals,[17] which includes ex-
amples of bimetallic complexes of Ni.[18] The terphenyldiphos-
phine ligands introduced by Agapie combine this property
with strongly s-donating phosphines. Notable examples in-
clude mono-,[19] di-,[15c] and trinuclear[20] complexes of Ni, dinu-
clear d9-Pd,[20b,21] M2-carbonyl cores of Fe, Co, and Ni,[22] and
complexes of Mo in which the ligand also functions as a two-
electron reservoir.[23]
with bimetallic (m-S)2Fe2 and (m-S)2NiFe[12] cores have provid-
[11]
ed detailed insight into the structure–function relationship of
the active-site chemistry of this class of metalloproteins.
Among the 3d-metals, mononuclear complexes of Ni have
been studied extensively for proton reduction,[13] whereas stud-
ies of homodinuclear complexes are rare.[14] Castillo reported
on an N,N-bis(2-thiophenol)methylisopropylamine that binds
NiII in the form of a dimeric thiolate-bridged complex. Electro-
chemical reduction (oxidation) of the di-NiII precursor in THF
has been suggested to afford di-NiI (di-NiIII) species that initiate
electrocatalytic proton reduction (evolution) from HBF4 (boro-
hydride),[14e] as shown in Scheme 1. Although neither of the Ni
species has been substantially characterized, these findings
We have reported on the properties of a 1,4-terphenyldithio-
phenol as a multidentate ligand for mono- and binuclear com-
plexes of Ni. k-S- and (m-S)2-coordination adjusts with s-donor/
p-acceptor bonding of the central p-system to stabilize NiI and
NiII as shown in Scheme 2.[24] Although the electronic ground
state structures are different, the total number of electrons is
the same. Whereas complexes 2Mes-L feature a (m-S)2NiI2 core,
the (m-h3:h3-C6H4)NiII2 core in 1R-PMe3 results from d–p* metal-
to-ligand electron transfer. Herein we report on the redox and
acid–base properties of these isomeric complexes. The results
[a] Dipl.-Chem. F. Koch, Dr. A. Berkefeld, Dr. H. Schubert, C. Grauer
Institut fꢀr Anorganische Chemie
Eberhard Karls Universitꢁt Tꢀbingen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
E-mail: andreas.berkefeld@anorg.uni-tuebingen.de
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201603060.
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p-system to those of the flanking phenyl rings that
enclose dihedral angles of ꢁ878 indicates m-h2:h2-
bonding to the Ni atoms, in agreement with Ni–C
distances of 2.139 and 2.150 ꢁ.
The electronic transition lmax(emax/ MÀ1 cmÀ1) de-
tected for 3R-L in THF solution most likely has a CT
character but the precise nature of the chromophore
is yet unknown. Notably, in the case of 3tBu-PMe3 lmax
is blueshifted by only 2 nm to 740(2700) in more-
polar 1,2-C6F2H4.
Solution magnetic moments of [3R-L]+ in C6D6 are
in the 1.8–2.3 range, slightly higher than the spin-
Scheme 2. Synthesis and structure of mixed-valent NiI,II cations [3R-L]+. Color scheme:
NiII =red, NiI/II =purple, NiI =blue; NTf2-anion not shown for clarity.
only value of 1.73 for a S=1= system. X-Band EPR
2
spectra in 2-Me-THF at 293 K show isotropic signals
show how the ligand cooperates in redox state changes and
competes as a kinetically slow proton acceptor with proton re-
duction at the di-Ni core.
Results and Discussion
Additional insight into the variability of metal–ligand bonding
has been obtained from studies of the one-electron oxidation
of either type of structures shown in Scheme 2. Using the fer-
rocene cation, oxidation reactions are facile and cleanly pro-
duce a series of isostructural mixed-valent cations [3R-L]+,
which were isolated as [N(SO2CF3)2]À-, NTf2-, salts in the form of
crystalline solids. In contrast to the starting materials, com-
Figure 1. Molecular structure of the bimetallic cation [3Mes-PMe3]+: Thermal
ellipsoids at 50% probability, H-atoms and the NTf2-anion are omitted for
clarity.
pounds 3R-L are fairly air and moisture stable, and can be ma-
nipulated in halogenated solvents other than 1,2-C6F2H4. Char-
acteristic electronic properties of complexes 3R-L are collected
in Table 1.
with giso =2.110–2.123 and line-widths in the 10–13 G range.
Frozen solutions at 77 K display rhombic spectra with g values
of the transition at lowest field in the 2.142–2.132 range and
Dgꢀ0.060. Hyperfine-splitting due to coupling with the 31P
nuclei is not resolved in any of the spectra, which indicates
a tetrahedral geometry at Ni as observed in crystalline sam-
ples.[26] While the g values support a Ni-centered electron spin,
the Dg values are markedly smaller than those reported for
mono- and binuclear systems with localized NiII/ NiI sites for
which Dg is >0.1.[18a,27] A single example has been reported for
a delocalized d8·5-Ni2(m-S)2 core with each Ni in a supposedly
square-planar coordination site of a pyridine-2,6-dimethane-
thiolate, and Dg=0.14.[28] Based on the EPR and solid-state
structural data, we propose a delocalized d8·5-Ni2 core structure
for [3R-L]+ to which (m-S)2-bonding likely contributes signifi-
cantly.[3] On the UV/Vis timescale, however, a localized d9-d8-Ni2
core structure appears to be the appropriate description since
emax is significantly smaller than expected for CT transitions
within delocalized electronic structures for which e typically is
ꢂ104 mÀ1 cmÀ1.[29]
Table 1. Characteristic electronic properties of cations [3R-L]+.[a]
[b]
[d]
[e]
1
1
2
R, L
E
= ,red/E = ,ox
lmax (emax
)
meff
giso(gx/Dg)[f]
2
tBu, PMe3
Mes, PMe3
Mes, PCy3
Mes, PPh3
À980/86
738(2400)
750(1600)
814(1600)
836(1400)
1.8
2.1
2.3
2.0
2.109 (2.133/ 0.054)
2.116 (2.142/ 0.060)
2.123 (2.136/ 0.047)
2.109 (2.132/ 0.053)
À962/148[c]
À947/177
À760/270
[a] In form of NTf2-salts. [b] In mV vs. Fc/[Fc]+ (0.03–0.4 mm, 0.1m
nBu4NPF6/ 1,2-C6H4F2, 296 K, GC disc electrode (0.039Æ0.002 cm2). [c] Re-
versible at v>500 mVsÀ1. [d]Æ2 nm (emax Æ300/ MÀ1 cmÀ1), THF, 296 K.
[e] C6D6, 278–343 K. [f] 2-Me-THF, 293 K (transition at lowest field, Dg in
rhombic spectra taken at 77 K).
The molecular structure of the [3Mes-PMe3]+ ion is shown in
Figure 1,[25] and details on isostructural cations [3tBu-PMe3]+
and [3Mes-PCy3]+ are provided in the Supporting Information,
Figures S14–S15.
The (m-S)2Ni2 core displays a local C2v symmetry. Each Ni
atom adopts a tetrahedral coordination geometry with bond
distances and angles to S-, P-, and C-atoms that are equal
within experimental accuracy (3sꢀ0.02 ꢁ). Ni-S-Ni angles of
638 and a Ni–Ni distance of 2.356 ꢁ are slightly decreased as
compared to the starting materials 2Mes-L[24] (658, 2.383 ꢁ). A
comparison of the bond lengths and angles within the central
CV analysis of 3R-L at a glassy carbon disc electrode in 0.1m
nBu4NPF6/ 1,2-C6F2H4 solution at 296 K showed that the cations
are stable over a potential range of ꢁ1100 mV. A cathodic po-
tential sweep vs. Fc/[Fc]+ displayed electrochemically reversi-
ble reductions of cations [3R-L]+ (E = ,red in Table 1) to neutral
1
2
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complexes in all cases, with peak-potential differences of less
than 70 mV and peak-current ratios ip(red)/ip(ox) close to unity.
In addition, a second electrochemically reversible oxidation
1
(E = ,ox in Table 1) to dicationic species occurs at potentials mod-
2
erately positive vs. Fc/[Fc]+. This second oxidation of [3R-L]+
presumably produces a di-NiII core that is stabilized through
tighter m-h2:h2-bonding of the ligand. In comparison to the
complexes with a trialkylphosphine donor, the relative shift of
half-potentials of ꢁ200 mV in the case of 3Mes-PPh3 reflects
that PPh3 is a weaker s-donor. Notably, CV data of neutral 1R-
PMe3 and 2Mes-L match those of 3R-L precisely, which proves
the identity of the species generated in electrochemical experi-
ments.
Figure 2. Molecular structure of [4Mes-PMe3]+: Thermal ellipsoids at 50%
probability, H-atoms, lattice-solvent, and the NTf2-anion are omitted for clari-
ty.
The fact that neutral di-Ni complexes can be oxidized twice
is important with regard to their acid–base reactivity. Treat-
ment with the cationic acid [C6H5NMe2H]+ in THF, with
a pKa(THF) of ꢁ7.39,[30] at room temperature affords the iso-
structural cations [4R-L]+ depicted in Scheme 3. The products
were isolated in the form of dark red or purple, crystalline
other four CÀC bonds (ꢀ1.43 ꢁ) that form a delocalized p-
system. The structural parameters of the cyclohexadienide
fragment are similar to those found in a mononuclear complex
of NiII reported by Agapie.[19] The local structure at the Ni
atoms is similar to that in [3R-L]+, although shorter Ni–C distan-
ces of 2.045(3) and 2.052(3) ꢁ are indicative of the higher oxi-
dation state of +2 in [4R-L]+.[18b,24] The diamagnetism of [4R-L]+
must result from antiferromagnetic coupling of the S=1 NiII
sites, and is probably mediated through (m-S)2- and m-h2:h2-
bonding to the ligand.
NTf2-salts, with lmax (emax/mÀ1 cmÀ1) that are 680 (3200) for 4tBu
-
PMe3, 673 (2200) for 4Mes-PMe3, and 742 (2300) for 4Mes-PPh3 at
296 K in THF. Compounds 4R-L are fairly stable to air and mois-
ture and tolerate chlorinated solvents.
2
Quantitative H NMR spectroscopic monitoring of the reac-
tion of 1Mes-PMe3 and >90 D-% [C6H5NMe2D]+ to [D1]-[4Mes
-
PMe3]+ in THF verified the regioselectivity of the H+-transfer
to the ligand. No introduction of the D-label into any of the
olefinic positions was observed. Alternative formation of a puta-
tive [Ni2D]+-species that undergoes intramolecular D-migration
to the proximal p-system of the ligand either is not favorable
or reversible. Addition of one equivalent of 1,4-diazabicy-
clo[2.2.2]octane (DABCO) produces an equilibrium mixture of
1
Mes-PMe3 (85%), [4Mes-PMe3]+ (15%), and rapidly exchanging
free and protonated base. The reverse reaction yields a mixture
of equal composition. Equilibrium concentrations of 1Mes-PMe3
and [4Mes-PMe3]+ were estimated at 253 K as a function of
added [DABCOH]+, which provided a pKa(THF) of ꢁ12.8 for
cations [4R-PMe3]+.
Scheme 3. Acid/base chemistry of di-Ni complexes toward cationic acids in
THF; NTf2-anion omitted for clarity, DABCO=1,4-diazabicyclo[2.2.2]octane.
Formation of [4R-L]+ results from an intermolecular proton
transfer to the Cipso-atom of the central p-system of the start-
ing material. Indicative of a CS-symmetric structure, 1H and
13C NMR spectra of, for example, [4tBu-PMe3]+ display resonan-
ces at 6.057/70.42 and 4.062/88.05 ppm for the m-h2:h2-cyclo-
hexadienide moiety as well as at 1.584/48.83 ppm for the exo-
Interestingly, solution X-band EPR spectra of crude 4R-L veri-
fied co-formation of 3R-L, albeit at low quantities of <5% as
1
judged from H NMR spectra. Substitution of the 4-position in
[C6H5NMe2H]+ for acetyl or NO2 increases pKa(THF) values of
the cationic acid by about three orders of magnitude.[30] As the
result, the molar fraction of [4Mes-PMe3]+ decreases in favor of
[3Mes-PMe3]+ as the ultimate product as shown in Table 2. The
formation of H2 as a byproduct was verified by monitoring the
reaction by 1H NMR spectroscopy. Ultrasound treatment and
readdition of H2 to NMR tubes by syringe verified its identity,
while use of a 1:2 mixture of HNTf2/DNTf2 resulted in H–D for-
mation. In other words, acids react as one-electron oxidants
with neutral 1R-PMe3, depending on their pKa. In fact, substitu-
tion of [Fc]NTf2 for HNTf2 in the bulk reaction of Scheme 2 pro-
vides salts 3R-PMe3 in identical isolated yields and purity.
1
CÀH bond. Characteristic sets of H-AA’X9X’9 and 13C{1H}-ABX N-
line patterns at 1.243/15.71 ppm imply trans-disposition of the
PMe3 ligands along the Ni–Ni vector and strong scalar P–P cou-
2,n
1,n
pling with JP–P
@
J
H–P
and
J .
C–P
The molecular structure of [4Mes-PMe3]+ is shown in Figure 2
and agrees with solution structural data. Isostructural [4Mes
PPh3]+ is shown in Figure S13, Supporting Information.[25]
-
The Ni atoms each adopt a tetrahedral coordination geome-
try and are chemically equivalent. The m-h2:h2-cyclohexadienide
moiety features a tertiary sp3-C-atom of distorted tetrahedral
geometry and bond distances of ꢂ1.48 ꢁ as compared to the
To explain the dependence of chemoselectivity on acid
strength in mechanistic terms requires knowledge as to wheth-
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Table 2. Chemoselectivity of the reaction of
acids[a]
1Mes-PMe3 with cationic
.
Acid (pKa(THF))[b]
n(4Mes-PMe3)[e] 1,5-COD %-isomerization[f]
[DABCOH]+ (ꢁ13.7)[30]
[C6H5NMe2H]+ (7.39)[30]
[4-Acetyl-C6H5NMe2H]+[c]
0.15
>0.95
0.43
2 (1 h), 50 (44 h), 65 (74 h)
6
not tested
not tested
>98
[4-NO2-C6H5NMe2H]+ (ꢁ5)[d] <0.05
HN(SO2CF3)2 (<5)[c]
<0.05
[a] [D8]THF, 10 equiv 1,5-COD, 1.3 equivalents of acid added at 193 K,
slow warming to room temperature within 2 h. [b] NTf2-salts. [c] No
pKa(THF) values available. [d] Estimated from pKa(THF) of anilinium
acids.[30] [e] Reactions in the absence of 1,5-COD, values calculated from
1H NMR spectra. [f] Molar fraction of 1,4-/ 1,3- vs. 1,5-COD derived from
1H NMR and GCMS data.
er [Ni2H]-species are accessible under the reaction conditions.
As an internal chemical probe for NiÀH bond formation, the
isomerization of additionally added 1,5-COD (1,5-COD = 1,5-cy-
clooctadiene) to the thermodynamically more stable 1,4- and
1,3-isomers was studied. Reactions were run in the presence of
10 equivalents of 1,5-COD under otherwise identical condi-
tions, and the reaction mixtures subject to mass spectrometric
(GCMS) analysis. As described in Table 2, the reaction of 1Mes
-
PMe3 with HNTf2 produced an 1:12:87 mixture of 1,5-, 1,4-, and
1,3-COD, [4Mes-PMe3]+ now being the major product (ꢁ90%)
along with some [3Mes-PMe3]+ (<10%). Most intriguingly,
a small but significant quantity of 6% of 1,4-COD was generat-
ed in the reaction with [C6H5NMe2H]+. An even continuous iso-
merization was observed for [DABCOH]+, reaching 65% of 1,4-
and 1,3-COD in a 4:1 ratio after 74 h at room temperature.
Control experiments verified that the isomerization of 1,5-COD
requires both 1Mes-PMe3 and acid. [Ni(cod)2] as a substitute for
Scheme 4. I: Reaction of cationic acids with selectively deuterated [D1]-[4tBu
PMe3]NTf2. II: Overview on the acid/base chemistry of 1R-PMe3.
-
Conclusions
1
Mes-PMe3 rapidly produced Ni-black and a sub-stoichiometric
Comparison of the electronic structures of isomeric com-
pounds 1R-PMe3 and 2Mes-L illustrates how a flexible adjust-
ment of donor/acceptor components allows for manipulating
the redox state of nickel. In the cations [3R-L]+ the m-h2:h2-co-
ordination of the p-system compensates for the lack of elec-
tron density at Ni. In contrast to compounds 2Mes-L that already
possess a rhombic (m-S)2Ni2 core structure, oxidation of di-NiII
complexes 1R-PMe3 involves a rearrangement of the ligand
field from square-planar to tetrahedral. However, the half-po-
quantity of 1,n-COD (n=3, 4). Formation of Ni-black is not sig-
nificant in reactions of 1Mes-PMe3.
The isomerization of 1,5-COD indicates that [Ni2H]-species
are present in mixtures of 1R-PMe3 and any of the cationic
acids, and likely form reversibly since COD-isomerization
ceases once the formation of [4Mes-PMe3]+ is complete. The de-
pendence of both the chemoselectivity of the oxidation of
1
Mes-PMe3 to 3Mes-PMe3 and the extent of COD-isomerization on
acid pKa requires that the formation of putative [Ni2H]+-species
is kinetically favored over that of ligand protonated 4Mes-PMe3
which is the thermodynamic product. To verify whether H2 for-
mation requires the intermediate formation of 4R-L, >90 D-%
selectively labelled [D1]-[4tBu-PMe3]+ was treated with the
strong cationic acid [4-NO2-C6H4NMe2H]+ as well as with an 1:1
mixture of the acid and its conjugate base at room tempera-
tentials for the first oxidation of neutral 1Mes-PMe3 and 2Mes-
1
PCy3 differ by only 15 mV (see E = ,red in Table 1), and in both
2
cases the reactions are diffusion controlled for scan rates
ꢀ5000 mVsÀ1. So despite being substantial, the reorganization
of the ligand field appears to impose only a small kinetic barri-
er to oxidation. The electrochemically reversible second oxida-
tion to dicationic species is assumed to further strengthen the
m-h2:h2-coordination of the p-system with Ni-arene bond met-
rics that presumably are similar to those determined in [4R-L]+.
The formation of ligand protonated [4R-L]+ results in a signifi-
cant change of the structural and electronic properties of the
central p-system, and restoration of aromaticity likely provides
a driving force for the reversibility of this reaction. On the
other hand, the rate of proton transfer to the p-system is
lower than that of the formation of putative [Ni2H]+-species.
1
ture as shown in Scheme 4, I. EPR and H NMR spectroscopic
monitoring showed no significant conversion into either [3tBu
-
PMe3]+, which is the ultimate product from H2 formation or
[4tBu-PMe3]+ from a net D–H exchange. As summarized in
Scheme 4, II these results provide a strong indication that H2
formation involves acid-generated [Ni2H]+-species and that re-
versible proton transfer to the ligand is a competitive yet slow
side reaction.
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2508C for 24–48 hours. NMR data were recorded on Bruker Avance
II 400 and DRX 250 instruments. VT NMR spectra were collected on
a Bruker AVII+500 spectrometer. d values are given in ppm, J
Since direct proton transfer to a low valent metal is expected
to be kinetically slow,[11b,31] we propose a rapid pre-equilibrium
proton transfer to thiolate prior to NiÀH bond formation, in an
analogous fashion to what has been described for mono- and
binuclear complexes of Ni and Fe with a pendant amine group
in close proximity to the metal site(s).[1c,11c]
Regarding the mechanism of H2-formation from [Ni2H]+-spe-
cies, homo- and heterolytic pathways have to be taken into ac-
count as summarized in Scheme 4, II. These mechanisms are
well established for Co–H fragments such as [CoIIIH]+ and neu-
tral [CoIIH],[32] and have also been evoked for Ni–H species,[13b]
but additional insight into the formation and properties of
acid-generated [Ni2H]+-species is required.
1
values in Hz. H and 13C{1H} NMR chemical shifts are referenced to
1
the residual H and naturally abundance 13C resonances of the sol-
vents: d=7.16/ 128.06 (C6D6), 1.72/ 67.21 ([D8]THF), 5.32/ 53.84
(CD2Cl2), and 7.26/ 77.16 ppm (CDCl3). 31P NMR chemical shifts are
referenced to an external standard sample of 85% H3PO4 set to
d=0 ppm. EPR spectra were collected using 4 mm O.D. Wilmad
quartz (CFQ) EPR tubes on a continuous wave X-band Bruker ESP
300E spectrometer, and are referenced to the Bruker Strong Pitch
standard giso =2.0028. Whenever applicable, reactions were moni-
tored by GCMS analysis on a HP 6890 instrument equipped with
a DB-5MS capillary column (JW, 30 mꢂ250 mmꢂ0.25 mm) and
a MSD 5973 mass detector. Samples were filtered through activat-
ed alumina prior to GCMS analysis. Evan’s method[35] was em-
ployed to determine meff in solution using a coaxial insert for 5 mm
NMR sample tubes, cMdia =À0.5ꢂM, M=unitless molecular weight
of the sample,[36] and cMdia(benzene)=À5.47ꢂ10À5 cm3 molÀ1[37]
were used to correct for diamagnetic susceptibilities of sample and
solvent.
In summary, the redox and acid–base properties of binuclear
complexes of Ni of 1,4-terphenyldithiophenol ligands have
been studied. The results reported herein demonstrate that
a fine balance of donor/ acceptor contributions in ligand-to-
metal bonding allows for stabilizing different redox states in
the context of chemical processes such as proton reduction, as
well as controlling the overall rates of competing reaction
pathways.
X-ray data were collected on a Bruker Smart APEXII diffractometer
with graphite-monochromated MoKa radiation. The programs used
were Bruker’s APEX2 v2011.8-0, including SADABS for absorption
correction and SAINT for structure solution, the WinGX suite of
programs version 2013.3,[38] SHELXS and SHELXL for structure solu-
tion and refinement,[39] PLATON,[40] and Ortep.[41] Crystals were,
unless otherwise noted, coated in a perfluorinated polyether oil
and mounted on a 100 mm MiTeGen MicroMountsꢃ loop that was
placed on the goniometer head under a stream of dry nitrogen at
100 K.
Experimental Section
General
All manipulations of air- and moisture-sensitive compounds were
carried out under an atmosphere of dry argon using standard
Schlenk or glove-box techniques. Starting materials 1Mes-PMe3 and
2
Mes-L,[24] (Me3P)2NiCl2,[33] and [Ni(cod)2][34] were prepared by follow-
ing literature procedures. Protocols for the preparation of tBu-sub-
stituted 1,4-terphenyldithiophenol, 1tBu-PMe3, and all cationic acids
are provided in the Supporting Information. N,N-Dimethylthiocar-
bamoyl chloride (Acros Organics) was used as received and stored
CV measurements were performed at room temperature (296 K)
under an argon atmosphere with an ECi 200 potentiostat (Nordic
Electrochemistry) in a gas-tight, full-glass, three-electrode cell
setup. nBu4NPF6 was used as electrolyte (Alfa Aesar), was recrystal-
lized 3 times from acetone/water and employed as a 0.1m solution
in 1,2-DFB and MeCN. The potentiostat was controlled using the
EC4ꢃ DAQ (version 4.1.90.1, Nordic Electrochemistry) software, and
data were treated with EC4ꢃ VIEW (version 1.2.36.1, Nordic Electro-
chemistry). A GC disc electrode (Metrohm, electro-active area=
0.039Æ0.002 cm2) and a 1 mm coiled Pt-wire were employed as
working and counter electrodes. The Ag/Ag+ redox couple, in the
form of a 0.5 mm Ag wire in a 0.01m AgClO4/ 0.1m nBu4NPF6
MeCN solution, served as a reference electrode. Voltammograms
were corrected for capacitive currents of electrolyte solutions and
in
a desiccator over P2O5. Bis(trifluoromethanesulfonyl)amine
(HNTf2, 99%, Acros Organics) was sublimed once before use.
AgClO4 and high-purity ferrocene (Fc) were obtained from Alfa
Aesar, and Fc was sublimed once prior to use. D2SO4 (>98% in
99.9+ % D2O) was obtained from ABCR. Benzoquinone was recrys-
tallized three times from ethanol, sublimed twice (room tempera-
ture, 10À3 mbar, static vacuum), and stored under argon. Solvents
were purified and dried prior to use. Dichloromethane and hexane
were dried using an MBraun solvent purification system (SPS). Ben-
zene, diethyl ether, pentane, THF, and toluene were predried over
activated 3 ꢁ molecular sieves and distilled from sodium benzo-
phenone ketyl or potassium metal under argon. 2-Methyl-tetrahy-
drofuran (2-Me-THF, >99%, inhibitor free, Sigma Aldrich) was dried
over and distilled from NaK alloy and stored in the glove box.
Methanol, N,N-dimethylformamide, N-methyl-2-pyrrolidone, and
1,2-difluorobenzene (1,2-DFB) were dried and purified by percola-
tion through a column of activated neutral alumina, and 1,2-DFB
was distilled onto fresh alumina prior to use in electrochemical ex-
periments. Acetonitrile (MeCN) for use in electrochemical experi-
ments was sequentially dried over and distilled from CaH2 and
P2O5, and finally percolated through activated neutral alumina.
C6D6 and [D8]THF were dried over and distilled from NaK alloy.
Hexamethyldisiloxane (HMDS), CDCl3, and CD2Cl2 were dried over
and vacuum transferred from 3 ꢁ molecular sieves. All solvents
were stored over 3 ꢁ molecular sieves or activated neutral alumina
(MeCN, 1,2-DFB) under argon. Molecular sieves and neutral alumina
were activated by heating under dynamic vacuum (10À3 mbar) at
overall cell resistance, and potentials are reported relative to Fc/
+
[Fc]+ in 1,2-DFB, with E = (Fc/[Fc] /0.1m nBu4NPF6/ 1,2-DFB,
1
2
296 K)=0.222Æ0.003 mV. The electroactive area of the GC disc
electrode was calculated from Fc/[Fc]+ measurements in 0.1m
nBu4NPF6 solution in MeCN at various concentrations and potential
sweep rates, using D(Fc/MeCN, 293 K)=2.40ꢂ10À5 cm2 sÀ1. The
working electrode was rinsed with acetone, polished very gently
with a paste of 0.3 mm alumina (Metrohm) in deionized water,
rinsed thoroughly with plenty of deionized water, and finally ace-
tone after each use and stored in a desiccator over P2O5. Periodic
Fc/[Fc]+ reference measurements verified the electroactive surface
area of the GC electrode, and the stability of the potential of the
Ag/Ag+ reference electrode.
Numeration scheme used for ligands [Mes2tBu2S2]2À and [tBu4S2]2À
in coordination compounds:
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room temperature afforded a red/purple solution. The solvent was
removed under dynamic vacuum, and the dark red residue washed
with 10 mL portions of hexane two times and dried. The product
was extracted into diethyl ether and the dark red solution was fil-
tered, concentrated until crystallization started, and kept at À358C
for 5 h. A pale red supernatant solution was removed by cannula,
and the dark red needles dried under vacuum. Yield: 113 mg
1
(0.106 mmol, 81%). H NMR (500 MHz, [D8]THF, 299 K): d=7.69 (d,
4J(HH)=2.1 Hz, 1H, H-3), 7.52 (d, J(HH)=2.1 Hz, 1H, H-16), 7.47 (d,
4
Synthesis and characterization
4J(HH)=2.3 Hz, 1H, H-5), 7.29 (d, 4J(HH)=2.3 Hz, 1H, H-18), 6.06
(m, 2H, H-8,-12), 4.06 (m, 2H, H-9,-11), 2.01 (s, 9H, H-24), 1.67 (s,
9H, H-23), 1.59 (brm, 1H, H-10), 1.41 (s, 9H, H-25), 1.34 (s, 9H, H-
26), 1.24 ppm (AA’X9X9’, J(PP)@J(HP), Dn (N-lines)=9.5 Hz, 18H,
P(CH3)3); 13C{1H} NMR (126 MHz, CDCl3, 298 K): d=152.74 (C-1),
151.72 (C-13), 150.44 (C-15), 150.16 (C-6), 149,47 (C-4), 148.06 (C17),
147.11 (brm, C-7), 142.94 (C-2), 127.87 (C-14), 126.53 (C18), 124.08
(C-16), 123.67 (C-5), 120.28 (quartet, 1J(CF)=322 Hz, NTF2-anion),
118.09 (C-3), 88.05 (C-8,-12), 70.42 (C-9,-11), 48.83 (C(sp3)-10), 37.71
(C-20), 36.8 (C-19), 34.77 (C-21), 34.46 (C-22), 30.89 (C-24), 30.62 (C-
25), 30.56 (C-26), 30.1 (C-23), 15.71 ppm (ABX, J(PP)@J(CP), Dn (N-
lines)=28.4 Hz, P(CH3)3); 19F NMR (376 MHz, [D8]THF, 299 K):
dÀ81.6 ppm (s, NTf2-anion); 31P{1H} NMR (202 MHz, [D8]THF, 299 K):
d=À20.53 ppm; UV/Vis (THF): lmax (e): 240 (27700), 264 (24200),
Preparation of ferrocenium bis(trifluoromethanesulfonyl)imide
[Fc]NTf2: To a solution of 4-benzoquinone (87 mg, 0.81 mmol) in
diethyl ether was first added dropwise a solution of HNTf2
(476 mg, 1.693 mmol) in diethyl ether at room temperature, fol-
lowed by
a diethyl ether solution of ferrocene (250 mg,
1.344 mmol); the total volume was 25 mL. A green microcrystalline
solid separated immediately after complete addition of ferrocene,
and the supernatant solution was removed by cannula. The result-
ing green solid was washed with diethyl ether and dried under dy-
namic vacuum to afford 574 mg (1.23 mmol, 92%) of blue/green
microcrystals.
UV/Vis
(THF):
lmax
(emax)=622 nm
(530 molÀ1dm3 cmÀ1); ESI MS: m/z: 185.9 [M+], 279.7 [MÀ]; ele-
mental analysis calcd (%) for C12H10F6FeNO4S2: C 30.92, H 2.16, N
3.00, S 13.75; found: C 31.00, H 2.14, N 3.10, S 13.78.
316
(16000),
367
(11300),
494
(10000),
680 nm
(3200 molÀ1 dm3 cmÀ1); elemental analysis calcd (%) for
C42H63NF6Ni2O4P2S2: C 47.25, H 5.95, N 1.31, S 12.01; found: C 47.12,
H 5.88, N 1.38, S 11.85.
Preparation of [3tBu-PMe3]NTf2
Procedure A: A solution of 1tBu-PMe3 (100 mg, 0.127 mmol) in 6 g of
THF at À408C was cannula transferred onto solid [Fc]NTf2 (59 mg,
0.127 mmol) precooled at À408C. The reaction mixture turned
dark red immediately and was slowly warmed to room tempera-
ture. After the solvent had been removed under dynamic vacuum,
the red solid residue was dissolved in 15 mL of toluene, and the re-
sulting dark red solution filtered and concentrated to 5 mL, which
deposited dark red needles. The mixture was kept at À358C for
3 h, and the orange supernatant was separated from the product
Complex [D1]-[4tBu-PMe3]NTf2 was prepared from 1tBu-PMe3 and
[C6H5NMe2D]NTF2 (>95 D-%) following the same protocol. Yield:
110 mg (0.103 mmol, 79%), >95% D-% as judged from 1H NMR
spectra by relative integration.
Preparation of [3Mes-PMe3]NTf2: A solution of 1Mes-PMe3 (145 mg,
0.148 mmol) in diethyl ether (40 mL) was cooled in a dry ice/ace-
tone bath to À788C. A solution of HNTf2 (55 mg, 0.195 mmol) in di-
ethyl ether (10 mL) was added. The resulting mixture was allowed
to gradually warm to r.t. in the cooling bath over a period of 3 h.
After this time, the color of the reaction mixture changed from
dark red to red brown. Crystallization was initiated by reducing the
solvent by half under reduced pressure. After 12 h at 68C, the crys-
tals were separated and the product recrystallized by slow diffu-
sion of hexane vapor into a concentrated solution of [3Mes-PMe3]+
in THF (98 mg, 55%). The same procedure was used to obtain
single crystals suitable for XRD analysis. UV/Vis (THF): l (e): 280
(sh), 322 (sh), 386 (7500), 475 (sh), 566 (2900), 750 nm
(1590 molÀ1 dm3 cmÀ1); meff =2.1 (Evans method, 278–318 K, C6D6
containing HMDS); elemental analysis calcd (%) for C52H67Ni2P2S4N
F6O4: C 52.46, H 5.59, S 10.77, N 1.18; found: C 52.14, H 5.64, S
12.18, N 1.19.
by cannula. Drying under dynamic vacuum produces [3tBu
-
PMe3]NTf2 ꢂ0.25C7H8 (109 mg, 0.100 mmol, 78%).
Procedure B: To a solution of 1tBu-PMe3 (50 mg, 0.064 mmol) in
10 mL of diethyl ether was added 10 mL of a diethyl ether solution
of HNTf2 (18 mg, 0.064 mmol) at room temperature. The color
changed immediately from red to dark red/purple and the mixture
was filtered after 30 min. The resulting filtrate was concentrated
until [3tBu-PMe3]NTf2 separated in the form of dark-red needles.
After 3 h at À358C, a pale-red supernatant solution was removed
by cannula, and the product dried under dynamic vacuum. Yield:
47 mg (0.044 mmol, 69%) of dark red/purple needles.
CW X-Band EPR (2-Me-THF, 293 K): Microwave frequency
9.7580 GHz, power 5.03 mW, modulation amplitude 0.951 G, modu-
lation frequency 100 kHz, center field 3279 G, sweep width 700 G,
resolution 1024 points: giso =2.11; 77 K, microwave frequency
9.5022 GHz: gx =2.133, gy =2.118, gz =2.079; UV/Vis (THF): l (e)=
284 (18000), 303 (sh), 335 (sh), 387 (11000), 470 (4700), 562 (4300),
738 nm (2400 molÀ1 dm3 cmÀ1); elemental analysis calcd (%) for
C42H62F6Ni2NO4P2S4 ꢂ0.25C7H8: C 48.23, H 5.92, N 1.29, S 11.77;
found: C 48.46, H 5.85, N 1.61, S 11.61; meff =1.8 (288–343 K, C6D6/
HMDS).
Preparation of [4Mes-PMe3]NTf2: 1Mes-PMe3 (150 mg, 0.164 mmol)
and [C6H5NMe2H]NTf2 (66 mg, 0.164 mmol) were combined in
a Schlenk flask and cooled in a dry ice/acetone bath to À788C. Di-
ethyl ether was added slowly and the resulting reaction mixture
was warmed up to room temperature over a period of 2.5 h in the
cooling bath. Over this time, the color of the solution changed
from dark red to red brown. The solvent was removed under
vacuum and the reddish brown solid was recrystallized by slow dif-
fusion of hexane vapor into a concentrated solution of [4Mes
-
Preparation of [4tBu-PMe3]NTf2: A solution of [C6H5NMe2H]NTf2
(51 mg, 0.127 mmol) in 10 mL THF was cooled to À408C and can-
nula transferred to a solution of 1tBu-PMe3 (100 mg, 0.127 mmol) in
10 mL THF precooled to À408C. After 5 min at À408C, warming to
PMe3]+ in THF (60 mg, 30%). The same procedure was used to
1
obtain single crystals suitable for XRD analysis. H NMR (500 MHz,
[D8]THF, 299 K): d=7.77 (d, 4J(H,H)=2.16 Hz, 1H, H-18), 7.38 (d,
4
4J(H,H)=2.26 Hz, 1H, 3-H), 7.14 (d, J(H,H)=2.15 Hz, 1H, H-16), 7.11
&
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4
(d, J(H,H)=2.26 Hz, 1H, H-5), 6.96 (s, 2H, H-23,-25), 6.86 (s, 2H, H-
S 8.20, N 0.90; found: C 62.72, H 5.36, S 7.68, N 0.89. [a] Partially
obscured by resonances of PPh3 ligands
29,-31), 6.15 (m, 2H, H-9,-11), 4.09 (m, 2H, 8-, H-12), 2.29 (s, 3H, H-
36), 2.23 (s, 3H, H-39), 2.06 (s, 6H, H-35,-37), 1.94, (s, 6H, H-38,-40),
1.40 (s, 1H, H-7), 1.37 (s, 9H, H-33), 1.30 (s, 9H, H-34) 1.13 ppm
(AA’X9X9’, J(PP)@J(HP), Dn (N-lines)=9.5 Hz, 18H, P(CH3)3);
13C{1H} NMR (126 MHz, [D8]THF, 299 K): d=154.0 (C-14), 150.1 (C-
Preparation of [3Mes-PCy3]NTf2: A solution of 2Mes-PCy3 (50 mg,
0.038 mmol) in THF (8 mL) was cooled down to À158C in an ice/
NaCl bath and an ice-cooled solution of FcNTf2 (17 mg,
0.038 mmol) in THF (8 mL) was cannulated to the complex. Instan-
taneously, the color of the solution turned yellow brown and the
reaction mixture was stirred for 30 min in the cooling bath and
then an additional 30 min. at room temperature. The solvent was
removed under vacuum and the product was washed with toluene
and recrystallized by slow diffusion of hexane vapor into a concen-
trated solution of [3Mes-PCy3]NTf2 in THF, which provided 37 mg
(0.023 mmol, 61%) of yellow brown crystals. UV/Vis(THF): l (e): 310
(13600), 353 (13300), 403 (13100), 490 (5100), 580 (3500), 814 nm
(1600 molÀ1 dm3 cmÀ1); meff =2.3 (Evans method, 278–318 K, CDCl3
containing HMDS); elemental analysis calcd (%) for
C82H114Ni2P2S4NF6O4: C 61.58, H 7.18, S 8.02, N 0.88; found: C 61.14,
H 7.24, S 7.74, N 0.91.
2
17), 149.7, 148.9 (C-4), 148.4 (t, J(C,P)=2.00 Hz, C-10), 142.6, 142.5,
141.5, 139.7 (C-21), 136.6, 136.3, 135.6, 135.5, 135.4 (C-27), 129.3 (C-
1), 127.7 (C-23,-25,-29,-31), 127.6 (C-16), 127.0 (C-5), 126.5 (C-3),
118.5 (C-18), 89.2 (C-9,-11), 70.0 (C-8,-12), 48.3 (C-7), 34.7 (C-19),
34.4 (C-20), 30.6 (C-33), 30.5 (C-34), 22.2 (C-36), 20.5 (C-35,-37), 20.4
(C-38,-40), 20.1 (C-39), 15.9 ppm (ABX, J(PP)@J(CP), Dn (N-lines)=
28.3 Hz, P(CH3)3); 31P{1H} NMR (202 MHz, [D8]THF, 299 K): d=
À19.41 ppm; 19F NMR (376 MHz, CDCl3, 298 K): d=À77.59 ppm (s,
NTf2-anion); UV/Vis (THF): l (e): 312 (sh), 375 (7700), 497 (6100),
673 nm (2200 molÀ1 dm3 cmÀ1); elemental analysis calcd (%) for
C52H67Ni2P2S4NF6O4: C 52.41, H 5.67, S 10.76, N 1.18; found: C 51.46,
H 5.71, S 10.77, N 1.24.
Preparation of [3Mes-PPh3]NTf2: A solution of 2Mes-PPh3 (50 mg,
0.039 mmol) in thf (8 mL) was cooled down to À158C in an ice/
NaCl bath and an ice-cooled solution of FcNTf2 (18 mg,
0.039 mmol) in THF (8 mL) was added by cannula-transfer. Instanta-
neously, the color of the solution turns yellow brown and the reac-
tion mixture was stirred for 30 min in the cooling bath and for an
additional 30 min. at room temperature. The solvent was removed
under vacuum and the product was recrystallized from a concen-
trated solution in toluene at À358C. Yield 25 mg (0.016 mmol,
41%) yellow brown crystals. UV/Vis(THF): l (e): 265 (sh), 315 (sh),
428 (12200), 505 (sh), 585 (sh), 836 nm (1300 molÀ1 dm3 cmÀ1);
meff =2.0 (Evans method, 278–318 K, C6D6 containing HMDS); ele-
mental analysis calcd (%) for C82H78Ni2P2S4NF6O4: C 63.01, H 5.03, S
8.20, N 0.90; found C 62.17, H 4.96, S 7.84, N 0.96.
Cyclic voltammetry
Stock solutions of compounds 1R-PMe3, 2Mes-L, and 3R-L were pre-
pared in 0.1m nBu4NPF6/1,2-DFB in 1 mL volumetric flasks, concen-
trations being in the range of 4–8ꢂ10À3 M. The electrochemical
cell containing the GC working, Pt-counter and a Pt-wire electrode
was evacuated and backfilled with argon three times, and charged
with 8 mL of electrolyte solution by syringe. The Ag/ Ag+-reference
electrode in MeCN was connected to the cell by a Haber-Luggin
capillary filled with electrolyte solution and was connected by
a 0,01-mF capacitor to the Pt-wire electrode completing the
double-reference electrode system. After a stable resting potential
was reached under open circuit conditions, background CV scans
in the appropriate potential range were run on the electrolyte so-
lution at sweep rates of 20, 50, 100, 200, 500, 1000, 5000, and
10000 mVsÀ1, and the correction for overall cell resistance was ap-
plied manually on the potentiostat. Sample aliquots (25–50 mL
range) were added to the cell through a septum cap using a gas-
tight 100 mL syringe, and CV data were collected at aforemen-
tioned sweep rates.
Preparation of [4Mes-PPh3]NTf2: A solution of 2Mes-PPh3 (50 mg,
0.039 mmol) in diethyl ether (20 mL) was cooled in a dry ice/ace-
tone bath to À788C and a solution of HNTf2 (14 mg, 0.051 mmol)
in diethyl ether (10 mL) was added. The resulting mixture was al-
lowed to gradually warm to room temperature in the cooling bath
over a period of 2.5 h. After this time, the color of the reaction mix-
ture changed from green to brown and the solvent was removed
under vacuum. Bulk crystallization by slow diffusion of hexane
vapor into a concentrated solution of [4Mes-PPh3]NTf2 in THF pro-
duced dark red crystals (47 mg, 77%). The same procedure was
used to obtain single crystals suitable for XRD analysis. 1H NMR
Acknowledgements
A.B. and F.K. are indebted to the Fonds der Chemischen Indus-
trie and the Eliteprogramme for Postdocs of the Baden-Wꢄrt-
temberg Stiftung for financial support. We are grateful to Dr. K.
Eichele for assistance with and discussion of NMR data, Dipl.
Chem. Nicole Mews for the preparation of [Fc]NTf2, and Umi-
core AG & Co. KG, Hanau, Germany, for a generous donation of
(tBu3P)2Pd.
4
(500 MHz, [D8]THF, 299 K): d=7.92 (d, J(H,H)=2.14 Hz, 1H, 18-H),
4
7.60 (d, J(H,H)=2.07 Hz, 1H, H-3), 7.41 (m, 6H, H-PPh3), 7.18 (2H,
5-H, 16-H,)[a]), 7.17 (m, 12H, H-PPh3), 6.96 (m, 12H, H-PPh3), 6.70 (s,
2H, H-23,-25), 6.58 (s, 2H, H-29,-31), 6.30 (m, 2H, H-9,-11) 4.35 (m,
2H, H-8,-12), 2.36 (s, 3H, H-36), 2.24 (s, 3H, H-39), 1.67 (s, 1H, H-7),
1.51 (s, 9H, H-33), 1.44 (s, 9H, H-34), 1.35 ppm (s, 6H, H-35,-37),
1.25 (s, 6H, H-38,-40); 13C{1H} NMR(126 MHz, [D8]THF, 299 K): d=
153.94 (C-14), 150.98 (C-17), 149.66 (C-4), 149.37, 149.19, 144.49,
143.87, 140.98, 138.91 (C-21), 136.26, 135.94, 135.88, 135.80, 134.79
(C-27), 133.18 (C-PPh3), 130.77 (C-PPh3), 130.51 (C-5), 128.59 (C-
PPh3), 128.59 (C-16), 127.6, 127.56 (C-23,-25), 127.45 (C-23,-29),
127.18 (C-3), 119.00 (C-18), 91.69 (C-9,-11), 74.79 (C-8,-12), 48.75 (C-
7), 34.90 (C-19), 34.65 (C-20), 30.77 (C-33), 30.61 (C-34), 20.2 (C-36),
20.08 (C-35,-37), 20.05 (C-38,-40), 20.07 (C-39) ppm; 31P{1H} NMR
(202 MHz, [D8]THF, 299 K): d=15.07 ppm; 19F NMR (162 MHz,
[D8]THF, 299 K): d=À79.49 ppm (s, NTf2-anion); UV/Vis(THF): l (e):
362 (8400), 417 (11000), 524 (7400), 742 nm (2300 molÀ1 dm3 cmÀ1);
elemental analysis calcd (%) for C82H79Ni2P2S4NF6O4: C 62.97, H 5.09,
Keywords: binuclear complexes
nickel · proton reduction · thiophenolato ligands
· ligand cooperativity ·
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Received: July 28, 2016
Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 9
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Acids’ choice: The redox and acid–base
properties of binuclear complexes of
nickel from 1,4-terphenyldithiophenol li-
gands are reported. Donor/acceptor
contributions to metal–ligand bonding
flexibly adjust to stabilize different
redox states at the metals, which is rele-
vant for redox reactions like proton re-
duction. Proton transfer to the [S2Ni2]
core and NiÀH bond formation are ki-
netically favored over the thermody-
namically favored yet unproductive
proton transfer to ligand (see scheme).
&
Binuclear Complexes
F. Koch, A. Berkefeld,* H. Schubert,
C. Grauer
&& – &&
Redox and Acid–Base Properties of
Binuclear 4-Terphenyldithiophenolate
Complexes of Nickel
Chem. Eur. J. 2016, 22, 1 – 9
www.chemeurj.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ