The reaction of potassium hydridopentacarbonylchromate with acrylates. X-ray structure of [PPN]+[CH3CH(COOEt)Cr(CO))5]-

Article abstract of DOI:10.1016/S0022-328X(01)01102-0

KHCr(CO)₅ reacts with methyl acrylate in acetonitrile to give methyl propionate after acid hydrolysis. Monitoring the reaction by IR and NMR spectroscopy showed that the reaction mainly proceeds through the 1,2-addition product K⁺ [CH₃CH(COOMe)Cr(CO)₅]^-. However, in situ formation of methyl propionate (ca. 15%) before hydrolysis suggests that part of the reaction also occurs through the 1,4-adduct K⁺ [CH₃CH=C(OMe)OCr(CO)₅]^-. The PPN⁺ salt of the 1,2-adduct obtained from ethyl acrylate was characterized by single crystal X-ray diffraction.

Full text of DOI:10.1016/S0022-328X(01)01102-0

Journal of Organometallic Chemistry 643–644 (2002) 27–31
www.elsevier.com/locate/jorganchem
The reaction of potassium hydridopentacarbonylchromate with
acrylates. X-ray structure of [PPN]+[CH3CH(COOEt)Cr(CO)5]−
Boris Andrieu, Jean-Jacques Brunet *, Ousmane Diallo, Bruno Donnadieu,
Jacques Lienafa, Emmanuel Roblou
Laboratoire de Chimie de Coordination du CNRS, Unite´ N° 8241,
lie´e par con6entions a` l’Uni6ersite´ Paul Sabatier et a` l’Institut National Polytechnique, 205 route de Narbonne,
31077 Toulouse Cedex 04, France
Received 17 May 2001; accepted 2 July 2001
Abstract
KHCr(CO)5 reacts with methyl acrylate in acetonitrile to give methyl propionate after acid hydrolysis. Monitoring the reaction
by IR and NMR spectroscopy showed that the reaction mainly proceeds through the 1,2-addition product K+
[CH3CH(COOMe)Cr(CO)5]−. However, in situ formation of methyl propionate (ca. 15%) before hydrolysis suggests that part of
the reaction also occurs through the 1,4-adduct K+ [CH3CHꢀC(OMe)OCr(CO)5]−. The PPN+ salt of the 1,2-adduct obtained
from ethyl acrylate was characterized by single crystal X-ray diffraction. © 2002 Published by Elsevier Science B.V.
Keywords: Hydridocarbonylchromates; Acrylates; 1,2-Adduct; Alkyl–chromium bonds; X-ray structure
1. Introduction
bis(triphenylphosphine)iminium (PPN+) salt (Eq. (1))
[2]. The structure of the latter was determined by X-ray
crystallography [5].
On the basis of the observed regioselectivity, further
investigations led us to design the first catalytic, re-
gioselective hydrocarboxylation of acrylic acid into
methylmalonic acid (Eq. (2)) [6,7].
Previously reported catalytic hydrocarboxylations of
acrylic acid derivatives were known to proceed with the
reverse regioselectivity, affording mainly succinic acid
derivatives [8]. Interestingly, the Fe(CO)5-based reac-
tion can be performed under much milder conditions;
however, its turnover frequency is low (TOFꢀ0.7
In the last decade, we have studied the reaction of
K+[HFe(CO)4]− with a,b-unsaturated carbonyl com-
pounds such as acrylic acid derivatives [1,2]. This reac-
tion promotes the selective reduction of the
carbon–carbon double bond, as previously stated by
Noyori et al. by using a slightly different reagent [3,4].
The mechanism of such reactions was examined in
more detail in the case of ethyl acrylate and showed to
involve the regioselective addition of [HFe(CO)4]− on
the activated carbon–carbon double bond, generating
an alkyltetracarbonylferrate which could be isolated as
(1)
(2)
* Corresponding author. Tel.: +33-561-333145; fax: +33-561-
553003.
E-mail address: brunet@lcctoul.lcc-toulouse.fr (J.-J. Brunet).
h
−1). This could be reflected by the reaction rate of the
reduction of ethyl acrylate by [HFe(CO)4]− [2].
0022-328X/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S0022-328X(01)01102-0

28
B. Andrieu et al. / Journal of Organometallic Chemistry 643–644 (2002) 27–31
Consequently, we recently decided to study the reac-
tion bands at 1901 (s, broad) and 1849 (w) cm−1). The
major absorptions of some remaining amounts of the
tivity of the corresponding potassium hydridopentacar-
bonylchromate K+[HCr(CO)5]− [9]. Indeed, on the
basis of studies by Darensbourg et al., this complex is
considered to be a better hydride transfer agent than
[HFe(CO)4]− [10].
starting complex 1 (wCO=1886 (s) and 1855 (m) cm−1
)
.
should be buried beneath the broad band at 1901 cm−1
The new absorptions at 1901 and 1849 cm−1 corre-
spond to a new carbonylchromium complex which may
have a geometry similar to that of [HCr(CO)5]−.
These observations led us to examine the reactivity of
the dinuclear hydride 2 for the reduction of methyl
acrylate. Complex 2 does not significantly react (B5%)
with methyl acrylate for 5 h in acetonitrile at room
temperature. It must be noted, however, that this com-
plex has been reported to selectively reduce the carbon–
carbon double bond of a,b-unsaturated carbonyl
compounds when used in refluxing THF for 4 h [14].
Thus it appears that [HCr(CO)5]− is the actual reducing
species in the reaction of Eq. (3).
We first designed a very simple method for a repro-
ducible, routine one-step preparation of KHCr(CO)5
from Cr(CO)6 [11]. As part of our study of the reactivity
of this complex towards carbonyl compounds, we ob-
served that KHCr(CO)5 selectively reacts with the CꢀC
bond of mesityl oxide to give 4-methylpentan-2-one in
more 80% (GC yield) [12]. Furthermore, this reaction
was much more rapid than that involving KHFe(CO)4,
an interesting observation in view of a possible use of
[HCr(CO)5]− for the functionalisation of a,b-unsatu-
rated carbonyl compounds (vide supra). From that
point of view, previous results from Darensbourg et al.
were rather discouraging since these authors reported
that the reaction of [PPN][HCr(CO)5] with acrylonitrile
needs 20 equivalents of acrylonitrile to consume the
chromium hydride because of the formation of large
amounts of oligomeric acrylonitrile side-products [13].
We wish to report here our first results on the
reactivity of KHCr(CO)5 with alkyl acrylates.
It was obviously interesting to get more insight into
the mechanism by which the reaction occurs. Indeed,
although the pentacoordinate [HFe(CO)4]− regioselec-
tively adds to acrylic esters to give the corresponding
alkylferrate (Eq. (1)), the reaction of the hexacoordinate
[HCr(CO)5]− could proceed by another way. For in-
stance, the reaction of ketones R2CꢀO with
[HFe(CO)4]− is believed to occur via nucleophilic attack
of the iron atom on the carbonyl group of the ketone,
generating [(CO)4Fe(H)ꢁCR2O]− species (FeꢁC bond
formation) [15]. In contrast, the reaction of
[HCr(CO)5]− with ketones is believed to occur via a
hydride transfer, generating the corresponding
chromium alkoxide [R2CHꢁOꢁCr(CO)5]− (CꢁH and
CrꢁO bonds formation) [16]. In other words, the reac-
tion of KHCr(CO)5, with methyl acrylate could a priori
proceed through any of three intermediates (Fig. 1).
The presence of the potassium enolate 5 is less likely,
but cannot be ruled out since the unsaturated species
‘Cr(CO)5’ is known to be easily quenched by [HCr-
(CO)5]− to give the dinuclear hydride [(m-H)Cr2-
(CO)10]−, a reaction which is involved in the degrada-
tion of [HCr(CO)5]− in the presence of traces of H+
sources [9].
2. Results and discussion
When KHCr(CO)5 (1), is reacted with one equivalent
of methyl acrylate in acetonitrile at room temperature,
GC–MS analysis of the reaction medium after hydroly-
sis indicates complete consumption of methyl acrylate
within 10 min and formation of methyl propionate in
ca. 90% yield (Eq. (3)).
KHCr(CO)5
+
CH CN, RT H O
3
+CH2ꢀCHꢁCOOMe ꢁꢁꢁꢁꢁꢁꢂ ꢁ3ꢁꢁꢂCH3CH2COOMe
10 min
ꢀ90%
(3)
The same reaction is observed in THF and in toluene
although in the latter solvent the reaction rate is much
lower, probably because of the very poor solubility of
KHCr(CO)5 in this solvent.
Monitoring the reaction in acetonitrile by IR analysis
(carbonyl ligand region) indicated the appearance of the
characteristic absorptions of K+[(m-H)Cr2(CO)10]− (2),
(w=2033 (vw), 1942 (m) cm−1) and of two new absorp-
The reaction of 1 with methyl acrylate was studied by
NMR (CD3CN as solvent). After 5 min reaction, the
1H-NMR (400.13 MHz) spectrum of the soluble frac-
tion exhibits three different methoxy singlets assigned to
(i) unreacted methyl acrylate (lMeO=3.73 ppm) (ca.
15%), (ii) methyl propionate (lMeO=3.63 ppm) (ca.
Fig. 1. Possible reaction intermediates.

B. Andrieu et al. / Journal of Organometallic Chemistry 643–644 (2002) 27–31
29
tra are identical to those of the potassium salt (vide
supra). The IR spectrum of 6 exhibits absorption bands
at 1903 (s, sharp) and 1849 (m) cm−1. Satisfactory
elemental analyses were obtained for the formula
C45H37NO7CrP2, indicating that the adduct contains
only one chromium atom, as expected.
Unfortunately, it has not been possible to obtain
satisfactory single crystals for 6. However, performing
the reaction of 1 with ethyl acrylate, followed by
metathesis with PPNCl as above, allowed isolation of
convenient single crystals of the adduct 7 (Fig. 2),
which were submitted to X-ray diffraction analysis,
confirming the proposed structure (see below).
Fig. 2. PPN+ salts of the 1,2-adducts of [HCr(CO)5]− with methyl
and ethyl acrylates.
Nevertheless, the 1H-NMR observation of methyl
propionate in the reaction medium before hydrolysis
(vide supra) raises some questions. Indeed, control ex-
periments indicated that 1 (tenfold excess) does not
react with the isolated adduct 3 for 5 h at room
temperature in acetonitrile. Furthermore, 3 was found
to be poorly reactive towards water in the same solvent.
Thus, the in situ formation of methyl propionate is
unlikely to arise from 3. In contrast, alkoxochromium
complexes [ROCr(CO)5]− are known to react easily
with [HCr(CO)5]− to give the stable dinuclear hydride
anion and ROH [17]. These chromium alkoxides are
also especially sensitive to weak acids such as traces of
water [17]. It is thus proposed that, in the reaction of 1
with methyl acrylate in acetonitrile, in situ generated
methyl propionate (ca. 15%) arises from 4 by either of
the above reactions.
On the basis of this hypothesis, it is proposed that
the reaction of 1 with methyl acrylate proceeds by
initial hydride transfer on the terminal sp2 carbon of
the carbonꢁcarbon double bond, generating an enolate
intermediate which further reacts with the unsaturated
‘Cr(CO)5’ species by either the carbon or the oxygen
site, as for the C/O alkylation of ambident enolate and
phenoxide ions [18]. (Note, however, that concerted
processes cannot be ruled out.) According to this pro-
posal, the reaction should occur without dissociation of
a carbonyl ligand from [HCr(CO)5]−. This was confi-
rmed by further experiments which showed that the
reaction proceeds exactly at the same rate (identical IR
and NMR spectra after 10 min reaction) when con-
ducted under carbon monoxide (1 atm, bubbling).
Fig. 3. Molecular structure of 7 with atom labelling. Ellipsoids
represent 50% probability.
15%) and (iii) another compound which exhibits a
methoxy signal at 3.48 ppm (ca. 70%). Others signals in
the 0–3 ppm region were difficult to assign in the
spectrum of the crude reaction mixture. Examination of
the hydride region indicates the presence of some
amounts of unreacted 1 (l= −6.99 ppm) and of the
dinuclear hydride 2 (l= −19.50 ppm).
After evaporation of the solvent (and low boiling
1
compounds) under reduced pressure, a new H-NMR
(400.13 MHz) spectrum was registered in CD3CN,
which indicated the presence of a nearly pure com-
pound exhibiting the following signals: (i) a singlet at
3.48 ppm, (ii) a quadruplet at 1.72 ppm (J=6.6 Hz)
and (iii) a doublet at 1.42 ppm (J=6.6 Hz). The
13C-NMR (100.6 MHz) spectrum exhibits signals at
14.10 ppm (d, J=135 Hz), 21.70 ppm (q, J=124 Hz),
48.65 ppm (q, J=144 Hz), 185.40 ppm (s) and 223.22
ppm (s, cis CO), 227.65 (s, trans CO) together with
some minor signals in the carbonyl ligands region,
assigned to 1 and 2. The above spectral data are in full
agreement with the formula of the 1,2-adduct, 3. Solu-
tions of 3 in CD3CN are stable for days at room
temperature under argon.
3. X-ray crystallographic study of 7
A molecular view of the anion with atom labelling
scheme is shown in Fig. 3. Crystal data and structure
refinement are provided in Table 1. It is interesting to
note that this anion is fluxional in the solid state since
one CO group and the alkyl fragment are statistically
distributed on two sites around the metal atom. Only
one of the conformation is shown in Fig. 3. To the best
The corresponding bis(triphenylphosphine)iminium
(PPN+) salt, 6, was easily obtained by metathesis with
PPNCl, followed by classical work-up. Its NMR spec-

30
B. Andrieu et al. / Journal of Organometallic Chemistry 643–644 (2002) 27–31
of our knowledge, there is only one structural report on
a similar alkylchromium derivative, the ylide [(CO)5-
CrCH(Me)(NC5H4Me)] [19]. A comparison of related
bond lengths and angles are presented in Table 2. The
geometry is roughly identical. However, although the
occurrence of a disordered distribution induces large
discrepancies and errors, the CrꢁC(alkyl) bond is sig-
nificantly shorter in 7. The rather long CrꢁC(alkyl)
bond determined for [(CO)5CrCH(Me)(NC5H4Me)] is
certainly related to the presence of the large pyridinium
substituent.
4. Experimental
4.1. General
All reactions were performed under Ar using stan-
dard Schlenk tube techniques. CH3CN (SDS) was dis-
tilled over P2O5 and stored under molecular sieves.
CD3CN (SDS) was used as received. All solvents were
deaerated by bubbling Ar before use. NMR spectra
were registered on a Bruker AC 200 or AMX 400
apparatus. IR spectra were registered on a Perkin–
Elmer 1725X FTIR spectrometer with CaF2 windows
(0.5 mm). GC analyses were performed on a Hewlett–
Packard HP 5890 apparatus equipped with a 30 m
capillary HP 5 column and GC–MS analyses on a
Hewlett–Packard HP 6890 apparatus (EC Wax, 30 m
capillary column) equipped with a HP 5973 M ion
detector. Elemental analyses (C, H, N) were performed
on a Perkin–Elmer 2400 apparatus.
Table 1
Crystal data and structure refinement for 7
Empirical formula
Formula weight
Temperature (K)
[C10H9CrO7](C36H30NP2)
825.67
160(2)
,
Wavelength (A)
0.71073
Triclinic
Crystal system
Space group
(
P1
Unit cell dimensions
,
a (A)
10.746(5)
12.679(5)
15.661(5)
75.546(5)
83.668(5)
83.221(5)
2044.5(14)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
KHCr(CO)5 [11] and KHCr2(CO)10 [20] were pre-
pared according to published procedures.
3
,
4.2. Reaction of 1 with methyl acrylate
Z
Crystal size (mm)
Theta range for data collection
(°)
0.46×0.28×0.18
2.25–24.71
A solution of methyl acrylate (0.2 mmol) and toluene
(internal standard, ca 0.1 mmol) in MeCN (1.5 ml) is
added to a suspension of KHCr(CO)5 in MeCN (0.5
ml). After 10 min stirring at room temperature, an
aliquot (0.1 ml) is withdrawn, and dropped into 0.2 ml
of Et2O. The resulting solution is acidified with ca. 1 ml
of trifluoroacetic acid and analysed by GC, indicating
complete conversion of methyl acrylate and formation
of methyl propionate in ca. 90% yield.
Index ranges
−125h512, −145k514,
−185l518
17 828
6565 [Rint=0.0497]
94.2%
Reflections collected
Independent reflections
Completeness to theta=24.22°
Refinement method
Full-matrix least-squares on
F2
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
6565/182/584
1.019
R1=0.0546, wR2=0.1419
R1=0.0924, wR2=0.1656
4.3. Synthesis of the adduct 7
Largest difference peak and hole 0.749 and −0.938
−3
,
(e A
)
A solution of KHCr(CO)5 (1 mmol) in THF (2 ml) is
added to ethyl acrylate (1 mmol) in THF (6 ml). After
1 h stirring, a solution of PPNCl (0.8 mmol) in CH2Cl2
(5 ml) is added. After 1 h, the solvents are evaporated
under reduced pressure and the resulting solid dissolved
in 5 ml CH2Cl2. This solution is filtered over celite 545,
and evaporated under vacuum at ca. −60 °C. After
washing with Et2O (2 ml), 7 is obtained as an orange
solid (yield: 87%, purity (NMR): 100%). Satisfactory
single crystals were obtained by very slow addition of
diisopropyl ether (3 ml) to a solution of the above solid
in CH2Cl2 (1 ml).
Table 2
Comparison of the CrꢁC(alkyl) geometries in complex 7 and in the
related [(CO)5CrCH(Me)(NC5H4Me)] complex [19]
[(CO)5CrCH(Me)-
(NC5H4Me)]
7
,
,
2.168(13); 2.188(12) A
,
1.527(15); 1.524(13) A
Cr(1)ꢁC(1)
C(1)ꢁC(10)
C(1)ꢁX(2)
2.250 (6) A
,
1.521(8) A
,
,
1.489(7) A [X=N] 1.441(13); 1.437(11)A
[X=CO2Et]
Cr(1)ꢁC(1)ꢁC(10) 115.5(4)°
112.1(8); 111.8(6)°
107.6(8); 107.0(6)°
[X=CO2Et]
110.9(11); 111.2(9)°
[X=CO2Et]
Cr(1)ꢁC(1)ꢁX(2)
111.1(3)° [X=N]
110.7(5)° [X=N]
4.4. Crystal structure determination of 7
C(10)ꢁC(1)ꢁX(2)
Data were collected on a Stoe IPDS diffractometer.
The final unit cell parameters were obtained by the

B. Andrieu et al. / Journal of Organometallic Chemistry 643–644 (2002) 27–31
31
least-squares refinement of 8000 reflections. Only statis-
tical fluctuations were observed in the intensity moni-
tors over the course of the data collections.
Wolff for his advice about crystallisation procedures.
Dr J.C. Daran is gratefully acknowledged for fruitful
discussions about crystallographic data.
The structure was solved by direct methods (SIR92)
[21] and refined by least-squares procedures on F2. All
H atoms attached to carbon were introduced in calcula-
References
,
tion in idealised positions [d(CH)=0.96 A] and treated
as riding models. One of the carbonyl group
C(15)ꢁO(15) and the alkyl CH(Me)(COOEt) fragment
exchange each other on two coordination sites around
the chromium leading to a disorder arrangement with
an occupancy ratio of 70/30. This disordered distribu-
tion was treated using the available tools in SHELXL-97
[22]. Least-squares refinements were carried out by
minimising the function Sw(F2o−Fc2)2, where Fo and Fc
are the observed and calculated structure factors. The
weighting scheme used in the last refinement cycles was
w=1/[|2(F2o)+(aP)2+bP] where P=(Fo2 +2F2c)/3.
Models reached convergence with R=S(ꢀꢀFoꢀ−ꢀFcꢀꢀ)/
S(ꢀFoꢀ) and wR2={Sw(F2o−Fc2)2/Sw(F2o)2}1/2, having
values listed in Table 1.
[1] For a review on [HFe(CO)4]−, see: J.J. Brunet, Chem. Rev. 90
(1990) 1041.
[2] J.J. Brunet, E. Passelaigue, J. Organomet. Chem. 375 (1989) 203.
[3] R. Noyori, I. Uemada, T. Ishigami, J. Org. Chem. 37 (1972)
1542 The exact nature of the reagent was later determined by
Collman et al. [4].
[4] J.P. Collman, R.G. Finke, P.L. Matlock, R. Wahren, R.G.
Komoto, J.I. Brauman, J. Am. Chem. Soc. 100 (1978) 1119.
[5] J.J. Brunet, F. Dahan, E. Passelaigue, Acta Crystallogr. Sect. C
48 (1992) 1103.
[6] J.J. Brunet, E. Passelaigue, Organometallics 9 (1990) 1711.
[7] J.J. Brunet, E. Passelaigue, FR Demande 89 13055 (Rhoˆne-Pou-
lenc), 1989.
[8] (a) A. Matsuda, Bull. Chem. Soc. Jpn. 42 (1969) 571;
(b) J.G. Reuvers, W. Richter, R. Krummer, Ger. Offen. DE.
3,332,018 1985. Chem. Abstr. 103 (1985) 177968.
[9] J.J. Brunet, Eur. J. Inorg. Chem. (2000) 1377.
[10] S.C. Kao, C.N. Spillet, C. Ash, R. Lusk, Y.K. Park, M.Y.
Darensbourg, Organometallics 4 (1985) 83.
[11] J.J. Brunet, R. Chauvin, B. Donnadieu, P. Leglaye, D.
Neibecker, J. Organomet. Chem. 571 (1998) 7.
[12] J.J. Brunet, R. Chauvin, P. Leglaye, Eur. J. Inorg. Chem. (1999)
713.
Calculations were carried out with the SHELXL-97
program using the integrated system WINGX(1.63) [23].
Molecular view was realised with the help of ORTEP
[24].
[13] M.Y. Darensbourg, B. Floris, K.A. Youndahl, Tetrahedron
Lett. 30 (1989) 1781.
5. Supplementary material
[14] G.P. Boldrini, A. Umani-Ronchi, Synthesis (1976) 596.
[15] L. Marko, M.A. Radhi, I. Otvo¨s, J. Organomet. Chem. 218
(1981) 369.
[16] L. Marko, Z. Nagy-Magos, J. Organomet. Chem. 285 (1985)
193.
[17] P.L. Gauss, S.C. Kao, K. Youngdahl, M.Y. Darensbourg, J.
Am. Chem. Soc. 107 (1985) 2428.
[18] O.A. Reutov, A.L. Kurts, Russ. Chem. Rev. 46 (1977) 1040.
[19] F. Cohen, R. Goumont, H. Rudler, J.C. Daran, R.A. Toscano,
J. Organomet. Chem. 431 (1992) C6.
Crystallographic data (excluding structure factors)
for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre, CCDC no. 163293. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 IEZ, UK
(Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
[20] M.D. Grillone, L. Palmisano, Transition Met. Chem. 14 (1989)
81.
[21] A. Altomare, G. Cascarano, G. Giacovazzo, A. Guagliardi,
M.C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 27
(1994) 435.
Acknowledgements
[22] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Go¨ttingen, Go¨ttingen, Germany,
1997.
[23] L.J. Farrugia, WINGX, J. Appl. Crystallogr. 32 (1999) 837.
[24] L.J. Farrugia, ORTEP3, J. Appl. Crystallogr. 30 (1997) 565.
The Centre National de la Recherche Scientifique
(France) is acknowledged for financial support. The
authors also wish to thank Y. Coppel, F. Lacassin and
G. Commenges for registration of NMR spectra and F.