Formation of a stable dicarbenoid and an unsaturated C2P 2S2 ring from two-electron oxidation of the [C(PPh 2S)2]2- dianion

Article abstract of DOI:10.1039/b810796c

Two-electron oxidation of the [C(PPh₂S)₂] ^2- dianion with iodine afforded an unexpected mixture of a dimeric Li-I carbenoid [(Et₂O)(μ-Li)][(μ₄-Li){IC(PPh ₂S)₂}₂] and a novel, unsaturated six-membered C₂P₂S₂ ring in [(SPh₂P) ₂C₂(PPh₂)₂S₂]. The Royal Society of Chemistry.

Full text of DOI:10.1039/b810796c

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Formation of a stable dicarbenoid and an unsaturated C2P2S2 ring
from two-electron oxidation of the [C(PPh2S)2]2ꢀ dianionw
Jari Konu and Tristram Chivers*
Received (in Berkeley, CA, USA) 25th June 2008, Accepted 19th July 2008
First published as an Advance Article on the web 12th September 2008
DOI: 10.1039/b810796c
Two-electron oxidation of the [C(PPh2S)2]2ꢀ dianion with iodine
afforded an unexpected mixture of a dimeric Li–I carbenoid
[(Et2O)(l-Li)][(l4-Li){IC(PPh2S)2}2] and a novel, unsaturated
six-membered C2P2S2 ring in [(SPh2P)2C2(PPh2)2S2].
Our recent systematic studies of the oxidation of dichalcogen-
idoimidodiphosphinate monoanions [(EPR2)2N]ꢀ (1, E = S, Se,
Te; R = iPr, tBu, Chart 1) have revealed new and unanticipated
aspects of the chemistry of these well-studied anions.1 We have
shown that one-electron oxidation with iodine produces dimeric
dichalcogenides EPR2NR2PE–EPR2NPR2E (2) with elongated
central E–E bonds.2,3 In one case (E = Te, R = tBu) a structural
isomer comprised of a contact ion pair in which a [(TePtBu2)2N]ꢀ
Chart 1
anion is Te,Te0-chelated to one Te atom of an incipient cyclic
cation [(TePtBu2)2N]+ was identified.3 The formally 6p-electron
i
five-membered cations [(EPR2)2N]+ (E = S, Se, Te; R = Pr,
d8-THF revealed a major component (ca. 75%) that exhibits
tBu) are obtained as iodide salts by two-electron oxidation of
a singlet at d 48.2 and a minor product (ca. 25%), which gives
rise to two mutually coupled doublets at d 46.4 and 42.1
the corresponding anions.3–5 By contrast, the oxidation of
the phenyl-substituted derivatives [(EPPh2)2N]ꢀ (E = Se, Te)
with one equivalent of iodine was found to be chalcogen-
dependent; the five-membered ring [(TePPh2)2N]+ was obtained
for tellurium, whereas the unusual six-membered ring
[(m-Se)(SePPh2)2N]+ was the major product in the case of
selenium.5
[2J(31P,31P) = 97 Hz]. Both of the new compounds were
identified in the solid state by their X-ray crystal structures.wy
Single crystals of the major component were obtained from a
toluene solution of the product mixture and identified as the
dicarbenoid [(Et2O)(m-Li)][(m4-Li){IC(PPh2S)2}2] (5) (Scheme 1).
In contrast to the monomeric structure of 4, the iodide derivative
5 adopts a dimeric arrangement of two molecules of the carbene
[:C(PPh2S)2] and two molecules of LiI. The two five-atom
SPCPS fragments in the C2-symmetric dicarbenoid 5, each of
which contains a C–I bond, are connected by a tetrahedral Li+
cation and a three-coordinate Li+ cation solvated by Et2O
(Fig. 1). The spirocyclic Li+ cation Li(1) has S–Li–S bond
angles spanning a range of 99.8(1) to 119.5(3)1, while the
In the context of these intriguing and diverse results we
turned our attention to the oxidative behaviour of the isoelec-
tronic [C(PPh2S)2]2ꢀ dianion (3), which is prepared from
bis(thiodiphenylphosphinoyl)methane upon treatment with
two equivalents of methyl-lithium.6 Le Floch and co-workers
have reported recently that mild oxidation of Li23 with hexa-
chloroethane produces the remarkably stable carbenoid
[ClC(PPh2S)2]Li(OEt2)2 (4).7,8 In this communication we give
details of our complementary investigations of the oxidation of
Li23 with iodine, which have led to the structural and spectro-
scopic characterisation of the stable dicarbenoid [(Et2O)-
(m-Li)][(m4-Li){IC(PPh2S)2}2] (5) and the novel, unsaturated
six-membered C2P2S2 ring in [(SPh2P)2C2(PPh2)2S2] (6).
The reaction between Li23 and one equivalent of I2 was
initially performed in a toluene–Et2O mixture at ꢀ80 1C
(Scheme 1).z The 31P NMR spectrum of the product in
3-coordinate lithium is in a trigonal planar environment
+Li(2) 359.91). The Li(2) centre also engages in significant
Liꢁ ꢁ ꢁI close contacts of 3.167(1) A which form an almost linear
P
(
University of Calgary, Department of Chemistry, 2500 University
Drive NW, Calgary, Alberta, Canada T2N 1N4. E-mail:
chivers@ucalgary.ca; Fax: +1-403-289-9488; Tel: +1-403-220-5741
w Electronic supplementary information (ESI) available: Experimental
and crystallographic data in pdf format. CCDC reference numbers
692935 (5) and 692936 (6). For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b810796c
Scheme 1 Formation of 5 and 6 by two-electron oxidation of the
dianion [C(PPh2S)2]2ꢀ (3) with iodine.
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Fig. 1 Crystal structure of 5 with thermal ellipsoid at 50% prob-
ability level. Hydrogen atoms have been omitted for clarity. Relevant
bond parameters (in A and 1): C(1)–I(1) 2.147(3), C(1)–P(1) 1.750(3),
C(1)–P(2) 1.745(3), P(1)–S(1) 1.997(1), P(2)–S(2) 2.022(1), Li(1)–S(1)
2.466(4), Li(1)–S(2) 2.512(5), Li(2)–S(2) 2.461(5), Li(2)ꢁ ꢁ ꢁI(1) 3.167(1),
P(1)–C(1)–P(2) 123.8(2), P(1)–C(1)–I(1) 111.8(2), P(2)–C(1)–I(1)
107.3(2). Symmetry operation (A): ꢀx, y, 0.5 ꢀ z.
Fig. 2 Molecular structure of 6ꢁEt2O with thermal ellipsoids at 50%
probability level. Hydrogen atoms and Et2O solvate have been omitted
for clarity. Pertinent bond lengths (in A): C(1)–P(1) 1.762(3),
C(1)–P(2) 1.692(3), C(1)–S(2) 1.775(3), C(2)–P(3) 1.766(3), C(2)–P(4)
1.692(3), C(2)–S(2) 1.769(3), P(1)–S(1) 1.975(1), P(2)–S(4) 2.133(1),
P(3)–S(3) 1.969(1), P(4)–S(4) 2.137(1).
is bent away from the ring. The structure exhibits both endo-
and exo-cyclic P–C and P–S bonds with inequivalent bond
lengths; the exocyclic P–C bonds are ca. 0.07 A longer and the
exocyclic P–S bonds are ca. 0.16 A shorter than the corres-
ponding endocyclic bond lengths, indicating significant double
bond character in the endocyclic PQC and exocyclic PQS
bonds, respectively, as illustrated in Scheme 1. The exocyclic
PQS distances are similar to those reported for the related
compounds [Ph2CQC(PPh2S)2]14 and [H(Cl)C(PPh2S)2].7 The
C(1) and C(2) atoms in 6 are only slightly distorted from
planarity with the sum of bond angles being 356.2 and 358.51,
respectively.
arrangement [+I(1)–Li(2)–I(1A) = 169.6(3)1]. Consequently,
the carbon atom in the PCP unit in 5 is pyramidally distorted
with the sum of bond angles being 342.91, cf. 359.91 in 4.7
Despite the disparity in the geometry around the carbon center
in 4 and 5, and the strength of the different carbon–halogen
bonds, the calculated bond orders for the C–Cl and C–I bonds
in 4 and 5, respectively, are essentially identical (0.90 and 0.92,
respectively).9 The endocyclic P–C and P–S bond distances in 5
are slightly elongated (by ca. 0.02 A), compared to those in 4.12
Notwithstanding the two inequivalent lithium and phosphorus
environments in 5, only a single resonance was observed in both the
7Li (d 0.73) and 31P NMR spectra at 23 1C indicating fluctionality
in solution. Indeed, when a CD2Cl2 solution of the product is
The formation of the C2P2S2 ring in 6ꢁEt2O from the two-
electron oxidation of 3 involves a sulfur transfer process to
generate a CSC unit. A tentative explanation of this observation
is shown in Scheme 2. The first step (i) invokes an intermolecular
nucleophilic attack of a sulfur atom of the initially formed
carbene [:C(PPh2S)2] on the electron-deficient carbon centre of
another carbene to create a C–S bond, accompanied by a
7
cooled to ꢀ90 1C, two singlets are observed in the Li NMR at
d 2.20 and 1.16. The broadness of the latter singlet, however,
suggests that unresolved fluctionality persists even at low tempera-
ture affording a broad singlet for the 3-coordinate lithium. Con-
sistently, the 31P NMR spectrum of 5 exhibits two broad,
overlapping singlets rather than the expected two mutually coupled
doublets at ꢀ90 1C. The 13C{1H} NMR spectrum of 5 in d8-THF
shows a broad triplet at d 2.4 ppm [1J(13C,31P) = ca. 60 Hz] for the
PCP-carbon at 23 1C. The triplet of the PCP-carbon in 5 is shifted
to higher field by ca. 36 ppm compared to that in 4 (38.5 ppm).7
This upfield shift can be attributed primarily to the greater shielding
effect of iodine compared to chlorine,13 although the change of
hybridization (towards sp3) in 5 compared to that in 4 (carbon lone
pair in a pure p orbital)7 may also be a contributing factor.
Single crystals of the minor product formed in the reaction
depicted in Scheme 1 were obtained from a diethyl ether
solution. This compound was identified by X-ray crystallo-
graphy as [(SPh2P)2C2(PPh2)2S2] (6), which contains a novel,
unsaturated six-membered C2P2S2 ring in a chair conforma-
tion (Fig. 2).wy This heterocycle can be viewed to result from
the union of two [:C(PPh2S)2] carbenes without the incorpora-
tion of LiI. The crystal structure of 6ꢁEt2O contains exocyclic
Ph2PQS units attached to each carbon, one of which is
inclined towards the centre of the ring while the second unit
Scheme 2 Suggested reaction mechanism for the formation of the
six-membered C2P2S2 ring in 6.
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reflections with I 4 2s(I)] and wR2 = 0.0629 (for all data). Crystal data
of 6ꢁEt2O: C54H50OP4S4, Mr = 967.06, monoclinic, space group P21/c,
a = 18.496(4), b = 11.907(2), c = 23.533(5) A, b = 108.68(3)1,
rearrangement to afford a five-membered CSPSP ring 7. Step (ii)
involves an intramolecular attack of a sulfur atom of the five-
membered ring on the exocyclic carbene center in 7 with
subsequent ring expansion to give the final product 6.
V = 4910(2) A3, Z = 4, rcalcd = 1.308 g cmꢀ3, m = 0.363 mmꢀ1
,
T = 173(2) K, 16 547 reflections collected (y range = 2.46–25.031),
8 634 unique (Rint = 0.0432), R1 = 0.0484 [for 6 137 reflections with
I 4 2s(I)] and wR2 = 0.1149 (for all data). The structures were solved
and refined by using SHELXS-97 and SHELXL-97.16
Attempts were made to improve the yield of 6 by carrying out
the oxidation of Li23 with a mixture of iodine and 12-crown-4
(to remove LiI) at ꢀ80 1C.w Although this procedure prevented
the formation of 5 and increased the yield of 6 to ca. 50% (31P
NMR) it was not possible to separate pure samples of 6 from the
other products H2C(PPh2S)2 and H(I)C(PPh2S)2. When
the reaction of Li23 with iodine was carried out at 23 1C the
dicarbenoid 5 was obtained in ca. 90% yield (31P NMR),
together with H2C(PPh2S)2 from which it is difficult to separate;
the six-membered ring 6 is not formed under these conditions.w
In summary, participation of the LiI by-product in the two-
electron oxidation of Li23 results in the formation of the
dimeric carbenoid 5, which exhibits distorted pyramidal car-
bon centers and a thermal stability comparable to that found
in the monomeric Li–Cl carbenoid 4.15 In the absence of LiI
incorporation, this oxidation process produces the novel
unsaturated six-membered C2P2S2 ring in 6.
1 For recent reviews, I. Haiduc, in Comprehensive Coordination
Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier
Ltd., Amsterdam, 2003, pp. 323–347; C. Silvestru and J. E. Drake,
Coord. Chem. Rev., 2001, 223, 117.
2 T. Chivers, D. J. Eisler, J. S. Ritch and H. M. Tuononen, Angew.
Chem., Int. Ed., 2005, 44, 4953.
3 T. Chivers, D. J. Eisler, J. S. Ritch and H. M. Tuononen,
Chem.–Eur. J., 2007, 13, 4643; T. Chivers, J. Konu, J. S. Ritch,
M. C. Copsey, D. J. Eisler and H. M. Tuononen, J. Organomet.
Chem., 2007, 692, 2658.
4 J. Konu, T. Chivers and H. M. Tuononen, Chem. Commun., 2006,
1634.
5 J. Konu, T. Chivers and H. M. Tuononen, Inorg. Chem., 2006, 45,
10678.
6 T. Cantat, N. Me
Angew. Chem., Int. Ed., 2004, 43, 6382; T. Cantat, L. Ricard, Y.
Jean, P. Le Floch and N. Mezailles, Organometallics, 2006, 25,
4965.
7 T. Cantat, X. Jacques, L. Ricard, X. F. Le Goff, N. Me
P. Le Floch, Angew. Chem., Int. Ed., 2007, 46, 5947.
8 For a review on stable PCP-carbenes, see: T. Cantat, N. Me
A. Auffrant and P. Le Floch, Dalton Trans., 2008, 1957.
9 The C–Cl and C–I bond orders were calculated by the Pauling
´
zailles, L. Ricard, Y. Jean and P. Le Floch,
´
´
zailles and
zailles,
Notes and references
´
z Formation of 5 and 6 from Li23 and I2: Li2[C(PPh2S)2] was
synthesized according to the literature.6,7 A solution of H2C(PPh2S)2
(0.287 g, 0.64 mmol) in 30 mL of toluene was cooled to ꢀ80 1C and
0.80 mL of MeLi (1.6 M in Et2O, 1.28 mmol) was added via syringe.
The reaction mixture was stirred for 15 min at ꢀ80 1C and 212 h at
room temperature.
The turbid solution of Li2[C(PPh2S)2] was cooled to ꢀ80 1C and a
solution of I2 (0.162 g, 0.64 mmol) in 30 mL of Et2O was added via
cannula. The reaction mixture was stirred for 12 h at ꢀ80 1C and 3 h at
room temperature giving an orange-yellow solution. The solvents were
evaporated under vacuum and the resulting tarry product was
dissolved to 50 mL of Et2O. White LiI powder was filtered with a
PTFE-disk and the solvent was evaporated in vacuo to give a yellow,
amorphous powder (0.335 g, 91% calculated as a 75 : 25 mixture of 5
and 6 based on 31P NMR data).
ꢀ
R)/0.71 10
equation N = 10(D
,
where R is the observed bond
length (A). The single bond length D is estimated from the sums
of appropriate covalent radii:10,11 C–Cl 1.75 and C–I 2.12 A. A
value of 0.73 A was used for sp2 hybridized carbons. If the covalent
radii of 0.76 A is used to indicate sp3 hybridized carbon, a bond
order of 1.01 is obtained for 5.
10 L. Pauling, The Nature of the Chemical Bond, Cornell University
Press, Ithaca, NY, 3rd edn, 1960.
11 B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves, J.
Echeverria, E. Cremades, F. Barragan and S. Alvarez, Dalton
Trans., 2008, 2832.
12 The crystal structure of 4 was determined at 150 K7 whereas the
structure of 5 was measured at 173 Kw.
13 For example, the 13C NMR chemical shift of CH3I appears ca.
46 ppm upfield from that of CH3Cl and the shift for
CH3CH2I is found ca. 40 ppm upfield from that of CH3CH2Cl:
J. W. Emsley, J. Feeney and L. H. Sutcliffe, High Resolution NMR
Spectroscopy, Pergamon Press Ltd., Oxford, 1966, vol. 2,
pp. 990–997.
NMR data of 5: 1H NMR (THF-d8, 23 1C): d 7.17–8.02 [m, 40H,
C6H5]. 13C{1H} NMR: d 139.5 [(dd) AXX0, J(13C,31P) = 101 Hz; Cipso
of C6H5], 133.8 [t, 2J(13C,31P) = 5.0 Hz; Cortho of C6H5], 130.2 [s; Cpara
of C6H5], 127.8 [t, 3J(13C,31P) = 6.1 Hz; Cmeta of C6H5], 2.4 [br, t,
1J(13C,31P) = ca. 60 Hz; PCP-carbon]. 31P NMR: d 48.2 ppm.
7Li NMR: d 0.73 ppm. 31P NMR (CD2Cl2, ꢀ90 1C): d 50.0 and
49.9 ppm (br, overlapping). 7Li NMR (CD2Cl2, ꢀ90 1C): d 2.20 and
1.16 (br) ppm.
14 T. Cantat, F. Jaroschik, F. Nief, L. Ricard, N. Mezailles and P. Le
Floch, Chem. Commun., 2005, 5178.
2
NMR data of 6: 31P NMR (THF-d8, 23 1C): d 46.4 [d, J(31P,31P) =
15 The monomeric carbenoid
4 was reported to be stable in
solution and in the solid state up to 60 1C.7 The dimeric
carbenoid 5 showed no decomposition when the solid material
was kept at 95 1C under vacuum (ca. 10ꢀ3 Torr) for several
hours.
97 Hz] and 42.1 [d, 2J(31P,31P) = 97 Hz] ppm. 1H and 13C{1H} NMR
data for 6 could not be obtained due to the small quantity of the
compound in the product mixture.
y Crystal data of 5: C54H50I2Li2OP4S4, Mr = 1234.74, monoclinic,
space group C2/c, a = 26.997(5), b = 10.517(2), c = 20.050(4) A,
16 G. M. Sheldrick, SHELXS-97, Program for solution of crystal
¨
b = 109.28(3)1, V = 5373(2) A3, Z = 4, rcalcd = 1.526 g cmꢀ3
,
structures, University of Gottingen, Germany, 1997; G. M.
Sheldrick, SHELXL-97, Program for refinement of crystal struc-
m = 1.483 mmꢀ1, T = 173(2) K, 8595 reflections collected (y range =
2.99–25.031), 4 707 unique (Rint = 0.0284), R1 = 0.0312 [for 3 694
tures, University of Gottingen, Germany, 1997.
¨
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4995–4997 | 4997