New and selective routes to functionalized biferrocenes and terferrocenes by [3 + 2] cycloadditions of alkynes with bridging C3 ligands in diiron complexes

Article abstract of DOI:10.1021/om101162r

The diiron bridging enaminoalkylidene complex [Fe₂{μ- η¹:η³-C(Fc)CHCH(NMe₂)}(μ-CO)(CO)(Cp) ₂] (2), containing a ferrocenyl group on the bridging ligand, reacts with HC≡CR, affording the corresponding 3-R-substituted 1,1′′-biferrocene (R = CPh₂OH, 4a; R = Tol, 4b; R = CO₂Me, 4c; R = CH₂OH, 4d). In a related reaction, the μ-vinylaminoalkylidene complex [Fe₂{μ-η¹: η³-C(NMe₂)CHCH(CO₂Me)}(μ-CO)(CO)(Cp) ₂] (1a), upon treatment with ethynylferrocene, leads to the formation of 2-NMe₂-4-CO₂Me-1,1′′-biferrocene (4f). Conversely, the μ-enaminoalkylidene complex [Fe₂{μ- η¹:η³-C(Tol)CHCH(NMe₂)}(μ-CO)(CO) (Cp)₂] (1b) reacts with ethynylferrocene, affording a mixture of monosubstituted biferrocene 4b and disubstituted 2-NMe₂-4-Tol-1, 1′′-biferrocene (4e), in comparable yields. Reaction of 2 with ethynylferrocene (HC≡CFc) leads to the formation of a mixture of 1,3-terferrocene (5a) and the 5-NMe₂-substituted 1,3-terferrocene (5b). Investigation of the reactivity of the vinyliminium complex [Fe ₂{μ-η¹:η³-C(Fc)CHCNMe ₂}(μ-CO)(CO)(Cp)₂][SO₃CF₃] (3) with HC≡CCPh₂OH is also reported: the reaction affords selectively the 3-NMe₂-4-R-disubstituted 1,1′′- biferrocene (6). The molecular structure of 5a has been determined by X-ray diffraction studies.

Full text of DOI:10.1021/om101162r

Organometallics 2011, 30, 1175–1181 1175
DOI: 10.1021/om101162r
New and Selective Routes to Functionalized Biferrocenes and
Terferrocenes by [3 þ 2] Cycloadditions of Alkynes with Bridging C3
Ligands in Diiron Complexes
Mauro Salmi, Luigi Busetto, Rita Mazzoni, Stefano Zacchini, and Valerio Zanotti*
ꢀ
Dipartimento di Chimica Fisica e Inorganica, Universita di Bologna, Viale Risorgimento 4, 40136 Bologna,
Italy
Received December 10, 2010
The diiron bridging enaminoalkylidene complex [Fe2{μ-η1:η3-C(Fc)CHCH(NMe2)}(μ-CO)(CO)-
(Cp)2] (2), containing a ferrocenyl group on the bridging ligand, reacts with HCtCR, affording the
corresponding 3-R-substituted 1,100-biferrocene (R = CPh2OH, 4a; R = Tol, 4b; R = CO2Me, 4c;
R = CH2OH, 4d). In a related reaction, the μ-vinylaminoalkylidene complex [Fe2{μ-η1:η3-C-
(NMe2)CHCH(CO2Me)}(μ-CO)(CO)(Cp)2] (1a), upon treatment with ethynylferrocene, leads to
the formation of 2-NMe2-4-CO2Me-1,100-biferrocene (4f). Conversely, the μ-enaminoalkylidene
complex [Fe2{μ-η1:η3-C(Tol)CHCH(NMe2)}(μ-CO)(CO)(Cp)2] (1b) reacts with ethynylferrocene,
affording a mixture of monosubstituted biferrocene 4b and disubstituted 2-NMe2-4-Tol-1,100-
biferrocene (4e), in comparable yields. Reaction of 2 with ethynylferrocene (HCtCFc) leads to
the formation of a mixture of 1,3-terferrocene (5a) and the 5-NMe2-substituted 1,3-terferrocene (5b).
Investigation of the reactivity of the vinyliminium complex [Fe2{μ-η1:η3-C(Fc)CHCNMe2}(μ-CO)-
(CO)(Cp)2][SO3CF3] (3) with HCtCCPh2OH is also reported: the reaction affords selectively the
3-NMe2-4-R-disubstituted 1,100-biferrocene (6). The molecular structure of 5a has been determined
by X-ray diffraction studies.
Scheme 1
Introduction
Ferrocenes are exceptionally valuable organometallic
scaffolds for the construction of functional molecules with
potential application in catalysis, materials science, and bio-
medicinal chemistry.1 In order to connect different ferrocene
units and/or to confer the desired properties, ferrocene cyclo-
pentadienyl (Cp) rings must be provided with appropriate
substituents and functionalities (polysubstituted ferrocenes).
Functionalization of the Cp ring can be challenging from a
synthetic point of view, particularly in the case of ferrocenes,
in which only one Cp ring contains different substituents.2
These difficulties might limit the design of molecular
architectures based on ferrocenes.3 A possible alternative
approach consists in the assembling of cyclopentadienyls, by
metal-mediated [3 þ 2] cycloaddition of molecular fragments
properly constructed in order to contain the desired substit-
uents and functionalities.4 We have recently shown that [3 þ
2] cycloaddition involving bridging C3 ligands in diiron
complexes and alkynes (C2) leads to the formation of poly-
substituted Cp ligands, directly affording ferrocenes.5 An
example, shown in Scheme 1, illustrates the reaction of the
bridging vinylaminoalkylidene complex 1a with PhCtCPh.
All the functionalities present on the parent C3 and C2
fragments are maintained in the resulting functionalized
Cp ring of the ferrocene product. Conversely, the dinuclear
*To whom correspondence should be addressed. E-mail: valerio.
zanotti@unibo.it.
(1) For a comprehensive overview of ferrocene and other metallo-
cenes see: (a) Ferrocenes: Homogeneous Catalysis, Organic Synthesis,
Materials Science; Togni, A., Hayashi, T., Eds.; Wiley-VCH: Weinheim,
Germany, 1995. (b) Metallocenes; Togni, A. Halterman, R. L., Eds.; Wiley-
VCH: Weinheim, Germany, 1998. (c) Ferrocenes: Ligands, Materials, and
ꢁ
ꢁ
ꢁ
Biomolecules; Stepnicka, P., Ed.; Wiley: West Sussex, U.K., 2008.
(2) (a) Metallinos, C.; Zaifman, J.; Dodge, L. Org. Lett. 2008, 10,
3527–3530. (b) Stoll, A. H.; Mayer, P.; Knochel, P. Organometallics
2007, 26, 6694–6697. (c) Wang, Y.; Weissensteiner, W.; Spindler, F.;
Arion, V. B.; Mereiter, K. Organometallics 2007, 26, 3530–3540. (d)
Christophe, P.; Odell, B.; Brown, J. M. Chem. Commun. 2004, 598–599.
(e) Steurer, M.; Tiedl, K.; Wang, Y.; Weissensteiner, W. Chem. Commun.
2005, 4929–4931. (f) Butler, D. C. D.; Richards, C. J. Organometallics
2002, 21, 5433–5436. (g) van Leusen, D.; Hessen, B. Organometallics
2001, 20, 224–226.
(4) (a) Lutsenko, Z. L.; Aleksandrov, G. G.; Petrovskii, P. V.;
Shubina, E. S.; Andrianov, V. G.; Struchkov, Yu. T.; Rubezhov, A. Z.
J. Organomet. Chem. 1985, 281, 349–364. (b) Blenkiron, P.; Breckenridge,
S. M.; Taylor, N. J. A.; Carty, J.; Pellinghelli, M. A.; Tiripicchio, A.;
Sappa, E. J. Organomet. Chem. 1996, 506, 229–240.
(3) (a) Debroy, P.; Roy, S. Coord. Chem. Rev. 2007, 251, 203–221. (b)
Osakada, K.; Sakano, T.; Horie, M.; Suzaki, Y. Coord. Chem. Rev. 2006,
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(5) Busetto, L.; Marchetti, F.; Mazzoni, R.; Salmi, M.; Zacchini, S.;
Zanotti, V. Organometallics 2009, 28, 3465–3472.
r
2011 American Chemical Society
Published on Web 02/07/2011
pubs.acs.org/Organometallics

1176 Organometallics, Vol. 30, No. 5, 2011
Salmi et al.
Scheme 2
Scheme 3
parent compound undergoes fragmentation, with elimination
of one Fe unit, formally corresponding to [Fe(H)(CO)2Cp].
The direct formation of polysubstituted ferrocenes, occur-
ring in a “one pot reaction”, compensates the synthetic effort
for the construction of a bridging C3 ligand containing the
appropriate functionalities, as well as the loss of an Fe-
containing fragment. Indeed, a variety of diiron complexes
bearing different bridging C3 ligands (such as vinylalkyli-
denes and vinyliminiums) are readily available by simple syn-
thetic procedures, and C3 substituents comprise a range of
groups, including NMe2, SMe, CO2R, CN, and SiMe3.6 Also,
ferrocenyls (Fc) are among the possible substituents, as shown
in Scheme 2, since 2 and 3 display ferrocenyl-vinylalkylidene
and ferrocenyl-vinyliminium ligands, respectively.7
In light of these results, we became interested in extending
studies on the [3 þ 2] cyclization reactions to C3 ligands
containing ferrocenyl substituents (e.g., 2 and 3). The aim
was to exploit the one-pot synthesis of functionalized ferro-
cenes for obtaining biferrocenes (including polysubstituted
biferrocenes). Indeed, biferrocenes are species of great inter-
est, due to the nature of interactions between metal centers,
and they have been extensively investigated; nevertheless,
synthetic methods for obtaining these types of complexes are
somewhat limited.8 The present report concerns a possible
alternative synthetic approach.
both possibilities (C-H and C-N cleavage) take place,
producing di- and trisubstituted ferrocenes.5 Conversely,
the reactions shown in Scheme 3 are chemoselective, but
unfortunately the C-N bond is selectively cleaved, depriving
the diferrocene products of a valuable NMe2 substituent.
Furthermore, the observed cycloaddition reactions are re-
giospecific, since the incorporation of primary alkynes oc-
curs selectively in one of the two possible modes, affording a
Cp ring where R and Fc substituents are in 1,3-positions. In
other words, the reactions selectively yield 3-substituted
1,100-diferrocenes. This point is evidenced in the NMR spec-
tra by the absence of any significant NOE effect between the
ferrocenyl substituent and the carbon atom bearing the R
group. The regioselectivity is presumably originated by steric
reasons, in that the formation of the observed isomer in-
volves minor repulsion between the substituents.
The most relevant aspect of the reaction is that it provides
a possible alternative approach to the synthesis of biferro-
cenes. Traditional methods are based on the Ullmann cou-
pling of haloferrocenes by copper, discovered by Rausch.9
Other similar methods, subsequently developed, require the
coupling with lithioferrocene or ferrocenyl Grignards.10
These strategies mainly produce 10,1000-disubstituted biferro-
cene compounds (Scheme 4, B), or similar centrosymmetric
compounds which are, by far, the most investigated biferro-
cene compounds.8,11 Conversely, 4a-d are 3-substituted
1,100-biferrocenes (Scheme 4 A). This is a rather uncommon
substitution pattern, which is available selectively neither by
coupling of ferrocenes nor by functionalization of diferrocenes.
Reaction of Enaminoalkylidene Complexes 1a,b with Ethy-
nylferrocene. The aforementioned cycloaddition reactions
result in the formation of biferrocenes for the reason that a
ferrocenyl group (Fc) is a substituent on the C3 fragment.
Thus, when the [3 þ 2] cycloaddition generates the substi-
tuted Cp ring of a ferrocene unit, a ferrocenyl results bound
to it. In theory, the same result should be obtained by placing
Fc as a substituent on the alkyne C2 unit instead of the C3
ligand. In order to verify this possibility, the bridging enami-
noalkylidene complex 1b has been treated with HCtCFc in
Results and Discussion
Reactions of the Enaminoalkylidene Complex 2 with Al-
kynes. The bridging enaminoalkylidene complex 2 reacts
with HCtCR (R = CPh2OH, Tol, CO2Me, CH2OH; Tol
=4-C6H4Me), in refluxing toluene, affording the 3-R-substi-
tuted 1,100-biferrocenes (R = CPh2OH, 4a; R = Tol, 4b; R =
CO2Me, 4c; R = CH2OH, 4d) in 40-60% yields (Scheme 3).
In Scheme 3, carbon atoms of the bridging chain of 2 have
been labeled to better identify them as constituents of the
cyclopentadienyl ring in the products 4a-d. Compounds
4a-d were purified by chromatography on alumina and
characterized by NMR spectroscopy and elemental analysis.
The prolonged heating required to perform the reaction
produces a considerable amount of decomposition, resulting
in modest yields; nevertheless, the reaction is selective, lead-
ing to the formation of a single product in a single isomeric
form, as indicated by NMR spectra. In theory, several
products might be produced, in that the formation of a Cp
ring by [3 þ 2] cycloaddition requires the loss of one of the
two substituents (H or NMe2) from the vinyl end of the
bridging C3 ligand. In similar reactions, previously reported,
(9) (a) Rausch, M. D. J. Am. Chem. Soc. 1960, 82, 2080–2081. (b)
Rausch, M. D. J. Org. Chem. 1961, 26, 1802–1805.
(10) (a) Schechter, H.; Helling, J. F. J. Org. Chem. 1961, 26, 1034–
1037. (b) Hata, K.; Motoyama, I.; Watanabe, H. Bull. Chem. Soc. Jpn.
1964, 37, 1719–1720.
(11) See for example: (a) Dong, T.-Y.; Hendrickson, D. N.; Iwai, K.;
Cohn, M. J.; Geib, S. J.; Rheingold, A. L.; Sano, H.; Motoyama, I.;
Nakashima., S. J. Am. Chem. Soc. 1985, 107, 7996–8008. (b) Sinha, U.;
Lowery, M. D.; Ley, W. W.; Drickamer, H. G.; Hendrickson, D. N. J.
Am. Chem. Soc. 1988, 110, 2471–2477.
(6) Zanotti, V. Pure Appl. Chem. 2010, 82, 1555–1568.
(7) Busetto, L.; Mazzoni, R.; Salmi, M.; Zacchini, S.; Zanotti, V. J.
Organomet. Chem. 2010, 695, 2519–2525.
(8) Barlow, S.; O’Hare, D. Chem. Rev. 1997, 97, 637–669.

Article
Organometallics, Vol. 30, No. 5, 2011 1177
Scheme 4
Scheme 6
present on the C3 fragment, including the NMe2 group, are
transferred to the resulting Cp ring of the ferrocene product.
Therefore, we also investigated the reaction of the vinylami-
noalkylidene 1a12 with ethynylferrocene. As expected, the
reaction leads to the formation of a single product: namely,
the 2,4-disubstituted 1,100-biferrocene 4f (Scheme 6).
In this case, the reaction is both chemo- and regioselective,
since alkyne incorporation occurs selectively, with alkyne
CH termination exclusively bound to the C-CO2Me moiety
of the μ-C3 frame (Scheme 6).
Scheme 5
Compound 4f was purified by chromatography on
alumina and characterized by NMR spectroscopy and
elemental analysis. Also in this case, the geometry of the
molecule and the substitution pattern at the cyclopenta-
dienyl ring were determined by NMR and NOE investi-
gations.
Reaction of Enaminoalkylidene Complex 2 with Ethynyl-
ferrocene. The synthetic approach described above can be
further extended to the preparation of terferrocenes. Indeed,
if both the C3 bridging ligand and C2 reagent (alkyne) con-
tain a ferrocenyl substituent, their assembly, through a [3 þ
2] cyclization, is expected to produce species with three
connected ferrocene units. Indeed, we have found that the
reaction of 2 with ethynylferrocene, in refluxing toluene,
affords a mixture of the terferrocenes 5a,b, in about 50% and
20% yields, respectively (Scheme 7).
refluxing toluene (Scheme 5). The reaction can be compared
to that of 2 with HCtCTol shown in Scheme 3: the differ-
ence is that the Fc and Tol have been inverted as substituents
at the C2 and C3 fragments, respectively. The reaction affords
a mixture of the monosubstituted biferrocene 4b and the 2,4-
disubstituted-1,100-biferrocene 4e, in comparable yields (ca.
30% for both 4b and 4e) (Scheme 5).
Thus, the reaction of 1b with ethynylferrocene exhibits a
very similar outcome compared to that of 2 with HCtCTol,
except for the lack of chemoselectivity. Both C-H and C-N
bonds at the vinyl end of the bridging C3 frame can undergo
cleavage, leading to the expected formation of 4b, and also of
the disubstituted biferrocene 4e, in comparable amounts.
Compounds 4b,e were separated by chromatography on alu-
mina and characterized by NMR spectroscopy and elemen-
tal analysis. In particular, the NMR data for 4e show a
significant NOE effect between the methyls of the amino
group and the ferrocenyl substituent, thus indicating the sub-
stitution pattern of 4e, which corresponds to a 2,4-disubsti-
tuted biferrocene.
The presence of a NMe2 substituent on a cyclopentadienyl
ring should be advantageously used to further modify or to
connect the biferrocene unit to other species. However, the
lack of selectivity, resulting in the formation of a product
mixture, is a limit for possible synthetic applications. We
have previously found that more selective [3 þ 2] cycloaddi-
tion reactions take place between alkynes and bridging
vinylalkylidenes in which the NMe2 group is connected
to the alkylidene carbon, instead of the vinyl end.5 This is
the case shown in Scheme 1, in which all the functionalities
Compounds 5a,b were separated by chromatography on
alumina and characterized by NMR spectroscopy and ele-
mental analysis. Moreover, the molecular structure of 5a has
been determined by X-ray diffraction. The ORTEP diagram
is shown in Figure 1, and the main bond lengths and angles
are reported in Table 1. The molecular structure of 5a,
as determined by X-ray diffraction, is composed of a 1,-
3-terferrocene molecule, where the central FeCp2 unit is 1,-
3-substituted at the same Cp ligand by two further ferrocene
moieties. Bonding parameters within the six Cp ligands are as
˚
expected. The inter-Cp contact (C(10)-C(11) = 1.462(3) A) is
essentially a single bond.
It has to be noted that 5a,b are 1,3-terferrocenes, which
corresponds to one of the possible constitutionally isomeric
forms of terferrocenes, that also include 1,2-terferrocenes
and 1,10-terferrocenes (Scheme 8). Isomeric mixtures of these
compounds, together with polyferrocenyls and other pro-
ducts, are usually obtained by random combination of
ferrocenyl radicals13 or by using related synthetic methods.14
More specific methods have been developed for the selective
(12) Busetto, L.; Salmi, M.; Zacchini, S.; Zanotti, V. J. Organomet.
Chem. 2008, 693, 57–67.
(13) Neuse, E. W. J. Organomet. Chem. 1972, 40, 387–392.
(14) (a) Watanabe, H.; Motoyama, I.; Hata, K. Bull. Chem. Soc. Jpn.
1966, 39, 790–801. (b) Rausch, M. D.; Roling, P. V.; Siegel, A. Chem.
Commun. 1970, 502–503.

1178 Organometallics, Vol. 30, No. 5, 2011
Salmi et al.
nonsubstituted external Cp rings (at 4.09 ppm) and one
resonance accounts for the central nonsubstituted Cp ring
(at 3.88 ppm). Analogously, the 13C NMR spectrum shows
only one resonance for the two external Cp rings (at 69.5 ppm)
and one resonance for the central Cp group (at 70.6 ppm).
In the case of 5b, the presence of the amino group causes a
loss of symmetry of the molecule and subsequently increases
1
the complexity of the NMR spectra. In particular, the H
NMR spectrum shows resonances due to the Cp rings in the
typical range 4-5 ppm and a single resonance due to the
methyls of the amino group at 2.62 ppm.
Finally, the reaction is accompanied by the formation, as
byproducts, of small amounts of FeCp2 and [Fe2Cp2(CO)4].
Reaction of the Vinyliminium Complex 3 with Propargyl
Alcohols. Cycloaddition reactions involving alkynes and
bridging C3 ligands in diiron complexes are not restricted
to bridging vinylalkylidenes: it has been shown that also
bridging vinyliminium ligands undergo [3 þ 2] cycloaddi-
tion, affording polysubstituted ferrocenes, although the
reaction is limited to propargyl alcohols.19 Interestingly,
these reactions also generate products derived from a [3 þ
2 þ 1] cycloaddition that involve CO and are reminiscent of
the classical Dotz benzannulation. In consideration of these
previous results, we investigated the reactivity of the vinyli-
minium complex 3 with HCtCCPh2OH. The reaction, per-
formed in refluxing toluene, leads to the formation of [3-
NMe2-4-CPh2OH-1,100-biferrocene] (6) in about 40% yield
(Scheme 9).
Figure 1. ORTEP drawing of 3a. Only the main images of the
Cp ligands bonded to Fe(1) and Fe(2) have been represented.
Thermal ellipsoids are at the 30% probability level. Only inde-
pendent atoms have been labeled.
Scheme 7
Compound 6 was purified by chromatography on alumina
and characterized by NMR spectroscopy and elemental anal-
ysis. In particular, the NMR spectra (in CDCl3) show a
significant NOE effect between the N-methyls and the pro-
tons of the CPh2OH, indicating that NMe2 and CPh2OH
substituents are in adjacent positions. Moreover, the ob-
served low-field resonance at 6.84 ppm, attributable to the
hydroxy group, suggests that the -OH is involved in a hydro-
gen-bonding interaction with the nitrogen atom of the amino
group.
In spite of the relatively low yield, the reaction is selective,
in that it generates only 6 and not other isomeric forms.
Noteworthy, compound 6 is a 3,4-disubstituted 1,100-bifer-
rocene, corresponding to a substitution pattern different
from that observed in 4e,f, which are 2,4-disubstituted
1,100-biferrocenes. This is simply the consequence of a differ-
ent distribution of the Fc, NMe2, and R substituents (R =
CPh2OH, Tol, CO2Me) on the C2 (alkyne) and bridging C3
fragments involved in the cycloaddition. This observation
implies that substitution patterns of the biferrocene products
can be predetermined, to a certain extent, by an appropriate
choice of bridging functionalized ligands and alkyne reagents.
Electrochemical Investigation. Electrochemical data con-
cerning our biferrocene and triferrocene complexes are of
interest in that biferrocenes, or in general conjugated ferro-
cenyl units, can find application in many fields, thanks to
their remarkable electrical, redox, and magnetic proper-
ties.20 In general, complexes containing conjugated ferroce-
nyls exhibit some degree of electronic communication be-
tween the Fc units, which can be evaluated by cyclic
preparation of 1,10-terferrocenes,15 1,2-terferrocenes,16 and
also 1,3-terferrocenes.17 In particular, access to this latter
isomer requires a laborious multistep procedure, which
involves the formation of 1,4-diferrocenylcyclopentadiene
as intermediate species.17
In other words, synthetic routes based on the assembly of
ferrocenyl units are simple but unselective and are mostly
directed to the formation of 1,10-terferrocenes. This is also
consistent with the fact that ferrocene oligomers and poly-
mers usually assume fulvene-based architectures.18 In the
absence of convenient and selective access to 1,3-terferro-
cenes, our reaction might represent an interesting and valu-
able synthetic approach.
NMR data of 5a are in agreement with the solid-state
1
structure and with published data.17 In particular, the H
NMR spectrum (in CDCl3) confirms the symmetry of the
molecule. Indeed, a single resonance is observed for the two
€
(15) Hilbig, H.; Kohler, F. H.; Mortl J. Organomet. Chem. 2001, 627,
€
71–79.
(19) Busetto, L.; Marchetti, F.; Mazzoni, R.; Salmi, M.; Zacchini, S.;
Zanotti, V. Chem. Commun. 2010, 46, 3327–3329.
(20) See for example: (a) Nijhuis, C. A.; Dolatowska, K. A.; Ravoo,
B. J.; Huskens, J.; Reinhoudt, D. N. Chem. Eur. J. 2007, 13, 69–80. (b)
Diallo, A. K.; Menuel, S.; Monflier, E.; Ruiz, J.; Astruc, D. Tetrahedron
Lett. 2010, 51, 4617–4619.
(16) (a) Goldberg, I. S.; Breland, J. G. J. Org. Chem. 1971, 36, 1499–
1503. (b) Roling, P. V.; Rausch, M. D. J. Organomet. Chem. 1977, 141,
195–204.
(17) Neuse, E. W. J. Organomet. Chem. 1972, 43, 385–392.
(18) Hudson, R. D. A. J. Organomet. Chem. 2001, 637-639, 47–69.

Article
Table 1. Selected Bond Lengths (A) for 5a
Organometallics, Vol. 30, No. 5, 2011 1179
˚
Fe(1)-Cp(1)a
Fe(1)-Cp(2)b
Fe(2)-Cp(3)c
Fe(2)-Cp(4)d
C(10)-C(11)
2.024(5)-2.053(5), av 2.037(15)
C-C Cp(1)a
C-C Cp(2)b
C-C Cp(3)c
C-C Cp(4)d
1.405(7)-1.416(8), av 1.409(16)
1.416(3)-1.427(3), av 1.421(7)
1.414(4)-1.427(3), av 1.424(7)
1.361(14)-1.413(12), av 1.40(2)
2.029(2)-2.040(2), av 2.034(5)
2.033(3)-2.055(2), av 2.047(5)
1.971(7)-2.052(7), av 2.019(13)
1.462(3)
a Cp(1) is defined by atoms C(1), C(2), C(3), C(4), and C(5). b Cp(2) is defined by atoms C(6), C(7), C(8), C(9), and C(10). c Cp(3) is defined by atoms
C(11), C(12), and C(13) and their symmetrical images. d Cp(4) is defined by atoms C(14), C(15), and C(16) and their symmetrical images.
Scheme 8
Scheme 9
render the oxidation easier); (iii) the substitution patterns
include a number of different situations (i.e., 2,4-disubstituted
1,100-biferrocene etc.) with possible consequences on the steric
hindrance and, therefore, on the coplanarity of the fulvene
system. This, in turn, can influence the electronic communica-
tion between the ferrocenyl units.
Concerning the terferrocene complexes, three distinct
redox processes are observed for 5a, which are consistent
with the data reported in the literature for the 1,10-terfer-
rocene,22 whereas only two are found in the case of 5b.
A better understanding of the redox processes taking
place in our complexes certainly requires more extended
and accurate electrochemical as well as structural investi-
gations, but this goes well beyond the scope of this work,
dealing with synthetic aspects, and will be the focus of
upcoming studies.
voltammograms.21 For example, biferrocene displays strongly
interacting metal centers and correspondingly undergoes two
reversible one-electron oxidations at separated potential values
(at about þ0.31 and þ0.65 V, respectively).22 On the other
hand, complexes 4a-f and 6 (see the Experimental Section)
exhibit a rather broad variety of cyclic voltammetric responses,
which virtually cover all classes (according to the so-called
Robin-Day classification) used to evaluate the degree of elec-
tronic communication.23 Complex 6 exhibits two apparently
reversible oxidations with a difference in redox potential of
about 240 mV. Likewise, compounds 4e,f give rise to separate
oxidations (ΔEp of about 290 mV), but these do not seem
reversible. The separation of the two redox processes is much
smaller in 4a, whereas the voltammograms of 4b,c,d exhibit a
single oxidation potential (typical for class I). These results
reflect the presence of several different factors influencing the
redox processes: (i) the two ferrocenyl units are nonequivalent
for the presence of one or two substituents in only one Fc unit;
(ii) the substituents display markedly different inductive effects
(there are both electron-donating and electron-withdrawing
substituents, in different combinations) with consequent effects
on the oxidation potential (i.e., electron-donating substituents
Conclusions
[3 þ 2] cycloaddition of alkynes with bridging C3 ligands in
diiron complexes can be exploited to form functionalized
biferrocenes, provided that the C3 or the C2 fragments
contain a ferrocenyl substituent, as well as other functional
groups. Bridging C3 ligands include vinylalkylidenes and
vinyliminium, which can be easily constructed so that they
contain the most appropriate substituents. As a consequence
of the cyclization and subsequent rearrangement, these
functions are transferred to one of the fulvene rings of the
biferrocene products. Thus, the reaction provides access to
biferrocenes with unusual substitution patterns: 3-substi-
tuted, 2,4-disubstituted, and 3,4-disubstituted 1,100-biferro-
cenes, which can be hardly obtained otherwise. It should be
remarked that these reactions are selective and that through
the appropriate design of the bridging C3 frame, and the
choice of most suitable alkyne reagents, it is possible to
control the type of functional groups and substitution pat-
tern of the biferrocene products. The same strategy can be
applied to the synthesis of terferrocenes, provided that both
C3 and C2 fragments contain a ferrocenyl substituent. Their
assembly occurs selectively, affording 1,3-terferrocenes,
which are less common among the possible terferrocene
isomeric forms.
(21) (a) Inorganic Electrochemistry: Theory, Practice, and Applica-
tions; Zanello, P., Ed.; RSC: London, 2003. (b) Morrison, W. H.; Krogsrud,
S; Hendrickson, D. N. Inorg. Chem. 1973, 12, 1998–2004.
(22) Brown, G. M.; Meyer, T. J.; Cowan, D. O.; LeVanda, C.;
Kaufman, F.; Roling, P. V.; Raush, M. D. Inorg. Chem. 1975, 14,
506–511.
(23) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10,
247–422.

1180 Organometallics, Vol. 30, No. 5, 2011
Salmi et al.
Scheme 10
Synthesis of 3-Tol-1,100-biferrocene (4b). Complex 4b was pre-
pared following the same procedure described for 4a, by reacting
2 with HCtCTol. Yield: 51%. Anal. Calcd for C27H24Fe2: C,
70.47; H, 5.26. Found: C, 70.69; H, 5.16. 1H NMR (CDCl3): δ
4
7.44-7.03 (m, 4H, C6H4Me), 4.83 (t, 1H, JHH = 1.4 Hz, H2,
Cp), 4.64 (m, 1H, Cp00), 4.49 (m, 1H, Cp00), 4.44 (m, 1H, Cp00),
4.37 (m, 1H, Cp00), 4.20 (m, 2H, H4 and H5, Cp), 4.03, 3.87 (s,
10H, Cp0 and Cp000), 2.34 (s, 3H, C6H4Me). 13C NMR (CDCl3):
δ 135.8 (Cipso Tol), 129.8, 128.8, 126.6, 125.9 (CH, Tol), 85.1 (C3,
Cp), 84.5 (C1, Cp00), 83.5 (C1, Cp), 71.2, 69.5 (Cp0 and Cp000), 68.0
(C4 and C5, Cp), 67.2 (Cp00), 66.8 (Cp00), 66.6 (Cp00 and C2, Cp),
65.2 (Cp00), 21.0 (C6H4Me).
Synthesis of 3-CO2Me-1,100-biferrocene (4c). Complex 4c was
prepared following the same procedure described for 4a, by
reacting 2 with HCtCCO2Me. Yield: 58%. Anal. Calcd for
1
C22H20Fe2O2: C, 61.73; H, 4.71. Found: C, 61.59; H, 4.83. H
NMR (CDCl3): δ 4.88 (dd, 1H, 3JHH = 2.44 Hz, 4JHH = 1.32
Hz, H4, Cp), 4.65 (m, 2H, Cp00 and H2, Cp), 4.50 (m, 1H, Cp00),
4.34 (m, 2H, Cp00), 4.21 (dd, 1H, 3JHH = 2.44 Hz, 4JHH = 1.32
Hz, H5, Cp), 4.04, 3.98 (s, 10H, Cp0 and Cp000), 3.69 (s, 3H,
CO2Me). 13C NMR (CDCl3): δ 174.4 (CO2Me), 85.5 (C1, Cp),
81.4 (C1, Cp00), 73.0 (C5, Cp), 72.1 (C2, Cp), 71.5 (C4, Cp), 70.8
(C3, Cp), 70.1, 68.7 (Cp0 and Cp000), 68.0, 67.3, 66.1, 64.9 (Cp00),
50.9 (CO2Me).
Experimental Section
General Data. All reactions were routinely carried out under a
nitrogen atmosphere, using standard Schlenk techniques. Sol-
vents were distilled immediately before use under nitrogen from
appropriate drying agents. Chromatographic separations were
carried out on columns of alumina. Glassware was oven-dried
before use. Infrared spectra were recorded at 298 K on a Perkin-
Elmer Spectrum 2000 FT-IR spectrophotometer, and elemental
analyses were performed on a ThermoQuest Flash 1112 Series
EA Instrument. All NMR measurements were performed on a
Varian Mercury Plus 400 instrument. The chemical shifts for 1H
and 13C were referenced to internal TMS. The spectra were fully
Synthesis of 3-CH2OH-1,100-biferrocene (4d). Complex 4d was
prepared following the same procedure described for 4a, by
reacting 2 with HCtCCH2OH. Yield: 37%. Anal. Calcd for
1
C21H20Fe2O: C, 63.04; H, 5.04. Found: C, 62.95; H, 5.13. H
NMR (CDCl3): δ 4.70 (m, 1H, Cp00), 4.55 (m, 1H, H5, Cp), 4.46
(m, 2H, Cp00), 4.35 (m, 3H, H4, Cp and CH2OH), 4.19 (t, 1H,
4JHH = 1.4 Hz, H2, Cp), 4.08 (m, 1H, Cp00), 4.04, 4.00 (s, 10H,
Cp0 and Cp000), 1.60 (br s, 1H, CH2OH). 13C NMR (CDCl3): δ
90.1 (C3, Cp), 86.1 (C1, Cp00), 83.2 (C1, Cp), 71.4 (C5, Cp), 70.8
(C4, Cp), 70.0 (C2, Cp), 69.5, 69.0 (Cp0 and Cp000), 68.2, 67.7,
67.0, 65.8 (Cp00), 60.9 (CH2OH).
1
assigned via DEPT experiments and H,13C correlation mea-
Synthesis of 4b and 2-NMe2-4-Tol-1,100-biferrocene (4e). To a
solution of 1b (250 mg, 0.531 mmol) in toluene (25 mL) was
added HCtCFc (200 mg, 0.95 mmol). The resulting solution
was stirred at reflux overnight and then was cooled to room
temperature and filtered on a Celite pad. Solvent removal and
chromatography of the residue on an alumina column with
petroleum ether (bp 40-60 °C) as eluent gave first a yellow
fraction, corresponding to nonsubstituted ferrocene (FeCp2). A
second orange fraction, corresponding to 4b, was collected by
using diethyl ether as eluent. Yield of 4b: 68 mg, 28%.
Then, a third red-orange fraction was collected using a 1/1
(v/v) diethyl ether/CH2Cl2 mixture, corresponding to 4e. Yield
of 4e: 85 mg, 32%. Anal. Calcd for C29H29Fe2N: C, 69.21; H,
5.81. Found: C, 69.38; H, 5.76. 1H NMR (CDCl3): δ 7.65-7.10
(m, 4H, C6H4Me), 5.01 (d, 1H, 4JHH = 1.2 Hz, H3, Cp), 4.80
(d, 1H, 4JHH = 1.2 Hz, H5, Cp), 4.66-4.59 (m, 3H, Cp00), 4.26
(m, 1H, Cp00), 4.10, 4.03 (s, 10H, Cp0 and Cp000), 2.62 (s, 6H,
NMe2), 2.34 (s, 3H, C6H4Me). 13C NMR (CDCl3): δ 135.2
(Cipso Tol), 130.5-125.9 (Carom), 112.5 (C2, Cp), 87.3 (C1, Cp00),
84.6 (C1, Cp), 82.0 (C4, Cp), 71.2 (C5, Cp), 70.0, 69.2 (Cp0 and
Cp000), 68.1, 67.0, 65.5 (Cp00), 66.4 (C3, Cp and Cp00), 45.7
(NMe2), 21.0 (C6H4Me).
Synthesis of 2-NMe2-4-CO2Me-1,100-biferrocene (4f). Com-
plex 4f was prepared following the same procedure described for
4a, by reacting 1a with HCtCFc. Yield: 52%. Anal. Calcd for
C24H25Fe2NO2: C, 61.18; H, 5.35. Found: C, 61.25; H, 5.23. 1H
NMR (CDCl3): δ 4.50 (d, 1H, 4JHH = 1.32 Hz, H3, Cp), 4.35 (m,
1H, H5, Cp), 4.35, 4.16, 4.04 (m, 4H, Cp00), 4.09, 3.94 (s, 10H, Cp0
and Cp000), 3.65 (s, 3H, CO2Me), 2.59 (s, 6H, NMe2). 13C NMR
(CDCl3): δ 175.1 (CO2Me), 114.0 (C2, Cp), 88.0 (C1, Cp), 84.6
(C1, Cp00), 74.0 (C3, Cp), 72.8 (C5, Cp), 69.6 (Cp00), 69.6, 69.0
(Cp0 and Cp000), 66.9, 65.1 (Cp00), 64.4 (C4, Cp and Cp00), 51.1
(CO2Me), 45.0 (NMe2).
sured through gs-HSQC and gs-HMBC experiments.24 All
NMR spectra were recorded at 298 K. NOE measurements
were recorded using the DPFGSE-NOE sequence.25 NMR reso-
nances are indicated according to the numbering scheme shown
above in Scheme 10. All the reagents were commercial products
(Aldrich) of the highest purity available and used as received.
Compounds 1a,12 1b,26 and 2 and 37 were prepared by published
methods.
Synthesis of 3-CPh2OH-1,100-biferrocene (4a). To a solution
of 2 (300 mg, 0.531 mmol) in toluene (25 mL) was added
HCtCCPh2OH (230 mg, 1.10 mmol). The resulting solution
was stirred at reflux overnight and then was cooled to room
temperature and filtered on a Celite pad. Solvent removal and
chromatography of the residue on an alumina column with pet-
roleum ether (bp 40-60 °C) as eluent gave a first yellow fraction,
corresponding to nonsubstituted ferrocene (FeCp2). A second
orange fraction, corresponding to 4a, was collected by using
diethyl ether as eluent. Yield: 123 mg, 42%. Anal. Calcd for
1
C33H28Fe2O: C, 71.77; H, 5.11. Found: C, 71.79; H, 5.23. H
NMR (CDCl3):δ7.62-7.12 (m, 10H, C6H5),4.46(dd,1H, 3JHH
=
2.48 Hz, 4JHH = 1.44 Hz, H5, Cp), 4.42 (m, 1H, Cp00), 4.33 (t,
1H, 4JHH = 1.44 Hz, H2, Cp), 4.31 (m, 1H, Cp00), 4.21 (m, 2H,
Cp00), 4.03, 4.00 (s, 11H, Cp0, Cp000 and H4, Cp), 3.48 (s, 1H,
OH). 13C NMR (CDCl3): δ 147.5, 147.0 (Cipso Ph), 129.8-
127.0 (Carom), 99.5 (C3, Cp), 85.3 (C1, Cp), 82.6 (C1, Cp00), 77.7
(C-OH), 70.2, 69.6 (Cp0 and Cp000), 69.7 (C4, Cp), 68.5, 68.3,
66.6, 66.5 (Cp00), 66.5 (C2, Cp), 65.6 (C5, Cp).
(24) Wilker, W.; Leibfritz, D.; Kerssebaum, R.; Beimel, W. Magn.
Reson. Chem. 1993, 31, 287–292.
(25) Stott, K.; Stonehouse, J.; Keeler, J.; Hwang, T. L.; Shaka, A. J.
J. Am. Chem. Soc. 1995, 117, 4199–4200.
Synthesis of 5a,b. Complexes 5a,b were prepared following the
same procedure described for 4a, by reacting 2 with HCtCFc.
(26) Albano, V. G.; Busetto, L.; Marchetti, F.; Monari, M.; Zacchini,
S.; Zanotti, V. Organometallics 2004, 23, 3348–3354.

Article
Organometallics, Vol. 30, No. 5, 2011 1181
Table 2. Crystal Data and Collection Details for 5a CHCl3
3
3.84 ppm. 13C NMR (CDCl3): δ 112.0 (C-NMe2), 84.0-82.6
(Cipso Cp), 72.3-66.3 (CH), 45.3 (NMe2).
formula
fw
T, K
λ, A
cryst syst
C31H27Cl3Fe3
673.43
296(2)
0.710 73
orthorhombic
Cmc21
18.391(2)
20.224(3)
7.7205(10)
2871.5(6)
4
Synthesis of 3-NMe2-4-CPh2OH-1,100-biferrocene (6). Com-
plex 6 was prepared following the same procedure described for
4a, by reacting 3 with HCtCCPh2OH. Yield: 40%. Anal. Calcd
for C35H33Fe2NO: C, 70.61; H, 5.59. Found: C, 70.75; H, 5.52.
1H NMR (CDCl3) δ 7.47-7.00 (m, 10H, C6H5), 6.84 (s, 1H,
OH), 4.55 (m, 1H, Cp00), 4.37 (m, 1H, Cp00), 4.24 (m, 1H, Cp00),
4.17 (m, 1H, Cp00), 4.08, 3.83 (s, 10H, Cp0 and Cp000), 3.96 (br s,
1H, H5, Cp), 3.90 (br s, 1H, H2, Cp), 2.55 (s, 6H, NMe2). 13C
NMR (CDCl3): δ 147.6 (Cipso Ph), 132.3-125.9 (Carom), 114.6
(C3, Cp), 87.5 (C1, Cp00), 87.0 (C4, Cp), 86.3 (C1, Cp), 77.4
(CPh2OH), 69.3, 69.1 (Cp0 and Cp000), 69.2 (C2, Cp), 69.0 (C5,
Cp), 68.1, 68.1, 66.9, 66.8 (Cp00), 43.4 (NMe2).
˚
space group
˚
a, A
˚
b, A
˚
c, A
3
˚
cell vol, A
Z
Dc, g cm-3
μ, mm-1
F(000)
1.558
1.794
1368
Electrochemical Measurements. Electrochemical data were
obtained from 10-3 M solutions in CH2Cl2 with 0.1 M [Bu4N]-
[PF6]. E1/2 (in mV, referenced to the SCE, at scan speed v = 100
mV s-1): 4a, 388, 430; 4e, 225, 520; 4f, 170, 460; 6, 192, 435; 5a,
290, 479, 880; 5b, 161, 680 mV.
cryst size, mm
θ limits, deg
index ranges
0.26 ꢀ 0.23 ꢀ 0.18
1.50-25.98
-22 e h e 22
-24 e k e 24
-9 e l e 9
14 125
no. of rflns collected
no. of indep rflns
completeness to θmax
X-ray Crystallography. Crystal data and collection details for
5a CHCl3 are reported in Table 2. The diffraction experiments
2924 (Rint = 0.0240)
100.0
3
%
were carried out on a Bruker APEX II diffractometer equipped
with a CCD detector using Mo KR radiation. Data were correc-
ted for Lorentz-polarization and absorption effects (empirical
absorption correction SADABS).27 Structures were solved by
direct methods and refined by full-matrix least squares on the
basis of all data using F2.28 All hydrogen atoms were fixed at
calculated positions and refined by a riding model. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. The asymmetric unit contains half of the complex
5a and half of the CHCl3 molecule (both on m). Similar U
restraints were applied to the C and Cl atoms (standard devia-
tion 0.005). The nonsubstituted Cp ligands bonded to Fe(1) and
Fe(2) are disordered over two positions. Disordered atomic
positions were split and refined using one occupancy parameter
per disordered group. The CHCl3 molecule is disordered over
four positions, two by two related by m symmetry. The two
independent atomic positions were split and refined using one
occupancy parameter per disordered group. The C-Cl dis-
no. of data/restraints/params
goodness on fit on F2
R1 (I > 2σ(I))
2924/303/286
1.062
0.0186
0.0481
0.050(16)
0.171/-0.195
wR2 (all data)
abs struct param
˚
largest diff peak/hole, e A
-3
In this case the two products 5a,b have been separated by
chromatography on alumina. 5a was collected using diethyl
ether as eluent, while 5b was collected with CH2Cl2.
Yield of 5a: 49%. Anal. Calcd for C30H26Fe3: C, 65.03; H,
1
4.73. Found: C, 64.95; H, 4.82. H NMR (CDCl3): δ 4.09 (s,
10H, Cpfree), 3.88 (s, 5H, Cpfree), 4.61-4.10 (m, 11H, Cp). 13
C
NMR (CDCl3): δ 82.0 (Cipso Cp), 70.6 (Cpfree), 69.5 (Cpfree),
71.9-65.0 (Cp). Crystals of 5a, suitable for X-ray analysis, were
obtained by slow evaporation of a CDCl3 solution.
Yield of 5b: 21%. Anal. Calcd for C32H31Fe3N: C, 64.36; H,
5.23. Found: C, 64.55; H, 5.08. 1H NMR (CDCl3): δ 4.14 (s, 5H,
Cpfree), 4.05 (s, 5H, Cpfree), 3.98 (s, 5H, Cpfree), 2.62 (s, 6H,
NMe2); the presence of the amino group causes the loss of
symmetry of the molecule and subsequently increases the com-
plexity of the NMR spectra. In particular, the signals due to the
protons of the Cp rings appear as a multiplet between 4.42 and
˚
tances in CHCl3 were restrained to 1.75 A (standard deviation
0.01).
Supporting Information Available: A CIF file giving crystal-
lographic data for 5a CHCl3. This material is available free of
charge via the Internet at http://pubs.acs.org.
3
(27) Sheldrick, G. M. SADABS, Program for Empirical Absorp-
€
€
tion Correction; University of Gottingen, Gottingen, Germany,
1996.
(28) Sheldrick, G. M. SHELX97, Program for Crystal Structure
€
€
Determination; University of Gottingen, Gottingen, Germany, 1997.