Angewandte
Chemie
further information on the reaction course(s) was obtained.
C2 nuclei of the anionic NHC derivatives 5 and 6; signals at
Compound 4, obtained by crystallization, showed a resonance
d = 140.3 ppm (d, 1JP, C = 66.5 Hz; 5) and d = 140.4 ppm (d,
1JP, C = 66.6 Hz; 6) were assigned to the C4 nuclei. It is evident
from the solution 31P{1H} and 13C{1H} NMR spectra that the
nature of the cation does not influence the chemical shift
significantly of the carbene carbon or the phosphanido
centers, thus indicating the existence of 5 and 6 in solution
as separated ion pairs.[25]
3
at d = ꢀ73.6 ppm (t, JP, H = 5.4 Hz) in the 31P NMR spectrum
as well as other diagnostic NMR spectroscopic resonances:
C2 (d = 140.8 ppm), C4 (d = 138.9 ppm; d, 1JP, C = 61.4 Hz),
and C2-H (d = 7.02 ppm). The X-ray structure confirmed the
molecular constitution of the zwitterion 4 (Figure 1), which
contains both an imidazolium and a dicoordinate phospha-
nide center.
The X-ray structure of the potassium salt (Figure 2) shows
that 6 exists as a monomer in the solid state, with the
potassium atom bound to the carbene carbon atom as well as
Figure 1. Molecular structure of compound 4 in the solid state. Hydro-
gen atoms (except C2-H) have been omitted for clarity (50% proba-
bility level). Selected atom distances [ꢃ] and bond angles [8] for
compound 4: C1-N2 1.412(19), C2-N1 1.386(19), P-C1 1.8068(16),
P-C9 1.8107(16), C1-C2 1.359(2), C3-N1 1.330(2), C3-N2 1.329(2);
N2-C3-N1 108.92(14), N2-C1-C2 104.04(12), C9-P-C1 101.07(7).
Ph
ꢀ
ꢀ
ꢀ
The P C bond lengths (P C1 1.8068(16) ꢀ and P C
Figure 2. Molecular structure of compound 6 in the solid state. Hydro-
gen atoms have been omitted for clarity (50% probability level).
Selected atom distances [ꢃ] and bond angles [8] for compound 6:
C1-N2 1.431(8), C2-N1 1.389(8), P-C1 1.823(6), P-C9 1.800(6), C1-C2
1.369(8), C3-N1 1.385(8), C3-N2 1.354(8); N2-C3-N1 102.1(5),
N2-C1-C2 103.4(5), C9-P-C1 103.3(3).
1
ꢀ
1.8107(16) ꢀ) are similar to the P C bond lengths of II (R ,
R2 = Mes, R3 = Ph; with 1.763 ꢀ and 1.839 ꢀ, respectively).[8b]
The slight difference is likely to be attributable to the
different rotational position of the phenyl substituents at the
phosphorus atom (see discussion in the Supporting Informa-
tion). Although the plane of the phenyl ring (containing the
C9 atom) is perpendicular to the imidazolium ring in 4, the
ipso carbon atom of the phenyl ring in II (R1, R2 = Mes, R3 =
Ph) is coplanar to the imidazolium ring.[8b] The N1-C3-N2
bond angle of 108.92(14)8 is in the typical range of the P-
functionalized imidazolium salt reported earlier.[19c,d] The N2-
C1-C2 bond angle of 104.04(12)8 is larger than that reported
for a crystalline aNHC (101.03(17)8).[18]
to two [12]crown-4 molecules. Despite being monomeric, the
C3-K distance of 3.066(6) ꢀ falls in the typical range of K-
Ccarbene interactions found in the literature for polymeric
potassium–imidazol-2-ylidene complexes.[26] The P C1 bond
ꢀ
ꢀ
of 1.823(6) ꢀ is slightly longer than the P C9 bond
(1.800(6) ꢀ), which is different from the situation in 4. The
most apparent change in the geometry of the imidazole ring
was seen from the N1-C3-N2 bond angle of 102.1(5)8 versus
108.92(14)8 (in 4), thus confirming the formation of a imida-
zol-2-ylidene.[8b] The N1-C3-N2 bond angle of 102.1(5)8 and
the N2-C1-C2 bond angle of 103.4(5)8 are larger than was
reported for an anionic N-heterocyclic dicarbene (NHDC;
100.5(2)8 and 99.8(2)8, respectively).[12f]
There is growing interest in the structural diversity of
NHCs, in general, and anionic NHC derivatives,[12e,24] in
particular. Therefore, 4 was deprotonated using nBuLi or
potassium hexamethyldisilazide (KHMDS) in the presence of
two equivalents of [12]crown-4 (Scheme 3); the obtained
products 5 and 6 displayed signals at d = ꢀ66.1 ppm (t, 3JP, H
=
5.4 Hz; 5) and d = ꢀ65.7 ppm (t, 3JP, H = 5.1 Hz; 6) in the
31P{1H} NMR spectrum. The identification of signals at d =
212.8 ppm (3+4JP, C = 2.0 Hz; 5) and 212.5 (brs; 6) in the
13C{1H} NMR spectrum is consistent with the presence of the
Computations (for details see the Supporting Informa-
tion) reveal that several close-lying rotational minima exist
for 4 and the rotational barriers are small. The same holds
true for other ylidic compounds, such as II, the deoxy-Breslow
intermediate III,[27,28] and methylene-phosphorane H3P
CH2.
=
[29]
Further calculations (at different levels of theory,
see the Supporting Information) on the isomeric structures
revealed that II is the most stable structure for the parent
system (R1, R2, R3 = H), while Vand IVare less stable by 16.6
and 18.6 kcalmolꢀ1, respectively, and VI is even more
destabilized (34.8 kcalmolꢀ1 less stable than II; Scheme 4).
The isodesmic reaction is nearly thermoneutral for the
parent system (R1, R2, R3 = H), thus indicating that the energy
difference between structures of types and II and IV is equal
Scheme 3. Synthesis of P-anionic NHC derivatives 5 and 6.
Angew. Chem. Int. Ed. 2013, 52, 10080 –10083
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim