Ansa [1]trochrocenophanes and their related unstrained 1,1′- disubstituted counterparts: Synthesis and electronic structure

Article abstract of DOI:10.1021/ja0724947

The heteroleptic sandwich complex [Cr(η⁵-C₅H ₅)(η⁷-C₇H₇)] (trochrocene) was prepared by subsequent treatment of CrCl₃ with NaCp and Mg in the presence of cycloheptatriene in yields of 40%. Selective dimetalation employing ^tBuLi/tmeda (N, N, N′, N′-tetramethylethylenediamine) afforded the highly reactive species [Cr(η⁵-C₅H ₄Li)(η⁷-C₇H₆Li)]·tmeda. An X-ray crystal-structure determination of its thf solvate revealed a symmetrical, dimeric composition in the solid state, that is, a formula of [Cr(η⁵-C₅H₄Li)(η⁷-C ₇H₆-Li)]₂·(thf)₈, where the C₅H₄ moieties of both units are connected by two bridging lithium atoms. Addition of different element dihalides to the dilithio precursor facilitated the isolation of ansa complexes with boron and germanium in the bridging position. Structural characterization by X-ray diffraction studies on [Cr(η⁵-C₅H₄)-BN(SiMe₃) ₂-(η⁷-C₇H₆)] and [Cr(η⁵-C₅H₄)-GeMe₂- (η⁷-C₇H₆)] emphasized the strained character with tilt angles of 23.87(13)° and 15.07(17)°, respectively. In contrast, the isolation of the appropriate [1]stannatrochrocenophane failed because of the thermal lability of the resulting product. However, the corresponding 1,1′-disubstitued derivatives [Cr(η⁵-C ₅H₄R)(η⁷-C₇H₆R)] (R = B(Cl)NⁱPr₂, SiMe₃, GeMe₃, SnMe ₃) were obtained by reverse addition of the dilithio precursor to an excess of the element (di)halide. The unstrained nature was proven by a crystal structure analysis of the 1,1′-diborylated species. The electronic structure of these substituted trochrocene derivatives, as well as of the [2]bora and [n]sila congeners (n = 1, 2), was investigated by means of UV-vis spectroscopy and DFT methods. As a consequence of the strong electronic influence of the B-N π-system on the LUMOs, the UV-vis studies revealed a complementary correlation of the lowest energy band maxima as a function of molecular distortion for the boron containing species on the one hand, and the boron-free compounds on the other hand. These trends were reproduced fairly well by time dependent DFT calculations.

Full text of DOI:10.1021/ja0724947

Published on Web 06/26/2007
Ansa [1]Trochrocenophanes and Their Related Unstrained
1,1′-Disubstituted Counterparts: Synthesis and Electronic
Structure
Holger Braunschweig,* Thomas Kupfer, Matthias Lutz, and Krzysztof Radacki
Contribution from the Institut fu¨r Anorganische Chemie, Julius-Maximilians-UniVersita¨t
Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received April 10, 2007; E-mail: h.braunschweig@mail.uni-wuerzburg.de
Abstract: The heteroleptic sandwich complex [Cr(η5-C5H5)(η7-C7H7)] (trochrocene) was prepared by
subsequent treatment of CrCl3 with NaCp and Mg in the presence of cycloheptatriene in yields of 40%.
Selective dimetalation employing tBuLi/tmeda (N, N, N′, N′-tetramethylethylenediamine) afforded the highly
reactive species [Cr(η5-C5H4Li)(η7-C7H6Li)]‚tmeda. An X-ray crystal-structure determination of its thf solvate
revealed a symmetrical, dimeric composition in the solid state, that is, a formula of [Cr(η5-C5H4Li)(η7-C7H6-
Li)]2‚(thf)8, where the C5H4 moieties of both units are connected by two bridging lithium atoms. Addition of
different element dihalides to the dilithio precursor facilitated the isolation of ansa complexes with boron
and germanium in the bridging position. Structural characterization by X-ray diffraction studies on [Cr(η5-
C5H4)-BN(SiMe3)2-(η7-C7H6)] and [Cr(η5-C5H4)-GeMe2-(η7-C7H6)] emphasized the strained character with
tilt angles of 23.87(13)° and 15.07(17)°, respectively. In contrast, the isolation of the appropriate
[1]stannatrochrocenophane failed because of the thermal lability of the resulting product. However, the
corresponding 1,1′-disubstitued derivatives [Cr(η5-C5H4R)(η7-C7H6R)] (R ) B(Cl)NiPr2, SiMe3, GeMe3, SnMe3)
were obtained by reverse addition of the dilithio precursor to an excess of the element (di)halide. The
unstrained nature was proven by a crystal structure analysis of the 1,1′-diborylated species. The electronic
structure of these substituted trochrocene derivatives, as well as of the [2]bora and [n]sila congeners (n )
1, 2), was investigated by means of UV-vis spectroscopy and DFT methods. As a consequence of the
strong electronic influence of the B-N π-system on the LUMOs, the UV-vis studies revealed a
complementary correlation of the lowest energy band maxima as a function of molecular distortion for the
boron containing species on the one hand, and the boron-free compounds on the other hand. These trends
were reproduced fairly well by time dependent DFT calculations.
Introduction
containing polymers via ring opening polymerization (ROP) has
been described in detail and can be considered as a funda-
Since the publication of ferrocene, [Fe(η5-C5H5)2], in 1951
by T. J. Kealy and P. L. Pauson1 and the discovery of bis
(benzene)chromium, [Cr(η6-C6H6)2], 4 years later by E. O.
Fischer and W. Hafner,2 the field of organometallic complexes
with cyclic π-ligands has been thoroughly investigated. Since
then, numerous derivatives were prepared and characterized,
including ansa complexes containing bridged carbocycles. These
highly strained organometallic molecules have recently been
the focus of attention owing to their unique structure, bonding,
and reactivity patterns as well as their potential utility as
precursors for metal-containing macromolecules.3 As a conse-
quence of the high stability, the cheap availability, and the well-
established dimetalation of ferrocene,4 [n]ferrocenophanes cur-
rently rank among the best investigated strained sandwich
complexes, and an entire string of different bridging atoms has
been realized.5 Their application in the synthesis of metal
mental highlight in both organometallic chemistry and mater-
ials science.6 It has been shown that the properties of the result-
ing polymers can easily be controlled depending on the bridg-
ing atom, the substituents on the carbocyclic rings, and the
(3) (a) Foucher, D. A.; Honeyman, C. H.; Nelson, J. M.; Tang, B. Z.; Manners,
I. Angew. Chem. 1993, 105, 1843-1845; Angew. Chem., Int. Ed. 1993,
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Chem. Soc. 2000, 122, 5765-5774. (e) Mizuta, T.; Imamura, Y.; Miyoshi,
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9
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J. AM. CHEM. SOC. 2007, 129, 8893-8906
8893

A R T I C L E S
Braunschweig et al.
bridging element(s), respectively, as well as on the polymeri-
zation technique employed during their synthesis. The intrinsic
molecular ring strain is also reflected by an enhanced reactivity
of the strained bond between the cyclopentadienyl ligand and
the bridging element(s),5f,7 between the bridging elements,3h,8
or the Fe-Cp bond,3d,e,9 which led to the isolation of several
insertion and ring opening products. As a striking example, the
transition metal mediated diboration of alkynes by strained
[2]metallocenophanes may be mentioned in this context, which
could be achieved for the first time under heterogeneous
conditions.8b In contrast to this mature field, examples of ansa
metallocenes containing other metal centers than iron are rare
and comprise only few derivatives of chromium,10 ruthenium,11
cobalt,12 and nickel.13 Similarily, the chemistry of the related
bis(benzene)metal complexes has been scarcely studied in
comparison to the ferrocene system, hence, the number of ansa
complexes is truncated to a few examples of bridged bis-
(benzene)vanadium, [V(η6-C6H6)2],14 and bis(benzene)chromium
derivatives.8b,14,15
metalation of the parent sandwich complexes [Ti(η5-C5H5)(η7-
C7H7)] (troticene)16 and [V(η5-C5H5)(η7-C7H7)] (trovacene)17
applying BuLi in the presence of N, N, N′, N′-tetramethyleth-
ylendiamine (tmeda) followed by treatment of the intermediate
dilithio complex with the appropriate element dihalides. Po-
lymerization experiments confirmed their susceptibility to strain
release by undergoing transition metal mediated ROP to yield
oligomeric materials.16 However, the highly reactive dilithiated
sandwich precursors are usually not isolated nor characterized,
e.g., by X-ray diffraction, even though their structural parameters
are of great interest. In fact, structural data of metalated
sandwich complexes are actually rare to date, though several
examples are reported in the literature, for instance the long
known dilithiated ferrocene derivatives [Fe(η5-C5H4Li)2]‚
pmdta18 (pmdta ) N, N′, N′, N′′, N′′-pentamethyldiethylentri-
amine) and [Fe(η5-C5H4Li)2]3‚(tmeda)2.19 Recently, Mulvey and
co-workers accomplished the selective tetrametalation of fer-
rocene and its higher homologues,20 the synergic monodepro-
tonation of bis(benzene)chromium,21 and the selective dimet-
alation of ferrocene22 by mixed alkali metal-magnesium amide
bases along with the appropriate crystal structure analyses.
Selective dimetalation of ferrocene was also achieved by alkali-
metal-mediated manganation, in which case manganese was for
In addition, heteroleptic sandwich compounds, for instance
those capped by cyclopentadienyl (Cp) and cycloheptatrienyl
(Cht) rings, have only been utilized very recently in the syn-
thesis of silicon- and boron-bridged derivatives of titanium and
vanadium, respectively. These complexes were obtained by
the first time directly attached to an aromatic framework.23a
A
related ferrocene derivative was prepared by Wagner et al. by
the transmetalation reaction of dilithiated ferrocene with FeCl2
that yielded an pentanuclear FeII cluster with two bridging iron
centers.23b The crystal structures of selectively mono- and
dimetalated ferrocenylcopper,24 dilithiated [Mn(η5-C5H5)(η6-
C6H6)],25 as well as dilithiated bis(benzene)molybdenum, [Mo-
(η6-C6H6)2],26 further contributed to the understanding of the
fundamentals that determine the conformation of this class of
organometallic compounds.
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8894 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

Ansa [1]Trochrocenophanes and Their Counterparts
A R T I C L E S
the [1]sila (2-5) and the [2]sila (6) derivatives we have
reported.27 The reactive dilithiated precursor has been prepared
by treatment of the parent sandwich complex with tBuLi/tmeda
in alipahtic solvents and has been isolated, as well as fully
characterized by NMR spectroscopy. The ansa-bridged species
exhibit weak to moderate molecular strain, as indicated by the
corresponding tilt angles R, ranging from 2.60° for 6 to 16.33°
for the silicon-bridged derivative 5. However, the ring strain
present in 1 and 2 was proven by a facile oxidative addition
reaction with [Pt(PEt3)3] that led to the insertion of the low-
valent transition metal into the bridging B-B bond and the ipso-
CCht-silicon bond, respectively. Additionally, [1]silatrochro-
cenophane 2 was successfully polymerized in the presence of
Karstedt’s catalyst to yield materials with moderate molecular
weights (MW ) 6.4 × 103).
Figure 1. Molecular structure of 7. Only one molecule of the asymmetric
unit is shown for clarity. Selected bond lengths [Å] and angles [deg]: Cr1-
C11, 2.164(3); Cr1-C12, 2.156(3); Cr1-C13, 2.169(3); Cr1-C14, 2.170-
(4); Cr1-C15, 2.171(4); Cr1-C16, 2.175(4); Cr1-C17, 2.168(3); Cr1-
C21, 2.196(4); Cr1-C22, 2.183(4); Cr1-C23, 2.193(4); Cr1-C24, 2.178(4);
Cr1-C25, 2.171(4); C21-C31, 1.477(5); Cr1-XCp, 1.819; Cr1-XCht
,
1.439; C22-C21-C31-C32, 22.34(57); XCp-Cr1-XCht, 178.1 (XCp, XCht
) centroids of the C5H4 and the C7H7 rings, respectively).
Scheme 1. Original Procedure for the Preparation of Trochrocene
The molecular ring strain, and hence the polymerization
behavior, can easily be tuned by the systematic variation of the
bridging elements. In this contribution, we report full details of
our work on the synthesis and structural characterization of the
first [1]bora-, [1]germa- and [1]stannatrochrocenophanes, as well
as on the preparation of the corresponding unstrained 1,1′-
disubstituted derivatives. We also discuss the electronic struc-
tures of these molecules, which were investigated by means of
UV-vis spectroscopy and theoretical calculations.
tion steps) this compound was identified by NMR spectroscopy
as the cyclopentadienyl ring-phenylated derivative 7.
Thus, the resonance signals of the ring protons in the 1H NMR
spectrum display a splitting pattern, which is in agreement with
a trochrocene derivative substituted exclusively at the C5H4
moiety. Hence, the R and â hydrogen atoms of the Cp ring show
two distinct multiplets (δ ) 3.75 and 4.17), whereas the Cht
hydrogen atoms exhibit a slightly broadend singlet at δ ) 5.33
with an integration ratio of 2:2:7. As expected for an unstrained
sandwich complex, these signals appear in the same region as
those found in the parent trochrocene complex (δ ) 3.66 and
5.45 ppm).
The proposed constitution was authenticated in the solid-state
structure of 7 (Figure 1). 7 crystallizes in the monoclinic space
group P21 with two independent molecules in the asymmetric
unit, whereas the structural parameters of both moieties differ
only marginally. Hence, for simplicity reasons only one of the
molecular structures is discussed below. Bond distances and
bond angles within the sandwich motif are unremarkable. The
Cr-C bond distances [C5H4, 2.171(4)-2.196(4) Å; C7H7, 2.156-
(3)-2.175(4) Å] strongly resemble the values found in the
unsubstituted sandwich precursor [2.147-2.167(4) Å].29 The
unstrained character of 7 is reflected by the following: (1) the
two aromatic ring moieties are planar, therefore a regular η5-
and η7-coordination can be assumed; (2) both rings are arranged
almost parallel with an angle between the planes of the C5H4
and the C7H7 moieties of 1.42(22)°; (3) the angle δ, defined by
Results and Discussion
Synthesis of [Cr(η5-C5H5)(η7-C7H7)]. The synthesis of
trochrocene was originally reported by E. O. Fischer and S.
Breitschaft in the 1960s by a multistep reaction sequence
(Scheme 1) with an overall yield of 14%, at best.28 However,
in our hands the yields were not reproducible at all, fluctuating
in a range of 0-10%. As a consequence, these preparative
drawbacks prompted us to reinvestigate the described procedure
and to further develop a more convenient protocol. As depicted
in Scheme 1, the yield determining step during the synthesis of
trochrocene comprises the formation of [Cr(η5-C5H5)(η6-
C6H6)],28b which is obtained in 22% yield at most. Hence, we
treated the crude product directly with cyclohepatriene in the
presence of AlCl3, and after workup we were able to isolate
trochrocene in essentially identical yields; that is, the isolation
of the intermediate is not a necessary prerequisite for this
reaction. However, the sublimation of crude trochrocene was
always accompanied by the appearance of a green rim right
above the heating source that could be separated mechanically
from the desired product. After a time-consuming purification
procedure (column chromatography and repeated recrystalliza-
(27) Bartole-Scott, A.; Braunschweig, H.; Kupfer, T.; Lutz, M.; Manners, I.;
Nguyen, T.-I.; Radacki, K.; Seeler, F. Chem.sEur. J. 2006, 12, 1266-
1273.
(28) (a) Fischer, E. O.; Breitschaft, S. Angew. Chem. 1963, 75, 94-95; Angew.
Chem., Int. Ed. 1963, 2, 44. (b) Fischer, E. O.; Breitschaft, S. Chem. Ber.
1966, 99, 2213-2226. (c) Fischer, E. O.; Breitschaft, S. Chem. Ber. 1966,
99, 2905-2916.
(29) Lyssenko, K. A.; Antipin, M. Yu.; Ketkov, S. Yu. Russ. Chem. Bull., Int.
Ed. 2001, 50, 130-141.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8895

A R T I C L E S
Braunschweig et al.
Scheme 2. Improved Synthesis of [Cr(η5-C5H5)(η7-C7H7)]
the ring centroids and the metal center, exhibits only a minimal
deviation from the ideal linear alignment found in the parent
compound (δ ) 178.1°). For sterical reasons, the phenyl
substitutent attached to the Cp ring is notedly distorted from a
coplanar arrangement [C22-C21-C31-C32 ) 22.34(57)°].
The formation of 7 as a side product must have occurred
during the first step of the trochrocene synthesis, that is, the
preparation of [Cr(η5-C5H5)(η6-C6H6)] (Scheme 1),28b and might
be explained by deprotonation of the five-membered ring and
subsequent reductive coupling with one molecule PhMgBr to
yield [Cr(η5-C5H4Ph)(η6-C6H6)], which is then converted to the
trochrocene derivative 7. In addition, the generation of other
ring-phenylated mono- and dinuclear species derived from the
biphenyl moiety, such as [(η5-C5H5)Cr(µ2,η6,η6-C12H10)Cr(η5-
C5H5)] and [Cr(η5-C5H5)(η6-C6H5Ph)], must be taken into
account as possible byproducts. Related biphenyl complexes
have been observed and isolated during the synthesis of the
corresponding manganese compound, [Mn(η5-C5H5)(η6-C6H6)],
in which case they even represent the main products.28b Thus,
the reason for the low yield synthesis of [Cr(η5-C5H5)(η6-C6H6)],
and consequently that of trochrocene, is given by the low
selectivity of the reaction and the formation of several side
products. Within the second step of the preparation of tro-
chrocene, the AlCl3-mediated ring exchange, the biphenyl
containing species are transmuted to trochrocene and, hence,
cannot be observed after workup.
To avoid the formation of the ring-phenylated side products
emerging during the original Fischer protocol and to further
develop a more convenient synthetic access to trochrocene, we
followed a different known reaction sequence that was already
successfully employed in the preparation of the analogous
titanium and zirconium sandwich species, [M(η5-C5H5)(η7-
C7H7)] (M ) Ti,30 Zr31). Accordingly, trochrocene has been
obtained in yields up to 40% by treatment of [(η5-C5H5)CrCl2]2,
prepared in situ from equimolar amounts CrCl3 and NaCp, with
magnesium turnings in the presence of excess cycloheptatriene
in THF (Scheme 2). In contrast to the syntheses of the Ti and
Zr analogues, the presence of catalytic amounts of FeCl3 had
no impact on this particular reaction.
Figure 2. Molecular structure of the [Cr(η5-C5H4Li)(η7-C7H6Li)]2‚(thf)8
(8) solvate. For clarity, only the oxygen atoms of the thf molecules are
shown and thf molecules incorporated in the crystal lattice are omitted.
Selected bond lengths [Å] and angles [deg]: Li1-Li2, 3.024(7); Li2-Li3,
2.416(6); Li3-Li4, 2.848(6); Li1-C1_1, 2.123(6); Li2-C1_1, 2.107(5);
Li2-C1_3, 2.179(5); Li2-C1_4, 2.138(5); Li3-C1_2, 2.182(5); Li3-C1_3,
2.135(5); Li3-C1_4, 2.207(5); Li4-C1_2, 2.134(5); Cr1-CCp, 2.159(3)-
2.272(2) [av. 2.084(3)]; Cr1-CCht, 2.109(2)-2.254(2) [av. 2.158(3)]; Cr2-
CCp, 2.109(3)-2.242(2) [av. 2.191(3)]; Cr2-CCht, 2.088(4)-2.233(2) [av.
2.162(3)]; Cr1-XCp, 1.838; Cr1-XCht, 1.419; Cr2-XCp, 1.777; Cr2-XCht
,
1.482; C1_1-Li2-C1_3, 101.75(18); C1_1-Li2-C1_4, 142.83(21); C1_3-
Li2-C1_4, 110.86(21); C1_2-Li3-C1_3, 141.72(25); C1_2-Li3-C1_4,
97.61(17); C1_3-Li3-C1_4, 109.94(21); XCp-Cr1-XCht, 178.0; XCp
-
Cr2-XCht, 176.3 (XCp, XCht ) centroids of the C5H4 and the C7H6 rings,
respectively).
by means of sublimation at 100 °C and 10-3 mbar, yielding
dark-blue crystals of [Cr(η5-C5H5)(η7-C7H7)]. Given the avail-
ability of gram quantities of trochrocene, our intention was to
further investigate the chemistry of this sandwich complex and
of strained [n]trochrocenophanes in particular.
Dimetalation. We recently reported on the facile dilithiation
of trochrocene to afford the highly reactive species [Cr(η5-C5H4-
Li)(η7-C7H6Li)]‚tmeda (8‚tmeda), which was fully characterized
by NMR spectroscopic experiments.3h The selectivity of the
dimetalation was confirmed by the 1H NMR data, which showed
the presence of a single compound with the expected splitting
pattern and integration ratio. The conformation of this species
was unambiguously demonstrated by the determination of the
solid-state structure. Thus, recrystallization of 8‚tmeda from a
saturated thf solution at -70 °C yielded pale brown crystals
that were formulated as 8‚(thf)3 on the basis of 1H NMR
spectroscopy; that is, dissolving in thf causes the replacement
of the tmeda ligand by coordinating thf molecules. As expected,
the NMR spectroscopic data in [D8]-thf solution are very similiar
to those of the tmeda adduct (8‚tmeda).3h The crystals obtained
were of sufficient quality; hence, the molecular structure of the
dilithiated precursor (8) was determined by crystal structure
analysis (Figure 2). In the solid state, 8 exhibits a symmetrical
dimeric structure, in which both molecules are connected by
two bridging lithium atoms (Li2 and Li3) bound to the ipso
carbons of both C5H4 moieties, as well as to one ipso carbon of
the C7H6 rings. In contrast, the terminal lithium atoms (Li1 and
Li4) are bound to the ipso carbon of the seven-membered rings
and are both further stabilized by the coordination of three thf
In our hands, the isolation of trochrocene directly from the
crude reaction mixture via sublimation turned out to be quite
difficult owing to polymeric organic side products arising from
the cycloheptatriene. In addition, the yields obtained were not
satisfactory at all (5-10%). We found it more valuable to follow
an oxidation-reduction cycle at this point (Scheme 2). Thus,
the crude trochrocene was oxidized to the corresponding cation
by refluxing the reaction mixture in acetone/H2O under air
exposure. The resulting water soluble green cation was subse-
quently reduced with Na2S2O4 in toluene/water under an inert
atmosphere to yield a dark blue solution of pure trochrocene.
Additional purification of the air-sensitive product was achieved
(30) Demerseman, B.; Dixneuf, P. H.; Douglade, J.; Mercier, R. Inorg. Chem.
1982, 21, 3942-3947.
(31) Tamm, M.; Kunst, A.; Bannenberg, T.; Herdtweck, E.; Schmid, R.
Organometallics 2005, 24, 3163-3171.
9
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Ansa [1]Trochrocenophanes and Their Counterparts
A R T I C L E S
[2.397 Å]33 and might suggest a direct lithium-lithium interac-
tion, in terms of stabilizing the unsaturated lithium centers.
However, according to earlier investigations on alkyllithium
reagents34 or the related dilithiated ferrocene derivative [Fe(η5-
C5H4Li)2]‚pmdta,18 this type of stabilization is weak at best.
[1]Trochrocenophanes. In our recent publications we suc-
cessfully demonstrated the conversion of 8‚tmeda to several
strained [1]trochrocenophanes containing silicon in the bridging
position.27 Hence, current work has focused on the incorporation
of other main-group elements into the ansa bridge. Accordingly,
the highly strained [1]boratrochrocenophanes 9 and 10, as well
as the germanium bridged derivative 11 were prepared by
stoichiometric treatment of 8‚tmeda at -78 °C with the
appropriate element dihalides in aliphatic or aromatic solvents,
respectively. These compounds were isolated after workup as
highly colored, crystalline materials in good yields of 60-70%
(Scheme 3).
Figure 3. Molecular structure of 9. Selected bond lengths [Å] and angles
[deg]: Cr1-C11, 2.069(3); Cr1-C12, 2.132(3); Cr1-C13, 2.182(3); Cr1-
C14, 2.176(3); Cr1-C15, 2.180(3); Cr1-C16, 2.181(3); Cr1-C17, 2.140-
(3); Cr1-C21, 2.126(3); Cr1-C22, 2.156(3); Cr1-C23, 2.213(3); Cr1-
C24, 2.219(3); Cr1-C25, 2.161(3); C11-B1, 1.620(4); C21-B1, 1.629(4);
B1-N1, 1.399(4); Cr1-XCp, 1.808; Cr1-XCht, 1.409; C11-B1-C21,
100.52(52); C11-B1-N1, 130.98(25); C21-B1-N1, 128.47(26); XCp
-
The introduction of the small boron bridge into 9 and 10
results in a splitting of the corresponding resonance signals of
the aromatic ring protons in the 1H NMR spectra in comparison
to the parent sandwich complex. The R and â protons of the
five-membered ring in 9 exhibit two distinct pseudotriplets (δ
) 3.51 and 3.55 ppm), which appear in a region similar to that
found for trochrocene (δ ) 3.66 ppm). The highly strained
character of this compound is best reflected by the signals of
the cycloheptatrienyl protons, which split up into a pseudodou-
blet at δ ) 4.68 ppm that can be assigned to the R CH groups
adjacent to the bridging boron center, as well as into two
multiplets for the â and γ protons (δ ) 5.51 and 5.89 ppm).
The large separation of the R and â protons (∆δ ) 0.83 ppm)
in combination with a significant highfield shift of the former
resonance with respect to the appropriate trochrocene signal (δ
) 5.45 ppm) is a characteristic feature for the presence of
molecular ring strain and has previously been described for the
corresponding [n]metalloarenophanes of chromium and manga-
nese.3h,25,27 Because of hindered rotation about the B-N double
bond, the SiMe3-groups are detected as two discrete signals at
room temperature (δ ) 0.37 and 0.66 ppm). The rotation barrier
has been estimated by variable-temperature (VT) NMR spec-
Cr1-XCht, 162.0 (XCp, XCht ) centroids of the C5H4 and the C7H6 rings,
respectively).
oxygen atoms. The presence of the thf molecules results in two
severe consequences: (1) even though the crystals showed an
absolutely proper morphology under the microscope, they did
not scatter any further than to a resolution of ∼1 Å; (2) the thf
molecules show extensive disorder, thus requiring a combination
of constraints and restraints in the refinement, and as a
consequence, the structural parameters within these moieties are
excluded from the following discussion. In agreement with the
molecular structures of [Cr(η5-C5H5)(η7-C7H7)]29 and 7, the
chromium CH carbon bond distances to the five-membered rings
[2.109(3)-2.184(2) Å] are comparable to those to the seven-
membered rings [2.088(4)-2.173(2) Å]. However, the chro-
mium ipso carbon bonds are significantly elongated both for
the C5H4 [2.242(2) and 2.172(2) Å] and the C7H6 moieties
[2.233(2) and 2.254(2) Å]. As expected, the two aromatic ring
substitutents of each chromium unit are arranged almost parallel
to each other; angles between the planes of the five- and seven-
membered rings amount to 1.23° and 1.61°, respectively. The
unstrained character of this structural motif is also demon-
strated by the angles δ (vide supra), which reflect almost linear
arrangements in both subunits [Cr1, 178.0° and Cr2, 176.3°].
The tetra-coordinated terminal lithium atoms Li1 and Li4 display
a tetrahedral environment, whereas the coordination sphere of
the unsaturated bridging lithium atoms Li2 and Li3 is best
described as a distorted trigonal planar geometry [ΣLi2 ) 355.4°
and ΣLi3 ) 349.3°]. Both the Li-C bond distances18,19 [2.107-
(5)-2.207(5) Å] and the Li-O bond lengths32 [1.856(4)-2.017-
(6) Å] lie within previously reported ranges. The Li‚‚‚Li
separation distance between the bridging lithium atoms [Li2‚‚
‚Li3 ) 2.416(6) Å] differs significantly from those to the
terminal lithium atoms [Li1‚‚‚Li2 ) 3.024(7) Å and Li3‚‚‚Li4
) 2.848(6) Å], which is in agreement with the structural
parameters found in the related dilithiated manganese complex
[Mn(η5-C5H4Li)(η6-C6H5Li)]‚pmdta.25 The relative short value
of the former is comparable to that found in cylohexyllithium
troscopy and amounts to a value of ca. 67.8 KJ mol-1
.
As expected, the key features of the 1H NMR spectrum of 9
are also present in the respective spectrum of 10. However, the
unsymmetrical substitution pattern of the nitrogen atom in 10
results in the presence of two isomers that do not interconvert
at room temperature, and hence, two sets of resonance signals
are observed. As a consequence, the R and â protons of the
five-membered ring give rise to three multiplets (δ ) 3.51, 3.56,
and 3.59 ppm) with an integration ratio of 4:2:2, whereas the
splitting pattern of the C7H6 moiety remains unmodified in
t
comparison to 9. In addition, both the Bu-groups (δ ) 1.53
and 1.82 ppm) and the SiMe3-groups (δ ) 0.42 and 0.73 ppm)
are detected as two well-separated signals, and their relative
intensities indicate a 1:1 mixture of the two isomers. In
agreement with Cs symmetry in solution, 9 shows five signals
for the ring CH carbon atoms (between δ ) 74.26 and 101.89
ppm), whereas, as expected, the isomers of 10 exhibit ten
(32) (a) Hoppe, I.; Marsch, M.; Harms, K.; Boche, G.; Hoppe, D. Angew. Chem.
1995, 107, 2328-2330, Angew. Chem. Int. Ed. 1995, 34, 2158-2160. (b)
Wiberg, N.; Hwang-Park, H.-S.; Mikulcik, P.; Mu¨ller, G. J. Organomet.
Chem. 1996, 511, 239-253. (c) Nanjo, M.; Nanjo, E.; Mochida, K. Eur.
J. Inorg. Chem. 2004, 2961-2967.
(33) Zerger, R.; Rhine, W.; Stucky, G. D. J. Am. Chem. Soc. 1974, 96, 6048-
6055.
(34) (a) Brown, T. L.; Seitz, L. M.; Kimura, B. Y. J. Am. Chem. Soc. 1968, 90,
3245. (b) Scovell, B. Y.; Kimura, B. Y.; Spiro, T. G. J. Coord. Chem.
1971, 1, 107.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8897

A R T I C L E S
Braunschweig et al.
Scheme 3. Syntheses of [1]Trochrocenophanes
resonances (between δ ) 72.53 and 100.49 ppm) in the
13C NMR spectra. The ipso carbons of both compounds cannot
be observed at room temperature, most probably because of the
quadrupolar moment of the boron atom.35 The 11B NMR
resonances (9, 49.9 ppm; 10, 46.6 ppm) lie within previously
reported ranges.3d,h,15b
boron center. Consequently, the C(arene)-B-N bond angles
are widened to values of 128.47(26)° and 130.98(25)°, respec-
tively. In addition, the highly strained geometry, particularly
of the cycloheptatrienyl unit, is imposingly illustrated by the
very short chromium ipso-CCht bond distance of 2.069(3) Å (vide
supra). In contrast to these variances within the structural
parameters deduced from the incorporation of an ansa bridge,
the Cr-C bond lengths to the C5H4 ring [2.123(3)-2.219(3)
Å], the chromium CH carbon bond distances to the C7H6 moiety
[2.132(3)-2.182(3) Å], as well as the chromium centroid
separation distances to the five-membered ring [1.808 Å] and
the seven-membered ring [1.409 Å] are not affected.
It should be mentioned here that the presence of at least one
silyl group at the nitrogen substituents is an essential prerequisite
for the successful preparation of the [1]boratrochrocenophanes,
most probably owing to their electronic impact.24,28 If alkyl-
substituted aminoboranes such as Cl2BNiPr2 (vide infra), Br2-
BNEt2, or Br2BNMe2 are employed during the synthesis, the
formation of the ansa complex cannot be observed.
The introduction of the larger germanium bridge in 11 leads
to the expected differences in the 1H NMR spectrum compared
to 9 and 10. Even though the splitting pattern for the resonances
of the aromatic ring protons has substantial similiarity to the
latter, that is, two pseudotriplets are observed for the C5H4 ring
(δ ) 3.69 and 3.78 ppm), as well as one pseudodoublet (δ )
5.24 ppm) and two multiplets (δ ) 5.69 and 5.83 ppm) for the
C7H6 ring, the reduced molecular ring strain present in 11 is
reflected by the diminished separation of the signals attributable
to the R and â protons of the C7H6 moiety (∆δ ) 0.45 ppm) in
combination with the minor highfield shift of the former with
respect to the corresponding resonance of trochrocene. As
expected, the 13C NMR spectrum displays five distinct resonance
signals for the aromatic CH carbon atoms (between δ ) 79.91
and 102.14 ppm) together with two more shielded signals, which
can be unambiguously assigned to the ipso-C5H4 carbon atom
(δ ) 54.49 ppm) and to the ipso-C7H6 carbon atom (δ )
61.35 ppm). Though the molecular ring strain is supposed to
be less pronounced than in 9 and 10, the NMR spectroscopic
data definitely suggest a notedly distorted molecular geometry.
Given that similar NMR parameters were obtained for the
appropriate [1]silatrochrocenophanes,27 the structural properties
of 11 should be more like those of the silicon species than those
of the boron-bridged complexes.
To confirm the formation of a strained ansa complex, a crystal
structure analysis of 11 was carried out (Figure 4). In accordance
with the NMR spectroscopic data, the introduction of a
monoatomic germanium bridge results in a less distorted
geometry in 11 than in 9. Whereas the Cr-C bond lengths are
not affected by the larger bridge [between 2.115(3) and 2.212-
(3) Å], the minor ring strain is manifested by a smaller tilt angle
R ) 15.07(17)°. As suspected, this value is comparable to those
found in the analogous [1]silatrochrocenophanes (R ) 15.6°-
16.6°),27 an effect that is related to the very similar covalent
To gain more detailed insight into the structural properties
of 9, the molecular structure was determined by single-crystal
X-ray diffraction analysis (Figure 3). The incorporation of the
small boron ansa bridge results in a substantial deviation of the
aromatic rings from the coplanar arrangement observed in the
unstrained precursor [Cr(η5-C5H5)(η7-C7H7)]. The most promi-
nent structural feature is given by the tilting of the two aromatic
ring moieties and hence, by the presence of molecular ring strain
that is represented by the tilt angle R ) 23.87(13)°. Because of
the small covalent radius of the boron atom, this value is
significantly larger than those of the corresponding [1]silatro-
chrocenophanes (R ) 15.6° to 16.6°)27 and suggests a notedly
larger ring strain present in 9. Yet, the values reported for the
analogous boron-bridged derivatives of ferrocene [32.4(3)°]3d
and trovacene [28.2°]16b are conspicuously higher, undoubtedly
because of the longer interannular distances in these systems
[cp. ferrocene (332 pm), trovacene (338 pm), and trochrocene
(326 pm)]. There is also evidence that the cycloheptatrienyl
systems distort in a different way to absorb strain energy than
the ferrocenophanes. For instance, the trovacene derivative
features a significantly lower value for a as expected, given
the larger interannular distance with respect to ferrocene.
Furthermore, the analogous bis(benzene)chromium species
exhibits a larger tilt angle [26.6(3)°]15b compared to 9, even
though the interannular distance in [Cr(η6-C6H6)2] is slightly
diminished (322 pm). The introduction of an ansa bridge into 9
results in a significant distortion of the cycloheptatrienyl unit
from planarity especially at the ipso carbon atom of the
cycloheptatrienyl unit [rms deviation for Cht ) 0.0553 Å and
for Cp ) 0.0073 Å], with the angles â between the carbocyclic
ring planes and the exocyclic C-B bond of âCp ) 29.9° and
âCht ) 43.1°. The latter appears notedly enlarged in comparison
to the ferrocene derivative [33.7(2)° and 34.0(2)°].3d Hence, the
flexibility of the seven-membered ring in 9 with respect to the
cyclopentadienyl and benzene ligands of the iron and chromium
congeners allows for the reduction of the molecular ring strain.
Directly associated with the tilted structure of 9, the angle δ is
essentially reduced to 162.0° compared to the unstrained species
7 and 8. The geometry around the bridging boron atom also
emphasizes the highly distorted character of 9. Even though
the B-N bond length [1.399(4) Å], as well as the trigonal planar
environment of the boron nucleus [ΣB ) 359.9°], are in
agreement with the formulation as a boron nitrogen double bond,
the CCp-B-CCht angle θ ) 100.52(22)° is perspicuously
diminished from the ideal angle of 120° for an sp2-hybridized
(35) Herberhold, M.; Do¨rfler, U.; Wrackmeyer, B. J. Organomet. Chem. 1997,
530, 117-120.
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8898 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

Ansa [1]Trochrocenophanes and Their Counterparts
A R T I C L E S
significantly from the tetrahedral angle for a sp3-hybridized
germanium atom. The smaller angle θ results in a slight
scissoring effect at germanium, with a widening of the CMe
-
Ge-CMe angle to 112.55(16)°, which has already been detected
within the silicon-bridged counterparts.27
In contrast to the facile access to [1]trochrocenophanes
containing boron, silicon, and germanium in the bridging
position, the synthesis of the corresponding [1]stanna derivative
caused much more difficulties. Hence, treatment of the dilithi-
ated precursor 8‚tmeda with Cl2SnMes2 (Mes ) 2,4,6-C6H2-)
under various conditions did not result in the isolation of any
pure material (Scheme 3). The NMR spectra obtained after
workup showed a whole string of broad signals in the majority
of cases, which could not be assigned to a distinct compound.
This might be explained by the high lability of the Sn-Caryl
bond in combination with the presence of molecular ring strain.
Consequently, even though the resulting species would be less
strained than the boron, silicon, and germanium bridged
analogues, 12 seems to be too labile for isolation. It is well
documented, that the corresponding [1]stannaferrocenophanes
tend to undergo thermal ROP even at room temperature
depending on the substituents at the tin center.5f However, the
desired ansa-bridged tin derivative 12 was unambiguously
identified spectroscopically in the reaction mixture by means
of multinuclear NMR spectroscopy. In addition, we succeeded
in the isolation of a small amount (∼10 mg) of pure 12 during
the workup procedure. The NMR spectroscopic data obtained
from the pure sample are in accordance with a Cs-symmetric
complex in solution. Thus, the aromatic ring protons of the five-
membered ring exhibit two distinct pseudotriplets for the R and
â positions (δ ) 3.68 and 3.89 ppm), whereas the cyclohep-
tatrienyl hydrogens are detected as two multiplets (δ ) 5.60
and 5.67 ppm) with a relative ratio of 2:2:2:4. The fact, that
the R protons of the C7H6 moiety are observed downfield shifted
with respect to the respective resonance of trochrocene indicates
the presence of only weak molecular ring strain in 12; similar
results were obtained with the [2]silatrochrocenophane that
displays a tilt angle R ) 2.60°.27 Additionally, the resonance
signals of the ipso-C5H4 (δ ) 70.36 ppm) and the ipso-C7H6
(δ ) 77.43 ppm) carbon atoms in the 13C NMR spectrum appear
deshielded in comparison to the analogous signals of the [1]-
sila and [1]germa derivatives and hence, also suggest a less
distorted molecular geometry.
Figure 4. Molecular structure of 11. Selected bond lengths [Å] and angles
[deg]: Cr1-C11, 2.115(3); Cr1-C12, 2.140(3); Cr1-C13, 2.181(3); Cr1-
C14, 2.173(3); Cr1-C15, 2.165(3); Cr1-C16, 2.174(3); Cr1-C17, 2.159-
(3); Cr1-C21, 2.170(3); Cr1-C22, 2.161(3); Cr1-C23, 2.216(3); Cr1-
C24, 2.209(3); Cr1-C25, 2.169(3); C11-Ge1, 1.994(3); C21-Ge1,
1.971(4); Cr1-XCp, 1.820; Cr1-XCht, 1.417; C1-Ge1-C2, 112.55(16);
C1-Ge1-C11, 114.46(15); C1-Ge1-C21, 112.37(15); C2-Ge1-C11,
114.17(15); C2-Ge1-C21, 110.94(15); C11-Ge1-C21, 90.45(13); XCp
-
Cr1-XCht, 168.3 (XCp,, XCht ) centroids of the C5H4 and the C7H6 rings,
respectively).
Figure 5. Molecular structure of 13. Only one molecule of the asymmetric
unit is shown for clarity. Selected bond lengths [Å] and angles [deg]: Cr1-
C11, 2.198(2); Cr1-C12, 2.171(2); Cr1-C13, 2.166(2); Cr1-C14, 2.161-
(2); Cr1-C15, 2.167(2); Cr1-C16, 2.168(2); Cr1-C17, 2.148(2); Cr1-
C21, 2.200(2); Cr1-C22, 2.185(2); Cr1-C23, 2.201(2); Cr1-C24, 2.194(2);
Cr1-C25, 2.188(2); C11-B1, 1.576(4); C21-B2, 1.559(3); B1-N1, 1.391-
(3); B1-Cl1, 1.808(3); B2-N2, 1.398(3); B2-Cl2, 1.825(3); Cr1-XCp
,
1.831; Cr1-XCht, 1.436; C11-B1-N1, 124.37(22); C11-B1-Cl1, 115.60-
(18); Cl1-B1-N1, 120.02(19); C21-B2-N2, 127.29(23); C21-B2-Cl2,
113.71(18); Cl2-B2-N2, 118.99(18); B1-C11-C21-B2, 89.9; XCp
-
Cr1-XCht, 178.0 (XCp, XCht ) centroids of the C5H4 and the C7H6 rings,
1,1′-Disubstituted Complexes. The syntheses of the un-
strained counterparts of the [1]trochrocenophanes described
above, that is, the 1,1′-disubstituted trochrocene derivatives [Cr-
(η5-C5H4R)(η7-C7H6R)] (R ) B(Cl)NiPr2 13, SiMe3 14, GeMe3
15, SnMe3 16), were accomplished by treating 8‚tmeda with
2.5 equiv of the appropriate element (di)halide at -30 °C in
aliphatic solvents (Scheme 4). These compounds were isolated
after workup as deeply colored solids in excellent yields of 70-
90%.
Compound 13 exhibits signal patterns in the 1H NMR
spectrum that emphasize the absence of molecular ring strain
in this 1,1′-disubstituted metallocene. For instance, the R and â
hydrogens of the five-membered ring, observed as two distinct
pseudotriplets (δ ) 4.34 and 4.44 ppm), appear significantly
downfield shifted with respect to the corresponding resonances
of both trochrocene (3.66 ppm) and the strained congeners
9-11. This attribute is further supported by the resonance
respectively).
radii of silicon and germanium, respectively. Again, the corre-
sponding germanium bridged derivatives of ferrocene (19.1°)36
and troticene (22.9°)16c both exhibit remarkably larger structural
distortion and consequently larger tilt angles than 11 (vide
supra). The distortion of the cycloheptatrienyl moiety from
planarity [rms deviation for Cht ) 0.0376 Å and for Cp )
0.0070 Å] is less pronounced compared to that found in 9,
whereas the angles âCp ) 29.8° and âCht ) 43.8° remain almost
unchanged. Additionally, the angle δ ) 168.3° is notedly
enlarged with respect to that observed in 9, whereas the silicon-
bridged derivatives feature an almost identical value (δ )
167.4-168.0°), in turn. The molecular strain is also reflected
by the CCp-Ge-CCht angle θ ) 90.45(13)°, which deviates
(36) Foucher, D. A.; Edwards, M.; Burrow, R. A.; Lough, A. J.; Manners, I.
Organometallics 1994, 13, 4959-4966.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8899

A R T I C L E S
Braunschweig et al.
Scheme 4. Syntheses of 1, 1′-Disubstituted Derivatives
signals of the cycloheptatrienyl protons, which show one
multiplet for the â and γ hydrogen atoms (δ ) 5.79 ppm), as
well as one pseudodoublet assignable to the R protons (δ )
6.01 ppm). As described above, the protons of the seven-
membered ring in R position to the substituents constitute an
excellent probe for the molecular ring strain present in substi-
tuted metallocenes. Thus, the pseudodoublet is detected notedly
deshielded in comparison to the appropriate trochrocene reso-
nance (δ ) 5.45 ppm) and hence, suggests an only slightly
distorted geometry of 13. In contrast, the corresponding signals
of the strained [n]trochrocenophanes are found conspicuously
downfield shifted compared to the parent sandwich complex
(vide supra), and the magnitude of the separation between these
signals and the resonances of the â ring protons indicates even
the dimension of the molecular distortion. The appearance of
As anticipated, the boron centers display a distorted trigonal
planar environment that differs only marginally from the ideal
geometry [ΣB1 ) 360.0° and ΣB2 ) 360.0°].
The NMR spectroscopic data of 14-16 can be interpreted
as being caused by 1,1′-disubstituted trochrocene derivatives
with time-averaged Cs-symmetry in solution. All compounds
1
exhibit very similar signal patterns in both the H NMR and
the 13C NMR spectrum that are in agreement with an unstrained
molecular structure as described for 13. Pertinent features in
the 1H NMR spectra include the splittings of the Cp resonances
into two pseudo triplets (14, δ ) 3.74 and 3.99 ppm; 15, δ )
3.74 and 3.98 ppm; 16, δ ) 3.70 and 4.02 ppm) and of the Cht
signal resonances into two multiplets (14, δ ) 5.61 and
5.68 ppm; 15, δ ) 5.58 and 5.63 ppm; 16, δ ) 5.51 and
5.64 ppm). Both the former and the R hydrogen atoms of the
latter appear downfield shifted with respect to the resonance of
trochrocene. In addition, the 13C NMR spectra exhibit signals
for the ipso carbon atoms of the C5H4 moieties (14, δ )
78.22 ppm; 15, δ ) 81.90 ppm; 16, δ ) 76.61 ppm) as well as
for the ipso carbon atoms of the C7H6 moieties (14, δ )
90.31 ppm; 15, δ ) 94.58 ppm; 16, δ ) 93.49 ppm), which
are found again significantly deshielded compared with those
of the corresponding [1]trochrocenophanes. These findings are
consistent with the NMR data obtained for the unstrained
complex 13 and indicate the absence of molecular ring strain
in these species. As expected, the 29Si NMR spectrum of 14
(δ ) -2.55 and 6.31 ppm) and the 119Sn NMR spectrum of 16
(δ ) 0.80 and 29.30 ppm) exhibit two distinct signal resonances
for the Me3E-substituents (E ) Si, Sn) owing to chemical
inequivalence of these groups.
i
four distinct resonances for the Pr-methyl groups both in the
1H and the 13C NMR spectrum as well as the detection of two
resonance signals in the 11B NMR spectrum (δ ) 36.6 and
39.1 ppm) definitely confirms the presence of two chemical
nonequivalent boron substituents and consequently the un-
strained character of 13.
The solid-state structure of 13 was determined by X-ray
diffraction (Figure 5). The asymmetric unit contains two
independent molecules, whereas the structural parameters of
both moieties differ only marginally. Hence, for simplicity
reasons only one of the molecular structures is discussed below.
The structural parameters of 13 are unremarkable with respect
to the crystal structures of 7 and trochrocene,29 save that the
boryl substituents do not adopt an anti arrangement because of
crystal packing effects; the angle between them amounts to
89.9°. However, both rings are virtually planar and can be
considered as η5- and η7-coordinated, respectively. The separa-
tion distances between the chromium center and the ring
centroids [Cr1-XCp, 1.831 Å; Cr1-XCht, 1.436 Å] lie within
the expected range and are almost identical to those found in 7
[Cr1-XCp, 1.819 Å; Cr1-XCht, 1.439 Å] and [Cr(η5-C5H5)(η7-
C7H7)] [Cr1-XCp, 1.830 Å; Cr1-XCht, 1.434 Å]. The absence
of molecular ring strain is further reflected by the tilt angle R
) 1.81(13)° and the angle δ ) 178.0°, both indicating an almost
ideal coplanar arrangement of the aromatic rings. Thus, in
agreement with the NMR spectroscopic data, the geometry of
the metallocene subunit exhibits only weak molecular distortion.
Electronic Structure: UV-Visible Spectroscopy. To obtain
information on the electronic structure of the substituted
trochrocene derivatives, solution UV-vis spectra in the range
of 200-800 nm were collected in thf for the species 9, 11, 13-
16, as well as for the parent compound [Cr(η5-C5H5)(η7-C7H7)],
and the prevoiusly reported ansa trochrocenophanes 1, 2, and 6
(Table 1). Unsubstituted trochrocene exhibits strong UV-
vis absorbances at 237 and 351 nm that can be assigned to a
ligand-to-metal charge-transfer transition and transitions arising
from the 1e2 molecular orbitals, respectively. In addition, a very
weak, broad visible band is observed at 559 nm that, in analogy
to ferrocene, is most probably related to excitations from the
Table 1. Experimental Determined UV-Visible Data and Calculated HOMO and LUMO Energies (eV)
1
2
6
9
11
12
13
14
15
16
Troa
λ
max (nm)
572
64
584
60
568
43
536
151
593
61
622
76
561
77
557
68
564
68
559
29
ꢀ (L mol-1 cm-1
)
R
E
E
calcd (deg)
HOMO (eV)
LUMO (eV)
10.0
-4.31
-0.43
3.89
16.2
-4.62
-0.51
4.11
3.3
24.8
15.1
-4.58
-0.45
4.13
11.9
4.48
-0.47
4.02
0.7
1.6
1.3
1.3
0.0
-4.59
-0.49
4.09
-4.41
-0.57
3.84
-4.61
-0.69
3.92
-4.62
-0.50
4.11
-4.5
-0.44
4.14
-4.56
-0.43
4.13
-4.66
-0.39
4.27
∆EHOMO-LUMO (eV)
a Tro ) [Cr(η5-C5H5)(η7-C7H7)].
9
8900 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

Ansa [1]Trochrocenophanes and Their Counterparts
A R T I C L E S
Table 2. Selected Calculated and Experimentally Determined (in Italics) NMR Chemical Shifts in ppm
1a
2b
6b
9
11
12
13
14
15
16
ipso-C5H4
ipso-C7H6
87.14
n. o.c
94.26
n. o.c
3.63
4.05
5.15
5.78
10.0
52.19
51.66
60.02
60.10
3.47
3.70
4.92
5.14
16.2
82.95
81.40
91.01
91.40
3.88
4.04
5.82
5.89
3.3
66.07
n. o.c
76.07
n. o.c
3.27
3.51
4.46
4.68
24.8
52.83
54.49
60.36
61.35
3.51
3.78
4.99
5.24
15.1
67.45
70.36
76,45
77.43
3.38
3.68
5.60
5.60
11.9
79.14
n. o.c
92.71
n. o.c
4.19
4.44
5.90
6.01
0.7
80.43
78.22
88.76
90.31
3.51
3.74
5.52
5.61
1.6
82.60
81.90
93.35
94.58
3.45
3.74
5.45
5.58
1.3
81.92
76.61
96.43
93.49
3.32
3.70
5.29
5.51
1.3
d
R C5H4
d
R C7H6
R (deg)
δ (deg)
9.0(8)
173.0
173.1
15.6(1)
168.7
167.5
2.6(2)
177.6
177.2
23.87(13)
162.5
162.0
15.07(17)
169.1
168.3
1.81(13)
179.2
178.0
171.3
178.9
179.1
179.1
a Experimental values see ref 3h. b Experimental values see ref 27. c Not observed (n. o.). d Average value.
1a1 molecular orbital (HOMO), which are predominantely d-d
in nature and Laporte forbidden. The two highest energy bands
are detected in the UV-vis spectra of all substituted complexes
in an almost identical region and hence, do not reveal any
differences in the electronic structure of the strained and
unstrained species with respect to trochrocene. However, the
corresponding lowest energy bands significantly differ from the
absorbance of trochrocene depending on the molecular distor-
tion, and the tilt angles in particular. Thus, the unstrained 1,1′-
disubstituted complexes 14-16 containing silicon, germanium,
and tin substitutents exhibit visible bands at 561, 557, and
564 nm that resemble the appropriate absorbance found in [Cr-
(η5-C5H5)(η7-C7H7)]. The increased molecular ring strain present
in the [n]sila- (n ) 1, 2; n ) 2, 6) and [1]germatrochro-
cenophanes (11) results in a significant red-shift of these visible
bands to values of 584, 568, and 593 nm, respectively. A similiar
trend has been reported for the corresponding [n]ferro-
cenophanes and has been explained by a lowering of the
HOMO-LUMO energy difference as the tilt angle increases.5e
By contrast, the species bearing boron substitutents deviate
notedly from this trend; in fact, they even exhibit a reverse
correlation. Thus, the unstrained derivative 13 displays a very
broad, conspicuously red-shifted visible band with a maximum
at 622 nm, whereas the absorbance of the highly strained [1]-
boratrochrocenophane 9 appears actually blue-shifted at 536 nm
compared to [Cr(η5-C5H5)(η7-C7H7)]. A related behavior was
observed in the UV-vis spectra of the corresponding [1]-
boraferrocenophanes, even though the effect was much less
pronounced. In this case, the smaller red-shift of these species
with respect to the less strained sulfur bridged [1]ferrocenophane
was attributed to the influence of subtle changes in the electronic
surrounding of the bridging atom, that is, the presence of a B-N
double bond.3d Hence, the electronic structure of substituted
trochrocenes strongly depends both on the geometric arrange-
ment as well as on the substitution pattern and the electronic
environment of the ligands.
which have not yet been investigated by X-ray diffraction, can
be predicted reliably, invoking the results of the calculations.
A comparison of selected experimental and calculated bond
lengths and angles can be found in the Supporting Information.
The molecular distortion of these complexes is emphasized by
the analysis of the Wiberg bond indeces (WBI) of the Cr-C
bonds (see Supporting Information). In accordance with a
contraction of the Cr-C bond distances to the ipso carbon atoms
of the aromatic carbocylces found in the substituted trochrocene
species, the corresponding WBI enlarge as the tilt angle
increases. However, the alterations for the chromium ipso carbon
bonds to the seven-membered ring are more significant than
those to the five-membered ring. Thus, the value of the former
in the highly strained derivative 9 is about 11% larger than the
value found in [Cr(η5-C5H5)(η7-C7H7)], whereas the latter
increases only about 5%. This effect is attributable to the closer
arrangement of the cycloheptatrienyl moiety and the chromium
center and, consequently, the larger deformation of the C7H6
ring with respect to the C5H4 ring. A similiar correlation is
observed within the WBI of the Cr-CR bonds, where the
dependency on the tilt angle is much less pronounced in this
case. In contrast, the calculated values for the Cr-Câ bonds
exhibit a complementary trend for all investigated species and
remain almost unaffected by the degree of molecular distortion
for the Cr-Cγ bonds; the average WBI for the former in 9 is
about 6% smaller for the Cht-ring and 4% for the Cp-ring
compared to the values found in trochrocene. The quality of
the DFT calculations is further supported by the calculated NMR
spectroscopic chemical shifts that reflect preeminently the most
prominent features associated with the molecular distortion of
the substituted trochrocene derivatives (Table 2). Thus, for
instance, the highfield shift of the ipso carbon atoms of both
aromatic ring moieties as well as of the R C7H6 protons as a
function of the tilt angle is very well reproduced.
To improve the understanding of the experimentally deter-
mined UV-vis spectra, we examined the molecular orbitals of
trochrocene and of the substituted derivatives in more detail.
In accordance with earlier experimental37 and theoretical
investigations,38 the ground-state electronic configuration of the
18-electron sandwich complex [Cr(η5-C5H5)(η7-C7H7)] is (1e1)4-
(2e1)4(e2)4(a1)2 (assuming infinite axes of rotation for both rings,
e.g., C2V). The first two virtual molecular orbitals are almost
Electronic structure: DFT Calculations. To assess the
differences in the electronic structure of strained and unstrained
trochrocene derivatives and, hence, to obtain deeper insight into
the relationship between the structural, spectroscopic, and
electronic properties, DFT calculations were performed on 1,
2, 6, 9, 11-16 and [Cr(η5-C5H5)(η7-C7H7)]. The geometries of
all complexes were optimized without symmetry constraints,
and the structural parameters obtained showed fairly good
agreement with the experimentally determined values available.
Hence, the molecular structure of the species 12 and 14-16,
(37) Evans, S.; Green, J. C.; Jackson, S. E.; Higgins, B. J. Chem. Soc., Dalton
Trans. 1974, 304-311.
(38) (a) Clack, D. W.; Warren, K. D. J. Organomet. Chem. 1978, 152, C60-
C62. (b) Menconi, G.; Kaltsoyannis, N. Organometallics 2005, 24, 1189-
1197.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8901

A R T I C L E S
Braunschweig et al.
Figure 6. Contour plots and eigenvalues (eV) of selected molecular orbitals in [Cr(η5-C5H5)(η7-C7H7)] (left), 15 (middle), and 11 (right).
degnerate and both exhibit strong contributions from the
chromium atomic orbitals (35% each) and the C7H7 molecular
orbitals (50% each). The HOMO is essentially chromium
localized (87%) with only minor contributions from the a1 C7H7
MOs (10%) and hence, can be described as principally metal d
in character. The next two levels are almost degenerate and arise
from δ bonding between the chromium center and the cyclo-
heptatrienyl ring. These molecular orbitals feature a significant
degree of metal-ring covalency, which is reflected by a
substantial contribution from both chromium (48% each) and
the e2 C7H7 MOs (48% each). On the contrary, the interaction
of the metal center with the cyclopentadienyl ring is mainly
ionic in nature. The degenerate molecular orbitals HOMO-3 and
HOMO-4 are made up predominantely of the e1 orbitals of the
Cp-moiety (81% each) and thus demonstrate π bonding of the
chromium and the cyclopentadienyl ring. Hence, the electronic
structure of trochrocene appears to be very similiar to that found
for the related titanium complex, [Ti(η5-C5H5)(η7-C7H7)], save
that the HOMO of trochrocne refers to the LUMO of the 16-
electron troticene complex.16a In principle, the introduction of
substituents to form the 1,1′-disubstituted species 13-16 and
the ansa-bridged complexes 1, 2, 6, 9, 11, and 12 does not
comprise the shape of the frontier orbitals described above
(exceptions will be discussed later). Contour plots of these
valence molecular orbitals together with their eigenvalues for
trochrocene as well as representative examples of both a 1,1′-
disubstituted complex (15) and a strained trochrocenophane (11)
are given in Figure 6. However, the fragment contribution of
the metal center, the five-membered ring and the seven-
membered ring to these orbitals deviates to some extent from
the values found in the unsubstituted [Cr(η5-C5H5)(η7-C7H7)].
In fact, a closer inspection of the contribution reveals a
significant correlation of the differences as a function of the
tilt angle within comparable occupied molecular orbitals.
Fragments with a strong contribution loose impact on the
correponding MOs as the molecular distortion increases, whereas
the contribution of the other fragments become more important;
however, the alterations are small and lie within a range of
1-9% (see Supporting Information). 12 seems to possess a
special character that can be attributed to the presence of two
additional aromatic aryl groups at the tin center. Consequently,
both the sequence of the occupied molecular orbitals as well as
that of the unoccupied orbitals changes substantially in com-
parison to the other species, and several ligand centered MOs
appear within these frontier orbitals. Owing to the substitution
of the two rings and the distortion of the sandwich structure,
the energy position of the occupied molecular orbitals changes
and the former isoenergetic MOs have given up their degen-
eracy. As a consequence, the HOMO-LUMO gap slightly
decreases in all of the substituted species in comparison to [Cr-
(η5-C5H5)(η7-C7H7)] (Table 1). Similar observations have been
reported for strained [1]ferrocenophanes. The red-shift of these
compounds in the UV-vis spectra has been explained by a
HOMO-LUMO transition that is lower in energy because of
the decreased HOMO-LUMO-gap, which is strongly dependent
on the molecular distortion.5e Complementary, an increase of
the HOMO-LUMO gap has been offered in the explanation
of the observed blue-shift in the visible spectra of strained [1]-
9
8902 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

Ansa [1]Trochrocenophanes and Their Counterparts
A R T I C L E S
the molecular orbitals involved in the transitions as well as the
appropriate wavelengths are summarized in Table 3. According
to the calculations, the absorbances observed in the UV-vis
experiments consist of four distinct excitations with several
orbital transitions involved in conjunction, whereas all of them
feature only small transition probabilities, which is in agreement
with the very weak intensities and the very broad shape of the
experimentally determined visible bands. In addition, the lower
energy maxima of the experiment, and consequently the
correlations of λmax as a function of the molecular distortion
for both the boron containing and the boron-free species, are
fairly well reproduced by the calculations. Hence, the lowest
energy electronic excitation of substituted metallocenes observed
in the UV-vis spectra is much more complicated than a simple
HOMO-LUMO transition; on the contrary, a large number of
transitions to a variety of LUMOs seems to be involved in these
processes.
Figure 7. Contribution of the B-N π system to the LUMO (bottom) and
the LUMO+3 (top) in 1, 9, and 13.
Summary and Conclusions
silatroticenophane with respect to unsubstituted troticene; these
findings are in agreement with the results obtained for strained
[n]ferrocenophanes considering the different electronic ground-
state configurations of these 16- and 18-electron species,
respectively (vide supra).17a In our hands, this simple explanation
cannot be applied; the calculated energies for the HOMO-
LUMO transitions do not show any correlation to the amount
of tilting present in the molecular structure of the substituted
trochrocene derivatives. In fact, the magnitude of the HOMO-
LUMO energy gap seems to be strongly related to the fragment
contribution of the ligand to the LUMO, which is not a function
of the molecular distortion (Similiar results were obtained at
the HF and the MP2 level of theory).
Nevertheless, the HOMO-LUMO energy differences for the
trochrocene series allow for the separation of the species into
three different groups, that is, the boron containing complexes
(∆E ) 3.84-3.92 eV), the compounds without boron (∆E )
4.09-4.13 eV), and the [1]stannatrochrocenophane 12 (∆E )
4.02 eV) (vide supra). Examination of the ten lowest lying
unoccupied molecular orbitals of 1, 9, and 13 showed that the
orbital sequence of the LUMOs in the boron containing species
fundamentally changed in comparison to the other complexes.
Hence, as depicted in Figure 7, the LUMO and the LUMO+3
of 1, 9, and 13 show significant contribution of the B-N π
bond (LUMO, 30, 14, and 15%; LUMO+3, 21, 13, and 27%),
whereas the fragment contribution of the substituents within the
boron-free complexes is negligible in orbitals lower in energy
than the LUMO+5. Thus, the complementary behavior of these
compounds in the UV-vis spectra is associated to the strong
electronic influence of the boron substituents and the interaction
with the B-N π-system in particular. Nevertheless, the UV-
vis spectoscopic data of the lowest energy band cannot be
explained in detail so far, because the HOMO-LUMO transition
criterion is not appropriate in the case of the substituted
trochrocene derivatives. To reveal reasons for the failure of this
criterion and to determine the factors that are associated with
the electronic excitations in the visible area, we determined the
first eight excitations of all complexes employing time-depend-
ent density functional theory. In all of the cases, the first four
calculated excitations are transitions arising from the HOMO
with transition wavelengths between 527 and 641 nm, all of
which can be assigned to the lowest energy band in the
corresponding UV-vis spectra. The excitations together with
In this paper, the derivatization of [Cr(η5-C5H5)(η7-C7H7)]
(trochrocene) via metalation and subsequent salt elimination
reaction with element dihalides has been described in detail.
To extend the chemistry of this sandwich complex, a more
convenient preparative protocol has been developed that allows
for the synthesis of gram quantities of [Cr(η5-C5H5)(η7-C7H7)].
t
The selectivity of its dimetalation employing BuLi/tmeda to
form the highly reactive species [Cr(η5-C5H4Li)(η7-C7H6Li)]‚
tmeda (8‚tmeda) was confirmed by X-ray diffraction of the thf
solvate [Cr(η5-C5H4Li)(η7-C7H6Li)]2‚(thf)8. Structural charac-
terization of metalated sandwich complexes is still a challenging
area owing to the high reactivity of these compounds, and hence,
the solution of the molecular structure of 8 contributes to the
understanding of the fundamentals that determine the conforma-
tion of this class of organometallic compounds. Starting from
this, novel [1]bora (9,10) and[1]germatrochrocenophanes (11)
have been synthesized and characterized, whereas the isolation
of the [1]stanna derivative 12 was hampered by its thermal
lability. X-ray diffraction studies of 9 and 11 revealed the
presence of strained, ring-tilted structures with tilt angles R
) 23.87(13)° and 15.07(17)°, respectively. The corresponding
unstrained 1,1′-disubstituted species 13-16 with broron, silicon,
germanium, and tin connected to both aromatic rings have been
prepared analogously; the molecular structure of 13 authenti-
cated the unstrained character. According to UV-vis experi-
ments of all compounds as well as of the previously described
strained derivatives 1, 2, and 6, the wave lengths of the lowest
energy band was strongly depending on the molecular distortion.
Within the boron-free species, these absorbances were found
red-shifted with respect to trochrocene as the tilt angle increased,
whereas the boron-containing derivatives showed a comple-
mentary correlation. DFT calculations revealed a strong frag-
ment contribution of the B-N double bond to the LUMO and
the LUMO+3, both involved in the excitation process. Hence,
the complementary trends appeared plausible considering the
strong electonic influence of the B-N π-system. Both correla-
tions were fairly well reproduced by time dependent DFT
calculations. Accordingly, the electronic excitation observed as
the lowest energy band in the UV-vis spectra is a quite
complicated process that is made up of four distinct excitations
consisting of several transitions and arising from the respective
HOMO.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8903

A R T I C L E S
Braunschweig et al.
Table 3. Calculated Excitations from the HOMOs and Corresponding Orbital Transitions and Transition Wave Lengths (nm)
1
2
6
9
11
12
13
14
15
16
Trob
first excitation
78f79
78f83
78f84
78f85
78f88
64f66
64f69
64f70
80f81
80f85
80f86
80f90
80f91
95f96
95f99
73f75
73f78
73f79
73f82
115f116
115f118
115f121
115f123
115f125
115f127
115f128
588.07a
127f128
127f129
127f131
127f134
127f135
127f136
89f90
89f93
89f94
89f95
107f108
107f111
107f112
107f113
79f80
79f83
79f85
49f53
49f55
λ1 (nm)
592.11
587.08a
599.3
621.42
587.45a
641.04a
605.45
596.09
599.97
563.31a
second excitation
78f79
78f82
78f83
78f85
78f86
64f68
64f71
80f84
80f87
80f88
95f97
95f98
95f100
95f102
73f77
73f80
115f117
115f124
115f126
127f128
127f131
127f132
127f133
127f134
127f135
127f136
584.36
89f90
89f93
89f94
89f96
107f108
107f111
107f112
107f113
107f114
79f80
79f83
79f84
79f86
79f87
49f54
49f56
λ2 (nm)
574.47a
580.96
570.37a
594.57
577.63
572.80
576.69
571.81
573.35a
562.02
third excitation
78f79
78f82
78f83
78f84
78f85
78f86
563.67
64f65
80f81
80f82
80f85
80f86
93f103
95f99
95f101
95f103
73f74
115f117
115f120
127f128
127f132
127f135
89f90
89f94
89f95
89f96
107f108
107f112
107f113
107f114
79f80
79f83
79f84
79f85
79f86
49f50
λ
3 (nm)
574.08
556.11
589.61
568.38
564.85
561.05
561.27a
555.91a
557.43a
536.13
fourth excitation
78f81
64f66
64f69
64f70
64f73
80f81
80f82
95f97
95f98
95f100
95f102
95f104
95f105
527.23a
527
73f75
73f76
73f78
73f79
73f82
115f116
115f118
115f119
115f123
115f128
127f129
127f131
89f91
89f92
107f109
107f110
79f81
79f82
49f51
λ
λ
4 (nm)
max (nm)
552.1
574
554.19
587
548.94
570
553.16
587
556.58
588
537.93
641
544.32
561
546.27
556
547.74
565
536.11
563
a Excitation(s) with the highest transition probability. b Tro ) [Cr(η5-C5H5)(η7-C7H7)].
Calculations were performed using DFT methods, applying the three
hybrid functional B3LYP43-45 using 6-31G(d, p) basis functions sets
for nonmetals and Stuttgart relativistic, small core ECP basis sets for
metals (see Supporting Information). NMR chemical shifts calculated
with the GIAO method were adjusted to TMS, Me4Sn, or diborane(6).
The latter was recalculated to the standard BF3‚Et2O scale. The Wiberg
bond indeces given in the text were obtained with the NBO 5 program.46
Illustrations of molecular orbitals were prepared with Molekel 4.3.47
Excitation energies were calculated using the TDDFT method included
in the Gaussian 03 package.
[Cr(η5-C5H5)(η7-C7H7)]. Solid CrCl3 (50.00 g, 316 mmol) was added
to a solution of NaCp (29.00 g, 330 mmol) in thf (200 mL), and the
reaction mixture was stirred at ambient temperature over a period of
24 h. Subsequently, excess Mg turnings (24.03 g, 1.00 mol) and
cycloheptatriene (50 mL, 482 mmol) were added, while stirring was
continued at room temperature for an additional 48 h. During this time,
the color of the suspension turned to deep blue. Excess magnesium
was separated by decantation; all volatiles were removed under reduced
pressure, and the resulting sticky solid was carefully poured into a
mixture of acetone/H2O (1:1) under air exposure. The mixture was
refluxed for 1 h, accompanied by the formation of a green solution
and the precipitation of an off-white solid. The solid was filtered and
treated again with refluxing acetone/H2O. This procedure was repeated
until the filtrate remained colorless. The aqueous fractions were
Experimental Section
General. All manipulations were performed under an inert atmo-
sphere of dry argon using standard Schlenk techniques or in a glovebox.
Solvents were dried according to standard procedures, freshly distilled
prior to use, degassed, and stored under argon over activated molecular
sieves. Deuterated solvents were distilled from potassium over a glass
bridge and subjected to several freeze-pump-thaw cycles. NaCp,39
Cl2BN(SiMe3)2,40 Cl2BN(SiMe3)(tBu),40 Cl2BNiPr2,40 and Mes2SnCl2
41
were prepared according to known methods. Cl2GeMe2, Me3SiCl, Me3-
GeCl, and Me3SnCl were obtained from Aldrich and distilled from
magnesium turnings or either sublimed before use, respectively. tBuLi
was purchased from Acros as 1.5 mol L-1 solutions in pentane. All
other chemicals were obtained commercially and were used without
further purification. tmeda was dried over K and distilled under argon
prior to use. The NMR spectra were recorded on a Bruker DRX 300
(7Li, 116.64 MHz) and on a Bruker AV 500 (1H, 500.13 MHz; 11B,
160.46 MHz; 13C, 125.76 MHz; 119Sn, 186.51 MHz) FT-NMR
spectrometer. 1H and 13C{1H} NMR spectra were referenced to external
TMS via the residual protio of the solvent (1H) or the solvent itself
7
(13C). Li{1H} NMR spectra were referenced to external LiCl, 11B-
{1H} NMR spectra to BF3‚OEt2, 29Si{1H} NMR spectra to external
TMS, and 119Sn{1H} NMR spectra to external Me4Sn. Microanalyses
for C, H, and N were performed on a Leco CHNS-932 elemental
analyzer. UV-vis spectra were recorded on a Shimadzu UV Mini
1240 UV-vis photometer.
(42) Frisch, M. J.; et al. Gaussian 03, revision B.04; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(43) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
All SCF computations reported herein were carried out using the
Gaussian 03 package running on a cluster of Linux workstations.42
(44) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-1211.
(45) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(46) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO5.0; Theoretical
Institute, Univeristy of Wisconsin: Madison WI, 2001.
(39) Panda, T. K.; Gamer, M. T.; Roesky, P. W. Organometallics 2003, 22,
877-878.
(40) Haubold, W.; Kraatz, U. Z. Anorg. Allg. Chem. 1976, 421, 105-110.
(41) Brown, P.; Mahon, M. F.; Molloy, K. C. J. Organomet. Chem. 1992, 435,
265-273.
(47) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber J. MOLEKEL 4.0; Swiss
Center for Scientific Computing: Manno, Switzerland, 2000.
9
8904 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

Ansa [1]Trochrocenophanes and Their Counterparts
A R T I C L E S
combined and concentrated by evaporation at elevated temperature to
about 300 mL yielding a bright-green solution of the trochrocene cation.
The aqueous solution was layered with toluene (300 mL) and degassed
with purified argon. The addition of Na2S2O4 (50 g) and KOH (50 g)
under vigorous stirring resulted in an immediate color change of the
organic layer to deep blue. The toluene fraction containing the air-
sensitive product was separated, and after insolubilities were separated
by filtration the solvent was removed in vacuo. The residue was
sublimed at 100 °C and 10-3 mbar to obtain 26.02 g (125 mmol, 40%)
of [Cr(η5-C5H5)(η7-C7H7)] as a crystalline blue solid. 1H NMR
(500 MHz, C6D6, 297 K): δ ) 3.66 (s, 5H, C5H5), 5.45 (s, 7H, C7H7).
13C{1H} NMR (126 MHz, C6D6, 297 K): δ ) 75.37 (C5H5), 87.09
(C7H7). λmax (ꢀ) ) 559 nm (29 L mol-1 cm-1). Full characterization of
trochrocene, including a crystal structure determination29 and an accurate
elemental analysis,28c has been reported previously.
C5H4), 4.68 (m, 2H, C7H6), 5.51 (m, 2H, C7H6), 5.89 (m, 2H, C7H6).
11B{1H} NMR (160 MHz, C6D6, 297 K): δ ) 49.9. 13C{1H} NMR
(126 MHz, C6D6, 297 K): δ ) 5.43, 5.56 (SiMe3), 74.26, 79.67, 86.81,
91.50, 101.89 (all CarylH). λmax (ꢀ) ) 536 nm (151 L mol-1 cm-1).
Anal. Calcd (%) for C18H28BCrNSi2 (377.40): C, 57.29; H, 7.48; N,
3.71. Found: C, 57.58; H, 7.20; N, 3.33.
[Cr(η5-C5H4)-BN(SiMe3)(tBu)-(η7-C7H6)], 10. The reaction was
analogously performed to that described for 9, using 8‚tmeda (0.82 g,
2.44 mmol) in heptane (15 mL) and Cl2BN(SiMe3)(tBu) (0.51 g,
2.44 mmol) in heptane (5 mL). During the reaction the color of the
mixture changed to deep brown and a white solid precipitated.
Insolubilities were removed by filtration, and the brown filtrate was
concentrated to about 4 mL in volume. Storage at -30 °C yielded a
deep-brown crystalline solid (0.62 g, 1.71 mmol, 70%), which was
isolated after washing with cold pentane (3 × 3 mL, -100 °C) and
drying in vacuo. Compound 10 exists as a 1:1 mixture of two isomers.
1H NMR (500 MHz, C6D6, 297 K): δ ) 0.42 (s, 9H, SiMe3), 0.73 (s,
9H, SiMe3), 1.53 (s, 9H, CMe3), 1.82 (s, 9H, CMe3), 3.51 (m, 4H, C5H4),
3.56 (m, 2H, C5H4), 3.59 (m, 2H, C5H4), 4.72 (m, 4H, C7H6), 5.52 (m,
4H, C7H6), 5.91 (m, 2H, C7H6). 11B{1H} NMR (160 MHz, C6D6,
297 K): δ ) 46.6. 13C{1H} NMR (126 MHz, C6D6, 297 K): δ )
6.46, 6.61 (SiMe3), 33.50, 33.53 (CMe3), 56.58, 57.16 (CMe3), 72.53,
72.58, 77.89, 78.97, 85.27, 85.36, 89.97, 90.80, 100.35, 100.49 (all
[Cr(η5-C5H4Ph)(η7-C7H7)], 7. Compound 7 was isolated as a side-
product formed during the synthesis of [Cr(η5-C5H5)(η7-C7H7)] fol-
lowing a slight modification of the original procedure reported by
E. O. Fischer.28 According to this, trochrocene was prepared from CrCl3
(20.00 g, 126 mmol), without isolating the intermediate [Cr(η5-C5H5)-
(η6-C6H6)]. Instead, the crude reaction was treated directly with AlCl3
and cycloheptatriene to yield the trochrocene cation after hydrolysis.
In addition to deep-blue crystalline trochrocene, the sublimation of the
residue, obtained after the reduction of the cation with Na2S2O4, at
100 °C and 10-3 mbar gave green 7 right above the heating source,
which was separated mechanically from the blue main product.
Pruification of crude 7 was achieved by columm chromatography (Alox
III, hexane) and three recrystallization steps from hexane at -60 °C.
The crystals that separated were washed with cold pentane (3 × 2 mL,
-80 °C) and dried in vacuo to afford 7 as a green solid (0.56 g,
CarylH). Anal. Calcd (%) for C19H28BCrNSi (361.33): C, 63.16; H, 7.81;
N, 3.88. Found: C, 63.17; H, 7.67; N, 4.23.
[Cr(η5-C5H4)-GeMe2-(η7-C7H6)], 11. A solution of Cl2GeMe2
(0.26 g, 1.49 mmol) in toluene (5 mL) was added over a period of
30 min to a well-stirred slurry of 8‚tmeda (0.50 g, 1.49 mmol) in toluene
(20 mL) at -78 °C. The deep-blue mixture was allowed to warm to
room temperature and was stirred for an additional 24 h. After the solid
had settled, the solution was filtered into another flask by a filter canula
and all volatiles were removed under reduced pressure. The residue
was extracted over a period of 48 h with heptane. In turn, insolubilities
were separated by filtration, and 11 was isolated as a dark blue solid
by crystallization at -30 °C (0.28 g, 0.89 mmol, 60%). Crystals suitable
for crystal structure analysis were obtained by recrystallization from
1
1.97 mmol, 2%). H NMR (500 MHz, C6D6, 297 K): δ ) 3.75 (m,
2H, C5H4), 4.17 (m, 2H, C5H4), 5.33 (s, 7H, C7H7), 7.35 (m, 5H, Ph).
13C{1H} NMR (126 MHz, C6D6, 297 K): δ ) 73.71 (C5H4), 75.26
(ipso-C5H4), 76.60 (C5H4), 88.23 (C7H7), 125.68, 126.02 (Ph), 128.23
(ipso-Ph), 128.63 (Ph). Anal. Calcd (%) for C18H16Cr (284.32): C,
76.04; H, 5.67. Found: C, 75.67; H, 5.81.
[Cr(η5-C5H4Li)(η7-C7H6Li)]‚(thf)3, 8‚(thf)3. Recrystallization of the
dilithiated complex [Cr(η5-C5H4Li)(η7-C7H6Li)]‚tmeda (8‚tmeda)
(0.10 g, 0.30 mmol) from a saturated thf solution (1 mL) afforded 8‚
(thf)3 as pale-brown crystals that were washed with cold pentane (2 ×
1 mL) and dried in vacuo (0.09 g, 0.21 mmol, 70%). 1H NMR
(500 MHz, [D8]THF, 297 K): δ ) 1.75 (m, 12H, CH2), 3.60 (m, 2H,
C5H4), 3.85 (m, 2H, C5H4), 3.89 (m, 12H, OCH2), 5.25 (m, 2H, C7H6),
1
thf at -80 °C. H NMR (500 MHz, C6D6, 297 K): δ ) 0.69 (s, 6H,
CH3), 3.69 (m, 2H, C5H4), 3.78 (m, 2H, C5H4), 5.24 (m, 2H, C7H6),
5.69 (m, 2H, C7H6), 5.83 (m, 2H, C7H6). 13C{1H} NMR (126 MHz,
C6D6, 297 K): δ ) 0.00 (CH3), 54.49 (ipso-C5H4), 61.35 (ipso-C7H6),
79.91, 83.30, 88.96, 94.97, 102.14 (all CarylH). λmax (ꢀ) ) 593 nm (61
L mol-1 cm-1). Anal. Calcd (%) for C14H16CrGe (308.88): C, 54.44;
H, 5.22. Found: C, 54.17; H, 5.01.
7
5.33 (m, 2H, C7H6), 5.80 (m, 2H, C7H6). Li{1H} NMR (117 MHz,
Attempted synthesis of [Cr(η5-C5H4)-SnMes2-(η7-C7H6)], 12. The
synthesis of 12 was attempted under various conditions. For instance,
a suspension of 8‚tmeda (0.50 g, 1.49 mmol) in hexane (10 mL) was
treated dropwise with a solution of Cl2SnMes2 (0.64 g, 1.49 mmol) in
thf (20 mL) at -78 °C. Upon the slurry being warmed to ambient
temperature the color changed to blue-brown and an off-white solid
deposited. After the slurry was stirred for a further 12 h, all volatiles
were removed in vacuo, the residue was extracted with Et2O, and
insolubilities were separated by filtration. The dark blue filtrate was
concentrated to about 7 mL in volume and stored at -30 °C in the
freezer. After several days, a small crop of a brownish, crystalline
material separated, which was subsequently isolated by decantation and
drying under high vacuum. This material turned out to be pure 12
(0.01 g, 17.75 µmol, 1%). However, the isolation of additional 12 failed
in all cases, and the materials isolated instead could not be identified.
1H NMR (500 MHz, C6D6, 297 K): δ ) 2.12 (s, 6H, CH3), 2.86 (s,
12H, CH3), 3.68 (m, 2H, C5H4), 3.89 (m, 2H, C5H4), 5.60 (m, 2H,
C7H6), 5.67 (m, 4H, C7H6), 6.91 (s, 4H, C6H2Me3). 13C{1H} NMR (126
MHz, C6D6, 297 K): δ ) 21.10, 25.34 (CH3), 70.36 (ipso-C5H4), 77.43
(ipso-C7H6), 82.91, 86.77, 95.02, 98.72 (all CarylH), 129.22, 137.32,
139.45, 145.57 (C6H2Me3). 29Sn{1H} NMR (187 MHz, C6D6, 297 K):
δ ) -61.92.
[D8]THF, 297 K): δ ) 2.40. 13C{1H} NMR (126 MHz, [D8]THF, 297
K): δ ) 25.32 (CH2), 68.04 (OCH2), 78.78, 85.73, 86.35, 91.45, 97.79
(all CarylH). Anal. Calcd (%) for C24H34CrLi2O3 (436.40): C, 66.05,
H, 7.85. Found: C, 64.95; H, 7.01. X-ray quality crystals of the thf
solvate [Cr(η5-C5H4Li)(η7-C7H6Li)]2‚(thf)8 showed additional thf mol-
ecules incorporated into the crystal lattice and were obtained during
the recrystallization procedure described above.
[Cr(η5-C5H4)-BN(SiMe3)2-(η7-C7H6)], 9. A slurry of 8‚tmeda
(0.75 g, 2.23 mmol) in hexane (10 mL) was cooled to -78 °C and
treated dropwise over a period of 2 h with a solution of Cl2BN(SiMe3)2
(0.54 g, 2.23 mmol) in hexane (5 mL). After complete addition, the
reaction mixture was stirred for an additional 5 h at -78 °C and
subsequently allowed to reach ambient temperature. During this time,
the color of the suspension turned from pale brown to intensive brown
and a white precipitate deposited. After the solid had settled, the solution
was filtered into another flask by a filter canula, and the filtrate was
concentrated to about 3 mL in volume. Cooling to -60 °C afforded 9
as a dark brown solid, which was obtained analytically pure after
washing with cold pentane (3 × 3 mL, -100 °C) and drying in vacuo
(0.53 g, 1.41 mmol, 63%). Crystals were grown from saturated hexane
solutions at -25 °C. 1H NMR (500 MHz, C6D6, 297 K): δ ) 0.37 (s,
9H, SiMe3), 0.66 (s, 9H, SiMe3), 3.51 (m, 2H, C5H4), 3.55 (m, 2H,
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8905

A R T I C L E S
Braunschweig et al.
[Cr{η5-C5H4B(Cl)NiPr2}{η7-C7H6B(Cl)NiPr2}], 13. A suspension
of 8‚tmeda (0.60 g, 1.78 mmol) in hexane (10 mL) was added rapidly
to a well-stirred solution of Cl2BNiPr2 (0.81 g, 4.46 mmol, 2.5 equiv)
in hexane (10 mL) at room temperature. Upon stirring for 18 h the
color of the slurry changed from brown to deep green. After the solid
had settled, the solution was filtered into another flask by a filter canula,
and the filtrate was concentrated to about 10 mL in volume to yield
greenish crystals of 13 (0.62 g, 1.25 mmol, 70%) upon cooling to
-35 °C, which were subsequently washed with cold pentane (3 ×
2 mL, -100 °C) and dried in vacuo. X-ray quality crystals were grown
from saturated heptane solutions at -25 °C. 1H NMR (500 MHz, C6D6,
297 K): δ ) 0.80 (d, 3J(H, H) ) 6.70 Hz, 6H, Me), 0.96 (d, 3J(H, H)
structures were solved using direct methods, refined with the Shelx
software package (Sheldrick, G. Shelx; University of Go¨ttingen:
Go¨ttingen, Germany, 1997) and expanded using Fourier techniques.
All non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were assigned idealized positions and were included in structure factor
calculations. The crystals of 7 were nonmerohedrical twins arising from
180° rotation about the reciprocal axis 001. Integration was performed
for two domains and an absorption correction was done with the twinabs
program. Refinement against HKLF5 formated F2 gave BASF ) 0.14.
Crystal Data for 7. C18H16Cr, Mr ) 284.31, green plate, 0.18 ×
0.11 × 0.03 Å3, monoclinic space group P21, a ) 10.7402(8) Å, b )
7.7821(6) Å, c ) 15.8286(11) Å, â ) 97.715(4)°, V ) 1311.00(17)
Å,3 Z ) 4, Fcalcd ) 1.440 g‚cm-3, µ ) 0.853 mm-1, F(000) ) 592, T
) 100(2) K, R1 ) 0.0336, wR2 ) 0.0879, 3943 independent reflections
[2θ e 52.14°], and 344 parameters.
3
) 6.70 Hz, 6H, Me), 1.59 (d, J(H, H) ) 6.63 Hz, 6H, Me), 1.60 (d,
3J(H, H) ) 6.63 Hz, 6H, Me), 3.24 (m, 2H, CHMe2), 4.05 (m, 1H,
CHMe2), 4.27 (m, 1H, CHMe2), 4.34 (m, 2H, C5H4), 4.44 (m, 2H,
C5H4), 5.79 (m, 4H, C7H6), 6.01 (m, 2H, C7H6). 11B{1H} NMR (160
MHz, C6D6, 297 K): δ ) 36.6, 39.1. 13C{1H} NMR (126 MHz, C6D6,
297 K): δ ) 21.48, 21.74, 23.70, 24.32 (Me), 46.53, 46.67, 50.89,
52.06 (CHMe2), 81.26, 84.05, 88.06, 88.90, 91.51 (all CarylH). λmax (ꢀ)
) 622 nm (76 L mol-1 cm-1). Anal. Calcd (%) for C24H38B2Cl2CrN2
(499.10): C, 57.76; H, 7.67; N, 5.61. Found: C, 58.05; H, 7.44; N,
5.83.
Crystal Data for 82‚(thf)8. C56H84Cr2Li4O8, Mr ) 1016.99, brown
block, 0.20 × 0.13 × 0.06 Å3, monoclinic space group P21/n, a )
10.2885(3) Å, b ) 28.3264(9) Å, c ) 18.4643(6) Å, â ) 94.224(2)°,
V ) 5366.5(3) Å3, Z ) 4, Fcalcd ) 1.259 g‚cm-3, µ ) 0.457 mm-1
,
F(000) ) 2176, T ) 100(2) K, R1 ) 0.0675, wR2 ) 0.1837, 11106
independent reflections [2θ e 53.08°], and 782 parameters.
Crystal Data for 9. C18H28BCrNSi2, Mr ) 377.40, brown plate,
0.18×.011 × 0.02, triclinic space group Pjı, a ) 6.4925(3) Å, b )
9.8498(4) Å, c ) 15.5959(7) Å, R ) 104.538(2)°, â ) 99.734(2)°, γ
) 93.165(2)°, V ) 946.52(7) Å3, Z ) 2, Fcalcd ) 1.324 g‚cm-3, µ )
0.729 mm-1, F(000) ) 400, T ) 293(2) K, R1 ) 0.0580, wR2 ) 0.1099,
3691 independent reflections [2θ e 52.2°] and 208 parameters.
Crystal Data for 11. C14H16CrGe, Mr ) 308.86, green needle, 0.24
× 0.06 × 0.03 Å3, monoclinic space group P21/n, a ) 10.9050(6) Å,
b ) 7.8237(5) Å, c ) 14.4488(8) Å, â ) 92.614(3)°, V ) 1231.45(12)
Å,3 Z ) 4, Fcalcd ) 1.666 g‚cm-3, µ ) 3.291 mm-1, F(000) ) 624, T
) 100(2) K, R1 ) 0.0402, wR2 ) 0.0818, 2433 independent reflections
[2θ e 52.16°], and 146 parameters.
Crystal Data for 13. C24H38B2Cl2CrN2, Mr ) 499.08, green plate,
0.12 × 0.08 × 0.03 Å3, monoclinic space group P21, a ) 10.8739(2)
Å, b ) 14.2066(3) Å, c ) 16.5281(4) Å, â ) 99.7530(10)°, V )
2516.38(9) Å,3 Z ) 4, Fcalcd ) 1.317 g‚cm-3, µ ) 0.682 mm-1, F(000)
) 1056, T ) 100(2) K, R1 ) 0.0302, wR2 ) 0.0709, 9914 independent
reflections [2θ e 52.04°], and 559 parameters.
Crystallographic data are also deposited with Cambridge Crystal-
lographic Data Centre. Copies of the data [7-9, 11, 13: CCDC
642176-642180] can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, by e-mailing data_request@ccdc.cam.ac.uk,
or by contacting the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB 1EZ, U.K. Fax +44 1223 336033.
[Cr(η5-C5H4SiMe3)(η7-C7H6SiMe3)], 14. In a procedure analogous
to the preparation of 13, employing 8‚tmeda (0.50 g, 1.49 mmol) in
pentane (10 mL) and Me3SiCl (0.40 g, 3.72 mmol, 2.5 equiv) in pentane
(10 mL) afforded 14 (0.41 g, 1.16 mmol, 78%) as a dark-blue solid
after recrystallization and drying in vacuo. 1H NMR (500 MHz, C6D6,
297 K): δ ) 0.26 (s, 9H, SiMe3), 0.48 (s, 9H, SiMe3), 3.74 (m, 2H,
C5H4), 3.99 (m, 2H, C5H4), 5.61 (m, 4H, C7H6), 5.68 (m, 2H, C7H6).
13C{1H} NMR (126 MHz, C6D6, 297 K): δ ) 0.10, 0.36 (SiMe3), 78.22
(ipso-C5H4), 78.38, 79.69, 87.49, 88.46, 90.14 (all CarylH), 90.31 (ipso-
C7H6). 29Si{1H} NMR (99.4 MHz, C6D6, 297 K): δ ) -2.55, 6.31.
λmax (ꢀ) ) 561 nm (77 L mol-1 cm-1). Anal. Calcd (%) for C18H28-
CrSi2 (352.58): C, 61.32; H, 8.00. Found: C, 60.97; H, 7.88.
[Cr(η5-C5H4GeMe3)(η7-C7H6GeMe3)], 15. In a procedure analogous
to the preparation of 13, employing 8‚tmeda (0.40 g, 1.19 mmol) in
pentane (10 mL) and Me3GeCl (0.46 g, 2.97 mmol, 2.5 eq.) in pentane
(10 mL) afforded 15 (0.45 g, 1.02 mmol, 86%) as a dark-blue solid
after recrystallization and drying in vacuo. 1H NMR (500 MHz, C6D6,
297 K): δ ) 0.39 (s, 9H, GeMe3), 0.57 (s, 9H, GeMe3), 3.74 (m, 2H,
C5H4), 3.98 (m, 2H, C5H4), 5.58 (m, 4H, C7H6), 5.63 (m, 2H, C7H6).
13C{1H} NMR (126 MHz, C6D6, 297 K): δ ) 0.03, 0.20 (GeMe3),
78.28, 79.76 (CarylH), 81.90 (ipso-C5H4), 87.79, 88.69, 90.31 (CarylH),
94.58 (ipso-C7H6). λmax (ꢀ) ) 557 nm (68 L mol-1 cm-1). Anal. Calcd
(%) for C18H28CrGe2 (441.63): C, 48.95; H, 6.39. Found: C, 49.14;
H, 6.40.
[Cr(η5-C5H4SnMe3)(η7-C7H6SnMe3)], 16. In a procedure analogous
to the preparation of 13, employing 8‚tmeda (0.35 g, 1.04 mmol) in
hexane (10 mL) and Me3SnCl (0.52 g, 2.60 mmol, 2.5 eq.) in hexane
(15 mL) afforded 16 (0.50 g, 0.94 mmol, 90%) as an analytically pure,
dark-blue solid. 1H NMR (500 MHz, C6D6, 297 K): δ ) 0.32 (s, 2JHCSn
Acknowledgment. This work was supported by the DFG.
T.K. thanks the FCI for a stipend. We are grateful to the BASF
AG for a generous donation of chemicals.
Supporting Information Available: Complete ref 42; crystal-
lographic data in CIF format; tables of selected calculated and
experimental determined geometrical and NMR spectroscopic
parameters for all compounds; plots of the calculated WBIs of
the Cr-C bonds as a function of the tilt angle R; dependency
of the calculated fragment contributions of the chromium center,
the C5H4-ring, and the C7H6-ring to selected frontier molecular
orbitals as a function of the tilt angle R. This material is available
free of charge via the Internet at http://pubs.acs.org.
2
) 52.49 and 54.85 Hz, 9H, SnMe3), 0.45 (s, JHCSn ) 50.60 and
52.88 Hz, 9H, SnMe3), 3.70 (m, 2H, C5H4), 4.02 (m, 2H, C5H4), 5.51
(m, 2H, C7H6), 5.64 (m, 4H, C7H6). 13C{1H} NMR (126 MHz, C6D6,
297 K): δ ) -8.04, -7.57 (SnMe3), 76.61 (ipso-C5H4), 78.77, 81.71,
87.80, 89.49, 92.93 (all CarylH), 93.49 (ipso-C7H6). 119Sn{1H} NMR
(187 MHz, C6D6, 297 K): δ ) 0.81, 29.30. λmax (ꢀ) ) 564 nm (68 L
mol-1 cm-1). Anal. Calcd (%) for C18H28CrSi2 (533.83): C, 40.50; H,
5.29. Found: C, 40.63; H, 5.44.
Crystal Structure Determinations. The crystal data of 7, 8‚(thf)4,
9, 11, and 13 were collected on a Bruker Apex diffractometer with a
CCD area detector and graphite monochromated Mo KR radiation The
JA0724947
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8906 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007