Binuclear osmium μ-oxocarboxylate complexes [Os2(μ-O) (μ-O2CR)2Cl4L2] (R = CH 3, CCl3; L = PPh3, AsPh3) and their electrochemical behavior in dichloromethan

Article abstract of DOI:10.1023/B:RUCO.0000022807.77697.ee

Cyclic voltammetry and galvanostatic coulometry techniques were used to determine how the redox properties of osmium binuclear μ-oxocarboxylates [Os₂^IV(μ-O)(μ-O₂CR)₂Cl ₄L₂] (R = CH₃

Full text of DOI:10.1023/B:RUCO.0000022807.77697.ee

Russian Journal of Coordination Chemistry, Vol. 30, No. 4, 2004, pp. 296–299. Translated from Koordinatsionnaya Khimiya, Vol. 30, No. 4, 2004, pp. 317–320.
Original Russian Text Copyright © 2004 by Belyaev, Simanova, Fisher, Eremin, Evreinova, Panina.
Binuclear Osmium m-Oxocarboxylate Complexes
[Os2(m-O)(m-O2CR)2Cl4L2] (R = CH3, CCl3; L = PPh3, AsPh3)
and Their Electrochemical Behavior in Dichloromethane
A. N. Belyaev, S. A. Simanova, A. I. Fisher, A. V. Eremin, N. V. Evreinova, and N. S. Panina
St. Petersburg State Technical University, ul. Politekhnicheskaya 29, St. Petersburg, 195251 Russia
Received April 16, 2003
Abstract—Cyclic voltammetry and galvanostatic coulometry techniques were used to determine how the redox
properties of osmium binuclear µ-oxocarboxylates [OsIV(µ-O)(µ-O2CR)2ël4L2] (R = CH3, CCl3; L = PPh3 and
2
R = CH3; L = AsPh3) are influenced by the nature of the bridging carboxylate ligand RCOO– and ligand L. It
was shown that all compounds in solution of dichloromethane undergo two single-electron reduction processes.
The data obtained were compared with the DFT calculations of the electronic structure of the model complexes
IV
'
[Os2 (µ-O)(µ-O2CR)2ël4L2 ] (R = CH3, CCl3; L' = PH3 and R = CH3; L' = AsH3).
The osmium compounds with the Os–(µ-O)–Os core
are poorly studied, although the fist structurally charac-
terized complex Cs4[Os2(µ-O)Cl10] was reported as far
as in 1973 [1]. Only four papers devoted to the osmium
compounds of this class were published for the last
30 years [2–5].
EXPERIMENTAL
The electrochemical behavior of complexes I–III
was studied by cyclic voltammetry (CV) and galvano-
static coulometry (GC) methods at T = 20 ± 2°ë using
a PI-50-1.1 potentiostat, a PR-8 programmer, the two-
coordinate XY 4130 Recorder. The experiments were
conducted in a glass (pyrex) three-electrode cell. The
platinum wire with a surface 22.6 mm2 served as a
working electrode in CV study and the platinum grid
was used as the working electrode in GC measure-
ments. The reference electrode was Ag|0.01 å AgNO3,
0.1 M [(n-Bu)4N]ClO4, CH2Cl2. Nickel plates were used
as the auxiliary electrodes. The working electrode com-
partment was partitioned from the reference electrode
and auxiliary electrode compartments by discharge
cocks. A 0.1 M [(n-Bu)4N]ClO4 solution saturated with
argon was used as a supporting electrolyte, the concen-
X-ray diffraction data reported in [3, 5] indicate that
the nature of ligand L in the starting mononuclear com-
pound, i.e., the trans-[OsO2X2L2] (X = Cl, Br), deter-
mines the formation of a linear or a bent binuclear Os–
O–Os fragment. If L = PPh3 or PEt2Ph in the initial
complex, then the [OsI2V(µ-O)(µ-O2CR)2ël4L2] com-
plexes (R = CH3, C2H5; X = Cl, Br) are formed with the
bent metal-containing Os2(µ-O) core and the bidentate
coordination of two bridging carboxylate groups [3]. In
the case where pyridine (Py) is a ligand, the trations of complexes were 1–5 and 5 mol/l for CV and
GC studies, respectively. The cyclic voltammograms
were recorded at the potential sweep rate 100 mV/s.
[(Py)2Cl2(CH3COO)Os(µ-O)Os(Py)2Cl3] compounds
are formed with the linear Os2(µ-O) core and a single
acetate group coordinated in the monodentate mode [5]
The electronic absorption spectra of solutions of the
complexes in chloroform were recorded on a SF-56
spectrophotometer in the range of 200–1100 nm in
quarts cells with l = 1 cm.
IR spectra were taken in the range of 400–4000 cm–1
using a Nicolet FT-360 spectrophotometer with KBr
pellets.
The aim of this work was to study the effect of the
nature of the R group in the bridging carboxylate ligand
RCOO– and ligand L in the osmium binuclear µ-oxo-
carboxylates [OsI2V(µ-O)(µ-O2CR)2ël4L2] (R = CH3, L
= PPh3 (I); R = CH3, L = AsPh3 (II), and R = CCl3, L =
PPh3 (III)) on their electrochemical behavior. We
selected and used PPh3 and AsPh3 as ligands because,
on the one hand, they act as donors and have free elec-
tron pair, and, on the other hand, they have unoccupied
d orbitals with sufficiently low energies, unlike the
pyridine ligand.
X-ray photoelectronic spectra were recorded on a
Perkin-Elmer PHI 5400 spectrophotometer with excita-
tion by X-ray radiation. The powdered samples were
pressed in indium, evacuated, and placed on manipula-
tor cooled with liquid nitrogen.The operating vacuum
in spectrophotometer was 10–8 mmHg.
1070-3284/04/3004-0296 © 2004 åÄIä “Nauka /Interperiodica”

BINUCLEAR OSMIUM µ-OXOCARBOXYLATE COMPLEXES
297
Cl
0.1 mA
P
–1.0
–0.5
0
0.5 E, V
Cl
Os
O
P
Os
Cl
Cl
Fig. 2. CV of a 1 mM solution of [Os (µ-O)(µ-
2
Fig. 1. The structure of binuclear Os(IV) oxocarboxylate
O CCël ) Cl (PPh ) ] in CH Cl (with a 0.1 M solution of
2
3 2
4
3 2
2
2
4+
complex with the [Os (µ-O)(µ-O CCCl ) ] core (phenyl
[(n-Bu) N]ClO in CH Cl as a supporting electrolyte and
2
2
3 2
4
4
2
2
rings are not shown for clarity).
a potential sweep rate 100 mV/s).
The binuclear Os µ-oxocarboxylates [OsOsI2V(µ-
RESULTS AND DISCUSSION
X-ray data for complex III showed that the Os2(µ-
O)(µ-O2CR)2ël4(PPh3)2] (R = CH3 (I) and R = CCl3
(III)) were synthesized following the procedure we
previously described in [6].
1
O) fragment is bent. The osmium atoms are bonded by
two bridging trichloroacetate ions. Two chlorine atoms
are in the trans-positions with respect to the bridging
oxygen atom of the Os2(µ-O) core. Two PPh3 mole-
cules lie opposite to the oxygen atoms of the bridging
trichloroacetate groups (Fig. 1).
A typical CV curve for complex III is shown in
Fig. 2, while the galvanostatic chronopotentiogram of
the complex III reduction is given in Fig. 3.
The comparison of the CV and GC results for solu-
tions of complexes I–III in dichloromethane makes it
possible to conclude that these compounds undergo two
subsequent single-electron reduction processes (n =
0.97–1.02).
The [Os2(µ-O)(µ-O2CCH3)2ël4(AsPh3)2] complex
(II) was prepared by boiling a suspension of 1 g of the
trans-[OsO2(AsPh3)2Cl2] in 20 ml of glacial acetic acid
until the initial complex dissolved completely. In this
case, the colorless solution became dark brown. A solid
residue, formed after evaporating the solution on air
bath, was dissolved in 40 ml of chloroform, and a small
amount of pentane was added to the obtained solution.
The precipitated complex was filtered off, washed with
ether, and kept in a desiccator over KOH to a constant
weight. Then it was dissolved in minimum quantity of
chloroform and purified on a chromatographic column
(silipore 600 (GC), 0.125–0.160 mm). The solution was
concentrated at room temperature, and a solid residue
was kept in vacuum-desiccator to a constant weight.
The product yield was ~70%. Complex II has dark
brown color, is readily soluble in chloroform, dichlo-
romethane, and benzene.
[OsI2V(µ-O)(µ-O2CR) Cl4L2] + e–
2
(1)
= [OsIIIOsIV(µ-O)(µ-O2CR) Cl4L2]–,
2
[OsIIIOsIV(µ-O)(µ-O2CR) Cl4L2]– + e–
2
(2)
= [OsI2II(µ-O)(µ-O2CR) Cl4L2]2–.
2
The first process is reversible, while the second one is
irreversible in both electrochemical (∆Ep ӷ 56 mV at
20°ë) and chemical (ip.c/ip.a ≥ 1) aspects (where ∆Ep is
the potential difference between the anode and cathode
For C40H36As2Cl4O5Os2
Anal. calcd. (%):
Found (%):
Cl, 11.2;
Cl, 11.4;
Os, 30.0.
Os, 29.8.
The [OsI2V(µ-O)(µ-O CCCl ) ël (PPh ) ] · CH Cl complex crys-
1
2
3 2
4
3 2
2
2
IR spectrum, ν, cm–1: 472 s, 690 s, 740 s, 778 w,
850 b, 1000 w, 1028 w, 1085 s, 1163 b, 1187 b, 1440 s,
1490 w, 1630 s, 3420 b. Electronic absorption spec-
trum, λmax, nm (ε, l mol−1 cm–1): 426 (7362).
tallizes in triclinic system: space group P1 ; a = 10.747(2), b =
19.291(4), c = 24.614(5) Å, α = 100,08(3)°, β = 90.63(3)°, γ =
97.05(3)°, V = 4983.5(17) Å , Z = 4; R = 0.0389. The Os–(µ-
O)–Os angle is 142.2(7)° [7].
3
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004

298
BELYAEV et al.
pseudopotential for the As and Os atoms [8] using
GAMESS-2001 program package [9].
E, V
0.5
0
–0.5
–1.0
–1.5
To estimate the difference in the strength of the
metal–carboxylate ligand bonds, the binding energies
(BE) were calculated from the differences of the total
energies of the compounds undergoing dissociation:
[Os2(µ-O)(µ-OOCR)2(PH3)2Cl4]
[Os2(µ-O)(PH3)2Cl4]2+ + 2RCOO–. In the complexation
of Os with carboxylates, the change in the electron den-
sity (∆q) for CH3COé– (0.651 e) is higher than for
τ, h
0.5 1.0 1.5 2.0 2.5
0
Fig. 3. Galvanostatic chronopotentiogram of a 5 mM solu-
CCl3COé– (0.545 e), and the value of BE in the com-
plex is also higher for the acetate anion (950 kJ/mol) as
compared to the trichloroacetate anion (809 kJ/mol).
The Mulliken redistribution of the electron density in
the course of complexation in a series of carboxylates
under study resulted in the charges on the Os atoms (q)
that are in a good agreement with the BE (Os4f7/2),
which follows from X-ray photoelectronic spectros-
copy: q = 0.616 e, Os4f7/2 = 52.8 eV (R = CH3) < q =
0.626 e, Os4f7/2 = 53.0 (R = CCl3).
tion of [Os (µ-O)(µ-O CCël ) Cl (PPh ) ] in CH Cl
2
2
3 2
4
3 2
2
2
(with 0.15 mA current).
peaks; ip.c is the reduction peak current, ip.a is the oxida-
tion peak current (Table 1).
As follows from the voltammetry data, the poten-
tials of the reduction processes (1) and (2) are mainly
determined by the nature of R in the bridging carboxy-
late ligand. Thus, the half-wave potentials (E1/2) of
these processes for the trichloroacetate complex III are
more positive as compared to the respective values for
the acetate analogue I (Table 1). At the same time, the
potential stability regions (∆E1/2) of the mixed-valence
form [OsIIIOsIV(µ-O)(µ-O2CR)2ël4(PPh3)2]– are almost
the same for complexes III and I (Table 1). On the con-
trary, the triphenylarsinic complex II has more narrow
region of thermodynamic stability of the mixed-valence
form as compared to those of the triphenylphosphine
complexes I and III. The calculated coproportionation
constants Kcoprop = exp(∆E1/2F/RT) correspond to the
following process
The replacement of the phosphine ligand by the ars-
ine ligand causes only insignificant reduction in the
energy of binding of the bridging acetate ligand with the
2+
'
coordinated [Os2(µ-O)L2 Cl4] fragment (948 kJ/mol).
The pattern of the electron density redistribution
obtained for the model complexes under consideration
cannot be directly applied to the compounds with PPh3
and AsPh3. The reason for this is that the energy of ion-
ization (I) of phosphine is higher than that of arsine
while the respective triphenyl derivatives exhibit the
reverse situation IPPh < IAsPh (Table 2). At the same
3
3
OsI2V + OsI2II
2OsIIIOsIV
time, insignificant differences in the I values and in the
electron affinity (A) (Table 2) calculated for these
ligands make it possible to assume that these ligands
have close donor–acceptor properties and similar elec-
and are equal to 6.2 × 1017 and 3.4 × 1018 for triphen-
ylphosphine complexes I and III, respectively and
1.1 × 1014 for the triphenylarsinic derivative II.
tronic
structures
of
acetates
[Os2(µ-O)(µ-
O2ëëH3)2L2Cl4].
In order to reduce the basis, the calculations of the
electronic structure of [Os2(µ-O)(µ-O2ëëH3)2L2Cl4]
Thus, the quantum-chemical calculations provide
were performed using the model systems, where L' = grounds to believe that the increased charge on the Os
PH3, AsH3 were used instead of L. These calculations atoms due to the weaker electron-donor properties of
were carried out by the B3LYP DFT method in the 6- the trichloroacetate ligands [11] can be the reason for
31G** basis for the H, C, O, P, Cl, atoms and with HW the higher E1/2 values of the electrode processes (1) and
Table 1. Voltammetry characteristics for Os binuclear complexes I–III*
OsI2V/OsIIIOsIV
OsIIIOsIV/OsI2II
Complex
∆E1/2, V
E1/2(∆Ep), mV
ip.c/ip.a
E1/2(∆Ep), mV
ip.c/ip.a
I
–23(65)
–28(58)
313(65)
0.9
1.0
1.0
–1058(115)
–845(110)
–765(120)
1.7
2.0
1.0
1.035
0.817
1.078
II
III
* E = (E + E )/2, ∆E = E – E and ∆E = E (OsIV/OsIIIOsIV) – E (OsIIIOsIV/Os2III).
1/2
p.a
p.c
p
p.a
p.c
1/2
1/2
1/2
2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004

BINUCLEAR OSMIUM µ-OXOCARBOXYLATE COMPLEXES
299
Table 2. The calculated (Icalcd) and experimental (Iexp) [10]
ionization energies and electron affinities for ligands L and L'
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Icalcd
A
L and L'
eV
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10.60
10.51
7.85
10.57
10.29
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–2.57
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–0.71
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This work was supported by the Russian Foundation
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004