Benzene-centered tri- and tetrametallacarborane sandwich complexes

Article abstract of DOI:10.1002/1521-3773(20001215)39:24<4562::AID-ANIE4562>3.0.CO;2-8

Multiple metallacarborane units bound to a central benzene ring are possible with the boron-recapping method. In the FeCo₃ complex 1, as well as its precursor, the benzene ring is used as a linchpin connecting the metallaboron units. These complexes are candidates for the generation of electron-delocalized polynuclear mixed-valence systems through redox processes.

Full text of DOI:10.1002/1521-3773(20001215)39:24<4562::AID-ANIE4562>3.0.CO;2-8

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[22] V. Papaefthymiou, M. M. Millar, E. Münck, Inorg. Chem. 1986, 25,
3010.
pounds. Although 1,3,5-tris;carboranyl)benzene derivatives
incorporating icosahedral 1,2- or 1,12-C2B10 polyhedra have
been reported,[5] no corresponding metallaborane or metal-
lacarborane species ;i.e., having metal atoms in the polyhe-
dral framework) have been described; to our knowledge, the
closest known metal system is the dimetallic 1,4-bis-
;7-cobaltacarboranyl)benzene complex 1 ;B ˆ BH, B), which
[23] The K4Ge4Se10 exhibits bands at 318 ;vs), 282 ;vs), 255 ;s, sh), and 208
;w) cmÀ1
.
[24] L. Que, R. H. Holm, L. E. Mortenson, J. Am. Chem. Soc. 1975, 97, 463.
[25] W. O. Gillum, L. E. Mortenson, J.-S. Chen, R. H. Holm, J. Am. Chem.
Soc. 1977, 99, 584.
[26] R. B. Frankel, T. Herskovitz, B. A. Averill, R. H. Holm, P. J. Krusic,
W. D. Phillips, Biochem. Biophys. Res. Commun. 1974, 58, 974.
[27] R. H. Holm, W. D. Phillips, B. A. Averill, J. J. Mayerle, T. Herskovits,
J. Am. Chem. Soc. 1974, 96, 2109.
[28] T. A. Rouault, R. D. Klausner, Trends Biochem. Sci. 1996, 21, 174.
[29] J. S. Kim, D. C. Rees, Nature 1992, 360, 553.
was recently prepared in one of our laboratories.[6] In contrast,
a number of benzene-centered trinuclear organometallic
compounds of the type 1,3,5-C6H3;XMLn)3 have been synthe-
sized,[7] in which transition metals M are linked to benzene
through groups X ;e.g. alkynyl). Here we report the designed
synthesis of the first metallacarborane systems of this class.
The approach utilized boron-recapping ;ªrecapitationº) of
nido-[1,2,3-Cp*Co;2,3-Et2C2B3H3)]2À ;Cp* ˆ C5Me5), a meth-
od recently developed in our groups for preparing mono- or
dimetallic species bearing substituents at the apex [B;7)]
boron atom.[6, 8] In the present work, we required a suitable
trifunctional benzene derivative as a precursor to the desired
metal complexes. Accordingly, 1,3,5-tris;diiodoboryl)benzene
;3), previously prepared but not isolated,[9] was generated
from the corresponding tris;trimethylsilyl)benzene as shown
in Scheme 1 and isolated in 30% yield as a colorless, air-
Benzene-Centered Tri- and
Tetrametallacarborane Sandwich Complexes**
Martin Bluhm, Hans Pritzkow, Walter Siebert,* and
Russell N. Grimes*
The special steric and electronic properties of polyhedral
borane and carborane clustersÐnotably their three-dimen-
sional geometry and electron delocalization ;aromatic char-
acter)Ðare being employed to advantage in the construction
of novel molecular architectures for use in a variety of
projected applications in materials synthesis, microelectron-
ics, optics, and medicine.[1] In recent work some remarkable
compound types have been produced in which boron clusters
serve as scaffolds or templates for attachment of organic
moieties; examples include ;1,7-C2B10H10)nHgn ªanti-crownsº
;n ˆ 3, 4),[1d, 2] peralkylated carboranes that mimic spherical
[3]
hydrocarbons, and benzene-m-carboranyl macrocycles.[4]
This paradigm can be reversed: Hydrocarbons may serve as
frameworks for attachment of multiple boron clusters. If an
aromatic system such as benzene is employed as a linchpin
connecting several metallaboron units, one might prepare
polynuclear, electron-delocalized, mixed-valence systems that
are not only interesting from a fundamental perspective, but
may also have practical potential as precursors or models for
new kinds of electronically tailorable organometallic com-
Scheme 1. Synthesis of 3.
sensitive solid. Treatment of Li2[Cp*Co;2,3-Et2C2B3H3)] ;4)
with one-third molar equivalent of 3 in toluene at 08C gave
the target compound [{Cp*Co;2,3-Et2C2B4H3-7)}3C6H3] ;5) as
[*] Prof. Dr. R. N. Grimes, Dr. M. Bluhm
Department of Chemistry
a
moderately air-stable yellow complex in 46% yield
University of Virginia
;Scheme 2).
Charlottesville, VA 22901 ;USA)
Fax : ;‡1)804-924-3710
Characterization of 5 by multinuclear NMR and IR
spectroscopy as well as mass spectrometry supports the
trigonally symmetric geometry shown ;the rotamer depicted
is arbitrary), and the solid-state structure has been confirmed
by X-ray crystallography ;Figure 1).[10] With the exception of
the benzene ring ;see the figure caption), the bond lengths
and angles are normal, and the metal atoms and apical boron
atoms are coplanar with the benzene ring. A notable feature
of the 1H NMR spectrum of 5 is the strong upfield shift of the
aromatic benzene resonances ;d ˆ 5.92 in CDCl3 versus 7.51
for benzene itself), reflecting the electron-donating character
of the carborane ligand ;for comparison, the corresponding
E-mail: rng@virginia.edu
Prof. Dr. W. Siebert, Dr. H. Pritzkow
Anorganisch-chemisches Insititut der Universität
Im Neuenheimer Feld 276
69120 Heidelberg ;Germany)
Fax : ;‡49)6221-54-5609
E-mail: ci5@ix.urz.uni-heidelberg.de
[**] Organotransition-Metal Metallacarboranes, part 56. This work was
supported in part by the National Science Foundation ;grant CHE
9980708 to R.N.G.) and the Deutsche Forschungsgemeinschaft ;SFB
247, to W.S. for M.B.). Part 55: T. Dodge, M. A. Curtis, J. M. Russell,
M. Sabat, M. G. Finn, R. N. Grimes, J. Am. Chem. Soc. 2000, 122,
10573.
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À
Figure 2. Molecular structure of 6. The mean benzene ring C C distance is
1.422 Š ;range 1.421 ± 1.425 Š). The mean benzene C-C-C angle centered
on C1, C3, and C5 is 117.48; that centered on C2, C4, and C6 is 122.68 ;the
numbering of the benzene ring is analogous to that in 5, Figure 1).
Tris;diiodoboryl)benzene ;3) can also serve as a precursor
to other trigonally substituted complexes. Scheme 3 shows the
preparation of the tris;nido-carboranyl)benzene species 8, a
potential building block for metal-connected trigonal sheet
Scheme 2. Synthesis of 5 and 6. B ˆ BH, B.
À
Figure 1. Molecular structure of 5. The mean benzene ring C C distance is
1.400 Š ;range 1.399 ± 1.402 Š). The mean benzene C-C-C angle centered
on C1, C3, and C5 is 117.18; that centered on C2, C4, and C6 is 122.98.
Scheme 3. Synthesis of 8. B ˆ BH, B.
signal in 1 appears at d ˆ 6.38, while those in the monocobalt
species [Cp*Co;2,3-Et2C2B4H3-7-C6H5)] are found at d ˆ
6.75 ± 7.08).[6]
A fourth metallacarborane unit was added to 5 by thermal
displacement of cyclooctatriene from [;h6-C8H10)Fe-
;Et2C2B4H4)][11] to give the heterotetranuclear complex
[{Cp*Co;Et2C2B4H3-7)}3{Fe;Et2C2B4H4)}C6H3] ;6, Scheme 2).
This species was isolated as a yellow solid in 30% yield and
characterized by spectroscopy and X-ray crystallography
;Figure 2).[10]
polymers. The structure of 8 is assigned from NMR and mass
spectra. Full details on this work, including electrochemical
and other data bearing on the electronic properties of the
polymetallacarborane complexes, will be presented in a later
publication.
Experimental Section
[12]
3: BI3 ;3.000 g, 7.66 mmol) and ;Me Si)3C6H3 ;0.752 g, 2.55 mmol) were
3
heated at 1008C for 1.5 h, following which the Me3SiI was removed by
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vacuum distillation. The solid residue was taken up in pentane ;30 mL), and
the suspension was cooled to À788C and filtered under N2 to obtain
colorless, air-sensitive 3 ;0.670 g, 0.77 mmol, 30%). 1H NMR ;300 MHz,
C6D6): d ˆ 9.27;s, 3H ar); 13C NMR ;75.5 MHz, C6D6): d ˆ 152.9 ;C6H3),
142.6 ;ipso-C6H3); 11B NMR ;115.8 MHz, CDCl3): d ˆ 47.7 ;s); UV/Vis
Colquhoun, D. F. Lewis, P. L. Herbertson, K. Wade, I. Baxter, D. J.
Williams in Contemporary Boron Chemistry ;Eds.: M. Davidson,
A. K. Hughes, T. B. Marder, K. Wade), Royal Society of Chemistry,
Cambridge ;UK), 2000, pp. 59 ± 66.
[5] a) W. Jiang, C. B. Knobler, M. F. Hawthorne, Inorg. Chem. 1996, 35,
3056; b) U. Schoeberl, T. F. Magnera, R. M. Harrison, F. Fleischer,
J. L. Pflug, P. F. H. Schwab, X. Meng, D. Lipiak, B. C. Noll, V. S.
Allured, T. Rudalevige, S. Lee, J. Michl, J. Am. Chem. Soc. 1997, 119,
3907.
[6] M. A. Curtis, M. Sabat, R. N. Grimes, Inorg. Chem. 1996, 35, 6703.
[7] Selected examples: a) H. Fink, N. J. Long, A. J. Martin, G. Opromolla,
A. J. P. White, D. J. Williams, P. Zanello, Organometallics 1997, 16,
2646; b) T. Weyland, C. Lapinte, G. Frapper, M. J. Calhorda, J.-F.
Halet, L. Toupet, Organometallics 1997, 16, 2024; c) C. Patoux, C.
Coudret, J.-P. Launay, C. Joachim, A. Gourdon, Inorg. Chem. 1997, 36,
5037; d) N. J. Long, A. J. Martin, F. Fabrizi de Biani, P. Zanello, J.
Chem. Soc. Dalton Trans. 1998, 2017; e) F. Cherioux, P. Audebert, P.
Hapiot, Chem. Mater. 1998, 10, 1984; f) P.-M. Pellny, V. M. Burlakov,
W. Baumann, A. Spannenberg, R. Kempe, U. Rosenthal, Organo-
metallics 1999, 18, 2906.
;CH2Cl2): l ;rel. intens.) ˆ 229 nm ;100); e ˆ 7014 cmÀ1mÀ1
.
5: [Cp*Co;Et2C2B3H5)] ;200 mg, 0.64 mmol) in toluene ;30 mL) at 08C was
treated with 1.6m n-butyllithium ;0.8 mL, 1.27mmol). The solution turned
red-orange during warming to room temperature ;RT) over a 6 h period. It
was cooled to 08C, and 3 ;185 mg, 0.21 mmol) was slowly added under
nitrogen, causing the solution to become cloudy and turn orange-brown as
the flask was warmed to RT. The mixture was stirred for 16 h, after which
the toluene was removed in vacuo. The residue was taken up in hexane and
washed through silica ;3 cm), first with hexane and then with CH2Cl2; the
hexane wash contained only [Cp*Co;Et2C2B3H5)]. Column chromatogra-
phy of the CH2Cl2 wash on silica with hexane/CH2Cl2 ;1/4) afforded 5 as a
major yellow band ;100 mg, 0.10 mmol, 46%). 1H NMR ;300 MHz,
CDCl3): d ˆ 5.92 ;s, 3Har), 2.18 ;m, ethyl CH2), 1.77 ;C5Me5), 1.27;t, ethyl
CH3); 13C NMR ;75.5 MHz, in C6D6): d ˆ 134.8 ;C6H3), 94.3 ;ipso-C6H3),
90.4 ;C5Me5), 22.1 ;CH2), 15.1 ;ethyl CH3), 10.1 ;C5Me5); 11B NMR
[8] M. A. Curtis, T. Mueller, V. Beez, H. Pritzkow, W. Siebert, R. N.
Grimes, Inorg. Chem. 1997, 36, 3602.
[9] a) W. Siebert, K.-J. Schaper, B. Asgarouladi, Z. Naturforsch. B 1974,
29, 642; b) W. Haubold, J. Herdtle, W. Gollinger, W. Einholz, J.
Organomet. Chem. 1986, 315, 1.
‡
À
;115.8 MHz, C6D6): d ˆ 5.0, 15.0 ;B H coupling not resolved); CI -MS:
‡
‡
m/z ;%): 1044.7;[ M ], 100), 721.8 ;[M À Cp*Co;Et2C2B4H3)], 0.9); UV/
Vis ;CH2Cl2): l ;rel. intens.) ˆ 229 ;41), 304 nm ;100); e ˆ 11435 cmÀ1mÀ1
.
6: Compound
5
;20 mg, 0.020 mmol) and [;h6-C8H10)Fe;Et2C2B4H4)]
;56 mg, 0.19 mmol) were heated in a Pyrex tube at 1858C for 20 min, after
which the black solid was taken up in CH2Cl2 and washed through silica
;3 cm), first with CH2Cl2 and then with ethyl acetate; the CH2Cl2 wash
contained only unchanged starting materials. The ethyl acetate wash was
purified by chromatography on a TLC plate with hexane/ethyl acetate
;3/1), affording a major yellow band that was characterized as 6 ;7mg,
30%). 1H NMR ;300 MHz, CDCl3): d ˆ 3.75 ;s, 3Har), 2.44 ;m, ethyl CH2)
[10] Crystal structure determinations: Data were collected on a Bruker
AXS SMART 1000 diffractometer ;MoKa radiation, l ˆ 0.71073 Š, w
scans), at 173 K, up to qmax ˆ 26.48. Structures were solved with direct
methods and refined by least-squares against F 2 ;SHELXTL V5.10);
non-hydrogen atoms were anisotropic, and hydrogen atoms were
located and refined isotropically. 5 ;C61H95B12Co3, crystal contains one
toluene molecule per complex): Mr ˆ 1134.9, orthorhombic, space
group Pbca, a ˆ 14.6958;3), b ˆ 24.7128;5), c ˆ 34.4880;8) Š, Vˆ
12525.2;5) Š3, Z ˆ 8, 1calcd ˆ 1.204 gcmÀ3. Of 81401 measured reflec-
tions, 12821 were independent ;Rint ˆ 0.071), 1033 parameters, R1 ˆ
0.041 ;for reflections with I > 2s;I)), wR2 ˆ 0.111 ;for all reflections),
max./min. residual electron density 0.72/ À 0.51 eŠÀ3. 6 ;C63H104B16Fe-
Co3, crystal contains 0.5 benzene molecules per complex): Mr ˆ 1267.1,
2.22 ;m, ethyl CH2), 2.07;m, ethyl CH ) 1.78 ;C5Me5), 1.19 ;t, ethyl CH3),
2
1.07;t, ethyl CH 3); 13C NMR ;75.5 MHz, C6D6): d ˆ 93.8 ;ipso-C6H3), 90.6
;C5Me5), 87.6 ;C H3), 25.5 ;CH2), 22.4 ;CH2), 15.7;ethyl CH 3), 15.3 ;ethyl
6
11
À
CH3), 9.9 ;C5Me5); B NMR ;115.8 MHz, C6D6): d ˆ 3.8 ;B H coupling
‡
‡
‡
not resolved); CI -MS: m/z ;%): 1229.9 ;[M ], 100), 1044.6 ;[M À
Fe;Et2C2B4H4)], 7); UV/Vis ;CH2Cl2): l ;rel. intens.) ˆ 230 ;57), 296 nm
;100); e ˆ 91138 cmÀ1mÀ1
.
Å
triclinic, space group P1, a ˆ 11.1196;3), b ˆ 13.9542;4), c ˆ
8: 2,3-Et2C2B4H6 ;118 mg, 0.9 mmol) in absolute diethyl ether ;30 mL) at
À788C was treated with 1.6m n-butyllithium ;1.1 mL, 1.8 mmol). The
solution was stirred for 4 h at RT. The Et2O was removed, the residue taken
up in absolute toluene ;30 mL), and the mixture cooled to À148C.
Compound 3 ;260 mg, 0.3 mmol) was slowly added under nitrogen while
warming up to RT The resulting solution became cloudy. The mixture was
stirred for 16 h, after which the toluene was removed in vacuo. The residue
was taken up in hexane and washed through silica ;3 cm), first with hexane
and then with CH2Cl2. The complete wash was purified by chromatography
on a TLC plate with CH2Cl2/hexane ;1/4), affording a colorless band that
was characterized as 8 in low yield. 1H NMR ;300 MHz, CDCl3): d ˆ 6.51 ;s,
23.2691;6) Š, a ˆ 85.959;2)8, b ˆ 79.768;2)8, g ˆ 77.633;2)8, Vˆ
3468.7;2) Š3, Z ˆ 2, 1calcd ˆ 1.213 gcmÀ3. Of 46579 measured reflec-
tions, 16911 were independent ;Rint ˆ 0.037), 1153 parameters, R1 ˆ
0.036 ;for reflections with I > 2s;I)), wR2 ˆ 0.096 ;for all reflections),
max./min. residual electron density 0.61/ À 0.38 eŠÀ3. Crystallograph-
ic data ;excluding structure factors) for the structures reported in this
paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC-147113 ;5) and
CCDC-147114 ;6). Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK
;fax: ;‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
‡
3Har), ethyl proton resonances from 8 and unchanged Et2C2B4H6; CI -MS:
[11] R. G. Swisher, E. Sinn, R. N. Grimes, Organometallics 1985, 4, 896.
[12] P. Boudjouk, C. A. Kepfer, J. Organomet. Chem. 1985, 296, 339.
‡
m/z ;%): 468.4 ;[M ], 100)
Received: July 12, 2000 [Z15441]
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