Mesoionic complexes of platinum(II) derived from rollover cyclometalation: A delicate balance between Pt-C(sp3) and Pt-C(sp2) bond cleavage as a result of different reaction conditions

Article abstract of DOI:10.1021/om300824d

Rollover cyclometalation is a particular case of metal-mediated C-H bond activation, and the resulting complexes constitute an emerging class of cyclometalated compounds. In the case of 2,2′-bipyridine rollover cyclometalation has been used to synthesize the complexes [Pt(bipy-H)(Me)(L)] (L = PPh₃, PCy₃, P(OPh)₃, P(p-tolyl)₃), whose protonation produces a series of stable corresponding pyridylenes [Pt(bipy)(Me)(L)]⁺. The unusual bipy* ligand may be described as an abnormal-remote heterocyclic chelated carbene or simply as a mesoionic cyclometalated ligand. These cationic species spontaneously convert in solution, through a retro-rollover reaction, to the corresponding isomers [Pt(bipy)(Me)(L)]⁺, where the 2,2′-bipyridine is coordinated in the classical N,N bidentate mode. Isomerization is achieved at different rates (ranging over three orders of magnitude), depending on the nature of the phosphane ligand, the most basic (PCy₃) providing the fastest reaction. The mesoionic species [Pt(bipy)(Me)(L)]⁺ contain two Pt-C bonds: the balance between the Pt-C(sp²) and Pt-C(sp³) bond rupture is subtle, and competition is observed according to the reaction conditions. In the presence of an external neutral ligand L′ methane is released to give the cationic derivatives [Pt(bipy-H)(L)(L′)]⁺, whereas reaction of the neutral [Pt(bipy-H)(Me)(L)] with HCl may follow different routes depending on the nature of the neutral ligand L. Assuming all reactions take place through the formation of a hydride intermediate, quantum chemical calculations show that computed energy barriers are qualitatively consistent with observed reaction rates.

Full text of DOI:10.1021/om300824d

Article
pubs.acs.org/Organometallics
Mesoionic Complexes of Platinum(II) Derived from “Rollover”
Cyclometalation: A Delicate Balance between Pt−C(sp3) and
Pt−C(sp2) Bond Cleavage as a Result of Different Reaction Conditions
Luca Maidich, Giuseppina Zuri, Sergio Stoccoro, Maria Agostina Cinellu, Marco Masia,
and Antonio Zucca*
̀
Dipartimento di Chimica e Farmacia, Universita di Sassari, via Vienna 2, 07100 Sassari, Italy.
S
* Supporting Information
ABSTRACT: “Rollover” cyclometalation is a particular case of metal-mediated C−H
bond activation, and the resulting complexes constitute an emerging class of cyclo-
metalated compounds. In the case of 2,2′-bipyridine “rollover cyclometalation” has
been used to synthesize the complexes [Pt(bipy-H)(Me)(L)] (L = PPh3, PCy3, P(OPh)3,
P(p-tolyl)3), whose protonation produces a series of stable corresponding pyridylenes
[Pt(bipy*)(Me)(L)]+. The unusual bipy* ligand may be described as an abnormal-
remote heterocyclic chelated carbene or simply as a mesoionic cyclometalated ligand.
These cationic species spontaneously convert in solution, through a retro-rollover
reaction, to the corresponding isomers [Pt(bipy)(Me)(L)]+, where the 2,2′-bipyridine
is coordinated in the classical N,N bidentate mode. Isomerization is achieved at
different rates (ranging over three orders of magnitude), depending on the nature of
the phosphane ligand, the most basic (PCy3) providing the fastest reaction. The
mesoionic species [Pt(bipy*)(Me)(L)]+ contain two Pt−C bonds: the balance
between the Pt−C(sp2) and Pt−C(sp3) bond rupture is subtle, and competition is ob-
served according to the reaction conditions. In the presence of an external neutral ligand L′ methane is released to give the
cationic derivatives [Pt(bipy-H)(L)(L′)]+, whereas reaction of the neutral [Pt(bipy-H)(Me)(L)] with HCl may follow different
routes depending on the nature of the neutral ligand L. Assuming all reactions take place through the formation of a hydride
intermediate, quantum chemical calculations show that computed energy barriers are qualitatively consistent with observed
reaction rates.
Scheme 1. “Rollover” Cyclometalation
INTRODUCTION
■
Classical spectator ligands such as 2,2′-bipyridine were used in
the past to simply complete the coordination sphere of a metal,
providing electronic and steric properties to the complex,
without being directly involved in chemical transformations. In
contrast, in recent years attention has been devoted to the
design of ligands with improved functions, able to enhance or
change the reactivity of the metal center in response to altera-
tions in the solution environment, such as changes in pH.
Nitrogen donors capable of forming N−H bonds, recently
defined “ligands with multiple personalities”,1 seem particularly
suitable for this purpose, showing interesting perspectives in
different fields, such as the design of molecular devices (e.g.,
molecular machines2 and organometallic sensors3) or C−H bond
activation.4
Among classical ligands 2,2′-bipyridine plays an important
role, being arguably the most studied ligand in inorganic chemistry;5
in its common coordinative behavior it acts as a chelated ligand even
if monodentate6 and bridging coordinations are also known.7
In addition to the classical coordination as a neutral N,N ligand,
2,2′-bipyidine may behave in a different way (see Scheme 1): start-
ing from a chelated complex, displacement of one of the nitrogen
atoms is followed by rotation of the pyridine ring, promoting the
activation of the C3−H bond to give a five-membered cyclo-
metalated complex.8
This unusual reaction, called “rollover” cyclometalation, is
raising a growing interest, as demonstrated by the publication
of a very recent review.9 It is worth noting that the reaction is
not restricted to 2,2′-bipyridines: other chelated heteroaromatic
ligands, such as pyrazolylmethanes,10 2-phenylpyridines,11 and
N-(2′-pyridyl)-7-azaindole,12 may react in a similar way. More-
over, cyclometalated complexes of 2,2′-bipyridine closely resemble
those of cyclometalated 2-phenylpyridine, whose platinum(II)
complexes showed interesting properties in catalysis, reactivity,
and medicinal chemistry,13 with the additional function of the
second nitrogen atom.
Received: August 27, 2012
© XXXX American Chemical Society
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In fact, compounds derived from “rollover” cyclometalation
represent a new emerging class of cyclometalated complexes,
due to the presence of the uncoordinated nitrogen atom, able
to strongly influence the reactivity of the complex. In the case
of platinum(II) these species have shown noteworthy poten-
tialities, e.g. in platinum-14 and palladium-mediated15 C−C
bond forming and in dehydrosulfurization and oxidative C−C
bond coupling of thioethers in the gas phase.16 Very recently,
two independent research groups reported catalytic applications
involving rhodium-mediated “rollover” cyclometalation.17
The first complex of this family appeared in the literature, an
iridium species with three coordinated 2,2′-bipyridines, was
completely characterized, through a single-crystal X-ray deter-
mination, only at the end of a long and controversial debate as
[Ir(bipy)2(bipy*)]3+.18 The zwitterionic bipy* ligand is an N,C-
bonded isomer of 2,2′-bipyridine, and this iridium complex has
been described only recently as the first abnormal pyridylene
complex.19 In recent years were called “abnormal”19 those
heterocyclic carbenes for which a canonical valence bond repre-
sentation requires additional formal charges on some nuclei,
whereas the term “remote” indicates that no heteroatom is
located in a position α to the carbene carbon.
characterized. The clean synthesis of the methyl species
[Pt(bipy-H)(Me)(L)] (1) has been reported only recently.23
The reaction occurs in two steps under strictly controlled
conditions and is not immune from problems. A new and more
effective route to such complexes has now been refined: the
new protocol does not rely on the isolation in the solid state
of the intermediate species 1a, a critical step, but follows a
“pseudo” one-pot reaction (see the Experimental Section),
where the desired ligand is added directly to the hot solution at
the end of the rollover process. This procedure permits us to
prepare in good yields even complexes that cannot be obtained
with the classical substitution reaction: i.e., with L = PCy3, P(OPh)3.
The new series of complexes [Pt(bipy-H)(Me)(L)] (L = PPh3
(1b), PCy3 (1c), P(OPh)3 (1d), P(p-tolyl)3 (1e)) was chosen in
order to furnish different steric and electronic properties to the
complexes, PPh3 and P(p-tolyl)3 lying in the middle for both
electronic and steric parameters.24 All synthesized complexes
The class of pyridylenes is receiving growing interest due to
their intriguing properties;20 among them, the real nature of
C-3 abnormal-remote pyridylenes has not yet been completely
defined and the distinction between the two forms, zwitterionic
and carbenic, may be subtle and, overall, only semantic. More-
over, according to IUPAC nomenclature abnormal carbenes are
included in the class of mesoionic compounds;21 however, the
term “carbene” is often used to highlight the relationship of
these species with normal heterocyclic and Fischer carbenes.
The unusual bipy* ligand may be obtained also by treatment
of the platinum(II) species [Pt(bipy-H)(Me)(DMSO)] (1a) with
acids having poorly coordinating anions, such as [18-crown-6-
H3O][BF4]. The reaction results in the synthesis of the cor-
responding mesoionic complex [Pt(bipy*)(Me)(DMSO)]+ (2a)
without involving the rupture of the Pt−C bonds present in the
molecule.
1
were characterized by H and 31P{1H} NMR spectroscopy and
elemental analysis.
Only one of the two possible geometric isomers for 1b−e is
formed, i.e. that with a P−Pt−C(sp2) trans geometry, as con-
firmed by NMR spectra, which show coupling constants in line
with phosphorus coordinated trans to the C3′ carbon (e.g.,
2
1
2JP−C3′ = 119.6 Hz, JP‑CH3 = 4.7 Hz, and JPt−P = 2229 Hz for
1b). Furthermore, in the 1H NMR spectrum the methyl
resonances appear as doublets, due to the phosphorus atom in a
cis position, with satellites.
Protonation of 1b with [18-crown-6-H3O][BF4] proceeds
smoothly to the corresponding cationic complex [Pt(bipy*)-
(Me)(PPh3)][BF4] (2b-BF4), which was isolated and charac-
terized. The same complex may also be obtained by a sub-
stitution reaction, from [Pt(bipy*)(Me)(DMSO)][BF4] (2a-BF4)
and PPh3.
In order to better describe the nature of 2b and its relation-
ship with 1b, we performed a deep NMR characterization of both
As a part of our continuous efforts in the study of cyclo-
metalated 2,2′-bipyridines22 we report here some aspects of the
chemical behavior of a series of mesoionic complexes derived
from “rollover” cyclometalation.
1
complexes. All H, 13C, and 31P NMR signals were attributed by
means of one- and two-dimensional experiments (1H, 13C, 31P,
1
1
1D-NOE, 2D H−1H COSY, and H−13C HETCOR).
First, the 13C NMR spectra allowed us to identify the cyclo-
metalated C3′ resonance with its satellites. Due to the protona-
tion the C3′ signal shifts downfield from δ 158.35 (1b) to δ
RESULTS AND DISCUSSION
■
Starting from the parent complex 1a, a series of complexes
of general formula [Pt(bipy-H)(X)(L)] (X = anionic ligand,
L = neutral ligand) can be isolated in the solid state and
1
163.41 (2b) while JPt−C increases from 970 (1b) to 997 Hz
(2b). These data may be compared to those recently reported
B
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for the analogous complex [Pt(L1*)(Me)(PPh3)]+ (δ 160.27
The reversibility of the rollover process opens up the
possibility to design catalytic cycles based on rollover and retro-
rollover reactions, with the bipy ligand playing an active role,
behaving as a hydrogen reservoir/acceptor.
At variance with 2b, the protonated complex [Pt(bipy*)-
(Me)(PCy3)]+ (2c) shows an enhanced reactivity and rapidly con-
verts into the corresponding adduct [Pt(bipy)(Me)(PCy3)]+ (3c)
(Chart 1). Even with short reaction times at room temperature a
1
ppm, JPt−C = 990 Hz, L1 = (5S,7S)-5,7-methane-6,6-dimethyl-
2-(pyridin-2-yl)-5,6,7,8-tetrahydroquinoline).25
The C3′ signal is not particularly shifted to high frequencies
for a carbon which may be defined as “carbenic”, but it can be
compared to the signals for platinum(II) C2-pyridylene
complexes reported by Bercaw and co-workers26 (δ ca. 160−
170 ppm). Furthermore, in contrast to C2 and C4 “normal”
pyridylenes, which often show chemical shift values above 190
ppm, “abnormal” C3-pyridylenes usually have values of
chemical shifts (δ ca. 160−170 ppm) lower than those of the
“normal” homologues. It has been recently concluded that
chemical shift values for C3-pyridylenes must be considered
with great care, being affected by a number of factors,19 and
overall cannot be simply described in terms of valence bond
theory.20
Chart 1
The enhancement of the Pt−C3 coupling constant from 1b
to 2b suggests an enhancement in the Pt−C3 bond strength
after protonation. A complete analysis of the 13C NMR spectra
enabled us to note that the coupling constant values of the
metalated pyridine ring carbons are greater than those of the N-
bonded pyridine ring. These values are in agreement with the
coordination geometry, i.e. nitrogen trans to methyl and C3′
trans to a less trans-influencing donor atom, such as phosphorus.
Complexes 1 and 2 have a good thermal stability; for example,
1b and 2b-BF4 are stable up to 215 and 183 °C, respectively, after
which decomposition occurs.
A new reaction was observed when complex 2b was left in
solution for several days: complex 2b slowly converts into a
new species with a completion time of ca. 30 days. The com-
pound formed was isolated in the solid state and characterized
as the adduct [Pt(bipy)(Me)(PPh3)][BF4] (3b-BF4) (Scheme 2).
mixture of 2c and 3c is formed (e.g., 9:1 molar ratio after 10 min)
so that complex 2c cannot be isolated in pure form but only
detected in solution: e.g., by means of NMR spectroscopy. Full
conversion is achieved in a couple of hours.
In contrast, the cationic complex [Pt(bipy*)(Me)(P(OPh)3)]+
(2d) is kinetically much more stable and can be easily characterized.
The subsequent isomerization to the adduct 3d is extremely slow
with a completion time of about 3 months at room temperature.
The last series of complexes, 1e−3e, was studied in order to
evaluate only the electronic factors by direct comparison with
the PPh3 series (1b−3b). The P(p-tolyl)3 ligand, according to
the Tolman cone angle θ, is equivalent to PPh3 but is more
basic due to the presence of the methyl groups.28
Scheme 2
Despite the close similarity between these two phosphines
the observed difference is striking when 10−2 M solutions in
1
CD2Cl2 of 2b,e are followed by means of H and 31P NMR
spectroscopy at room temperature: after 6 days a 90% conver-
sion was observed for 2e, in contrast to the 33% conversion
observed for 2b.
These data show a clear dependence of the reaction rate from
the donor properties of the phosphane and are in line with
Tolman electronic parameters.
In the 31P NMR spectrum a strong 1JPt−P coupling constant (4351 Hz)
On the whole, our experimental data show that the time
requested to complete the retro-rollover reaction may range
over almost 3 orders of magnitude by simply changing the
nature of the fourth ligand. The reaction seems to be closely
related to the electronic properties of the phosphane: the more
basic donor, PCy3, is kinetically more active than the less basic
P(OPh)3, with PPh3 and P(p-tolyl)3 in the middle, a trend com-
patible with a hypothetical mechanism where the proton
migrates to the Pt(II) center to give a Pt(IV) hydride, from
which, after reductive elimination of C3−H, the final product is
formed. On the basis of our experimental data we cannot rule
out the possibility of a concerted electrophilic attack at the
metal−carbon bond; moreover, a different mechanism may be
operating for different phosphane donors.
is in agreement with a phosphorus trans to a pyridine nitrogen.
3
2
The JP−H and JPt−H values in the adduct 3b are particularly
small: ca. 3 and 69 Hz, respectively (vs 7.1 and 82 Hz in 2b).
Complexes 2b and 3b are isomers; therefore, this reaction
corresponds to an isomerization process. The rate of the
reaction may be influenced by addition of DMSO to the
solution: the reaction evolves completely in 2 days either in
CD2Cl2 with 10 drops of DMSO or in DMSO-d6 solution. It is
also worth noting that excess [18-crown-6-H3O][BF4] does not
influence the rate of the process.
The reaction is unprecedented for 2,2′-bipyridine, being the
very first example of a “retro-rollover” reaction in solution of a
bipy complex with isolation of the product. Reversibility of the
rollover process has been recently invoked in the gas phase to
explain the exchange of hydrogen between the bipy fragment
and dimethyl sulfide.27 A related reversible reaction, driven by a
change of solvent, has also been recently reported by Rourke
and co-workers for a cyclometalated phenylpyridine.11
The most electron-rich Pt(II) center in complex 2c is likely
to undergo the oxidative addition pathway more easily than
2b,d; however, it is important to note that steric factors may
also play a role in the process and should not be completely
ruled out.
C
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Table 1. Selected Distances, in Å, for the Complexes Optimized at the PBE0/def2-SVP Level
Pt−P
Pt−CH3
Pt−C3′
Pt−N1
N1−C2
C2−C2′
C2′−C3′
N1′−H
Pt−H
1b
2b
Hb
2.34920
2.36436
2.46720
2.27258
2.40027
2.40918
2.51660
2.29789
2.25061
2.27117
2.38265
2.19685
2.35137
2.36572
2.46324
2.27449
2.04446
2.03797
2.04693
2.03849
2.04067
2.03327
2.04334
2.03400
2.06163
2.05302
2.06188
2.05451
2.04418
2.03767
2.04638
2.03797
2.03727
2.02873
2.04123
2.12163
2.03832
2.02920
2.04468
2.13049
2.04933
2.04268
2.03600
2.12177
2.03725
2.02858
2.04338
2.12314
2.19397
2.20444
2.21437
2.18233
2.21738
2.23252
2.23873
2.20572
2.17416
2.18241
2.18655
2.16400
2.19300
2.20293
2.21426
2.18154
1.35079
1.35299
1.34998
1.35094
1.35093
1.35285
1.34975
1.35081
1.35008
1.35286
1.35048
1.35100
1.35083
1.35298
1.34998
1.35088
1.47052
1.46847
1.47216
1.47742
1.46757
1.46587
1.47012
1.47454
1.47131
1.46877
1.47228
1.47653
1.47055
1.46842
1.47236
1.47758
1.41345
1.40741
1.41048
1.35180
1.41271
1.40664
1.40850
1.34985
1.41211
1.40528
1.41045
1.35130
1.41361
1.40770
1.41052
1.35162
1.01689
1.50983
a
3b
1c
2c
Hc
1.01677
1.01712
1.01686
1.50872
1.51744
1.50942
a
3c
1d
2d
Hd
a
3d
1e
2e
He
a
3e
a
Distances involve the N1′ instead of the C3′ atom.
Table 2. Selected Angles, in deg, for the Complexes Optimized at the PBE0/def2-SVP Level
P−Pt−CH3
CH3−Pt−C3′
C3′−Pt−N1
N1−Pt−P
N1−Pt−CH3
P−Pt−C3′
1b
2b
Hb
92.77
91.90
90.59
90.44
91.77
90.47
90.51
87.31
88.29
86.25
85.85
84.10
92.74
91.88
90.55
90.59
90.12
90.64
92.38
92.85
88.19
88.95
89.87
92.24
89.89
91.10
92.05
93.24
90.20
90.74
92.44
92.79
79.47
79.03
79.67
76.91
78.84
78.27
79.03
76.06
79.41
78.96
79.98
76.92
79.47
79.04
79.62
76.89
97.68
98.47
169.39
169.49
171.93
168.91
166.63
166.29
168.85
161.80
169.29
170.03
170.83
170.09
169.47
169.62
171.93
169.14
176.84
177.30
175.78
175.83
178.74
177.43
178.24
171.18
177.39
177.21
177.46
177.33
176.80
177.22
175.79
176.08
97.27
a
3b
1c
2c
Hc
99.99
101.26
102.52
100.54
106.39
102.42
103.68
102.22
105.74
97.64
a
3c
1d
2d
Hd
a
3d
1e
2e
He
98.37
97.31
a
3e
99.85
a
Angles involve the N1′ instead of the C3′ atom.
Under these experimental conditions, therefore, selective Pt−
species involved: i.e., 1−3. On the basis of the results of
previous studies on similar reactions,31 we hypothesize that the
formation of a hydride intermediate is the key step for the
reaction mechanism of all species studied. Therefore, we
optimized the geometry of reagents, (proposed) intermediates,
and products for the three reactions discussed above. All
calculations were performed with the hybrid functional PBE0,32
using the Firefly QC package.33 The def2-SVP34 basis set was
used for all atoms; for Pt an effective core potential was used to
take into account 60 core electrons. Convergence criteria for
energy minimization and geometry optimization were set to 10−6.
All optimized geometries were validated as minima by inspecting
the eigenvalues of the normal-mode analysis. Harmonic fre-
quencies were then used to compute the reported energies of
all species. Calculations with a larger basis set (def2-TZVP)
performed on the 1b−3b series of complexes agree with a maxi-
mum discrepancy of 2% in bond distances, thus confirming the
validity of the results obtained with a smaller basis set (see the
Supporting Information).
C(sp2) vs Pt−C(sp3) bond breaking is observed in compounds
2. Protonolysis of Pt−C bonds has been the subject of
extensive research. Significant mechanistic insights were gained
from this reaction, regarded as the microscopic reverse of the
C−H activation step of hydrocarbons, a process of great
practical implications. A particular attention has been devoted
to Pt−alkyl systems, whose protonolysis follows two alternative
mechanisms: (1) a concerted electrophilic attack at the metal−
carbon bond and (2) protonation at the metal to generate a
platinum(IV) hydride, from which, after reductive elimination,
alkane is lost. In general, the distinction between the two
pathways is problematic, but it has been shown that for Pt(II)
complexes with good electron donor ligands protonation of the
metal center is favored.29 Pt(IV) hydrido alkyl species have
been observed at low temperature, and in many cases it has
been asserted that the metal is the preferred site of
protonation.30
Ab Initio Calculations. In order to get more insights into
the observed reaction kinetics of the retro-rollover process, we
performed density functional theory (DFT) calculations on the
Selected data for the optimized geometries can be found in
Tables 1 and 2.
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Protonation of the neutral species 1b−e has very little
influence on the bond distances and angles; it is worth noting
that there are no differences above 0.02 Å, the largest ones
involving the elongation of Pt−P and Pt−N1 bonds. As
expected, all the bonds connected to the protonation site, e.g.
the N1′ atom, are shorter after protonation.
In Table 3 we report the (zero point energy corrected)
enthalpies for all the species involved in the experimental study.
Table 3. Calculated Enthalpies, in kJ/mol at 298.15 K and
101325 Pa, for the Complexes Optimized at the PBE0/def2-
SVP Level
Bonds in the metallacycle not directly connected to platinum
undergo very slight and probably not meaningful changes. All
species are distorted square planar (see Table 2), with the
heteroaromatic ligand lying on the plane; the lowest values for
the C3′−Pt−CH3 and C3′ −Pt−N1 angles are found for the
PCy3 ligand.
2
H
3
PPh3
0.00
0.00
0.00
0.00
78.75
72.71
93.01
73.75
−92.78
−96.57
−91.66
−95.01
PCy3
P(OPh)3
P(p-tolyl)3
A comparison of the proposed hydride intermediates Hb−e
permits us to note that all the bonds involving platinum are
elongated with respect to both the protonated (2b−e) and neutral
(1b−e) corresponding species, with the Pt−P bond experienc-
ing the greatest change in absolute value. Pt−H distances follow
a reverse trend in comparison to those of Pt−P; these ob-
servations are consistent not only with the change in geometry
and coordination number around the Pt center but also with
the steric requirements of each phosphane.
Finally, for species 3b−e, we notice that, by comparison with
the corresponding species 1, 2, and H, (a) the Pt−P distance
decreases possibly because the trans influence of the nitrogen is
weaker in comparison to that of the C(sp2) carbon of the
cyclometalated bipyridine, (b) the distances Pt−CH3 and Pt−
N1 increase and decrease respectively, (c) the C2−C2′ distances
remain substantially unchanged from Hb−e but increase in com-
parison to those in 3b−e, and (d) P−Pt−CH3 and C3′ −Pt−N1
angles tighten while C3′−Pt−CH3 and N1−Pt−P widen. Figure 1
reports the geometry-optimized minimum structures of com-
plexes 1b, 2b, Hb, and 3b.
In order to qualitatively correlate numerical results to experi-
mental findings, since we do not have any data for the transi-
tion states (TS), we refer to the Hammond−Leffler postulate:35
assuming that the intermediates are structurally similar, and
thus close in energy to the respective TSs, we hypothesize that
the activation energies and enthalpies follow the same trend
as the calculated values for the intermediate species. We notice
that the smallest barrier is found for the complex containing the
most basic ligand (PCy3), while the highest barrier is found for
the complex containing the most acidic ligand, in fair agree-
ment with experimental observations. This is also coherent with
experimental findings that P(p-tolyl)3 is faster than PPh3, the
former having a smaller energy barrier, thus suggesting that
electronic factors effectively have a role in speeding up the
reaction.
Looking at the final product of the process, we can see
another factor which drives the reaction that is purely thermo-
dynamic: the energy content of the adducts 3 is considerably
higher, in absolute value, than that of the protonated complexes
2, and this can give a good explanation why we do not observe
an equilibrium. From a closer point of view we can note that
the sum of the energies of Pt−C and N−H bonds is less than
that of Pt−N and C−H bonds, all other bonds being kept
constant.
The energy profile for the retro-rollover reaction is reported
in Figure 2.
The synthesis of adduct species 3b−e is not trivial. To the
best of our knowledge, only the adduct [Pt(bipy)(Me)(PPh3)]
Figure 2. (a) Energy profile for the retro-rollover reaction: (green)
PCy3; (blue) P(p-tolyl)3; (orange) PPh3; (red) P(OPh)3. (b) Expansion
of the hydride intermediate region.
Figure 1. Geometry-optimized minimum structures of complexes 1b,
2b, Hb, and 3b.
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Cl has been reported in the literature,36 being detected only in
solution but not isolated as a pure species in the solid state. The
diimino cationic complexes [Pt(N,N)(R)(L)]+ are of interest
for their activity in C−H bond activation,37 and their synthesis
is not so straightforward. Complex 3b has been previously
obtained and fully characterized in our laboratory38 following a
different synthetic procedure.
simply accelerates the reaction; in contrast, the presence of a strong
donor in a solution of 2b may drive the reaction toward a different
route: addition of a second equivalent of PPh3 results in formation
of methane and coordination of PPh3 on the fourth coordination
site, to give the complex [Pt(L-H)(PPh3)2]+ (4).
NMR Data of Neutral and Protonated Species 1 and 2.
The analysis of 1H, 31P, and 13C NMR spectra of species 1 and
2 shows, after protonation, an enhancement of the coupling
constants related to the mutually trans Pt−P and Pt−C(sp2)
bonds (e.g., 1JPt−C3 in 1b and 2b, 3JPt−H4 = 46.4 Hz in 1d and 49
1
Hz in 2d; JPt−P = 2229 Hz in 1b and 2500 Hz in 2b).
In contrast, coupling constants related to Pt−L bonds cis to
C3 show a small decrease after protonation (e.g., the Pt−CH3
bond: 1JPt−C = 725 Hz in 1b and 716 Hz, 2b). Pt−N bonds may
The same result was obtained by starting from 2a and 2
equiv of PPh3. Complex 4 was characterized in solution and in
the solid state. In particular, the presence of two signals in the
31P NMR spectrum with different Pt−P coupling constants
3
be evaluated by comparison of the JPt−H6 values (e.g., 16.0 Hz
in 1d and 15 Hz in 2d).
On the whole, the rationale of the observed NMR trends for
species 1 and 2 is not as straightforward and great care should
be taken in the analysis of NMR data in solution, because several
factors may have effects on spin−spin coupling constants.
One-bond coupling constants are dominated by the Fermi
contact interaction of nuclei with s electrons and are usually
taken as an estimate of bond strength,39 provided such bonds
involve hybrid orbitals with some s character;40 as an example, a
correlation of Pt−P bond lengths with Pt−P coupling constants
has been recently reported by Wollins and co-workers.41
In those cases in which greater values of 1JPt−P are associated
with shorter Pt−P bonds, the trend is likely to derive from the
sensitivity of both parameters to the s orbital bond order.42
In addition, charge effects may operate and the presence of a
protonated nitrogen in species 2 should influence the coupling
constant values; however, back-donation in Pt(II) complexes
may be considered of little relevance43 and it has also been
reported that PtII−P bonds are weaker when the metal atom is
less positively charged41 with a negligible Pt−P π donation in
cationic complexes.
accounts for P−Pt−C (JPt−P = 2105 Hz) and P−Pt−N (JPt−P
=
3938 Hz) trans arrangements; the relative cis coordination of
the two phosphorus atoms is forced by the presence of the
cyclometalated chelating ligand and is also confirmed by the
value of the P−P coupling constant (2JP−P = 19 Hz).
1
Furthermore, when the reaction is followed by means of H
NMR spectroscopy a signal at 0.22 ppm, due to free methane,
is observed.44 The same behavior is observed with bidentate
donors, such as 1,2-bis(diphenylphosphino)ethane (dppe) and
2,2′-bipyridine (bipy). Addition of dppe or bipy to a solution of
2a results in Pt−C(sp3) bond cleavage, and [Pt(bipy-H)(dppe)]-
[BF4] (5-BF4) and [Pt(bipy-H)(bipy)][BF4] (6-BF4)
may be isolated in the solid state and characterized.
Complex 6 has some points of interest, having two
coordinated 2,2′-bipyridines, one in the classical κ2N,N and
the other in the κ2N,C mode.
Complex 5 was easily characterized by means of NMR
spectroscopy. In particular, the 31P NMR spectrum shows two
signals at high frequency, as is usual for chelated dppe,
attributable to a P trans to C (δ 51.81 ppm, 1JPt−P = 1949 Hz)
and a P trans to N (δ 42.64 ppm, 1JPt−P = 3691 Hz). The signals
1
in the H NMR spectrum appear as broad lines, probably due
to a dynamic process in solution.
The presence of a good donor in solution seems to be of
paramount importance in driving the reaction toward Pt−C(sp3)
instead of Pt−C(sp2) protonolysis (Scheme 3). The vacant
coordination site created in the first case is occupied by the external
ligand, whereas in the second case the pyridine itself contains a
nitrogen donor center.
Both of the observed Pt−C bond breaking reactions are
irreversible, and in the retro-rollover process the second
nitrogen coordinates only after the reductive elimination of
C(sp2)−H; thus, the differentiating factor between M−C sp2
and sp3 bond breaking operates before the reductive
elimination step. For this reason we may assume coordination
of the external ligand before the reductive elimination step.
As for chemical shift value, protonation of species 1a−e
produces a shift to high frequency of most of the metalated
bipyridine hydrogens, as may be expected from the presence of
a positive charge. This effect is more evident in the metalated-
protonated pyridine ring, and it is clearly associated with the
electron deficiency of the complex.
Competition between Pt−C(sp2) and Pt−C(sp3) Bond
Breaking. The isomerization process from species 2 to 3
shows that, under the experimental conditions described, only
the Pt−C(sp2) bond is attacked, leaving the Pt−C(sp3) bond
unaltered. However, the balance between the two Pt−C bond
ruptures is subtle. Addition of a weak donor such as DMSO
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activation step: i.e., protonolysis of alkylplatinum(II) model sys-
tems. The protonation reaction may occur with different
mechanisms, but it has been demonstrated that in electron-rich
platinum(II) diiminoalkyl complexes the metal is both the
thermodynamic and kinetic site of protonation.46
Scheme 3
In the protonation reaction of the so-called “rollover com-
plexes” a competition between two Brønsted−Lowry bases, i.e.
the uncoordinated nitrogen and the platinum lone pairs, may
occur, in addition to electrophilic attack at the Pt−C(sp2)
and Pt−C(sp3) bonds. The nitrogen is likely to be the kinetic
protonation site to give the uncommon cationic species 2,
[Pt(bipy*)(Me)(L)]+, which contains the unusual bipy* ligand
described as an abnormal-remote pyridylene or as a mesoionic
cyclometalated ligand.
The protonated species converts in solution with proto-
nolysis of one of the two Pt−C bonds present in the complex.
According to the reaction conditions Pt−C(sp2) or Pt−C(sp3)
bond rupture may take place. In the first case, in the absence of
a good external donor, a unique retro-rollover reaction occurs
to give the cationic adducts [Pt(bipy)(Me)(L)]+. The rate of
this isomerization process is dramatically dependent on the
nature of the neutral ligand L, the more basic PCy3 ligand
providing the fastest reaction among the phosphanes used in
this study. In contrast, in the presence of an external good
donor elimination of methane is observed through Pt−C(sp3)
bond protonolysis. Theoretical studies are in qualitative agree-
ment with the experimental observations, suggesting that the
key factor influencing the kinetics is the (de-) stabilization of
the electron density on the metal center.
Several factors are active in the course of the protonolysis
reaction, and a subtle balance between them may drive the
reaction toward different routes.
The comprehension of the factors leading the rollover and
retro-rollover processes would give the possibility of designing
catalytic cycles where the bipy and bipy* ligands will no longer
operate as spectator ligands but will be active actors in the
process: for example, acting as a hydrogen reservoir/acceptor
system.
Reaction with HCl. As previously reported, the reaction of
1a with an acid having a coordinating anion, such as HCl, gives
the corresponding chloride [Pt(bipy-H)Cl(DMSO)] (7a) and
free methane.8b
The same product, 7a, can be obtained with good yields by
reaction of 1a with [18-crown-6-H3O][BF4] in acetone in the
presence of LiCl. The reaction likely proceeds through protona-
tion of the uncoordinated nitrogen atom, followed by a
mechanism similar to that operating in the presence of 2 equiv
of PPh3, with the anionic ligand Cl− in place of neutral PPh3.
The reaction of the species [Pt(bipy-H)(Me)(L)] with HCl,
however, is not as straightforward: the corresponding reaction
of 1b, having PPh3 in place of DMSO, shows a competition
between the two protonolysis processes, and a mixture of
[Pt(bipy-H)(Cl)(PPh3)] (7b) and [Pt(bipy)(Me)(PPh3)]Cl
(3b-Cl) in a 1:4 molar ratio is formed. Their identification in
solution was ascertained through NMR spectroscopy by
comparison with literature data.23,26
EXPERIMENTAL SECTION
■
cis-[Pt(Me)2(DMSO)2] was synthesized according to ref 47. All of the
solvents were purified and dried according to standard procedures.48
Elemental analyses were performed with a Perkin-Elmer 240B ele-
mental analyzer by Mr. Antonello Canu (Dipartimento di Chimica,
̀
Universita degli Studi di Sassari, Italy).
In contrast, the reaction of 1c with HCl gives the cationic
adduct [Pt(bipy)(Me)(PCy3)]Cl (3c-Cl) as the main species,
in addition to a minor species (in a 1:3 molar ratio) not cor-
responding to the expected rollover complex [Pt(bipy-H)Cl-
(PCy3)]. Despite several efforts, we were not able to separate
and characterize the second species.
The role of the nature of the neutral fourth ligand appears
therefore to be of paramount importance in driving the Pt−C
bond breaking, with a marked propensity of phosphane
complexes to give Pt−C(sp2) protonolysis followed by a
retro-rollover reaction: overall, several factors may play a role in
the reaction and will be the subject of future investigations.
Infrared spectra were recorded with a FT-IR Jasco 480P using Nujol
1
mulls. H, 13C{1H}, and 31P{1H} NMR spectra were recorded with
Varian VXR 300 and Bruker Avance III 400 spectrometers. Chemical
shifts are given in ppm relative to internal TMS for H and 13C{1H}
1
and external 85% H3PO4 for 31P{1H}; J values are given in Hz. NOE
difference, 2D-COSY and 13C apt (attached proton test) experiments
were performed by means of standard pulse sequences. The numerical
scheme for NMR data is as follows:
CONCLUSIONS
■
Great efforts have been devoted to the comprehension of the
mechanisms operating in alkane C−H activation by platinum
complexes.45 Significant mechanistic insights have been gained
through investigations of the microscopic reverse of the C−H
Preparations. [Pt(bipy-H)(Me)(PPh3)] (1b). Method A. For the
reaction of [Pt(bipy-H)(Me)(DMSO)] with PPh3, see ref 8b (yield
67%).
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Method B. To a solution of cis-[Pt(Me)2(DMSO)2] (50.1 mg, 0.131
mmol) in anhydrous toluene an excess of 2,2′-bipyridine (63.5 mg,
0.406 mmol) was added under a nitrogen atmosphere. The solu-
tion became suddenly red and was heated to reflux for 3 h. At the end of
the reaction PPh3 (37.8 mg, 0.144 mmol) was added to the hot solution
and left to react for about 30 min; then the solution was concentrated to a
small volume and treated with n-hexane to form a precipitate. The solid
was filtered off, washed with n-hexane, and vacuum-pumped to give the
analytical sample as a yellow solid. Yield: 87%. Mp: 215 °C. Anal. Calcd
for C29H25N2PPt·1/2H2O: C, 54.72; H, 4.12; N, 4.40. Found: C, 54.67; H,
3.79; N, 4.51. 1H NMR (CDCl3): 8.40 (m, 1H, H6′); 8.31 (dd, 1H, JH−H
= 8.0 Hz, JH−H = 1.5 Hz, H3); 8.24 (ddd with sat, 1H, 3JPt−H = 48.0 Hz,
JH−H = 7.5 Hz, JH−H = 5.4 Hz, JH−H = 1.8 Hz, H4′); 7.79−7.72 (m, 7H, H4
+ Ho PPh3); 7.46−7.35 (m, 10H, H6 + Hm + Hp PPh3) 7.22 (ddd, 1H,
JH−H = 7.8 Hz, JH−H = 7.5, JH−H = 1.7, H5′); 6.67 (td, 1H, JH−H = 5.4 Hz,
JH−H = 1.5 Hz, H5); 0.74 (s with sat, 3H, 2JPt−H = 83 Hz, 3JP−H = 7.7 Hz,
Pt-CH3). 13C NMR (CDCl3): 165.71 (s with sat, 2JPt−C = 19.6 Hz, C2 or
heated to reflux for 3 h; then 69.0 mg (0.227 mmol) of P(p-tolyl)3 was
added to the hot solution and left to react for about 1 h. Complex 1e
was extracted with water (3 × 10 mL), and the aqueous phase was
back-extracted with CH2Cl2 (2 × 5 mL). All the organic phases were
then mixed together and treated with Na2SO4 for 15 min with stirring,
filtered, concentrated to a small volume, and then treated with n-
hexane to give a vivid yellow precipitate that was filtered off and
vacuum-dried. Yield: 50%. Anal. Calcd for C32H31N2PPt: C, 57.39; H,
1
4.66; N, 4.18. Found: C, 57.22; H, 4.85; N, 4.03. H NMR (CD2Cl2,
400 MHz): 8.38 (d br, 1H, JH−H = 4.9 Hz, H6′); 8.33 (dd, 1H, JH−H
=
3
4
8.2, 1.3 Hz, H3); 8.18 (ddd sat, 1H, JPt−H = 48 Hz, JP−H = 5.4 Hz,
JH−H = 7.6, 1.6 Hz, H4′); 7.86−7.79 (m, 2H, H4 + H5′); 7.64 (dd, 6H,
3JP−H = 10.2 Hz, JH−H = 8.1 Hz, Ho P(p-tol)3); 7.24 (d br, 7H, JH−H
=
8.3 Hz, H6 + Hm P(p-tol)3); 6.73 (ddd, 1H, JH−H = 7.2, 5.8, 1.5 Hz,
2
H5); 2.41 (s, 9H, CH3 P(p-tol)3); 0.66 (d sat, 3H, JPt−H = 84 Hz,
3JP−H = 7.7 Hz, Pt-CH3). 31P NMR (ppm, CD2Cl2, 161.9 MHz): 30.0
(s sat, 1P, JPt−P = 2245 Hz, P(p-tol)3).
2
3
C2′); 164.69 (d with sat, JPt−C = 48.3 Hz, JP−C = 3.5 Hz, C2 or C2′);
[Pt(bipy*)(Me)(PPh3)][BF4] (2b-BF4). Method A. To a stirred pale
yellow solution of [Pt(bipy-H)(Me)(PPh3)] (50.3 mg, 0.080 mmol) in
10 mL of CH2Cl2 was added at room temperature, under a nitrogen
atmosphere, 29.5 mg (0.081 mmol) of [18-crown-6-H3O][BF4]. The
solution was stirred for 30 min; then it was concentrated to small
volume and treated with diethyl ether. The precipitate that formed was
filtered off, washed with diethyl ether, and vacuum-pumped to give the
analytical sample as a yellow solid. Yield: 86%.
2
155.35 (d with sat, JPt−C = 970.3 Hz, JP−C = 119.6 Hz, C3′); 150.49 (d
with sat, 3JPt−C = 13.7 Hz, 4JP−C = 3.8 Hz, C6); 144.98 (s, C4); 140.08 (s
2
with sat, JPt−C = 82.4 Hz, C4′); 137.51 (s, C6′); 134.99 (d with sat,
3JPt−C = 17.1 Hz, 2JP−C = 11.9 Hz, Co PPh3); 132.14 (d with sat, 2JPt−C
=
17.0 Hz, JP−C = 44.0 Hz, Cipso PPh3); 130.26 (d, 4JP−C = 1.7 Hz, Cp PPh3);
126.30 (d, 3JP−C = 9.8 Hz, Cm PPh3); 124.48 (d with sat, 2JPt−C = 53.4 Hz,
3JP−C = 5.6 Hz, C5′); 123.73 (s with sat, 3JPt−C = 11.1 Hz, C5); 121.38 (s
3
1
Method B. To a solution of [Pt(bipy*)(Me)(DMSO)][BF4] (65.1 mg,
0.1226 mmol) in CH2Cl2 (30 mL) was added under a nitrogen
atmosphere 32.3 mg of PPh3 (0.1233 mmol). The solution was stirred
for 1 h 30 min; then it was concentrated to a small volume and treated
with diethyl ether. The precipitate that formed was filtered off, washed
with diethyl ether, and vacuum-pumped to give the analytical sample
as a yellow solid. Yield: 85%. Anal. Calcd for C29H28BF4N2OPPt·H2O:
C, 47.49; H, 3.85; N, 3.82. Found: C, 47.88; H, 3.98; N, 3.87. Mp: 183
with sat, JPt−C = 20.1 Hz, C3); −12.37 (d with sat, JPt−C = 725.3 Hz,
2JP−C = 4.7 Hz, CH3). 31P NMR (CDCl3): 33.59 (s with sat, 1JPt−P = 2229
Hz, PPh3).
[Pt(bipy-H)(Me)(PCy3)] (1c). To a solution of cis-[Pt(Me)2-
(DMSO)2] (209.1 mg, 0.548 mmol) in anhydrous toluene was
added an excess of 2,2′-bipyridine (267.4 mg, 1.71 mmol) under a
nitrogen atmosphere. The solution became suddenly red and was
heated to reflux for 3 h. At the end of the reaction was added 268.3 mg
(0.956 mmol) of PCy3 to the hot solution, and the mixture was left to
react for about 30 min. The resulting solution was concentrated to a
small volume and treated with n-pentane. The precipitate that formed
was filtered off, washed with n-hexane, and vacuum-pumped to give
the analytical sample, a vivid yellow solid. Yield: 76%. Mp: 229 °C.
Anal. Calcd for C29H43N2PPt: C, 53.94; H, 6.71; N 4.34. Found: C,
1
°C. H NMR (CDCl3): 13.5 (s broad, 1H, N−H); 8.97 (m with sat,
3
4
1H, JPt−H = 50 Hz, JP−H = 6.3 Hz, JH−H = 7.5 Hz, JH−H = 1.2 Hz,
H4′); 8.69 (dd, 1H, JH−H = 5.4 Hz, JH−H = 1.2 Hz, H6′); 8.48 (d, 1H,
JH−H = 8.2 Hz, H3); 8.16 (td, 1H, JH−H = 7.9 Hz, H4); 7.88 (dd with
3
sat, 1H, JPt−H = 10 Hz, JH−H = 5.3 Hz, H6); 7.78 (m, 1H, JH−H = 7.2
Hz, JH−H = 5.4 Hz, H5′); 7.64−7.71 (m, 6H, Hm (PPh3)); 7.42−7.50
(m, 9H, Ho + Hp (PPh3)); 6.96 (ddd, 1H, JH−H = 7.6 Hz, H5); 0.83 (d
1
54.28; H, 6.32; N, 4.70. H NMR (CDCl3): 8.75 (d with sat, 1H,
2
3
with sat, 3H, JPt−H = 82 Hz, JP−H = 7.1 Hz, Pt-CH3). 13C NMR
(CDCl3): 163.41 (d with sat, JPt−C = 997 Hz, JP−C = 122.6 Hz, C3′);
156.75 (s with sat, JPt−C = 24 Hz, C2 or C2′); 155.44 (d with sat,
JPt−C = 49 Hz, JP−C = 4.0 Hz, C2′ or C2); 151.83 (d with sat, JPt−C = 12
Hz, JP−C = 3.5 Hz, C6); 149.83 (s with sat, JPt−C = 75.8 Hz, C4′);
139.77 (s, C4); 136.80 (s, C6′); 134.75 (d with sat, JPt−C = 17.4 Hz,
JP−C = 11.9 Hz, Co PPh3); 130.89 (d, JP−C = 2.4 Hz, Cp PPh3); 130.66
(d with sat, JPt−C = 19 Hz, JP−C = 48.0 Hz, Cipso PPh3); 128.63 (d,
JP−C = 9.9 Hz, Cm PPh3); 127.39 (s with sat, JPt−C = 5.1 Hz, C5);
125.63 (d with sat, JPt−C = 54.7 Hz, JP−C = 5.6 Hz, C5′); 121.79 (s with
sat, JPt−C = 15.5 Hz, C3); −11.59 (d with sat, JPt−C = 716 Hz, JP−C = 5.5
Hz, Pt-CH3). 31P NMR (CDCl3): 32.13 (s with sat, JPt−P = 2500 Hz,
3JPt−H = 12.4 Hz, JH−H = 5.6 Hz, H6); 8.33−8.40 (m, 2H, H3 + H6′);
3
4
8.20 (ddd with sat, 1H, JPt−H = 45.2 Hz, JP−H = 5.1 Hz, JH−H = 7.6
Hz, JH−H = 1.7 Hz, H4′); 7.91 (ddd, 1H, JH−H = 8.4 Hz, JH−H = 5.6 Hz,
JH−H = 1.3 Hz, H4); 7.13−7.26 (m, 2H, H5 + H5′); 1.20−2.40 (m,
2
3
33H, Cy); 1.00 (d with sat, 3H, JPt−H = 84.3 Hz, JP−H = 5.5 Hz, Pt-
CH3). 31P NMR (CDCl3): 19.30 (s with sat, 1JPt−P = 2083 Hz, PCy3).
[Pt(bipy-H)(Me)(P(OPh)3)] (1d). To a solution of cis-[Pt(Me)2-
(DMSO)2] (103.0 mg, 0.270 mmol) in anhydrous toluene was
added an excess of 2,2′-bipyridine (123.5 mg, 0.791 mmol) under a
nitrogen atmosphere. The solution became suddenly red and was
heated to reflux for 3 h. At the end of the reaction was added to the
hot solution 92 μL (0.351 mmol) of P(OPh)3, and the mixture was left
to react for about 30 min. The resulting solution was evaporated to
dryness and treated with with n-pentane to obtain a pale yellow solid,
which was filtered on a Hirsch funnel, washed with n-pentane, and
vacuum-pumped to give the analytical sample. Yield: 78%. Mp:
90−95 °C. Anal. Calcd for C29H25N2O3PPt: C, 51.56; H, 3.73; N,
−
PPh3). IR (Nujol, νmax/cm−1): 3563 N−H; 1060 (broad) BF4 .
[Pt(bipy*)(Me)(PCy3)][BF4] (2c-BF4). To a stirred solution of
[Pt(bipy-H)(Me)(PCy3)] (0.116 mmol) in 10 mL of distilled CH2Cl2
was added at room temperature, under a nitrogen atmosphere,
43.5 mg (0.118 mmol) of [18-crown-6-H3O][BF4]. After 15 min of
reaction at room temperature the dark solution was concentrated to
give an oil: treatment with diethyl ether gave, after stirring, a pale
1
4.15. Found: C, 52.02; H, 3.49; N, 3.62. H NMR (CDCl3): 9.25 (d
with sat, 1H, 3JPt−H ≈ 16 Hz, H6); 8.34 (dt broad, 1H, JH−H = 4.6 Hz,
H6′); 8.30 (d broad, 1H, JH−H = 7.6 Hz, H3); 8.02 (ddd with sat, 1H,
3JPt−H = 23.2 Hz, JP−H = 9.3 Hz, JH−H = 7.6 Hz, JH−H = 1.7 Hz, H4′);
1
yellow solid, which was filtered off and washed with diethyl ether. H
and 31P NMR spectra show a mixture of two species, 2c and 3c, in a
1
2:1 molar ratio. Selected NMR data for 2c are as follows. H NMR
7.91 (td, 1H, JH−H = 7.6 Hz, JH−H = 1.6 Hz, H4); 7.37−7.08 (m, 17H,
(CDCl3): 13.6 (br, 1H, N−H); 9.06−8.90 (m, 2H, H6 + H6′); 8.62
(m, 1H, H4′); 8.49 (d, JH−H = 8.1 Hz, H3); 8.25 (t br, JH−H = 7.14 Hz,
H4); 7.71 (ov, H5′ or H5); 7.57 (t br, JH−H = 6.22 Hz, H5 or H5′);
2
H5 + H5′ + Ho + Hm + Hp P(OPh)3); 0.83 (d with sat, 3H, JPt−H
=
83.6 Hz, 3JP−H = 7.1 Hz, Pt-CH3). 31P NMR (CDCl3): 117.96 (s with
1
2
sat, JPt−P = 3848 Hz, P(OPh)3).
2.45−1.20 (m, 33H, Cy); 1.09 (d with sat, 3H, JPt−H = 83.1 Hz,
3JP−H = 4.9 Hz, Pt-CH3). 31P NMR (CDCl3): 19.32 (s with sat, JPt−P
=
Synthesis of [Pt(bipy-H)(Me)(P(p-tolyl)3)] (1e). To a solution of
cis-[Pt(Me)2(DMSO)2] (66.2 mg, 0.174 mmol) in anhydrous toluene
was added an excess of 2,2′-bipyridine (57.3 mg, 0.367 mmol) under
a nitrogen atmosphere. The solution became suddenly red and was
2337 Hz, PCy3).
[Pt(bipy*)(P(OPh)3)(Me)][BF4] (2d-BF4) and [Pt(bipy)(P(OPh)3)-
(Me)][BF4] (3d-BF4). In an NMR tube 13.3 mg (0.020 mmol) of
H
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[Pt(bipy-H)(Me)(P(OPh)3)][BF4] was dissolved in ca. 1 mL of
CD2Cl2, giving a light yellow solution. A 7.4 mg portion (0.020 mmol)
of [18-crown-6-H3O][BF4] was added at room temperature, and the
solution became slightly darker. The reaction was followed by means
of NMR spectroscopy. After a few minutes complex 2d was formed in
an almost quantitative yield (1H and 31P NMR criteria). After 3
months complete conversion of 2d to 3d occurred. Selected NMR
3JP−H = 3.4 Hz, Pt-CH3). 31P NMR (CDCl3): 20.36 (s with sat, JPt−P
=
−
4351 Hz, PPh3). IR (Nujol, νmax/cm−1): 1058 s br, BF4 .
[Pt(bipy)(Me)(PCy3)][BF4] (3c-BF4). Under an N2 atmosphere
71.0 mg (0.11 mmol) of [Pt(bipy-H)(Me)(PCy3)] was dissolved in
10 mL of CH2Cl2, and 44.4 mg (0.12 mmol) of [18-crown-6-
H3O][BF4] was added. The solution was stirred under a nitrogen
atmosphere at 30 °C for 48 h. Afterward the solution was evaporated
to dryness and diethyl ether was added. The suspension was filtered off
and washed with diethyl ether to give the analytical sample. Yield:
76%. Mp: 254−256 °C. Anal. Calcd for C29H44N2PPtBF4·H2O: C,
1
data for 2d are as follows. H NMR (CD2Cl2): 9.58 (d with sat, 1H,
3
3JPt−H = ca. 15 Hz, JH−H = 5.4 Hz, H6); 8.73 (dd, 1H, JPt−H = 49 Hz,
JH−H = 7.8 Hz, H4′); 8.56 (d, 1H, JH−H = 5.0 Hz, H6′); 8.45 (d, 1H,
JH−H = 7.4 Hz, H3); 8.24 (t, 1H, JH−H = 7.4 Hz, H4); 7.75 (m, 1H, H5);
7.63 (m, 1H, H5′); 7.09−7.36 (m, 15H, Ho + Hm + Hp P(OPh)3); 0.90
1
46.35; H 6.17; N, 3.73. Found: C, 46.32; H 5.73; N, 3.85. H NMR
(CDCl3): 8.84 (m, 2H, H6 + H6′); 8.73 (d br, 2H, H3 + H3′); 8.38 (t
2
3
br, 2H, H4 + H4′); 8.74 (t br, 2H, H5 + H5′); 1.18−2.42 (m, 33H, Cy);
(d with sat, 3H, JPt−H = 82.0 Hz, JP−H = 6.3 Hz, Pt-CH3). 31P NMR
(CD2Cl2): 111.6 (s with sat, JPt−P = 4254.8 Hz, P(OPh)3). Selected
NMR data for 3d are as follows. 1H NMR (CDCl3): 9.47 (d with sat,
1H, H6 JH−H = 5.2 Hz, JPt−H not resolved); 8.68 (m with sat, 1H, JPt−H
not resolved); 8.52 (m, 2H); 8.35 (m, 2H); 7.74 (m, 2H); 7.10−7.41
(m, 14H); 6.83 (m, 1H); 0.84 (s with sat, 3H, JPt−H = 66.3 Hz, JP−H
not resolved). 31P NMR (CDCl3): 69.72 (s with sat, JPt−P = 7164.9 Hz,
P(OPh)3).
2
3
1.00 (d with sat, 3H, JPt−H = 71.6 Hz, JP−H = 1.5 Hz, Pt-CH3). 31P
NMR (CDCl3): 16.41 (s with sat, JPt−P = 4068 Hz, PCy3).
[Pt(bipy-H)(PPh3)2][BF4] (4-BF4). To a solution of [Pt(bipy-
H)(Me)(DMSO)][BF4] (36.2 mg; 0.0683 mmol) in CH2Cl2 was
added at room temperature 36.0 mg of PPh3 (0.137 mmol). The
mixture was stirred under an inert atmosphere for 4 h; then it was
concentrated and treated with diethyl ether. The precipitate that
formed was filtered off, washed with diethyl ether, and vacuum-
pumped to give the analytical sample as a yellow solid in almost
quantitative yield. Mp: 183 °C. Anal. Calcd for C46H37BF4N2P2Pt: C,
Synthesis of [Pt(bipy*)(Me)(P(p-tolyl)3)][BF4] (2e-BF4). At room
temperature, in an NMR tube, 6.7 mg (0.01 mmol) of 1e was
dissolved in 1.0 mL of CD2Cl2 and 5.3 mg of [18-crown-6-H3O][BF4]
(0.014 mmol) was added. The solution instantly changed color,
becoming bright yellow and remaining clear. The reaction was then
followed by NMR spectroscopy.
1
57.45; H, 3.88; N, 2.91. Found: C, 57.22; H, 3.96; N, 3.29. H NMR
(CDCl3): 6.67 (dd, 1H), 6.84 (t, 1H), 7.26−7.62 (m, 18H), 7.94 (m,
1H), 8.35 (m, 1H). 31P NMR (CDCl3) 17.68 (d, JPt−P = 2104.6 Hz,
2JP−P = 19 Hz, P trans C); 24.63 (d, JPt−P = 3937.6 Hz, 2JP−P = 19 Hz,
P trans N).
[Pt(bipy-H)(bipy)][BF4] (5-BF4). Method A. To a solution of
[Pt(bipy*)(Me)(DMSO)][BF4] (54.1 mg; 0.102 mmol) in CH2Cl2
was added 16.1 mg of 2,2′-bipyridine (0.103 mmol). The solution was
stirred for 3 h at 35 °C under a nitrogen atmosphere. The resulting
orange solution was concentrated to a small volume and treated with
pentane; the precipitate that formed was filtered off, washed with
pentane, and vacuum-pumped to give the analytical sample as an
orange solid. Yield: 65%.
Complex 2e-BF4 slowly converts to the adduct 3e-BF4 (ca. 90% of
conversion after 6 days, almost complete conversion after two weeks).
1
2e-BF4. H NMR (ppm, CD2Cl2, 400 MHz): 8.99 (ddd sat, 1H,
3JPt−H = 50 Hz, 4JP−H = 5.1 Hz, JH−H = 7.8,1.5 Hz, H4′); 8.66 (d br, 1H,
JH−H = 5.7 Hz, H6′); 8.41 (d, 1H, JH−H = 8.0 Hz, H3); 8.12 (td, 1H,
3
JH−H = 7.8,1.4 Hz, H4); 8.00 (d br sat, 1H, JPt−H = 14 Hz, H6); 7.81
5
(ddd, 1H, JP−H = 1.7 Hz, JH−H = 7.8,5.8 Hz, H5′); 7.60 (dd, 4H,
3JP−H = 10.6 Hz, JH−H = 8.1 Hz, Ho P(p-tolyl)3); 7.27 (dd, 4H, 4JP−H
1.5 Hz, JH−H = 7.9 Hz, Hm P(p-tolyl)3); 7.02 (ddd, 1H, JH−H
=
=
7.6,5.6,1.0 Hz, H5); 2.42 (s, 9H, CH3 P(p-tolyl)3); 0.82 (d sat, 3H,
Method B. To a solution of [Pt(bipy-H)(Me)(DMSO)][BF4] (81.3 mg;
0.183 mmol) in CH2Cl2 was added 44.4 mg of 2,2′-bipyridine
(0.284 mmol) and 68.3 mg of [18-crown-6-H3O][BF4] (0.184 mmol).
The solution was stirred for 2 h 30 min at room temperature; then it
was concentrated to a small volume and treated with pentane. The
precipitate that formed was filtered off, washed with pentane, and
vacuum-pumped to give the analytical sample as an orange solid. Yield:
55%. Anal. Calcd: C, 40.49; H, 2.55; N, 9.44. Found: C, 39.63; H,
3
2JPt−H= 83 Hz, JP−H = 7.0 Hz, Pt-CH3). 31P NMR (ppm, CD2Cl2,
161.9 MHz): 28.7 (s sat, 1P, JPt−P = 2502 Hz, P(p-tolyl)3).
3e-BF4. 1H NMR (CD2Cl2): 9.02 (dm with sat, 1H, 3JPt−H ≈ 34 Hz,
H6); 8.56 (d, 1H, JH−H = 8.0 Hz); 8.46 (d, 1H, JH−H = 8.0 Hz); 8.41 (t,
1H, JH−H = 7.6 Hz); 8.18 (t, 1H, JH−H = 7.6 Hz); 7.84 (t, 1H, JH−H
=
6.0 Hz); 7.64 (dd, Hortho, p-tolyl, 6H, JH−H = 7.8 Hz, JP−H = 11.2 Hz);
7.31 (d, Hmeta, p-tolyl, 6H, JH−H = 7.8 Hz); 7.26 (d, 1H, JH−H = 7.8
Hz); 7.08 (m, 1H, JH−H = 6.8 Hz); 2.44 (s, CH3 p-tolyl, 9H); 0.80 (d
1
2.39; N, 9.46. Mp: 232 °C. H NMR (CD2Cl2): 9.35 (d, 1H); 9.10−
2
3
with sat, 3H, JPt−H = 72 Hz, JP−H = 3.3 Hz, Pt-CH3). 31P NMR
9.20 (m, 1H); 8.96−9.08 (d, 1H); 8.71−8.77 (m, 2H); 8.39−8.51 (m,
3H); 8.26−8.38 (m, 1H); 8.17−8.25 (m, 1H); 7.95−8.17 (m, 1H);
7.86−7.88 (m, 1H); 7.78- 7.84 (d, 1H); 7.62−7.66 (m, 1H); 7.21−
7.25 (q, 1H).
1
(CDCl3): 16.76 (s with sat, JPt−P = 4343 Hz, P(p-tolyl)3).
[Pt(bipy)(Me)(PPh3)][BF4] (3b-BF4). Method A. To a solution of
[Pt(bipy-H)(Me)(PPh3)] (57.9 mg, 0.090 mmol) in 10 mL of CH2Cl2
was added, under a nitrogen atmosphere, 47.0 mg (0.127 mmol) of
[18-crown-6-H3O][BF4]. The reaction mixture was stirred under
nitrogen at 30 °C for 1 week. The resulting solution was concentrated
to a small volume and treated with diethyl ether. The precipitate that
formed was filtered off, washed with diethyl ether, and vacuum-
pumped to give the analytical sample as a pale yellow solid. Yield: 76%.
Method B. To a solution of [Pt(bipy*)(Me)(DMSO)][BF4] (48.0 mg,
0.0904 mmol) in CH2Cl2 was added with stirring 24.0 mg of PPh3
(0.0916 mmol) and 3 drops of DMSO. The solution was stirred under
a nitrogen atmosphere at room temperature for 5 h; then it was
concentrated to a small volume, and pentane was added. The
precipitate that formed was filtered off and washed with pentane to
give the analytical sample as a yellow solid. Yield: 70%. Mp: 187 °C.
Anal. Calcd: C, 48.77; H, 3.50; N, 3.92. Found: C, 49.05; H, 3.58; N,
[Pt(bipy-H)(dppe)][BF4] (6-BF4). To a solution of [Pt(bipy*)(Me)-
(DMSO)][BF4] (39.4 mg, 0.074 mmol) in CH2Cl2 was added 29.4 mg
of 1,2-bis(diphenylphosphino)ethane (dppe; 0.074 mmol). The
solution was stirred at room temperature under a nitrogen atmosphere
for 1 h and then concentrated to a small volume and treated with
diethyl ether. The precipitate that formed was filtered off, washed with
diethyl ether, and vacuum-pumped to give the analytical sample as a
whitish solid. Yield: 55%. Mp: 162 °C. Anal. Calcd for
C36H31BF4N2P2Pt: C, 51.75; H, 3.74; N, 3.35. Found: C, 51.45; H,
1
3.49; N, 3.07. H NMR (CD2Cl2): 8.39 (br, 1H); 8.34 (br, 1H); 8.17
(br, 1H); 7.98 (br, 1H); 7.82 (br, 6H); 7.58 (br, 10H); 7.30 (br, 1H);
7.08 (br, 1H); 6.73 (br, 1H); 2.57 (br, 2H). 31P NMR (CDCl3): 51.81
(s, P trans C, JPt−P = 1949 Hz); 42.64 (s, P trans N, JPt−P = 3691 Hz).
Reaction of [Pt(bipy-H)(Me)(L)] Complexes with HCl (L =
DMSO, PPh3, PCy3). L = DMSO. The reaction of [Pt(bipy-H)(Me)-
(DMSO)] (1a) with HCl has been reported in ref 18.
L = PPh3, PCy3. To a solution of [Pt(bipy-H)(Me)(L)] (0.10 mol)
in acetone (20 mL) was added an equimolar amount of aqueous 0.1 M
HCl. The mixture was stirred at room temperature for 24 h and then
evaporated to dryness, extracted with CH2Cl2, and analyzed by means
of NMR spectroscopy.
1
2
3.59. H NMR (CDCl3): 8.95 (m, 1H, JPt−H = ca. 32 Hz, H6′); 8.75
(d, 1H, JH−H = 8.0 Hz, H3 or H3′); 8.66 (d, 1H, JH−H = 8.0 Hz, H3′ or
H3); 8.42 (m, 1H, JH−H = 8.0 Hz, JH−H = 7.6 Hz, H4 or H4′); 8.20 (m,
1H, JH−H = 8.0 Hz, JH−H = 7.6 Hz, H4′ or H4); 7.83 (m, 1H, H5′);
7.71−7.89 (m, 6H, Ho (PPh3)); 7.58−7.40 (m, 10H, H6 + Hm + Hp
2
(PPh3)); 7.03 (m, 1H, H5); 0.78 (d with sat, 3H, JPt−H = 69.0 Hz,
I
dx.doi.org/10.1021/om300824d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
L = PPh3. 1H NMR (CDCl3): the spectrum shows a series of
overlapping signals attributable to two species in a 4:1 molar ratio,
identified as the cationic adduct [Pt(bipy)(Me)(PPh3)]Cl (main
species) and [Pt(bipy-H)(Cl)(PPh3)] (minor species) on the basis of
literature data (ref 38 for [Pt(bipy)(Me)(PPh3)]Cl and ref 18 for
[Pt(bipy-H)(Cl)(PPh3)]). 31P NMR (CDCl3): 23.43, JPt−P = 4285 Hz
([Pt(bipy-H)(Cl)(PPh3)]); 20.40, JPt−P = 4347 Hz ([Pt(bipy)(Me)-
(PPh3)]Cl).
(8) (a) Skapsky, A. C.; Sutcliffe, V. F.; Young, G. B. J. Chem. Soc.,
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L = PCy3. 1H NMR (CDCl3): the spectrum shows a series of
overlapping signals attributable to two species in a 3.5:1 molar ratio,
identified as the cationic adduct [Pt(bipy)(Me)(PCy3)]Cl (main
species) and a minor species that was not characterized; 9.60 (m, 1H,
minor species, H4′); 9.50 (d, 1H, minor species, JH−H = 7.8 Hz);
9.40 (d, 1H, minor species, JH−H = 7.9 Hz); 9.31 (d, 1H, main species,
JH−H = 8.0 Hz); 8.35−8.91 (m, overlapping signals); 7.69−7.95 (m,
overlapping signals); 7.30−7.55 (m, overlapping signals); 1.14−2.65
(m, overlapping signals); 0.99 (broad s with satellites, 3H, main
2
species, JPt−H = 67.0 Hz, CH3). 31P NMR (CDCl3): 16.30 (s with
satellites, main species, JPt−P = 4064 Hz); 14.40 (s with satellites,
minor species, JPt−P = 3431 Hz).
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ASSOCIATED CONTENT
* Supporting Information
Tables and figures giving details of the calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
(11) Crosby, S. H.; Clarkson, G. J.; Rourke, J. P. Organometallics
2011, 30, 3603−3609.
S
(12) Zhao, S.-B.; Wang, R.-Y.; Wang, S. J. Am. Chem. Soc. 2007, 129,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: zucca@uniss.it.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Universita
■
̀
di Sassari (FAR) and the
(15) Petretto, G. L.; Zucca, A.; Stoccoro, S.; Cinellu, M. A.;
Minghetti, G. J. Organomet. Chem. 2010, 695, 256−259.
Ministero dell’Universita
̀
e della Ricerca Scientifica (MIUR,
(16) (a) Butschke, B.; Schlangen, M.; Schroder, D.; Schwarz, H. Int. J.
̈
PRIN 2007) is gratefully acknowledged. We thank Johnson
Matthey for a generous loan of platinum salts. The research
institution INSTM is acknowledged by M.M., as well as the
resources given by the Cybersar Project managed by the
“Consorzio COSMOLAB”. L.M. gratefully acknowledges a
Ph.D. fund, financed on POR/FSE 2007-2013, from the
Regione Autonoma della Sardegna.
Mass Spectrom. 2009, 283, 3−8. (b) Butschke, B.; Schlangen, M.;
Schroder, D.; Schwarz, H. Chem. Eur. J. 2010, 16, 3962−3969.
̈
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dx.doi.org/10.1021/om300824d | Organometallics XXXX, XXX, XXX−XXX