Substitution reactions of (C5Ph5)Cr(CO)3: Structural, electrochemical, and spectroscopic characterization of (C5Ph5)Cr(CO)2L (L = PMe3, PMe2Ph, P(OMe)3)

Article abstract of DOI:10.1021/om960374u

The radical complex (C₅Ph₅)Cr(CO)₃ reacts with small, neutral, monodentate Lewis bases (PMe₃, PMe₂Ph, P(OMe)₃) in THF at -78 °C (PMe₂Ph reacts at ambient temperature) to yield th

Full text of DOI:10.1021/om960374u

Organometallics 1996, 15, 4791-4797
4791
Su bstitu tion Rea ction s of (C5P h 5)Cr (CO)3: Str u ctu r a l,
Electr och em ica l, a n d Sp ectr oscop ic Ch a r a cter iza tion of
(C5P h 5)Cr (CO)2L (L ) P Me3, P Me2P h , P (OMe)3)
D. J ohn Hammack,1a Mills M. Dillard,1a Michael P. Castellani,*,1a
Arnold L. Rheingold,*,1b Anne L. Rieger,1c and Philip H. Rieger*,1c
Departments of Chemistry, Marshall University, Huntington, West Virginia 25755,
University of Delaware, Newark, Delaware 19716, and Brown University,
Providence, Rhode Island 02912
Received May 20, 1996X
The radical complex (C5Ph5)Cr(CO)3 reacts with small, neutral, monodentate Lewis bases
(PMe3, PMe2Ph, P(OMe)3) in THF at -78 °C (PMe2Ph reacts at ambient temperature) to
yield the monomeric substitution products (C5Ph5)Cr(CO)2L‚THF as thermally stable solids.
Electrochemical and spectroscopic data are provided. An X-ray crystal structure of the
hemisolvate (C5Ph5)Cr(CO)2PMe3‚0.5THF was obtained. Frozen-solution ESR spectra of (C5-
Ph5)Cr(CO)2L in toluene are comparable to those of other low-spin d5 “piano-stool” complexes.
Rotation of the Cr(CO)2L moiety relative to the C5Ph5 ring is rapid on the ESR time scale in
low-temperature liquid solutions and leads to axial powderlike spectra. Analysis of this
effect leads to significant insights into the electronic structure.
In tr od u ction
studied extensively the substitution reactions of (C5R5)-
Cr(CO)3 (R ) H,9 Me9e,10) with phosphines. Where R
) H (Cp) isolation of a product complex requires larger
phosphines, while for R ) Me (Cp*) only smaller
phosphines replace CO in the starting complex.
Over the past two decades, the study of paramagnetic
organometallic complexes has greatly expanded.2 These
complexes are generally highly reactive, and many have
been postulated as reaction intermediates. In particu-
lar, the (C5R5)Cr(CO)3 (R ) H, Me, Ph) family of
complexes 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,3 while for R ) Ph the
complex exists solely as a 17e monomer both in solution
and the solid state.4
Seventeen electron complexes containing CO ligands
frequently undergo substitution reactions under mild
conditions.5,6 The reactions tend to proceed via associa-
tive mechanisms7 because of incompletely filled sets of
bonding molecular orbitals.8 Baird and co-workers have
The very large size of the C5Ph5 ligand should
significantly restrict the size of substituents that can
substitute CO in (C5Ph5)Cr(CO)3, 1. Three small,
monodentate Lewis bases, PMe3, PMe2Ph, and P(OMe)3,
react with 1 to yield isolable products, (C5Ph5)Cr(CO)2L.
These compounds have been spectroscopically and elec-
trochemically characterized.
There have been many ESR studies of low-spin d5
“piano-stool” complexes such as (C5R5)Cr(CO)3-xLx (R
) H, Me), [(arene)Cr(CO)3-xLx]+, and Mn(II) analogs.11
As we will show, the ESR spectra of (C5Ph5)Cr(CO)2L
fit comfortably into the general scheme for these com-
plexes and are thus rather unremarkable. However, the
unique steric bulk of the C5Ph5 ligand leads to selective
averaging of anisotropies in the ESR spectra of low-
temperature liquid solutions, and parameters obtained
from such spectra provide insights into the electronic
structure which were unavailable in previous studies.
X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) (a) Marshall University. (b) University of Delaware. (c) Brown
University.
(2) (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, ll89. (d) Organometallic Radical Processes; Trogler, W.
C., Ed.; Elsevier: Amsterdam, 1990.
(3) (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 (Russ.
Chem. Bull., Engl. Transl. 1995, 44, 1306).
(4) 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.
(5) (a) Kochi, J . K. Organometallic Mechanisms and Catalysis; Aca-
demic: New York, 1978; Chapter 8. (b) Reference 2d, Chapters 4 and
9.
(7) (a) Atwood, J . D. Inorganic and Organometallic Reaction Mech-
anisms; Brooks/Cole: Monterey, CA, 1985; p 123. (b) Crabtree, R. H.
The Organometallic Chemistry of the Transition Metals; J ohn Wiley
and Sons: New York, 1988; pp 80-81.
(8) (a) Therien, M. J .; Trogler, W. C. J . Am. Chem. Soc. 1988, 110,
4942. (b) Lin, Z.; Hall, M. B. Inorg. Chem. 1992, 31, 2791.
(9) (a) Cooley, N. A.; Watson, K. A.; Fortier, S.; Baird, M. C.
Organometallics 1988, 7, 2563. (b) Cooley, N. A.; MacConnachie, P. T.
F.; Baird, M. C. Polyhedron 1988, 7, 1965. (c) Cooley, N. A.; Baird, M.
C.; Morton, J . R.; Preston, K. F.; Le Page, Y. J . Magn. Reson. 1988,
76, 325. (d) Watkins, W. C.; Macartney, D. H.; Baird, M. C. J .
Organomet. Chem. 1989, 377, C52. (e) Fortier, S.; Baird, M. C.; Preston,
K. F.; Morton, J . R.; Ziegler, T.; J aeger, T. J .; Watkins, W. C.; MacNeil,
J . H.; Watson, K. A.; Hensel, K.; Le Page, Y.; Charland, J .-P.; Williams,
A. J . J . Am. Chem. Soc. 1991, 113, 542. (f) O’Callaghan, K. A. E..;
Brown, S. J .; Page, J . A.; Baird, M. C.; Richards, T. C.; Geiger, W. E.
Organometallics 1991, 10, 3119. (g) Watkins, W. C.; Hensel, K.; Fortier,
S.; Macartney, D. H.; Baird, M. C.; McLain, S. J . Organometallics 1992,
11, 2418.
(6) Huang, Y.; Carpenter, G. B.; Sweigart, D. A.; Chung, Y. K.; Lee,
B. Y. Organometallics 1995, 14, 1423.
(10) J aeger, T. J .; Baird, M. C. Organometallics 1988, 7, 2074.
(11) Rieger, P. H. Coord. Chem. Rev. 1994, 135, 203.
S0276-7333(96)00374-3 CCC: $12.00 © 1996 American Chemical Society

4792 Organometallics, Vol. 15, No. 22, 1996
Hammack et al.
Ta ble 1. Cr ysta l a n d Refin em en t Da ta for
Exp er im en ta l Section
(C5P h 5)Cr (CO)2P Me3‚0.5THF
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 nitrogen or argon in a Vacuum
Atmospheres glovebox equipped with a HE-493 dri-train.
Solvents (Fisher) were distilled from the appropriate drying
agent under argon: toluene, hexane (sodium/benzophenone),
benzene, tetrahydrofuran (THF) (potassium/benzophenone),
and dichloromethane (CaH2). (C5Ph5)Cr(CO)3‚C6H6 was pre-
pared according to a literature procedure.4 NMR solvents were
vacuum distilled from CaH2 and placed under an argon
atmosphere. PPh3 (PCR) was recrystallized from 95% ethanol.
PMe3, PMe2Ph, P(OMe)3, P(OPh)3 (Strem), CDCl3, CD2Cl2
(Aldrich), 2,2′-bipyridine (Matheson), diphenylacetylene (East-
man), and all other solvents (Fisher) were used without further
purification. Elemental analyses were performed by Schwartz-
kopf Microanalytical Laboratory, Woodside, NY, and Mick-
roanalytisches Labor Pascher, Remagen, Germany. 1H (200.06
MHz) and 31P (80.962 MHz) NMR spectra were obtained on a
Varian XL-200 NMR spectrometer equipped with a Motorola
data system upgrade.
a. Crystal Data
formula
fw
cryst system
space group
a, Å
C40H34CrO2P
629.6
triclinic
P1h
12.834(2)
13.271(3)
22.536(5)
90.96(2)
94.29(2)
112.55(1)
3530.6(12)
4
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
color
cryst size, mm
D(calcd), g/cm3
abs coeff, cm-1
dark red
0.44 × 0.58 × 0.74
1.185
0.401 mm-1
b. Data Collection
diffractometer
radiation
temp, K
2θ scan range, deg
scan type
Siemens P4
Mo KR (λ ) 0.710 73 Å)
293
1.00
Wycoff
Electr och em istr y. Electrochemical data were obtained on
a EG&G PAR VersaStat Model 250-1 electrochemical analysis
system. The apparatus was maintained on a bench top under
constant nitrogen purge. Freshly distilled CH2Cl2 was em-
ployed as the solvent, with a supporting electrolyte of 0.1 M
nBu4NPF6 (recrystallized from 95% ethanol). Solutions were
ca. 3 mM in complex.12 Decamethylferrocene was added as
an internal reference. Potentials are referred to the ferrocene/
ferrocenium couple. All data were obtained with a Pt disk
working electrode (r ) 1.6 mm) and either a Ag/AgCl reference
electrode or a AgCl-coated Ag wire reference electrode.
ESR Sp ectr oscop y. Electron spin resonance spectra were
obtained using a Bruker ER-220D X-band spectrometer
equipped with a Bruker variable-temperature accessory, a
Systron-Donner microwave frequency counter, and a Bruker
gaussmeter. Samples for ESR study were prepared in an
argon-filled glovebox by shaking the compound with degassed
toluene to obtain a saturated solution; the solution was
syringed into an ESR tube which was sealed with Parafilm
before removal from the glovebox. One series of spectra was
obtained with 5 mg of the P(OMe)3 complex in 3 mL of 1:1
1,2-C2H4Cl2/CH2Cl2 (dce/dcm); the solution was prepared in a
glovebox as before.
X-r a y Str u ctu r a l Deter m in a tion . Crystallographic data
are summarized in Table 1. A specimen mounted on a glass
fiber was found photographically to possess only triclinic
symmetry. The centrosymmetric space group was initially
assumed and later supported by the reasonable results of
refinement. Variation in azimuthal scans were less than 10%,
and corrections for absorption were ignored. The structure
was solved by direct methods. The asymmetric unit is
composed of two crystallographically independent but chemi-
cally very similar molecules of the Cr complex and one
molecule of THF. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Selected bond distances
and angles are collected in Tables 2 and 3, respectively.
Phenyl dihedral angles are presented in Table 4. All computa-
tions used SHELXTL 4.2 programs (G. Sheldrick, Siemens
XRD, Madison, WI).
reflns collcd
obsd rflns
14 234
6465 (F > 5.0σ(F))
c. Solution and Refinement
solution
direct methods
full-matrix least-squares
∑w(Fo - Fc)2
w-1 ) σ2(F) + 0.0015F2
838
R ) 5.62, wR ) 6.67
R ) 12.75, wR ) 8.93
1.11
refinement method
quantity minimized
weighting scheme
no. of params refined
final R indices (obsd data),%
R indices (all data), %
GOF
data-to-param ratio
largest diff peak, e Å-3
largest diff hole, e Å-3
7.7:1
0.34
-0.39
Ta ble 2. Selected Bon d Dista n ces (Å) in
(C5P h 5)Cr (CO)2P Me3‚0.5THF
conformer
conformer
A
B
A
B
Cr-C(1)
Cr-C(2)
Cr-C(3)
Cr-C(4)
Cr-C(5)
2.254(5) 2.269(5) Cr-P
2.383(2) 2.372(2)
2.288(5) 2.256(4) C(1)-C(2) 1.430(7) 1.424(7)
2.241(6) 2.236(5) C(2)-C(3) 1.441(8) 1.425(8)
2.198(7) 2.212(5) C(3)-C(4) 1.425(6) 1.430(7)
2.218(6) 2.221(4) C(4)-C(5) 1.429(8) 1.432(8)
Cr-CNTa 1.881
1.881
C(1)-C(5) 1.429(7) 1.429(8)
Cr-C(6)
Cr-C(7)
1.837(6) 1.836(7) C(6)-O(6) 1.162(7) 1.156(9)
1.834(6) 1.812(7) C(7)-O(7) 1.149(8) 1.158(9)
a
CNT ) centroid of the cyclopentadienyl ring.
Ta ble 3. Bon d An gles (d eg) in
(C5P h 5)Cr (CO)2P Me3‚0.5THF
conformer
conformer
A
B
A
B
C(6)-Cr-C(7) 78.7(3) 78.0(4) C(6)-Cr-CNTa 125.8 124.8
P-Cr-C(6)
P-Cr-C(7)
OC-Cr-CO
(av)
89.4(2) 86.9(2) C(7)-Cr-CNT 122.4 126.1
89.8(2) 90.4(2) P-Cr-CNT
134.8 133.3
175.8(6) 176.8(7)
a
CNT ) centroid of the cyclopentadienyl ring.
Low -Tem p er a tu r e IR Sp ectr oscop y. In a glovebag, a
dilute solution of 1 in THF was cooled to -78 °C. Two
equivalents of PMe3 were added, and the solution was allowed
to warm until the blue solution turned to a green color. The
solution was recooled to -78 °C and then transferred to a
precooled IR cell via a precooled syringe (both at -78 °C). The
color changes observed are the same as occur in synthetic scale
reactions.
(12) Solutions at higher concentrations than normal were used
because of the high air-sensitivity of the complexes in solution. Under
the conditions employed, 0.5 mM solutions rarely maintained their
integrity for more than a few minutes. Typical values of ∆Ep were in
the range 150-180 mV. A 3 mM solution of (C5Ph5)Cr(CO)3 gave the
same E°(0/1-) value as that reported for a much more dilute solution.4
(C5P h 5)Cr (CO)2P Me3‚THF (2). (C5Ph5)Cr(CO)3‚C6H6 (0.50
g, 0.76 mmol) was dissolved in 10 mL of THF. The solution
was then cooled to -78 °C, and 0.21 mL PMe3 (2.0 mmol) was
added. The solution was allowed to warm to room temperature
with stirring (ca. 1 h). As the dark blue solution warmed, it

Substitution Reactions of (C5Ph5)Cr(CO)3
Organometallics, Vol. 15, No. 22, 1996 4793
Ta ble 4. P h en yl Rin g Tor sion An gles (d eg) in
the electrochemical data are noteworthy. Replacing Cp
by C5Ph5 in a complex usually results in potential shifts
of ca. 0.2 V to more positive values.4,14 In contrast, the
-1/0 couples for the PMe3 and PMe2Ph complexes show
roughly the opposite trend. Hershberger and Kochi
examined a variety of (MeCp)Mn(CO)2L complexes by
cyclic voltammetry and found that replacing CO by PEt3
and P(OMe)3 resulted in potential shifts of -0.70 and
-0.42 V, respectively.15 For complexes 2 and 4 the
shifts are -0.85 and -0.54 V, respectively. Thus, the
shifts in the reduction potentials of 2 and 4 relative to
1 are consistent with precedent. At ambient tempera-
ture, each complex also undergoes an irreversible oxida-
tion approximately 1.3 V to more positive potential than
the reversible reduction. The anodic waves equaled the
cathodic waves in height within 20% in all cases and
are also assigned as one-electron processes.
A low-temperature (-78 °C) infrared spectrum of the
reaction mixture of 1 with excess PMe3 shows four
absorptions (Table 5). The spectrum is consistent with
a compound of the formula [(C5Ph5)Cr(CO)3PMe3][(C5-
Ph5)Cr(CO)3].16,17 It is well established that 17e com-
plexes tend to undergo substitution reactions via asso-
ciative pathways.2d,7 Thus, a plausible reaction mech-
anism is shown in eqs 2 and 3. When the reaction
mixture is warmed to ambient temperature, 2 is pro-
duced quantitatively (eq 4). Further studies of this and
similar low-temperature reactions are underway.
(C5P h 5)Cr (CO)2P Me3‚0.5THF
conformer
conformer
Cp carbon
A
B
Cp carbon
A
B
1
2
3
51.9
48.0
51.1
47.7
55.6
45.6
4
5
54.7
50.0
56.8
55.0
initially turned a jade green color and then deep cherry-red.
The solution was filtered via cannula and layered with 12 mL
of hexane to yield 2 (0.48 g, 0.76 mmol) in 88% yield as dark
red crystals: Mp 211-218 °C (dec); 1H NMR (C6D6) δ 7.32 (m,
C5Ph5, br) 5.76 (s, PMe3, br); visible λmax (CH2Cl2) 516 nm. Anal.
Calcd for C44H42CrO3P: C, 75.31; H, 6.03. Found: C, 75.69;
H, 5.79.
(C5P h 5)Cr (CO)2P Me2P h ‚THF (3). (C5Ph5)Cr(CO)3‚C6H6
(0.50 g, 0.76 mmol) was dissolved in 10 mL of THF, and 0.50
mL of PMe2Ph (3.7 mmol) was added. The solution was stirred
overnight. The resulting red solution was filtered via cannula
and layered with 12 mL of hexane to yield 3 (0.48 g, 0.76 mmol)
1
in 72% yield as dark red crystals: Mp 198-200 °C (dec); H
NMR (C6D6) δ 7.18 (m, C5Ph5 and P(Me2Ph)3, br), 5.49 (s,
P(Me2Ph)3, br); visible λmax (CH2Cl2) 470 nm (sh). Anal. Calcd
for C49H44CrO3P: C, 77.05; H, 5.81. Found: C, 76.54; H, 5.50.
(C5P h 5)Cr (CO)2P (OMe)3‚THF (4). The procedure is the
same as for 2 except that a magenta colored product is
obtained in 90% yield: Mp 188-192 °C (dec); 1H NMR (C6D6)
δ 7.34 (m, C5Ph5, br), 5.32 (s, P(OMe)3, br); visible λmax (CH2-
Cl2) 532 nm. Anal. Calcd for C44H42CrO6P: C, 70.49; H, 5.65.
Found: C, 71.05; H, 5.06.
(C5Ph5)Cr(CO)3 + PMe3 -78 °C8
Resu lts a n d Discu ssion
Syn th eses. Reaction of the (C5Ph5)Cr(CO)3 radical
with a variety of neutral, monodentate Lewis bases
resulted in substitution products or no reaction between
the materials depending on the ligand. The small, soft
ligands PMe3 and P(OMe)3 react with (C5Ph5)Cr(CO)3
in THF solution at low temperatures to yield the
substitution products, (C5Ph5)Cr(CO)2L (eq 1), as highly
(C5Ph5)Cr(CO)3PMe3 (2)
(C5Ph5)Cr(CO)3PMe3 + (C5Ph5)Cr(CO)3 -78 °C8
[(C5Ph5)Cr(CO)3PMe3][(C5Ph5)Cr(CO)3] (3)
PMe3
[(C5Ph5)Cr(CO)3PMe3][(C5Ph5)Cr(CO)3]
8
(C5Ph5)Cr(CO)3 + L -78 °C8
2(C5Ph5)Cr(CO)2PMe3 + 2CO (4)
THF
(C5Ph5)Cr(CO)2L + CO (1)
PMePh2 reacts with 1 at ambient temperature to yield
solutions which display CO absorptions in the infrared
where expected but from which very little substitution
product can be isolated. The following ligands do not
react with 1 even at elevated temperatures (e.g. reflux-
ing THF or benzene): PPh3, P(OPh)3, 2,2′-bipyridine,
and PhCtCPh. The data for PMe3, PMe2Ph, and
PMePh2 suggest that steric effects are probably very
important in the lack of reactivity of PPh3 and P(OPh)3.
Molecu la r St r u ct u r e of (C5P h 5)Cr (CO)2P Me3‚
0.5THF . The X-ray crystal structure of (C5Ph5)Cr(CO)2-
PMe3‚0.5THF is displayed in Figure 2. Bond distances
colored, crystalline materials in high yields (compounds
2 and 4, respectively). PMe2Ph reacts with 1 at ambient
temperature to yield this product (3) in slighly lower
yields. The former reactions proceed very rapidly at
ambient temperature; however isolated yields of the
products are somewhat reduced. Unlike for CpMn-
+ 6,13
(CO)3
,
no evidence for disubstitution of 1 was
observed. All are air-sensitive, both in solution and in
the solid state. 1H NMR spectra were very broad, and
no resonances were observed in 31P NMR spectra of
these compounds.
Infrared spectral and electrochemical data for these
complexes are collected in Tables 5 and 6, respectively.
A cyclic voltammogram of 2 is presented in Figure 1.
The CO stretching frequencies and complex reduction
potentials both follow the expected trends for the
ligands. In the cyclic voltammetry, the cathodic waves
are assigned to a reversible, one-electron reduction,
consistent with literature precedent.9f Two aspects of
(14) (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.
(15) Hershberger, J . W.; Kochi, J . K. Polyhedron 1983, 2, 929.
(16) Two absorptions, 1892 and 1791 cm-1, are associated with
- 4
(C5Ph5)Cr(CO)3
. The infrared spectrum of this complex is qualita-
tively similar to [(C5Ph5)Cr(CO)2(depe)][(C5Ph5)Cr(CO)3] (J arrell, C. S.;
Castellani, M. P. Unpublished results).
(17) To our knowledge no such Cr complexes are known; however,
several similar Mo complexes have been isolated. The infrared spectra
of the Mo complexes are very similar to those obtained for the Cr
system. (a) Nolte, M. J .; Reimann, R. H. J . Chem. Soc., Dalton Trans.
1978, 932. (b) Asdar, A.; Tudoret, M.-J .; Lapinte, C. J . Organomet.
Chem. 1988, 349, 353.
(13) It is interesting that Baird and co-workers do not report
observing evidence of disubstitution reactions occurring in isoelectronic
CpCr(CO)3.6 While the large size of the C5Ph5 ligand probably precludes
disubstitution in 1 for the ligands studied here, Baird’s work suggests
that a steric argument may not be necessary.

4794 Organometallics, Vol. 15, No. 22, 1996
Ta ble 5. In fr a r ed Sp ectr a l Da ta for (C5P h 5)Cr (CO)2L Com p lexes
Hammack et al.
complex
solvent
ν(CtO),a cm-1
ref
(C5Ph5)Cr(CO)3
THF
2005, 1897
4
(C5Ph5)Cr(CO)2PMe3
CpCr(CO)2PMe3
(C5Ph5)Cr(CO)2PMe2Ph
(C5Ph5)Cr(CO)2P(OMe)3
[(C5Ph5)Cr(CO)3PMe3][(C5Ph5)Cr(CO)3]b
THF
CH2Cl2
THF
THF
THF
1911, 1797
1910, 1778
1911, 1792
1923, 1816
this work
9f
this work
this work
this work
2025, 1956, 1892, 1791
a
b
All absorptions are strong. Spectrum taken at -78 °C.
Ta ble 6. Electr och em ica l Da ta for (C5P h 5)Cr (CO)2L in CH2Cl2
c
complex
Epa(0/1+),a,b
ca. 0.9e
V
E°(0/1-),a
V
ipa/ipc
ref
(C5Ph5)Cr(CO)3
(C5H5)Cr(CO)2PMe3
(C5Ph5)Cr(CO)2PMe3
-0.69
-1.42
-1.56
-1.36
-1.48
-1.26
1.0
4
9f
d
-0.07
-0.37
-0.06
0.05
0.98
this work
9f
this work
this work
(C5H5)Cr(CO)2PMe2Ph
(C5Ph5)Cr(CO)2PMe2Phd
0.86
0.95
d
(C5Ph5)Cr(CO)2P(OMe)3
a
b
d
Potential vs Fc. Irreverisible. c Determined according to ref 25. Scan rate 100 mV/s; 0.1 M (nBu4N)PF6; ca. 3 mM complex. e Appears
as a very broad peak. This work.
F igu r e 1. Cyclic voltammogram of (C5Ph5)Cr(CO)2PMe3
(2) (scan rate 100 mV/s; 0.1 M (nBu4N)PF6; ca. 3 mM
complex in CH2Cl2).
F igu r e 3. ESR spectra of (C5Ph5)Cr(CO)2PMe3 (2) in
toluene solution at 125, 165, and 200 K. The low-field
portions of the 125 and 165 K spectra are shown magnified
by a factor of 4.
9a
a staggered conformation, but for CpCr(CO)2PPh3 the
P atom eclipses a carbon atom of the C5 ring. As we
will show below, the conformational energy difference
for 2 is small, less than or on the order of kT at 200 K
(0.02 eV). One further feature of the structure warrants
1
comment. Elemental analysis, H NMR, and thermo-
gravimetric analysis18 all support formulation of the
solid phase as a monosolvate. Since all atoms in this
structure were at full occupancy, it is likely that the
structure was obtained of a rare crystal of an unrepre-
sentative solvation number.
ESR Sp ectr a . ESR spectra of 2-4 in frozen toluene
solution are rhombic with three distinct g-components.
Spectra of 2 and 3 are shown in Figures 3 and 4. The
spectrum of 4 is very similar to that of 2. Interpretation
of the spectra is straightforward, and the resulting
parameters are given in Table 7a. In all cases, the low-
field features (gmax) are much broader than those
F igu r e 2. Molecular structure and labeling scheme for (C5-
Ph5)Cr(CO)2PMe3‚0.5THF (2).
and angles are listed in Tables 2 and 3, respectively.
Phenyl dihedral angles are given in Table 4. There are
two conformers in the unit cell that do not differ in any
chemically significant way. As in other, similar para-
magnetic systems, the tripodal angles deviate signifi-
cantly from 90°.4,9a,e Fortier and co-workers have
reported calculations describing the origin of this effect.9e
The P atom lies below a C-C bond of the C5 ring (a
staggered conformation). Cp*Cr(CO)2PMe39e also adopts
(18) A TGA of this compound shows that the complex loses its THF
solvate well before decomposition begins. Desolvation begins at ca. 150
°C and results in the crystals losing their luster with no significant
decoloration occurring. Complexes 3 and 4 undergo similar visual
changes for, presumably, the same reason. A copy of the TGA of 2 is
provided in the Supporting Information.

Substitution Reactions of (C5Ph5)Cr(CO)3
Organometallics, Vol. 15, No. 22, 1996 4795
Ta ble 7. ESR P a r a m eter s for (C5P h 5)Cr (CO)2L
(a) Frozen-Solution Spectra
a
a
a
L
T/K (solvent)
g1
g2
g3
A1
A2
A3
PMe3 (2)
125-160 (tol)c
105-120 (tol)
1.9941(3)
1.9940(2)
1.9944(3)
1.9940(2)
2.0130(3)
2.0130(2)
2.0147(5)
2.0140(2)
2.104(2)
2.1060(2)
2.130(3)
b
34.2(2)
32.6(2)
40.4(2)
40.2(3)
35.8(4)
34.8(2)
45.9(2)
45.5(2)
34(1)
34.2(2)
35(2)
b
PMe2Ph (3)
P(OMe)3 (4)
P(OMe)3 (4)
125-145 (tol)
125-160 (dcm/dce)
(b) Liquid-Solution Spectra
a
L
T/K (solvent)
g
g|
A
A|a
PMe3 (2)
200 K (tol)
190 K (tol)
180-195 K (tol)
185 K (dcm/dce)
2.012(1)
2.011(1)
2.006(2)
2.012(1)
2.090(1)
2.091(1)
2.112(1)
2.095(1)
35(1)
36(1)
46(3)
45(1)
35(1)
32(2)
45(1)
41(1)
PMe2Ph (3)
P(OMe)3 (4)
P(OMe)3 (4)
a
b
31P hyperfine coupling in units of 10-4 cm-1
.
Features poorly resolved. c tol ) toluene.
solution spectra, albeit with significant shifts in the
positions of features.
The complexes (C5Ph5)Cr(CO)2L have nominal Cs
symmetry so that the SOMO could belong to either the
a′ or a′′ representation. Previous work on related
systems9e,20,21 and extended Hu¨ckel MO calculations9e
suggest a SOMO of a′′ symmetry. Taking xz as the
plane of symmetry, the SOMO is given by eq 5.
|SOMO ) a1|xy + a2|yz + ...
(5)
Components of the g-matrix are given by eqs 6,11
2
gxx ) ge + 2[a1 δxz + a22(δx2-y2 + 3δz2)]
(6a)
(6b)
2
2
gyy ) ge + 2(a1 δyz + a2 δxy)
F igu r e 4. ESR spectra of (C5Ph5)Cr(CO)2PMe2Ph (3) in
toluene solution at 120, 160, and 190 K.
2
2
gzz ) ge + 2(4a1 δx2-y2 + a2 δxz)
(6c)
(6d)
gxz ) -2a1a2(δxz + 2δx2-y2
)
corresponding to the two smaller g components. For 2
and 4, the low-field features increase in width with
increasing temperature whereas, in spectra of 3, these
features are not as broad and sharpen slightly with
increasing temperature. Spectra of 4 in dcm/dce were
essentially identical to those in toluene except that the
low-field features were broader and could not be located
accurately, even at 125 K. These line width effects will
be discussed elsewhere.19
The g-matrices have one component close to the free-
electron g-value, ge, a second component slightly larger
than ge, and a third component substantially larger than
ge. This pattern is characteristic of low-spin d5 sys-
tems11 and can be understood qualitatively in terms of
a simple ligand-field theory model. The degeneracy of
the octahedral ligand-field configuration, t2g5, is lifted
in lower symmetry, but strong spin-orbit coupling of
the singly-occupied orbital (SOMO) with the other two
components of the t2g set leads to two g-components
greater than ge; the third g-component differs from ge
through spin-orbit coupling with one of the eg orbitals
which is empty and at much higher energy. Although
the spectra of (C5Ph5)Cr(CO)2L fit this qualitative
pattern, they exhibit a temperature-dependent line
width effect which requires a more detailed analysis.
Furthermore, the spectra in liquid solution at low
temperatures are not isotropic but resemble the frozen
2
2
where, for example, δx -y is given by eq 7, in which úCr
2
ck,x2-y2
δx2-y2 ) úCr
(7)
∑
E - E
k*0
0
k
is the spin-orbit coupling constant for Cr, E0 - Ek is
the energy of the kth MO relative to the energy of the
SOMO, and ck,x -y is the LCAO coefficient of dx -y in
the kth MO. EHMO calculations19 suggest that the two
highest doubly-occupied MO’s, just below the SOMO in
energy (the other members of the t2g set), are predomi-
nantly dx -y and dz in character so that δxz, δyz, δxy
x -y , δz . Thus gyy is expected to be close to ge, but the
other components should be larger. The g-matrix is
diagonalized by rotation about the y-axis by the angle
â, given by eq 8, where R ) a2/a1 and Q ) δz /δx -y . The
2
2
2
2
2
2
2
,
2
2
2
δ
2
2
2
-4R
tan 2â )
(8)
4 - R2(1 + 3Q)
X and Z principal values of the g-matrix then are given
by eq 9. Since, experimentally, gX is close to ge and gZ
(20) Morton, J . R.; Preston, K. F.; Cooley, N. A.; Baird, M. C.; Krusic,
P. J .; McLain, S. J . J . Chem. Soc., Faraday Trans. 1 1987, 83, 3535.
(21) Pike, R. D.; Rieger, A. L.; Rieger, P. H. J . Chem. Soc., Faraday
Trans. 1 1989, 85, 3913.
(19) Castellani, M. P.; Connelly, N. G.; Rieger, A. L.; Rieger, P. H.
To be published.

4796 Organometallics, Vol. 15, No. 22, 1996
Hammack et al.
2
gX,Z ) ge + a1 δx2-y2[4 + R2(1 + 3Q)] ×
48R2Q
1 (
1 -
(9)
2
2
{
}
x
[4 + R (1 + 3Q)]
is much larger than ge, the square root term of eq 9 is
apparently close to 1.22
Spectra in liquid solution 10-20 K above the melting
point of the solvent appear as approximately axial
powder patterns. Spectra of 2 and 3 are shown in
Figures 3 and 4; again the spectrum of 4 is qualitatively
similar to that of 2, with features significantly sharper
than for 3. In all cases, the features broaden at higher
temperatures and eventually coalesce into a single
broad line. Although the line narrows somewhat near
room temperature, 31P splitting is never resolved.
Parameters for the approximately axial spectra are
given in Table 7b.
The parallel features in the axial spectra are shifted
upfield from the frozen-solution gZ features, and the
perpendicular features are close to the position of the
gY features of the frozen-solution spectra. So far as we
are aware, this behavior is unprecedented in organo-
metallic ESR studies although it is similar to effects
observed in liquid crystal solvents.23 At temperatures
just above the melting point, the viscosity of toluene or
dce/dcm is high, and it is not surprising that molecular
rotation is too slow to produce an isotropic spectrum.
Apparently there is some degree of averaging, however,
such that the gX and gY features are merged and the gZ
features somewhat shifted. The most likely explanation
of this behavior is that the Cr(CO)2L moiety is nearly
freely rotating relative to the C5Ph5 group. In other
words, the very bulky “seat” of the “piano stool” is
essentially stationary on the ESR time scale while the
“legs” rotate freely. The bulkier PMe2Ph ligand would
be expected to impede this averaging process, and
features in the approximately axial spectra of 3 are
significantly broader than those of 2 or 4.
This behavior can be simulated using the program
described by Schneider and Freed.23 Shown in Figure
5 are computer simulations using the spin Hamiltonian
parameters for 2 and a 5-G Lorentzian line width. For
the simulations in Figure 5a, isotropic rotational diffu-
sion is assumed with Dx ) Dy ) Dz ranging from 107 to
5 × 108 s-1 whereas, in Figure 5b, rotational diffusion
is anisotropic with Dx ) Dy ) 106 s-1 and Dz ranging
from 107 to 5 × 108 s-1. Although isotropic rotational
diffusion can lead to an approximately axial spectrum,
the parallel features are very broad and both the
parallel and perpendicular features shift significantly
from the frozen-solution positions. We can obtain an
order-of-magnitude estimate of the isotropic rotational
diffusion coefficients from eqs 10. Extrapolating litera-
F igu r e 5. Simulated spectra based on the spin Hamilto-
nian parameters of 2: (a) Isotropic rotational diffusion with
Dx ) Dy ) Dz ) D; (b) anisotropic rotational diffusion with
Dx ) Dy ) 106 s-1 and Dz ) D ) (i) 1 × 107, (ii) 2 × 107, (iii)
5 × 107, (iv) 1 × 108, (v) 2 × 108, and (vi) 5 × 108 s-1
.
Ta ble 8. Va lu es of th e An gle â (d eg) Com p u ted
fr om Axia l Sp ectr a in Tolu en e
â
L
eq 11a
eq 11b
av
PMe3
PMe2Ph
P(OMe)3
15.3 ( 1.0
15.7 ( 0.9
15.8 ( 1.0
16.6 ( 1.3
15.4 ( 1.4
5.8 ( 4.8
15.8 ( 0.6
15.6 ( 0.1
15.4 ( 2.0
obtain η ≈ 0.43 kg m-1 s-1
.
Assuming that 2 is
approximately spherical with a radius of about 7 Å, τr
≈ 2 × 10-7 s, with D ≈ 7 × 105 s-1, about 2 orders of
magnitude slower than required to obtain an ap-
proximately axial spectrum from isotropic motion.
On the other hand, anisotropic rotational diffusion
with Dx ) Dy , Dz ≈ 2 × 108 s-1 gives a reasonable
account of the experimental results. This rate is
considerably faster than might have been expected for
rotational diffusion of the Cr(CO)2PMe3 moiety in
toluene at 200 K (Dz ≈ 3 × 107 s-1, assuming a volume
1
about /10 that of the whole complex and accounting for
rotation about one axis). The most likely explanation
is that the “piano-stool legs” rotate in a nearly solvent-
free cavity created by the C5Ph5 ligand.
Assuming that anisotropic rotational diffusion is fast
enough to completely average gx and gy, that the parallel
axis corresponds to the Cr-Cp vector (the z-axis), and
that the Z principal axis of the g-matrix differs from
this axis by the angle â, the parallel and perpendicular
components of the averaged g-matrix are given by eqs
11. These equations were used to compute the values
D ) 1/6τr
(10a)
(10b)
τr ) Vhη/kT
2
2
2
2
2g| ) gZ + gX + (gZ - gX2) cos2 2â (11a)
ture values of the viscosity of toluene24 to 200 K, we
2
2
2
2
2
4g ) gZ + gX + 2gY - (gZ - gX2) cos2 2â (11b)
(22) Since EHMO calculations19 suggest that R is negative and Q
is positive, eq 9 predicts gZ, gX > ge. The observed values of gX, ca.
1.994, suggest that the δxz terms in eqs 6 are not entirely negligible.
(23) Schneider, J . H.; Freed, J . H. In Spin Labeling: Theory and
Application, Biological Magnetic Resonance, Vol. 8; Berliner, L. J .,
Reuben, J ., Eds.; Plenum: New York, 1989.
of â listed in Table 8. Except for 4, the agreement
(24) International Critical Tables; Washburn, E. W., Ed.; McGraw-
Hill: New York, 1926-1930; Vol. VII, p 218.

Substitution Reactions of (C5Ph5)Cr(CO)3
Organometallics, Vol. 15, No. 22, 1996 4797
between values of â computed from g| (eq 11a) and those
computed from g (eq 11b) is quite good, suggesting that
the model is at least qualitatively correct. Extended
Hu¨ckel MO calculations19 suggest that Q ≈ 1.4; with
this value, â ) 16° and eq 8 gives R ) -0.46, in
reasonable agreement with the EHMO prediction of
0.34. The values of â listed in Table 8 also may be
compared with those obtained from ESR studies of
CpCr(CO)2PPh3 and (C5Me5)Cr(CO)2PMe3 diluted
into single crystals of the Mn analogs. For CpCr(CO)2-
PPh3, four paramagnetic sites were found with slightly
different principal values of the g-matrix and â ranging
from 3 to 8°; for (C5Me5)Cr(CO)2PMe3, only one site was
found with â ) 2.4°. These angles refer to the orienta-
tion of the gmax principal axis relataive to the Mn-CNT
axis in the host crystal and so may not be exactly equal
to those relative to the Cr-CNT axis. Nonetheless, the
angles are considerably smaller than those found in the
present work; whether this reflects an error in our
analysis or a true difference between the C5Ph5 ligand
and the Cp and C5Me5 ligands is unclear.
Ack n ow led gm en t. The research at Marshall Uni-
versity was supported by the National Science Founda-
tion (Grant NSF CHE-9123178) and the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society. The NSF provided funds toward
the University of Delaware diffractometer. We thank
Dr. R. J . Hoobler and Prof. J . W. Larson for experimen-
tal assistance, Dr. Michael Iglehart for obtaining and
interpreting the TGAs, Prof. W. E. Geiger for helpful
suggestions regarding both electrochemical experiments
and interpretation of our results, Prof. J . H. Freed and
Dr. R. Crepeau for supplying the ESR simulation
program, and Prof. M. B. Zimmt for assistance in
implementing the program at Brown.
9c
9e
Su p p or tin g In for m a tion Ava ila ble: For (C5Ph5)Cr(CO)2-
PMe3‚0.5THF, listings of atomic coordinates and equivalent
isotropic displacement coefficients, anisotropic displacement
coefficients, and hydrogen atom coordinates (Tables 1S-3S)
and a TGA diagram of 2 (12 pages). Ordering information is
given on any current masthead page.
OM960374U