A 4,5-diphosphino-substituted imidazolium salt: A building block for the modular synthesis of mixed diphosphine-NHC heterometallic complexes

Article abstract of DOI:10.1002/chem.201200031

Fused in the same molecule! An N-heterocyclic carbene and a diphosphine are generated and used in the modular synthesis of a variety of target heterometallic complexes. The experimental approach involves formation of a unique 4,5-bis(diphenylphosphino)-1,3-dimethyl-imidazolium salt, simply by starting from 1-methylimidazole (see scheme; M¹=Cr, Mn; M ²=Au, Rh, Pd; LiHMDS=LiN(SiMe₃)₂).

Full text of DOI:10.1002/chem.201200031

COMMUNICATION
DOI: 10.1002/chem.201200031
A 4,5-Diphosphino-Substituted Imidazolium Salt: A Building Block for the
Modular Synthesis of Mixed Diphosphine–NHC Heterometallic Complexes
Javier Ruiz* and Alejandro F. Mesa[a]
Phosphine ligands functionalized with imidazolium salts
have recently received a widespread interest owing to the
multiple applications that the corresponding transition-
metal complexes have found in organometallic chemistry
and catalysis. Most known examples contain the phosphino
substituent linked to the nitrogen atoms of the imidazolium
cycle through a carbon spacer (Figure 1, I), and these imida-
ligand[6] and hence better considered as an NHC–phospheni-
um adduct.[6b,c]
To the best of our knowledge, imidazolium-4-phosphines
that are unsubstituted at the 2-position are unknown. A
recent breakthrough in this chemistry has been the synthesis
of the corresponding deprotonated derivatives, 4-phosphino-
substituted N,N’-diaryl-imidazol-2-ylidenes, from 4,5-unsub-
stituted NHCs by the groups of Bertrand (Va)[7] and Gates
(Vb).[8] These new molecules behave as bifunctional ligands
that allow the synthesis of a variety of dimetallic com-
plexes.[8,9] Another remarkable example of NHC functional-
ized at the backbone, containing an imidato functionality,
has recently been described by Cꢀsar and Lavigne and used
as a ditopic ligand in the preparation of polymetallic deriva-
tives.[10]
Considering the ubiquity of diphosphine and NHC ligands
in organometallic chemistry and catalysis,[11] we aimed to
prepare diphosphino-functionalized imidazole-2-ylidenes, or
the corresponding imidazolium cations, to achieve the first
molecule with both functionalities.[12] We have successfully
found a straightforward approach for the synthesis of the
4,5-bis(diphenylphosphino)-1,3-dimethyl-imidazolium
salt
([5](CF3SO3); Figure 1) and proved its suitability for easy
modular formation of mixed diphosphine–NHC heterome-
tallic complexes.
AHCTUNGTRENNUNG
Figure 1. Examples of phosphino-functionalized imidazolium cations (I–
IV), 4-phosphino-substituted NHCs (Va, Vb), and the 4,5-diphosphino-
substituted imidazolium cation 5.
For the preparation of [5]ACHTNUGTRNEUNG(CF3SO3) we have followed
a one-pot, multi-step, experimental procedure that starts
from 1-methylimidazole (1). As summarized in Scheme 1,
imidazole 1 was reacted with nBuLi at À788C in THF and
then treated with ClPPh2 to form 2-diphenylphosphino-1-
methylimidazole 2.[13] This was reacted with an equivalent
amount of methyl triflate, allowing for the selective methyl-
ation of the N3 atom of the imidazole ring, leaving the phos-
phino substituent unreacted, to give the 2-imidazolium phos-
zolium salts have been widely used in the synthesis of com-
plexes with chelating or pincer N-heterocyclic carbene
(NHC)–phosphine ligands,[1] and as ionophilic phosphines
for ionic-liquid, biphasic catalysis.[2] A few examples of imi-
dazolium salts substituted at the 2-position with a phosphino-
methyl group have also been prepared (II) and used in the
design of valuable recyclable catalysts.[3] Imidazolium salts
containing a phosphino group directly bonded to a nitrogen
atom (III)[4] or to the C2 atom (IV)[5] are also known, the
latter is a good example of a strong p-acceptor phosphorus
phine 3. Subsequent addition of LiN
ClPPh2 (1 equiv) and quenching with water afforded the di-
phosphinoimidazole salt [5](CF3SO3), which was isolated as
ACHTUNGTREN(UNGN SiMe3)2 (2 equiv) and
AHCTUNGTRENNUNG
colorless crystals in high yield (85%). Based on the pioneer-
ing work of Bertrand on mesoionic carbenes[14] and double
functionalization of NHCs at the carbon–carbon double
bond,[7] we propose that the reaction is very likely to pro-
ceeds through the formation of mesoionic carbenes A and
D (Scheme 1), which spontaneously evolve to the normal
NHC isomers B and 4, before protonation.
[a] Prof. J. Ruiz, A. F. Mesa
Departamento de Quꢁmica Orgꢂnica e Inorgꢂnica
Universidad de Oviedo
Facultad de Quꢁmica, 33006 Oviedo (Spain)
Fax : (+34)985-103-446
E-mail: jruiz@uniovi.es
Very interestingly, Arduengo has described a double-
chlorination reaction at the carbon backbone of the imida-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200031.
Chem. Eur. J. 2012, 18, 4485 – 4488
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4485

Scheme 2. Deprotonation of 5 to afford diphosphino-functionalized NHC
4.
phosphino-functionalized NHC 4 (assumed as an intermedi-
ate in Scheme 1), which was fully spectroscopically charac-
terized. The signal of the phosphino groups in the
31P{1H} NMR spectrum of 4 is only slightly shifted high-field
(3 ppm) with respect to the imidazolium precursor 5. In the
13C{1H} NMR spectrum the resonance of the carbene carbon
appears at 208.4 ppm,[17] that is 6 ppm lower in frequency
than the unsubstituted 1,3-dimethyl-imidazol-2-ylidene
(214.4 ppm).[18]
Note that this experimental approach, for the functionali-
zation of the imidazole ring with phosphino groups at posi-
tions 4 and 5, involves air- and moisture-stable imidazoles
and imidazolium salts to avoid the use of free carbenes as
starting material. 1,3-Dialkyl-substituted NHCs, such as 1,3-
dimetyl-imidazol-2-ylidene, are usually liquid at room tem-
perature, very moisture sensitive, thermally labile, and de-
compose after a few days in room temperature.[18] In addi-
tion, the preparation of these carbenes involves the use of
corrosive and toxic ammonia as solvent.
Scheme 1. Proposed mechanism for the formation of imidazolium cation
5, starting from 1-methylimidazole.
zole ring by treatment of 1,3-dimesitylimidazol-2-ylidene
with two equivalents of CCl4,[15] suggesting that the reaction
proceeds through mesoionic intermediates similar to A and
D.
Compound [5]ACHTUNGTRENNUNG(CF3SO3) was fully characterized by spec-
troscopic methods (see the Experimental Section; note the
presence of a singlet signal at À30.4 ppm for the two equiva-
lent phosphino groups in the 31P{1H} NMR spectrum). The
The potential coordination capability of the imidazolium
salt [5]ACTHNUGTRNEUNG(CF3SO3) combined with the stability and easy han-
dling, prompted us to study their behavior as ligand in tran-
sition-metal complexes. We first checked the reaction with
prototypic carbonyl complexes, such as [MnBr(CO)5] and
[Cr(CO)6]. The treatment of these species with an equiva-
lent amount of 5 in refluxing toluene afforded the cationic
complexes 6 and 7, respectively, which contain the new imi-
dazolium–diphosphine as a chelating ligand (Scheme 3).
crystal structure of [5]ACHTNUGTRNEUGN(CF3SO3) has been determined by an
X-ray diffraction study (Figure 2).[16] The endocyclic bond
lengths, ranging from 1.322(3) to 1.396(3) ꢄ, reflect the ex-
pected electronic delocalization within the heterocycle. The
À
C2 N bond lengths (1.324(3) ꢄ on average) and the N1-C2-
N3 bond angle (109.5(2)8) are typical of imidazolium salts.
The imidazolium cation 5 can be deprotonated by treat-
ment with LiNACHTUNGTRENNUNG(SiMe3)2 (Scheme 2) to afford the new di-
Scheme 3. Coordination of 5 to MnI (6) and Cr0 (7) carbonyl complexes.
The cationic nature of 5 makes this ligand a poorer elec-
tron-donor than classical, neutral, diphosphine ligands, such
as 1,2-bis(diphenylphosphino)ethane (dppe); this is reflected
in the IR spectra of 6 and 7 (see the Experimental Section)
that show n˜(CO) bands at higher frequencies (14 cmÀ1 on
average) than the neutral counterparts [MnBr(CO)3-
Figure 2. Molecular structure of imidazolium cation 5 shown with 50%
À
thermal ellipsoids. Hydrogen atoms (except C2 H) are omitted for clari-
À
ty. Selected interatomic distances (ꢄ) and angles (8): C2 N1 1.322(3),
A
(dppe)].[20] The structure of 6 was
4ACHTUNGTRENNUNG
À
À
À
À
À
C2 N3 1.325(3), N1 C5 1.396(2), N3 C4 1.392(2), C4 C5 1.373(3), C5
À
P1 1.823(2), C4 P2 1.827(3); N1-C2-N3 109.5(2).
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Chem. Eur. J. 2012, 18, 4485 – 4488

A 4,5-Diphosphino-Substituted Imidazolium Salt
COMMUNICATION
Figure 3. Molecular structure of complex 6 shown with 50% thermal el-
À
lipsoids. Hydrogen atoms (except C2 H) are omitted for clarity. Selected
À
À
interatomic distances (ꢄ) and angles (8): C2 N1 1.335(4), C2 N3
À
À
À
À
1.327(4), N1 C5 1.388(4), N3 C4 1.380(4), C4 C5 1.363(4), C5 P1
1.808(3), C4 P2 1.800(3); N1-C2-N3 109.3(3).
Figure 4. Molecular structure of the heterometallic complex 8a shown
with 50% thermal ellipsoids. Hydrogen atoms (except those of the allyl
group) are omitted for clarity. Selected interatomic distances (ꢄ) and
À
ing that the structural parameters of the ligand remain es-
sentially constant upon coordination.
À
À
À
À
angles (8): C2 N1 1.352(4), C2 N3 1.358(4), N1 C5 1.388(4), N3 C4
À
À
À
À
1.386(4), C4 C5 1.360(4), C5 P1 1.804(3), C4 P2 1.803(3), Pd1 C2
2.035(3); N1-C2-N3 105.5(3).
The metal-containing imidazolium salts [6]
[7](CF3SO3) readily react with a variety of metal complexes,
in the presence of a deprotonating agents, such as LiN-
(SiMe3)2 (LiHMDS), to yield mixed, diphosphine–NHC, het-
ACHTUGNERTN(NUNG CF3SO3) and
ACHTUNGTRENNUNG
G
using the methodology above. We also anticipate that the
use of other imidazoles and chlorophosphines as starting
materials can further expand the synthetic utility of the
present experimental approach.
erometallic derivatives. Selected examples are shown in
Scheme 4 and involve coordination of the in situ generated
In summary, we have described herein the synthesis of the
unique 4,5-diphosphino-1,3-dimethyl-imidazolium salt [5]-
AHCTUNGTREN(GNUN CF3SO3), which generates, in a modular manner, different
heterometallic complexes with the mixed diphosphine–NHC
ligand 4. For future applications in organometallic synthesis
and dual catalysis, these results could be extended to the
preparation of a great variety of target heterometallic com-
plexes containing carbene 4. The new imidazolium–diphos-
phine ligand 5 is also promising as an ionophilic diphosphine
for ionic-liquid, biphasic catalysis.
Scheme 4. Formation of heterodimetallic complexes 8a–c with diphosphi-
no-functionalized NHC 4 ([Mn]=[MnBr(CO)3]).
Experimental Section
Selected spectroscopic data for the new compounds:
Ligand [5]ACTHNUTRGNEUNG
(CF3SO3): 1H NMR (400 MHz, CD2Cl2, 258C): d=9.50 (s, 1H,
carbene to [PdCl
fragments to give complexes 8a, 8b and 8c, respectively.
The most significant spectroscopic features of the new com-
G
E
N2CH), 3.47 ppm (s, 6H, Me); 13C{1H} NMR (100.61 MHz, CD2Cl2,
258C): d=144.8 (s, C2), 139.9 (m, C4 and C5), 36.6 ppm (s, Me);
31P{1H} NMR (162.14 MHz, CD2Cl2, 258C): d=À30.4 ppm (s, PPh2).
Ligand 4: 1H NMR (400 MHz, CD2Cl2, 258C): d=3.48 ppm (s, 6H, Me);
13C{1H} NMR (100.61 MHz, CD2Cl2, 258C): d=208.4 (s, C2), 136.7 (br,
C4 and C5), 37.6 ppm (s, Me); 31P{1H} NMR (162.14 MHz, CD2Cl2,
258C): d=À33.3 ppm (s, PPh2).
pounds are the disappearance of the C2 H resonances in
1
the H NMR spectra and the presence of the low-field car-
bene-carbon resonances around 200 ppm in the
13C{1H} NMR spectra (for full spectroscopic data see the Ex-
perimental Section and the Supporting Information).[17] Ad-
ditionally, the crystal structure of 8a, as a representative ex-
ample, has been determined (Figure 4).[16]
Considering the extraordinary coordination capability of
diphosphines and NHCs, we envisage that a plethora of pre-
designed heterodimetallic complexes could be prepared by
Complex [6]ACTHNUTRGNEUNG
(CF3SO3): 1H NMR (300 MHz, CD2Cl2, 258C): d=9.41 (s,
1H, N2CH), 3.48 ppm (s, 6H, Me); 13C{1H} NMR (75.5 MHz, CD2Cl2,
258C): d=218.5 (s, CO), 217.2 (s, CO), 144.9 (s, C2), 134.7 (br, C4 and
C5), 43.0 ppm (s, Me); 31P{1H} NMR (121.4 MHz, CD2Cl2, 258C): d=
47.3 ppm (s, PPh2); IR (CH2Cl2): n˜(CO)=2036 (vs), 1975 (s), 1936 cmÀ1
(s).
Complex [7]
1H, N2CH), 3.37 ppm (s, 6H, Me); 13C{1H} NMR (100.61 MHz, CD2Cl2,
(CF3SO3): 1H NMR (400 MHz, CD2Cl2, 258C): d=9.64 (s,
ACHTUNGTRENNUNG
Chem. Eur. J. 2012, 18, 4485 – 4488
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J. Ruiz and A. F. Mesa
258C): d=225.7 (s, CO), 218.1 (s, CO), 150.9 (s, C2), 147.2 (br, C4 and
C5), 36.5 ppm (s, Me); 31P{1H} NMR (162.14 MHz, CD2Cl2, 258C): d=
56.2 ppm (s, PPh2); IR (CH2Cl2): n˜(CO)=2022 (s), 1936 (s), 1913 cmÀ1
(vs).
Complex 8a: 1H NMR (300 MHz, CD2Cl2, 258C): d=3.47 ppm (s, 6H,
Me); 13C{1H} NMR (75.5 MHz, CD2Cl2, 258C): d=217.9 (br, CO), 203.0
(s, C2), 143.5 (m, C4 and C5), 38.7 ppm (s, Me); 31P{1H} NMR
(121.4 MHz, CD2Cl2, 258C): d=38.1 ppm (s, PPh2); IR (CH2Cl2):
n˜(CO)=2031 (vs), 1968 (s), 1929 cmÀ1 (s).
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Complex [8b]ACHTUNGTRENNUNG
(CF3SO3): 1H NMR (400 MHz, CD2Cl2, 258C): d=
3.39 ppm (br, 6H, Me); 13C{1H} NMR (100.61 MHz, CD2Cl2, 258C): d=
219.5 (s, CO), 217.5 (s, CO), 201.5 (s, C2), 144.1 (br, C4 and C5),
39.1 ppm (s, Me); 31P{1H} NMR (162.1 MHz, CD2Cl2, 258C): d=41.9 (s,
PPh3), 39.7 ppm (s, PPh2); IR (CH2Cl2): n˜(CO)=2028 (vs), 1963 (s),
1932 cmÀ1 (s).
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Complex 8c: 1H NMR (400 MHz, CD2Cl2, 258C): d=3.69 ppm (s, 6H,
Me); 13C{1H} NMR (100.61 MHz, CD2Cl2, 258C): d=219.2 (s, CO), 217.6
(s, CO), 205.4 (br, C2), 143.5 (br, C4 and C5), 38.3 ppm (s, Me);
31P{1H} NMR (162.1 MHz, CD2Cl2, 258C): d=37.3 ppm (s, PPh2); IR
(CH2Cl2): n˜(CO)=2029 (vs), 1967 (s), 1929 cmÀ1 (s).
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Received: January 4, 2012
Published online: March 7, 2012
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Chem. Eur. J. 2012, 18, 4485 – 4488