Synthesis and characterization of a first generation organorhenium dendrimer

Article abstract of DOI:10.1016/S0022-328X(99)00543-4

A new, more efficient route to the functionalized Cp compound [η⁵-(C₅H₄)CH₂CH₂CH₂I]Re(CO)₃, (2) is reported. The reaction of 2 with [(LiC₅H₄)Re(CO)₃] yields the new binuclear complex (CO)₃Re[(η⁵-C₅H₄)(CH₂)₃(η⁵-C₅H₄)]Re(CO)₃ (3). The crystal structure of 3 has been determined. Crystals of 3 are triclinic, space group P1?, with a=8.241(1) A?, b=11.154(1) A?, c=11.699(1) A?, α=106.383(10)°, β=102.512(10)^o, γ=105.067(10)°, Z=2 and R=0.045. Compound 2 has also been used to build up a first generation organometallic dendrimer containing six rhenium units by the convergent method.

Full text of DOI:10.1016/S0022-328X(99)00543-4

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 593–594 (2000) 361–368
Synthesis and characterization of a first generation organorhenium
dendrimer
Ipe J. Mavunkal, John R. Moss *, John Bacsa
Department of Chemistry, Uni6ersity of Cape Town, Rondebosch 7701, South Africa
Received 28 June 1999; accepted 20 September 1999
Dedicated to Professor Fausto Calderazzo on his 70th birthday.
Abstract
A new, more efficient route to the functionalized Cp compound [h5-(C5H4)CH2CH2CH2I]Re(CO)3, (2) is reported. The reaction
of 2 with [(LiC5H4)Re(CO)3] yields the new binuclear complex (CO)3Re[(h5-C5H4)(CH2)3(h5-C5H4)]Re(CO)3 (3). The crystal
(
,
,
structure of 3 has been determined. Crystals of 3 are triclinic, space group P1, with a=8.241(1) A, b=11.154(1) A, c=11.699(1)
o
,
A, h=106.383(10)°, i=102.512(10) , k=105.067(10)°, Z=2 and R=0.045. Compound 2 has also been used to build up a first
generation organometallic dendrimer containing six rhenium units by the convergent method. © 2000 Elsevier Science S.A. All
rights reserved.
Keywords: Dendrimer; Rhenium; Convergent synthesis
ent organometallic units including Fe, Ru and Co
functionalities [1,7]. Having established the generality
that can be adopted for the construction of
organometallic dendrimers we have turned our atten-
tion to the synthesis of cascade molecules for possible
applications. The rich chemistry of various oxo-rhe-
nium compounds in homogeneous catalysis is well doc-
umented [8]. Our first aim was to investigate the
feasibility of synthesizing a first generation dendrimer
with CpRe(CO)3 (1) units in the dendritic wedges and
to attempt subsequently the oxidation of the carbonyl
groups to generate oxo-rhenium units in the dendritic
periphery.
1. Introduction
Recent advances in dendritic molecules have estab-
lished synthetic strategies for the systematic construc-
tion of various organometallic dendrimers [1]. Lately
the focus has shifted from the synthetic aspects to their
properties and applications. Synthesis of dendrimers
with components of specific functions have been
targeted [2]. Properties are being modified increasingly
with functional centers. The multiplication of func-
tional components attached to a dendritic skeleton
moves into the forefront, not the dendrimer itself, and
new materials with specific properties are anticipated
[3].
A wide range of organometallic dendrimers have
been synthesized in the past decade with varying molec-
ular architecture incorporating different metal atoms. It
has been demonstrated that the metal atom can be
incorporated at the periphery [4] or at the core [5] or
throughout the dendritic skeleton [6]. We have been
successful in our laboratory in the past to build up
various organometallic dendrimers incorporating differ-
2. Experimental
2.1. General procedures
All reactions were carried out under a purified nitro-
gen atmosphere using standard Schlenk line techniques.
Solvents were distilled from appropriate drying agents
under nitrogen prior to use. Potassium carbonate was
dried before use and Re2(CO)10 (Strem), 3,5-dihydroxy-
* Corresponding author.
E-mail address: jrm@psipsy.uct.ac.za (J.R. Moss)
benzyl
alcohol
(Aldrich),
1,1,1-tris(4-hydroxy-
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00543-4

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I.J. Ma6unkal et al. / Journal of Organometallic Chemistry 593–594 (2000) 361–368
phenyl)ethane (Aldrich), n-BuLi (1.5 M hexane solu-
tion, Merck) were used as received. Diiodopropane
(Aldrich) was refluxed over CaH2 and was distilled
(under reduced pressure) prior to use. CpRe(CO)3 (1)
was synthesized by literature methods [9]. Silica gel
(Merck, 60) was used for column chromatography elut-
ing the products with appropriate solvents/solvent mix-
tures. NMR spectra were recorded at ambient
temperature on a Varian Unity-400 spectrometer in
were chromatographed using the procedure described in
Section 2.2 but using a silica gel column prepared with
10% ethylacetate/hexane. After eluting the column with
10% ethylacetate/hexane, compound 4 was then eluted
with 30% ethyl acetate/hexane. Mononuclear 5 was
collected as the last fraction eluting with 50% ethyl
acetate/hexane mixture. Compound 4 was obtained by
adding pentane to a CH2Cl2 solution of the residue
obtained above to give the required product as a color-
less oil. Yield, 0.15 g (74%). IR (CH2Cl2), w (CO), 2021,
1
CDCl3 at the following frequencies: H, 399.952 MHz;
13C, 100.577 MHz. Chemical shifts are reported relative
to the residual signals of CDCl3. Infrared spectra were
recorded on a Perkin Elmer Paragon 1000 FT-IR spec-
trometer as hexane or CH2Cl2 solutions. Mass spectra
were obtained on a VG-70SEQ mass spectrometer us-
ing xenon gas in the FAB mode and a Voyager-DE
PRO machine was used for MALDI TOF analysis.
1924 cm−1; H-NMR l 6.51 (d, J(H,H)=1.6 Hz, 2H,
1
Ar), (t, J(H,H)=1.99 Hz, 1H, Ar), 5.26 (m, 8H, Cp),
4.63 (s, Ar-CH2OH), 3.98 (t, J(H,H)=5.99 Hz, 4H,
ꢀCH2Oꢀ), 2.63 (m, 4H, CpCH2ꢀ), 1.96 (m, 4H,
ꢀCH2CH2Oꢀ); 13C-NMR l 194.2 (CO), 160.0 (Ar),
143.4 (Ar), 110.2 (ipso-Cp), 105.1(Ar), 100.5 (Ar), 83.6
(Cp), 83.0 (Cp), 66.5 (ꢀCH2ꢀOꢀ), 65.2 (ꢀCH2OH), 31.0
(CpCH2ꢀ), 24.7 (ꢀCH2ꢀ); MS (FAB) m/z 890 (M+),
862 (MꢀCO+), 806 (Mꢀ3CO+), 778 (Mꢀ4CO+), 750
(Mꢀ5CO+), 722 (Mꢀ6CO+). Anal. Calc. for
C29H26O9Re2.CH2Cl2: C 36.92, H 2.89. Found: C 36.76,
H 2.76.
2.2. Synthesis of [(p5-C5H4)(CH2)3I]Re(CO)3(2)
n-BuLi ( 0.62 ml, 1.5 M, 1.0 mmol) was added to a
degassed THF (15 ml) solution of CpRe(CO)3, (0.28 g,
0.83 mmol) at −78°C and was stirred at that tempera-
ture for 3 h. Diiodopropane (0.2 ml, 1.7 mmol) was
added and the reaction mixture brought to room tem-
perature. It was then stirred for an additional 2 h at this
temperature. Solvent was removed under vacuum and
the residue dissolved in CH2Cl2. The CH2Cl2 solution
was filtered and the volume reduced to ꢀ10 ml. Silica
gel (1 g) was added into the solution and the solvent
pumped off. The residue was then loaded onto a
column of silica gel prepared with hexane. Excess di-
iodopropane was removed by first eluting the column
with hexane. Unreacted 1 was then collected with 5%
CH2Cl2/hexane and compound 2 was eluted with 10%
CH2Cl2/hexane. Small amounts of the dimer (3) were
also collected in the last fraction on eluting with 20%
CH2Cl2/hexane mixture. Compound 2 was obtained by
adding pentane to a CH2Cl2 solution of the residue
obtained above to give the required product as a color-
less oil. Yield 0.21 g (50%). IR (CH2Cl2) w (CO) 2021,
Solvent was removed from the last fraction and the
compound (5) was obtained by adding pentane into a
CH2Cl2 solution of the residue obtained: Yield, 0.047 g
(20%); IR (CH2Cl2), w (CO), 2021, 1924 cm−1 1H-
;
NMR l 6.48 (s,1H, Ar), 6.45 (s,1H, Ar), 6.31 (t, 1H,
J(H,H)=2.19 Hz, Ar), 5.26 (m, 4H, Cp), 4.61 (s, 2H,
ArCH2ꢀ), 3.96 (t, 2H, CH2CH2Oꢀ, J(H,H)=5.98 Hz),
2.62 (m, CpCH2ꢀ, 2H, J(H,H)=7.59 Hz), 1.96 (m,
ꢀCH2CH2O, 2H); 13C-NMR l 194.2 (CO), 160.2 (Ar),
156.8 (Ar), 143.6 (Ar), 110.2 (Ar), 109.2 (ipso-Cp),
106.2 (Ar), 105.1 (Ar), 101.0 (Ar), 83.6 (Cp), 83.0 (Cp),
66.5 (ꢀCH2ꢀOꢀ), 65.0(ꢀCH2OH), 31.0 (CpCH2ꢀ), 24.7
(ꢀCH2ꢀ); MS (FAB) m/z 516 (M+), 488 (MꢀCO+), 460
(Mꢀ2CO+), 432 (Mꢀ3CO+).
2.4. Synthesis of compound 6
Bromination of 4 was carried out using CBr4/PPh3.
The hydroxy wedge 4 ( 0.13 g, 0.14 mmol) was stirred
with a mixture of CBr4 (0.064 g, 0.19 mmol) and PPh3
(0.051 g, 0.19 mmol) in THF (10 ml). The reaction was
monitored by TLC. Additional amounts of CBr4/PPh3
(2×0.19 mmol) were added to drive the reaction to
completion. The reaction was then quenched by adding
few drops of distilled water. Solvent was evaporated
under vacuum and the residue extracted with CH2Cl2
(3×10 ml). The combined extracts were chro-
matographed using a procedure described in Section 2.2
but using a silica gel column prepared with hexane. The
column was first eluted with hexane and the benzyl
bromide 6 was eluted with 20% ethyl acetate/hexane
mixture. Solvent was removed under reduced pressure
and compound 6 was obtained by adding pentane to a
CH2Cl2 solution of the residue obtained to give the
1924 cm−1 1H-NMR l 5.26 (m, 4H Cp), 3.19 (t,
;
J(H,H)=6.79 Hz, 2H, –CH2I), 2.55 (m, 2H, CpCH2–
), 1.99 (m, 2H, –CH2CH2I); 13C-NMR l 194.2 (CO),
108.9 (ipso-Cp), 83.8 (Cp), 83.2 (Cp) 34.9 (CpCH2–),
28.8 (–CH2–), 5.2 (–CH2I).
2.3. Synthesis of compound 4
A mixture of 2 (0.23 g, 0.45 mmol), 3,5-dihydroxy-
benzyl alcohol (0.031 g, 0.22 mmol), K2CO3 (0.18 g, 1.3
mmol), 18-crown-6 (0.014 g, 0.053 mmol) was refluxed
in acetone (15 ml) for 72 h. The reaction was monitored
periodically by TLC. The solvent was evaporated under
reduced pressure and the residue was extracted with
CH2Cl2 (3×10 ml). The combined CH2Cl2 extracts

I.J. Ma6unkal et al. / Journal of Organometallic Chemistry 593–594 (2000) 361–368
363
required product as a colorless oil. Yield, 0.13 g (92%).
was warmed to room temperature and stirred for an
additional 2 h. The solvent was removed under reduced
pressure and the residue was extracted with CH2Cl2
(3×10 ml). The combined extracts were then chro-
matographed using a procedure described in Section
2.2. Complex 3 was eluted with 20% CH2Cl2/hexane
and the solvent removed under reduced pressure. The
residue was dissolved in CH2Cl2 and white crystals of 3
were obtained by the slow diffusion of pentane into the
CH2Cl2 solution. Yield 0.053 g (22%). m.p.: 108–
1
IR (CH2Cl2), w (CO), 2021, 1924 cm−1; H-NMR l
6.52 (d, 2H, J(H,H)=2.39 Hz, Ar), 6.36 (t, J(H,H)=
2.39 Hz, 1H, Ar,), 5.26 (m, 8H, Cp), 4.40 (s, 2H,
ꢀCH2Br), 3.97 (t, J(H,H)=5.59 Hz, 4H, ꢀCH2Oꢀ),
2.62 (m, J(H,H)=7.59 Hz, 4H, CpCH2ꢀ,), 1.96 (m,
4H, CH2CH2Oꢀ); 13C-NMR l 194.2 (CO), 159.9 (Ar),
139.8 (Ar), 110.2 (ipso-Cp), 107.5 (Ar), 101.4 (Ar), 83.6
(Cp), 83.0 (Cp), 66.6 (ꢀCH2ꢀOꢀ), 33.4 (ArCH2Br), 31.0
(CpCH2ꢀ), 24.7 (ꢀCH2ꢀ); MS (FAB) m/z, 954 (M+),
926 (MꢀCO+), 870 (Mꢀ3CO+), 842 (Mꢀ4CO+), 814
(Mꢀ5CO+), 786 (Mꢀ6CO+); Anal. Calc. for
110°C; IR (CH2Cl2), w (CO), 2021, 1924 cm−1 1H-
;
NMR l 5.25 (m, 8H, Cp), 2.47 (t, J(H,H)=7.99 Hz,
4H, CpCH2ꢀ), 1.70 (m, ꢀCH2ꢀ, 2H,); 13C-NMR, l
194.2 (CO), 110.0 (ipso-Cp), 83.6 (Cp), 82.9 (Cp) 33.5
(ꢀCH2ꢀ), 27.6 (CpCH2ꢀ); MS (FAB) m/z 710 (M+),
682 (MꢀCO+), 654 (Mꢀ2CO+), 626 (Mꢀ3CO+), 598
(Mꢀ4CO+), 570 (Mꢀ5CO+), 542 (Mꢀ6CO+);
C29H25O8BrRe·2CH3COOC2H5:
Found: C 39.49, H 2.72.
C
39.19,
H
3.64.
2.5. Synthesis of compound 7
A mixture of 6 (0.10 g, 0.10 mmol), 1,1,1-tris(4-hy-
droxyphenyl)ethane (0.010 g, 0.033 mmol), K2CO3
(0.05 g, 0.36 mmol) and 18-crown-6 (0.010 g, 0.038
mmol) was refluxed in acetone (15 ml) for 72 h. The
reaction was monitored by TLC. The solvent was re-
moved under reduced pressure and the residue was
extracted with CH2Cl2 (3×10 ml). The combined ex-
tracts were chromatographed using a procedure de-
scribed in Section 2.2 but using a silica gel column
prepared with 10% ethylacetate/hexane. The column
was first eluted with 10% ethylacetate/hexane and the
first generation rhenium dendrimer was then eluted
with 35% ethylacetate/hexane. Solvent was removed
under reduced pressure and the compound 7 was ob-
tained by adding pentane to a CH2Cl2 solution of the
residue obtained, to give the required product as a
colorless oil. Yield 0.084 g, (82%). IR (CH2Cl2), w (CO),
2.7. Crystal structure determination of 3
A colorless crystal selected from a batch obtained by
slow diffusion of pentane into a CH2Cl2 solution of
compound 3 at −30°C, was used for the X-ray diffrac-
tion study. Crystal and refinement data is given in
Table 1. Data were collected at room temperature using
a
Nonius Kappa CCD with 1.5 kW graphite
monochromated Mo radiation. The strategy for the
data collection was evaluated using the COLLECT Soft-
ware [10]. Exposure times of 80 s per frame and scan
widths of 1° were used throughout the data collection.
Four sets of data were collected: a 180° € scan and 3 ꢀ
scans. The four sets of data were scaled and reduced
using Denzo-SMN [11]. Unit cell dimensions were
refined on all data. The data were treated for absorp-
tion corrections using the program SORTAV [12]. The
structure was solved and refined using SHELX97 [13].
Hydrogen atoms were placed in calculated positions
and included in the model during later stages of the
refinement.
1
2021, 1924 cm−1; H-NMR l 7.00 (d, J(H,H)=8.79
Hz, 6H, Ar), 6.84 (d, J(H,H)=8.79 Hz, 6H, Ar), 6.49
(d, J(H,H)=2.39Hz,6H, Ar), 6.39 (t, J(H,H)=1.99
Hz, 3H, Ar), 5.26 (m, 24H, Cp), 4.95 (s, ArCH2Oꢀ,
6H), 3.97 (t, J(H,H)=5.59 Hz, 12H, ꢀCH2Oꢀ), 2.62
(m, J(H,H)=7.59 Hz,12H, CpCH2ꢀ), 2.17 (s, 3H,
ꢀCCH3) 1.96 (m, 12H, CH2CH2Oꢀ); 13C-NMR, l 194.3
(CO), 160.0 (Ar), 156.7 (Ar), 142.0 (Ar), 139.6 (Ar),
129.5 (Ar), 113.9 (Ar), 110.3 (ipso-Cp), 105.8 (Ar),
100.7 (Ar), 83.6 (Cp), 83.11 (Ar), 69.8 (ꢀCH2Oꢀ), 66.6
(ArCH2Oꢀ), 53.7 (ꢀCCH3), 31.0 (CpCH2ꢀ), 29.2
(ꢀCCH3), 24.7 (ꢀCH2ꢀ); MS (MALDI TOF) 3083
(MꢀCO+matrix+), 3214 (Mꢀ3CO+2matrix+), 3345
(Mꢀ5CO+3 matrix+).
3. Results and discussion
Scheme 1 depicts the synthetic strategy involved in
the build-up of a first generation dendrimer containing
six CpRe(CO)3 units at the periphery.
Previously we have synthesized organometallic den-
drimers in which the metal atom is s-bonded to the
dendritic skeleton at the periphery. Using this strategy
we have synthesized dendrimers containing up to 48
metal atoms at the dendritic periphery [7a]. Recently we
prepared a first generation dendrimer in which the
linking unit is primarily derived from one of the ancil-
lary ligands which is p-bonded to the metal atom (see
Scheme 1). Herein we report the systematic construc-
tion of a first generation organometallic rhenium den-
2.6. Synthesis of compound 3
The compound CpRe(CO)3 (0.25 g, 0.74 mmol) was
reacted with n-BuLi (0.6 ml, 1.5 M, 0.96 mmol) at
−78°C in THF (15 ml). The reaction mixture was
stirred at this temperature for 2.5 h and diiodopropane
(0.04 ml, 0.37 mmol) was added. The reaction mixture

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drimer using standard convergent methodology [14] to
investigate the potential of the rhenium functionality
incorporated in the dendrimer.
Compound 2 was formed in 50% yield along with
small amounts (ꢀ2%) of the dimer (CO)3Re[(h5-
C5H4)(CH2)3(h5-C5H4)]Re(CO)3 (3). Compound 3 can
be independently synthesized in 22% yield by treating
the lithiated species 1a with 0.5 equivalents of
diiodopropane.
3.1. Synthesis of [(p5-C5H4)CH2CH2CH2I]Re(CO)3 (2)
and (CO)3Re[(p5-C5H4)(CH2)3(p5-C5H4)]Re(CO)3 (3)
Both 2 and 3 have been characterized by spectro-
scopic methods. The two bands observed in the CO
stretching region of the IR spectra of both 2 and 3 are
in similar positions to those of the parent compound 1.
Three sets of methylene protons at l 3.19, 2.55 and 1.99
ppm and a multiplet due to the Cp ring at l 5.26 ppm
are seen in the 1H-NMR spectrum of 2, and as ex-
pected, six peaks are observed in the 13C-NMR spec-
trum. Two sets of multiplets from the methylene
protons at l 2.47 and 1.70 ppm and a multiplet arising
from the Cp ring is observed at l 5.25 ppm in the
1H-NMR spectrum of 3. In the 13C-NMR spectrum five
signals are seen. In the mass spectrum a molecular ion
peak is observed at 710 corresponding to M+ and
peaks due to subsequent loss of CO groups are in
Compound 2 had been synthesized previously by
Casey and coworkers [15] from CpRe(CO)3 by intro-
ducing an allyl unit on the Cp ring and subsequently
converting the double bond of the allyl group into a
halo-alkyl functionality. We find that this compound
can be synthesized in a single step and with better
overall yields. Thus 2 was synthesized by the addition
of excess of diiodopropane to a THF solution of
[Li(C5H4)Re(CO)3] (1a) [16] Eq. (1).
agreement
with
a
molecular
formula
(CO)3Re(C5H4)(CH2)3(C5H4)Re(CO)3 for 3. Crystals of
3 suitable for X-ray analysis were grown by the diffu-
sion of pentane into a CH2Cl2 solution of 3. An X-ray
crystal structure determination confirms the molecular
structure of the dimer formed. The bond angles and
bond lengths are comparable with the parent com-
pound 1 [17] and other related compounds [18]. The
(1)
Table 1
Crystallographic and structure refinement data for compound 3
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
C19H14O6Re2
710.70
Triclinic
,
average ReꢀC(Cp) bond length (2.304 A) is slightly
(
P1
longer than the average ReꢀC(Cp) bond length (2.284
,
A) in 1. However it is similar to the related compounds
,
a (A)
8.241(1)
11.154(1)
11.699(1)
106.38(1)
102.51(1)
105.07(1)
946.2(2)
2, 2.495
having substitution at the Cp ring (see Ref. [12]). A
PLATON diagram of 3 shown in Fig. 1 depicts its
molecular structure.
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3.2. Synthesis of
{(CO)3Re(C5H4)(CH2)3(O)}2(C6H3)CH2OH (4)
3
,
V (A )
Z, Calculated density (Mg m−3
)
Temperature (K)
293(2)
,
In previous experiments to synthesize organometallic
dendrimers we have made use of a haloalkyl chain
which is s-bonded to the metal atom as the linking unit
in the ‘wedge’ synthesis [7]. Dendrimers up to genera-
tion four have been made in our laboratory using the
organometallic synthon CpRu(CO)2(CH2)3Br. The
haloalkyl substituent in our current procedure is not
directly s-bonded to the metal as in the earlier cases,
but is a substituent on the Cp ligand which is p-bonded
to the metal atom. Related organometallic dendrimers
where the organometallic fragments are bonded to the
dendrimer via a substituent on the Cp ligand have been
reported before. Ferrocene terminated dendrimers [19]
and ruthenium terminated dendrimers [20] have thus
been prepared by using appropriate organometallic
units in which the metal units are linked to the polyaro-
matic ether skeleton via an alkyl chain derived from the
p-bonded Cp ring.
Wavelength (A)
0.71070
12.814
652
0.23×0.35×0.45
Enraf–Nonius Kappa CCD
1° € and ꢀ scans
1.91 to 30.53
Absorption coeffcient (mm−1
F(000)
)
Crystal size (mm)
Diffractometer
Data collection method
Theta range for data collection
(°)
Index ranges
−11BhB11,−15BkB5,
−16BlB16
13452/5635 [Rint=0.047]
97.3%
Numerical (SORTAV)
Full-matrix least-squares on F2
5635/0/245
Reflections collected/unique
Completeness to 2q=30.53
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
0.982
Final R indices [I\2|(I)]
R indices (all data)
Extinction coefficient
R1=0.045, wR2=0.1173
R1=0.0513, wR2=0.1245
0.0106(9)
3
,
Largest different peak and hole
6.12 and −3.35 e A

I.J. Ma6unkal et al. / Journal of Organometallic Chemistry 593–594 (2000) 361–368
365
Scheme 1.
The reaction of 2 with 3,5-dihydroxybenzyl alcohol
in the presence of potassium carbonate and 18-crown-6
in refluxing acetone for 72 h yielded 4. The reaction was
periodically monitored by TLC and the product formed
was isolated by column chromatography as a colorless
oil (yield ꢀ80%). Small amounts of the monosub-
stitued product, (CO)3Re(C5H4)(CH2)3(O)(C6H3)(OH)-
(CH2OH) (5) were also formed in the reaction. Both
products were identified by 1H-NMR spectroscopy.
Two resonances arising from the aromatic protons of
the benzylic group are observed at l 6.51 and 6.35 ppm
in the ratio 2:1 for compound 4. Whereas three reso-
nances are observed at l 6.48, 6.45 and 6.31 ppm in the
ratio 1:1:1 for compound 5 for the same group. The
triplet at l 4.63 ppm, which is due to one of the CH2
groups of the C3 chain, is shifted downfield compared
to the parent organometallic unit 2. The ratio of the
methylene protons to that of the aromatic protons and
the CH2 protons of the benzylic group confirm the
formation of the bis-substituted hydroxy wedge 4. The
relative intensities of all the protons in compound 5 are
also in good agrelement with the formation of the
monosubstituted product.
Both 4 and 5 are further characterized by 13C-NMR,
IR and mass spectral analyses. As expected, five addi-
tional peaks are seen in the 13C-NMR spectrum of 4
and seven additional peaks in the case of 5 compared
with the starting organometallic compound 2. There is
no change in w(CO) from precursor to products in the
IR spectra. A peak corresponding to the molecular ion

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I.J. Ma6unkal et al. / Journal of Organometallic Chemistry 593–594 (2000) 361–368
was observed at m/z 890 (M+) in the FAB mass
spectrum of 4. Peaks corresponding to subsequent loss
of CO groups provide further support for the formation
of the hydroxy dendritic wedge. FAB mass spectrum of
5 also conclusively support the formation of the mono-
substituted compound. Peaks corresponding to the
molecular ion (m/z 516, M+) and subsequent loss of
CO groups are observed in the spectrum.
molecule 1,1,1-tris(4-hydroxyphenyl)ethane (CORE) in
the presence of potassium carbonate and 18-crown-6 in
refluxing acetone for 72 h produced the anticipated first
generation dendrimer 7 containing six rhenium atoms
at the periphery in good yields (82%). The reaction was
1
monitored by TLC. Additional resonances in the H-
NMR spectrum of 7 support the formation of the first
generation rhenium containing organometallic den-
drimer. Two sets of peaks, at l 7.00 and 6.84 ppm
arising from the CORE molecule, are in agreement with
the resulting dendrimer. The benzylic CH2 protons
show a downfield shift to l 4.95 ppm. Furthermore, the
relative intensities of various resonances in the 1H-
NMR spectrum of 7 are in good agreement with the
expected number of protons and thus provide ample
support for the proposed dendrimer. Six carbon reso-
nances due to the CORE molecule, in addition to the
peaks due to the ‘wedge’, are seen in the 13C-NMR
spetrum of 7. The shift observed for the benzylic CH2
carbon atom is as expected due to the replacement of
the bromine atom by the ethereal oxygen from the
CORE molecule. Our attempts to observe a molecular
ion peak in the FAB and electrospray mass spectral
experiments carried out thus far have been unsuccess-
ful. However, the fragmentation peaks observed in
some of the experiments provide valuable information.
The peak observed at m/z 872 in the chemical ioniza-
tion process corresponds to the dendritic wedge
[{(CO)3Re(C5H4)(CH2)3(O)}2(C6H5)CH2]+ and pro-
vides some support for the suggested first generation
dendrimer. We further carried out a MALDI TOF
experiment on 7 in an effort to obtain a molecular ion
peak, using indoleacrylic acid (IAA) as the matrix.
Even though we could not observe the expected molec-
ular ion peak at m/z 2922, a number of peaks
were detected in the mass range close to it. Three
3.3. Synthesis of compound 6
The bis-substituted hydroxy ‘wedge’ 4 was converted
to the corresponding benzyl bromide complex 6 in high
yield (90%) by treatment with CBr4/PPh3 in THF (see
Scheme 1). The reaction was monitored by TLC and it
was found that excess of brominating reagent is re-
quired for the completion of the reaction. The product
formed was separated by column chromatography and
was isolated as a colorless oil.
Compound 6 was charcterized by various spectro-
scopic methods. The notable change in the proton, as
well as carbon spectrum, are that due to the CH2
signals from the benzylic group. An upfield shift of the
CH2 protons (l 4.40 ppm) of the benzylic group
provide ample support for the formation of the bromi-
nated product. A similar shift was seen in the 13C-NMR
spectrum for the benzylic CH2 carbon resonance. A
molecular ion peak at m/z 954 (M+) and peaks corre-
sponding to subsequent loss of CO groups in the FAB
mass spectrum further support the formation of the
brominated product.
3.4. A first generation dendrimer containing six
rhenium atoms
Reaction of three molar equivalents of the benzyl
bromide wedge 6 with one molar equivalent of the core
Fig. 1. Molecular structure of 3.

I.J. Ma6unkal et al. / Journal of Organometallic Chemistry 593–594 (2000) 361–368
367
Fig. 2. Selected portions of 1H-NMR spectra of 2, 4, 6 and 7.
distinct peaks were observed at m/z 3083, 3214 and
3341 with the peak at m/z 3083 showing the highest
intensity of these three peaks. These peaks observed
may be due to adducts formed with the matrix. Thus
the peaks at 3083, 3214 and 3341 can be assigned to
to the nucleii at the reaction sites. In the first step, the
triplet due to the ꢀCH2I group shifts downfield when
the halogen atom is replaced by the oxygen atom from
the building block 3,5-dihydroxybenzyl alcohol. The
three CH2 protons from the C3 chain on the Cp ring
show more or less similar chemical shift, in the remain-
ing steps of the dendrimer construction. The reaction
site now is the CH2 of the benzylic group and it can be
seen from the spectra, that the peaks due to these CH2
protons shift upfield or downfield in the subsequent
steps, depending on the functionality attached to this
CH2 group. Similar changes can be observed in the
13C-NMR as well, and can be effectively used to moni-
tor the progression of the reaction in each step of the
dendrimer build up.
We made several attempts to obtain good elemental
analyses for the compounds reported. However, in most
cases the percentage carbon and hydrogen found were
not in good agreement with calculated values. This we
believe is due to the presence of small amounts of
impurities, or solvents, which we could not remove by
repeated column chromatography or prolonged drying.
However, IR and NMR spectroscopic data along with
FAB and MALDI TOF mass spectrometry experiments
(MꢀCO+matrix)+,
(Mꢀ3CO+2matrix)+
and
(Mꢀ5CO+3 matrix)+, respectively.
We carried out additional MALDI TOF experiments
with different matrices. A similar pattern of multiple
adduct formation was observed with hydroxypicolinic
acid (HPA) and anthranilic acid matrices. Furthermore,
the spectrum obtained for 7 with anthranilic acid as the
matrix showed a peak at 3034 corresponding to
(MꢀCO+matrix)+ and peaks at 3006, 2978, 2950 and
2922 due to subsequent CO losses. Thus, MALDI TOF
experiments along with spectroscopic data strongly sup-
port the formation of the proposed first generation
dendrimer 7.
Multinuclear NMR is particularly useful for the
characterization of the various dendritic ‘wedges’ and
dendrimers. Fig. 2 illustrates the changes that can be
followed in the 1H-NMR spectra in the systematic
construction of the dendrimer using the convergent
approach. The chemical shift changes occur primarily

368
I.J. Ma6unkal et al. / Journal of Organometallic Chemistry 593–594 (2000) 361–368
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strongly support the suggested formulation for all the
new compounds reported.
Having prepared the first generation dendrimer we
turned our attention to its oxidation. Our initial at-
tempts to oxidize the carbonyl groups on the metal
atom in the dendrimer were unsuccessful. We are pur-
suing alternative methods to generate oxo-rhenium
functionalities at the periphery of the dendrimer.
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4. Conclusions
A first generation organometallic dendrimer contain-
ing six rhenium atoms at the periphery can be synthe-
sized by convergent methodology in good yields. By
using an alternative route, we have prepared 2 in good
yields. A dinuclear rhenium compound 3 was formed
during the synthesis of 2 in small quantities. Although
our initial attempts to oxidize the carbonyl groups on
the rhenium dendrimer 7 were unsuccessful, alternative
strategies may be incorporated to synthesize the oxida-
tion products.
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5. Supplementary material
Atomic coordinates, thermal parameters, and bond
lengths and angles for compound 3 are available from
the authors.
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Acknowledgements
We thank the Claude Harris Leon Foundation,
British Council, the University of Cape Town and the
NRF for support. We also thank Professor Derek
Sutton, Dr Colin White and Dr Selwyn Mapolie for
useful discussions.
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