Synthesis and characterization of organoplatinum dendrimers with 1,3,5-triethynylbenzene building blocks

Article abstract of DOI:10.1021/om980337y

The multinuclear complexes 1,3,5-[trans-I(Et₃P)₂PtC≡C]₃C ₆H₃ (1) and 1,2,4,5-[trans-I(Et₃P)₂PtC≡C]₄C ₆H₂ (2) were prepared via σ-acetylide synthetic methodologies by reaction of tri- and tetraethynylbenzene with diiodobis(triethylphosphine)platinum. The combination of this synthetic coupling strategy with the divergent synthetic principle for the formation of dendrimers led to organometallic first- and second-generation dendrimers with ethynyl-platinum units in the main chain. The outer spheres of the dendrimers consist of either terminal acetylenic groups or bis(triethylphosphine)platinum iodide groups. All compounds were fully characterized by elemental analysis, mass spectrometry, NMR, and vibrational spectroscopy. The single-crystal structure of a first generation dendrimer 3 with terminal acetylene groups was determined by X-ray analysis. It shows an out-of-plane twisting of all three terminal 1,3,5-triethynylbenzene groups as well as a twisting of one of the coordination planes around a platinum atom, relative to the central aromatic ring.

Full text of DOI:10.1021/om980337y

Organometallics 1998, 17, 3981-3987
3981
Syn th esis a n d Ch a r a cter iza tion of Or ga n op la tin u m
Den d r im er s w ith 1,3,5-Tr ieth yn ylben zen e Bu ild in g
Block s
Stefan Leininger* and Peter J . Stang*
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Songping Huang
Department of Chemistry, University of Puerto Rico, San J uan, Puerto Rico 00931
Received May 1, 1998
The multinuclear complexes 1,3,5-[trans-I(Et3P)2PtC≡C]3C6H3 (1) and 1,2,4,5-[trans-
I(Et3P)2PtC≡C]4C6H2 (2) were prepared via σ-acetylide synthetic methodologies by reaction
of tri- and tetraethynylbenzene with diiodobis(triethylphosphine)platinum. The combination
of this synthetic coupling strategy with the divergent synthetic principle for the formation
of dendrimers led to organometallic first- and second-generation dendrimers with ethynyl-
platinum units in the main chain. The outer spheres of the dendrimers consist of either
terminal acetylenic groups or bis(triethylphosphine)platinum iodide groups. All compounds
were fully characterized by elemental analysis, mass spectrometry, NMR, and vibrational
spectroscopy. The single-crystal structure of a first generation dendrimer 3 with terminal
acetylene groups was determined by X-ray analysis. It shows an out-of-plane twisting of
all three terminal 1,3,5-triethynylbenzene groups as well as a twisting of one of the
coordination planes around a platinum atom, relative to the central aromatic ring.
In tr od u ction
symmetry can have nonzero â values and are therefore
under investigation for potential NLO applications.6
In recent years there has been a growing interest in
the design of oligomeric and polymeric transition metal
complexes containing σ-bonded acetylene units because
of their significantly altered physical properties com-
pared to conjugated organic oligomers and polymers.1
The ability of the transition metal d orbitals to interact
with p orbitals of conjugated alkynyl ligands is one
major factor in stabilizing vinylidene intermediates2 as
well as catalyzing cross coupling reactions.3
Moreover, the electronic delocalization across the
transition metal center4 influences the magnetic and the
nonlinear optical properties and behavior.5 Although
the classical nonlinear optical (NLO) materials have
donor-bridge-acceptor character, it has recently been
shown that nonpolar molecules with a 3-fold rotation
Highly branched systems with metal centers on the
surface or within the dendrimers encompassing all these
characteristics are of considerable current interest.1c,5a,7
Furthermore, the attractive advantage of organometallic
dendrimers over their polymeric counterparts lies ad-
ditionally in their precise molecular architecture as well
as in their predetermined chemical composition.8 Al-
though many studies have been done on surface-
modified dendrimers,8,9 only a few organometallic den-
drimers with interior metal centers are known due to a
lack of appropriately designed building blocks.8,10 Re-
cent results show that numerous bimetallic and poly-
metallic complexes linked by rigid alkynyl and alkynyl-
aryl backbones with a large extent of π-delocalization
(1) (a) Lavastre, O.; Plass, J .; Bachmann, P.; Guesmi, S.; Moinet,
C.; Dixneuf, P. H. Organometallics 1997, 16, 184-189. (b) Manners,
I. Chem. Br. 1996, 32, 46-49. (c) Faust, R.; Diederich, F.; Gramlich,
V.; Seiler, P. Chem. Eur. J . 1995, 1, 111-117.
(2) (a) Touchard, D.; Morice, C.; Cadierno, V.; Haquette, P.; Toupet,
L.; Dixneuf, P. H. J . Chem. Soc., Chem. Commun. 1994, 859-860. (b)
Rappert, T.; Nu¨rnberg, O.; Mahr, N.; Wolf, J .; Werner, H. Organome-
tallics 1992, 11, 4156-4164.
(3) (a) Stang, P. J .; Kowalski, M. H. J . Am. Chem. Soc. 1989, 111,
3356-3362. (b) Onitsuka, K.; Yanai, K.; Takai, F.; J oh, T.; Takahashi,
S. Organometallics 1994, 13, 3862-3867. (c) Klein, H. F.; Zettel, B.;
Florke, U.; Haupt, H.-J . Chem. Ber. 1992, 125, 9-14.
(4) (a) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J . Am.
Chem. Soc. 1993, 115, 3276-3285. (b) Lichtenberger, D. L.; Renshaw,
S. K.; Wong, A. Organometallics 1993, 12, 3522-3526.
(5) (a) Whittall, I. R.; Humphrey, M. G.; Houbrechts, S.; Maes, J .;
Persoons, A.; Schmid, S.; Hockless, D. C. R. J . Organomet. Chem. 1997,
544, 277-283. (b) Blau, W. J .; Byrne, H. J .; Cardin, D. J .; Davey, A.
P. J . Mater. Chem. 1991, 1, 245-249. (c) Nalwa, H. S. Adv. Mater.
1993, 5, 341-358. (d) Kanis, D. R.; Ratner, M. A.; Marks, T. J . Chem.
Rev. 1994, 94, 195-242.
(6) (a) Wortmann, R.; Glania, C.; Kra¨mer, P.; Matschiner, R.; Wolff,
J . J .; Kraft, S.; Treptow. B.; Barbu, E.; La¨ngle, D.; Go¨rlitz, G. Chem.
Eur. J . 1997, 3 (11), 1765-1772. (b) Zyss, J .; Ledoux, I. Chem. Rev.
1994, 94, 77-105. (c) Stadler, S.; Bra¨uchle, C.; Brandle, S.; Gompper,
R. Chem. Mater. 1996, 8, 414-417.
(7) Balzani, V.; Campagna, S.; Denti, G.; J uris, A.; Serroni, S.;
Venturi, M. Acc. Chem. Res. 1998, 31, 26-34.
(8) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Mol-
ecules: Concepts, Syntheses, Perspectives; VCH: New York, 1996; pp
201-222.
(9) (a) Alonso, B.; Cuadrado, I.; Mora´n, M.; Losada, J . J . Chem. Soc.,
Chem. Commun. 1994, 2575-2576. (b) Seyferth, D.; Son, D. Y.;
Rheingold, A. L.; Ostrander, R. L. Organometallics 1994, 13, 2682-
2690. (c) Miedaner, A.; Curtis, C.J .; Barkley, R. M.; DuBois, D. L. Inorg.
Chem. 1994, 33, 5482-5490. (d) Hawker, C. J .; Frechet, J . M. J . J .
Chem. Soc., Chem. Commun. 1990, 1010-1013. (e) Liao, Y.-H.; Moss,
J . R. Organometallics 1995, 14, 2130-2132.
(10) (a) Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani, V.
J . Am. Chem. Soc. 1992, 114, 2944-2950. (b) Newkome, G. R.; Gu¨ther,
R.; Moorefield, C. N.; Cardullo, F.; Echegoyen, L.; Pe´rez-Cordero, E.;
Luftmann, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2023-2026.
S0276-7333(98)00337-9 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/07/1998

3982 Organometallics, Vol. 17, No. 18, 1998
Leininger et al.
Sch em e 1
and 2 was achieved by column chromatography on silica
gel and afforded the products as slightly yellow crystals.
In addition, all of the excess diiodobis(triethylphos-
phine)platinum was quantitatively recovered in analyti-
cally pure form and was therefore used in further
reactions.
Both platinum compounds were characterized by
NMR (1H, 31P{1H}, 13C{1H}), vibrational spectroscopy
(IR), and mass spectrometry (FAB-MS), as well as
elemental analysis and physical means (vide infra).
These multinuclear complexes are air- and moisture-
stable solids and are highly soluble in benzene, chloro-
form, and methylene chloride, insoluble in hexanes, and
partly soluble in diethyl ether. They can be stored over
a long period of time without decomposition.
Although, we never observed a partly substituted
product,5a the yield of the final complexes dropped with
smaller excesses of diiodobis(triethylphosphine)platinum.
A plausible explanation for this can be found in the high
reactivity of the remaining iodoplatinum bond, which
is still activated by the triethylphosphine ligands and
not sterically hindered. Therefore, competing oligomer-
ization reactions, which are well-known in this field of
chemistry,13,16,17 can occur and lower the overall yield.
The 31P{1H} NMR spectra of 1 and 2 are very similar
in that each compound shows one singlet resonance for
the triethylphosphine ligands at δ ) 11.3 (1) and 11.1
ppm (2), respectively. Compared to the resonance of the
starting material, the substitution of one iodo ligand
with an acetylenic unit results in a downfield shift of
almost 8 ppm. The phosphorus-platinum single-bond
spin-spin couplings of 2327 (1) and 2333 Hz (2) are in
the expected range for a trans-PtP2 system. The 1H
NMR spectra show only one single resonance in the
aromatic region, beside the signals for the triethylphos-
phine ligands, at δ ) 7.02 (1) and 7.10 (2), respectively,
strongly suggesting that the complexes 1 and 2 have a
symmetrical structure. No signals in the range for
terminal acetylenes were detected, excluding the exist-
ence of partly substituted products within the detection
limits of NMR.
F or m a tion of a F ir st-Gen er a tion Den d r im er via
a Diver gen t Str a tegy. The concept of the divergent
iterative strategy, defined by Moorefield and New-
kome,12 involves an initial monomer (core or zeroth-
generation) which possesses one or more reactive site(s),
and then a new generation of building blocks is added
to form the first-generation dendrimer. Continuing to
use this building block after transformation of the
terminal groups into functionalized groups will result
in the formation of the next generation dendrimer. The
number of terminal groups depends on the number of
reactive sites provided by the building blocks (Scheme
2).
Applying this strategy to our systems, we were able
to form the first-generation dendrimer 3 by reacting the
trinuclear complex 1 with an excess of 1,3,5-triethynyl-
benzene under the same reaction conditions already
described for the formation of complexes 1 and 2
(Scheme 3).
constitute the most efficient link between organome-
tallic building blocks.11
Herein we report the molecular assemblies of a
variety of neutral multinuclear platinum complexes
with a backbone of σ-bonded tri- and tetraethynylben-
zene units. In a stepwise divergent synthetic ap-
proach12 the synthesis of a first- and a second-generation
dendrimer is facilitated where the trans-bis(triethyl-
phosphine)platinum units serve as functionalities and
are covalently connected through conjugated organic
building blocks. The structure of the first-generation
dendrimer was established by an X-ray determination.
Resu lts a n d Discu ssion
P r epar ation ofth e P r ecu r sor s 1,3,5-[tr a n s-I(Et3P )2-
P tC≡C]3C6H3 (1) an d 1,2,4,5-[tr a n s-I(Et3P )2P tC≡C]4-
C6H2 (2). The polycondensation between metal halides
and acetylenic compounds has been shown to proceed
smoothly in the presence of a cuprous halide catalyst
and an amine base.13a This method developed for the
synthesis of polyyne polymers could successfully be
transferred to the formation of two-dimensional
systems.5a,11b,13b,14
Reaction of the tri- and tetraethynylbenzene with 9
and 12 equiv, respectively, of diiodobis(triethylphos-
phine)platinum in a mixture of toluene and diethyl-
amine generates, in the presence of cuprous(I) iodide,
the 1,3,5-tris- and 1,2,4,5-tetrakis-substituted complexes
1 and 2 in good yields (Scheme 1). The use of the
diiodoplatinum compound was shown to be more reac-
tive and to give higher yields than the corresponding
dichloroplatinum compound due to a more labile halo-
gen-metal bond.15-17 Purification of the complexes 1
(11) (a) Crescenzi, R.; Sterzo, C. L. Organometallics 1992, 11, 4301-
4305. (b) Weyland, T.; Lapinte, C.; Frapper, G.; Calhorda, M. J .; Halet,
J .-F.; Toupet, L. Organometallics 1997, 16, 2024-2031. (c) Rappert,
T.; Nu¨rnberg, O.; Werner, H. Organometallics 1993, 12, 1359-1364.
(d) Le Narvor, N.; Lapinte, C. Organometallics 1995, 14, 634-639.
(12) Moorefield, C. N.; Newkome, G. R. Advances in Dendritic
Macromolecules; J ai Press: Greenwich, CN, 1994; Vol. 1, Chapter 1.
(13) (a) Takahashi, S.; Kariya, M.; Yatake, T.; Sonogashira, K.;
Hagihara, N. Macromolecules 1978, 11, 1063-1066. (b) Kahn, M. S.;
Davies, S. J .; Kakkar, B. L.; Schwartz, D. J .; Lin, B.; J ohnson, B. F.
G.; Lewis, J . J . Organomet. Chem. 1992, 424, 87-91.
(14) (a) Ohshiro, N; Takei, F.; Onitsuka, K.; Takahashi, S. Chem.
Lett. 1996, 871-872. (b) Kahn, M. S.; Schwartz, D. J .; Pasha, N. A.;
Kakkar, B. L.; Raithby, P. R.; Lewis, J . Z. Anorg. Allg. Chem. 1992,
616, 121-124.
(15) (a) Turco A.; Vettori U.; Giancaspro C. Gazz. Chim. Ital. 1986,
116, 193-199. (b) Chatt, J .; Wilkins, R. G. J . Chem. Soc. 1951, 2532-
2533.
(16) Sonogashira, K.; Ohyama, Y.; Murata, E.; Takahashi, S.;
Hagihara, N. J . Polym. Chem. Ed. 1980, 18, 349-353.
(17) Sonogashira, K.; Fujikura, Y.; Toyoshima, N.; Takahashi, S.;
Hagihara, N. J . Organomet. Chem. 1978, 145, 101-108.

Organoplatinum Dendrimers
Sch em e 2
Organometallics, Vol. 17, No. 18, 1998 3983
the three core protons. The signal at δ ) 7.35 ppm was
assigned to the six protons in the ortho position and the
resonance at δ ) 7.37 to the three protons in the para
position of the outer aromatic rings.
Fast atom bombardment mass spectrometric (FAB-
MS) analysis of 3 gave the molecular peak at 1888 amu
as well as an isotopic distribution pattern which was
consistent with the calculated composition.18
All attempts to prepare the corresponding dendrimer
from the 1,2,4,5-tetrasubstituted complex 2 and 1,3,5-
triethynylbenzene have failed so far. Only a mixture
of partly substituted products was observed. A reason-
able explanation for this fact can be seen in the higher
steric demand of the hypothetical ortho-substituted
system compared to the meta-substituted system of 3.
Sin gle-Cr ysta l X-r a y Str u ctu r e An a lysis of Den -
d r im er 3. The geometrical features of dendrimers are
of considerable interest since they influence the chemi-
cal and physical properties of these compounds. Bond
angles and twisting affect the overlapping of orbitals,
the size and accessibility of cavities are important
features for potential guests, and the shape and acces-
sibility of the outer sphere may influence catalytic
reactivities. Therefore we attempted to grow crystals
of all dendrimers. To date, we were only successful in
obtaining X-ray quality crystals for the smallest den-
drimer, 3. Crystallographic data for this crystal struc-
ture are summarized in Table 1, and important geo-
metric parameters are shown in Table 2. An ORTEP
plot of the first-generation dendrimer 3 is shown in
Figure 1.
Sch em e 3
Because of the marginal precision of this crystal
structure, we will only discuss the reliable features that
are apparent. As a result of the monoclinic cell and the
determined space group (C2/c), the molecule shows a
2-fold symmetry axis passing through C(16), C(15),
C(19), C(20), Pt(2), C(26), C(25), C(22), C(21), and C(24).
Again, a 9-fold excess of the building block triethy-
nylbenzene is necessary to avoid side reactions and a
concomitant decrease in overall yield. Separation of
dendrimer 3 from the crude reaction mixture was
achieved by standard column chromatography on silica
gel. The bis-ethynyl substitution of the bis(trieth-
ylphosphine)platinum function results in a downfield
shift of the 31P{1H} NMR resonance for 3 from δ ) 11.3
to 14.3 ppm compared to the monosubstituted complex
1. The P-Pt coupling constant increases slightly from
2327 to 2357 Hz but is still consistent with a trans
configuration at the transition metal.
All three platinum atoms show the expected square
planar coordination geometry with cis bond angles close
to 90°. The Pt(2)-P(3) bond distance of 2.39(1) Å is
longer than the Pt(1)-P(1) and Pt(1)-P(2) bond dis-
tances of 2.25(2) and 2.26(1) Å. On the other hand the
Pt(1)-C(8) as well as the Pt(1)-C(18) bond distances
of 2.28(6) and 2.31(4) Å are much longer than the
corresponding Pt(2)-C(20) and Pt(2)-C(26) bonds length
of 1.83(5) and 2.09(4) Å. Due to the size of the estimated
standard deviations any detailed discussion of a possible
alkyne-metal π-bonding is not reasonable.
The most interesting structural feature of this struc-
ture analysis is an out-of-plane twisting of all three
terminal 1,3,5-triethynylbenzene groups as well as a
twisting of the coordination plane around the platinum
atom Pt(1) relative to the central aromatic ring. The
coordination plane around Pt(2) is almost parallel to the
inner aromatic ring, whereas the adjacent outer tri-
ethynylbenzene ring is located almost perpendicular to
the inner aromatic ring. These findings for the respec-
tive orientation of the aromatic ring systems in the solid
state suggest a three-dimensional rather than a two-
dimensional polyhexagonal graphite-like structure for
the polyyne polymers postulated by Lewis.13b
An analogous downfield shift was observed in the 13C
NMR spectra for the resonances of the core acetylene
nuclei. The signal for the carbon atom σ-bonded to the
platinum shifted from δ ) 100.1 to 109.8 ppm, and the
signal for the other acetylenic carbon shifted from δ )
90.2 to 107.3 ppm on conversion of 1 to 3. The
corresponding acetylenic resonances for the newly formed
ethynyl platinum unit were found at δ ) 107.8 and
122.3 ppm, whereas the signals at δ ) 83.0 and 78.0
ppm were assigned to the terminal acetylenic units.
1
In the H NMR spectra the characteristic signal for
the terminal acetylene protons was found at δ ) 3.14
ppm, whereas for the magnetically and chemically
different aromatic protons of the inner and outer
benzene rings, three distinguishable resonances were
found with an integration ratio of 3:6:3. Based upon
complex 1 the signal at δ ) 6.98 ppm was assigned to
(18) Whiteford, J . A.; Rachlin E. M.; Stang, P. J . Angew. Chem., Int.
Ed. Engl. 1996, 35, 2524-2529.

3984 Organometallics, Vol. 17, No. 18, 1998
Ta ble 1. Cr ysta llogr a p h ic Da ta for th e X-r a y Diffr a ction An a lysis of 3
Leininger et al.
empirical formula
crystal color, habit
a (Å)
C84H108P6Pt3
yellow, needle
28.595(1)
15.0764(9)
8966.5(8)
4
fw
1888.89
cryst syst
b (Å)
â (deg)
monoclinic
20.978(1)
97.511(2)
C2/c (No. 15)
1.399
c (Å)
V (Å3)
space group
density (g/cm3)
2θmax
Z value
F(000)
no. of reflections
3744.00
54.50
0.0300
total: 20 954
unique: 8744
Patterson
methods
p-factor
structure solution
no. of variables
210
no. of observations
(1 > 3.00σ(I))
reflection/parameter
ratio
1918
R
0.089
0.064
-2.28
9.13
2.63
0.61
Rw
GOF
minimum peak in
final diff map (e/Å3)
maximum peak in
final diff map (e/Å3)
Ta ble 2. Selected Bon d Dista n ces a n d Bon d
An gles for 3
Bond Lengths (Å)
Pt(1)-P(1)
Pt(1)-C(8)
Pt(2)-P(3)
Pt(2)-C(20)
P(1)-C(29)
P(1)-C(33)
P(2)-C(37)
P(3)-C(41)
P(3)-C(45)
C(17)-C(18)
C(7)-C(8)
2.25(2)
2.28(6)
2.39(1)
1.83(5)
1.96(5)
2.00(8)
1.94(5)
1.80(6)
1.94(5)
1.17(5)
1.16(7)
1.31(6)
1.36(4)
1.39(4)
Pt(1)-P(2)
Pt(1)-C(18)
Pt(2)-P(3)
Pt(2)-C(26)
P(1)-C(31)
P(2)-C(35)
P(2)-C(39)
P(3)-C(43)
C(21)-C(25)
C(1)-C(7)
C(25)-C(26)
C(13)-C(14)
C(13)-C(17)
C(15)-C(19)
2.26(1)
2.31(4)
2.39(1)
2.09(4)
1.68(6)
1.98(4)
1.77(5)
2.09(8)
1.42(6)
1.47(6)
1.27(6)
1.36(5)
1.33(5)
1.34(7)
C(19)-C(20)
C(13)-C(16)
C(14)-C(15)
Bond Angles (deg)
P(1)-Pt(1)-P(2)
P(1)-Pt(1)-C(18)
P(2)-Pt(1)-C(18)
P(3)-Pt(2)-P(3)
P(3)-Pt(2)-C(26)
P(3)-Pt(2)-C(26)
C(1)-C(7)-C(8)
178.1(6) P(1)-Pt(1)-C(8)
93(2)
88(2)
177(2)
89(1)
90(1)
P(2)-Pt(1)-C(8)
C(8)-Pt(1)-C(18)
177.0(8) P(3)-Pt(2)-C(20)
88.5(4) P(3)-Pt(2)-C(20)
88.5(4) C(20)-Pt(2)-C(26)
91.5(4)
91.5(4)
180.0(2)
158(6)
F igu r e 1. ORTEP plot of the first-generation dendrimer
3. Hydrogen atoms omitted for clarity.
169(7)
Pt(1)-C(8)-C(7)
C(14)-C(13)-C(17) 114(4)
C(14)-C(13)-C(16) 133(4)
fully characterized by NMR (1H, 31P{1H}, 13C{1H}),
vibrational spectroscopy (IR), and mass spectrometry
(FAB-MS), as well as elemental analysis.
Diver gen t Ap p r oa ch to th e F or m a tion of a Sec-
on d -Gen er a tion Den d r im er . In the divergent con-
struction of dendrimers, functionalization of the outer
sphere is a key step. One of the prime considerations
is the complete conversion of all the terminal sites of
the growing cascade.12 Incomplete conversion generally
results in branching defects that can affect topology and
physical properties.
The step-by-step construction of the second-genera-
tion dendrimer 5 first affords a functionalization of the
terminal acetylenic groups of the first-generation den-
drimer 3. By reacting compound 3 with an excess of
diiodobis(triethylphosphine)platinum, all terminal acety-
lene groups of 3 were converted into the corresponding
[trans-iodobis(triethylphosphine)ethynyl]platinum units,
giving rise to the formation of compound 4, which
provides the reactive site for further substitution reac-
tions (Scheme 4).
Two different phosphorus resonances were observed
in the 31P{1H} NMR spectrum for the inner and outer
diethylphosphineplatinum units of the dendrimer 4. The
signal at δ ) 14.1 ppm was assigned to the phosphine
ligands of the bis-ethynyl-substituted bis(triethylphos-
phine)platinum units, and the resonance at δ ) 11. 4
ppm was assigned to the phosphine ligands of the [trans-
iodobis(triethylphosphine)ethynyl]platinum units in the
outer sphere. The phosphorus-platinum spin-spin
coupling constants of 2365 Hz for the inner and 2321
Hz for the outer platinum units are similar to those
observed for the complexes 1 and 3 and are character-
istic for trans-substituted PtP2 systems.
In addition, the two chemically different triethylphos-
phine ligands show inseparable resonances in the 1H
NMR spectrum for the methylene as well as the methyl
groups. For the three sets of different aromatic protons
three distinguishable signals were detected with an
integration ratio of 3:6:3. The singlet at δ ) 6.94 ppm
was assigned to the protons of the core ring, whereas
the AB2 spin system at δ ) 6.99 and 6.97 ppm with a
4J HH ) 2.8 Hz was assigned to the ortho and para
Purification of complex 4 was done by column chro-
matography on aluminum oxide, as attempts to use
silica gel resulted in a loss of most of the product. The
complex was obtained in 44% yield as an air- and
moisture-stable pale yellow powder. Compound 4 was

Organoplatinum Dendrimers
Sch em e 4
Organometallics, Vol. 17, No. 18, 1998 3985
drimer 6, which is also supported by vibrational spec-
troscopy as well as elemental analysis as given in the
Experimental Section.
Con clu sion s
We have established that 1,3,5-triethynylbenzene and
diiodobis(triethylphosphine)platinum are effective build-
ing blocks for the synthesis of several dendrimers. The
step-by-step divergent strategy is an effective methodol-
ogy for the formation of transition metal dendrimers
with an alkynyl-aryl backbone. Considering that,
compared to the all-carbon analogue dendrimeres, the
transition metal fragments serve as a spacer that
effectively prevents proximal groups from interfering,
higher generation dendrimers can be generated. By
intermolecularly connecting the branches of the den-
drimers, we are currently investigating transition metal
linked polycondensated systems with honeycomb-like
structures.
Exp er im en ta l Section
protons of the outer aromatic rings. Fast atom bom-
bardment mass spectrometric (FAB-MS) analysis gave
the molecular peak at 5231 amu as well as an isotopic
distribution pattern that was consistent with the cal-
culated composition for compound 4.18
To generate the second-generation dendrimer 5, the
functionalized first-generation dendrimer 4 was reacted
with an 18-fold excess of 1,3,5-triethynylbenzene. Al-
though we were able to confirm the product by 31P{1H}
NMR spectroscopy, so far we could not isolate the
dendrimer in a pure form from the reaction mixture via
this procedure.
However, a functionalized second-generation den-
drimer 6 was prepared by reaction of the first-genera-
tion dendrimer 3 with the trinuclear platinum com-
pound 1 (Scheme 4). The reaction was performed in a
mixture of diethylamine and toluene in the presence of
cuprous iodide, and the product isolated in 43% yield
by column chromatography on aluminum oxide.
Only two different phosphorus resonances were ob-
served in the 31P{1H} NMR spectrum for the two inner
and one outer diethylphosphineplatinum units of the
dendrimer 6. The signal at δ ) 14.2 ppm was assigned
to the phosphine ligands of the bis-ethynyl-substituted
bis(triethylphosphine)platinum units of the inner core
sphere as well as to the bis(triethylphosphine)platinum
units of the inner first sphere. No splitting of the signal
was observed, which maybe explained by the nearly
magnetical equivalence of the chemically different phos-
phine ligands. The resonance at δ ) 11.5 ppm was
assigned to the phosphine ligands of the [trans-iodobis-
(triethylphosphine)ethynyl]platinum units in the outer
sphere. The phosphorus-platinum spin-spin coupling
constants of 2357 Hz for all inner and 2319 Hz for the
outer platinum units are similar to those observed for
the first-generation dendrimer 4.
Gen er a l Meth od s. All experiments were performed under
an inert atmosphere of nitrogen utilizing standard Schlenk
techniques. Solvents used were of reagent or HPLC grade and
purified in the following manner: benzene was distilled over
sodium/potassium (2:1) alloy, toluene was distilled over sodium
metal, diethyl ether was distilled over sodium/benzophenone,
triethylamine was distilled over potassium hydroxide, and
hexanes and methylene chloride were distilled over calcium
hydride.19 All NMR solvents (CD2Cl2 and CDCl3) were stored
over 4 Å molecular sieves prior to use.
Triethylphosphine, 1,3,5-tribromobenzene, 1,2,4,5-tetrabro-
mobenzene, and tetrakis(triphenylphosphine)palladium were
purchased from the Aldrich Chemical Co. and used without
further purification. Trimethylsilylacetylene was purchased
from Lancaster Synthesis Inc. trans-Diiodobis(triethylphos-
phine)platinum was prepared by standard literature proce-
dure.20
All NMR spectra were obtained at room temperature with
a
Varian XL-300 or VXR-500 spectrometer employing a
deuterium sample as an internal lock unless noted otherwise.
The operating frequencies of the former spectrometer for the
1H, 13C{1H}, and 31P{1H} NMR spectra were 300.0, 75.4, and
121.4 MHz, respectively. For the latter spectrometer, the
operating frequencies of the 1H and 13C{1H} NMR spectra were
499.8 and 125.7 MHz, respectively. The 1H chemical shifts
are reported relative to the residual nondeuterated solvent of
CD2Cl2 (5.32 ppm) or CDCl3 (7.27 ppm). The 13C{1H} chemical
shifts are reported relative to CDCl3 (77.0 ppm) or CD2Cl2 (54.0
ppm). The 31P{1H} NMR spectra were referenced to sealed
external standards of 85% phosphoric acid.
Mass spectra were obtained with a Finnigan MAT 95 mass
spectrometer with a Finnigan MAT ICIS II operating system
under positive ion fast atom bombardment (FAB) conditions
at 8 keV. 3-Nitrobenzyl alcohol was used as a matrix in CH2-
Cl2; polypropylene glycol and cesium iodide were used as a
reference for peak matching.
Elemental analyses were performed by Atlantic Microlab
Inc., Norcross, GA. IR spectra were obtained with a Mattson
Polaris FT-IR spectrometer at 2 cm-1 resolution. Reported
melting points were obtained with a Mel-Temp capillary
apparatus and are uncorrected.
In contrast to complex 4, separate signals in the
1
aromatic region of the H NMR spectrum could not be
assigned to the five different sets of aromatic protons,
as only one broad signal for the 30 aromatic protons was
observed. These data are consistent with the expected
structure for the functionalized second-generation den-
(19) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals: Pergamon Press: Tarrytown, NY, 1988.
(20) J ensen, K. A. Z. Anorg. Allg. Chem. 1936, 229, 225-232.

3986 Organometallics, Vol. 17, No. 18, 1998
Leininger et al.
1,3,5-Tr ieth yn ylben zen e.21 In a 100 mL round-bottom
Schlenk flask 1,3,5-tribromobenzene (2.66 g, 8.4 mmol) and
trimethylsilylacetylene (11.7 mL, 80 mmol) were dissolved in
60 mL of freshly distilled THF and 15 mL of dry triethylamine.
A mixture of tetrakis(triphenylphosphine)palladium (1.470 g,
1.26 mmol) and copper(I) iodide (193 mg, 1.01 mmol) was
added, and the suspension stirred for 36 h at room tempera-
ture in the absence of light. After removal of the solvents the
residue was suspended in 100 mL of diethyl ether and washed
twice with 100 mL of water, and the organic phase was dried
over magnesium sulfate. Removing the solvent and separation
by column chromatography on silica gel afforded 2.26 g (74%)
of the 1,3,5-tris(trimethylsilylethynyl)benzene eluting as the
second fraction with hexanes. Desilylation of 1,3,5-tris(tri-
methylsilylethynyl)benzene (2.276 g, 6.22 mmol) was achieved
by stirring the compound in a saturated KOH solution of
methanol for 4 h. Hydrolysis and extraction of the organic
product with diethyl ether and subsequent column chroma-
tography on silica gel with hexanes/diethylether (1:1) yielded
912 mg (98%) of the 1,3,5-triethynylbenzene as a white powder
(overall yield: 72%).
0.195 mmol) were dissolved in 30 mL of toluene and 20 mL of
dry diethylamine. A 20 mg (0.11 mmol) portion of copper(I)
iodide was added, and the reaction mixture stirred for 4 h at
room temperature. The solvent was then removed in vacuo,
and the remaining yellow residue was separated by column
chromatography on silica gel. The excess diiodobis(trieth-
ylphosphine)platinum was nearly quantitatively recovered
(1.050 g) with hexanes/benzene (3:1), whereas the product was
isolated with hexanes/benzene (1:4). Yellow crystals of the
complex 2 were obtained after crystallization from dichlo-
romethane. Yield: 205 mg (43.8%); mp 312 °C dec; IR (thin
film; CD2Cl2; cm-1) 2963, 2933, 2914 (C-H), 2113 (C≡C), 1471,
1
1456, 1410 (Ar); H NMR (CD2Cl2) 7.10 (s, 2 H, aromatic-H),
2.21 (m, 48 H, CH2), 1.18 (m, 72 H, CH3); 13C{1H} NMR (CD2-
3
Cl2) 136.5 (s, aromatic-CH), 125.8 (s, J CPt ) 33 Hz, ipso-C),
104.2 (s, 1J CPt ) 422 Hz, C≡C-Pt), 93.4 (s, 3J CP ) 29 Hz, C≡C),
17.2 (pseudo quin, J CP ) 17Hz, PCH2), 8.8 (pseudo t, J CP ) 14
1
Hz, PCH2CH3); 31P{1H} NMR (CD2Cl2) 11.1 (s, J PPt ) 2333
Hz); FAB-MS m/z (relative intensity) 2403 (M+, 1), 119 (Et3P,
100). Anal. Calc for
C62H122I4P8Pt4: C, 30.98; H, 5.12.
Found: C, 31.24; H, 4.98.
1,2,4,5-Tetr a eth yn ylben zen e.22 1,2,4,5-Tetrabromoben-
zene (3.17 g, 8.05 mmol) and trimethylsilylacetylene (15.5 mL,
100.3 mmol) were reacted with a mixture of tetrakis(tri-
phenylphosphine)palladium (1.950 g, 1.68 mmol) and copper-
(I) iodide (260 mg, 1.73 mmol) in 50 mL of freshly distilled
THF and 15 mL of triethylamine. After stirring the mixture
for 4 h at room temperature in the dark, the solvents were
removed under reduced pressure. Workup was performed
analogous to the procedure for the 1,3,5-tris(trimethylsilyl-
ethynyl)benzene and afforded 2.715 g (73%) of 1,2,4,5-tetrakis-
(trimethylsilylethynyl)benzene. Desilylation of the product
and separation by column chromatography finally yielded 987
mg (97%) of 1,2,4,5-tetraethynylbenzene as plates possessing
a metallic luster (overall yield: 71%).
F ir st-Gen er a tion Den d r im er : 1,3,5-Tr is[[tr a n s-(1′,3′,5′-
t r ie t h yn ylb e n ze n e )b is(t r ie t h ylp h osp h in e )p la t in u m ]-
eth yn yl]ben zen e (3). In a 25 mL round-bottom Schlenk
flask 1,3,5-tris[[trans-iodobis(triethylphosphine)platinum]-
ethynyl]benzene (1) (400 mg, 0.22 mmol) and 1,3,5-triethy-
nylbenzene (295 mg, 2.00 mmol) were dissolved in 10 mL of
toluene and 10 mL of diethylamine. After 5 min 10 mg (0.05
mmol) of cuprous(I) iodide was added, and the mixture was
stirred at room temperature for 3 h. The solvent was then
removed in vacuo, and the remaining pale yellow residue was
separated by column chromatography on silica gel. The excess
1,3,5-triethynylbenzene was isolated as the first fraction with
hexanes as eluent, whereas subsequent elution with benzene
yielded the pure product 3. After crystallization from dichlo-
romethane the dendrimer 3 was obtained as bright yellow
colored crystals. Yield: 0.315 g (76%); mp 156-158 °C; IR
(thin film; CD2Cl2; cm-1) 3301, 3226 (C-H), 2964, 2932, 2906
1,3,5-Tr is[[tr a n s-iod obis(tr ieth ylp h osp h in e)p la tin u m ]-
eth yn yl]ben zen e (1). To a 100 mL round-bottom Schlenk
flask were added trans-diiodobis(triethylphosphine)platinum
(2.060 g, 3.01 mmol) and 1,3,5-triethynylbenzene (75 mg, 0.50
mmol). Then, 30 mL of toluene and 20 mL of dry diethylamine
were added under nitrogen. The solution was stirred for 10
min at room temperature, and 20 mg (0.11 mmol) of cuprous-
(I) iodide was added in one portion. After 3 h at room
temperature, a small amount of diethylammonium iodide
started precipitating out of solution. The solvent was then
removed in vacuo, and the remaining yellow residue was
separated by column chromatography on silica gel. The first
fraction containing excess diiodobis(triethylphosphine)platinum
was recovered almost quantitatively (1.050 g, 1.60 mmol) by
elution with hexanes/benzene (3:1). The second fraction with
benzene as eluent gave the product 1 as slightly yellow crystals
after recrystallization from dichloromethane. Yield: 0.510 g
(56%); mp 302 °C dec; IR (thin film; CD2Cl2; cm-1) 2964, 2931,
2908 (C-H), 2112 (C≡C), 1567, 1454, 1410 (Ar); 1H NMR (CD2-
Cl2) 7.02 (s, 3 H, aromatic-H), 2.22 (m, 36 H, CH2), 1.18 (m,
54 H, CH3); 13C{1H} NMR (CD2Cl2) 130.5 (s, Cortho), 128.8 (s,
1
(C-H), 2093 (CC), 1567, 1452, 1410 (Ar); H NMR (CD2Cl2)
7.37 (m, 3 H, aromatic-H), 7.35 (m, 6 H, aromatic-H), 6.98 (s,
3 H, aromatic-H), 3.14 (s, 6 H, C≡C-H), 2.18 (m, 36 H, CH2),
1.22 (m, 54 H, CH3); 13C{1H} NMR (CD2Cl2) 135.1 (s, Cortho),
132.0 (s, Cortho), 130.7 (s, Cmeta), 130.1 (s, Cipso), 128.7 (s, Cipso),
3
122.7 (s, Cpara), 122.3 (s, J CP ) 29 Hz, C≡C), 109.8 (s, C-Pt),
3
107.8 (s, C-Pt), 107.3 (s, J CP ) 29 Hz, C≡C), 83.0 (s, C≡C-
H), 78.0 (s, C-H), 16.6 (pseudo quin, J CP ) 13 Hz, PCH2), 8.6
(pseudo t, J CP ) 13 Hz, PCH2CH3); 31P{1H} NMR (CD2Cl2) 14.3
(s, 1J PPt ) 2357 Hz); FAB-MS m/z (relative intensity) 1888 (M+,
6), 119 (Et3P, 100). Anal. Calc for C84H108P6Pt3: C, 53.36; H,
5.71. Found: C, 53.18; H, 5.71.
F u n ction a lized F ir st-Gen er a tion Den d r im er (4). To a
25 mL round-bottom Schlenk flask were added trans-diiodobis-
(triethylphosphine)platinum (760 mg, 1.112 mmol), 1,3,5-tris-
[[tra ns-(1′,3′,5′-triethynylbenzene)bis(triethylphosphine)-
platinum]ethynyl]benzene (3) (116 mg, 0.061 mmol), 7 mL of
toluene, and 5 mL of dry diethylamine under nitrogen. Next,
10 mg (0.05 mmol) of cuprous(I) iodide was added, and the
solution was stirred at room temperature for 4 h. The solvent
was then removed in vacuo, and the remaining yellow residue
was separated by column chromatography on aluminum oxide.
The excess diiodobis(triethylphosphine)platinum was recov-
ered nearly quantitatively (450 mg) as the first fraction with
hexanes/benzene (3:1) as eluent, whereas the product was
eluted with benzene. Recrystallization from dichloromethane
afforded the functionalized first-generation dendrimer 4 as a
pale yellow powder. Yield: 132 mg (41%); mp 290-294 °C
dec; IR (thin film; CD2Cl2; cm-1) 2968, 2945 (C-H), 2109, 2095
(C≡C) 1566, 1459, 1410 (Ar); 1H NMR (CD2Cl2) δ 6.99 (d, 6 H,
1
3
C
ipso), 100.1 (s, J CPt ) 415 Hz, C-Pt), 90.2 (s, J CP ) 28 Hz,
C≡C), 17.1 (pseudo quin, J CP ) 13 Hz, PCH2), 8.6 (pseudo t,
1
J CP ) 13 Hz, PCH2CH3); 31P{1H} NMR (CD2Cl2) 11.3 (s, J PPt
) 2327 Hz); FAB-MS m/z (relative intensity) 1821 (M+, 5), 119
(Et3P, 100). Anal. Calc for C48H93I3P6Pt3: C, 31.60; H, 5.14.
Found: C, 31.63; H, 5.14.
1,2,4,5-Tetr a kis[[tr a n s-iod obis(tr ieth ylp h osph in e)pla t-
in u m ]eth yn yl]ben zen e (2). In a 100 mL round-bottom
Schlenk, trans-diiodobis(triethylphosphine)platinum (1.602
mg, 2.34 mmol) and 1,2,4,5-tetraethynylbenzene (33.9 mg,
(21) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara,
N. Synthesis 1980, 627-630. (b) Trumbo D. L.; Marvel C. S. J . Polym.
Sci. Part A, Poly. Chem. 1986, 24, 2311-2326.
(22) Berris, B. C.; Hovakeemian, G. H.; Vollhardt, P. C. J . Chem.
Soc., Chem. Commun. 1983, 9, 502-503.
4
4J HH ) 2.8 Hz, aromatic-H), 6.97 (t, 3 H, J HH ) 2.8 Hz,
aromatic-H), 6.94 (s, 3 H, aromatic-H), 2.21 (m, 108 H, CH2),
1.21 (m, 162 H, CH3); 13C{1H} NMR (CD2Cl2) 17.3 (m, PCH2),

Organoplatinum Dendrimers
Organometallics, Vol. 17, No. 18, 1998 3987
8.7 (m, PCH2CH3): 31P{1H} NMR (CD2Cl2) 14.1 (s, 1J PPt ) 2365
reflections that were collected, 8744 were unique. The linear
absorption for Mo KR radiation was 47.9 cm-1. The data were
corrected for Lorentz and polarization effects; however due to
the small size of the crystal, no further absorption corrections
were done. The structure was solved by heavy-atom Patterson
methods and expanded using Fourier techniques. Some non-
hydrogen atoms were refined anisotropically, while the rest
were refined isotropically. Hydrogen atoms were included but
not refined. Neutral atom scattering factors were taken from
Cromer and Waber. Anomalous dispersion effects were in-
cluded in Fcalc: the values for f′ and f′′ were those of Creagh
and McAuley. The values for the mass attenuation coefficients
are those of Creagh and Hubbel. All calculations were
performed using the teXsan crystallographic software package
of Molecular Structure Corporation.23 The structure was
refined to values of 0.089 (R) and 0.064 (Rw) based on full-
matrix least-squares refinement on F2 of 1918 reflections (I >
3.00σ(I)) and 210 variable parameters.
1
Hz, Pinner), 11.4 (s, J PPt ) 2321 Hz, Pouter); FAB-MS m/z
(relative intensity) 5231 (M+, 2), 119 (Et3P, 100). Anal. Calc
for C156H282I6P18Pt9: C, 35.81; H, 5.43. Found: C, 36.16; H,
5.49.
Secon d -Gen er a tion Den d r im er (6). 1,3,5-Tris[[trans-
(1′,3′,5′-triethynylbenzene)bis(triethylphosphine)platinum]-
ethynyl]benzene (3) (506 mg, 0.277 mmol) and 1,3,5-tris[[trans-
iodobis(triethylphosphine)platinum]ethynyl]benzene (1) (29.1
mg, 0.015 mmol) were dissolved in 5 mL of toluene and 4 mL
of dry diethylamine under nitrogen. Next 4 mg (0.02 mmol)
of copper(I) iodide was added in one portion, and the solution
was stirred for 4 h at room temperature followed by removal
of the solvents under reduced pressure. The remaining yellow
residue was separated by column chromatography on alumi-
num oxide. With hexanes/benzene (1:1) the excess 1,3,5-tris-
[[trans-iodobis(triethylphosphine)platinum]ethynyl]benzene was
nearly quantitatively recovered (412 mg), whereas the product
was eluted with hexanes/benzene (1:3). Recrystallization from
dichloromethane/hexanes (2:1) afforded 79 mg of an almost
white powder. Yield: 79 mg (43%); mp 284-285 °C dec; IR
(thin film; CD2Cl2; cm-1) 2967, 2940 (C-H), 2109 (C≡C) 1562,
1455, 1410 (Ar); 1H NMR (CD2Cl2) δ 7.03-6.91 (bs, 30 H,
aromatic-H), 2.31-2.12 (m, 252 H, CH2), 1.32-1.11 (m, 378
H, CH3); 31P{1H} NMR (CD2Cl2) 14.2 (s, 1J PPt ) 2357 Hz, Pinner),
Ack n ow led gm en t. Financial support from NSF
(CHE-9529093) is gratefully acknowledged. We thank
J ohnson-Matthey for a generous loan of PtCl2 and the
Alexander von Humboldt Stiftung for a Feodor-Lynen
Postdoctoral Fellowship to S.L.
11.5 (s, 1J PPt ) 2319 Hz, Pouter). Anal. Calc for C372H660I12P42
Pt21: C, 37.06; H, 5.51. Found: C, 36.86; H, 5.38.
-
Su p p or tin g In for m a tion Ava ila ble: Crystal structure
data for 3 including tables of calculated positional parameters
and U for the hydrogen atoms, non-hydrogen positional
parameters and anisotropic thermal parameters, complete
bond lengths, angles, and torsional angles are provided (11
pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS. See
any current masthead page for ordering information.
X-r a y An a lysis of 3. A yellow needle crystal having
approximate dimensions of 0.11 × 0.03 × 0.02 mm was
mounted on a glass fiber. All measurements were made on a
CCD area detector with graphite monochromated Mo KR
radiation. The space group C2/c (No. 15) was unambiguously
determined from the systematic absence in the data as well
as a statistical analysis of intensity distribution and the
successful solution and refinement of the structure. The data
were collected at a temperature of 23 °C to a maximum 2θ
value of 54.5°. Data were collected in 0.00° oscillations with
25.0 s exposures. The crystal-to-detector distance was 5.04
mm with the detector at the zero swing position. Of the 20 954
OM980337Y
(23) TeXscan: Single Crystal Structure Analysis Software, Version
4.2; Molecular Structure Corporation: The Woodlands, TX, 1995.