Synthesis and optical and electrochemical properties of glucose-cored ferrocenyl dendrimers

Article abstract of DOI:10.1080/00397911.2016.1245752

Triazole-based ferrocenyl glycoconjugates 1, 2, and 3 were synthesized by regiospecific copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) of azidoferrocenyl derivatives with glucose pentaacetylide. Higher generation ferrocenyl glycoconjugates form the stable ferrocenium cation and exhibit increased light harvesting property as revealed from cyclic voltammogram studies and ultraviolet–visible spectrum respectively due to the presence of more ferrocenyl and triazolyl units than the lower generation dendrimer.

Full text of DOI:10.1080/00397911.2016.1245752

Synthesis and Optical, Electrochemical Properties of Glucose Cored Ferrocenyl
Dendrimers
Ayyavoo Kannan1,2, Ramasamy Anandhan1, Perumal Rajakumar1
1Department of Organic Chemistry, University of Madras, Chennai, India 2Department of
Chemistry, University of the Free State, Bloemfontein, South Africa
Corresponding author Perumal Rajakumar E-mail: perumalrajakumar@gmail.com
Abstract
Triazole based ferrocenyl glycoconjugates 1, 2 and 3 were synthesized by regiospecific
copper (I) catalyzed azide–alkyne cycloaddition (CuAAC) of azidoferrocenyl derivatives
with glucose pentaacetylide. Higher generation ferrocenyl glycoconjugates form the
stable ferrocenium cation and exhibit increased light harvesting property as revealed from
CV studies and UV-Vis spectrum respectively due to the presence of more number of
ferrocenyl and triazolyl units, then the lower generation dendrimer.
Graphical Abstract
1

KEYWORDS: Glucose, ferrocene, Click Chemistry, dendrimer and cyclic
voltammogram
INTRODUCTION
Ferrocenyl dendrimers1 play important role in the field of synthetic, material and
biological chemistry. Ferrocenyl units mainly play an important role in electron-transfer
reaction, which is involved in many living systems as well as in synthetic chemistry as
multi-redox nanosystems, especially in polymers,2 dendrimers,3 nanoparticles,4 vectors,5
biosensors,6 biological redox processes,7 molecular conductors and semiconductors,8
mixed-valence stabilizers,9 catalysis,10 redox recognition11 as redox reagents,12 redox
catalysts,13 electron transfer catalysts,14 redox sensors,15 and as green catalysts.16 Among
all the redox systems, the ferrocenyl dendritic framework becomes important due to Fe2+
and Fe3+ reversible system. Generally, carbohydrates play an important role in living
system because of their interaction with the cell-surface protein which is vital for the
explanation of many biological activities, such as viral and bacterial infections, cell–cell
adhesion, inflammatory and immune response, fertilisation and cancer metastasis.17
Generally, carbohydrate-protein interactions are detected based on labelled proteins or
carbohydrates with a fluorescent or biotin moiety, so that the recognition event induces a
measurable signal.18 The ferrocenyl glycoconjugates are used as electrochemical
biosensor for concanavalin A (Con A)19 through the electro active ferrocenyl ring and
multivalent carbohydrate interacts with protein. In few cases, the biosensors are
recognized from carbohydrates attached with electroactive moieties such as ferrocene.
This triggers electrochemical signals upon binding with the complementary
2

supramolecular partner such as lectin.20 Ferrocenyl-carbohydrate bioconjugates21 have
enormous applications in biology such as anti-tumor,22 anti-inflammatory,23 cytotoxic and
DNA cleaving-activity towards cancer cells,24 antimicrobial and anti-malarial activities,25
and glycoferrocenyl hybrid molecule has been also used as electrochemical probes.26
The regiospecific copper (I) catalyzed azide–alkyne cycloaddition (CuAAC), known as
click chemistry is mainly used for the synthesis of triazolyl bridged ferrocenyl
dendrimers.27 Triazole nucleus is an ideal choice for its interaction with many anions and
transition metals. Thus, the introduction of triazole unit in dendrimers would rather
enable the encapsulation of guest molecules. During recent years, we have reported the
synthesis and properties of glycodendrimers,28a,b carbazole dendrimers,28c m-terphenyl
dendrimers,28d phenothiozine thethered dendrimers,28e quinoline dendrimers,28f
benzothiazole dendrimers,28g pyrene dendrimers28h and chalcone linked dendrimers.28i
Particularly, synthesis and photophysical, electrochemical properties of 1,2,3-triazolyl
−
bridged ferrocenyl dendrimers reveal that they are used for sensing H2PO4 and Pd2+ ion
and also exhibit encapsulation of gold nanoparticles.29 The present investigation mainly
deals with synthesis and optical, electrochemical properties of novel ferrocenyl sugar
hybrid dendrimers 1, 2 and 3 (Fig. 1) using click chemistry.
RESULTS AND DISCUSSION
Reaction of 3,5-bis(propargyloxy)benzyl chloride 5 with 2.1 equiv. of the ferrocenyl
azide 4 under Cu(I)-catalyzed click reaction conditions gave the first generation dendritic
chloride 6 in 92% yield, which on further treatment with NaN3 in a mixture of acetone-
3

water (4:1) at 60 oC gave the dendritic azide 7 in 96% yield. The second generation
dendritic azide 9 was obtained by a similar way by the repeting the above reaction
sequence namely Cu(I)-catalyzed click reaction. Reaction of 1.0 equiv. of 3,5-
bispropargyloxy benzyl chloride 5 with 2.1 equiv. of the azidodendron 7 under click
reaction conditions gave the dendritic chloride 8 in 83% yield, which was further
converted into the dendritic azide 9 in 85% yield using NaN3 in a mixture of acetone-
water (4:1) at 60 oC (Scheme 1).
The 1H NMR spectrum of the dendritic azide 9 displayed three different singlets at
4.16, 4.21 and 4.27 for the ferrocenyl protons, two different singlets at 5.06 and 5.28
for the O-methylene and N-methylene protons, a singlet at 7.51 for triazolyl proton. The
13C NMR spectrum of the dendritic azide 9 displayed the ferrocenyl carbon at 68.6,
68.8, 68.9 and 69.1, and two different triazolyl carbons at 143.3 and 144.1 respectively.
The appearance of molecular ion peak at m/z 723 also confirmed the structure of the
dendritic azide 9. Further the constitution of the dendritic azide 9 was confirmed from
elemental analysis.
Propargylated D-glucose tetraacetate 10 on treatment with sodium methoxide in MeOH-
THF-CHCl3 (6:1:1) at pH 9-10 for 1 h (Zemplen condition) gave the deacetylated
propargyl glucose 11 in 56 % yield, which on further treatment with 4.1 equiv. of
propargyl bromide in presence of NaH in DMF at 0 °C for 10 min gave the penta
propargylated glucose core 12 in 68% yield (Scheme 2).
4

The 1H NMR spectrum of pentaacetylide 12 displayed a multiplet at 2.48 for acetylenic
protons in addition to other aliphatic protons. The 13C NMR spectrum of the penta
acetylide derivative 12 displayed five O-CH2 carbons at 55.9, 58.6, 59.2, 60.0 and 60.2
in adition to other aliphatic carbons. The appearance of molecular ion peak at m/z 393
(M++Na) also confirmed the structure of the pentaacetylide derivative 12. The
constitution of penta acetylide 12 was further confirmed from elemental analysis.
The synthesis of ferrocenyl dendrimers 1, 2 and 3 is shown in Scheme 2. Reaction of 1.0
equiv. pentapropargylated glucose core 12 with 5.1 equiv. of azidomethyl ferrocene 4/
ferrocenyldendritic azide 7/ ferrocenyl dendritic azide 9 under click reaction conditions
for 12 h at room temperature, gave the zeroth, first and second generation ferrocenyl
dendrimers 1, 2 and 3 in 89 %, 86% and 81% yields, respectively (Scheme 2).
The 1H NMR spectrum of the ferrocenyl dendrimer 1 showed three different signals for
the ferrocenyl protons at 4.17, 4.28 and 4.31 and five sharp singlets at 7.55, 7.68,
7.75, 7.84 and 7.89 for five different triazolyl protons in addition to the signal for the
other aliphatic proton. The 13C NMR spectrum of 1 showed three different ferrocenyl
carbons at
69.1, 69.0, 69.7 and the triazole carbons at 144.3, 144.5, 144.7 and 144.9
in addition to other aliphatic and aromatic carbon signals. The appearance of molecular
ion peak at m/z 1575 [M+] also confirmed the structure of the ferrocenyl dendrimer 1.
Further, the constitution of the ferrocenyl dendrimer 1 was confirmed from elemental
analysis. In the 1H NMR spectrum, the ferrocenyl dendrimer 2 should the ferrocenyl
protons as a singlet at 4.16 and the triazole protons as another singlet at 7.56 in
5

addition to the signals for the other aliphatic and aromatic protons. In the 13C NMR
spectrum, the ferrocenyl dendrimer 2 displayed the ferrocenyl carbons at 69.0 and 69.2
and the triazole carbon at 143.3 in addition to the signals for the other aliphatic and
aromatic carbons. The appearance of molecular ion peak at m/z 3987[M+] also confirmed
the structure of the ferrocenyl dendrimer 2. Further, the structure of the ferrocenyl
dendrimer 2 was also confirmed from elemental analysis. The 1H NMR spectrum of the
ferrocenyl dendrimer 3 showed ferrocenyl protons as singlets at 4.13, 4.14, 4.15, 4.20
and 4.28, the triazolyl protons as a singlet at 7.53, 7.58 and 7.71 in addition to the
signals for the other aliphatic and aromatic proton. The 13C NMR spectrum of 3 showed
three different ferrocenyl corbons at
68.9, 69.1 and 69.7 and the triazolyl carbons at
143.2 and 143.6 in addition to the signals for other aliphatic and aromatic carbons. The
appearance of molecular ion peak at m/z 8810 [M+] also confirmed the structure of the
ferrocenyl dendrimer 3. Further, the constitution of the ferrocenyl dendrimer 3 was
confirmed from elemental analysis.
OPTICAL AND ELECTROCHEMICAL PROPERTIES OF FERROCENYL
DENDRIMERS 1, 2 AND 3
Optical Studies
The UV-Vis absorption spectra of all the ferrocenyl dendrimers 1, 2 and 3 were obtained
in DCM (1 X 10-5 M) and the absorption maxima is listed in Table 1. In the absorption
spectra though λ-max remains almost constant between 435 to 438 nm, the absorption
intensity of the UV-visible band increases from the dendrimer 1 to 3.
6

The molar absorptivity coefficient of dendrimer 3 is greater than that of 1 and 2. When
the UV-vis spectrum is recorded for all the dendrimer at the same concentration,
dendrimer 3 showed the highest light absorption coefficient value due to the presence of
greater number of ferrocenyl and triazolyl units, which is usually called valence effect in
dendrimer chemistry.30 In conclusion, the absorption coefficient increases on increasing
of the generation of the dendrimer from 1, 2 and 3 though λ-max remains almost
constant.
Electrochemical Studies
The redox property of the ferrocenyl dendrimers 1, 2 and 3 was studied using cyclic
voltammetry (CV) technique. The cyclic voltammogram for dendrimer 1 to 3 was
obtained in the potential range of 0.0 V to 1.0 V at room temperature in CH2Cl2
containing 0.1 M TBAP as the supporting electrolyte at the scan rate 50 mV/s (Figure 3).
The reversible oxidation potential for the ferrocenyl dendrmer 1-3 was observed at 530,
577 and 636 mV and the reversible reduction peak at 425, 356 and 345mV respectively.
Thus, in the anodic region single reversible oxidation process corresponding to the
generation of ferrocenium cation was observed. Among all the dendrimers, the ferrocenyl
dendrimer 3 undergoes electrochemical oxidation much easily than the other dendrimers
indicating that the presence of more triazole31 unit which is responsible for easier
oxidation. Similarly on increasing the dendrimer generation from 1 to 3 there is decreases
in the redox potential due to the presence of more number of triazolyl units in 3 than in 1
and 2. The variation of the oxidation and reduction potentials in dendrimers 1, 2 and 3
could be due to presence of more ferrocenyl units and bridging triazoly units in the higher
7

generation dendrimer. The Epa and Epc values are shown in Table 1 and the potential
difference ΔE increases from dendrimer 1 to 3.
The variation of ΔE = Epa-Epc values reveals that the dendrimer 3 undergoes anodic
oxidation in the usual potential range as similar to that of the other dendrimers, but the
ferrocenium cation is relatively stable and hence the cathodic reduction value is less and
probably ΔE for dendrimer 3 is slightly higher (0.29) than the value of ΔE for the
dendrimer 1 and 2 (≈0.11 and 0.22). Further the current intensity values obtained for
dendrimer 3 (Figure 3) is much greater than that of dendrimer 1 and 2 which shows that
the ferrocenyl moieties are placed more closely and hence high current intensity is
observed. Further, the increases in the value of ΔE on going from dendrimer 1 to 3
indicates that the electrochemical reversibility decreases from 1 to 3, which further
supports the formation of stable ferrocenyl cation for dendrimer 3. In conclusion, the
electrochemical behavior is altered on increasing the number of ferrocenyl and triazole
moieties in the dendritic architectures.
EXPERIMENTAL
Materials And Methods
All reagents were commercially available and used as such unless otherwise stated.
Analytical TLC was performed on commercial Merk plates coated with Silica Gel
GF254. Analytical samples were obtained from silica gel chromatography, using silica
gel of 100-200 mesh and elution with the solvent system as mentioned under each
experiment section. The melting points were determined by using a Metler Toledo
8

melting point apparatus by open capillary tube method and were uncorrected.1H and 13C
NMR spectra were recorded on a 300 MHz BRUKER AVANCE (75 MHz for 13C
NMR,) spectrometer and Mass spectra (MS) were recorded using Voyager-DE PRO mass
spectrometer by either Fast Atom Bombardment or MALDI-TOF technique. UV-Vis
absorption spectra were measured with a Perkin–Elmer Lambda 35 UV-vis spectrometer
Cyclic voltammetry measurements were performed in a conventional three electrode
system on a CHI model 1100A series electrochemical analyzer (CH Instrument, Austin,
TX). The measurements were based on a three electrode system, with a glassy carbon
(GC) electrode (of geometric area 0.07 cm2) being used as the working electrode, a Pt
wire in the form of a spiral (with a high geometrical surface area B20 cm2) being used as
the auxiliary electrode and Ag/AgCl as the reference electrode with a scan rate of 100
mV s-1. The supporting electrolyte of 0.1 M TBAP.
General Procedure For The Cu (I) Catalyzed Click Reaction (Procedure A)
A mixture of acetylenic derivative (1.0 mmol, 1equiv. ) and azido derivative (5.1 mmol,
5.1equiv.) was dissolved in THF/H2O (1:1; 8 mL) and added sodium ascorbate (0.4
mmol, 0.4 equiv.) followed by the addition of CuSO4 (0.2 mmol, 0.2 equiv.). The
reaction mixture was stirred overnight at room temperature. The solvent was evaporated
and the crude product was dissolved with EtOAc (2 X 100 mL), washed with NH4Cl
solution (50 mL) and brine solution (50 mL), dried over Na2SO4 and concentrated to give
a residue, which was purified by column chromatography (SiO2), using the eluent as
mentioned under each compound
FERROCENYL DENDRIMER 1
9

Ferrocenyl dendrimer 1 (0.38 g) was obtained as brownish solid from propargyl 2,3,4,6-
tetra-O-propargyl-β-D-glucopyranoside 12 (0.1 g, 0.27 mmol) and azidomethyl ferrocene
4 (0.33 g, 1.4 mmol) under click reaction conditions following the general procedure A.
Yield: 89%, MP: 144-146 oC 1H NMR: (300 MHz, CDCl3): H 3.25-3.27 (m, 1H), 3.29-
3.31(m, 1H), 4.17, 4.28 (s, 45H, cp), 4.31 (s, 10H ) 4.62 (s, 1H), 4.74-4.78 (m, 2H), 4.80-
4.87 (m, 2H), 5.25 (s, 10H), 7.55 (s, 1H), 7.68 (s, 1H), 7.75 (s, 1H), 7.84 (s, 1H), 7.89 (s,
1H). 13C NMR: (75 MHz, CDCl3): C 50.0, 68.9, 69.0, 69.7, 81.4, 84.0, 122.5, 123.0,
123.1, 123.2, 123.4, 141.0, 144.3, 144.5, 144.7, 144.9, 145.4, 147.9. MS (ES): m/z =
1575 [M+]. Elemental Anal. Calcd for C76H77Fe5N15O6: C, 57.93, H, 4.93, N, 13.33%.
Found: C, 57.86, H, 4.88, N, 13.26%.
CONCLUSION
In conclusion, glycoferrocenyl conjugated dendrimers 1, 2 and 3 with triazole as the
bridging unit has been obtained through click chemistry in excellent yield. The presence
of more triazolyl and ferrocenyl units increases the light absorbing capacity of the
dendrimer. The electrochemical behaviour of the ferrocenyl-sugar conjugated dendrimers
also indicate that the presence of a greater number of ferrocenyl and triazolyl units
increases the current potential in the cyclic voltammogram curve and all the ferrocenyl
dendrimers shows oxidation–reduction curve. Applications of such dendrimer for the
synthesis of nanoparticles of transition metals such as Au, Pd and Ag are underway.
SUPPORTING INFORMATION
10

The full experimental part, 1H NMR, 13C NMR and Mass spectra details are present in
the electronic supporting information (ESI).
ACKNOWLEDGMENT
The authors thank CSIR, New Delhi, India, for financial assistance and DST-FIST for
providing NMR facility to our department. AK thanks CSIR and UGC, New Delhi, for
fellowship.
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Table 1. Optical and electrochemical parameters of ferrocenyl dendrimers 1, 2 and 3 in
DCM (1 X 10-5 M). Ferrocenyl dendrimers 1, 2 and 3 are almost identical and exhibit an
intense absorption band at 435-438 nm (Figure 2).
Dendrimers λabs
Optical
density
Molar absorptivity
Epc Epa E1/2 ∆E =
Epa-
max
coefficient (ɛ ) L mol-1
(nm)
cm-1)
6000
Epc
438
437
435
0.06
0.13
0.25
0.42 0.53 0.47 0.11
0.36 0.58 0.47 0.22
0.35 0.64 0.49 0.29
1
2
3
13000
25000
15

Scheme 1. Reagents and conditions: (i) CuSO4 (5 mol%), sodium ascorbate (10 mol%),
H2O/THF (1:1), rt, 12 h, 6 (92%), 8 (83%) (ii) 1.5 equiv. NaN3, CH3COCH3/H2O, 60 ºC,
1-3 h, 7 (96%), 9 (85%).
16

Scheme 2. Reagents and conditions: (i) NaOMe (0.1N), MeOH/CH2Cl2/THF (6:1:1), 11
(56%). (ii) 4.1 equiv of Propargyl bromide, NaH, anhydrous THF, N2 for 24 h. 12 (68 %).
(iii) 5.1 equiv. of 4/ 7/ 9, CuSO4 (5 mol%), sodium ascorbate (10 mol%), H2O/THF (1:1),
rt, 12 h, 1 (89%), 2 (86%) and 3 (81%).
17

Figure 1. Structure of the ferrocenyl dendrimers 1, 2 and 3
18

Figure 2. UV-Vis absorption spectra of ferrocenyl dendrimers 1, 2 and 3 in DCM (1 X
10-5 M) at room temperature.
19

Figure 3. Cyclic votammogram of ferrocenyldendrimers 1, 2 and 3 in DCM at room
temperature. (scan rate at 50 mV s-1) and 0.1 m tetrabutylammonium perchlorate as a
supporting electrolyte in dry DCM.
20