Regioselective C-H activation preceded by Csp2 -Csp3 reductive elimination from cyclometalated platinum(IV) complexes

Article abstract of DOI:10.1021/om400398g

Reductive elimination reactions of the cyclometalated platinum(IV) compounds [PtMe₂Cl{C₆H₄CHi - NCH ₂(4-ClC₆H₄)}L] and [PtMe₂Br{C ₆H₄CHi - NCH₂(C

Full text of DOI:10.1021/om400398g

Article
pubs.acs.org/Organometallics
2
3
Regioselective C−H Activation Preceded by Csp −Csp Reductive
Elimination from Cyclometalated Platinum(IV) Complexes
Craig M. Anderson,*,† Margarita Crespo,*,‡ Nicole Kfoury,† Michael A. Weinstein,†
and Joseph M. Tanski§
†Department of Chemistry, Bard College, P.O. Box 5000, Annandale-on-Hudson, New York 12504, United States
‡
́
́
Departament de Quımica Inorganica, Facultat de Quımica, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain
̀
§Department of Chemistry, Vassar College, Poughkeepsie, New York, United States
S
* Supporting Information
ABSTRACT: Reductive elimination reactions of the cyclometalated platinum-
(IV) compounds [PtMe2Cl{C6H4CHNCH2(4-ClC6H4)}L] and [PtMe2Br-
3
2
{C6H4CHNCH2(C6H5)}L] (L = SMe2, PPh3) to form Csp −Csp bonds,
2
followed by either exclusive Csp −H bond activation (L = SMe2) or competition
2
3
between Csp −H and Csp −H bond activation (L = PPh3), are reported.
Isomerization to give endo products instead of the expected exo complex was
observed for the ligand C6H4CHNCH2(2-BrC6H5), and formation of an endo
six-membered platinacycle occurs for the ligand 2,4,6-Me3C6H2CHNCH2(2-
BrC6H4).
subject of paramount importance in the area of obtaining value-
INTRODUCTION
■
added products from inexpensive fuel stocks and in the organic
synthesis of complex molecules: for example, in the area of
pharmaceuticals. In this study, which is an extension of our
recently published communication,12 we examined the
Reductive elimination is often viewed as the most important
step in a catalytic cycle or in stoichiometric processes, as it
often is the step where the desired product is formed, usually in
an irreversible reaction of C−C or C−H bond formation.
There has been much interest in reductive eliminations where
2
3
reductive elimination of Csp −Csp bonds from Pt(IV)
intermediates, which are then held tethered to the metal center
by the ligand’s nitrogen atom. This allows for subsequent
competitive regioselective C−H bond formation to give the
final Pt(II) cyclometalated product. Figure 1 shows the
molecular structures of the ligands which were examined in
this study along with structures of the platinum(IV)
intermediates either isolated or suspected to be part of an
overall mechanism. The intramolecular oxidative addition of
C−X bonds (X = Br, Cl) of these appropriately chosen
nitrogen ligands to [Pt2Me4(μ-SMe2)2] (1) gives initially
cyclometalated Pt(IV) products with fac carbon stereo-
chemistry,11 which then can be studied for reductive
elimination reactions. We have probed the factors affecting
the products in the elimination reactions and the subsequent
C−H activation step. The final products have three variables
Csp −Csp elimination is concerned1 and many others where
competition between C−C bond formation and C−O,2 C−N,3
and C−I4 bond formation exists. Methane elimination of a
methyl group and a hydrido ligand is yet another much studied
reaction,5 the reverse of which has been studied in the area of
C−H activation and which has gained a great deal of attention
in recent decades. Reductive elimination studies have also been
3
3
2
performed for arylplatinum(IV) compounds, including Csp −H
6
2
2
2
bond formation and competition between Csp −Csp and Csp −
7
2
2
halide bond formation. In addition, Csp −Csp coupling from
cyclometalated platinum(IV) compounds leading to five-, six-,
or seven-membered platinacycles containing a biaryl linkage has
also been reported.8 Examples of Csp −Csp reductive
2
3
elimination are more rare than those listed above.9 Recently,
2
2
a catalytic process for conversion of a Csp −F bond into a Csp −
3
2
that are important in their formation: Csp −H or Csp −H bond
activation, endo (ring contains the imine double bond) versus
exo (ring does not contain the double bond) platinacycle
formation, and five-membered versus six-membered platina-
cycle ring formation.
3
Csp bond involving reductive elimination from platinum(IV)
compounds has been reported.10 With respect to the reverse
reaction, oxidative addition, many reports have included the use
of chelate-assisted additions where an appropriately designed
ligand has first a heteroatom coordinated to the metal, allowing
for the C−X or C−H bond to be added to be in close proximity
to the metal and hence facilitating the reaction.11 Regioselective
bond activation, specifically in the case of C−H bonds, is a
Received: May 8, 2013
Published: July 26, 2013
© 2013 American Chemical Society
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methyl ligands. The original chelate C∧N ligand would remain
attached to the metal through the Pt−N bond after the C−C
coupling; thus, it would be in position to activate a second
bond, in this case a C−H bond, which would allow for a choice
between an sp3 C−H of the methyl on the ring or an sp2 C−H
bond on the phenyl ring itself.
Scheme 1 shows the two routes examined, both generating
3
2
the formation of complexes with Csp −Csp coupling: one route
includes the isolation of the Pt(IV) intermediate L1B, and the
other route, that through L1F, consists of a one-pot synthesis
where only the platinum(II) final products were isolated. The
reaction was also run at room temperature, under very mild
conditions, in order to isolate the platinum(IV) intermediate
L1A. In any case the labile dimethyl sulfide ligand can be easily
substituted by triphenylphosphine. This varies the sterics and
electronics of the ancillary ligand as well as allowing for better
crystallization. Scheme 1 includes many of the reactions
Figure 1. (top) Ligands used in this study. (bottom) Platinum(IV)
intermediates.
3
observed. In addition to the compounds resulting from Csp −
2
Csp coupling, minor products, tentatively assigned to those
3
3
2
resulting from either Csp −Csp (elimination of ethane) or Csp −
Cl reductive elimination, were observed in the 1H NMR spectra
of the crude reaction mixtures and were not pursued. Recent
RESULTS AND DISCUSSION
■
Reactions Related to Ligand L1. In this report we present
results for the thermolysis reactions of platinum(IV) com-
pounds containing different cyclometalated ligands. To begin,
2-ClC6H4CHNCH2(4-C6H4Cl) (L1; see Figure 1 for
ligands) was studied in its reactions with the platinum(II)
dimer [Pt2Me4(μ-SMe2)2] (1). Scheme 1 summarizes the
reactions and conditions investigated for ligand L1. The
reaction is assumed to go through a platinum(IV) intermediate
by oxidative addition of the C−Cl bond followed by a C−C
coupling reaction of the ortho carbon on the ring and one of the
3
2
papers have reported on both Csp −Cl and Csp −Cl coupling
from platinum(IV) cyclometalated compounds.13 In the case
3
2
involving cleavage of the metallacycle through Csp −Csp
reductive elimination (L1C, a suspected intermediate),
subsequent cyclometalation at the available ortho position of
3
the aryl ring or cyclometalation of the Csp −H of the methyl
group from the newly formed bond (followed by reductive
elimination of methane) was observed. The results for L1 are
analogous to those obtained for the ligand 2-BrC6H4CH
a
Scheme 1. Syntheses of the Cyclometalated Platinum(II) and -(IV) Compounds using L1
a
Proposed intermediates are given in brackets. In order to follow the fate of the methyl groups initially bound to platinum, these are indicated in
boldface.
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NCH2(4-C6H4Cl), reported in our recent communication.12
Once the bulky phosphine was added to a solution of L1A, a
platinum(IV) species was observed. With the chloro analogue,
in contrast to the bromo analogue, we were able to isolate and
characterize the Pt(IV) intermediate L1B by XRD (Figure 2),
Figure 3. Molecular structure of compound L1E. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: Pt−
C(1), 2.057(3); Pt−N, 2.105(2); Pt−P, 2.2159(7); Pt−Cl(1),
2.4139(6); N−C(8), 1.288(3); N−C(9), 1.470(3); C(1)−Pt−N,
83.99(10); C(1)−Pt−P, 88.90(8); N−Pt−P, 172.85(6); C(1)−Pt−
Cl(1), 173.83(7); N−Pt−Cl(1), 90.15(6); P−Pt−Cl(1), 96.98(2);
C(8)−N−C(9), 117.9(2); C(8)−N−Pt, 123.44(19); C(9)−N−Pt,
118.36(18).
Figure 2. Molecular structure of compound L1B. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: Pt−
C(2), 2.0073(17); Pt−C(1), 2.0704(17); Pt−C(34), 2.0923(19), Pt−
N, 2.1467(14); Pt−P, 2.4150(4); Pt−Cl(1), 2.4298(4); N−C(8),
1.284(2); N−C(9), 1.474(2); C(8)−N−C(9), 122.41(15); C(8)−N−
Pt, 112.82(12); C(9)−N−Pt, 124.40(11); C(2)−Pt−C(1), 93.47(7);
C(2)−Pt−C(34), 87.13(7); C(1)−Pt−C(34), 86.79(8); C(2)−Pt−N,
80.21(6).
suitable crystals of the product expected for L2 were obtained
when using C6H5CHNCH2(2-C6H4Br) (L3) (after phos-
phine substitution for dimethyl sulfide). L3 was originally
employed in our study in order to force the initial oxidative
addition away from the imine double bond, thus giving an exo
intermediate, which was to be studied in order to compare the
elimination reactions to the known endo cases. The reaction of
1 with L3 was followed under various conditions. Initially, the
thus confirming its structure and indicating that the mechanism
likely goes through an initial oxidative addition intermediate,
even in the one-pot synthesis generating L1F, shown in Scheme
1. The platinum atom displays an octahedral coordination with
a fac-PtC3 arrangement and the bulky triphenylphosphine in
the less hindered position, which is trans to a methyl group.
This arrangement is consistent with previous studies indicating
that the intramolecular oxidative addition of a C−Cl is initially
cis and is followed by an isomerization process upon
substitution of SMe2 for the bulkier PPh3.14 We were also
able to characterize one of our final platinum(II) products by
XRD for the reactions with L1. The structure of L1E (Figure 3)
indicated that the methyl group that had been eliminated by the
C−C coupling with the phenyl group had then had one of its
sp3 C−H bonds activated. In addition to the six-membered
platinacycle containing the imine functionality, a chloride and a
PPh3 ligand complete the square-planar coordination around
the platinum(II). This was the major product (Scheme 1) as
determined by NMR integration, in agreement with the
previously reported results12 indicating that the presence of
1
reaction was run under mild conditions and the H and 31P
NMR data for the compound obtained after reaction with PPh3
were consistent with the formation of a platinum(IV)
cyclometalated compound (L3B). Although formation of this
type of compound containing an exo-metallacycle had been
previously described and characterized only by NMR,15 the
molecular structure determination indicated formation of an
endo-platinacycle. Crystals of this Pt(IV) product were studied
by XRD and shown clearly to have the endo structure with the
imine double bond included in the platinacycle with an
arrangement analogous to that observed for L1B (Figure 4).
This is clearly evident from the lengths of the carbon−nitrogen
bonds. The ligand L3, once coordinated, either subsequent to
the initial oxidative addition reaction that produced the Pt(IV)
species or immediately thereafter, isomerizes to give the endo
product. Scheme 2 details the reactions and illustrates a
possible mechanism for the transformation where, immediately
after the C−Br oxidative addition, the isomerization occurs.
The presence of a bromo substituent in the saturated arm of
the free N-benzylidenebenzylamine mandates initial formation
of an exo-platinacyle arising from C−Br bond activation,
followed by isomerization to a more stable endo-platinacycle,
which can take place before or after the substitution of SMe2 by
PPh3. In principle, the isomerization could take place either
through a 1,3-proton shift mechanism as described for N-
benzylamines16 or through an intramolecular exchange reaction
as described for cyclopalladated compounds.17 The second
mechanism is more likely, since the ligand itself did not
isomerize when left in solution for days or when refluxed for
several hours with no metal complex.
3
the bulky triphenylphosphine ligand favors Csp −H versus
2
Csp −H bond activation.
Reactions Related to Ligands L2 and L3. 2-
BrC6H4CHNCH2(C6H5) (L2), when refluxed in toluene
for several hours, gave results similar to those reported in our
earlier communication for the ligand 2-BrC6H4CHNCH2(4-
ClC6H4), and the isolated products were characterized by NMR
spectroscopy.
Suitable crystals for XRD were not obtained for this product,
with this ligand and reaction conditions, and the results
observed were straightforward. However, surprisingly, a very
unexpected result occurred with ligand L3. In this system,
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Figure 5. Molecular structure of compound L3D. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: Pt−
C(1), 2.020(3); Pt−Br, 2.4968(3); Pt−N, 2.077(2); Pt−P, 2.2298(7);
N(1)−C(8), 1.287(4); N(1)−C(9), 1.468(4); N−C(8)−C(7),
117.7(3); N−C(9)−C(10), 111.3(2); C(8)−N−C(9), 119.1(3);
C(8)−N−Pt, 114.6(2); C(9)−N−Pt, 126.11(19); C(1)−Pt−N,
80.16(11); C(1)−Pt−P, 94.62(8); N−Pt−P, 174.69(7); C(1)−Pt−
Br, 170.68(8).
Figure 4. Molecular structure of compound L3B. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: Pt−
C(1), 2.017(3); Pt−C(16), 2.075(3); Pt−C(15), 2.080(3); Pt−N,
2.151(3); Pt−P, 2.4265(8); Pt−Br, 2.5791(4); N−C(7), 1.287(4);
N−C(8), 1.486(4); C(1)−Pt−C(16), 94.66(13); C(1)−Pt−C(15),
85.65(13); C(16)−Pt−C(15), 85.24(14); C(1)−Pt−N, 80.25(11);
C(16)−Pt−N, 172.16(12); C(15)−Pt−N, 88.41(12); C(1)−Pt−P,
93.77(9); C(16)−Pt−P, 93.73(10); C(15)−Pt−P, 178.77(10); N−
Pt−P, 92.57(7).
The thermolysis of compound [PtMe2Br{C6H4CH2N
CHC6H5}PPh3] (L3B) was carried out in refluxing toluene
for 4 h and gave, in addition to trans-[PtMeBr(PPh3)2], the
single cyclometalated platinum(II) compound [PtBr-
{CH2C6H4CHNCH2(C6H5)}PPh3] (L3E) containing a six-
membered metallacycle, which was also characterized by XRD
(Figure 6) and showed features analogous to those of L1E.
This result indicates that Csp −Csp reductive elimination
followed by a fast cyclometalation, with subsequent loss of
methane, takes place. The cyclometalation step consists of
A one-pot procedure from ligand L3 followed by reaction
with PPh3 produced crystals of L3D. The NMR spectra of these
crystals matched the spectra of those from L2D; thus, the two
different ligands eventually gave the same product. The
compound was characterized by XRD (Figure 5), and once
again the CN bond, easily identified by its length, is included
in the five-membered platinacycle and the methyl group is
3
2
2
attached to the remaining ortho Csp of the benzyl ring.
Scheme 2. Proposed Mechanism and Reaction for L2 and L3
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Scheme 3. Reactions of Ligand L4
Figure 6. Molecular structure of compound L3E. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: Pt−
C(1), 2.072(3); Pt−N, 2.099(3); Pt−P, 2.2168(8); Pt−Br, 2.5193(4);
N−C(8), 1.284(4); N−C(9), 1.481(4); C(1)−C(2), 1.496(4); C(1)−
Pt−N, 82.10(7); C(1)−Pt−P, 89.03(9); N−Pt−P, 172.49(8); C(1)−
Pt−Br, 173.11(9); N−Pt−Br, 90.32(8); P−Pt−Br, 97.15(2); C(8)−
N−C(9), 117.5(3); C(8)−N−Pt, 124.2(2); C(9)−N−P, 118.0(2);
C(2)−C(1)−Pt, 108.5(2).
3
activation of a Csp −H bond of the methyl group previously
reductively eliminated, leading to the six-membered platina-
cycle. As stated above, when an alternative reaction was run,
where L3 and 1 were refluxed for several hours and the Pt(IV)
intermediate was not isolated, the final Pt(II) product isolated,
after phosphine substitution and recrystallization, was the
product shown in Scheme 2, which included the isomerization
2
of the ligand but instead the Csp −H had been activated to form
the five-membered platinacycle L3D. As indicated above for
ligand L1, the presence of the triphenylphosphine ligand favors
3
Figure 7. Molecular structure of compound L4E. Selected bond
lengths (Å) and angles (deg) with estimated standard deviations: Pt−
C(1), 2.0460(18); Pt−N, 2.0957(15); Pt−P, 2.2074(5); Pt−Br,
2.5421(2); N−C(8), 1.290(2); N−C(9), 1.484(2); C(1)−C(2),
1.495(3); C(1)−Pt−N, 82.10(7); C(1)−Pt−P, 88.19(5); N−Pt−P,
167.84(5); C(1)−Pt−Br, 168.39(5); N−Pt−Br, 90.70(4); P−Pt−Br,
100.000(14); C(8)−N−C(9), 118.08(16); C(8)−N−Pt, 123.35(13);
C(9)−N−Pt, 117.61(12); C(2)−C(1)−Pt, 104.58(12).
cyclometalation at the Csp −H bond over cyclometalation at the
2
Csp −H bond.
Reactions Related to Ligand L4. Finally, the ligand 2,4,6-
C6H2(CH3)3CNCH2(2-C6H4Br) (L4) was studied in order
to examine a ligand which was predicted to give a product with
the reductively eliminated methyl group not included in the
final platinacycle and would be predicted not to isomerize due
to the two ortho methyl groups poised to form an endo, six-
membered platinacycle. Therefore, the predicted product was
in this case, given the propensity for these systems to form endo
3
products, an endo six-membered ring resulting from the Csp −H
for determining the final product. Generally, previous reports
have stated that five-membered rings form more readily than
six-membered rings18 and that six-membered platinacycles are
somewhat of a rarity.19 A recent example has been reported for
activation. This ligand reacts according to Scheme 3: that is,
oxidative addition of C−Br, followed by C−C bond coupling/
3
reductive elimination, ending with a Csp −H bond activation of
a mesityl methyl group to give the endo six-membered
platinacycle ring. No isomerization was observed in this case.
In this system, it is one methyl group of the mesityl ligands that
had its C−H activated to give the desired endo product, without
the need for exo to endo isomerization, as was observed for
ligand L3, which had the methyl originally on the platinum that
was activated. XRD study of a single crystal clearly shows the
original platinum/methyl group attached to the phenyl of the
benzyl ring (Figure 7), even though the ring shows a slight
disorder. The same product was observed whether the
platinum(IV) intermediate was isolated prior to reflux or in a
one-pot synthesis from the platinum dimer and ligand L4.
Concluding Remarks. Given our results for these systems,
it appears that endo formation is the paramount driving force
a fluorinated ligand.19f Furthermore, Csp −H bonds are
2
3
preferred over Csp −H bonds on activation by a metal center.
However, our results indicate that a six-membered ring with
3
2
Csp −H activation is preferred over a Csp −H five-membered
exo ring, overriding both of these accepted rules in order to
form an endo product with the imine bond in the platinacycle.
Therefore, of the three variables for the final C−H activation
that are covered in this study (endo vs exo, sp2 vs sp3, five-
membered ring vs six-membered ring), the most important
appears to be formation of the platinacycle ring including the
imine double bond, thus rendering the endo structure. This
feature appears to override the other variables.
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1H NMR (CDCl3, 400 MHz): δ 0.88 (d, 3J(H−P) = 8.0, 2J(H−Pt)
EXPERIMENTAL SECTION
■
3
2
= 58.4, 3H, CH3); 1.41 (d, J(H−P) = 8.0; J(H−Pt) = 68.8, 3H,
General Considerations. The solvents and reagents were
purchased from Sigma Aldrich unless otherwise noted. K2PtCl4 was
purchased from the Pressure Chemical Co. NMR spectra were
performed at Bard College using a Varian MR 400 MHz spectrometer
(1H, 400 MHz; 13C, 100.6 MHz; 31P{1H}, 161.8 MHz) or at the
Unitat de RMN d’Alt Camp de la Universitat de Barcelona using
Varian Unity 300 (31P{1H}, 121.4 MHz) and Mercury 400 (1H, 400
MHz; 1H−1H NOESY) spectrometers, referenced to SiMe4 (1H, 13C)
and H3PO4 (31P). δ values are given in ppm and J values in Hz.
Abbreviations used: s, singlet; d, doublet; t, triplet; m, multiplet.
Electrospray mass spectra were performed at Vassar College and at the
Servei d’Espectrometria de Masses (Universitat de Barcelona) using an
LC/MSD-TOF spectrometer and 1/1 H2O/CH3CN to introduce the
sample. The labeling scheme for selected compounds is given in Figure
8.
2
2
CH3); {4.51 (d, J(H−H) = 17.8), 5.23 (d, J(H−H) = 17.8), AB
system, 2H, CH2}; 6.61 (d, J(H−H) = 7.6, 1H); 6.76 (t, J(H−H) =
7.6, 1H); 6.84 (d, J(H−H) = 8.4, 2H); 6.92 (t, J(H−H) = 7.2, 1H);
7.10 (d, J(H−H) = 7.6, 1H); 7.28−7.37 (m, 11H); 7.46 (t, J(H−H) =
3
9.2, 6H); 7.69 (s, J(H−Pt) = 48.4, 1H, CHN). 31P NMR (CDCl3,
161.8 MHz): δ −4.04 (s, 1J(P−Pt) = 999.8). Anal. Calcd for
C34H32Cl2NPPt: C, 54.33; H, 4.29; N, 1.86. Found: C, 53.54; H, 3.31;
N, 1.96. Mp (°C): 112−116 dec.
[PtCl{2-MeC6H3CHNCH2(4-ClC6H4)}PPh3] (L1D). A 12 mg
amount (0.022 mmol) of L1F and 6 mg (0.022 mmol) of PPh3
were combined in acetone (10 mL), and the mixture was stirred at
room temperature for 30 min. The solvent was evaporated and the
brownish yellow solid washed with pentane and vacuum-dried. Yield:
9.1 mg (55%).
1H NMR (CDCl3, 400 MHz): δ 2.39 (s, 3H, CH3); 5.37 (s, 2H,
CH2); 6.43 (t, J(H−H) = 7.6, 1H); 6.62 d, J(H−H) = 7.2, 1H); 7.31−
7.41 (m, 14H); 7.70−7.76 (m, 6H); 8.53 (d, 4J(H−P) = 9.2, 3J(H−Pt)
= 96, 1H, CHN). 31P NMR (161.8 MHz, CDCl3): δ 23.11 (s, 1J(P−
Pt) = 4189). Anal. Calcd for C33H28Cl2NPPt: C, 53.89; H, 3.84; N,
1.90. Found: C, 54.46; H, 3.80; N, 1.99. HR-ESI-(+)-MS: m/z
699.1289; calculated for C33H28ClNPPt, [M − Cl]+ 699.1296. Mp
(°C): 164−168 dec.
[PtCl{CH2C6H4CHNCH2(4-ClC6H4)}PPh3] (L1E). Compound L1B
was dissolved in toluene (10 mL), and the solution was refluxed for 4
h. The solvent was evaporated, and the residue was washed with
Figure 8. Labeling scheme for selected compounds. The arrows
indicate the most relevant NOE interactions.
1
pentane and cold diethyl ether to afford a yellow solid. The initial H
NMR spectrum showed the presence of both L1D and L1E in an
approximate 1/3 ratio. Crystals of LIE were obtained by
recrystallization from acetone/hexane.
X-ray Diffraction. X-ray diffraction data were collected on a
Bruker APEX 2 CCD platform diffractometer (Mo Kα (λ = 0.71073
Å)) at 125 K. Crystals were mounted in a nylon loop with Paratone-N
cryoprotectant oil. The structure was solved using direct methods and
standard difference map techniques, and was refined by full-matrix
least-squares procedures on F2 with SHELXTL (version 6.14).20 All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms on
carbon were included in calculated positions and were refined using a
riding model. See Figures 2−7 and their captions for ORTEP
drawings, labels, bond lengths, and bond angles. CIFs for all six
structures are included in the Supporting Information.
1H NMR (CDCl3, 400 MHz): δ 2.34 (d, 3J(H−P) =3.2, 2J(H−Pt) =
78.4, 2H, CH2Pt), 5.60 (s, 2H, CH2); 6.26 (m, 1H); 7.05 (m, 2H);
7.14 (m, 1H); 7.36−7.40 (m, 9H); {7.50 (d, J(H−H) = 8.4), 7.66 (d,
J(H−H) = 8.4), AB system, 4H}; 7.71−7.75 (m, 6H); 8.14 (d, 4J(H−
3
P) = 12.8, J(H−Pt) = 52, 1H, CHN). 31P NMR (161.8 MHz,
CDCl3): δ 17.36 (s, 1J(P−Pt) = 4470.3). HR-ESI-(+)-MS: m/z
699.1292; calcd for C33H28ClNPPt, [M − Cl]+ 699.1296.
[PtCl{2-MeC6H3CHNCH2(4-ClC6H4)}SMe2] (L1F). A 100 mg
portion (0.17 mmol) of 1 and 92 mg (0.35 mmol) of L1 were
combined in toluene (15 mL), and the mixture was refluxed for 4 h.
The solvent was evaporated, and the residue was washed with pentane
and vacuum-dried to afford an orange solid. Yield: 134 mg (72%).
Alternatively, compound L1F could be obtained by refluxing L1A (46
mg) in toluene for 4 h. Yield: 24 mg (49%).
Preparation of the Compounds. The platinum dimer cis-
[Pt2Me4(μ-SMe2)2], ligands L1−L4, and compounds L2A and L3A
were prepared as reported elsewhere.8e,15,21,22
PtMe2Cl{C6H4CHNCH2(4-ClC6H4)}SMe2] (L1A). A 50 mg portion
(0.087 mmol) of 1 and 44 mg (0.17 mmol) of L1 were combined in
acetone (15 mL), and the mixture was stirred at room temperature for
16 h. The solution was filtered and the solvent evaporated to afford an
oil which was triturated with pentane to afford a reddish powder.
Yield: 84 mg (87%).
1H NMR (CDCl3, 400 MHz): δ 2.41 (s, 3H, CH3); 2.68 (s, 3J(H−
Pt) = 52.8, 6H, SMe2); 5.27 (s, 2H, CH2); 6.81 (d, J(H−H) = 7.6,
3
1H); 7.01 (t, J(H−H) = 8, 1H); 7.16−7.40 (m, 5H); 8.29 (s, J(H−
Pt) = 124.8, 1H, CHN). 13C NMR (CDCl3, 100.6 MHz): δ 176.62
(CHN); 133.06; 130.18 (2C); 128.97 (2C); 128.13; 125.83; 60.91
(CH2); 22.93 (J(C−Pt) = 12, SCH3); 19.69 (CH3). HR-ESI-(+)-MS:
m/z 499.0564; calcd for C17H19ClNPtS, [M − Cl]+ 499.0574. Anal.
Calcd for C17H19Cl2NPtS: C, 38.14; H, 3.58; N, 2.62, Found: C,
38.82; H, 3.21; N, 2.77. Mp (°C): 84−89 dec.
2
1H NMR (CDCl3, 400 MHz): δ 0.96 (s, J(H−Pt) = 69.6, 3H,
2
3
CH3); 1.26 (s, J(H−Pt) = 67.2, 3H, CH3); 1.93 (s, J(H−Pt) = 12,
6H, SMe2); {5.18 (d, 2J(H−H) = 15.0), 5.25 (d, 2J(H−H) = 15.0), AB
system, 2H, CH2}; 7.10 (t, J(H−H) = 9.2, 1H); 7.20−7.42 (m, 7H);
8.20 (s, J(H−Pt) = 45.6, 1H, CHN). 13C NMR (CDCl3, 100.6
3
[PtMe2Br{C6H4CHNCH2(C6H5)}SMe2] (L2A). A 100 mg portion
(0.17 mmol) of 1 and 96 mg (0.35 mmol) of L2 were combined in
acetone, and the mixture was stirred at room temperature for 16 h.
The solution was filtered and the solvent evaporated to afford a yellow
oil. Yield: 195 mg (75%). The NMR spectra were identical with those
obtained for L3A and that previously reported.22
[PtBr{2-MeC6H3CHNCH2(C6H5)}PPh3] (L2D). A 62 mg portion
(0.11 mmol) of L2F and 30 mg (0.11 mmol) of PPh3 were dissolved
in acetone (12 mL), and the mixture was stirred at room temperature
for 30 min. The solvent was evaporated and the residue washed with
pentane and vacuum-dried to afford a brown solid. Yield: 63 mg
(74%). The NMR spectra were identical with those obtained for L3D;
see below.
MHz): δ 171.86 (J(C−Pt) = 54, CHN); 143.86 (J(C−Pt) = 970,
C
metalated); 132.52 (J(C−Pt) = 65); 131.52 (2C); 129.86 (J(C−Pt) =
33); 129.07 (2C); 124.35 (J(C−Pt) = 7); 59.23 (CH2); 18.43
(SCH3); 1.52 (J(C−Pt) = 620, PtCH3); −3.53 (J(C−Pt) = 662,
PtCH3). Anal. Calcd for C18H23Cl2NPtS: C, 39.21; H, 4.20; N, 2.54.
Found: C, 39.61; H, 3.72; N, 3.26. HR-ESI-(+)-MS: m/z 499.0565;
calcd for C17H19ClNPtS, [M − Cl − CH3 − H]+ 499.0575. Mp (°C):
73−77.
[PtMe2Cl{C6H4CHNCH2(4-ClC6H4)}PPh3] (L1B). An 83 mg
amount (0.15 mmol) of L1A and 39 mg (0.15 mmol) of PPh3 were
combined in acetone, and the mixture was stirred at room temperature
for 30 min. The solution was filtered and the solvent evaporated to
afford an oil which was triturated with pentane to afford a gray-green
powder. Yield: 64%. Crystals were obtained by recrystallization from
dichloromethane/methanol.
[PtBr{2-MeC6H3CHNCH2(C6H5)}SMe2] (L2F). A 100 mg portion
(0.17 mmol) of 1 and 96 mg (0.35 mmol) of L2 were dissolved in
toluene (20 mL), and the solution was refluxed for 4 h. The solvent
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was evaporated and the residue washed with pentane and cold diethyl
ether and vacuum-dried to afford a brown solid. Yield: 137 mg (72%).
Alternatively, compound L2F could be obtained by refluxing L2A in
toluene for 4 h. Yield: 90 mg (48%). The NMR spectrum was identical
with that obtained for L3F; see below.
[PtBr{2-MeC6H3CHNCH2(C6H5)}SMe2] (L3F). A 50 mg portion
(0.087 mmol) of 1 and 48 mg (0.17 mmol) of L3 were combined in
toluene (15 mL), and the mixture was refluxed for 4 h. The solvent
was evaporated and the residue was washed with pentane and vacuum-
dried to afford a brown solid. Yield: 87 mg (90%). Alternatively,
compound L3F can be obtained by refluxing compound L3A in
toluene (15 mL) for 4 h. Yield: 92 mg (49%).
[PtMe2Br{C6H4CH2NCHC6H5}PPh3] (L3B). This compound was
obtained from 95 mg (0.35 mmol) of imine 2-BrC6H4CH2N
CHC6H5 and 100 mg (0.17 mmol) of cis-[Pt2Me4(μ-SMe2)2] in 20 mL
of acetone. The mixture was stirred at room temperature for 16 h. L3A
was formed and characterized as reported.22 After this time, 90 mg
(0.34 mmol) of PPh3 was added and the mixture was stirred for 2 h at
room temperature. The solvent was removed, and the residue was
recrystallized in CH2Cl2/MeOH. Yield: 140 mg (53%).
1H NMR (CDCl3, 400 MHz): δ 2.37 (s, 3H, CH3); 2.76 (s, 3J(H−
Pt) = 51.6, 6H, SMe2); 5.47 (s, 2H, CH2); 6.82 (d, J(H−H) = 7.6,
1H); 7.09 (t, J(H−H)= 7.6); 7.34−7.43 (m, 6H); 8.26 (s, 3J(H−Pt) =
126.0, 1H, CHN). 13C NMR (CDCl3, 100.6 MHz): δ 176,43 (J(C−
Pt) = 102, CHN); 132.85; 129.04 (2C); 128.83 (2C); 127.91; 127.51;
125.95; 63.54 (J(C−Pt) = 45, CH2); 24.06 (J(C−Pt) = 16, SCH3);
19.83 (CH3). HR-ESI-(+)-MS: m/z 465.0951, calcd for C17H20NPtS,
[M − Br]+ 465.0964. Anal. Calcd for C17H20BrNPtS·0.4C5H12: C,
39.74; H, 4.35; N, 2.44. Found: C, 40.12; H, 3.52; N, 2.72. Mp (°C):
78−83 dec.
1H NMR (400 MHz, CDCl3): δ 7.67 (d, 4J(H−P) = 4.0, 3J(H−Pt)
= 52.0, 1H, Hd); 7.47−7.42 (m, 6H); 7.35−7.30 (m, 6H); 7.24−7.22
3
4
(m, 6H); 7.10 (dd, J(H−H) = 8.0; J(H−H) = 2.0, 1H); 6.93 (t,
3J(H−H) = 7.0, 1H); 6.90 (m, 2H); 6.79 (td, J(H−H) = 7.0; J(H−
3
4
H) = 2.0, 1H); 6.58 (d, J(H−H) = 8.0, J(H−Pt) = 44.0, 1H, He);
3
3
2
3
2
{5.37 (dd, J(H−H) = 16.0; J(H−H) = 4.0); 4.60 (dd, J(H−H) =
[PtMe2Br{2,4,6-(CH3)3C6H2CHNCH2(C6H4)}SMe2] (L4A). A 50 mg
portion (0.087 mmol) of 1 and 55 mg (0.17 mmol) of L4 were
combined in acetone, and the mixture was stirred at room temperature
for 16 h. The solution was filtered and the solvent evaporated to afford
a whitish- green powder. Yield: 43 mg (39%).
3
3
16.0; J(H−H) = 4.0), AB system, 2H, Hc}; 1.49 (d, J(H−P) = 8.0,
2J(H−Pt) = 72.0, 3H, Meb); 1.04 (d, 3J(H−P) = 8.0, 2J(H−Pt) = 60.0,
3H, Mea). 13C NMR (CDCl3, 100.6 MHz): δ 170.59 (CHN); 141.90;
135.97; 134.07 (d, J(C−P) = 13, Cortho PPh3); 132.18; 131.00; 130.61;
130.41; 129.82 (Cpara PPh3); 129.02; 128.86; 128.41; 128.07; 127.95
(d, J(C−P) = 6, Cmeta PPh3); 123.20 ; 59.90 (CH2); 10.35 (PtCH3);
−4.94 (PtCH3). 31P NMR (121.4 MHz, CDCl3): δ −5.80 (s, 1J(P−Pt)
= 1005.2). HR-ESI-(+)-MS: m/z 681.1993, calcd for C34H33NPPt, [M
− Br]+ 681.1998. Anal. Calcd for C34H33BrNPPt: C, 53.62; H, 4.37; N,
1.84. Found: C, 53.93; H, 3.72; N, 2.01. Mp (°C): 91−95 dec.
[PtBr{2-MeC6H3CHNCH2(C6H5)}PPh3] (L3D). A 15 mg portion of
(0.028 mmol) of L3F and 10 mg (0.028 mmol) of PPh3 were
combined in acetone (10 mL), and the mixture was stirred at room
temperature for 30 min. The solvent was evaporated and the residue
washed with pentane to afford a yellow solid. Recrystallization from
acetone/hexane gave single crystals, which were studied by XRD and,
when redissolved, were shown to be identical with compound L2D by
NMR characterization.
1H NMR (CDCl3, 400 MHz): δ 0.75 (s, 2J(H−Pt) = 73.6, 3H, Me);
2
1.38 (s, J(H−Pt) = 71.2, 3H, Me); 2.12 (s, 3H, Me); 2.31 (m, 6H,
Me); 2.43 (s, 3J(H−Pt) = 78, 6H, SMe2); {4.83 (d, 2J(H−H) = 10.4),
2
6.00 (d, J(H−H) = 10.4), 2H, CH2}; 6.83 (s, 1H); 6.91 (s, 1H);
3
7.02−7.17 (m, 3H); 9.08 (s, J(H−Pt) = 29.6, 1H, CHN). 13C
NMR (CDCl3, 100.6 MHz): δ 168.55 (CHN); 130.54; 128.35;
127.64; 126.71 (J(C−Pt) = 65); 124.70; 122.06 (J(C−Pt) = 30);
73.27 (CH2 J(C−Pt) = 13); 22.35 (CH3); 21.11 (SCH3); 20.24
(CH3); 17.49 (CH3); −0.41 (PtCH3); −0.73 (PtCH3). Anal. Calcd for
C21H30BrNPtS: C, 41.79; H, 5.01; N, 2.32. Found: C, 40.67; H, 4.06;
N, 2.44. Mp (°C): 68−74 dec.
[PtBr{CH2-2,4-(CH3)2C6H2CHNCH2(2-CH3C6H4)}PPh3] (L4E). A 20
mg portion (0.034 mmol) of L4F and 9 mg (0.034 mmol) of PPh3
were dissolved in acetone (10 mL), and the mixture was stirred at
room temperature for 30 min. The solvent was evaporated, and the
residue was washed with pentane and vacuum-dried to afford a gray
solid. Yield: 11.5 mg (44%).
1H NMR (CDCl3, 400 MHz): δ 2.32 (s, 3H, CH3); 5.56 (s, 2H,
CH2); 6.41 (t, J(H−H) = 8, 1H); 6.58 (d, J(H−H) = 8, 1H); 7.34−
7.44 (m, 12H); 7.53−7.57 (m, 3H); 7.70−7.75 (m, 6H); 8.44 (d,
3
4J(H−P) = 9.2; J(H−Pt) = 95.2, 1H, CHN). 13C NMR (CDCl3,
100.6 MHz): δ 175.83 (CHN); 137.68; 135.48 (d, J(C−P) =11, Cortho
PPh3); 132.12; 130.06 (br, Cpara PPh3); 129.40 (2C); 128.74 (2C);
127.80 (d, J(C−P) = 13, Cmeta PPh3); 124.96; 122.99; 62.55 (CH2);
1H NMR (CDCl3, 400 MHz): δ 2.04 (s, 3H, Me); 2.17 (s, 3H, Me);
4
3
2.31 (d, J(H−P) = 4.8, J(H−Pt) = 82, 2H, CH2Pt); 2.47 (s, 3H,
CH3); 5.68 (d, 4J(H−P) = 8.4, 2H, CH2); 6.60 (s, 1H); 7.26−7.42 (m,
1
20.11 (CH3). 31P NMR (161.8 MHz, CDCl3): δ 22.87 (s, J(P−Pt)
3
13H); 7.55−7.59 (m, 6H); 7.72 (d, J(H−H) = 7.6, 1H); 8.07 (d,
=4153). HR-ESI-(+)-MS: m/z 665.1667, calcd for C33H29NPPt, [M −
Br]+ 665.1685. Anal. Calcd for C33H29BrNPPt·0.5C5H12: C, 54.55; H,
4.51; N, 1.79. Found: C, 54.19; H, 4.57; N, 1.83. Mp (°C): 95−100.
[PtBr{CH2C6H4CHNCH2(C6H5)}PPh3] (L3E). This compound was
obtained from 50 mg of [PtMe2Br{C6H4CH2NCHC6H5}PPh3]
(L3B) in 20 mL of toluene. The solution was refluxed for 4 h, the
4J(H−P) = 13.2, J(H−Pt) = 61.6, 1H, CHN). 31P NMR (161.8
3
1
MHz, CDCl3): δ 18.48 (s, J(P−Pt) = 4147.7). HR-ESI-(+)-MS: m/z
707.2145; calcd for C36H35NPPt, [M − Br]+ 707.2155. Anal. Calcd for
C36H35BrNPPt: C, 54.90; H, 4.48; N, 1.78. Found: C, 54.45; H, 3.78;
N, 1.68. Mp (°C): 95−100 dec.
1
solvent was removed in a rotary evaporator, and H and 31P NMR
[PtBr{CH2-2,4-(CH3)2C6H2CHNCH2(2-CH3C6H4)}SMe2] (L4G). A
70 mg portion (0.12 mmol) of 1 and 78 mg (0.25 mmol) of L4
were dissolved in toluene (15 mL), and the mixture was refluxed for 4
h. The solution was evaporated and the residue washed with pentane
and cold diethyl ether to afford a grayish brown solid. Yield: 80 mg
(55%). Alternatively, compound L4F can be obtained by refluxing L4A
in toluene for 4 h. Yield: 43 mg (40%).
spectra of the residue indicated the presence of equimolar amounts of
the expected compound L3E and trans-[PtMeBr(PPh3)2]. Recrystal-
lization in CH2Cl2/MeOH yielded 25 mg (51%) of the yellow solid
L3E. 1H NMR (400 MHz, CDCl3): δ 8.10 (d, 4J(H−P) = 16.0, 3J(H−
3
4
Pt) = 60.0, 1H, Hf); 7.72 (dd, J(H−H) = 8.0, J(H−H) = 4.0, 2H);
7.60−7.53 (m, 6H, PPh3); 7.45−7.30 (m, 12H); 7.20 (m, 1H, He);
7.03 (m, 2H, Hc,d); 6.17 (m, 1H); 5.71 (d, J(H−P) = 4.0, 2H, Hg);
4
1H NMR (CDCl3, 400 MHz): δ 2.05 (s, 3H, CH3); 2.21 (s, 3H,
2.37 (d, J(H−P) = 4.0, J(H−Pt) = 80.0, 2H, Ha). NOESY cross
4
2
3
CH3); 2.41 (s, 3H, CH3); 2.53 (s, J(H−Pt) = 52.0, 6H, SMe2); 2.87
peaks: Ha, Hb; Ha, ArPPh3; Hf, Hg; He, Hf. 31P NMR (121.4 MHz,
2
(s, J(H−Pt) = 92.4, 2H, CH2Pt); 5.57 (s, 2H, CH2); 6.65 (s, 1H);
CDCl3): δ −17.92 (s, J(P−Pt) = 4476.0). 13C NMR (CDCl3, 100.6
1
3
6.83 (s, 1H); 7.21−7.25 (m, 3H); 7.44 (d, J(H−H) = 7.2, 1H); 7.78
MHz): δ 162.89 (CHN); 137.29; 134.78 (d, J(C−P) =10, Cortho
PPh3); 132.28; 130.75; 130.27 (d, J(C−P) = 2, Cpara PPh3); 129.99
(2C); 128.58 (2C); 127.79 (d, J(C−P) = 13, Cmeta PPh3); 126.76;
124.11; 67.00 (CH2); 14.64 (PtCH2). HR-ESI-(+)-MS: m/z 665.1673,
calcd for C33H29NPPt, [M − Br]+ 665.1685. Anal. Calcd for
C33H29BrNPPt: C, 53.16; H, 3.92; N, 1.88. Found: C, 53.73; H,
3.85; N, 1.81. Mp (°C): 71−76.
(s, 3J(H−Pt) = 87.6, 1H, CHN). 13C NMR (CDCl3, 100.6 MHz): δ
160.23 (CHN); 130.93; 130.67; 128.93; 126.34; 65.88 (CH2); 23.89
(SCH3); 21.57 (CH3); 19.58 (CH3); 18.64 (CH3); 11.97 (CH2Pt).
HR-ESI-(+)-MS: m/z 507.1421; calce for C20H26NPtS, [M − Br]+
507.1434. Anal. Calcd for C20H26BrNPtS: C, 40.89; H, 4.46; N, 2.38.
Found: C, 41.71; H, 3.59; N, 2.88. Mp (°C): 105−110 dec.
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ASSOCIATED CONTENT
ACKNOWLEDGMENTS
■
■
This material is based upon work supported by the National
Science Foundation under CHE-1058936 (C.M.A., P.I.) and by
the Ministerio de Ciencia y Tecnologıa under project
S
* Supporting Information
CIFs giving crystallographic data for the six complexes
characterized by XRD. This material is available free of charge
via the Internet at http://pubs.acs.org.
́
CTQ2009-11501. We also thank Bard College (BSRI) for
support and the Dreyfus Foundation for supporting C.M.A.
with a Henry Dreyfus Teacher-Scholar Award. Mass spectrom-
etry at Vassar College was performed using instrumentation
supported by funding from a National Science Foundation
Major Research Instrumentation Grant (No. 1039659, Teresa
A. Garrett, P.I.). Bard College students Matthew Greenberg,
David Goldberg, and Leila Duman are also acknowledged for
their help preparing samples and running melting points.
AUTHOR INFORMATION
■
Corresponding Author
*C.M.A.: e-mail, canderso@bard.edu; tel, 845-752-2356; fax,
845-752-2339. M.C.: e-mail, margarita.crespo@qi.ub.es; tel,
34934039132; fax, 34934907725.
Notes
The authors declare no competing financial interest.
REFERENCES
■
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Poster presentation of Michael Weinstein (center, Bard
̈
College), with Franco̧ is P. Gabbaı (left) and John A. Gladysz
(right)
This article is based upon a poster presented by Michael Weinstein
and coauthored by Craig M. Anderson, Margarita Crespo, Nicole
Kfoury, and Joseph M. Tanski at the first Organometallics Symposium
at the ACS meeting in Philadelphia, PA, on August 21, 2012. Michael
Weinstein will receive his B.A. from Bard College this year. He is
currently working under the direction of Craig Anderson, the
corresponding author of this manuscript. Michael Weinstein’s research
interests encompass the synthesis of cyclometalated organoplatinum
complexes by C−H activation and the study of their photophysical
properties. After his undergraduate degree is complete, Mike will
attend Tufts University to obtain an M.S. in biomedical engineering.
After obtaining his graduate degree, he plans to open up his own
engineering concern in the greater New York area.
Craig Anderson published his first paper in Organometallics in 1987
when he was an undergraduate student with Dick Puddephatt at UWO
in London, ON, Canada. At Western University Craig met Margarita
Crespo. They share a common interest in organoplatinum chemistry
and have been working together for many years. Michael Weinstein
represents the 44th undergraduate student he has mentored over a 12-
year academic career at Bard College (an undergraduate college with
no graduate program, but a strong undergraduate research program in
chemistry). Professor Anderson is the Wallace Benjamin Flint and L.
May Hawver Professor of Chemistry at Bard College and is currently a
Henry Dreyfus Teacher-Scholar, which was awarded for his research
with undergraduates at Bard College.
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