Computational and carbon-13 NMR studies of Pt-C bonds in P-C-P pincer complexes

Article abstract of DOI:10.1039/c5dt02049b

A ¹³C{¹H} NMR based investigation was conducted to examine the electronic properties of C(aryl)-M bonds and their trans influence in P-C(aryl)-P pincer complexes. A series of structurally related platinum pincer complexes were ration

Full text of DOI:10.1039/c5dt02049b

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Computational and carbon-13 NMR studies of
Pt–C bonds in P–C–P pincer complexes†
Cite this: DOI: 10.1039/c5dt02049b
Yu-Xiang Jia, Xiang-Yuan Yang, Wee Shan Tay, Yongxin Li, Sumod A. Pullarkat,
Kai Xu, Hajime Hirao and Pak-Hing Leung*
A
13C{1H} NMR based investigation was conducted to examine the electronic properties of C(aryl)–M
bonds and their trans influence in P–C(aryl)–P pincer complexes. A series of structurally related platinum
pincer complexes were rationally designed and their corresponding 13C–195Pt coupling constants were
systematically examined. By methodical substitution of the ligand trans to the organometallic C(aryl)–Pt
bond, this study revealed the significant influence of the ligands on the nature of the C(aryl)–M bonds.
The single crystal X-ray analysis of the complexes and computational studies further confirmed the obser-
vations that the C–M bond exhibits significant π-character.
Received 30th May 2015,
Accepted 7th October 2015
DOI: 10.1039/c5dt02049b
www.rsc.org/dalton
Introduction
The important role played by phosphorus ligands in transition
metal catalyzed organic synthesis is well established. Many
well-established diphosphine ligands, such as BINAP, DIPAMP
and CHIRAPHOS are able to efficiently project their well-
defined structural features to control the stereochemistry
during the course of the asymmetric reaction.1 It needs to be
noted that the soft phosphorus atoms of these diphosphines
readily function as both σ-donors as well as π-acceptors. This
particular donor–acceptor property renders phosphine ligands
as ideal catalyst-supporters. This dual role is especially critical
Fig. 1 PCP- and PC-type Pt complexes.
site can be rationally controlled by a specially designed pincer
ligand. In contrast to the aforementioned diphosphine
for reactions in which the mechanisms involve vast changes in ligands, the active catalytic site in the P–C–P complex (type 1)
electronic density, such as oxidative addition of substrates and
reductive elimination of products. Noticeably, the active cata-
is the coordination position trans to the aromatic carbon. The
two phosphorus donors in the P–C–P complex occupy the posi-
lytic sites of diphosphine–metal catalysts are typically located tions cis relative to the catalytic site. Due to the symmetry of
trans to the phosphorus donors.
the d-orbitals, it is well established that square–planar and
Similar to their diphosphine analogues, square–planar
octahedral transition metal complexes exert much stronger
metal complexes (Fig. 1, type 1) supported by P–C–P ligands trans electronic influences than the analogous cis interactions.
frequently demonstrate high stability and attractive catalytic
properties.2 The pincer catalysts can direct their stereocontrol
Therefore the (C)aryl–M bonds in type 1 complexes are the
most important and direct contributors to the activation of
efficiently via their pincer arms or from the chirality of the substrates in catalytic processes.
phosphorus donors. Therefore, for a catalytic process that does
Despite the substantial development of P–C(aryl)–P pincer
not involve changes in the coordination geometry of the P–C–P
complexes since the late 1970s, relatively little attention has
pincer catalyst, the stereochemistry of the sole M–X catalytic been directed towards the electronic properties of the M–C
bonds or their trans influences in catalysis. Roddick and co-
workers used ν(CuO) vibrational spectroscopy to study the
electronic properties of trans-C–M–CuO pincer complexes
bearing different phosphorus substituents in the cis posi-
tions.3 Several groups also studied the electrochemistry of
similar complexes.3a A review of the literature revealed that,
based on the solid state structural features, monodentate (C)aryl–
M bonds are often described as simple C → M σ-bonds.4
Division of Chemistry & Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637371.
E-mail: pakhing@ntu.edu.sg
†Electronic supplementary information (ESI) available: Experimental and spec-
tral data, crystallographic refinement data and computational data. CCDC
1401358 (6a) and 1401359 (6c). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5dt02049b
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The aryl ligands in these reported C(aryl)–M complexes are
inherently free to rotate around the organometallic bond. In
contrast, the central aryl-rings in (type 1) pincer complexes are
structurally rigid, particularly when the pincer side arms
contain substituents which restrict the 5-membered P–C ring
conformations. Consequently, the central aromatic ring and
the square–planar pincer complexes are expected to be locked
into co-planar geometries. We believe that this structurally
rigid framework makes it possible for the appropriate π or π*
orbitals of the aromatic ring to overlap effectively with the
metal d-orbitals, thus directly influencing the electronic pro-
perties at the M–X catalytic site. Indeed, we have recently estab-
lished such π-bonding characters in a group of related ortho-
metalated (Fig. 1, type 2) complexes.5 In these complexes, the
π* orbitals of the highly conjugated aromatic rings overlap
with the metal orbitals, thus rendering a strong trans-elec-
tronic withdrawing effect directed towards the organometallic
M–C bonds. Accordingly, the trans C–M–X coordination posi-
tion shows a preferential affinity towards π-donating moieties,
such as oxygen atoms in sulfoxides, ketones, amides and even
those in the classic non-coordinating perchlorate anion.6
Notably, the cyclometalated (type 2) complexes also exhibit
typical Lewis acid properties and can be utilized effectively for
the catalytic activation of dienophiles and Michael acceptors.7
We therefore decided to conduct a detailed study in order to
determine whether similar π-C–M bonding characters indeed
occur in the less conjugated (type 1) pincer complexes.
Insights gained from such a detailed investigation is central to
understanding the electronic properties of these organometal-
lic bonds and consequently to the future design of pincer com-
plexes as chiral catalysts for asymmetric transformations.
Scheme 1 Syntheses of the pincer ligands and their Pt complexes.
Table 1 Selected NMR data for the pincer-Pt complexesa
Complexes
[6/7/8]
13C{1H} and 31P{1H}
NMR data
6 (Z = H) 7 (Z = Me) 8 (Z = Br)
[a], X = Cl
δ
13C–195Pt (ppm)
145.9
936
142.0
944
145.1
955
1JPt–C (Hz)
1JPt–PPh (Hz)
2967
157.8
688
2974
154.6
688
2935
157.0
703
2
[b] (ClO4),
δ
13C–195Pt (ppm)
X = PPh3
1JPt–C (Hz)
trans-(2JC –P) (Hz)
90
90
91
ipso
1JPt–PPh (Hz)
1JPt–PPh (Hz)
2055
2768
159.5
677
2057
2775
155.9
677
2085
2743
158.7
688
3
2
[c], X = CN
δ
13C–195Pt (ppm)
1JPt–C (Hz)
1JPt–PPh (Hz)
δ
13C–195Pt (ppm)
1JPt–C (Hz)
1JPt–PPh (Hz)
2781
135.3
951
2790
131.3
947
2757
134.8
966
2
[d], X = NO3
3086
3099
3057
2
a All NMR spectra were recorded in CDCl3 except for complexes 6d–8d,
which were recorded in acetone-d6.
Results and discussion
In view of the fact that most of the catalytic asymmetric synth-
eses are conducted in homogeneous solutions, it is our judg-
ment that the determination of the C–M bond properties in
solution is of paramount importance. For this specific
purpose, 13C{1H} NMR spectroscopy is an ideal and powerful
tool for the direct investigation of the organometallic bonds in
diamagnetic pincer complexes. It should be noted that
although the natural abundance of 13C is only about 1.1%, the
simple nuclear spin of this isotope generates readily interpret-
able signals. Furthermore, the carbon atom involved in the
C–M bond in these pincer complexes is located at the central
position of the two conjoined 5-membered chelates. As routi-
nely observed from the analogous 31P{1H} NMR spectroscopy,
this unique structural feature allows the C–M (13C{1H}) reson-
ance signals to exhibit characteristic high “coordination
shifts” and thus renders them generally discernible from the
other aromatic carbon signals within the pincer framework.8
In order to focus on the nature of the C–M bonds, we designed
and prepared a series of platinum(II) pincer complexes for 13C
ments serve as the most direct and reliable indicators for the
bond strengths of these organometallic bonds in solution.
The designed pincer ligands can be obtained in high yields
with excellent optical purities from the hydrophosphination
reaction of the corresponding diketones by using a P–C cyclo-
metalated catalyst (Scheme 1). The pincer ligands are powerful
sequesters for platinum(II) ions. The reactions between
PtCl2(MeCN)2 or PtCl2(PPh3)2 and the generated ligands in the
presence of triethylamine generated highly stable chloro com-
plexes 6a–10a in high yields.2c,7b For a systematic investigation
of the trans electronic influence on the organometallic C–Pt
bond, the chloro ligands in complexes 6a–8a were subsequently
replaced by the anionic CN, NO3 and the neutral PPh3 ligands.
As anticipated, the 13C–195Pt signals of the synthesized
series of platinum pincer complexes are clearly identified in
the 13C{1H} NMR spectra. Interestingly, in the 100 MHz 13C
{1H} NMR spectra, the pincer carbon donors of almost all
these platinum complexes do not show any spin–spin coupling
with the two identical adjacent P atoms in the square–planar
complexes. Complex 8c is the only complex that shows a very
{1H} NMR investigations (Scheme
1 and Table 1). The
13C–195Pt coupling constants determined from NMR measure-
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small coupling constant (2JP–C = 2 Hz) between the cis orien- signals. It is interesting to note that in the corresponding 31P
tated donors.
{1H} NMR study of PPh3 complexes 6b, 7b and 8b, all three
Typically, the pincer carbon signals appear as a singlet complexes exhibited visibly weaker Pt–P coupling constants
associated with a pair of platinum satellites at distinctly high (2055–2085 Hz) for the Pt–PPPh fragments than their Pt–PPPh
3
2
chemical shifts. For example, the 13C{1H} NMR spectrum of counterparts (2743–2775 Hz). Clearly, the aromatic rings
complex 6a exhibits the C–Pt signal as a singlet signal at δ induce stronger trans influence than the phosphorus atoms.
145.9 which is, importantly, accompanied by a pair of clearly
Consistent with the solution phase NMR observations, the
visible platinum satellites (1JPt–C = 936 Hz) (Fig. 2a). However, solid state crystallographic study showed that the Cipso–Pt bond
the pincer aryl carbon shows the expected coupling to the sole in CN complex 6c [2.062(5) Å] is longer than the corresponding
trans phosphorus donor in complex 6b. The cationic complex organometallic bond of the two crystallographically indepen-
6b was prepared by replacing the chloro ligand in complex 6a dent molecules found in the single crystals of chloro complex
with a PPh3 ligand. The 13C{1H} NMR spectrum of complex 6b 6a [2.001(12) Å and 2.022(13) Å]. The observations from the
clearly shows Cipso–P coupling (90 Hz) between the two trans solution NMR and solid state crystallographic studies indi-
1
donor atoms (Fig. 2c). Interestingly, the JPt–C coupling cated that some π back bonding may indeed be operating
observed from PPh3 complex 6b (1JPt–C = 688 Hz) is markedly within the pincer Cipso–Pt bonds (Fig. 3 and 4).
weaker than that recorded for complex 6a (936 Hz). The 13C
A classic approach towards tuning the electronic properties
1
{1H} NMR study shows that the JPt–C coupling constant is of a particular carbon atom within an aromatic ring is to intro-
further reduced in complex 6c (1JPt–C = 677 Hz) in which a CN duce different substituents at the para-carbon positions. As
anion is coordinated to the pincer complex (Fig. 2b). On the shown in Table 1, the three cationic PPh3 complexes 6b, 7b
1
1
other hand, NO3 complex 6d shows a JPt–C coupling constant and 8b exhibit very similar JPt–C coupling constants (688, 688
(1JPt–C = 951 Hz) which is similar to the corresponding signal and 703 Hz, respectively). Likewise, JPt–C coupling constants
1
recorded for chloro complex 6a. As a further test, when the are recorded within a small range (936–955 Hz) for the three
chloro ligand in complex 6a was replaced by the anionic OAc neutral chloro complexes 6a, 7a and 8a. The lack of noticeable
counterpart, the resulting platinum complex 6e (X = OAc) electronic influence from the para-substituents is rather sur-
shows a singlet at δ 139.0 (13C{1H} NMR) with a 1JPt–C coupling prising. In order to confirm this observation, a very strong elec-
constant of 883 Hz.
These NMR studies clearly indicate that the organometallic
C–Pt bonds in the pincer complexes are significantly influ-
enced by the nature of trans donor atoms employed. A poten-
tial π-electron donor ligand, such as NO3, tends to somewhat
stabilize the organometallic Pt–C bond as determined by its
respective Pt–P coupling constants. On the other hand, the Pt–
C bonds are noticeably weakened by the trans-positioned CN
and PR3 ligands which are typically considered as electronic
13
π-acceptors. Technically, it should be noted that no 13
C
ipso
– CCN
couplings could be detected from CN complexes 6c, 7c
and 8c, due to the extremely low abundance of the required
13
13Cipso–13CCN fragments. Furthermore, the
C
signals are
CN
Fig. 3 Molecular structure of complex 6c. All hydrogen atoms except
H(C6) and H(C6A) are omitted for clarity.
usually not discernible from the other aromatic carbon
Fig. 4 Molecular structure of one independent molecule of complex
6a. All hydrogen atoms except H(C7) and H(C28) are omitted for clarity.
Another independent molecule which differs only slightly in bond
angles and bond lengths, is shown in Fig. 45 of the ESI.†
Fig. 2 13C{1H} NMR signals for the Pt–C bonds in (a) complex 6a, (b)
complex 6c, and (c) complex 6b.
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tron-withdrawing fluorine atom was placed at the para-position estingly, both chiral catalysts required the presence of an
to form complex 9a. The resulting complex 9a displays a additional base to activate the P–H bond in the asymmetric
singlet at δ 140.7 (13C{1H} NMR) with a 1JPt–C coupling constant addition reaction. An external base such as triethylamine can
of 951 Hz. A further confirmation was done by the introduc- be introduced to initiate the reaction, as illustrated in
tion of the π-electron donating OMe group to form complex Scheme 1. Alternatively, an acetate anion associated with the
10a. This para-substituted OMe complex exhibits a singlet at δ catalysts can also serve as an effective internal base for the
136.6 (13C{1H} NMR) with a 1JPt–C coupling constant of 952 Hz. AHP process. From a mechanistic viewpoint, it is evident that
Clearly, the electronic properties of the para-substituents in the type 1 and 2 complexes operate via different catalytic path-
complexes 6–8 do not have significant effects on the organo- ways during the course of the AHP. A PC-cyclometalated
metallic C–Pt bonds.
complex (type 2) allows both the phosphine nucleophile and
To further investigate the subtle Pt–C interactions that the reacting substrate to coordinate simultaneously with the
cannot be evaluated directly by NMR spectroscopy, we con- metal center in the transition state. As a result, an intra-
ducted natural bond orbital (NBO)9 analyses on complexes 6a molecular P–C bond formation mechanism is adopted.7b
and 6c at the B3LYP/[SDD(Pt),6-31G*(others)] level,10,11 using However, a pincer (type 1) catalyst offers only one easily acces-
the Gaussian 09 software package.12 The NBO analysis identi- sible catalytic site; hence it is necessary for pincer complexes
fied two pairs of orbitals corresponding to the d(Pt)-to-π*(C) to adopt an intermolecular mechanism for an AHP reac-
interactions for both 6a and 6c (Table 2). The sums of the π tion.14a,15 In view of the current finding that oxy and phos-
delocalization energies are 8.02 and 6.65 kcal mol−1 for 6a and phorus donors are able to coordinate to these pincer
6c, respectively, as evaluated in a second-order perturbation complexes, the pincer-catalyzed AHP could be triggered by
fashion (for the identities of the NBO used for the second- either the P → Pt or the carbonyl–O → Pt interaction. In order
order calculations, see Fig. 47 and 48 in the ESI†). The larger to determine which mode of activation is involved when the
delocalization energy in 6a is consistent with its shorter Pt–C platinum pincer complexes are used as catalysts in the P–H
bond distance.
addition reaction with chalcone 11, a series of closely moni-
As seen from the synthesized examples, the current series tored 1H and 31P{1H} NMR experiments were conducted (see
of PCP pincer platinum complexes show a high degree of flexi- Fig. 42–44 in the ESI†).
1
bility for different types of monodentate ligands, such as NO3
Based on the H NMR spectrum, the addition of chalcone
and PPh3, to coordinate with the trans position of the C–M 11 to pincer complex (R,R)-6e does not result in any visible
organometallic bonds. The above NMR and computational chemical shifts in the proton signals of both the catalyst and
investigations clearly reveal the involvement of Pt → C π* back substrate (see Fig. 42 in the ESI†). Furthermore, the proton
donation in these pincer complexes. This is probably the main signal arising from the –OAc group on 6e remains unchanged
driving force for the coordination of the classical “hard” oxy at 1.92 ppm. Similarly, the 31P{1H} NMR studies indicate that
ligands to the typical soft platinum(II) centers.
no changes in the 31P{1H} chemical shift of complex 6e was
It should be noted that the analogues of both type 1 and 2 observed (when a stoichiometric amount of chalcone 11 was
complexes have been applied successfully in asymmetric added to the NMR sample). However, in a separate sample,
hydrophosphination (AHP) reactions (Scheme 2).5,7,13–15 Inter- when a stoichiometric amount of Ph2PH was introduced into
complex 6e, significant changes to the 31P{1H} chemical shifts
are observed (see Fig. 43 and 44 in the ESI†). Subsequently,
the reaction proceeded to completion upon addition of chal-
cone 11 to the NMR sample containing complex 6e and
HPPh2. It should be noted that the same experimental obser-
Table 2 NBO-based π-delocalization energy and DFT-derived r(Pt–C)
for 6a and 6c
vations were made in a previous study involving the analogous
Pd(II) pincer complex.15 In that report, it was also demon-
strated that when PCP pincer ligand 1 was coordinated to the
palladium(II) metal, these resulting Pd(II) pincer complexes
could be used as reactive catalysts for hydrophosphination of
activated olefins. It was observed that complex 6e exhibits
lower catalytic activity as compared to the analogous Pd(II)
complex (X = OAc), indicating that the Pt–P (from Ph2PH)
bond formed during the course of the catalytic cycle is kineti-
cally more stable than the corresponding Pd–P counterpart.
The palladium catalysts also generated much higher ee (up to
43%) than complex 6e (3% ee) when applied to the AHP of
chalcone 11.13
π Delocalization
Species
energy (kcal mol−1
)
r(Pt–C) (Å)
6a
6c
8.02
6.65
2.045
2.086
The conducted mechanistic investigations thus show that
the platinum(II) pincer catalyst prefers to bind to phosphorus
instead of the keto-moiety during the course of P–H addition
Scheme 2 Catalytic P–H addition reaction. (Note: A cat. loading of
5 mol% with chalcone 11 and HPPh2 in DCM afforded the product 12 in
95% yield.)13
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reaction. This experimental observation reveal that while the Kappa CCD spectrometer. Structure solution and refinement
M → C π* character is inherently present in the pincer M–C were done on a computer using the SHELX package.16 PC-
bond, the back bonding nature is insufficient to render a high cyclometalated catalyst A,7b and complexes 9a and 10a10 were
oxophilicity for the pincer catalyst to activate chalcone 11 via prepared according to the literature methods. Pincer com-
keto-O → M interactions. This is in contrast to the observation plexes 6a–8a were prepared by the combination of
that the chalcone is activated via O → metal coordination hydrophosphination protocol7b and a metalation procedure.17
when the ortho-metalated naphthalene (type 2) complexes were
a
General procedure for the synthesis of complexes 6a, 7a
used as the catalysts for the same addition reaction.7j In fact
and 8a
the naphthalene (type 2) complexes prefer to form trans
C(aryl)–M ← O bonds rather than the analogous C(aryl)–M ← To a solution of HPPh2 (0.218 mmol, 1.0 eq.) in THF (3 mL)
P bonds.
was added catalyst A (0.0109 mmol, 5 mol%) (see Scheme 1).
The reaction mixture was stirred for complete dissolution and
cooled to −80 °C. Dienone (0.107 mmol, 0.49 equiv.) was
added, followed by a solution of NEt3 (0.218 mmol, 1.0 equiv.)
in THF (1 mL) dropwise. The reaction mixture was stirred over-
Conclusions
In this work, a series of rationally designed Pt(II) pincer com- night and monitored by 31P{1H} NMR for its completion. Upon
plexes were systematically examined to determine the elec- completion, the reaction mixture was allowed to stand at room
tronic properties of the C(aryl)–M bonds. A study of the temperature and the solvent was removed under reduced
13C–195Pt coupling constants of these complexes reveals the pressure protected by nitrogen. The residue was dissolved in
presence of significant π back-bonding operating within the chloroform (10 mL) and PtCl2(PPh3)2 (0.107 mmol, 0.49 eq.)
pincer C–Pt bonds. The X-ray analysis and computational was added. The reaction was stirred under reflux overnight.
studies of selected complexes further substantiate the 13C{1H} The reaction mixture was condensed to 2 mL and diluted with
NMR observations. NMR investigation conducted into the acetone (8 mL). KCl (0.214 mmol, 0.99 equiv.) and sulfur
mode of activation (either P → Pt or carbonyl–O → Pt) in the (0.214 mmol, 0.99 equiv.) were added. The mixture was
pincer-catalyzed AHP reaction clearly indicates that the inter- refluxed for 2 h and evaporated under reduced pressure to give
molecular addition of HPPh2 to chalcone 11 proceeds via P → the crude product, which was then purified by silica gel
Pt interaction. Based on the current experimental and compu- column chromatography to afford the pure complex.
tational findings, we are currently developing a series of P–C–P
General procedure for the synthesis of complexes 6b, 7b
and 8b
transition metal pincer complexes with various functional
groups on the side arms to allow for the fine-tuning of reactiv-
ity and stereoselectivity. These valuable complexes will be To
a solution of pincer-Pt–Cl complex 6a, 7a or 8a
employed as catalysts for different types of asymmetric organic (0.0505 mmol, 1.0 equiv.) in DCM (5 mL) and water (1 mL)
transformation reactions.
were added PPh3 (0.0505 mmol, 1.0 equiv.) and AgClO4
(0.101 mmol, 2.0 equiv.) The reaction mixture was stirred for
2 h. The residue was removed and the filtrate was washed with
water (2 × 20 mL), dried over Na2SO4 and concentrated to give
the crude product, which was purified by silica gel column
Experimental
All reactions were carried out under a positive pressure of chromatography.
nitrogen using the standard Schlenk technique. Solvents were
General procedure for the synthesis of complexes 6c, 7c and 8c
purchased from the respective companies (DCM, THF: Fisher;
toluene, n-hexanes: Avantor; acetone: Sigma-Aldrich) and used To
a
mixture of pincer-Pt–Cl complex 6a, 7a or 8a
as supplied. Where necessary, solvents were degassed prior to (0.042 mmol, 1.0 equiv.) in DCM (5 mL) and water (1 mL) was
use. A Low Temp Pairstirrer PSL-1800 was used for controlling added AgCN (0.084 mmol, 2.0 equiv.). The reaction mixture
low temperature reactions. Column chromatography was per- was stirred overnight, filtered through celite and the filtrate
formed on Silica gel 60 (Merck). Melting points were measured was washed with water, dried over Na2SO4 and concentrated to
using the SRS Optimelt Automated Point System SRS MPA100. give the crude product, which was purified by silica gel
Optical rotation was measured with an Atago automatic polari- column chromatography.
meter (AP-300) in the specified solvent in a 0.1 dm cell at
589 nm. NMR spectra were recorded on Bruker AV 300 and AV
400 spectrometers at 300 K. Chemical shifts were reported in
General procedure for the synthesis of complexes 6d, 7d
and 8d
ppm and referenced to an internal SiMe4 standard (0 ppm) or To
a solution of pincer-Pt–Cl complex 6a, 7a or 8a
chloroform-d (7.26 ppm) for 1H NMR, chloroform-d (0.213 mmol, 1.0 equiv.) in chloroform (10 mL) and water
(77.23 ppm) for 13C{1H} NMR, and an external 85% H3PO4 for (2 mL) was added AgNO3 (0.850 mmol, 4.0 equiv.). The reaction
31P{1H} NMR. All other reactants and reagents were used as mixture was stirred overnight. The residue was removed by fil-
supplied. The X-ray crystallographic examination and data col- tration and the filtrate was washed with water, dried over
lection were performed with Mo Kα radiation on a Bruker Na2SO4 and concentrated to give the pure product.
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Synthesis of complex 6e
Topics in Organometallic Chemistry, ed. G. van Koten and D.
Milstein, Springer-Verlag, Berlin, Heidelberg, 2013;
(d) N. Selander and K. J. Szabó, Chem. Rev., 2011, 111,
2048; (e) Phosphorous(III) Ligands in Homogeneous Catalysis:
Design and Synthesis, ed. P. C. J. Kamer and P. W. N. M. van
Leeuwen, Wiley-VCH, Weinheim, Germany, 2012;
(f) B. Rybtchinski and D. Milstein, Angew. Chem., Int. Ed.,
1999, 38, 870; (g) M. E. van der Boom and D. Milstein,
Chem. Rev., 2003, 103, 1759.
3 For a recent review, see: (a) D. M. Roddick, Top. Organomet.
Chem., 2013, 40, 49; For examples of (PCP)Pd systems, see:
(b) J. L. Bolliger, O. Blacque and C. M. Frech, Angew. Chem.,
Int. Ed., 2007, 46, 6514; (c) R. Gerber, T. Fox and
To a solution of pincer-Pt–Cl complex 6a (0.213 mmol, 1.0
equiv.) in DCM (10 mL) was added AgOAc (0.320 mmol, 1.5
equiv.). The reaction mixture was stirred overnight, filtered
through a plug of silica gel and extracted into DCM (25 mL).
The organic layer was washed with water (2 × 25 mL), dried
over Na2SO4 and concentrated to give the pure product 6e.
General procedure for catalytic addition of diphenylphosphine
to chalcone
Catalyst 6e (25 μmol, 5 mol%) was added to a solution of
diphenylphosphine (0.5 mmol, 1.0 equiv.) in DCM (1 mL) and
stirred at RT followed by subsequent addition of chalcone 11
(0.5 mmol, 1.0 equiv.). Completion of the reaction was deter-
mined by the disappearance of the phosphorous signal attribu-
ted to diphenylphosphine (−40 ppm) in the 31P{1H} NMR
spectrum. Upon completion of the reaction, aq. H2O2 (0.1 mL,
31% v/v) was added to form the respective product. The vola-
tiles were removed under reduced pressure and the crude
product was directly loaded onto a silica gel column (3EA : 2n-
hexane) to afford the pure product. The data obtained is con-
sistent with the literature.7c
C. M. Frech, Chem.
–
Eur. J., 2010, 16, 6771;
(d) J. L. Bolliger, O. Blacque and C. M. Frech, Chem. – Eur.
J., 2008, 14, 7969; For examples of (PCP)Pt systems, see:
(e) J. J. Adams, A. Lau, N. Arulsamy and D. M. Roddick,
Inorg. Chem., 2007, 46, 11328; (f) D. Vuzman, E. Poverenov,
Y. Diskin-Posner, G. Leitus, L. J. W. Shimon and D. Milstein,
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Computational methods
DFT calculations and NBO analyses were performed on com-
plexes 6a and 6c, which show distinct C–Pt bond lengths (see
above). The B3LYP functional was used in conjunction with
the SDD effective core potential basis set (for Pt) and the
6-31G* basis set (for the other atoms).10,11 This level of theory
is referred to here as B3LYP/[SDD(Pt),6-31G*(others)]. Calcu-
lations were performed using the Gaussian 09 software
package.12 NBO analyses were performed on DFT optimized
geometries using the NBO program version 3.1 implemented
in Gaussian 09.
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Acknowledgements
We are grateful to Nanyang Technological University for sup-
porting this research and the research scholarships to Y.-X. J
and X.-Y. Y.
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