New [2]Boraferrocenophane and Diferrocenyldiborane(4) Derivatives

Article abstract of DOI:10.1002/ejic.201000705

The new [2]boraferrocenophane derivative [Fe(η⁵-C ₅H₄)- B(Mes)-B(Mes)-(η⁵-C₅H ₄)] (2) was prepared by reaction of 1,1′-dilithioferrocene with B₂(Mes)2Cl₂. In addition, the diborane( 4) B ₂(NMe₂)2Fc₂ (3) was isolated from the reaction of B₂(NMe₂)2Cl₂ and monolithioferrocene. Both species were fully characterized including X-ray diffraction analyses. While B₂(NMe₂)2Fc₂ (3) adopts an unstrained geometry, [2]boraferrocenophane 2 features notable molecular ring strain manifested in the tilt-angle a = 10.5(2)°.

Full text of DOI:10.1002/ejic.201000705

SHORT COMMUNICATION
DOI: 10.1002/ejic.201000705
New [2]Boraferrocenophane and Diferrocenyldiborane(4) Derivatives
Holger Braunschweig,*[a] Alexander Damme,[a] and Thomas Kupfer[a]
Keywords: Sandwich complexes / Boron / Ferrocene / ansa-Complexes / [2]Boraferrocenophane / Diborane(4)
The new [2]boraferrocenophane derivative [Fe(η5-C5H4)–
B(Mes)–B(Mes)–(η5-C5H4)] (2) was prepared by reaction of
1,1Ј-dilithioferrocene with B2(Mes)2Cl2. In addition, the dibo-
rane(4) B2(NMe2)2Fc2 (3) was isolated from the reaction of
B2(NMe2)2Cl2 and monolithioferrocene. Both species were
fully characterized including X-ray diffraction analyses.
While B2(NMe2)2Fc2 (3) adopts an unstrained geometry,
[2]boraferrocenophane 2 features notable molecular ring
strain manifested in the tilt-angle a = 10.5(2)°.
zation of the reactive diboron-bridge via π-donation from
nitrogen. Along with these results, we were also able to syn-
thesize and fully characterize a related, symmetric dibo-
rane(4) derivative bearing two ferrocenyl substituents,
namely B2(NMe2)2Fc2 (3; Fc = (η5-C5H4)Fe(η5-C5H5)).
Introduction
Strained [n]metalloarenophanes have sparked tremen-
dous interest during the last decades due to their highly
interesting reactivity patterns and promising applications in
catalysis and materials science.[1] While the large molecular
ring strain present in [1]metalloarenophanes has been ex-
tensively exploited in the synthesis of transition metal con-
taining macromolecules, the preeminent example being
poly(ferrocenylsilanes),[1,2] strained [2]metalloarenophanes
have been shown to possess a copious reactivity at the E–E
bond of the ansa-bridge.[3] To date, diatomic ansa-bridges
have been realized with a variety of bridging elements such
as B,[3c,3d,4] C,[5] Si,[3f,4e,6] Ge,[7] Sn,[4f,8] and P.[9] With the
isolation of [Fe(η5-C5H4)–B(NMe2)–B(NMe2)–(η5-C5H4)]
(1) in 1997 Herberhold and Wrackmeyer successfully ac-
complished the incorporation of boron into a [2]ferroceno-
phane for the first time.[4a] During the last five years, ad-
ditional [2]borametalloarenophanes have appeared in the
literature derived from other sandwich systems containing
Results and Discussion
[2]Boraferrocenophane 2 was readily prepared by salt-
elimination reaction of 1,1Ј-dilithioferrocene, [Fc]-
Li2·tmeda, with equimolar amounts of 1,2-dichloro-1,2-di-
mesityldiborane(4), B2(Mes)2Cl2, in pentane. After work-
up, [Fe(η5-C5H4)–B(Mes)–B(Mes)–(η5-C5H4)] (2) was iso-
lated as a red solid in moderate yields of 37% (Scheme 1).
Scheme 1. Preparation of [Fe(η5-C5H4)–B(Mes)–B(Mes)–(η5-
Ti,[4f] V,[3d,4e] Cr,[3c,4b] and Mn[3c] as the transition metal C5H4)] (2).
center. In 2005 (and later on) we demonstrated that the B–
The solution structure, as determined by NMR spec-
B bond in [2]borametalloarenophanes is susceptible to oxi-
dative addition to low-valent transition metals such as plati-
num,[3c,3d,4f] a reactivity that has subsequently been em-
ployed in the stoichiometric and catalytic functionalization
of organic molecules such as alkynes, isocyanides or diazo
compounds.[3e,4e,10] However, the diborane component of
the ansa-species has been limited to the amino-substituted
B(NMe2)–B(NMe2) bridge so far. We now report on the
isolation of the new [2]ferrocenophane 2, [Fe(η5-C5H4)–
B(Mes)–B(Mes)–(η5-C5H4)], which does not require stabili-
troscopy, confirms the presence of a strained ansa complex.
The Cp ring protons show the characteristic splitting
1
pattern in the H NMR spectrum, and are detected as dis-
tinct pseudo triplets at δ = 3.70, 4.18, 4.36 and 4.89. The
signals of the mesityl groups appear at lower field with re-
spect to the starting material B2(Mes)2Cl2 (δ = 2.16 (p-
CH3), 2.67 (o-CH3) and 6.86 (CH)). The 11B NMR spec-
trum of 2 features one single resonance at δ = 66.0, which
is shifted to higher field by 21 ppm upon incorporation into
the ansa complex [B2(Mes)2Cl2 δ = 87.0],[11] but is found
down-field shifted by 22 ppm in comparison to the related
[2]boraferrocenophane 1 bearing dimethylamino groups at
boron.[4a]
[a] Institut für Anorganische Chemie, Julius-Maximilians-
Universität Würzburg,
Am Hubland, 97074 Würzburg, Germany
Fax: +49-931-31-84623
Single crystals of 2 suitable for X-ray diffraction analysis
were grown from benzene/pentane (1:3) at room tempera-
ture (Figure 1). 2 crystallizes in the monoclinic space group
View this journal online at
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000705.
wileyonlinelibrary.com
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H. Braunschweig, A. Damme, T. Kupfer
SHORT COMMUNICATION
C2/c. The B–B bond [1.698(4) Å] is shortened with respect trum (δ = 2.91, 2.97). As expected, the aromatic protons of
to the unstrained species 3 [1.725(3) Å; see below], which the unsubstituted Cp rings give rise to only one resonance
might be ascribed to the reduced π-bonding at the boron at δ = 4.05, while signals of the other Cp ring protons can-
center caused by the presence of nitrogen donors in 3. The not be detected separately, but feature a single multiplet at
B–C1 bond length [1.581(3) Å] is similar to those reported δ = 4.25.
previously for related [2]boraferrocenophanes and -metal-
The proposed constitution was ascertained in the solid
loarenophanes.[3,4] The strained character of 2 is readily de- state by crystal structure analysis on single crystals of 3
rived from the deviation of the Cp rings from the coplanar obtained by crystallization from pentane at –30 °C (Fig-
arrangement, which is usually expressed in terms of the tilt- ure 2). 3 crystallizes in the monoclinic space group P21/c.
angle α. Thus, 2 [α = 10.5(2)°] features comparable, but less The molecular structure combines typical features of di-
strongly pronounced molecular ring strain than the [2]bora- boranes(4) as well as unstrained metallocene derivatives.
ferrocenophane 1 [α = 12.8(2)°],[4d] which is further sup- Thus, the B1–B2 bond length [1.719(2) Å] is similar to that
ported by other geometrical parameters typically used to observed in B2(NMe2)2Ph2 [1.714(4) Å],[13] and the B–N
characterize ansa complexes[1d] such as angles β = 25.3°, γ bond lengths [1.405(2) and 1.401(2) Å] are found in the ex-
= 63.0(2)° and δ = 173.4° [cf. 1: α = 12.8(2)°, γ = 42.27(22)°, pected range as well. The torsion angle N1–B1–B2–N2
δ = 170.1°]. The smaller extent of tilting in 2 is also reflected [–95.61(17)°] is widened by 7° compared to that in
by its UV/Vis absorbance at 453 nm in benzene, which is B2(NMe2)2Ph2 (88.7°),[13] which is most likely a result of the
slightly blue-shifted compared to 3 (465 nm), but appears steric requirements of the bulkier ferrocenyl ligands. The Cp
red-shifted with respect to that of unstrained ferrocene rings are arranged almost parallel to each other, which is
(441 nm).[1d]
demonstrated by tilt-angles α = 2.39 and 3.62°, respectively.
These values are significantly smaller than those found for
the strained species 1[4d] and 2 (see above). The unstrained
character of 3 is also manifested by small values for the
following angles: βB1 = –1.64°, βB2 = –0.46°, δFe1 = 178.06°,
δFe2 = 177.29°.
Figure 1. Molecular structure of 2 in the solid state. Hydrogen
atoms are omitted for clarity. Symmetry-related positions (–x + y, –
z + 3/2) are labelled with Ј. Selected bond lengths [Å] and bond
angels [°]: B1–B1Ј 1.698(4), B1–C1 1.581(3), B1–C2 1.574(3), C1–
B1–B1Ј 107.20(12), C2–B1–B1Ј 129.50(16), C1–B1–B1Ј–C1Ј
63.0(2), C2–B1–B1Ј–C2Ј 99.7(3).
Figure 2. Molecular structure of 3 in the solid state. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and bond
angels [°]: B1–B2 1.719(2), B1–N1 1.405(2), B1–C1 1.575(2), B2–
N2 1.401(2), B2–C2 1.578(2), N1–B1–B2 121.44(13), C1–B1–B2
115.33(12), N2–B2–B1 121.62(13), C2–B2–B1 114.76(12), N1–B1–
B2–N2 –95.61(17).
Stoichiometric salt-elimination reaction of 1,2-bis(dime-
thylamino)-1,2-dichlorodiborane(4), B2(NMe2)2Cl2, with
two equivalents of ferrocenyllithium, FcLi, in toluene at
room temperature afforded the unstrained diborane(4) de-
rivative B2(NMe2)2Fc2 (3) as a red crystalline material in
poor yields (Scheme 2).
Conclusions
In this communication, we report on the successful isola-
tion of a new strained [2]boraferrocenophane, [Fe(η5-
C5H4)–B(Mes)–B(Mes)–(η5-C5H4)] (2) that lacks any stabi-
lization of the electrophilic boron centers by nitrogen donor
substituents. The crystal structure determination confirms
the presence of a strained ansa complex, supported by a
Scheme 2. Synthesis of B2(NMe2)2Fc2 (3).
3 was unambiguously identified by NMR spectroscopy, tilt-angle α = 10.5(2)°. A related, albeit unstrained species,
which is in agreement with the presence of a C2-symmetric was isolated via salt elimination of B2(NMe2)2Cl2 with two
species in solution. The 11B NMR spectrum shows one sig- equivalents of ferrocenyllithium, namely B2(NMe2)2Fc2 (3).
nal at δ = 47.7, which appears shifted by 10 ppm to lower The unstrained character was unequivocally validated by X-
field with respect to the starting material B2(NMe2)2Cl2 (δ ray diffraction analysis. The suitability of the two dibo-
= 37.7).[12] In addition, two distinct resonances are ob- rane(4) species for further functionalization reactions at the
1
served for the dimethylamino groups in the H NMR spec- B–B bond is currently tested in our laboratories.
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Eur. J. Inorg. Chem. 2010, 4423–4426

Boraferrocenophane and Diferrocenyldiborane
Experimental Section
Acknowledgments
General: All manipulations were performed under an inert atmo-
sphere of argon using standard Schlenk techniques or in a glove
box. Solvents were dried according to standard procedures and
stored under argon over molecular sieves. B2Mes2Cl2,[11] B2-
(NMe2)2Cl2,[12] [Fc]Li2·tmeda[14] and [Fc]Li[15] were prepared ac-
cording to literature procedures. All other chemicals were obtained
commercially and used without further purification. The UV/Vis
spectra were measured on a Shimadzu UV mini-1240-spectrometer
(suprasil-quartz cuvette). NMR experiments were performed on a
Bruker Avance 500 (1H, 500.130 MHz; 11B, 160.462 MHz; 13C,
125.758 MHz). 1H NMR and 13C NMR spectra were referenced
externally to TMS. 11B NMR spectra were referenced externally to
BF3·OEt2. Elemental analyses were performed on a Vario Micro
Cube elemental analyzer.
Financial support was provided by the Deutsche Forschungs-
gemeinschaft (DFG).
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[Fe(η5-C5H4)-B(Mes)-B(Mes)-(η5-C5H4)] (2): A cooled solution of
B2Mes2Cl2 (0.48 g, 1.47 mmol) in pentane (30 mL) was added to
[Fc]Li2·tmeda (0.45 g, 1.43 mmol) via syringe at –70 °C. The mix-
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solution changed to dark red after stirring for 16 h at room tem-
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under vacuum to afford a red solid, which was recrystallized from
pentane at –30 °C overnight. The resulting crystals were isolated
and dried in vacuo; yield 37% (0.26 g, 0.58 mmol). 1H NMR
(23.7 °C, 500.130 MHz, C6D6): δ = 2.16 (s, 6 H, p-CH3), 2.67 (s, 12
H, o-CH3), 3.70 (m, 2 H, C5H4), 4.18 (m, 2 H, C5H4), 4.36 (m, 2
H, C5H4), 4.89 (m, 2 H, C5H4), 6.86 s, 4 H, Mes-CH) ppm. 11B{1H}
NMR (23.7 °C, 160.462 MHz, C6D6): δ = 66.0 (br) ppm. 13C{1H}
NMR (23.7 °C, 125.758 MHz, C6D6): δ = 21.38 (s, 2 C, p-CH3),
25.94 (s, 4 C, o-CH3), 69.76 (s, 2 C, C5H4), 77.17 (s, 2 C, C5H4),
78.13 (s, 2 C, C5H4), 78.59 (s, 2 C, C5H4), 141.41 (s, 4 C, Mes-CH)
ppm. C28H30B2Fe (444.00): calcd. C 75.74, H 6.81; found C 75.46,
H 7.21.
1,2-Bis(dimethylamino)-1,2-diferrocenyldiborane(4) (3): To a stirred
suspension of FcLi (0.29 g, 1.55 mmol) in toluene (10 mL) was
added B2(NMe2)2Cl2 (0.14 g, 0.77 mmol) via syringe at room tem-
perature. The color of the solution changed from orange-red to
dark red after stirring overnight at room temperature. The solution
was filtered and all volatiles were removed under vacuum. The red
solid was dissolved in hexanes and cooled to –30 °C overnight. The
resulting red crystals were isolated and dried under vacuum; yield
10% (0.10 g, 0.35 mmol). 1H NMR (23.7 °C, 500.130 MHz, C6D6):
δ = 2.91 (s, 6 H, CH3), 2.97 (s, 6 H, CH3), 4.05 (s, 10 H, C5H5),
4.25 (m, 8 H, C5H4) ppm. 11B{1H} NMR (23.7 °C, 160.462 MHz,
C6D6): δ = 47.7 (br) ppm. 13C{1H} NMR (23.7 °C, 125.758 MHz,
C6D6): δ = 40.34 (s, 2 C, CH3), 45.86 (s, 2 C, CH3), 68.60 (s, 10 C,
C5H5), 70.51 (s, 2 C, C5H4), 70.88 (s, 2 C, C5H4), 75.10 (s, 2 C,
C5H4), 75.38 (s, 2 C, C5H4) ppm. 14N{1H} NMR (23.7 °C,
50.697 MHz, C6D6): δ = 283.06 [N(CH3)2] ppm. C24H30B2Fe2N2
(479.82): calcd. C 60.08, H 6.30, N 5.84; found C 59.21, H 6.34, N
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H. Braunschweig, A. Damme, T. Kupfer
SHORT COMMUNICATION
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Received: June 29, 2010
Published Online: August 18, 2010
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Eur. J. Inorg. Chem. 2010, 4423–4426