Redox-active dirhodium(II,II) species covalently entrapped into a methylmethacrylate backbone

Article abstract of DOI:10.1039/b509281g

The synthesis of novel copolymers based on redox-active dirhodium (II,II) systems covalently attached to the PMMA backbone was carried out. The copolymers showed glass transition temperature due to low content of dirhodium. The dirhodium complex content in the copolymers was determined by UV-vis spectroscopy. The results show that the synthetic approach based on a conventional radical polymerization of an acrylate-functionalized dirhodium complex with MMA, has been demonstrated to be a convenient route for the introduction of functional metal system into the PMMA polymer backbone.

Full text of DOI:10.1039/b509281g

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C O M M U N I C A T I O N
Redox-active dirhodium(II,II) species covalently entrapped into a
methylmethacrylate backbone†
Sandra Lo Schiavo,*a Placido Mineo,b Giuseppe Tresoldi,a Paola Cardianoa and
Pasquale Piraino*a
a Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` di
Messina, Salita Sperone n. 31, 98166, Vil. S. Agata, Messina, Italy.
E-mail: loschiavo@chem.unime.it; Fax: 39 90 393756; Tel: 39 90 6765721
b Consiglio Nazionale delle Ricerche, Istituto di Chimica e Tecnologia dei Polimeri
c/o Dipartimento di Scienze Chimiche, Universita` degli Studi di Catania, Viale A. Doria 6,
95125, Catania, Italy. E-mail: gmineo@dipchi.unict.it; Fax: 39 95 221541; Tel: 39 95 7385033
Received 5th July 2005, Accepted 20th July 2005
First published as an Advance Article on the web 1st August 2005
A novel class of polymers was obtained by insertion of
dirhodium(II,II) metal systems into a methacrylate back-
bone; the synthesis was realized by free radical poly-
merization of an appropriate methacrylate-functionalized
dirhodium polymer precursor, namely [Rh2(form)2(MA-
dirhodium(II,II) core, followed, through a sol–gel process, by
hydrolysis and condensation of the silanole groups to form a
siloxane network.9 As a continuation of this study we planned
the synthesis of dirhodium(II,II) complexes covalently anchored
to a polymethylmethacrylate (PMMA) backbone. Methacry-
lates constitute a class of polymers of relatively straightforward
synthetic access, which offer, with respect to siloxanes, advan-
tages in terms of solubility and hence of processability. The
preparation of the above materials involves the copolymerization
of MMA, via a free radical process, with a dirhodium(II,II) com-
plex functionalized with a methacrylate fragment, which acts as
a comonomer. This last target was achieved by introducing in the
equatorial positions of the lantern structure of dirhodium(II,II)
two carboxylate groups with dangling methacrylate fragments.
By exploiting the peculiar equatorial reactivity of
dirhodium(II,II) species, the dirhodium-MA polymer precursor,
[Rh2(form)2(MA-COO)2] (1), featured by the presence of two
formamidinates and two MA-functionalized carboxylate groups
in a cis-arrangement around the dirhodium core, was readily
realized by reaction of [Rh2(form)2(CF3COO)2(H2O)2]3a or
COO)2] (form
=
N,Nꢀ-di-p-tolylformamidinate) (MA-
COO = 2-(methacroyloxy)ethyl-phthalate), with methyl-
methacrylate (MMA); the new copolymers, in solution,
show reversible CO-absorption, connected to the axial
reactivity of dirhodium(II,II) species.
The design and synthesis of new functional polymers with
defined properties and high processability constitute a topic
a current interest because of their impact in the large field
of materials science and polymer technology. Transition metal
(TM) containing polymers appear particularly promising in this
context, as they combine the intrinsic properties of the metal
complexes (catalytic, electrical, optical, etc.) with the processing
advantages of organic polymers.1,2 The incorporation, through
complexation, of metal complexes into a polymer backbone
constitutes today the most pursued synthetic approach to these
materials. It may be realized: i) by the covalent attachment of
the metal to a functionalized polymer; ii) by the introduction of
a polymerizable ligand in the coordination sphere of the metal
system, which will act as a polymeric precursor.
Dirhodium(II,II) species with a lantern structure represent
a versatile class of compounds characterized by a rich chem-
istry. This spans from axial and/or equatorial reactivity,3
redox,4 catalytic5 and biological activity,6 to the formation of
supramolecular species and coordination polymers.7 Their redox
activity is connected to the reversible one-electron oxidation
dirhodium(II,II) ꢀ dirhodium(II,III) process and may be con-
trolled by varying, to some extent, the bridging ligands around
the dirhodium core.3,4
Our experience in the chemistry of dirhodium(II,II)
complexes3,4,7b,c led us to explore the potential of this class of
compounds in producing funtional TM-based polymers. Due to
their peculiar redox-properties they could be particularly useful
in the development of novel modified electrodes, as electrocata-
lyst and electrochemical sensors.8 We recently succeeded in the
preparation of dirhodium(II,II) hybrid-nanostructured materials
via the covalent entrapment of the metal complex [Rh2(form)2-
(CH3COO)2] (form = N,Nꢀ-di-p-tolylformamidinate) into a
siloxane network. The synthetic route involved the coordination
of two amino-organo alkoxysilanes to the axial sites of the
10
[Rh2(form)2(CH3CN)6]-[BF4]2 with potassium 2-(methacroyl-
oxy)ethylphthalate (MA-COOK). The proposed structure,†
was accomplished by elemental analysis, IR, and NMR
spectroscopic data, and confirmed by MALDI-TOF mass
spectrometry.‡ The mass spectrum shows, in fact, three base
signals at m/z = 1206, 1228, and 1244 relative to the molecular
ion peaks [MH+], [MNa+], and [MK+], respectively.† 1 is a
stable green solid, which dissolves well in all the most common
solvents, such as diethyl ether, chloroform, acetone, and less
well in alcohols. CV studies show that, analogous to other
dirhodium(II,II) systems, the [Rh2(form)2(MA-COO)2] complex
exhibits two distinct, reversible, one-electron oxidations at
0.36 and 1.22 V, arising from dirhodium(II,II)/dirhodium(II,III)
and dirhodium(II,III)/dirhodium(III,III) oxidation processes,
respectively.
Polymerization of 1 with MMA leading to copolymers 2–
ꢀ
4 (Fig. 1) was performed in net MMA by using AIBN (a,a -
azoisobutyronitrile) as radical initiator.§
† Electronic supplementary information (ESI) available: Experimen-
tal details; synthesis and positive MALDI-TOF (experimental and
calculated) for [Rh2(form)2(MA-COO)2]; GPC traces of 2; posi-
tive MALDI-TOF of 3; UV-vis spectra of 2–4 and PMMA. See
http://dx.doi.org/10.1039/b509281g
Fig. 1 Schematic representation of copolymers 2–4.
All copolymers dissolve well in chlorinated solvents and THF,
have good average molecular weights and show a good film
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 2 9 7 9 – 2 9 8 1
2 9 7 9
©

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Table 1 Macromolecular properties of copolymers 2–4
Complex content (w/w) (%)
Samples
Mw/Da
Mn/Da
Experimental
Theoretical
Tg/◦C
Copolymer 2
Copolymer 3
Copolymer 4
50700
50300
100000
30000
22000
43100
1.2
2.0
2.5
0.53
0.78
1.18
113
124
125
formation ability when casting from CHCl3 solutions leading to
green-colored films. Conventional spectroscopic methods, GPC,
MALDI-TOF spectrometry and DSC were used to characterize
the new materials. GPC-average molecular weight values of
copolymers and other features are summerized in Table 1.
The dirhodium complex content in the copolymers was deter-
mined by UV-vis spectroscopy. Analogously to the dirhodium-
methacrylate precursor, 1, copolymers 2–4 show in the UV-vis
spectra a maximum absorption at 334 nm, which is not observed
in that of the PMMA homopolymer.† The spectroscopic data
reveal a metal complex content which significantly differs from
that present in the initial reaction mixtures (Table 1). Such a
discrepancy may be the consequence of the purification treat-
ment from CHCl3/EtOH, which resulted in loss of unreacted
MA monomer and low molecular weight oligomers. Washing of
the crude precipitate with diethyl ether ensured the absence of
unreacted starting complex in the copolymer samples analyzed.
GPC experiments, performed using both differential refrac-
tometer and UV-visible (set at 334 nm) detectors connected
in parallel, exhibit for copolymers 2–4 superimposable RI and
UV traces pointing out the insertion of the dirhodium units
along the macromolecule chains in all the molecular mass
distribution. All the copolymers display a bimodal trend for
the RI distributions,† indicative of the presence of structures
with different hydrodynamic volumes, which may be attributed
to the contemporary presence of PMMA homopolymer and
dirhodium-MMA branched copolymers. Due to low content of
dirhodium, all the copolymers show glass transition temperature
(Tg) values (Table 1) consistent with the literature data reported
for PMMA polymers. This demonstrates that the copolymer
thermal properties are not influenced by the incorporation of
the metal complexes and that the processing features (extrusion,
at 2062 cm−1 consistent with CO axially coordinated to the
dirhodium(II,II) species.3b It was observed that CO release, quite
rapid in solution (ca. 1 h.), occurs, in the solid state, in ca. 60 h
◦
at r.t. and in 24 h at 40 C. The same experiments performed
on solid copolymer samples of 2–4 did not give any evidence of
CO absorption as a consequence, very likely, of the low content
of dirhodium moieties in the samples. In contrast, concentrated
chloroform solutions of 2–4 flushed with CO afford IR and UV-
vis spectra comparable with that of the CO-[Rh2(form)2(MA-
COO)2] adduct displaying a signicant vis-absorption at 552 nm
(Fig. 2).
Fig. 2 UV-vis spectra of CO adducts of the MA-dirhodium(II,II)
comonomer, 1, and copolymer 4.
In conclusion, novel copolymers based on redox-active
dirhodium(II,II) systems covalently attached to the PMMA
backbone were synthesized for the first time. The synthetic
approach used, based on a conventional radical polymerization
of an acrylate-functionalized dirhodium complex with MMA,
has been demonstrated to be a convenient route for the intro-
duction of functional metal systems into the PMMA polymer
backbone. The presence of metal complexes inside the polymer
network was unambiguously confirmed by combined GPC and
UV-vis analyses. Further studies including the morphological
characterization and the potential applications of these new
materials are under way. Analogous to the dirhodium precursor,
the new copolymers, in solution, display a good reversible
CO-binding capability, connected to the axial activity of the
dirhodium(II,II) species. New studies are planned to enhance
such a property, which could be of interest for applications in
the context of gas sensors.
1
stamping, etc.) are maintained. Consistently IR and H NMR
spectra display only bands associated with the PMMA polymer
backbone.
At the MALDI-TOF mass spectrometric analysis no signal
attributable to the dirhodium-MMA copolymers was identified.
Copolymers 2 and 3 afforded similar spectra, which consisted
essentially of a series of peaks corresponding to PMMA units,
while copolymer 4, which exhibits the highest metal complex
content and molecular mass (Table 1), did not afford any
MALDI-TOF signals.† The absence of copolymer signals may
be explained by the fact that i) the dirhodium-MMA copolymers
give signals partially overlapping those of the PMMA units,
or, ii) the low content of the dirhodium complex and the
presence of two MA groups in the comonomer 1 can lead to
branched systems having molecular masses too high and molar
abundances too low to be detected by MALDI-TOF analysis.
As a consequence of the free axial sites 1–4 exhibit the
well known axial reactivity of dirhodium(II,II) complexes with
a lantern structure. This allows the introduction, at the axial
sites, of a large number of Lewis bases, even water, leading to
the formation of mono- and bis-adducts, whose deep color is
dependent on the donor atoms. In particular, for its interesting
applications, we, preliminarly, paid attention to the reversible
CO binding ability of 1–4. The dirhodium-MA comonomer
1 displays, at atmospheric pressure and room temperature,
reversible CO axial binding both in solution and as a solid as
revelead by the color change from green to red. The process was
followed by IR spectroscopy which shows a diagnostic band
Acknowledgements
This work was supported by MIUR.
Notes and references
‡ C58H56N4O12Rh2: calcd. C 57.72, H 4.68, N 4.64; found C 58.12, H 4.83,
N 4.73. Yield 85%. IR (KBr pellet, cm−1): 1718 (C O), 1620 (CO2 asym),
1590 (N–C–N). H NMR (CDCl3): d 1.86 (s, 6H), 2.23 (s, 12H), 4.16
=
1
(unresolved AX system, 8H), 5.77 (d, 4H, J = 1.7 Hz), 6.94 (m, 16H),
7.36 (4, m), 7.43 (m, 4H), 7.60 (t, br, 2H). UV-vis (CHCl3, nm) kmax
(e, M−1 cm−1): 334 (11060). MALDI-TOF MS (m/z): 1206 [MH+], 1228
[MNa+], 1244 [MK+].
§ 50 mg (0.042 mmol) of the dirhodium(II,II)-MA complex, 1, were
dissolved in variable amounts of net MMA under a nitrogen atmosphere,
9.36 g (0.0935 mol) for copolymer 2, 6.32 g (0.0630 mol) for 3, and 4.22 g
(0.042 mol) for 4, respectively), and AIBN added in 2 wt% with respect to
MMA. The resulting solution was slowly heated to 80 ◦C, and left to stir
until polymerization occurred (10 min), as evidenced by the formation
2 9 8 0
D a l t o n T r a n s . , 2 0 0 5 , 2 9 7 9 – 2 9 8 1

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of a light green solid. This was dissolved in chloroform (300 ml) and the
resulting solution added to ethanol (400 mL) inducing the precipitation
of the copolymers 2–4 as a green solid. This was washed with diethyl
ether to remove unreacted starting comonomer 1 and dried in vacuo.
Yield 60%. IR (KBr pellet, cm−1): 1731, 1480, 1434, 1370, 1268, 1242,
1189, 1148, 1061, 989, 842, 750, 722. 1H NMR (CDCl3) d 0.84, 0.86, 1.02
(s, a-CH3); 1.2–1.9 (br, overlapped, CH, CH2, CH3); 3.60 (s, OCH3). UV-
vis (CHCl3, nm): kmax: 334.
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2 9 8 1