31P NMR chemical shifts of carbene-phosphinidene adducts as an indicator of the π-accepting properties of carbenes

Article abstract of DOI:10.1002/anie.201209109

Beyond the Tolman electronic parameter: The ³¹P NMR chemical shifts of easily synthesized carbene-phenylphosphinidene adducts allow the determination of the relative π-acceptor properties of carbenes. In combination with the Tolman electronic p

Full text of DOI:10.1002/anie.201209109

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DOI: 10.1002/anie.201209109
Stable Carbenes
31P NMR Chemical Shifts of Carbene–Phosphinidene Adducts as an
Indicator of the p-Accepting Properties of Carbenes**
Olivier Back, Martin Henry-Ellinger, Caleb D. Martin, David Martin, and Guy Bertrand*
Since the discovery of the first stable carbene in 1988[1] and
the seminal report by Arduengo and co-workers in 1991 of the
synthesis of a stable imidazol-2-ylidene,[2] the number of
stable carbenes has increased tremendously.[3] Of particular
interest is their use as ligands for transition metals, which has
led to numerous breakthroughs in the field of homogenous
catalysis.[4]
It was first postulated that the excellent catalytic activity
of transition-metal complexes bearing N-heterocyclic carbene
(NHC) ligands was due to their strong s-donating ability,
coupled with almost no p-accepting character.[5] However,
recent computational studies as well as experimental evi-
dence have shown that non-negligible p back donation occurs
in the bonding between NHCs and transition metals.[6] In
a noteworthy example, Fꢀrstner et al. showed that the p-
acceptor properties of NHC ligands influence the outcome of
gold-catalyzed reactions.[6i]
mainly based on the 31P NMR chemical shift of the corre-
sponding easily synthesized phenylphosphinidene–carbene
adducts. These compounds can be represented by two
extreme canonical structures: the resonance form A corre-
=
sponds to a typical phosphaalkene featuring a formal P C
double bond, whereas form B corresponds to a carbene–
À
phosphinidene adduct featuring a P C dative bond with two
lone pairs of electrons at phosphorus (Figure 1).[13,14]
Figure 1. The two extreme canonical structures and bonding descrip-
tion of the carbene–phosphinidene adducts.
Several techniques have been developed to evaluate the
donor properties of NHCs,[7] such as calorimetric measure-
ments of [Cp*RuCl] complexes,[8] the 13C NMR chemical
shifts of palladium(II) complexes,[9] and the electrochemical
E0 value for various redox couples in a series of RuIII/RuII
complexes containing the NHC of interest.[10] The most
widely used method relies on the measure of the A1 stretching
frequency of CO ligands in nickel complexes [Ni(CO)3(L)]
(Tolman electronic parameter, TEP),[11] or the average CO
stretching frequencies of cis-[IrCl(CO)2(L)] and cis-
[RhCl(CO)2(L)] complexes, which are easier to handle.[12]
These methods rely on the fact that the electron density
from a ligand cannot only be passed on to the metal, but also
onto the p* orbital of CO ligands. Although all of these
techniques provide a convenient way to evaluate the overall
donor properties of carbenes, they are limited by the fact that
they do not deconvolute the s donation and the p back
donation of carbene ligands.
Some examples of carbene–phosphinidene adducts have
been previously reported (Scheme 1).[15,16] They were directly
prepared in one step by reacting the free carbene with
pentaphenylcyclopentaphosphane for 1 and 4 (Table 1) or
with dichlorophenylphosphine for 6.[15] The main feature of
these compounds is the very high-field 31P NMR chemical
Herein we report a convenient method that allows the
evaluation of the p-acceptor properties of carbenes (and
therefore the evaluation of their s-donating properties). It is
Scheme 1. Previously reported syntheses of carbene–phosphinidene
adducts 1·PPh, 4·PPh, and 6·PPh.
shift of the phosphorus center (1·PPh d = À53.5, 4·PPh À23.0,
6·PPh À10.4 ppm) in comparison with the chemical shift
usually displayed by typical, non-polarized phosphaalkenes
(d = 230–420 ppm).[16] These high-field chemical shifts indi-
cate that the phosphorus atom in these adducts is electron-
rich, in agreement with resonance form B. This is confirmed
by single-crystal diffraction studies, as these adducts display
[*] Dr. O. Back, M. Henry-Ellinger, Dr. C. D. Martin, Dr. D. Martin,
Prof. G. Bertrand
UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
[**] We gratefully acknowledge financial support from the NIH (R01 GM
68825), the DOE (DE-FG02-09ER16069), and Dr. C. Moore for
assistance with X-ray crystallography.
À
Ccarb P bonds (1·PPh 1.794, 4·PPh 1.763, 6·PPh 1.746 ꢁ) that
are significantly longer than those in nonconjugated phos-
phaalkenes (1.65–1.67 ꢁ).[16,17] Moreover, the P CPh bonds
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209109.
are not coplanar with the imidazole rings, and according to
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Table 1: 31P NMR and 13C NMR chemical shifts (P C) for the adducts 1·PPh–16·PPh (recorded in C6D6 unless otherwise indicated), Tolman electronic
parameter (TEP), and 13C NMR chemical shifts for the corresponding carbenes 1–16.
=
Carbene·PPh adduct
Carbene
d13C
Carbene·PPh adduct
Carbene
Carbene
d31P
d13C
TEP
Carbene
d31P
d13C
TEP
d13C
1[18a]
2[18b]
3[18c]
À53.5[a]
169.1[a]
2051.7[c] 213.7
2051.7[c] 210.5
2054.0[c] 221.6
9[18f]
34.9
199.9
2047[e]
2050[e]
2037[f]
282.0
À61.2
À34.6
À23.0
À18.9
167.8
177.4
170.0[a]
172.9
10[18g]
39.7
69.5
57.0
56.2
180.3
199.7
192.0
191.3
260.56
255.5
254.4
11[18h]
4[18a]
2050.7[d] 219.7
2051.5[d] 220.6
12[18i]
2054[e]
5[18d]
13[18j]
2042.2[e] 319.0
2048.5[f] 309.4
2043.8[f] 330.5
6[18d]
À10.4
184.3[a]
2051.5[d] 243.8
2052.2[d] 244.0
14[18j]
68.9
208.1
7[18d]
À10.2[b]
186.9[b]
186.5
15[18k]
126.3[b]
217.2[b]
138.5
8[18e]
14.8
2044
245.1
16[18l]
À34.9
2045[f]
185.5
[a] The NMR spectrum was recorded in [D8]thf. [b] The NMR spectrum was recorded in CDCl3. [c] Computational value.[23] [d] Determined
experimentally from LNi(CO)3 complex.[24] [e] Value calculated by linear regression from the experimentally measured nCO of the [IrCl(CO)2L]
av
complex.[12b] [f] Value calculated by linear regression taking the experimentally measured nCO of the [RhCl(CO)2L] complex.[7]
av
1
the H and 13C NMR spectra recorded in solution at room
corresponding
(Scheme 2).
carbene–phenylphosphinidene
adduct
À
temperature, there is free rotation around the Ccarb P bonds.
It is obvious that an increase of the p-accepting property
of the carbene favors the back donation of the lone pair of the
phosphorus atom to the vacant p orbital of the carbene
center, therefore increasing the contribution of resonance
form A. Consequently, the 31P NMR chemical shift of
carbene–phosphinidene adducts should provide a straightfor-
ward method to evaluate the p-acceptor property of the
carbene: the more p-accepting the carbene is, the further
downfield the chemical shift of the phosphorus nucleus will
be.
Table 1 and Figure 2 show that the 31P NMR chemical
shifts of the carbene–phosphinidene adducts vary over a wide
range (À61.2 to + 126.3 ppm). This is already apparent when
Scheme 2. Preparation of carbene–phosphinidene adducts using
dichlorophenylphosphine. 2·PPh has been prepared from the free
carbene 2 and pentaphenylcyclopentaphosphane; the preparation of
1·PPh, 4·PPh, and 6·PPh was already reported, as shown in Scheme 1.
The carbenes 1–16[18] selected for this study are listed in
Table 1. All of the corresponding adducts (except 1·PPh,
2·PPh, 4·PPh, and 6·PPh) have been prepared using a synthetic
pathway that can be applied to a wider range of carbenes than
the methods described in Scheme 1. In the first step, the
carbene is reacted with one equivalent of dichlorophenyl-
phosphine in hexanes or benzene, and the resulting salt is then
filtered out from the solution. Then, without further purifi-
cation, this salt is treated with two equivalents of KC8 or
magnesium in THF, and subsequent workup affords the
considering the diaminocarbene series 1–11. The 31P NMR
signal of the unsaturated NHC adducts 4·PPh and 5·PPh
(À23.0 and À18.9 ppm) are at significantly higher field than
those of the saturated NHC adducts 6·PPh and 7·PPh (À10.4
and À10.2 ppm), which have the same substituents on the
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of the nitrogen atoms of NHC 9.[18f] The
carbonyl group of 10 is of course a p ac-
ceptor and withdraws electron density
from the carbene p orbital. Because of
the difference in ring size, it is risky to
draw a firm conclusion when comparing
five-membered NHC 7·PPh and six-mem-
bered NHC 8·PPh. However, the apparent
increased electrophilicity of 8, indicated
by the 31P NMR chemical shift, might
result from the increased flexibility of
the six-membered compared to the five-
membered ring skeleton, which allows for
the pyramidalization of the nitrogen cen-
ters. Along this line, the 31P NMR signal of
carbene adduct 11·PPh appears to be the
most deshielded of the diaminocarbene
series 1–11, which argues for a substantial
electrophilic character of 11. Indeed, cal-
culations showed that for an acyclic dia-
minocarbene, one of the nitrogen centers
of its adducts is pyramidalized, preventing
the lone pair to interact with the carbene
vacant orbital.[19] Based on the different
Figure 2. Plots of observed 13C NMR chemical shifts of the adducts 1·PPh–16·PPh against the
31
^
P NMR chemical shift. The plots represented by have been considered in the straight-line
^
correlation, whereas 16·PPh ( ) has been omitted.
nitrogen atoms. This is consistent with the increased p-
accepting properties of the latter, as already observed by
Nolan using a series of [(NHC)Pt(dmso)(Cl)2]complexes.[6h]
This was confirmed by variable-temperature NMR experi-
ments performed on 5·PPh and 7·PPh. Indeed, compound
reactivity of NHC–gold and acyclic diaminocarbene–gold
complexes, Hong et al.[20] drew the same conclusion.
As expected from the presence of only one p donor
substituent, and in agreement with previous studies on their
reactivity,[19,21] (alkyl)(amino)carbenes 13–15 are much more
p-accepting than diaminocarbenes, and indeed their phenyl-
phosphinidene adducts give 31P NMR signals at much lower
fields. Note that there is a noticeable difference between the
31P NMR chemical shifts observed for the adducts 13·PPh and
14·PPh, which are obtained from electronically very similar
carbenes. This is mainly due to the difference of stereochem-
7·PPh displays broad signals in the room temperature 1H and
13
=
C NMR spectra that are due to slow rotation around the P
C bond; in contrast, the room-temperature NMR spectra of
5·PPh contain only sharp signals. By lowering the temper-
ature, we were able in both cases to observe the splitting of
the signals attributed to the iPr groups of the Dipp substitu-
=
=
ents (the absence of rotation around the P C bond makes the
istry around the P C double bond. It has been reported that
Dipp group unsymmetrical). The coalescence temperatures
allowed us to determine the enthalpy of activation for the
rotation process, and indeed the value is much higher in the
case of 7·PPh (5·PPh, Tc = 180 K, DG° = 34 kJmolÀ1; 7·PPh,
Tc = 293 K, DG° = 58 kJmolÀ1). Even subtle changes in the
electronic structure of the carbenes give noticeable differ-
ences in the 31P NMR chemical shift of the corresponding
phenylphosphinidene adducts. This is apparent when 2·PPh is
compared to 3·PPh. As observed by Heinicke et al. using
rhodium complexes,[6f] extension of the p system of the NHCs
by annelation increases the p-accepting property of the
carbene, resulting here in a significant downfield shift of the
31P NMR signal (Dd(3·PPhÀ2·PPh) = 26.6 ppm). N-alkyl-sub-
stituted NHCs 1 and 2 give 31P NMR signals at higher field
than those of corresponding N-aryl substituted NHCs 4 and 5,
as expected from replacement of inductive donor groups by p-
accepting groups.
in phosphaamidines, E isomers give rise to a higher chemical
shift in the 31P NMR spectra than Z isomers.[17,22] Owing to the
steric hindrance of the menthyl moiety, the phosphaalkene
13·PPh possesses a Z configuration, whereas 14·PPh, pre-
pared from the less sterically demanding carbene 14 possesses
an E configuration, as confirmed by x-ray diffraction stud-
ies.[25] Note also that the adduct of the acyclic (alkyl)-
(amino)carbene 15 appears at even lower field
(+ 126.3 ppm), as observed in the diaminocarbene series,
and for the same reasons.
Finally, 16·PPh gives a 31P NMR signal at high field
(À34.9 ppm), which might indicate that cyclopropenylidene
16 has p-acceptor properties similar to unsaturated NHCs 1–
5. Although this assumption is reasonable owing to the
aromaticity of this carbene, the ring strain could have
a tremendous influence on the observed chemical shift.
Table 1 and more clearly Figure 2 show that in the
carbene·PPh adducts, the variation of the 13C NMR chemical
shift of the former carbene carbon follows the same trend as
the 31P NMR chemical shift. Indeed, when the point corre-
sponding to 16·PPh is omitted, a linear correlation can be
obtained with a fair correlation coefficient R2 of 0.91.
However, the range of the 13C NMR chemical shifts is of
The trend observed for the 31P NMR chemical shifts of
phosphinidene adducts of six-membered ring carbenes 8–10 is
in excellent agreement with their expected p-accepting
properties. Indeed, the superior electrophilicity of 9 com-
pared to 8 has been demonstrated computationally and
experimentally; it is attributed to the pyramidalization of one
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course narrower than the 31P NMR chemical shifts, and the
time of acquisition on the spectrometer is much longer.
Table 1 shows that the 31P NMR chemical shift of the
carbene–phenylphosphinidene adducts does not correlate
with the TEP of the corresponding carbenes. This confirms
that these two parameters are representative of two different
properties, and an examination of the data strongly suggests
that indeed the TEP and the 31P NMR chemical shift gives
a good indication of the overall donating and p-accepting
properties, respectively. Importantly, a comparison of the two
scales allows for an evaluation of the relative pure s-donor
abilities of carbenes. As a first illustration, the 31P NMR signal
of the benzoimidazol-2-ylidene adduct 3·PPh (À34.6 ppm) is
at higher field than that of the thiazol-2-ylidene adduct
12·PPh (+ 57.0 ppm), in line with the expected poorer p-
accepting properties of 3 (which is due to the better
p donation of nitrogen compared to sulfur). Concomitantly,
benzoimidazol-2-ylidene 3 and thiazol-2-ylidene 12 have the
same TEP value (2054 cmÀ1), arguing that they have similar
overall donating properties. As a consequence, it can be
concluded that 12 is significantly more s-donating than 3 as
expected, because sulfur is more electropositive than nitro-
gen. In another good example, according to the TEP values,
the acyclic diaminocarbene 11 (2037 cmÀ1) is a better overall
donor than the acyclic (alkyl)(amino)carbene 15
(2043.8 cmÀ1). However, because 11 is a much weaker
p acceptor than 15, it is quite reasonable to believe that 11
is a weaker s donor than 15, in line with the relative
electronegativity of nitrogen and carbon. This is confirmed
when comparing the data for classical NHCs 1–7 and cyclic
(alkyl)(amino)carbenes 13 and 14. From the TEP values of
NHCs 1–7 and CAACs 13 and 14 (2050.7–2054.0 versus
2042.2–2048.5 cmÀ1), it is clear that 13 and 14 are stronger
overall donors; however, 31P NMR chemical shifts indicate
that 13 and 14 (+ 56.2 and + 68.9 ppm) are also stronger
p acceptors than 1–7 (À61.2 to À10.2 ppm). Therefore these
data allow concluding that CAACs 13 and 14 are much
stronger s donors than classical NHCs 1–7, in perfect agree-
ment with calculations.[19]
Experimental Section
Representative synthesis of 11·PPh: Dichlorophenylphosphine
(0.186 g, 1.04 mmol) was added at room temperature to a solution
of carbene 11 (0.220 g, 1.04 mmol) in hexane (10 mL). Immediately,
a bright yellow precipitate was generated. The mixture was then
stirred overnight and the mixture filtered using a cannula. The solids
were washed with diethyl ether (2 ꢂ 10 mL) and dried under reduced
pressure. Magnesium (0.050 g, 2.08 mmol) was added, followed by the
addition of THF (8 mL). The mixture was then stirred at room
temperature overnight and the volatiles removed under reduced
pressure. The product was extracted with benzene (15 mL). After
removing the volatiles under reduced pressure, compound 11·PPh was
obtained as a bright yellow oil; yield 74% (0.246 g, 0.77 mmol).
Further experimental details are available in the Supporting Infor-
mation.
Received: November 13, 2012
Revised: December 18, 2012
Published online: February 12, 2013
Keywords: adducts · N-heterocyclic carbenes ·
.
NMR spectroscopy · phosphinidenes
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In summary, this work demonstrates that the 31P NMR
chemical shift of carbene–phenylphosphinidene adducts gives
a good indication of the relative p-accepting properties of
carbenes. These adducts can be easily prepared in two steps,
by reaction of the carbene with dichlorophenylphosphine
followed by reduction with Mg or KC8, and they do not have
to be purified. In marked contrast with the narrow range of
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carbenes. For example, noticeable differences are observed
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adduct allows for an evaluation of the relative pure s-
donating properties of carbenes.
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