Ni, Pd, and Pt complexes of a tetradentate dianionic thiosemicarbazone-based O^N^N^S ligand

Article abstract of DOI:10.1039/d1dt00272d

New tetradentate phenolate O^N^N^S thiosemicarbazone (TSC) ligands and their Ni(ii), Pd(ii) and Pt(ii) complexes were studied. The diamagnetic and square planar configured orange or red complexes show reversible reductive electrochemistry and in part reversible oxidative electrochemistry at very moderate potentials. DFT calculations show essentially pyridyl-imine centred lowest unoccupied molecular orbitals (LUMO) while the highest occupied molecular orbitals (HOMO) receive contributions from the phenolate moiety, the metal d orbitals and the TSC thiolate atom in keeping with UV-vis spectroelectrochemistry. DFT calculations in conjunction with IR spectra showed details of the molecular structures, the UV-vis absorptions were modelled through TD-DFT calculation with very high accuracy. UPS is fully consistent with UV-vis absorption and TD-DFT calculated data and shows decreasing HOMO-LUMO gaps along the series Pd > Pt > Ni.

Full text of DOI:10.1039/d1dt00272d

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Ni, Pd, and Pt complexes of a tetradentate
dianionic thiosemicarbazone-based O^N^N^S
ligand†
Cite this: Dalton Trans., 2021, 50,
4311
a
Alexander Haseloer, a Luca Mareen Denkler, a Rose Jordan,
Max Reimer, b Selina Olthof, b Ines Schmidt, b Klaus Meerholz,
b
Gerald Hörner *c and Axel Klein
*
a
New tetradentate phenolate O^N^N^S thiosemicarbazone (TSC) ligands and their Ni(II), Pd(II) and Pt(II)
complexes were studied. The diamagnetic and square planar configured orange or red complexes show
reversible reductive electrochemistry and in part reversible oxidative electrochemistry at very moderate
potentials. DFT calculations show essentially pyridyl-imine centred lowest unoccupied molecular orbitals
(LUMO) while the highest occupied molecular orbitals (HOMO) receive contributions from the phenolate
moiety, the metal d orbitals and the TSC thiolate atom in keeping with UV-vis spectroelectrochemistry.
DFT calculations in conjunction with IR spectra showed details of the molecular structures, the UV-vis
absorptions were modelled through TD-DFT calculation with very high accuracy. UPS is fully consistent
with UV-vis absorption and TD-DFT calculated data and shows decreasing HOMO–LUMO gaps along the
series Pd > Pt > Ni.
Received 26th January 2021,
Accepted 2nd March 2021
DOI: 10.1039/d1dt00272d
rsc.li/dalton
O^N^N^S coordination was reported by Mak in 1997.30 Here,
Ru(II) is coordinated to a tetradentate TSC ligand derived from
Introduction
Thiosemicarbazones (TSC) are very versatile ligands in coordi- 1,5-diacetylpyridine (Scheme 1D). From the 1980s on the tetra-
nation chemistry with applications in catalysis and medical dentate 1-(2-pyridyl-methylideneamino)-3-(salicyl-ideneamino)
chemistry.1–6 A lot of research on TSC is devoted to their thiourea (PST) ligand (E) was used for the spectrometric deter-
chemical modification. Thus, the basic N^S chelate binding of mination of divalent metal ions like Pd(II), Fe(II), Cd(II), and Hg
TSC can be enlarged to tri-,7–12 tetra-,13–16 and polydentate (II).31,32 This ligand and the related resorcinol derivative
binding chelators to further expand the coordination chemist’s (PPRT)33 owe their use to the phenolate function in addition
toolbox.17–22 Some of these higher coordination numbers are to the TSC thiolate function compensating for the charge of
achieved by bis-thiosemicarbazone ligands. Good examples for the divalent metal. Five years ago Ni(II) complexes containing
the tetradentate coordination of a bis-TSC ligand are the Cu-
ATSM (Scheme 1A)23–27 and the bis-TSC Ni(II) complex (B)
reported recently as an electrocatalyst for H2 evolution.28 In
contrast to these symmetric S^N^N^S-coordinating bis-TSCs,
unsymmetric N^N^N^S coordination of Pt(II) was very recently
accomplished with a phenanthroline-TSC ligand (C).29 An
early example of a tetradentate mono-TSC complex with an
aUniversität zu Köln, Department für Chemie, Institut für Anorganische Chemie,
Greinstraße 6, D-50939 Köln, Germany. E-mail: axel.klein@uni-koeln.de;
Tel: +49-221-470-4006
bUniversität zu Köln, Department für Chemie, Institut für Physikalische Chemie,
Greinstrasse 4-6, D-50939 Köln, Germany
Scheme 1 Structure of tetradentate TSC ligands and complexes. Cu-
ATSM (A),23–27 a very similar Ni(II) complex (B)28 neutral Pt(II) complex
consisting of a phenanthroline backbone (C)29 the [Ru(ONNS)(PPh3)2]+
complex (D),30 the ligand 1-(2-pyridylmethylideneamino)-3-(salicyl-
idene-amino)thiourea (PST) (E),31,32 and a Ni(II) complex with an amino-
phenol modified TSC ligand (F).34
cInstitut für Chemie, Anorganische Chemie IV, Universität Bayreuth,
Universitätsstraße 30, D-95440 Bayreuth, Germany.
E-mail: gerald.hoerner@uni-bayreuth.de
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1dt00272d
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Scheme 2 Synthesis of the metal complexes [M(L)] and [M(tBuL)] with
the 6-(2-hydroxyphenyl)pyridyl-2-carbaldehyde-TSC ligands.
salicylidene and aminophenol modified TSC ligands making
up a O^N^N^S dianionic coordination (F) were reported.34
To our surprise no TSC ligands with a 2-(pyridin-2-yl)
phenol side arm allowing for tetradentate O^N^N^S coordi-
nation (Scheme 2) were ever reported. Such tetradentate dia-
nionic ligands are well suited to coordinate divalent metals
with a planar coordination preference.
Herein, we report the synthesis of the first examples of such
tetradentate O^N^N^S ligands based on TSC containing the
chiral (S)-N4-(α-methylbenzyl) moiety (Scheme 2), H2L and
H2tBuL. The chirality of this residue is not of importance for
the present study but was relevant for a recent biochemical
study using related TSC ligands.35 We also studied the coordi-
nation of the group 10 divalent metals Ni(II), Pd(II), and Pt(II)
and report their complexes [M(L)] and [M(tBuL)] with struc-
tural, spectroscopic and electrochemical characterisation and
detailed density functional theory (DFT) calculations.
Scheme 4 Synthesis routes and yields. Conditions: (a) 2-methoxyphe-
nyl or 2-methoxy(3,5-di-tert-butyl-phenyl) boronic acid, (b) 2-hydroxy-
phenyl or 2-hydroxy(3,5-di-tert-butyl-phenyl) boronic acid. All with [Pd
(PPh3)4], K2CO3 in boiling toluene/EtOH for 20 h; (c) N4-methylbenzyl
thiosemicarbazide in MeOH, glacial acetic acid at 60 °C in ultrasonic
bath for 2 h; (d) BBr3 in CH2Cl2 at 298 K for 4 h.
(upper reaction sequence) was 18 times higher than the direct
route (middle reaction sequence) which gave only 2.5% overall
yield for the protoligand (ligand prior to deprotonation) H2L.
For the 3,5-di-tert-butylphenol derivative H2tBuL we followed
exclusively the “methoxy route” with an overall yield of 13%.
Despite the better yield in the Suzuki coupling (Scheme 4, a),
the conversion of OMe to OH (d) turned out to be less favour-
able for sterically hindered substituents.
Results and discussion
Synthesis of the complexes
Ligand synthesis
The complexes were obtained through mixing of equimolar
amounts of 6-(2-hydroxyphenyl)-2-formyl-pyridine N4-methyl-
benzyl TSC (H2L) dissolved in MeCN with aqueous solutions of
NiCl2, K2PdCl4, or K2PtCl4 in the presence of NEt3 as base
(Scheme 2). Upon mixing, a rapid colour change to orange (Pd)
or red (Ni, Pt) occurred, indicating a successful coordination
of the metal ions. All complexes were purified by column
chromatography on silica with ethyl acetate as eluent. In
keeping with a conserved complex polarity, all products eluted
with very similar Rf values. In the same manner, Ni, Pd and Pt
complexes of the sterically more demanding ligand 6-(3,5-di-
tert-butyl-2-hydroxyphenyl)-2-formyl-pyridine N4-methyl-benzyl
TSC (H2tBuL) were obtained as red or orange solids. All
attempts failed to receive single crystals of the products of
sufficiently high quality for X-ray structure elucidation.
However, elemental analyses and MS are in keeping with the
overall composition of neutral complexes with fully deproto-
nated ligands. IR spectra show clearly the C–S stretching fre-
quency in line with a single bond (see more below). Sharp reso-
nances in the 1H NMR spectra in the range of 1.0 < δ <
9.0 ppm, clearly support the diamagnetic d8 electron configur-
ation with square planar coordination. All resonances could be
unambiguously assigned to the proton sites of the complexes.
(NMR spectra in the electronic ESI, Fig. S4–S6,† data in the
Experimental section.)
2,6-Dibromopyridine was reacted with n-BuLi and DMF giving
6-bromo-2-pyridinecarbaldehyde in 24% yield (Scheme 3,
details in the Experimental section).
Subsequent Suzuki coupling using 2-methoxyphenyl
boronic acid gave 6-(2-methoxyphenyl)picolinaldehyde in 88%
or quantitative yield (Scheme 4, a), for R = H or t-Bu, respect-
ively. In the same way, we tried 6-(2-hydroxyphenyl)picolinalde-
hyde but obtained only 5% yield (b). Both were converted into
the thiosemicarbazones (TSC) through reaction with (S)-N4-
α-methylbenzyl thiosemicarbazide in 84 or 67% yield, respect-
ively (c). The protecting methoxy group was successfully con-
verted to the hydroxy function using BBr3 (d). Alternatively, we
also tested a route in which the 2-formyl-6-bromo-pyridine was
reacted with the thiosemicarbazide (Scheme 4, bottom, c).
However, the final Suzuki reactions (a and b) did not work with
this substrate and we received only traces of the hydroxy and
methoxy products. The overall yield of the “methoxy route”
Scheme 3 Synthesis of 6-bromo-2-pyridinecarbaldehyde.
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Molecular structures of the complexes
Lacking suitable single crystals for an XRD structure determi-
nation, we embarked on modelling the geometric and elec-
tronic structures with Density Functional Theory (DFT)
methods (Fig. 1, data in Tables S2 and S3, ESI†).
In order to substantiate and validate the quality of the struc-
ture prediction, we included to the study related systems; that is,
we re-visited compound F (Scheme 1), which has been reported
by Thomas et al.34 and the bis-TSC Ni(II) complex reported by
Artero and Orio et al. (B, metrical data in Table S1, ESI†).28
We found excellent agreement of predicted and experi-
mental metrics across the entire reference set (error in bond
lengths <2 pm). Taken together with the general close compli-
ance between model and experiment of planar d8 configured
complexes,36–39 the predicted metrics of [M(L)] and [M(tBuL)]
can be interpreted on reliable grounds. This notion is corrobo-
rated by the close match of experimental and computed IR
spectra, as is exemplarily shown for [Ni(L)] (Fig. 2; spectra of
all [M(L)] and [M(tBuL)] are provided in Fig. S20 and S21, ESI†).
As could be concluded from strictly diamagnetic behaviour
of the complexes in the 1H NMR spectra, all complexes exhibit
close-to-ideal square planar coordination of the S^N^N^O
donor set. The bond length of the coordinated C–S moiety
cluster at 1.78 0.01 Å, indicating dominant ene-thiolate char-
acter. It is noted that the structural changes along the series
do not follow the formal order 3d < 4d < 5d. Instead we con-
sistently find the metrics of the Pt complex rather close to
those of the Ni case, suggesting an ordering 3d ≪ 5d < 4d.
Similar observations have been previously made for the d8
triade in planar coordination.36,40–42
Fig. 2 Experimental (ATR; grey) and DFT-calculated (red; line: broad-
ened convolution; sticks: modes) IR spectrum of [Ni(L)]; asterisks
denote modes due to contamination with ethyl acetate (diagnostic
CvO mode at 1730 cm−1, not shown).
UV-vis absorption spectroscopy
The electronic structure of parent protoligands and of the
metal complexes has been addressed through UV-vis absorp-
tion spectroscopy with dilute solutions in CH3CN at ambient
temperature. Spectra recorded for the series deriving from H2L
are shown in Fig. 3 (for the tBuL series, see Fig. S24, ESI†). The
spectra of both protoligands H2L and H2tBuL are dominated by
intense absorptions in the near UV regime, peaking around
330 nm (Fig. 3). These transitions are safely assigned to π–π*
Fig. 3 UV-vis absorption spectra of H2L black, [Ni(L)] dark red, [Pd(L)]
orange and [Pt(L)] red, in MeCN.
transitions carrying some CT character. When coordinating
the metals, these broad bands are replaced by sharper bands
of similar peak intensity. In case of Pd, the band is split into
two bands of lower intensity.
The intense UV bands peaking at 220 nm and 250 nm show
the typical coordination-induced red-shift of ligand-borne π–π*
transitions (Table 1). The relative intensity of these two bands
clearly responds to metal; i.e., the transitions at ca. 270 nm is
dominant for Ni but only features as a low-energy shoulder for
Pd and Pt. More diagnostic is the metal-dependent response
in the visible region >400 nm which defines the orange or red
colour of the complexes. Energies, intensities and the partial
recovery of fine-structure point to conserved phenolate-to-(pyr-
idine-imine) charge transfer (ILCT) character. Nevertheless,
Fig. 1 Optimised structures and selected bond lengths of the com-
plexes [Ni(tBuL)] (a), [Pd(tBuL)] (b), [Ni(tL)] (c), and [Pt(tBuL)] (d) (further
figures in Fig. S22, ESI.†
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Table 1 Selected absorption maxima of the protoligands H2L and
H2tBuL and the complexes [M(L)] and [M(tBuL)] (M = Ni, Pd, Pt)a
λ1
λ2
λ3
λ4
H2L
[Ni(L)]
249
261
229
236
248
267
231
239
274
337
316/360
333
279
339
316/366
335
327
413
441
405
331
387/420
455
560
497
547
[Pd(L)]
[Pt(L)]
H2tBuL
[Ni(tBuL)]
[Pd(tBuL)]
[Pt(tBuL)]
577
518
567
408
a Measured in MeCN. Full data in Table S4, ESI.†
the maxima and overall structure of these bands are highly
depending on the metal. Clearly the energy and the intensity
(1200 > ε [M−1 cm−1] > 600) of the leading visible transitions
decrease along the series Pt (405 nm) > Ni (413 nm) > Pd
(441 nm) (Table 1). This sequence is counter-intuitive on first
view as it places the values for Pd not between Ni and Pt.
However, this seemingly deviating ordering has been observed
before along the d8 triade and is usually explained in terms of
energy matching of the metal-borne binding orbitals in the
case of Ni and Pt.36,43–46
Fig. 4 TD-DFT calculated optical spectrum of [Ni(L)]; line: convoluted
spectrum; verticals: transitions. Inset: difference densities of selected
transitions.
Pd. The complete spectral material is assembled in the ESI
(Fig. S25–S29; Table S5, ESI†).
The low-energy tailing of these bands reveals additional
weaker transitions in the case of Ni and Pt which extend to λ =
650 nm. Based on the similarity to the recently studied Pt(II)
complexes of the pyridinecarbaldehyde (S)-N4-(α-methyl-
benzyl)thiosemicarbazone (HTSCmB) ligands,35 these broad
bands are preliminarily assigned to metal-to-ligand charge
transfer (MLCT) transitions. Results from TD-DFT studies
support this assignment.
Ultraviolet photo electron spectroscopy (UPS)
UPS measurements of thin films of three complexes [M(L)]
with M = Pt, Pd or Ni (Fig. 5) were used to determine the work
function Wf and the ionisation energy IE. Alluding to
Koopmans theorem the latter represents the binding energy of
electrons in the highest occupied molecular orbital (HOMO).
The work function Wf was calculated from the difference of
the energy of the excitation source hν and the difference of the
high binding energy cut-off (HBEC) and the Fermi level EF
(eqn (1)).
The optical spectra of [M(L)] and [M(tBuL)] were modelled
with TD-DFT methods using the TPSSh hybrid functional.
These settings gave excellent agreement with experiment in
several previous studies on square-planar d8-based
systems.37–39 In keeping with this, the experimental transition
energy and intensity could be satisfyingly matched for all com-
plexes in this study (cf., Fig. 4 for Ni; for more spectra see
Fig. S25–S29, ESI†). Accordingly, the leading vis bands in the
experimental spectrum [Ni(L)] at λ = 413 and 560 nm can be
attributed to intense LLCT transitions calculated at λ = 420
and 540 nm (an additional weak band toward the NIR is pre-
dicted for all complexes). Difference densities of these tran-
sitions indicate substantial CT character with the diimine unit
acting as the acceptor. Ni(II) generally features prominently as
an electron source in all transitions in the visible range, giving
mixed ILCT/MLCT character.
Wf ¼ hν ꢀ ðHBEC ꢀ EFÞ
ð1Þ
By contrast, the broad near UV transition envelope from
280–360 nm is identified as a convolute of numerous individ-
ual transitions of varying character; nevertheless, the dominat-
ing transitions possess sharply decreased metal character and
utilise donor orbitals located rather on the TSC moiety, render-
ing the transition(s) ILCT like. Similar conclusions and assign-
ments hold for all complexes. However, the metal contribution
Fig. 5 Ultraviolet photoelectron spectra (UPS) of [M(L)] with M = Pt, Pd,
Ni showing the high binding energy cut-off (HBEC, left) and the HOMO
to the leading transitions decreases along the series Pt > Ni > region (right).
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The ionisation energy is calculated by adding the Wf and HOMO and HOMO−1 both are largely ligand-like, whereas
the difference of the HOMO onset and the Fermi level EF non-bonding d-like metal orbitals feature ca. 1 eV below the
which is positioned at 0 eV.
HOMO in all cases (HOMO-2 for Ni and HOMO-3 for the
others).
IE ¼ Wf þ ðHOMO onset ꢀ EFÞ
ð2Þ
The HOMO is largely localised on the phenolate moiety in
all cases, but subtle differences point out the influence of the
metal on the energy and the nature of the HOMO. Firstly, in
the Pt and Ni complexes significant contributions to the
HOMO arise from the S atom of the TSC moiety and from the
metal itself; all of these metal-borne components are
π-antibonding in nature. Whereas such contributions persist
in the Pd compounds, they are substantially smaller. In
keeping with the diminished metal-orbital contribution of Pd,
the HOMO of [Pd(L)] is stabilised by ca. 100 mV with respect
to both, [Pt(L)] and [Ni(L)].
The values for the HBEC and the HOMO onset are deter-
mined by using a linear fit of the cut off and the background
and of the HOMO onset and the background, respectively.
The ionisation energy IE decreased in the series Pd > Ni > Pt
(Table 2). This notion is fully corroborated by DFT-based ana-
lysis of the electronic structure. The optical HOMO–LUMO
gaps derived from the above absorption spectra (Fig. 3) are cal-
culated as 2.494 (Pd), 2.266 (Pt), and 2.214 (Ni) eV, respectively.
Oxidation and reduction – DFT computation and cyclic
Thus, DFT nicely captures the discontinuous order which
became evident also experimentally from the UV-vis absorp-
tion spectra and the UPS ionisation energies. Interestingly,
analysis of the series [M(tBuL)] echoes this ordering but indi-
cates a much closer range of HOMO energies. In keeping with
the clustered HOMO energies of [M(tBuL)], electrochemical oxi-
dation reveals only minor differences in the redox potentials
(Table 3). Due to irreversible electrochemical oxidation of [M
(L)] the computed HOMO energies cannot be correlated to the
CV pattern in quantitative terms. It is nevertheless noted that
the anodic peak potentials indeed follow the trends suggested
by DFT.
voltammetry
The occupied frontier orbitals (blue bars in Fig. 6 for [M(L)]
are qualitatively quite similar, irrespective of the metal centre.
Table 2 Calculated values for work function and HOMO level (ionis-
ation energy) for the [M(L)] complexes
[Ni(L)]
[Pd(L)]
[Pt(L)]
Work function Wf/eV
HOMO onset vs. EF/eV
Ionisation energy IE/eV
3.99
1.67
5.66
3.84
1.92
5.76
3.94
1.67
5.61
In their cathodic voltammetric scans, the six complexes
show two one-electron reduction waves and one or two one-
electron oxidation waves in the range +1.3 to −2.7 V (vs. ferro-
cene/ferrocenium) (Table 3 and Fig. 7, further material,
Fig. S31–S33 and Table S6, ESI.† The introduction of the tert-
butyl substituents resulted in slightly more negative reduction
potentials in keeping with the electron-donating character of
these groups.47,48 It is noted that the Ni(II) compounds proved
to be hardest to reduce, in agreement with the computed
LUMO energies. With respect to the protoligands, reduction
potentials of the complexes are shifted anodically by 0.4 to 0.6
V pointing to a substantial stabilisation of the LUMOs upon
Table 3 Selected electrochemical data of the protoligands H2L and
H2tBuL and the complexes [M(L)] and [M(tBuL)] (M = Ni, Pd, Pt)a
E1/2-Red2
E1/2-Red1
E1/2-Ox1
Epa-Ox2
H2L
[Ni(L)]
−2.36 irr
−2.37
−1.94 irr
−1.70
0.94 irr
0.66 irr
0.72 irr
0.69 irr
0.59
0.56
0.55
—
—
—
—
1.14 irr
0.99 irr
>1.3
0.96 irr
[Pd(L)]
[Pt(L)]
−2.34
−1.66
−2.36
−1.65
H2tBuL
[Ni(tBuL)]
[Pd(tBuL)]
[Pt(tBuL)]
−2.40 irr
−2.42
−2.02 irr
−1.75
−2.35
−2.36
−1.70
−1.69
0.55
a From cyclic voltammetry, electrochemical potentials in V (uncertain-
ties ∼1–3 mV), half-wave potentials E1/2 for reversible and partially
reversible redox waves, anodic (Epa) or cathodic (Epc) peak potentials
for irreversible (irr) waves; measured in 0.1 M n-Bu4NPF6/MeCN at
298 K, scan rate 100 mV s−1. Full data in Table S6, ESI.†
Fig. 6 DFT calculated energies of virtual frontier MOs (top) and occu-
pied orbitals (bottom) for [Ni(L)] (left), [Pd(L)] (centre) and [Pt(L)] (right).
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wave shifted markedly in potential for the Pt and Ni complexes
compared with the ligand H2tBuL, but nor for Pd. As these
second oxidation events are irreversible for all compounds,
however, we refrain from deep-going interpretation.
Spectroelectrochemical UV-vis absorption spectroscopy
The spectroscopic signature of reduced and oxidised states of
the parent complexes has been studied through UV-vis absorp-
tion spectroscopy during reductive and oxidative electrolysis in
dilute MeCN solution (spectroelectrochemistry). As is exempla-
rily shown for [Pt(tBuL)] (Fig. 8), the intense near UV pattern at
320 nm is completely bleached upon oxidation. The oxidised
species, presumably the cation [Pt(tBuL)]+, gives no new diag-
nostic bands in the vis region but rather echoes the pattern of
the parent neutral complex, though slightly shifted and with
tailing to the NIR. Bands ranging between 300 and 500 nm,
akin to those in Fig. 8 (and Fig. S34–S41, ESI†) were previously
assigned to the oxidation of the phenolate group forming a co-
ordinated phenoxy radical.34,47–52 In keeping with this, the
spin density computed for doublet [Pt(tBuL)]+ (inset in Fig. 8)
is largely localised on the phenolate/phenoxy moiety. A similar
Fig. 7 Cyclic voltammogramms of H2tBuL and [Pt(tBuL)] in 0.1 M
n-Bu4NPF6 MeCN solution.
coordination. In fact, the cyclic voltammograms of the protoli-
gands H2L and H2tBuL showed two reduction waves in the
range from −1.9 to −2.4 V that were irreversible (Fig. 7). For
the complexes, the first reduction wave gains markedly in
reversibility and both were shifted anodically by about 0.25
V. From the high similarity of the reduction behaviour of the
protoligands and their complexes we conclude, that all
reduction processes occurred essentially at the pyridine-imine
moiety of the TSC. This conclusion is in line with the behav-
iour of the recently studied pyridinecarbaldehyde (S)-N4-
(α-methylbenzyl)-thiosemicarbazones (HTSCmB) and their Pt
(II) complexes.35 It is corroborated also by DFT computation.
On the TPSSh/TZVP level of theory, the series [M(L)] gives
indeed similar virtual orbital pattern (red in Fig. 6). In particu-
lar, the LUMO energies vary by only 80 mV. The shape of the
LUMO which is essentially localised on the diimine moiety in
all cases and the conserved LUMO energy is in line with the
similarity that prevailed in the UV-vis absorption spectra.
Thus, it also matches the close similarity in the electro-
chemical reduction potentials.
For H2L one irreversible oxidation was observed at 0.94
V. For H2tBuL this wave became reversible, shifted to 0.59 V,
and a second oxidation wave appeared. This is clearly due to
the chemical stabilisation of the oxidised ligand through intro-
duction of the tert-butyl groups at the 3,5-phenolate
positions.47,48
The oxidation potentials recorded in the complex series [M
(L)] are shifted cathodically by about 0.3 V when compared
with the protoligand. For the series [M(tBuL)] this shift is very
small (∼0.03 V). For the Ni and Pt complexes [M(tBuL)] the
second oxidation is slightly shifted to lower potentials when
compared with the protoligand H2tBuL. By contrast, Pd does
not give a second oxidation wave in the covered potential
window and an oxidation potential of >1.15 V must be
assumed. With a view to the HOMOs depicted in Fig. 6, we
associate the first oxidation mainly with a phenolate/phenoxy
(PhO−/PhO•) redox couple. For M = Pd, the oxidation site is
particularly well localised on the phenolate with only little con-
tributions from the metal and other donor sites. Inspection of
the HOMO−1 suggests the second oxidation in the series
derived from H2tBuL to be another phenolate-based event with
significant metal contributions. Evidence for a metal contri-
Fig. 8 UV-vis absorption spectra recorded during electrolysis of [Pt
(
tBuL)] in 0.1 M n-Bu4NPF6 MeCN solution; top: oxidation; bottom:
bution comes from the observation that the second oxidation reductions.
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pattern prevails for the other complexes [M(tBuL)]+. While the observed in the range from +1.3 to −2.7 V. The introduction of
assignment of the oxidised species as planar mono-cation is the tert-butyl substituents in [M(tBuL)] led to several effects. As
probably justified for Pd and Pt, reconstruction of the coordi- expected, the first oxidation gets reversible and the potentials
nation sphere upon oxidation must be expected for Ni. are lowered, thus allowing also the observation of a second oxi-
Expansion of the coordination sphere of the oxidised complex dation in the same potential range. In turn, tert-butyl substi-
through axial ligation has been previously found to switch the tution of the ligand led also to completely metal indifferent
nature from ligand-radical like to formal Ni(III). Indeed, com- oxidation potentials for these complexes, while the unsubsti-
putation of [Ni(tBuL)(CH3CN)2]+ indicates localised metal oxi- tuted complexes [M(L)] show a metal-dependence of the oxi-
dation. The second, irreversible oxidation of H2tBuL and its dation potentials Ni < Pt < Pd in keeping with the sequence
complexes leads to a general bleaching of the entire visible from the MLCT energies and the UPS. Thus, the tert-butyl sub-
and near UV spectral range.
stitution lead to a shift of the character of the highest occupied
The first reduction of the protoligands and the complexes molecular orbitals from having a small (L) to almost no (tBuL)
leads to pronounced absorption bands around 500 nm. They metal character. The assignment of the loci for oxidation and
are largely insensitive to the presence of a metal and were thus reduction was further substantiated by UV-vis spectroelectro-
assigned to π*–π* transitions in the TSC pyridine-imine chemistry and the calculation of the frontier orbitals of the
moiety. In much the same way, we observed them in the com- assumed radical cations [M(L)]+ or [M(tBuL)]+ and anions
plexes [Pt(R-TSCmB)Cl].35 Computed spin densities of mono- [M(L)]− or [M(tBuL)]−.
anionic [Pt(tBuL)]− (inset in Fig. 8, bottom) fully support this
interpretation.
In future work we will explore potential applications of
these complexes. As a first idea we can state that the electro-
The second reduction leads to a complete bleach of the chemical data of the three [M(tBuL)] complexes looks promis-
intense visible feature, which is replaced by a blue-shifted ing in view of Negishi type of C–C cross coupling catalysis.53
much weaker band (Fig. 8). We assign also these bands to The potentials are quite similar and the variation of the metals
π*–π* transitions in the TSC pyridine-imine moiety. The long- Ni–Pd–Pt is very interesting concerning differences in their
wavelength feature for the complexes compared with the proto- specific reactivity towards C–C coupling.
ligands is probably due to the planarisation through
coordination.
Experimental section
Materials
Conclusions
All solvents and substances were purchased from commercial
sources and used without further purification if not otherwise
stated. N4-methylbenzyl thiosemicarbazide was prepared as
previously reported.35
We herein present the synthesis of square planar Ni(II), Pd(II)
and Pt(II) complexes [M(L)] and [M(tBuL)] of dianionic tetraden-
tate O^N^N^O coordinating 6-(2-phenolato)picolinaldehyde
thiosemicarbazone ligands. The ligands differ in their substi-
tution pattern of the 3,5-R-phenolate group with R = H or t-Bu.
The complexes are all air stable and can be purified by column
Syntheses
Synthesis of 6-bromo-2-pyridinecarbaldehyde. 13 mL n-BuLi
chromatography. Lacking of single crystals we were able to (1.6 M in hexane, 20.8 mmol) dissolved in 20 mL THF were
study the molecular structures by a combination of DFT calcu- added slowly to solution of 2,6-dibromopyridine
a
5 g
lations and IR and NMR spectroscopy. With the aid of TD-DFT (21 mmol) in 20 mL THF at −78 °C. 30 min later 2.5 mL DMF
the weak long-wavelength absorptions between 500 and (32.5 mmol) were added dropwise. The mixture was left to
600 nm in the UV-vis spectra were assigned to metal-to-ligand warm up to to 0 °C, then stirred for additional 3 h at ambient
(pyridine-imine) charge transfer (MLCT) transition, while the temperature before 30 mL MeOH was added. 40 mL of sat.
strong absorptions in the 300–500 nm range, which are aqueous NaHCO3 solution was added and the product was
responsible for the orange (Pd) and red colour (Ni, Pt) of the extracted with CH2Cl2. The crude product was purified by
materials, are traced to metal-perturbed intraligand(phenol- column chromatography (silica, c-hexane (c-hex): ethyl acetate
ate)-to-ligand(pyridine-imine) charge transfer (ILCT) tran- (EtOAc), v : v = 5 : 1). The product was isolated as a pale yellow
sition. For both ILCT and MLCT band energies the Ni and the solid. Yield: 0.94 g (186.01 g mol−1, 5.05 mmol, 24%). Rf: 0.56
Pt complexes are far more similar and differ largely from the (c-hex : EtOAc, v : v = 5 : 1). 1H NMR (300 MHz, CDCl3) δ = 10.01
Pd complexes. The series of the MLCT energies is clearly Pd ≫ (s, 1H), 7.93 (dt, 1H, J = 6.3, 1.8 Hz), 7.80–7.70 (m, 2H) ppm.
Pt > Ni. Akin to previous findings, we ascribe this to better
Synthesis of 2-bromo-4,6-di-tert-butylphenol. 20.0
g
orbital overlap for Ni and Pt. The work function Wf from UV (0.097 mol) 2,4-di-tert-butylphenol were dissolved in 30 mL
photoelectron spectroscopy (UPS) sets the series Pd ≪ Pt < Ni CHCl3. The solution was cooled to 0 °C and 5.0 mL (15.514 g,
in complete agreement with this idea. From the three possible 0.097 mol) bromine was added dropwise over a course of 1.5 h.
redox centres of the complexes, the divalent metal M(II), the The solution was stirred for 19 h, after which the solvent was
phenolate group and the pyridine-imine only the latter two removed. The crude product was purified via silica gel column
were addressed in the oxidation and reduction processes chromatography using c-hex. The product was isolated as a col-
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ourless solid. The NMR data is in agreement with literature.52 2.7 Hz), 7.94–7.85 (m, 3H), 7.43 (ddd, 1H, J = 8.3, 7.4, 1.9 Hz),
Yield: 26.18 g (285.23 g mol−1, 91.8 mmol, 95%). Rf: 0.67 7.13 (td, 1H, J = 7.5, 1.1 Hz), 7.04 (dd, 1H, J = 8.3, 1.0 Hz), 3.88
(c-hex). 1H NMR (300 MHz, CDCl3): δ = 7.32 (d, 1H, J = 2.3 Hz), (s, 3H) ppm.13C NMR (75 MHz, CDCl3): δ = 194.1, 136.5, 131.1,
7.24 (d, 1H, J = 2.3 Hz), 5.64 (s, 1H), 1.40 (s, 9H), 1.28 (s, 9H) 130.6, 129.4, 121.2, 119.4, 114.5, 113.9, 111.5, 110.7, 60.4 ppm.
ppm.
Synthesis of 2-bromo-4,6-di-tert-butylanisol. 1.44
(5.01 mmol) 2-bromo-4,6-di-tert-butylphenol and 1.40
6-(3,5-di-tert-butyl-2-Methoxyphenyl)picolinaldehyde. Yield:
250 mg (325.2 g mol−1, 0.79 mmol, 100%). Rf = 0.87
(c-hex : EtOAc, v : v = 3 : 1). 1H NMR (300 MHz, CDCl3): δ =
g
g
(10.13 mmol) K2CO3 were dissolved in 25 mL of acetone. 10.19 (s, 1H), 8.03 (dd, 1H, J = 7.0, 2.0 Hz), 7.94–7.86 (m, 2H),
1.25 mL (2.84 g, 20.07 mmol) methyl iodide was added, and 7.55 (d, 1H, J = 2.5 Hz), 7.44 (d, 1H, J = 2.5 Hz), 3.32 (s, 3H),
the reaction mixture was brought to reflux at 65 °C. After stir- 1.45 (s, 9H), 1.36 (s, 9H) ppm. 13C NMR (75 MHz, CDCl3): δ =
ring under reflux for 21 h, the reaction mixture was cooled to 194.1, 159.1, 155.4, 153.1, 137.2, 132.3, 130.7, 129.3, 126.6,
ambient temperature and filtrated. The residue was washed 125.6, 119.7, 61.7, 35.5, 31.7, 31.3 ppm.
with acetone. The filtrate and washing fraction were combined
6-Phenyl-picolinaldehyde. Yield: 420 mg (183.21 g mol−1
,
1
and the solvent was removed under reduced pressure. 20 mL 2.3 mmol, 85%). Rf = 0.52 (c-hex : EtOAc, v : v = 2 : 1). H NMR
of H2O were added and the mixture extracted three times with (300 MHz, CDCl3): δ = 10.17 (s, 1H), 8.14–8.05 (m, 2H),
20 mL of CH2Cl2. The combined organic phases were dried 7.99–7.88 (m, 3H), 7.55–7.40 (m, 3H) ppm. 13C NMR (75 MHz,
over MgSO4 and the solvents were removed under reduced CDCl3): δ = 194.1, 158.1, 152.9, 138.3, 137.9, 129.8, 129.1,
pressure. The crude product was purified via silica gel column 127.1, 124.6, 119.9 ppm.
chromatography (c-hexane), affording the product as a colour-
6-(2-Hydroxyphenyl)picolinaldehyde. Yield: 27 mg (199.21 g
less solid. The NMR data is consistent with that given in litera- mol−1, 0.14 mmol, 5%). Rf = 0.25 (EtOAc). H NMR (300 MHz,
ture.54 Yield: 0.92 g (299.23 g mol−1, 3.07 mmol, 61%). Rf: 0.47 CDCl3): δ = 10.11 (d, 1H, J = 0.7 Hz), 8.17 (dd, 1H, J = 8.2, 1.2
(c-hex). 1H NMR (300 MHz, CDCl3): δ = 7.40 (d, 1H, J = 2.4 Hz), Hz), 8.11–8.00 (m, 1H), 7.91 (dd, 1H, J = 7.5, 1.1 Hz), 7.85 (dd,
7.27 (d, 1H, J = 2.4 Hz), 3.91 (s, 3H), 1.39 (s, 9H), 1.29 (s, 9H) 1H, J = 8.0, 1.7 Hz), 7.39 (ddd, 1H, J = 8.6, 7.2, 1.6 Hz), 7.09
1
ppm.
(dd, 1H, J = 8.3, 1.3 Hz), 6.99 (ddd, 1H, J = 8.3, 7.2, 1.3 Hz)
Synthesis of 3,5-di-tert-butyl-2-methoxyphenyl boronic acid. ppm. 13C NMR (75 MHz, CDCl3): δ = 191.1, 159.7, 158.7, 138.8,
In an inert Schlenk flask, 0.9 g (3 mmol) 2-bromo-4,6-di-tert- 138.6, 132.4, 126.6, 123.7, 119.9, 119.4, 118.9 ppm.
butylanisol were dissolved in 15 mL THF and the solution was
Synthesis of the thiosemicarbazones – general procedure
cooled to −78 °C. 2.3 mL n-BuLi (1.6 M in hexane, 3.68 mmol)
were added. After stirring for 15 min, 1.0 mL (0.815 g, 1 eq. of the corresponding aldehyde (about 2 mmol) were dis-
4.30 mmol) tri(isopropyl)borate were added, and the reaction solved in 3 mL MeOH. To that solution 1 eq. of N4-methyl-
mixture was stirred for another 2 h. After letting the solution benzyl thiosemicarbazide and 0.1 mL HOAc were added. The
warm to ambient temperature, 20 mL 1 M HCl were added mixture was sonicated at 60 °C for 2 h and then cooled to
slowly. The aqueous phase was extracted three times with ambient temperature. The solvent was removed under reduced
30 mL of diethyl ether, the combined organic phases were pressure and the crude product was purified by column
dried over MgSO4, and the solvents were removed under chromatography (silica; c-hex : EtOAc v : v = 2 : 1). The products
reduced pressure. The boronic acid was obtained as a colour- were isolated as yellow solids.
1
less solid. Yield: 0.50 g (264.17 g mol−1, 1.89 mmol, 63%). H
6-(2-Methoxyphenyl)pyridinecarbaldehyde
TSC.
Yield:
NMR (300 MHz, CDCl3): δ = 7.66 (d, 1H, J = 2.6 Hz), 7.47 (d, 320 mg (390.51 g mol−1, 0.82 mmol, 59%). Rf = 0.40
1H, J = 2.4 Hz), 5.74 (s, 2H), 3.79 (s, 3H), 1.41 (s, 9H), 1.32 (s, (c-hex : EtOAc, v : v = 2 : 1). 1H NMR (300 MHz, CDCl3): δ =
9H) ppm. 11B NMR (96 MHz, CDCl3): δ = 33.19, 29.81 ppm.
8.09–7.90 (m, 1H), 7.88–7.72 (m, 3H), 7.48–7.23 (m, 6H),
7.13–6.96 (m, 2H), 5.77 (dt, 1H, J = 8.5, 6.5 Hz), 4.12 (q, 1H, J =
7.1 Hz), 3.86 (d, 3H, J = 9.9 Hz), 1.66 (dd, 3H, J = 10.7, 7.0 Hz).
13C NMR (75 MHz, CDCl3): δ = 143.3, 137.1, 136.1, 133.0,
Syntheses of 6-phenyl substituted picolinaldehydes – general
description
1 eq. 6-bromo-picolinaldehyde, 2 eq. K2CO3, 0.02 eq. [Pd 131.6, 131.1, 131.0, 130.3, 128.8, 128.6, 128.6, 127.5, 127.3,
(PPh3)4] and 1.1 eq. of the corresponding boronic acid were 126.4, 125.9, 125.5, 123.2, 121.6, 118.4, 60.4, 26.9. EI-MS [m/z]
dissolved in 15 mL abs. toluene and 10 mL EtOH and heated = 390 [M]+.
to 90 °C for 20 h. The mixture was cooled to ambient tempera-
6-(3,5-di-tert-butyl-2-Methoxyphenyl)picolinaldehyde TSC.
ture before 30 mL of H2O were added and the mixture was Yield: 240 mg (502.72 g mol−1, 0.48 mmol, 62%). Rf = 0.37
1
stirred for additional 30 min. The aqueous layer was extracted (c-hex : EtOAc, v : v = 5 : 1). H NMR (499 MHz, CDCl3): δ = 9.41
with CH2Cl2. The combined organic phases were dried over (s, 1H), 7.99 (s, 1H), 7.79 (d, 1H, J = 8.6 Hz), 7.78–7.75 (m, 3H),
Na2SO4 and the solvent was removed under reduced pressure. 7.47–7.37 (m, 6H), 7.30 (t, 1H, J = 7.3 Hz), 5.76 (quint, 1H, J =
The crude product was purified by column chromatography 6.9 Hz), 3.31 (s, 3H), 1.69 (s, 3H), 1.43 (s, 9H), 1.34 (s, 9H)
(silica, EtOAc). The products were isolated as yellow oils.
6-(2-Methoxyphenyl)picolinaldehyde. Yield: 940
ppm.
Picolinaldehyde TSC. Yield: 1.37
mg
g (284.38 g ,
mol−1
1
(213.24 g mol−1, 4.41 mmol, 88%). Rf = 0.67 (EtOAc). H NMR 4.82 mmol, >99%). C15H16N4S (284.38 g mol−1). 1H NMR:
(300 MHz, CDCl3): δ = 10.15 (s, 1H, H13), 8.09 (dd, 1H, J = 6.3, (300 MHz, DMSO-d6): δ = 11.77 (s, 1H), 8.76 (d, 1H, J = 8.6 Hz),
4318 | Dalton Trans., 2021, 50, 4311–4322
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8.57 (d, 1H, J = 4.2 Hz), 8.31 (d, 1H, J = 7.9 Hz), 8.76 (s, 1H), The ligand was isolated as a yellow solid. Yield: 40 mg
7.85 (t, 1H, J = 7.3 Hz), 7.32–7.44 (m, 5H), 7.24 (t, 1H, J = 7.1 (C29H36N4OS, 488.69 g mol−1, 0.082 mmol, 20%). Rf: 0.18
Hz), 5.75 (quint, 1H, J = 6.8 Hz), 1.58 (s, 3H) ppm.
(c-hex : EtOAc, v : v = 5 : 1). Anal. calcd (%): C, 71.28; H, 7.43; N,
11.46. Found: C, 71.22; H, 7.41; N, 11.44. H NMR (499 MHz,
1
6-Bromo-picolinaldehyde TSC. Yield: 0.35 g (363.28 g mol−1
,
1
0.97 mmol, 82%). Rf = 0.35 (c-hex : EtOAc, v : v = 2 : 1). H NMR CDCl3): δ = 14.69 (s, 1H), 9.59 (s, 1H), 7.88 (d, 1H, J = 8.4 Hz),
(300 MHz, CDCl3): δ = 13.59 (s, 1H), 9.25 (s, 1H), 7.78 (d, 1H, 7.86 (m, 1H), 7.80–7.77 (m, 2H), 7.61 (d, 1H, J = 2.1 Hz),
J = 1.0 Hz), 7.72–7.64 (m, 1H), 7.58 (t, 1H, J = 7.8 Hz), 7.52–7.46 7.39–7.35 (m, 4H), 7.29–7.26 (m, 2H), 7.20–7.17 (m, 1H), 5.69
(m, 1H), 7.45–7.35 (m, 4H), 7.34–7.27 (m, 1H), 5.83–5.64 (m, 1H), 1.65 (d, 3H, J = 6.9 Hz), 1.41 (s, 9H), 1.29 (s, 9H) ppm.
(quint, 1H, J = 7.2 Hz), 1.68 (d, 3H, J = 6.9 Hz) ppm.
13C NMR (75 MHz, CDCl3): δ = 158.42, 156.83, 138.80, 138.16,
128.68, 127.41, 126.87, 126.33, 120.83, 120.37, 120.21, 53.73,
6-Phenyl-picolinaldehyde TSC. Yield: 0.28 g (360.48 g mol−1
,
1
0.78 mmol, 84%). Rf = 0.42 (c-hex : EtOAc, v : v = 2 : 1). H NMR 31.58, 29.61, 22.33 ppm. HR-ESI-MS [m/z] = 489.26828 ([M +
(300 MHz, CDCl3): δ = 14.88 (s, 1H), 9.05 (s, 1H), 8.01 (d, 2H, H]+, calc. 489.2682590), 511.25050 ([M
J = 8.1 Hz), 7.93 (d, 1H, J = 1.0 Hz), 7.83–7.68 (m, 4H), 511.2502037).
7.56–7.31 (m, 7H), 5.77 (p, 1H, J = 7.6, 7.2, 6.8 Hz), 1.70 (d, 3H,
+
Na]+, calc.
Synthesis of the metal complexes – general procedure
J = 7.0 Hz) ppm.
6-(2-Hydroxyphenyl)picolinaldehyde TSC. Yield: 27.7 mg 5.0 mL of a stock solution of H2L or H2tBuL (c = 0.01 M) in
(376.48 g mol−1, 0.07 mmol, 51%). Rf = 0.29 (c-hex : EtOAc, v : v MeCN was mixed first with Et3N (5 mmol, 1.0 eq.) before
1
= 2 : 1). H NMR (300 MHz, CDCl3): δ = 10.42 (s, 1H), 8.03 (d, aqueous solutions of NiCl2, K2PdCl4 or K2PtCl4 were added.
1H, J = 8.5 Hz), 7.90–7.76 (m, 4H), 7.56–7.48 (m, 2H), 7.42–7.21 After stirring for 12 h at ambient temperatures, the solvent was
(m, 5H), 7.03 (dd, 1H, J = 8.3, 1.3 Hz), 6.91 (ddd, 1H, J = 8.3, removed, and the crude products were purified by column
7.2, 1.3 Hz), 5.83–5.61 (m, 1H), 1.73 (d, 3H, J = 6.9 Hz) ppm. chromatography (silica, EtOAc for [M(L)], c-hex : EtOAc, v : v =
13C NMR (75 MHz, CDCl3): δ = 176.64, 160.63, 157.96, 148.11, 3 : 1 for [M(tBuL)]).
142.67, 138.41, 137.99, 131.97, 128.73, 127.47, 126.54, 126.26,
120.63, 119.34, 118.88, 118.70, 118.17, 53.98, 22.28 ppm. tion. The product was isolated as red solid. Yield: 18.6 mg
EI-MS [m/z] = 376 [M]+. (C21H18N4ONiS, 433.16 g mol−1, 0.43 mmol, 86%). Rf = 0.8
[Ni(L)]. From 2.0 mL of an aqueous NiCl2 (c = 0.01 M) solu-
Deprotection of 6-(2-methoxyphenyl) picolinaldehyde thiose- (EtOAc). Anal. calcd (%): C, 58.23; H, 4.19; N, 12.93. Found: C,
micarbazone (H2L). 330 mg of the 2-methoxy-phenyl thiosemi- 58.31; H, 4.10; N, 12.95.1H NMR (300 MHz, CDCl3): δ = 8.07 (d,
carbazone (0.85 mmol) was dissolved in 5 mL CH2Cl2. 160 µL 1H, J = 8.5 Hz,). 7.81–7.92 (m, 2H), 7.31–7.40 (m, 4H),
(426 mg, 1.7 mmol) BBr3 were added slowly at ambient temp- 7.17–7.31 (m, 3H), 7.15 (m, 1H), 7.02 (dd, 1H, J = 8.5, 1.0 Hz),
erature. The mixture was stirred for 22 h and slowly quenched 6.60–6.75 (m, 1H), 5.04 (m, 1H), 1.56 (d, 3H, J = 6.8 Hz) ppm.
with 20 mL H2O. The aqueous layer was extracted with CH2Cl2 13C NMR (75 MHz, CDCl3): δ = 161.90, 151.51, 142.77, 137.13,
and the combined organic phases were dried over Na2SO4. The 137.29, 131.50, 128.84, 127.63, 127.04, 126.25, 120.76, 118.90,
crude product was purified by column chromatography (silica, 117.84, 115.62, 55.09, 29.84, 22.63 ppm. HR-ESI-MS(+) [m/z] =
c-hex : EtOAc, v : v = 1 : 1) yielding a yellow solid. Yield: 201 mg 433.06305 ([M + H]+, calc. 433.06276), 455.04496 ([M + Na]+,
(C21H20N4OS, 376.48 g mol−1, 0.53 mmol, 62%). Rf = 0.29 calc. 455.04470).
(c-hex : EtOAc, v : v = 2 : 1). Anal. calcd (%): C, 67.00; H, 5.35; N,
[Pd(L)]. From 2.0 mL of an aqueous K2PdCl4 (c = 0.01 M)
1
14.88. Found: C, 67.04; H, 5.38; N, 14.91. H NMR (300 MHz, solution. The product was isolated as orange solid. Yield:
CDCl3): δ = 10.42 (s, 1H), 8.03 (d, 1H, J = 8.5 Hz), 7.90–7.76 (m, 21.6 mg (C21H18N4OPdS, 480.88 g mol−1, 0.45 mmol, 90%).
4H), 7.56–7.48 (m, 2H), 7.42–7.21 (m, 5H), 7.03 (dd, 1H, J = 8.3, Rf = 0.8 (EtOAc). Anal. calcd (%): C, 52.45; H, 3.77; N, 11.65.
1
1.3 Hz), 6.91 (ddd, 1H, J = 8.3, 7.2, 1.3 Hz), 5.83–5.61 (m, 1H), Found: C, 52.44; H, 3.81; N, 11.65. H NMR (300 MHz, CDCl3):
1.73 (d, 3H, J = 6.9 Hz) ppm. 13C NMR (75 MHz, CDCl3): δ = δ = 8.05 (d, 1H, J = 8.6 Hz). 7.72–7.96 (m, 2H), 7.33–7.41 (m,
176.64, 160.63, 157.96, 148.11, 142.67, 138.41, 137.99, 131.97, 4H), 7.22–7.28 (m, 2H), 7.28–7.33 (m, 2H), 7.18 (dd, 1H, J = 8.6,
128.73, 127.47, 126.54, 126.26, 120.63, 119.34, 118.88, 118.70, 1.5, 1.0 Hz), 6.69 (ddd, 1H, J = 8.2, 6.5, 1.5 Hz), 5.07 (m, 1H),
118.17, 53.98, 22.28 ppm. EI-MS [m/z] = 376 [M]+.
1.56 (d, 3H, J = 6.8 Hz) ppm. 13C NMR (75 MHz, CDCl3): δ =
Deprotection of 6-(3,5-di-tert-butyl 2-methoxyphenyl)-picoli- 163.25, 150.91, 143.12, 137.60, 137.80, 132.23, 128.90, 128.66,
naldehyde thiosemicarbazone (H2tBuL). 0.20 g (0.40 mmol) 127.74, 126.32, 124.30, 120.51, 119.13, 115.65, 53.93,
6-(3,5-di-tert-butyl-2-methoxyphenyl)-2-(formyl)pyridine N4-(1- 21.34 ppm. EI-MS(+) [m/z] = 480 [M]+, 465 [M − CH3]+.
methyl-benzyl)thiosemicarba-zone were dissolved in 9 mL
[Pt(L)]. From 5.0 mL of an aqueous K2PtCl4 (c = 0.009 M)
CH2Cl2 in an inert Schlenk flask equipped with a dropping solution. The product was isolated as a red solid. Yield:
funnel. To this solution, 95 μL (1 mmol) BBr3 in 15 mL CH2Cl2 26.3 mg (C21H18N4OPtS 569.55 g mol−1, 0.46 mmol, 92%). Rf =
were added, upon which the solution turned red. After stirring 0.8 (EtOAc). Anal. calcd (%): C, 44.29; H, 3.19; N, 9.84. Found:
1
overnight, 20 mL of H2O were added. The aqueous phase was C, 44.24; H, 3.20; N, 9.86. H NMR (300 MHz, CDCl3): δ = 8.16
extracted three times with 25 mL CH2Cl2 and the organic (d, 1H, J = 8.6 Hz), 7.99 (d, 1H, J = 8.1 Hz), 7.91 (t, 1H, J = 8.0
phases were dried over Na2SO4. The solvent was removed Hz), 7.63 (s, 1H), 7.33–7.41 (m, 4H), 7.25–7.34 (m, 3H), 7.20 (d,
under reduced pressure. The crude product was purified via 1H, J = 7.4 Hz), 5.19 (m, 1H), 6.67–6.85 (m, 1H), 1.58 (d, 3H, J =
silica gel column chromatography (c-hex : EtOAc v : v = 5 : 1). 6.9 Hz). ppm. 13C NMR (75 MHz, CDCl3): δ = 161.62, 147.25,
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143.30, 136.32, 131.58, 128.88, 128.02, 127.70, 126.32, 124.49, 2D NMR experiments were performed using standard pulse
122.51, 121.76, 119.08, 116.37, 58.81, 22.82 ppm. EI-MS(+) [m/ sequences from the Bruker pulse program library. Chemical
z] = 569 [M]+.
shifts were relative to TMS (1H, 13C), H2PtCl6 (195Pt), and BF3-
[Ni(tBuL)]. From 2.0 mL of an aqueous NiCl2·6H2O (c = 0.01 OEt2 (11B). UV-vis absorption spectra were measured using a
M) solution. The product was isolated as dark red solid. Yield: Varian 50 Scan UV-visible photometer. Electrochemical experi-
11.0 mg (C29H34N4ONiS, 545.37 g mol−1, 0.020 mmol, 100%). ments were carried out in 0.1 M n-Bu4NPF6 solutions using a
Rf = 0.86 (EtOAc). Anal. calcd (%): C, 63.87; H, 6.28; N, 10.27. three-electrode configuration (glassy carbon working electrode,
1
Found: C, 63.84; H, 6.19; N, 10.24. H NMR (499 MHz, CDCl3): Pt counter electrode, Ag/AgCl pseudo reference) and
a
δ = 8.17 (d, 1H, J = 8.6 Hz), 7.89 (dd, 1H, J = 8.5, 7.5 Hz), 7.75 BioLogic SP-150 or
a
Metrohm µStat400 potentiostat.
(d, 1H, J = 2.2 Hz), 7.43–7.32 (m, 5H), 7.31–7.26 (m, 1H), 7.19 Experiments were run at a scan rate of 100 mV s−1, at ambient
(d, 1H, J = 7.2 Hz), 5.09 (m, 1H), 1.56 (d, 3H, J = 5.6 Hz), 1.34 temperature. The ferrocene/ferrocenium couple served as
(s, 9H), 1.30 (s, 9H). 1H NMR (300 MHz, DMSO-d6): δ = 8.52 (d, internal reference. The number of electrons per wave was
1H, J = 7.3 Hz), 8.46 (d, 1H, J = 8.8 Hz), 8.14 (dd, 1H, J = 8.6, gauged using weighted samples of ferrocene. UV-vis spectro-
7.4 Hz), 7.86 (d, 1H, J = 2.3 Hz), 7.75 (s, 1H), 7.41–7.29 (m, 5H), electrochemical measurements were performed with an
7.26–7.25 (m, 1H), 7.23 (d, 1H, J = 2.4 Hz), 4.96 (m, 1H), 1.41 optical transparent thin-layer electrochemical (OTTLE)
(d, 3H, J = 6.2 Hz), 1.30 (s, 9H), 1.21 (s, 9H) ppm. 13C NMR cell.55,56 EI(+)-Mass spectra were recorded on a Thermo Quest
(75 MHz, CDCl3): δ = 152.51, 136.58, 136.55, 128.83, 127.61, Finnigan MAT 95 at 70 eV.
126.84, 126.20, 121.60, 120.80, 118.17, 117.11, 49.33, 31.68,
For ultraviolet photoelectron spectroscopy (UPS) and
inverse photoelectron spectroscopy (IPES) measurements thin
29.69, 22.50 ppm. EI-MS(+) [m/z] = 544 [M]+, 529 [M − CH3]+.
[Pd(tBuL)]. From 2.0 mL of an aqueous K2PdCl4 (c = 0.01 M) films were prepared on ITO-coated glass substrates (15 Ω per
solution. The product was isolated as red solid. Yield: 10.0 mg sq sheat resistance), which were cleaned with CHCl3, acetone,
(C29H34N4OPdS, 593.10 g mol−1, 0.017 mmol, 84%). Rf = 0.87 a 2% Mucasol soap solution and deionised H2O before use.
(EtOAc). Anal. calcd (%): C, 58.73; H, 5.78; N, 9.45. Found: C, Thin films were prepared out of CH2Cl2 solutions via spin-
1
58.74; H, 5.77; N, 9.41. H NMR (499 MHz, CDCl3): δ = 8.13 (d, coating at a rate of 4000 rpm for 35 s, to yield layers with thick-
1H, J = 8.7 Hz), 7.87 (dd, 1H, J = 8.7, 7.4 Hz), 7.71 (d, 1H, J = nesses of about 10 nm. Solid state UV-vis absorption spectra
2.5 Hz), 7.44 (d, 1H, J = 2.5 Hz), 7.39–7.31 (m, 4H), 7.30–7.27 were recorded using a Varian Cary 50 Bio spectrometer.
(m, 1H), 7.25 (d, 1H, J = 7.5 Hz), 5.14 (m, 1H), 1.57 (d, 3H, J = Photoelectron spectroscopy measurements were performed in
6.8 Hz), 1.40 (s, 9H), 1.35 (s, 9H) ppm. 1H NMR (300 MHz, an ultra-high vacuum chamber using a Phoibos 100 hemi-
DMSO-d6): δ = 8.62 (d, 1H, J = 8.0 Hz), 8.44 (d, 1H, J = 8.7 Hz), spherical analyser by Specs. For ultraviolet photoelectron spec-
8.17 (t, 1H, J = 8.0 Hz), 7.92 (s, 1H), 7.82 (d, 1H, J = 2.3 Hz), troscopy (UPS) a monochromatic helium discharge lamp (He I
7.69 (d, 1H, J = 7.5 Hz), 7.44–7.33 (m, 3H), 7.32 (d, 1H, J = 2.3 at 21.22 eV) with a probing depth of around 2 nm at a pass
Hz), 7.25 (m, 1H), 5.03 (m, 1H), 1.45 (d, 3H, J = 6.3 Hz), 1.31 (s, energy of 2 eV and a resolution of 110 meV was used.
9H), 1.24 (s, 9H) ppm. 13C NMR (75 MHz, CDCl3): δ = 150.09,
136.98, 128.71, 127.40, 126.50, 122.50, 114.47, 47.03, 31.52,
Computational details (DFT)
29.69, 22.41 ppm. EI-MS(+) [m/z] = 592 [M]+, 577 [M − CH3]+.
[Pt(tBuL)]. From 5.0 mL of an aqueous K2PtCl4 (c = 0.009 M) Electronic structure calculations on the complexes have been
solution. The product was isolated as a red solid. Yield: 6.7 mg performed through density-functional theory (DFT) methods
(C29H34N4OPtS, 681.76 g mol−1, 0.0098 mmol, 49%). Rf = 0.92 using the ORCA program package.57 For all optimisations
(EtOAc). Anal. calcd (%): C, 51.09; H, 5.03; N, 8.22. Found: C, triple-ξ-valence TZVP58 basis sets were used with the general-
1
51.11; H, 5.09; N, 8.22. H NMR (300 MHz, CDCl3): δ = 8.33 (d, ised gradient approximated functional BP86.59 Harmonic fre-
1H, J = 8.6 Hz, JPt,H = 21.0 Hz), 8.01 (td, 1H, J = 7.5, 1.0 Hz), quencies were computed along the series [ML] (a computed IR
7.92 (d, 1H, J = 2.4 Hz), 7.84 (s, 1H), 7.52 (d, 1H, J = 2.4 Hz), spectrum of [NiL] is shown in Fig. 2); the absence of imaginary
7.43–7.32 (m, 4H), 7–32–7.27 (m, 2H), 5.28 (m, 1H), 1.60 (d, modes proved the optimised geometries to be stationary
3H, J = 6.8 Hz), 1.44 (s, 9H), 1.37 (s, 9H) ppm. 13C NMR points. Pertinent metrical data is arranged in Tables S2 and
(75 MHz, CDCl3): δ = 157.03, 136.01, 128.98, 127.67, 126.03, S3, ESI.† Molecular orbitals and electronic properties were
126.24, 119.81, 51.82, 31.71, 29.77, 18.22 ppm. EI-MS(+) [m/z] = extracted from single-point calculations in the optimised posi-
681 [M]+, 666 [M − CH3]+.
tions with the global hybrid functional TPSSh60 and triple-ξ-
valence TZVP basis sets. Grimme’s third generation D3 correc-
tion of dispersion was used;61,62 medium effects were approxi-
Instrumentation
NMR spectra were recorded on a Bruker Avance II 300 MHz mated in a dielectric continuum approach (COSMO), parame-
1
spectrometer, using a triple resonance H, nBB inverse probe terised for THF.63 Coordinates of the computed structures are
head or a Bruker Avance III spectrometer at 499 MHz with a assembled in the ESI† file COORDINATES, frontier orbital
TCI prodigy 5 mm probe head with z-gradient (1H/19F 13C 15N landscapes are shown in Fig. 6 and Fig. S25–S29 (ESI).† For
2H). The unambiguous assignment of the 1H and 13C reso- each complex the 80 lowest optical electronic transitions were
nances was obtained from 1H NOESY, 1H COSY, gradient assessed with ORCA implemented TD-DFT methods within the
selected 1H, 13C HSQC and 1H, 195Pt HMBC experiments. All Tamm-Dancoff approximation.
4320 | Dalton Trans., 2021, 50, 4311–4322
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14 J. C. Lima, R. D. Nascimento, L. M. Vilarinho, A. P. Borges,
L. H. F. Silva, J. R. Souza, L. R. Dinelli, V. M. Deflon,
A. E. da Hora Machado, A. L. Bogado and P. I. S. Maia,
J. Mol. Struct., 2020, 1199, 126997, 1–10.
Conflicts of interest
There are no conflicts to declare.
15 C. González-García, A. Mata, F. Zani, M. A. Mendiola and
E. López-Torres, J. Inorg. Biochem., 2016, 163, 118–130.
16 T. B. Y. Nguyen, C. T. Pham, T. N. Trieu, U. Abram and
H. H. Nguyen, Polyhedron, 2015, 96, 66–70.
Acknowledgements
Prof. Dr Mathias Schäfer and Michael Neihs, Department
of Chemistry, University of Cologne are acknowledged 17 M.-X. Li, M. Yang, J.-Y. Niu, L.-Z. Zhang and S.-Q. Xie,
for the HR-ESI-MS measurements. The Deutsche Inorg. Chem., 2012, 51, 12521–12526.
Forschungsgemeinschaft DFG KL1194/15-1 and KL1194/16-1 18 N. C. Kasuga, K. Sekino, M. Ishikawa, A. Honda,
[DFG Priority Programme 2102 “Light-controlled Reactivity of
Metal Complexes”] is acknowledged for funding of this
M. Yokoyama, S. Nakano, N. Shimada, C. Koumo and
K. Nomiya, J. Inorg. Biochem., 2003, 96, 298–310.
project. G.H. thanks DFG for financial support within SFB 840 19 K. Nomiya, K. Sekino, M. Ishikawa, A. Honda, M. Yokoyama,
(TP A10 and A2). Prof. Dr Bernd Neumaier, Department of
Chemistry, Institute for Nuclear Chemistry, Cologne University
N. C. Kasuga, H. Yokoyama, S. Nakano and K. Onodera,
J. Inorg. Biochem., 2004, 98, 601–615.
Clinics, Institute of Radiochemistry, and Research Centre 20 M. R. Bermejo, R. Pedrido, M. I. Fernandez,
Jülich for financial support and consulting. We also like to
thank the Regional Computing Centre of the University of
Cologne (RRZK) for providing computing time on the DFG-
A. M. González-Noya, M. Maneiro, M. J. Rodríguez,
M. J. Romero and M. Vázquez, Inorg. Chem. Commun.,
2004, 7, 4–8.
funded High Performance Computing (HPC) system CHEOPS 21 B. M. Paterson, J. M. White and P. S. Donnelly, Dalton
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