Heterobimetallic rollover derivatives

Article abstract of DOI:10.1021/om200660a

Heterobimetallic complexes with metal centers connected by a small delocalized ligand constitute an interesting class of compounds. Here we report that starting from the mononuclear platinum(II) rollover complexes [Pt(bipy-H)(L)Cl] (bipy-H = 2,2′-bipyridine C(3′)-N cyclometalated, L= DMSO, PPh₃) a second rollover cyclometalation may produce a series of Pt(II)/Pd(II) heterobimetallic complexes where the two metals are linked by the planar, highly delocalized, doubly deprotonated 2,2′-bipyridine.

Full text of DOI:10.1021/om200660a

Article
pubs.acs.org/Organometallics
Heterobimetallic Rollover Derivatives
Giacomo L. Petretto,†,‡ Jonathan P. Rourke,‡ Luca Maidich,† Sergio Stoccoro,† Maria Agostina Cinellu,†
Giovanni Minghetti,† Guy J. Clarkson,‡ and Antonio Zucca*,†
†Dipartimento di Chimica e Farmacia, Universita
̀
di Sassari, via Vienna 2, I-07100 Sassari, Italy
‡Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K.
S
* Supporting Information
ABSTRACT: Heterobimetallic complexes with metal centers
connected by a small delocalized ligand constitute an interesting
class of compounds. Here we report that starting from the
mononuclear platinum(II) rollover complexes [Pt(bipy-H)(L)Cl]
(bipy-H = 2,2′-bipyridine C(3′)-N cyclometalated, L= DMSO, PPh3)
a second rollover cyclometalation may produce a series of Pt(II)/
Pd(II) heterobimetallic complexes where the two metals are linked
by the planar, highly delocalized, doubly deprotonated 2,2′-
bipyridine.
conditions this system catalyzes the selective dimerization of α-
INTRODUCTION
■
methylstyrene.5
The synthesis of bimetallic complexes has had a long and
venerable history. The interest in these systems arises from the
idea that two metal centers in close proximity may influence
each other and affect both the chemical and physical properties
of the molecule, often with surprising results.1 The mutual
influence may produce significant modifications of the
individual properties of the metal centers or even develop
novel characteristics which do not occur in the analogous
mononuclear compounds.
One of the most fascinating aspects of bimetallic systems is
the electronic contact created when a π-conjugated ligand is
used to connect the metal centers. Interesting developments
may arise when the system contains metal atoms with different
properties: examples are mixed-valence compounds, i.e.
compounds with two metals in different oxidation states,
which are particularly suitable for electron transfer processes,2
and heterobimetallic complexes which are expected to display
unique properties due to the cooperation of metal centers with
different physical and chemical characteristics.
We sought to investigate the properties of complexes where
two different metals are held in close proximity via an
unsaturated organic linker, building on our early work with
“rollover” Pt(II) complexes;6 the so-called “rollover” com-
pounds originate from a particular coordinating behavior of
2,2′-bipyridine, which, under well-defined conditions, is able to
activate the C(3′)−H bond and act as a cyclometalated ligand.7
Heterobimetallic complexes where the metal centers are
connected by a delocalized carbon chain are of great interest in
the fields of materials science, biology, and homogeneous
catalysis.3
This family of compounds includes polypyridyl complexes,
such as those derived from 2,2′-bipyrimidine, which have been
studied as models for energy and electron transfer processes.4
Applications in photocalysis are particularly promising: a recent
paper reports a dinuclear Ru/Pd complex with bridging
substituted bipyrimidines containing a photosensitizing Ru(II)
fragment that operates as an antenna device able to collect solar
light and transfer energy to a Pd(II) center. Under irradiating
Only a few metals, such as Pt(II),8 Pd(II),9 Au(III),10
Rh(III),11 Ir(III),12 and Cu(II)13 are, to the best of our
knowledge, able to display this behavior, which has interesting
potentialities due, inter alia, to the presence of an
Received: July 21, 2011
Published: March 22, 2012
© 2012 American Chemical Society
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uncoordinated nitrogen atom. Platinum(II) rollover com-
pounds have recently received attention due to their activity
in the gas phase, with promising applications in catalysis and
C−C and C−H bond formation.14
than the sum of van der Waals radii (1.09 + 1.55 Å). According
to these data the uncoordinated nitrogen may not be easily
available for coordination, at least on the basis of steric
considerations.
As a starting point we reacted [Pt(bipy-H)(DMSO)Cl] (2a)
with palladium acetate in refluxing benzene to obtain the
tetranuclear species [(DMSO)ClPt(μ-bipy-2H)Pd(μ-OAc)]2
(4) (reaction 2).
In the case of platinum(II), reaction of 1 equiv of the
electron-rich [Pt(Me)2(DMSO)2] with 2,2′-bipyridine (bipy)
results in the high yield of complex 1a (Scheme 1), which can
Scheme 1
1
The H NMR spectrum of the isolated product shows two
sets of resonances, indicating the presence of two species in a
4:1 molar ratio. The subspectra of these species are very similar:
in both cases the aromatic region contains six protons,
suggesting a second C−H bond activation. In the aliphatic
region the main species has two signals ascribable to
coordinated DMSO ligands (δ 3.83 ppm, 6H, JPt−H = 17 Hz;
δ 3.49 ppm, 6H, JPt−H = 19 Hz) and only one for the acetato
groups (δ 2.22 ppm, 6H). In contrast, the minor species
possesses two signals for the DMSO methyls (3.79 and 3.49
ppm, 6H + 6H partially overlapping) and two singlets for the
acetato ligands (2.64 and 2.17 ppm, 3H + 3H). This is in line
with tetranuclear species of general formula [(DMSO)(Cl)Pt-
(μ-bipy-2H)Pd(μ-OAc)]2, present as the trans and cis isomers
4trans and 4cis. The main species, 4trans, should have two
equivalent acetato bridging ligands; the DMSO ligands should
be chemically equivalent as well.
be converted to the corresponding chloride [Pt(bipy-H)-
(DMSO)Cl] (2a) by reaction with HCl. The coordinated
DMSO in 1a and 2a can be easily displaced by neutral ligands
to give complexes with an array of electronic and steric
properties: e.g., [Pt(bipy-H)(L)Me] (L = PPh3, 1b; L = CO,
1c) and [Pt(bipy-H)(L)Cl] (L = PPh3, 2b; L = CO, 2c).
The “rollover” geometry allows for further reaction with
either the same or a different metal source. Thus,
biorganometallic complexes can potentially be prepared with
the two metals in very close proximity. Such complexes might
potentially have many uses, with the adjacent metals being in
effective electronic communication with each other.
RESULTS AND DISCUSSION
■
The rollover cyclometalated compounds closely resemble the
analogous derivatives of 2-phenylpyridine.15 A striking differ-
ence between the two families of complexes is the presence (or
absence) of the uncoordinated nitrogen atom. For this reason
compounds 1 and 2 are able to further react to give dinuclear
species 3 of general formula [L(X)Pt(μ-bipy-2H)Pt(X′)L′] (X,
X′ = Me, Cl; L, L′ = PPh3, DMSO, CO, etc.), where the doubly
deprotonated 2,2′-bipyridine acts as a planar, delocalized
dianionic bridging ligand (reaction 1).
Acetato dimeric species usually have a folded structure,16
which in our case should render diastereotopic the methyls of
the same DMSO (planar chirality). The minor species 4cis
possesses chemically inequivalent acetato ligands but equivalent
coordinated DMSO. Due to the folded conformation of the
complex, in this case, again the methyl groups in the same
DMSO should be diastereotopic.
This synthetic approach has made possible the synthesis of
both symmetric (L = L′, X = X′; e.g. [(PPh3)(Me)Pt(μ-bipy-
2H)Pt(Me)(PPh3)] (3a)) and unsymmetric (L ≠ L′, X ≠ X′;
e.g. [(PPh3)(Me)Pt(μ-bipy-2H)Pt(Cl)(CO)] (3b)) species.6 A
logical consequence of this approach is that the second
cyclometalation may involve a different metal from the first one
and hence may produce heterodinuclear species where the
metal centers are strictly connected by the delocalized bridging
ligand.
The synthesis of these dinuclear complexes, however, is not
trivial. As shown by the X-ray crystal structure of [Pt(bipy-
H)(PPh3)Cl] (1b),6 the distance between the hydrogen atom
bonded to C(3′) and the uncoordinated nitrogen is 2.579 Å
(average distance in two independent molecules), slightly less
In the absence of a structural characterization we investigated
the nature of complexes 4 through mono- and multidimen-
17
1
sional H NMR spectroscopy (COSY and GOESY experi-
ments). For complex 4trans the 1H GOESY experiments show,
inter alia, enhancement of the signals at 7.04 (H4) and 7.55
ppm (H6′) after irradiation of the acetato signal, confirming that
they are adjacent to the Pd atom. Irradiation of aromatic signals
confirms that peaks that in a planar conformation would not be
expected to be in close proximity to each other actually are: e.g.,
irradiation at 7.04 ppm (H4) enhances the signal at 7.55 ppm
(H6′) and vice versa. In addition an extremely small, but
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significant, enhancement is present between the H5 and H5′
protons.
heterodinuclear “symmetric” species 6a (L = L′ = PPh3) and 6b
(L = L′ = lutidine) (reaction 4).
The DMSO signal at 3.85 ppm enhances the peaks at 8.26
(H4′) and 8.55 (H6), whereas the other signal, at 3.49 ppm,
enhances only the H4′ peak. All these data fit with a bent
molecule having, on the NMR time scale, a nonrotating
DMSO, with the SO bond in the plane, one methyl (at 3.85
ppm) inside the folded molecule, and the other one (at 3.49
ppm) pointing out.
In addition, the spectra seem to indicate that there may be
some exchange between the two Me groups on the same
DMSO, as both are diminished in intensity upon irradiation of
the other one, rather than each enhancing the other. This is
consistent with a dynamic behavior of the molecule which
inverts the structure, so that the methyl pointing out becomes
in and vice versa.
The 1H and 31P NMR spectra of complex 6a clearly show the
different influences of palladium and platinum on the chemical
shifts. In the 31P NMR spectrum the phosphorus coordinated
to platinum is unambiguously assigned by the presence of
satellites. The coupling constant value (1JPt−P = 4155 Hz) is in
agreement with a trans P−Pt−N arrangement. The chemical
shifts of the two phosphorus atoms are very different (41.89
ppm, P−Pd; 19.80 ppm, P−Pt), as previously observed in
corresponding cyclometalated palladium and platinum com-
plexes.20
The protons adjacent to the nitrogen, H6 and H6′, are slightly
deshielded by coordination; as is often observed, the
deshielding effect of the platinum is greater than that of
palladium (δPt 9.05 ppm, δPd 8.85 ppm). A comparison between
analogous mono- and dinuclear rollover species (Scheme 2,
Furthermore, irradiation of some of the peaks of the major
compound causes a small amount of negative enhancement of
the peaks of the minor component. This would imply that they
are in slow chemical exchange.
In agreement with the proposed formulation the IR spectrum
shows bands in the region 1390−1570 cm−1 (see the
Experimental Section) consistent with bridging acetato
ligands.18
Scheme 2
The reaction of 2a with palladium acetate may be followed
by treatment with LiCl, to give the corresponding chloride
[(DMSO)ClPt(μ-bipy-2H)Pd(μ-Cl)]2 (5a). Analogously, start-
ing from 2b, [(PPh3)ClPt(μ-bipy-2H)Pd(μ-Cl)]2 (5b) may be
isolated in the solid state in good yields and characterized. In
contrast to the acetato species 4, the chloride-bridged
1
complexes 5a,b are flat, with evidence coming from the H
NMR spectra which show only one resonance for the DMSO
protons (reaction 3).
compounds 6a, 3c, and 2b) evidences the shielding of the H6′
proton upon the second metalation (9.85, 9.06, and 9.05 ppm
in 2b, 3c, and 6a, respectively), apparently independent from
the nature of the second metal. The second metalation has
consequences also in the 31P NMR spectra, in both chemical
shift and coupling constant values.
Operating under mild conditions, starting from 5b (L =
PPh3), the “unsymmetric” complex 6c (L = PPh3, L′ = 3,5-
lutidine) can be isolated in the solid state by reaction with 3,5-
lutidine. As expected, 3,5-lutidine opens the chloride bridge but
does not displace PPh3.
The NMR spectra of 6c confirm in solution the coordination
of the neutral ligands trans to the bipy nitrogen, as indicated by
1J(Pt−P) and 3J(Pt−H) (ortho lutidine protons) coupling
constants (4155 and 55.8 Hz, respectively). Furthermore, the
position of H4 and H4′ in the spectrum (multiplet at ca. 6.6−6.8
ppm) is due to the shielding effect of the adjacent PPh3 and
lutidine, respectively. The structure of complex 6c was
definitely confirmed by the resolution of the crystal structure
in the solid state by X-ray diffraction (Figure 1).
1
The H NMR spectra of 5a,b show only one set of signals
and support the given formulation. In particular, the coupling
1
constants of the DMSO signal in the H spectrum of 5a
(3J(Pt−H) = 25.2 Hz) and of the coordinated phosphorus in
5b (1J(Pt−P) = 4154 Hz) agree with neutral ligands
coordinated trans to a nitrogen atom.19
Starting from the tetranuclear species 5a,b subsequent
reactions lead to cleavage of the anionic bridge between the
two palladium centers and the generation of relatively simple
bimetallic species. Starting from 5a, apart from opening the
chloride bridge, the neutral ligand may also displace the
coordinated DMSO: the reactions with PPh3 or 3,5-lutidine,
driven with at least a 4:1 ligand:complex molar ratio, gave the
Distances and angles are in line with those previously
reported for the analogous homodinuclear rollover complex
[Pt2(bipy-2H)(PPh3)2Cl2] (3c; e.g., Pt−Cl = 2.352 Å, Pt−C =
2.113 Å, Pd−C = 2.004 Å, Pd−Cl = 2.394 Å).6 The bipy-2H
bridging ligand is almost completely planar.
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The characterization of 7 mainly rests on NMR spectrosco-
py. In particular, in the 31P NMR spectrum a singlet at 17.5
ppm flanked by satellites (1JPt−P = 4136 Hz) is in line with a P
trans to N.
An HMQC 195Pt−1H spectrum shows a 195Pt signal at
−4029 ppm, coupled to 31P. The marked difference of 195Pt
chemical shifts between 7 and 4 (δ −3562 ppm) is an
indication of the significant electronic effect of a different
palladium center to the platinum. A mass spectrum confirms
the proposed formulation, showing a peak at m/z 817,
corresponding to protonation and loss of Cl.
Figure 1. Solid-state structure of one of the crystallographically
independent but chemically identical molecules in the asymmetric unit
of 6c. Hydrogens and disordered solvent have been removed for
clarity. Thermal parameters are drawn at the 50% probability level.
As mentioned above, the second nitrogen atom is not
completely available for coordination, and up to now only Pt−
Pt double-rollover cyclometalated species have been reported
for 2,2′-bipyridine. Furthermore, the second cyclometalation
reaction requires the use of a mononuclear rollover species as a
ligand, with the drawback that reaction conditions often are not
compatible with a reactive metal−carbon bond or a metal
center. With the intention to extend the study to other metals,
the reactions of the mononuclear derivative [Pt(bipy-H)-
(PPh3)Cl] (2b) with a series of Au(III), Hg(II), and Rh(I)
metal salts were tested as a part of a preliminary study.
In no cases were heterodinuclear doubly cyclometalated
species formed: the reaction of [Pt(bipy-H)(DMSO)Cl] (2a)
with NaAuCl4 was previously reported6 and resulted in the
oxidation of the Pt(II) center to give a Pt(IV) complex,
[Pt(bipy-H)(DMSO)Cl3].24
Electronic delocalization through the two planar deproto-
nated bipyridine rings and the cyclometalated rings (metal-
loaromaticity) may be deduced by the short C2−C2′ bond,
whose length decreases from 1.490 Å in free 2,2′-bipyridine21 to
1.463/1.468 Å in 2b, 1.438 Å in 3c, and 1.445 Å in 6c (see
Scheme 3). Similar trends were used to identify metal-
Scheme 3. Variation of Selected C−C and C−N Bond
Lengths (Å) in the Free bipy and in Mono- and Dinuclear
a
Rollover Complexes
The reaction of 2b with mercury(II) acetate was performed
in order to obtain an organomercurial, useful for trans-
metalation reactions. Under standard experimental conditions
no reaction was observed and only the starting complex 2b was
recovered from the reaction mixture.
NMR and MS Spectra. The 195Pt NMR spectra of 2b, 4,
1
6a−c, and 7 were obtained by means of H−195Pt correlation
spectra using a variant of the HMBC pulse sequence. A first
comparison of the spectra shows, as expected, that the major
influence on the chemical shift is provided by the nature of the
ligands on the platinum center, so that the species having the
same Pt(N∧C)(PPh3)Cl environment (2b, 6a,c, and 7) have
similar chemical shifts (see Table 1), whereas complex 6b,
having a 3,5-lutidine instead of a PPh3, shows a deshielding of
more that 1000 ppm. This is in line with literature data, which
indicate the same trend when a nitrogen donor is replaced by a
a
For complex 3c, M = Pt(PPh3)Cl; for complex 6c, M = Pd(3,5-Me2-
py)Cl.
loaromaticity in palladium(II) five-membered cyclometalated
species.22 It should be noted, however, that in species 2b, 3c,
and 6c no clear corresponding elongation is observed in C2−N
and C2′−C3 bond lengths and the shortening of the C2−C2′
bond may be due to double chelation effects of the bipy-2H
ligand (see Scheme 2).
The crystal structure of 6a was partially solved by X-ray
diffraction,23 but extensive disorder, in particular that arising
from the interchange of Pd and Pt, meant that a satisfactory
solution could not be found.
The acetato species [{(PPh3)(Cl)Pt(μ-bipy-2H)Pd}μ-OAc]2
was not isolated in the solid state but was directly treated with
LiCl to give the corresponding chloride [{(PPh3)(Cl)Pt(μ-
bipy-2H)Pd}μ-Cl]2 (5b). If Na(acac) is added instead of LiCl,
the dinuclear species [{(PPh3)(Cl)Pt(μ-bipy-2H)Pd(acac)] (7)
may be isolated in good yields and characterized (reaction 5).
Table 1. Selected NMR Data
a
δ(195Pt), ppm
δ(31P), ppm
JPt−P, Hz
δ(H6′), ppm
2b
−4195
−3562
−3568
−4050
−2953
−4034
−4136
23.60
4285
9.85 (31)
4trans
4cis
6a
19.80
4155
9.05 (30)
8.81 (29)
9.07 (30)
8.95 (30)
6b
6c
18.87
17.51
4155
4136
7
a
3JPt−H (in Hz) in parentheses.
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phosphine.25 The same behavior is observed for complexes 4
(trans and cis), which have a Pt−S instead of a Pt−P bond.
The differences in chemical shift values between the
dinuclear complexes 6a,c and 7 is only attributable to the
different environments around the palladium (ca. 100 ppm
between 6a and 7), reflecting the influence of the palladium
center on the platinum atom, connected via the delocalized
planar doubly cyclometalated 2,2′-bipyridine. The substitution
of a PPh3 on the palladium with a 3,5-lutidine results in a 16
ppm shift to lower field of the 195Pt, following the trend
expected for a P/N substitution, but with a smaller effect due to
the Pt−Pd distance.
EXPERIMENTAL SECTION
■
[Pt(Me)2(DMSO)2],26 [Pt(bipy-H)(DMSO)Cl] (2a),6 and [Pt(bipy-
H)(PPh3)Cl] (2b)6 were synthesized according to the literature. All
reactions were carried out under an inert atmosphere. The solvents
were purified and dried before use according to standard procedures.27
Elemental analyses were performed with a Perkin-Elmer 240B
elemental analyzer by Mr. Antonello Canu (Dipartimento di Chimica,
̀
Universita degli studi di Sassari, Sassari, Italy) or by the Warwick
Analytical Service (University of Warwick, Coventry, U.K.). Infrared
spectra were recorded with a FT-IR Jasco 480P using Nujol mulls. 1H,
13C{1H}, and 31P{1H} NMR spectra were recorded with Bruker AV
400, 500, or 600 MHz and Varian VXR 300 MHz spectrometers.
1
Chemical shifts are given in ppm relative to internal TMS for H and
1
A comparison between the H and 31P spectra of 2b, 6a,c,
13C{1H} and external 85% H3PO4 for 31P{1H}. Coupling constant
values are given in Hz. NOE difference (GOESY) and 2D-COSY
experiments were performed by means of standard pulse sequences. Pt
and 7, all having a Pt(N∧C)(PPh3)Cl coordination, shows a
clear difference between the H6 chemical shifts in mono- and
dinuclear species (9.85, 9.05, 9.07, and 8.95 ppm in 2b, 6a,c,
and 7, respectively), a sharp effect due to the second
cyclometalation. Conversely, the second metalation has minor
consequences on the chemical shift of the H6′ proton but
greater effects on its 195Pt−H coupling constants (see Scheme
1), showing also a clear dependence on the nature of the
palladium center. Furthermore, the 31P NMR spectra also show
consequences, both in chemical shift and coupling constant
values (Scheme 4), due to the second cyclometalation.
NMR spectra were recorded on a Bruker Avance 500 using H−195Pt
1
correlation spectra; these were recorded using a variant of the HMBC
pulse sequence with the 195Pt chemical shifts quoted taken from the
2D HETCOR spectra and referenced to external Na2PtCl6. All
accurate mass spectra were run on a Bruker MaXis mass spectrometer.
Scheme 4
Complex 2b. 195Pt NMR (CDCl3): −4195 ppm. Accurate mass:
m/z 612.1173, calculated for C28H22N2P195Pt (M − Cl)+ 612.1165.
Synthesis of [(DMSO)(Cl)Pt(μ-bipy-2H)Pd(μ-OAc)]2 (4). To a
solution of [Pt(bipy-H)(DMSO)(Cl)] (2a; 131 mg, 0.39 mmol) in 30
mL of benzene was added 91.8 mg of [Pd(OAc)2] (0.41 mmol). The
solution was heated to 80 °C for 12 h and then filtered and
concentrated to dryness under reduced pressure. The solid residue was
dissolved in CH2Cl2, filtered over Celite, concentrated to a small
volume, and treated with diethyl ether. The precipitate that formed
was filtered, washed with diethyl ether, and vacuum-pumped to give
the analytical sample as an orange-yellow solid. Yield: 60%. Anal. Calcd
for C28H30Cl2N4O6Pd2Pt2S2: C, 26.76; H, 2.41; N, 4.46. Found: C,
26.48; H, 2.45; N, 4.49.
Further evidence for the characterization of the complexes
were provided by high-resolution mass spectra of species 2b,
6a−c, and 7, showing in all cases the [M − Cl]+ peak, with an
excellent match between calculated and experimental values
(e.g., 6b: m/z found 704.0373, calculated for
C24H24ClN4106Pd195Pt, (M − Cl)+, 704.0373).
The 1H NMR spectrum shows the presence of two species in a 4:1
1
molar ratio. H NMR (CDCl3; main species, 4trans): 8.53 (dd, 2H,
JH−H = 5.6, 1.2 Hz, JPt−H = 30 Hz, H6), 8.26 (dd, 2H, JH−H = 7.9, 1.2
Hz, JPt−H = ca. 26 Hz, H4′), 7.55 (dd, 2H, JH−H = 5.5, 1.1 Hz, H6′), 7.04
(dd, 2H, JH−H = 7.6, 1.2 Hz, H4), 6.67 (dd, 2H, JH−H = 7.9, 5.5 Hz,
H5′), 6.38 (dd, 2H, JH−H = 7.6, 5.6 Hz, H5), 3.83 (s, 6H, JPt−H = ca. 17
Hz, CH3 DMSO), 3.49 (s, 6H, JPt−H = ca. 19 Hz, CH3 DMSO), 2.22
(s, 6H, CH3 OAc). 195Pt NMR (CDCl3): −3562.
CONCLUSIONS
■
Rollover compounds constitute a new emerging class of
cyclometalated complexes having the additional key feature of
an uncoordinated donor atom, which can be exploited in a way
not feasible for classic cyclometalated species. One of the
possibilities is the double-cyclometalation reaction, which may
be used to obtain heterobimetallic species where the metals are
linked by the highly delocalized, doubly deprotonated
bipyridine. The coordination of the second metal center is
not a trivial task: the uncoordinated nitrogen doublet in
mononuclear rollover species is engaged in an N- - -H
interaction, and hence it is not easily feasible for coordination.
Furthermore, the cyclometalation reaction conditions are often
not compatible with the presence of a metal−carbon bond. The
new heterobimetallic Pt−Pd species reported here have been
thoroughly characterized, revealing electronic influence be-
tween the metal centers, mediated by the bipyridine spacer, as
indicated by NMR and IR data.
1H NMR (CDCl3; minor species, 4cis): 8.63 (dd, 2H, JH−H = 5.6,
1.2 Hz, JPt−H n.r., H6), 8.35 (dd, 2H, JH−H = 7.8, 1.2 Hz, JPt−H n.r., H4′),
7.47 (dd, 2H, JH−H = 5.5, 1.1 Hz, H6′), 6.94 (dd, 2H, JH−H = 7.6, 1.2
Hz, H4), 6.57 (dd, 2H, JH−H = 7.8, 5.5 Hz, H5 or H5′), 6.51 (dd, 2H,
JH−H = 7.6, 5.6 Hz, H5 or H5′), 3.79 (s, 6H, JPt−H = n.r., CH3 DMSO),
3.49 (s, overlapping, 6H, JPt−H n.r., CH3 DMSO), 2.64 (s, 3H, CH3
OAc), 2.17 (s, 3H, CH3 OAc). 195Pt NMR (CDCl3): −3568.
Synthesis of [(DMSO)(Cl)Pt(bipy-2H)Pd(μ-Cl)] (5a). To a
solution of [Pt(bipy-H)(DMSO)(Cl)] (2a; 131 mg, 0.39 mmol) in
30 mL of benzene was added 91.8 mg of [Pd(OAc)2] (0.41 mmol).
The solution was stirred for 8 h at 80 °C and then filtered through
Celite and evaporated to dryness under reduced pressure. The solid
residue was treated with a 3/1 water/acetone mixture and an excess of
LiCl. The mixture was stirred for 12 h: the precipitate that formed was
filtered, washed with diethyl ether, and vacuum-pumped to give the
analytical sample as a yellow solid. Yield: 70%. Anal. Calcd for
C12H12Cl2N2OPdPtS: C, 23.83; H, 2.00; N, 4.63. Found: C, 23.87; H,
1.65; N, 4.68. 1H NMR (acetone-d6): 8.83 (d, 1H, JH−H = 5.4 Hz, H6′),
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Organometallics
Article
8.75 (d, 1H, JH−H = 5.4 Hz, JPt−H = 36 Hz, H6), 8.51 (dd, 1H, JH−H
=
occupied by either the chloroform or the two waters. Additionally,
both of the partially occupied chloroforms were modeled as disordered
over two positions related by rotation about the C−H bond. The two
positions were fixed at 50:50 for C1−Cl3 but tied to a free variable for
chlorofom C2−Cl6 and refined to an occupancy of 60:40. No
hydrogens were found for the two partially occupied water molecules
O100 and O200, but these were included in the formula so as to
calculate the correct density. DFIX and SIMU restraints were used to
give both disordered chloroform molecules chemically reasonable
bond lengths and thermal parameters. SIMU restraints were used to
give both 3,5-dimethylpyridine molecules and one of the phenyls
(C119−C124) of the PPh3 reasonable thermal parameters.
7.6 Hz, JPt−H = 42 Hz, H4′), 8.24 (dd, 1H, JH−H = 7.6 Hz, H4), 7.04
(dd, 1H, JH−H = 5.5, 7.6 Hz, H5 or H5′), 7.00 (dd, 1H, JH−H = 5.5, 7.6
Hz, H5 or H5′) 3.61 (s, 6H, JPt−H = 25.2 Hz, DMSO).
Synthesis of [(PPh3)(Cl)Pt(bipy-2H)Pd(μ-Cl)] (5b). Complex 5b
was synthesized as for complex 5a, starting from [Pt(bipy-H)(PPh3)-
(Cl)] (2b) instead of [Pt(bipy-H)(DMSO)(Cl)] (2a). Yield: 85%
(yellow solid), Anal. Calcd for C28H21Cl2N2PPdPt·0.5H2O: C, 42.15;
H, 2.78; N, 3.51. Found: C, 42.04; H, 2.83; N, 3.44. 1H NMR
(CDCl3): 9.03 (m, 1H, H6), 7.99 (d, 1H, JH−H = 5.3 Hz, H6′), 7.78−
7.72 (m, 6H, HoPPh ) 7.47−7.37 (m, 10H, Hm+pPPh , H4′), 6.97 (m, 1H,
3
3
broad, H5 or H5′), 6.69 (d, 1H, JH−H = 7.5 Hz, JPt−H n.r. H4′), 6.33 (m,
1H, broad, H5′ or H5′). 31P NMR (CDCl3): 18.15 (s, JPt−P = 4154 Hz).
Synthesis of [PtPd(bipy-2H)(PPh3)2(Cl)2] (6a). To a solution of
5a (50 mg, 0.083 mmol) in 20 mL of CH2Cl2 was added 43.3 mg of
PPh3 (0.165 mmol). The solution was stirred for 8 h and then filtered,
concentrated to a small volume, and treated with diethyl ether. The
precipitate that formed was filtered, washed with diethyl ether, and
vacuum-pumped to give the analytical sample as a yellow solid. Yield:
85%. Mp: >260 °C. Anal. Calcd for C46H36Cl2N2P2PdPt·0.5H2O: C,
Crystal data: C36H32.25Cl3.50N3.25O0.50PPdPt, Mr = 974.94, triclinic,
space group P1, a = 9.8002(2) Å, b = 19.5809(5) Å, c = 20.0382(5) Å,
̅
α = 94.331(2)°, β = 94.5328(19)°, γ = 93.6782(19)°; V = 3812.77(16)
Å3 (by least-squares refinement on 19 833 reflection positions); T
=150(2) K; λ = 0.710 73 Å; Z = 4, D(calcd) = 1.698 Mg m−3, F(000)
= 1894; μ(Mo Kα) = 4.454 mm−1; yellow block, dimensions 0.16 ×
0.14 × 0.12 mm, no decay; θmax = 27.50°; hkl ranges −12 to +12, −25
to +25, −25 to +26; 64 011 reflections measured, 17 410 unique
(R(int) = 0.0602); goodness of fit on F2 1.166; R1 (for 14 509
reflections with I > 2σ(I)) = 0.0895, wR2 = 0.1980; 17 410/153/856
data/restraints/parameters; largest difference Fourier peak and hole
5.595 and −2.262 e Å−3.
1
52.11; H, 3.52; N, 2.64. Found: C, 51.85; H, 3.67; N, 2.82. H NMR
(CDCl3): 9.05 (m, 1H, JPt−H = ca. 30 Hz, H6), 8.85 (dd, 1H, JH−H
=
5.7 Hz, H6′), 7.81−7.75 (m, 12H, HoPPh ), 7.47−7.38 (m, 18H,
3
The structure was solved by direct methods using SHELXS,28 with
additional light atoms being found by Fourier methods. Hydrogen
atoms were added at calculated positions and refined using a riding
model with freely rotating methyl groups. Anisotropic displacement
parameters were used for all non-H atoms; H atoms were given
isotropic displacement parameters equal to 1.2 (or 1.5 for methyl
hydrogen atoms) times the equivalent isotropic displacement
parameter of the atom to which the H atom is attached.
H
m+pPPh ), 6.77 (dd, 1H, JPt−H = 42, JH−H = 7.5 Hz, H4′), 6.72 (dd, 1H,
3
J
H−H = 1.2, 7.5 Hz, H4), 6.50 (dd, 1H, JH−H = 5.7, 7.8 Hz, H5 or H5′),
6.43 (dd, 1H, JH−H = 5.7, 7.8 Hz, H5 or H5′). 31P NMR CDCl3: 41.89
(s, 1P, P−Pd), 19.80 (s, 1P, JPt−P= 4155 Hz, P−Pt). 195Pt NMR
(CDCl3): −4050 ppm (doublet). Accurate mass: m/z 1014.0761, calcd
for C46H36ClN2P2106Pd195Pt (M − Cl)+ 1014.0732.
Synthesis of [PtPd(bipy-2H)(3,5-Me2py)2(Cl)2] (6b). To a
solution of 5a (37.3 mg, 0.062 mmol) in acetone (20 mL) was
added 0.62 mmol of 3,5-dimethylpyridine. The solution was heated to
reflux for 8 h, filtered, concentrated to a small volume, and treated with
diethyl ether. The precipitate that formed was filtered, washed with
diethyl ether, and vacuum-pumped to give the analytical sample as a
yellow solid. Yield: 80% Mp: >260 °C Anal. Calcd for
C24H24Cl2N4PdPt·H2O: C, 37.98; H, 3.45; N, 7.38. Found: C,
37.81; H, 3.08; N, 7.18. 1H NMR (CD2Cl2): 8.81 (dd, 1H, JPt−H = 29
Hz, JH−H = 1.1, 5.6 Hz, H6), 8.64 (dd, 1H, JH−H = 1.4, 5.4 Hz, H6′),
8.56 (s, 2H, HopyN−Pt), 8.52 (s, 2H, HopyN−Pd), 7.53 (s, 2H, Hppy),
6.85−6.72 (m, 3H, H5, H5′, H4′, or H4), 6.65 (dd, 1H, JH−H = 6.6, 7.6
Hz, H4′ or H4), 2.37 (s, 12H, CH3 py). 195Pt NMR (CDCl3): −2953
ppm. Accurate mass: m/z 704.0373, calcd for C24H24ClN4106Pd195Pt
(M − Cl)+ 704.0373.
Synthesis of [(PPh3)(Cl)Pt(bipy-2H)Pd(acac)] (7). To a solution
of [Pt(bipy-H)(PPh3)(Cl)] (2b; 112 mg, 0.24 mmol) in benzene (15
mL) was added 65.1 mg of [Pd(OAc)2] (0.28 mmol). The solution
was heated to reflux under an argon atmosphere and then filtered and
evaporated to dryness. The resulting solid was dissolved in acetone
and treated with an excess of sodium acetylacetonate. The mixture was
stirred for 24 h, and the precipitate that formed was filtered and
washed with acetone and diethyl ether to give the analytical sample.
Yield: 86%. Mp: >260 °C. Anal. Calcd for C33H28ClN2O2PPdPt: C,
1
46.49; H, 3.31; N, 3.29. Found: C, 46.39; H, 3.34; N, 3.32. H NMR
(CDCl3, 400 MHz): 8.95 (m, 1H, JPt−H = 30 Hz, H6), 8.02 (dd, 1H,
JH−H = 5.4 Hz, H6′), 7.82 (d, 1H, JH−H = 7.7 Hz, H4), 7.72−7.68 (m,
6H, HoPPh ), 7.41−7.30 (m, 9H, Hm+pPPh ), 7.00 (m, 1H, JH−H = 1.5,
3
3
Synthesis of [(PPh3)(Cl)Pt(μ-bipy-2H)Pd(3,5-Me2py)(Cl)] (6c).
To a solution of 5b (51.7 mg, 0.066 mmol) in CHCl3 (30 mL) was
added 0.66 mmol of 3,5-dimethylpyridine. The solution was stirred at
room temperature for 8 h, concentrated to a small volume, filtered,
and treated with diethyl ether. The precipitate that formed was filtered,
washed with diethyl ether, and vacuum-pumped to give the analytical
sample as a yellow solid. Yield: 75%. Mp: >260 °C. Anal. Calcd for
C35H30Cl2N3PPdPt2·H2O: C, 45.10; H, 3.68; N, 4.51. Found: C,
5.4, 7.7 Hz, H5), 6.61 (dd, 1H, JPt−H = 36.5 Hz, JH−H = 7.7 Hz, H4′),
6.27 (ddd, 1H, JH−H = 6.2, 7.7 Hz, H5), 5.33 (s, 1H, Hacac), 2.01 (s, 3H,
CH3), 1.95 (s, 3H, CH3). 31P NMR (CDCl3): 17.51 (s, PPh3, JPt−P
=
4136 Hz). 195Pt NMR (CDCl3): −4029 ppm, ESI-MS (CH3CN/H2O
80/20): m/z 817 [MH − Cl].
ASSOCIATED CONTENT
* Supporting Information
■
1
S
44.82; H, 3.62; N, 4.42. H NMR (CDCl3): 9.07 (m, 1H, JPt−H = 30
Hz, H6), 8.66 (dd, 1H, JH−H = 5.4 Hz, H6′), 8.54 (s, 2H, Hopy), 7.79−
CIF files and tables giving crystal data for 6c and a figure giving
the structure of 6a. This material is available free of charge via
the Internet at http://pubs.acs.org.
7.73 (m, 6H, HoPPh ), 7.50−7.38 (m, 10H, Hm+pPPh + Hppy), 6.89 (dd,
3
3
1H, JH−H = 7.5 Hz, H5 or H5′), 6.68 (m, 2H, JPt−H = ca. 45 Hz, H4 +
H4′ partially overlapping), 6.34 (dd, 1H, JH−H = 5.4, 7.5 Hz, H5 or H5′)
2.38 (s, 6H, CH3 py). 31P NMR (CDCl3): 18.87 (s, JPt−P= 4155 Hz).
195Pt NMR (CDCl3): −4034 ppm (doublet). Accurate mass: m/z
859.0557, calcd for C35H30ClN3P106Pd195Pt (M − Cl)+ 859.0553.
Single crystals of 6c suitable for X-ray crystallography were grown
from CHCl3, diisopropyl ether, and (wet) acetonitrile.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zucca@uniss.it.
■
Notes
The asymmetric unit contained two mixed metal complexes which
were crystallographically independent but chemically identical (4
complexes in the unit cell). Other electron density in the cell modeled
as two chloroforms at 50% occupancy, two waters at 50% occupancy,
and a molecule of acetonitrile at 50% occupancy. The two waters were
modeled as disordered over the same position as chloroform C1−Cl3,
with the occupancies fixed at 50% so that the region of the crystal is
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Universita
■
̀
di Sassari (FAR) and the
Ministero dell’Universita
PRIN 2007) is gratefully acknowledged. We thank Johnson
̀
e della Ricerca Scientifica (MIUR,
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Organometallics
Article
Organometallics 2010, 29, 1966−1976. (e) Crosby, S. H.; Clarkson, G.
J.; Rourke, J. P. J. Am. Chem. Soc. 2009, 131, 14142−14143.
Matthey for a generous loan of platinum salts. We are grateful
for support from Advantage West Midlands (AWM) (partially
funded by the European Regional Development Fund) for the
purchase of a high-resolution mass spectrometer and the XRD
system that was used to solve the crystal structures. L.M.
gratefully acknowledges Regione Autonoma della Sardegna for
a Ph.D. fund (POR/FSE 2007−2013).
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̈
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NOTE ADDED AFTER ASAP PUBLICATION
■
In the version of this paper published on March 22, 2012, one
graphic appeared in duplicate and another had an incorrect
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2012 has the correct structures.
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