Solution, structural and catalytic studies of neutral MCl2 (M = Pd, Pt) complexes of the N/E mixed-donor ligands 2-(RECH2)C 5H4N (RE = MeS, PhS, MeSe)

Article abstract of DOI:10.1002/ejic.200400514

The reaction of MCl₂ (M = Pd, Pt) with one mole-equivalent of L [L = 2-(MeSCH₂)C₅H₄N (L¹), 2-(PhSCH₂)C₅H₄N (L²), 2-(MeSeCH ₂)C₅H₄

Full text of DOI:10.1002/ejic.200400514

FULL PAPER
Solution, Structural and Catalytic Studies of Neutral MCl2 (M = Pd, Pt)
Complexes of the N/E Mixed-Donor Ligands 2-(RECH2)C5H4N
(RE = MeS, PhS, MeSe)
Roderick C. Jones,[a] Rachael L. Madden,[a] Brian W. Skelton,[b] Vicki-Anne Tolhurst,*[a]
Allan H. White,[b] Alan M. Williams,[a] Adele J. Wilson,[a] and Brian F. Yates[a]
Keywords: Palladium / Platinium / N,S ligands / Homogeneous catalysis / Fluxional processes
The reaction of MCl2 (M = Pd, Pt) with one mole-equivalent
were confirmed, by DFT calculations, to be (E)-pyramidal in-
version at the chalcogen centre rather than ring-flip or (E)-
of L [L = 2-(MeSCH2)C5H4N (L1), 2-(PhSCH2)C5H4N (L2), 2-
(MeSeCH2)C5H4N (L3)] in MeCN gave the monomeric com- dissociation processes. The calculations showed that the bar-
plexes [MCl2L] in good yields. Single-crystal X-ray diffrac-
tion studies of [MCl2L2] confirmed the complexes to be mo-
nomeric with square-planar geometry about the metal cen-
tre. Variable-temperature NMR spectroscopy showed that
the complexes undergo fluxional processes in solution, which
rier to inversion increases in the order SϽSeϽTe. All com-
plexes were characterised using 1H NMR, UV/Vis and IR
spectroscopy and microanalysis.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
nor ligands 2-(RECH2)C5H4N [E = S, R = Me (L1), Ph
(L2); E = Se, R = Me (L3)] and the single-crystal structure
determinations of isomorphous [PdCl2L2] and [PtCl2L2].
We have investigated the solution behaviour of these com-
plexes using variable-temperature NMR spectroscopy. Den-
sity Functional Theory calculations have been employed to
determine the likely processes being observed. Trends in
(E)-pyramidal inversion barriers for the complexes have
been compared with those associated with PdII and PtII
complexes containing homoleptic dithio- and diselenoether
ligands. Studies into the solid-state structures and solution
properties are important for understanding the usefulness
of these complexes as catalysts as they infer information on
the strength of the ligand-metal interactions. We also report
the catalytic activity of the PdII complexes [PdCl2L1] and
[PdCl2L2] in the Heck reaction, by using various aryl ha-
lides and olefins. The efficiencies of these complexes in this
reaction are compared to those for the analogous com-
plexes containing the homoleptic ligands 2,5-dithiahexane
(MeS(CH)2SMe) and bipyridine (bipy).
Introduction
Palladium(ii) and platinum(ii) complexes containing di-
thioether ligands have been extensively studied in recent
years,[1–3] leading to an understanding of the S-pyramidal
inversion process in solution. Reports of similar compounds
containing diselenoether and ditelluroether ligands are now
more prevalent and include solution properties ascertained
by using multinuclear NMR spectroscopy.[4] In contrast,
complexes containing mixed-N/S or N/Se donor groups
have not been as extensively studied, nor have these studies
generally included systematic investigations into the com-
plexes’ behaviour in solution.
The use of hemilabile ligands in catalysis is gaining inter-
est, as these ligands are thought to give better catalyst sta-
bility and higher yields compared to catalysts containing
homoleptic ligands.[5] It has been suggested that the enantio-
selectivity observed in Pd-catalysed enantioselective allylic
alkylation may be related to the pyramidal inversion at the
sulfur donor atom in complexes containing thioether li-
gands.[6] Organo-palladium(ii) complexes containing the
mixed-donor ligand 2-(RSCH2)C5H4N (R = Me, Ph) un-
dergo allene insertion into Pd–C bonds.[7,8]
Results and Discussion
Herein we report the preparation of a number of palladi-
um(ii) and platinum(ii) complexes containing the mixed-do-
Neutral [MCl2L] Complexes (M = Pd, Pt)
The reactions of L1, L2 and L3 with PdCl2 or PtCl2 in
[a] School of Chemistry, University of Tasmania,
Private Bag 75, Hobart TAS, 7001, Australia
Fax: +61-3-6226-2858
MeCN at ambient temperature afforded the complexes
E-mail: VickiAnne.Tolhurst@utas.edu.au
[b] Department of Chemistry, University of Western Australia,
Crawley WA, 6009, Australia
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400514
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MCl2 (M = Pd, Pt) Complexes of the N/E Mixed-Donor Ligands 2-(RECH2)C5H4N
FULL PAPER
[MCl2L] {M = Pd, L = L1 (1a), L2 (2a), L3 (3a); M = Pt, Table 1. Coalescence temperatures for compounds.
L = L1 (1b), L2 (2b), L3 (3b)} in good to high yields as
Complex
L1 2-(MeSCH2)C5H3N
PdCl2L
Coalescence temperature [°C]
yellow powders (Scheme 1). All complexes were air stable
and had poor solubility in most solvents except DMSO.
Liquid-secondary-ionisation (LSI) mass spectra of all com-
plexes exhibited ions corresponding to the molecular ion L2 2-(PhSCH2)C5H3N
95
Ͼ160
PtCl2L
PdCl2L
90
157
with loss of one halide atom. The IR spectra for all com-
PtCl2L
pounds showed all observed stretches corresponding to
L3 2-(MeSeCH2)C5H3N
those of the coordinated ligand. The UV/Vis spectra of
PdCl2L
82
PtCl2L
110[a]
these compounds exhibited peaks consistent with square-
planar geometries.
[a] Decomposes at this temperature in solution.
Density Functional Theory (DFT) calculations confirm
that the fluxional process observed for the complexes de-
scribed here is not a ring-flip process, which has a calcu-
lated activation energy of 2.7 kJmol–1 for the complex
PdCl2L1 (in contrast, sulfur-pyramidal-inversion and sul-
fur-dissociation processes for PdCl2L1 have calculated acti-
vation energies of 71.7 and 92.0 kJmol–1, respectively). This
suggests that the ring inversion process is already occurring
at room temperature, thus explaining the presence of only
one AB spin system rather than two, as would be expected
for two possible conformers given the similar energies of
the two.
The calculated energy difference between the two con-
formers, the barrier to (E)-pyramidal inversion and the bar-
rier to (E)-dissociation for [MCl2L] {M = Pd, Pt; L = 2-
(MeECH2)C5H4N where E = S (L1), Se (L3), Te (L4)} are
listed in Table 2. All ΔH‡ values listed here have been zero-
point corrected for enthalpy. Structures II and III are en-
antiomers, so the energy for only one enantiomer was calcu-
lated, as both are of equal energy.
Scheme 1. Preparation of Pd(II) and Pt(II) chloride complexes.
1
Room-temperature H NMR spectra of the complexes
in [D6]DMSO exhibit resonances between 9.13–6.63 ppm,
ascribed to the pyridyl rings of the ligands. A second-order
AB spin system is observed for the methylene protons of the
α-carbon upon coordination to the metal centre for each
compound. This is due to the formation of a new ste-
reogenic centre at each chalcogen upon coordination. The
methyl protons in complexes 1(a,b) and 3(a,b) are obscured
by the [D6]DMSO resonance.
Trends observed in the calculated energy barriers to (E)-
pyramidal inversion for the PdII and PtII complexes con-
taining the mixed-donor ligands are similar to those seen
Fluxional Behaviour and Theoretical Calculations
The appearance of a second-order AB spin system at am- for the analogous complexes containing dichalcogenoether
bient temperature in the 1H NMR spectra for all complexes ligands. Pyramidal inversion at the chalcogen centres of the
1
led us to undertake variable-temperature H NMR studies. palladium complexes occurs at lower energies relative to
On warming the complexes in solution, the resonances of those of the platinum compounds, consistent with a
the methylene protons coalesced for most complexes to stronger Pt–E bond compared with Pd–E, as is also ob-
form broad singlets. Coalescence temperatures of the meth- served in the solid-state structures of [MCl2L2] (2a, 2b).
ylene protons in the complexes are listed in Table 1. Three There is also an increase in the barrier to chalcogen pyrami-
fluxional processes can occur in these complexes: (a) con- dal inversion in the order S Ͻ Se Ͻ Te. This trend is also
formational changes of the chelate ring (ring flip, Figure 1a) observed in the compounds containing the homoleptic li-
where the transition state contains a nearly flat five-mem- gands. However, the relative increase in the energy barrier
bered chelate ring which is quasi-coplanar with the coordi- is considerably larger for the heteroleptic complexes com-
nation plane of the metal, (b) pyramidal inversion at the pared with the homoleptic complexes. For instance, the en-
chalcogen, where the transition state has a planar five-mem- ergy difference between the complexes [PtClMe{MeS(CH2)
bered ring, and the terminal substituent is also in the same 2SMe}] and [PtClMe{MeSe(CH2)2SeMe}] is 10.9 kJmol–1
plane as the coordination plane of the metal centre (Fig- where inversion occurs trans to the coordinated methyl
ure 1b), and (c) dissociation of the chalcogen donor atom group,[2] whereas the increase in inversion energy between
from the palladium centre (Figure 1c). Variable-tempera- the complexes [PtCl2L1] and [PtCl2L3] calculated here is
ture NMR studies of palladium(ii) and platinum(ii) com- 29.8 kJmol–1. However, this could be consequent on the
plexes containing chelating dichalcogenoether ligands have difference in the trans-effect of the methyl compared to the
established that the coalescence of resonances at high tem- chloride group on the chalcogen.
peratures is due to pyramidal inversion at the chal-
Energies associated with the full dissociation of the chal-
cogen donor atom from the metal centre are generally
cogen.[2,3,9,10]
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V.-A. Tolhurst et al.
FULL PAPER
Figure 1. Calculated structures of conformers and transition states for a) ring inversion, b) pyamidal S-inversion and c) S-dissociation
for [PdCl2L1]. Structures are viewed side-on and through the coordination planes of the metals with the pyridine rings distant from the
reader.
Table 2. DFT calculated (E)-pyramidal inversion barriers and (E)-dissociation energies.
Energy of III [kJmol–1][a]
ΔH‡ [(E)-Pyramidal Inversion]
ΔH (Dissociation Energy)
Complex
[kJmol–1]
[kJmol–1]
2-(MeSCH2)C5H3N (L1)
PdCl2L
1.5
71.7
92.1
92.0
130.7
PtCl2L
0.4[c]
2-(MeSeCH2)C5H3N (L3)
PdCl2L
2.2
2.0
97.5
119.1
95.3
131.1
PtCl2L
2-(MeTeCH2)C5H3N (L4)
[b]
PdCl2L
PtCl2L
3.5
1.75
118.7
141.2
119.2
150.8
[a] Energy relative to I (0 kJmol–1). [b] Synthesis of L4 and its compounds have not been reported. [c] Energy of I relative to III
(0 kJmol–1).
higher than those of the (E)-inversion processes by 10– Structures of [PdCl2L2] (2a) and [PtCl2L2] (2b)
40 kJmol–1 except for [PdCl2L] (L = L3, L4) where the two
processes have similar energies. This is consistent with the
The solid-state structures of 2a and 2b are isomorphous,
better orbital overlap between the softer selenium and tel- with both complexes crystallising in the monoclinic space
lurium donor atoms with platinum compared to the harder group P21/n, and the molecules being discrete monomers,
palladium centre affording stronger M–Se and M–Te bonds one comprising the asymmetric unit of each. The structure
for platinum.
of [PdCl2L2] is shown in Figure 2. Crystallographic data of
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MCl2 (M = Pd, Pt) Complexes of the N/E Mixed-Donor Ligands 2-(RECH2)C5H4N
FULL PAPER
2a and 2b are listed in Table 3, with important bond dis- Table 4. Selected bond distances and angles of [MCl2L3] {M = Pd
(2a), Pt (2b)}.
tances and angles given in Table 4. The molecules of 2a and
2b contain four-coordinate metal centres with square-
planar geometry with slight tetrahedral distortion, with the
ligand acting as a bidentate.
M = Pd
M = Pt
Bond distances [Å]
M–Cl(1)
M–Cl(2)
M–N(1)
M–S(21)
2.2887(6)
2.3162(5)
2.045(1)
2.296(1)
2.316(1)
2.034(3)
2.235(1)
2.2540(5)
Bond angles [°]
Cl(1)–M–Cl(2)
Cl(1)–M–N(1)
Cl(1)–M–S(21)
Cl(2)–M–N(1)
Cl(2)–M–S(21)
N(1)–M–S(21)
90.90(2)
172.95(3)
88.74(1)
94.50(3)
174.40(1)
86.32(3)
89.79(4)
174.73(9)
89.70(4)
94.2(1)
175.15(3)
86.6(1)
Dihedral angles [°]
M–N(1)–C(6)–C(5)
N(1)–M–S(21)–C(211)
M–N(1)–C(2)–C(21)
N(1)–C(2)–C(21)–S(21)
C(2)–C(21)–S(21)–M
–179.71(1)
–93.12(6)
–2.7(2)
16.9(2)
–20.1(1)
179.5(3)
–92.5(2)
–3.1(5)
18.0(5)
–21.4(3)
Figure 2. Molecular projection of [PtCl2L2] showing the atom
labelling scheme. Thermal ellipsoids are drawn at the 50% prob-
ability level. [PdCl2L2] is isostructural.
atom is trans to nitrogen and sulfur, respectively.[14] They
are also similar to those seen in the palladium complex 2a,
which is not surprising given that the ionic radii of PdII and
PtII are the same within experimental error. The Pt–N bond
distance of 2.034(3) Å is also similar to that in complex 2a
within experimental error. Interestingly, the Pt–S bond dis-
tance of 2.235(1) Å is significantly shorter than the Pd–S
bond distance observed in 2a [ΔS = –0.019 Å, where ΔS =
d(Pt–S)-d(Pd–S)] while the Pt–Cl bond for the chloride
trans to the sulfur atom is the same as the analogous Pd–
Cl bond [2.316(1) vs 2.3162(5) Å]. Similar trends in bond
distances in isomorphous structures of palladium and plati-
num complexes containing homoleptic and heteroleptic li-
gands have been observed, and theoretical calculations have
been performed to explain the trend.[14–19] Results from
theoretical calculations for the geometry optimisations of
the complexes [MMe3{(H2C=N–NH)3CH}]+ compared
well with the X-ray studies of isostructural [MMe3{(pz)3-
CH}]+ (M = Pd, Pt), where the M–C bond distances are
identical or slightly shorter for palladium and platinum
while the Pd–N bonds are longer than the Pt–N bonds.[19]
The explanation for this trend is attributed to the softer
acidity of platinum compared with palladium. Steric influ-
Table 3. Crystallographic data of [MCl2L3] {M = Pd (2a), Pt (2b)
}.
[PdCl2L2] (2a)
[PtCl2L2] (2b)
Formula
M
Crystal system
Space group
a [Å]
C12H11NSPdCl2
378.62
monoclinic
P21/n
C12H11NSPtCl2
467.28
monoclinic
P21/n
9.232(2)
9.176(2)
b [Å]
c [Å]
β [°]
12.997(3)
10.847(3)
96.306(5)
13.035(3)
10.895(2)
95.822(5)
Dc [gcm–3]
Z
1.944
4
1.98
2.394
4
11.4
µ [mm–1]
Tmin/max
Nt
0.71
0.65
26827
6828 (0.027)
5776
0.025
0.032
1.09
9548
6017 (0.028)
4500
0.032
0.034
1.12
N(Rint
NO
)
R
Rw
GooF
The Pd–N bond distance of 2.045(1) Å is shorter than ences on the trends in bond lengths, which were previously
that found in [Pd(η3-allyl)L2](ClO4) (2.088(5) Å).[11] This suggested as having an additional effect,[16,17] were elimin-
can be attributed to the difference in the trans effect of the ated after similar trends were found in the optimised geom-
chloride ion compared to that of the allyl group. The Pd–S etries of trans-[M(H)2(CH3)(NH3)]–.[19]
bond distance of 2.2540(5) Å is similar to those observed in
The five-membered chelate ring formed by the bidentate
the similar compounds [PdClMe(2-(MeSCH2)C5H4N)] ligand in both the palladium and the platinum complexes
(2.257(5), 2.255(6) Å][8] and [PdClMe(2-(tBuSCH2)C5H4N)] is puckered, with the sulfur atom lying out of the M, N,
(2.2589(13) Å][7], where the sulfur atom is trans to a chlo- C(2), C(21) plane [χ2 205 (Pd), 28 (Pt)] by 0.415(2) and
ride group. The Pd–Cl bond distances of 2.2887(6) and 0.434(5) Å, respectively. This distortion and the larger
2.3162(5) Å, are similar to those seen in complexes where Cl(2)–M–N(1) bond angle [94.50(3) (Pd), 94.2(1)° (Pt)] off-
chloride is trans to nitrogen and sulfur, respectively.[12–14]
set the smaller angles about the metal centre, including the
Pt–Cl bond distances of 2.296(1) and 2.316(1) Å, are small bite angle of the ligand [86.32(3) (Pd), 86.6(1)° (Pt)].
again similar to those seen in complexes where the chloride The ligand chelates the metal centre, with the pyridyl ring
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V.-A. Tolhurst et al.
FULL PAPER
of the ligand lying almost in the same plane as the coordi- tively; however, the turnover number for neither reaction
nation plane of the metal centre {τ[M–N–C(6)–C(5)] was as good as those for 4-bromoacetophenone.
179.70(1) (Pd), 179.5(3)° (Pt)} and the phenyl substituent
More significantly, the reaction between 4-bromoace-
of the thioether almost perpendicular to this plane {τ[N– tophenone and ethyl trans-cinnamate was undertaken to see
M–S(21)–C(211)] –93.12(6) (Pd), –92.5(2)° (Pt)}.
if the complexes facilitated the reaction using 1,2-disubsti-
tuted alkenes (Scheme 2). Intermolecular Heck reactions of
internal alkenes are not often due to their low reactivity
using typical catalysts.[21–23] The difficulty of this reaction
is reflected in the high catalyst loadings and long reaction
times required as well as other complications such as the
possibility of both (E)- and (Z) isomers of the product be-
ing formed because of the high temperatures often required
to undertake the reaction. Full conversion of 4-bromoace-
tophenone is achieved within 3 hours with [PdCl2L2], re-
sulting in the formation of (E)-ethyl-3-(4-acetylphenyl)-3-
phenyl-2-propenoate regio- and stereoselectively. This oc-
curs at lower catalyst loadings relative to other similar reac-
tions reported in the literature.[21,22] Fu and Littke have re-
ported the reaction of deactivated aryl bromides with
methyl trans-cinnamate at room temperature achieving
80 % isolated yield after 72 h with Ͼ20:1 E/Z selectivity by
using 1.0 mol-% Pd2(dba)3 and 2.0 mol-% P(tBu)3.[21]
Catalytic Activity of [PdCl2L1] (2a) and [PdCl2L2] (2b)
The catalytic activity of [PdCl2L] in the Heck reaction
was investigated. The versatility of the compounds in this
reaction was tested by using both an activated and deacti-
vated aryl bromide, and terminal mono- and disubstituted
alkenes, and an internal alkene. The catalytic results are
listed in Table 5 and Table 6.
Table 5. Catalytic data for the Heck reaction of various aryl halides
and alkenes.[a]
[a] The reactions were carried out with 1.5 mol aryl halide, excess
alkene, 2.2 mol NaOAc in 7 mL DMAc for 48 h. [b] Determined
by GC (standard: diethylene glycol monobutyl ether). [c] Reaction
carried out at 100 °C with 0.001 mol-% [PdCl2L2]. [d] Reaction car-
ried out at 120 °C with 1 mol-% [PdCl2L2].
Scheme2. Reaction of 4-bromoacetophenone and ethyl trans-cinna-
mate.
Modification of the hemilable ligands also results in sig-
nificant differences in activity. When the reaction shown in
Scheme 2 is conducted without the aid of phase-transfer
agents and reducing agents, the phenylthioether-containing
complex, [PdCl2L2], achieves higher conversion of the 4-
bromoacetophenone (83 %) compared to the methylthio-
ether-containing complex, [PdCl2L1] (68 %) after 16 hours
at 140 °C, suggesting that the presence of a softer S-donor
atom enhances the reactivity. This is consistent with theo-
retical calculations investigating the electronic and steric li-
gand effects on the activity of the Heck reaction, by using a
substituted phenylpalladium(ii) diimine complex.[24] Similar
reactivity comparisons have been observed for the insertion
reaction of allenes into Pd–C bonds.[7,8]
Table 6. Catalytic data for the Heck reaction of 4-bromoacetophe-
none and ethyl trans-cinnamate.[a]
Catalyst
Time [h] Conversion [%][b] E/Z Ratio[c]
[PdCl2L1]
6
68
4.3:1
[PdCl2L2]
6 (3)
6 (6)
6 (6)
83 (Ͼ99[d]
56 (46)
64 (90)
)
4.2:1 (Ͼ99:1)
4.3:1 (Ͼ99:1)
4.6:1 (Ͼ99:1)
[PdCl2(bipy)]
[PdCl2(MeS(CH2)2SMe)]
[a] The reactions were carried out with 1.5 mol 4-bromoacetophe-
none, 4.2 mol ethyl trans-cinnamate, 1 mol-% catalyst, 2.2 mol
NaOAc in 7 mL DMAc at 140 °C for 6 h. Data in brackets corre-
spond to reactions carried out with addition of 1.1 equiv. nBu4NCl,
1.1 equiv. HCO2Na at 80 °C for 6 h. [b] Determined by GC (stan-
dard: diethylene glycol monobutyl ether). [c] Determined by GC.
[d] Conversion after 3 h.
We also compared the catalytic activity in the Heck reac-
tion of the heteroleptic complex [PdCl2L2] with that of
[PdCl2(bipy)] (bipy = 2,2Ј-dipyridyl), a complex containing
The complex [PdCl2L2] was found to facilitate high turn-
a
homoleptic
bidentate
N-donor
ligand,
and
over numbers (1,000,000) in the reaction between 4-bromo- [PdCl2(MeS(CH2)2SMe)], a complex containing a homolep-
acetophenone and both n-butylacrylate and n-butylmethac- tic bidentate S-donor ligand, again using the reaction
rylate. In both reactions, only one product was observed by shown in Scheme 2. As mentioned previously, the complex
using GC. These turnover numbers are comparable to those [PdCl2L2] facilitates the full conversion of 4-bromoace-
of Herrmann’s palladacycle, {Pd2[P(o-tol)3]2(μ-OAc)2}, al- tophenone in 3 hours to form (E)-ethyl-3-(4-acetylphenyl)-
though this reaction is undertaken at 130 °C.[20] The reac- 3-phenyl-2-propenoate regio- and stereoselectively. In con-
tion between 4-bromotoluene and n-butylacrylate and n-bu- trast, complexes containing the homoleptic nitrogen and
tylmethacrylate resulted in excellent conversion of the aryl sulfur ligands only achieved 46 and 90 % conversion,
halide with 94 and Ͼ99 % conversion achieved, respec- respectively, after 6 hours under the same conditions. If the
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MCl2 (M = Pd, Pt) Complexes of the N/E Mixed-Donor Ligands 2-(RECH2)C5H4N
FULL PAPER
reaction is done at higher temperatures, Pd black is pro- difference in catalytic activities is due to a difference in in-
duced in the two homoleptic systems, with significant duction period and/or the efficiency in the reduction of the
amounts in the [PdCl2(MeS(CH2)2SMe)] catalysed system, complexes to a Pd0 catalytically active species. This is under
which indicates that catalyst degradation occurs readily. De- further investigation.
gradation of catalysts containing the hemilable ligands was
also observed, but probably occurred after the reaction was
complete.
Experimental Section
General Remarks: All reactions were done under nitrogen gas using
standard Schlenk techniques. Palladium(ii) chloride and plati-
num(ii) chloride were obtained as a loan from Johnson Matthey,
Conclusions
and used without purification. The ligands 2-(RECH2)C5H4N [E
= S, R = Me (L1), Ph (L2); E = Se, R = Me (L3)] were prepared
Neutral palladium(ii) and platinum(ii) complexes have
according to literature procedures.[25,11] All solvents were degassed
been prepared containing the mixed-N/E donor ligands 2-
prior to use. Infrared spectra were recorded as a KBr disc on a
Bruker IF55 Infra-Red Spectrometer. LSIMS mass spectra of the
complexes were recorded on a Kratos Analytical Concept ISQ mass
(RECH2)C5H4N [E = S, R = Me (L1), Ph (L2); E = Se, R
= Me (L3)].
spectrometer. 1H NMR spectra were recorded on a Varian Gemini
(200 MHz) or Unity (400 MHz) NMR spectrometer, in deuterated
dimethylsulfoxide and referenced to the residual resonances of the
solvent (δ = 4.33 ppm). Microanalyses (C,H,N,S) were determined
by The Microanalysis Services, Central Science Laboratory, Uni-
versity of Tasmania.
[PdCl2L1] (1a): L1 (43 mg, 0.309 mmol) was added to a degassed
solution of PdCl2 (50 mg, 0.28 mmol) in MeCN (20 mL), and the
reaction was stirred at room temperature for 16 hours. An orange
precipitate formed over this time. The MeCN volume was reduced
in vacuo ( to about 2 mL) to afford a yellow precipitate. The solid
was collected and washed with Et2O (56 mg, 62 %). 1H NMR
(200 MHz, [D6]DMSO): δ = 4.51, 4.85 (AB spin system, 2 H, CH2),
7.60 (m, 1 H, C6H4N), 7.78 (m, 1 H, C6H4N), 8.13 (m, 1 H,
C6H4N), 9.13 (m, 1 H, C6H4N) ppm. IR (KBr): ν˜ = 3079 (m), 2987
(m), 2945 (vs), 2902 (s), 1606 (vs), 1481 (s), 1446 (s), 1381 (m), 1320
(w), 1274 (s), 1166 (s), 1111 (m), 1060 (m), 1030 (m), 978 (m), 863
(w), 825 (w), 769 (vs), 657 (w), 428 (w) cm–1. m/z = 280 (M-Cl)+
DFT calculations have shown that the process observed
in variable-temperature H NMR studies is most probably
1
due to either the (E)-pyramidal inversion process or (E)-
dissociation processes rather than ring flip. For all platinum
complexes and the palladium complex [PdCl2L1] the flux-
ionality is most probably due to the (E)-pyramidal inversion
process, which is 10–40 kJmol–1 lower in energy than the
(E)-dissociation process. The cause of the fluxional process
for the complexes [PdCl2L] (L = L2, L3) is not as clear
based on the DFT calculations. Both the (E)-pyramidal in-
version and (E)-dissociation processes involve a weakening
of the M–E bond. In the case of the heavier chalcogeno-
ether-containing palladium complexes, the orbital overlap
between the softer chalcogen atom and the harder palla-
dium atom becomes mismatched, leading to a destabilising
effect on the Pd–E bond during the processes. As a result,
it is plausible that full (E)-dissociation may occur in these
complexes rather than just a partial weakening of the Pd–
E bond during the (E)-pyramidal inversion process. The
trends observed in the barrier to (E)-pyramidal inversion
for the complexes containing the mixed-donor ligand com-
plexes reported here are the same as those observed for
complexes containing homoleptic dichalcogenoether li-
gands. These include the barrier to (E)-inversion increasing
in the order SϽSeϽTe, and PdϽPt.
[
106Pd(12C7H9N32S)35Cl 280]. UV/Vis (DMSO)/cm–1: ν˜max (εmol
/
dm3 mol–1 cm–1) = 26170 (1113), 33600 (3537). C7H9Cl2PdNS
(316.51): calcd. C 26.56, H 2.87, N 4.43; found C 26.99, H 2.83, N
4.63.
[PtCl2L1] (1b): The procedure described above using PtCl2 afforded
an off-white solid (75 %). 1H NMR (200 MHz, [D6]DMSO): δ =
4.50, 4.78 (AB spin system, 2 H, CH2), 7.60 (m, 1 H, C6H4N), 7.87
(m, 1 H, C6H4N), 8.22 (m, 1 H, C6H4N), 9.45 (m, 1 H,
C6H4N) ppm. IR (KBr): ν˜ = 3116 (w), 3087 (m), 3000 (m), 2959
(vs), 2903 (s), 1612 (s), 1475 (s), 1427 (s), 1397 (m), 1321 (m), 1272
(m), 1168 (s), 1113 (w), 1063 (w), 996 (m), 977 (m), 901 (w), 773
The isomorphous solid-state structures of [MCl2L2] (M
= Pd, Pt) show that the thioether functional group is more
strongly bound to the platinum than to the palladium as
seen by a shorter Pt–S bond compared to the Pd–S bond,
probably due to the softness of the platinum centre.
(vs), 710 (w), 473 (w), 435 (w) cm–1. m/z
=
369 (M-Cl)+
[
195Pt(12C7H9N32S)35Cl 369]. UV/Vis (DMSO)/cm–1: ν˜max (εmol
/
dm3 mol–1 cm–1) = 34400 (2502). C7H9Cl2NPtS (405.21): calcd. C
20.75, H 2.43, N 3.46; found C 20.91, H 2.26, N 3.44.
The complexes [PdCl2L] (L = L1, L2) were found to facil-
itate the Heck reaction of various aryl bromides with ter-
minal and internal alkenes with excellent conversions. In
the case of the reaction of 4-bromoacetophenone with ethyl
trans-cinnamate, full conversion of the aryl bromide is
achieved to form (E)-ethyl-3-(4-acetylphenyl)-3-phenyl-2-
propenoate regio- and stereoselectively. The complex
[PdCl2L2] was found to have higher catalytic activity com-
pared to [PdCl2L1], suggesting that activity is promoted by
the presence of the softer ligand. Both complexes contain-
ing the hemilabile ligands had higher catalytic activity than
complexes containing homoleptic N- and S-donor contain-
[PdCl2L2] (2a): Procedure as for [PdCl2L1]using L2 (64 %). 1H
NMR (200 MHz, [D6]DMSO): δ = 4.84, 5.40 (AB spin system, 2
H, CH2), 7.47–7.52 (m, 4 H, C6H4N, C6H5), 7.77–7.85 (m, 3 H,
C6H4N, C6H5), 8.12 (m, 1 H, C6H4N), 9.13 (m, 1 H, C6H4N) ppm.
IR (KBr): ν˜ = 3085 (w), 3058 (w), 2969 (s), 2902 (s), 1604 (m), 1475
(s), 1439 (s), 1400 (m), 1276 (m), 1165 (m), 1060 (w), 1030 (w),
1002 (w), 914 (w), 827 (w)m 781 (s), 738 (s), 705 (w), 684 (m),
656 (w), 600 (w), 480 (m) cm–1. UV/Vis (DMSO)/cm–1: ν˜max (εmol
dm3 mol–1 cm–1) = 25650 (1163), 33900 (6415). m/z = 344 (M-Cl)+
106Pd(12C12H11N32S)35Cl 344]. C12H11Cl2NPdS (378.58): calcd. C
/
[
38.07, H 2.93, N 3.70; found C 38.00, H 3.03, N 3.46.
[PtCl2L2] (2b): Procedure as for [PtCl2L1] using L2 (79 %). 1H
ing ligands. At this stage it can not be discounted that the NMR (200 MHz, [D6]DMSO): δ = 4.85, 5.21 (AB spin system, 2
Eur. J. Inorg. Chem. 2005, 1048–1055
www.eurjic.org © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1053

V.-A. Tolhurst et al.
FULL PAPER
H, CH2), 7.46–7.63 (m, 4 H, C6H4N, C6H5), 7.76–7.85 (m, 3 H, tial and double-zeta valence basis set on palladium, platinum, sul-
C6H4N, C6H5), 8.19 (m, 1 H, C6H4N), 9.45 (m, 1 H, C6H4N) ppm. phur, selenium and tellurium together with the Dunning/Huzi-
IR (KBr): ν˜ = 3085 (w), 3052 (w), 2974 (m), 2905 (m), 2854 (w),
1608 (m), 1476 (s), 1439 (s), 1396 (m), 1310 (w), 1273 (w), 1163
naga[31] double zeta basis set on other atoms). Sets of five d-func-
tions were used in the basis sets throughout these calculations. For
(m), 1113 (w), 1063 (w), 1022 (w), 998 (w), 914 (w), 780 (s),m 739 the optimised geometries, harmonic vibrational frequencies were
(s), 704 (w), 684 (m), 611 (w), 484 (m) cm–1. UV/Vis (DMSO)/cm–1: calculated at the B3LYP level. All transition structures possessed
ν˜max (εmol/dm3 mol–1 cm–1) = 33950 (4698). m/z = 432 (M-Cl)+
[
30.84, H 2.37, N 3.00; found C 30.99, H 2.40, N 2.97.
one and only one imaginary frequency, and they were further char-
acterised by following the corresponding normal mode towards
each product and reactant. Single-point energies on B3LYP/
LANL2DZ optimised geometries were calculated at the B3LYP
level with the LANL2augmented:6-311+G(2d,p) basis set,[32] which
incorporates the LANL2 effective core potential and augmented
basis sets on the metals (the large f-polarised valence basis set of
Bauschlicher and coworkers[33] was used for palladium and the
LANL2TZ+(3f) basis set for platinum[34]) together with the 6-
311+G(2d,p) basis set[35–37] on all other atoms (3–11G basis set was
used for tellurium). Energies from these single-point calculations
were combined with the zero-point vibrational energy corrections
from the lower level of theory to yield ΔH0 numbers. Unless other-
wise noted all computed energies quoted in this paper refer to these
final ΔH0 numbers. All calculations were carried out with the
Gaussian 03[38] program.
195Pt(12C12H11N32S)35Cl 432]. C12H11Cl2NPtS (467.28): calcd. C
[PdCl2L3] (3a): Procedure as for [PdCl2L1] using L3 (82 %). 1H
NMR (200 MHz, [D6]DMSO): δ = 4.52, 4.75, (AB spin system, 2
H, CH2), 7.55 (m, 1 H, C6H4N), 7.71 (m, 1 H, C6H4N), 8.08 (m,
1 H, C6H4N), 9.21 (m, 1 H, C6H4N) ppm. IR (KBr): ν˜ = 3076 (m),
3000 (w), 2983 (w), 2958 (s), 2908 (m), 1652 (w), 1606 (s), 1558 (w),
1540 (w), 1480 (s), 1446 (s), 1421 (m), 1381 (m), 1265 (m), 1111
(m), 1060 (w), 1029 (w), 924 (m), 839 (w), 815 (w), 762 (vs), 650
(w), 467 (w), 425 (w) cm–1. UV/Vis (DMSO)/cm–1: ν˜max (εmol
/
dm3 mol–1 cm–1) = 25920 (1100), 32800 (4295). m/z = 328 (M-Cl)
+
[
106Pd(12C7H9N80Se)35Cl 328]. C7H9Cl2NPdSe (363.41): calcd. C
23.13, H 2.50, N 3.86; found C 23.41, H 2.83, N 3.58.
[PtCl2L3] (3b): Procedure as for [PtCl2L1] using L3 (45 %). 1H
NMR (200 MHz, [D6]DMSO): δ = 4.47, 4.70 (AB spin system, 2
H, CH2), 7.53 (m, 1 H, C6H4N), 7.81(m, 1 H, C6H4N), 8.15 (m, 1
H, C6H4N), 9.50 (m, 1 H, C6H4N) ppm. IR (KBr): ν˜ = 3111 (w),
3080 (w), 3005 (w), 2987 (w), 2964 (m), 2910 (m), 1610 (s), 1480
(s), 1447 (s), 1421 (m), 1382 (m), 1270 (m), 1113 (m), 1064 (w), 924
(m), 838 (m), 819 (w), 651 (w)m 560 (w), 468 (w), 433 (w) cm–1.
UV/Vis (DMSO)/cm–1: ν˜max (εmol/dm3 mol–1 cm–1) = 33450 (2071).
CCDC-228118 and CCDC-228119 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
XYZ Cartesian coordinates for all optimised structures described
in this article are available as Supporting Information (see also the
footnote on the first page of this article).
m/z = 416 (M-Cl)+ 195Pt(12C7H9N80Se)35Cl 416]. C7H9Cl2NPtSe
[
(452.11): calcd. C 18.59, H 2.01, N 3.10; found C 18.56, H 1.99, N
3.01.
Acknowledgments
General Procedure for Heck Reactions: A degassed solution of aryl
halide in dimethylacetamide (5.0 mL, 0.30 molL–1) was added to
the alkene (0.70 mL), NaOAc (2.2 mmol), catalyst, nBu4NCl
(1.1 equiv.) and HCO2Na (1.1 equiv.) in a Schlenk vessel under ar-
gon. The reaction was heated at the appropriate temperature for
3–48 hours. The reaction was then cooled to room temperature,
after which 0.20 mL diethylene glycol monobutyl ether was added.
An aliquot (1.00 mL) was taken from the reaction, diluted with
CH2Cl2, and washed three times with a saturated aqueous solution
of NaCl. The organic layer was dried over MgSO4. The conversion
of aryl halide and the E/Z product ratio of ethyl 3-(4-acetylphenyl)-
3-phenyl-2-propenoate were then determined by GC.
We acknowledge the Australian Research Council for funding this
project. We also thank Johnson-Matthey for the generous loan of
PtCl2 and PdCl2, and Prof. W. Levason and Dr. G. Reid for the
[PdCl2(MeS(CH2)2SMe)] sample.
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1054
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MCl2 (M = Pd, Pt) Complexes of the N/E Mixed-Donor Ligands 2-(RECH2)C5H4N
FULL PAPER
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Received: June 15, 2004
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