Synthesis and characterization of an amphiphilic cobalt cage complex with aza-crown spacer

Article abstract of DOI:10.1016/j.inoche.2010.09.036

The synthesis, spectrophotometric and surfactant properties of a novel amphiphilic cobalt cage metallosurfactant with an aza-oxa crown ether spacer and dodecyl hydrocarbon tail are reported. Surface pressure isotherms with Group I and Group II metal solut

Full text of DOI:10.1016/j.inoche.2010.09.036

Inorganic Chemistry Communications 14 (2011) 79–82
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization of an amphiphilic cobalt cage complex with
aza-crown spacer
⁎
Gina E. Jaggernauth, Richard A. Fairman
Department of Chemistry, The University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2010
Accepted 30 September 2010
Available online 20 October 2010
The synthesis, spectrophotometric and surfactant properties of a novel amphiphilic cobalt cage metallo-
surfactant with an aza-oxa crown ether spacer and dodecyl hydrocarbon tail are reported. Surface pressure
isotherms with Group I and Group II metal solution subphases indicate increasing interaction of the crown
moiety with Ca2+, Na+, K+, Li+/Ba2+ and Mg2+ ions in that order, with mean molecular areas of the effective
head group at the surface ranging from 141 Å2 to 219 Å2. The critical micelle concentrations determined by
surface tension measurements of the unmetallated ligand and the metallosurfactant are 1.98 mM and
0.66 mM respectively. Wormlike micelles and vesicles are observed with atomic force microscopy after
multilayer Langmuir Blodgett film deposition on a glass substrate.
Keywords:
Diamsar
Metallosurfactant
Amphiphile
Aza-crown spacer
Langmuir isotherms
Wormlike micelles
© 2010 Elsevier B.V. All rights reserved.
Introduction
monoacid was isolated by ion exchange/sephadex chromatography
in 45% yield. In our laboratory, negligible amounts of disubstituted
The total encapsulation of transition metal ions into cage-type
ligands derived from 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane,
commonly called sarcophagines [1,2], offers several advantages.
Foremost, is the preservation of chromophore and redox character-
istics of the metal in a range of chemical environments [3,4] and the
associated potential for applications as inert but easily detectable
molecular sensors [5], electron transfer agents [6,7] and radio-
pharmaceuticals [8,9]. If the amine cage is functionalized with
lipophilic pendant groups [10–12], the resulting amphiphiles have
added surfactant, catalytic and biological activity capabilities [13–15].
As part of our interest in delineating structure activity relationships in
mesophases and membrane transport processes, we have synthesized
and herein report the properties of the first example of a [Co(sar)]3+
derivative which combines a lipophilic tail and an aza-crown ether
spacer group for the investigation of ionic transport across cell
membranes.
cage complex were obtained even after extending the reaction time to
five days. The unsymmetrically functionalized diaza-18-crown-6 [G]
was prepared in a parallel synthetic route in 47% yield after silica gel
chromatography (rf =0.5; CHCl3:5% MeOH). Functionalization of the
free amine followed by a condensation reaction with the cage
monoacid and treatment with dilute hydrochloric acid afforded L2
as an orange hydrochloride salt in 34% yield. We have also prepared
the organic amine without the cobalt cage appended (L1) for
comparative analysis. All complexes have been characterized by
standard NMR, IR, and mass spectral analysis (Supplementary data).
Compound L2 is soluble in organic and aqueous solvents, readily
dissolving in water, methanol and chloroform. The UV/VIS spectrum
(Fig. 1) of a 2 mM solution (orange) shows a hypsochromic shift to
458 nm in both water and chloroform as compared to 470 nm for the
1A1g←1T1g transition of Co3+ in aqueous sar [1]. In preparing samples
of L2 for NMR analysis in dimethylsulfoxide (DMSO), a bright green
colour was noted (λmax =689 nm) that may be attributable to the
effects of DMSO on the aggregation behavior of the ligand or on the
reversible oxidation/reduction process for the cobalt (II) ion. We have
explored the aggregation of L1 and L2 further with surface tension
and surface pressure measurements where the differences in
solvation, counterion interactions and hydrogen bonding are
expected to influence the derived mean molecular areas [17].
Results and discussion
The target compound was prepared in a convergent route
according to Scheme 1 using literature methods for the preparation
of monofunctionalized cobalt(III)daimsar [16]. Typically, a 10-fold
excess of chloroacetate was added to diaminosarcophagine(diamsar)
and after constant pH adjustment of the reaction mixture, the
The discontinuity in the graph of surface tension against log
concentration [18] for L2 at 28 °C (Fig. 2) gives a critical micelle
concentration (cmc) of 0.66 mM and an effective surface pressure of
44 mNm−1. Without the cage cation (L1), the corresponding values
are 1.9 mmol and 41 mNm−1 respectively. Relative to the surfactant
⁎
Corresponding author. Tel.: +868 662 2002x3570; fax: +868 645 3771.
E-mail address: richard.fairman@sta.uwi.edu (R.A. Fairman).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.036

80
G.E. Jaggernauth, R.A. Fairman / Inorganic Chemistry Communications 14 (2011) 79–82
Scheme 1. Synthesis of cobalt cage amphiphile. (i) DMF, K2CO3, 4d, (ii) p-TsCl, NaOH /THF, 0 °C, (iii) BzNH2, MeCN, Na2CO3, 3d, (iv) H2, Pd/C, 50 °C, 8 h, (v) CH2CHCN, 12 h, (vi)=(iv),
(vii) SOCl2, 3 h and (viii) K2HPO4, RT, 12 h.
properties of a similar but unmetallated crown [19] (cmc=0.16 mM),
and a metal cage amphiphile which lacks the crown subunit [11]
(cmc=1.3 mM), L2 shows intermediate surfactant properties. How-
ever, the contribution of the crown to the aggregate and interfacial
organization remains unclear. We therefore estimated mean molec-
ular areas (mma) from surface pressure isotherms with, alternately,
the unmetallated ligand, the cage functionalized ligand and with
metal ion subphases that may possibly coordinate to the aza-crown
subunits of both (Table 1).
Fig. 3 presents the π-A isotherms for the pure ligand and the
metals which exert the greatest influence on the surface pressure of
the monolayer from the aqueous subphase. Potassium and calcium
lower the surface pressure as the monolayer is compressed, but
lithium, magnesium and barium cause significant increases. The
selectivity of the aza-crown moiety for magnesium ions gives a
dramatic doubling of surface pressure with the ligand L2, suggesting
strong electrostatic repulsion between ions of like charge and
concomitant increase in the spacing requirement between the polar
components at the air–water interface. In this conformational
hypothesis, the amide functionality enhances the metal-ligand
stability but restricts the orientation of the cobalt cage (Scheme 2).
The role of ionic solvation is also apparent in the matching profiles
obtained for L2 with lithium ion in the subphase (high charge density
and degree of hydration) versus the larger but doubly charged
magnesium ion.
0.3
Water
DMSO
0.2
For comparable cage amphiphiles which lack the aza-crown
spacer, Karaman et al. [13] and Wanless et al. [20] have reported
mma of 85 Å2 and 154 Å2 with straight and branched tails respec-
tively. Despite having longer tail lengths, we may bracket this analysis
with the surfactant properties of functionalized diaza crown ether
ligands (no caged-metal head group) and the rationale provided by
Klein Gebbink et al. [21] whereby values of mma up to 118 Å2 are
obtained due to the crown adopting a flat extended shape parallel to
the air–water interface. In summary, the components of L2 have
limited impact on the cmc but dramatically increase the mma at
the interface due to the combined effects of the cobalt cage and the
aza-crown ether.
0.1
0
400
500
600
700
Wavelength (nm)
Fig. 1. Visible absorption spectrum of l2 in water and dimethylsulfoxide.
56
Without the cobalt cage pendant group, monolayers of ligand L1
are much more easily compressed (Fig. 4), showing an onset of surface
L1
L2
52
48
44
40
Table 1
Ligand mean molecular areas from surface pressure isotherms.
In subphase
Ligand/(Å2 2.6)
L1
L2
Water
Li
Na
K
Mg
Ca
Ba
59.4
–
153.5
171.6
151.1
157.7
218.5
140.9
170.9
48.9
51.7
67.2
–
-6
-5
-4
-3
-2
-1
Log Concentration
–
Fig. 2. DuNouy ring surface tension measurements for L2 and L1.

G.E. Jaggernauth, R.A. Fairman / Inorganic Chemistry Communications 14 (2011) 79–82
81
25
25
20
15
10
5
L2-Mg(II)
L1-Mg(II)
L1-H2O
L1-Na+
L1-K+
20
L2-Li+
L2-H2O
15
L2-K+
10
L2-Ca(II)
L1-H2O
5
0
0
20
20
40
60
80
100
70
120
170
220
270
320
370
Fig. 4. Surface pressure isotherms of l1 with varying subphase.
Fig. 3. Surface pressure isotherms with varying ligand and subphase.
Conclusion
pressure increase only after the mean molecular area is reduced to
about half of the value of ligand L2. The surface pressure exerted by
monolayers of ligand L1 is also marginally higher when magnesium is
present in the subphase, consistent with the coordination modes
suggested above for L2.
We report the synthesis, amphiphilic properties and selectivity of a
new metallosurfactant molecule with a diaza crown ether bridging
the cobalt cage head group and the lipophilic tail. The critical micelle
concentration, aggregate morphologies and surface pressure iso-
therms for this multicomponent amphiphile suggest that an expand-
ed family of such compounds should afford a greater understanding of
the theoretical constructs relating molecular dimensions, aggregation
number, activity coefficients in solution and electroneutrality of ionic
surfactants to macroscopic observables like surface tension [23]. We
are therefore preparing analogues with varying tail lengths and
macrocycle ring sizes to investigate these relationships.
Evidence of the aggregation motifs was sought in tapping mode
atomic force microscopy of glass substrates coated with up to four
monolayers in a standard dipping experiment using the Langmuir
Blodgett trough. After drying the films, the amphiphile is observed as
vesicles, ranging from 60 nm to 250 nm in a cross sectional area
(Fig. 5) and as wormlike micelles (inset) which are similar to the
morphologies observed by Koutsantonis et al. [12]. We are exploring,
with molecular mechanics and dynamic simulations, correlations of
the mean molecular areas to an effective surfactant packing
parameter (N) that will adequately model the polar head group
proxy of amphiphile L2. Determining whether the simplified
geometrical treatment [22,23] of volume per molecule (V), head
group effective area (A) and extended chain length (l) formulation
obtains (N=V /Al) will be an important finding for this new class of
amphiphiles.
Acknowledgements
This work was supported by the Research and Publication Fund of
the University of the West Indies (UWI), St. Augustine. The authors
are grateful to Dr. Richard Taylor (UWI) for providing atomic force
microscopy.
Scheme 2. Metal induced amphiphile reorientation at the air water interface.

82
G.E. Jaggernauth, R.A. Fairman / Inorganic Chemistry Communications 14 (2011) 79–82
Fig. 5. Tapping mode atomic force micrograph of multilayered l2 on glass substrate. *Open arrows highlight wormlike micelles.
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Appendix A. Supplementary material
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Supplementary data to this article can be found online at
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