Boosting catalyst activity in cis -selective semi-reduction of internal alkynes by tailoring the assembly of all-metal aromatic tri-palladium complexes

Article abstract of DOI:10.1039/c6dt01840h

Highly symmetric [Pd₃]⁺ clusters that present delocalized metal-metal bonds can catalyse the selective semi-reduction of internal alkynes to cis-alkenes. Studies on factors governing the formation of all-metal aromatics enabled the design of an optimised catalytic system that delivers cis-alkenes with almost complete selectivity on a gram scale with very low catalyst loadings (0.03 mol%).

Full text of DOI:10.1039/c6dt01840h

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DOI: 10.1039/C6DT01840H
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Boosting Catalyst Activity in Cis-Selective Semi-Reduction of
Internal Alkynes by Tailoring the Assembly of All-Metal Aromatic
Tri-Palladium Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Anna Monfredini,a† Veronica Santacroce,a† Pierre-Alexandre Deyris,b Raimondo Maggi,a Franca
DOI: 10.1039/x0xx00000x
Bigi,a Giovanni Maestri*a and Max Malacria*b,c
www.rsc.org/
Highly symmetric [Pd3]+ clusters that present delocalized metal- hydrogenation conditions. The novel complex delivers desired
metal bonds can catalyse the selective semi-reduction of internal cis-alkene almost quantitatively, in shorter times and a with
alkynes to cis-alkenes. Studies on factors governing the formation thirty-three-times lower catalytic charge (Scheme 1, bottom).
of all-metal aromatics enabled the design of an optimised catalytic
Me
system that delivers cis-alkenes with almost complete selectivity
on a gram scale with very low catalyst loadings (0.03 mol%).
1 eq. Pd(dba)2
S
0.33 eq.
AgSbF6
PR3
R3P
Me
Pd Pd
1 eq. PR3
(R = p-tolyl)
S
S
CHCl3
Pd
PR3
Me
0.5 eq. (Me-S)2
All-metal aromatic molecules feature delocalised metal-metal
bonds that parallel bonding of their well-known main-group
counterparts.1 While they are often synthetically challenging at
best,2 we recently developed a simple route3 to a family of
-
Previous
work
SbF6
I
1 mol% cat. I
HCO2H/NEt3
10 hours
bench-stable [Pd3]+ complexes whose core represent
a
1
2, 93%
prototypical well-defined sub-nanometric metal surface.4
Ph
P
Me
S
These complexes could be decorated with a variety of organic
fragments and represent therefore a valuable asset to assess
the consequences of their bonding mode on coordination
chemistry and catalysis. Internal alkynes were semi-reduced to
alkenes with good cis-selectivity under transfer-hydrogenation
conditions,5 which avoids the use of hazardous H2 gas.6 The
Ph
1 eq. Pd(dba)2
R3P
Me
Pd Pd
CHCl3
-NaCl
0.66 eq. PR3
0.33 eq. PPh2C6H4SO3Na
S
S
Pd
PR3
Me
-
SO3
0.5 eq. (Me-S)2
This
work
D
0.03 mol% cat. D
HCO2H/NEt3
6 hours
best first generation catalyst (
I, 1 mol%) delivered full
conversion of phenylpropyne upon 10 hours in refluxing THF.
1
1
2, 98%
No alkane was detected and the ratio between semi-reduced
alkenes was 93:3:4 (cis-, trans- and isomerised-products
respectively, Scheme 1, top).
Scheme 1. First vs second generation metal-aromatic transfer hydrogenation catalysts
We report herein preliminary results on factors triggering the
formation of triangular all-metal aromatic [Pd3]+ complexes.
Through this approach we were able to decorate their metal
core with two different phosphinic ligands, a neutral and an
anionic one. This in turn enabled to develop a catalytic system
Wondering to gain insights on factors governing the formation
of delocalized metal-metal bonds, we tried to monitor these
reactions by NMR. In a 50 mL Schlenk we thus mixed 0.2 mmol
of Pd(dba)2 (dba = dibenzylideneacetone), 0.2 mmol of PPh3
and 0.1 mmol of diphenyldisulphide in 20 mL of CDCl3, taking
1
0.5 mL samples at regular intervals to collect H and 31P NMR
for the selective semi-reduction of
1 to cis-alkene 2 with
unparalleled activity among Pd-based catalysts under transfer
spectra (Figure 1, spectra in ESI).
Surprisingly, the broad and diagnostic 31P resonance of the
[Pd3]+ cation at 15.5 ppm was already visible within few
minutes (Figure 1, red line) and free phosphine at -5 ppm was
no longer present. From the proton spectrum, we noticed the
consumption of Pd(dba)2 and evaluated 18% of the desired
[Pd3]+ cation using dba resonances as internal standard. Two
other broad 31P resonances were observed at 24 and 27 ppm
respectively. We were so far unable to unambiguously assign
these signals, although their intensity slowly fades through
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time with a concomitant increase of the desired resonance at We performed our usual reaction in degassed VTieHwFArtwicleitOhnoliunet
15.5 ppm. Upon 60 minutes, the amount of the [Pd3]+ complex addition of any additive. Expectedly, no traces of the desired
DOI: 10.1039/C6DT01840H
is around 40% (green line). Addition of 0.33 equiv. of AgSbF6 [Pd3]+ cation were observed upon 24 hours at room
resulted in a further increase of the desired resonance at the temperature. On the contrary, mixing reagents for 1 hours and
expense of the two unassigned species (cyan line). Upon celite adding then 0.33 equiv. of AgSbF6 allowed the mixture to turn
filtration under nitrogen and concentration in vacuum, the deep red, eventually delivering complex II in 73% yield.
[Pd3]SbF6 complex II became the almost exclusive species We then performed the same experiment adding a weaker
present in the crude mixture (Figure 1, blue line, 86% isolated Cu(II) oxidant (+0.28 V vs SCE, +0.6 V for Ag(I)).
yield). Taken together, these observation showed that addition Thus, 0.2 mmol of Pd(dba)2, 0.2 mmol of P(p-tol)3 and 0.1
of a silver salt is not necessary to trigger the formation of mmol of dimethyldisulphide were stirred for 1 hour at RT in
metal-aromatic [Pd3]+ clusters. This in turn suggested that the degassed THF. Two main resonances at 23 and 26 ppm were
chlorinated solvent played a key role in the formation of the observed in the corresponding 31P spectra, which likely
[Pd3]+ core.
correspond to species observed in chloroform presented in
Figure 1 (spectra in ESI). Addition of 0.066 mmol of Cu(OTf)2
triggered a slow turning of the solution towards the red and,
furthermore, the appearance of the diagnostic 31P resonance
of the [Pd3]OTf complex III at 15 ppm. The reaction was slower
than the previous one but once again a single resonance was
eventually detected upon 24 hours stirring at RT. These
findings paralleled observations made using chlorinated
solvents and provide preliminary insights on the mechanism of
formation of all-metal aromatic [Pd3]+ complexes.
We thus attempted the catalytic semireduction of
phenylpropyne
Triethylammonium formate and
1
preparing in-situ the [Pd3]+ complex.
1
were added as a THF
solution to eventually provide the same concentration
previously reported (0.1 M). Sadly, the reaction proved
significantly slower, requiring 24 hours to achieve full
conversion. The selectivity towards desired cis-alkene
2
remained similar (91%). Partial decomposition to Pd black was
observed through time although no alkane has been detected.
The reduced activity may be due to dba, which can likely
compete with the substrate slowing down its semi-reduction.
Figure 1. 31P spectra of crude samples from the synthesis of [Pd3]SbF6 complex
highlighting the diagnostic resonance of its three equivalent PPh3 units.
Table 1. Synthesis of [Pd3]+ complexes A-E
The overall process involves the formal oxidation of Pd(0) to
Pd(4/3). Considering the stoichiometry of the reaction, the
disulphide can oxidise the noble metal on average to Pd(I).
Radical palladium species7 might be sufficiently strong
reductant to trigger SET (single electron transfer) on a
chloroform molecule (-1.09 V vs SCE).8 This would form the
corresponding radical anion, which eventually collapses into a
radical and a chloride ion. The formal addition of a cationic
Pd(II) fragment on a neutral Pd(I) dimer9 would eventually
deliver the [Pd3]+ cation, isolable as a chloride salt, in analogy
to the step-wise assembly of [Au3]+ complexes from an Au(I)
cation and an Au(0) dimer reported by Bertrand.10
Ph
P
R'
S
Pd Pd
1 eq. Pd(dba)2
0.66 eq. PR3
Ph
CHCl3, N2,
RT, 3 hours
R3P
R'
S
S
Pd
PR3
0.33 eq.
PPh2C6H4SO3Na
R'
-NaCl
-
SO3
0.5 eq. (R'-S)2
A-E, 55-76%
Entry
R3
R’
Complex
Yield (%)
1
2
3
4
5
(4-F-C6H4)3
(4-F-C6H4)3
(4-Me-C6H4)3
(4-Me-C6H4)3
(Ph)2C6H4SO3Na
4-Me-C6H4
Me
A
B
C
D
E
71
55
59
76
72
The [Pd3]+ cation barely forms in CH2Cl2 while the reaction is
almost instantaneous in CCl4, being limited by the
solubilisation of Pd(dba)2 only. These results fit with the trend
of C-Cl BDE and reduction potentials among chlorinated
solvents.8
Ph
Me
Me
Reaction conditions: on 0.2 mmol scale, 0.01 M in degassed chloroform.
We then turned to another strategy to design better catalysts
for this reaction. Reasoning that [Pd3]+ is the major species in
chloroform even without the addition of any other metal salts,
we run the reaction with 0.33 equiv. of a sodiumsulfonate
phosphine. This should provide a zwitterionic complex by
filtering off coproduced NaCl. We prepared accordingly a small
library of complexes (Table 1).
Likely, the soluble AgX salt just favours anion metathesis
steered by the precipitation of AgCl in chloroform. This
suggests that [Pd3]+ complexes might be synthesised in any
given solvent if a suitable mild stoichiometric oxidant is added.
This is of particular interest for catalysis as it would allow the
in-situ preparation of [Pd3]+ complexes in any medium.
2 | J. Name., 2012, 00, 1-3
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Several organic fragments varying both electronic and steric the opposite edge of the metal kernel. It can cVoierwreAlratitcele Ownliitnhe
31
features could be easily introduced. Products
purified through a series of solvent washings3 (55-76% yield). could rationalise the activity of complexes with p-fluorinated
They were characterized via HRMS analyses and multinuclear phosphines ( and , entries 1-2). Their corresponding C3-
NMR experiments. In contrast to C3-symmetric complexes, as symmetric [Pd3]+ peers are indeed fairly active only for the
salt , not even attaining full conversion upon 96
, which showed [Pd3]+ cations in ESI+ experiments, we semi-reduction of
invariably observed protonated molecular adducts only for
hours.5 DFT modeling supports this hypothesis. The Pd-P
. This confirms their zwitterionic nature. Regarding NMR, two distance with the anionic phosphine shortens in compared
, while those with neutral ones are slightly longer (Figure 2,
A-E
were (slight) upfield shifts of the latter obseDrvOeI:d10b.1y039/PC6NDMT0R184a0nHd
A
B
I
1
A
-
D
D
pattern of resonances in a 2:1 ratio were observed for bridging to
I
thiolate fragments in 1H spectra. 31P signals followed suite. For details in ESI).
instance, a broad triplet and a broad doublet in a 1:2 ratio
Me
S
Me
S
were observed for
We then tested these new complexes on the catalytic semi-
reduction of phenylpropyne in the presence of a five-fold
A (at 17 and 13 ppm respectively).
PR3
Me
R3P
Me
R3P
Me
PR3
Me
cation I
zwitterion D
Pd Pd
Pd Pd
Pd-P
S
S
S
S
equivalent
Pd-P
1
Pd
PR3
Pd
P
(2.3021 Å)
excess of triethylammonium formate as hydrogen source
(Table 2). This excess ensures that the chemoselectivity
towards semi-reduction of alkynes is due to the catalytic
system rather than to a mere shortage of hydrogen donor.11
(2.2979 Å)
Ph
Ph
Pd-P
(2.2901 Å)
-
SO3
Table 2. Catalytic semi-reduction of phenylpropyne 1 with [Pd3]+ complexes A-E
Ph
P
R'
S
Pd Pd
Ph
R3P
R'
S
S
Pd
PR3
1 mol% cat.
2
3
4
R'
-
optimised at
M11/lacvp(d)
level
SO3
1
5 equiv HCO2H/NEt3
N2, THF, reflux
Figure 2. Comparison of optimised structures of cation I and zwitterion D; the Pd-P
distances in D at the M11/lacvp(d) level vary by +0.0041 and -0.0078 Å with the netural
and anionic phosphines respectively.
X
Entry
R
R’
t (h)
Conv. of
Sel. to
1 (%)a
2 (%)a
In all cases we noticed formation of traces of Pd black when
1
2
3
4
5
(4-F-C6H4)3
(4-F-C6H4)3
(4-Me-C6H4)3
(4-Me-C6H4)3
(Ph)2C6H4SO3Na
4-Me-C6H4
Me
8
5
6
3
3
97
85
87
78
77
77
conversion of passed 70%, suggesting that zwitterionic
1
quant.
quant.
quant.
quant.
complexes A-E could be less robust than their cationic
analogues. Nonetheless, no trace of over-reduced alkane has
been ever detected, even prolonging warming after full
Ph
Me
Me
conversion. In these cases, partial isomerization to
however noticed (below 5% upon 24 hours).
3 and 4 was
Conditions: 0.35 mmol of 1, 0.1 M in THF, 1.75 mmol of triethylammonium
formate, 1 mol% of complexes A-E; a: 0.1 mL samples were periodically taken and
analysed by GC and GC-MS using p-xylene (0.3 mmol) as internal standard.
Table 3. Addition of ligands on the semi-reduction of 1 with complex D
Complexes with fluorinated phosphines proved active with
either aromatic or aliphatic bridging thiolates (entries 1 and 2).
The latter proved a better substitution pattern providing
complete conversion of substrate upon five hours. In both
cases, cis-selectivity resulted lower than that previously
observed (85% and 87% respectively). No traces of alkane
1 mol% of cat. D
X mol% of L
5 equiv HCO2H/NEt3
N2, THF, reflux
2
1
were detected. Alkenes
amount (6-8%). Complexes bearing two p-tolylphosphines
were even more active (entries 3 and 4). In combination with
3 and 4 formed in a comparable
Entry
L
mol%
t (h)
Conv. of
1 (%)a
quant.
quant.
quant.
quant.
Sel. to 2
(%)a
96
96
92
1
2
3
4
P(4-Me-C6H4)3
P(4-Me-C6H4)3
dba
3
10
6
4
24
24
4
bridging methylthiolates, complete conversion of
achieved within 3 hours, albeit at the expense of selectivity
towards (77%). and formed in 10% and 13% yield
respectively. The same outcome was observed using complex
(entry 5, 77%). Interestingly, the catalytic system using any
complex of the series proved both more active and less
1 was
P(Ph)2C6H4SO3Na
3
96
2
3
4
E
Conditions: as Table 2; a: by GC using p-xylene as internal standard.
Taken together, these results confirmed the complete
chemoselectivity of the catalytic system towards alkynes and
let us speculate that isomerisation of
A-E
selective than the best one previously developed based on C3-
symmetric [Pd3]+ complexes (Scheme 1, top).
2 could be due to Pd
We reasoned that this might be a trans-like effect12 of the
anionic ligand that would make more labile the phosphine on
species generated from the decomposition of the actual
catalyst in the reducing reaction medium.13 We wondered
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whether addition of a suitable ligand might slow down these clearly visible throughout the reaction, indicating that the
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undesired processes without quenching activity (Table 3).
We were delighted that addition of mol% of p- These results suggest that addition of a suitable ligand can be a
tolylphosphine proved decisive (entry 1). Full conversion of tool to stabilise the active species and to minimise undesired
was achieved upon 4 hours and the selectivity towards was isomerization of cis-olefins. We thus tried to exploit these
almost complete (96%). Isomers and formed in 2% yield hypotheses by reducing the catalyst loading (Table 4).
each. Furthermore, the homogeneous orange solution of
complex did not show any trace of formation of black metal
colloids before consumption of (photos in ESI). We then
added 10 mol% of phosphine (entry 2). Conversion of
complex remains stable in solution until DfuOllI:c1o0n.1v0e39rs/Cio6nDTo0f1840H
1.
3
1
2
3
4
Table 4. Reduction of catalyst loadings on semi-reduction of 1
D
1
X mol% of cat. D
Y mol% of L
(L = P(C7H8)3)
1
required 24 hours, in agreement with a stiffer competition
between substrates and ligands for metal coordination.
5 equiv HCO2H/NEt3
N2, (THF), 70 °C
Selectivity towards
2 remained 96% despite the prolonged
2
1
warming. As in the previous case, no traces of visible
heterogeneous aggregates appeared throughout this (longer)
experiment. The use of a coordinating electron-poor olefin as
dba confirmed the outcome previously observed generating a
[Pd3]+ complex in-situ (vide supra), requiring 24 hours to
Entry
X
(mol%)
1
0.3
0.1
0.05
0.05
0.03
Y
t (h)
Conv. of
Sel. to
2 (%)a
77
90
96
98
98
98
TOF
(h-1)b
18
137
329
582
1026
1242
(mol%)
1 (%)a
quant.
quant.
quant.
77
1
2
3
-
-
-
3
4
7
6
5
6
achieve full conversion of
black were observed together with a slightly more pronounced
formation of and (4% each). Addition of 3 mol% of the
sodium salt of a sulfonated phosphine gave with 96%
1 (entry 3). In this case, traces of Pd
4c
5c
6c,d
-
0.15
0.09
quant.
quant.
3
4
2
Conditions: 5.0 mg of complex D, 0.0035 mmol; THF was progressively reduced
among the series to keep a fixed concentration of D of 0.001 M; a: by GC; b:
determined from the linear slope of a time/moles of 2 curve, typically between
0.5 and 2-4 hours; c: without THF; d: without p-xylene as internal GC standard.
selectivity within 4 hours (entry 4) and the reaction mixture
remained once more a red homogeneous solution.
Ph
P
Me
S
Pd Pd
Ph
1 mol% of cat. D
(C7H8)3P
Me
3 mol% of L
Keeping in mind the practical viability of the synthetic method,
S
S
Pd
Me
-
we decided to fix the concentration of complex
mixture, reducing progressively the amount of THF to hold for
a higher amount of . This is possible thanks to the increased
solubility in apolar media of zwitterion compared to salt I.
Reducing the catalyst loading to 0.3 mol% proved beneficial for
selectivity (90%, entry 2) and full conversion was attained
upon four hours. Further reduction of
96% of upon 7 hours (entry 3). The reaction with 0.05 mol%
did not reach full conversion, and the consumption of ended
at 77% upon 6 hours (entry 4). In all these cases, formation of
traces of Pd black were observed. For comparison, first
D in the
SO3
P(C7H8)3
2, 96%
upon 4 hours
5 equiv HCO2H/NEt3
N2, THF, reflux
D
1
1,8
0,3
0,25
0,2
1
0 min (red)
30 min (orange)
1 h (cyan)
2 h (purple)
3 h (green)
4 h (blue)
1,6
1,4
1,2
1
D
D (2.4·10-5 M)
D (1.4·10-5 M) + 3 eq. L
0,15
0,1
D to 0.1 mol% provided
0,8
0,6
0,4
0,2
0
2
1
0,05
0
300
600
400
500
generation complexes, as I, were no longer active before full
300
350
400
450
500
550
650
700
750
800
conversion of alkyne at catalyst loadings lower than 1 mol%.
We then repeated our last attempt adding 0.15 mmol% of
phosphine (entry 5). To our delight, the solution remained
λ (nm)
Figure 3. UV-Vis absorption spectra of samples collected from the catalytic reaction of
Table 3, entry 1; 100 µl samples were taken via syringe and diluted with 4.9 mL of
degassed THF prior to analyses; the right box presents absorption spectra of pure D
(blue) and D with added tritolylphosphine (green) in THF.
homogeneous and
delivering almost exclusively
loading of to 300 ppm, observing a comparable outcome
(entry 6). Full conversion of 1.20 g of was observed upon 6
hours, with 98% selectivity using commercial reagents without
any previous purification. Pure has been isolated adsorbing
1
was completely converted upon 5 hours,
2
(98%). We further reduced the
D
1
We performed ESI-MS analyses on samples collected at regular
intervals to check the stability of
D during the reaction. This
2
method proved useful to detect a fading concentration of
the crude on a 2 cm silica pad and eluting with 5 mL of
pentane (1.15 g, 96% yield). This highlights the practical and
environmental viability of present method.14
cation
I throughout the reaction, in agreement with the
formation of visible Pd black.5 However, no Pd-containing ion
has been detected in this case, likely owing to a difficult
We are unaware of Pd-based catalytic systems active at so-low
ionisation of zwitterion
UV-Vis absorption to monitor the diagnostic bands of
D
(spectra in ESI). We thus resorted to
in the
loadings under transfer-hydrogenation conditions.6,11,13-14
D
region between 300 and 550 nm (Figure 3, box), which is not
affected by any reagent during the catalytic reaction. Figure 3
shows absorption spectra of reaction samples. Both bands in
the near UV (320-380 nm) and the saddle at 450 nm remains
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6DT01840H
Conclusions
We reported preliminary studies on the mechanism of
formation of triangular Pd complexes with delocalized metal-
metal bonds. Through this approach, the panel of readily
accessible structures has been further expanded, allowing to
access zwitterionic tripalladium complexes. These clusters
form in one-step from commercial reagents, are definitely
stable at the solid state and proved remarkable precatalysts
under transfer-hydrogenation conditions, highlighting the
great potential of all-metal aromatics in catalysis.
Notes and references
‡ We thank MIUR, UPCM and CNRS for funding. Research on
metal aromaticity is funded through SIR Project “AROMA-TriP”
(Grant No.: RBSI14NKFL, GM).
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Zwitterionic all-metal aromatic tri-Palladium complexes can catalyse the cis-selective semi-
reduction of internal alkynes at ppm levels