Zwitterionic ring-borylated ansa-chromocene complexes

Article abstract of DOI:10.1021/ja026633q

Ring borylation of [Me₄C₂(η⁵-C₅H₄)₂CrCO] by B(C₆F₅)₃ affords the zwitterionic complex {Me₄(η⁵-C₅H₄)(η⁵

Full text of DOI:10.1021/ja026633q

Published on Web 08/24/2002
Zwitterionic Ring-Borylated ansa-Chromocene Complexes
Piet-Jan Sinnema, Pamela J. Shapiro,* David Min Jin Foo, and Brendan Twamley
Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343
Received April 22, 2002
Very little is known about the chemistry of bent-sandwich
chromocene complexes.1 This fact is surprising given the importance
of early transition metal-containing bent-metallocene complexes to
homogeneous catalysis, particularly alkene polymerization.2 High-
spin chromocene’s recalcitrance toward assuming a bent-sandwich
geometry and its propensity toward cyclopentadienyl ring loss are
the dominant factors that interfere with the isolation of bent-
sandwich derivatives. Theoretical calculations indicate that the
interannular bridges stabilize the carbonyl derivatives by destabiliz-
ing the 16e- chromocene fragment through enforced bending.3
Brintzinger and co-workers have demonstrated that carbonyl adducts
of ethano- and silylene-bridged ansa-chromocene complexes are
isolable,4 unlike Cp2CrCO, which is unstable to CO loss.5 These
findings have persuaded us to pursue the synthesis of additional
ansa-chromocene derivatives6 to uncover the previously elusive
chemistry of bent-sandwich chromocene and to establish useful
catalytic applications for these systems.
Figure 1. Molecular structure of 1. Thermal ellipsoids are shown at 30%
probability. Selected bond distances (Å) and angles (deg): av Cp-Cr
1.823(2); Cr1-C17O17 1.873(3); Cr1C17-O17 1.139(4); Cr1-H1′ 1.44-
(3); Cr1-F23 3.735(2); Cp-Cr-Cp 140.0(3); Cp-Cp dihedral 42.5(3).
Electrophilic addition of B(C6F5)3 to a cyclopentadienyl ring
followed by proton transfer to the metal has been demonstrated
for a variety of unbridged early transition metal bent-metallo-
cene complexes.7 Therefore, as one approach to an unprecedented
Cr(4+) hydride derivative of the bent-chromocene system, we
examined the reaction between [Me4C2(η5-C5H4)2CrCO] and B(C6F5)3.
The reaction was monitored by 1H NMR spectroscopy. Complex 1
formed immediately upon mixing the two reagents in toluene-d8 at
-70 °C (Scheme 1).
Scheme 1
A characteristic metal hydride signal was observed at δ -7.66,
and the remainder of the spectrum was consistent with the assigned
structure (see Supporting Information). As the temperature of the
sample was raised to 25 °C, the signals for 1 diminished, and a
very broad signal grew in at δ -1.6, indicating the formation of
paramagnetic species. Toepler pump measurements8 revealed no
gas evolution at this stage. This was surprising in light of the
instability of the related cationic species [Me4C2(η5-C5H4)2CrH-
(CO)]+ to the nonselective loss of CO and H2.6c
X-ray quality crystals of 1 were isolated for a molecular structure
determination (Figure 1).9 Significantly, this is the first X-ray
crystallographic characterization of a chromocene complex with
chromium formally in a +4 oxidation state.10 The gross geometry
of the ansa-chromocene fragment is very similar to that of the
neutral complex Me4C2(C5H4)2CrCO3a and the cation in [Me4C2-
(η5-C5H4)2CrCO][B(3,5-(CF3)2C6H3)4].6c The cyclopentadienyl rings
of 1, with a dihedral angle of 42.5°, are slightly more tilted than
those in the other two species (with corresponding angles of
38.5-38.6°). The Cr-CO (1.873(3) Å) and the CrC-O (1.139(4)
Å) distances of 1 do not differ substantially from those of the neutral
Cr(II) species [Me4C2(η5-C5H4)2CrCO] (1.85(1) Å, 1.16(1) Å) or
the cationic Cr(III) species in [Me4C2(η5-C5H4)2CrCO][B(3,5-
(CF3)2C6H3)4] (1.893(6) Å, 1.133(7) Å). The C-O stretching
frequencies of the three complexes (1992, 1905, 1990 cm-1
respectively), on the other hand, are quite sensitive to variations in
the electronic properties of the metal center.
,
Upon being heated from room temperature to 80 °C, the red-
brown toluene solution containing the paramagnetic decomposition
products of 1 turned green and evolved 0.48 ( 0.02 equiv of carbon
monoxide gas. The thermolysis product consisted of two new
species, one green and the other red, both of which were separated
by selective crystallization and structurally characterized. The green
species is the zwitterionic ring-borylated Cr(III) complex, {Me4C2-
(η5-C5H4)(η5-C5H3B(C6F5)3}Cr (2). The red species is the ionic
Cr(III) complex, [Me4C2(η5-C5H4)2CrCO][HB(C6F5)3] (3). X-ray
crystal structures of both 211 and 3 were obtained. An ORTEP
* To whom correspondence should be addressed. E-mail: shapiro@uidaho.edu.
9
10996
J. AM. CHEM. SOC. 2002, 124, 10996-10997
10.1021/ja026633q CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
No. DE-FG02-98ER45709) for their generous financial support.
The establishment of a Single-Crystal X-ray Diffraction Laboratory
and the purchase of a 500 MHz NMR spectrometer were supported
by the M. J. Murdock Charitable Trust of Vancouver, WA, the
National Science Foundations, and the NSF-Idaho EPSCoR Pro-
gram.
Supporting Information Available: Crystallographic data for 1,
2, and 3 (CIF) and synthetic and experimental details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) For reviews on the chemistry of chromocene, see: (a) Davis, R.; Kane-
Maguire, L. A. P. In ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel., E. W., Eds.; Pergamon: Oxford, 1982;
Vol. 3, Chapter 26.2. (b) Morris, M. J. In ComprehensiVe Organometallic
Chemistry; Abel, E. W., Gordon, F., Wilkinson, G., Eds.; Pergamon:
Oxford, 1992; Vol. 5, Chapter 7.
Figure 2. Molecular structure of 2. Thermal ellipsoids are shown at 30%
probability. Selected bond distances (Å) and angles (deg): Cp-Cr1
1.851(5); BCp-Cr1 1.821(5); Cr-F18 2.675(4); Cr-F30 3.173(4);
Cp-Cr-Cp 149.3(3); Cp-Cp dihedral 36.9(3).
(2) (a) Metalorganic Catalysts for Synthesis and Polymerization; Kaminsky,
W., Ed.; Springer-Verlag: Berlin, 1999. (b) Ziegler Catalysts; Fink, G.,
Mu¨lhaupt, R., Brintzinger, H. H., Eds.; Springer-Verlag: Berlin, 1995.
(3) (a) Brintzinger, H. H.; Lohr, L. L., Jr.; Tang Wong, K. L. J. Am. Chem.
Soc. 1975, 97, 5146-5155. (b) Simpson, K. M.; Rettig, M. F.; Wing, R.
M. Organometallics 1992, 11, 4363-4364. (c) Green, J. C.; Jardine, C.
N. J. Chem. Soc., Dalton Trans. 1999, 3767-3770.
drawing of the molecular structure of 2 is shown (Figure 2).
Compound 3 is related to an ansa-chromocenium carbonyl salt
whose structure has been reported.6c Details about its structure are
provided in the Supporting Information. The structure of 2 is
reminiscent of that of a related zwitterionic boryl-titanocene
complex, (η5-C5H5)(η5-[C5H4B(C6F5)3])Ti.7c As in the titanium
complex, there are close contacts between the metal center in 2
and two ortho-fluorine atoms on the pendant boryl group
(Cr-F18 2.675(4) Å; Cr-F30 3.173(4) Å) that are shorter than
the sum of the van der Waal radii for both elements (Cr-F ) 3.47
Å).12 These distances are substantially longer than those in the
titanocene complex (2.248(2) Å and 2.223(3) Å), even though the
single-bonded metallic radius of Ti(1.32 Å) is larger than that of
Cr(1.19 Å).13 This is probably due to the fact that the titanium
complex has only one unpaired electron in its frontier orbitals,
whereas the high-spin chromium complex has three unpaired
electrons (µeff ) 3.8 µB).
(4) (a) Schaper, F.; Rentzsch, M.; Prosenc, M. H.; Rief, U.; Schmidt, K.;
Brintzinger, H.-H. J. Organomet. Chem. 1997, 534, 67-79. (b) Schwem-
lein, H.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem.
1983, 256, 285-289.
(5) Wong, K. L. T.; Brintzinger, H. H. J. Am. Chem. Soc. 1975, 97, 5143-
5146.
(6) (a) Foo, D. M. J.; Shapiro, P. J. Organometallics 1995, 14, 4957-4959.
(b) Matare, G. J.; Foo, D. M.; Kane, K. M.; Zehnder, R.; Wagener, M.;
Shapiro, P. J. Organometallics 2000, 19, 1534-1539. (c) Foo, D. M. J.;
Sinnema, P.-J.; Twamley, B.; Shapiro, P. J. Organometallics 2002, 21,
1005-1007.
(7) (a) Liu, S.; Liu, F.-C.; Renkes, G.; Shore, S. G. Organometallics 2001,
20, 5717-5723. (b) Burlakov, V. V.; Pellny, P.-M.; Arndt, P.; Baumann,
W.; Spannenberg, A.; Shur, V. B.; Rosenthal, U. Chem. Commun. 2000,
241-242. (c) Burlakov, V. V.; Troyanov, S. I.; Letov, A. V.; Strunkina,
L. I.; Minacheva, M. K.; Furin, G. G.; Rosenthal, U.; Shur, V. B. J.
Organomet. Chem. 2000, 598, 243-247. (d) Doerrer, L. H.; Graham, A.
J.; Haussinger, D.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 2000,
813-820. (e) Ruwwe, J.; Erker, G.; Fro¨lich, R. Angew. Chem., Int. Ed.
Engl. 1996, 35, 80-82.
(8) Burger, B. J.; Bercaw, J. E. In Experimental Organometallic Chemistry;
Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society:
Washington, D.C., 1985; Chapter 4.
(9) Crystallographic data for 1‚toluene: triclinic, space group P-1; a )
11.0780(11) Å, b ) 12.1571(12) Å, c ) 16.0434(16) Å, R ) 106.75(1)°,
â ) 101.15(1)°, γ ) 108.26(1)° at 203(2)K; V ) 1866.8(3) Å3; Z ) 2;
Dcalc ) 1.595 Mg/m3, µabs ) 0.417 mm-1, F(000) ) 904, refl. collected
) 23 214, indep. refl. ) 8550, R1 ) 0.0671 (I > 2σ(I), wR2 ) 0.1259).
Anal. Calcd for C42H28BCrF15O: C, 56.27; H, 3.15. Found: C, 54.66:
H, 2.89. We attribute the low carbon value to the presence of less than a
stoichiometric amount of toluene in the analytical sample.
Experiments in toluene-d8 exclude the involvement of the solvent
in the formation of the hydroborate anion of 3, indicating that the
hydride must originate from a cyclopentadienyl ring. Loss of CO
in the formation of 2 is reversible, and exposure of a solution of 2
in toluene-d8 to either an atmosphere of CO or an equivalent of
xylyl isocyanide (XylNC) at room temperature produces the
corresponding adducts, the structures and properties of which will
be described in a full paper.
In summary, complex 1 represents the first structurally character-
ized bent-metallocene complex of Cr(4+). This species is metastable
and decomposes thermally in toluene to species that contain Cr in
its preferred +3 oxidation state. The chirality conferred to com-
plexes 1 and 2 by the boryl substituent poises them for application
in the asymmetric catalysis of reactions such as the Nozaki-
Hiyama-Kishi reaction, for which Cp2Cr14 and our ansa-chro-
mocene derivatives15 have demonstrated activity.
(10) Although paramagnetic complexes of the type Cp2CrXY (X ) halide,
CN, NCSe, NCS; Y ) CN, NCSe, NCS) were previously reported and
characterized by elemental analysis, ESR, and magnetic susceptibility,
X-ray structural confirmation was not obtained: (a) Mora´n, M.; Ferna´ndez,
V. J. Organomet. Chem. 1979, 165, 215-223. (b) Mora´n, M.; Gayoso,
M. Z. Naturforsch. 1983, 38b, 177-180.
(11) Crystallographic data for 2: monoclinic, space group P2(1)/c; a ) 15.952-
(3) Å, b ) 8.9873(15) Å, c ) 21.434(4) Å, â ) 102.47(3°) at 203(2)K;
V ) 3000.5(9) Å3; Z ) 4; Dcalc ) 1.716 Mg/m3, µabs ) 0.502 mm-1
,
F(000) ) 1548, refl. collected ) 25 522, indep. refl. ) 5287, R1 ) 0.0651
(I > 2σ(I), wR2 ) 0.1116). Anal. Calcd for C34H19BCrF15: C, 52.67; H,
2.47. Found: C, 52.40; H, 2.52.
(12) Bondi, A. J. Phys. Chem. 1964, 68, 441-551.
(13) Wulfsberg, G. Inorganic Chemistry; University Science Books: Sausalito,
CA, 2000; p 32.
(14) Fu¨rstner, A. J. Am. Chem. Soc. 1996, 118, 12349-12357.
(15) Shapiro, P. J.; Ho¨hn, B.; Sinnema, P.-J.; Bandini, M.; Cozzi, P. G.
Abstracts of Papers, 221st National Meeting of the American Chemical
Society, San Diego, CA; American Chemical Society: Washington, D.C.,
2001; ORGN 603.
Acknowledgment. The authors are grateful to the donors of
the Petroleum Research Fund, administered by the American
Chemical Society, the National Science Foundation (grant No. CHE-
9816730), and the Department of Energy EPSCoR program (grant
JA026633Q
9
J. AM. CHEM. SOC. VOL. 124, NO. 37, 2002 10997