Divergent assembly of a starburst tetracosacobalt compound

Article abstract of DOI:10.1039/a807838f

The syntheses ofC[{p-C₆H₄C≡C}(n)H]₄ (n = 1,2,3) and their reactions with Co₂(CO)₈ have led to the formation of starburst molecules with rigid, cluster-containing arms; up to twelve cluster units have

Full text of DOI:10.1039/a807838f

Divergent assembly of a starburst tetracosacobalt compound
Edwin C. Constable,* Oliver Eich, Catherine E. Housecroft* and Lesley A. Johnston
Institut für Anorganische Chemie, Universität Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland.
E-mail: housecroft@ubaclu.unibas.ch
Received (in Cambridge, UK) 8th October 1998, Accepted 30th October 1998
The syntheses of C[{p-C6H4C·C}nH]4 (n = 1 ,2 ,3) and their
reactions with Co2(CO)8 have led to the formation of
starburst molecules with rigid, cluster-containing arms; up
to twelve cluster units have been incorporated.
materials. Here, we report initial studies on the synthesis of
metal-rich species containing low oxidation state carbonyl
clusters with precise spatial distribution. We hope to use
molecules of this type, which exhibit distinct onion-like shells
of ‘organic’ and ‘inorganic’ components, for the formation of
metal oxide nanospheres with precise dimensions.
Very recently, there has been active interest in the use of
multifunctional alkynes containing 1,3,5-triethynylbenzene
We are currently developing synthetic strategies for the
preparation of monodispersed dendritic and starburst molecules
containing metal centres,1–3 with a view to developing novel
(CO)3
Co
Co2(CO)8
C
H
C
H
(i)
–2CO
C
H
C
H
Co
(CO)3
(ii)
n
4
4
n
4
4
n
n + 1
1 n = 1
2 n = 2
3 n = 3
4 n = 1
5 n = 2
6 n = 3
1 n = 1
2 n = 2
2 n = 1
3 n = 2
Scheme 1 Reagents and conditions: i, p-IC6H4C·CSiMe3; ii, NaOH (aq).
Scheme 2
H
(OC)3Co
Co(CO)3
(OC)3Co Co(CO)3
(OC)3Co Co(CO)3
(CO)3
Co
(CO)3
Co
(OC)3Co
Co
(CO)3
(CO)3
Co
Co(CO)3
(OC)3Co
Co
(CO)3
Co(CO)3
(OC)3Co
H
Co
(CO)3
Co(CO)3
(OC)3Co
H
Co(CO)3
Co(CO)3
(OC)3Co
Co(CO)3
(OC)3Co
H
Fig. 1 Proposed structure of compound 6.
Chem. Commun., 1998, 2661–2662
2661

cores as building blocks for starburst systems.4–7 Here, we focus
on multifunctional alkynes radiating from a tetrahedral carbon
centre; the compound C(p-C6H4I)4 has previously been
‡ 2: Alkyne 1 (83 mg, 0.20 mmol), p-IC6H4C·CSiMe3 (251 mg, 0.837
mmol), CuI (15 mg, 0.080 mmol) and [(PPh3)2PdCl2] (56 mg, 0.080 mmol)
were stirred in dry, degassed NEt3 (10 ml) under argon for 42 h at 41 °C.
Chromatographic work-up [alumina, hexane–CH2Cl2 (4:1)] gave the TMS-
protected intermediate as a yellow crystalline solid; it was dissolved in THF
(30 ml), and 1 M NaOH (30 ml) added; the solution was stirred for 1 h at
room temperature. Water was added and after extraction with CH2Cl2, the
residue was purified by column chromatography [SiO2, hexane–CH2Cl2
8
recognised as a precursor for the assembly of nanostructures
such as organometallic tripodaphyrins.9,10 We report here the
reaction of alkynes 1–3, (prepared by a divergent strategy), with
dicobalt octacarbonyl to give starburst molecules 4–6 which
possess rigid, cluster-containing arms. The synthetic procedure
may be extended to allow the incorporation of any desired
number of alkyne substituents. Reactions between alkynes and
Co2(CO)8 are well established, and provide a useful means of
entry into this novel area.11–13 More recently, Diederich and
coworkers have shown that cyclic systems containing
{C2Co2(CO)6} units are stabilised with respect to the corre-
sponding polyalkynes.14,15
1
(1:1)] to give a yellow crystalline product (107 mg, 66 %). H NMR (250
MHz, CDCl3) d 7.47–7.43 (m, 24H), 7.20 (d, 8H, J 8.3 Hz), 3.18 (s, 4H);
13C NMR (63 MHz, CDCl3) d 146.1, 132.1, 131.5, 131.2, 130.9, 123.6,
122.0, 121.1, 90.9 (Calkyne), 89.3 (Calkyne), 83.3 (Calkyne), 78.9 (Calkyne), 65.0
(Cquat); MS (MALDI-TOF) m/z 816 [M]+, 614 [M 2 2C6H4CCH]+, 413 [M
2 4C6H4CCH]+.
3: Alkyne 2 (47 mg, 0.058 mmol), p-IC6H4C·CSiMe3 (72.5 mg, 0.242
mmol), CuI (4.4 mg, 0.023 mmol) and [(PPh3)2PdCl2] (16.2 mg, 0.023
mmol) were stirred in dry, degassed NEt3 (5 ml) under argon for 12 h at
35 °C. Work-up similar to that for 2 gave a white crystalline solid (37 mg,
53 %). 1H NMR (250 MHz, CDCl3) d 7.50–7.44 (m, 40H), 7.22 (d, 8H, J
8.3 Hz), 3.18 (s, 4H); 13C NMR (75 MHz, CDCl3) d 146.1, 132.1, 131.6,
131.5, 131.2, 130.9, 123.5, 123.3, 122.1, 121.2, 91.0 (Calkyne), 90.9
(Calkyne), 90.7 (Calkyne), 89.6 (Calkyne), 83.2 (Calkyne), 79.0 (Calkyne), 65.0
(Cquat); MS (MALDI-TOF) 1217 [M]+, 915 [M 2 {C6H4C2}3H]+, 613 [M
2 2{C6H4C2}3H]+.
§ 5 and 6: As for 4. Isolated yields of 5 and 6 were 26 and 45% respectively.
5: IR (CHCl3, cm21) nCO 2089s, 2057vs, 2028vs. 1H NMR (250 MHz,
CDCl3) d 7.60–7.29 (m, 32H), 6.39 (s, 4H); 13C NMR (75 MHz, CDCl3) d
199.1 (CO), 145.7, 138.4, 137.3, 136.3, 131.6, 130.6, 129.6, 128.5, 91.7
(Ccluster), 91.3 (Ccluster), 89.1 (Ccluster), 72.8 (Ccluster), 65.0 (Cquat). 6: IR
(KBr pellet, cm–1) nCO 2090s, 2052vs, 2020vs. 1H NMR (300 MHz, CDCl3)
d 7.61–7.31 (m, 48H), 6.40 (s, 4H); 13C NMR (75 MHz, CDCl3) d 199.1
(CO), 145.7, 138.3, 138.1, 137.4, 136.3, 131.7, 130.7, 129.8, 129.7, 129.6,
128.6, 91.7 (Ccluster), 91.6 (Ccluster), 91.4 (Ccluster), 91.3 (Ccluster), 89.1
(Ccluster), 72.9 (Ccluster), 65.0 (Cquat); MS (MALDI-TOF) m/z 4653 [M]+.
Compound 1 was prepared as previously reported,8–10 and
reaction of 1 with an excess of Co2(CO)8 gave 4 as a brown solid
in 46% isolated yield after chromatographic purification.† The
IR spectrum showed the expected absorptions in the carbonyl
1
region. In the H NMR spectrum, the change in the chemical
shift of the alkyne proton from d 3.06 in 1 to d 6.37 in 4 was
consistent with the incorporation of the RC·CH group into a
1
Co2(CO)6(RCCH) cluster unit. Both the H and 13C NMR
spectra were in accord with a symmetrical product, i.e. reaction
of all four alkyne functionalities with Co2(CO)8 and this was
supported by the observation in the MALDI-TOF mass
spectrum of a parent ion corresponding to 4.
Systematic growth of each polyyne chain from the carbon
core of 1 was achieved‡ by sequential divergent reaction as
shown in Scheme 1. Compounds 2 and 3 were characterised by
NMR spectroscopy and mass spectrometry. Reaction of each
alkyne generation with an excess of Co2(CO)8 (Scheme 2) gave
moderately good yields of compounds 5 and 6.§ The spectro-
scopic data for 5 and 6, and mass spectrometric data for 6 (no
parent ion could be obtained for 5) were fully in accord with
cluster formation at each of the alkyne functionalities of the
respective precursors. 13C NMR chemical shift correlations
across the series of compounds 1–6 provided additional support
for their formulation. A schematic representation of compound
6 is given in Fig. 1. In going from 3 to 6, each carbon
incorporated into a Co2C2-cluster core undergoes a change in
local geometry and the Cring–Ccluster–Ccluster–Cring is no longer
linear. Crystallographic determinations of [1,4-{Co2-
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(CO)6CHC}2C6H4]16 and [Co2(CO)6C2Ph2]17 show the Cring
–
Ccluster–Ccluster angles to be ca. 140°. We estimate from
molecular modelling studies that the radius of compound 6 is in
the range 1.6–2 nm.
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We are currently extending these studies to higher genera-
tions and to the oxidative degradation of the compounds.
This work was supported by the Schweizerischer Natio-
nalfonds zur Förderung der wissenschaftlichen Forschung, the
University of Sydney (Eleanor Sophia Wood Travel Scholar-
ship, L. A. J.), and the University of Basel. We thank Dr W.
Amrein (ETH, Zürich) for obtaining mass spectrometric data
for 4.
Notes and references
† 4: Alkyne 1 (19 mg, 0.046 mmol) and Co2(CO)8 (94 mg, 0.27 mmol) were
stirred in CH2Cl2 (or acetone) for 1 h, the solvent removed, and the residue
purified by column chromatography [SiO2, hexane:CH2Cl2 (1:1)] to give a
dark brown solid (33 mg, 46%). IR (CHCl3, cm21) nCO 2092s, 2057vs,
2027vs. 1H NMR (300 MHz, CDCl3) d 7.44 (d, 8H, J 8.1 Hz), 7.22 (d, 8H,
J 8.1 Hz), 6.37 (s, 4H); 13C NMR (101 MHz, CDCl3) d 199.4 (CO), 145.8,
135.5, 131.4, 129.5, 89.5 (Ccluster), 73.0 (Ccluster), 64.9 (Cquat); MS
(MALDI-TOF) m/z 1532 [M 2 CO]+.
Communication 8/07838F
2662
Chem. Commun., 1998, 2661–2662