Facile displacement of cyclohexadienide ligand from K[Cr(η5-C6H7)(CO)3] to give crystallographically characterized trans-[Cr(CO)4(PPh3)2]

Article abstract of DOI:10.1016/S0022-328X(01)00929-9

Substitution of K[Cr(η⁵-C₆H₇)(CO)₃] with triphenylphosphine resulted in cyclohexadienide loss and formation of an unexpected product, trans-[Cr(CO)₄(PPh₃)₂]. The structure of thi

Full text of DOI:10.1016/S0022-328X(01)00929-9

Journal of Organometallic Chemistry 631 (2001) 188–192
www.elsevier.com/locate/jorganchem
Note
Facile displacement of cyclohexadienide ligand from
K[Cr(h5-C6H7)(CO)3] to give crystallographically characterized
trans-[Cr(CO)4(PPh3)2]
Jian Bao, Steven J. Geib 1, N. John Cooper *
Department of Chemistry, Uni6ersity of Pittsburgh, Pittsburgh, PA 15260, USA
Received 31 October 2000; received in revised form 1 May 2001; accepted 2 May 2001
Abstract
Substitution of K[Cr(h5-C6H7)(CO)3] with triphenylphosphine resulted in cyclohexadienide loss and formation of an unexpected
product, trans-[Cr(CO)4(PPh3)2]. The structure of this complex was determined by single crystal X-ray diffraction. Crystals
contain two conformational isomers, differing mainly in the orientation of PPh3 ligands. © 2001 Published by Elsevier Science
B.V.
Keywords: Cyclohexadienyl; Chromium; Triphenylphosphine; Carbonyl complexes; Ligand replacement
1. Introduction
Hieber and co-workers in 1959 [3a], and has long been
recognized as the more stable one of the two possible
isomers of [Cr(CO)4(PPh3)2] [3b]; surprisingly, however,
this archetypal disubstituted phosphine carbonyl com-
plex has never been crystallographic characterized. Our
procedure has provided access to crystalline samples of
the trans-[Cr(CO)4(PPh3)2] that has now allowed crys-
tallographic characterization of this classic complex.
Cyclohexadienyl complexes of Cr(0) are readily pre-
pared by exo-addition of soft carbanionic nucleophiles
to the aromatic ring in [Cr(h6-C6H6)(CO)3] (1) and its
many substituted derivatives, and the potential of these
species as intermediates in manipulations of the aro-
matic ring that can be of value in organic syntheses has
been extensively explored [1].
Less attention has been paid to the inorganic chem-
istry of these systems, but our convenient access [2] to
the parent cyclohexadienyl complex [Cr(h5-C6H7)-
(CO)3]− (2−) through protonation of the reductively
2. Results and discussion
Samples of K[Cr(h5-C6H7)(CO)3] (K2) were prepared
by two equivalent KNap reduction of 1 in THF at
−78°C followed by in situ protonation with H2O.
Substitution of the cyclohexadienyl complex K2 with
PPh3 was carried out at room temperature by addition
of 3 equivalents of PPh3 as shown in Scheme 1.
Infrared spectroscopy showed the disappearance of
the CꢀO stretches of the starting complex and the
appearance of a new band at 1880 cm−1 indicating the
formation of a new carbonyl complex. After work-up
as described in Section 4, the product was obtained as
yellow cubic crystals. These were shown to be
activated h4-benzene complex [Cr(h4-C6H6)(CO)3]2−
,
recently led us to explore substitution reactions of
K[Cr(h5-C6H7)(CO)3]. We now wish to report that this
results in an unexpected displacement of the cyclohexa-
dienide anion to provide access to the Cr(0) carbonyl
phosphine complex trans-[Cr(CO)4(PPh3)2] (3). The
trans isomer of [Cr(CO)4(PPh3)2] was first reported by
* Corresponding author. Fax: +1-412-624 6089.
E-mail address: cooper@pitt.edu (N.J. Cooper).
1 Address: Crystallographic correspondence to their author.
0022-328X/01/$ - see front matter © 2001 Published by Elsevier Science B.V.
PII: S0022-328X(01)00929-9

J. Bao et al. / Journal of Organometallic Chemistry 631 (2001) 188–192
189
Scheme 1.
,
phenyl ring (45°), and has a Cr–P bond at 2.371(1) A
[Cr(CO)4(PPh3)2] by IR, NMR and elemental analysis;
the single band in the IR spectrum suggested that we
had obtained 3 as the trans isomer.
,
,
as compared with 2.347(1) A. The 0.024 A difference
between these values is more than 20|, but their aver-
,
age value of 2.359 A is close to the Cr–P distance in the
Zero-valent complexes [Cr(CO)4(PR3)2] have been
known since early work by Hieber, and have been
subjected to electrochemical, photochemical, and spec-
troscopic investigations [4]. The structure of the
analogous molybdenum complex [5] has been reported
in both cis [5a] and trans [5b] configurations, but the
only structural studies of bisphosphine chromium car-
bonyl phosphine complexes [6] are of cis-
[Cr(CO)4(PH3)2] [6a] and [Cr(CO)4(Ph2PCH2)2] [6b].
The PPh3 complexes have a much more extensive chem-
istry than the PH3 complexes, and it is of obvious value
to determine structural parameters for 3, particularly
given recent theoretical interest in this molecule [7]. Our
procedure provided ready access to crystallographic
quality crystals of 3 and we determined the structure of
the molecule as described in Section 4. Crystallographic
data and structural parameters for 3 are summarized in
Tables 1 and 2. The diffraction study confirmed that 3
had been formed in the trans form.
Crystals of 3 contain molecules in two conforma-
tional isomers, one of which has twofold symmetry and
two symmetric triphenylphosphine ligands. The Cr
atom of this conformer (Cr1%) is at a molecular center
of inversion, while the other conformer has
triphenylphosphine ligands with slightly different orien-
tations of the phenyl groups (Fig. 1). The two conform-
ers are present in a 1 to 2 ratio, with the asymmetric
conformer as the major species. The unit cell contains
three molecules with a crystallographic inversion center
at the Cr1% atom of the symmetric molecule.
symmetric molecule, suggesting that there is a soft
potential energy surface for trans-[Cr(CO)4(PPh3)2] that
allows marked changes in molecular parameters in re-
sponse to crystal packing forces.
3. Conclusion
Reaction of the cyclohexadienyl complex K[Cr(h5-
C6H7)(CO)3] with excess Ph3P does not result in the
anticipated carbonyl substitution reaction but instead
Table 1
Crystal data and structure refinement for 3
Empirical formula
Formula weight
Temperature (K)
C40H30CrO4P2
688.58
208(2)
,
Wavelength (A)
0.71073
Triclinic, P1
(
Crystal system, space group
Unit cell dimensions
,
a (A)
10.184(2)
10.621(2)
25.756(5)
88.01(3)
83.45(3)
65.35(3)
2515.2(9)
3, 1.364
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
Volume (A )
Z, Calculated density (Mg m−3
)
Absorption coefficient (mm−1
F(000)
)
0.478
1068
Crystal size (mm)
q range for data collection (°)
0.21×0.24×0.31
2.11–25.00
The inversion center in the symmetric molecule ren-
ders the two phosphorus atoms directly trans to one
another (P1%–Cr1%–P1%A=180°). The metal center in
this unit is close to an ideal octahedral geometry, with
the P–Cr–C angles ranging from 88.1(1) to 91.1(1)°.
Limiting indices
05h512, −125k512,
−305l530
9414/8865 [Rint=0.0222]
99.9
Reflections collected/unique
Completeness to q=25.00 (%)
Absorption correction
Psi-scan
Refinement method
Full-matrix least-squares on
F2
,
The Cr–P bond length is 2.356(1) A. Cr–P distances in
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
8865/0/637
1.005
R1=0.0465, wR2=0.1043
R1=0.0859, wR2=0.1231
all crystallographically characterized [Cr(CO)4(PR3)2]
complexes are compared in Table 3.
In the asymmetric molecule the four carbonyl carbon
atoms and the Cr have a rms average deviation of 0.039
Largest difference peak and hole 0.487 and −0.235
−3
,
A from the least square plane, and the two Cr–P bond
,
(e A
)
lengths differ significantly. One PPh3 has a rotated

190
J. Bao et al. / Journal of Organometallic Chemistry 631 (2001) 188–192
Table 2
complexes [8] like [Mn(h5-C5H5)(CO)3] [8a,b] and
[Mn(h5-C6H7)(CO)3] [8c]. We attribute this difference
primarily to the fact that loss of a cyclohexadienide
group from [Cr((h5-C6H7)(CO)3]− does not involve the
charge separation that would be inherent in analogous
reactions of [Mn(h5-C5H5)(CO)3] or [Mn(h5-
C6H7)(CO)3]. We note also that the higher electron
density on the anionic Cr center in 2− does imply that
increased back donation to the CO ligands in 2− would
bind them more tightly than in complexes of Mn(1) like
[Mn(h5-C5H5)(CO)3] and [Mn(h5-C6H7)(CO)3], and it is
well established in phosphine carbonyl complexes that
increased back donation decreases the lability of the
carbonyl ligands.
,
Selected bond lengths (A) and angles (°) for symmetric [Cr(1%)] and
asymmetric [Cr(1)] 3
Cr1–C2
Cr1–C3
Cr1–C1
Cr1–C4
Cr1–P2
Cr1–P1
C4–O4
O3–C3
1.873(3)
1.878(3)
1.881(3)
1.896(3)
2.3474(14)
2.3714(14)
1.143(4)
1.152(4)
Cr1%–C1%
Cr1%–C2%
Cr1%–P1%
O1–C1
O1%–C1%
O2–C2
1.884(3)
1.885(3)
2.3562(13)
1.154(4)
1.149(4)
1.147(4)
1.150(4)
O2%–C2%
C2–Cr1–C3
C2–Cr1–C1
C3–Cr1–C4
C1–Cr1–C4
C2–Cr1–P2
C3–Cr1–P2
C1–Cr1–P2
C4–Cr1–P2
C2–Cr1–P1
91.89(14)
83.79(15)
92.19(14)
92.21(14)
87.53(11)
89.82(10)
91.32(10)
88.55(10)
96.15(11)
C2%–Cr1%–P1%
C1%–Cr1%–P1%
C4–Cr1–P1
P2–Cr1–P1
C1%A–Cr1%–C2%
C1%–Cr1%–C2%
C3–Cr1–P1
C1–Cr1–P1
90.82(10)
88.84(10)
87.86(10)
176.15(4)
89.28(13)
90.72(13)
88.93(10)
90.20(10)
4. Experimental
4.1. General procedures
Symmetry transformations used to generate equivalent atoms: c1
−x+1, −y+1, −z+1.
All reactions and manipulations were carried out
under an atmosphere of dry, oxygen-free nitrogen by
means of standard Schlenk techniques or a Vacuum
Atmospheres drybox as described in recent publications
from this group [9].
in loss of a cyclohexadienide anion and, after carbonyl
redistribution, formation of trans-[Cr(CO)4(PPh3)2].
This contrasts sharply with the corresponding chem-
istry of neutral cyclopentadienyl and cyclohexadienyl
Fig. 1. ORTEP drawing of [Cr(CO)4(PPh3)2] (35% probability ellipsoids).
Table 3
Comparison of the Cr–P bond lengths in octahedral [Cr(CO)4(PR3)2] complexes {[M]=Cr(CO)4}
,
Distance (A)
[M] [Ph2P(CH2)2]2
Cis-[M](PH3)2
2.349(4)
[6a]
Trans-[M](POPh3)2
Symmetric 3
Asymmetric 3
Cr–P
P–Cr–P
Reference
2.360
[6b]
2.252(1)
4.504(2)
[11]
2.356(1)
4.712(2)
This work
2.347(1), 2.371(1)
4.718(2)

J. Bao et al. / Journal of Organometallic Chemistry 631 (2001) 188–192
191
All solvents were freshly distilled under nitrogen
before use. Tetrahydrofuran (THF) and diethyl ether
(Et2O) were pre-dried over sodium ribbon. THF was
distilled from potassium, and Et2O was distilled from
sodium–benzophenone ketyl. n-Pentane was stirred
over concentrated H2SO4 for more than 24 h, neutral-
ized with K2CO3, and distilled from CaH2. Deuterated
NMR solvents were purchased from Cambridge Iso-
tope Labs and used as received. Chromium carbonyl
[Cr(CO)6] was purchased from Strem Chemicals and
used as received.
69.96; H, 4.33%. The IR spectrum of the product is
comparable to that in the literature [3]. IR (wCꢀO, Nujol
1
mull) 1980 (vs) cm−1. H-NMR (300 MHz, CD2Cl2, rt)
l 7.57 (m, 2H), 7.39 (m, 3H). 31P-NMR (75 MHz,
CD2Cl2, rt) l 74.7. Crystallographic quality crystals
were formed by slowly adding a layer of Et2O (3
volumes) onto a saturated THF solution of 3. Crystals
were formed in ca. 1 week, and collected in 30% yield
after removal of the solution through a cannula.
Potassium naphthalenide [K(C10H8)] was prepared by
dissolution of freshly cut potassium metal (Aldrich) in a
solution (0.2 M) of naphthalene (1.0 equivalent) in
THF. The mixture was stirred for 8 h at room temper-
ature, and potassium naphthalenide solutions were
stored at −70°C.
5. Supplementary material
Crystallographic data for the structure analyses have
been deposited with the Cambridge Crystallographic
Data Center, CCDC no. 163156. Copies of this infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2,
1EZ, UK (Fax: +44-1223-336033; email: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Samples of [Cr(h6-C6H6)(CO)3] (1) were prepared
from [Cr(CO)6] in 70% yield by a local modification of
the literature procedure [10].
Infrared spectra were recorded on a Perkin–Elmer
1
model 783 spectrophotometer. H-NMR spectra were
recorded on a Bruker AC 300 spectrometer at 300 Hz.
13C-NMR spectra were recorded on a Bruker AC 300
at 75 MHz. Microanalyses were performed by Atlantic
Microlab, Norcross, GA.
Acknowledgements
This work was financially supported by the NSF
through grant number CHE 9632202 to N.J.C.
Crystals for X-ray diffraction studies were coated
with fluorolube then mounted on a glass fiber and
coated with epoxy. X-ray data were collected on a
Siemens P3 diffractometer using graphite monochroma-
References
,
tized Mo–Ka radiation (u=0.71073 A). Data process-
ing and graphics were done with the Siemens SHELXTL
package program. All non-hydrogen atoms were lo-
cated and refined anisotropically. All hydrogen atoms
in [Cr(CO)4(PPh3)2] were located and refined
isotropically.
[1] (a) For reviews, see: M.F. Semmelhack, in: B. Trost, L. Fleming
(Eds.), Comprehensive Organic Synthesis, vol. 4, Pergamon,
Oxford, 1992, p. 521 and references cited therein;
(b) M.F. Semmelhack, H. Schmalz, Tetrahedron Lett. 37 (1996)
3089;
(c) F. Rose-Munch, V. Gagliardini, C. Renard, E. Rose, Coord.
Chem. Rev. 180 (1998) 249;
(d) K. Schellhaas, H.G. Schmalz, J.W. Bats, Chem-A Euro. J. 4
(1998) 57;
4.2. Reaction of K[Cr(p5-C6H7)(CO)3] with Ph3P
(e) L.S. Hegedus, Coord. Chem. Rev. 168 (1998) 49;
(f) F. Rose-Munch, E. Rose, Curr. Org. Chem. 3 (1999) 445.
[2] (a) V.S. Leong, N.J. Cooper, J. Am. Chem. Soc. 110 (1988) 2644
and references cited therein;
(b) J.A. Corella, N.J. Cooper, J. Am. Chem. Soc. 112 (1990)
2832.
[3] (a) W. Hieber, J. Peterhans, Z. Naturforsch. Teil B 14 (1959)
462;
A 0.2 M solution of freshly prepared KNap in THF
(12.8 ml, 2.57 mmol, 2.2 equivalents) was added by
syringe to a vigorously stirred solution of 1 (0.25 g, 1.17
mmol) in 50 ml THF at −78°C. Addition of 1 equiva-
lent of H2O (21 ml, 1.17 mmol) gave the adduct anion
[Cr(h5-C6H7)(CO)3]− (2−) instantly. The reaction was
monitored by infrared monitoring of the fingerprint wCO
absorptions of the complexes. The reaction was
warmed to room temperature before Ph3P (0.92 g, 3.51
mmol, 3 equivalents) was added and then stirred
overnight. The solvent was then removed under vac-
uum and the residue was washed with pentane (30
ml×3) and then Et2O (30 ml×3). This procedure left
trans-[Cr(CO)4(PPh3)2] (3) as the reaction product (0.31
g, 0.45 mmol, yield 38% relative to the metal). Anal.
Calc. for C40H30CrO4P2: C, 69.77; H, 4.39. Found: C,
(b) A. Magee, C.N. Matthews, T.S. Wang, J.H. Wotiz, J. Am.
Chem. Soc. 83 (1961) 3200.
[4] (a) R.N. Bagchi, A.M. Bond, R. Colton, D.L. Luscombe, J.E.
Moir, J.E. Kevekordes, Organometallics 3 (1984) 4;
(b) S.W. Kirtley, in: G. Wilkinson, F.G.A. Stone, E.W. Abel
(Eds.), Comprehensive Organometallic Chemistry, vol. 3, Perga-
mon, Oxford, 1982, p. 783.
[5] (a) F.A. Cotton, D.J. Darensbourg, S. Klein, B.W.S. Koltham-
mer, Inorg. Chem. 21 (1982) 294;
(b) G. Hogarth, T. Norman, Inorg. Chim. Acta 254 (1997) 167.
[6] (a) L.S. Guggenberger, U. Klabunde, R.A. Schunn, Inorg.
Chem. 12 (1973) 1143;

192
J. Bao et al. / Journal of Organometallic Chemistry 631 (2001) 188–192
(b) M.J. Bennett, F.A. Cotton, M.D. LaPrade, Acta Crystallogr.
Sect. B 27 (1971) 1899.
[7] D.A. Cummings, J. McMaster, A.L. Rieger, P.H. Rieger,
Organometallics 16 (1997) 4362.
(c) J.B. Sheridon, R.S. Padda, K. Chaffee, C.J. Wang, Y.Z.
Huang, R. Lalancette, J. Chem. Soc. Dalton Trans. (1992) 1539.
[9] J.M. Veauthier, A. Chow, G. Fraenkel, S.J. Geib, N.J. Cooper,
Organometallics 19 (2000) 661.
[8] (a) R.J. Angelici, E. Efraty, J. Organomet. Chem. 23 (1970) 527;
(b) M.E. Rerek, L.-N. Ji, F. Basolo, J. Chem. Soc. Chem.
Commun. (1983) 1208;
[10] C.A.L. Mahaffy, P.L. Paulson, Inorg. Synth. 19 (1979) 154.
[11] H.S. Preston, J.M. Stewart, S.O. Grim, Inorg. Chem. 11 (1972)
161.
.