Unprecedented rearrangement during the formation of P-P homoatomic N-phosphino formamidine complexes

Article abstract of DOI:10.1016/j.jorganchem.2008.10.033

A variety of homoatomic P-P donor-acceptor homoleptic (R = R′) and heteroleptic (R ≠ R′) N-phosphino formamidine complexes [iPr₂N-C(H){double bond, long}N-PR₂-PR′₂]Cl were synthesized from the addition of N-phosphino forma

Full text of DOI:10.1016/j.jorganchem.2008.10.033

Journal of Organometallic Chemistry 694 (2009) 229–236
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Unprecedented rearrangement during the formation of P–P homoatomic
N-phosphino formamidine complexes
Thanh D. Le a, Damien Arquier a, Karinne Miqueu b, Jean-Marc Sotiropoulos b, Yannick Coppel a,
Stéphanie Bastin a, Alain Igau a,
*
a Laboratoire de Chimie de Coordination, CNRS, 205 Route de Narbonne, 31077 Toulouse, Cedex 4, France
b Université de Pau et des Pays de l0Adour, UMR CNRS 5254, IPREM, 2 avenue du Président Angot 64053 Pau cedex 09, France
a r t i c l e i n f o
a b s t r a c t
A variety of homoatomic P–P donor–acceptor homoleptic (R = R0) and heteroleptic (R – R0) N-phosphino
formamidine complexes [iPr2N–C(H)@N–PR2–PR02]Cl were synthesized from the addition of N-phosphino
formamidine (phosfam) donor reagent iPr2N–C(H)@N–PR2 on halogenophosphane compounds R02PCl
which are synthetic sources for the corresponding phosphenium derivatives R2P+. We have demonstrated
that the dynamic equilibrium observed between the different species is shifted either completely to the
side of the free species or to the side of the donor–acceptor adduct [iPr2N–C(H)@N–PPh2–PPh2]Cl by
Article history:
Received 5 September 2008
Received in revised form 15 October 2008
Accepted 16 October 2008
Available online 25 October 2008
Keywords:
Phosphorus
Formamidine
Donor–acceptor systems
NMR spectroscopy
DFT theoretical calculations
changing the solvent or by varying the temperature. Activation parameters of
D
S– = (ꢀ130 7.2)
J molꢀ1 Kꢀ1 H– = (8.4 0.6) kJ molꢀ1 and G– (298.15 K) = (53.6 2.3) kJ molꢀ1 were determined by
,
D
D
an Eyring analysis over the temperature range of 193–293 K. The negative entropy of activation is con-
sistent with an associative pathway and the low value of
D
H– suggests that the energy barrier for this
reaction is entropically controlled. Phosphine–phosphenium adducts is the most appropriate term to
describe the dynamic process observed at variable temperature for complexes [iPr2N–C(H)@N–PR2
? PR02]+, but the 31P NMR chemical shift and the calculated electronic charges are more in favor of a
phosphinophosphonium Lewis drawing [iPr2N–C(H)@N–PR2–PR02]+. Formation of the homoatomic P–P
heteroleptic formamidine complexes [iPr2N–C(H)@NPR02PR2]Cl (R = Ph, R0 = Et, iPr) results in the formal
insertion of the phosphino group of the corresponding alkyl chlorophosphanes R02PCl into the N–P bond
of the starting phosfam ligand iPr2N–C(H)@N–PR2. Computed data are in agreement with the transient
formation of a heteroatomic N–P intermediate [iPr2N–C(H)@N(PR2)PR02]Cl, which then rearranges to
the more thermodynamically favored homoatomic P–P compound [iPr2N–C(H)@N–PR2–PR02]Cl.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
compounds IV [5b,e,g] (Fig. 1). In the field of monocationic a-
diphosphorus compounds the P–P bond is known to display both
dative and covalent characters, illustrated by forms I0 and I00
according to the classical Lewis formalism.
Phosphenium ligands can be divided into two main classes: (i)
those where the phosphorus atom is stabilized by p-donor substit-
In p-block coordination chemistry, electron-rich and electron-
deficient centers form donor–acceptor bonds; these electron pair
donor (EPD)-electron pair acceptor (EPA) systems have been
known for a long time [1]. EPD/EPA complexes with phosphenium
ions R2P+ as Lewis acceptors have been largely reviewed [2]. The
quantitative nature of the reactions and the structural simplicity
of the complexes contribute to the new development of p-block
coordination chemistry [3]. Homoatomic P–P [4] and heteroatomic
N–P [5] coordination compounds introduce novel bonding possi-
bilities and new synthetic opportunities. Electrophilic phospheni-
um acceptors R2P+ form adducts with neutral donor bases, as
exemplified by the formation of phosphane I [2b,4], amine II
[5c,d,e], pyridine III [5a,c,h], and cyclic amidinephosphenium
uents [2b,5b–g,6] such as amino substituents (R2N)2P+ and (ii)
those bound to two aryl Ar2P+ or alkyl R2P+ substituents [4,5a].
The former can be isolated as base-free phosphenium cations
which may explain why extensive experimental and theoretical
studies [7] have appeared since their discovery. Diaryl- and dial-
kyl-phospheniums are usually prepared by halide abstraction from
their corresponding halophosphane precursors and stabilized by
the coordination of a Lewis base [4,5a]. Isolation of free diaryl-
and dialkyl-phosphenium cations has to date remained elusive.
Since their first identification, these phospheniums have been
experimentally under-explored, except the diphenylphosphenium
Lewis acidic phosphorus center Ph2P+. Experimental solid-state
structures in conjunction with computational data indicate that
* Corresponding author. Fax: +33 5 61 55 30 03.
E-mail addresses: jean-marc.sotiro@univ-pau.fr (J.-M. Sotiropoulos), alain.
igau@lcc-toulouse.fr (A. Igau).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.033

230
T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
Fig. 1. Homo-PP and hetero-NP atomic p-block complexes with alkyl R2P+ and aryl Ar2P+ phosphenium ions as Lewis acid acceptors.
Ph2P+ is a better Lewis acid than its diaminophosphenium ana-
logue (R2N)2P+. The preparation of the first derivatives of catena-
polyphosphorus cations from homoatomic P–P donor–acceptor
aryl cations has been achieved [3b]. Very recently, diaryl- and dial-
kyl-phosphinophosphonium homoleptic (R = R0) and heteroleptic
(R – R0) compounds [R3P–PR02]+ gave access to 1,2-diphosphonium
derivatives [R3PPR03]2+ representing prototypical phosphorus ana-
logues of ethane [3a].
While investigating the protonation reaction of the N-phos-
phino formamidine (phosfam) iPr2N–C(H)@N–PPh2 1a, we
identified the novel phosphine–phosphenium adduct, [iPr2N–
C(H)@N–PPh2–PPh2]Cl 3a [8]. Complex 3a was subsequently pre-
pared via an independent route as reported herein (Scheme 1). In
this work, we expand the preparation of homoatomic homoleptic
P–P derivatives [iPr2N–C(H)@N–PR2–PR2]Cl to R = Et (3b) and
R = iPr (3c). We used NMR spectroscopy to explore the dynamic
equilibrium 1a + 2a ¢ 3a enabling the formation of a formal
P ? P coordinative bond (Fig. 1, I0) which can also be Lewis drawn
as a P–P covalent bond (Fig. 1, I00). We also report an unprecedented
rearrangement which occurs with the phosfam ligand 1a along the
formation of heteroleptic (R – R0) P–P homoatomic complexes
[iPr2N–C(H)@N–PR2–PR02]Cl. These experimental studies are sup-
ported by quantum-chemical calculations to unravel the structural
and electronic properties of the formamidine P–P homoatomic
complexes.
of 3a–c. The 31P NMR chemical shifts for 3a–c at respectively d
31.7, 59.0, and 55.0 ppm are distinctive for the phosphonium cen-
þ
ters and are consistent with the characteristic shift for @N–PR3
(d ꢁ 32 ppm (R = Ph), d ꢁ 53 ppm (R = iPr)) [10]. The large 1JPP cou-
pling constant values (ca. 280–340 Hz) of the new donor–acceptor
adducts 3a–c fall in the range for P–P complexes of type I reported
in the literature [4a,c,e] (Fig. 1). The 1H and 13C NMR spectra of 3a–
c showed the presence of the proton and the carbon of the
formamidine framework >N–C(H)@N in the range of 8.40–
2
8.80 ppm and 156–161 ppm respectively. The low JCP observed
between the imino carbon atom and the phosphorus fragment
(2.5 < 2JCP < 7.5 Hz) is characteristic for tetracoordinated
r
4–P N-
phosphorus formamidino derivatives [8,10]. 2D HMBC 1H–15
N
and HMQC 31P–15N{1H} NMR experiments monitored on 3a (d
31P 31.7 and ꢀ17.6 ppm) allowed to identify the imino nitrogen
atom of the formamidine fragment at d 15N ꢀ244.0 (1JNP = 51.0
Hz) ppm which correlates with the phosphorus atom at d 31P
31.0 ppm (Fig. 2). All these NMR data suggest that the homoatomic
P–P derivatives 3a–c are best described by a covalent ‘phosphino-
phosphonium’ structure illustrated by I00 (Fig. 1).
Phosphenium cations R2P+ generated from their halophosphane
precursors R2PCl 2a–c, are formally related to their synthons via a
dynamic equilibrium whose position depends intimately upon the
nature of the counter ion, the R groups coordinated to phosphorus
and to some extent the solvents. In solution P–Cl bonds in alkyl and
aryl R2PCl species may be described as covalently interacting ion
pairs or separated ions depending on the experimental reaction
conditions. The formation of adduct 3a at 293 K was tested in dif-
ferent solvents from the protonic and non-protonic slightly polar
solvents as toluene, CHCl3, CH2Cl2, and THF to the highly polar
CH3CN solvent (Scheme 2). According to 31P NMR, at 293 K, the ma-
jor species present in THF and toluene are the N-phosphino form-
amidine derivative 1a and Ph2PCl 2a. The broad resonances are
consistent with a dynamic equilibrium with 3a which is in the
2. Results and discussion
In the presence of an appropriate donor reagent, halogenophos-
phane compounds R2PCl (2) are synthetic sources of the corre-
sponding phosphenium derivatives R2P+. In dichloromethane,
phosfams iPr2N–C(H)@N–PR2 1a–c react with chlorophosphanes
R2PCl 2a–c to give the corresponding homoatomic P–P donor–
acceptor adducts 3a–c in quasi quantitative yield based on 31P
NMR (Scheme 1). The other compound observed as a by-product
in the reaction is R2P(O)–PR2 produced by the reaction of R2PCl
with traces of water [9]. When R = NiPr2, formation of the corre-
sponding homoatomic P–P complex [iPr2N–C(H)@N–P(NiPr2)2–
P(NiPr2)2]Cl was not observed. This is most probably due to the
strong covalent nature of the P–Cl bond in P-chlorodiamino
phosphanes as they dissociate in solution only to a small extent
[5b] and to the extreme steric crowding at the phosphorus
atoms.
Oily solids were obtained in most attempts and crystalline or
microcrystalline materials could not be obtained for 3a–c. Never-
theless, spectroscopic features allow for definitive identification
Scheme 1. Formation of homoatomic P–P homoleptic phosphinophosphonium
formamidines 3a–c.
Fig. 2. 2D HMQC-nd 31P–15N{1H} NMR spectrum showing correlations between
phosphorus and imino nitrogen atoms in [iPr2N–C(H)@N–PPh2–PPh2]Cl 3a.

T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
231
Fig. 3b displays simulations of the spectra obtained at different
temperatures with the WINDNMR software [12]. The model totally
validates the hypothesis of a concomitant change of the 1a:2a:3a
ratio. From the DNMR simulation we were able to establish the
rate of exchange between the different species 1a, 2a, and 3a. An
Eyring analysis over the temperature range 193–293 K
Scheme 2. Solvent-dependent formation of homoatomic P–P adduct 3a at 293 K
(S1 = CH2Cl2, CHCl3, CH3CN; S2 = toluene, THF).
afforded activation parameters of
D
S– = (ꢀ130 7.2) J molꢀ1 Kꢀ1
,
D
H– = (8.4 0.6) kJ molꢀ1 G– (298.15 K) = (53.6 2.3)
,
and D
kJ molꢀ1. The negative entropy of activation is consistent with an
intermediate range on the NMR timescale. At the same tempera-
ture, in CHCl3, CH2Cl2 and CH3CN, the major species is the adduct
3a. The large resonances suggest a rapid equilibrium between 3a
and the free species 1a and 2a, which is shifted almost completely
towards the formation of adduct 3a. There is undoubtedly a close
connection between phosphenium reactivity and the nature of
the solvent employed in the formation of 3a in solution. It seems
likely that the trend of the formation of 3a in condensed phases
arises from the interplay of several competing factors (e.g., Cou-
lombic, directional, inductive, dispersion and hydrogen bonding
associative pathway and the small value of
D
H– suggests that
the energy barrier for this reaction is entropically controlled. All at-
tempts to measure analogous data using other P–P adducts were
unsuccessful. However, the characteristic broadness of the 31P–
{1H} NMR signals associated with free and coordinated species is
observed in all samples, suggesting that exchange is rapid what-
ever the R substituents.
We extended the formation of P–P compounds to the prepara-
tion of heteroleptic N-phosphino formamidine complexes. The
phosfam 1a was reacted at 293 K in CH2Cl2 with Et2PCl 2b to give
the heteroleptic phosphinophosphonium derivative 3d (Scheme 3).
Only traces of Ph2P(O)–PPh2 were observed as the unique side
product of the reaction.
interaction forces) [11] rather than from
interaction.
a single dominant
Variable-temperature 31P NMR experiments recorded in THF in
the range of 293–193 K exhibited a pronounced temperature-
dependent equilibrium between the phosfam 1a, Ph2PCl 2a, and
3a. At 293 K the major species present are the dissociated 1a and
Ph2PCl 2a phosphorus derivatives as the two phosphorus reso-
nances are close to the corresponding free species. However, the
line-shapes of the signals of these species are broad, especially
the one of Ph2PCl 2a, which is characteristic of the existence of
an equilibrium. At 193 K, the major species is the donor–acceptor
adduct 3a. The formation of this complex is illustrated by the
chemical shift of the Ph2P fragment (d 31P of free Ph2PCl
2a = 82 ppm) shielded to high field by 100 ppm (d 31P (Ph2P–) =
ꢀ17.6 ppm) and the large P–P coupling constant value
(1JPP = 282.5 Hz). At 243 K, the spectrum shows two broad signals
In the 31P NMR spectrum of the reaction mixture, the initial
chemical shifts at 54.3 ppm (1a) and 118.1 ppm (2b) are replaced
by two doublets centered at 50.5 and ꢀ27.2 ppm (3d) with a cou-
1
pling constant value JPP = 279.5 Hz consistent with P–P homo-
atomic adducts[4a,c,e]. The 1H and 13C NMR spectra of 3d
3
showed the presence of the proton (d 8.10 ppm, JHPEt = 20.6 Hz)
and the carbon (d 159.0 ppm) of the formamidine framework
2
>N–C(H)@N. The absence of a JCP coupling constant was already
observed in tetracoordinated N-phosphorus formamidine deriva-
tives [10]. A combination of 2D homo- and heteronuclear NMR
experiments with selective 31P decoupling allowed complete
assignments of the atoms connectivity profile for the formamidine
phosphinophosphonium framework molecule 3d. 2D HSQC 1H–13
C
at 39.4 (
D
m
1/2 = 650 Hz) ppm and ꢂꢀ3.0 (
Dm1/2 = 4000 Hz) ppm.
and HMBC 1H–31P NMR experiments allowed us to assign the ethyl
groups attached to the phosphorus atom at d 31P 50.5 ppm. 2D
HMBC 1H–15N and HMQC 31P–15N{1H} NMR experiments moni-
These signals are close to an average position between the reso-
nances of the donor–acceptor adduct 3a and the dissociated 1a
and 2a species.
Fig. 3. (a) Experimental and (b) simulated variable-temperature 31P NMR experiments showing the equilibrium between the adduct 3a and the free 1a and Ph2PCl 2a species
( Ph2PP(O)Ph2).
*

232
T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
Scheme 3. Formation of the homoatomic heteroleptic phosphinophosphonium
formamidine 3d.
tored on 3d allowed to identify the imino nitrogen atom of the
formamidine framework at ꢀ237.7 ppm and to show the correla-
1
tion with the phosphorus atom at d 31P 50.5 ppm. The JNPEt2 cou-
pling constant value of 50.5 Hz definitely confirmed the presence
of the diethyl phosphino group directly connected to the formam-
idine fragment. The formation of the phosphinophosphonium 3d
results from the formal insertion reaction of Et2P+ into the phos-
phorus–nitrogen bond N–PPh2 of the starting donor N-phosphino
formamidine 1a. Interestingly, the heteroleptic P–P compound 3d
was also prepared when 1b was reacted with 2a (Scheme 3).
The same type of rearrangement was observed when 1a was re-
acted with 2c to give the phosphinophosphonium compound 3e,
which formally resulted from the insertion reaction of iPr2P+ into
the phosphorus–nitrogen bond N–PPh2 of the starting donor
N-phosphino formamidine 1a (Scheme 4). Starting from the
N-phosphino formamidine 1c and after addition of Ph2PCl 2a for-
mation of the thermodynamic stable heteroleptic P–P adduct 3e
was observed (Scheme 4).
Fig. 4. Molekel plot of the calculated molecular structure of 3a with hydrogen
atoms omitted and numbering atoms.
Table 1
0
Selected calculated bond lengths (ÅA) and angles (°) for 3a–c. NPA charges (qX).
X
3a
3b
3c
C1–N1
N1–P1
P1–P2
N1–P1–P2
1.312
1.656
2.261
103.81
123.03
ꢀ27.4
ꢀ0.97
1.54
1.316
1.649
2.248
111.74
124.93
ꢀ53.4
ꢀ0.98
1.51
1.314
1.655
2.261
111.83
125.63
ꢀ36.7
ꢀ0.98
1.51
C1–N1–P1
a
D
Greaction
qN1
qP1
qP2
0.65
0.56
0.55
The 31P NMR spectrum of 3e showed two doublets at 52.4 and
a
1
Calculated energies in kcal molꢀ1 for the reaction ‘2a–c + R2P+ ? 3a–c’.
ꢀ31.5 ppm with a JPP = 311.2 Hz. The isopropyl substituents are
connected to the phosphorus atom resonating at d 31P 52.4 ppm
which is coupled to the proton of the formamidine skeleton >N–
C(H)@N at d 7.50 (3JHPiPr2 = 18.8 Hz) ppm. The 1JNPiPr2 coupling con-
stant value of 51.8 Hz of the imino nitrogen atom at ꢀ238.1 ppm
definitely confirmed the presence of the diisopropyl phosphino
group directly attached to the formamidine fragment.
from a partial delocalization of the positive charge along the form-
amidine framework (Scheme 5).
The N1–P1 bond lengths of ꢁ1.65 Å in 3a–c are considerably
shorter than those in the corresponding phosfams 1a–c (ꢁ1.75 Å)
and the phosphorus–phosphorus bond lengths (ꢂ2.25 Å) are in
the range of those reported for P–P donor–acceptor adducts
[4c,e]. The CNP bond angles are around 125°. Besides the strong
stabilizing interaction between the P amino nitrogen lone pair
and the p*C@N orbital (ꢂ90 kcal/mol), consistent with the delocaliza-
tion, NBO calculations show significant negative hyperconjugations
In order to have a better insight into the geometrical and elec-
tronic structures of the N-phosphino formamidine complexes 3a–c,
DFT calculations at the B3LYP/6-31G level of theory were carried
**
out. In agreement with the experimental data, the formamidine
homoatomic P–P derivatives were found as global minima on the
potential energy surface. The E configuration corresponds to the
most thermodynamically stable structure in which the P–P bond
and the imino nitrogen lone pair are in trans position, except for
3a where the P–P–C–N dihedral angle value is 106° (Fig. 4). The
main geometrical parameters for 3a–c are shown in Table 1.
The calculated C1–N1 bond lengths of ꢁ1.31 Å in 3a–c are
slightly longer than those observed in 1a–c [8] (vide infra) and fall
in the upper limit for double carbon–nitrogen bond [13]. This C1–
N1 bond length elongation is commonly observed for tetracoordi-
nated N-phosphonium formamidine derivatives [10] and results
(ꢂ7.5–11 kcal/mol) between the imino nitrogen lone pair and
r
ꢃCH
and
r
ꢃPP (or
r
ꢃPC for the phenyl substituents) orbitals, leading to
a decrease of the PN bond lengths. All these stabilizing interactions
are in favor for the formation of the P–P adducts. Considering the cal-
culated
DGreaction (Table 1), it is noteworthy that the reaction is exo-
thermic and that the calculated values are consistent with the
experimental data since the driving force of the reaction is the for-
mation of the P–P adducts. Moreover the NPA charges, obtained from
the NBO analysis, are consistent with a phosphinophosphonium
covalent structure (type I00) with the phosphorus atom P1 next to
the imino nitrogen atom bearing
a strong positive charge,
qP1 = 1.51–1.54, (Table 1) and the phosphorus atom P2 at the termi-
nal position a less positive one qP2 ꢁ 0.5. The calculated geometrical
parameters as well as the NBO analyses clearly indicate that the ad-
ducts 3a–c can be represented by the phosphinophosphonium Lewis
structure I00.
In order to provide more information on the complexes 3a–c,
the donor N-phosphino formamidines 1a–c and acceptor phosphe-
nium cations R2P+ (R = Ph, Et, iPr) were theoretically studied. As
previously reported [8] for the phosfams iPr2N–C(H)@N–PR2 1a,c,
for R = Et, the E configuration corresponds to the most thermody-
namically stable structure in which the imino nitrogen and phos-
Scheme 4. Formation of the homoatomic heteroleptic phosphinophosphonium
formamidine 3e.

T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
233
Scheme 5. Mesomeric forms in the formamidine framework of the phosphinophosphonium 3.
phorus lone pairs are in trans position. The C@N and P–N bond
lengths are respectively about 1.29 Å and 1.75 Å. The phosphorus
and imino nitrogen charges are similar for all the compounds.
The energetic positions of the bonding combination (nP + nN) with
an important weight on phosphorus vary from ꢀ5.1 to ꢀ5.3 eV and
from ꢀ7.1 to ꢀ7.8 eV for the antibonding combination (nP ꢀ nN)
with an important weight on imino nitrogen (Table 2).
tals are relatively close for the Ph2P+ (ꢀ8.07 eV) and iPr2P+
(ꢀ7.56 eV) cations, whereas the LUMO (ꢀ9.62 eV) of Et2P+ is more
accessible by about 1.5 eV. The distinct difference observed be-
tween the phosphenium cations can be explained by strong inter-
p
actions between the rCH and rCH
/
rCC orbitals and the 3p (P)
orbital respectively for Et2P+and iPr2P+ (Table 3), which destabilize
the unoccupied phosphorus p orbital.
In the determination of acceptor properties of the free phosphe-
In addition to the orbital consideration, the p-acceptor effect in
nium cations R2P+ (R = Ph, Et, iPr), the most important frontier orbi-
the phosphenium species R2P+ (R = Ph, Et, iPr) can be estimated by
p
p
tal is the lowest unoccupied molecular 3p (P) orbital (LUMO).
regarding the electron population in the 3p (P) orbital. The NBO
Table 3 summarizes the main geometrical parameters, the ener-
analysis indicates that the occupation in iPr2P+ is slightly more
r
p
getic positions of the HOMO [n (P)] and the LUMO [3p (P)], the
important than the one in Et2P+. For the phenyl cation Ph2P+
higher occupation of the phosphorus 3p orbital is observed in
a
p
p
NPA charges and the 3p (P) occupation for the two alkyl (R = Et,
iPr) and aryl (Ph) substituents.
comparison with the alkyl analogues, illustrating a slight p-delo-
The results are consistent with those previously reported by
Macdonald et al. [14] for the diphenyl and dimethylamino pho-
sphenium cations. The P–C bond lengths vary from 1.77 Å to
1.85 Å (Ph2P+ > Et2P+ > iPr2P+) and the C–P–C angles from 104 to
109.5°. It is noteworthy that the two P–C bond lengths are surpris-
ingly inequivalent with iPr substituents. The energetic position of
calization. Nevertheless the lowest energy structure found for
Ph2P+ shows non coplanar phenyl substituents, limiting the corre-
sponding MO overlap. In this case, the energetic position of the
p
3p (P) orbital is intermediate between those of the two alkyl
substituents because of interaction between this latter orbital
and one of the p*C@C orbitals of the phenyl ring, leading to a
stabilization of the unoccupied p phosphorus orbital. Consequently,
the bis(ethyl)phosphenium cation is the best acceptor of this
series.
p
the most accessible 3p (P) orbital depends on the substituents
p
and reflects their electron acceptor ability (Fig. 5). The 3p (P) orbi-
Overall, the formation of the heteroleptic P–P compounds 3d
and 3e starting from the phosfam ligand 1a and the corresponding
phosphenium precursors 2b and 2c, respectively, cannot be ratio-
nalized from the strict consideration of the position of the molec-
ular orbitals in donor derivatives 1a–c and acceptor
phosphenium cations R2P+ (R = Ph, Et, iPr). From the computed data
it is evident that the charges should have a decisive contribution in
the formation of phosphinophosphonium formamidines 3d,e. Con-
sequently, the imino nitrogen of the formamidine fragment may
interact with the Lewis acid phosphorus center of the phospheni-
um cations during the course of the reaction. In fact, on the poten-
tial energy surface, we found that the heteroatomic N–P
formamidine isomer [iPr2N–C(H)@N(PR2)PR2]+ exists as a mini-
mum, which is thermodynamically less stable than the corre-
sponding homoatomic P–P formamidine [iPr2N–C(H)@N–PR2–
PR2]+ compound with an energetic difference in the range of 15–
25 kcal molꢀ1 (see Supplementary material). Moreover, a transi-
tion state has been located on the potential energy surface with
an energy barrier allowing the heteroatomic N–P (4)/homoatomic
P–P (3) rearrangement. This rearrangement energy profile has been
computed with alkyl substituents in order to reduce calculation
time. In the gas phase, for R = R0 = Et, iPr, the value of the energetic
barrier for the heteroatomic/homoatomic rearrangement ranges
from 22 to 30 kcal molꢀ1 (see Supplementary material). Overall,
considering the experimental (condensed phase) and computed
data (gas phase), it is reasonable to propose in a first step the
formation of the transient heteroatomic N–P intermediates
Table 2
Selected calculated bond lengths (Å) and angles (°) for N-phosphino formamidine 1a–
c
at the B3LYP/6-31G level. NPA charges (qX). Energetic positions (eV) of the
**
bonding and antibonding P and N lone pair combination, calculated at the B3LYP/6-
31G and MP2/6-31G level of theory.
** **
X
1a
1b
1c
C1–N1
P1–N1
C1–N1–P1
qN1
qP1
(nP + nN) B3LYP
(nP + nN) MP2
(nP + nN) B3LYP
(nP + nN) MP2
1.293
1.737
115.56
ꢀ0.89
1.03
ꢀ5.33
ꢀ8.00
ꢀ7.78
ꢀ11.46
1.292
1.746
116.13
ꢀ0.89
1.00
ꢀ5.12
ꢀ8.14
ꢀ7.21
ꢀ10.99
1.292
1.746
116.13
ꢀ0.89
1.00
ꢀ5.10
ꢀ8.01
ꢀ7.13
ꢀ10.90
Table 3
0
Selected calculated bond lengths (AÅ) and angles (°) for the phosphenium cations R2P+
(R = Ph, Et, iPr) at the B3LYP/6-31G level. Energetic positions (eV) of the frontier
orbitals calculated at the B3LYP/6-31G and MP2/6-31G level of theory. Phospho-
**
**
**
p
rus NPA charges (qP), occupation of the 3p (P) and main stabilizing interactions (Kcal
molꢀ1) involving the 3p (P) orbital.
p
X
Ph2P+
Et2P+
iPr2P+
P–C
C–P–C
qP
Pop. 3p (P)
EHOMO
1.766
104.08
1.07
1.801
103.72
0.24
1.846/1.824
109.42
0.32
[iPr2N–C(H)@N(PR2)PR02]Cl
4 which then rearrange to the
corresponding more thermodynamically stable P–P formamidine
adducts 3 (Scheme 6).
The heteroatomic N–P intermediates [iPr2N–C(H)@N(PR2)-
PR02]Cl 4 may be represented by several mesomeric Lewis struc-
tures such as the formamidinium species 4A or 4B. Theoretical cal-
culations have evidenced that the preparation of compounds 3
goes through the formation of a 3-membered transition state 5.
p
0.58
1.31
1.15
ꢀ12.15
ꢀ8.07
ꢀ4.98
ꢀ13.28
ꢀ9.62
ꢀ5.84
ꢀ12.53
ELUMO
ꢀ7.56
MP2
E
ꢀ3.67
LUMO
pPCh@1 C ? 3p (P):
r
CH ? 3p (P):
p
p
p
r
CH ? 3p (P):
Main stabilizing interactions
54.5
64.5
CC ? 3p (P):
(Kcal molꢀ1
)
pPCh@2 C ? 3p (P):
r
p
16.1
p
54.6
25.7

234
T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
Fig. 5. Molekel plots and energetic positions (Kohn–Sham energies in eV) of the LUMO and HOMO for phosphenium species R2P+ with R = Ph, Et, iPr.
Scheme 6. Proposed mechanism for the formation of P–P homoatomic phosphinophosphonium formamidines 3.
electronic charges are more in favor of the phosphinophosphonium
covalent Lewis drawing [iPr2N–C(H)@N–PR2–PR02]+ illustrated by
form I”. Computed data are in agreement with the proposed tran-
3. Conclusions
In the present study we report the preparation of a series of
homoatomic P–P homoleptic (R = R0 = Ph, Et, iPr) and heteroleptic
(R = Et, R0 = Ph; and R = iPr, R0 = Ph) N-phosphino formamidine
complexes [iPr2N–C(H) = N–PR2–PR02]Cl. We have demonstrated
that changing the solvent or the temperature allowed to control
the formation of the [iPr2N–C(H) = N–PPh2–PPh2]Cl adduct. The
phosphenium cations R2P+ in the presence of the N-phosphino
formamidine (phosfam) donor derivatives give the corresponding
electron pair donor (EPD)-electron pair acceptor (EPA) complexes.
The dynamic equilibrium observed in the different condensed
phases involves P–P dissociation in [iPr2N–C(H)@N–PR2–PR02]Cl
adducts. The unprecedented rearrangement which occurs along
the formation of the homoatomic P–P heteroleptic (R = Et, iPr;
R0 = Ph) complexes [iPr2N–C(H)@N–PR2–PR02]Cl results in the for-
mal insertion of the phosphino group of the corresponding alkyl
chlorophosphanes R2PCl into the N–P bond of the starting phosfam
ligand iPr2N–C(H)@N–PPh2. Phosphine–phosphenium illustrated
by I0 is the most appropriate term to describe the dynamic process
observed at variable temperature for complexes [iPr2N–C(H)@N–
PR2 ? PR02]+, but the 31P NMR chemical shift and the calculated
sient formation of
a heteroatomic N–P intermediate [iPr2N–
C(H)@N(PR2)PR02]Cl illustrated by 4, which then rearranges to the
more thermodynamically favored homoatomic P–P formamidine
compound [iPr2N–C(H)@N–PR2–PR02]Cl 3.
4. Experimental
4.1. General
All reactions were conducted under an inert atmosphere of dry
argon using standard Schlenk-line techniques. Chemicals were
treated as follows: pentane and CH2Cl2 distilled from CaH2; CDCl3
distilled from P2O5; C6D6, CD2Cl2 (Euriso-top) and other solvents
stored on 4 Å molecular sieves. Solvents were degassed by stan-
dard methods before use. Chlorodiphenylphosphane (97%) was ob-
tained from ALFA AESAR and distilled prior to use. All other
commercial chemicals were from Aldrich (N,N-diisopropylcyana-
mide, 97%), ACROS (chlorodiisopropylphosphane, 96%) and STREM
(bis(cyclopentadienyl)zirconium dichloride, 99%) and were used as

T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
235
received. [Cp2ZrHCl]n was prepared following the procedure re-
ported by Buchwald et al. [15]. Infrared spectra were performed
in solution (KBr windows) on a Perkin–Elmer GX 2000 spectrome-
ter. Mass spectra were recorded on a TSQ7000 Thermo Electron (EI)
and on a Q Trap (ES–MS) mass spectrometer. Melting points were
obtained using an Electrothermal Digital Melting Point apparatus
and are uncorrected. Elemental analyses were carried out by the
‘‘Service d’ Analyse du Laboratoire de Chimie de Coordination” in
Toulouse.
(5 mL). The mixture was stirred for 5 min. The solvent was re-
moved by oil pump vacuum to give 3a in quasi quantitative yield
as a white residue (0.256 g, 0.480 mmol). Compound 3a was char-
acterized by NMR without any further treatment. 31P NMR
1
(81 MHz, CDCl3): [d/ppm] 31.7 (d, JPP = 282.5 Hz, N–P–P), ꢀ17.6
1
(d, JPP = 282.5 Hz, N–P–P). 1H NMR (200 MHz, CDCl3): [d/ppm]
3
7.58–7.03 (m, 21H, HPh and HC@N), 4.37 (sept, 1H, JHH = 6.8 Hz,
3
NCHCH3), 3.54 (sept, 1H, JHH = 6.7 Hz, NCHCH3), 1.12 (d, 6H,
3
3JHH = 6.8 Hz, NCHCH3), 0.92 (d, 6H, JHH = 6.7 Hz, NCHCH3). 13C
2
NMR (50 MHz, CDCl3): [d/ppm] 156.5 (d, JCP = 3.4 Hz, HC@N),
1
1
4.2. NMR experiments
135,1 (d, JCP = 7.0 Hz, i-PCPh), 134.7 (d, JCP = 10.4 Hz, i-PCPh),
134,5 (s, CHPh), 134.2(s, CHPh), 131.9 (d, JCP = 8.8 Hz, CHPh), 131.3
(s, CHPh), 129.7 (d, JCP = 11.8 Hz, CHPh), 129.3 (d, JCP = 7.3 Hz, CHPh),
50.1 (s, NCHCH3), 47.8 (s, NCHCH3), 23.1 (s, NCHCH3), 19.3 (s,
NCHCH3). 15N NMR (51 MHz, [toluene-d8]): [d/ppm] ꢀ210.0
(NiPr2), ꢀ244.0 (1JNP = 51.0 Hz, C@N–P).
1H, 1H–{31P}, 31P–{1H}, 13C–{1H} and 13C–{1H, 31P} NMR spectra
were recorded on a Bruker AV500, AV 400, and AV 300 spectrom-
eters equipped with a 5 mm triple resonance inverse probe with
dedicated 31P channel. All chemical shifts for 1H and 13C are rela-
tive to TMS using residual peak of the solvent as a secondary stan-
dard. 31P chemical shifts were referenced to an external 85% H3PO4
sample. The 15N resonances were referenced to neat CH3NO2. Tem-
perature calibration was determined using a methanol chemical
shift thermometer. All the 1H and 13C signals were assigned on
the basis of chemical shifts, spin–spin coupling constants, splitting
patterns and signal intensities, and by using 2D 1H–1H COSY45,
1H–13C HSQC, 1H–13C HMBC, 1H–15N HMBC and 31P–15N HMQC-
{1H} with broadband or selective 31P decoupling when necessary.
All spectra were recorded at ambient probe temperature unless
stated otherwise. NMR simulations were run with WINDNMR-Pro
7.1.12 software [12].
Compound 3b: 31P NMR (81 MHz, CDCl3): [d/ppm] 55.5 (d,
1
1JPP = 281.8 Hz, N–P–P), ꢀ36.7 (d, JPP = 281.8 Hz, N–P–P). 1H NMR
3
(200 MHz, CDCl3): [d/ppm] 8.75 (d, 1H, JHP = 20.8 Hz, HC@N),
4.41 (sept, 1H, 3JHH = 6.8 Hz, NCHCH3), 3.99 (sept, 1H, 3JHH = 6.8 Hz,
NCHCH3), 3.52 (m, 2H, PCH2CH3), 2.47 (m, 2H, PCH2CH3), 1.97 (m,
2H, PCH2CH3), 1.76 (m, 2H, PCH2CH3), 1.14–1.42 (m, 24H, NCHCH3
et PCH2CH3). 13C NMR (50 MHz, CDCl3): [d/ppm] 160.2 (d,
2JCP = 2.5 Hz, HC@N), 50.7 (s, NCHCH3), 46.9 (s, NCHCH3), 22.9 (s,
NCHCH3), 19.8 (s, NCHCH3), 18.0 (dd, JC,P = 47,6 Hz, JCP = 8.0 Hz,
NPCH2CH3), 14.1 (dd, JCP = 17.2 Hz, JCP = 2.9 Hz, PPCH2CH3), 11.7
(dd, JCP = 17.0 Hz, JC.P = 9,8 Hz, PPCH2CH3), 11.7 (dd, JCP = 17.0 Hz,
JCP = 9.8 Hz, PPCH2CH3), 6,4 (dd, JCP = 5.5 Hz; JCP = 5.5 Hz;
NPCH2CH3).
4.3. Preparation of N-phosphino formamidines iPr2NC(H)@NPEt2 (1b)
Compound 3c: 31P NMR (81 MHz, CD2Cl2): [d/ppm] 59.0 (d,
1
1JPP = 340.9 Hz, N–P–P), ꢀ6.2 (d, JPP = 340.9 Hz, N–P–P). 1H NMR
3
Following the procedure described for 1a,c [8], 1b and 1d were
prepared after addition of a solution of iPr2NCN (7.814 mmol) in
CH2Cl2 (5 mL) to a suspension of [Cp2Zr(H)Cl]n (7.814 mmol) in
CH2Cl2 (15 mL) followed by Et2PCl and (iPr2N)2PCl respectively
(7.814 mmol) to give 1b in 85% yield (1.436 g, 6.642 mmol) and
(200 MHz, CD2Cl2): [d/ppm] 8.42 (d, 1H, JHP = 19.3 Hz, HC@N),
3
4.49 (sept, 1H, JHH = 6.8 Hz, NCHCH3), 3.02 (m, 2H, PCHCH3),
2.43 (m, 2H, PCHCH3), 1.09–1.37 (m, 36H, CHCH3). 13C NMR
2
(50 MHz, CD2Cl2): [d/ppm] 160.4 (d, JCP = 7.5 Hz, HC@N), 50.5 (s,
NCHCH3), 49.8 (s, NCHCH3), 47.1 (d, JCP = 4.5 Hz, PCHCH3), 46.8
(d, JCP = 3.3 Hz, PCHCH3), 27.6 (dd, JCP = 41.7 Hz, PCHCH3), 23.4 (s,
NCHCH3), 21.8 (dd, JCP = 21.4 Hz, JCP = 2.4 Hz, PCHCH3), 21.5 (d,
JCP = 2.8 Hz, PCHCH3), 21.5 (s, NCHCH3), 19.7 (d, JCP = 3.1 Hz,
PCHCH3), 15.5 (d, JCP = 2.0 Hz, PCHCH3).
1d in 90% yield (2.502 g, 6.989 mmol). 1b: IR (KBr, THF):
m
(C@N) = 1607 cmꢀ1 31P NMR (121 MHz, C6D6): [d/ppm] 91.0 (s).
.
3
1H NMR (300 MHz, CD2Cl2): [d/ppm] 7.95 (d, 1H, JHP = 19.1 Hz,
HC@N), 4.56 (m, 1H, NCHCH3), 3.52 (m, 1H, NCHCH3), 1.42 (m,
3
4H, PCH2CH3), 1.35 (d, 6H, JHH = 6.8 Hz, NCHCH3), 1,32 (d, 6H,
Compound 3d: 31P NMR (202 MHz, CD2Cl2): [d/ppm] 50.5 (d,
3
3
1
3JHH = 6.7 Hz, NCHCH3), 0.99 (td, 6H, JHP = 14.8 Hz, JHH = 7.5 Hz,
PCH2CH3). 13C NMR (75 MHz, CD2Cl2): [d/ppm] 157.1 (d,
2JCP = 47.4 Hz, HC@N), 50.3 (s, NCHCH3), 47.1 (s, NCHCH3), 23.2 (s,
1JPP = 279.5 Hz, PEt2), ꢀ27.2 (d, JPP = 279.5 Hz, PPh2). 1H NMR
3
(500 MHz, CD2Cl2): [d/ppm] 8.10 (d, 1H, JHP = 20.6 Hz, HC@N),
3
7.72 ꢀ7.49 (m, 10H, HPh), 4.36 (sept, 1H, JHH = 6.8 Hz, NCHCH3),
2
3
3
NCHCH3), 19.7 (s, NCHCH3), 25.3 (d, JCP = 8.8 Hz, PCH2CH3), 9,0
3.79 (sept, 1H, JHH = 6.8 Hz, NCHCH3), 2.43 (m, 4H, JHH = 7.8 Hz,
2
3
(d, JCP = 13.5 Hz, PCH2CH3). EI MS m/z (%): 216 [M+]. C11H25N2P:
PCH2CH3), 1.21 (d, 6H, JHH = 6.8 Hz, NCHCH3), 1.20 (d, 6H,
3
3
calcd. C 61.08, H 11.65, N 12.95; found C, 61.96, H 12.08, N 12.48.
3JHH = 6.8 Hz, NCHCH3), 1,19 (td, 6H, JHP = 18.3 Hz, JHH = 7.6 Hz,
Compound 1d: IR (KBr, THF):
m .
(C@N) = 1602 cmꢀ1 31P NMR
PCH2CH3). 13C NMR (126 MHz, CD2Cl2): [d/ppm] 159.0 (s, HC@N),
(121 MHz, C6D6): [d/ppm] 71.3 (s). 1H NMR (300 MHz, CD2Cl2):
135.0 (dd, JCP = 21.4 Hz, JCP = 6.5 Hz, o-CHPh), 131.1 (s large,
2
3
3
5
3
[d/ppm] 8.14 (d, 1H, JHP = 18.0 Hz, HC@N), 4.72 (m, 1H, NCHCH3),
4JCP < 1.0 Hz, JCP = 2.3 Hz, p-CHPh), 129.7 (dd, JCP = 8.2 Hz,
3
3
1
2
3.92 (d sept; 4H; JHH = 6.7 Hz; JHP = 10,4 Hz; PNCHCH3), 3.11 (m,
4JCP < 2.0 Hz, m-CHPh), 126.8 (dd, JCP = 14.8 Hz, JCP = 4.0 Hz,
3
1
2
1H, NCHCH3), 1.48 (d; 12H; JHH = 6.7 Hz; PNCHCH3), 1.44 (d;
i-PCPh), 18.1 (dd, JCP = 48.2 Hz, JCP = 10.0 Hz, PCH2CH3), 6,1 (dd,
3
3
12H; JHH = 6.7 Hz; PNCHCH3), 1.19 (m, 6H, NCHCH3), 0.97 (m,
2JCP = 5.9 Hz, JCP = 5.8 Hz, PCH2CH3). 15N NMR (51 MHz, [toluene-
6H, NCHCH3). 13C NMR (75 MHz, CD2Cl2): [d/ppm] 152.3 (d,
d8]): [d/ppm] ꢀ213.9 (d, JNP = 10.0 Hz, NiPr2), ꢀ237.7 (d,
3
2JCP = 65.4 Hz, HC@N), 46.3 (s, NCHCH3), 45.5 (d; JCP = 11,4 Hz;
1JNP = 50.5 Hz, C@N–P).
2
2
PNCHCH3), 44.5 (s, NCHCH3), 24.9 (d, JCP = 8.8 Hz, PNCHCH3),
Compound 3e: 31P NMR (202 MHz, CD2Cl2): [d/ppm] 52.4 (d,
JPP = 311.2 Hz, PiPr2), ꢀ31.5 (d, JPP = 311.2 Hz, PPh2). 1H NMR
3
24.7 (d, JCP = 5.6 Hz; PNCHCH3), 24.0 (s, NCHCH3), 20.5 (s,
NCHCH3). EI MS m/z (%): 358 [M+]. Anal. Calc. for C19H43N4P: C,
(500 MHz, CD2Cl2): [d/ppm] 7.50 (d, 1H, JHP = 18.8 Hz, HC@N),
3
3
63.65; H, 12.09; N, 15.63. Found: C, 63.87; H, 12.23; N, 15.32%.
7.80–7.20 (m, 10H, HPh), 4.51 (sept, 1H, JHH = 6.8 Hz, NCHCH3),
3
3.63 (sept, 1H, JHH = 6.8 Hz, NCHCH3), 2.89 (sept d, 2H,
2
3
4.4. Representative experimental procedure for the preparation of P–P
homoatomic phosphinophosphonium formamidines [iPr2N–C(H)@N–
PR2–PR2]Cl (3a–e)
3JHH = 7.2 Hz, JHP = 12.6 Hz, PCHCH3), 1.32 (dd, 6H, JHH = 7.2 Hz,
3JHP = 17.3 Hz, PCHCH3), 1.30 (d, 6H, JHH = 6.8 Hz, NCHCH3), 1.24
3
3
3
(dd, 6H, JHH = 7.2 Hz, JHP = 16.8 Hz, PCHCH3), 1.14 (d, 6H,
3JHH = 6.8 Hz, NCHCH3). 13C NMR (126 MHz, CD2Cl2): [d/ppm]
157.7 (s, HC@N), 136.0–126.4 (broad resonances), 50.1 (s,
A Schlenk flask was charged with iPr2N–C(H)@N–PPh2 (1a,
0.151 g, 0.480 mmol), Ph2PCl (2a, 0.106 g, 0.480 mmol) and CH2Cl2
1
NCHCH3), 47.0 (s, NCHCH3), 27.1 (d, JCP = 43.7 Hz, PCHCH3), 22.9

236
T.D. Le et al. / Journal of Organometallic Chemistry 694 (2009) 229–236
(h) D.C.R. Hockless, M.A. McDonald, M. Pabel, S.B. Wild, J. Organomet. Chem.
529 (1997) 189;
(i) F. Sh. Shagvaleev, T.V. Zykova, R.L. Tarasova, T. Sh. Sitdikova, V.V. Moskva,
Zh. Obshch. Khim. 60 (1990) 1775.
(s, NCHCH3), 19.4 (s, NCHCH3), 17.0 (s, PCHCH3), 16.7 (d,
2JCP = 3.6 Hz, PCHCH3). 15N NMR (51 MHz, [toluene-d8]): ꢀ238.1
1
3
(d, JNP = 51.8 Hz, C@N–P), ꢀ213.3 (s, JNP = 9.8 Hz, NiPr).
[5] (a) N. Burford, P. Losier, A.D. Philips, P.J. Ragogna, T.S. Cameron, Inorg. Chem.
42 (2003) 1087;
4.5. Computational details
(b) D. Gudat, A. Haghverdi, H. Hupfer, M. Nieger, Chem. Eur. J. 6 (2000) 3414;
(c) V.A. Jones, S. Sriprang, M. Thornton-Pett, T.P. Kee, J. Organomet. Chem. 567
(1998) 199;
(d) N. Burford, P. Losier, P.K. Bakshi, T.S. Cameron, J. Chem. Soc., Chem.
Commun. (1996) 307;
(e) G. Bouhadir, R.W. Reed, R. Reau, G. Bertrand, Heteroat. Chem. 32 (1995)
399;
(f) T. Kaukorat, I. Neda, R. Schmutzler, Coord. Chem. Rev. 137 (1994) 53;
(g) R.W. Reed, R. Reau, F. Dahan, G. Bertrand, Angew. Chem., Int. Ed. 32 (1993)
399;
Calculations were performed with the Gaussian 03 suite of pro-
grams [16], using the density functional method [17]. The hybrid
exchange functional B3LYP in conjunction with the 6-31G [18]
**
basis set was used. B3LYP [19] is a three parameter functional
developed by Becke which combines the Becke gradient-corrected
exchange functional and the Lee–Yang–Parr and Vosko–Wilk–Nus-
air correlation functionals with part of exact HF exchange energy.
Geometry optimisations were carried out without any symmetry
restrictions, the nature of the extrema (minimum or transition state)
was verified with analytical frequency calculations. All free Gibbs
energies have been zero-point energy (ZPE) and temperature cor-
rected using unscaled density functional frequencies. Natural Bond
Orbital (NBO) [20] analysis was used to calculate the natural
charges, determine the stabilizing interactions and the occupancy
(h) C. Payrastre, Y. Madaule, J.G. Wolf, T.C. Kim, M.-R. Mazières, R. Wolf, M.
Sanchez, Heteroat. Chem. 3 (1992) 157.
[6] (a) S. Burck, D. Gudat, K. Nättinen, M. Nieger, M. Niemeyer, D. Schmid, Eur. J.
Inorg. Chem. (2007) 5112;
(b) S. Burck, D. Gudat, F. Lissner, K. Nättinen, M. Nieger, T. Scheid, Z. Anorg,
Allg. Chem. 631 (2005) 2738;
(c) C.J. Carmalt, V. Loleli, B.G. McBurnett, A.H. Cowley, Chem. Commun. (1997)
2095;
(d) M.K. Denk, S. Gupta, R. Ramachandran, Tetrahedron Lett. 37 (1996) 9025.
[7] (a) D. Gudat, Eur. J. Inorg. Chem. (1998) 1087;
p
(b) R.J. Boyd, N. Burford, C.L.B. Macdonald, Organometallics 17 (1998) 4014.
[8] T.D. Le, M.-C. Weyland, Y. El-Harouch, D. Arquier, L. Vendier, K. Miqueu, J.-M.
Sotiropoulos, S. Bastin, A. Igau, Eur. J. Inorg. Chem. (2008) 2577.
[9] (a) Similar behaviour has been previously reported: Z. Fei, R. Scopelliti, P.J.
Dyson, Eur. J. Inorg. Chem. (2004) 530;
of the 3p P) orbital. Molecular Orbitals have been plotted with
the Molekel package [21].
Acknowledgements
(b) E.J. Nurminen, J.K. Mattinen, H. Lönnberg, J. Chem. Soc., Perkin Trans. 2
(2001) 2159. and references therein.
We thank the CNRS for financial support and the Institut du
Developpement de Ressources en Informatique Scientifique
administered by the CNRS, for the calculation facilities. T. D. L. is
grateful to the AUF for a doctoral fellowship. D. A. thanks the Euro-
pean Community for a post-doctoral fellowship (FSE).
[10] T.D. Le, Ph.D. Thesis, Université de Toulouse, 2008.
[11] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 2nd ed., VCH,
Weinheim, 1990.
[12] H.J. Reich, WINDNMR-Pro: NMR Spectrum Calculation, Version 7.1.12.
[13] CN bonds in organic formamidine derivatives have only partial double bond in
character, see: (a) T.M. Krygowski, K. Wozniak, in: S. Patai, Z. Rappoport (Eds.),
The Chemistry of Amidines and Imidates, John Wiley Sons Ltd., Chichester,
1991, p. 101;
(b) A.G. Orpen, L. Brammer, F.H. Allen, O. Kennard, D.G. Watson, R. Taylor, in:
H.-B. Bürgi, J.D. Dunitz (Eds.), Structure Correlation, Vol. 2, VCH, Weinheim,
1994, p. 770.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.10.033.
[14] B.D. Ellis, P.J. Ragogna, C.L.B. Macdonald, Inorg. Chem. 43 (2004) 7857.
[15] S.L. Buchwald, S.J. LaMaire, R.B. Nielsen, B.T. Watson, S.M. King, Tetrahedron
Lett. 28 (1987) 3895.
[16] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, T. Vreven, Jr., K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. X. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann,
O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 03, Revision D-02, Gaussian, Inc., Pittsburgh, PA, 2003.
[17] R.G. Parr, W. Yang, in: R. Breslow, J.B. Goodenough (Eds.), Functional Theory of
Atoms and Molecules, Oxford University Press, New York, 1989.
[18] P.C. Hariharan, J.A. Pople, Theor. Chim. Acta 28 (1973) 213.
[19] (a) A.D. Becke, Phys. Rev. A 38 (1988) 3098;
References
[1] W.W. Schoeller, Top. Curr. Chem. 229 (2003) 75.
[2] (a) D. Gudat, Coord. Chem. Rev. 163 (1997) 71;
(b) M. Sanchez, M.R. Mazières, L. Lamandé, R. Wolf, in: M. Regitz, O.J. Scherer
(Eds.), Multiple Bonds and Low Coordination in Phosphorus Chemistry,
Thieme, Stuttgart, 1990, p. 129;
(c) A.H. Cowley, R.A. Kemp, Chem. Rev. 85 (1985) 367.
[3] (a) J.J. Weigand, S.D. Riegel, N. Burford, A. Decken, J. Am. Chem. Soc. 129 (2007)
7969;
(b) C.A. Dyker, N. Burford, M.D. Lumsden, A. Decken, J. Am. Chem. Soc. 128
(2006) 9632.
[4] (a) J.M. Slattery, C. Fish, M. Green, T.N. Hooper, J.C. Jeffery, R.J. Kilby, J.M.
Lynam, J.E. McGrady, D.A. Pantazis, C.A. Russell, C.E. Willans, Chem. Eur. J. 13
(2007) 6967;
(b) N. Burford, C.A. Dyker, A. Decken, Angew. Chem., Int. Ed. 44 (2005) 2364;
(c) N. Burford, D.E. Herbert, P. Ragogna, R. McDonald, M.J. Ferguson, J. Am.
Chem. Soc. 126 (2004) 17067;
(d) N. Burford, P. Ragogna, R. McDonald, M.J. Ferguson, J. Am. Chem. Soc. 125
(2003) 14404;
(e) N. Burford, P. Ragogna, R. McDonald, M.J. Ferguson, Chem. Commun. (2003)
2066;
(f) N. Burford, T.S. Cameron, P.J. Ragogna, E. Ocando-Mavarez, M. Gee, R.
McDonald, R.E. Wasylishen, J. Am. Chem. Soc. 123 (2001) 7947;
(g) N. Burford, T.S. Cameron, D.J. LeBlanc, P. Losier, S. Sereda, G. Wu,
Organometallics 16 (1997) 4712;
(b) A.D. Becke, J. Chem. Phys. 98 (1993) 5648;
(c) C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[20] (a) A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899;
(b) J.P. Foster, F. Weinhold, J. Am. Chem. Soc. 102 (1980) 7211.
[21] (a) Molekel 4.3, P. Flükiger, H.P. Lüthi, S. Portmann, J. Weber, Swiss Center for
Scientific Computing, Manno, Switzerland, 2000–2002.;
(b) S. Portmann, H.P. Lüthi, Molekel: An Interactive Molecular Graphics Tool,
Chimia 54 (2000) 766.