On the p-coordinating limit of NHC-phosphenium cations toward rh icenters

Article abstract of DOI:10.1002/chem.201104046

Two types of imidazoliophosphane with additional electron-withdrawing substituents, such as alkoxy or imidazolio groups, are experimentally described and theoretically studied. Diethyl N,N'-2,4,6-methyl(phenyl) imidazoliophosphonite is shown to retain a P-coordinating ability toward a {RhCl(cod)} (cod=cycloocta-1,5-diene) center, thus competing with the cleavage of the labile C-P bond. Derivatives of N,N'-phenylene-bridged diimidazolylphenylphosphane were isolated in good yield. Whereas the dicationic phosphane proved to be inert in the presence of [{RhCl(cod)}2], the monocationic counterpart was shown to retain the P-coordinating ability toward a {RhCl(cod)} center, thus competing with the N-coordinating ability of the nonmethylated imidazolyl substituent. The ethyl phosphinite version of the dication, thus possessing an extremely electron-poor PIII center, was also characterized. According to the difference between the calculated hemolytic and heterolytic dissociation energies, the N₂C P bond of imidazoliophosphanes with aryl, amino, or alkoxy substituents on the P atom is shown to be of dative nature. The P-coordinating properties of imidazoliophosphanes with various combinations of phenyl or ethoxy substituents on the P atom and those of six diimidazolophosphane derivatives with zero, one, or two methylium substituents on the N atom, were analyzed by comparison of the corresponding HOMOs and LUMOs and by calculation of the IR C=O stretching frequencies of their [RhCl(CO) ₂] complexes. Comparison of the n_CO values allows the family of the electron-poor Im⁺PRR' (Im=imidazolyl) potential ligands to be ranked in the following order versus (R,R'): P(OEt)₃< (Ph,Ph)<(Ph,OEt)<(OEt,OEt)< PF₃<(Ph,Im)<(Ph,Im ⁺)<(OEt,Im⁺). The (Ph,Im) representative is therefore the least electron-donating phosphane for which coordinating behavior toward a RhI center has been experimentally evidenced to date. Ultimate applications in catalysis could be envisaged.

Full text of DOI:10.1002/chem.201104046

FULL PAPER
DOI: 10.1002/chem.201104046
On the P-Coordinating Limit of
Centers**
ACHUTNGTRENNUGNHC–Phosphenium Cations toward ACHTUNGTRENNUNG
RhI
Carine Maaliki,[a, b] Christine Lepetit,[a, b] Yves Canac,*[a, b] Christian Bijani,[a, b]
Carine Duhayon,[a, b] and Remi Chauvin*[a, b]
Abstract: Two types of imidazoliophos-
phane with additional electron-with-
drawing substituents, such as alkoxy or
imidazolio groups, are experimentally
described and theoretically studied. Di-
ethyl N,N’-2,4,6-methyl(phenyl)imida-
zoliophosphonite is shown to retain
stituent. The ethyl phosphinite version
of the dication, thus possessing an ex-
tremely electron-poor PIII center, was
also characterized. According to the
difference between the calculated ho-
molytic and heterolytic dissociation en-
ergies, the N2C···P bond of imidazolio-
phosphanes with aryl, amino, or alkoxy
substituents on the P atom is shown to
be of dative nature. The P-coordinating
properties of imidazoliophosphanes
with various combinations of phenyl or
ethoxy substituents on the P atom and
those of six diimidazolophosphane de-
rivatives with zero, one, or two methyl-
ium substituents on the N atom, were
analyzed by comparison of the corre-
sponding HOMOs and LUMOs and by
calculation of the IR C=O stretching
frequencies of their [RhCl(CO)2] com-
plexes. Comparison of the nCO values
allows the family of the electron-poor
Im+PRR’ (Im=imidazolyl) potential li-
gands to be ranked in the following
a
a
P-coordinating
ability
toward
{RhCl(cod)} (cod=cycloocta-1,5-
ACHTUNGTRENNUNG
diene) center, thus competing with the
order
versus
(R,R’):
PACHTUNGTRENNUNG(OEt)3 <
ꢀ
cleavage of the labile C P bond. De-
(Ph,Ph)<(Ph,OEt)<(OEt,OEt)<
PF3 <(Ph,Im)<(Ph,Im+)<(OEt,Im+).
The (Ph,Im) representative is therefore
the least electron-donating phosphane
for which coordinating behavior
toward a RhI center has been experi-
mentally evidenced to date. Ultimate
applications in catalysis could be envis-
aged.
rivatives of N,N’-phenylene-bridged di-
imidazolylphenylphosphane were iso-
lated in good yield. Whereas the dicat-
ionic phosphane proved to be inert in
the presence of [{RhClACHTNUTRGNEUNG(cod)}2], the
monocationic counterpart was shown
to retain the P-coordinating ability
toward a {RhClACHTUNGTRENNUNG(cod)} center, thus com-
peting with the N-coordinating ability
of the nonmethylated imidazolyl sub-
Keywords: carbene ligands · dative
bonds · donor–acceptor systems ·
heterocycles · phosphane ligands ·
rhodium
Introduction
proposition is illustrated by the isolobal analogy between
phosphenium ions R2PD+ and carbenes R2CD in the singlet
spin state,[4] which are both prone to be stabilized by coordi-
nation with either a Lewis acid (LA) or a Lewis base (LB).
In amidiniophosphanes,[5] which constitute a particular class
Beyond the vertical periodicity based on the similarity of
the valence-electron shell, thus making phosphorus the ni-
trogen “analogue”, diagonal periodicity, based on the simi-
larity of electronegativity,[1] has also been invoked for
making phosphorus the carbon “copy”.[2] The diagonal anal-
ogy becomes particularly relevant in terms of reactivity for
low-coordinate phosphorus and carbon compounds.[3] This
+
ꢀ
of carbeniophosphanes represented by
a
R2N2 C PR’2
Lewis structure, the carbon and phosphorus centers were
shown to preserve their Lewis acido–basic characters, and
thus their common ambiphilic chemical nature in a comple-
mentary manner for LA = R2PD+ and LB = R2CD. The strict
ꢀ
[a] C. Maaliki, Dr. C. Lepetit, Dr. Y. Canac, Dr. C. Bijani, C. Duhayon,
Prof. R. Chauvin
donor–acceptor character of a given X Y bond is indeed
not formal:[6] it is determined by the energetic preference
for a heterolytic dissociation mode over the homolytic
Laboratoire de Chimie de Coordination (LCC), CNRS
205, Route de Narbonne, 31077 Toulouse (France)
Fax : (+33)561-553-003
+
mode. The dative nature of the R2N2CD!PR’2 bond was
thus early suggested[7] and recently demonstrated both ex-
perimentally and theoretically.[8] The Lewis base character
of the phosphenium Lewis acid was also shown to persist in
the adduct by coordination of Ph2PD+ motifs to Lewis acidic
transition-metal centers (RhI, PdII),[5] thus giving rise to
carbon–phosphorus–metal ternary complexes.[8c] A natural
question addresses the limit of the coordinating ability of
the R2PD+ lone pair of electrons versus the electron-with-
drawing character of the R substituents. Upon substitution
E-mail: yves.canac@lcc-toulouse.fr
chauvin@lcc-toulouse.fr
[b] C. Maaliki, Dr. C. Lepetit, Dr. Y. Canac, Dr. C. Bijani, C. Duhayon,
Prof. R. Chauvin
Universitꢀ de Toulouse
UPS, INP; LCC
31077 Toulouse (France)
[**] NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201104046.
Chem. Eur. J. 2012, 18, 7705 – 7714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7705

at the P atom by electron-withdrawing groups, weakening of
the intrinsic coordinating ability of the PD+ lone pair of elec-
trons on a free phosphenium ion is accompanied by
strengthening of the NHC!PD+ bond (NHC=N-heterocy-
clic carbene). Conversely, indeed, free R2PD+ phosphenium
ions are stabilized by p-electron-donating R substituents,
such as amino groups, in the same way, and to a similar
both in the diimidazolium series (adducts of type B)[12] and
in the
cross-vinylogous
trisACTHNGUTERNNU(G diamino)cyclopropenium
+
series,[13] the dative nature of the N2 C P bonds is not
a priori guaranteed; the homolytic dissociation mode could,
indeed, be favored by the corresponding electrostatic relaxa-
tion (Scheme 1).[6] Although the coordinating ability of tris-
ꢀ
AHCTUNGTREG(NNUN diamino)cyclopropeniophosphanes toward the anionic tri-
extent, as in stable diaminocarbenes,[9] with concomitant de-
chloropalladate center has been reported, the electrostatic
driving force is certainly the determining factor.[13] On the
other hand, the coordinating ability of diimidazoliophos-
phanes toward neutral metallic centers remains unknown,
and thus deserves investigation from a fundamental view-
point, keeping mind the possibility of applications in cataly-
sis. In the following, the intrinsic stability and coordination
chemistry of imidazoliophosphanes of type A and B are
studied by using experimental and theoretical investigation
tools. The results will be discussed on the basis of the elec-
tronic and electrostatic features of the systems.
+
stabilization of the NHC!P
(NR’2)2 bond. A NHC–diami-
nophosphenium adduct was however reported by Baker and
co-workers[10] who showed that the addition of an electron-
rich platinum(0) complex (i.e., [PtACHTNUTRGEN(UNG PPh3)3]) resulted in the
ꢀ
instantaneous cleavage of the anyway labile C P bond,
likely via a NHC–diaminophosphenium/platinum complex
(which was however neither isolated nor spectroscopically
detected). Complementary to the case of the aryl and amino
R substituents is the case for the oxy R substituents. Where-
as the p-donating character (+M effect) of oxygen atoms is
weaker than that of nitrogen atoms, the s-withdrawing char-
acter (ꢀI effect) varies in the opposite sense: the overall
effect on the stability of the corresponding free phospheni-
um ions has to be compared with that of aryl substituents.
Inversion of the periodicity can be anticipated (both diaryl-
and dioxyphosphenium ions are expected to be less stable
than the diaminophosphenium paradigms), but the P-donat-
ing character of NHC–dioxyphosphenium adducts A re-
mains an open issue (Scheme 1).
Results and Discussion
Experimental results
Systems of type A: Amidiniophosphonites (R=alkoxy:
Scheme 1): Two complementary approaches were envisaged
to access the amidiniophosphonites. The first approach
starts from the readily available
1-(1-phenyl)-1H-imidazole 1a
or 1-(2-diphenylphosphino)-1H-
imidazole 1b.[14] Subsequent ad-
dition of one equivalent of
nBuLi in Et2O and a stoichio-
metric amount of chlorodiethyl-
phosphite allowed phosphonite
2a and phosphane–phosphonite
2b to be obtained in 60 and
79%
yield,
respectively
single
(Scheme 2). Finally,
a
equivalent of methyl triflate
(MeOTf) reacted selectively
with 2a and 2b in toluene to
give the targeted amidiniophos-
phonites 3a and 3b in quasi-
Scheme 1. Possible dissociation modes of imidazoliophosphanes
(botton).
A (top) and diimidazoliophosphanes B
quantitative
yields.
The
31P NMR chemical shift of 3a
(dp =+142.4 ppm) is in the typi-
More electron-withdrawing than alkoxy substituents are
cationic amidinio substituents, in which the positive charge
is p-conjugated to the PIII
cal range for phosphonite compounds.[15] Whereas the
31P NMR resonance of the phosphonite fragment of 3b is
center (in most reported cation-
ic phosphanes, the positive
charge is indeed conjugatively
insulated in a remote position
from the P center).[11] Although
the stability of polyACTHUNTGRNEUNG(amidinio)-
phosphanes has been illustrated Scheme 2. Synthesis of the amidiniophosphonites 3a and 3b from the imidazole precursors 1a and 1b.
7706
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 7705 – 7714

P-Coordination of Imidazoliophosphanes toward Rh Centers
FULL PAPER
observed in the same range (dp =+141.1 ppm, doublet, JPP
=
atom to the Rh atom, thus confirming the weakness, and
+
ꢀ
65.8 Hz), the second resonance (dp =ꢀ17.4 ppm, doublet,
perhaps the dative nature, of the N2 C P bond, as observed
J
PP =65.8 Hz) is in good agreement with the triarylphos-
for the free ligand. It has been recently reported that a rela-
tively weak nucleophile, such as the chloride ion, induced
the heterolytic cleavage of imidazoliophosphanes to the
chlorodiphenylphosphane and corresponding NHC moieties
at 508C.[8] The same cleavage was also observed in the coor-
dination sphere of a Pd complex, with subsequent trapping
of the NHC moiety by the PdII center.[8] A similar reactivity,
albeit at lower temperature, is thus observed for imidazolio-
phosphonite analogues.
phane fragment. The ionic nature of the products was indi-
cated by a very low solubility in nonpolar solvents and by
a deshielded 1H NMR signal that corresponds to the N-
methyl substituent (dH =+4.1 ppm; Scheme 2).
The alternative strategy consists of the generation of the
free NHCs 4a and 4c (with IMes and IPr substituents, re-
spectively; Scheme 3) from the corresponding imidazolium
Systems of type B: diamidinio-
phosphanes (R=aryl, alkoxy:
Scheme 1): The preparation of
diamidiniophosphanes started
from the readily available 1,2-
di(N-imidazolyl)benzene 8.[17]
Double deprotonation of 8 with
two equivalents of nBuLi in
THF followed by addition of
one equivalent of dichlorophen-
Scheme 3. Preparation of the amidiniophosphonites 5a–5c from the free NHCs 4a–4c generated in situ.
salts by addition of a suitable base.[16] Subsequent addition
of one equivalent of chlorodiethylphosphite in Et2O afford-
ed the cationic phosphonites 5a and 5c in 70 and 89%
yield, respectively (Scheme 3). The 31P NMR spectrum of
the products confirmed the formation of the imidazoliophos-
phane moieties (5a: dP =+149.9 ppm; 5c: dP =+156.6 ppm).
The addition of IMes 4c to 2-chloro-1,3-dioxaphospholane
gave amidiniophosphonite 5b in 83% yield. In comparison
to 5a, the more-shielded 31P NMR signal of 5b (dP =+
134.6 ppm) was attributed to the presence of a five-mem-
bered cyclic phosphonite. Although the amidiniophosphon-
ites appeared to be stable whatever the anion (3a,b: TfOꢀ;
5a–c: Clꢀ), they decomposed over time in solution (t1/2 ꢁ10–
20 h at 208C in CH2Cl2), thus releasing the corresponding
imidazolium salt and primary phosphane oxide by hydrolytic
ylphosphane afforded the cyclic diimidazolophosphane 9a in
49% yield (Scheme 5). The 31P{1H} NMR spectrum of 9a
displayed
a singlet signal at very high field (dP =
ꢀ60.8 ppm), consistent with a tris-annelated cyclic environ-
ment of the PIII atom. The exact structure of 9a was con-
firmed by X-ray diffraction analysis of colorless crystals de-
posited from a CH2Cl2/Et2O mixture (Figure 1).[18]
The addition of two equivalents of MeOTf to the neutral
diimidazolophosphane 9a in CH2Cl2 afforded the dicationic
diimidazoliophosphane 11a in 98% yield. The use of one
equivalent of MeOTf allowed the monocationic imidazolo–
imidazoliophosphane 10a to be isolated in 95% yield
(Scheme 5). Both products were fully characterized by mul-
tinuclear NMR spectroscopy, and in particular by using
31P{1H} NMR spectroscopic analysis (10a: dP =ꢀ68.5 ppm
(singlet); 11a: dP =ꢀ76.2 ppm (singlet)). The successive
methylation steps in the series 9a!10a!11a resulted in
+
[8]
ꢀ
cleavage of the N2 C P bond).
The amidiniophosphonite 5a was treated with 0.5 equiva-
lents of [{RhCl(cod)}2] (cod=cycloocta-1,5-diene) in
G
a shielding of approximately Dd=8 ppm each in the
CH2Cl2. The 31P NMR spectrum of the reaction mixture dis-
played a doublet signal at high field, thus revealing the coor-
dination of the P atom to the Rh center (dP =+121.2 ppm,
doublet, 1JPRh =241.0 Hz). The formation of complex 6
shows that in spite of the presence of two electron-with-
drawing ethoxy substituents at the P atom, phosphonite 5a
retains some donor character toward a moderately Lewis
acidic RhI center (Scheme 4).
31P NMR spectra. The structure of dication 11a was finally
confirmed by X-ray diffraction analysis of single crystals ob-
tained from a MeCN/Et2O mixture (Figure 1).[18] In both
cases (i.e., 9a and 11a), the seven-membered phosphoracy-
cle is boat-shaped, the prow occupied by the P atom and the
ꢀ
stern by the C C bond of the phenylene bridge. Although
the conformation is mainly dictated by steric constraints, an
The
phosphonite/rhodium
complex 6 was however ob-
tained in poor yield (7%),
along with the NHC/rhodium
complex 7 (24%) and the start-
ing imidazolium salt IMes·HCl
(69%). The formation of 7 can
be explained by displacement
Scheme 4. Preparation of the rhodium(I) complex 6 of the amidiniophosphonite ligand 5a accompanied by
ꢀ
of the NHC moiety from the P cleavage of the P C bond.
Chem. Eur. J. 2012, 18, 7705 – 7714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7707

R. Chauvin, Y. Canac et al.
methylation of 1,2-diimidazolyl-
benzene 8.[14b] Subsequent addi-
tion of one equivalent of
dichloroACHTNUTRGNEUNG(ethyl)phosphite in the
presence of two equivalents of
Et3N afforded diamidiniophos-
phinite 11b in 32% yield. As
expected, the unique 31P NMR
signal of 11b is more deshield-
ed (dP =+31.4 ppm) than that
of the related diamidiniophos-
phane 11a (dP =ꢀ76.2 ppm)
and more shielded than those
of acyclic imidazoliophosphon-
ites 3a,b (dP ꢁ141–142 ppm).
The potential ligands 9a,
10a, and 11a were investigated
by cyclic voltammetry, and each
of them exhibited a single irre-
versible oxidation peak at the
following potentials versus the
saturated calomel electrode
(SCE): Epox =+1.02, +2.28, and
+2.83 V for 9a, 10a, and 11a,
respectively. These data confirm
the increasing electron deficien-
cy of the oxidation site (likely
the PPh group) in the series
9a!10a!11a. As the oxida-
tion potential of phosphanes is
recognized as a measure of the
electronic endowment of the P
atom, coordination of diimida-
zoliophosphane 11a to a Lewis
acid center is, henceforth, an-
ticipated to be a difficult chal-
lenge. Whatever the conditions
used (solvent, temperature,
stoichiometry, and so forth), the
treatment of 11a with [{RhCl-
Scheme 5. The principle of the preparation of the diimidazolophosphane derivatives 9–11 from 1,2-diimidazo-
lylbenzene (8) through either successive N-methylation of the intermediate 9a in the phosphane series a or via
the elusive bis(diaminocarbene) of the diimidazolium salt 12 in the phosphinite series b (route a via the puta-
tive phosphinites 9b and 10b was not attempted).
AHCTUNGERTG(NNUN cod)}2] did not lead to any co-
Figure 1. ORTEP views of the X-ray crystal structures of neutral and dicationic phosphanes 9a (left) and 11a
(right). The thermal ellipsoids are drawn at the 30% probability level (the triflate anions and H atoms are
ordination and the starting di-
cation was recovered, as indi-
cated by multinuclear NMR
ꢀ
ꢀ
omitted for clarity). Selected bond lengths [ꢂ] and angles [8] of 9a: C1 P1 1.8084(12), C10 P1 1.8078(12),
ꢀ
ꢀ
N1 C1 1.3229(16), N2 C1 1.3818(14); C1-P1-C10 95.75(5), N1-C1-N2 111.00(10), N2-C1-P1 125.50(9); and
11a: C1-P1 1.818(3), C10-P1 1.822(3), N1-C1 1.339(3), N2-C1 1.352(4); C1-P1-C10 92.61(12), N1-C1-N2
106.7(2), N2-C1-P1 128.0(2).
spectroscopic
analysis.
At-
tempts at treating 11a with
other Lewis acids ([PdCl2-
auxiliary driving force is p-stacking of the P-phenyl substitu-
ent with the phenylene bridge. From the neutral phosphane
9a to the dimethylated version 11a, no significant differen-
ces concerning the bond lengths and angle values were ob-
served.
CATHNUGTREN(NUNG PhCN)2], CuI, CuBr2, BH3) or oxidizing agents (S8, H2O2)
were unsuccessful as well. These results indicate that the
electron deficiency of 11a, and all the more of 11b, is
beyond the coordinating limit to any neutral Lewis acid.
Nevertheless, the monocationic imidazolo-imidazoliophos-
Ethyl diimidazoliophosphinite 11b was targeted as
“poorer analogue” of imidazoliophosphane 11a
(Scheme 5). Its preparation was attempted by alkylation of
the sp2 N atoms of the imidazole fragments prior to the
phosphan-diylation step. The dication 12 was obtained by di-
phane 10a reacted with a slight excess of [{RhClACTHGUNETRNNU(G cod)}2] in
a
CH2Cl2. After 24 hours at 408C, the 31P NMR spectrum of
the mixture and other analyses indicated that complexes 13
and 14 were formed in a 1:1 ratio and 73% overall yield.
The 31P{1H} NMR resonance of 13 at dp = +15.8 ppm (dou-
7708
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 7705 – 7714

P-Coordination of Imidazoliophosphanes toward Rh Centers
FULL PAPER
interaction between the P and
Rh atoms. As observed for
phosphanes 9a and 11a, the P
atom is strongly pyramidalized
(sum of the angles: 300.2, 295.5,
and 295.28, for 9a, 11a, and 14,
respectively), with the phenyl
substituent on the P atom locat-
ed on the same side as the
ortho-phenylene bridge.
The P-coordinated rhodium
complex 13 thus indicates that
the lone pair of electrons on
the P atom of the monocationic
phosphane 10a retains suffi-
cient donating character. Never-
theless, the concomitant forma-
tion of the N-coordinated rho-
dium complex 14 suggests that
the monocationic phosphane
10a is “on the good side” of the P-coordination limit with
a neutral RhI center.
Scheme 6. The outcome of the addition of the [{RhClACTHNUTRGNENG(U cod)}2] complex to the phenylphosphanes 9a–11a.
blet, JPRh =166.1 Hz) is in agreement with the coordination
of the lone pair of electrons on the P atom to a RhI center
(Scheme 6). In contrast, the 31P{1H} NMR singlet signal of
14 (at dp =ꢀ69.1 ppm) is similar to that of 10a, thus ruling
out any direct P···Rh interaction. The exact structure was fi-
nally determined by X-ray diffraction analysis of single
yellow crystals of 14 deposited from CH2Cl2 (see Scheme 6
and Figure 2).[18]
To confirm the latter hypothesis, neutral phosphane 9a
was treated with [{RhClACHTNUGTRNE(UGN cod)}2] in CH2Cl2. A reaction was
observed to occur readily at room temperature, thus afford-
ing rhodium complex 15 in 88% yield. The doublet
31P{1H} NMR signal at dp = +7.7 ppm (JPRh =160.0 Hz) gave
clear evidence that the P atom of 15 is directly connected to
the RhI center (Scheme 6). Finally, the exact structure was
confirmed by X-ray diffraction analysis of yellow crystals of
15 deposited from a CH2Cl2/Et2O mixture (Figure 3).[18] No
N-coordinated rhodium complex was detected, thus confirm-
ing the stronger P-donating character of phosphane 9a rela-
tive to 10a.
Figure 2. ORTEP view of the X-ray crystal structure of rhodium complex
14. The thermal ellipsoids are drawn at the 30% probability level (the
triflate anions and H atoms are omitted for clarity). Selected bond
ꢀ
ꢀ
ꢀ
lengths [ꢂ] and angles [8]: C1 P1 1.8120(17), C10 P1 1.8234(18), N1 C1
ꢀ
ꢀ
ꢀ
1.325(2), N2 C1 1.365(2), N1 Rh1 2.0944(14), Rh1 Cl1 2.3905(4); C1-
P1-C10 94.78(7), N1-C1-N2 110.09(14), N2-C1-P1 128.20(13).
The structure of 14 evidences the coordination of a sp2 N
atom to a RhI atom located at the center of a quasi-square-
planar environment. The lone pair of electrons on the P
atom is directed toward the metallic center at a P···Rh dis-
tance of approximately 3.43 ꢂ, which is much greater than
the sum of the corresponding covalent radii (ꢁ2.31 ꢂ), but
smaller than the sum of corresponding van der Waals radii
(ꢁ3.8 ꢂ). This outcome suggests the existence of a residual
Figure 3. ORTEP view of the X-ray crystal structure of the rhodium com-
plex 15. The thermal ellipsoids are drawn at the 30% probability level
(the H atoms are omitted for clarity). Selected bond lengths [ꢂ] and
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: C1 P1 1.815(3), C12 P1 1.819(3), N1 C1 1.316(3), N2 C1
ꢀ
ꢀ
1.376(3), P1 Rh1 2.2854(6), Rh1 Cl1 2.3636(6); C1-P1-C12 95.80(11),
N1-C1-N2 111.9(2), N2-C1-P1 119.02(17).
Chem. Eur. J. 2012, 18, 7705 – 7714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7709

R. Chauvin, Y. Canac et al.
Theoretical studies
dissociation mode prefered, at least in the polar acetonitrile
medium (e.g., by D(DG)=+26.2 kcalmolꢀ1 for Cl PPh2).
ꢀ
The electron-donating properties of imidazoliophosphanes
For monocationic NHC–phosphenium systems (XDꢀ =
of type A and diimidazoliophosphanes of type B (Scheme 1)
NHC=1,3-bisACTHUGNETRNNU(G methyl)imidazol-2-ylidene), the D(DG) val-
and the nature of the corresponding N2C+ P bonds were in-
ues range from ꢀ20.8 to 42.3 kcalmolꢀ1 in the following
order: +PCl2 < +PPh2 < +PPh(OH)< +P(O)Ph2 < +P(OR)2 <
+P(NMe2)2. Thus, the more stable the phosphenium cation,
the greater the D(DG) value, namely, the dative character of
ꢀ
vestigated by DFT calculations.
Imidazoliophosphanes of type A: Characterization of the
ꢀ
ꢀ
dative N2C P bond of imidazoliophosphane models: On the
the C P bond (approaching the neutral H3N!BH3 para-
basis of experimental and theoretical results, imidazolio-
phosphanes were recently demonstrated to be NHC–phos-
phenium adducts involving a “true” dative N2C!PR1R2+
bond (“true” is understood to be according to a quantitative
criterion).[8a] More generally, the degree of dative character
of generic X···P bonds varies from the purely covalent case
digm). Indeed, whereas the homolytic dissociation energy
does not vary much over the series (DGhomo =(65ꢂ7) kcal
molꢀ1), the heterolytic counterpart is the main variation
factor, which ranges from DGhetero =25.3 kcalmolꢀ1 for the
most stable phosphenium (+P
ACTHNURTGNEG(UN NMe2)2) to DGhetero =79.4 kcal
molꢀ1 for the least stable one (+PCl2).
ꢀ
+
For the dicationic diaminophosphane [(NHC)2PPh]2+ (an
acyclic model of 11a; Scheme 5), a preference for the homo-
lytic mode (D(DG)=ꢀ13.7 kcalmolꢀ1) is restored due to the
electrostatic strain in the released [(NHC)PPh]2+ moiety
that results from a heterolytic dissociation. This feature is
discussed in detail below.
ꢀ
(X P) to the purely ionic case (XD , P ) depending on the
C
C
difference in energy between the homolytic (to X + P) and
heterolytic (to XDꢀ +P+) dissociation modes.[6] This character
is addressed below in a systematic manner by computation
of the corresponding Gibbs dissociation energies D(DG) at
298.15 K in the acetonitrile continuum dielectric medium
(polarizable continuum model (PCM) method) at the
B3PW91/6-31G** level (Table 1).
Electron-donating properties of imidazoliophosphane
models: The near-frontier molecular orbitals (FMOs) of li-
gands of type A were investigated in the representative
Table 1. Quantitative variation of the dative character of C···P bonds over
a representative series of neutral, cationic, and dicationic “donor–acceptor
adducts”.
series [NHC PR R ] (Ri =Ph or OEt, i=1,2; Figure 4).
Upon replacement of a phenyl substituent by an ethoxy sub-
stituent, both the HOMO (strongly polarized at the lone
pair of electrons on the P atom) and the LUMO (which re-
sults from the overlap of the antibonding p* orbital of the
NHC fragment with the p orbital of the original sp2 phos-
phenium fragment) are shifted deeper in energy. The PIII
atom of imidazoliophosphonites (R1 =R2 =OEt) is therefore
expected to be less s-donating and more p-accepting, and
1
2 +
ꢀ
[a]
[b]
Overall Donor
charge
Acceptor
DGhomo
DGhetero
D(DG)[c]
[kcalmolꢀ1
]
[kcalmolꢀ1
]
[kcalmolꢀ1
]
0
Phꢀ
+PPh2
+PPh2
BH3
57.2
60.9
133.5
112.5
34.7
27.1
ꢀ55.3
26.2
106.4+1
Clꢀ
H3N
+PCl2
58.6
65.8
66.3
71.1
70.5
72.2
67.6
79.4
57.4
50.6
55.0
45.6
44.4
25.3
ꢀ20.8
8.4
15.7
16.1
24.9
+1
+PPh2
+PPh(OH)
+P(O)Ph2
(NHC)
+P
+P
+P
N
27.8
42.3+2
+2
NHC
2+P
(NHC)Ph 52.6
66.3
ꢀ13.7
[a] Homolytic Gibbs dissociation energy at 298.15 K. [b] Heterolytic Gibbs
The PCM-
dissociation energy at 298.15 K. [c] D(DG)=DGhomoꢀGhetero
.
(U)B3PW91/6-31G** level of calculation in the acetonitrile continuum (e=
ꢀ
35.688) was used. The values for C P bonds with a dominant covalent charac-
ter are in bold.
For neutral systems, due to the absence of opposite
charge separation,[6] heterolytic dissociation to neutral
closed-shell systems is strongly favored, which is the case for
the amine–borane adduct H3N!BH3 (D(DG)=+106.4 kcal
molꢀ1). For neutral systems in which heterolytic dissociation
to closed-shell species entails a separation of opposite charg-
es, the resulting electrostatic cost generally makes the homo-
lytic dissociation mode to radical species more favored (e.g.,
by D(DG)=ꢀ55.3 kcalmolꢀ1 for Ph PPh2). Nevertheless,
ꢀ
ꢀ
Figure 4. Near-frontier molecular orbitals of representative [NHC
PR1R2]+ adducts (Ri =Ph or OEt, i=1,2). PCM-B3PW91/6-31G** level
of calculation in the acetronitrile continuum (e=35.688).
the relative electronegativity of the charged closed-shell
moieties may reverse the trend and make the heterolytic
7710
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 7705 – 7714

P-Coordination of Imidazoliophosphanes toward Rh Centers
FULL PAPER
thus to be globally a weaker donor toward transition-metal
centers, than the PIII atoms of imidazoliophosphinites (R1 =
Ph, R2 =OEt) and imidazoliophosphanes (R1 =R2 =Ph).
The actual donating character of these ligands was estimated
by the average of the calculated IR C=O stretching frequen-
1
2
+
ꢀ
cies in their corresponding [(NHC PR R )Rh(CO)2Cl]
complexes (Table 2). The corresponding n˜CO values were
Table 2. Quantification of the overall donating character of representa-
tive ligands L by the average n˜CO values of the two IR C=O stretching
frequencies in [RhLCl(CO)2] complexes.[a]
[a]
nCO [cmꢀ1
]
˜
L
R1 =R2 =Ph
2071.6
2072.7
2075.9
2077.9
1
2 +
R1 =Ph; R2 =OEt
R1 =R2 =OEt
ꢀ
[NHC PR R ]
PF3
(2,6-Me2C6H3CH2CH2)PtBu2
2034.9 (2042.5)[b]
[a] Calculated at the PBEPBE/6-31G**/LANL2DZ*(Rh) level. [b] Ex-
perimentally obtained from reference [21].
Figure 5. Selected near-frontier molecular orbitals of diimidazolophos-
phane derivatives in phenylphosphanes 9a–11a and ethyl phosphinite
11b series (Scheme 5). PCM-B3PW91/6-31G** level of calculation in the
acetronitrile continuum (e=35.688). For 9a, 11a, and 11b, the selected
orbitals with the highest contribution at the P atom and quasi-degenerate
with the HOMO are displayed.
indeed widely used to scale the donor properties of phos-
phanes and NHCs either experimentally or computationally
(i.e., the larger the n˜CO value, the weaker the donation).[19]
The PBEPBE/6-31G**/LANL2DZ*(Rh) calculation level
was selected for its ability to reproduce the experimental
n˜CO measurements (Table 2). In the present series, the large
calculated n˜CO values, which range from n˜CO =2071.6 to
2075.9 cmꢀ1 for R1 =R2 =Ph and OEt, respectively, are indi-
cative of a weak donating character analogous to the one of
PF3 (n˜CO =2077.9 cmꢀ1).[20] In agreement with the above
FMO analysis (Figure 4), replacement of phenyl groups of
ꢀ
[NHC PPh
(OEt)a]+ (a=0, 1, 2; see above), both varia-
2ꢀaACHTGNUTRENNUNG
tions suggest a weaker P-donating character of 11b with re-
spect to 11a. A similar trend is observed in the phenylated
series going from the neutral diimidazolophosphane 9a to
the dicationic diimidazoliophosphane 11a through the
monocationic
(Figure 5).
imidazolo–imidazoliophosphane
10a
+
NHC!PPh2 by ethoxy substituents induces a weakening
of the net donating character of the P atom.
The actual donating character in the completed series of
ligands (L=9a,b, 10a,b, and 11a,b; Scheme 5) was evaluat-
ed by calculating the average IR C=O stretching frequencies
in the corresponding [RhCl(L)(CO)2] complexes (Table 3).
This character decreases in the following order: 9a<9b<
10a<10b<11aꢃ11b. Denoting the overall charge as q and
the varying substituent on the P atom as R, the phosphane
Diamidiniophosphanes of type B: Characterization of the
ꢀ
N2C P bond of diamidiniophosphanes. The nature of the
ꢀ
N2C P bond of diamidiniophosphane 11a (Scheme 5) was
(NHC)Ph]2+ ion. In
ꢀ
investigated using the model [NHC P
a valence-bond or mesomeric approach, the preference for
a homolytic C P bond cleavage by 13.7 kcalmol (see
above and Table 1) suggests a slightly dominating contribu-
ꢀ1
ꢀ
ꢀ
tion of the purely covalent form (C P). As emphasized
Table 3. Characteristics of [RhCl(L)(CO)2] complexes calculated at the
PCM-B3PW91/6-31G**/LANL2DZ*(Rh) level in the acetonitrile contin-
uum (e=35.688).
above, the reversal of the relative covalent/dative character
with respect to the amidiniophosphanes of type A is due to
the electrostatic strain of the released [(NHC)PPh]2+ moiety
that results from a heterolytic dissociation mode.
[c]
[d]
L[a]
Rh···P[b]
Ediss [kcalmolꢀ1
]
n˜CO [cmꢀ1
]
q
R
9a
9b
10a
10b[e]
11a
11b[e]
0
0
+1
+1
+2
+2
Ph
OEt
Ph
OEt
Ph
OEt
2.386
2.357
2.377
2.344
2.358
2.352
14.27
14.41
6.32
2053.6
2056.9
2082.7
2088.2
2102.2
2102.5
Electron-donating properties of diamidiniophosphanes. The
FMOs of the phenylated phosphanes 9a–11a and ethyl
phosphinite 11b (Scheme 5) were calculated at the PCM-
B3PW91/6-31G** level (Figure 5). Upon replacement of the
phenyl substituent of diamidiniophosphane 11a by the
ethoxy substituent in 11b, the selected HOMOs, polarized
on the lone pair of electrons on the P atom (quasi-degener-
ate with the HOMO for 9a and 11b), and the LUMO, polar-
6.75
ꢀ2.67
ꢀ4.52
[a] Variable ligands with formal charge q and monovalent P substituent
ꢀ
R (Scheme 5). [b] Rh P bond lengths in ꢂ. [c] Zero-point corrected
bond dissociation energies. [d] Average of the two IR C=O stretching fre-
quencies calculated at the PBEPBE/6-31G**/LANL2DZ*(Rh) level in
the gas phase. [e] Geometry optimization performed with looser conver-
gence criteria than the default (calculation could not be achieved with
default criteria).
ꢀ
ized along the NHC P bond, are shifted down to lower
energy. As in the case of the model imidazoliophosphanes
Chem. Eur. J. 2012, 18, 7705 – 7714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7711

R. Chauvin, Y. Canac et al.
and phosphinites subsequences a (R=Ph) and b (R=OEt)
thus alternate perfectly in the main charge sequence q=0,
+1, and +2 for 9, 10, and 11, respectively, thus showing
that the electron-withdrawing effect of substituting an
ethoxy group (R=OEt) for a phenyl group (R=Ph) is
much weaker (0.3<n˜CO = <5.5 cmꢀ1) that the effect of
adding a positive charge through a methylium cation (14.3<
n˜CO = <31.3 cmꢀ1). This outcome is in agreement with the
previous observation in the [RhCl(L)2(CO)] series that the
overall donating character of two imidazoliophosphane li-
gands is equivalent to that of two phosphite ligands (and not
phosphinite ligands), namely, that the effect of one imidazo-
lio P-substituent is equivalent to that of three alkoxy
groups.[5c] In the present [RhCl(L)(CO)2] series (Table 3), it
is however noteworthy that the first N-methylation is twice
as efficient at weakening the donating character of the
ligand (Dn˜CO ꢁ30 cmꢀ1 for 9!10) than the second one
(Dn˜CO ꢁ15 cmꢀ1 for 10!11). These results are in perfect
agreement with the above FMO analysis (Figure 5).
Scheme 7. Relative stability of the isomeric complex ions 13 and 14 ac-
cording to the zero-point corrected energies calculated at the PCM-
B3PW91/6-31G**/LANL2DZ*(Rh) level in the dichloromethane contin-
uum (e=8.93). Gibbs energies at 298.15 K are given in parenthesis.
Conclusion
In a comparative approach, the electron-poor ligands of the
amidiniophosphane (a) and amidiniophosphinite (b) series
of compounds can be classified according to the averaged
n˜CO values in the corresponding [RhCl(L)(CO)2] complexes
(Table 4). Substitution of a phenyl group by an imidazolio
group was thus found to be more efficient at weakening the
donating character of the ligand than substitution of
a phenyl group by an ethoxy group. In spite of a slight struc-
tural difference due to the presence of the phenylene bridge
between the imidazolyl moieties in 10a, the relative ranks of
the ethyl imidazoliophenylphosphinite (n˜CO =2072.7 cmꢀ1)
and 10a (n˜CO =2082.7 cmꢀ1) suggests that the electron-with-
drawing effect on a PIII center of a neutral imidazolyl group
is also superior to that of an ethoxy group. The scale also
shows that the phosphane 11a and phosphinite 11b (n˜CO
ꢁ2102 cmꢀ1) are the least electron-donating potential li-
gands reported to date.[23] Finally, the monocationic phos-
phane 10a (n˜CO ꢁ2082.7 cmꢀ1) appears as the most electron-
poor ligand of the series that preserve actual P-coordinating
properties toward a RhI center (in complex 13). The coordi-
nating properties of the extreme ligands 10a, 11a, and 11b
toward stronger Lewis acidic centers certainly deserve fur-
ther investigation that could open more applied perspec-
tives, in particular in catalysis.
Coordinating ability of diamidiniophosphanes. The weaken-
ing of the donation along the series 9a–11b induces a vanish-
ing coordinating ability to the RhI center. The correspond-
ing dissociation energy Ediss decreases,[22] and spontaneous
dissociation is even predicted for the two last ligands 11a
and 11b (Table 3). This finding is in line with the experi-
mental results; that is, no [RhClACTHNUTRGNEUNG(cod)] complex could
indeed be obtained from 11a, in contrast to complexes 13–
14 and 15, which were readily formed from the phosphanes
9a and 10a, respectively (Scheme 6). From the monocation-
ic phosphane 10a, the P-coordinated complex 13 was how-
ever obtained in a 50:50 mixture with the N-coordinated
complex 14 (Scheme 6). The FMOs of 9a and 10a (Figure 5)
are similar and cannot account for the difference in the P-
versus N-coordination selectivity. In both case, the HOMO
is mainly polarized at the lone pair of electrons on the P
atom, whereas molecular orbitals related to the lone pairs of
electrons on the N atom are much lower in energy. Isomers
13 and 14 are calculated to be close in energy at the PCM-
B3PW91/6-31G**/LANL2DZ*(Rh) level in the CH2Cl2 con-
tinuum (e=8.93), the N-coordinated isomer 14 is more
stable than the P-coordinated isomer 13 by only 1.76 kcal
molꢀ1. These findings are therefore in favor of thermody-
namic control of the coordination selectivity (Scheme 7).
Table 4. Scale of the electron-donating properties of potential ligands L based on the average IR C=O stretching frequencies (in cmꢀ1) in the corre-
sponding [RhCl(L)(CO)2] complexes (calculated at the PBEPBE/6-31G**/LANL2DZ*(Rh) level in the gas phase).[a]
L
P
G
P
G
PF3
3a model
10a
11a
11b
~
nCO
2058.5
2060.4
2071.6
2072.7
2075.9
2077.9
2082.7
2102.2
2102.5
[a] The actual coordinating limit of electron-poor ligands is between 10a and 11a.
7712
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 7705 – 7714

P-Coordination of Imidazoliophosphanes toward Rh Centers
FULL PAPER
[2] a) K. B. Dillon, F. Mathey, J. F. Nixon in Phosphorus: The Carbon
Copy, Wiley, Chichester, 1998; b) R. Zurawinski, B. Donnadieu, M.
Mikolajczyk, R. Chauvin, J. Organomet. Chem. 2004, 689, 380.
[3] Multiple Bonds and Low Coordination in Phosphorus Chemistry
(Eds.: M. Regitz, O. J. Scherer), Thieme, Stuttgart, 1990.
[4] a) J. F. Harrison, J. Am. Chem. Soc. 1981, 103, 7406; b) D. Gudat,
Eur. J. Inorg. Chem. 1998, 1087; c) D. Gudat, A. Haghverdi, H.
Hupfer, M. Nieger, Chem. Eur. J. 2000, 6, 3414; d) C. A. Caputo,
M. C. Jennings, H. M. Tuononen, N. D. Jones, Organometallics 2009,
28, 990; e) D. Gudat, Acc. Chem. Res. 2010, 43, 1307.
[5] a) N. Kuhn, J. Fahl, D. Blꢄser, R. Boese, Z. Anorg. Allg. Chem.
1999, 625, 729; b) M. Azouri, J. Andrieu, M. Picquet, P. Richard, B.
Hanquet, I. Tkatchenko, Eur. J. Inorg. Chem. 2007, 4877; c) Y.
Canac, N. Debono, L. Vendier, R. Chauvin, Inorg. Chem. 2009, 48,
5562; d) Y. Canac, N. Debono, C. Lepetit, C. Duhayon, R. Chauvin,
Inorg. Chem. 2011, 50, 10810; e) J. J. Weigand, K. O. Feldmann, F. D.
Henne, J. Am. Chem. Soc. 2010, 132, 16321; f) J. Petuskova, H.
Bruns, M. Alcarazo, Angew. Chem. 2011, 123, 3883; Angew. Chem.
Int. Ed. 2011, 50, 3799.
Experimental Section
Computational details: Geometries were fully optimized at the PCM-
B3PW91/6-31G**/LANL2DZ*(Rh) level of calculation using Gaussi-
an09.[24] LANL2DZ*(Rh) means that f-polarization functions derived by
Ehlers et al.[25] for Rh have been added to the LANL2DZ(Rh) basis set.
Vibrational analysis was performed at the same level as the geometry op-
timization. Solvent effects were included using the polarizable continuum
model (PCM) implemented in Gaussian09 either for acetonitrile (e=
35.688) or dichloromethane (e=8.93). Gibbs energies were calculated at
298.15 K. IR C=O stretching frequencies were calculated in the gas phase
at the PBEPBE/6-31G**/LANL2DZ*(Rh) level. Molecular orbitals were
plotted using the GABEDIT program.[26]
Crystal structure determination of 9a, 11a, 14, and 15: X-ray diffraction
data for the crystals were collected at low temperature on a Bruker
Apex2, an Oxford Diffraction Xcalibur, or an Oxford Diffraction Gemini
diffractometer using a graphite-monochromated MoKa radiation source
(9a, 11a, and 15: l=0.71073 ꢂ) or CuKa radiation source (14: l=
1.54180 ꢂ). Multiscan absorption corrections were applied. The struc-
tures were solved by direct methods using SIR92[27] or SUPERFLIP[28]
and refined by means of least-square procedures with the programs of
the PC version of CRYSTALS.[29] Atomic scattering factors were taken
from the International tables for X-ray crystallography.[30] All non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms were refined
with riding constraints.
[6] Although a dative bond can be considered as intermediate between
a pure (homopolar) covalent bond and a pure ionic bond (in the va-
lence bond approach), thus varying with polarity so that very polar
bonds are definitely nondative; for example, in the gas phase, the
heterolytic bond dissociation energy (BDE) of the highly polar
+q
ꢀq
ꢀ
H F
bond (m=1.826 D, q=0.225 e) is two times greater
than the corresponding homolytic BDE
(1548 kJmolꢀ1
)
9a: C18H13N4P, Mr =316.30 gmolꢀ1
12.0665(4), c=10.0953(4) ꢂ, b=107.309(4)8, V=1508.62(10) ꢂ3, T=
180 K, space group Cc, Z=4,
(MoKa)=0.187 mmꢀ1, 7963 reflections
,
monoclinic, a=12.9720(5), b=
(569 kJmolꢀ1); this homolytic dissociation preference for such a neu-
tral motif is actually due to the electrostatic cost of the H+···Fꢀ
charge separation in the heterolytic mode; in the case of ionic
motifs, such as amidiniophosphanes, the electrostatic cost is cancel-
led, thus possibly confering a dative nature to bonds between atoms
of similar electronegativities (even if the bond is weakly polar with
respect to the bond center when the charge is uniformly delocalized
between the two atoms in the ground state); for relevant discussions,
see: a) R. S. Mulliken, W. B. Person, Ann. Pharm. Belg. Ann. Rev.
Phys. Chem. 1962, 13, 107; b) R. S. Mulliken, J. Am. Chem. Soc.
1952, 74, 811; c) A. Haaland, Angew. Chem. 1989, 101, 1017; Angew.
Chem. Int. Ed. Engl. 1989, 28, 992; d) V. I. Minkin, Pure Appl.
Chem. 1999, 71, 1919; the observable global dative character of
a bond could however be analyzed in more detail by “subtracting”
the electrostatic contribution from the crude BDE values, which
could be achieved by natural bond order (NBO) analysis (e.g.,
through the Wiberg indexes) or by topological analysis of the elec-
tron density (e.g., through the sign of the Laplacian at the bond crit-
ical point) or of the electron-localization function (e.g., through the
position of the attractor in the bond-valence basin).
mACHTUNGTRENNUNG
measured, 3888 unique (Rint =0.017), 209 parameters, refinement on F,
3557 reflections used in the calculations [I>3s(I)], R1=0.0265, wR2=
0.0314.
11a: C20H19N4P, 2
a=10.3919(6) b=23.7534(14), c=24.5833(14) ꢂ, V=6068.2(6) ꢂ3, T=
ACHTUNGTRENNUNG
(CF3O3S), C2H3N, Mr =685.56 gmolꢀ1, orthorhombic,
180 K, space group Pbca, Z=8, mACHTNUGRTNEUNG
(MoKa)=0.312 mmꢀ1, 141761 reflections
measured, 8925 unique (Rint =0.044), 397 parameters, refinement on F2,
5441 reflections used in the calculations [I>3s(I)], R1=0.0555, wR2=
0.1435.
14: C27H28ClN4PRh, CF3O3S, CH2Cl2, Mr =811.88 gmolꢀ1, triclinic, a=
9.5244(4) b=13.4771(8), c=14.1632(7) ꢂ, a=64.578(5), b=85.402(4),
3
¯
g=78.637(4)8, V=1609.77(16) ꢂ , T=100 K, space group P1, Z=2, m-
ACHTUNGTRENNUNG , 19499 reflections measured, 4842 unique (Rint =
(CuKa)=8.150 mmꢀ1
0.043), 406 parameters, refinement on F, 4367 reflections used in the cal-
culations [I>3s(I)], R1=0.0245, wR2=0.0254.
15: C26H25ClN4PRh, Mr =562.84 gmolꢀ1
, triclinic, a=8.6872(3), b=
9.6723(3), c=15.2200(5) ꢂ, a=108.511(3), b=94.614(3), g=104.558(3)8,
V=1155.73(7) ꢂ3, T=180 K, space group P1, Z=2,
mACHTNGUTRENNU(G MoKa)=
¯
[7] a) B. D. Ellis, P. J. Ragogna, C. L. B. Macdonald, Inorg. Chem. 2004,
43, 7857; b) N. Debono, Y. Canac, C. Duhayon, R. Chauvin, Eur. J.
Inorg. Chem. 2008, 2991.
[8] a) I. Abdellah, C. Lepetit, Y. Canac, C. Duhayon, R. Chauvin,
Chem. Eur. J. 2010, 16, 13095; b) I. Abdellah, M. Boggio-Pasqua, Y.
Canac, C. Lepetit, C. Duhayon, R. Chauvin, Chem. Eur. J. 2011, 17,
5110; c) Y. Canac, C. Maaliki, I. Abdellah, R. Chauvin, New J.
Chem. 2012, 36, 1727.
0.947 mmꢀ1
,
50571 reflections measured, 5882 unique (Rint =0.066),
298 parameters, refinement on F, 4694 reflections used in the calculations
[I>3s(I)], R1=0.0306, wR2=0.0330.
[9] a) A. H. Cowley, R. A. Kemp, Chem. Rev. 1985, 85, 367; b) M. San-
chez, M. R. Maziꢃres, L. Lamandꢀ, R. Wolf, “Phosphenium cations”
in Multiple Bonds and Low Coordination In Phosphorus Chemistry,
(Eds.: M. Regitz, O. J. Scherer), Thieme, Stuggart, Germany, 1990,
p129–148; c) D. Gudat, Coord. Chem. Rev. 1997, 163, 71.
[10] N. J. Hardman, M. B. Abrams, M. A. Pribisko, T. M. Gilbert, R. L.
Martin, G. J. Kubas, R. T. Baker, Angew. Chem. 2004, 116, 1989;
Angew. Chem. Int. Ed. 2004, 43, 1955.
[11] a) J. J. Brunet, R. Chauvin, G. Commenges, B. Donnadieu, P. Le-
glaye, Organometallics 1996, 15, 1752; b) N. Kuhn, M. Gçhner, M.
Steimann, Z. Anorg. Allg. Chem. 2002, 628, 896; c) P. Leglaye, B.
Donnadieu, J. J. Brunet, R. Chauvin, Tetrahedron Lett. 1998, 39,
9179; d) L. Viau, R. Chauvin, J. Organomet. Chem. 2002, 654, 180;
e) V. Cadierno, J. Francos, J. Gimeno, Chem. Eur. J. 2008, 14, 6601;
Acknowledgements
The authors thank the Ministꢃre de l’Enseignement Supꢀrieur de la Re-
cherche et de la Technologie and the Universitꢀ Paul-Sabatier, the
Centre National de la Recherche Scientifique and the ANR program
(ANR-08-JCJC-0137-01) for the doctoral fellowship of C.M. The theoret-
ical studies were performed using HPC resources from CALMIP (Grant
2010 and 2011 [0851]) and from GENCI-[CINES/IDRIS] (Grant 2010
and 2011 [085008]).
[1] According to L. C. Allen, electronegativity is the “third dimension”
of the Periodic Table; a) L. C. Allen, J. Am. Chem. Soc. 1989, 111,
9003; b) X. Sun, The Chemical Educator 2000, 5, 54.
Chem. Eur. J. 2012, 18, 7705 – 7714
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7713

R. Chauvin, Y. Canac et al.
f) D. J. M. Snelders, G. v. Koten, R. J. M. Klein Gebbink, Chem. Eur.
J. 2011, 17, 42.
[12] M. Azouri, J. Andrieu, M. Picquet, H. Cattey, Inorg. Chem. 2009,
48, 1236.
[13] J. Petuskova, M. Patil, S. Holle, C. W. Lehmann, W. Thiel, M. Alcar-
azo, J. Am. Chem. Soc. 2011, 133, 20758.
[14] a) Y. Canac, C. Duhayon, R. Chauvin, Angew. Chem. 2007, 119,
6429; Angew. Chem. Int. Ed. 2007, 46, 6313; b) Y. Canac, C. Lepetit,
M. Abdalilah, C. Duhayon, R. Chauvin, J. Am. Chem. Soc. 2008,
130, 8406.
[24] Gaussian 09, Revision A.1, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vre-
ven, Jr., J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark,
J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi,
J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyen-
gar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E.
Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Dan-
iels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J.
Fox, Gaussian, Inc., Wallingford CT, 2009.
[15] A. B. Salah, D. Zargarian, Dalton Trans. 2011, 40, 8977.
[16] S. P. Nolan, US Patent Application Serial No 10/653,688 (September
2, 2003).
[17] Y. H. So, Macromolecules 1992, 25, 516.
[18] CCDC-858972 (9a), CCDC-858973 (11a), CCDC-858974 (14), and
CCDC-863842 (15) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[25] A. W. Ehlers, M. Bçhme, S. Dapprich, A. Gobbi, A. Hçllwarth, V.
Jonas, K. F. Kçhler, R. Stegmann, A. Veldkamp and G. Frenking,
Chem. Phys. Lett. 1993, 208, 111.
[19] A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller, R. H. Crabtree, Or-
ganometallics 2003, 22, 1663.
[20] The given values correspond to the most stable calculated conform-
ers.
[21] G. Canepa, C. D. Brandt, H. Werner, Organometallics 2004, 23,
1140.
[26] http://sites.google.com/site/allouchear/Home/gabedit.
[27] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C.
Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 1994, 27, 435.
[28] L. Palatinus, G. Chapuis, J. Appl. Crystallogr. 2007, 40, 786.
[29] P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout, D. J.
Watkin, J. Appl. Crystallogr. 2003, 36, 1487.
ꢀ
[22] It is noteworthy that the calculated Rh P bond lengths are not con-
[30] International Tables for X-ray Crystallography, Vol. IV, Kynoch
Press, Birmingham, England, 1974.
sistently correlated with Ediss; secondary conformational and/or elec-
tronic effects might be sought for.
Received: December 24, 2011
[23] D. Martin, D. Moraleda, T. Achard, L. Giordano, G. Buono, Chem.
Eur. J. 2011, 17, 12729.
Revised: February 27, 2012
Published online: May 31, 2012
7714
www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 7705 – 7714