Aromatic PCN pincer palladium complexes: forming and breaking C[sbnd]C bonds

Article abstract of DOI:10.1016/j.jorganchem.2017.04.025

Through a salt metathesis reaction, (^t?BuPCN)Pd-ONO₂ (2) was prepared and used as a precursor for producing (^t?BuPCN)Pd-OH (3) and (^t?BuPCN)Pd-aryl acetylide complexes 4 (phenyl acetylide) and 5 (p-tolyl acetylide). The aryl acetylide complexes could also be prepared through another synthetic route: by condensation of 3 with the corresponding aryl acetylene. The reactivity of complexes 3 and 4 toward carbon dioxide was studied and it was found that both reactions give the hydrogen carbonate complex (6). The low reactivity of the Pd-acetylide bond was further confirmed by the fact that the propiolate complex undergoes decarboxylation to give 4. PCN palladium complexes are good catalysts for the decarboxylative cross coupling reactions between acetylene carboxylic acids and aryl halides. The yield of the cross coupling product was improved by adding a catalytic amount of CuI.

Full text of DOI:10.1016/j.jorganchem.2017.04.025

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Journal of Organometallic Chemistry xxx (2017) 1e8
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Aromatic PCN pincer palladium complexes: forming and breaking CeC
bonds
*
ꢀ
Abdelrazek H. Mousa, Andre Fleckhaus, Mikhail Kondrashov, Ola F. Wendt
Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P. O. Box 124, S-221 00 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Through a salt metathesis reaction, (tꢀBuPCN)Pd-ONO2 (2) was prepared and used as a precursor for
producing (tꢀBuPCN)Pd-OH (3) and (tꢀBuPCN)Pd-aryl acetylide complexes 4 (phenyl acetylide) and 5 (p-
tolyl acetylide). The aryl acetylide complexes could also be prepared through another synthetic route: by
condensation of 3 with the corresponding aryl acetylene. The reactivity of complexes 3 and 4 toward
carbon dioxide was studied and it was found that both reactions give the hydrogen carbonate complex
(6). The low reactivity of the Pd-acetylide bond was further confirmed by the fact that the propiolate
complex undergoes decarboxylation to give 4. PCN palladium complexes are good catalysts for the
decarboxylative cross coupling reactions between acetylene carboxylic acids and aryl halides. The yield
of the cross coupling product was improved by adding a catalytic amount of CuI.
Article history:
Received 9 February 2017
Received in revised form
12 April 2017
Accepted 19 April 2017
Available online xxx
Dedicated to Gerard van Koten on the
occasion of his 75th birthday.
Keywords:
© 2017 Elsevier B.V. All rights reserved.
Aromatic pincer complexes
PCN pincer ligand
Palladium
Carboxylation
Decarboxylative cross coupling
1. Introduction
many catalytic reactions, their actual demonstration mainly include
metal-alkyl bonds; there are very few examples of stoichiometric
Transition metal pincer complexes have found great applica-
tions in catalysis due to their high thermal stability compared to
other organometallic complexes as a result of the tridentate che-
lation of the pincer ligand [1]. The most common examples in the
literature are based on symmetrical pincer architectures, e.g. PCP
[2], NCN [3] or POCOP [4], which are all accessible through
straightforward synthetic routes. Complexes of unsymmetrical
pincer ligands are much less common and include e.g. PCN [5], PCO
[6] and POCN [7] ligand frameworks. Although relatively long
synthetic approaches are required to prepare these complexes they
sometimes display different reactivities compared to the symmet-
ric ones through, for example, the hemilabile character of the O/N
pincer arm. One area where pincer complexes have been utilized is
the stoichiometric and catalytic activation of CO2 and it is notable
that pincer ligands together with carbenes seem particularly suit-
able to induce a high reactivity in insertions into the metal e carbon
bond to form the corresponding metal carboxylate complexes
[4e,8]. However, although such insertions have been proposed in
insertions of CO2 into late metal-carbon sp2- and sp-bonds [9].
Recently we published the first (PCN) Pd complexes [5f] and here
we continue to explore their reactivity reporting the synthesis of
hydroxide and acetylide derivatives and their reactivity towards
CO2 and phenyl iodide. Furthermore, the decarboxylation of (PCN)
Pd phenyl propiolate complex at room temperature is reported
together with a catalytic decarboxylative cross coupling reaction
between the propiolic acids and aryl halides.
2. Experimental section
General Procedures and Materials. All experiments were car-
ried out under an atmosphere of argon or nitrogen using standard
Schlenk or high vacuum techniques [10] unless otherwise noted.
Anhydrous solvents were obtained from a Braun SPS-800 system or
distilled from sodium/benzophenone ketyl radical. NMR experi-
ments were carried out using J. Young NMR tubes. All chemicals
were purchased from Acros, Alfa Aesar or Sigma-Aldrich. 1H, 31P
{1H}, and 13C{1H} NMR were recorded on a Varian Unity INOVA 500
spectrometer operating at 499.77 MHz (1H) using C6D6 unless
noted. Chemical shifts are given in ppm downfield from TMS using
residual solvent peaks (1H and 13C{1H} NMR) or H3PO4 as reference.
* Corresponding author.
E-mail address: ola.wendt@chem.lu.se (O.F. Wendt).
http://dx.doi.org/10.1016/j.jorganchem.2017.04.025
0022-328X/© 2017 Elsevier B.V. All rights reserved.
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Multiplicities are abbreviated as follows: (s) singlet, (d) doublet, (t)
triplet, (q) quartet, (m) multiplet, (br) broad. Elemental analyses
were performed by Mikroanalytisches Laboratorium KOLBE (Mül-
heim an der Ruhr, Germany). The (PCN)H ligand, the [PCN]Pd-Cl
complex (1) and [PCN]Pd-I complex (9) were prepared according
to previously published procedures [5f].
2-[(N,N-Dimethylamino)methyl]-6-[(di-tert-butylphos-
phino)-metyl]phenylpalladium p-methylphenylacetylide [PCN]
Pd-CC-Tol (5). Method A: In a J. Young NMR tube 3 (4.2 mg,
10.1 mmol, 1.00 eq.) and p-tolylacetylene (2.3 mg, 19.8 mmol, 1.96
eq.) were dissolved in C6D6 inside the glove box. After less than
5 min full conversion to the product can be observed in NMR
spectra. Evaporation, filtration with hexane, and evaporation yiel-
ded 5.0 mg (96%) of the product. Crystallization from a hexane
solution at e 20 ꢁC gave single crystals suitable for X-ray diffraction.
Method B: In a screw capped vial 2 (46.3 mg, 0.100 mmol, 1.00 eq.),
K2CO3 (72.0 mg, 0.522 mmol, 5.22 eq.) and p-tolylacetylene (2.3 mg,
0.200 mmol, 2.00 eq.) were stirred for one day. The solvent was
evaporated, it was filtered with benzene and evaporated again
yielding 57.0 mg of a NMR-pure product as a black solid. Crystal-
lization from hexane at e 20 ꢁC gave the product as colourless
2-[(N,N-Dimethylamino)methyl]-6-[(di-tert-butylphos-
phino)-metyl]phenylpalladium nitrate [PCN]Pd-ONO2 (2). 1
(64.0 mg, 0.150 mmol, 1.00 eq.) and AgNO3 (26.8 mg, 0.158 mmol,
1.05 eq.) were stirred in 5 mL THF for 2 days. After evaporation the
residue was dissolved in toluene, filtered and the filtrate was
evaporated yielding 69.1 mg (99%) of the product as a pale yellow
solid. Solvent vapour diffusion (benzene/hexane) at 4 ꢁC gave single
crystals suitable for X-ray analysis. 1H NMR (500 MHz, C6D6)
d 6.94
(td, J ¼ 7.5,1.4 Hz,1H), 6.77 (d, J ¼ 7.8 Hz,1H), 6.58 (d, J ¼ 7.3 Hz,1H),
3.23 (s, 2H), 2.72 (d, J ¼ 9.4 Hz, 2H), 2.36 (d, J ¼ 2.1 Hz, 6H), 1.12 (d,
needles (33.3 mg, 56%). 1H NMR (500 MHz, C6D6)
d 7.66 (d,
J ¼ 14.1 Hz, 18H). 13C{1H} NMR (126 MHz, C6D6)
d
153.1 (s), 148.1 (d,
J ¼ 8.0 Hz, 2H), 7.08 (t, J ¼ 7.4 Hz, 1H), 7.02 (d, J ¼ 7.0 Hz, 1H)#, 7.01
(d, J ¼ 8.1 Hz, 2H)#, 6.83 (d, J ¼ 7.2 Hz, 1H), 3.57 (s, 2H), 3.13 (d,
J ¼ 9.1 Hz, 2H), 2.70 (d, J ¼ 2.1 Hz, 6H), 2.11 (s, 3H), 1.34 (d,
J ¼ 13.9 Hz, 18H), #signals are overlapping. 13C{1H} NMR (126 MHz,
J ¼ 15.1 Hz),147.9 (d, J ¼ 1.1 Hz),124.8 (s),122.1 (d, J ¼ 21.1 Hz),120.5
(s), 70.6 (d, J ¼ 2.5 Hz), 48.8 (d, J ¼ 2.3 Hz), 34.3 (d, J ¼ 16.3 Hz), 32.9
(d, J ¼ 29.1 Hz), 28.3 (d, J ¼ 4.6 Hz). 31P{1H} NMR (202 MHz, C6D6)
d
90.94. Anal. Found (calc. for (C18H31N2O3PPd) C, 46.55 (46.91); H,
C6D6)
d
170.9 (s), 149.6 (s), 147.9 (d, J ¼ 15.2 Hz), 133.5 (s), 131.0 (s),
6.83 (6.78); N, 5.99 (6.08).
128.6 (s), 126.9 (d, J ¼ 15.5 Hz), 124.3 (s), 121.0 (d, J ¼ 21.0 Hz), 119.3
(s), 109.1 (s), 74.2 (d, J ¼ 2.4 Hz), 50.7 (d, J ¼ 2.5 Hz), 37.7 (d,
J ¼ 27.8 Hz), 34.3 (d, J ¼ 17.3 Hz), 29.0 (d, J ¼ 4.7 Hz), 20.9 (s) (one
carbon missing/hidden under C6D6). 31P{1H} NMR (202 MHz, C6D6)
2-[(N,N-Dimethylamino)methyl]-6-[(di-tert-butylphos-
phino)-metyl]phenylpalladium hydroxide [PCN]Pd-OH (3). In a
Straus flask 2 (231 mg, 0.502 mmol, 1.00 eq.), KOH (560 mg,
9.98 mmol, 19.9 eq.) and 25 mL of THF were combined inside the
glove box and the mixture was sonicated and then stirred for 19 h.
After evaporation, swivel filtration with benzene and evaporation
of the filtrate the crude product was obtained as a white powder
(208 mg, 99%). Single crystals suitable for X-ray analysis were ob-
tained from a hexane/benzene solution at ꢀ20 ꢁC inside the glo-
d
98.40. Anal. Found (calc. for (C27H38NPPd) C, 63.21 (63.09); H, 7.46
(7.45); N, 2.70 (2.73).
[PCN]Pd-OCO2H (6). In a J. Young NMR tube, 6.0 mg (15.0 mmol)
of 3 was dissolved in 0.5 ml C6D6 inside the glove box. The tube was
degassed (three freeze-pump-thaw cycles) using the high vacuum
line and the solution was pressurized with 8 atm of CO2. The re-
action was monitored by 1H and 31P{1H} NMR and resulted in 95%
of (PCN)PdOCO2H and 5% of (PCN)PdONO2. Single crystals suitable
for X-ray analysis were obtained by slow fusion of n-hexane into a
concentrated solution of the crude product in C6D6 at 5 ꢁC. 1H NMR
vebox. 1H NMR (500 MHz, C6D6)
d
7.03 (td, J ¼ 7.4, 1.2 Hz, 1H), 6.95
(d, J ¼ 7.5 Hz, 1H), 6.76 (d, J ¼ 7.5 Hz, 1H), 3.56 (s, 2H), 2.95 (d,
J ¼ 9.2 Hz, 2H), 2.64 (d, J ¼ 2.0 Hz, 6H), 1.22 (d, J ¼ 13.6 Hz,
18H), ꢀ1.32 (s, 1H). 13C{1H} NMR (126 MHz, C6D6)
d 154.9 (s), 148.6
(s), 147.7 (d, J ¼ 15.3 Hz), 123.7 (s), 121.8 (d, J ¼ 20.8 Hz), 120.0 (s),
72.9 (d, J ¼ 2.5 Hz), 48.9 (d, J ¼ 2.6 Hz), 35.8 (d, J ¼ 29.0 Hz), 34.4 (d,
J ¼ 16.0 Hz), 29.2 (d, J ¼ 5.2 Hz). 31P{1H} NMR (202 MHz, C6D6)
(500 MHz, C6D6)
d
6.95 (t, J ¼ 7.4 Hz, 1H), 6.81 (d, J ¼ 7.4 Hz, 1H),
6.62 (d, J ¼ 7.3 Hz, 1H), 3.34 (s, 2H), 2.80 (d, J ¼ 9.2 Hz, 2H), 2.56 (s,
6H), 1.24 (d, J ¼ 13.9 Hz, 18H). 13C{1H} NMR (126 MHz, C6D6)
d 163.4
d
91.44. The compound is very hygroscopic and failed to give a
(s), 155.6 (s), 148.7 (s), 148.5 (d, J ¼ 15.3 Hz), 124.7 (s), 122.2 (d,
satisfactory elemental analysis.
2-[(N,N-Dimethylamino)methyl]-6-[(di-tert-butylphos-
phino)-metyl]phenylpalladium phenylacetylide [PCN]Pd-CC-Ph
J ¼ 21.0 Hz), 120.6 (s), 71.5 (s), 49.4 (s), 34.7 (d, J ¼ 16.0 Hz), 33.8 (d,
J ¼ 28.7 Hz), 29.0 (d, J ¼ 4.6 Hz). 31P{1H} NMR
d 89.74.
{[PCN]Pd}2-(m
-CO3)(7). Heating 6 to 100 ꢁC and removing the
(4). Method A: 2 (4.6 mg, 10.0
lene (2.2 L, 2.0 mg, 20.0 mol, 2.00 eq.) were stirred under ni-
trogen together with KOH (8.60 mg, 150 mol, 15.0 eq.) in 0.5 mL of
dry THF for 16 h. After evaporation and filtration with benzene,
evaporation yielded 4.8 mg (96%) of the product. Slow solvent
evaporation from a hexane solution gave single crystals suitable for
X-ray diffraction. Method B: 2 (138 mg, 0.3 mmol, 1.00 eq.) and
m
mol, 1.00 eq.) and phenyl acety-
volatiles under high vacuum or reacting 3 with 4 atm of CO2 at
room temperature gave 7. The complex was characterised in situ by
1H and 31P{1H} NMR spectroscopy and was 77% pure (with 23% 2).
m
m
m
1H NMR (500 MHz, C6D6)
d
7.03 (t, J ¼ 7.4 Hz, 2H), 6.92 (d, J ¼ 7.2 Hz,
2H), 6.77 (d, J ¼ 7.3 Hz, 2H), 3.62 (s, 4H), 2.91 (d, J ¼ 9.4 Hz, 4H)#,
2.88 (br s, 12H)#, 1.36 (d, J ¼ 13.7 Hz, 36H), #signals are overlapping.
31P{1H} NMR
d 88.64.
phenyl acetylene (66
m
L, 0.6 mmol, 2.00 eq.) were stirred under
Reaction of [PCN]Pd-CC-Ph (4) with CO2. In a J. Young NMR
tube, 5.0 mg (10.0 mol) of 4 was dissolved in 0.5 mL C6D6. The tube
nitrogen together with K2CO3 (207 mg, 1.5 mmol, 5.0 eq.) in 15 mL
of dry THF for 24 h. After evaporation and filtration with benzene,
evaporation yielded 120 mg (80%) of the product. 1H NMR
m
was degassed (three freeze-pump-thaw cycles) using the high
vacuum line and the solution was pressurized with 8 atm of CO2.
The reaction was followed by 1H and 31P{1H} NMR spectroscopy.
Pressurizing with CO2 was repeated at least three times until full
conversion to 6 was achieved.
(500 MHz, C6D6)
d
7.72 (d, J ¼ 8.2, 2H), 7.20e7.17 (m, 2H), 7.08 (t,
J ¼ 7.0, 1H), 7.04e6.99 (m, 2H), 6.83 (d, J ¼ 7.3 Hz, 1H), 3.57 (s, 2H),
3.13 (d, J ¼ 9.1 Hz, 2H), 2.69 (d, J ¼ 1.7 Hz, 6H), 1.33 (d, J ¼ 13.9 Hz,
18H). 13C{1H} NMR (126 MHz, C6D6)
d
171.2 (d, J ¼ 1.9 Hz), 150.0 (s),
Reaction of [PCN]Pd-ONO2 (2) with sodium phenyl-
148.3 (d, J ¼ 15.2 Hz), 131.5 (s), 130.6 (s), 128.8 (d, J ¼ 15.4 Hz), 128.3
(s), 124.7 (d, J ¼ 6.6 Hz), 121.4 (d, J ¼ 21.0 Hz),119.8 (s), 109.7 (s), 74.6
(d, J ¼ 2.3 Hz), 51.1 (d, J ¼ 2.2 Hz), 38.1 (d, J ¼ 27.9 Hz), 34.7 (d,
J ¼ 17.3 Hz), 29.4 (d, J ¼ 4.7 Hz). 31P{1H} NMR (202 MHz, C6D6)
propiolate. In a J Young NMR tube, 9.22 mg (20.0
2 was dissolved in 0.5 mL C6D6 then (3.40 mg, 20.0
m
m
mol, 1.00 eq.) of
mol,1.00 eq.) of
sodium phenyl propiolate was added. The tube was sonicated and
the reaction was followed by 1H and 31P{1H} NMR spectroscopy. A
slight excess of sodium phenyl propiolate was added after 8 h of
sonication to achieve full conversion to the product.
d
98.44. Anal. Found (calc. for (C26H36NPPd)) C, 62.59(62.46); H,
7.23 (7.26); N, 2.76(2.80).
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Reaction of [PCN]Pd-CC-Ph (4) with aryl halide. In a J Young
NMR tube, 5.0 mg (10.0 mol, 1.00 eq.) of 4 was dissolved in
0.45 mL C6D6 and phenyl bromide (10.1 L, 100 mol, 10.0 eq.) or
mol, 10.0 eq.) was added. The tube was
m
m
m
phenyl iodide (11.2 mL, 100 m
heated to 50e100 ꢁC and the reaction was followed by 1H and 31P
{1H} NMR spectroscopy. The cross coupling product, diphenylace-
tylene was observed in the 1H NMR spectrum.
General procedures for the catalytic decarboxylative
coupling reactions of phenylpropiolic acid with aryl halides.
29.2 mg (0.2 mmol) phenyl propiolic acid, 0.2 mmol aryl halide, and
276.4 mg (2.0 mmol) K2CO3 were mixed together in a small vial and
3 mL of the solvent was added to the vial; then the catalyst was
added in 2.5% mol ratio. The reaction mixture was heated to 135 ꢁC
for 48 h. The resulting mixture was poured into 10 mL of saturated
aqueous NH4Cl solution and the organic product was extracted
with 3 ꢂ 5 mL Et2O, dried over anhydrous MgSO4 and filtered over
Celite.
Fig. 1. Molecular structure of 2 at the 30% probability level. Hydrogen atoms and the
benzene solvent are omitted for clarity. Selected bond lengths (Å) and bond angles (ꢁ):
Pd1-C1 ¼ 1.976(4), Pd1-P1 ¼ 2.2702(11), Pd1-N1 ¼ 2.192(3), Pd1-O1 ¼ 2.167(3), O1-
N2 ¼ 1.260(6), O2-N2 ¼ 1.253(7), O3-N2 ¼ 1.229(7), C1- Pd1-P1 ¼ 82.97 (12), N1-
Pd1-P1 ¼ 164.97 (10), O1- Pd1-P1 ¼ 101.28 (10), O1- Pd1-N1 ¼ 92.68 (14), C1- Pd1-
O1 ¼ 172.48 (16).
Crystallography. Intensity data were collected with an Oxford
Diffraction Excalibur
3 system, using u-scans and MoKa
(
l
¼ 0.71073 Å) radiation [11]. The data were extracted and inte-
grated using Crysalis RED [12]. The structures were solved by direct
methods and refined by full-matrix least-squares calculations on F2
using SHELXT [13], SHELXL [14] and OLEX2 [15]. Molecular graphics
were generated using Crystal Maker 8.7 [16].
structure was determined by X-ray diffraction analysis showing a
monomeric hydroxide complex (Fig. 2). The complex is very hy-
groscopic and therefore failed to give a proper elemental analysis; it
crystallised with three molecules of water. The hydroxide complex
showed no tendency, neither in solution nor in the solid state, to
form bridging dinuclear complexes as was recently reported by
Goldberg for the analogous tꢀBuPCO pincer palladium hydroxide
complex and the pyrazole-based PCN pincer palladium hydroxide
complex which, however, were prepared using a different synthetic
route [5g,6b]. The Pd-O bond distance of 2.115(4) Å in 3 is slightly
longer than the corresponding distance in the pyrazole-based PCN
pincer palladium hydroxide complex (2.078(7) and 2.080(7) Å for
the two molecules in the asymmetric unit) [5g]. As previously
observed [5g,17e18] the water molecules in the structure of 3 form
a hydrogen bonded network connecting two terminal hydroxide
complexes.
3. Results and discussions
3.1. Synthesis of (PCN)Pd complexes
The (PCN)Pd-Cl(1) complex was synthesized through cyclo-
metallation of the (PCN)H pincer preligand with (PhCN)2PdCl2 us-
ing our previously published procedure [5f]. A salt metathesis
reaction with AgNO3 produced the complex (tꢀBuPCN)Pd-ONO2(2)
in 99% yield and this was used as a precursor for preparing the
hydroxide and the aryl acetylide complexes as illustrated in Scheme
1. The molecular structure of 2 is given in Fig. 1. The hydroxide
complex (3) was synthesized by a procedure analogous to the one
used for (PCP)Pd-OH [17a]. Sonication of 2 with an excess of KOH in
THF under an inert atmosphere offered complex 3, which shows a
single 31P NMR peak at 91.44 ppm and a 1H NMR spectrum with an
upfield singlet at ꢀ1.32 ppm, which agrees well with the few
palladium hydroxide complexes based on pincer ligands that have
been reported [5g,6b,17e18]. Complex 3 was further characterised
by 1H, 31P{1H}, and 13C{1H} NMR spectroscopy and the molecular
There are only a few previous examples of palladium pincer
acetylide complexes and they were synthesized either through
base assisted ligand substitution [2a] or metathesis of free acety-
lene with hydride or alkyl complexes [18b,19]. Here, we obtained
Scheme 1. Synthesis of (PCN)Pd-hydroxide and arylacetylide complexes.
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3.2. Reaction of CO2 with 3
The reactivity of the hydroxide complex with carbon dioxide
was investigated. Compound 3 was pressurized with approxi-
mately 8 atm of CO2 using C6D6 as a solvent. In the 31P NMR
spectrum, two new signals at 89.74 ppm (95%) and 90.93 ppm (5%)
appeared within minutes. The 13C NMR spectrum displays a sharp
singlet for the major product at 163.4 ppm which is consistent with
formation of the expected hydrogen carbonate pincer complex (6),
cf. Scheme 2. Free CO2 was observed as well in the 13C NMR spec-
trum at 124.8 ppm [20]. Heating up to 100 ꢁC had no effect on the
product distribution. The 1H, 31P and 13C NMR spectra of the minor
product confirm its identity as complex 2, which is probably
formed due to presence of trace amounts of an NMR silent inor-
ganic nitrate salt in the hydroxide complex; the nitrate anion dis-
places the weakly bound hydrogen carbonate anion producing
complex 2. The structure of complex 6 was further confirmed by an
X-ray diffraction analysis (Fig. 4). Single crystals were obtained by
slow diffusion of n-hexane into a concentrated benzene-d6 solution
of the crude product, and the recrystallization also allowed for a
separation of 6 and 2. When a lower pressure of CO2 (4 atm) was
used complex 6 was not observed, and instead the 31P NMR spec-
trum displayed a new broad signal at 88.63 ppm in addition to the
signal for complex 2 at 90.93 ppm. Similarly to above reaction,
heating had no effect on the product distribution.
Fig. 2. Molecular structure of 3 at the 30% probability level. Hydrogen atoms and three
water molecules are omitted for clarity. Selected bond lengths (Å) and bond angles (ꢁ):
Pd1-C1 ¼ 1.980(4), Pd1-P1 ¼ 2.2539(11), Pd1-N1 ¼ 2.174(4), Pd1-O1 ¼ 2.115(4), C1-
Pd1-P1 ¼ 82.82 (13), N1- Pd1-P1 ¼ 164.47 (12), O1- Pd1-P1 ¼ 103.93 (11), O1- Pd1-
N1 ¼ 91.31 (15), C1- Pd1-O1 ¼ 173.02 (16).
Crystallization by slow diffusion of n-hexane into the crude
benzene-d6 solution led to the formation of small crystals, which
were not suitable for X-ray diffraction. However, the 31P NMR
spectrum of the crystals displayed a sharp signal corresponding to
complex 6 at 89.74 ppm with trace amount of the original complex.
To confirm the identity of the second insertion product, 7, and study
its relation with the hydrogen carbonate complex, 6 was synthe-
sized as we described above and the solution was heated at 100 ꢁC
under vacuum. The solid residue was kept for drying on the high
arylacetylide complexes 4 and 5 either via the former pathway or
through a new synthetic route involving CeH activation of the free
acetylene where the hydroxide ligand acts as an internal base. Both
reactions take place at room temperature, which is advantageous to
the metathesis pathways which generally suffer from using ther-
mally unstable complexes as starting materials and which require
high temperatures in case of the alkyl metal complexes. Moreover,
formation of byproducts such as styrene was observed in the
metathesis reactions. The new arylacetylide complexes 4 and 5
were fully characterised with 1H, 31P{1H}, and 13C{1H} NMR spec-
troscopy, elemental analysis and X-ray diffraction analysis, cf. Fig. 3.
vacuum line for approximately 5 h and redissolved in C6D6. The 31
P
{1H} NMR spectrum showed a broad signal at 88.63 ppm as
observed above. This indicates that the insertion of CO2 into the
hydroxide complex is reversible and that 7 can be formulated as a
dimeric compound with a bridging carbonate; this was confirmed
Fig. 3. Molecular structures of 4 and 5 at the 30% probability level. Hydrogen atoms are omitted for clarity.
Scheme 2. Insertion of CO2 into (PCN)Pd-hydroxide complex (3).
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