Synthesis and redox properties of dinuclear rhodium(II) carboxylates with 2,6-di-tert-butylphenol moieties

Article abstract of DOI:10.1016/j.ica.2010.01.029

By exploiting the peculiar reactivity of Rh₂(μ-O₂CBu^t)₄ (H₂O)₂ (1) the examples of dinuclear rhodium(II) carboxylates containing N-donor axial ligands (2, 3) Rh₂(μ-O₂CBu^t)₄L ₂ where L = benzonitrile (2), 3,5-di-tert-butyl-4-hydroxybenzonitrile (3) were synthesized and characterized by elemental analysis, IR, multinuclear NMR spectroscopy, MALDI-TOF mass spectrometry. It was found by X-ray diffraction that pairs of 3 in crystals are associated through H atoms of phenol groups to produce a dimer of dimers. The chemical oxidation of dirhodium complexes with 2,6-di-tert-butyl-4-cyano{cyrillic}phenol pendants studied by means of ESR method in solutions leads to the formation of phenoxyl radicals 3′ in dirhodium system. The ESR data show the interaction of the unpaired electron with ligand nuclei (¹H, ¹⁴N) and ¹⁰³Rh. The stability of radical complexes with phenoxyl fragments in axial position is influenced by the temperature. The enthalpy of the 3′ decomposition followed by the formation of cyanophenoxyl radical as 20 ± 1 kJ/mol was estimated. Redox transformations in dirhodium system including both metal and axial ligands were investigated by electrochemistry. CV experiments confirm the assumption of the metal oxidation (Rh^II→Rh^III) as the first step following by the oxidation of the ligand.

Full text of DOI:10.1016/j.ica.2010.01.029

Inorganica Chimica Acta 363 (2010) 1455–1461
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and redox properties of dinuclear rhodium(II) carboxylates with
2,6-di-tert-butylphenol moieties
a
a
a
Elena R. Milaeva a, , Natalia N. Meleshonkova , Dmitry B. Shpakovsky , Kirill V. Uspensky ,
Alexander V. Dolganov a, Tatiana V. Magdesieva a, Alexander V. Fionov a, Alexey A. Sidorov b,
Grigory G. Aleksandrov b, Igor L. Eremenko b
*
a Chemistry Department Moscow State Lomonosov University, Lenin Hill 1-3, Moscow, 119991, Russian Federation
b N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky pr., 31, Moscow, 119991, Russian Federation
a r t i c l e i n f o
a b s t r a c t
-O2CBut)4(H2O)2] (1) the examples of dinuclear rhodium(II)
-O2CBut)4L2] [where L = benzonitrile (2), 3,5-
Article history:
Received 16 July 2009
Received in revised form 15 January 2010
Accepted 22 January 2010
Available online 29 January 2010
By exploiting the peculiar reactivity of [Rh2(
carboxylates containing N-donor axial ligands (2, 3) [Rh2(
l
l
di-tert-butyl-4-hydroxybenzonitrile (3)] were synthesized and characterized by elemental analysis, IR,
multinuclear NMR spectroscopy, MALDI-TOF mass spectrometry. It was found by X-ray diffraction that
pairs of 3 in crystals are associated through H atoms of phenol groups to produce a dimer of dimers.
The chemical oxidation of dirhodium complexes with 2,6-di-tert-butyl-4-cyanophenol pendants studied
by means of ESR method in solutions leads to the formation of phenoxyl radicals 30 in dirhodium system.
The ESR data show the interaction of the unpaired electron with ligand nuclei (1H, 14N) and 103Rh. The
stability of radical complexes with phenoxyl fragments in axial position is influenced by the temperature.
The enthalpy of the 30 decomposition followed by the formation of cyanophenoxyl radical as 20 1 kJ/
mol was estimated. Redox transformations in dirhodium system including both metal and axial ligands
were investigated by electrochemistry. CV experiments confirm the assumption of the metal oxidation
(RhII?RhIII) as the first step following by the oxidation of the ligand.
Keywords:
Dirhodium carboxylates
Rhodium(II)
2,6-Di-tert-butyl-4-cyanophenol
Phenoxyl radical
ESR
Cyclic voltammetry
Electron transfer
Ó 2010 Published by Elsevier B.V.
1. Introduction
Although the precise anti-tumor mechanism of dirhodium car-
boxylate compounds has not been elucidated, it is known that they
Dirhodium tetracarboxylates, Rh2(
l
-O2CR)4L2 with various axial
bind to DNA nucleobases and exhibit a strong preference for axial
binding of guanine due to the hydrogen bonding interactions be-
tween the exocyclic amine group and a carboxylate oxygen atom
[15–18]. A systematic variation of the axial and equatorial ligands
has shed light on the structure-activity relationship in this family
of compounds. It has been shown, for instance, that the anti-tumor
ligands containing a variety of strong N, O, P bases L have long been
known [1], and their paddlewheel structure, bonding, and reactiv-
ity have been extensively investigated [2–5]. Dirhodium com-
plexes are of interest because of their applications in the field of
material science and nanotechnology [6,7]. The use of dirhodium
salts as catalysts for C–C bond formation in cycloaddition reactions
of olefins [8], acetylenes [9] or in ring-forming C–H insertion reac-
tions [10] has become such powerful synthetic tool as the develop-
ment of highly enantioselective chiral Rh(II) catalysts [11].
Additionally, recent results achieved in the molecular design
and focused synthesis of metal-based drugs suggest that transition
metal complexes are promising anti-tumor agents [12]. Studies of
the biological activity of dirhodium complexes support the
assumption that dirhodium tetracarboxylates act as antibacterial
agents, exhibit cytostatic activity against tumors [13] and a signif-
icant in vivo antitumor activity [14].
activity increases in the series Rh2(l-O2CR)4 (R = Me, Et, Pr) with
the increase of lipophilicity of the R group [19]. Moreover, since
there is the increasing evidence that the oxidative stress plays a
critical role in certain pathological states including cancer the
application of antioxidants in cancer therapy is strongly needed
[20].
The metal complexes containing antioxidative groups of 2,6-
di-tert-butylphenols have shown the activity as antioxidants,
inflammatory agents, scavengers for reactive oxygen species
[21–25] which makes them promising agents in complex cancer
therapy.
We believe that a systematic research is needed in order to get
more insight into complexation of lipophilic phenolic pendants by
dirhodium(II) carboxylate molecules, as these interactions play an
important role in design of new polytopic redox systems.
* Corresponding author. Tel.: +7 495 9393864; fax: +7 495 9395546.
E-mail address: milaeva@org.chem.msu.ru (E.R. Milaeva).
0020-1693/$ - see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.ica.2010.01.029

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E.R. Milaeva et al. / Inorganica Chimica Acta 363 (2010) 1455–1461
Previously we demonstrated that the introduction of 2,6-di-
dried in the air. Yield 0.197 g (85%). Anal. Calc. for C34H46N2O8Rh2:
C, 50.01; H, 5.68; N, 3.43. Found: C, 50.07; H, 5.39; N, 3.64%.
IR (KBr pellet, cmꢀ1): 1583 (vs), 1483 (m), 1414 (s), 1223 (s),
tert-butulphenols into the ligand environment of transition metal
increases the stability of corresponding phenoxyl radicals due to
the interaction of the unpaired electron with the metal center [26].
In the present paper we report on the synthesis, characteriza-
tion and redox properties of dirhodium complexes with 2,6-di-
m
(CN) 2249 (w),
m(CH) 2871–2962 (s). MS (MALDI-TOF), m/z: 610
[C20H36O8Rh2]+.
1H NMR (CDCl3): 1.03 (s, 36 H, C(CH3)3); 7.56 (t, 4H, Ph), 7.70 (t,
2H, Ph), 7.85 (d, 4H, Ph).
tert-butulphenol moiety in the ligand [Rh2(
L = 3,5-di-tert-butyl-4-hydroxybenzonitrile (3) and its analogue
[Rh2(
-O2CBut)4L2], where L = benzonitrile (2).
l
-O2CBut)4L2], where
13C NMR: 27.74 (OOCC(CH3)3); 40.55 (OOCC(CH3)3); 112.46;
117.27; 129.18; 132.67; 133.13 (C-arom); 117.27 (CN), 198.48
(C@O).
l
The X-ray crystal structure of phenol containing complex (3) is
described as well. We found by ESR that the chemical oxidation of
3 leads to phenoxyl radicals, the stability of which depends on
temperature. We also studied electrochemical properties of dirho-
dium complexes.
UV–Vis spectrum (EtOH), lg
2.62 (590).
e (kmax/nm): 4.32 (249); 2.63 (422);
2.2.2. [Rh2(-O2CBut)4L2] (3)
To a solution of 1 (0.265 g, 0.410 mmol) in 3 ml toluene 3,5-di-
tert-butyl-4-hydroxybenzonitrile (0.200 g, 0.865 mmol) was
added. The color of solution immediately changed from pale green
to dark blue. The solution was stirred for 15 min. The solvent was
partially removed by evaporation, and then 2 ml of benzene were
added and left for crystallization on the air. Purple crystals of 2
were collected after precipitation by filtration, washed with hex-
ane and dried in the air. Yield 0.303 g (69%). Anal. Calc. for
C50H78N2O10Rh2: C, 55.97; H, 7.33; N, 2.61. Found: C, 56.27; H,
6.97; N, 2.40%.
2. Experimental
2.1. Materials and equipment
The starting complex [Rh2(l
-O2CCBut)4(H2O)2] (1) [27] and 3,5-
di-tert-butyl-4-hydroxybenzonitrile [28] were prepared according
to literature procedures. All other chemicals are commercially
available and used as supplied. CDCl3 (Merck) was used without
further purification; other solvents were routinely distilled and
dried prior to use. Elemental analysis was performed in the Labo-
ratory of Microanalysis at the Moscow State Lomonosov University
(Moscow), Russia.
IR (KBr pellet, cmꢀ1): 1585 (vs), 1482 (m), 1414 (s), 1223 (s),
m
m
(CN) 2246 (w),
(OH non-associated) 3623 (w).
1H NMR (CDCl3): d (ppm): 1.03 (s, 18 H, C(CH3)3); 1.48 (s, 18 H,
m(CH) 2873–2960 (s), m(OH associated) 3546 (w),
Infrared spectra were recorded on KBr pellets or in CH2Cl2 solu-
tion with a ‘‘IR200 (Thermo Nicolet)” spectrometer. Electronic
C(CH3)3); 5.62 (s, 1 H, OH); 7.36 (s, 2H, C6H2).
13C NMR: 27.69 (OOCC(CH3)3); 29.89 (ArC(CH3)3); 34.43
(OOCC(CH3)3); 40.41 (ArC(CH3)3); 103.01, 129.96, 136.98, 158.00
(C-arom.); 118.01 (CN), 198.48 (C@O).
absorption spectra were measured on
spectrophotometer.
a Cari 219 Varian
The 1H, 13C NMR measurements were performed with a Bruker
Avance-400 spectrometer operating on 400 or 100 MHz, respec-
tively in CDCl3.
The MALDI-TOF spectra were acquired using a Autoflex II (Bru-
ker daltonics) mass spectrometer. The samples were dissolved in
CHCl3 with 2,5-dihydroxybenzoic acid (DHB) as matrix and put
at target.
ESR spectra were recorded on the Bruker EMX spectrometer at
X-band frequency (9.8 GHz). The measurements were carried out
after pre-evacuation 10ꢀ2 Torr) of tubes with solutions of samples
(concentration 1ꢁ10ꢀ4 Mol Lꢀ1). The oxidant PbO2 (Aldrich) was ta-
ken in a tenfold excess.
Cyclic voltammetry experiments were performed in CH3CN
solution with 0.05 M Bu4NBF4 as supporting electrolyte using a
model IPC-Win potentiostat in a conventional one - compartment
three-electrode cell (10 ml of solution). A platinum working elec-
trode, platinum wire auxiliary electrode and aqueous Ag/AgCl/
KCl reference electrode were used. The electrode was thoroughly
polished and rinsed before measurements. The measurements
were performed at scan rate equal to 100 mV sꢀ1. The formal po-
tential of the ferrocene Fc/Fc+ couple versus RE in the dichloro-
methane solution is equal to 550 mV. All solutions were
thoroughly deaerated before and during the CV experiments using
argon gas.
Table 1
Crystallographic data and structure refinement for [Rh2(
(3ꢁ4C6H6, L= 3,5-di-tert-butyl-4-hydroxybenzonitrile).
l
-O2CBut)4L2]ꢁ4C6H6.
Formula
Formula weight
C74H102N2O10Rh2
1385.40
120(2)
T (K)
0
0.71073
k (AÅ)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
P1
12.755(6)
12.939(9)
13.725(6)
66.93(4)
67.18(3)
81.67(4)
a
(°)
b (°)
c
(°)
0
1920.7(18)
V (AÅ3)
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
Crystal size (mm)
Theta range for data collection (°)
1.198
0.482
730
)
0.35 ꢂ 0.23 ꢂ 0.15
1.71–28.25
Limiting indices
ꢀ8 6 h 6 12, ꢀ12 6 k 6 17,
ꢀ18 6 l 6 16
Reflections collected/unique
Completeness to theta
Maximum and minimum
transmission
13 655/9421 [Rint = 0.0625]
28.25 93.5%
2.2. Syntheses
0.9312 and 0.8494
2.2.1. [Rh2(-O2CBut)4(PhCN)2] (2)
Refinement method
Full-matrix least-squares on F2
9421/0/424
To a suspension of 1 (0.200 g, 0.310 mmol) in 10 ml toluene
benzonitrile (0.070 g, 0.679 mmol) was added. The solid phase
immediately dissolved after addition of benzonitrile. The color of
solution immediately changed from pale green to purple. The solu-
tion was left for 1 h. The solvent was slowly removed in vacuum,
the resulting brown-red crystals were washed with hexane and
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
1.014
Final R indices [I > 2
R indices (all data)
Largest difference in peak and hole
(e Aꢀ3
r
(I)]
R1 = 0.0671, wR2 = 0.1794
R1 = 0.0724, wR2 = 0.1827
1.352 and ꢀ1.640
)

E.R. Milaeva et al. / Inorganica Chimica Acta 363 (2010) 1455–1461
1457
UV–Vis spectrum (EtOH), lg
2.5 (594).
e (kmax/nm): 3.7 (250); 2.3 (449);
Mass spectrum (MALDI-TOF), m/z: 610 [C20H36O8Rh2]+.
The crystals of 3 were suitable for a single-crystal X-ray struc-
ture determination.
2.3. Crystallographic data collection and structure determination
Crystallogrphic data for 3 were collected by usual procedure
[29] using a Bruker AXS SMART 1000 diffractometer equipped with
CCD detector (graphite-monochromated Mo
Ka radiation,
k = 0.7107 Å, T = 120 K, scan mode, scan step – 0.3°, time of
x
Fig. 2. Molecular structure of Complex 3.
frames collection – 30 s, 2hmax 6 60°). The semiempirical absorp-
tion correction was applied [30]. The structure was solved by direct
methods using the SHELXS97 [31] program and refined using the
SHELXL97 package [32] by least-squares method in the full-matrix
anisotropic (atoms C(8), C(9) and C(10) are disordered and every
atom occupied two equivalent position with populated ꢃ0.5)
approximation. The crystallographic data for 3 are summarized
in Table 1.
L = benzonitrile (2) and its analogue [Rh2(l
-O2CBut)4L2], where
L = 3,5-di-tert-butyl-4-hydroxybenzonitrile (3) were synthesized.
Complexes 2 and 3 were obtained in high yield by treatment of 1
with two equivalents of benzonitrile or 3,5-di-tert-butyl-4-hydrox-
ybenzonitrile, respectively (Scheme 1).
Compounds 2, 3 were isolated as purple solids stable in the air
and in solutions. They were characterized by means of elemental
analyses, MALDI-TOF mass spectra, IR and 1H, 13C NMR spectros-
copy, the complex 3 was analyzed by X-ray diffraction.
3. Results and discussion
3.1. Synthesis
The solid IR spectra of complexes 1–3 exhibit similar
m
asym(COO)
(CN)
According to the typical axial reactivity of [Rh2[l
-O2CBut)4
bands at 1583–1585 cmꢀ1
.
The shifts of characteristic
m
(H2O)2] (1) novel dirhodium complexes [Rh2(
l
-O2CBut)4L2], where
absorption bands for 2, 3 at 2249–2246 cmꢀ1 to a region of higher
frequencies (
D
m
(CN) ꢄ 21 cmꢀ1) when compared with non-coordi-
But
But
nated ligands confirm the N-coordination of axial ligands in dirho-
But
But
dium complexes.
O
O
Rh
O
O
O
Rh
O
In the IR spectrum of 3 in KBr there are two absorptions in the
region of 3623–3546 cmꢀ1 (Fig. 1). The first characteristic narrow
band at 3623 cmꢀ1 is assigned to the free hindered phenol group
and another wide one at 3546 cmꢀ1 is related to hydrogen bonds
formation between phenol group and neighboring carboxylate
moiety of the complex molecule that is nontrivial for hindered
phenols. On the contrary, the only one free O–H group absorption
at 3620 cmꢀ1 was observed in IR spectrum of CH2Cl2 solution of 3.
As it is well known in non-coordinating and non-polar solvents
the phenol group in sterically hindered phenols is not involved in
hydrogen bond formation and the bands corresponding to non-
associated OH group in the IR spectra are registered in the region
O
O
O
O
O
O
L
OH2
H2O
But
Rh
L
L
Rh
PhCH3, 20oC
O
O
O
O
But
L=
But
But
2, 3
(2)
CN
1
But
HO
But
(3)
CN
Scheme 1.
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
3700
3600
3500
3400
3300
3200
3100
3000
2900
2800
Frequency, cm-1
Fig. 1. IR spectrum of 3 in KBr pellets (solid line) and in CH2Cl2 solution (dashed line).

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E.R. Milaeva et al. / Inorganica Chimica Acta 363 (2010) 1455–1461
Fig. 3. Crystal packing of Complex 3.
of 3600–3650 cmꢀ1 [33]. Therefore, 3 does not associate in non-po-
lar solvents.
atom of phenol group is bonded to oxygen atom O(1) of neighbor-
ing carboxylate group (O5–H5 0.84 Å, Hꢁ ꢁ ꢁO 2.20 Å, O–Hꢁ ꢁ ꢁO 140°,
O(5)ꢁ ꢁ ꢁO1 [ ꢀx, ꢀy, ꢀz + 1] 2.901 (5) Å) .
On the contrary, the only one free O–H group absorption at
3620 cmꢀ1 was observed in IR spectrum of CH2Cl2 solution of 3.
Therefore, 3 does not associate in non-polar solvents.
These data was supported by X-ray diffraction investigation of
complex 3 (Figs. 2 and 3).
Selected bond length and bond angles of 3 are summarized in
Table 2. The unit N–Rh–Rh–N has appeared slightly distorted from
linear geometry. The values of Rh–Rh and Rh–O bonds distances
for 3 correspond to that of 1. The length of Rh–Rh and Rh–O in
dirhodium complex 1 with H2O as axial ligands are 2.371 (1) Å
and 2.036 (2), respectively [4,27]. The complex Rh2(OOCC-
The structure of binuclear complex [Rh2(l
-O2CBut)4L2] (3) was
determined for solvate with 4 molecules of the benzene. All four
solvate molecules have crystallographic symmetry Ci, Complex 3
have crystallographic symmetry Ci also. In the crystal molecules
of 3 are assembled in infinite ribbons along axis z: each hydrogen
ꢀ
Me3)4(H20)2 crystallizes in the triclinic space group P1 with one
formula weight in the unit cell. Four pivalate groups bridge the
dirhodium(II) unit which is located about a crystallographic center
of inversion. Two oxygen atoms of water molecules coordinate to
the Rh atoms via the axial positions with Rh–O distances of
2.295 (2) Å. There are several examples of dirhodium complexes
with distorted from linear geometry of L–Rh–Rh–L unit such as
dirhodium pivalates with hindered axial ligands [1] and complexes
with non-equal equatorial ligands, e.g., Rh2(ButCO2)3(TTFCO2)L2
and Rh2(ButCO2)2(TTFCO2)L2, where TTFCO2 = tetrathiafulvalene
carboxylate [34].
Table 2
Selected bond length (Å) and angles (°) for 3.
Bond lengths
Bond angles
Rh–Rh0 2.3897(14)
Rh–N 2.237(5)
Rh–O 2.008(4)
2.019(4)
RhRhN 177.11(12)
RhNC 168.0(5)
ORhN 89.34(16)
91.89(17)
The data of electronic absorption spectroscopy show the pres-
ence of
p–p* transition in the aromatic systems of 2 and 3 in the re-
2.038(4)
91.34(17)
gion of 220–230 nm and transitions
p*?Rh–O r* (420–450 nm)
2.041(4)
95.03(16)
But
O
But
But
But
NC
O
O
O
PbO2, PhCH3
.
3
L
Rh
O
Rh
O
O
3'
- e, - H+, 295 K
O
O
But
But
But
t
Bu
HO
CN
L =
t
But
B
u
But
O
O
O
O
.
NC
+
O
L
Rh
Rh
O
O
O
O
But
But
But
Scheme 2.

E.R. Milaeva et al. / Inorganica Chimica Acta 363 (2010) 1455–1461
1459
and in region of 590–595 nm assigned to
was denoted in the case of similar complex [Rh2(02CMe)4(H2O)2].
The location of the Rh–Rh d* orbital just above the * implies that
there should be a dipole-forbidden vibronically allowed transition
just below the Rh–Rh * ? Rh–Rh * transition at 578 nm assigned
to d* ? * [4,35].
As it follows from the spectral data, no oxidation of phenol sub-
stituents to the quinoid derivatives occurs in the synthesis of 3.
The chemical oxidation of phenol fragment in complex 3 was
carried out by lead dioxide in toluene to generate phenoxyl radical
30 (Scheme 2).
p
*(Rh2) ?
r
* (Rh2) as it
reported. The spectrum of the complexed nitroxide demonstated a
remarkable g-value shift, compared to the uncomplexed base, and
as well as a rhodium hyperfine coupling of 1.7 G with only one rho-
dium [4,37].
The second spectrum (g = 2.0084, a(N) = 2.06 G, a(2H) = 2.06 G,
a(103Rh) = 0.8 G) was assigned to a spectrum of paramagnetic
complex 30. The experimental and simulated ESR spectra are in
good correlation to each other (Fig. 4). So, ESR spectrum of 30
showed that the spin density is distributed between the organic
moiety (including nitrogen atom of nitrile group and two meta-
protons of the phenoxyl ring) and the metal (one rhodium atom).
The radical 30 is stable at room temperature in the absence of oxy-
gen for several days.
p
p
r
r
The X-band ESR measurements (at 295 K) have shown the
superposition of two spectra for oxidation product of 3 (Fig. 4a).
The first spectrum (g = 2.0043, a(N) = 1.345 G, a(2H)= 2.18 G) cor-
responds to free 2,6-di-tert-butyl-4-cyanophenoxyl radical de-
scribed in the literature [28]. On the other hand, it was reported
that the oxidation of Rh(III) complex with N-coordinated 4-hydro-
xy-3,5-di-tert-butylbenzonitrile leads to the formation of the cor-
responding phenoxyl-centered radical [36]. No interaction of the
unpaired electron with the metal nucleus was observed and the
hyperfine coupling (hfc) constants [a(2H), a(N)] are typical for
the organic phenoxyl radical.
It is worth to note that despite the presence of two phenol
groups the only monoradical species were detected after the oxida-
tion of 3 as it has been shown previously for dimers of p-allyl com-
plexes [38] and tetraphenol substituted metallophthalocyanines
[39]. Therefore no spin-spin interaction was detected in this case
as well.
According to the ESR data the stability of radical 30 confirms the
possible cleavage of metal-ligand bond and formation of free
cyanophenoxyl radical in solution, that has been observed previ-
ously for mononuclear rhodium nitrile complexes [36].
In the case of radical 30 we have observed the interaction of the
unpaired electron with only one Rh nuclei and the number of lines,
intensity, hyperfine coupling constant do not correspond to the
signal observed for Rh52þ-centered species [34].
It was also reported that the hyperfine splitting constant from
the second rhodium nuclei may be several time smaller than that
of first one, so it could not be detected in spectrum of radical.
The ESR spectrum of the Lewis acid-base adduct of the rhodium tri-
fluoroacetate dimer and 2,2,6,6-tetramethylpiperidine-N-oxyl was
Fig. 4. ESR spectra (T = 295 K) observed after oxidation of 3 by PbO2 in toluene (a –
experimental; b – simulated with parameters g = 2.0084, a(N)= 2.06 G, a(2H)= 2.06
G, a(103Rh)= 0.8 G; c – simulated with parameters g = 2.0043, a(N) = 1.345 G,
a(2H) = 2.18 G).
Fig. 5. Temperature dependence of the ESR spectrum of oxidation product of 3
(PbO2, toluene, 190–330 K).

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E.R. Milaeva et al. / Inorganica Chimica Acta 363 (2010) 1455–1461
This allowed to estimate the enthalpy of the 30 decomposition fol-
lowed by the formation of cyanophenoxyl radical as 20 1 kJ/mol.
Thus, the oxidation of dirhodium complex 3 with redox frag-
ments leads to the temperature-controlled equilibrium between
dirhodium moiety and N-donor paramagnetic ligand. It is worth
to note that g-factor of radical 30 is also temperature dependent
and increases with the temperature decrease. At 190 K the wide
signal was observed with g = 2.0093. It could be proposed that at
low temperature this signal corresponds to biradical species.
Important information about the properties of complexes 1–3
can be obtained from their electrochemical investigation. Electro-
chemical oxidation and reduction potential values for phenol-con-
taining complex 3 as well as for model complexes 1 and 2
measured using cyclic voltammetry are presented in Table 3.
The first oxidation peak for complexes 1–3 is one-electron one
and reversible, as follows from the ratios of peak currents for direct
(anodic) and reverse (cathodic) peaks Ia/Ic which are equal to 1 at
Table 3
Oxidation and reduction peak potential values for 1–3. (Pt, C = 0.7 mM, 0.05 M
Bu4NBF4, vs. Ag/AgCl/KCl, 0.1 V sꢀ1).
Compound
E1ox (V)
E2ox (V)
Ered (V)
1
2
3
1.20/1.15
1.20/1.15
1.23/1.15
ꢀ1.14
ꢀ1.21
ꢀ1.08
2.04
2.07
But
HO
But
CN
I, µA
200
the applied potential scan rates. The peak separation values (DEp)
fall within the interval of 50–80 mV (in CH3CN). The linear depen-
dence Ip ꢀ v1/2
(v = scan rate) indicates that the process is diffu-
sionally controlled. The peak can be assigned to the formation of
mixed-valence Rh2+–Rh3+ complex with high delocalization of a
charge over dirhodium fragment [40], without destruction of the
dimeric cage structure, as it is confirmed by the CV data. In the case
of 3, an additional two-electron peak at the potential of 2.04 V
(CH3CN) can be observed (Fig. 6a), which more likely corresponds
to the oxidation of 2,6-di-tert-butylphenol fragment. The irrevers-
ibility of the peak observed means that the electron transfer is fol-
lowed by chemical reaction (Scheme 3). More likely it is a proton
elimination from the initially formed phenoxyl radical-cation
yielding phenoxyl radical which is easily further oxidized at a
potential applied. The phenoxonium cation formed during the oxi-
dation is unstable and can be easily involved in different chemical
processes [41].
100
0
-100
-2000
-1000
0
1000
2000
E, mV
Fig. 6. Cyclic voltammogram of 3 in Ar saturated solution (Pt, CH3CN, C = 0.7 mM,
0.05 M Bu4NBF4, vs. Ag/AgCl/KCl, scan rate 0.1 Vsꢀ1).
The reductive electrochemical behaviour of 1–3 is in a good
agreement with that observed for analogous Rh2(NHCOCF3)4 in
which the LUMO is localized at the orbitals of Rh centers [42].
The reduction of complexes 1–3 is one-electron process and irre-
versible. The electrochemical experiments were carried out in
coordinating polar CH3CN. As follows from Table 3, both oxidation
and reduction potential values do not differ significantly for 1–3
in CH3CN, allowing to assume that the substitution of axial li-
gands by the molecules of solvent takes place in this case. The po-
tential value for the second oxidation peak for 3 is very close to
that observed for the free 3,5-di-tert-butyl-4-hydroxybenzonitrile
taken for comparison. This is one more indication of the substitu-
tion of axial ligands in complexes 1–3 with the molecules of the
solvent which proceeds in CH3CN. Possibly, the redox properties
The intensities of both signals in ESR spectrum depend on tem-
perature. The intensity of signal of 30 decreases with the increase of
free cyanophenoxyl radical signal during the heating of the
solution. The decomposition of radical 30 is accompanied by the
elimination of 2,6-di-tert-butyl-4-cyanophenoxyl radical and,
probably, the compensation of rhodium vacant axial position by
coordination of donor atoms of neighboring carboxylate groups
(Scheme 2).
The temperature dependence of ESR spectra has been studied in
the range of temperatures from 330 K to 190 K (Fig. 5). The depen-
dence of two radicals integral intensities ratio on Tꢀ1 has demon-
strated a linear function (in the temperature range 260–330 K).
t
t
Bu
Bu
t
t
t
Bu
Bu
t
Bu
Bu
O
O
O
O
O
O
O
+
O
-e, -H
-e
(i)
L
Rh
L
O
Rh
O
NC
OH
Rh
Rh NC
OH
t
(ii)
O
t
O
t
O
t
t
O
O
O
t
Bu
t
Bu
t
Bu
Bu
Bu
Bu
t
Bu
Bu
t
t
t
Bu
Bu
Bu
Bu
Bu
O
Bu
O
O
O
O
O
O
O
-e
+
O
.
O
L
Rh
NC
L
NC
Rh
Rh
Rh
O
(iii)
O
t
O
O
t
O
t
t
O
O
O
t
t
Bu
Bu
Bu
Bu
destruction products
Scheme 3.

E.R. Milaeva et al. / Inorganica Chimica Acta 363 (2010) 1455–1461
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orbitals of Rh centers in the electrochemical oxidation and reduc-
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4. Conclusion
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The novel coordination compound of dirhodium, containing
2,6-di-tert-butylphenol fragment in nitrile ligands was synthe-
sized. The oxidation of this compound that leads to the phenoxyl
radical formation in the ligand environment was studied. The rel-
atively stable paramagnetic species formed in the redox transfor-
mations are ligand-centered radicals. The influence of the
unpaired electron upon the coordination center, metal and me-
tal–ligand bond has been observed. The intramolecular transfor-
mations in paramagnetic species formed in the oxidation of
binuclear rhodium carboxylates were investigated by ESR.
It was found by electrochemical method (CVA) that the dirho-
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centers – both metal and ligand.
This result opens up the possibility for the design of novel bio-
mimetic systems that might serve as electron and proton transfer
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We gratefully acknowledge financial support from Russian
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CCDC 660536 contains the supplementary crystallographic data
for 3. These data can be obtained free of charge from The Cam-
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c.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.ica.2010.01.029.
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