Halogen oxidation reactions of (C5Ph5)Cr(CO)3 and lewis base addition to [(C5Ph5)Cr(μ-X)X]2: electrochemical, magnetic, and Raman spectroscopic characterization of [(C5Ph5</

Article abstract of DOI:10.1021/om000653o

The 17-electron complex (C₅Ph₅)Cr(CO)₃ reacts with halogens (C₆H₅I·Cl₂, Br₂, and I₂) in C₆H₆ to yield the dimeric oxidation products [(C₅

Full text of DOI:10.1021/om000653o

734
Organometallics 2001, 20, 734-740
Ha logen Oxid a tion Rea ction s of (C5P h 5)Cr (CO)3 a n d
Lew is Ba se Ad d ition to [(C5P h 5)Cr (µ-X)X]2:
Electr och em ica l, Ma gn etic, a n d Ra m a n Sp ectr oscop ic
Ch a r a cter iza tion of [(C5P h 5)Cr X2]2 a n d (C5P h 5)Cr X2(THF )
(X ) Cl, Br , I) a n d X-r a y Cr ysta l Str u ctu r e of
[(C5P h 5)Cr (µ-Cl)Cl]2
Marc A. Hutton, J ames C. Durham, Robert W. Grady, Brett E. Harris,
Carter S. J arrell, J . Matthew Mooney, and Michael P. Castellani*
Department of Chemistry, Marshall University, Huntington, West Virginia 25755
Arnold L. Rheingold*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, 19716
Ulrich Ko¨lle*
Institute for Inorganic Chemistry, Technical University at Aachen, Prof.-Pirlet-Strasse 1,
D-52074 Aachen, Germany
Brenda J . Korte, Roger D. Sommer, and Gordon T. Yee*
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309
J . Matthew Boggess and Roman S. Czernuszewicz*
Department of Chemistry, University of Houston, Houston, Texas 77204
Received J uly 28, 2000
The 17-electron complex (C5Ph5)Cr(CO)3 reacts with halogens (C6H5I‚Cl2, Br2, and I2) in
C6H6 to yield the dimeric oxidation products [(C5Ph5)Cr(µ-X)X]2 as thermally stable solids.
Reactions with other chlorinating agents similarly yield [(C5Ph5)CrCl2]2. An X-ray crystal
structure of [(C5Ph5)Cr(µ-Cl)Cl]2 was obtained. The magnetic properties of the Cl2-bridged
dimer have been determined and modeled using the usual isotropic Hamiltonian H ) -2J Sˆ1‚
Sˆ2, which yields J / k ) -30 K. Low-temperature (77 K) Raman spectra of solid [(C5Ph5)-
CrX2]2 (X ) Cl, I) allow assignments to be made for the metal-ring and metal halogen
stretching modes in the low-frequency region (<600 cm-1). Tetrahydrofuran (THF) cleaves
these dimers to yield complexes of the form (C5Ph5)CrX2(THF).
In tr od u ction
as a comparison of the physical properties of the three
radical species (C5R5)Cr(CO)3 (R ) H, Me, Ph). In many
ways the physical properties of these species are similar;
however in some notable ways (e.g., reduction poten-
tials) they differ significantly. A question that naturally
follows such an examination is how the large size and
electron-withdrawing capabilities of the pentaphenyl-
Over the past two decades, the study of paramagnetic
organometallic complexes has greatly expanded.1 These
complexes are generally highly reactive, and many have
been postulated as reaction intermediates. In particular,
the (η5-C5R5)Cr(CO)3 (R ) H, Me, Ph) family of com-
plexes recently has received much attention. The R )
H and Me complexes both exist in equilibria between
17e monomers and 18e dimers in solution and as dimers
in the solid state,2 while for R ) Ph the complex exists
solely as a 17e monomer both in solution and in the solid
state.3
(2) (a) Adams, R. D.; Collins, D. E.; Cotton, F. A. J . Am. Chem. Soc.
1974, 96, 749. (b) McLain, S. J . J . Am. Chem. Soc. 1988, 110, 643. (c)
Goh, L. Y.; Khoo, S. K.; Lim, Y. Y. J . Organomet. Chem. 1990, 399,
115. (d) Goh, L. Y.; Hambley, T. W.; Darensbourg, D. J .; Reibenspies,
J . J . Organomet. Chem. 1990, 381, 349. (e) Watkins, W. C.; J aeger,
T.; Kidd, C. E.; Fortier, S.; Baird, M. C.; Kiss, G.; Roper, G. C.; Hoff,
C. D. J . Am. Chem. Soc. 1992, 114, 907. (f) The fluorenyl complex
(C13H9)Cr(CO)3 has also been prepared. It is also unstable and
undergoes a Cr(CO)3 shift from the C5 ring to a C6 ring, followed by a
dimerization through the methine C5 carbon. Novikova, L. N.; Ustynyuk,
N. A.; Tumanskii, B. L.; Petrovskii, P. V.; Borisenko, A. A.; Kukharen-
ko, S. V.; Strelets, V. V. Isv. Akad. Nauk, Ser. Khim. 1995, 1354 (Engl.
Transl. Russ. Chem. Bull. 1995, 44, 1306).
The isolation of (C5Ph5)Cr(CO)3 allowed comparison
of predicted and actual structural parameters, as well
(1) (a) Connelly, N. G.; Geiger, W. C. Adv. Organomet. Chem. 1984,
23, 1. (b) Baird, M. C. Chem. Rev. 1988, 88, 1217. (c) Astruc, D. Chem.
Rev. 1988, 88, 1189. (d) Organometallic Radical Processes; Trogler,
W. C., Ed.; Elsevier: Amsterdam, 1990. (e) Astruc, D. Electron Transfer
and Radical Processes in Transition-Metal Chemistry; Wiley-VCH:
New York, 1995.
(3) Hoobler, R. J .; Hutton, M. A.; Dillard, M. M.; Castellani, M. P.;
Rheingold, A. L.; Rieger, A. L.; Rieger, P. H.; Richards, T. C.; Geiger,
W. E. Organometallics 1993, 12, 116.
10.1021/om000653o CCC: $20.00 © 2001 American Chemical Society
Publication on Web 01/20/2001

Halogen Oxidation Reactions of (C5Ph5)Cr(CO)3
Organometallics, Vol. 20, No. 4, 2001 735
(Mallinckrodt), and all other materials (Fisher) were used as
received without further purification. Elemental analyses were
performed by Mikroanalytisches Labor Pascher, Remagen,
Germany.18
cyclopentadienyl ligand affect the reactivity of com-
plexes incorporating it.
Previous work has shown that reaction of the [(C5R5)-
Cr(CO)3]2 (R ) H (Cp),4 Me (Cp*)5) dimers with halogens
yields complexes of the form (C5R5)Cr(CO)3X (X ) Cl,
Br, I). Likewise, Baird5-8 and co-workers have shown
that alkyl and allyl halides also produce (C5R5)Cr(CO)3X
complexes. Herrmann and co-workers found that react-
ing excess chlorine or bromine with [(C5H5)Cr(CO)3]2 in
acetonitrile yielded the complexes CpCrX2(CH3CN)
(X ) Cl, Br).9 Theopold and co-workers have prepared
[Cp*Cr(µ-X)X]2 by direct reaction of LiCp* with CrCl3‚
3THF,10 while several other methods have been reported
by other groups.11-15
Electr och em istr y. Cyclic voltammetry was performed with
EG&G equipment (175 programmer and 173 potentiostat) as
previously described19 at a Pt electrode on 10-3 M substrate
solutions in dichloromethane and THF using tetrabutylam-
monium hexafluorophosphate as the supporting electrolyte.
Scan rates were generally 0.1 V s-1. Data were stored on a
Nicolet 3091 digital oscilloscope and transferred to a PC.
Simulations were done with a Windows adaptation of the
Gosser program.20 The potential scale is versus aqueous SCE,
which is converted to the ferrocene scale by subtracting 0.40
V from the figures given.
Ra m a n Sp ectr oscop y. Raman spectra were obtained using
a Coherent Innova 90-6 Ar+ ion laser by collecting backscat-
tered photons directly from the surface of a KCl-supported
pellet held under vacuum in a liquid-N2 dewar.21 The radiation
used was 514.5 nm. Laser power was varied from 150 to 400
mW. Under these conditions, no decomposition of the samples
was observed. A Spex 1403 double monochromator equipped
with a pair of 1800 grooves/mm holographic gratings and a
Hamamatsu 928 photomultiplier detector was used to record
the spectra under the control of a Spex DM3000 microcom-
puter system, as described in detail elsewhere.22 Multiple scans
(15-20) were averaged to improve the signal-to-noise ratio.
Each scan was obtained with 6-8 cm-1 slit widths at 1 cm-1/s
intervals. Manipulation of Raman data was performed using
LabCalc software (Galactic Industries, Inc.).
Ma gn etic Su scep tibility. Magnetization versus temper-
ature data were obtained on a Quantum Design MPMS 5T or
7T SQUID magnetometer in 1000 G applied field. Samples
were loaded between two cotton plugs in gelatin capsules or
in glass holders as previously described.23 Diamagnetic cor-
rections for the samples were calculated from Pascal’s con-
stants. The corrections for the holders were calculated from
the measured average gram susceptibility of several nominally
identical holders.
X-r a y Str u ctu r a l Deter m in a tion . Crystallographic data
for [(C5Ph5)Cr(µ-Cl)Cl]2‚2CH2Cl2 are collected in Table 1. Dark
green block crystals were photographically characterized and
determined to belong to the triclinic crystal system. The
centrosymmetric space group alternative was initially chosen
by its frequency of occurrence and the distribution of E values;
the choice was confirmed by subsequent refinement behavior.
An empirical correction for absorption was applied to the data.
The structure was solved by direct methods, completed from
difference Fourier maps, and refined with anisotropic thermal
parameters for all nonhydrogen atoms. Hydrogen atoms were
placed in idealized positions, except for the disordered (two
positions, 70/30 distribution) solvent molecule (CH2Cl2). All
computations used SHELXTL (4.2) software (G. Sheldrick,
Siemens XRD, Madison, WI).
Interestingly, Fischer reports that allyl bromides
convert [CpCr(CO)3]2 to [CpCrBr2]2 at elevated temper-
atures,14 while Baird reports that alkyl and benzyl
bromides react with [CpCr(CO)3]2 to yield CpCr(CO)3Br
at or below ambient temperatures.7 The two most
reasonable pathways for [CpCrX2]2 formation are CpCr-
(CO)3X compounds decomposing or further reaction with
RX. Although all known (C5R5)Cr(CO)3X complexes
decompose slowly both in solution and in the solid state,
to our knowledge no determination of the decomposition
products has been reported. Herein, we report the
synthesis and characterization of (C5Ph5)Cr(CO)3X and
[(C5Ph5)Cr(µ-X)X]2 (X ) Cl, Br, I) compounds by a
variety of pathways and show the halide-bridged dimer
results from the decomposition of the tricarbonyl halide
in both solution and the solid state.
Exp er im en ta l Section
Gen er a l Da ta . All reactions of air- and moisture-sensitive
materials were performed under a nitrogen atmosphere em-
ploying standard Schlenk techniques unless otherwise stated.
Solids were manipulated under argon in a Vacuum Atmo-
spheres glovebox equipped with a HE-493 dri-train. Solvents
(Fisher) were distilled from the appropriate drying agent under
nitrogen: toluene, hexane (sodium/benzophenone), benzene,
tetrahydrofuran (THF) (potassium/benzophenone), carbon tet-
rachloride, chloroform, and dichloromethane (CaH2). (C5Ph5)-
Cr(CO)3‚C6H6 and CrCl3‚3THF16 were prepared according to
3
literature procedures. NMR solvents were vacuum distilled
from CaH2 and placed under a nitrogen atmosphere. C6H5I‚
Cl2 was prepared following a literature procedure,17 and I2 was
sublimed prior to use. Br2 (Fisher), SOCl2 (Acros), PCl3
(4) Hackett, P.; O’Neill, P. S.; Manning, A. R. J . Chem. Soc., Dalton
Trans. 1974, 1625.
(5) J aeger, T. J .; Baird, M. C. Organometallics 1988, 7, 2074.
(6) Goulin, C. A.; Huber, T. A.; Nelson, J . M.; Macartney, D. H.;
Baird, M. C. J . Chem. Soc., Chem. Commun. 1991, 798.
(7) Huber, T. A.; Macartney, D. H.; Baird, M. C. Organometallics
1995, 14, 592.
[(C5P h 5)Cr (µ-Cl)Cl]2 (1). Meth od 1. (C5Ph5)Cr(CO)3 (1.00
g, 1.72 mmol) was dissolved in benzene (50 mL) and then
transferred via cannula to a separate flask containing C6H5I‚
(8) MacConnachie, C. A.; Nelson, J . A.; Baird, M. C. Organometallics
1992, 11, 2521.
(9) Scheer, M.; Nam, T. T.; Schenzel, K.; Herrmann, E.; J ones, P.
G.; Fedin, V. P.; Ikorski, V. N.; Fedorov, V. E. Z. Anorg. Allg. Chem.
1990, 591, 221.
(10) Richeson, D. S.; Mitchell, J . F.; Theopold, K. H. Organometallics
1989, 8, 2570.
(11) Ko¨hler, F. H.; de Cao, R.; Ackermann, K.; Sedlmair, J . Z.
Naturforsch. 1983, 38B, 1406.
(12) Ko¨hler, F. H.; Lachmann, J .; Mu¨ller, G.; Zeh, H.; Brunner, H.;
Pfauntsch, J .; Wachter, J . J . Organomet. Chem. 1989, 365, C15.
(13) Mora´n, M. Trans. Met. Chem. 1981, 6, 173.
(14) Fischer, E. O.; Ulm, K.; Kuzel, P. Z. Anorg. Allg. Chem. 1963,
319, 253.
(18) In our experience, compounds containing the C5Ph5 ligand
frequently exhibit poor elemental analyses. This occurs even for
samples where other analytical techniques are consistent with a pure
sample. Duplicate analyses can sometimes differ by more than 5% in
carbon. For this reason, no elemental analyses are reported for the
iodo compounds. The syntheses and the physical and spectroscopic
properties measured were consistent with the formulations given.
(19) Ko¨lle, U.; Kossakowski, J . Inorg. Chim. Acta 1989, 164, 23.
(20) Gosser, D. K. In Cyclic Voltammetry; VCH: Weinheim, 1994.
(21) Czernuszewicz, R. S.; J ohnson, M. K. Appl. Spectrosc. 1983, 37,
297-300.
(22) Czernuszewicz, R. S. In Methods in Molecular Biology; J ones,
C.; Mulloy, B.; Thomas, A. H., Eds.; Humana Press: Totawa, 1993;
Vol. 17, p 345.
(15) Manning, A. R.; Thornhill, D. J . J . Chem. Soc. (A) 1971, 637.
(16) Collman, J . P.; Kittleman, E. T. Inorg. Synth. 1966, 8, 149.
(17) Lucas, H. J .; Kennedy, E. R. Organic Syntheses; Wiley: 1955;
Collect. Vol. 3, p 482.
(23) Sellers, S. P.; Korte, B. J .; Fitzgerald, J . P.; Reiff, W. M.; Yee,
G. T. J . Am. Chem. Soc. 1998, 120, 4662.

736 Organometallics, Vol. 20, No. 4, 2001
Hutton et al.
Ta ble 1. Cr ysta l a n d Refin em en t Da ta for
containing (C5Ph5)Cr(CO)3 (1.00 g, 1.72 mmol). The mixture
was refluxed overnight, cooled to room temperature, and
filtered via cannula. The solid was then dried in vacuo to yield
3 as a dark green powder (0.75 g, 58%). Mp: 335 °C (dec).18
(C5P h 5)Cr Cl2(THF ) (4). [(C5Ph5)Cr(µ-Cl)Cl]2 (0.25 g, 0.22
mmol) was dissolved in THF (25 mL). After filtering via
cannula, the solvent was removed in vacuo to yield the product
as an olive green powder in near quantitative yield. Micro-
crystalline samples may be obtained by layering a THF
solution of 4 with hexane. Anal. Calcd for C39H33Cl2CrO: C,
73.12; H, 5.19. Found: C, 72.56; H, 6.32.
(C5P h 5)Cr Br 2(THF ) (5) a n d (C5P h 5)Cr I2(THF ) (6). These
compounds are prepared in an analogous fashion to 4. Anal.
Calcd for C39H33Br2CrO: C, 64.21; H, 4.56. Found: C, 64.27;
H, 4.98.18
(C5P h 5)Cr (CO)3Br (7). (C5Ph5)Cr(CO)3 (0.50 g, 0.86 mmol)
was dissolved in CH2Cl2 (30 mL) and cooled to -90 °C using
a liquid nitrogen/acetone bath. Adding bromine (0.041 mL, 1.59
mmol) immediately caused the solution to change from dark
blue to dark reddish-brown. This solution was filtered into a
second flask and mixed with cold hexane (40 mL). This solution
was transferred to a -50 °C freezer overnight to yield 7 (0.31
g, 55%) as a thermally sensitive, reddish-brown powder.
[(C5P h 5)Cr (µ-Cl)Cl]2‚2CH2Cl2
a. Crystal Data
formula
fw
C72H54Cl8Cr2
1306.8
cryst syst
space group
a, Å
b, Å
c, Å
triclinic
P1h
12.072(6)
12.290(6)
12.638(6)
67.23(4)
65.28(4)
75.99(2)
1565(5)
1
R, deg
â, deg
γ, deg
V, Å3
Z
color
dark green
0.42 × 0.44 × 0.48
1.362
cryst size, mm
D(calcd), g/cm3
abs coeff, mm-1
0.732
b. Data Collection
diffractometer
radiation
temp, K
Siemens P3m/V
Mo KR (λ ) 0.710 73 Å)
296
2θ scan range, deg
scan type
4.0-48.0
Wycoff
no. of reflns collcd
no. of obsd rflns
5191
2868 (F > 5.0σ(F))
Resu lts a n d Discu ssion
c. Solution and Refinement
Syn th eses. Reaction of the (C5Ph5)Cr(CO)3 radical
with C6H5I‚Cl2, Br2, or I2 yielded olive green complexes
of the form [(C5Ph5)Cr(µ-X)X]2 (X ) Cl (1), Br (2), I (3))
in high yields (eq 1).
solution
direct methods
full-matrix least-squares
∑w(Fo - Fc)2
w-1 ) σ2(F) + 0.0010F2
311
R ) 6.73, wR ) 8.25
R ) 11.19, wR ) 9.32
1.58
refinement method
quantity minimized
weighting scheme
no. of params refined
final R indices (obsd data), %
R indices (all data), %
GOF
C6H6
2 (C5Ph5)Cr(CO)3 + X2
8
[(C5Ph5)Cr(µ-X)X]2 + 6 CO (1)
data-to-param ratio
largest diff peak, e Å-3
largest diff hole, e Å-3
9.2:1
0.75
-0.61
Complex 1 can also be prepared from the reaction of
CrCl3‚3THF and Na(C5Ph5) in refluxing benzene or from
the reaction of CHCl3, CCl4, SOCl2, or PCl3 with (C5-
Ph5)Cr(CO)3. All three complexes are poorly soluble in
noncoordinating solvents, although solubility increases
in the order 1 < 2 < 3. Crystalline materials are
produced by layering a solution of the appropriate Me3-
SiX in hexane above a solution of a dichloromethane
solution of the THF adduct (vida infra).24
Cl2 (0.53 g, 2.02 mmol). The stirred solution immediately
turned yellowish brown, then quickly to olive green with gas
evolution. After stirring overnight, a green solid precipitated.
The mixture was filtered via cannula and the solid dried in
vacuo to yield 0.86 g (0.75 mmol, 88%) of 1 as an olive green
powder. Crystals were grown by dissolving (C5Ph5)CrCl2(THF)
(vide infra) in CH2Cl2 and layering with a 1:1 mixture of Me3-
SiCl/hexane. Mp: 335 °C (dec). Anal. Calcd for C70H50Cl4Cr2:
C, 73.95; H, 4.43. Found: C, 73.55; H, 4.38.
Meth od 2. Na[C5Ph5] (2.50 g, 5.34 mmol) and CrCl3‚3THF
(2.00 g, 5.34 mmol) were combined in dry benzene (80 mL) to
give a dark olive green colored solution. The mixture was
refluxed overnight, cooled to room temperature, and filtered
via cannula. The resulting solid was dried in vacuo to yield
2.13 g (1.87 mmol, 70%) of 1 as an olive green powder. The
identity of the product in this reaction (and that in method 3)
was confirmed by comparison of its IR spectrum (KBr pellet)
with that of a sample generated by method 1.
Dissolving these complexes in THF results in their
cleavage to monomeric species, (C5Ph5)CrX2(THF) (eq
2). All solids of these monomers are very similar in color
[(C5Ph5)Cr(µ-X)X]2 + 2 THF f
2 (C5Ph5)CrX2(THF) (2)
to their parent dimers, but form blue-green solutions.
They are much more soluble than their parent dimers.
Initial reaction of (C5Ph5)Cr(CO)3 and each halogen
results in formation of the expected 18-electron (C5Ph5)-
Cr(CO)3X, which then further reacts with additional
halogen to yield [(C5Ph5)CrX2]2. At ambient tempera-
ture, solutions of (C5Ph5)Cr(CO)3I decompose over a
period of days to 3, those of (C5Ph5)Cr(CO)3Br, 7,
decompose to 2 in less than 10 min, and those of (C5-
Ph5)Cr(CO)3Cl decompose in seconds to 1. CO stretching
frequencies for (C5Ph5)Cr(CO)3X (X ) Br, I) are collected
in Table 4. Attempts to isolate (C5Ph5)Cr(CO)3I at room
temperature always resulted in substantial conversion
to [(C5Ph5)CrI2]2. Solutions of (C5Ph5)Cr(CO)3Br pre-
Meth od 3. (C5Ph5)Cr(CO)3 was dissolved in chloroform to
give a blue solution. This reaction mixture was stirred
overnight. Stirring was stopped, and the suspended 1 was
allowed to settle out.
[(C5P h 5)Cr (µ-Br )Br ]2 (2). Bromine (0.082 mL, 3.18 mmol)
was added to a stirred solution of (C5Ph5)Cr(CO)3 (1.00 g, 1.72
mmol) in benzene (30 mL). The resulting reddish-brown
solution changed to a dark olive green in ca. 10 min. Stirring
was stopped and a dark olive green solid precipitated. The
mixture was filtered via cannula, and the resulting solid was
dried in vacuo to yield 2 (0.99 g, 87%) as an olive green powder.
Visible (CH2Cl2): λmax 410 (sh), 480 (sh), 655. Mp: 330-333
°C (dec). Anal. Calcd for C70H50Br4Cr2: C, 63.95; H, 3.83.
Found: C, 64.00; H, 3.82.
[(C5P h 5)Cr (µ-I)I]2 (3). I2 (0.38 g, 3.0 mmol) was dissolved
in benzene (50 mL) and transferred via cannula to a flask
(24) van der Heijden, H.; Schaverien, C. J .; Orpen, A. G. Organo-
metallics 1989, 8, 255.

Halogen Oxidation Reactions of (C5Ph5)Cr(CO)3
Organometallics, Vol. 20, No. 4, 2001 737
Ta ble 2. Selected Bon d Dista n ces (Å) in [(C5P h 5)Cr (µ-Cl)Cl]2‚2CH2Cl2
a
b
c
[(C5Ph5)Cr(µ-Cl)Cl]2
[(C5H5)Cr(µ-Cl)Cl]2
[(C5Me5)Cr(µ-Cl)Cl]2
Cr-C(1)
Cr-C(2)
Cr-C(3)
Cr-C(4)
Cr-C(5)
Cr-CNTd
Cr-Cl(1)
Cr-Cl(2)
Cr-Cl(2A)
Cr(A)-Cl(2)
Cr-Cr
2.270(5)
2.285(5)
2.266(6)
2.240(7)
2.242(7)
1.911
2.247(3)
2.380(3)
2.360(3)
2.360(3)
3.375
1.867
1.88
2.274(2)
2.377(1)
2.374(1)
2.291(2)
2.393(1)
2.383(1)
3.362(1)
a
b
d
This work. Ref 11. c Ref 12. There were two conformers in the unit cell. Averaged values are presented. CNT ) centroid of the
cyclopentadienyl ring.
Ta ble 3. Selected Bon d An gles (d eg) in [(C5P h 5)Cr (µ-Cl)Cl]2‚2CH2Cl2
a
b
c
[(C5Ph5)Cr(µ-Cl)Cl]2
[(C5H5)Cr(µ-Cl)Cl]2
[(C5Me5)Cr(µ-Cl)Cl]2
Cl(1)-Cr-Cl(2)
Cl(1)-Cr-Cr(2A)
Cl(2)-Cr-Cl(2A)
Cr-Cl(2)-Cr(A)
CNT-Cr-Cl(1)
CNT-Cr-Cl(2)d
CNT-Cr-Cl(2A)d
92.2 (1)
92.2 (1)
89.2 (1)
92.2 (1)
124.4
96.12 (6)
97.0 (1)
96.5 (1)
87.7 (1)
92.3 (1)
123.2
89.89 (5)
123.6
121.8
120.5
124.6
124.2
122.0
122.4
a
b
d
This work. Ref 11. c Ref 12. There were two conformers in the unit cell. Averaged values are presented. CNT ) centroid of the
cyclopentadienyl ring.
Ta ble 4. In fr a r ed Sp ectr a l Da ta for
(C5P h 5)Cr (CO)3X Com p lexes
The C5Ph5 ligand probably decreases the stability of
the (C5Ph5)Cr(CO)3X complexes relative to their Cp and
Cp* analogues because its large size increases crowding
and because it releases far less electron density to the
metal than either Cp or Cp*.3,28 Furthermore seven-
coordinate first-row transition metal complexes are
uncommon because of the small size of the metal atoms/
ions.29
complex
solvent
ν(CtO), cm-1
reference
(C5H5)Cr(CO)3Br
(C5Me5)Cr(CO)3Br
(C5Ph5)Cr(CO)3Bra
(C5H5)Cr(CO)3I
(C5Me5)Cr(CO)3I
(C5Ph5)Cr(CO)3I
CS2
hexane
THF
CS2
hexane
THF
2041, 1986, 1954
2032, 1979, 1935
2031, 1980, 1939
2029, 1975, 1953
2018, 1967, 1934
2016, 1965, 1937
15
5
this work
15
5
this work
The aforementioned reports on (C5R5)Cr(CO)3X (X )
halide), coupled to reports on the thermal stabilities of
X ) H30 and the [(C5R5)Cr(CO)3]2 dimers,2b,e suggest
that the crowding associated with the seven-coordinate
geometry significantly reduces the stabilities of these
molecules to a point of marginal viability. However, if
only steric factors affected complex stability, the order
of stability would be Cl > Br > I. The actual ordering
is I > Br > Cl for both R ) H and Ph, consistent with
electronic factors also playing an important role. A
reasonable explanation for this observation posits that
as the halogens become more electronegative, the
complexes more readily decompose because of decreased
electron density available for π-back-bonding to the CO
ligands.
a
A cold solution of (C5Ph5)Cr(CO)3Br was rapidly transferred
to an IR cell at ambient temperature, and the spectrum was
immediately taken.
pared and stored below -30 °C are sufficiently stable
that 7 may be isolated as a thermally sensitive, red-
brown solid. On standing for several weeks at ambient
temperature, this solid converts to (C5Ph5)Cr(CO)3 and
2, presumably via a solid-state disproportionation reac-
tion.
The other known (C5R5)Cr(CO)3X (R ) H, Me) com-
pounds also exhibit poor stability. King25 and Baird26
report that CpCr(CO)3Cl is quite unstable. Likewise,
although Cp*Cr(CO)3X (X ) Br, I) can be isolated, they
decompose slowly in the solid state.5 Despite being
sufficiently stable to isolate, Manning found CpCr-
(CO)3Br and CpCr(CO)3I decompose on gentle heating
(76 and 91 °C, respectively),15 while King found them
too unstable to isolate pure.25 No decomposition product
was identified for any of these compounds. In all cases,
the (C5Ph5)Cr(CO)3X compounds are less stable than
either their Cp or Cp* analogues. Finally, cyclic voltam-
metric studies of [CpCr(CO)3]2 suggest that under
appropriate conditions the very unstable CpCr(CO)3L+
(L ) THF, CH2Cl2) ion forms, although it was not
directly observed.27
Molecu la r St r u ct u r e of [(C5P h 5)Cr (µ-Cl)Cl]2‚
2CH2Cl2. The X-ray crystal structure of [(C5Ph5)Cr(µ-
Cl)Cl]2‚2CH2Cl2 is displayed in Figure 1. Bond distances
and angles are listed in Tables 2 and 3, respectively.
The geometry is that of an edge-shared bioctahedron
with C5Ph5 ligands capping opposite faces and bridging
(28) (a) Broadley, K.; Lane, G. A.; Connelly, N. G.; Geiger, W. E. J .
Am. Chem. Soc. 1983, 105, 2486. (b) Connelly, N. G.; Geiger, W. E.;
Lane, G. A.; Raven, S. J .; Rieger, P. H. J . Am. Chem. Soc. 1986, 108,
6219. (c) Lane, G. A.; Geiger, W. E.; Connelly, N. G. J . Am. Chem.
Soc. 1987, 109, 402. (d) Connelly, N. G.; Manners, I. J . Chem. Soc.,
Dalton Trans. 1989, 283.
(29) Huheey, J . E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and Reactivity, 4th ed.; Harper Collins: New
York, 1993; p 502.
(30) (a) Fischer, E. O. Inorg. Synth. 1963, 7, 136. (b) Wormsba¨cher,
D.; Nicholas, K. M.; Rheingold, A. L. J . Chem. Soc., Chem. Commun.
1985, 721. (c) Leoni, P.; Landi, A.; Pasquali, M. J . Organomet. Chem.
1987, 321, 365.
(25) King, R. B.; Efraty, A.; Douglas, W. M. J . Organomet. Chem.
1973, 60, 125.
(26) MacConnachie, C. A.; Nelson, J . M.; Baird, M. C. Organome-
tallics 1992, 11, 2521.
(27) O’Callaghan, K. A. E.; Brown, S. J .; Page, J . A.; Baird, M. C.;
Richards, T. C.; Geiger, W. E. Organometallics 1991, 10, 3119.

738 Organometallics, Vol. 20, No. 4, 2001
Hutton et al.
F igu r e 1. Molecular structure and labeling scheme for [(C5Ph5)Cr(µ-Cl)Cl]2‚2CH2Cl2 (1) (30% thermal ellipsoid probability).
chloride ligands occupying the shared edge. As in
[(C5H5)CrCl2]2 and [(C5Me5)CrCl2]2,12 the chromium
11
atoms and bridging chlorines in 1 form a square in the
center of the complex. The chromium atoms lie too far
apart to bond, and their separation is nearly identical
in the C5H5 and C5Ph5 complexes. Bond distances differ
only slightly from those in the C5H5 and C5Me5 com-
plexes. Most bond angles differ between these complexes
by 0-3°; however, the Cl(bridging)-Cr-Cl(terminal)
angles on 1 are 4-5° smaller than those on the other
complexes. Inspection of a space-filling model of 1
generated from the crystal structure shows close con-
tacts between the terminal chlorine atoms and phenyl
rings bound to the Cp on the same chromium atom.
These contacts may cause the contraction of the Cr-
Cr-Cl bond angle.
Electr och em istr y. The electrochemistry of com-
F igu r e 2. Cyclic voltammograms of (C5Ph5)CrBr2(THF):
plexes 2-6 was investigated by cyclic voltammetry in
THF and dichloromethane. [(C5Ph5)CrCl2]2 did not give
informative electrochemistry in any solvent. Reduction
occurs outside the solvent/electrolyte window, and the
observed rather positive, irreversible oxidation probably
arises from bound chloride.
[(C5Ph5)CrBr2]2 exhibits a quasi-reversible reduction
at -0.45 V in dichloromethane. Incomplete electro-
chemical reversibility does not allow the number of
transferred electrons to be determined from the peak
separation. The THF complex, (C5Ph5)CrBr2(THF), dis-
plays two successive reductions in the same solvent: the
first one reversible and the second irreversible at about
-0.5 and -1.4 V, respectively. This pattern more clearly
develops after the solution has been kept at ambient
temperature for about 30 min prior to data collection.
Since the first reduction is very close to the one found
for the dimeric complex and much different from the
reduction pattern of the mononuclear complex in THF
(vide infra), we suggest complex 5 reverts to dimeric
(A) in THF at v ) 100 mV/s, (B) same after addition of
[PPh3Me]Br.
complex 2 in dichloromethane. The observation of two
successive reduction peaks then argues for stepwise
reduction of each Cr(III) to Cr(II) with concomitant loss
of bromide, slower for the first reduction and faster for
the second. A large peak at 1.1 V on the successive
anodic scan is assigned to the liberation of Br-.
The mononuclear THF complex, (C5Ph5)CrBr2(THF),
in THF instead gave one chemically irreversible reduc-
tion, Epc ) -1.04 V, with close lying follow-up peaks at
-0.23 and 0.03 V, respectively (Figure 2). Simulation
clearly shows these to be direct electrode products of
the reduction. Loss of bromide in the course of the
reduction is a plausible assumption in this case too,
where the follow-up peaks are assigned to the reoxida-
tion of the dehalogenated reduction product with and
without incorporation of bromide. This conclusion is
drawn from the observation that only one single reoxi-

Halogen Oxidation Reactions of (C5Ph5)Cr(CO)3
Organometallics, Vol. 20, No. 4, 2001 739
F igu r e 3. Plot of øT vs T for [(C5Ph5)CrCl2]2 and fit to
Heisenberg Hamiltonian model.
dation (at -0.166 V) follows the -1.04 V reduction on
addition of excess Br- (as [PPh3Me]Br).
Reduction of (C5Ph5)CrI2(THF) in dichloromethane
gave very similar electrochemistry to the bromide
complex with potentials of -0.75 and -0.33 V for
reduction and reoxidation, respectively. In addition,
there are two oxidation peaks at 0.31 and 0.57 V that
occur on the initial anodic scan. The potential range and
the fact that the final oxidation is partly reversible
argue in favor of a bound iodide, which is liberated at
two successive potentials.
Ma gn etic Su scep tibility. The values of øT for all
of the mononuclear species, (C5Ph5)CrX2(THF), X ) Cl
(1.94), Br (1.72), I (1.83), are essentially consistent with
the expected spin only value of 1.875 emu-G/mol for d3
Cr(III) in a pseudo-octahedral field.31 The data are
temperature independent from 25 to 250 K with a slight
downturn below 25 K and warrant no further comment.
The magnetic properties of [(C5Ph5)CrCl2]2 are il-
lustrated in the plot of øT versus T (Figure 3). At room
temperature, the value of øT is less than that expected
for two uncoupled S ) 3/2 centers (3.75 emu-G/mol),
indicating the presence of antiferromagnetic coupling.
This fact has previously been noted by Ko¨hler and co-
workers for the corresponding Cp and Cp* analogues,
but was apparently never quantified.12 The data were
fit (eq 3) to the analytical solution of the Heisenberg
Hamiltonian32 H ) -2J Sˆ1‚Sˆ2.
F igu r e 4. Low-temperature (77 K) Raman spectra (100-
600 cm-1) of 1 (bottom) and 3 (top) excited at 514.5 nm.
Both spectra obtained via backscattering on KCl pellets
in a liquid-N2 dewar: (1) 400 mW laser power, 6 cm-1 slit
widths, average of 16 scans; (3) 150 mW laser power, 8 cm-1
slit widths, average of 15 scans.
Ta ble 5. Ra m a n Low F r equ en cies (cm -1) a n d
Assign m en ts for [(C5P h 5)Cr (µ-X)X]2 Com p lexes
(X ) Cl-, I-)
[(C5Ph5)CrCl2]2
[(C5Ph5)CrI2]2
assignment
157
231
255
272, 311
351
421
401
482 br
536
131
177
198
240
283, 275
414
400 sh
490 br
530
δ(X-Cr-X)
νb(Cr-X)
νb(Cr-X)
νb(Cr-X)
νt(Cr-X)
ν(Cr-Cp), A
ν(Cr-Cp), B
Cp ring tilt
Cp ring tilt
ples were used because the solid precipitates from
solution in liquid samples upon laser exposure. The
strongest peaks obtained for each sample are observed
in the region containing chromium-halogen and Cp-
ring tilting vibrations as seen in Figure 4. All band
identifications and assignments are given in Table 5.
Samples of [(C5Ph5)Cr(µ-Br)Br]2 (2) gave very high
scattering backgrounds and did not provide Raman
spectra.
øT )
2Ng2µB
2
exp[x] + 5 exp[3x] + 14 exp[6x]
1 + 3 exp[x] + 5 exp[3x] + 7 exp[6x]
(3)
{
}
k
2J
where x )
kT
The skeletal vibrational modes are designated by the
symbol ν and represent stretching motions. The metal-
halogen stretching modes were assigned after those of
trans-planar M2X4L2 and tetrahedral Cp2MX2 and Cp-
MX3 compounds (M ) a transition metal).34-37 The νb
stretches represent stretching of the Cr-X bonds in the
Cr2(µ-X)2 bridge and occur in the 175-240 and 230-
315 cm-1 regions in 3 (X ) I) and 1 (X ) Cl), respec-
tively. These are lower in frequency compared to the
terminal Cr-X stretching modes34 represented by νt,
The two-parameter fit gives g ) 2.1 and J / k ) -30
K. The g value is slightly high compared to the expected
value of 2, but the coupling is in the range of other
doubly hydroxy-bridged chromium dimers.33
Ra m a n Sp ectr oscop y. Due to the sensitivity of the
[(C5Ph5)CrX2]2 samples to air and prolonged exposure
to high temperature, Raman spectra could only be
obtained under low-temperature conditions. Solid sam-
(31) Bender-Gresse, M.; Collange, E.; Poli, R., Mattamana, S.
Polyhedron 1998, 17, 1115.
(32) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, 1986;
p 94.
(33) Michelsen, K.; Pederson, E.; Wilson, S. R.; Hodgson, D. J . Inorg.
Chem. Acta 1982, 63, 141.
(34) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 5th ed.; J ohn Wiley: New York, 1997; Part
B, p 188.
(35) Maslowsky, E.; Nakamoto, K. Appl. Spectrosc. 1971, 25, 187.

740 Organometallics, Vol. 20, No. 4, 2001
Hutton et al.
which occur at ∼283 cm-1 in 3 and ∼350 cm-1 in 1.
Other halogen-sensitive bands in this region at 131 (3)
and 157 cm-1 (1) appear to be X-Cr-X bending (δ)
vibrations.35
to release electron density to metals to which it binds
makes the seven-coordinate (C5Ph5)Cr(CO)3X com-
pounds much less stable than their Cp or Cp* ana-
logues. A crystal structure of [(C5Ph5)CrCl2]2 shows the
molecule to exhibit only limited intramolecular crowding
consistent with the reduced strain in a six-coordinate
environment. Magnetic susceptibility measurements
show the chromium centers to couple antiferromagneti-
cally. All complexes exhibit complex electrochemical
behavior, and Raman spectra for the X ) Cl and I
dimers are assigned. The dimers react with Lewis bases
(e.g., THF) to yield monomeric species: (C5Ph5)CrX2-
(THF).
The second Raman region of interest is that contain-
ing metal-cyclopentadienyl (Cp) ring stretching and
tilting vibrations (400-550 cm-1). Due to the fact that
the [(C5Ph5)CrX2]2 compounds are classified in the C2
point group, the ν(Cr-Cp) stretches can be easily
identified as the A- and B-symmetry modes.35,36 These
two ν(Cr-Cp) modes occur in the narrow range of 400-
425 cm-1 and are distinguishable in the chloride dimer
1 as seen in Figure 4 and Table 5. However, the B mode
(∼400 cm-1) for the iodide dimer 3 cannot be clearly
identified due to close overlap with the A mode at 414
cm-1. The Cp ring tilting motions36 are found in the
450-550 cm-1 region. As expected, these Raman bands
are almost independent of X and quite similar for both
compounds (Figure 4). Finally, the Raman spectra of 1
and 3 taken above 600 cm-1 (not shown) also contain
internal C5Ph5 ligand vibrational modes associated with
the Cp and Ph rings at the frequencies typical of
centrally π-bonded complexes34 and monosubstituted
benzenes.38
Ack n ow led gm en t. We thank the generous support
of the National Science Foundation (Grant NSF CHE-
9630094) and the donors of the Petroleum Research
Fund, administered by the American Chemical Society
for the research at Marshall University. The NSF
provided funds toward the University of Delaware
diffractometer. G.T.Y. thanks the National Institute of
Standards and Technology for the use of the magne-
tometers and the NSF for support. R.S.C. thanks the
Robert A. Welch Foundation (Grant E-1184) for support.
We also thank Prof. J . W. Larson and Mr. M. M. Dillard
for experimental assistance.
Su m m a r y. (C5Ph5)Cr(CO)3 rapidly reacts with halo-
gens to initially yield (C5Ph5)Cr(CO)3X complexes (X )
Cl, Br, I), which then decompose to [(C5Ph5)CrX(µ-X)]2
dimers in solution. The larger size of C5Ph5 (as com-
pared to Cp or Cp*) combined with its reduced tendency
Su p p or tin g In for m a tion Ava ila ble: For [(C5Ph5)Cr(µ-
Cl)Cl]2‚2CH2Cl2 as follows: Table 1S (bond distances), Table
2S (bond angles), Table 3S (atomic coordinates and equivalent
isotropic displacement coefficients), Table 4S (anisotropic
displacement coefficients), and Table 5S (hydrogen atom
coordinates). This material is available free of charge via the
Internet at http://pubs.acs.org.
(36) Samuel, E.; Ferner, R.; Bigorgne, M. Inorg. Chem. 1973, 12,
881.
(37) Balducci, G.; Bencivenni, L.; DeRosa, G.; Gigli, R.; Martini, B.;
Cesaro, S. N. J . Mol. Struct. 1980, 64, 163.
(38) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic
Raman Frequencies of Organic Compounds; Wiley-Interscience: New
York, 1974; p 162.
OM000653O