Isolation of a stable ansa-chromocene(III) carbonyl cation and characterization of a transient, cationic ansa-chromocene(IV) carbonyl hydride

Article abstract of DOI:10.1021/om011066l

Protonation of the metal occurs upon reaction of Me₄C₂(C₅H₄)₂CrCO with [H(Et₂O)₂][B(3,5-(CF₃)₂C₆ H₃)₄] at -70 °C in thf-d₈. The diamagnetic Cr(IV) hydride intermediate was characterized by ¹H NMR spectroscopy. Its decomposition to a paramagnetic product mixture upon warming to room temperature is accompanied by the evolution of 0.5 equiv of gas that is 54% H₂ and 46% CO. Crystals of the borate salt of the 17e^- ansa-chromocenium carbonyl cation [Me₄C₂(C₂H₄)₂CrCO]⁺ were isolated from the product mixture for a molecular structure determination.

Full text of DOI:10.1021/om011066l

© Copyright 2002
Volume 21, Number 6, March 18, 2002
American Chemical Society
Communications
Isola tion of a Sta ble a n sa -Ch r om ocen e(III) Ca r bon yl
Ca tion a n d Ch a r a cter iza tion of a Tr a n sien t, Ca tion ic
a n sa -Ch r om ocen e(IV) Ca r bon yl Hyd r id e
David M. J . Foo, Piet-J an Sinnema, Brendan Twamley, and Pamela J . Shapiro*
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343
Received December 17, 2001
Summary: Protonation of the metal occurs upon reaction
of Me4C2(C5H4)2CrCO with [H(Et2O)2][B(3,5-(CF3)2C6H3)4]
at -70 °C in thf-d8. The diamagnetic Cr(IV) hydride
intermediate was characterized by 1H NMR spectroscopy.
Its decomposition to a paramagnetic product mixture
upon warming to room temperature is accompanied by
the evolution of 0.5 equiv of gas that is 54% H2 and 46%
CO. Crystals of the borate salt of the 17e- ansa-
chromocenium carbonyl cation [Me4C2(C2H4)2CrCO]+
were isolated from the product mixture for a molecular
structure determination.
geometry. This makes the pairing of electrons required
for the coordination of ligands such as carbon monoxide
thermodynamically unfavorable.2 Another problem is
the lability of chromocene toward the loss of one or both
cyclopentadienyl rings in its reaction with small
molecules.1a,3
Brintzinger and co-workers demonstrated that in-
cluding an ansa-bridge between the rings stabilizes the
bent-sandwich chromocene carbonyl derivative so that
it can be isolated and structurally characterized.4 The
enhanced thermal stability of the ansa-chromocene
carbonyl adduct has been confirmed in theoretical
calculations by Simpson et al.5 and by Green and
J ardine.6 These results have encouraged us to pursue
The reaction chemistry of bent-sandwich dicyclopen-
tadienylchromium complexes has been conspicuously
absent from the literature despite the significant role
that early transition metal-containing metallocenes
have played and continue to play in our understanding
of fundamental organo-transition-metal reactivity and
their current prominence as homogeneous catalysts,
particularly for olefin polymerization. Some of the
earliest investigations into bent-metallocene chemistry
involved molybdenocene and tungstenocene, which form
a wide variety of derivatives and display a host of
thermal and photochemical reactions including [2+2]
additions and eliminations, the oxidative addition and
reductive elimination of small molecules, R and â
migrations and eliminations, and C-H bond activation.1
The relative paucity of information on bent-chromocene
complexes may be traced to a couple of problems. One
is the preference of chromocene for the high-spin
electronic configuration associated with a parallel ring
(1) (a) Morris, M. J . In Comprehensive Organometallic Chemistry;
Abel, E. W., Gordon, F., Wilkinson, G., Eds.; Pergamon: Oxford, 1992;
Vol. 5, Chapter 7, and references therein. (b) Davis, R.; Kane-Maguire,
L. A. P. In Comprehensive Organometallic Chemistry; Wilkinson, G.,
Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 3,
Chapters 27.2 and 28.2, and references therein.
(2) (a) Brintzinger, H. H.; Lohr, L. L., J r.; Tang Wong, K. L. J . Am.
Chem. Soc. 1975, 97, 5146-5155. (b) Wong, K. L. T.; Brintzinger, H.
H. J . Am. Chem. Soc. 1975, 97, 5143-5146.
(3) (a) Sneeden, R. P. A. Organochromium Compounds; Academic
Press: New York, 1975. (b) Davis, R.; Kane-Maguire, L. A. P. In
Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G.
A., Abel., E. W., Eds.; Pergamon: Oxford, 1982; Vol. 3, Chapter 26.2,
and references therein.
(4) (a) Schaper, F.; Rentzsch, M.; Prosenc, M. H.; Rief, U.; Schmidt,
K.; Brintzinger, H.-H. J . Organomet. Chem. 1997, 534, 67-79. (b)
Schwemlein, H.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. J .
Organomet. Chem. 1983, 256, 285-289.
(5) Simpson, K. M.; Rettig, M. F.; Wing, R. M. Organometallics 1992,
11, 4363-4364.
10.1021/om011066l CCC: $22.00 © 2002 American Chemical Society
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1006 Organometallics, Vol. 21, No. 6, 2002
Communications
Sch em e 1
the ansa-chromocene derivatives as a means of gaining
entry into the chemistry of the elusive bent-sandwich
chromocene system. The development of a more conve-
nient and higher-yielding synthetic route to ansa-
chromocene carbonyl and isocyanide derivatives in our
laboratory7 has allowed us to prepare the complexes in
sufficient quanitities for further chemical investigation.
Described herein is the first structurally characterized
bent-metallocene complex of Cr(III) and the NMR
characterization of its Cr(IV) hydride precursor, which
are formed through the protonation and subsequent one-
electron oxidation of the neutral, 18 e- Cr(II) carbonyl
derivative.
Brønsted acids have been employed previously for the
one-electron oxidation of neutral Cp2M complexes to
form the corresponding parallel-ring metallocenium
ions.8 Since reversible electrochemical oxidation of ansa-
chromocene carbonyl and isonitrile complexes has been
demonstrated,7a,9 this appeared to be a straightforward
chemical approach to the corresponding cations. Indeed,
reaction of Me4C2(C5H4)2CrCO (1) with the oxonium acid
[H(Et2O)2][B(3,5-(CF3)2C6H3)4]10 afforded crystals of the
Cr(III) species [Me4C2(C5H4)2CrCO][B(3,5-(CF3)2C6H3)4]
(3), the structure of which was determined by X-ray
crystallography.11
3 are 143.3(5)° and 142.2(2)°, respectively, with associ-
ated dihedral angles between the Cp ring planes of 38.5°
and 38.6°, respectively. The average Cr-Cp centroid
distance of 1.808(5) Å for 3 is close to that of 1 (1.78(1)
Å). The longer Cr-CO distance in 3 (1.893(6) vs 1.85-
(1) Å in 1) and shorter CrC-O distance (1.133(7) vs 1.16-
(1) Å in 1) reflect the higher oxidation state of the
chromium center in 3. The only other chromocene
carbonyl cations that have been isolated and structur-
ally characterized, Cp(2,4-C7H11)CrCO+ and Cp*(3-
C6H9)CrCO+, contain an open η5-pentadienyl ligand in
place of one of the cyclopentadienyl rings.12 Accurate
structural details could only be obtained for the Cp*
complex, for which the Cr-CO distance (1.858(7) Å) was
slightly longer and the CrC-O distance (1.145(7) Å)
slightly shorter than the corresponding distances in 3,
in keeping with the greater electron-releasing properties
of the Cp* ring relative to Cp.
The intermediacy of a diamagnetic chromium(IV)
hydride derivative (2) in the formation of 3 (Scheme 1)
was established by monitoring the reaction by 1H NMR
spectroscopy. The 1H NMR spectrum of the C1-sym-
metric chromocenium carbonyl hydride derivative13 is
observed immediately upon mixing the reagents at -70
°C in thf-d8. Complex 2 is stable in solution at -70 °C
for at least 14 h and at -40 °C for at least 9 h. Upon
warming the NMR sample above -25 °C, the signals
An ORTEP drawing of the ansa-chromocenium car-
bonyl portion of salt 3 is shown in Figure 1. The
geometry of the cation is similar to that of the neutral
species, 1. The centroid-Cr-centroid angles for 1 and
(6) Green, J . C.; J ardine, C. N. J . Chem. Soc., Dalton Trans. 1999,
3767-3770.
(7) (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.
(8) (a) Curphey, T. J .; Sater, J . O.; Rosenblum, M.; Richards, J . H.
J . Am. Chem. Soc. 1960, 82, 5249-5250. (b) Bitterwolf, T. E.; Ling, A.
C. J . Organomet. Chem. 1972, 40, C29-C32. (c) Bitterwolf, T. E.; Ling,
A. C. J . Organomet. Chem. 1973, 57, C15-C18. (d) Doerrer, L. H.;
Green, M. L. H. J . Chem. Soc., Dalton Trans. 1999, 4325-4329.
(9) van Raaij, E. U.; Mo¨nkeberg, S.; Kiesele, H.; Brintzinger, H. H.
J . Organomet. Chem. 1988, 356, 307-314.
(10) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992,
11, 3920-3922.
(11) Crystallographic data for 3: monoclinic, space group P21; a )
9.9402(9) Å, b ) 16.4924(14) Å, c ) 15.1391(13) Å, b ) 105.4240(10)°
at 203(2) K; V ) 2392.5(4) Å3; Z ) 2; Dcalc ) 1.604 Mg/m3, mabs ) 0.371
mm-1, F(000) ) 1158, reflns collected ) 14 377, indep reflns ) 7487,
R1 ) 0.0534 (I > 2σ(I), wR2 ) 0.1427). Anal. Calcd for C49H36
-
F igu r e 1. Top and side views of the cation portion of the
molecular structure of 3. Thermal ellipsoids are shown at
30% probability. Selected bond lengths and angles (Å,
deg): Cr-C(17) ) 1.893(6); C(17)-O(1) ) 1.133(7); Cr-
cent (av) ) 1.808(5); Cr-C(17)-O(1) ) 178.8(5); cent-Cr-
cent ) 142.2(2).
BCrOF24: C, 50.74; H, 3.13. Found: C, 50.43: H, 3.28.
(12) Shen, J . K.; Freeman, J . W.; Hallinan, N. C.; Rheingold, A. L.;
Arif, A. M.; Ernst, R. D.; Basolo, F. Organometallics 1992, 11, 3215-
3224
(13) 1H NMR (THF-d8, -70 °C, 500 MHz): δ 7.90, 7.50 (2s, aryl-B,
12H), 6.00, 5.82, 5.80, 5.53 (4s, 8H, C5H4), 1.31, 1.26 (2s, 12H,
(CH3)4C2), -7.99 (1H, Cr-H).

Communications
Organometallics, Vol. 21, No. 6, 2002 1007
due to 2 disappear, leaving only the signals for the
borate counteranion and a new, very broad signal at
-1.5 ppm.
to date were formed by oxidative addition of XCN (X )
halide), (CN)2, and (SCN)2 to Cp2Cr.19 These pseudoha-
lide derivatives are paramagnetic and have not been
crystallographically characterized.
Toepler pump measurements14 showed the evolution
of 0.5 ( 0.02 equiv of a noncondensable gas per
chromium when the reaction was performed in toluene;
however, IR analysis of the gas indicated that it
contained a substantial amount of carbon monoxide.
Oxidation of the gas to CO2 and H2O over a heated
column of CuO revealed that the gas was 46% CO and
54% H2. Thus, other paramagnetic species are formed
along with 3, possibly the 16e- chromocenium hydride
ion and/or the 15e- chromocenium cation.15 As a result,
the reaction product exhibits a higher magnetic mo-
ment16 (µeff ) 2.85 µB) than that expected for a low-spin,
17e- complex. One-electron oxidation of 1 by [CPh3]-
[B(C6F5)4]17 in toluene affords the ansa-chromocene
carbonyl salt [Me4C2(C5H4)2CrCO][B(C6F5)4] (4) with
only a small amount of CO gas evolution (0.08 equiv
per Cr). The X-ray crystal structure of 4 was also
determined. The geometric features of the chromoce-
nium carbonyl cation fragment are very similar to that
of 3 and will be reported elsewhere. The frequency of
the carbonyl stretch in 4 (2002 cm-1) is 100 cm-1 higher
than that of neutral 1 (1905 cm-1). A similar increase
in C-O stretching frequencies is observed upon one-
electron oxidation of half-open chromocene carbonyl
complexes. A µeff ) 1.83 µB was measured for 4,
consistent with the spin-only value expected for a low-
spin, 17e- species. Both 3 and 4 exhibit normal Curie
magnetic behavior from 188 to 300 K and are ESR active
at room temperature with g values close to 2.
In summary, we have isolated and structurally char-
acterized the first stable chromocene carbonyl cation,
which is formed by one-electron oxidation of the neutral
chromocene carbonyl species by either a Brønsted acid
or a trityl salt. The diamagnetic, cationic Cr(IV) hydride
intermediate that is formed en route to 3 is a rare
example of a bent-sandwich bis(cyclopentadienyl)chro-
mium(IV) derivative and demonstrates that this oxida-
tion state can play a role in the reaction chemistry of
these systems. We are currently examining further the
physical properties and reactivity of these bent-sand-
wich derivatives of Cr(III) and Cr(IV) and exploring
their potential application to homogeneous catalysis.
Ack n ow led gm en t. 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 Depart-
ment of Energy EPSCoR program (grant 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 Sci-
ence Foundations, and the NSF-Idaho EPSCoR Pro-
gram. The authors thank Dr. Alex Blumenfeld (Univer-
sity of Idaho) for his assistance with the NMR experi-
ments, Professor Roger Willett (Washington State Uni-
versity) for the use of his ESR spectrometer, and Ms.
Suh-J ane Lee for preparing the [H(Et2O)2][B(3,5-
(CF3)2C6H3)4] and [CPh3][B(C6F5)4].
Our ability to isolate and structurally characterize 3
and 4 is remarkable in light of the instability of neutral
Cp2CrCO (let alone its one-electron oxidation product)
and highlights the influence of the ansa-bridge in
stabilizing derivatives that are not isolable with the
parent, unbridged chromocene system.18 Detection of the
diamagnetic Cr(IV) hydride intermediate 2 is also
significant since it means that bent-sandwich chromo-
cene(IV) derivatives of potential catalytic utility are
chemically accessible. The only other dicyclopentadi-
enylchromium(IV) derivatives that have been reported
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for 3, NMR spectra of 2, ESR spectra of 3 and 4, and
synthetic and experimental details (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
OM011066L
(18) For more examples of the ansa effect see: (a) Churchill, D. G.;
Bridgewater, B. M.; Parkin, G. J . Am. Chem. Soc. 2000, 122, 178-
179. (b) Churchill, D.; Shin, J . H.; Hascall, T.; Hahn, J . M.; Bridge-
water, B. M.; Parkin, G. Organometallics 1999, 18, 2403-2406. (c)
Shin, J . H.; Parkin, G. Chem. Commun. 1999, 887-888. (d) Lee, H.;
Desrosier, P. J .; Guzei, I.; Rheingold, A. L.; Parkin, G. J . Am. Chem.
Soc. 1998, 120, 3255-3256. (e) Conway, S. L. J .; Dijkstra, T.; Doerrer,
L. H.; Green, J . C.; Green, M. L. H.; Stephens, A. H. H. J . Chem. Soc.,
Dalton Trans. 1998, 2689-2695. (f) Chernega, A.; Cook, J .; Green, M.
L. H.; Stephens, A. H. H. J . Chem. Soc., Dalton Trans. 1997, 3225-
3243. (g) Labella, L.; Chernega, A.; Green, M. L. H. J . Chem. Soc.,
Dalton Trans. 1995, 395-399. (h) Heinekey, D. M.; Radzewich, C. E.
Organometallics 1999, 18, 3070-3074.
(14) Burger, B. J .; Bercaw, J . E. In Experimental Organometallic
Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical
Society: Washington, DC, 1985; Chapter 4.
(15) Robbins, J . L.; Edelstein, N.; Spencer, B.; Smart, J . C. J . Am.
Chem. Soc. 1982, 104, 1882-1893.
(16) Magnetic susceptibility measurements were performed by the
Evan’s NMR method; see: Girolami, G. S.; Rauchfuss, T. B.; Angelici,
R. J . Synthesis and Technique in Inorganic Chemistry; University
Science Books: Sausolito, 1999; pp 120-126.
(17) Ihara, E.; Young, V. G.; J ordan, R. F. J . Am. Chem. Soc. 1998,
120, 8277-8278, Supporting Information, page 3.
(19) (a) Mora´n, M.; Ferna´ndez, V. J . Organomet. Chem. 1979, 165,
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