A novel coordinated inorganic benzene: Synthesis and characterization of {η5-C5Me5Re}2{μ-η6:η6-B4H4CO2(CO)5} [24]

Full text of DOI:10.1021/ja991306c

J. Am. Chem. Soc. 1999, 121, 7451-7452
7451
Chart 1
A Novel Coordinated Inorganic Benzene: Synthesis
and Characterization of
{η5-C5Me5Re}2{µ-η6:η6-B4H4Co2(CO)5}
Sundargopal Ghosh, Maoyu Shang, and Thomas P. Fehlner*
Department of Chemistry and Biochemistry
UniVersity of Notre Dame
Notre Dame, Indiana 46556-5670
ReceiVed April 23, 1999
Since the time of Kekule’s dream, chemists have been
fascinated with the structures of systems possessing the property
called aromaticity both in two1 and three dimensions.2,3 These
include heterobenzenes, e.g., C5H5As, in which other p-block
elements particpate in the 6 π electron aromatic system4 and
metallabenzenes, e.g., 1-Ir(PEt3)3-3,5-Me2-C5H3, which has been
characterized as a free species5 as well as when coordinated to
another metal fragment, e.g., (CO)3Mo{η6-[1-Ir(PEt3)3-3,5-Me2-
C5H3]}.6 Even the purely inorganic analogue, H3B3N3H3, and
coordinated forms, e.g., (CO)3Cr{η6-H3B3N3H3}, have received
considerable attention.7
Metal-coordinated benzenes and heterobenzenes are found to
possess ring structural parameters nearly identical to those of the
free species even for triple-decker complexes.6,8-10 This means
that unusual benzene analogues, too reactive to be isolated as
free species, may be sought as metal-stabilized adducts. In this
vein, we reported a coordinated tetraborabenzene (CpCr)2{µ-η6:
η6-B4H4C2R2}, 1, Cp ) η5-C5H5.11 Consistent with past observa-
tions, the ring geometry of free [C2B4H6]4-, derived from accurate
calculations, is the same as that of the coordinated ring derived
from the solid-state structure determination of 1. Now we report
the directed synthesis of (Cp*Re)2{µ-η6:η6-B4H4Co2(CO)5}, 2,
Cp* ) η5-C5Me5, an isolobal analogue of 1, which contains a
novel inorganic benzene.
we attempted to close the central open ring by reaction with main
group and transition element fragments. However, with the
exception of one successful closure in which a nonplanar B4S2C
ring was formed by reaction of 3 with CS2,15 only ring expansion
has been observed until now, e.g., Chart 1.16-18 Extension of our
synthetic method to Re has resulted in the isolation of Cp*2-
Re2B4H8, 4, the saturated 44 cve analogue of 3. Curiously, the
solid-state structure of 4 shows that it possesses the same cluster
geometry as 3; however, the endo hydrogen atoms, which are
mainly interacting with boron in 3, are now found to be principally
interacting with the metal (1H chemical shift and coupling
constant: δ -3.9, JBH ) 70 Hz for 3 and δ -11.5, JBH ≈ 15 Hz
for 4). Similar derivative reactions have been surveyed for 4, and
reaction with Co2(CO)8 results in the replacement of the four endo
hydrogen atoms with a 4 electron Co2(CO)5 fragment and in clean
closure of the open face to generate 2.19
Compound 2 contains a planar (0.032 Å rms deviation, sum
of internal angles 719.6°), six-membered B4Co2 ring sandwiched
between two Cp*Re fragments (Figure 1).20 However, the
necessity of packing the five CO ligands of the Co2 fragment
between the Cp* ligands of Re causes the Cp* ligands to tilt and
produces a conrotatory twist of the two Co(CO)2(CO)br fragments
around their pseudo C3 axes such that the bridging CO lies out
of the plane of the B4Co2 ring. Within the ring the two types of
BB distances are equal to those in the [C2B4H6]4- aromatic ring
mentioned above (av 1.719(15) vs av 1.718(8) Å); the BCo
distances are within the range found for cobaltaborane clusters
After developing a good route to unsaturated 42 cluster valence
electrons12 (cve) Cp*2Cr2B4H8, 3,13 and a related compound,14
(1) Binsch, G. Naturwissenschaften 1973, 60, 369.
(2) Michel, H.; Steiner, D.; Wacadlo, S.; Allwohn, J.; Stamatis, N.; Massa,
W.; Berndt, A. Angew Chem., Int. Ed. Engl. 1992, 31, 607.
(3) Schleyer, P. v. R.; Najafian, K. Inorg. Chem. 1998, 37, 3454.
(4) Ashe, A. J., III. Acc. Chem. Res. 1978, 11, 153.
(5) Bleeke, J. R.; Behm, R.; Xie, Y.-F.; Chiang, M. Y.; Robinson, K. D.;
Beatty, A. M. Organometallics 1997, 16, 606.
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Soc. 1992, 114, 4213.
(7) Gaines, D. F.; Borlin, J. In Boron Hydride Chemistry; Muetterties, E.
L., Ed.; Academic Press: New York, 1995; p 241.
(8) Elschenbroich, C.; Salzer, A. Organometallics; VCH: New York, 1989.
(9) Lamanna, W. M.; Gleason, W. B.; Britton, D. Organometallics 1987,
6, 1583.
(12) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry;
Prentice Hall: New York, 1990.
(13) Ho, J.; Deck, K. J.; Nishihara, Y.; Shang, M.; Fehlner, T. P. J. Am.
Chem. Soc. 1995, 117, 10292.
(14) Nishihara, Y.; Deck, K. J.; Shang, M.; Fehlner, T. P.; Haggerty, B.
S.; Rheingold, A. L. Organometallics 1994, 13, 4510.
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118, 8164.
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P. Inorg. Chem. 1998, 37, 928.
10.1021/ja991306c CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/29/1999

7452 J. Am. Chem. Soc., Vol. 121, No. 32, 1999
Communications to the Editor
There is additional circumstantial evidence supporting the view
of 2 as a coordinated 6 π electron [B4H4Co2(CO)5]6- ring.
Compound 2 can be considered a cluster; however, the geometry
expected for an 8 vertex closo cluster is a dodecahedron rather
than a hexagonal bipyramid.24,25 Further 9 skeletal electron pairs
(sep) are expected although 2 possesses only 6.26 Alternatively,
as 2 contains a ReRe bond (2.6393(5) Å), the compound can be
related to the hypoelectronic clusters of tungsten described
earlier,27 but the structural patterns of these new clusters remain
to be fully defined. Thus, there is no established cluster structure
principle that requires the observed geometry of 2.
On the other hand, 2 is easily seen to be a 24 valence electron
triple-decker complex28 analogous to 1 with a 6 π electron
[B4H4Co2(CO)5]6- central ring. That is, adding 4 electrons to
closo-C2R2B4H6 gives planar, arachno [C2R2B4H6]4-, whereas
adding 4 electrons to closo-[B4H4Co2(CO)5]2-, equivalent to
known closo-H2B4H4Co2Cp2,29,30 yields the planar arachno co-
baltaborane ring found in 2. Like 1, and 24 valence electron CpTi-
(µ-η3:η3-P6)TiCp,31 2 contains a metal-metal bond. Again like
1, but unlike CpTi(µ-η3:η3-P6)TiCp, the central ring is planar.
With the C2R2B4H6 and B4H4Co2(CO)5 rings, but not with the
P6 ring, there is a good match between the energies of the
Cp*M fragment orbitals and the out of plane orbitals of the
central ring leading to a good HOMO-LUMO gap without the
necessity of ring distortion.11,32 Further examination of the
electronic structure of planar [B4H4Co2(CO)5]6- as found in 2 is
in progress.
Figure 1. Molecular structure of {η5-C5Me5Re}2{µ-η6:η6-B4H4Co2-
(CO)5}, 2. Selected bond distances (Å) and angles (deg): Re1-Re2
2.6393(5), Re1-Co1 2.6478(12), Re1-Co2 2.6854(13), Re2-Co2
2.6157(12), Re2-Co1 2.6460(12), Re1-B3 2.130(10), Re1-B2 2.150-
(11), Re1-B1 2.189(10), Re1-B4 2.194(11), Re2-B3 2.142(10), Re2-
B2 2.161(10), Re2-B1 2.237(10), Re2-B4 2.255(11), Co1-Co2 2.4192-
(17), Co1-B1 2.010(11), Co2-B4 2.036(11), B1-B2 1.712(15), B2-
B3 1.770(15), B3-B4 1.712(15), B1-B2-B3 120.4(7), B4-B3-B2
122.9(8), B3-B4-Co2 130.6(7), B2-B1-Co1 132.8(7), B4-Co2-Co1
106.4(3), B1-Co1-Co2 106.5(3).
Cp*2Re2B4H8 appears to be a versatile starting material. For
example, reaction of 4 with an excess of BH3THF leads to the
formation of Cp*2Re2B7H7, a hypoelectronic metallaborane
described earlier.33 Additional reaction chemistry of 4, including
a detailed comparison of its geometric and electronic structure
with that of 3, will be reported in the full paper.
but distinctly shorter than that in (CO)2(η1-dppm)Co(µ-dppm)-
BH2, dppm ) Ph2PCH2PPh2, which contains a CoB single bond
(av 2.023(11) vs 2.227(6) Å);21 and the CoCo distance is
significantly shorter than that accepted for a CoCo single bond
(2.4192(17) vs 2.49-2.52 Å).22,23 Thus, the planarity of the ring
and its edge lengths are consistent with considering 2 an analogue
of a coordinated benzene just like isolobal 1.
Acknowledgment. The generous support of the National Science
Foundation is gratefully acknowledged.
(19) A 2-fold excess of Co2(CO)8 was added to 0.07 g (0.1 mmol) of 4 in
10 mL of hexane in a 100-mL Schlenk tube and stirred at room temperature.
After 15 min the solvent was removed under vacuum, and the residue was
extracted in hexane, concentrated, and kept at -40 °C to remove Co4(CO)12
by fractional crystallization. The mother liquor was filtered quickly through
silica gel to give a yellowish brown solution which generated two types of
crystals at -40 °Cswhite, Cp*Re(CO)3 (resulting from a Cp*ReH6 impurity
in 4) and brown 2 (75% yield by NMR). X-ray quality crystals were obtained
Supporting Information Available: Description of the synthesis and
characterization of 4 and X-ray structural information on 2 (PDF). An
X-ray crystallographic file for 2 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA991306C
by recrystallizing selected brown crystals overnight at -4 °C. MS (EI), P+
)
949, isotope pattern for 2Re, 2Co, and 4B atoms; calcd for weighted average
of isotopomers lying within instrument resolution, 950.0558, obsd, 950.0586.
11B NMR (C6D6, 22 °C) δ 87.8 d, JB-H ) 172 Hz, 2B; δ 86.7 d, JB-H ) 162
Hz, 2B. 1H NMR (C6D6, 22 °C) δ 9.31 partially collapsed quartet (pcq), 2BHt;
δ 4.38 (pcq), 2BHt; δ 1.67 s, 30H, 2Cp*; IR (KBr, cm-1): 2536 w, 2453 w
(B-Ht ), 2022 s (Co-CO), 1997 vs (Co-CO), 1972 s (Co-CO), 1820 w
(Co-(µ-CO)).
(20) Crystallographic data: orthorhombic Pbca, a ) 23.282(2) Å, b )
16.787(2) Å, c ) 15.0481(14) Å, V ) 5881.3(10) Å3, Z ) 8, 20 °C, CAD4
diffractometer, 5186 unique data, θ up to 25°, empirical psi-scan absorption
correction, direct methods (Shelxs-86), Shelxl-97, H atoms found from
difference Fourier map, refined with riding models, R. (I g 2σ(I)) ) 0.0368,
Rw ) 0.0757.
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N.; Magnuson, V. R.; Moser, I. M. Inorg. Chem. 1990, 29, 5014.
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W. R. Chem. ReV. 1979, 79, 91.
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D., Adams, R. D., Eds.; VCH: New York, 1990.
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