Crystal structures of the dimeric cobalt compounds [Co(PPhMe2)(CO)3]2 and [Co2(PPhMe2)3(CO)5]

Article abstract of DOI:10.1016/j.ica.2005.11.005

The molecular structures of the dimeric cobalt compounds [Co(PPhMe₂)(CO)₃]₂ and [Co₂(PPhMe₂)₃(CO)₅] have been characterised by X-ray crystallography. The compound [Co₂

Full text of DOI:10.1016/j.ica.2005.11.005

Inorganica Chimica Acta 359 (2006) 3785–3789
www.elsevier.com/locate/ica
Note
Crystal structures of the dimeric cobalt
compounds [Co(PPhMe2)(CO)3]2 and [Co2(PPhMe2)3(CO)5]
*
Simon A. Llewellyn , Malcolm L.H. Green, Andrew R. Cowley
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK
Received 10 October 2005; received in revised form 4 November 2005; accepted 7 November 2005
Available online 4 January 2006
Dedicated to Professor Michael Mingos, FRS, in recognition of his outstanding contributions to chemistry.
Abstract
The molecular structures of the dimeric cobalt compounds [Co(PPhMe2)(CO)3]2 and [Co2(PPhMe2)3(CO)5] have been characterised
by X-ray crystallography. The compound [Co2(PPhMe2)3(CO)5] contains both bridging and terminal carbonyl groups in the solid state.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Cobalt; Tertiary phosphine; Bridging carbonyl; Dimer
1. Introduction
2. Results and discussion
The hydroformylation [1] reaction represents one of the
most industrially significant applications of homogeneous
catalysis. Typically, catalysts consist of Rh or Co carbonyl
compounds. In the case of Co, catalysts are formed in situ
from tertiary phosphine ligands and [Co(CO)4]2, to give
dimeric cobalt compounds of the form [Co(L)(CO)3]2
(L = tertiary phosphine). These dimers are subsequently
cleaved under a syngas (CO/H2) atmosphere to give pre-cat-
alysts of the form [Co(L)(CO)3(H)] (L = tertiary phos-
phine). However, the presence of excess phosphine allows
for the formation of more highly substituted compounds
such as [Co(L)2(CO)2(H)] [2] and presumably [Co2(L)3
(CO)5]. The importance of this class of dimeric cobalt com-
pounds in the hydroformylation of alkenes has ensured con-
tinued interest in their structural and chemical properties
[2–6] Here, we report the X-ray crystal structures of two
related cobalt dimers, illustrating the structural conse-
quences of increasing the number of coordinated tertiary
phosphine ligands.
The addition of two equivalents of the tertiary phosphine
PPhMe2 to a toluene solution of [Co(CO)4]2, followed by
heating, gave the compound [Co(PPhMe2)(CO)3]2 (1), as
an orange solid, in which two of the CO groups have been
replaced by tertiary phosphine ligands. The compound 1
was recrystallised from dichloromethane/hexane, forming
crystals that were moderately air-stable, though solutions,
in wet solvents, decomposed over several hours. Compound
1 is soluble in most common organic solvents. Compound 1
has been reported previously [7], though minimal analytical
data were provided, it has now been fully characterised by
1H, 31P{1H}, 13C{1H} NMR and IR spectroscopies, by mass
spectrometry, by microanalysis and by X-ray structure
determination.
Crystals of 1, suitable for X-ray structure determination,
were grown by layering a saturated dichloromethane solu-
tion of 1 with hexane. Crystal data are summarised in
Table 1. A view of the solid state structure is shown in
˚
Fig. 1, while selected bond lengths (A) and angles (ꢁ), are
given in Table 2. Compound 1 has a crystallographic centre
of inversion, midway along the Co–Co bond, and exhibits a
staggered conformation about this bond. The molecular
structure of 1 displays a dinuclear arrangement, of trigonal
*
Corresponding author. Tel.: +44 7787157064.
E-mail address: simon.llewellyn@chem.ox.ac.uk (S.A. Llewellyn).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.11.005

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S.A. Llewellyn et al. / Inorganica Chimica Acta 359 (2006) 3785–3789
Table 1
C7
Crystallographic data for compounds 1 and 2
C8
Compound
1
2
O2
C2
C6
C9
O3
Chemical formula
Formula weight
T (K)
C22H22Co2O6P2
562.23
150
0.71073
triclinic
C29H33Co2O5P3
672.37
150
0.71073
orthorhombic
P212121
9.7009(2)
11.0947(2)
28.4616(4)
90
C3
C5
˚
C4
k (A)
Co1
Crystal system
Space group
P1
ꢀ
P1
˚
a (A)
7.2208(3)
8.5301(4)
11.0362(6)
88.523(2)
72.590(2)
71.824(2)
614.54(5)
1
˚
b (A)
C1
˚
C10
C11
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
90
O1
3
˚
V (A )
3063.28(9)
4
1.458
Fig. 1. Molecular structure of [Co(PPhMe2)(CO)3]2 (1). Thermal ellip-
soids are drawn at 40% probability. Hydrogen atoms have been omitted
for clarity.
Z
Dcalc (Mg/m3)
Absorption
1.519
1.514
1.275
coefficient (mmꢀ1
F(000)
)
286
1384
Crystal size (mm)
Description of crystal
Absorption correction
0.14 · 0.22 · 0.22
dark-red fragment
semi-empirical from
0.14 · 0.20 · 0.22
orange block
semi-empirical from
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) in [Co(PPhMe2)(CO)3]2 (1)
Co(1)–Co(1)*
Co(1)–C(1)
Co(1)–C(2)
Co(1)–C(3)
Co(1)–P(1)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
P(1)–C(4)
2.6663(8)
1.772(4)
1.782(4)
1.758(4)
2.1750(9)
1.145(5)
1.149(5)
1.156(4)
1.814(3)
P(1)–C(10)
P(1)–C(11)
C(4)–C(5)
C(4)–C(9)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
P(1)–C(10)
1.809(4)
1.805(4)
1.400(5)
1.401(5)
1.389(5)
1.380(6)
1.379(6)
1.396(5)
1.809(4)
equivalent reflections equivalent reflections
Transmission coefficients 0.72, 0.81
(minimum, maximum)
h Range for
data collection (ꢁ)
Index ranges
0.76, 0.84
5.0 6 h 6 27.5
5.0 6 h 6 27.5
ꢀ8 6 h 6 9,
ꢀ10 6 k 6 10,
0 6 l 6 14
8528
2781
0.061
ꢀ12 6 h 6 12,
0 6 k 6 14,
0 6 l 6 36
22255
6797
0.043
Reflections measured
Unique reflections
Rint
Co(1)–P(1)–C(10)
Co(1)*–Co(1)–C(1)
114.1(12)
89.0(11)
Co(1)*–Co(1)–C(2)
Co(1)*–Co(1)–C(3)
89.1(11)
83.6 (11)
Observed reflections
[I > 3r(I)]
1723
5429
Refinement method
full-matrix
least-squares on F
145
Chebychev
3-term polynomial
1.1165
0.0347
0.0330
ꢀ0.35, 0.27
full-matrix
least-squares on F
353
Chebychev
3-term polynomial
1.1002
0.0353
0.0359
ꢀ0.38, 0.57
P–Co–Co0 bond angles of 180.0ꢁ are observed. However,
P–Co–Co0 angles of <180ꢁ have been reported for the com-
pounds [Co(L)(CO)3]2 (L = P(OiPr)3 [9] and P(OPh)3 [4]).
NMR and IR data were consistent with the molecular
structure of the crystal.
Parameters refined
Weighting scheme
Goodness-of-fit
R
wR
Heating a toluene solution of [Co(CO)4]2, with three
equivalents of PPhMe2, led to the formation of [Co2(PPh-
Me2)3(CO)5]. This compound was purified by recrystallisa-
tion from dichloromethane/pentane at ꢀ20 ꢁC, to afford
the product as orange/red crystals. The synthesis of 2, by
the controlled-potential electrolysis of [Co(PPh-
Me2)(CO)3]2, has been reported [10]. However, minimal
characterising and structural data were given, though IR
data are shown. Compound 2 has thus been characterised
Residual electron
density (minimum,
ꢀ3
˚
maximum) (e A
)
[Co(PPhMe2)(CO)3] units, in which the two PPhMe2
ligands are observed in trans positions, with respect to
the Co–Co bond. The molecular structure further shows
that all the carbonyl groups are terminal. This is typical
for tertiary phosphine substituted dimers of this type, and
contrasts with the compound [Co(CO)4]2, in which six of
the carbonyl groups are found in terminal configurations,
while the remaining two are in bridged configurations, in
the solid state [8]. A Co–Co bond length of 2.6663(8) A,
an average Co–(CO) bond length of 1.771(4) A, and a
Co–P length of 2.1750(9) A, are all in the range typical
for this class of compound (see Table 3). The P–Co–Co0
angle of 175.6(4)ꢁ is lower than has been reported for
[Co(L)(CO)3]2 (L = PMe3 and P(nBu)3), for which
1
by H, 31P{1H}, 13C{1H} NMR, and IR spectroscopies,
by mass spectrometry, by microanalysis, and by X-ray
structure determination.
Crystals of the compound 2, suitable for X-ray structure
determination, were grown by the cooling of a saturated
dichloromethane/pentane solution. Crystal data are sum-
marised in Table 1. A view of the molecular structure of
2 in the solid state is given in Fig. 2. Selected bond lengths
˚
˚
˚
˚
(A) and angles (ꢁ), are given in Table 4. The molecular
structure of 2 shows both CObridging and COterminal groups.
The two CObridging groups have Co–C bond lengths of

S.A. Llewellyn et al. / Inorganica Chimica Acta 359 (2006) 3785–3789
3787
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) for compounds 1 and 2 of the form [Co(L)(CO)3]2
˚
Ligand
Bond lengths (A)
Co–Co
Bond angles (ꢁ)
P–Co–Co0
Co–(CO)
Co–P
CO [8]
PMe3 [12]
2.522(1)
2.669(1)
1.820(1), 1.930(1)
1.772(3)
2.175(1)
180.0a
PPhMe2
2.6663(8)
2.654(1)
2.6544(10)
2.6722(4)
2.5216(6)
1.770(4)
1.75(3)
1.769(2)
1.783(2)
2.1750(9)
2.178(1)
2.1350(10)
2.1224(4)
2.2320(10), 2.1952(10)
175.6(4)
P(nBu)3 [13,14]
P(OiPr)3 [9]
P(OPh)3 [4]
[Co2(PPhMe2)3(CO)5]
180.0a
177.0(6)
177.3(2)
116.8(3), 119.2(3)
1.788(3)–1.978(3)
a
esd not given.
Fig. 2. ORTEP view of [Co2(PPhMe2)3(CO)5] (2). Thermal ellipsoids are drawn at 40% probability. Hydrogen atoms have been omitted for clarity.
Table 4
where L = alkyl, aryl or alkoxy tertiary phosphine, in
which all appended carbonyl groups are observed in termi-
˚
Selected bond lengths (A) and angles (ꢁ) in [Co2(PPhMe2)3(CO)5] (2)
Co(1)–Co(2)
Co(1)–C(1)
Co(1)–C(2)
Co(1)–C(3)
Co(1)–C(4)
Co(1)–P(1)
Co(2)–C(1)
Co(2)–C(2)
2.5216(6)
1.938(3)
1.987(3)
1.788(3)
1.780(4)
2.2194(10)
1.908(4)
1.878(3)
C(2)–O(2)
C(3)–O(3)
C(4)–O(4)
C(5)–O(5)
Co(2)–C(5)
Co(2)–P(2)
Co(2)–P(3)
C(1)–O(1)
1.180(4)
1.143(4)
1.146(4)
1.145(4)
1.785(4)
2.2320(10)
2.1952(10)
1.183(4)
nal configurations. The Flack [11] enantiopole parameter
calculated for 2 was 0.355. This value, coupled with a small
esd, indicates that the crystal was a twin by inversion, con-
taining both enantiomers. This may suggest that the two
enantiomers interconvert in solution, perhaps via exchange
of terminal and bridging CO groups.
1
The H NMR and 31P{1H} NMR spectra of 2 indicate
Co(2)–Co(1)–P(1)
C(1)–Co(2)–P(2)
Co(1)–Co(2)–P(3)
Co(2)–Co(1)–C(1)
116.8(3)
Co(2)–Co(1)–C(2)
Co(2)–Co(1)–C(3)
Co(2)–Co(1)–C(4)
Co(1)–Co(2)–C(5)
47.4(10)
107.9(11)
122.7(11)
113.3(11)
two chemically distinct PPhMe2 groups, in a ratio of 2:1.
The solid state structure of 2 would suggest the presence
of three chemically distinct 31P nuclei. That only two dis-
tinct 31P nuclei are observed in the 31P{1H} NMR spec-
trum of 2 suggests that an exchange between COterminal
and CObridging groups may occur in solution. The
13C{1H} NMR spectrum shows a broad single resonance
d 225.9, which may be attributed to the COterminal groups.
The absence of other resonances attributable to other
COterminal/CObridging groups, also suggests that the
COterminal and CObridging groups observed in the solid state
structure of 2, may exchange in solution. A similar
163.6(11)
119.2(3)
48.5(11)
˚
˚
1.938(3) and 1.987(3) A (cf. 1.930(1) A for [Co(CO)4]2, see
Table 3). The average Co–COterminal bond length in 2 is
˚
˚
1.784(4) A. This compares with a 1.770(4) A average Co–
COterminal bond length, observed for the compound
[Co(PPhMe2)(CO)3]2. The presence of CObridging groups,
in the solid state structure, contrasts with [Co(L)(CO)3]2,

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S.A. Llewellyn et al. / Inorganica Chimica Acta 359 (2006) 3785–3789
situation has been reported for the compound
[Co2(PEt3)3(CO)5] [12]. The difference between the solid-
state/solution behaviour of 2 when contrasted with that
of 1, suggests the existence of a delicate balance between
the terminal and bridging carbonyl groups that is per-
turbed by changes in the electronic and steric properties
of the metal centres.
m/z (FI) 138 (100%) [PPhMe2]+. Anal. Calc. for 1: C,
47.1; H, 3.9; N, 0. Found: C, 47.0; H, 3.9; N, 0%.
3.2. Synthesis of [Co2(PPhMe2)3(CO)5] (2)
The compound [Co2(CO)8] (1.00 g, 2.93 mmol) was dis-
solved in 10 ml of toluene. PPhMe2 (1.63 ml, 8.78 mmol)
was then added and the resulting suspension was stirred
at 60 ꢁC for 48 h. Solvents were then removed in vacuo,
and the resulting dark red/brown solid was dissolved in
20 ml of CH2Cl2. Addition of 10 ml of pentane led to the
precipitation of an orange/red solid. The resulting suspen-
sion was concentrated to ca. 10 ml and filtered. The mother
liquor was then cooled to ꢀ25 ꢁC, which led to the precip-
itation of red crystals that were isolated by filtration and
dried under vacuum. Crystals suitable for single-crystal
X-ray diffraction were grown by cooling a saturated dichlo-
romethane/pentane solution to ꢀ25 ꢁC. Yield of 2, 1.20 g
(60.9%).
3. Experimental
All manipulations of air- and/or moisture-sensitive com-
pounds were carried out under an inert atmosphere of dini-
trogen, in either an inert-atmosphere box or using standard
Schlenk line techniques. Solvents used in the preparation of
air- and/or moisture-sensitive compounds were dried by
passage through an alumina column under a positive pres-
sure of dinitrogen. Dinitrogen was passed through the
dried solvents for 20 min before use. C6D6 was degassed
using three freeze–pump–thaw cycles and was vacuum dis-
tilled from sodium. Solution NMR spectra were recorded
with either a Varian UNITYplus (1H: 500.0 MHz, 13C:
125.7 MHz, 31P: 202.4 MHz) or a Varian Mercury 300
(1H: 300.2 MHz, 13C: 75.5 MHz, 31P: 121.5 MHz). The
spectra were either internally referenced relative to the
residual protio-solvent (1H) and solvent (13C) resonances
relative to trimethylsilane (1H, 13C, d = 0 ppm), or were
referenced externally, relative to H3PO4 (31P, d = 0 ppm).
Infrared spectra were recorded on either a Perkin–Elmer
1710 FT-IR spectrometer or a Perkin–Elmer 1600 FT-IR
instrument in the range 4000–400 cmꢀ1. Samples were pre-
pared either as Nujol mulls between KBr plates, or as solu-
tions in CH2Cl2. Data are quoted in wave numbers (cmꢀ1).
The compound [Co2(CO)8] was purchased from Strem, and
PPhMe2 was purchased from Aldrich. Both were used as
received.
2
1H NMR (C6D6) (d): 1.22 [d, JPH = 4.04 Hz, 12H,
2
PPh(CH3)2], 1.46 [d, JPH = 7.92 Hz, 6H, PPh(CH3)2],
7.15 [m, 15H, ArH]; 13C{1H} NMR (d): 14.78 [t,
1
1JPC = 13.28 Hz, PPh(CH3)2], 15.75 [d, JPC = 4.96 Hz,
PPh(CH3)2], 129.79 [m, ArC], 139.45 [s, ArC], 139.92 [s,
ArC], 140.95 [s, ArC], 141.20 [s, ArC], 141.45 [s, ArC],
225.9 [s, Co(CO)terminal]; 31P{1H} NMR (d): 9.85 [s,
PPh(CH3)2], 13.01 [s, PPh(CH3)2]. IR (CH2Cl2) 1744 m,
1942 s, 1994 s. m/z (FI) 335 (50%) [Co(PPh(CH3)2)]+. Anal.
Calc. for 2: C, 51.8; H, 4.9; N, 0. Found: C, 51.4; H, 4.8; N,
0%.
4. Crystallography
Data collections were performed by Dr. A.R. Cowley,
using an Enraf-Nonius KappaCCD diffractometer (graph-
˚
ite-monochromated Mo Ka radiation, k = 0.71073 A).
3.1. Synthesis of [Co(PPhMe2)(CO)3]2 (1)
Intensity data were processed using the DENZO-SMN package
[13]. The crystal structures were solved using direct-meth-
ods program SIR-92 [14], which located all non-hydrogen
atoms. Subsequent full-matrix least-squares refinement
was carried out using the CRYSTALS [15] program suite.
Coordinates and anisotropic thermal parameters of all
non-hydrogen atoms were refined. Hydrogen atoms were
positioned geometrically after each cycle of refinement. A
3-term Chebychev polynomial weighting scheme was
applied. Crystal structure diagrams were produced using
the CRYSTALS [15] program suite. Crystal data are summa-
rised in Table 1.
The compound [Co2(CO)8] (2.00 g, 5.85 mmol) was dis-
solved in 20 ml of toluene to give a deep purple solution.
PPhMe2 (2.18 ml, 11.70 mmol) was then added portion-
wise by syringe. Effervescence was observed immediately.
The resulting suspension was then heated at 60 ꢁC until
the reaction was complete as judged by IR (CH2Cl2). Sol-
vents were removed under reduced pressure to give a pale
brown solid, which was recrystallised from dichlorometh-
ane and pentane. The resulting red/brown crystals were
isolated by filtration and dried under vacuum. Crystals
suitable for single-crystal X-ray diffraction were grown by
layering hexane onto a saturated dichloromethane. Yield
of 1, 2.15 g (65.4%).
Acknowledgements
2
1H NMR (C6D6) (d): 1.23 [d, JPH = 8.79 Hz, 12H,
We thank the EPSRC for a grant (S.A.L).
Appendix A. Supplementary data
PPh(CH3)2], 6.97 [m, 6H, ArH], 7.29 [s, 4H, ArH];
13C{1H} NMR (d): 9.16 [d, JPC = 30.65 Hz, PPh(CH3)2],
128.20 [s, ArC], 128.82 [m, ArC], 129.87 [m, ArC], 137.31
2
[d, ArC], 203.32 [t, JPC = 9.78 Hz, Co(CO)3]; 31P{1H}
Crystallographic data for the structures reported in
this paper have been deposited with the Cambridge
NMR (d): 35.33 [s, PPh(CH3)2]. IR (Nujol, KBr): 1938 s.

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3789
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