Oxidative addition chemistry of tetrathiocines: Synthesis, structures and properties of group 10 dithiolate complexes

Article abstract of DOI:10.1039/c3dt52802b

Oxidative addition of the electron-rich tetra-methoxy-dibenzo-1,2,5,6- tetrathiocine [(MeO)₂C₆H₂S₂] ₂ to zero-valent group 10 transition metal complexes in the presence of diphenylphosphinoethane (dppe) affords the corresponding dithiolate complexes, [(DMOBD)M(dppe)] (DMOBD = dimethoxybenzenedithiolato, (MeO) ₂C₆H₂S₂^2-; M = Ni, Pd, Pt) in high yield which were characterized by single crystal X-ray diffraction. Whereas the Pd and Pt complexes exhibit two quasi-reversible 1e^- oxidation processes, the nickel species undergoes a quasi-reversible 1e ^- reduction.

Full text of DOI:10.1039/c3dt52802b

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Oxidative addition chemistry of tetrathiocines:
Synthesis, structures and properties of Group
10 dithiolate complexes.
Cite this: DOI: 10.1039/x0xx00000x
Justin D. Wrixon, John J. Hayward, Osman Raza and Jeremy M. Rawsona*
Received 00th January 2012,
Accepted 00th January 2012
Oxidative addition of the electron-rich tetra-methoxy-dibenzo-1,2,5,6-tetrathiocine [(MeO)2C6H2S2]2 to
zero-valent group 10 transition metal complexes in the presence of diphenylphosphinoethane (dppe)
DOI: 10.1039/x0xx00000x
www.rsc.org/
affords
the
corresponding
dithiolate
complexes,
[(DMOBD)M(dppe)]
(DMOBD
=
dimethoxybenzenedithiolato, (MeO)2C6H2S22-; M = Ni, Pd, Pt) in high yield which were characterized by
single crystal X-ray diffraction. Whereas the Pd and Pt complexes exhibit two quasi-reversible 1e-
oxidation processes, the nickel species undergoes a quasi-reversible 1e– reduction.
Introduction
a a potentially versatile
oneꢀpot reaction,15 makes this
methodology to prepare transition metal dithiolate complexes.
Here we report the oxidative addition of the tetramethoxyꢀ
dibenzoꢀ1,2,5,6ꢀtetrathiocine (T, Scheme 1) to zeroꢀvalent
group 10 metals in the presence of the chelate dppe ligand
(dppe = bisꢀdiphenylphosphinoethane), affording the dithiolate
Transition metal dithiolene chemistry has continued to be an
active field of study since it first originated in the early 1960s.1
Recent studies have focused on their potential applications in
the areas of nonꢀlinear optics,2 dyes,3 catalysis4 and as
conducting5 and magnetic materials.6 In particular the potential
to tailor the electronic properties of the dithiolene ligand
through modification of the substituents is central to future
applications.1 Such dithiolene complexes are typically prepared
complexes
1 – 3 (Scheme 1) as crystalline solids in high
isolated yields (>75%) in just two steps from readily available
starting materials.
from ligand exchange reactions of
sꢀblock metal dithiolates
with ꢀblock metal salts or via condensation of the free thiol
d
with transition metal oxo, alkoxo, and amido precursors.1
Alternative strategies include reaction of metal sulfides with
alkynes7 and the oxidative addition of 1,2ꢀdithietes (RC)2S2 and
dithiins to lowꢀvalent metals.8
Despite the potential for
πꢀdelocalisation and conjugation,
few benzoꢀfused derivatives have been reported and the
chemistry of benzoꢀfusedꢀ1,2ꢀdithiolato complexes focuses
extensively on derivatives of commercially available 1,2ꢀ
benzenedithiol and tolueneꢀ3,4ꢀdithiol,9 though a small number
of other derivatives have been crystallographically
characterised.10ꢀ13 Our recent studies on the chemistry of
tetrathiocines14 prompted us to examine the oxidative addition
chemistry of such tetrathiocines to low oxidation state transition
metals as an alternative strategy to synthesise transition metal
dithiolate complexes. In particular, ready access to a range of
alkoxyꢀfunctionalised tetrathiocines in multiꢀgram quantities in
Scheme 1. Two-step synthesis of group 10 dithiolenes.
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Table 1. Crystallographic details for [(DMOBD)Ni(dppe)] (1), [(DMOBD)Pd(dppe)] (2), and [(DMOBD)Pt(dppe)] (3).
Compound
Chemical formula
Formula weight
Crystal System
Space group
a (Å)
1
C34H32O2P2S2Ni
657.37
Orthorhombic
P212121
11.9151(12)
12.7964(11)
20.0546(19)
90.00
2
C34H32O2P2S2Pd∙CH2Cl2
789.98
3
C34H32O2P2S2Pt∙CH2Cl2
878.67
Tetragonal
I-42d
20.8660(21)
20.8660(21)
15.5435(16)
90.00
Tetragonal
I-42d
20.864(6)
20.864(6)
15.544(5)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
90.00
90.00
90.00
γ (deg)
90.00
3057.7(5)
4
1.428
0.907
90.00
6767.5(15)
8
1.551
0.956
90.00
6766(3)
V (Å3)
Z
8
Dcalcd (g cmꢀ3)
ꢁ (mmꢀ1)
1.725
4.556
2.55 to 30.03
1.95 to 24.99
1.38 to 28.46
θ range (deg)
Reflns collected
Unique reflns
Data/restraints/parameters
R1, wR2 (I > 2σ(I))
Residual electron density (e-/Å3)
20051
8721
8721/0/373
0.0426, 0.0881
+0.527/ꢀ0.346
32531
4126
4126/6/205
0.0289, 0.0666
+0.596/ꢀ0.538
39594
4142
4142/0/205
0.0172, 0.0407
+0.647/ꢀ0.487
Synthesis of 1 – 3 (Scheme 1)
Experimental Section
General Considerations
Synthesis of [(DMOBD)Ni(dppe)], 1. Ni(COD)2 (0.150 g,
0.545 mmol), dppe (0.218 g, 0.545 mmol) and tetrathiocine
0.109 g, 0.273 mmol) were combined in an oven dried 5 mL
.
T
Ni(COD)2 was obtained commercially from Sigma Aldrich
and used as received. Pd2dba3 and Pt(dppe)2 were prepared
according to the literature methods16,17 using starting materials
acquired from Strem. The bisꢀ(dimethoxybenzo)ꢀtetrathiocine
(
microwave vial in the glove box. Dry toluene (5 mL) was added
and the suspension was heated in the microwave for 40 min at
150 °C. The resultant dark brown solid was isolated from a pale
(T
) was prepared according to the literature method.15 Standard
yellow solution by filtration. The precipitate was washed with
hexanes and dried in air (0.320 g, 89% yield). The solid was
recrystallized from a saturated CH2Cl2 solution layered with
hexanes to produce dark purple needleꢀshaped crystals suitable
solvents were dried and deoxygenated using an Innovative
Technology Solvent Purification System and manipulation of
airꢀsensitive materials carried out under an atmosphere of dry
nitrogen using standard Schlenk techniques and a dryꢀnitrogen
glove box (Braun Labmaster). Microwave syntheses were
carried out in sealed vessels using a Biotage Initiator 1
microwave.
for Xꢀray diffraction. NMR: δH = 7.83 (8H, 7.86ꢀ7.80, m,
7.47 (12H, 7.54ꢀ7.43, m, o,pꢀH ), 6.95 (2H, s, benzo CꢀH), 3.74
(6H, s, CH3), 2.36 (4H, d, = 17.4 Hz, PCH2); δP = 59.83.
mꢀH),
J
HRMS (ESIꢀTOF) m/z: [M+H]+ calcd for C34H33O2P2S2Ni+
657.0745; found 657.0723. Elemental Analysis calcd for
C34H32O2P2S2Ni: C 62.12; H 4.91%; found: C 61.59; H 4.71%.
IR νmax (cmꢀ1): 3052(w), 2997(w), 2929(w), 2829(w), 1584(w),
1483(s), 1469(s), 1434(vs), 1343(m), 1244(vs), 1199(s),
1174(m), 1100(s), 1043(s), 815(m), 782(m), 746(m), 690(vs),
531(vs), 482(m).
Physical Measurements.
NMR spectra were recorded on
UltraShield 300MHz spectrometer with a Broadband AX Probe
using CDCl3 (1H
= 7.26 ppm, s) as an internal reference point
relative to Me4Si (
= 0 ppm). 31P NMR spectra were
referenced to 85% H3PO4 ( = 0 ppm). IR spectra were obtained
a Bruker DPX300
δ
δ
δ
Synthesis of [(DMOBD)Pd(dppe)], 2. Pd2dba3 (0.100 g, 0.109
using a Bruker Alpha FTꢀIR spectrometer equipped with a
Platinum single reflection diamond ATR module. Elemental
compositions were determined on a PerkinElmer 2400 Series II
Elemental Analyzer. Mass spectra were recorded on a Waters
Micromass LCT Classic Electrospray Ionization Time of Flight
(ESIꢀTOF) mass spectrometer operated in positive mode.
mmol),dppe (0.087 g, 0.218 mmol) and tetrathiocine
T (0.044
g, 0.109 mmol) were combined in an oven dried 5 mL
microwave vial in the glove box. Dry toluene (5 mL) was added
and the suspension heated in the microwave for 30 min at 150
°
C. The resultant bright pink solid was isolated from the clearꢀ
yellow solution by filtration. The precipitate was washed with a
small amount of hexanes and dried in air (0.131 g, 85% yield).
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The solid was recrystallized from a saturated CH2Cl2 solution (0.094(10)) permitted the correct absolute structure to be
layered with toluene and hexanes to produce redꢀpurple needles determined (as
suitable for Xꢀray diffraction. NMR: δH = 7.83 (8H, 7.86ꢀ7.79, study).
S for the crystal selected for the crystallographic
m,
CꢀH), 3.75 (6H, s, CH3), 2.50 (4H, d,
51.94. HRMS (ESIꢀTOF) m/z
m
ꢀH), 7.46 (12H, 7.51ꢀ7.44, m,
o
,
J
p
ꢀH), 6.91 (2H, s, benzo
= 20.7 Hz, PCH2); δP =
[M+H]+ calcd for
:
C34H33O2P2S2Pd+ 705.0427; found 705.0462. Elemental
Analysis calcd for C34H32O2P2S2PdꢂCH2Cl2: C 53.20; H 4.87%;
found: C 53.26; H 4.39%. IR νmax (cmꢀ1): 3358(w), 3050(w),
2922(m), 2850(w), 2829(w), 1483(s), 1470(s), 1434(vs),
1339(m), 1240(vs), 1198(s), 1174(m), 1100(s), 1039(s),
846(m), 744(m), 703(s), 690(vs), 524(vs), 486(m).
Synthesis of [(DMOBD)Pt(dppe)], 3. Pt(dppe)2 (0.100 g,
0.101 mmol) and tetrathiocine
T (0.020 g, 0.050 mmol) were
combined in an oven dried 5 mL microwave vial in the glove
box. Dry toluene (5 mL) was added to the vial and the
suspension heated in the microwave for 30 min at 150 °C. The
resultant bright yellow microcrystalline solid was isolated from
a clearꢀyellow solution by gravity filtration. The precipitate was
washed with a small amount of hexanes dried in air (0.062 g,
77% yield). The solid was recrystallized from a saturated
CH2Cl2 solution layered with hexanes to produce bright yellow
plateꢀlike crystals suitable for Xꢀray diffraction. NMR: δH
=
7.84 (8H, 7.87ꢀ7.81, m, ꢀH), 7.45 (12H, 7.46ꢀ7.44, m, o,pꢀH) ,
m
7.07 (2H, s, benzo CꢀH), 3.76 (6H, s, CH3), 2.47 (4H, 2.54ꢀ
2.39, m, PCH2); δP = 45.90 (1JPtꢀP = 3427 Hz). HRMS (ESIꢀ
TOF) m/z: [M+H]+ calcd for C34H33O2P2S2Pt+ 794.1039; found
794.1037.
Elemental
Analysis
calcd
for
C34H32O2P2S2Pt
∙CH2Cl2: C 47.84, H 3.90%; found: C 47.53, H
3.66%. IR νmax (cmꢀ1): 3053(w), 2988(w), 2900(w), 2830(w),
1485(s), 1435(vs), 1341(m), 1241(vs), 1200(m), 1103(s),
1042(m), 748(m), 691(vs), 531(vs), 481(m).
X-Ray Crystallography
Crystals were mounted on a cryoloop with paratone oil and
examined on
a
Bruker SMART or Bruker APEXꢀII
diffractometer equipped with a CCD area detector and an
Oxford Cryostream cooler. Data were measured at 150(2) K
using graphiteꢀmonochromated MoꢀKα radiation (λ = 0.71073
Å) using the APEXꢀII software.18 Final cell constants were
determined from full least squares refinement of all observed
reflections. The data were collected for absorption (sadabs)19
and the structures solved by direct methods to reveal most nonꢀ
H atoms. Remaining heavy atom positions were located in
subsequence difference maps and the structure refined with full
least squares refinement on F2 within the SHELXTL suite.20
Hydrogen atoms were placed at calculated positions and refined
isotropically with a riding model. In the case of
2 and 3 both the
complex and the CH2Cl2 solvate lay about a crystallographic 2ꢀ
fold axis with the CH2Cl2 exhibiting some evidence for disorder
which was modeled over two sites. In addition all three
complexes crystallized in acentric space groups. In all cases
Figure 1. Crystal structure and selected atom labelling of complexes
Hydrogen atoms (and solvate molecules for and ) have been omitted for
clarity. Thermal ellipsoids are drawn at the 75% probability level.
1 – 3.
2
3
there was some degree of merohedral twinning. In the case of
and the twin was ca. 50:50 making assignment of the absolute
structure meaningless. However, in the case of , the crystal
2
3
1
chosen exhibited a 90:10 twin and the Flack parameter
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1
In all cases the twinning was treated with the appropriate TWIN revealed singlet spectra (with satellites from JPtꢀP coupling in
law and BASF parameter to refine the twin components. the case of
Structure solution, refinement and preparation of final cif files NMR chemical shifts comparable with other squareꢀplanar
were undertaken using the SHELXTL
package. group 10 MII complexes with MP2S2 ligand sets.27,28
3
) consistent with a unique 31P environment and 31P
Crystallographic data are summarized in Table 1. CCDC
965075 – 965077.
Table 2. Structural and electrochemical data
1
2
3
Electrochemistry
2.1619(7)
2.1661(7)
2.1490(7)
2.1613(7)
86.64(3)
92.88(3)
ꢀ1.61
MꢀP bond length (Å)
MꢀS bond length (Å)
2.2739(6)
2.2552(7)
Cyclic voltammetry measurements were made on solutions of
1
2.2977(6)
2.3042(7)
–
3
in CH2Cl2 using 0.1 M [nBu4N][PF6] as the supporting
PꢀMꢀP bond angle (o)
SꢀMꢀS bond angle (o)
E1/2 (V)red
84.98(3)
88.85(3)
ꢀꢀꢀ n
+0.51,
+1.35
85.31(4)
88.46(4)
ꢀꢀꢀ n
+0.58,
+1.36
electrolyte (electrochemical grade, Fluka) using a BAS 100B
Electrochemical Analyzer with BAS 100W software using a
sweep rate of 100 mVsꢀ1 with glassy carbon working electrode,
Pt wire auxiliary electrode and a Ag/AgCl reference electrode
against which the Fc/Fc+ couple appeared at +0.73 V.
E1/2 (V)ox
+0.73i
0.37
0.33,0.29
0.29,0.26
∆Epp (V)
Computational studies
Single point DFT calculations were undertaken on the
1
structures of
1 – 3 determined by Xꢀray diffraction using the
LACVP3* basis set and both B3LYP and BP86 functionals21
within Jaguar.22 The LACVP3* basis set uses an effective core
potential for the metal atom and provides triple splitꢀvalence
zeta quality (6ꢀ311+G*) for all atoms with an additional
polarization function.23 Such basis sets have been previously
employed to study a range of organometallic/coordination
chemistry complexes including Pd and Ru phosphines.24
Changes in the functional did not lead to significant changes to
the energies of the HOMOs and LUMOs or indeed the nature of
the frontier orbitals.
2
Results and Discussion
Ligand exchange of dithiolate anions with simple metal salts
has proved to be the most common route to dithiolate
complexes.1 However oxidative addition of dithietes and
disulfides to lower oxidation state metals has proved an
alternative if not extensively used synthetic strategy.8,25 For
example, recent studies by Corrigan and coꢀworkers revealed
oxidative addition of cyclic disulfides to Pt(PPh3)4 occurred in
moderate isolated yield (ca. 50%) after stirring at ambient
temperature for 20 h.25 In our current studies we have
investigated tetrathiocines as a readily accessible alternative to
dithiete and disulfides.15 However the low solubility of the
tetrathiocines in a range of organic solvents led to very slow
reactivity with zeroꢀvalent group 10 transition metals in the
presence of dppe under ambient conditions and,
disappointingly, yielded a mixture of products (31P NMR) upon
heating. Conversely microwave irradiation has been shown to
afford higher selectivity and fast reaction times in many
3
transformations26 and we found that reaction of tetrathiocine
T
with the zeroꢀvalent group 10 metal complexes Ni(COD)2 or
Pd2(dba)3 in the presence of 1 equivalent of dppe per metal
o
centre or with Pt(dppe)2 in toluene at 150 C under microwave
Figure 2. CV scans of complexes
electrolyte in CH2Cl2 (scan rate 100 mV/s)
1 – 3
using 0.1 M [nBu4N][PF6] supporting
irradiation for 30ꢀ40 min afforded 1, 2 and 3 respectively in
very good (77 – 89%) recovered yields (Scheme 1). The 31P
NMR spectra of the resultant solids dissolved in CDCl3
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Complexes
1 – 3
all revealed [M+H]+ ions by positive ion oxidation of ferrocene (+0.73 V). These are marginally easier
ESIꢀTOF mass spectroscopy and were further characterized by oxidations than those observed for [(Ph2C2S2)Pd(dppe)] and
Xꢀray diffraction and compositions confirmed by elemental [(Ph2C2S2)Pt(dppe)] (+0.43 and +0.45 V respectively).30 Again
analysis.
the peakꢀtoꢀpeak separation indicated electrochemically
generated oxidation products with a short lifetime. Attempts to
X-Ray Crystallographic studies.
selectively oxidize both
2 and 3 in the presence of the mild
In all cases crystals of 1, 2 and 3 were grown by slow oxidant I2 or the stronger oxidant [NO][BF4] also failed to
diffusion of hexane into a concentrated CH2Cl2 solution and generate any longꢀlived EPRꢀactive species.
their structures determined by Xꢀray diffraction (Figure 1).
Computational Studies.
Whilst
without any solvent molecules,
(tetragonal I-42d), crystallizing as CH2Cl2 solvates with half a processes observed for
molecule in the asymmetric unit. The crystal structures of point calculations on 1 – 3
1
crystallises in the orthorhombic space group P212121
2
and are isomorphous
3
In order to evaluate the differing nature of the redox
in relation to and , a series of single
were undertaken using hybrid
1
2
3
1
–
3
each reveal squareꢀplanar coordination geometries at the group density functional theory methods using both B3LYP and BP86
10 metal with unexceptional bond lengths (Figure 1, Table 2). functionals and tripleꢀzeta quality basis set with additional
In particular the lengths of the MꢀS, CꢀS and aryl CꢀC bonds polarisation (LACV3P*) which implements an effective core
coupled with the squareꢀplanar geometries at the metal centres potential for the dꢀblock metal. No significant change to the
confirms that the best representation for these complexes is as relative energies or characteristics of the frontier orbitals were
an M2+ꢀdithiolate complex as opposed to the alternative observed with changes to the functional employed.
representation as a M0ꢀdithioketone.
In all three cases the LUMO is predominantly based around the
metalꢀphosphine moiety Both the nickel and palladium LUMOs
Electrochemical Studies.
involve substantial metalꢀsulfur
contribution, whereas the Pt complex the LUMO
in order to compare and contrast the effects of comprises predominantly dppe * character. Conversely the
changing the metal centre on the redox properties of the HOMOs of the series are all highly dithiolate ligandꢀbased, as
complexes and to probe the nature of the frontier orbitals in shown for (Figure 3b) with a small amount of electron density
conjunction with DFT studies. Cyclic voltammagrams for all on the metal comprising a dxz dyz admixture. Thus reduction of
three complexes are illustrated in Figure 2, and the resulting is likely to be a metalꢀbased reduction and lead to weakening
and are best
Epp) are summarized in considered to be ligandꢀbased redox processes, consistent with
σ* character (Figure 3a) with a
Cyclic voltammetry (CV) studies were carried out on metal dx –y
2 2
complexes
1
ꢀ
3
π
2
–
1
data in the form of halfꢀwave potentials (E1/2) relative to of the NiꢀS bonds whereas oxidation of
2
3
Ag/Ag+, and peakꢀtoꢀpeak potentials (
Table 2.
ꢀ
−
conversion of the dithiolate DMOBDT2ꢀ to the DMOBDT•
Previous studies on nickel dithiolate complexes has shown that radical anion, reflecting the nonꢀinnocent nature of this ligand.
ox
the redox chemistry is sensitive to the dithiolate ligand.28a For The similarity in E for both
2
and
3 is also consistent with a
½
example [Ni(mnt)(dppe)] [mnt
= maleonitrile dithiolato, predominantly ligand based oxidation process.
(NC)2C2S2)2ꢀ] has an E1/2 value of ꢀ1.20 V and a peakꢀtoꢀpeak
potential of ca. 0.06 V, consistent with the 59 mV peakꢀtoꢀpeak Conclusions
separation expected for a 1eꢀ process and the [Ni(mnt)(dppe)]ꢀ
We have shown that, under microwave conditions, oxidative
addition of tetrathiocines to zeroꢀvalent group 10 metals occurs
in very good yields. This is a potentially versatile route to a
anion was detected by EPR spectroscopy. Conversely
[Ni(tdt)(dppe)] [tdt = tolueneꢀ3,4ꢀdithiolato, MeC6H3S22ꢀ] was
reported to have a E1/2 of ꢀ1.50 V with a much larger peakꢀtoꢀ
peak potential which was scanꢀrate dependent (130 mV at 100
mV/s and 200 mV at 200 mV/s). The latter was assigned to
electron transfer being a rateꢀlimiting step in the redox process
with the electroꢀgenerated product insufficiently stable to be
detected by EPR, and undergoing rapid disproportionation at a
rate comparable with the timescale of the electrochemical
broad crossꢀsection of
dꢀblock metal complexes, permitting
tuning of phosphine, transition metal and tetrathiocine. Further
systematic studies are underway to probe the effects of the
tetrathiocine and coꢀligand employed on such substitutions and
will be the subject of future reports.
Acknowledgements
experiment.29 Thus, whilst
1 was found to exhibit an apparently
We thank NSERC and the Canada Research Chair (J.M.R.)
program for funding as well as CFI and ORF and the University
of Windsor for instrumentation support. In addition we thank
Prof. M. Pilkington (Brock University) for diffractometer time
reversible oneꢀelectron reduction, the peakꢀtoꢀpeak potential
(0.37 V) was consistent with a shortꢀlived species, analogous to
that generated from [Ni(tdt)(dppe)]. Attempts to identify a
paramagnetic species by in situ EPR spectroscopy using either
Cp2Co or Mg as reducing agents also proved unsuccessful,
supporting the transient nature of the electrochemically
generated species.
to measure complex
1. We would also like to thank Dr. J. Auld
(University of Windsor) for elemental analysis and mass
spectroscopy measurements of complexes
University of Windsor for a Scholarship.
1ꢀ3. O.R. thanks the
Both complexes
2 and 3 exhibited two quasiꢀreversible oneꢀ
electron oxidations, the first of which appeared close to the
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Notes and references
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