Carbosilane dendrimers containing complexes N,N′-pyridylimine of molybdenum and platinum at their periphery

Article abstract of DOI:10.1016/j.jorganchem.2007.10.050

Pyridylimine ligands of general formula CS-{O-4-(2,5-C₆H₂R₂)-N{double bond, long}CH-2-Py}_n, where CS is a trimethylsilyl group (n = 1, R = H, Ia or Me, Ib) or a carbosilane dendritic framework (IIa,b, n = 4; IIIa, n = 8), have been coordinated to platinum(II) and molybdenum(0) centers to give the mononuclear [(Ia,b){PtCl₂}], tetranuclear [(IIb){PtCl₂}₄] and [(IIa){Mo(CO)₃(MeCN)}₄], and octanuclear [(IIIa){Mo(CO)₃(MeCN)}₈] complexes. The poor solubility of the polymetallic platinum compounds impedes the preparation of higher-generation dendrimers, although such a limitation is not found in the case of the more soluble molybdenum dendrimers.

Full text of DOI:10.1016/j.jorganchem.2007.10.050

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Journal of Organometallic Chemistry 693 (2008) 278–282
www.elsevier.com/locate/jorganchem
Carbosilane dendrimers containing complexes N,N0-pyridylimine
of molybdenum and platinum at their periphery
*
*
´
´
´
Jose M. Benito, Ernesto de Jesus , F. Javier de la Mata, Juan C. Flores , Rafael Gomez
Departamento de Quı´mica Inorga´nica, Universidad de Alcala´, Campus Universitario, 28871 Alcala´ de Henares, Madrid, Spain
Received 6 July 2006; received in revised form 30 October 2007; accepted 30 October 2007
Available online 4 November 2007
Abstract
Pyridylimine ligands of general formula CS-{O-4-(2,5-C6H2R2)-N@CH-2-Py}n, where CS is a trimethylsilyl group (n = 1, R = H, Ia
or Me, Ib) or a carbosilane dendritic framework (IIa,b, n = 4; IIIa, n = 8), have been coordinated to platinum(II) and molybdenum(0)
centers to give the mononuclear [(Ia,b){PtCl2}], tetranuclear [(IIb){PtCl2}4] and [(IIa){Mo(CO)3(MeCN)}4], and octanuclear [(IIIa)
{Mo(CO)3(MeCN)}8] complexes. The poor solubility of the polymetallic platinum compounds impedes the preparation of higher-
generation dendrimers, although such a limitation is not found in the case of the more soluble molybdenum dendrimers.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: N ligands; Dendrimers; Molybdenum; Platinum
1. Introduction
Nickel and palladium complexes with N,N0-pyridylimine
ligands have been used as catalysts in different polymeriza-
tion processes [12], and, recently, we have synthesized a ser-
ies of dendrimers containing such metal complexes and
found that the dendrimer generation affects the nature of
the polymerization products [13]. Platinum complexes con-
taining the same type or closely related ligands have played
an important role in the study of C–H bond activation [14],
or showed promising results as anticancer agents [15].
Other reported applications of N,N0-pyridylimine ligands
include the immobilization of molybdenum carbonyls or
copper halides for solid-phase organometallic synthesis
and radical polymerization, respectively [16].
Since high metal loadings in metallodendrimers,
together the presence of the dendritic matrix itself, might
bring about uncovered behaviors in practical applications
(e.g., therapeutic agents), we set out to explore the chemis-
try of platinum carbosilane dendrimers. Here, we describe
the synthesis and characterization of new carbosilane
dendrimers decorated with N,N0-pyridylimine complexes
of molybdenum or platinum at their periphery. Related
to this work, there are a few reports in the literature
describing alkylpyridyliminopalladium moieties bound to
the periphery of poly(propyleneimine) – also known as
Dendrimers form a special type of materials because of
the nanosized, hyperbranched, and well-defined nature
of their macromolecular structures [1]. Functionalization
of the core, branches, or periphery of such macromolecules
with transition metals or organic synthons has attracted
much attention for applications in areas such as catalysis
[2,3] or in the biomedical arena [4]. In our ongoing pro-
gram in this field, we have contributed to the chemistry
of dendrimers, mostly of a carbosilane nature, that contain
early transition metal complexes bonded to the dendritic
periphery or focal point through aryloxy [5], siloxy [6], or
cyclopentadienyl ligands [7], or using N-donor anchoring
ligands such as diketiminato [8], imido [9], or scorpionate
linkages [10]. Additionally, we have evidenced the good
biocompatibility of ammonium-functionalized carbosilane
dendrimers and their capability to form dendriplexes with
oligonucleotides, making them potential candidates as
drugs nanocarriers [11].
*
Corresponding authors. Tel.: +34 91 8854607; fax: +34 91 8854683.
E-mail address: juanc.flores@uah.es (J.C. Flores).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.10.050

J.M. Benito et al. / Journal of Organometallic Chemistry 693 (2008) 278–282
279
DAB – dendrimers [17], or iron-bis(imino)pyridyl-termi-
nated carbosilane dendrimers [18].
and polymetallated polymers derived from this type of
complexes have already been prepared [16a]. Accordingly,
molybdenum dendrimers 3a and 4a were efficiently pre-
pared by treatment of [Mo(CO)3(1,3,5-C6H3Me3)] in aceto-
nitrile with the dendritic ligands IIa or IIIa. The initial
pale-yellow solutions turned deep blue immediately and
complexes 3a and 4a were isolated from these solutions
as purple solids that are soluble in polar solvents such as
acetonitrile and fairly air-stable, although decomposing in
long exposures to air. The complete metalation of the den-
2. Results and discussion
The dichloroplatinum complexes 1a,b were prepared in
good yields by reaction of the corresponding pyridylimine
ligand Ia,b with a mixture of cis and trans isomers of
[PtCl2(SMe2)2] in dichloromethane (Scheme 1) [19]. They
were isolated as orange or red solids, that are quite air-sta-
ble but that should be stored under an inert atmosphere for
prolonged storage. Complex 1a is slightly whereas 1b is
scarcely soluble in chlorinated solvents but both complexes
are insoluble in alkanes, diethyl ether, or toluene. The posi-
tive-ion electrospray mass spectra (ESI+/MS) of com-
pounds 1a,b were measured in acetonitrile and exhibit the
fragments due to the replacement of a chloride by an ace-
tonitrile molecule [MꢀCl+MeCN]+, thus showing the
characteristic ionization reported for related group 10
metal complexes [13,20]. While related palladium com-
plexes were previously prepared by the displacement of a
coordinated COD ligand (COD = 1,5-cyclooctadiene)
[13c], the reaction of [PtX2(COD)] (X = Cl, I) with Ia,b
afforded complicated mixtures of compounds in agreement
with the relative resistance toward diolefin dissociation of
platinum(II) compared with palladium(II) complexes [21].
In the reaction of IIb and [PtCl2(SMe2)2], the tetranu-
clear platinum 2b precipitated out from the CH2Cl2 solu-
tion as a red solid insoluble in all common solvents. The
elemental analysis and IR spectrum of the solid are consis-
tent with the coordination of four platinum moieties per
molecule. The solubility of these pyridylimine complexes
and dendrimers generally decreases when the number of
methyl substituents at the pyridylimine ligand is reduced
or the dendrimer generation is increased [13]. The lack of
solubility rends complete metalation of the dendrimer
periphery difficult, as observed in our attempts to prepare
2a or higher-generation derivatives.
1
dritic branches was confirmed by H and 13C NMR spec-
troscopy in CD3CN. Only resonances of uncoordinated
1
CH3CN molecules are observed in the H and 13C NMR
spectra, indicating exchange between the coordinated ace-
tonitrile and the deuterated solvent. The three carbonyl
ligands are coordinated fac to the octahedral metal center
as demonstrated by the appearance of three carbonyl reso-
nances in the 13C NMR spectra and the observation of two
m(CO) IR absorptions at around 1900 and 1780 cmꢀ1
(a1 + e modes). The IR spectra of complexes 1–4 in KBr
pellets also show a medium-to-strong absorption at around
1575 cmꢀ1, together with a much weaker band at around
1610 cmꢀ1 for Pt, or 1590 cmꢀ1 for Mo instead of the
group of three intense absorptions observed in the range
1565–1627 cmꢀ1 for the corresponding free ligands I–III.
The disappearance or shift of the C@N vibration in the
IR spectra of related complexes have been attributed to a
reduction of electron density in the C@N bonds after coor-
dination of the ligand to the metal center [12d].
Important CIS effects (CIS = coordination-induced
shift) are observed for the proton and carbon-13 NMR res-
onances of the pyridylimine ligands after coordination to
the metal centers, especially for the imine group and the
pyridine ring (Table 1). The negative CIS effect observed
for H9 (Dd ꢁ ꢀ0.2 to ꢀ0.4 ppm) is a result of the change
from the transoid conformation of the free ligands in solu-
tion, where H9 is in the proximity of the imine lone-pair, to
the cisoid conformation required to coordinate to the metal
center (Scheme 1) [16a,22]. The negative CIS effect
observed for the imine proton (H7) in the palladium ana-
The molybdenum(0) complex [Mo(CO)3(MeCN)(Ia)],
reported by Heinze, is much more soluble in polar solvents
n [PtCl2(SMe3)2] or
R
N
R
n [Mo(CO)3(1,3,5-C6H3Me3)]
Carbosilane
O
N
Carbosilane
O
N
N
CH2Cl2 or MeCN
[M]
R
R
n
n
n = 1
n = 4
n = 8
I
II
III
1–4
a: R = H
b: R = Me
R
R
Me
Me
Si
Me3Si
O
N
N
Si
O
N
N
O
N
N
Si
Si
Si
Me
Me
[M]
[M]
4
Me
[M]
2 4
R
R
[M] = PtCl2 1a,b
[M] = PtCl2 2b
[M] = Mo(CO)3(MeCN) 3a
[M] = Mo(CO)3(MeCN) 4a
Scheme 1.

280
J.M. Benito et al. / Journal of Organometallic Chemistry 693 (2008) 278–282
Table 1
Relevant CIS effects observed by NMR spectroscopy for compounds 1, 3 and 4a
9
10
2 R'13
3
7
8
11
12
4
1
RO
N
N
6
14R' 5
R' = H (1a, 3a, 4a), Me (1b)
Complex
Ddb
H12
C12
H11
C11
H10
C10
H9
C9
H7
C7
+8.2
+10.5
+4.8
+4.8
1a
1b
3a
4a
a
+1.03
+1.05
+0.31
+0.31
+0.2
+0.9
+3.1
+3.2
+0.34
+0.42
+0.18
+0.18
+3.2
+4.3
+2.9
+2.8
+0.43
+0.43
+0.22
+0.21
+2.7
+2.9
+2.0
+2.2
ꢀ0.36
ꢀ0.27
ꢀ0.22
ꢀ0.23
+6.7
+6.5
+7.7
+7.7
+0.00
+0.25
+0.04
+0.01
In CDCl3 at 20 ꢁC.
Dd in ppm with respect to the corresponding free ligands I–III.
b
logues (Dd ꢁ ꢀ0.3 ppm) [13c] becomes zero or positive in
the platinum and molybdenum complexes 1–4. On the
other hand, the largest coordination shifts of the H12 pro-
ton in platinum complexes (Dd ꢁ +1.0) is ascribed to the
strong deshielding effect of the chloro ligand cis to the pyr-
idyl ring. The most affected protons in the molybdenum
complexes are H12 and H2,6 (Dd ꢁ +0.3 ppm) probably as
a result of a deshielding contribution of the carbonyl
groups coplanar with the chelate ligand. The most signifi-
cant CIS effects in the 13C NMR spectra are found for
C9, and the imine carbon C7 (Table 1).
to the literature procedures. Solvents were dried prior to
use and distilled under argon as described elsewhere [26].
NMR spectra were recorded with Varian Unity VR-300
or Varian Unity 200 NMR spectrometers. Chemical shifts
(d) are reported in ppm relative to SiMe4, and were mea-
1
sured relative to the 13C and residual H resonances of
the deuterated solvents. Assignments for the pyridylimine
fragments are given according to the numbering of the
1
positions depicted in Table 1. For dendrimers, H NMR
integrals are given relative to one of the four arms of the
molecule. IR spectra were recorded with a Perkin–Elmer
FT-IR Spectrum-2000 spectrophotometer. Elemental anal-
yses were performed by the Microanalytical Laboratories
3. Conclusion
´
of the University of Alcala with a Heraeus CHN-O-Rapid
We have described the preparation of carbosilane den-
drimers containing PtCl2 and Mo(CO)3(MeCN) moieties
coordinated to pyridylimine ligands linked at the dendritic
periphery. We have previously reported the synthesis of
NiBr2, PdCl2, and PdClMe analogues. The procedures
used for the preparation of metal-pyridylimine dendrimers
are efficient, with the only limitation being the solubility of
the final products. Thus, pure dendrimers of the less solu-
ble complexes, for example platinum compounds, can only
be obtained in the lowest generations, however, such an
obstacle is not met for carbonyl molybdenum dendrimers
described here. Therefore, modification of the ligand and/
or the complex, or the dendritic matrix itself, will be
required to obtain more soluble compounds in the case
of platinum.
or a Perkin–Elmer 2400 Serie II C, H, N, S/O microanalyz-
er. Mass spectra were recorded with a Thermo Quest Fin-
ningan Automass Multi mass spectrometer.
4.2. Preparation of [PtCl2(Me3SiO-4-C6H4-N@CH-2-Py)]
(1a)
[PtCl2(SMe2)2] (80 mg, 0.20 mmol) was added to a
solution of ligand Ia (60 mg, 0.22 mmol) in dichloro-
methane (10 mL), and the resulting reaction mixture
was stirred at room temperature for 12 h. Removal of
the solvent, followed by washing of the residue with pen-
tane (4 · 10 mL) gave compound 1a as an orange solid.
Yield: 85 mg (80%). Anal. Calc. for C15H18Cl2N2OPtSi
(536.39): C, 33.59; H, 3.38; N, 5.22. Found: C, 33.89;
H, 3.50; N, 5.44%. 1H NMR (CDCl3): d 0.28 (s, 9H,
SiMe3), 6.89 (AA0 part of an AA0XX0 spin system, 2H,
H3,5), 7.40 (XX0 part of an AA0XX0 spin system, 2H,
H2,6), 7.64 (pt, 1H, H11), 7.80 (d, JH,H = 7.9 Hz, 1H,
H9), 8.19 (pt, 1H, H10), 8.60 (s, 1H, H7), 9.70 ppm (d,
JH,H = 6.3 Hz, 1H, H12). 13C{1H} NMR (CDCl3): d 0.4
(SiMe3), 119.4 (C3,5), 125.9 (C2,6), 128.3 (C9), 128.4
(C11), 139.6 (C10), 140.7 (C1), 149.7 (C12), 154.6 (C4),
157.1 (C8), 167.3 ppm (C7). IR (KBr): m 1610 (w),
1570 cmꢀ1 (s, C@N). MS (ESI+ in CH3CN): m/z 542
[MꢀCl+CH3CN]+.
4. Experimental
4.1. Reagents and general techniques
All operations were performed under argon using
Schlenk or dry-box techniques. Unless otherwise stated,
reagents were obtained from commercial sources and
used as received. [Mo(CO)3(1,3,5-C6H3Me3)] [23], [PtCl2-
(SMe2)2] [24], [PtX2(COD)] (X = Cl, I) [25], and the pyri-
dylimine ligands I–III [13,16a] were prepared according

J.M. Benito et al. / Journal of Organometallic Chemistry 693 (2008) 278–282
281
4.3. Preparation of [PtCl2{(Me3SiO-4-(2,5-C6H2Me2)-
N@CH-2-Py)}] (1b)
156.5 (C8), 163.0 (C7), 218.6 (CO trans to MeCN), 228,6
and 230.8 ppm (CO cis). 29Si{1H} NMR (CD3CN): d
21.5 (SiMe2), 2.1 ppm (central Si). IR (KBr): m 1903 and
1774 (vs. CO), 1593 cmꢀ1 (s, C@N and py-ring).
Compound 1b was synthesized as described for 1a from
Ib (144 mg, 0.48 mmol) and [PtCl2(SMe2)2] (173 mg,
0.44 mmol) as starting substrates, and was isolated as a
red solid. Yield: 195 mg (78%). Anal. Calc. for
C17H22Cl2N2OPtSi (564.45): C, 36.17; H, 3.93; N, 4.96.
4.6. Preparation of [(IIIa){Mo(CO)3(MeCN)}8] (4a)
Compound 4a was synthesized as described above for
3a, starting from dendrimer IIIa (275 mg, 0.10 mol) and
[Mo(CO)3{1,3,5-C6H3Me3}] (241 mg, 0.80 mmol), and
was isolated as a purple solid. Yield: 307 mg (68%). Anal.
Calc. for C192H228Mo8N24O32Si13 (4516.7): C, 51.06; H,
5.09; N, 7.44. Found: C, 50.03; H, 5.50; N, 6.99%. 1H
NMR (CD3CN): d ꢀ0.08 (s, 3H, SiMe), 0.23 (s, 12H,
SiMe2), 0.59 (m, 8H, SiCH2), 0.84 (m, 4H, CH2SiMe2),
1.27 and 1.42 (2 · m, 2H and 4H, CH2CH2CH2), 1.93 (s,
3H, CH3CN), 6.90 (AA0 part of an AA0XX0 spin system,
2H, H3,5), 7.48 (m, 1H, H11), 7.54 (XX0 part of an AA0XX0
spin system, 2H, H2,6), 7.91 (m, 1H, H9), 7.96 (m, 1H, H10),
8.59 (bs, 1H, H7), 8.98 ppm (bs, 1H, H12). 13C{1H} NMR
(CD3CN): d ꢀ4.4 (SiMe), ꢀ0.9 (SiMe2), 18.5, 19.1, 19.6,
21.2 and 21.9 (CH2), 120.9 (C3,5), 124.5 (C2,6), 127.5
(C11), 129.2 (C9), 138.6 (C10), 146.5 (C1), 152.6 (C12),
155.5 (C4), 156.5 (C8), 162.9 (C7), 218.5 (CO trans to
MeCN), 228.6 and 230.8 ppm (CO cis). 29Si{1H} NMR
(CD3CN): silent after 48 h. IR (KBr): m 1903 and 1789
(vs, CO), 1593 cmꢀ1 (s, C@N and py-ring).
1
Found: C, 36.61; H, 3.89; N, 5.44%. H NMR (CDCl3):
d 0.27 (s, 9H, SiMe3), 2.09 (s, 3H, Me14), 2.33 (s, 3H,
Me13), 6.60 (s, 1H, H3), 6.93 (s, 1H, H6), 7.73 (pt, 1H,
H11), 7.94 (d, JH,H = 7.7 Hz, 1H, H9), 8.20 (pt, 1H, H10),
8.76 (s, 1H, H7), 9.71 ppm (d, JH,H = 6.6 Hz, 1H, H12).
13C{1H} NMR (CDCl3): d 0.7 (SiMe3), 16.2 and 18.4
(Me13,14), 120.3 (C3), 125.5 (C6), 126.8 (C5), 127.7 (C9),
128.8 (C11), 130.2 (C2), 139.3 (C10), 140.2 (C1), 150.3
(C12), 154.0 (C4), 156.4 (C8), 168.0 ppm (C7). IR (KBr): m
1615 (w), 1580 cmꢀ1 (m, C@N). MS (ESI+ in CH3CN):
m/z 570 [MꢀCl+CH3CN]+.
4.4. Preparation of [(IIb)(PtCl2)4] (2b)
[PtCl2(SMe2)2] (70 mg, 0.18 mmol) was added to a
solution of the carbosilane dendrimer IIb (60 mg,
0.045 mmol) in dichloromethane (15 mL), and the reac-
tion mixture was stirred at room temperature for 18 h.
A red solid precipitated during the course of the reaction.
Removal of the solvent was followed by washing of the
crude residue with pentane (2 · 30 mL) to yield
compound 2b as a red solid that is insoluble in common
solvents. Yield: 80 mg (74%). Anal. Calc. for C76H100Cl8-
N8O4Pt4Si5 (2394.1): C, 38.13; H, 4.21; N, 4.68. Found:
C, 38.30; H, 4.21; N, 5.06%. IR (KBr): m 1615 (w),
1571 cmꢀ1 (m, C@N).
Acknowledgements
We gratefully acknowledge financial support from the
´
Spanish Ministerio de Educacion y Ciencia (Project
CTQ2005-00795/BQU) and the Comunidad de Madrid
(Project S-0505/PPQ/0328-03).
4.5. Preparation of [(IIa){Mo(CO)3(MeCN)}4] (3a)
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