Dinuclear organoplatinum(II) complexes containing N-methylbenzamide

Article abstract of DOI:10.1139/V08-116

The preparation and characterization of cationic, dinuclear complexes of the type trans-[Pt(PPh₃)₂(σ C₆H ₄NHMe)-μ-L-Pt(PPh₃)₂(σ-C ₆H₄NHMe)](OTf)₂(L = 4,4′-b

Full text of DOI:10.1139/V08-116

2
12
Dinuclear organoplatinum(II) complexes containing
1
N-methylbenzamide
Michael G. Crisp and Louis M. Rendina
Abstract: The preparation and characterization of cationic, dinuclear complexes of the type trans-[Pt(PPh ) (s-
3
2
C H NHMe)-m-L-Pt(PPh ) (s-C H NHMe)](OTf) (L = 4,4¢-bipyridine, 4,7-phenanthroline, 4,4¢-dipyrazolylmethane, and
6
4
3 2
6
4
2
3
1
,1¢-diphenyl-4,4¢-dipyrazolylmethane; OTf = trifluoromethanesulfonate (triflate)) containing two C -N-methylbenzamide
ligands are described. The key structural feature of the cationic complexes is the presence of two convergent amide
groups that may allow for charge-assisted, H-bonding interactions in solution with suitable heteroaromatic guest mole-
cules such as nucleobases.
Key words: organoplatinum(II) complex, organometallic host, dinuclear complex, N-methylbenzamide.
Résumé : On décrit la préparation et la caractérisation de complexes cationiques dinucléaires du type trans-[Pt(PPh3)2
(
1
s-C H NHMe)-m-L-Pt(PPh ) (s-C H NHMe)](OTf) [L= 4,4¢-bipyridine; 4,7-phénanthroline; 4,4¢-dipyrazolylméthane; et
,1¢-diphényl-4,4¢-dipyrazolylméthane; OTf = trifluorométhanesulfonate (triflate)] contenant deux ligands C -N-méthyl-
6
4
3 2
6
4
2
3
benzamide. La caractéristique structurale clé des complexes cationiques est la présence de deux groupes amides conver-
gents qui, en solution, peuvent favoriser des interactions de liaisons hydrogènes assistées par des charges avec des
molécules hôtes hétéroaromatiques appropriées, telles des nucléobases.
Mots-clés : complexe organique du platine(II), hôte organométallique, complexe dinucléaire, N-méthylbenzamide.
[
Traduit par la Rédaction] Crisp and Rendina 216
Introduction
its derivatives. Theoretical studies have also complemented
the work (10, 11).
There is great interest in the study of synthetic receptors for
the selective binding of nucleobases (1, 2), owing to their
many important functional roles in biological systems, e.g.,
intracellular energy transfer, signal transduction, and nucleic-
acid synthesis. Various modes of hydrogen-bonding recogni-
tion have been shown to occur between nucleobases and
simple carboxylic acids, for example, and it has been demon-
strated that many aromatic carboxylic acids preferentially
bind Hoogsteen sites, whereas aliphatic carboxylic acids bind
preferentially at the Watson–Crick site (3, 4). In addition to
hydrogen-bonding, p-stacking is usually found to augment the
interaction involving aromatic hosts and nucleobase guests,
and both macrocyclic and non-macrocyclic molecular recep-
tors containing hydrogen-bonding and (or) p-stacking binding
domains have been studied previously (5–18), all of which
have the capacity to target nucleobases such as adenine and
Macrocyclic nucleobase receptors can discriminate their
guest molecule by attributes such as size, electronic proper-
ties, nature of the hydrogen-bonding groups, and the p-
stacking surface area (5, 6, 12). In contrast, non-macrocyclic
“
molecular tweezers” possess two binding sites with conver-
gent functionality, which are linked together by a spacer unit
7–9, 13–18). Molecular tweezers usually possess one or
(
more hydrogen-bonding functionalities, such as carboxylic
acids or amides, and, in some cases, converging aromatic
surfaces that have the capacity to undergo p-stacking inter-
actions. Organometallic complexes in which direct coordina-
tion of adenine to the metal centre is augmented by H-
bonding and p-stacking interactions are also known (19, 20),
but to our knowledge there exist no examples of where metal
coordination does not play a key role in the recognition mo-
tif. Instead, the formation of strong H-bonding interactions
with the guest molecule is the result of the complementary
shape of the cationic host, which can be further augmented
by charge assistance (21).
Received 5 May 2008. Accepted 14 June 2008. Published on
the NRC Research Press Web site at canjchem.nrc.ca on
1
6 September 2008.
We have previously reported the preparation, pK data,
a
Dedicated to Prof. R.J. Puddephatt, FRS, on the occasion of
his retirement and in recognition of his many outstanding
contributions to organometallic chemistry.
and X-ray structures of s-arylplatinum(II) complexes bear-
ing carboxylic acid functionalities (22–24) some of which
have been shown to possess interesting supramolecular prop-
erties in non-aqueous solution, e.g., the formation of dis-
crete, multimeric cyclic arrays with nanoscale dimensions
M.G. Crisp. School of Chemistry and Physics, The
University of Adelaide, Adelaide SA 5005, Australia.
2
L.M. Rendina. School of Chemistry, The University of
(
24). The use of a protection–deprotection strategy was re-
Sydney, Sydney, NSW 2006, Australia.
quired in the oxidative addition reactions involving aryl io-
dides bearing a carboxylic acid functionality owing to their
high reactivity with platinum(0) precursor compounds. Fur-
thermore, in some cases, we found that carboxylic acids
1This article is part of a Special Issue dedicated to Professor
R. Puddephatt.
2
Corresponding author: (e-mail: rendina@chem.usyd.edu.au).
Can. J. Chem. 87: 212–216 (2009)
doi:10.1139/V08-116
© 2008 NRC Canada

Crisp and Rendina
213
Scheme 1.
readily H-bonded to counter-ions (e.g., triflate) both in low-
polarity solvents and in the solid-state, thus markedly com-
plicating the assembly and (or) molecular-recognition stud-
ies (22, 23). The use of N-substituted s-benzamide ligands
instead of pyridine carboxylic acids was expected to address
many of these issues largely because of the markedly lower
acidity of the amide N–H group compared with that of a
carboxylic acid. Herein, we describe the synthesis and char-
acterization of dinuclear organoplatinum(II) derivatives
bearing convergent N-methylamide functionalities for poten-
tial application as molecular receptors for nucleobases.
as a distinct signal, which is assigned to the XX¢ portion of a
AA¢XX¢ spin system. The NMR spectra of 3 were measured
in CD Cl solution instead of CDCl as the complex was
2
2
3
found to dissociate to afford free 4,7-phenanthroline over a
period of a few hours at room temperature in CDCl but re-
3
mained stable for indefinite periods in CD Cl solution. This
2
2
effect has been previously observed in the ESI-MS of these
and related complexes (vide infra) (22–25), and it is most
probably the result of the strong trans effect associated with
the s-aryl ligand. Interestingly, a Pd hexagon prepared from
6
4,7-phenanthroline and dinuclear organopalladium(II) pre-
cursor complexes appears to remain intact both in solution
and in the gas phase despite the presence of a s-aryl ligand
located trans to the coordinated N-atom (26).
Results and discussion
The oxidative addition of the C–I bond of 3-iodo-N-
The assignment of the aromatic proton resonances in 2–5
was facilitated by 2D H COSY (correlation spectroscopy)
1
methylbenzamide to Pt(PPh ) proceeded cleanly to afford
3
4
the iodoplatinum(II) complex 1 in good yield (Scheme 1).
The H NMR spectrum of 1 displayed four distinct signals at
NMR experiments at 600 MHz, whereby distinct cross-peaks
were observed for coupled protons of the s-aryl ring systems, as
seen in the NMR spectrum of 4 (Fig. 1). For this complex, pro-
1
d 6.67, 6.72, 6.24, and 6.96 ppm, which are assigned to the
2
4
5
6
4
6
H , H , H , and H protons of the s-aryl ligand, respectively.
tons H and H appeared as doublets at d 6.77 and d 7.00 ppm,
5
3
The methyl protons of the NHMe group appeared as a dou-
respectively. The H proton appeared as a triplet ( JHH =
3
2
blet at d 2.76 ppm ( J = 4.8 Hz), and the NH proton ap-
7.2 Hz) at d 6.35 ppm. The signal due to H appeared as a
HH
peared as a broad multiplet at d 4.98 ppm. Complex 1 was
readily converted to the labile triflato derivative by the use
of AgOTf. Two equivalents of this species were then treated
immediately with 1 equiv. of the bridging bidentate, N-donor
ligand 4,4¢-bipyridine, 4,7-phenanthroline, 4,4¢-dipyrazolyl-
methane, or 1,1¢-phenyl-4,4¢-dipyrazolylmethane to form the
desired diplatinum(II) complexes (2–5) in good yield and
purity (Scheme 1).
sharp singlet at d 6.10 ppm. The N–Me protons appeared as a
3
doublet at d 2.83 ppm ( JHH = 4.8 Hz), and the amide N–H
proton gave rise to a broad singlet at d 6.01 ppm. The signal
due to the methylene bridge protons appeared as a sharp sin-
glet at d 2.53 ppm, and the N–H signal of the pyrazolyl ring
was located downfield at d 12.45 ppm.
1
Compared to the H NMR spectrum of complex 4, the re-
lated complex 5 had many broad unresolved signals, with all
the s-aryl signals appearing as broad singlets. This effect is
most likely related to the dynamic phenomena of analogous
platinum(II) species (23), whereby free rotation of the 1,1¢-
diphenyl-4,4¢-dipyrazolylmethane ligand about the Pt–N
bond is restricted owing to the considerable steric interac-
1
Complex 2 displayed a doublet signal in the H NMR
3
spectrum at d 7.03 ppm ( J = 7.5 Hz), which is attributed
HH
6
to the H proton of the s-aryl ligand. The signal due to the
H proton appeared as a doublet at d 6.81 ppm ( J
.5 Hz), and the H proton gave rise to a triplet signal at
d 6.39 ppm ( J = 7.8 Hz). The ortho protons of the 4,4¢-
4
3
=
HH
5
7
3
tions between the N–Ph group and the bulky PPh ligands.
HH
3
bipyridine were masked by the PPh3 signals at d 7.34–
This proposal is also supported by the fact that the related
4,4¢-dipyrazolylmethane complex 4 does not exhibit this re-
7
.19 ppm, whilst the meta hydrogens appeared at d 6.83 ppm
©
2008 NRC Canada

2
14
Can. J. Chem. Vol. 87, 2009
1
Fig. 1. Expanded H COSY NMR spectrum (600 MHz) of 4 in CDCl solution showing key couplings and peak assignments for the
3
s-aryl protons.
lated NMR behaviour, suggesting that in this case there ex-
ists free rotation around the Pt–N bond.
of the amide N–H group ensures that unwanted side reac-
tions involving the N–H bond do not occur, and C–I bond
cleavage is favoured exclusively. We are in the process of
investigating the “molecular chelation” properties of se-
lected complexes with various guest molecules, such as de-
rivatives of purine bases, including adenine and guanine.
Indeed, preliminary molecular modelling studies (SYBYL
force field) demonstrate that a stable 1:1 species between 4
and adenine is feasible in the gas phase in which the
All complexes prepared in this work displayed a sharp
3
1
1
singlet between d 20–25 ppm in the P{ H} NMR spectrum,
1
95
and it was always flanked by
Pt satellite signals. The
1
magnitude of J (~3000 Hz) for all complexes falls in the
PtP
range that is expected for trans-substituted s-arylplatinum(II)
complexes (28, 29). Only minor differences were observed
1
in the magnitude of J upon substitution of the iodo ligand
PtP
1
95
31
with N-donor ligands, largely because the Pt– P coupling
is predominantly influenced by the nature of the trans ligand
exocyclic NH , N-7, and N-1 atoms of the guest molecule
2
are all involved in strong H-bonding interactions with both
amide groups of the complex, and we are currently investi-
gating the host–guest chemistry of 2–5 and related com-
plexes in various solvents. The results of this work will be
reported in due course.
(
PPh3).
Positive ion ESI-MS studies failed to show any evidence
of an intact species in the spectra of 2–5 and, instead, the N-
donor ligands (L) were readily lost, and peaks corresponding
+
to [M–OTf–L] were observed. The strong trans-effect asso-
ciated with the s-aryl ligand would account for the facile
labilization of the N-donor ligand in the gas phase, and simi-
lar behaviour has also been observed in the ESI-MS of re-
lated systems (22–25).
Experimental
All complexes were synthesised under a N atmosphere
using standard Schlenk techniques. All solvents were dried
and purified in the following manner: CH Cl was distilled
2
2
2
from CaH , and toluene was pre-dried over CaSO followed
by distillation from sodium. The solvents were stored under
2
4
Conclusion
In this work, we have demonstrated the facile synthesis of
organplatinum(II) complexes bearing convergent H-bonding
amide functionalities by exploiting an oxidative addition re-
action involving a platinum(0) precursor. The low reactivity
N over 4 Å molecular sieves.
All 1D NMR spectra were recorded at 298 K by means of
2
a Varian Gemini 2000 NMR spectrometer with an Oxford
1
31
300 MHz magnet ( H at 300.10 MHz, P at 121.50 MHz);
©
2008 NRC Canada

Crisp and Rendina
215
1
3JHH = 7.8 Hz, H ), 6.38 (t, 2H, J = 8.1 Hz, H ), 5.96
14
3
15
2
6
D NMR ( H COSY) spectra were recorded by using a
HH
1
3
31
1
00 MHz magnet. H NMR chemical shifts were reported in
(br s, 2H, NH), 2.84 (d, 6H, J = 4.8 Hz, NMe). P{ H}
NMR (CD Cl ) d: 19.5 ( J = 3003 Hz). Anal. calcd. for
C102H F N O P Pt S : C, 56.04; H, 3.87; N, 2.56. Found:
C, 55.94; H, 3.45; N, 2.18.
HH
3
1
1
1
ppm relative to tetramethylsilane (TMS). P{ H} NMR
spectra were referenced to sealed external standard of 85%
phosphoric acid. IR spectra were recorded as Nujol mulls in
the range 4000–400 cm–1 on a PerkinElmer FTIR spectro-
photometer. ESI-MS were obtained by means of a Finnegan
LCQ mass spectrometer in the positive-ion mode using
HPLC-grade MeOH as the solvent. Elemental analysis was
performed by CMAS (Chemical and Microanalytical Ser-
vices, Pty. Ltd.), Belmont, Victoria.
2
2
PtP
84
6
4
8
4
2 2
Trans-␮-4,4¢-dipyrazolylmethanebis[(N-
methylbenzamide-C )bis(triphenylphosphine)plati-
num(II)] bis(triflate) (4)
3
Following a procedure similar to that described for 2,
complex 1 (0.400 g, 0.401 mmol) was treated with AgOTf
(0.103 g, 0.401 mmol) followed by the addition of 4,4¢-
dipyrazolylmethane (0.029 g, 0.201 mmol) to afford 4 as a
colourless, microcrystalline solid (0.369 g, 81%). IR
(Nujol): 1643 n(C=O), 3400 n(N–H) cm . H NMR
CDCl ) d: 12.45 (s, 2H, pyrazolyl NH), 7.45–7.25 (m, 60H,
PPh3), 7.04 (s, 2H, H ), 7.00 (d, 2H, JHH = 7.8 Hz, H ),
3
-Iodo-N-methylbenzamide
(27),
tetrakis(triphenyl-
phosphine)platinum(0) (30), 4,4¢-dipyrazolylmethane (31),
and 1,1¢-phenyl-4,4¢-dipyrazolylmethane (32) were prepared
and purified according to the literature procedures.
–
1
1
(
3
9
3
6
Trans-iodo(N-methylbenzamide-
C )bis(triphenylphosphine)platinum(II) (1)
-Iodo-N-methylbenzamide (0.110 g, 0.402 mmol) was
placed in a Schlenk flask with Pt(PPh3)4 (0.500 g,
.402 mmol). Toluene (20 mL) was added to the flask, and
3
3
2
6.77 (t, 2H, JHH = 7.2 Hz), 6.40 (s, 2H, H ), 6.35 (t, 2H,
3
5
10
3
JHH = 7.2 Hz, H ), 6.10 (s, 2H, H ), 6.01 (m, 2H, amide
3
NH), 2.83 (d, 6H, JHH = 4.8 Hz, NHMe), 2.53 (s, 2H, CH2).
3
1
1
1
0
P{ H} NMR (CDCl3) d: 20.8 ( JPtP = 3022 Hz). Anal.
the solution was stirred for 16 h at 80 °C. Hexane (20 mL)
was slowly added to the solution to precipitate the product,
which was collected by filtration to afford 1 as a colourless,
microcrystalline solid (0.344 g, 88%). IR (Nujol): 1632
calcd. for C97H84F6N6O8P4Pt2S2: C, 54.09; H, 3.93; N, 3.90.
Found: C, 53.98; H, 4.09; N, 3.71.
Trans-␮-(1,1¢-phenyl-4,4¢-dipyrazolylmethane)bis[(N-
–
1
1
3
n(C=O) cm . H NMR (CDCl ) d: 7.56–7.22 (m, 30H,
methylbenzamide- C )bis(triphenylphosphine)plati-
3
3
1
6
PPh ), 6.96 (d, 1H, J = 7.5 Hz, J = 29.3 Hz, H ), 6.72
d, 1H, J = 8.4 Hz, H ), 6.67 (s, 1H, J = 28.8 Hz, H ),
num(II)] bis(triflate) (5)
3
HH
PtH
3
4
1
2
(
6
(
2
5
Following a procedure similar to that described for 2,
complex 1 (0.400 g, 0.401 mmol) was treated with AgOTf
(0.103 g, 0.401 mmol) followed by the addition of 1,1¢-
phenyl-4,4¢-dipyrazolylmethane (0.060 g, 0.201 mmol) to af-
ford 5 as a colourless, microcrystalline solid (0.129 g, 54%).
HH
PtH
3
5
.24 (t, 1H, J = 7.8 Hz, H ), 4.98 (br m, 1H, NH), 2.76
HH
3
31
1
d, 3H, J = 4.8 Hz, NCH3). P{ H} NMR (CDCl ) d:
H
H
3
1
2.0 ( J = 3029 Hz). Anal. calcd. for C H INOP Pt: C,
PtP 44 38 2
3.89; H, 3.91; N, 1.43. Found: C, 53.37; H, 3.85; N, 1.45.
1
6
2
H NMR (CDCl ) d: 6.94 (br s, 2H, H ), 6.69 (br s, 2H, H ),
3
5
6
.58 (br s, 2H), 6.25 (br s, 2H, H ), 5.70 (m, 2H, NH), 3.11
Trans-␮-4,4¢-bipyridinebis[(N-methylbenzamide-
3
3
(
s, 2H, CH ), 2.84 (d, 6H, J = 4.5 Hz, NHMe), 7.64–7.28
C )bis(triphenylphosphine) platinum(II)] bis(triflate) (2)
2 HH
3
1
1
1
Complex 1 (0.400 g, 0.401 mmol) was treated with
(m, 60H, PPh3). P{ H} NMR (CDCl3) d: 25.0 ( JPtP =
3084 Hz). Anal. calcd. for C109H92F6N6O8P4Pt2S2: C, 56.77;
H, 4.02; N, 3.64. Found: C, 56.57; H, 4.09; N, 3.49.
AgOTf (0.103 g, 0.401 mmol) in CH Cl (25 mL), and the
2
2
reaction mixture was stirred at room temperature for 3 h in
the absence of light. AgI was then removed by filtration
through Celite filter aid. 4,4¢-Bipyridine (0.031 g, 0.201 mmol)
was added to the clear solution and then stirred overnight at
room temperature. The solvent was reduced in vacuo to af-
ford 2 as a colourless, microcrystalline solid (0.260 g, 60%).
Acknowledgements
We thank Mr Phil Clements (The University of Adelaide)
for assistance with the NMR experiments. We also thank
Johnson–Matthey for the generous loan of platinum salts.
The authors wish to acknowledge funding of the work by the
Australian Research Council (ARC).
–
1 1
IR (Nujol): 1655 n(C=O) cm . H NMR (CDCl ) d: 7.34–
3
3
6
7
(
.19 (m, 60H, PPh ), 7.03 (d, 2H, J = 7.5 Hz, H ), 6.83
3 HH
2
s, 2H, H ), 6.83 (XX¢ portion of AA¢ XX¢, 4H, H ), 6.81 (d,
H, J = 7.5 Hz, H ), 6.39 (t, 2H, J = 7.8 Hz, H ).
HH HH
P{ H} NMR (CDCl ) d: 20.9 ( J
m
3
4
3
5
2
3
1
1
1
= 3008 Hz). Anal.
References
3
PtP
calcd. for C H INOP Pt: C, 55.56; H, 3.92; N, 2.59.
4
4
38
2
1
. S.R. Perumalla, E. Suresh, and V.R. Pedireddi. Angew. Chem.,
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