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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Homolytic cleavage of Lawesson’s reagent: N-heterocyclic carbene
complexes of ArPS2 (Ar = 4-CH3O-C6H4)
Ebrahim Soleimani,a,b* Katherine N. Robertson,b Cory C. Pyeb and Jason A. C. Clyburneb*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Lawesson’s reagent has been shown to react with the N-
heterocyclic carbenes [1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-
ylidene (SIPr)] to give adducts of the general form NHC•P(S)2-
C6H4OCH3. Full characterizations, including X-ray crystal structures,
are provided. The reaction of Woollin’s reagent with IMes gave
the known selanone, (IMes)Se.
O
S
S
S
P
P
S
P
S
S
S
P
2
S
O
O
O
2
1
Ar
N
S
P
S
Se
Se
P
Lawesson's reagent (LR)
1 is the compound of choice for
N
P
Se
Se
Ar
the direct conversion of a wide array of carbonyl containing
species into the corresponding thiocarbonyls.1,2,3 It is
commercially available, easy to handle, and usually gives the
desired product in high yield. In spite of its versatility, reports
regarding its actual mechanism(s) of reaction are few.4,5 It is
accepted that the initial step of the thionation reactions occur
via cleavage of LR to give two equivalents of the neutral but
O
3
4
Ar = 2,4,6-trimethylphenyl
Ar
S
P
Ar
N
S
N
N
Se
N
Ar
Ar
reactive species
with the equilibrium lying far in favour of LR remaining intact.
Species contains an unusual (and presumably highly reactive)
2. LR and 2 exist in equilibrium in solution but
O
5
6
Ar = 2,6-diisopropylphenyl
Ar = 2,4,6-trimethylphenyl
2
been reacted with many main group and transition metal
complexes; see for examples the work of Carmalt et al.11 and
Weng et al.12 Such reactions normally begin with cleavage of
LR. The products formed most often feature interactions with
the sulfur atoms of the monomer and less frequently involve
interaction with the phosphorus centre. Very few simple
pentavalent, tricoordinate phosphorus centre. It can be drawn
in either neutral or zwitterionic form, the latter highlighting
the Lewis acidic nature of the phosphorus centre. It is this
Lewis acidity of phosphorus that facilitates the S/O exchange
reactions of LR with organic carbonyl compounds.
The crystal structure of LR has been studied by both Kempe
et al. (1992)6 and Grossman et al. (1995),7 the latter as a
toluene solvate. However, the structure of the monomeric
adducts of
2 bonded through phosphorus have been
crystallographically characterized and only one of these is an
overall neutral molecule (the reaction of ethylenediamine with
LR gives a zwitterionic product) rather than a salt.13
cleavage product
2
has never been reported. There are three
that have been published but these all
structures similar to
2
There are
a number of NHC-phosphorus compounds
contain aromatic rings stabilized by very bulky and/or
8
electron-withdrawing groups (2,6-CF3 ; 2,4,6-t-Bu9; 2,4-t-Bu
known, isolation of which began with the compounds
described by Arduengo’s group in 1997. All of the complexes
then isolated feature the NHC IMes bonded to a phosphorus
centre bearing electron withdrawing atoms or groups (PF4Ph14,
P(BH3)2Ph15, CF3 and Ph16). Their incorporation of Lewis acidic
phosphorus centres to form adducts with NHCs led to our
conjecture that LR should also form neutral adducts with
NHCs. A search of the literature revealed only one paper
where the reactions of NHCs with LR or WR had been
attempted.17 Bockfeld et al. observed (1H and 77Se NMR) the
and 6-Me10) which likely limits their reactivities. LR has also
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formation of the diselenide (IMe)P(Se)2Ph in ca. 25 % yield
after addition of 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe)
to WR in [D8]THF. However, the main product, which was
formed in 75 % yield, was identified as the selenone, (IMe)Se.
Although largely unsuccessful in their attempts to react
NHCs with LR or WR, Bockfeld et al. were successful in isolating
compounds of the formula (IMe)P(E)2Ph (E = S or Se) by an
alternate route. (IMe)PPh was found to react readily with
sulfur (S8) and selenium (Se grey) in THF to afford the
corresponding NHC-phosphinidene disulfide and diselenide as Figure 1. Solid state structure of 4. Hydrogen atoms and co-
colourless solids. Reactions of more sterically demanding NHC- crystallized solvent (CH2Cl2) have been omitted for clarity.
phosphinidenes (IMes and IPr) with elemental sulfur or Thermal ellipsoids are drawn at the 50% probability level.
selenium did not lead to the isolation of single products. Here
we build on this previous work and report the successful
preparation and isolation of crystalline complexes formed in dichloromethane solvate) and
the reaction of LR with the NHCs IMes and SIPr. have been determined and the final results are shown in
Solutions of LR in either THF or toluene solution exhibited a Figures 1 and 2, respectively. In addition, Table 1 summarizes
31P resonance for
at 15.9 ppm and did not show a resonance selected metrical parameters and there are further diagrams
that could be attributed to . This is not unexpected given the and tables of crystallographic data in the Supplementary
high Lewis acidity of the phosphorus site in coupled with the Information. Our results can be compared to the reported
strong phosphorus - sulphur bonds in . Furthermore, 1
in crystal structures for Lawesson’s reagent.6,7 In addition to the
The X-ray crystal structures of both
4
(as
a
5
(as a toluene hemi-solvate)
1
2
2
1
pyridine-containing solutions did not exhibit any new 31P NMR Arduengo structures previously mentioned,14–16 and the
features suggesting that it does not form a stable complex IMePS2Ph structure of Bockfeld et al.,17 a search of the
with pyridine in solution. This is in contrast to both
might have been expected to be stabilized by pyridine based revealed only one other structure, IMes=P-Mes, suitable for
on the work of Meisel et al.,18 and , which has been shown to comparison to 4 21
There are even fewer structures with which
form pyridine stabilized PhPSe2 in solution (Ascherl et al.19).
to compare ; all of the relevant SIPr-P containing species have
2, which Cambridge Structural Database (Version 5.38, November 2017)
3
.
5
A stirred solution of LR in THF was treated with the NHC additional P-P rather than P-C bonds.22
IMes in THF solution. The resulting solution was stirred for 30
minutes at room temperature under a nitrogen atmosphere.
An aliquot of this solution exhibited a single phosphorus NMR
resonance at 53 ppm. The solution was stirred overnight and
the solvent was removed in vacuo. The residue was dissolved
in CH2Cl2 and slow evaporation of the solvent resulted in the
formation of crystalline material that was isolated in 91% yield.
Elemental analysis and HR-MS are consistent with the
formation of compound 4
. The 31P NMR spectrum of crystalline
4
dissolved in CD2Cl2 solution exhibits a single resonance at
1
52.6 ppm. The H and 13C NMR spectra suggest the formation
of a species composed of the NHC moiety and a fragment
derived from 2. The equivalence of the Mes methyl groups is Figure 2. Solid state structure of 5. Hydrogen atoms and co-
consistent with free rotation around a new dative C-P single crystallized solvent (C6H5CH3) have been omitted for clarity.
bond. All of the NMR data is in agreement with that reported Thermal ellipsoids are drawn at the 50% probability level.
for (IMe)P(S)2Ph by Bockfeld et al., particularly the 31P
resonance they observed at 52.9 ppm in CD2Cl2.17
Following a similar procedure, but using LR and SIPr, nitrogen atoms lie slightly further out of the defined plane in
complex was prepared and characterized, including an X-ray (0.0500 Å) than in (0.0005 Å). The average C1-N bond
crystal structure. Lastly, and under similar conditions, reaction lengths are slightly longer in
of IMes with (Woollins’ Reagent) resulted in the immediate (1.338(3) Å). The N-C1-N angle in
formation of a purple solution and subsequent isolation of a those in either or IMeP(S)2Ph. The angle in
The carbene rings in 4 and 5 are planar, although the
5
5
4
4
(1.357(2) Å) than they are in
5
3
5 is considerably larger than
4
4
is typical of that
crystalline solid. X-ray diffraction showed this compound to be in similar IMes carbene phosphorus complexes.14,15 The angle
the known selanone, 6, by comparison of the experimental in 5 is larger than other such angles reported for SIPr-P
unit cell to that reported by Vummaleti et al.20 Bockfeld et al. complexes in the literature (~107°)22 but this is perhaps
had observed similar reactivity in their investigation of IMe unsurprising as the structures are not that similar.
with WR, having isolated the related product (IMe)Se.17
Table 1. Selected bond lengths [Å] and angles [°] for
compounds 4 and 5
and IMeP(S)2Ph.17
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also planar (including the methoxy oxygen and carbon atoms)
and almost perpendicular to the carbene ring (83.73(5)°). It lies
directly below one of the IMes groups in the solid state, and is
oriented almost parallel to it. There is no steric interference
and the two rings can approach each other quite closely. This
4
5
IMeP(S)2Ph
1.9780(4)
1.9688(4)
1.8659(12)
1.8164(12)
120.01(2)
103.41(5)
106.62(10)
S(1)-P(1)
1.9741(5)
1.9620(5)
1.8822(14)
1.8139(15)
119.79(3)
103.71(6)
105.27(12)
1.9674(10)
1.9577(10)
1.905(2)
1.817(3)
120.70(5)
106.98(11)
110.3(2)
S(2)-P(1)
P(1)-C(NHC)
P(1)-C(Ph)
results in
4 having an interplanar angle of only 12.29(8)° and a
S(1)-P(1)-S(2)
C(NHC)-P(1)-C(Ph)
N(1)-C(NHC)-N(2)
distance of only 3.5072(9) Å between the ring centroids. Close
intramolecular interactions thus form between the two rings.
This is in contrast to the geometry adopted in
5, where
steric effects result in a more distorted structure. The
The P-C bond between the carbene and the phosphorus
centre is slightly longer in than in . This again is reflective of
the unsaturation of the carbene ring in and substantiated by
isopropyl groups of the di-isopropylphenyl (dipp) ligands do
not lie in the plane of the aryl rings, but protrude above and
below them. This would bring one of them into close contact
5
4
4
the similarity to the length of the same bond in IMeP(S)2Ph. All
three bond lengths are indicative of predominantly single bond
character and are similar to other P-C carbene single bond
lengths reported in the literature.14,15 In addition to the
carbene P-C bond, each complex also forms a second P-C bond
to a carbon atom of an aromatic ligand. These bonds are more
invariant in the three compounds, being almost equal in each.
The bond to the C6H4OCH3 group is shorter than that to the
with the C6H4OCH3 group below it, if the geometry of
5 did not
adapt. In 5, the NHC aryl ring planes lie at angles of 76.3(1) and
88.7(1)° relative to the carbene ring plane. The dipp group
lying above the C6H4OCH3 ligand is the one that has closed up
as the two rings (dipp and C6H4OCH3) rotate away from each
other. This results in an angle of 22.8(1)° between the rings
and an increased distance (relative to 4) of 3.889(2) Å between
the ring centroids. The plane of the C6H4OCH3 ligand is also no
longer perpendicular to the carbene ring plane, this angle
having decreased to 55.3(1)°.
carbene in
4 and 5, but the distances are still indicative of
single bonds. These P-C bonds are, however, slightly longer
than those reported in LR itself.6,7 The remainder of the bonds
and angles in the C6H4OCH3 groups are unsurprisingly very
This has consequences in the packing and intermolecular
interactions of
4 and 5. In 4, the alignment of the planar Mes
similar in 4, 5 and both LR structures.
and C6H4OCH3 groups leaves the sulfur atoms relatively
unshielded. There is also a solvent molecule with potential
hydrogen bonding acceptors (Cl atoms) and donors (C-H).
In each compound the phosphorus centre forms two P-S
bonds in addition to the two P-C bonds. The two bond lengths
are roughly equal and they are similar between the two
compounds. The bonds are intermediate in length between
those reported in LR,6,7 where the P-Sterminal bonds are shorter
and the P-Sbridge bonds are longer than all of the P-Sterminal
Compound
4 forms 14 contacts that are less than the sum of
the van der Waals radii, 9 of which go to the best acceptors
available (Cl or S), and 7 of which are intermolecular in nature.
These are all listed in Table S2 (and Table S3 for 5) of the
bonds in
4 and 5. They are very similar to the bond lengths
Supplementary Information. Of note are the contacts made by
the protons on the back side of the central carbene ring and
the contacts from the solvent to the sulfur atoms, which bind
everything together in an extended network. There are very
reported for IMeP(S)2Ph by Bockfeld et al. and are longer than
P=S double bonds.17
Each phosphorus centre forms a total of four bonds (2 P-C
and 2 P-S) with most of the angles at phosphorus falling in the
range of 103-111°. The range of angles is in agreement with
that reported for the tetrahedral phosphorus centre in the
IMeP(S)2Ph complex.17 There are notable outliers from ideal
few short C-H…C or C-H…H-C contacts in 4. The sulfur atoms
accept both inter- and intra-molecular contacts, the latter
particularly with protons on the IMes methyl groups and with
aryl protons of the C6H4OCH3 ligand.
tetrahedral angles in the two structures. Both
S(1)-P(1)-S(2) angles that lie well outside the expected
tetrahedral values and in the C(1)-P(1)-S(1) angle (99.13(8)°)
4 and 5 have
Compound
5 forms many more short contacts than 4, but
almost all of them are intramolecular in nature. The bulky dipp
groups shield the sulfur atoms, so sulfur accepts only 3
5
is also smaller than expected. Thus they should be described as
having distorted tetrahedral geometries at phosphorus. The S-
contacts and all of these are intramolecular. Compound
forms many more (usually weaker) C-H…C and C-H…H-C type
contacts than , and almost all of these are also intramolecular
in nature. The dipp groups themselves, and the change in
geometry relative to to accommodate steric interactions,
result in being less available to neighbouring molecules to
5
P-S angles in 4 and 5 are larger than those found in LR, which is
4
not surprising given that the latter are geometrically
constrained. The observed S-P-S angles are again best
compared to that in IMeP(S)2Ph. Overall, they bracket the
angle reported by Bockfeld et al. and all are notably similar.17
4
5
form intermolecular type interactions, and thus, intra-type
contacts are favoured.
The unexpectedly small C(1)-P(1)-S(1) angle in
5, relative to
4
, highlights their major structural differences. These are also
At the MP2/6-31G* level of theory,23 the calculated
visibly evident if Figures 1 and 2 are contrasted and is further
substantiated by examining the interplanar angles in the two
standard gas-phase dissociation enthalpy (free energy) of
1 to
produce 2 is calculated to be 108 (46) kJ/mol. For the simpler
compounds. In 4, the aryl rings of both IMes groups are planar,
model systems, RPS2 (R = H, Me, Ph), the corresponding
numbers are 74 (21), 114 (43), and 110 (51) kJ/mol,
respectively. The electronic-only dimerization energies at the
including the methyl carbon atoms, and lie almost
perpendicular to the carbene ring plane (88.90(5) and
87.38(5)°, respectively). The aryl ring of the C6H4OCH3 ligand is
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MP2/6-311+G* level are higher than those at MP2/6-31G* by
20-30 kJ/mol. The frontier MOs of reveal that the compound
1 A. Saeed, H. Mehfooz, F. A. Larik, M. Faisal and P. A. Channar, J.
2
has the potential to be amphoteric, with Lewis acidity at
phosphorus and Lewis basicity at sulphur. The electrostatic
Asian Nat. Prod. Res., 2017, 19, 1112-1123.
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M. Jesberger, T. P. Davis and L. Barner, Synthesis, 2003, 13
,
potential plot for
can see the probability of
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2
is shown (Figure 3 left) and from this one
1929-1958.
3
T. Ozturk, E. Ertas and O. Mert, Chem. Rev. 2007, 107, 5210-
2
engaging bases at the phosphorus
4,
5278.
4
combination of the model systems RPS2 (above) with the
simplified carbene 1,3-dimethylimidazol-2-ylidene is predicted
to be exothermic (exergonic) by 179 (130), 172 (108), and 185
(127) kJ/mol, respectively. The overall process from dimer to
two adduct molecules is therefore favourable. Combination of
the model systems with pyridine, is predicted to be much less
exothermic (exergonic) by 73 (26), 79 (21), and 78 (29) kJ/mol,
and the overall reaction is nearly thermoneutral. Lastly, the
L. Legnani, L. Toma, P. Caramella, M. A. Chiacchio, S. Giofrè, I.
Delso, T. Tejero, and P. Merino, J. Org. Chem., 2016, 81, 7733-
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M.A. Chiacchio, L. Legnani, P. Caramella, T. Tejero and P. Merino,
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R. Kempe, J. Sieler, H. Beckmann and G. Ohms, Z. Kristallogr.,
1992, 202, 159.
7
G. Grossmann, G. Ohms, K. Kruger, G. Jeschke, P.G. Jones and A.
electrostatic potential plot for the complex
(Figure right), highlighting the changes induced by
coordination of the carbene to Consonant with the
electrostatic potential plot, possesses large dipole
4 is also shown
Fischer, Phosphorus, Sulfur, Silicon, Rel. Elem., 1995, 107, 57.
8
3
M. Nieger, E. Niecke and R. Serwas, CSD Communication (Private
2
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Communication), 2002, refcode BADKOH.
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4
a
R. Appel, F. Knoch and H. Kunze, Angew. Chem., Int.Ed., 1983, 22,
moment, 10.5 D, when calculated at the B3LYP/6-31G* level.
1004.
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Z. Weng, W.K. Leong, J.J. Vittal and L.Y. Goh, Organometallics,
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Devillanova, F. Isaia, V. Lippolis, L. Loddo and G.Verani, Chem.
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Pyati, Chem. Commun., 1997, 981-982.
Figure 3. Electrostatic potentials [atomic units (au)] super-
imposed on the total electron density isosurface (3x10-3 au) of
(left)
2
and (right)
4
calculated using DFT [B3LYP/6-31G*].24
16
A.J. Arduengo III, J.C. Calabrese, A.H. Cowley, H. V.R. Dias, J.R.
Goerlich, W.J. Marshall and B. Riegel, Inorg. Chem., 1997, 36, 2151-
2158.
Compounds
4
and 5 are examples of a rare type of
17
D. Bockfeld, T. Bannenberg, P. G. Jones and M. Tamm, Eur. J.
Inorg. Chem., 2017, 3452–3458.
M. Meisel, P. Lönnecke, A.-R. Grimmer and D. Wulff-Molder,
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Commun., 2014, 50, 6214-6216.
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complex in which an NHC is bonded to a low coordinate and
high oxidation state phosphorus centre. They represent the
ultimate products formed in the stepwise oxidation of the
arylphosphinidene complexes, NHC•PAr, species that can be
considered to be formed between a neutral carbene and a
phosphinidene. A full discussion is beyond the scope of this
report, but we note that during the preparation of this
manuscript the first phosphinidene structure, the intermediate
oxidation product ArP(S)•NHC, was published.25 In summary,
the first adducts between NHCs and the cleavage product of LR
18
19
20
1895–1904.
21
K. Pal, O.B. Hemming, B.M. Day, T. Pugh, D.J. Evans and R.A.
have been prepared. These have been fully characterized, with Layfield, Angew. Chem. Int. Ed., 2016, 55, 1690-1693.
22
X-ray crystal structures and ab initio calculations included.
We thank the Natural Sciences and Engineering Research
Council of Canada, the Canadian Foundation for Innovation
and the Nova Scotia Research and Innovation Trust Fund. CCP
thanks ACEnet for computational support.
J.D. Masuda, W.W. Schoeller, B. Donnadieu and G. Bertrand, J.
Am. Chem. Soc., 2007, 129, 14180-14181.
Gaussian 03, Revision D.02, M. J. Frisch et al., Gaussian, Inc.,
23
Wallingford CT, 2004.
24
Spartan 14, Wavefunction Inc., Irvine, CA, USA, 2013.
C.M.E. Graham, T.E. Pritchard, P.D. Boyle, J. Valjus, H.M.
25
Tuononen and P.J. Ragogna, Angew. Chem. Int. Ed. 2017, 56, 6236 –
6240.
Conflicts of interest
The authors have no conflicts to declare.
Notes and references
4 | J. Name., 2012, 00, 1-3
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