Platinum(II) complexes with novel diisocyanide ligands: Catalysts in alkyne hydroarylation

Article abstract of DOI:10.1021/om400859q

A series of novel diisocyanide ligands (o-CNC₆H ₄O)₂Y (diNC-1: Y = P(O)Ph; diNC-2: Y = o-C(O)C ₆H₄C(O); diNC-3: Y = m-C(O)C₆H₄C(O); diNC-4: Y = C(O)C₂H₄C(O); diNC-5: Y = trans-C(O)C ₂H₂C(O)) was successfully synthesized by reaction of lithium 2-isocyanophenate (generated in situ from benzoxazole and n-BuLi) and a diacylic or phosphonic dichloride. The corresponding platinum(II) complexes of general formula [PtX₂(diNC)]_1,2 (X = Cl, Me; diNC = diisocyanide ligand) were isolated by simple substitution of 1,5-cyclooctadiene in the starting [PtX₂(COD)] complexes. The structure of the complexes, mononuclear or dinuclear, was confirmed by single-crystal X-ray analysis. A dinuclear complex of formula {(μ-diNC)[cis-PtCl ₂(PPh₃)]₂} could also be obtained with the diisocyanide ligand having a rigid fumaryl bridge and consequently the isocyanide moieties pointing in opposite directions. All the complexes were employed as catalysts in the hydroarylation of alkynes, showing generally good activity and selectivity toward the trans-hydroarylation product. With N-methylindole as aromatic substrate the major product was instead a heterocycle:alkyne 2:1 adduct.

Full text of DOI:10.1021/om400859q

Article
pubs.acs.org/Organometallics
Platinum(II) Complexes with Novel Diisocyanide Ligands: Catalysts in
Alkyne Hydroarylation
Daniele Vicenzi,† Paolo Sgarbossa,*,‡ Andrea Biffis,† Cristina Tubaro,*,† Marino Basato,†
Rino A. Michelin,‡ Arianna Lanza,† Fabrizio Nestola,§ Sara Bogialli,† Paolo Pastore,† and Alfonso Venzo∥
†Dipartimento di Scienze Chimiche, Universita
‡Dipartimento di Ingegneria Industriale, Universita
̀
di Padova, Via Marzolo 1, 35131 Padova, Italy
di Padova, Via Marzolo 9, 35131 Padova, Italy
̀
di Padova, Via Gradenigo 6, 35131 Padova, Italy
̀
§Dipartimento di Geoscienze, Universita
∥IENI-CNR, Dipartimento di Scienze Chimiche, Universita
̀
di Padova, Via Marzolo 1, 35131 Padova, Italy
S
* Supporting Information
ABSTRACT: A series of novel diisocyanide ligands (o-
CNC6H4O)2Y (diNC-1: Y = P(O)Ph; diNC-2: Y = o-
C(O)C6H4C(O); diNC-3: Y = m-C(O)C6H4C(O); diNC-4:
Y = C(O)C2H4C(O); diNC-5: Y = trans-C(O)C2H2C(O))
was successfully synthesized by reaction of lithium 2-
isocyanophenate (generated in situ from benzoxazole and n-
BuLi) and a diacylic or phosphonic dichloride. The
corresponding platinum(II) complexes of general formula
[PtX2(diNC)]1,2 (X = Cl, Me; diNC = diisocyanide ligand) were isolated by simple substitution of 1,5-cyclooctadiene in the
starting [PtX2(COD)] complexes. The structure of the complexes, mononuclear or dinuclear, was confirmed by single-crystal X-
ray analysis. A dinuclear complex of formula {(μ-diNC)[cis-PtCl2(PPh3)]2} could also be obtained with the diisocyanide ligand
having a rigid fumaryl bridge and consequently the isocyanide moieties pointing in opposite directions. All the complexes were
employed as catalysts in the hydroarylation of alkynes, showing generally good activity and selectivity toward the trans-
hydroarylation product. With N-methylindole as aromatic substrate the major product was instead a heterocycle:alkyne 2:1
adduct.
chiral phosphonium ester based diisocyanide ligands.16
Interestingly Reiser was the first to apply diisocyanide
complexes in catalysis.
INTRODUCTION
■
Isocyanides continue to attract considerable attention by
organometallic chemists as a result of their versatility in both
organic synthesis1,2 and coordination chemistry.1 In particular,
for this latter aspect, they are used both as simple ligands3,4 and
as synthons of coordinated carbenes,5−8 generated upon
addition of a nucleophile to the isocyanide carbon atom.
Remarkably, the majority of the studies on isocyanides involve
monodentate compounds, probably because of the long
multistep synthesis required for the bidentate derivatives and
the stringent steric requirement for chelation.9 After the first
examples of Rh(I) complexes bearing long-chain aliphatic
diisocyanide ligands, reported in the late 1970s,10 Angelici
described the synthesis of a series of potentially chelating
diisocyanides by a well-defined multistep procedure.11 The
derivatives were produced in low overall yield but led to a series
of transition metal complexes of both octahedral and square
planar geometry.11−13 In 1984 Angelici reported also the
synthesis of two new examples of chelating diisocyanides
obtained by a two-step process consisting in the opening of the
oxazole ring of benzoxazole, after connecting two function-
alized benzoxazoles to an ethylene bridge.14 Recently a similar
approach, starting from oxazole and acetyl chlorides, has been
applied to obtain new monoisocyanides by Pirrung in 200615
and then exploited by Reiser in the single-step synthesis of
In fact, the use of isocyanides as supporting ligands in
transition metal based catalysis has been much less investigated
than their coordination behavior and reactivity, even though
their strong σ-donor and π-acceptor ability would suggest
interesting effects on the electronic properties of the metal
centers; some recent applications have been however reported
and are mainly centered on Pd(II) C−C cross-coupling
reactions and on Rh(I) hydrosilylation of ketones.7,17 As
regards the diisocyanide ligands reported so far, they are
generally used as building blocks for the synthesis of
metallopolymers or functional materials,18 because the two
NC moieties are spatially oriented to bridge two metal centers
rather than chelate on a single metal. However, chelating
diisocyanide ligands are expected to give more stable transition
metal complexes than the corresponding bis(isocyanide) ones.
On the basis of the above, we report here the synthesis and
characterization of novel diisocyanide ligands (diNC) and their
Pt(II) complexes, which analyze as [PtCl2(diNC)] and
[PtMe2(diNC)]; these complexes are simply indicated here as
Received: August 26, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of the Diisocyanide Ligands
mononuclear, although there is clear evidence of a mono-
nuclear−dinuclear equilibrium in solution. Furthermore the
existence of mononuclear and dinuclear complexes is confirmed
by their isolation in the solid state. The Pt(II) complexes were
employed in the catalytic hydroarylation of alkynes, a reaction
that, as we have demonstrated, is efficiently catalyzed by Pd(II)
Table 1. Synthesis of the Platinum(II) Complexes with
Diisocyanide Ligands
and Pt(II) complexes with N-heterocyclic dicarbene ligands,19
class of ligands with which isocyanides are often compared.
a
RESULTS AND DISCUSSION
■
Synthesis of the Diisocyanide Ligands. The diisocya-
nide ligands were synthesized as depicted in Scheme 1 by (i)
deprotonation of the C2−H position of benzoxazole with n-
BuLi, in order to exploit the equilibrium between the carbanion
A and the 2-isocyanophenate form B;15 (ii) nucleophilic attack
of the open form, which has been demonstrated to be
dominant in the case of benzoxazole,20 on a diacylic or
phopshonic dichloride Y−Cl2.
The products are white solids, soluble in common organic
solvents, and have been obtained with yields in the range 24−
34%; such values are comparable with those achieved by
Angelici et al. with diisocyanides characterized by an ethereal
bridge,11b but with the advantage of a reduced number of
synthetic steps and of the use of commercially available
reagents. The FTIR spectra of the compounds present a band
around 2129 cm−1, associated with the ν(NC) stretching, in the
typical range for similar functionalized phenyl isocyanides;21
the presence of a unique ν(NC) band, together with a single
signal around 170 ppm in the 13C NMR spectra related to the
isocyanide carbon, indicates the equivalence of the two
isocyanide groups.
Synthesis of the Platinum(II) Diisocyanide Complexes.
The platinum(II) dichloro or dimethyl complexes were
synthesized by reaction of the diisocyanide ligand and the
platinum(II) precursor cis-[PtX2(COD)] (X = Cl, Me) in a 1:1
ratio (Table 1) in dichloromethane. The complexes are white
solids, stable in both the solid state and solution; the dichloro
complexes are slightly soluble in chlorinated solvents, alcohols,
and acetonitrile, while the dimethyl species exhibit good
solubility in all the commonly used organic solvents.
The experimental data suggest for the diisocyanide ligand a
cis coordination. In particular, in the infrared spectra, all the
complexes exhibit two bands associated with the symmetric and
asymmetric Pt−Cl stretching, typical of a cis arrangement;
accordingly, there are also two NC stretching bands shifted
to higher wavenumbers with respect to the free ligand (Table
2). As it can be observed in Table 2, the ν(NC)coordinated and
̃
Δν values for the dichloro complexes are in the range 2203−
̃
2244 cm−1 and 91−100 cm−1, while for the dimethyl complexes
they are in the range 2120−2184 cm−1 and 14−34 cm−1
respectively. It is well known that a higher value of Δν =
̃
(ν(NC)
̃
− ν(NC)free) reflects a greater electro-
̃
coordinated
philicity of the coordinated isocyanide carbon, which is
therefore more susceptible to nucleophilic attack.5a,22 The
highest values are generally observed for metal centers in
medium-high oxidation state and/or with coordinated electron-
B
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dimethylplatinum(II) complexes.23 The JPt−H value is
indicative of the electron-donating properties of the ligand
trans to the methyl group: the value recorded for the isocyanide
ligands is in between the phosphino ligands (≤71 Hz for
complexes such as cis-[PtMe2(PPh3)2], [PtMe2(dppe)], and cis-
[PtMe2(PMe2Ph)2]) and the amino ones (≥81 Hz for
complexes such as cis-[PtMe2(Py)2], [PtMe2(en)], and
[PtMe2(tmen)]).24
2
Table 2. Main Characteristic FTIR Bands in the
Dichloro(diisocyanide) and Dimethyl(diisocyanide) Pt(II)
Complexes
a
complex
ν
̃
(Pt−Cl) (cm−1
)
ν
̃
(NC) (cm−1
)
Δν
̃
(NC) (cm−1
)
1
328/348
328/347
324/340
325/349
2203/2237
2214/2244
2210/2243
2210/2242
2132/2177
2143/2184
2120/2167
2133/2176
91
100
97
2
3
4
97
The 13C NMR spectra of complexes 1′−4′ present a signal at
ca. 155 ppm (1JPt−C ca. 930 Hz) corresponding to the
coordinated isocyanide carbon. The signal is shifted upfield
with respect to the free ligand (169 ppm), as usually observed
in the coordination of isocyanide ligands to electron-poor Lewis
acidic metal centers,1 which indicates a predominant σ-donating
character of the ligands. This is confirmed by the aforemen-
tioned shift in the IR stretching band, indicating the
strengthening of the CN triple bond due to the stabilization,
upon coordination to the metal center, of the lone pair present
on the isocyanide carbon, which is slightly antibonding in
character in the free ligand.
The structure of the complexes, with the cis-chelated
coordination of the diisocyanide ligand, was confirmed also
by the X-ray crystal structure determination of complexes 2 and
1′. The ORTEP views of the complexes together with the
atomic labeling scheme are reported in Figure 1 and Figure 2,
respectively; a list of the most important bond distances and
angles is given in the caption.
1′
2′
3′
4′
25
34
14
25
a
Δν
̃
= (ν
̃
(NC)coordinated − ν
̃
(NC)free); ν
̃
(NC)free = 2129 cm−1,
ν
̃
(NC)coordinated is the mean value between the two observed bands.
withdrawing groups; as expected, the Δν
̃
(NC) is higher for
the series of dichloro complexes (electron-withdrawing groups)
than for the dimethyl complexes (electron-donating groups).
Finally, the carbonyl stretching in the coordinated ligands
with a diester bridge also presents two different bands both in
the solid state and in solution, in contrast with the unique band
observed for the free ligand; a possible explanation can be
found in the distortion of the diester bridge due to the chelated
coordination of the ligand, as it is well evident in the solid X-ray
structure of 2 and 2′ (vide infra).
1
The H and 13C NMR spectra of the complexes show the
In both complexes the platinum(II) atom is tetracoordinated
in a square planar geometry, as expected for a heavy metal
center in a d8 configuration.
In complex 2, the bond angles Cisocyanide−Pt−Cl are nearly
linear (C1−Pt1−Cl1 178.67(11)°, C22−Pt1−Cl2
expected resonances for the functional groups present in the
molecules. In particular, in the H NMR spectra of complexes
1
1′−4′, the signal associated with the coordinated methyl groups
2
is detectable around 0.6−0.8 ppm with JPt−H values of 74 Hz,
similar to those reported for related cis-bis(isocyanide)-
Figure 1. ORTEP view of complex 2. Ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected bond
distances (Å) and angles (deg): C1−Pt1 1.912(4), C1−N1 1.142(4), C2−N1 1.405(4), C22−Pt1 1.910(4), C22−N2 1.142(4), C21−N2 1.394(4),
Cl1−Pt1 2.3111(10), Cl2−Pt1 2.3004(10); C22−Pt1−Cl1 91.18(11), C22−Pt1−Cl2 177.71(12), C1−Pt1−Cl2 88.05(11), C22−Pt1−C1
89.91(15), C1−Pt1−Cl1 178.67(11), Cl2−Pt1−Cl1 90.86(4), N1−C1−Pt1 179.0(4), N1−C22−Pt1 175.3(4), C1−N1−C2 174.7(4), C22−N2−
C21 174.9(4).
C
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Figure 2. ORTEP view of complex 1′. Ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): C1−Pt1 1.947(6), C1−N1 1.158(6), C11−N1 1.378(6), C2−Pt1 1.960(5), C2−N2 1.146(6), C21−N2
1.392(6), C3−Pt1 2.086(6), C4−Pt1 2.085(6); C1−Pt1−C2 91.90(19), C1−Pt1−C4 176.3(2), C2−Pt1−C4 91.3(2), C2−Pt1−C3 177.9(2), C4−
Pt1−C3 86.6(3), C4−Pt1−C3 90.1(2), N1−C1−Pt1 177.6(5), N2−C2−Pt1 177.5(4), C1−N1−C11 169.2(5), C2−N2−C21 174.7(5).
177.71(12)°). The bond distances Pt−Cl (2.3111(10) and
2.3004(10) Å) and Pt−Cisocyanide (1.912(4) and 1.910(4) Å) are
comparable to those reported for analogous dichloro-bis-
(isocyanide) platinum(II) complexes.25 The linearity of the
RNC fragment suggests a negligible π electron density back-
donation from the metal and consequently an isocyanide
behavior as pure σ-donor ligand;4 the isocyanide bond lengths
N−C (1.142(4) Å) are also comparable to those reported so far
for bis-isocyanide platinum(II) complexes.25 The square planar
coordination of the Pt ion forces the ligand to assume a
nonsymmetric distorted conformation, which is responsible for
the splitting of the IR absorption bands. In particular, one of
the ester groups appears to be coplanar and the other
perpendicular to the central phenyl group, while the terminal
phenyl rings are both incident to the Pt coordination plane. No
strong intermolecular contacts are present, and the crystal
packing is achieved solely through weak interactions (π
stacking, C−H···O or C−H···Cl weak hydrogen bonds).
Regarding the dimethyl complex 1′, the bond distances Pt−
Me (2.086(6) and 2.085(6) Å) and Pt−Cisocyanide (1.947(6) and
1.960(5) Å) are comparable to those reported for analogous
dimethyl-bis(isocyanide) platinum(II) complexes.26 Also in this
case, the isocyanide behaves as a σ-donor ligand, as suggested
by the linearity of the RNC fragment. The ligand is in an
Figure 3. ORTEP view of complex 2′. Ellipsoids are drawn at the 30%
almost symmetrical conformation, with the two terminal phenyl
rings approximately parallel to the Pt coordination plane. The
crystal packing is again achieved via π stacking and C−H···O
hydrogen bonds.
probability level. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): Pt1−C1 1.946(7),
Pt1−C22 1.975(8), Pt1−C24 2.092(6), Pt1−C23 2.098(5), N1−C1
1.162(6), N1−C2 1.397(7), N2−C22 1.136(7), N2−C21 1.394(8);
C1−Pt1−C22 97.4(2), C1−Pt1−C24 88.0(2), C22−Pt1−C24
174.3(2), C1−Pt1−C23 172.1(3), C22−Pt1−C23 89.9(2), C24−
Pt1−C23 84.8(2), N1−C1−Pt1 175.2(6), N2−C22−Pt1 178.0(8),
C1−N1−C2 178.7(7), C22−N2−C21 174.9(6).
Suitable crystals for X-ray analysis were obtained also by slow
diffusion of diethyl ether into a dichloromethane solution of
complex 2′. In this case a platinum(II) dinuclear complex forms
(Figure 3), in which two diNC ligands bridge two PtMe2 units.
Overall, the geometric parameters around the platinum(II)
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Scheme 2. Synthesis of the Dinuclear Complex 5
centers (see figure caption) are comparable with those of
complex 1′.
diisocyanide ligand and the platinum(II) precursor [Pt(μ-
Cl)Cl(PPh3)]2 in a 1:1 ratio (Scheme 2) in dichloromethane.
Complex 5 is a dinuclear compound in which the
diisocyanide ligand bridges two {PtCl2(PPh3)} units; the
isocyanide functionality is coordinated trans to a chloro ligand.
The FTIR spectrum shows a unique absorption band at 2199
cm−1 corresponding to the NC bond stretching, thus
suggesting a symmetric bridging coordination of the ligand.
Moreover, the presence of two Pt−Cl stretching bands at 304
and 341 cm−1 indicates a cis coordination of the two chloride
ligands, as already observed for similar complexes with other
aryl isocyanide ligands.27 Finally the 31P NMR spectrum shows
When compared to complex 2, the ester groups of the ligand
are likewise one coplanar and the other perpendicular to the
central phenyl group, but the orientation of the terminal phenyl
rings is different. In 2′, the latter are one parallel to the central
phenyl and perpendicular to the Pt plane and the other
perpendicular to the central phenyl and parallel to the Pt
coordination plane. The dimer contains two such moieties
related by one center of symmetry. The crystal structure
contains in addition one crystallization dichloromethane
molecule per dimer, which assists the packing of the dimers
into planes via C−H···O or C−H···Cl weak hydrogen bonds.
Such planes are packed in the third dimension via π stacking.
In order to understand if the dinuclear structure of complex
2′ represents an exception in the series of the dimethyl
complexes, we have further characterized these compounds.
1
a signal at 8.72 ppm with a JPt−P of 3319 Hz; this value is
comparable with that reported in the literature for platinum(II)
complexes with a triphenylphosphine ligand trans to a chloride;
for an isocyanide group in trans position to the phosphine a
lower value was actually expected due to the higher trans
influence of the isocyanide ligand compared with a chloride.28
Catalytic Tests in the Hydroarylation of Alkynes. We
started our work on the catalytic application of the novel
diisocyanide complexes by optimizing the catalytic conditions
in a standard test reaction between pentamethylbenzene and
ethyl propiolate (Scheme 3), using complex 1 as model catalyst.
1
The H NMR spectrum in a deuterated chlorinated solvent of
complex 2′ (dinuclear in the solid state) is practically
indistinguishable from that of the parent complex 2
(mononuclear), apart from the signals of the coordinated
methyl groups; for this reason it is not possible to discriminate
between the mono- and dinuclear form solely on the basis of
the NMR and FTIR spectra.
On the other hand, the high-resolution mass spectrometric
analysis of the dimethyl complexes 1′−4′ revealed the presence
of signals corresponding to the mononuclear
{ [ P t M e 2 ( d i N C ) ] + Na } + a s w e l l a s d i n u c l e a r
{[PtMe2(diNC)]2+Na}+ species, whose ratio depends on
sample concentration (10−4 M to 10−6 M). In particular for
complex 1′ the mononuclear/dinuclear ratio varies from ca. 1/5
at 10−4 M to 1/2 at 10−5 M and ca. 3/1 at 10−6 M.
Scheme 3. Hydroarylation Reaction of Ethyl Propiolate and
Pentamethylbenzene
2[PtMe2(η2‐diNC)] ⇌ {[PtMe2]2(μ2‐diNC)2}
This behavior has been tentatively explained by the
coexistence of both mono- and dinuclear forms in equilibrium
in solution, and the predominance of one form could possibly
depend on sample concentration and employed solvents.
The high-resolution mass spectra of complexes 1′ and 2′
were registered also in acetonitrile solution in the presence of
trifluoroacetic acid ([complex] = 10−5 M; [HTFA] = 10−3 M),
in order to use conditions similar to the catalytic ones (vide
infra). In this case, both complexes present a single signal
corresponding to the ion [PtMe(MeCN)(diNC)]+, thus
suggesting that in the catalytic conditions the equilibrium
between the mono- and dinuclear species is shifted toward a
mononuclear one and allowing a reliable comparison between
the catalytic results of the different complexes.
Pentamethylbenzene has a unique C−H bond that can be
functionalized, so that on the basis of our previous knowledge
of this reaction three possible hydroarylation products are
expected: the trans hydroarylation product a, the E isomer b,
and the product of double alkyne insertion c (Scheme 3). The
diisocyanide complex 1 was found to exhibit very low activity
under the reaction conditions usually employed by us with
dicarbene Pd(II) and Pt(II) complexes19 (1 equiv of alkyne, 1
equiv of arene, 0.1 mol % catalyst, 0.2 mol % silver
trifloroacetate (AgTFA), 1 equiv of trifluoroacetic acid
(HTFA), chlorinated solvent to a total volume of 6.3 mL,
The ligand diNC-5 with the rigid fumaryl bridge has the two
isocyanide functionalities that point in opposite directions,
which makes the obtainment of a chelate complex impossible.
Conversely, complex 5 was prepared by reaction of the
E
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room temperature): after 48 h the conversion of arene was only
6% with exclusive formation of the trans hydroarylation product
a. However, by increasing the amount of trifluoroacetic acid
and the reaction temperature up to 50 °C, it was possible to
reach high conversions and selectivities (Table 3, tests 1−4) in
complexes and 90% for the dimethyl derivatives (Table 4). The
reaction is very selective toward the Z olefin a, with the only
minor product being the double insertion derivative c.
Table 4. Catalyst Screening in the Hydroarylation of Ethyl
a
Propiolate and Pentamethylbenzene
Table 3. Optimization of the Reaction Conditions in the
Hydroarylation of Ethyl Propiolate and
b
entry
catalyst
yield of a (%) (alkyne conversion (%))
a
1
2
3
4
5
6
7
8
9
1
86 (90)
86 (86)
60 (62)
90 (94)
76 (76)
91 (93)
85 (85)
86 (90)
58 (58)
Pentamethylbenzene
c
1′
2
T
time
acid/arene
molar ratio
yield of a (%) (alkyne
b
entry
catalyst
(°C) (h)
conversion (%))
c
2′
3
c
1
2
3
4
5
6
7
1
25
25
50
50
50
50
50
48
48
24
24
24
24
24
1
4
1
4
4
4
4
6 (11)
1
73 (73)
c
3′
4
c
1
13 (32)
d
1
86 (90)
c
4′
5
e
1′
PtCl2
86 (86)
f
d
45 (53)
a
Reaction conditions: 13.2 μmol of catalyst, 26.4 μmol of AgTFA, 13.2
f
d
K2PtCl4
87 (>99)
mmol of pentamethylbenzene, 13.2 mmol of ethyl propiolate, 52.8
a
Reaction conditions: 13.2 μmol of catalyst, 26.4 μmol of AgTFA, 13.2
mmol of HTFA, chloroform in order to reach a total volume of 6.3
b
mL, 50 °C, 24 h. The yield was determined by 1H NMR
mmol of pentamethylbenzene, 13.2 mmol of ethyl propiolate, 13.2 or
52.8 mmol of HTFA, chloroform in order to reach a total volume of
spectroscopy. The difference between alkyne conversion and yield of
b
c
1
c
6.3 mL. The yield was determined by H NMR spectroscopy. The
a is due to the formation of product c. No AgTFA was used in this
d
alkyne in part polymerizes. The alkyne in part gives product c,
stemming from its double insertion. No AgTFA was used in this case.
In these cases AgOTf was used as silver cocatalyst (26.4 μmol).
case.
e
f
Complex 5 displays the lowest catalytic efficiency (58%), but
it should be remarked that its structure is different from that of
the other complexes; in this case the diisocyanide ligand is
bridging two cis-[PtCl2(PPh3)] units.
reasonable time (24 h). It should be noted that no products
were formed in the absence of the platinum(II) catalyst or in
the absence of HTFA. The dimethyl complex 1′ displayed a
performance comparable to 1 over 24 h (test 5). The actual
catalytic species is assumed to be the same in both cases, i.e., a
platinum(II) complex with weakly coordinating anions,
generated through Cl−/TFA− (eq 1) or Me−/TFA− (eq 2)
exchange.
As a first general remark, the catalytic performance of these
diisocyanide complexes appears somewhat lower than that of
dicarbene platinum(II) and palladium(II) complexes, which are
active at room temperature and without the necessity of excess
trifluoroacetic acid.19 On the other hand, the dimethyl
diisocyanide complexes are very efficient catalysts in the
hydroarylation reaction of ethyl propiolate and pentamethyl-
benzene, without the necessity of adding silver cocatalyst.
The scope of the hydroarylation reaction was then explored
with respect to both the arene and the alkyne, using
respectively complex 2′ or 3′ under the previously optimized
experimental conditions. The obtained results are reported in
Table 5.
[PtCl2(diNC‐1)] + 2AgTFA
→ [Pt(TFA)2(diNC‐1)] + 2AgCl
(1)
[PtMe2(diNC‐1)] + 2HTFA
→ [Pt(TFA)2(diNC‐1)] + 2MeH
(2)
The formation of [Pt(TFA)2(diNC-1)] has been evidenced
by independent synthetic experiments, which show the
disappearance of the Pt−Cl stretching in the FTIR for 1
upon treatment with the silver salt and the presence of the
The screening of arenes was performed by reacting ethyl
propiolate with variously substituted methylbenzenes. In all
cases, the main product was the trans hydroarylation product,
i.e., the Z olefin. The observed reactivity was not directly
related to the number of methyl groups, but depended also on
their relative position, the reactivity of 1,2,4,5-tetramethylben-
zene being for example lower than that of pentamethylbenzene
and 1,3,5-trimethylbenzene. Since the hydroarylation reaction
under these conditions is believed to proceed through an
electrophilic aromatic substitution mechanism,30 the applica-
tion of the Hammett treatment on electrophilic aromatic
substitution may help in justifying the observed trend. In fact,
the calculated activation parameter for the C3−H in 1,2,4,5-
tetramethylbenzene (ca. 450) is much lower than that for the
C2−H in 1,3,5-trimethylbenzene (ca. 15 000). Regarding the
alkyne screening, differently substituted alkynes were reacted
with pentamethylbenzene. All the alkynes are very reactive,
reaching almost quantitative conversions and better selectivities
than with the palladium(II) dicarbene catalysts under similar
reaction conditions.19 Polymerization and alkyne hydration are
competitive reactions when phenylacetylene is used as alkyne,
1
methane signal in the H NMR spectra of 1′ after the addition
of trifluoroacetic acid.
The efficiency of complexes 1 and 1′ was comparable with
that displayed by simple platinum salts such as PtCl2 and
K2PtCl4 (tests 9 and 10), in the presence of AgOTf
cocatalyst.29
We then turned our attention to the comparison of the
catalytic efficiency of the series of platinum complexes with
different diisocyanide ligands in order to find a possible
relationship between the structure of the ligand and the
catalytic activity of the complexes. We used the previously
optimized reaction conditions, i.e., arene/alkyne/acid 1:1:4, 0.1
mol % catalyst, 0.2 mol % AgTFA (only for the dichloro
complexes), 50 °C, 24 h, and chloroform as solvent.
All the platinum(II) complexes are efficient catalysts for the
hydroarylation reaction of ethyl propiolate and pentamethyl-
benzene, reaching conversions around 80% for the dichloro
F
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alkyne. The reaction conditions adopted in this case were those
reported in the literature for Pd(OAc)2 as catalyst,31 namely,
platinum complex 3 (1 mol %), AgBF4 as cocatalyst (2 mol %),
heteroarene (2 mmol), phenyl ethylpropiolate (1 mmol), and
acetic acid (1 mL), 50 °C, 24 h. With 1- or 2-methylpyrrole and
2-methylindole as aromatic substrates the observed selectivity
for the trans hydroarylation product was not very satisfactory:
in fact, products of monofunctionalization at different indole
positions as well as arene:alkyne 2:1 adducts were obtained. A
similar behavior has been observed also for Pd(II) and Pt(II)
dicarbene complexes.19e On the other hand, by using N-
methylindole as substrate, the catalytic efficiency improves and
besides the 3-monofunctionalization product the only other
major product is the 2,2-bis(N-methylindol-3-yl)-2-phenyl-
ethane (Table 6, entry 1). This product derives from the
double addition of N-methylindole to the phenyl ethyl-
propiolate, followed by hydrolysis of the ester function and
successive decarboxylation. It is interesting to note that this last
product is the unique product when phenylacetylene is used as
alkyne instead of phenyl ethylpropiolate (Table 6, entry 2).
Generally these types of arene:alkyne 2:1 adducts are the main
products when Lewis acidic salts, such as metal triflates, are
employed as catalysts.32
Table 5. Arene and Alkyne Screening in the Hydroarylation
Reaction
CONCLUSIONS
■
In conclusion, we have exploited and extended an efficient
synthetic procedure to obtain novel diisocyanide ligands upon
nucleophilic substitution of isocyanophenate on diacylic
dichloride, demonstrating that this reaction sequence has a
quite broad scope. These diisocyanides have shown good
coordinating properties, and, for example, platinum(II)
complexes are readily obtained in good yield via substitution
of the cyclooctadiene ligand in cis-[PtX2(COD)] (X = Cl, Me).
The resulting complexes cis-[PtX2(diNC)] are stable com-
pounds in which the diisocyanide ligand coordinates in a cis-
chelated fashion. The infrared and 13C NMR data seem to
indicate good σ-donor and limited π-acceptor properties for
these ligands.
a
b
The yield was determined by 1H NMR spectroscopy. The additional
product c, stemming from the double insertion of alkyne, is formed.
c
An additional product (10%), stemming from the hydrolysis of the
ester function, is formed, together with 9% alkyne hydration product.
d
Acetophenone (45%), stemming from alkyne hydration, and alkyne
e
polymerization products are formed. Reaction conditions: 13.2 μmol
of catalyst, 13.2 mmol of arene, 13.2 mmol of alkyne, 52.8 mmol of
HTFA, chloroform in order to reach a total volume of 6.3 mL, 50 °C,
24 h.
as indicated by the large difference between the yield in the
trans hydroarylation product and the alkyne conversion. Traces
of water lower the yield also in the case of phenyl
ethylpropiolate, due to hydrolysis of the ester function and
alkyne hydration.
Finally, complex 3 was tested in the hydroarylation reaction
using heteroarenes as substrate and phenyl ethylpropiolate as
The positive role of these diisocyanides on the stability and
reactivity of their platinum(II) complexes has been proved in
selected catalytic tests. The metal complexes turned out to be
good precatalysts in the hydroarylation of alkynes, showing
high yields and selectivity toward the production of Z olefins.
Table 6. Hydroarylation Reaction between N-Methylindole and Phenyl Ethyl Propiolate or Phenylacetylene
a
b
The yield was determined by 1H NMR spectroscopy. An additional 10% of alkyne reacts to give the E olefin and the double arene:alkyne adduct.
c
Reaction conditions: 0.01 mmol of [PtCl2(diNC-3)] 3, 0.02 mmol of AgBF4, 2 mmol of N-methylindole, 1 mmol of alkyne, 1 mL of HOAc, 50 °C,
24 h.
G
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Found: C, 67.24; H, 3.76; N, 8.84. FTIR (CH2Cl2, cm−1): 2129
Only with N-methylindole as aromatic substrate, the main
product is an arene:alkyne 2:1 adduct.
We are currently exploring the coordination of these new
ligands to other transition metal centers with different
coordination geometries and studying their reactivity toward
nucleophilic attack on the CN triple bond.
1
[ν(NC)], 1772 [ν(CO)]. H NMR (CDCl3): δ 3.17 (s, 4H, CH2),
7.20−7.50 (m, 8H, Ar). 13C NMR (CDCl3): δ 29.0 (CH2), 120.2,
123.5, 126.8, 127.6, 130.5, 146.2 (Ar), 169.2 (CO), 169.5 (NC). ESI-
MS (CH3OH): m/z 342.91 (100%) [M + Na]+.
Synthesis of diNC-5. The procedure is similar to that followed for
ligand diNC-1 starting from benzoxazole (0.89 g, 7.5 mmol), n-BuLi
(5.0 mL of a 1.6 M solution in hexane, 7.5 mmol), and fumaroyl
chloride (0.85 g, 3.9 mmol). White solid (yield 33%). Mp: 142−143
°C (dec). Anal. Calcd for C18H10N2O4: C, 67.93; H, 3.17; N, 8.80.
Found: C, 67.68; H, 3.24; N, 8.54. FTIR (CH2Cl2, cm−1): 2129
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out using
standard Schlenk techniques under an atmosphere of argon or
dinitrogen. The reagents were purchased as high-purity products and
generally used as received; all solvents were dried and purified using
standard procedures.33 The complexes [PtCl2(COD)],22
1
[ν(NC)], 1753 [ν(CO)]. H NMR (CDCl3): δ 7. 0.36 (s, 2H, CH),
7.28−7.60 (m, 8H, Ar). 13C NMR (CDCl3): δ 120.0, 123.1, 127.2,
127.8, 130.5, 145.7 (Ar), 134.2 (CH), 161.5 (CO), 169.9 (NC). ESI-
MS (CH3OH): m/z 340.88 (85%) [M + Na]+.
[PtMe2(COD)],22 and [PtCl(μ-Cl)(PPh3)]2 were prepared accord-
34
Synthesis of the Dichloro(diisocyanide)platinum(II) Com-
plexes. Synthesis of 1. A solution of the diisocyanide ligand diNC-1
(100 mg, 0.27 mmol) in dichloromethane (5 mL) was added to a
solution of [PtCl2(COD)] (96 mg, 0.27 mmol) in dichloromethane
(30 mL). The mixture was maintained under stirring for 2 h; then n-
hexane (50 mL) was added to precipitate the product as a white solid,
which was filtered, washed with n-hexane, and dried in vacuo. White
solid (yield 99%). Mp: >250 °C (dec). Anal. Calcd for
C20H13Cl2N2O3PPt·0.25C6H14: C, 39.86; H, 2.57; N, 4.32. Found:
C, 39.89; H, 2.36; N, 4.36. FTIR (CsI, cm−1): 2237 and 2203
[ν(NC)], 926 [ν(POR)], 348 and 328 [ν(PtCl)]. 1H NMR (CD2Cl2):
δ 7.20−8.15 (m, 13H, Ar and Ph). 31P NMR (CD2Cl2): δ 16.96. ESI-
MS (CH3CN): m/z 649.07 (90%) [M + Na]+, 1274.51 (40%) [2 M +
Na]+. It was not possible to acquire the 13C NMR because of the low
solubility of the complex.
ing to literature procedures. Details on laboratory instruments are
reported in the Supporting Information.
Synthesis of the Diisocyanide Ligands diNC-1−diNC-5.
Synthesis of diNC-1. A 1.6 M solution of n-BuLi in hexanes (5 mL,
7.5 mmol) was added dropwise with stirring to a solution of
benzoxazole (0.92 g, 7.5 mmol) in dry tetrahydrofuran (18 mL) at
−78 °C and maintained under stirring for 1.5 h. Phenylphosphonic
dichloride (0.64 mL, 3.8 mmol) was then added at −78 °C; the
mixture was further stirred at −78 °C for 30 min, then allowed to
warm to room temperature in 2 h and finally added to 100 mL of
diethyl ether. The organic phase was washed with a saturated
NaHCO3 solution (50 mL) and water (2 × 50 mL) and finally dried
over anhydrous Na2CO3. The volatiles were removed in vacuo to give
a brown solid, which was washed several times with cold methanol,
giving ligand diNC-1 as a white solid (yield 34%). Mp: 135−140 °C
(dec). Anal. Calcd for C20H13N2O3P: C, 66.67; H, 3.64; N, 7.77.
Found: C, 66.21; H, 3.54; N, 7.68. FTIR (CH2Cl2, cm−1): 2129
Synthesis of 2. The procedure is similar to that followed for
complex 1 starting from [PtCl2(COD)] (100 mg, 0.27 mmol) and the
ligand diNC-2 (94 mg, 0.27 mmol). White solid (yield 90%). Mp:
165−175 °C (dec). Anal. Calcd for C22H12Cl2N2O4Pt·2H2O: C, 39.42;
H, 2.41; N, 4.18. Found: C, 39.65; H, 2.07; N, 3.78. FTIR (CsI, cm−1):
2224 and 2214 [ν(NC)], 1770 and 1746 [ν(CO)], 347 and 328
1
[ν(NC)], 936 [ν(POR)], 924. H NMR (CDCl3): δ 7.10−7.80 (m,
11H, ArNC and Ph), 8.05−8.30 (m, 2H, Ph). 13C NMR (CDCl3): δ
119.4, 121.6, 125.6, 128.1, 130.4, 145.5 (Ar), 129.0 (3JC−P = 16 Hz,
Ph), 132.7 (2JC−P = 11 Hz, Ph), 134.2 (4JC−P = 3.3 Hz, Ph), 169.9
(NC), the aromatic carbon bonded to the phosphorus atom was not
detected. 31P NMR (CDCl3): δ 13.69. ESI-MS (CH3OH): m/z 382.90
(100%) [M + Na]+.
1
[ν(PtCl)]. H NMR (CD2Cl2): δ 7.40−7.55 (m, 2H, Ar), 7.60−7.80
(m, 6H, Ar), 7.85−7.95 (m, 2H, Ar), 8.10−8.20 (m, 2H, Ar). It was
not possible to acquire the 13C NMR and ESI-MS spectra because of
the low solubility of the complex.
Synthesis of diNC-2. The procedure is similar to that followed for
ligand diNC-1 starting from benzoxazole (0.89 g, 7.5 mmol), n-BuLi
(5.0 mL of a 1.6 M solution in hexane, 7.5 mmol), and o-phthaloyl
dichloride (0.60 mL, 3.9 mmol). The obtained brown oil was dissolved
in dichloromethane and purified through column chromatography
(silica gel 230−400 mesh, eluent diethyl ether, Rf = 0.75). The removal
of the solvent gives the diisocyanide diNC-2 as a white solid (yield
24%). Mp: 100−101 °C (dec). Anal. Calcd for C22H12N2O4: C, 71.74;
H, 3.28; N, 7.61. Found: C, 71.40; H, 3.29; N, 7.70. FTIR (CH2Cl2,
Synthesis of 3. The procedure is similar to that followed for
complex 1 starting from [PtCl2(COD)] (100 mg, 0.27 mmol) and the
ligand diNC-3 (110 mg, 0.27 mmol). White solid (yield 90%). Mp:
>250 °C (dec). Anal. Calcd for C22H12Cl2N2O4Pt·2H2O: C, 39.42; H,
2.41; N, 4.18. Found: C, 39.65; H, 2.07; N, 3.78. FTIR (CsI, cm−1):
2243 and 2210 [ν(NC)], 1758 and 1751 [ν(CO)], 340 and 324
[ν(PtCl)]. 1H NMR (CD2Cl2): δ 7.20−7.70 (m, 8H, ArNC), 7.76 (m,
1H, Ar), 8.46 (m, 2H, Ar), 9.13 (m, 1H, Ar). ESI-MS (CH3CN): m/z
657.12 (100%) [M + Na]+, 1290.58 (80%) [2 M + Na]+. It was not
possible to acquire the 13C NMR because of the low solubility of the
complex.
1
cm−1): 2129 [ν(NC)], 1752 [ν(CO)]. H NMR (CDCl3): δ 7.20−
7.60 (m, 8H, ArNC), 7.70−7.85 (m, 2H, Arpht), 8.10−8.25 (m, 2H,
Arpht). 13C NMR (CDCl3): δ 120.3, 123.5, 126.9, 127.6, 129.9, 130.5,
130.7, 132.6, 146.2 (Ar), 164.4 (CO), 169.6 (NC). ESI-MS
(CH3OH): m/z 390.91 (100%) [M + Na]+.
Synthesis of 4. The procedure is similar to that followed for
complex 1 starting from [PtCl2(COD)] (100 mg, 0.27 mmol) and the
ligand diNC-4 (90 mg, 0.27 mmol). White solid (yield 94%). Mp:
215−225 °C (dec). Anal. Calcd for C18H12Cl2N2O4Pt·0.1C6H14: C,
37.55; H, 2.27; N, 4.71. Found: C, 37.49; H, 2.11; N, 4.76. FTIR (CsI,
cm−1): 2242 and 2210 [ν(NC)], 1777 and 1758 [ν(CO)], 349 and
Synthesis of diNC-3. The procedure is similar to that followed for
ligand diNC-2 starting from benzoxazole (0.89 g, 7.5 mmol), n-BuLi
(5.0 mL of a 1.6 M solution in hexane, 7.5 mmol), and m-phthaloyl
dichloride (0.85 g, 3.9 mmol). Light yellow solid (Rf = 0.77, yield
25%). Mp: 125−128 °C (dec). Anal. Calcd for C22H12N2O4: C, 71.74;
H, 3.28; N, 7.61. Found: C, 71.42; H, 3.49; N, 7.18. FTIR (CH2Cl2,
1
325 [ν(PtCl)]. H NMR (CD2Cl2): δ 3.20 (s, 4H, CH2), 7.15−7.80
(m, 8H, Ar). 13C NMR (CDCl3): δ 29.7 (CH2), 119.1, 123.4, 127.1,
128.6, 132.4, 145.1 (Ar), 168.5 (CO), isocyanide carbon not detected.
ESI-MS (CH3CN): m/z 609.06 (100%) [M + Na]+]+, 1194.60 (40%)
[2 M + Na]+. It was not possible to acquire the 13C NMR because of
the low solubility of the complex.
1
cm−1): 2129 [ν(NC)], 1752 [ν(CO)]. H NMR (CDCl3): δ 7.27−
7.85 (m, 9H, ArNC and Arpht), 8.55 (dd, 3JHH = 7.8 Hz and 4JHH = 1.7
Hz, 2H, Arpht), 9.10 (t, 4JHH = 1.7 Hz, 1H, Arpht). 13C NMR (CDCl3):
δ 120.4, 123.5, 126.9, 127.7, 129.4, 129.6, 130.5, 132.4, 135.7, 146.3
(Ar), 162.8 (CO), 169.6 (NC). ESI-MS (CH3OH): m/z 390.91
(100%) [M + Na]+.
Synthesis of 5. The procedure is similar to that followed for
complex 1 starting from [PtCl(μ-Cl)(PPh3)]2 (210 mg, 0.20 mmol)
and the ligand diNC-5 (64 mg, 0.20 mmol). Orange solid (yield 90%).
Mp: 155−159 °C (dec). Anal. Calcd for C54H40Cl4N2O4PPt2·C6H14:
C, 49.32; H, 3.73; N, 1.92. Found: C, 49.41; H, 3.37; N, 2.22. FTIR
(CsI, cm−1): 2199 [ν(NC)], 1753 [ν(CO)], 341 and 304 [ν(PtCl)].
Synthesis of diNC-4. The procedure is similar to that followed for
ligand diNC-1 starting from benzoxazole (0.89 g, 7.5 mmol), n-BuLi
(5.0 mL of a 1.6 M solution in hexane, 7.5 mmol), and succinoyl
chloride (0.85 g, 3.9 mmol). White solid (yield 28%). Mp: 132−133
°C (dec). Anal. Calcd for C18H12N2O4: C, 67.50; H, 3.78; N, 8.75.
5
1H NMR (CD2Cl2): δ 6.44 (d, 2H, JPt−H = 8 Hz, Ar), 7.36 (s, 2H,
H
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Article
CH), 7.32−7.95 (m, 36H, Ar). 13C NMR (CDCl3): δ 119.7, 124.3,
CCD and CrysAlis RED35 were used for data collection, unit cell
determination, data reduction, and empirical absorption correction.
The subsequent steps were carried out with WinGX36 using Sir201137
for structure solution and SHELXL201338 for least-squares refinement
against all F2. Anisotropic ADPs were refined for all non-hydrogen
atoms, while hydrogen atoms were placed in idealized positions and
then refined with a riding model.
126.2, 128.8, 131.6, 131.7, 134.5, 134.8, 136.6, 146.6 (Ar), 134.3
(CH), 155.8 (NC), 161.7 (CO). 31P NMR (CDCl3): δ 8.72 (1JPt−P
=
3319 Hz). It was not possible to acquire the ESI-MS spectra because of
the low solubility of the complex.
Synthesis of the Dimethyl(diisocyanide)platinum(II) Com-
plexes. Synthesis of 1′. A solution of the diisocyanide ligand diNC-1
(110 mg, 0.31 mmol) in dichloromethane (5 mL) was added to a
solution of [PtMe2(COD)] (100 mg, 0.31 mmol) in dichloromethane
(30 mL). The mixture was maintained under stirring for 2 h; then n-
hexane (50 mL) was added to precipitate the product as a white solid,
which was filtered, washed with n-hexane, and dried in vacuo. White
solid (yield 78%). Mp: 165−170 °C (dec). Anal. Calcd for
C22H19N2O3PPt: C, 45.13; H, 3.27; N, 4.78. Found: C, 44.84; H,
3.13; N, 4.69. FTIR (CsI, cm−1): 2177 and 2132 [ν(NC)], 952 and
General Procedure for the Catalytic Tests. General Procedure
for the Catalytic Tests Reported in Tables 3−5. The arene (13.2
mmol, if solid), the platinum(II) complex (13.2 μmol), and silver
trifluoroacetate (26.4 μmol, only for the dichloro complexes) were
placed in a 100 mL round-bottomed flask, previously evacuated and
filled with nitrogen. Trifluoroacetic acid (1 or 4 mL, 13.2 or 52.8
mmol), the quantity of chlorinated solvent (dichloromethane for the
room-temperature tests, chloroform for the tests at 50 °C) necessary
to reach a total volume of 6.3 mL, and the arene (if liquid) were then
added; the resulting solution was stirred at 25 or 50 °C for 5 min.
Finally the alkyne (13.2 mmol) was added, and the reaction mixture
was further stirred at the desired temperature for the time indicated in
the tables. Portions of the solution (0.2 mL) were drawn off from the
1
926 [ν(POR)]. H NMR (CDCl3): δ 0.80 (2JPt−H = 74.4 Hz, 6H,
CH3), 7.30−7.75 (m, 9H, Ar and Ph), 8.00−8.20 (m, 2H, Ph). 13C
NMR (CDCl3): δ −5.85 (1JPt−C = 590 Hz, CH3), 120.6, 121.4, 126.2,
126.9, 130.2, 146.3 (Ar), 126.0 (1JC−P = 191 Hz, Ph), 129.2 (3JC−P = 16
Hz, Ph), 132.5 (2JC−P = 11 Hz, Ph), 134.4 (4JC−P = 3.2 Hz, Ph), 154.5
(1JPt−C = 932 Hz, NC). 31P NMR (CDCl3): δ 14.33. ESI-HRMS
(CH3CN): m/z 608.0661 [M + Na]+, calcd for C22H19N2O3PPt (Δ
−1.4 mDa); 1193.1437 [2 M + Na]+, calcd for C44H38N4O6P2Pt2 (Δ
−1.3 mDa). ESI-HRMS (CH3CN + HTFA): m/z 611.0817 [M −
CH3 + CH3CN]+, calcd for C23H19N3O3PPt (Δ +0.9 mDa).
1
reaction mixture and analyzed by H NMR.
General Procedure for the Catalytic Tests Reported in Table 6. N-
Methylindole (2 mmol), complex 3 (0.01 mmol), and AgBF4 (0.02
mmol) were placed in a Schlenk tube. After three vacuum/argon
cycles, acetic acid (1 mL) was added through the rubber septum, and
the reaction mixture was stirred at room temperature for 5 min before
addition of the alkyne (1 mmol). Portions of solutions were drawn off
from the reaction mixture and analyzed by 1H NMR to determine the
yield. The products were identified by comparison with character-
ization data found in the literature.19e
Synthesis of 2′. The procedure is similar to that followed for
complex 1′ starting from [PtMe2(COD)] (140 mg, 0.43 mmol) and
the ligand diNC-2 (160 mg, 0.43 mmol). White solid (yield 69%). Mp:
167−170 °C (dec). Anal. Calcd for C24H18N2O4Pt: C, 48.57; H, 3.06;
N, 4.72. Found: C, 48.16; H, 2.98; N, 4.84. FTIR (CsI, cm−1): 2184
1
and 2143 [ν(NC)], 1770 and 1742 [ν(CO)]. H NMR (CDCl3): δ
0.62 (2JPt−H = 74.0 Hz, 6H, CH3), 7.27−7.60 (m, 9H, Ar), 7.70−7.85
(m, 2H, Ar), 8.00−8.10 (m, 2H, Ar). 13C NMR (CDCl3): δ −6.12
(1JPt−C = 594 Hz, CH3), 121.1, 123.2, 126.8, 127.2, 129.4, 129.7, 130.0,
132.7, 145.8 (Ar), 152.0 (1JPt−C = 923 Hz, NC), 163.7 (CO). ESI-
HRMS (CH3CN): m/z 616.0810 [M + Na]+, calcd for C24H18N2O4Pt
(Δ +0.1 mDa); 1209.1708 [2 M + Na]+, calcd for C48H36N4O8Pt2 (Δ
−0.9 mDa). ESI-HRMS (CH3CN + HTFA): m/z 619.0947 ([M −
CH3 + CH3CN]+, calcd for C25H18N3O4Pt (Δ +0.5 mDa).
ASSOCIATED CONTENT
■
S
* Supporting Information
Details on laboratory instruments, X-ray crystallographic
details, NMR spectra of all the compounds, X-ray crystallo-
graphic data in CIF format for 2, 1′, and 2′. This material is
available free of charge via the Internet at http://pubs.acs.org.
Synthesis of 3′. The procedure is similar to that followed for
complex 1′ starting from [PtMe2(COD)] (100 mg, 0.30 mmol) and
the ligand diNC-3 (120 mg, 0.30 mmol). White solid (yield 73%).
Anal. Calcd for C24H18N2O4Pt: C, 48.57; H, 3.06; N, 4.72. Found: C,
48.60; H, 3.07; N, 4.38. FTIR (CsI, cm−1): 2167 and 2120 [ν(NC)],
1748 and 1740 [ν(CO)]. 1H NMR (CDCl3): δ = 0.69 (s, 2JPt−H = 73.8
Hz, 6H, CH3), 7.28−7.65 (m, 9H, Ar), 7.77 (m, 1H, Ar), 8.50 (m, 2H,
Ar), 9.27 (m, 1H, Ar). 13C NMR (CDCl3): δ −5.04 (1JPt−C = 574 Hz,
CH3), 120.7, 123.9, 126.7, 127.3, 129.4, 129.8, 129.9, 131.3, 136.3,
145.4 (Ar), 152.9 (1JPt−C = 940 Hz, NC), 162.4 (CO). ESI-MS
(CH3OH): m/z 615.87 (100%) [M + Na]+.
AUTHOR INFORMATION
■
Corresponding Authors
*(P. Sgarbossa) Tel: +39 049 8275733. Fax: +39 049 8275522.
E-mail: paolo.sgarbossa@unipd.it.
*(C. Tubaro) Tel: +39 049 8275655. Fax: +39 049 8275223. E-
mail: cristina.tubaro@unipd.it.
Notes
The authors declare no competing financial interest.
Synthesis of 4′. The procedure is similar to that followed for
complex 1′ starting from [PtMe2(COD)] (100 mg, 0.30 mmol) and
the ligand diNC-4 (100 mg, 0.30 mmol). White solid (yield 76%). Mp:
158−165 °C (dec). Anal. Calcd for C20H18N2O4Pt: C, 44.04; H, 3.33;
N, 5.14. Found: C, 44.24; H, 3.12; N, 5.13. FTIR (CsI, cm−1): 2176
REFERENCES
■
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Synthesis and Material Science; Wiley-VCH: Weinheim, 2012.
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dx.doi.org/10.1021/om400859q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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