A digalla[1.1]ferrocenophane and its coordination chemistry: Synthesis and structure of [{Fe(η5-C5H4)2}2 {GaMe}2] and of the adducts [{Fe(η5-C5H4)2}2 {GaMe(D)}2] (D = monodentate donor)

Article abstract of DOI:10.1021/om030115m

Gentle warming of 1,1′-bis(dimethylgally)ferrocene (1) leads to the formation of trimethylgallium and the thermolabile compound [{Fe(η⁵-C₅H₄)₂}₂{GaMe }₂] (2), a [1.1]ferrocenophane featuring group 13 elements in bridging positions. While NMR data for 2 prove a dynamic structure in solution, X-ray data reveal an anti conformation of the ferrocenophane framework in the solid state. The anti conformation is maintained in the thermolabile adducts 2a-g, which are obtained from 2 and the donors diethyl ether (2a), pyridine (2b), pyrimidine (2c), quinoxaline (2d), DMSO (2e), pyrazine (2f), and dioxane (2g), by donor-exchange reactions (2b-g) or on gentle warming of the respective donor adducts of 1. Rodlike polymers are formed either by interaction of 2 with bidentate donors (2f,g) or by π-stacking effects of aromatic molecules acting as monodentate donors (2b-d). Steric requirements inhibit the complex formation between 2 and the donor phenazine. A cyclic voltammogram of 2b in pyridine reveals two reversible oxidation steps at -314 and -114 mV, indicating only weak electron delocalization in the cationic species. The formation of 2 from 1 has been shown to be reversible and thus is an example of an application of dynamic covalent chemistry as synthetic strategy.

Full text of DOI:10.1021/om030115m

2766
Organometallics 2003, 22, 2766-2774
A Diga lla [1.1]fer r ocen op h a n e a n d Its Coor d in a tion
Ch em istr y: Syn th esis a n d Str u ctu r e of
[{F e(η5-C5H4)2}2{Ga Me}2] a n d of th e Ad d u cts
[{F e(η5-C5H4)2}2{Ga Me(D)}2] (D ) Mon od en ta te Don or )
a n d [{F e(η5-C5H4)2}2{Ga Me}2D] (D ) Bid en ta te Don or )
Alexander Althoff, Peter J utzi,* Norman Lenze, Beate Neumann,
Anja Stammler, and Hans-Georg Stammler
Fakulta¨t fu¨r Chemie, Universita¨t Bielefeld, 33615 Bielefeld, Germany
Received February 20, 2003
Gentle warming of 1,1′-bis(dimethylgallyl)ferrocene (1) leads to the formation of trimeth-
ylgallium and the thermolabile compound [{Fe(η5-C5H4)2}2{GaMe}2] (2), a [1.1]ferrocenophane
featuring group 13 elements in bridging positions. While NMR data for 2 prove a dynamic
structure in solution, X-ray data reveal an anti conformation of the ferrocenophane framework
in the solid state. The anti conformation is maintained in the thermolabile adducts 2a -g,
which are obtained from 2 and the donors diethyl ether (2a ), pyridine (2b), pyrimidine (2c),
quinoxaline (2d ), DMSO (2e), pyrazine (2f), and dioxane (2g), by donor-exchange reactions
(2b-g) or on gentle warming of the respective donor adducts of 1. Rodlike polymers are
formed either by interaction of 2 with bidentate donors (2f,g) or by π-stacking effects of
aromatic molecules acting as monodentate donors (2b-d ). Steric requirements inhibit the
complex formation between 2 and the donor phenazine. A cyclic voltammogram of 2b in
pyridine reveals two reversible oxidation steps at -314 and -114 mV, indicating only weak
electron delocalization in the cationic species. The formation of 2 from 1 has been shown to
be reversible and thus is an example of an application of “dynamic covalent chemistry” as
synthetic strategy.
In tr od u ction
an intermediate, which then goes on to form [{Fe-
(C5H4)2}3{Ga(C5H5N)}2]. 2b is the first group 13 bridged
[1.1]ferrocenophane. Shortly after publication of the
synthesis of 2b another digalla[1.1]ferrocenophane,
namely the compound [{Fe(C5H4)2}2{GaCH(SiMe3)2}2],
has been reported.5
The favorable electrochemical properties make fer-
rocene fragments especially promising candidates for
incorporation into the backbone of polymeric com-
pounds. Such materials have been shown to possess
interesting electrical, magnetic, and optical properties
as a result of electron delocalization.6 One route to
polyferrocenyl species is based on the formation of
donor-acceptor bonds between difunctional nitrogen
bases and Fe(η5-C5H4EMe2)2 (E ) B, Ga) units. Among
these substances, the compounds [Fe(η5-C5H4GaMe2)2L]n
(L ) phenazine)7 and [Fe(η5-C5H4BMe2)2L]n (L ) 4,4′-
bipyridyl, pyrazine)8 have been structurally character-
ized.
One of the rich areas of metallocene chemistry is that
of ferrocenophanes. Particularly interesting are those
compounds in which the ferrocene units are fixed in a
mutually coplanar geometry. This criterion is met
ideally by the binuclear [m.m]ferrocenophanes [0.0]-
ferrocenophane ([Fe(C5H4)2]2) and [2.2]ferrocenophane-
1,13-diyne ([{Fe(C5H4)2}2(CtC)2]) and to a lesser extent
by the [1.1]ferrocenophanes [{Fe(C5H4)2}2(CR2)2].1,2 Un-
til recently only a few heteroatom-bridged compounds
[{Fe(C5H4)2}2X2] (X ) SiMe2, SnBu2, PbPh2, PMen;
Men ) menthyl) have been reported. They contain group
14 or 15 atoms at the 1- and 12-positions.3
Recently we reported on the novel trinuclear ferro-
cenophane [{Fe(C5H4)2}3{Ga(C5H5N)}2], which was
formed in a highly selective reaction by heating a
solution of 1,1′-bis(dimethylgallyl)ferrocene (1) in toluene/
pyridine.4 In the reaction sequence, the digalla[1.1]-
ferrocenophane [{Fe(C5H4)2}2{GaMe(C5H5N)}2] (2b) is
In the present paper, we report in more detail on the
synthesis and structure of the digallaferrocenophane-
pyridine adduct 2b, of further adducts (2a ,c-g), and of
(1) For metal-metal interactions in linked metallocenes see: Bar-
low, S.; O’Hare, D. Chem. Rev. 1997, 97, 637.
(2) Review on [m.m]ferrocenophanes: Mueller-Westerhoff, U. T.
Angew. Chem. 1986, 98, 700; Angew. Chem., Int. Ed. Engl. 1986, 25,
702.
(3) (a) Zechel, D. L.; Foucher, D. A.; Pudelski, J . K.; Yap, G. P. A.;
Rheingold, A. L.; Manners, I. J . Chem. Soc., Dalton Trans. 1995, 1893.
(b) Clearfield, A.; Simmons, C. J .; Withers, H. P.; Seyferth, D. Inorg.
Chim. Acta 1983, 75, 139. (c) Utri, G.; Schwarzhans, K.-E.; Allmaier,
G. M. Z. Naturforsch. 1990, 45b, 755.
(4) J utzi, P.; Lenze, N. Neumann, B.; Stammler, H.-G. Angew. Chem.
2001, 113, 1470; Angew. Chem., Int. Ed. 2001, 40, 1424.
(5) Uhl, W.; Hahn, I.; J antschak, A.; Spies, T. J . Organomet. Chem.
2001, 637-639, 300.
(6) Manners, I. Angew. Chem. 1996, 108, 1712; Angew. Chem., Int.
Ed. 1996, 35, 1602.
(7) Althoff, A.; J utzi, P.; Lenze, N.; Neumann, B.; Stammler, A.;
Stammler, H.-G. Organometallics 2002, 21, 3018.
(8) (a) Grosche, M.; Herdtweck, E.; Peters, F.; Wagner, M. Organo-
metallics 1999, 18, 4669. (b) Dinnebier, R. E.; Wagner, M.; Peters, F.;
Shankland, K.; David, W. I. F. Z. Anorg. Allg. Chem. 2000, 626, 1400.
10.1021/om030115m CCC: $25.00 © 2003 American Chemical Society
Publication on Web 05/23/2003

Digalla[1.1]ferrocenophane
Sch em e 1. F or m a tion of 2
Organometallics, Vol. 22, No. 13, 2003 2767
the donor-free digallaferrocenophane 2. The adducts
2c,d ,f,g possess rodlike polymeric structures in the solid
state.
F igu r e 1. Molecular structure of 2, with thermal ellipsoids
given at the 50% probability level.
Resu lts
Syn th esis a n d Str u ctu r e of 2. The donor-free [1.1]-
ferrocenophane [{Fe(η5-C5H4)2}2{GaMe}2] (2) has been
synthesized by two slightly different pathways. In the
first variant, 2 could be obtained directly from 1 in the
form of orange crystals together with trimethylgallium
(Scheme 1). This reaction took already place at room
temperature, after 1 had been dissolved in trichloro-
methane. Over the course of 1 week X-ray-quality crys-
tals of 2 formed. In the second variant, 2 was prepared
via the diethyl ether adduct 2a (vide infra), which was
formed from 1,1′-bis(dimethylgallyl)ferrocene (1) in
toluene/diethyl ether solution with concomitant forma-
tion of the diethyl ether adduct of trimethylgallium
(Scheme 2). This reaction took place at room tempera-
ture after 1 had been dissolved in a mixture of toluene
and diethyl ether. When dried in vacuo, 2a decomposed
to give 2 in the form of an orange powder. The overall
yield of 2 was 46%. Compound 2 is sparingly soluble in
nondonor solvents and is air sensitive. It is a thermo-
labile compound, and heating to 80 °C results in the
formation of the trinuclear ferrocenophane [{Fe(C5H4)2}3-
Ga2] and trimethylgallium. Therefore, amorphous 2
cannot be transformed into a crystalline material by
recrystallization from hot solution. An NMR spectrum
was recorded in DMSO-d6; the spectrum shows data for
the DMSO adduct 2e (Scheme 2).
Ta ble 1. Selected Dista n ces (Å) a n d An gles (d eg)
for 2
Distances
Ga(1)-C(1)
Ga(1)-C(6A)
Ga(1)-C(11)
Ga(1)-Fe(1)
1.9442(15)
1.9456(15)
1.9502(18)
3.481
Ga(1)-Fe(1A)
Ga-Ga
Fe-Fe
3.536
4.414
5.455
Angles
118.60(6)
120.00(8)
120.00(8)
14
C(1)-Ga(1)-C(6A) (R)
C(1)-Ga(1)-C(11)
C(6A)-Ga(1)-C(11)
Cp-Cp rotation (â)
Cp-Cp tilt (γ)
dip angle (δ)
∑C-Ga-C
4
5
359
The structure analysis shows that compound 2 crys-
tallizes in the monoclinic space group P21/n. Two
ferrocene-1,1′-diyl groups are linked together by two
GaMe groups with formation of a [1.1]ferrocenophane
frame in an anti conformation. Interestingly, only weak
van der Waals interactions are observed between the
molecular units, so that 2 is regarded as a monomeric
species. A drawing of the molecule in the form of a
thermal ellipsoid plot is given in Figure 1. Table 1
summarizes selected distances and angles.
F igu r e 2. Definition of structural parameters in 2.
The main structural features of 2 are given by the
bridge (R), the rotation (â), the tilt (γ), and the dip angles
(δ) (see Figure 2). These angles in 2 amount to R )
118.60(6)°, â ) 14°, γ ) 4°, and δ ) 5°. The nonbonding
Ga-Ga distance is 4.414 Å, and the nonbonding Fe-
Fe distance is 5.455 Å. The coordination geometry at
the gallium atoms is trigonal planar (angle sum 359°).
The Ga(1)-C(1), Ga(1)-C(6A), and Ga-CMe bond dis-
tances are typical for gallium organyls.10 The shortest
nonbonding distance between the R-H(Cp) atoms within
the ferrocene-1,1′-diyl units is 2.308 Å (rvdw of the H
atom ∼1.0 Å). The C(Cp)-Ga bonds are bent toward the
iron centers with reference to the Cp plane (δ ) 5°; see
Figure 2). This bonding situation can be compared with
that in borylferrocenes,11 gallylferrocenes,7 and ferro-
cenyl carbocations.12 The smallest dip angle is observed
in 2, thus indicating only weak attractive interactions.
Compound 2 reveals structural parameters similar to
(11) (a) Wrackmeyer, B.; Do¨rfler, U.; Milius, W.; Herberhold, M.
Polyhedron 1995, 11, 1425. (b) Appel, A.; J a¨kle, F.; Priemeier, T.;
Schmid, R.; Wagner, M. Organometallics 1996, 15, 1188.
(12) Kreidlin, A. Z.; Dolgushin, F. M.; Yanovsky, A. I.; Kerzina, Z.
A.; Petrovskii, P. V.; Rybinskaya, M. I. J . Organomet. Chem. 2000,
616, 106.
(9) (a) Lo¨wendahl, J .-M.; Håkansson, M. Organometallics 1995, 14,
4736. (b) Lo¨wendahl, J .-M.; Davidsson, O¨ .; Ahlberg, P.; Håkansson,
M. Organometallics 1993, 12, 2417.
(10) Cambridge Crystallographic Structural Database.

2768 Organometallics, Vol. 22, No. 13, 2003
Sch em e 2. F or m a tion of Ad d u cts fr om Don or Molecu les a n d 1 a n d 2, Resp ectively
Althoff et al.
those of the digalla[1.1]ferrocenophane [{Fe(C5H4)2}2-
{GaCH(SiMe3)2}2].5 The three-coordinate gallium atoms
in 2 allow the formation of coordination compounds. In
the following, donor-acceptor compounds with quite
different association behaviors in the solid state will be
described.
Syn th esis a n d Str u ctu r e of Coor d in a tion Com -
p ou n d s [2‚2D] (2a -d ). As already mentioned, the
diethyl ether adduct 2a could be prepared from 1 and
excess diethyl ether (Scheme 2). After the diethyl ether
solution was cooled to +6 °C, the adduct 2a precipitated
in the form of air-sensitive orange crystals suitable for
X-ray crystal structure analysis. Compound 2a turned
out to be rather thermolabile and decomposes even at
room temperature with formation of 2 and diethyl ether.
It is sparingly soluble in nondonor solvents.
The coordination compounds 2b-d have been pre-
pared by two different methods (Scheme 2). The first
one starts with the synthesis of 2a as described above.
When aromatic nitrogen donors D were added to 2a ,
substitution of the Et2O molecule immediately took
place. The air-sensitive adducts 2b-d were formed in
rather good yields, but no crystals suitable for X-ray
diffraction analysis were formed. Recrystallization of
2b-d turned out to be impossible because of their
thermolability. Heating of 2b-d to 80 °C resulted in
the formation of the respective donor-stabilized tri-
nuclear ferrocenophanes [{Fe(C5H4)2}3{Ga(D)}2] and
donor adducts of trimethylgallium. Following the second
synthetic method, 1 and the donor molecules were
treated with toluene and then warmed to 40 °C until a
clear solution had formed. After a few days at room
temperature the adducts 2b-d were obtained suitable
for X-ray diffraction analysis. The crystals are sparingly
soluble in nondonor solvents and also only sparingly
soluble in diethyl ether, pyridine, and pyrimidine. This
procedure resulted in lower yields than the first one.
The compounds 2a -d have been characterized using
X-ray structure determination, NMR spectroscopy, and
elemental analysis. The solubility of 2 and 2a -d in
DMSO stems from the formation of the monomeric
adduct 2e. The NMR spectra were recorded in DMSO-
d6; thus, the spectra show data for the DMSO adduct
2e and for the free donor molecules.
The diethyl ether complex 2a crystallizes in the
monoclinic space group P21/n. The compound maintains
the anti conformation and the framework parameters
of 2. A drawing of the molecular structure in form of a
thermal ellipsoid plot is given in Figure 3. Table 2
summarizes selected distances and angles for 2a -d .
The gallium atoms show an only slightly distorted
trigonal pyramidal coordination geometry (C-Ga-C
angle sum 355°). In general, the geometry of compounds
with tetracoordinated gallium atoms varies between
trigonal pyramidal and tetrahedral.10 Due to the small
deviation from planarity at the gallium atoms, the
methyl groups are only slightly bent out of the plane of
the GaCp2 unit toward the iron centers. The dip angle

Digalla[1.1]ferrocenophane
Organometallics, Vol. 22, No. 13, 2003 2769
F igu r e 4. Solid-state structure of 2b, including the unit
cell. A drawing of two molecular units of 2b is shown on
the right side (two different views).
F igu r e 3. Molecular structure of 2a , with thermal el-
lipsoids given at the 50% probability level.
Ta ble 2. Selected Dista n ces (Å) a n d An gles (d eg)
for 2a -d
[2c‚(tol-
uene)]
2a
2b
2d
Distances
Ga(1)-C(1)
Ga(1)-C(6A)
Ga(1)-C(11)
Ga(1)-N/O
Ga(1)-Fe(1)
Ga(1)-Fe(1A)
Ga-Ga
1.962(4)
1.957(3)
1.977(4)
2.153(2)
3.618
3.51
4.648
5.432
1.981(6) 1.970(2)
1.957(6) 1.961(2)
1.976(7) 1.976(2)
1.9630(19)
1.965(2)
1.978(2)
2.144(5) 2.1508(16) 2.2192(16)
3.682
3.54
4.768
5.427
3.571
3.608
4.631
5.485
3.626
3.593
4.734
5.448
Fe-Fe
donor-donor
3.515(8) 3.528(3)
3.458(3)
Angles
C(1)-Ga(1)-C(6A) 116.44(14) 115.6(2) 115.92(9) 116.96(8)
(R)
C(1)-Ga(1)-C(11) 121.82(15) 117.9(3) 119.42(11) 115.39(9)
C(6A)-Ga(1)-C(11) 116.64(16) 118.3(3) 117.69(11) 120.44(9)
N/O-Ga(1)-C(1)
97.60(10) 97.5(2) 97.10(7)
N/O-Ga(1)-C(6A) 97.56(11) 100.0(2) 97.88(7)
95.49(7)
97.20(7)
F igu r e 5. Solid-state structure of [2c‚(toluene)], including
the unit cell (the disordered toluene molecules are not
shown). The π-stacking between two pyrimidine rings is
shown in more detail on the right side of the picture.
N/O-Ga(1)-C(11) 97.48(14) 101.2(2) 101.49(8) 104.17(8)
Cp-Cp rotation (â)
Cp-Cp tilt (γ)
dip angle (δ)
9
2
10
3
9
1
10
2
0
3
1
2
between the aromatic rings is 3.515 Å, which is typical
for weak interactions.10 The solid-state structure of
uncoordinated pyridine reveals no π-stacking interac-
tions.13
∑C-Ga-C
355
352
353
353
is δ ) 3°, thus indicating only weak Fe-Ga interactions.
The ether molecules are coordinated in the axial posi-
tion.
The pyrimidine complex 2c crystallizes as [2c‚
(toluene)] in the triclinic space group P1h (Figure 5). It
shows a disorder of toluene molecules located at an
inversion center. The [2‚2(pyrimidine)] unit reveals
structural parameters similar to those of the [2‚2(pyri-
dine)] unit. [2c‚(toluene)] shows Ga-N bonds (2.1508-
(16) Å) only slightly longer than those found in 2b
(2.144(5) Å). The molecular units of [2c‚(toluene)] form
polymer strands in a manner similar to that for 2b.
They are aligned parallel to the crystallographic c axis.
A distance of 3.528 Å between the pyrimidine planes is
observed. Regarding the position of the pyrimidine
molecules, the N atoms are arranged with the greatest
possible distance. The solid-state structure of uncoor-
The pyridine complex 2b crystallizes in the monoclinic
space group C2/c (Figure 4). The Ga-CMe bond distances
are comparable to those found in 2. The Ga-N bond
distances of 2.144(5) Å are typical for donor-acceptor
adducts formed from sp2-nitrogen donors and three-
coordinated gallium acceptors.10 The C-Ga-C angle
sum at each gallium atom is 352° and thus shows a
slightly more pronounced distortion than in 2a . The
pyridine molecules are coordinated in the axial position.
Complex 2b is regarded as a polymeric compound which
consists of [2‚2(pyridine)] units (see Figure 4). Each
polymer strand is surrounded by six other strands and
aligned parallel to the crystallographic a-c diagonal.
The molecular units are linked by π-stacking interac-
tions between the pyridine molecules. The distance
(13) Mootz, D.; Wussw, H.-G. J . Chem. Phys. 1981, 75, 1517.

2770 Organometallics, Vol. 22, No. 13, 2003
Althoff et al.
Ta ble 3. Selected Dista n ces (Å) a n d An gles (d eg)
for 2f,g
[2f‚1.5(pyrazine)‚
1.5(toluene)]
2g
Distances
Ga(1)-C(1)
Ga(1)-C(6A)
Ga(1)-C(11)
Ga(1)-N/O
Ga(1)-Fe(1)
Ga(1)-Fe(1A)
Ga-Ga
1.9605(17)
1.9656(17)
1.9775(19)
2.1854(14)
3.612
3.541
4.637
5.448
1.955(2)
1.961(3)
1.986(3)
2.2000(17)
3.457
3.595
4.476
5.451
Fe-Fe
Angles
C(1)-Ga(1)-C(6A) (R)
C(1)-Ga(1)-C(11)
C(6A)-Ga(1)-C(11)
N/O-Ga(1)-C(1)
N/O-Ga(1)-C(6A)
N/O-Ga(1)-C(11)
Cp-Cp rotation (â)
Cp-Cp tilt (γ)
116.10(7)
116.69(8)
121.03(8)
98.80(6)
96.54(6)
99.62(7)
9
1
3
354
117.81(11)
117.31(11)
120.73(11)
97.31(8)
96.31(8)
96.78(10)
11
3
5
356
dip angle (δ)
∑C-Ga-C
as donor, the formation of the coordination polymer [2‚
(quinoxaline)] using both nitrogen donors in quinoxaline
seemed possible. Instead, 2d was formed, even when a
1:1 stoichiometry of the reactants was applied.
F igu r e 6. Solid-state structure of 2d , including the unit
cell. The π-stacking between two quinoxaline rings is
shown in more detail on the right side of the picture. A
drawing of one molecular unit of 2d , viewed along the
crystallographic b-c diagonal, is shown in the upper right
corner.
Syn th esis a n d Str u ctu r e of Coor d in a tion Com -
p ou n d s [2‚D] (2f,g). Coordination compounds of the
type [2‚D] were prepared using the same two methods
as described above for the coordination compounds 2b-
d . The adducts 2f,g, with pyrazine (2f) and dioxane (2g)
as donors, are only sparingly soluble in donor and
nondonor solvents. They have been characterized by
X-ray structure and elemental analysis. Selected bond
lengths and bond angles of 2f,g are given in Table 3.
Solution NMR spectra could not be measured, due to
their low solubility at room temperature even in donor
solvents. Gentle warming resulted in thermal decom-
position with formation of the donor-stabilized tri-
nuclear ferrocenophanes [{Fe(C5H4)2}3{Ga(D)}2] and the
donor adducts of trimethylgallium.
dinated pyrimidine reveals the N atoms stacked upon
each other with a distance of 3.806 Å between the ring
planes.14 Accordingly, there are stronger π-stacking
interactions in [2c‚(toluene)] than in uncoordinated
pyrimidine. It could be expected that 2 would react with
pyrimidine to give the polymeric coordination compound
[2‚(pyrimidine)] (using both nitrogen atoms as donors).
Instead, the formation of [2c‚(toluene)] was observed.
Even a reaction in a 1:1 stoichiometry did not result in
the formation of [2‚(pyrimidine)].
The quinoxaline complex 2d crystallizes in the tri-
clinic space group P1h (Figure 6). The molecular unit
reveals structural parameters similar to that of the [2‚
(pyrimidine)] unit. It is evident from the orientation of
the quinoxaline ligands that the annelated benzene ring
in the quinoxaline molecule is directed away from the
ferrocenophane fragment. The structure of 2d consists
of polymer strands similar to those of 2b and [2c‚
(toluene)]. They are aligned parallel to the crystal-
lographic b-c diagonal. The distance between the
aromatic rings is 3.458 Å and is thus shorter than in
2b and [2c‚(toluene)]. The solid-state structure of
uncoordinated quinoxaline reveals π-stacking interac-
tions with the nitrogen-containing rings positioned on
top of each other, while the rings in 2d are twisted by
180°, resulting in a displacement of the ferrocenophane
compared to the orientation in 2c. The distance between
the aromatic rings is 3.901 Å in uncoordinated quinoxa-
line, thus indicating weaker π-stacking interactions
than in 2d .15 Similar to the situation with pyrimidine
The pyrazine complex 2f crystallizes as [2f‚1.5-
(pyrazine)‚1.5(toluene)] in the triclinc space group P1h
(Figure 7). The crystals show a disorder of pyrazine and
toluene molecules. The Ga-N bond distances are 2.1854-
(14) Å. The compound consists of polymer strands
aligned parallel to the crystallographic a-b-c diagonal.
The ferrocenophane molecules form sheets with all units
of 2 arranged parallel to the similar units of the
neighbored strands; the shortest intramolecular Fe-Fe
distance is 5.448 Å, and the shortest intermolecular Fe-
Fe distance is 5.609 Å. The sheets alternate with layers
of the pyrazine donors containing also the disordered
toluene and pyrazine molecules. The pyrazine ring
planes are not arranged parallel to each other, so that
π-stacking interactions between the pyrazine rings
cannot arise.
The dioxane complex 2g crystallizes in the monoclinic
space group P21/c (Figure 8). The Ga-O bond distances
are slightly longer than those found in 2a . The dioxane
donors are fixed in a chair conformation. Regarding the
dip angle (δ ) - 5°), a bending toward the Fe atoms is
observed, as described for [2f‚1.5(pyrazine)‚1.5(toluene)].
Similar to [2f‚1.5(pyrazine)‚1.5(toluene)], 2g consists of
(14) Wheatley, P. J . Acta Crystallogr. 1960, 13, 80.
(15) Anthony, A.; Desiraju, G. R.; J etti, R. K. R.; Kuduva, S. S.;
Madhavi, N. N. L.; Nangia, A.; Thaimattam, R.; Thalladi, V. R. Cryst.
Eng. 1998, 1, 1.

Digalla[1.1]ferrocenophane
Organometallics, Vol. 22, No. 13, 2003 2771
Ta ble 4. UV/Vis Sp ectr oscop ic Da ta
compd
λmax (nm)
compd
λmax (nm)
2
466
482
464
549
2f
2g
ferrocene
518
469
469
2b
2c
2d
tions. A comparable dynamic behavior has been ob-
served for the Sn,Sn′-tetra-n-butyldistanna[1.1]ferro-
cenophane, also possessing an anti conformation in the
solid state.2,3b In contrast, the more rigid carbon-bridged
compound exo,exo,anti-1,12-dimethyl[1.1]ferrocenophane
1
displays four H NMR signals, two signals for the R-CH
units and two signals for the â-CH units of the fer-
rocene-1,1′-diyl fragments.9b The dynamic behavior of
[1.1]ferrocenophanes in solution has been already dis-
cussed in more detail, including a proposal for an anti-
twist-syn-transfer mechanism.2,9a
Electr och em istr y of 2b. A cyclic voltammogram of
2b was recorded using pyridine as solvent and tetrabu-
tylammonium fluoride (TBAPF) as the supporting elec-
trolyte.16 Reversible oxidation potentials are observed
at E1/2(1) ) -314 mV and E1/2(2) ) -114 mV (versus
ferrocene/ferrocenium), with a peak separation of 90
F igu r e 7. Solid-state structure of [2f‚1.5(pyrazine)‚1.5-
(toluene)], including the unit cell (the disordered pyrazine
and toluene molecules are not shown). The bonding situ-
ation between the ferrocenophane and pyrazine is shown
in more detail on the right side of the picture.
mV. The difference in the oxidation potentials (∆E1/2
)
200 mV) indicates only a partial electron delocalization
in the cationic species (presumably, class II in the Robin
and Day classification1). In Figure 9, the first oxidation
potentials of ferrocene and decamethylferrocene are
compared with those of boron- and gallium-substituted
ferrocenes, measured in pyridine as solvent. The lowest
potential is observed for Fe(C5Me5)2. The electron-
donating ability decreases in the order Fe(C5Me5)2 >
7
Fe(C5H4BMe2‚Py)2 > Fe(C5H4GaMe2‚Py)2] (1‚2Py)7 >
Fe{[GaMe(Py)H4C5]2Fe} (2b).
Ma ss Sp ectr om etr y of 2 a n d 2b-d ,f,g. In the mass
spectrum of 2, the peak of the molecular ion 2+ was not
observed; interestingly, the molecular ion of the con-
densation product [{Fe(C5H4)2}3Ga2]+ could be detected.
Similarly, in the spectra of the coordination complexes
2b-d ,f,g, peaks for the molecular ions [2‚2D]+, [2‚D]+,
and 2+ were not observed; again, the molecular ion of
the condensation product [{Fe(C5H4)2}3Ga2]+ could be
found. Thus, condensation reactions take place very
easily under mass spectrometric conditions. A compa-
rable behavior has been described for 1;7 the [{Fe-
(C5H4)2}3Ga2]+ peak but not the 1+ peak could be
detected.
F igu r e 8. Solid-state structure of 2g, including the unit
cell.
UV/Vis Sp ectr oscop y of 2 a n d 2b-d ,f,g. UV/vis
spectroscopic data for 2 and its adducts 2b-d ,f,g in the
solid state are given in Table 4. The compounds 2 and
2b,c,g exhibit a yellow to orange color (λmax 464-482
nm); only small differences in the spectra can be
observed. Compounds 2d (λmax 549 nm) and 2f (λmax 518
nm) exhibit an intense purple color, and single crystals
of these compounds are almost black. The observed
absorptions can be attributed mainly to internal transi-
tions within the molecular units. One may assume
electron delocalization to occur between molecular units
along the polymer chains of the coordination compounds
2d ,f, but this assumption is not supported by band
structure calculations.
polymer strands. They are aligned parallel to the
crystallographic a-c diagonal.
Rea ction of 1 a n d 2 w ith P h en a zin e. The reaction
of 1 with phenazine did not result in the formation of
[2‚2(phenazine)], even after prolonged heating. Instead,
the coordination polymer [1‚(phenazine)] 1a was formed
as green crystals (Scheme 2). We have reported the
preparation and the solid-state structure of 1a in a
previous publication.7 With regard to the observation
described above, it is not surprising that 2a also does
not react with phenazine.
Dyn a m ic Beh a vior of 2e in Solu tion . The 1H NMR
spectrum of a solution of 2e in DMSO-d6 displays one
signal for all R-CH units and one signal for all â-CH
units of the ferrocene-1,1′-diyl fragments. This observa-
tion indicates an averaged, highly symmetrical structure
in solution with equilibrating anti and syn conforma-
(16) A solution of 2b in pyridine was prepared by dissolving 2 in
pyridine. Compound 2b is insoluble in pyridine at room temperature.
Heating results in the formation of [{Fe(C5H4)2}3{Ga(pyridine)}2] and
the pyridine adduct of trimethylgallium.

2772 Organometallics, Vol. 22, No. 13, 2003
Althoff et al.
F igu r e 9. First oxidation potentials (in pyridine) of substituted ferrocenes (functional groups given in boldface letters).
Rever sibility of F er r ocen op h a n e F or m a tion . We
have reported that the ferrocenophane 2 can be pre-
pared in quantitative yield by elimination of trimeth-
ylgallium from the 1,1′-bis(dimethylgallyl)ferrocene 1.
Similarly, the adducts of 2 can be synthesized by
starting from 1 and the corresponding donor. In sepa-
rate experiments we could demonstrate that also the
reverse reactions are feasible. Thus, 2 reacts with excess
trimethylgallium at 100 °C in a closed flask with
quantitative formation of 1. Similarly the compounds
2a -d ,f,g react with excess trimethylgallium to give of
1 and the donor adduct of trimethylgallium. The re-
versibility of the formation of 2 is described in eq 1.
as described in eq 1 allows the correction of mistakes
concerning the specifity of exchange reactions (“proof-
reading” process17). In general, reversibility is a pre-
requisite for reactions performed under the concept of
“dynamic covalent chemistry”.17
The dynamic structure of 2 and of its adducts in
solution is based on fast rearrangements between syn
and anti conformations of the ferrocenophane fragments
and on fast coordination/decoordination processes of the
respective donor molecules. In the solid state, these
compounds are present in the anti conformation with
the donor molecules in an axial position.
Several rodlike polymeric coordination compounds
possessing a digalla[1.1]ferrocenophane as a molecular
building block could be prepared (2b-d ,f,g) and struc-
turally characterized. These polymers are formed inde-
pendent from the stoichiometry of the reactants. Two
bonding situations have been observed for the bridging
difunctional donor units: (i) Difunctional donors such
as pyrazine and dioxane act, as expected, as bidentate
ligands (formation of 2f,g), (ii) bidentate ligand pairs
are formed by π-stacking interactions between mono-
and difunctional donors such as pyridine, pyrimidine,
and quinoxaline (formation of 2b-d ). The coordination
modes of the donor molecules in 2b-d ,f,g are presented
in Figure 10. Steric reasons might prevent the function
of pyrimidine as a bidentate donor. The reason quinoxa-
line does not act as a bidentate donor is not obvious.
It is interesting to regard in more detail the coordina-
tion behavior of pyrazine and its benzannelated deriva-
tives quinoxaline and phenazine. No steric hindrance
between pyrazine and the ferrocenophane framework
and, thus, a free rotation around the Ga-N bond
without any restrictions concerning possible conforma-
tions is observed in complex 2f. However, the quinoxa-
line donor molecules of complex 2d show a special
conformation, with an orientation of the annelated
benzene ring directed away from the Cp rings of the
ferrocenophane unit (see Figure 6). This observation is
an indication for steric repulsion as a structure-directing
factor. This conclusion is strongly supported by the
coordination behavior of the bidentate donor phenazine,
possessing two annelated benzene rings. Due to steric
reasons, this compound does not react with the ferro-
2 1 h 2 + 2GaMe3
(1)
Discu ssion
A novel route to [1.1]ferrocenophanes has been found,
with the synthesis of the gallium-bridged ferrocenophane
[{Fe(C5H4)2}2{GaMe}2] (2) and of its donor adducts 2a -
g. The synthetic strategy depends on rather labile
ferrocenyl-gallium and methyl-gallium bonds, so that
substituent exchange reactions can take place easily.
Thus, gentle warming of Fe(C5H4GaMe2)2 (1) leads, with
elimination of trimethylgallium in quantitative yields,
to the formation of 2. Donor adducts of 2 are formed
under comparable conditions. Interestingly, 2 can be
transferred back to 1 in the presence of trimethylgal-
lium. The equilibrium situation as described in eq 1 is
characterized by only small enthalpy differences; the
number of Ga-C bonds is the same on both sides, and
the Ga-ferrocenyl and the Ga-methyl bond strengths
are regarded as comparable. Entropic factors shift the
equilibrium to the right side.
The lability of the Ga-C bonds in substrates and
products is caused by the easy formation of dimeric
transition state complexes in exchange reactions. The
coordinative unsaturation of the gallium atoms allows
the formation of electron-deficient bonds in dimeric
units. In the ground-state structure of 1, weakly bonded
dimers with methyl groups in bridging positions are
observed (see the solid-state structure of 17). In transi-
tion-state structures necessary for exchange reactions,
ferrocenyl groups also have to be placed in bridging
positions.
(17) Review on dynamic covalent chemistry: Rowan, S. J .; Cantrill,
S. J .; Cousins, G. R. L.; Sanders, J . K. M.; Stoddart, J . F. Angew. Chem.
2002, 114, 938; Angew. Chem., Int. Ed. 2002, 41, 898.
Highly stereo- and regiospecific substitution reactions
are necessary to transform 1 into 2. The reversibility

Digalla[1.1]ferrocenophane
Organometallics, Vol. 22, No. 13, 2003 2773
F igu r e 10. Coordination modes of the donor molecules in 2b-d ,f,g (the π electrons of the aromatic nitrogen donors are
not shown).
CH2), 4.03 (s, 8 H, ring C2/5-H or C3/4-H), 4.17 (s, 8 H, ring
C2/5-H or C3/4-H).19
cenophane 2. As expected, it reacts with 1,1′-bis(dimeth-
ylgallyl)ferrocene (1).7
P r ep a r a tion of 2b-d ,f,g. (a) In a Schlenk flask (25 mL),
1 (300 mg, 0.78 mmol) was treated with diethyl ether (1 mL)
and toluene (3 mL) at room temperature. To the resulting
solution was added a solution of the donor (2 mmol) in toluene
(0.5 mL). After a short time a microcrystalline solid was
formed. The solid was washed with n-hexane and dried in
vacuo. All yields are reported according to this procedure.
(b) In an NMR tube 1 (35 mg, 0.09 mmol) and the donor
(0.5 mmol) were dissolved in toluene (0.7 mL) by gentle heating
(maximum 40 °C). Single X-ray-quality crystals had grown
after 1 day.
The ferrocenophane 2 turns out to be an interesting
building block in supramolecular chemistry. We are
currently trying to prepare suitably substituted digal-
laferrocenophanes with the aim of preparing better
soluble coordination polymers.
Exp er im en ta l Section
Gen er a l Com m en ts. All manipulations were carried out
under a purified argon atmosphere using standard Schlenk
techniques. The solvents were commercially available, purified
by conventional means, and distilled immediately prior to use.
The NMR spectra were recorded in DMSO-d6 using a Bruker
Advance DRX 500 spectrometer (1H, 500.1 MHz). Chemical
shifts are reported in ppm and were referenced to the solvent
resonances as an internal standard. The elemental analyses
were performed by the Microanalytical Laboratory of the
Universita¨t Bielefeld. Cyclic voltammograms were recorded on
an EG&G potentiostat, Model 273A, controlled by M 250/270
software. The solvent was pyridine, and the supporting
electrolyte was tetrabutylammonium fluoride (TBAPF), which
was purchased from Fluka and used without further purifica-
tion. The electrolyte concentration was 0.1 M. The voltam-
metric measurements were performed using a platinum-disk
electrode (d ) 2 mm), which was polished prior to use.
Potentials were calibrated by the method of Gagne´ and are
quoted vs the ferrocenium-ferrocene couple as an internal
standard.18 A platinum wire was used as a counter electrode.
UV-vis data were collected using a Shimadzu UV-3101PC
spectrometer (250-800 nm). The samples were measured as
barium sulfate triturates for solid-state reflection spectroscopy.
Sta r tin g Ma ter ia ls. 1,1′-Bis(dimethylgallyl)ferrocene (1)
was prepared using a literature procedure.7
2b. Yield: 170 mg (0.24 mmol, 61%). UV: λ 260, 482 nm.
1H NMR (DMSO-d6): δ -0.17 (s, 6 H, CH3), 4.05 (s, 8 H, ring
C2/5-H or C3/4-H), 4.18 (s, 8 H, ring C2/5-H or C3/4-H), 7.38
(s, 4 H, pyridine), 7.78 (s, 2 H, pyridine), 8.55 (s, 4 H, pyridine).
Anal. Calcd for C32H32Fe2Ga2N2 (Mr ) 695.76): C, 55.24; H,
4.64; N, 4.03. Found: C, 55.58; H, 4.70; N, 4.09. CV: E1/2(1) )
-314 mV, E1/2(2) ) -114 mV (peak separation 90 mV).
2c. Yield: 176 mg (0.25 mmol, 64%). UV: λ 259, 464 nm.
1H NMR (DMSO-d6): δ -0.17 (s, 6 H, CH3), 4.05 (s, 8 H, ring
C2/5-H or C3/4-H), 4.18 (s, 8 H, ring C2/5-H or C3/4-H), 7.54
(s, 2 H, pyrimidine), 8.82 (s, 4 H, pyrimidine), 9.20 (s, 2 H,
pyrimidine). Anal. Calcd for C30H30Fe2Ga2N4 (Mr ) 697.73):
C, 51.64; H, 4.33; N, 8.03. Found: C, 51.45; H, 4.46; N, 8.06.20
2d . Yield: 207 mg (0.26 mmol, 67%) of 2d . UV: λ 256, 549
1
nm. H NMR (DMSO-d6): δ -0.18 (s, 6 H, CH3), 4.04 (s, 8 H,
ring C2/5-H or C3/4-H), 4.17 (s, 8 H, ring C2/5-H or C3/4-H),
7.87 (dd, 4J ) 3.1 Hz, 3J ) 6.1 Hz, 4 H, quinoxaline), 8.11 (dd,
4J ) 3.1 Hz, 3J ) 6.1 Hz, 4 H, quinoxaline), 8.96 (s, 4 H,
quinoxaline). Anal. Calcd for C38H34Fe2Ga2N4 (Mr ) 797.86):
C, 57.21; H, 4.30; N, 7.02. Found: C, 57.05; H, 4.31; N, 6.81.
2f. Yield: 179 mg (0.29 mmol; 74%). UV: λ 261, 519 nm.
Anal. Calcd for C26H26N2Fe2Ga2 (Mr ) 617.65): C, 50.56; H,
4.24; N, 4.54. Found: C, 50.36; H, 4.26; N, 4.30.21
P r ep a r a tion of 2. (a) In a Schlenk flask (25 mL) 1 (300
mg, 0.78 mmol) was treated with diethyl ether (1 mL) and
toluene (3 mL) at room temperature. Then the reaction
mixture was cooled to +6 °C. After 1 day an orange, crystalline
solid had formed, which was washed with n-hexane and dried
in vacuo to give 98 mg (0.18 mmol, 46%) of 2.
2g. Yield: 168 mg (0.27 mmol; 69%). UV: λ 256, 469 nm.
Anal. Calcd for C26H30Fe2Ga2O2 (Mr ) 625.66): C, 49.91; H,
4.83. Found: C, 49.69; H, 4.68.
Rea ction of 2 a n d 2b-d ,f,g w ith Tr im eth ylga lliu m .
Trimethylgallium ((pyrophoric!) 0.10 g, 0.9 mmol) was added
to a suspension of 2 (40 mg, 0.07 mmol) in toluene-d8 (0.5 mL)
in a NMR tube. The reaction mixture was heated to 100 °C in
the tightly closed flask until all components had dissolved.
When the mixture was cooled to room temperature, 1 formed
as an orange microcrystalline solid. The supernatant solution
was decanted, and the solid residue was washed with hexane.
It was identified using NMR spectroscopy. Only GaMe3 could
be identified in the reaction mixture by NMR spectroscopy.
(b) In an NMR tube 1 (35 mg, 0.09 mmol) was dissolved in
trichloromethane. The solution was cooled to +6 °C. After 1
week, single X-ray-quality crystals had formed. UV: λ 258,
1
466 nm. H NMR (DMSO-d6): δ -0.17 (s, 6 H, CH3), 4.05 (s,
8 H, ring C2/5-H or C3/4-H), 4.18 (s, 8 H, ring C2/5-H or
C3/4-H). Anal. Calcd for C22H22Fe2Ga2 (Mr ) 537.54): C, 49.16;
H, 4.13. Found: C, 49.04; H, 4.05.
P r ep a r a tion of 2a . In an NMR tube 1 (35 mg, 0.09 mmol)
was treated with diethyl ether (0.2 mL) and toluene (0.7 mL)
at room temperature. The resulting solution was cooled to +6
°C. After 1 day orange X-ray-quality crystals of 2a had formed,
which are thermolabile and decomposed even at room tem-
(19) A single crystal was coated with a layer of hydrocarbon oil,
attached to a glass fiber, and cooled to 173 K for X-ray structure
determination. Because of the thermolability of 2a no elemental
analysis could be performed.
(20) Using method B single crystals of [2c‚(toluene)] were obtained.
When the solvent was removed in vacuo, the crystals decomposed to a
yellow powder.
1
perature. H NMR (DMSO-d6): δ -0.18 (s, 6 H, CH3), 1.08 (t,
3
3J ) 6.9 Hz, 12 H, Et2O CH3), 3.37 (q, J ) 6.9 Hz, 8 H, Et2O
(21) Using method B, single crystals of [2f‚1.5(pyrazine)‚1.5(toluene)]
were obtained. When pyrazine and toluene were removed in vacuo,
the crystals decomposed to a dark red powder.
(18) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854.

2774 Organometallics, Vol. 22, No. 13, 2003
Althoff et al.
The coordination compounds 2b-d ,f,g react with trimeth-
ylgallium in a similar manner. The donor adduct of GaMe3
and uncoordinated GaMe3 could be identified in the reaction
mixture.
University of Marburg, for a gift of trimethylgallium and
Professor Mu¨ller, University of Bielefeld, for recording
the UV/vis spectra.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, positional and thermal parameters, and bond lengths
and angles. This material is available free of charge via the
Internet at http://pubs.acs.org.
Ack n ow led gm en t. The support of this work by the
Deutsche Forschungsgemeinschaft, the Universita¨t
Bielefeld, and the Fonds der Chemischen Industrie is
gratefully acknowledged. We thank Professor Lorberth,
OM030115M