Redox reactions via outer sphere charge transfer complexation: The interaction of ferrocenes with σ- and π-type acceptors

Article abstract of DOI:10.1016/j.inoche.2004.07.023

Interactions of ferrocenes with iodine as a typical σ-acceptor or with DDQ and CHL as π-acceptors always led to the oxidation of these ferrocenes to the corresponding ferricinium salts. This type of oxidation is proposed to occur via initial formation of outer sphere charge transfer complexes. Decamethylferrocene Cp₂*Fe, ferrocene (Cp ₂Fe), 1,1′-dimethylferrocene. ((MeCp)₂Fe), and 1,2-diferrocenylethane (Fc₂C₂H₄) were studied as donors for charge transfer complexation with iodine (I₂) as a typical σ-type CT acceptor and with 2,3-dichloro-5,6-dicyano-p- benzoquinone (DDQ), and p-chloranil (CHL) as typical π-type CT acceptors. Oxidation of the ferrocenes to the corresponding ferricinium salts is proposed to occur via initial formation of outer sphere charge transfer complexes. UV-Vis spectra for the mixtures of any of the ferrocene donors with I₂ or DDQ in CH₂Cl₂ showed the characteristic bands of the triiodide ion, I3- and the DDQ^- anion radical. With CHL, the same behavior occurred only for Cp₂*Fe to give Cp ₂*Fe+ and CHL^- radicals. Elemental analyses of the isolated solid complexes indicated the formation of [ferrocene]I₃ for ferrocenes and I₂, except for (Fc₂C₂H ₄) which gave [(Fc₂C₂H₄)]I ₅. Ferrocene-DDQ complexes were 1:1 except for (MeCp)₂Fe and (Fc₂C₂H₄) which gave 1:2 complexes. By using n-hexane as a non-polar solvent instead of CH₂Cl₂, the CT band of the Cp₂*Fe adduct has been observed and the formation constant for this CT complexation was found to be 3900 (±500) dm³ mol^-1.

Full text of DOI:10.1016/j.inoche.2004.07.023

Inorganic Chemistry Communications 7 (2004) 1209–1212
www.elsevier.com/locate/inoche
Redox reactions via outer sphere charge transfer complexation:
the interaction of ferrocenes with r- and p-type acceptors
a,
b
a
H.M.A. Salman *, M.R. Mahmoud , M.H.M. Abou-El-Wafa ,
a
U.M. Rabie , R.H. Crabtree
c
a
Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt
b
Department of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt
c
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, CT 06520-8107, USA
Received 30 May 2004; accepted 18 July 2004
Available online 13 October 2004
Abstract
Decamethylferrocene Cpꢀ2Fe, ferrocene (Cp2Fe), 1,10-dimethylferrocene. ((MeCp)2Fe), and 1,2-diferrocenylethane (Fc2C2H4)
were studied as donors for charge transfer complexation with iodine (I2) as a typical r-type CT acceptor and with 2,3-dichloro-
5,6-dicyano-p-benzoquinone (DDQ), and p-chloranil (CHL) as typical p-type CT acceptors. Oxidation of the ferrocenes to the cor-
responding ferricinium salts is proposed to occur via initial formation of outer sphere charge transfer complexes. UV–Vis spectra for
the mixtures of any of the ferrocene donors with I2 or DDQ in CH2Cl2 showed the characteristic bands of the triiodide ion, Iꢁ3 and
the DDQÅ- anion radical. With CHL, the same behavior occurred only for Cp2ꢀFe to give Cpꢀ2Feþ and CHLÅ- radicals. Elemental
analyses of the isolated solid complexes indicated the formation of [ferrocene]I3 for ferrocenes and I2, except for (Fc2C2H4) which
gave [(Fc2C2H4)]I5. Ferrocene–DDQ complexes were 1:1 except for (MeCp)2Fe and (Fc2C2H4) which gave 1:2 complexes. By using
n-hexane as a non-polar solvent instead of CH2Cl2, the CT band of the Cpꢀ2Fe adduct has been observed and the formation constant
for this CT complexation was found to be 3900 ( 500) dm3 molꢁ1
.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Ferrocenes; Redox reactions; CT complexes; IR; UV–Vis spectroscopy
Interesting and important properties of ferrocene
moieties have attracted many researchers for extensive
studies. Thus, ferrocenes play a fundamental role in
many redox-interactions [1] and synthesis of organome-
tallic compounds characterized by biological [2] and
optical activities [3]. Further, Ferrocenes are well known
by the intra- [4,5] and intermolecular charge transfer [6].
Outer sphere association of metal complexes has been
relatively little studied, although Taube [7] showed that
a variety of redox reactions of metal complexes proceed
by outer sphere association of oxidant and reductant.
Understanding and controlling such associations might
have useful applications in catalysis and in molecular
recognition.
However, we found that photolysis of decamethylfer-
rocene with perfluoroalkanes leads to defluorination to
give perfluoroalkenes [8] and postulated that this pro-
ceeds via initial formation of an outer sphere charge
transfer (CT) complex. The Cpꢀ2Fe/per-F–alkane com-
plexes were too weakly bound to be studied directly by
UV–Vis spectroscopy, so we have now moved to much
powerful acceptors, elemental iodine,
a
typical
r-acceptor, and 2,3-dichloro-5,6-dicyano-p-benzoqui-
none (DDQ) and p-chloranil (CHL), typical p-acceptors.
Charge transfer interactions between some ferrocenes, as
*
Corresponding author. Tel.: +2096 341289; fax: +2096 211279.
E-mail address: hsalman16@yahoo.com (H.M.A. Salman).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.07.023

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H.M.A. Salman et al. / Inorganic Chemistry Communications 7 (2004) 1209–1212
electron donors, and r- and p-acceptors (I2, DDQ,
TCNE, TCNQ, CHL, etc.) are well-known [9,10], but
no CT interactions have been reported for r-acceptors,
such as iodine, with ferrocenes under investigation.
As early as 1966, Brandon et al. [11] reported the io-
nic nature of the salts formed from Cp2Fe/DDQ and
Cp2Fe/TCNE, indicating the occurrence of redox reac-
tions. Gebert [12] proposed that Cpꢀ2Fe and DDQ yield
the ionic salt [Cpꢀ2Feþ][DDQHꢁ]. Recently, Miller [13]
reformulated the salt as [Cpꢀ2Feþ][DDQÅ-]. In the course
of our own work, we found that CT complexes can in-
deed undergo outer sphere charge transfer to give D.A
adducts and then undergo complete electron transfer
to give the D+Aꢁ salt.
All ferrocenes, DDQ, and CHL were purchased from
Aldrich Co., and used without further purification, iodine
was supplied by Alfa and was resublimed. High purity
spectral-grade solvents were used (J.T. Baker). Electronic
UV–Vis spectra were recorded with a Cary 3E Varian
spectrometer with a temperature-controlled cell holder.
IR spectra were recorded with a Midac M1200 FT-IR
spectrometer. Microanalyses were performed by Atlantic
Microlabs, Norcross, GA, USA and Robertson Microlit
Laboratories, Madison, NJ, USA.
Equimolar amounts (0.5 mmol) of Cpꢀ2Fe or Fc2C2H4
and I2 were dissolved separately in the minimum volume
of n-hexane and the solutions were warmed and mixed.
An immediate deep green (from Cpꢀ2Fe) or dark reddish
brown (from Fc2C2H4) precipitate was collected, fil-
tered, and washed several times with Et2O. The solid
products were recrystallized from CH2Cl2-n-hexane
and dried in vacuo. For the Cp2Fe and (MeCp)2Fe
cases, CH2Cl2 was used as solvent and the solution of
the mixtures was refluxed for 1/2 h. On cooling, deep
green precipitates, formed in each case, were collected,
filtered, and washed several times with Et2O. The solid
products were recrystallized from CH2Cl2 and dried in
vacuo. Anal. Calc. for Cpꢀ2Fe I3: C, 33.98; H, 4.28.
Found: C, 34.98; H, 4.22. For Cp2FeI3, Calc.: C,
21.19; H, 1.78. Found: C, 21.23; H, 1.75. For (MeCp)2
FeI3, Calc.: C, 24.23; H, 2.37. Found: C, 24.55; H,
2.37. For Fc2C2H4I5, Calc.: C, 25.59; H, 2.15. Found:
C, 25.06; H, 2.07.
C, 50.33; H, 2.11; N, 8.38; Cl, 21.22. Found: C, 49.52;
H, 2.68; N, 7.29; Cl, 20.04. For Fc2C2H4(DDQ)2, Calc.:
C, 53.52; H, 2.58; N, 6.57; Cl, 16.67. Found: C, 53.36; H,
2.78; N, 6.52; Cl, 16.80.
The reaction of the ferrocenes with I2 in CH2Cl2 al-
ways led to the formation of salts via electron transfer.
All attempts to identify charge transfer band(s) for mix-
tures of ferrocenes and I2 in CH2Cl2 failed. Instead, the
characteristic [15] bands of the triiodide ion ðIꢁ3 Þ at 292
and 363 nm (Fig. 1) grew in, while the yellow color of
the ferrocenes turned to the green color of the ferrici-
nium salts. The electronic spectra of the redissolved so-
lid complexes in MeOH indicate the presence of
ferricinium [16] and Iꢁ3 ions [15].
For Cpꢀ2Fe/I2 in CH2Cl2, the triiodide bands ap-
peared immediately on mixing indicating rapid interac-
tion, but for Cp2Fe/I2, (MeCp)2Fe/I2 (Fig. 1), and
Fc2C2H4/I2, this oxidation took about five to ten min-
utes. The stoichiometric ratios of these complexations
in CH2Cl2 were ascertained by applying the continuous
variation method [17] which gave a symmetrical curve
with a maximum at a mole fraction of 0.40 of the ferro-
cene donors with I2, indicating that the complexes are of
the form ferrocene–I3. Early evidence was reported [18]
for the transformation of the associative outer com-
plexes to the dissociative inner complexes. The only
reaction pathway previously reported [19] for triiodide
formation via the interaction of iodine as a r-acceptor
with strong donors is:
D þ I2
!
DI2
outer complex
! DþI ꢂ Iꢁ
ð1Þ
donor
inner complex
acceptor
DþI ꢂ Iꢁ þ I2 ! DþI ꢂ I3ꢁ ðtriiodide complexÞ
ð2Þ
This pathway requires a lone pair on the donor, not
present in the ferrocene case. We believe that the follow-
ing pathway can better explain the interaction of ferro-
cene donors with iodine.
ðferroceneÞ þ I2
¡ðferroceneÞ ꢂ I2ðouter CT complexÞ
ð3Þ
In a simpler modification of a prior route [14], equi-
molar amounts (0.5 mmol) of the ferrocenes and DDQ
were dissolved separately in the minimum volume of
CH2Cl2 and the solutions mixed giving rise to an imme-
diate color change. The resulting mixtures were refluxed
for 1 h. Precipitated solid products were collected, fil-
tered, and washed several times with Et2O. The solid
products were recrystallized from CH2Cl2-n-hexane
and dried in vacuo. Anal. Calc.: for Cpꢀ2Fe(DDQ): C,
60.78; H, 5.46; N, 5.06; Cl, 12.81. Found: C, 59.99; H,
5.84; N, 4.55; Cl, 11.77. For Cp2Fe(DDQ), Calc.: C,
52.34; H, 2.44; N, 6.78; Cl, 17.18. Found: C, 52.34, H,
2.57; N, 6.81; Cl, 17.36. For (MeCp)2Fe(DDQ)2, Calc.:
Fig. 1. Effect of time on the electronic spectra of Cp2Fe/I2 solution
,
mixture in CH2Cl2 at 25 °C; [Cp2Fe] = [I2] = 0.0002 mol dmꢁ3
blank = CH2Cl2.

H.M.A. Salman et al. / Inorganic Chemistry Communications 7 (2004) 1209–1212
1211
ðferroceneÞ ꢂ I2 ! ðferriciniumÞþIꢁ þ 1=2 I2
ðferriciniumÞþIꢁ þ I2 ! ðferriciniumÞþIꢁ3
ð4Þ
ð5Þ
We propose fast formation of the CT complex fol-
lowed by a slow step (Eq. (4)) to give the ferricinium
ion, followed by formation of triiodide.
Although the system Fc2C2H4/I2 gave both the triio-
dide and the ferricinium bands in CH2Cl2 solution, indi-
cating the formulation (Fc2C2H4)I3, the elemental
analyses of the isolated solid complex indicated the for-
mulation (Fc2C2H4)I5; pentaiodide compounds are well
known [20]. This formulation implies the presence of
one Fe(III) and one Fe(II) in the complex. Accordingly,
the solution UV–Vis spectra of (Fc2C2H4)I3 has a shoul-
der corresponding to the Fe(II) center.
Fig. 2. Electronic absorption spectra of (MeCp)2Fe/DDQ solution
mixture in CH2Cl2 at 25 °C; [(MeCp)2Fe] = 0.001 mol dmꢁ3
,
[DDQ] = 0.0002 mol dmꢁ3, blank = [(MeCp)2Fe].
Chlorinated solvents, CHCl3, CH2Cl2, and even CCl4
only gave salts, indeed ferrocenes are known [21] to be
oxidized by such solvents. With n-hexane, a non-polar
solvent, a CT band was formed at 374 nm for the case
of Cpꢀ2Fe/I2 at least initially. The donor at the same con-
centration as in the test solutions was taken as a refer-
ence to eliminate any possible interference. Spectra
were collected directly after mixing to avoid precipita-
tion which occurred after few minutes for some of these
mixtures in the higher concentrations; no thermody-
namic data could be obtained in this case. The well
known Benesi–Hildebrand plot [22]:
posable on those of the corresponding free ferrocenes
and free DDQ, the spectra of the complexes did not ex-
hibit any new bands indicating that no chemical reac-
tions occurred, other than electron transfer from the
iron (II) to the DDQ. This suggestion is supported by
the following three observations. (a) The complexes dis-
played the important IR spectral bands of the radical
anion DDQÅ- [25]; namely, the vibration frequencies of
C„N, C@O, C–C–CN, and C–Cl observed at 2230,
1680, 895, and 803 cmꢁ1 for DDQ are shifted to 2198–
2216, 1592–1552, 882–852, and 772–728 cmꢁ1, respec-
tively, for the radical anion DDQÅ- of the isolated
complexes of DDQ with the ferrocene donors. (b) The
elemental analyses indicated a 1:1 ratio of Cpꢀ2Fe or
Cp2Fe with DDQ and a 1:2 ratio for (MeCp)2Fe or
Fc2C2H4 with DDQ. (c) The structural characteristic
bands of the radical anion DDQÅ- are seen in the UV–
Vis spectra along with those of the dark green ferrici-
nium ions. Moreover, we agree with Miller [13] with
the formation of the radical anion DDQÅ- rather than
hydride anion DDQHꢁ as the result of the interaction
of ferrocenes with DDQ.
½I2ꢃ=Abs: ¼ 1=KCTeCT:1=½CpꢀFeꢃ þ 1=eCT
2
ðwhere ½CpꢀFeꢃ ꢄ ½I2ꢃÞ
2
was used to find the association constant (KCT) and
extinction coefficient (eCT). The high values of the asso-
ciation constant, KCT = 3900 ( 500) dm3 molꢁ1 and
extinction coefficient, eCT = 1800 ( 200) dm3 molꢁ1
cmꢁ1 indicate that this n ! r CT complex is of the
strong type. This is supported by the ease of transforma-
tion into the salt. Because Cpꢀ2Fe has only a slightly
lower oxidation potential than the other ferrocenes
[23], the latter should also give CT bands in hexane,
but the incomplete solubility of these ferrocenes
prevented their detection.
The CT complexes formed are therefore of the strong
n ! p type; i.e., the contribution of the dative structure
ꢁ
wave function YD+–A in the ground state of the CT
complexes is large. In polar solvents, this leads to the
complete electron transfer to yield oxidation of Fe(II)
to Fe(III); ferrocenes to ferricinium cations via MLCT
interaction.
In CH2Cl2, the ferrocenes/DDQ system showed the
characteristic bands of the DDQÅ- anion radical [24] at
k
max = 346, 435, 458, 546 and 587 nm (Fig. 2). This
means that the complexes of these ferrocenes with
DDQ exist predominately in the dissociated state: radi-
cal cations and anions, D+Aꢁ. Thus, the recorded spec-
tra of these complexes are perturbed by those of the
radical anions, they do not exhibit well defined CT
band(s), whereas all attempts to obtain definite CT
band(s) did not succeed, and so the determination of
the formation constants of these CT complexes is not
possible.
UV–Vis spectral studies also showed the characteris-
tic bands [24] of the CHLÆꢁ anion radical at kmax = 425
and 450 nm, for the mixture Cpꢀ2Fe/CHL in CH2Cl2,
while solutions of the other ferrocene donors in the pres-
ence of CHL did not exhibit any new bands. This behav-
ior is consistent with decamethlylferrocene being the
most powerful electron donor of the series. Moreover,
the ability of DDQ to form complexes with all the ferr-
ocenes in contrast with CHL, which only interacts with
decamethylferrocene, is consistent with the higher elec-
tron affinity of DDQ [26].
Even though the IR spectra of the isolated solid com-
plexes from the ferrocenes and DDQ are not superim-

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H.M.A. Salman et al. / Inorganic Chemistry Communications 7 (2004) 1209–1212
[12] E. Gebert, A.H. Reis, J.S. Miller, H. Rommelmann, A.J. Epstein,
J. Am. Chem. Soc. 104 (1982) 4403.
In conclusion, oxidation of organometallic species by
halogens is well known, but here we show that in a non-
polar solvent, the I2/decamethylferrocene system forms
a charge transfer complex and propose that oxidation
occurs via prior CT complexation. DDQ also interacts
with ferrocenes to give ferricinium salts of the
DDQÅ- radical anion.
[13] J.S. Miller, P.J. Krusic, D.A. Dixon, W.M. Reif, J.H. Zhang,
E.C. Anderson, A.J. Epstein, J. Am. Chem. Soc. 108 (1986) 4459.
[14] (a) M. Rosenblum, R.W. Fish, C. Bennett, J. Am. Chem. Soc. 86
(1964) 5166;
(b) O.W. Webster, W. Mahler, R.E. Benson, J. Am. Chem. Soc.
84 (1962) 3678.
[15] (a) K. Kaya, N. Mikami, Y. Udagawa, M. Ito, Chem. Phys. Lett.
16 (1972) 151;
(b) W. Kiefer, H.J. Bernstein, Chem. Phys. Lett. 16 (1972) 5;
(c) L. Andrews, E.S. Prochask, A. Loewenschuss, Inorg. Chem.
19 (1980) 463.
References
[1] J. Moutet, E. Saint-Aman, G. Royal, S. Tingry, R. Ziessel, Eur.
J. Inorg. Chem. (2002) 3692.
[16] G. Wilkinson, M. Rosenblum, M.C. Whiting, R.B. Woodward, J.
Am. Chem. Soc. 74 (1952) 2125.
[17] P. Job, Ann. Chim. 10 (1928) 113.
[2] S. Huikai, W. Qingmin, H. Runqiu, L. Heng, L. Yonghong, J.
Organomet. Chem. 655 (1–2) (2002) 182.
[18] (a) R.S. Mulliken, J. Phys. Chem. 56 (1952) 801;
(b) R.S. Mulliken, C. Reid, J. Am. Chem. Soc. 76 (1954) 3869.
[19] (a) M.M.A. Hamed, E.M. Abdalla, M.R. Mahmoud, Can. J.
Appl. Spect. 39 (1993) 24;
[3] H. Nishihara, Bull. Chem. Soc. Jpn. 74 (2001) 119.
[4] D.F. Perepichka, I.F. Perepichka, A.F. Popov, M.R. Bryce, A.S.
Batsanov, A. Chesney, J.A.K. Howard, N.I. Sokolov, J. Organ-
omet. 637 (2001) 445.
(b) N.S. Rao, G.B. Rao, Spectrochim. Acta 46A (1990) 1107;
(c) S.N. Bhat, C.N.R. Rao, J. Am. Chem. Soc. 88 (1966) 3216.
[20] (a) C.L. Raston, A.H. White, D. Petridis, D. Taylor, J. Chem.
Soc., Dalton Trans. (1980) 1928–1931;
[5] A.J. Moore, A. Chesney, M.R. Bryce, A.S. Batsanov, J.F. Kelly,
J.A.K. Howard, I.F. Perepichka, D.F. Perepichka, G. Meshulam,
G. Berkovic, Z. Kotler, et al., Eur. J. Inorg. Chem. 14 (2001)
2671.
(b) D.W. Larsen, S.A. Mestemacher, J. Inorg. Nucl. Chem. 33
(1970) 869–874.
[6] H.B. Tatistcheff, L.F. Hancock, M.S. Wrighton, J. Phys. Chem.
99 (1995) 7689.
[21] J.C.D. Brand, W. Snedden, Trans. Faraday Soc. 53 (1957) 894.
[22] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.
[23] A. Togni, Ferrocenes, VCH, New York, 1995, pp. 436.
[24] (a) Ref. [9a];
[7] H. Taube, E.S. Gould, Acc. Chem. Res. 2 (1969) 321.
[8] J. Burdeniuc, R.H. Crabtree, J. Am. Chem. Soc. 118 (1996) 2525.
[9] (a) J.S. Miller, A.J. Epstein, W..M. Reiff, Chem. Rev. 88 (1988)
201;
(b) Y. Iida, Bull. Chem. Soc. Jpn. 44 (1971) 2615;
(c) Y. Iida, Y. Matsunga, Bull. Chem. Soc. Jpn. 40 (1967) 231.
[25] (a) T. Yamaoka, S. Nagakura, Bull. Chem. Soc. Jpn. 44 (1971)
1780;
(b) E. Adman, M. Rosenblum, S. Sullivan, T.N. Margulis, J. Am.
Chem. Soc. 89 (1967) 4540;
(c) J.C. Goan, E. Berg, H.E. Podall, J. Org. Chem. 29 (1964) 975.
[10] S.K. Pal, K. Alagesan, A.G. Samuelson, J. Pebler, J. Organomet.
Chem. 575 (1999) 1108.
(b) N. Sakai, I. Shirotani, S. Minomura, Bull. Chem. Soc. Jpn. 44
(1971) 675;
[11] R.L. Brandon, J.H. Osiecki, A. Ottenberg, J. Org. Chem. 31
(1966) 1214.
(c) Y. Iida, Bull. Chem. Soc. Jpn. 43 (1970) 2772.
[26] P.G. Briegleb, Ang. Chem. Int. Ed. 13 (1964) 617.