Reaction of (PPh3)2C→CO2 with halogenated hydrocarbons; Formation and crystal structure of a cationic ester with 1, 2-dichloroethane

Article abstract of DOI:10.1002/zaac.201200122

The carbodiphosphorane CO₂ adduct (2) reacts slowly with 1, 2-dichloroethane to give (HC{PPh₃}₂)Cl (5) as result of HCl abstraction along with the ester-like salt (ClCH₂CH ₂O(O)CC{PPh₃}₂)Cl (4) from nucleophilic substitution of one Cl^- by 2. Both compounds could be separated by fractional crystallization. Attempts to dissolve 2 in 1, 2-difluorobenzene leads to small amounts of the hydrolysis product (HC{PPh₃} ₂)(HCO₃)·H₂O (6·H₂O) caused by some humidity in the solvent. All compounds could be crystallized and the structures studied by X-ray analyses and ³¹P NMR spectroscopy. Copyright

Full text of DOI:10.1002/zaac.201200122

ARTICLE
DOI: 10.1002/zaac.201200122
Reaction of (PPh3)2CǞCO2 with Halogenated Hydrocarbons; Formation
and Crystal Structure of a Cationic Ester with 1,2-Dichloroethane
Wolfgang Petz*[a] and Bernhard Neumüller*[a]
Keywords: X-ray diffraction; Ylides; Carbodiphosphorane CO2 adduct; Alkylation; Ester-like cation
Abstract. The carbodiphosphorane CO2 adduct (2) reacts slowly with
1,2-dichloroethane to give (HC{PPh3}2)Cl (5) as result of HCl abstrac-
1,2-difluorobenzene leads to small amounts of the hydrolysis product
(HC{PPh3}2)(HCO3)·H2O (6·H2O) caused by some humidity in the
tion along with the ester-like salt (ClCH2CH2O(O)CC{PPh3}2)Cl (4) solvent. All compounds could be crystallized and the structures studied
from nucleophilic substitution of one Cl– by 2. Both compounds could
be separated by fractional crystallization. Attempts to dissolve 2 in
by X-ray analyses and 31P NMR spectroscopy.
transition metal[8] or main group Lewis acids[9] via the two
oxygen atoms;[10,11] coordination of both oxygen atoms each
at different beryllium atoms was also achieved.[12] During
attempts to study the chemistry of 2 with several Lewis acids
we found that the betain is insoluble in benzene, toluene, and
other non polar hydrocarbons but is not stable in solutions of
halogenated hydrocarbons. Upon reacting of 2 with Lewis ac-
ids in these solvents proton abstraction and loss of CO2 was
observed with formation of the cation (HC{PPh3}2)+ either as
main product or as byproduct of the desired addition com-
pound.
1 Introduction
It has recently been shown that the bonding situation in the
double ylide hexaphenylcarbodiphosphorane C(PPh3)2 (1),[1]
which has a bent equilibrium geometry[2] is best described in
terms of donor-acceptor interactions between two phosphane
ligands and a bare carbon(0) atom. For those compounds of
the general type CL2 the term carbones has been suggested,
in which L is represented by various neutral electron donor
molecules.[3]
Herein we report on the behavior of 2 in various halogenated
hydrocarbons and the formation and the X-ray structure of a
cationic ester-like compound upon reacting 2 with 1,2-dichlor-
oethane.
Due to the presence of two electron lone pairs (HOMO and
HOMO-1) carbones have first and second proton affinities,[4]
which are very large for 1. Carbones exhibit a chemical behav-
ior that is distinctively different from carbenes such as NHC’s,
which have only one lone pair at a carbon(II) atom.[5] Com-
pound 1 reacts with the electron poor heterocumulene CO2 to
give the betain 2 as shown in Equation (1).[6] The molecular
structure of 2 was established by X-ray analysis by us re-
cently.[7]
2 Results and Discussion
In an earlier report the alkylation of 2 with MeI was de-
scribed with result
of
the cationic compound
({PPh3}2CǞC(O)OMe)I (3), which was characterized by IR
and 31P NMR spectroscopy.[13] Further ester-like compounds
of the type ({PPh3}2CǞC(O)OR)X were not reported so far.
The 31P NMR spectrum of a freshly prepared solution of 2
in DCM exhibits two singlet at δ = 13.50 and 19.04 ppm in a
1:008 ratio, which were assigned to 2 and the cation
(HC{PPh3}2)+, respectively. On standing of the probe at room
temperature under exclusion of air the ratio has changed to
1:0.24 after 6 h and to 1:3.4 after 4 h; after one day standing
the signal of 2 has disappeared.
If 2 was dissolved in 1,2-dichloroethane a clear solution was
obtained. The 31P NMR spectrum after 1 h showed three sing-
let signals at δ = 20.8, 19.9, and 14.3 ppm in a 0.2:0.4:1 ratio,
respectively. After 3 h the ratios changed to 0.6:0.9:1 ratio. 22
h later the initial signal of 2 at δ = 14.3 ppm has disappeared
in favor of a 1:1 ratio of the signals at δ = 20.8 and 19.9 ppm.
1 + CO2 Ǟ (PPh3)2CǞCO2 (2)
(1)
Similar adducts are formed with CS2 and COS. Compound
2 can act as a monodentate or as a chelating ligand towards
* Prof. Dr. W. Petz
E-Mail: petz@staff.uni-marburg.de
* Prof. Dr. B. Neumüller
Fax: +49-6421-2825653
E-Mail: neumuell@chemie.uni-marburg.de
[a] Fachbereich Chemie der Philipps-Universität
Hans-Meerwein-Strasse
35032 Marburg, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200122 or from the au-
thor.
Z. Anorg. Allg. Chem. 2012, 638, (6), 987–991
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
987

W. Petz, B. Neumüller
ARTICLE
The NMR signals were assigned to (ClCH2CH2O(O) of the C–C bond and formation of the salt 6; an intermediate
CC{PPh3}2)Cl (4) and (HC{PPh3}2)Cl (5), respectively. Lay- protonated version of 2 could not be found (Scheme 2).
ering the solution with n-pentane generated two sorts of crys-
talline materials. On fractional crystallization the first part of
crystals turned out to be 5. Further colorless crystals of
3 Crystal Structure Determination
4·2C2H4Cl2 were obtained as the result of a nucleophilic sub-
stitution of one chlorine atom by the oxygen atom of the betain
2. If 4 was dissolved in DCM the signal of 4 slowly disap-
peared in favor of the signal of 5; no alkylation with DCM
was observed (Scheme 1).
Crystals suitable for X-ray analyses were obtained from
4·2C2H4Cl2, 5, and 6·H2O. Crystallographic details were sum-
marized in Table 1. Molecular structures of 4 and 6 are shown
in Figure 1 and Figure 2, respectively; the molecular structure
of 5 is presented as Supporting Information (Figure S1). In
4·2C2H4Cl2 the solvent molecules were strongly disordered.
The residual electron density of these molecules was omitted
with SQUEEZE.[16]
3.1 Molecular Structure of 4·2C2H4Cl2
The molecular structure of 4·2C2H4Cl2 is depicted in Fig-
ure 1. The environments at C(1) and C(2) are exactly planar
indicating sp2 hybridization of the carbon atoms. No contacts
exist to the Cl– ion or to the disordered solvent molecules. The
C(1)–C(2) bond length has decreased from 1.494(3) in 2 to
1.459(4) in 4 indicating a slight increase in double bond char-
acter upon alkylation. The cation of 4 can be compared with
organic esters and with the neutral compounds [Cl2SnO(O)
CC{PPh3}2] (7)[9] and [(CO)5WO(O)CC{PPh3}2] (8),[8] in
which ClCH2CH2+ are replaced by SnCl2 and W(CO)5, respec-
tively. The planes OCO and PCP form a dihedral angle of 29°
which is larger than the related angle in 2 (10°) or even in 7
(13°) and 8 (9°). Similar as in 4 the two C–O bond lengths are
different and the longer one belongs to the coordinating CO
function [7: 1.251(7)/1.297(7), Δ = 0.046; 8: 1.241(6)/
1.290(6), Δ = 0.049; 4: 1.221(5)/1.363(5), Δ = 0.142 Å) and
the largest difference Δ of C–O bond lengths is recorded for
4. The related pairs of bond length in typical esters are 1.202/
1.333 Å; both values are slightly shorter than those in 4. The
elongated CO bond in 4 corresponds to the low ν(CO) vi-
bration found by IR spectroscopic studies.
Scheme 1. Reaction of 2 in 1,2-dichloroethane to give 4 and 5.
In the IR spectrum of 4 a strong band at 1670 cm–1 was
found, which was assigned to the ν(CO) vibration of the free
carbonyl function; a further strong band at 1230 cm–1 may be
attributed to the vibration of the C–O–C group. A comparison
with typical organic esters shows that the carbonyl vibration
of 4 has shifted to lower frequencies; ester carbonyl groups
absorb in a narrow range between 1735 and 1770 cm–1.[14]
A
similar cationic ester was obtained from alkylation of the be-
tain NHCǞCO2; the ν(CO) vibration of (NHCǞC(O)OEt)+
amounts to 1745 cm–1[15] being in the range of organic esters.
The shift to lower frequency in 4 can be explained with a
higher π electron density at the carboxylate carbon atom due
to some π donation from the occupied p orbital of the CL2
carbon atom.
Attempts to dissolve 2 in 1,2-difluorobenzene gave a sus-
pension. Compound 2 is nearly insoluble in this solvent, but
the 31P NMR spectrum of the supernatant solution showed a
singlet at δ = 20.0 ppm. Filtration after several hours stirring
at room temperature and layering of the solution with n-pent-
ane gave colorless crystals, which turned out to be the hydro-
gen carbonate (HC{PPh3}2)(HCO3)·H2O (6·H2O). The forma-
tion of the hydrogen carbonate can be explained with a not
completely dried 1,2-difluorobenzene causing part of 2 to hy-
drolyze by traces of water to give 6. The additional free pair
of electrons at the carbon atom of 2 is basic enough to attract a
proton, which weakens the C–C bond. Subsequent nucleophilic
attack of OH– at the CO2 carbon atom finally leads to splitting
3.2 Molecular Structure of (HC{PPh3}2)(HCO3)·H2O
(6·H2O)
The molecular structure of 6·H2O is depicted in Figure 2.
The unit cell of the salt contains a centrosymmetric dimer of
–
6·H2O. Two HCO3 anions are linked by O–H···O bridges
forming a planar eight-membered ring. As expected three dif-
ferent C–O bond lengths are found in the dimeric anion. The
unbridged C–O bond lengths amounts to 1.233(3) Å and is
about 0.12 Å shorter than the C–O–H bond and about 0.04 Å
shorter than the C–O····H bond. No contacts to the cation exist
and its parameters are close to those of the cation of 5. The
bridged oxygen atoms are separated by 2.664(2) Å. Further
OH bridges exist to water molecules with O···O separations of
2.904(2) and 2.896(2) Å. A similar dimeric structure was
2–
found in the salt (C{NMe2}3)(HCO3). The related (HCO3)2
unit has no further contacts and the corresponding bridged C–
O bond lengths are about 0.01 Å shorter than those in 6·H2O
[17]
Scheme 2. Proposed mechanism of the hydrolysis of 2.
.
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Z. Anorg. Allg. Chem. 2012, 987–991

Reaction of (PPh3)2CǞCO2 with Halogenated Hydrocarbons
Table 1. Crystallographic data of (ClCH2CH2O(O)CC{PPh3}2)Cl·2C2H4Cl2 (4·2C2H4Cl2), (HC{PPh3}2)Cl (5), and (HC{PPh3}2)(HCO3)·H2O
(6·H2O).
4·2C2H4Cl2
(HC{PPh3}2)Cl (5)
6·H2O
Formula
C44H42Cl6O2P2
877.48
20.577(1)
9.749(1)
40.326(2)
90
C37H31ClP2
573.05
9.470(1)
19.070(2)
16.942(2)
90
C38H34O4P2
616.63
13.594(1)
16.642(1)
14.934(1)
90
Mw /g·mol–1)
a /Å
b /Å
c /Å
α /°
β /°
γ /°
93.04(1)
90
102.88(1)
90
112.27(1)
90
Crystal size /mm
Volume /Å3
Z
0.23ϫ0.18ϫ0.08
8078(1)
8
0.29ϫ0.27ϫ0.07
2982.6(6)
4
0.23ϫ0.19ϫ0.18
3126.5(1)
4
dcalc /g·cm–3
Crystal system
Space group
Diffractometer
Radiation
Temperature /K
μ /cm–1
1.443
1.276
1.31
monoclinic
I2/a (Nr. 15)
IPDS II (Stoe)
Mo-Kα
193
5.4
monoclinic
P21/n (Nr. 14)
IPDS II (Stoe)
Mo-Kα
193
2.6
monoclinic
P21/n (Nr. 14)
IPDS II (Stoe)
Mo-Kα
193
1.8
2θmax /°
52.43
52.53
52.55
Index range
–25 Յ h Յ 25
–12 Յ k Յ 12
–50 Յ l Յ 50
57093
8079 (0.0892)
5330
–11 Յ h Յ 11
–23 Յ k Յ 23
–19 Յ l Յ 20
21440
5890 (0.0989)
3482
–16 Յ h Յ 16
–20 Յ k Յ 20
–18 Յ l Յ 18
44810
6282 (0.0656)
4094
Number of rflns collected
Number of indep. rflns (Rint
Number of observed rflns
with Fo Ͼ 4σ(Fo)
)
Parameters
417
366
411
Absorption correction
Structure solution
numerical
numerical
numerical
direct methods SHELXL-97[19]
direct methods SHELXS-97[20]
direct methods SHELXL-97[19]
Refinement against F2
Hydrogen atoms
SHELXL-97[19]
SHELXL-97[19]
SHELXL-97[19]
calculated positions with common
displacement parameter
calculated positions with common calculated positions with common
displacement parameter
displacement parameter; H(1) was
refined freely
0.0523
0.1069
R1
0.0752
0.2254
0.54
0.034
0.077
0.52
wR2 (all data)
Max. electron density left /
0.19
e·Å–3
CHCl after proton abstraction from DCM.[18]. A possible
doublealkylationof2togivethedication[(RO)2CǟC(PPh3)2]2+
could not be observed so far; a challenge for further studies.
4 Conclusions
The ester-like salt 4 is the first alkylation product of 2,
which could be confirmed by X-ray diffraction analysis. The
related NHC addition compound with NHCǞCO2 was studied
by X-ray analysis[15] but to the best of our knowledge no struc-
tural details of related ester-like [NHCǞC(O)OR]+ com-
pounds were reported so far. Addition compounds between the
carbone 1 and Lewis acidic heteroallenes such as CO2 in 2 can
act as monodentate or chelating ligands towards various Lewis
acids. Attempts to alkylate 2 led either to R+ transfer to one
oxygen atom or to proton abstraction with loss of CO2 and
formation of the cation (HC{PPh3}2)+. This competition reac-
tion will be explored more intensively during further studies.
With 1,2-dichloroethane nucleophilic substitution of Cl– to 4
and HCl abstraction to give 5 take place simultaneously. How-
ever, if 2 is dissolved in CH2Cl2 abstraction of HCl occurs and
slow formation of the (HC{PPh3}2)+ and no alkylation product
[e.g. ClCH2O(O)CC{PPh3}2] could be found. With
OSCC(PPh3)2 and CH2Cl2 a similar pathway occurred and we
could identify the olefin ClHC=CHCl by gas chromatographic
5 Experimental Section
General: All operations were carried out in an argon atmosphere in
dried and degassed solvents using Schlenk techniques. The solvents
were thoroughly dried and freshly distilled prior to use. The IR spectra
were run with a Nicolet 510 spectrometer in Nujol mull (the Nujol
mull bands are omitted). For the 31P NMR spectra we used the instru-
ment Bruker AC 300. The carbodiphosphorane C(PPh3)2 (1) was pre-
pared according to a modified literature procedure.[21] 2 was obtained
as colorless precipitate upon admitting CO2 to a solution of 1 in tolu-
ene at room temperature.
Reaction of 2 with 1,2-Dichloroethane: Compound 2 (0.20 g,
0.34 mmol) was dissolved in 1,2-dichloroethane (about 2 mL). The re-
action was monitored by 31P NMR spectroscopy. After 1 h at room
temperature signals at δ = 20.8, 19.9, and 14.3 ppm in a 0.2:0.4:1 ratio,
respectively, were found. After 3 h the ratios changed to 0.6:0.9:1 and
after 22 h the signal of 2 at δ = 14.3 ppm has disappeared in favor of
studies; the olefin formed upon dimerization of the carbene the signals at δ = 20.8 and 19.9 ppm in a 1:1 ratio. Layering of the
Z. Anorg. Allg. Chem. 2012, 987–991
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W. Petz, B. Neumüller
ARTICLE
solution with n-pentane (0.5 mL) gave crystals of (HC{PPh3}2)Cl (5).
After removal of the crystals the solution showed only the signal at δ
= 20.8 ppm in the 31PNMR spectrum. New colorless crystals of
4·2C2H4Cl2 have grown after layering again with n-pentane. 31P NMR
(in 1,2-dichloroethane): δ = 20.8 ppm. IR (Nujol mull): ν˜ = 1671 vs,
1585 w, 1481 s, 1451 m, 1437 s, 1261 w, 1312 w, 1284 m, 1263 m,
1229 s, 1198 m, 1162 w, 1103 s. 1181 m, 1054 w, 997 m, 925 m, 879
w, 755 s, 745 s, 717 s, 699 s, 669 m, 650 m, 542 m, 526 s, 503 s, 437
w, 404 w cm–1.
Formation of 6·H2O: Compound 2 (0.24 g, 0.41 mmol) was sus-
pended in o-difluorobenzene (about 2.5 mL) and the mixture was
stirred for five days at room temperature and filtered. The IR spectrum
of the residue was similar to that of 2. The 31P NMR of the solution
showed a single signal at δ = 20.0 ppm. Layering of the solution with
n-pentane caused separation of colorless crystals suitable for an X-ray
analysis, which turned out to be 6·H2O. Traces of water in the solvent
probably were responsible for the hydrolysis of 2. Further experiments
with more dried o-difluorobenzene showed that 2 is only slightly solu-
ble in this solvent and a signal at δ = 13.4 ppm was obtained for 2.
After 4 h on standing in the NMR tube, the signal of 2 has diminished
in favor of that of the cation (HC{PPh3}2)+ (1:2 ratio). After three
days only the signal of the cation was recorded.
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-867347 (4·2C2H4Cl2), CCDC-867348 (5), and
CCDC-867349 (6·H2O) (Fax: +44-1223-336-033; E-Mail: deposit@-
ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Figure 1. Molecular structure of (ClCH2CH2O(O)CC{PPh3}2)Cl in
4·2C2H4Cl2 showing the atom numbering scheme. The ellipsoids are
drawn at a 40% probability level and the phenyl rings are presented
as thin lines. The hydrogen atoms are omitted for clarity. Selected bond
lengths /Å and angles /°: C(1)–C(2) 1.459(6), P(1)–C(1) 1.738(4),
P(2)–C(1) 1.745(4), O(1)–C(2) 1.363(5), O(2)–C(2) 1.221(5), O(1)–
C(3) 1.430(5), C(3)–C(4) 1.497(7), Cl(1)–C(4) 1.787 (5); P(1)–C(1)–
P(2) 127.5(2), P(1)–C(1)–C(2) 116.9(3), P(2)–C(1)–C(2) 115.5(3),
O(1)–C(2)–O(2) 121.6(4), O(1)–C(2)–C(1) 111.7(4), O(2)–C(2)–C(1)
126.7(4).
Supporting Information (see footnote on the first page of this article):
Structural view and information on compound 5.
Acknowledgement
We thank the Deutsche Forschungsgemeinschaft for financial support.
W. P. is also grateful to the Max-Planck-Society, Munich, Germany,
for supporting this research project.
Figure 2. Molecular structure of the salt (HC{PPh3}2)(HCO3)·H2O (6·H2O) showing the atomic numbering scheme. The ellipsoids are drawn
at a 40% probability level and the phenyl groups are represented as thin lines. Selected bond lengths /Å and angles /°: P(1)–C(1) 1.704(2),
P(2)–C(1) 1.707(2), O(1)–C(38) 1.355(3), O(2)–C(38) 1.269(2), O(3)–C(38) 1.233(3), O(1)–H(2) 0.89(2); P(1)–C(1)–P(2) 128.9(1), O(1)–C(38)–
O(2) 116.8(2), O(1)–C(38)–O(3) 117.1(2), O(2)–C(38)–O(3) 126.1(2).
990
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Reaction of (PPh3)2CǞCO2 with Halogenated Hydrocarbons
[10] W. Petz, B. Neumüller, R. Tonner, Eur. J. Inorg. Chem. 2010,
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Received: March 19, 2012
Published Online: Mai 15, 2012
Z. Anorg. Allg. Chem. 2012, 987–991
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
991