Structure of Co2(CO)6(dppm) and Co2(CO)5(CHCO2Et)(dppm) (dppm = Ph2PCH2PPh2) and exchange reaction with 13CO: An experimental and computational study

Article abstract of DOI:10.1016/j.ica.2007.09.037

Crystal structures of Co₂(CO)₆(dppm) (1) and Co₂(CO)₅(CHCO₂Et)(dppm) (2) (dppm = Ph₂PCH₂PPh₂) show asymmetry with respect to the orientation of the phenyl groups in 1

Full text of DOI:10.1016/j.ica.2007.09.037

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 1832–1842
www.elsevier.com/locate/ica
Structure of Co2(CO)6(dppm) and Co2(CO)5(CHCO2Et)(dppm)
13
(dppm = Ph2PCH2PPh2) and exchange reaction with CO: An
experimental and computational study
a
a
a
b
c
a,
*
}
´
´
´ ´
´
E. Fordos , N. Ungvari , T. Kegl , L. Parkanyi , G. Szalontai , F. Ungvary
¨
a
Department of Organic Chemistry, University of Pannonia, Egyetem u. 10, 8200 Veszpre´m, Hungary
Institute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, 1525 Budapest, P.O. Box 17, Hungary
b
c
NMR Laboratory, University of Pannonia, Egyetem u. 10, Veszpre´m, Hungary
Received 23 July 2007; received in revised form 17 September 2007; accepted 20 September 2007
Available online 5 October 2007
Abstract
Crystal structures of Co2(CO)6(dppm) (1) and Co2(CO)5(CHCO2Et)(dppm) (2) (dppm = Ph2PCH2PPh2) show asymmetry with
respect to the orientation of the phenyl groups in 1 and owing to the bridging ethoxycarbonylcarbene ligand in 2. The effect of this asym-
metry was recognized in the solid-state 31P NMR spectra of 1 and 2 and in the solid-state and solution 13C NMR spectra of 2 as well, but
not in the solid-state and solution 13C NMR spectra of 1. In CH2Cl2 solution under an atmosphere of 13CO, the CO ligands of both
complexes exchange with 13CO. The overall rate of 13CO exchange at 10 ꢀC was found to be kobs = 0.107 · 10ꢀ3 sꢀ1 for 1 and
kobs = 0.243 · 10ꢀ3 sꢀ1 for 2. Two-layered ONIOM(B3LYP/6-31G(d):LSDA/LANL2MB) studies revealed fluxional behavior of 1 with
rather small barriers of activation of the rearrangements. Four possible isomers have been computed for 2, close to each other
energetically.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; Solid-state NMR spectra; Isotopic labeling; Carbonyl and carbene ligands; Cobalt
1. Introduction
Obviously the depiction of complex 1 as having a symmet-
rical structure [2,3,7], where a plane formed by the two
cobalt and the PCH2P fragment bisecting the planes of
the two bridging carbonyls, is wrong.
Complex 1 was reported to give in the dediazotation
reaction of ethyl diazoacetate complex 2 (Eq. (2)) in which
reaction one of the bridging carbonyl of 1 is replaced by an
ethoxycarbonylcarbene unit.
The cobalt complex Co2(CO)6(dppm) (1) is smoothly
formed from Co2(CO)8 and dppm (dppm = Ph2PCH2PPh2)
in benzene [1] or CH2Cl2 solution [2,3] (Eq. (1)).
M
Co ðCOÞ þ dppm
Co2ðCOÞ6ðdppmÞ þ 2CO
ƒƒƒƒƒƒƒƒ!
2
8
C6H6 or CH2Cl2
1
ð1Þ
M
1 þ EtO2CCHN2
þ CO þ N2
Co2ðCOÞ5ðCHCO2EtÞðdppmÞ
ƒƒ!
The structure of complex 1 has been assumed [2] to be anal-
ogous to that of Co2(CO)6L (L = l-[1,2-bis(dimethylarsino)
tetrafluorocyclobutene-AsAs]) [4,5], which was confirmed
recently by an X-ray crystal structure determination [6].
CH2Cl2
2
ð2Þ
Based on the large three-bond 3J(PH) coupling of the
l-alkylidene resonance in the low-temperature H NMR
1
spectra of 2 (d 4.64, J(PH) 21 Hz) a Newman projection of
the structure of complex 2 was proposed [8] (see Scheme 1).
*
Corresponding author. Tel.: +36 88 624 156; fax: +36 88 624 469.
´
E-mail address: ungvary@almos.vein.hu (F. Ungvary).
0020-1693/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.09.037

}
E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1833
Complex 1 was found to be a suitable catalyst for the
carbonylation of ethyl diazoacetate in the presence of eth-
anol to give diethyl malonate product [9] (Eq. (3)).
Co2(CO)6(dppm)
EtO2CCHN2
EtO2CCH2CO2Et
1
2 mol % 1
EtO2CCHN2 þ CO þ EtOH
þ N2
!
EtO2CCH2CO2Et
50 bar;45 ꢁC;4 d
N2 + CO
2CO + EtOH
ð3Þ
Co2(CO)5(CHCO2Et)(dppm)
Complex 2 is probably involved in the catalytic cycle (see
Scheme 2) since both its formation from 1 and ethyl diazo-
acetate [8] and the reaction of complex 2 with carbon mon-
oxide and ethanol [10] were demonstrated.
2
Scheme 2.
We report here the results of various solid-state and
solution spectroscopic investigations of 1 and 2 and the
results of an X-ray structure determination of 2. The results
indicate the non-equivalence of the phosphorus atoms in
the complexes. In addition we studied the exchange reac-
tions of 1 and 2 with 13CO in methylene chloride solution.
Structures and possible transition states of the exchange
reactions were computed.
(dCDCl3 = 77 ppm was used for the conversion). For
solid-state work for referencing and set-up purposes poly-
crystalline glycine was used (diso = 176.5 ppm (a-poly-
morph) relative to TMS) by the substitution method. In
case of 31P 90ꢀ the pulse width was about 3.6 ls. For refer-
encing and set-up purposes, polycrystalline PPh3 was
applied (diso = ꢀ6 ppm relative to the 85% H3PO4) by the
substitution method. 13CO (99% isotope purity) was
obtained from Sigma–Aldrich.
2. Experimental
X-ray crystal structure determination. Intensity data
collections were performed on a Rigaku R-axis Rapid IP
diffractometer at ambient temperature with MoKa radia-
All operations were performed under exclusion of air
and moisture using the usual Schlenk technique [11]. IR
spectra were recorded with a Thermo Nicolet Avatar 330
FTIR spectrometer in KBr pellets or using 0.00765,
0.02095, or 0.05097 cm CaF2 solution cells, calibrated by
the interference method [12]. Both the liquid-phase and
the solid-state NMR experiments were performed on a
Bruker Avance 400 NMR spectrometer equipped with
a 5 mm broad-band variable temperature liquid and a
4 mm variable temperature MAS probe. The solution-
phase spectra were recorded in CDCl3 at room temperature
or in CD2Cl2 at 200 K. Approximately 5–30 mg samples
were used for the solution studies. The 13C and 31P
resonance frequencies, x0/2p were ꢀ100.613 and
ꢀ161.976 MHz, respectively. The MAS spectra were
recorded with high-power (ꢂ100 W) proton decoupling.
Sample spinning speeds were typically varied between
10000 and 12500 1 Hz. 90–100 mg sample quantities
were used in 4 mm o.d. zirconia or Si3N4 rotors. The num-
ber of scans varied between 32 and 128 for the 31P spectra
and 1500–2500 for the 13C spectra. For phosphorus both
MAS (with recycling delays of 12–15 s) and CP/MAS spec-
tra (with contact times of 0.7–1.5 ms and recycling delays
of 6 s) were recorded and gave practically identical results.
The 13C chemical shifts are referred to external TMS
˚
tion (k = 0.7107 A). The structure was solved by direct
methods [13] and refined by full matrix least-squares [14]
against F2. A survey [15] showed that a fraction of non-cen-
trosymmetric crystal structures published in Inorg. Chim.
Acta are in fact centrosymmetric. Compound 2 was treated
as a racemic twin and the absolute structure parameter (at
97.5% Friedel pair coverage) is 0.510(18), typical of a cen-
trosymmetric structure. The structure indeed, can be solved
in space group Pbca but it is non-refinable (R = 0.1275,
wR2 = 0.4647, with unusual ADP ellipsoids in the phenyl
rings). Therefore the Pca21 space group was retained.
Complex 2 also contains unidentifiable solvent molecule(s).
The SQUEEZE procedure [16] was applied to eliminate
3
˚
these atoms (volume of solvent accessible area: 1041.0 A ,
electron count: 59/cell). Crystal data, data collection and
refinement parameters are listed in Table 1.
2.1. Preparation of Co2(CO)6(dppm) (1)
The preparation of Co2(CO)6(dppm) followed published
procedures [1–3]. Analytically pure 1 was obtained by slow
diffusion of n-pentane into a solution of 1 in CH2Cl2
(120 mg/cm3) at 5 ꢀC. IR (KBr pellets) m(C„O) 2042,
1998, 1982, 1971 cmꢀ1, m(C@O) 1809, 1797 cmꢀ1, IR
(CH2Cl2) m(C„O) 2045 (eM = 2808 cm2/mmol), 2012
(eM = 3406 cm2/mmol),
cmꢀ1
(eM = 1066 cm2/mmol) cmꢀ1
1985
m(C@O) 1820 (eM = 823 cm2/mmol), 1794
13C NMR (293 K, CDCl3):
(eM = 3751 cm2/mmol)
,
O
O
C
EtO2C
C
,
O
C
C
d 216.3 ppm (terminal and bridging carbonyls, in fast
exchange); 134.3 ppm (ipso carbons, pseudo triplet);
131.8 ppm (meta carbons, pseudo triplet); 130.4 ppm (para
carbons, singlet); 128.6 ppm (ortho carbons, pseudo trip-
let); 29.4 ppm (P–CH2–P carbons, pseudo triplet). 31P
H
P
C
O
O
H
H
P= PPh2
C
P
Scheme 1.

}
E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1834
Table 1
JPCHB = 7–8 Hz), 3.16 (dt, 1H, HA, JAB = 14 Hz,
JPCHA = 10 Hz), 4.06 (q, 2H, CH3CH2), 4.53 (t, 1H,
JPCHmethin = 21.5 Hz), 7.2–7.7 (m, 10H, Ph-H). 13C
NMR: (293 K, CDCl3): d ꢂ239.0 ppm (bridging carbonyl);
205.2 and 202.7 ppm (terminal carbonyls); 181.3 ppm
(–C(@O)O–carbonyl); 136.4 and 134.5 ppm (ipso phenyl
carbons, pseudo triplets); 132.6 and 131.0 ppm (ortho phe-
nyl carbons, poorly resolved pseudo triplets); 130.4 and
130.0 ppm (para phenyl carbons, singlets); 128.6 ppm (meta
phenyl carbons, poorly resolved pseudo triplet); 94.5 ppm
(bridging –CH–carbon); 60.0 ppm (–O–CH2–); 53.4 ppm
(CH2Cl2); 27.3 ppm (P–CH2–P, triplet); 14.2 ppm (–CH3).
31P NMR (293 K, CDCl3): d 57.6 (s) ppm, 31P NMR
(200 K, CD2Cl2): d 58.2 (s) ppm. 13C SS-NMR (293 K,
polycrystalline sample): d ꢂ244.7 ppm (bridging carbon-
yls); ꢂ204.3 ppm (terminal carbonyls); 181.4 ppm
(–C(@O)O–); 138.6–124 ppm (phenyl carbons, not suffi-
ciently resolved); ꢂ105.4 ppm (bridging –CH–carbon);
59.8 ppm (–O–CH2–); 53.5 ppm (most probably CH2Cl2
is present in the crystals); 28.3 ppm (P–CH2–P); 15.6 ppm
(–CH3). 31P SS-NMR (293 K, polycrystalline sample): d
ꢂ52.3 (m) and 50.2 (m) ppm.
Crystal data and structure refinement of Co2(CO)5(CHCO2Et)(dppm) (2)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C34H28Co2O7P2
728.36
293(2)
orthorhombic
Pca21
18.235(3)
18.121(4)
21.843(4)
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢀ)
Volume (A )
3
˚
7218(2)
8
1.340
1.052
Z
Dcalc (Mg/m3)
Absorption coefficient, l (mmꢀ1
F(000)
)
2976
Crystal size (mm)
Absorption correction
Maximum/minimum transmission
h-Range for data collection (ꢀ)
Reflections collected
Independent reflections [R(int)]
Reflections I > 2r(I)
Data/restraints/parameters
Goodness-of-fit on F2
0.529 · 0.516 · 0.482
multi-scan
0.600/0.520
3.12 6 h 6 27.48
58096
16329 [0.0548]
13053
16329/1/811
1.052
R1 = 0.0571, wR2 = 0.1397
R1 = 0.0732, wR2 = 0.1482
0.005; 0.001
0.836 and ꢀ0.528
Final R indices [I > 2r(I)]
R indices (all data)
Maximum and mean shift/e.s.d.
2.3. Reaction of Co2(CO)7(CHCO2Et) with dppm
ꢀ3
˚
Largest difference in peak/hole (e A
)
To a solution of Co2(CO)7(CHCO2Et) [17] (10 mg,
0.025 mmol) in CH2Cl2 (2.0 cm3) under argon dppm
(9.6 mg, 0.025 mmol) was added at room temperature
and stirred for 30 min. In accord with the TLC analysis,
the IR spectrum of the reaction mixture at 1822 and
1794 cmꢀ1 showed the presence of a 1:3 mixture of complex
1 and complex 2. Repeating the experiment in the presence
of ethanol (0.025 mmol) diethyl malonate (0.007 mmol,
28% yield) was formed beside complexes 1 and 2, according
to quantitative infrared analysis, using the molar absor-
NMR (293 K, CDCl3): d 60.8 (s) ppm, 31P NMR (200 K,
CD2Cl2): d 62.3 (s) ppm.
13C SS-NMR (293 K, polycrystalline sample): 134.2–
121 ppm (phenyl carbons, not sufficiently resolved);
27.4 ppm (P–CH2–P). 31P SS-NMR (293 K, polycrystalline
sample): d ꢂ63.5 (m) and ꢂ59.6 ppm (m).
2.2. Preparation of Co2(CO)5(CHCO2Et)(dppm) (2)
bance of diethyl malonate eM(CH2Cl2), 1749 cmꢀ1
=
579 cm2/mmol, and eM(CH2Cl2), 1749 cmꢀ1 = 666 cm2/
To a solution of Co2(CO)6(dppm) (1) (268 mg, 0.40
mmol) in CH2Cl2 (7.8 cm3) ethyl diazoacetate (50.2 mg,
0.44 mmol) was added and refluxed for 2.5 h. Progress
of the reaction was followed by IR (disappearance of
the m(N„N) band of ethyl diazoacetate at 2112
(eM = 800 cm2/mmol) cmꢀ1), and by TLC on Silica gel/
CH2Cl2: by disappearance of complex 1, Rf(1) = 0.785,
Rf(2) = 0.354. By removing the solvent in vacuum below
0 ꢀC complex 2 was obtained as red crystalline solid
(295 mg, 0.39 mmol, 97.5% yield). Analytically pure 2 suit-
able for a single-crystal structure determination was
obtained by slow diffusion of n-pentane into a solution of
2 in CH2Cl2 (120 mg/cm3) at 5 ꢀC. IR (KBr pellets)
m(C„O) 2044, 2010, 1987 cmꢀ1, m(C@O) 1813, m(C@O)org.
1677 cmꢀ1, IR (CH2Cl2) m(C„O) 2045 (eM = 2953 cm2/
mmol.
2.4. Preparation of 13CO enriched samples of complex 2
To a solution of complex 2 (34.0 mg, 0.045 mmol) in
CH2Cl2 (0.85 cm3) under argon in a closed Schlenk flask
fitted with a silicon-rubber injection port (total inner vol-
ume: 12 cm3) 13CO (6 cm3) was added using a gas-tight syr-
inge at room temperature. After stirring for an hour, the
solvent was removed under vacuum, and the residue dis-
solved under argon in CH2Cl2 (0.85 cm3). After closing
the flask a new portion of 13CO (6 cm3) was injected
and the reaction mixture was stirred for an hour. Repeating
this procedure four-times resulted in a 39.4% 13CO-enrich-
ment of the carbonyl ligands in complex 2 as could be
calculated based on the intensities of the bridging
carbonyl bands m(12C@O) at 1822 (eM = 946 cm2/mmol)
and m(13C@O) at 1783 (eM = 909 cm2/mmol) cmꢀ1. The
eM of the m(13C@O) band was calculated from the experi-
mentally measured eM value of the m(12C@O) band by
mmol), 2015 (eM = 4001 cm2/mmol), 1989 (eM
3358 cm2/mmol) cmꢀ1 m(C@O) 1822 (eM = 946 cm2/
mmol) cmꢀ1 m(C@O)org. 1689 (eM = 204 cm2/mmol),
1650 (eM = 230 cm2/mmol) cmꢀ1 1H NMR: (303 K,
CDCl3): d 1.23 (t, 3H, CH3), 2.6 (dt, 1H, HB, JAB = 14 Hz,
=
,
,
.

}
E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1835
multiplying it with the mass correction value (12/
13)ꢀ0.5 = 0.96077.
the calculations the GAUSSIAN 03 program package was used
[25].
2.5. Measurement of the rate of 13CO incorporation into
complex 1 and 2
3. Results and discussion
3.1. Crystal structure of 2
For the measurements a magnetically stirred thermo-
statted glass reactor (35 cm3 total volume) was used,
equipped with a gas inlet and with a silicon disk port.
The gas inlet was connected through a two-way stopcock
to a vacuum pump and a 13CO-filled gas burette, respec-
tively. A stainless-steel cannula connected to a 3-port T-
valve was immersed close to the bottom of the reactor
through the silicon disk. To the two other ports of the
valve a Hamilton TLL syringe (2.5 cm3 volumes) and
the IR cell (through a PTFE tubing) were connected.
Samples from the reaction mixture for IR analyses
were withdrawn through the stainless steel cannula,
and pumped into the IR cell continuously by using the
Hamilton TLL syringe, allowing the liquid sample to
return from the IR cell to the reactor through a second
PTFE tubing. The solvent and the solution of the
reactant were added to the 13CO-filled reactor through
the silicon disk using Hamilton TLL syringes. In typical
experiments the reactor and its connected parts were
first evacuated and then filled with 13CO (740 mmHg
total pressure) and CH2Cl2 (6.5 cm3) was added. After stir-
ring at 10 ꢀC for 10 min the reaction was started by
injection of a precooled solution of complex 1 or complex
2 (0.066 mmol) in CH2Cl2 (1.5 cm3). Infrared spectra
were recorded with four scans after 2, 4, 6, 8, 12, 16,
20, 30,. . . min, and so on until the 13CO enrichment of
30.1% (complex 1) or 44.2% (complex 2) has been
reached in ca. 24 or 14 h, respectively. The rate constant
of the overall 13CO incorporation (0.107 · 10ꢀ3 sꢀ1 for
complex 1, and 0.243 · 10ꢀ3 sꢀ1 for complex 2) was calcu-
lated from the initial changes of the concentration of com-
plex 1 or complex 2, as obtained from the intensity values
measured at 1820 cmꢀ1 (1) or 1822 cmꢀ1 (2), or that
The crystal structure of 2 (Fig. 1) shows that the ethoxy-
carbonylcarbene replaces one of the bridging carbonyls in
complex 1, which was closer to the phosphorus atoms.
The crystal data and structure refinement, and selected
bond lengths and angles are compiled in Tables 1 and 2.
3.2. NMR and IR spectra of 1 and 2
NMR spectra of complexes 1 and 2 show symmetrical
structures when recorded in liquid phase. The solution
31P NMR spectra of complexes 1 and 2 exhibit singlet
absorptions between 200 and 293 K (see Section 2). In
the 13C NMR spectra all carbon atoms but the carbonyls
and the phenyl para carbons (these are not coupled to
chemically equivalent but magnetically inequivalent P
atoms) show the expected pseudo triplets corresponding
to the AXX0 spin systems (A = 13C, X = X0 = 31P) [26].
As an instructive example see the aromatic region of com-
plex 2 (Fig. 2). It proves unambiguously that in this com-
pound two kinds of P-bound phenyl rings can be
distinguished at room temperature. While the ipso carbons
having sizeable P–C coupling exhibit well resolved pseudo
triplets, likewise the meta and ortho carbons exhibit also
triplet-like fine structures (though much less resolved due
to the different P–C couplings), the para carbons lacking
measurable P–C couplings show singlets. Note that there
is no plane of symmetry between the CO groups trans to
ofCo2(13CO)x(CO)6ꢀx(dppm) or Co2(13CO)x(CO)5ꢀx
-
(CHCO2Et)(dppm) from the intensity values measured at
1758 cmꢀ1 (1) or 1783 cmꢀ1 (2) by dividing the observed
initial rates with the initial concentration of complex 1 or
complex 2.
2.6. Computational details
All the structures (minima and transition states) were
fully optimized without symmetry constraints within the
framework of the density functional theory using the ONI-
OM method developed by Morokuma and co-workers [18].
The whole system was divided into two layers. The B3LYP
functional [19] was used for the inner layer in combination
with the 6-31G(d) basis set [20] while the remainder, con-
taining only the phenyl rings of the dppm ligand, was trea-
ted with the local spin density approximation (LSDA)
[21,22] utilizing the LANL2MB basis set [23,24]. For all
Fig. 1. Molecular diagram of molecule 1 of the asymmetric unit of
complex 2 with the atomic numbering (hydrogen atoms are omitted for
clarity).

}
E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1836
Table 2
1, only one set of the two methylene protons was observed
˚
Selected bond lengths (A) and angles (ꢀ) with e.s.d.s of Co2(CO)5(CH-
CO2Et)(dppm) (2)a
[3] in form of a triplet at 3.15 ppm. One of the reviewers
1
pointed out that in contrast to 1 in H NMR of 2, there
Co(1)–Co(2)
2.4202(13)
2.4268(12)
2.2595(17)
2.2405(16)
2.2466(17)
2.2559(16)
1.972(7)
1.994(7)
1.963(7)
1.907(6)
1.965(7)
1.966(7)
1.931(6)
1.938(6)
97.97(5)
98.60(5)
98.16(5)
98.06(5)
113.0(3)
113.7(3)
shall be two sets of multiplets for the two methylene pro-
tons, since a back and forth motion around the dicobalt
fragment does not average out the differences in their
chemical environments. Indeed, we found two sets for
those protons as doublets of triplets at 2.6 and 3.16 ppm
(see Section 2), supporting this expectation.
Co(1)–P(1)
Co(2)–P(2)
Co(1)–C(1)
The symmetry is lost in solid phase (see Figs. 4 and 5 for
Co(1)–C(2)
the 31P and 13C CPMAS spectra of 2, respectively). The 31
P
CPMAS spectra exhibit complicated unsymmetrical multi-
plets between 65 and 45 ppm. The multiplet structure of the
signals observed only in the solid-state is due the residual
dipolar coupling existing between the 31P and quadrupolar
57Co nuclei. Similar cases for the 31P(I = 1/2)–57Co(I = 7/
2) pair have already been reported [27,28]. The fine struc-
ture of the signals (two heavily overlapped, distorted
octets) can be taken as an unambiguous proof for the direct
31P–57Co bonds and for the two different 31P sites. The iso-
tropic chemical shifts can be estimated from the centre of
gravity of the observed transitions.
Co(2)–C(1)
Co(2)–C(2)
P(1)–Co(1)–Co(2)
P(2)–Co(2)–Co(1)
P(2)–C(7)–P(1)
a
Numbering of atoms according to Fig. 1.
The 13C CPMAS spectrum is even more informative.
Based on the same phenomenon signals of carbon atoms
attached directly to one or two 57Co nuclei show multiplets.
Although not all transitions of the octets can be identified
the bridging and terminal CO groups and the bridging
–CH–carbon atom can be clearly assigned to the signals
at 244.7, ꢂ206, and at ꢂ104 ppm, respectively. At the same
the P atoms and those of quasi-trans to the bridging CH
group. On the other hand, in complex 1 all the phenyl
and the carbonyl groups are equivalent (see Fig. 3 for the
aromatic region of complex 1) indicating fast exchange
between the axial and equatorial and bridging and terminal
positions, respectively. Owing to the fluxional behavior of
138
137
136
135
134
133
132
131
130
129
128
127
ppm
Fig. 2. 13C NMR spectrum of 2 (aromatic region only) recorded in CDCl3 at room temperature at 9.39 T. The two meta carbons overlap accidentally at
128.59 ppm.

}
E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1837
138
137
136
135
134
133
132
131
130
129
128
127
ppm
Fig. 3. 13C NMR spectrum of 1 (aromatic region only) recorded in CDCl3 at room temperature at 9.39 T.
130
120
110
100
90
80
70
60
50
40
30
20
10
0
-10
-20
ppm
Fig. 4. 31P CPMAS spectrum of 2 recorded at 9.39 T, spinning speed 12500 Hz, room temperature. The isotropic transitions are indicated by numbers.
For clarity only the first pair (+1,ꢀ1) of the spinning side band manifold is shown.

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E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1838
260
240
220
200
180
160
140
120
100
80
60
40
20
0 ppm
Fig. 5. 13C CPMAS spectrum of 2 recorded at 9.39 T, spinning speed 12500 Hz, room temperature. The isotropic signals are indicated by numbers. The
signal at 53.5 ppm is tentatively assigned to CH2Cl2 present in the crystals.
time all other carbons not attached to cobalt atom exhibit
Co2ðCOÞ7ðCHCO2EtÞ
þ dppm ! Co2ðCOÞ5ðCHCO2EtÞðdppmÞ þ 2CO
singlets (the observed line widths are larger than the one,
r:t:
two or three-bond 13C–31P spin–spin couplings therefore
these couplings cannot be seen).
ð4Þ
The 31P solid-state spectra of complex 1 (not discussed
in details) showed similar features, however, in the 13C
CPMAS spectrum, unlike 2, practically only the proton-
ated carbons gave signals. This can be explained by the
lack of protons in vicinity of the carbonyls, but also the
exchange of bridging and terminal carbonyls, still going
on in the solid-state, can scale down the dipolar interac-
tions responsible to the magnetization transfer.
The solid-state IR spectrum of complex 1 in KBr show
four terminal and two bridging carbonyl absorptions in
accord with the asymmetric structure revealed by the crys-
tal structure determination and the solid-state NMR spec-
tra. In CH2Cl2 solution, however, only three (but broad)
terminal carbonyl absorptions were found, indicating a
more symmetrical environment for those m(C@O) vibra-
tions. In the case of complex 2 both in the solid-state and
in solution three terminal and one bridging carbonyl
stretching vibrations were detected.
Applying a 1:1 molar ratio of Co2(CO)7(CHCO2Et) and
dppm at room temperature in CH2Cl2 solution, a 1:3 mix-
ture of Co2(CO)6(dppm) and Co2(CO)5(CHCO2Et)(dppm)
is formed. Under these reaction conditions 25% of the eth-
oxycarbonylcarbene ligand from the complexes is lost
probably by its diphosphane-induced coupling with carbon
monoxide. The highly reactive product of this coupling
reaction, ethoxycarbonylketene can be trapped with etha-
nol in form of diethyl malonate [17].
3.4. 13CO exchange experiments
Using 13CO atmosphere over solutions of complex 1 in
CH2Cl2 at 10 ꢀC, incorporation of 13CO can be recognized
according to Eq. (5) from the beginning of the reaction by
the shifts of the m(CO) bands of the coordinated CO ligands
of complex 1 to lower wave numbers in the solution infra-
red spectra. In the terminal m(C„O) range the various
m(12C„O) and m(13C„O) bands are strongly overlapping
during the progress of the exchange reaction. In parallel
to that, ꢀh1owever, one of the bridging m(12C@O) bands at
1820 cm and one of the bridging m(13C@O) bands at
1758 cmꢀ1 are separated and are suitable for quantitative
determination the rate of the exchange reaction. The over-
all rate of 13CO exchange of complex 1 was calculated as
3.3. An alternative pathway for the preparation of complex 2
Although the dediazotation reaction (Eq. (2)) of ethyl
diazoacetate with Co2(CO)6(dppm) offer the best practical
way to prepare complex 2 we checked an alternative path-
way according to Eq. (4).

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E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1839
kobs = 0.107 · 10ꢀ3 sꢀ1 which is about 56 times less than
that found for Co2(CO)8 [29 and references therein].
Co2ðCOÞ6ðdppmÞ
10 ꢁC
13
þ x13CO ! Co2ðCOÞ6ꢀxð COÞxðdppmÞ þ xCO
ð5Þ
CH2Cl2
Under the conditions above, complex 2 behave similarly.
Incorporation of 13CO can be recognized according to
Eq. (6) from the beginning of the reaction by the shifts of
the m(CO) bands of the coordinated CO ligands of complex
2 to lower wave numbers in the solution infrared spectra.
For quantitative purposes the bridging m(12C@O) band at
1822 cmꢀ1 and the bridging m(13C@O) band at 1783 cmꢀ1
was used to calculate the overall rate of 13CO exchange.
The overall rate of 13CO exchange of complex 2 was calcu-
lated as kobs = 0.243 · 10ꢀ3 sꢀ1 which is more than twice of
that for complex 1. This rate is two orders of magnitude
slower than that observed under the same conditions for
the complex Co2(CO)5(CHCO2Et)(PPh3)2 [30].
10 ꢁC
13
2 þ x13CO ! Co2ðCOÞ5ꢀxð COÞxðCHCO2EtÞ
CH2Cl2
ꢃ ðdppmÞ þ xCO
ð6Þ
3.5. Computed structures of complexes 1 and 2
The optimized structures of 1 and 2 are depicted in
Fig. 6. Both geometries are in reasonable agreement with
the corresponding crystal structures, however the orienta-
tion of the phenyl rings are somewhat different than in con-
densed phase. The cobalt–carbonyl carbon bond distances
are shorter than those in the analogous Co2(CO)8 and
Co2(CO)7(l-CH2) complexes containing no P-donor
ligands [31], thus larger dissociation energies for the Co–
Ccarbonyl bonds are expected. The axial CO ligands are
more strongly bound to cobalt than the equatorial ones.
Fig. 6. Computed structures of
OM(B3LYP:LSDA) level of theory. The hydrogen atoms of the phenyl
rings are omitted for clarity.
1
and
2
optimized at the ONI-
account the higher thermal stability of 1, the more pre-
ferred pathway of the CO dissociation is 1 ! 3a, directly.
The coordinative unsaturated 3a can also readily isom-
erize to the semi-bridged 3b via 3abTS with a free energy
of activation of 1.5 kcal/mol (see Fig. 8). Both isomers
are almost degenerate energetically, but 3b is more stable
by 0.2 kcal/mol. The other transition state found on the
potential energy hyper surface describes the migration of
one of the terminal CO ligands from one cobalt to the
other via transition state 3aaTS. The free energy of activa-
tion for this process is 5.7 kcal/mol.
The infrared data of the carbonyl complexes containing
dppm ligand are collected in Table 3. Since no scaling fac-
tor to offset the systematic error caused by neglecting of
anharmonicity for the ONIOM(B3LYP/6-31G(d):LSDA/
LANL2MB) model chemistry is available the wavenum-
bers are given unscaled. As expected, the deviation between
the experimental and computed frequencies was quite sim-
ilar to that for the B3LYP/6-31G(d) method [33].
3.6. Isomerization of 1 and CO dissociation in
Co2(CO)6(dppm) complexes
As discussed in Section 3.1, the NMR spectra of 1 indi-
cated a very fast exchange of the carbonyl groups, similarly
to that in Co2(CO)8, which is generally explained with the
very facile interconversion between the double-bridged C2v
and the unbridged D2d and D3d structures [32]. For
Co2(CO)6(dppm) only two isomers have been located (see
Fig. 7), namely with a dibridged (1) and a semi-bridged
(1a) structure, respectively. The semi-bridged structure is
higher in free energy by 4.6 kcal/mol and the barrier of
the interconversion is 6.1 kcal/mol.
Dissociation of an equatorial CO from 1 leads to the
coordinative unsaturated dibridged complex 3a with the
free energy of dissociation of 19.7 kcal/mol. The looser
bound equatorial CO in 1a results in a lower free energy
of dissociation, namely 14.9 kcal/mol and structure 3b pos-
sessing a semi-bridged CO ligand. However, taking into

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E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1840
Fig. 7. Free energy profile of the isomerization of Co2(CO)6(dppm).
Fig. 8. Free energy profile of the isomerization reactions of Co2(CO)5(dppm).
Table 3
to 1, thus the CO dissociation of 2 resulting in 4 is not
expected to be involved in the preferred pathway for CO
exchange. Complex 2a is very close to 2 in energy, however,
the free energy of dissociation is 24.0 kcal/mol, still higher
than an expected value being in accordance with the higher
rate of the CO exchange of 2. The CO dissociation in com-
plex 2b is smaller than in any of the four isomers (7.5 kcal/
mol), however, the reaction takes place via a transition
state; the free energy barrier is 20.8 kcal/mol. Complex
2c, which is higher in free energy by 5.6 kcal/mol and
may be originated from 2 by an arm-off dissociation of
the phosphane ligand followed by an isomerization, con-
tains a loosely bound terminal carbonyl ligand with a sig-
nificantly lower free energy of dissociation (12.2 kcal/mol)
compared to 2 and 2a. Thus, this pathway is predicted to
be the possible route in the 13CO exchange reaction of com-
plex 2. The harmonic vibrational data of complexes con-
taining ethoxycarbonyl carbene ligand are given in Table 4.
Selected harmonic vibrational frequencies for complexes and transition
states of Co2(CO)n(dppm) type (n = 5,6)
Structure
m (cmꢀ1
)
1
1840, 1943, 2080, 2087, 2106, 2137
21i, 2000, 2021, 2062, 2076, 2094, 2132
2005, 2028, 2062, 2073, 2093, 2132
1869, 1923, 2079, 2083, 2122
1TS
1a
3a
3aaTS
3abTS
3b
34i, 1924, 2030, 2048, 2081, 2114
103i, 1875, 2008, 2064, 2083, 2117
1923, 2040, 2075, 2082, 2130
3.7. CO dissociation in Co2(CO)5(CHCO2Et)(dppm)
complexes
As possible isomers of Co2(CO)5(CHCO2Et)(dppm),
four complexes have been considered. Two with the bridg-
ing ethoxycarbonylcarbene group in cis-position to dppm
and two with trans-position, both with two possible orien-
tations of the ethoxycarbonyl group, as depicted in Fig. 9.
The free energy of the CO dissociation of 2 is 30.1 kcal/
mol, significantly higher than that of 1. However, the rate
of the 13CO exchange is more than twice for 2 compared
4. Conclusions
In this paper the structures of Co2(CO)6(dppm) (1)
and Co2(CO)5(CHCO2Et)(dppm) (2) and the rate of their

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E. Fo¨rdos et al. / Inorganica Chimica Acta 361 (2008) 1832–1842
1841
structure of 2 revealed that the ethoxycarbonylcarbene
group replaces one of the bridging carbonyls in complex
1, which was closer to the phosphorus atoms. According
to the computational results the structure with this position
and orientation of the ethoxycarbonylcarbene group
proved to be the lowest energy structure among the possi-
ble Co2(CO)5(CHCO2Et)(dppm) isomers. In CH2Cl2 solu-
tion under an atmosphere of 13CO, the CO ligands of
both complexes exchange with 13CO. The overall rate of
13CO exchange at 10 ꢀC was found to be 2.27 times higher
for 2 than for 1. According to ONIOM studies the domi-
nant pathway of CO dissociation of complex 2 is originated
from the slightly higher energy isomer 2c bearing the
ethoxycarbonylcarbene group trans to the dppm ligand.
The solution NMR spectra of 1 indicate a fast exchange
between the axial and equatorial and bridging and terminal
positions of the carbonyl groups, respectively. Accordingly,
the free energy barrier of the interconversion of 1 into the
semi-bridged structure 1a is 6.1 kcal/mol.
5. Supplementary material
CCDC 648429 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Cen-
tre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
The authors thank the Hungarian Academy of Sciences
and the Hungarian Scientific Research Fund for financial
support under Grant No. OTKA F046959 and for the
Supercomputer Center of the National Information Infra-
structure Development (NIIF) Program.
Fig. 9. Geometries and Gibbs free energies of CO dissociation of isomers
of Co2(CO)5(CHCO2Et)(dppm). Free energy values are given in kcal/mol.
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