Electrochemical and spectral study of intramolecular charge transfer in dinuclear and polynuclear ladder complexes

Article abstract of DOI:10.1007/s11172-006-0334-8

The potentials of electrochemical oxidation and reduction of the polynuclear ladder complexes Cp(CO)LM-η ¹, η⁵- C₅H₄Mn(CO)₂L (M = Fe or W(CO); L = PPh ₃ or CO), μ-(C≡C)[C₅H₄

Full text of DOI:10.1007/s11172-006-0334-8

Russian Chemical Bulletin, International Edition, Vol. 55, No. 5, pp. 788—792, May, 2006
788
Electrochemical and spectral study of intramolecular
charge transfer in dinuclear and polynuclear ladder complexes
T. V. Magdesieva,aꢀ O. M. Nikitin,a M. G. Ezernitskaya,b T. Yu. Orlova,b A. G. Ginzburg,b and Yu. S. Nekrasovb
aDepartment of Chemistry, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 932 8846. Eꢀmail: tvm@org.chem.msu.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (495) 135 5085
The potentials of electrochemical oxidation and reduction of the polynuclear ladꢀ
1
5
der complexes Cp(CO)LMꢀη ,η ꢀC5H4Mn(CO)2L (M = Fe or W(CO); L = PPh3
1
5
1
5
or CO), µꢀ(C≡C)[C5H4(CO)2Feꢀη ,η ꢀC5H4Mn(CO)3]2, and MeSi[C5H4(CO)2Feꢀη ,η ꢀ
C5H4Mn(CO)3]3 were measured, and the mechanism of these processes is proposed. The
change in the electron density at the atom of one metal (Fe or W) is transferred along the σꢀ and
πꢀbond chain in the cyclopentadienyl bridge to the atom of another metal (Mn) and, on the
contrary, the perturbing effect of the substituent is somewhat weakened.
Key words: electrochemical oxidation and reduction; iron, tugsten, and manganese comꢀ
plexes; bridging cyclopentadienyl ligand.
Dinuclear and polynuclear transition metal complexes
Oxidation of complexes 1—6 is accompanied by
the appearance of several peaks in the cyclic voltammoꢀ
grams (CV). In most cases, the transfer of the first elecꢀ
tron occurs reversibly and leads to a change in the oxiꢀ
dation state Mn+/2+. The transfer of the second electron
causes, apparently, the cleavage of the M—C σꢀbond
in the bridging ligand. Reduction of the [CpFe(CO)L]+
and [CpW(CO)2L]+ cations can be observed (L = CO
or PPh3) on the reverse scan after the second oxidaꢀ
tion peak. Apparently, the oxidation peak recorded
at high anodic potentials corresponds to oxidation of
fulvaleneꢀtype complexes that are formed. This is consisꢀ
tent with the fact that the potentials of the longꢀrange
redox transitions are rather similar for all the complexes
under study.
Therefore, oxidation occurs with the involvement of
the highest occupied molecular orbital, to which the orꢀ
bitals of the metal atom πꢀcoordinated by the bridging
Cp ligand, viz., the Mn atom, make the major contribuꢀ
tion (Scheme 1).
Reduction of the complexes occurs with the involveꢀ
ment of the orbitals localized on the σꢀbond of the bridgꢀ
ing ligand, the electron transfer being followed by the fast
cleavage of this bond (Scheme 2). This is evidenced by
the fact that the first reduction peak of the complexes is
irreversible. Reoxidation of the [CpFe(CO)L]– and
[CpW(CO)2L]– anions (L = CO or PPh3) is observed on
the reverse scan (see Scheme 2).
with the bridging η1,η5ꢀcyclopentadienyl or η1,η6ꢀarene
ligands (soꢀcalled ladder complexes) are of interest
as models of intramolecular charge transfer along the
πꢀligand—metal—σ,πꢀligand—metal bond system.
Studies of the electronic structures of complex orgaꢀ
nometallic compounds by electrochemical methods alꢀ
low one to quickly estimate the influence of the replaceꢀ
ment of one or several ligands in the coordination sphere
of one metal atom on the electronic properties of another
metal center and the ligands coordinated to the latter
metal. Earlier,1 we have studied the electrochemical propꢀ
erties of a series of diꢀ, triꢀ, and pentanuclear organomeꢀ
tallic chains containing bridging ligands σꢀcoordinated to
one of the metal atoms and πꢀcoordinated to another
metal atom.
In the present study, we investigated the electrochemiꢀ
cal properties of new dinuclear ladder complexes 1—6,
which contain the Fe—Mn or W—Mn sequences and in
which one of the CO ligands in the coordination sphere of
the metal atom is replaced by PPh3, as well as the properꢀ
ties of complexes 7 and 8 in which the dinuclear fragꢀ
ments C5H4(CO)2Feꢀη1,η5ꢀC5H4Mn(CO)3 are linked to
each other by the acetylene or silyl bridge.
All the compounds under study proved to be electroꢀ
chemically active at both anodic and cathodic potentials,
the localization sites of the electronic changes for oxidaꢀ
tion being different from those for reduction.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 761—765, May, 2006.
1066ꢀ5285/06/5505ꢀ0788 © 2006 Springer Science+Business Media, Inc.

Electrochemical study of ladder complexes
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 5, May, 2006
789
Scheme 1
M = Fe, W(CO); L = CO, PPh3
Scheme 2
The measured oxidation and reduction potentials are
given in Table 1.
As can be seen from Table 1, the replacement of the
πꢀacceptor ligand CO by the σꢀdonor ligand PPh3 has a
much stronger effect on the oxidation potentials of the
dinuclear complexes than on their reduction potentials;
these potentials vary within the ranges of 470 and 200 mV,
respectively. In the case of CpFe(CO)2Ph, the replaceꢀ
ment of one CO group by PPh3 leads to a slight change in
the reduction potential accompanied by the cleavage of
the Fe—Ph σꢀbond (cf. –1.96 and –2.01 V relative to the
saturated calomel electrode (SCE)).2 The same tendency
is observed for the bridged dinuclear complexes, e.g., ERed
is slightly shifted to cathodic potentials (see Table 1). If
the replacement by the donor ligand occurs at the Mn
atom, the shift of the potential is less pronounced comꢀ
pared to that caused by the replacement of the ligand at
the Fe or W atom, i.e., the transfer through the bridging
Cp ligand leads to a "damping" of the electronic effect of
one reaction center on another center. On the contrary,
the oxidation potentials of the dinuclear complexes are
more sensitive to the replacement of the ligand at the
i. Rapidly.
M = Fe, W(CO); L = CO, PPh3
Mn atom comapred to those at the Fe (W) atom. This is
quite reasonable taking into account that oxidation ocꢀ
curs at the Mn atom.
The electrochemical data agree well with the data on
metallation of complexes containing the PPh3 ligand.
Earlier,3 we have demonstrated that metallation of
the dinuclear complex Cp(CO)2FeC5H4Mn(CO)3 (1)

790
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 5, May, 2006
Magdesieva et al.
Table 1. Oxidation peak potentials (Е Ox), reduction peak potentialsa (ЕRed), and CO stretching
frequenciesb ν(CO) of complexes 1—8
Compoundc
Е Оx
–ERed
ν(СО)/cm–1
V
M(CO)nL
Mn(CO)nL
Cp(CO)2FeC5H4Mn(CO)3
1.07, 1.32,
1.93
2.20
2.40
2.35
2.22
2.45
2.20
2032,
1978
2003,
1916
(1)
Cp(CO)LFeC5H4Mn(CO)3
0.75, 1.35,
1.95
1932
1998,
1906
(2)
Cp(CO)2FeC5H4Mn(CO)2L
0.6, 1.25,
2.05
2025,
1971
1914,
1848
(3)
Cp(CO)3WC5H4Mn(CO)3
1.06, 1.36,
1.63, 1.96
2033,
1940
2007,
1923
(4)
Cp(CO)2LWC5H4Mn(CO)3
0.98, 1.52,
1.75, 2.05
1940,
1885
2003,
1913
(5)
Cp(CO)3WC5H4Mn(CO)2L
0.85, 1.39,
2.02
2022,
1945
1920,
1877
(6)
µꢀC2[C5H4(CO)2FeC5H4Mn(CO)3]2 1.08, 1.38,
(7) 2.18
2.20
2029,
1976
2003,
1918
d
MeSi[C5H4(CO)2FeC5H4Mn(CO)3]3 1.12, 1.36,
(8) 1.92
2033,
1982
2004,
1918
a Conditions: MeCN, a Pt electrode, 0.05 M Bu4NPF6, Ag/AgCl/KCl, 20 °C, 200 mV s–1
.
b CH2Cl2 as the solvent.
c L = PPh3.
d A weakly pronounced peak at a potential <–2.3 V.
with BunLi (THF, –78 °C) occurs selectively and exꢀ
clusively at the Cp ring coordinated to the Fe atom,
and we have isolated the monodeuterium derivative
DC5H4(CO)2FeC5H4Mn(CO)3 by treatment of the
lithium derivative with D2O.
Under the same conditions, no metallation of the
Cp(CO)PPh3FeC5H4Mn(CO)3 complex (2) occurs. The
mononuclear complex Cp(CO)(PPh3)FePh does not unꢀ
dergo metallation as well.4 This is attributed to the fact
that the strong electronꢀdonating ligand PPh3 at the Fe
atom substantially decreases the CHꢀacidity of the Cp
ligand. Metallation of the Cp(CO)3FeC5H4Mn(CO)2PPh3
complex (3), in which the electronꢀdonating ligand PPh3
is far removed from the reaction center (Cp ring), gives
rise to the lithium derivative. Treatment of the latter
with D2O affords the monodeuterium derivative
DC5H4(CO)2FeC5H4Mn(CO)2PPh3.
tial difference is ~130 mV, which is typical of weakly
interacting biferrocenes.
For complex 7, the first oxidation peak at anodic poꢀ
tentials corresponds to two weakly resolved successive
electron transfers, the potential difference between which
is as small as 60 mV.
For the Siꢀcontaining complex (8), no separation of
the peaks was observed, which could be evidence that the
Mn+ atoms of each propeller blade are successively oxiꢀ
dized. The first oxidation peak current of complex 8,
which corresponds to the threeꢀelectron transfer, and the
fact that the oxidation potential of complex 8 is similar to
that of complex 1 suggest that oxidation of the Mn atoms
of all three propeller blades occurs virtually simultaꢀ
neously. We failed to obtain clear reduction peaks at caꢀ
thodic potentials because reduction of the Siꢀcontaining
complex occurs at high negative potentials.
The complexes, in which the dinuclear Fe—Mn fragꢀ
ments are linked to each other by the acetylene (comꢀ
plex 7) or silyl (complex 8) bridges, are of particular interꢀ
est. The properties of the acetylene and polyyne spacers
have been studied in sufficient detail. For example, elecꢀ
trochemical oxidation of two ferrocene fragments in
the CpFe(η5ꢀC5H4—C≡C—C5H4ꢀη5)FeCp complex was
demonstrated5 to occur successively, although the potenꢀ
These data provide evidence that the MeSiꢀcontainꢀ
ing bridge, which links three η5ꢀC5H4Fe(CO)2(η1,η5ꢀ
C5H4)Mn(CO)3 fragments, serves as a poor conductor of
the electronic effects. Oxidation of the Mn atom in one
fragment gives rise to a positive charge, which, however,
has almost no effect on the oxidation potentials of the
other Mn atoms. Because of this, the cyclic voltammogram
shows one threeꢀelectron peak.

Electrochemical study of ladder complexes
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 5, May, 2006
791
The electrochemical data can be compared with the
CO stretching frequencies, which also provide evidence
for the electron density distribution in the molecules of
the complexes in question. Earlier, we have demonstrated6
that the ν(CO) frequencies of ladder diꢀ and trinuclear
carbonyl complexes containing the Fe—Fe, Fe—Mn,
Fe—Fe—Mn, Fe—Fe—Cr, W—Fe—Cr, etc. sequences
are sensitive to changes in the electron density at the
metal atoms.
The ν(CO) frequencies for complexes 1, 7, and 8 (see
Table 1) are virtually identical, i.e., the introduction of
several identical η5ꢀC5H4Fe(CO)2(η1,η5ꢀC5H4)Mn(CO)3
fragments, which are linked to each other by the acetyꢀ
lene or silyl bridge, into one molecule has virtually no
effect on their properties.
Experimental
The CO stretching frequencies for complexes 1—8 are
given in Table 1. The assignment of the observed bands to
ν(CO) vibrations of particular metal carbonyl fragments,
which is presented in Table 1, was made according to an
approach proposed in our earlier study.6 The replacement
of the CO group at the Mn atom by the electronꢀdonating
PPh3 ligand (complexes 3 and 6) decreases ν(CO) at the
Mn atom by ~90 cm–1, whereas ν(CO) for the groups at
the Fe (complex 3) and W (complex 6) atoms decreases to
a lesser extent (by ~7 cm–1). The introduction of the
PPh3 ligand into the coordination sphere of the Mn atom
also leads to a slight increase in the electron density at the
Fe or W atom, i.e., as in the previously studied comꢀ
plexes, the electronic effect is transferred from the atom
of one metal to the atom of another metal through the
bridging Cp ligand. A comparison of the ν(CO) frequenꢀ
cies of complexes 1 and 2 and of complexes 4 and 5 shows
that the replacement of the CO group at the Fe (comꢀ
plex 2) and W (complex 5) atom by Ph3P leads to a
substantial (by 70 cm–1) decrease in ν(CO) at these metal
atoms and to a slight (by ~5 cm–1) decrease in ν(CO) at
the Mn atom, i.e., the electronic effect in these comꢀ
plexes is also transferred from the Fe or W atom to the
Mn atom through the bridging Cp ligand.
The influence of the nature of the metal atom on the
ν(CO) frequencies can be seen from a comparison of the
IR spectra of complexes 1 and 4. The ν(CO) frequencies
of the groups at the Mn atom of complex 4 are 4—7 cm–1
higher than the corresponding frequencies of complex 1,
which indicates that the electronꢀdonating ability of the
CpW(CO)3 fragment, which causes a decrease in the
ν(CO) frequency, is slightly lower than that of the
CpFe(CO)2 fragment.
Therefore, the spectroscopic data agree well with the
results of electrochemical studies. The results obtained by
both methods reflect the same tendency, i.e., an increase
in the electron density at the atom of one metal (Fe or W)
is transferred along the chain of σꢀ and πꢀbonds in the
Cp bridge to the atom of another metal (Mn). This effect
is less pronounced in the opposite direction (from Mn to
Fe or W). It should be noted that the total estimation of
the electronic perturbations introduced into the molecule
due to the replacement of one CO ligand by PPh3 at any
metal center, which was calculated as (∆EOx + ⏐∆ERed⏐)
or ∆ν(CO)Fe(W) + ∆ν(CO)Mn, gave similar values regardꢀ
less of at which metal atom the ligand is replaced.
The electrochemical oxidation and reduction potentials were
measured on an IPCꢀWin digital potentiostat/galvanostat conꢀ
nected to a personal computer. The voltammetric curves were
recorded by cyclic voltammetry on a stationary Pt electrode at a
potential scan rate of 200 mV s–1 using 0.05 M Bun4NPF6 as the
supporting electrolyte in acetonitrile at 20 °C in a 10 mL elecꢀ
trochemical cell. Oxygen was removed from the cell by purging
with dry argon. The measured potentials were corrected for
ohmic losses. Platinum served as the auxiliary electrode, and a
saturated silver chloride electrode was used as the reference
electrode.
The IR spectra (solutions in CH2Cl2) were recorded on a
Nicolet Magna IR 750 Fourierꢀtransform spectrometer at 2 cm–1
1
resolution. The H NMR spectra (C6D6) were measured on a
Bruker AMXꢀ400 spectrometer (400.13 MHz).
Tetrahydrofuran (chemical purity) was distilled over sodium
benzophenone ketyl under Ar immediately before use. Acetoniꢀ
trile (chemical purity) was stirred over CaH2 for 12 h, distilled,
refluxed over P2O5 for 2 h, and again distilled, the fraction with
the b.p. of 81—82 °C (760 Torr) being collected.
The Cp(CO)2FeC5H4Mn(CO)3,7 Cp(CO)PPh3FeC5H4ꢀ
Mn(CO)3,8 Cp(CO)2FeC5H4Mn(CO)2PPh3,9 Cp(CO)3WC5H4ꢀ
Mn(CO)3, Cp(CO)2PPh3WC5H4Mn(CO)3, Cp(CO)3WC5H4ꢀ
Mn(CO)2PPh3,10 µꢀC2[C5H4(CO)2FeC5H4Mn(CO)3]2,11 and
MeSi[C5H4(CO)2FeC5H4Mn(CO)3]312 complexes were syntheꢀ
sized according to known procedures. All reactions were carried
out under dry argon.
5
1
5
The reaction of η ꢀcyclopentadienyldicarbonylironꢀη ,η ꢀ
cyclopentadienyldicarbonyltriphenylphosphinemanganese,
Cp(CO)2FeC5H4Mn(CO)2PPh3 (3), with BunLi and D2O. A 1.6 M
BunLi solution in hexane (0.4 mL, 0.6 mmol) was added to a
solution of Cp(CO)2FeC5H4Mn(CO)2PPh3 (0.1 g, 0.16 mmol)
in THF (10 mL) at –78 °C. The reaction mixture was stirred at
–78 °C for 1.5 h, and D2O (0.5 mL) was added. The reaction
mixture was stirred at –78 °C for 30 min, and then the temperaꢀ
ture was gradually raised to ~20 °C. The solvent was removed,
and the residue was chromatographed on Al2O3 (Brockmann
5
activity II). η ꢀMonodeuterocyclopentadienyldicarbonylironꢀ
1
5
η ,η ꢀcyclopentadienyldicarbonyltriphenylphosphinemanganese
was eluted with a 1 : 1 benzene—hexane mixture. The yield
of DC5H4(CO)2FeC5H4Mn(CO)2PPh3 was 0.03 g (30%).
1H NMR, δ: 4.10 (2 H, CpMn); 4.27 (4 H, C5H4DFe); 4.43
(2 H, CpMn); 7.17 (10 H, Ph); 7.84 (5 H, Ph).
5
The reaction of η ꢀcyclopentadienylcarbonyltriphenylphosꢀ
1
5
phineironꢀη ,η ꢀcyclopentadienyltricarbonylmanganese,
Cp(CO)PPh3FeC5H4Mn(CO)3 (2), with BunLi and D2O. A 1.6 M
BunLi solution in hexane (1 mL, 1.6 mmol) was added to a
solution of Cp(CO)PPh3FeC5H4Mn(CO)3 (0.6 g, 1 mmol) in
THF (50 mL) at –78 °C. The reaction mixture was stirred at
–78 °C for 2 h, and D2O (1 mL) was added. The reaction
mixture was stirred at –78 °C for 30 min, and then the temperaꢀ

792
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 5, May, 2006
Magdesieva et al.
ture was gradually raised to ~20 °C. The solvent was removed,
6. M. G. Ezernitskaya, B. V. Lokshin, T. Yu. Orlova, V. N.
Setkina, and S. Nunziante Cesaro, Izv. Akad. Nauk, Ser.
Khim., 1994, 1948 [Russ. Chem. Bull., 1994, 43, 1837 (Engl.
Transl.)].
and the residue was chromatographed on Al2O3 (Brockmann
5
activity II). η ꢀCyclopentadienylcarbonyltriphenylphosphineꢀ
1
5
ironꢀη ,η ꢀcyclopentadienyltricarbonylmanganese was eluted
with a 1 : 1 benzene—hexane mixture. The yield of the starting
complex 2 was 0.55 g (92%). H NMR, δ: 3.72, 4.25, and 4.31
(all, 1 H each, CpMn); 4.32 (5 H, CpFe); 4.43 (1 H, CpMn);
7.06 (8 H, Ph); 7.40 (6 H, Ph). The ratio of the signals
CpFe : CpMn = 5 : 1.
7. A. N. Nesmeyanov, E. G. Perevalova, L. I. Leontґeva, E. A.
Eremin, and O. V. Grigorґeva, Izv. Akad. Nauk SSSR, Ser.
Khim., 1974, 2645 [Bull. Acad. Sci. USSR, Div. Chem. Sci.,
1974, 23 (Engl. Transl.)].
8. A. N. Nesmeyanov, E. G. Perevalova, L. I. Leontґeva, and
E. V. Shumilina, Izv. Akad. Nauk SSSR, Ser. Khim., 1977,
1142 [Bull. Acad. Sci. USSR, Div. Chem. Sci., 1977, 26 (Engl.
Transl.)].
9. A. N. Nesmeyanov, E. G. Perevalova, L. I. Leontґeva, and
E. V. Shumilina, Izv. Akad. Nauk SSSR, Ser. Khim., 1977,
1156 [Bull. Acad. Sci. USSR, Div. Chem. Sci., 1977, 26 (Engl.
Transl.)].
1
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 05ꢀ03ꢀ
32869).
References
10. A. N. Nesmeyanov, E. G. Perevalova, E. V. Shumilina, and
L. I. Leontґeva, Izv. Akad. Nauk SSSR, Ser. Khim., 1977,
2328 [Bull. Acad. Sci. USSR, Div. Chem. Sci., 1977, 26 (Engl.
Transl.)].
1. T. V. Magdesieva, S. V. Milovanov, T. Yu. Orlova, and K. P.
Butin, Izv. Akad. Nauk, Ser. Khim., 1995, 362 [Russ. Chem.
Bull., 1995, 44, 354 (Engl. Transl.)].
2. L. I. Denisovich, S. P. Gubin, and Yu. A. Chapovskii, Izv.
Akad. Nauk SSSR, Ser. Khim., 1965, 2378 [Bull. Acad. Sci.
USSR, Div. Chem. Sci., 1965, 14 (Engl. Transl.)].
3. T. Yu. Orlova, V. N. Setkina, P. V. Petrovskii, A. I. Yanovsky,
A. S. Batsanov, and Yu. T. Struchkov, Metalloorg. Khim.,
1988, 1, 1327 [Organomet. Chem. USSR, 1988, 1, 725 (Engl.
Transl.)].
11. T. Yu. Orlova, Yu. S. Nekrasov, P. V. Petrovskii, and M. G.
Ezernitskaya, Izv. Akad. Nauk, Ser. Khim., 2004, 1860 [Russ.
Chem. Bull., Int. Ed., 2004, 53, 1938].
12. T. Yu. Orlova, Yu. S. Nekrasov, P. V. Petrovskii, M. G.
Ezernitskaya, Z. A. Starikova, and A. I. Yanovsky, Izv. Akad.
Nauk, Ser. Khim., 1998, 2056 [Russ. Chem. Bull., 1998, 47,
2000 (Engl. Transl.)].
4. M. E. Wright and V. W. Dey, J. Organomet. Chem.,
1987, 329, 4.
5. C. La Vanda, D. Cowan, C. Leitch, and K. Bechgaard,
J. Am. Chem. Soc., 1974, 96, 6788.
Received January 10, 2006;
in revised form March 28, 2006