Adaptive Behavior of a Ditopic Phosphine Ligand

Article abstract of DOI:10.1002/ejic.201900571

Synthetic, structural and computational studies have been performed to investigate ligand interchange in the fluxional chelate complex [RhCl₃{Ph₂P^ACH₂C(O^A)OEt-κ²PO^A}{Ph₂P^BCH₂C(O^B)OEt-κP}], which contains two hybrid phosphine-ester ligands, one acting as P,O chelator, the other as a P-monodentate ligand. The observed ligand exchange may occur according to two pathways which both involve four elementary movements: a) oxygen dissociation with formation of a lacunary octahedral RhCl₃P₂ intermediate; b) migration of the Cl atom trans to P^A towards the position trans to P^B; c) rotations of the phosphine moieties about the Rh–P bonds, these occurring either concomitantly with the Cl displacement or in a separate step; d) coordination of an oxygen atom of the second phosphine. The two pathways thus differ by conformational changes within two distinct steps. In each pathway the rate-limiting step is the one involving a movement of the two phosphines, which generates steric frictions between the two PPh₂ groups. The calculated theoretical energetic spans of both pathways (ΔG^≠ ≈ 17 kcal mol^–1) is close to the energy barrier obtained from a variable temperature NMR study carried out in C₂D₂Cl₄ (ΔG^≠ = 15.5 kcal mol^–1). While one of the pathways leads to an isomer with a Rh-bound ethoxy O atom, the other results in the isomer having the metal coordinated to the adjacent C=O group. Exchange between the two O atoms of the coordinated ester group occurs readily (ΔG_TS = 12.5 kcal mol^–1).

Full text of DOI:10.1002/ejic.201900571

A Journal of
Accepted Article
Title: Adaptive behaviour of a ditopic phosphine ligand
Authors: Nicolas Renard, Eric Brenner, Dominique Matt, and
Christophe Gourlaouen
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European Journal of Inorganic Chemistry
10.1002/ejic.201900571
FULL PAPER
Adaptive behaviour of a ditopic phosphine ligand
[
a]
[a]
[a]
[b]
Nicolas Renard, Eric Brenner, Dominique Matt,* , and Christophe Gourlaouen,*
2
Abstract: Synthetic, structural and computational studies have been
[RuCl
2
(LOMe- PO)(LOMe-P)(CO)] and [RuCl
2
(LOMe-PO)
2
(CO)
2
],
with only the first involving a chelating LOMe ligand.[ Some time
after this report, Braunstein, Matt and Mathey found that opening
of a P,O-metallocycle does not require the presence of an
exogeneous reagent as in the above example, but may occur in
complexes that accommodate two identical P,O ligands, a
chelating and a P-monodentate one, each alternating between
4]
performed to investigate ligand interchange in the fluxional chelate
A
A
2
A
B
B
complex
[RhCl
3
(Ph
2
P CH
2
C(O )OEt- PO )(Ph
2 2
P CH C(O )OEt-
P)], which contains two hybrid phosphine-ester ligands, one acting
as P,O chelator, the other as a P-monodentate ligand. The observed
ligand exchange may occur according to two pathways which both
involve four elementary movements : a) oxygen dissociation with
[
5]
formation of
a
lacunary octahedral RhCl
3
P
2
intermediate; b)
these binding modes in solution. Thus, the two ester groups in
A
B
2
migration of the Cl atom trans to P towards the position trans to P ;
c) rotations of the phosphine moieties about the Rh-P bonds, these
occurring either concomitantly with the Cl displacement or in a
separate step; d) coordination of an oxygen atom of the second
phosphine. The two pathways thus differ by conformational changes
within two distinct steps. In each pathway the rate limiting step is the
one involving a movement of the two phosphines, which generates
[RhCl
3
(Ph
2
PCH
2
CO
2
Et- PO)(Ph
2
PCH
2
CO
2
Et-P)]
alternately
bind the rhodium centre in a fluxional manner (Scheme 1-B). In
1979, Jeffrey and Rauchfuss carefully re-investigated the
steric frictions between the two PPh
2
groups. The calculated
≠
-1
theoretical energetic spans of both pathways (G ~ 17 kcal.mol ) is
close to the energy barrier obtained from a variable temperature
≠
-1
2 2 4
NMR study carried out in C D Cl (ΔG = 15.5 kcal mol ). While one
of the pathways leads to an isomer with a Rh-bound ethoxy O atom,
the other results in the isomer having the metal coordinated to the
adjacent C=O group. Exchange between the two O atoms of the
-
1
coordinated ester group occurs readily (ΔGTS = 12.5 kcal mol ).
Introduction
Hemilability is a well-established concept that refers to hybrid
bidentate ligands containing a soft and a hard donor atom and
which, when acting as chelates, undergo ready dissociation of
one donor atom from the metal centre, while the other remains
strongly bound.[1] Chelate opening, when occurring, may either
create a vacant site which subsequently binds an incoming
substrate[
1b, 2]
or merely induce a change of the metal
[
3]
coordination number and geometry. The history of hemilabile
ligands stretches back to the discovery by Roundhill and
Rauchfuss in 1975 that o-diphenylphosphinoanisole (LOMe) forms
with hydrated ruthenium trichloride the bis-chelate complex
2
[
RuCl
2
(LOMe- PO)
2
]. Reaction of the latter with CO (1 atm)
(
Scheme 1-A) produced two equilibrating octahedral complexes,
[
[
a]
b]
N. Renard, Dr. E. Brenner, Dr. D. Matt
Laboratoire de Chimie Inorganique Moléculaire et Catalyse
Institut de Chimie UMR 7177 CNRS-Université de Strasbourg
Scheme 1. Early literature examples of hemilabile P,O ligands[2a, 4-6]
4
6
rue Blaise Pascal
7070 Strasbourg cedex (France)
solution behaviour of the earlier reported complex [RuCl
2
(LOMe
-
2
E-mail: dmatt@unistra.fr
http://lcimc.u-strasbg.fr
Dr. C. Gourlaouen
Laboratoire de Chimie Quantique
Institut de Chimie UMR 7177 CNRS-Université de Strasbourg
 PO)(LOMe-P)(CO)], and confirmed that the presence in a
complex of two identical P,O ligands with different binding
2
modes (PO and  PO) may trigger reversible O-binding and
O-decoordination (Scheme 1-C).[ On this occasion, the
authors introduced the term hemilabile, which was attributed to
hybrid phosphine-oxygen or phosphine-amine chelating ligands
able to readily dissociate selectively their hard ligand
6]
1
6
rue Blaise Pascal
7008 Strasbourg cedex (France)
E-mail: gourlaouen@unistra.fr
http://quantique.u-strasbg.fr
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European Journal of Inorganic Chemistry
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FULL PAPER
component. Interestingly, to date, to the best of our knowledge, Results and Discussion
spectroscopic detection of P,O-hemilability has only been
achieved with complexes for which internal or external
competitor ligands (including solvents) were available for
replacement of the O donor. In other terms, spontaneous
opening of a P,O-metallocycle with a simple change of the metal
coordination number has not been reported yet. Note, however,
that as early as 1974 Shaw had postulated that
anisylphosphines P-bound to iridium(I) may intermittently bind
the metal centre in a P,O chelating fashion, this facilitating
oxidative addition steps.[7] There was, however, no firm evidence
for ether binding during such a process. Today, the term
hemilabile is commonly used for any hybrid chelator able to
dissociate one of its coordinating atoms, whatever the nature of
the binding atoms and whatever the origin of this dissociation. In
principle, it may therefore also be attributed to the so-called
hemichelating ligands in which the weakly binding moiety is an
entity that coordinates a metal centre through non-covalent
bonds.[8] The observation of a dynamic exchange between a
monodentate P,O ligand and a chelating one within the same
complex, when occurring in the absence of any external
competitor ligand, poses the question: what is the driving force
for the ring opening step to occur ? Is it a competitive process in
which the free oxygen atom approaches the metal centre before
The phosphine-ester Ph
2
PCH
2
CO
2
Et (2), first reported by
Issleib,[
13]
is a colourless liquid which is best prepared by
reduction of the corresponding phosphine oxide (1) (See
experimental section). Reaction at room temperature of 2 equiv.
of 2 with hydrated rhodium trichloride (40% Rh content) in
ethanol afforded instantly a yellow-orange precipitate (Scheme
2). 31P-NMR analysis of the crude powder revealed the presence
of two complexes, 3 and 4, formed in a 57:43 ratio, together with
free phosphine as well as trace amounts of an unidentified
product. Chromatographic purification on a silica gel column with
1% MeOH in CH
Having isolated 3, further chromatographic separation with a
10% MeOH-CH Cl mixture resulted in the isolation of a distinct
orange-brown fraction, which was composed of (major
2 2
Cl afforded 3 as a yellow product in 25 % yield.
2
2
4
compound, which had not been described in the original paper),
3, and free phosphine. This finding suggested that during
chromatography 4 had slowly converted into 3 with release of
phosphine. Some crystals of (pure) 4 could be obtained upon
displacing the coordinated one
–
this involving an
hypercoordinated intermediate –, or does it merely result from a
spontaneous metal-oxygen dissociation with formation of a
relatively stable, under-coordinated species that may then
readily reform a M-O bond ? Beyond these questions one should
also bear in mind that in non-bonding solvents and competitor-
free solutions, spontaneous hemilability has never been
observed for complexes with a single P,O chelator.
With the aim of exploring the above issues, we decided to
reinvestigate the synthesis and solution behaviour of
2
[
RhCl
3
(Ph
2
PCH
2
CO
2
Et- PO)(Ph
2
PCH
2
CO
2
Et-P)],
and
to
Scheme 2. Synthesis of complexes 3 and 4
complement it with a DFT study. In the original paper, this
complex was mistakenly formulated as having
a
fac
configuration.[ In fact, as revealed by an X-ray structure
determination (this study, vide infra), the three chlorido ligands
of the complex display a mer arrangement. The present study
has been performed with the aim of proposing a pathway to
explain the occurrence of the alternating opening-closing
mechanism taking place in solution concomitant with an
apparent cis migration of one of the chlorido ligands.
Surprisingly, despite the relevance of hemilability in transition
metal chemistry, no theoretical study on the hemilabile
behaviour of hybrid P,O ligands had been carried out before.
5]
slow crystallisation (CH
fraction, but once redissolved in CDCl
transformed into 3. Overall, the above results constitute a clear
2
Cl
2
/hexane) from the orange-brown
3
, 4
was slowly
3
indication that the mer-RhCl unit prefers to coordinate a set of
one O and two PIII atoms, rather than three PIII atoms. The solid
state structures of 3 and 4 were determined by X-ray diffraction
measurements (Figure
1
and 2), which revealed
a mer
configuration for the RhCl
3
moieties in both complexes.
Consistent with the corresponding 31P NMR spectra (see
experimental section), the two P atoms of 3 are cis-disposed,
while the three P atoms of 4 adopt a meridional arrangement.
Selected structural data for 3 and 4 are given in the legends of
Fig. 1 and Fig. 2, respectively. It is worth noting that the
Cambridge Crystallographic Data Bank reports only two mer-
This is rather surprising in view of the many interesting catalytic
applications that were found for this ligand class.[1b, 9] Finally, it is
worth reminding that the concept of ligand hemilabilty is not
restricted to that of P,O ligands. Many other hybrid ligands,
including phosphine-amines,[10] substituted cyclopentadienyl
[14]
rings[11] and N-heterocyclic carbenes functionalised with side
chains[12] have also recently found applications as hemilabile
3 3 3 2 3
[RhCl (phosphane) ] compounds, namely [RhCl (PEt Ph) ]
[15]
and [RhCl (PMe ) ].
3
3 3
Careful examination of the two latter
structures did not allow to propose an explanation for the
observed instability of 4, the three complexes showing similar
ligands in a variety of catalytic reactions.
(
slightly distorted) octahedral coordination geometries, as well
as Rh-P and Rh-Cl bond lengths all lying in the expected range.
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European Journal of Inorganic Chemistry
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concerted opening/closing of the two P,O ligands. Note that the
two interconverting species are identical, being related by a
A variable temperature solution NMR study ( H and 31P)
was carried out for bis(phosphine) complex 3 in 1,2-C Cl on a
00 MHz instrument (this solvent can be regarded as a non
coordinating solvent, as the corresponding complexation energy
1
2
D
2
4
3
was found to be endothermic by 3.6 kcal mol ). The 31P NMR
-1
- 20 °C
spectrum at 25 °C showed an ABX pattern with δ
A
= 32.2
2
5 °C
5 °C
A
A
B
B
(
J(P Rh) = 126 Hz, J(P P ) = 22 Hz) and δ
11 Hz).
B
= 24.2 (J(P Rh) =
3
1
4
5
6
8
5 °C
5 °C
5 °C
5 °C
O1
Cl1
Cl3
P1
90 °C
1
00 °C
Cl2
110 °C
P2
120 °C
ppm
Figure 1. Molecular structure of complex 3. Selected distances (Å) and angles
°): Rh–P1 2.310(3), Rh–P2 2.260(3), Rh–O(1) 2.184(7), Rh–Cl1 2.350(2),
Figure 3. Variable temperature 31P NMR spectra (C
complex 3
2
D
2
Cl
4
, 121.51 MHz) of
(
Rh–Cl2 2.342(2), Rh–Cl3 2.387(3); P1–Rh–Cl3 168.5(1), P1–Rh–O1 82.7(2),
P1–Rh–P2 105.3(1)
twofold rotation about an axis perpendicular to the trans-Cl-Rh-
Cl unit, but that the interchange dynamics involve the cis-
displacement of a chlorido ligand. Calculation (using the Eyring
equation[16]) of the free energy for the first process occurring
≠
-1
above 25 °C gave ΔG = 15.5 kcal mol at coalescence (Tc =
00 °C). This activation parameter is similar to that found for
P2
1
Cl3
P3
Cl1
Cl2
P1
Scheme 3. Ligand exchange within 3
Figure 2. Molecular structure of complex 4. Selected distances (Å) and angles
°): Rh–P1 2.426(1), Rh–P2 2.364(1), Rh–P3 2.299(1), Rh–Cl1 2.431(3), Rh–
(
Cl2 2.423(1), Rh–Cl3 2.434(2); P1–Rh–P2 163.3(1), P3–Rh–Cl2 166.7(1),
Cl1–Rh–Cl3 174.7(1), P1–Rh–P3 97.41(4), P2–Rh–P3 98.43(4)
the
RhCl
related
{PhCH P(CH
phosphine-ether
CH OEt) (15.0
complex
0.1 kcal mol ).
1
mer,cis-
-
1 [17]
[
3
2
2
2
2
}
2
]
±
Determination of the energy barrier from the H NMR spectra,
led, within experimental error, to the same value (15.4 kcal mol ;
Tc = 65 °C, based on the CH CH signals). Careful examination
2 3
of the 31P NMR spectrum revealed that by gradually lowering the
A
The signal at lower field ( P ~ 34 ppm; Figure 3) was assigned
-1
to the chelating phosphine on the basis of a HMQC(P/H)
experiment (see SI). Raising the temperature induced
a
broadening of the signals, which coalesced at T = 90 °C, before
c
temperature to -20°C, the phosphorus signal of the chelating unit
merging into a unique, broad signal at higher temperatures (its
broadness reflecting J(RhP) coupling). These observations,
which were found concentration independent, are consistent
with a fluxional behaviour of the complex as shown in Scheme 3,
A
(
P ) remained close to 33 ppm, while the broader signal of the
monodentate ligand was quite temperature sensitive and moved
towards that of P . The unexpected gradual decrease of the AB
separation may be indicative of a concomitant fast equilibrium
between 3 and another species, likely an isomer in which the
monodentate phosphine changes its conformation thereby
A
this intramolecular movement being characterised by
a
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European Journal of Inorganic Chemistry
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enabling the formation of one (or several) weak CH...Cl bond(s).
However, such an equilibrium could not be frozen. We noticed
also that similar changes occurred when recording the spectrum
pentacoordinate complex does not favour coordination of a
-1
CH
2
Cl
2
solvent molecule (G = 4.5 kcal mol ), CH
3
CN would
-1
readily bind the rhodium atom (G(CH
3
CN) = - 2.8 kcal mol ).
in CDCl
3
(see SI, spectrum at -60°C). The fact that the
The step following O1 dissociation (pathway A, Scheme 4;
see also Fig. 4 for structural details) was found to be a
displacement without bond breaking of the equatorial Cl atom
from the position trans to P(1) to that trans to P(2), this resulting
in the pentacoordinate intermediate O-I3. The higher stability of
coalescence temperature did not change when operating at a
tenfold lower concentration is in accord with an intramolecular
exchange phenomenon. In the following, the chelating
phosphine will be termed phosphine 1 (containing P1 and O1),
the other one phosphine 2 (containing P2 and O2).
-1
-1
O-I3
3.1 kcal.mol ) vs. O-I2
7.1 kcal.mol ) mainly
arises from the two phosphine ligands adopting different
orientations in both complexes. In fact O-I3 displays a set of
hydrogen bonds that was not found in the pentacoordinate
complex O-I2. Note, in the transition state of the O-I2  O-I3
How can the ligand interchange in 3 be rationalised ? To
address this issue we undertook a series of DFT calculations
using the B3LYP functional in which the solvent was treated as a
continuum.[18] In the following, the Gibbs free energy of each
calculated intermediate will be given with respect to that of the
most stable conformer of 3 (termed hereafter 3REF, see ESI), the
structure of which was found to be close to the solid state
structure of 3. Preliminary investigations showed that rotation of
the monodentate phosphine 2 about the Rh-P2 bond results in
three possible minima (O-I1; R1; R2) close to that of the
reference complex (G = 1.0 (O-I1), 1.1 (R1), and 1.5 (R2) kcal
≠
isomerisation (involving the transition state O-TS2; G = 5.6
-1
kcal mol ) the metal adopts a trigonal bipyramidal geometry (Fig.
4).
-1
mol , see SI). Analysis of the non covalent interactions revealed
the presence in each of these three conformers of Cl...H
interactions involving either methylenic (PCH ) or aromatic (PPh)
2
hydrogen atoms of the non chelating phosphine. These findings
justified consideration of possible rotamers in all subsequent
calculations and thus to introduce appropriate dispersion
corrections. O-I1 was arbitrarily chosen as the starting point of
the present study.
Subsequently, we examined whether the observed ligand
interchange may occur through an associative mechanism. We
found that in an hypothetical pathway in which the free ester
group approaches the metal atom, heptacoordination is
prevented, the resulting complex evolving towards an unstable
species having the chlorido anion located in the second
coordination sphere. An associative mechanism involving two
coordinated ester groups was thus ruled out. However, it should
be mentioned that associative substitution mechanisms at Rh(III)
have already been proposed in the literature, notably for
complexes in which the entering ligand replaces the leaving one
at the same binding site.[19]
Considering that the ligand interchange necessarily occurs
via a dissociative mechanism, we assessed the energetics of the
following three steps: a) dissociation of one of the axial chlorine
atoms (Cl1 or Cl2); b) dissociation of the equatorial Cl3 atom; 3)
cleavage of the Rh-O1 bond. The calculations showed that
dissociation of an axial Cl atom not only involves a very high-
Scheme 4. Energy profiles for the ligand interchange in 3 (Top, pathway A;
-1
bottom, pathway B). Energies in kcal mol . Complex O-I1 is a conformer of
REF
3
(not drawn) making the formation of O-I2 more easily accessible. The P1
and P2 atoms are represented in red and blue, respectively. The energy spans
-
1
for the processes are 16.2 (A) and 16.1 (B) kcal mol . For a more accurate
view of the different structures, see Fig. 4.
≠
-1
energy transition state (G = 34.1 kcal mol ), but also leads to
-1
an unstable product (G = 24.0 kcal mol ). Dissociation of the
In O-I3 the ester group of phosphine 2 is not in a position
suitable for forming a metal-oxygen bond. Ester coordination
requires a preliminary rotation[20] by ca. 110° of the entire
phosphine about the Rh–P(2) bond leading then to intermediate
≠
-1
=
19.7 kcal mol ), but
this process leads again to a rather unstable product due to the
-1
associated charge separation (G = 17.8 kcal mol ). As
expected, dissociation of O1 turned out to have by far the lowest
-1
O-I4 (G
= 6.4 kcal.mol ). This transformation actually
≠
-1
barrier (G = 10.1 kcal mol ), thereby leading to the relatively
constitutes the rate determining step of the whole ligand
exchange. The corresponding relatively high activation energy
-1
stable intermediate O-I2 (G = 7.1 kcal mol ), which can be
regarded as a "lacunary" octahedron (shortest P(1)CH...Rh
separation in OI-2: 3.41 Å) (Scheme 4). Note that while this
≠
-1
(
O-TS3, G = 14.2 kcal mol ) merely reflects steric constraints
between phenyl rings attached to P(1) and P(2) during the
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European Journal of Inorganic Chemistry
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Figure 4. Calculated transtion states and intermediates on going from 3 to O-
I4. The arrows in TS-3 show the sterically interacting phenyl groups which are
responsible for the high energetic span of pathway A.
rotation (see ESI). The corresponding non-covalent interactions
are shown in the SI. The final coordination step starting from O-
I4 occurs easily, the barrier for oxygen binding being only 5.2
kcal mol . Interestingly, this step results in binding of the OEt
group instead of the associated C=O function. The energy of the
final product, O-I5, lies only 5.0 kcal mol-1 above 3REF (not
drawn). From O-I5 a single step mechanism (Scheme 5)
-1
consisting of a rotation about the PC–CO
2
Et bond allows
≠
coordination of the carbonyl oxygen atom O(2) (via O-TS5, G
-1
=
7.5 kcal mol ). Note that examples of the less common ether-
vs. carbonyl O-coordination of esters have been documented.[21]
During our investigations, an alternative exchange
pathway (B, Scheme 4) was found that starts from O-I2bis, a
conformer of O-I2. Unlike in the above mechanism,
displacement of the equatorial Cl atom from the position trans to
P(1) to that trans to P(2) occurs here concomitantly with a
movement involving the two phosphine ligands (rotation about
-1
the Rh-P bonds), leading to O-I3bis
8.8 kcal mol ). In this
intermediate, phosphine 2 has already achieved the Rh–P
rotation that is required to go from O-I3 to O-I4. The activation
energy associated with this step is G≠ = 9.3 kcal mol-1
(
corresponding transition state: O-TS2bis, see Scheme 4). The
next two steps consist in conformational changes within
phosphine 2, this placing the carbonyl group of the ester group
in a conformation suitable for coordination. Unlike in O-I5, in that
case, the product obtained after oxygen coordination, O-I5bis,
which is structurally identical to O-I1, has a carbonyl group
bonded to the rhodium.
[22]
Overall, both pathways have energetic barriers of ca. 17
-1
kcal mol , a value which is close to that obtained experimentally
Figure 5. Calculated transtion states and intermediates on going from 3 to O-
I4bis. The arrows in O-TS2bis show the sterically interacting phenyl groups
which are responsible for the high energetic span of pathway B.
2 2 4
from a study carried out in 1,2-C D Cl . The highest energetic
transition state is found in distinct steps of each pathway. In
pathway A the critical step is the third one, which consists
essentially in a ligand rotation about the Rh–P bonds, whereas
in B the slowest step is step 2, which combines a displacement
of the chlorine and a rotation of the phosphines about the
respective Rh–P bonds. Interestingly, whatever the pathway, the
most difficult step is the one during which the phosphine
moieties rotate, thereby generating strong steric frictions
2
between the two PPh groups (see O-TS3 in Fig. 4, and O-
TS3bis in Fig. 5; see also the SI for a visualization of the
corresponding steric interactions). If such steric factors were
absent, the exchange process would probably be easier and
therefore possibly not detectable by NMR, as is often the case
for highly active complexes containing potential P,O chelators.[
It is worth mentioning here that the idea that a monodentate-
1a]
bound P,O ligand may exist in distinct rotameric forms when
2
forming a mer,cis-[RhCl
3
(P,O)( P,O] complex was previously
suggested by Cole-Hamilton et al. for a camphor-derived
phosphine,[ and each rotamer may then be the starting point
23]
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European Journal of Inorganic Chemistry
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FULL PAPER
for a new interconversion mechanism, as illustrated by paths A
and B.
essentially due to steric factors, which retard the exchange
process, rather than to the sole O-dissociation/O'-coordination
steps. This latter finding may, at least in part, explain why the
hemilabile behaviour of P,O ligands in sterically non-
encumbered mono-phosphine complexes is usually not
observable. The above investigations constitute the first study
aiming to rationalise the adaptive behaviour of a mixed P,O
ligand in a transition metal complex.
Cl
OEt
Ph2P
O
Rh
EtOOC
P
Cl
Ph2
Cl
O-TS5
12.5
Experimental Section
DG≠ = 11.4
General methods All manipulations were performed in Schlenk-type
flasks under dry nitrogen. Solvents were dried and distilled under
5.0
nitrogen before use. CDCl
3
was passed down a 5 cm thick alumina
Cl
O
column and stored under nitrogen over molecular sieves (4 Å). Routine
1
.1
Ph2P
P
OEt
1H, 13C{ H} and P{ H} NMR spectra were recorded with FT Bruker AV-
1
31
1
Rh
Cl
EtOOC
Cl
Cl
OEt
O
spectrometers (AV-400, AV-500, and AV-600). The variable temperature
studies were performed with a FT Bruker AV-300 spectrometer. 1
H
Ph2
Ph2P
Rh
Cl
EtOOC
P
Ph2
Cl
O-I5
chemical shifts are referenced to residual protiated solvents (δ = 7.26
), 13C chemical shifts are reported
ppm for CDCl ; 6.00 ppm for C Cl
relative to deuteriated solvents (δ = 77.16 ppm for CDCl ; 73.78 ppm for
Cl
) and the 31P NMR spectroscopic data are given relative to
external H PO . Chemical shifts and coupling constants are reported in
3
D
2 2
4
O-I5bis
3
C D
2 2
4
Scheme 5. Facile exchange between the two oxygen atoms of the chelating
3
4
phosphine-ester ligand
ppm and Hz, respectively. Elemental analyses were performed by the
Service de Microanalyse, Institut de Chimie, Université de Strasbourg.
PhSiH
3
was prepared according to a literature procedure.[24] Ph
PCl in Et O with EtOH in the
2
POEt
was obtained by reacting a solution of Ph
2
2
Conclusions
presence of NEt
31P NMR :
3
, followed by product distillation (Bp 179 °C, 14 mm Hg;
ydrate (40% Rh; 11-
Through this study, the configuration of complex 3, improperly
formulated when first published in 1978, has definitely been
established. The present investigation has further provided clear
evidence that the ligand interchange in the fluxional complex
1
2
2% H O) was purchased from Fluka. We noted that rhodium trichloride
provided by other suppliers gave considerably lower yields. Ethyl
(diphenylphosphoryl)acetate (1) and ethyl (diphenylphosphanyl)acetate
(2) were prepared by improving the previously reported procedures (vide
infra).
A
2
B
mer,cis-[RhCl
3
(Ph
2
P CH
2
CO
2
2 2 2
Et- PO)(Ph P CH CO Et-P)] in
CH Cl occurs via a dissociative mechanism and not through an
2
2
Ethyl (diphenylphosphoryl)acetate (1): A solution of BrCH
14.506 g, 86.86 mmol, ca. 9.6 mL) in xylene (150 mL) was heated up to
00 °C, then Ph POEt (20.000 g, 86.86 mmol, ca. 18.8 mL) was added.
2 2
CO Et
associative one, i.e. via an heptacoordinated intermediate. In
fact, two dissociative exchange mechanisms may be envisaged,
both involving a TBP transition state. In the first one, a lacunary
(
1
2
The resulting mixture was heated for 0.5 h at 140 °C. After cooling down
the solution to 50 °C, EtBr was evaporated within 10 min under vacuum.
After addition of hexane (150 mL) the solution was cooled to -20 °C,
affording 1 as colourless needles. These were filtered off, washed with
3 2
RhCl P octahedron is formed as first step, followed by non-
dissociative migration of the equatorial Cl atom between cis-
located coordination sites. The phosphine ligands need then to
rotate about the corresponding Rh–P bonds, this movement
being the rate determining step as a result of significant steric
1
hexane, and dried under vacuum. Yield: 20.010 g, 80%. H NMR (400.13
MHz, CDCl
3
):
= 7.77-7.72 (m, 4H, arom. H), 7.49-7.26 (m, 6H, arom.
3
2
A
B
H), 3.93 (q, 2H, J = 7 Hz, OCH
2
), 3.48 (d, 2H, JPH = 15 Hz, PCH
2
), 0.96
): δ
PC = 104 Hz), 132.02 (d,
para-C, JPC = 2 Hz), 130.91 (d, ortho-C, 2JPC = 10), 128.39 (d, meta-C,
3JPC = 12 Hz), 61.29 (s, CH , 1JPC = 61 Hz), 13.64 (s,
CH ), 39.02 (d, PCH
). 31P{ H} NMR (161.97 MHz, CDCl
): δ = 26.5 (s, PPh
solid state, cm-1): 1730.0 (vs,
CO), 1120.6 (vs,
interactions between a pair of P Ph/P Ph phenyl rings. This
pathway results in an intermediate in which the OEt group of
phosphine B is bound to the rhodium, which then readily
converts into an isomer having the adjacent carbonyl group
coordinated. In the second pathway, the step with chlorine
migration occurs concomitantly with a rotation of the phosphine
1
(
1
t, 3H, 3J = 7 Hz, CH
3
) ppm. 13C{ H} NMR (100.62 MHz, CDCl
3
=
65.90 (d, 2JPC = 6, C=O), 131.71 (d, ipso-C, 1
J
4
2
3
2
1
CH
3
3
2
) ppm. IR
(
PO
, tentative
moieties, this again resulting in steric frictions between the PPh
2
17 3
assignment); elemental analysis calcd (%) for C16H O P (Mr = 288.28):
groups and therefore rendering this step the rate limiting one. In
contrast to pathway A, this mechanism leads to an isomer
C, 66.66; H, 5.94; found (%): C, 66.66 ; H, 5.98.
featuring a carbonyl bonded group. The energetic spans of both
Ethyl (diphenylphosphanyl)acetate (2).
(diphenylphosphoryl)acetate (20.000 g, 69.4 mmol) and phenylsilane
37.540 g, 346.9 mmol, ca. 43 mL) in toluene (10 mL) were heated at
10 °C for 18 h. The solvent was removed in vacuo. The phosphine was
distilled under high vacuum at 185 °C, yielding a colourless oil 16.200 g,
A
solution of ethyl
exchange processes (G≠
-1
≈
17 kcal.mol ) are in good
(
1
agreement with the energy barrier obtained from a variable
temperature NMR study (ΔG≠ = 15.5 kcal mol ). Overall, the
facile spectroscopic observation of the hemilabile character of
the two P,O ligands of 3 (which constituted the first P,O-complex
for which spontaneous phosphine hemilability was detectable) is
-1
6%). 1H NMR (400.13 MHz, CDCl
8
3
) : δ = 7.50-7.45 (m, 4H, arom. H),
, 3J = 7), 3.12 (s, 2H,
.37-7.33 (m, 6H, arom. H), 4.04 (q, 2H, OCH CH
7
2
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201900571
FULL PAPER
1
2
PCH
CDCl
2
), 1.12 (t, 3H, CH
):
= 170.66 (d, 2
3
, 3J = 7 Hz) ppm. 13C{ H} NMR (100.62 MHz,
the full-matrix least-square techniques (use of F ; x, y, z, ij for C, Cl, O,
PC = 8 Hz, C=O), 137.49 (d, ipso-C, 1
Hz), 132.89 (d, ortho-C, JPC = 20 Hz), 129.22 (s, para-C, 4JPC = 0 Hz),
J
PC = 14
P, and Rh atoms; x, y, z in riding mode for H atoms); 375 variables and
3
J
2
2
2
5826 observations with I > 2.0 σ(I); calcd. w = 1/[σ (Fo ) + 39.3621P]
where P = (Fo2 + 2Fc )/3, with the resulting R = 0.1047, Rw = 0.2294,
and S
W
= 1.377, Δρ < 3.661 e Å–3. The somewhat high R value reflects
1
=
28.71 (d, meta-C, 3JPC = 7 Hz), 61.05 (s, CH
22 Hz), 14.24 (s, CH
2
CH
3
), 35.46 (d, PCH
2
, JPC
1
2
). 31P{ H} NMR (161.97 MHz, CDCl
1
- 16.5
3
3
): δ
(
2
s, PPh
2
) ppm; elemental analysis calcd (%) for C16
H
17
O
2
P
(Mr =
the use of poorly diffracting crystals. It could not be improved after a
second diffraction experiment; CCDC-1505778 contains the
supplementary crystallographic data for this report. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam. ac.uk/data_request/cif.
72.28): C, 70.58; H, 6.29; found (%): C, 70.56 ; H, 6.32.
2
mer,cis-[Ethyl(diphenylphosphinyl)acetate- PO]-
[ethyl(diphenylphosphanyl)acetate-P] rhodium trichloride (3) and
mer,mer-tris-[ethyl (diphenylphosphinyl) acetate-P] rhodium
trichloride (4): To a solution of rhodium(III) chloride hydrate (40% Rh)
X-ray Crystal Structure Determination for 4. Single crystals of 4
suitable for X-ray diffraction were obtained by slow diffusion of hexane
into a dichloromethane solution of the complex. Crystal data: Mr =
1026.05 g mol–1, monoclinic, space group P2/c, a = 22.3269(4) Å, b =
(0.150 g, 0.58 mmol) in absolute ethanol (10 mL) was added a solution of
Ph PCH CO Et (0.317 g, 1.16 mmol) in absolute ethanol (5 mL). An
2
2
2
orange-yellow powder precipitated immediately. After stirring for 1.5 h,
the product was filtered off and washed with diethyl ether (20 mL), then
dried under vacuum. The orange-yellow product was chromatographed
3
11.2858(2) Å, c = 18.6471(2) Å, = 101.817(1)°, V = 4599.1 (1) Å , Z =
4, Dx = 1.482 mg m–3, μ = 0.699 mm–1, F(000) = 2112, T = 173(2) K. The
sample (0.450 × 0.400 × 0.200 mm) was studied on a Kappa APEX II
diffractometer (graphite monochromated Mo-Kα radiation, λ = 0.71073 Å).
The data collection (2θmax = 54.9°, omega scan frames by using 0.7°
omega rotation and 30 s per frame, range hkl: h –28,28 k –14,13 l –
23,24) gave 10525 reflections. The structure was solved with SIR-97,[25]
which revealed the non-hydrogen atoms of the molecule. After
anisotropic refinement, all of the hydrogen atoms were found with a
on silica gel 60 (40-63 m) using CH
Complex 3 was obtained first as a yellow powder (R
g, 25%). Pursuing the chromatographic separation with CH
90 :10 v/v) gave an orange fraction (R = 0.5; yield: 0.020 g), which
contained 4, free phosphine and complex 3. Pure crystals of 4 were
obtained by slow cristallisation of this mixture from a CH Cl /hexane
mixture. However, NMR data of pure 4 could not be obtained. Complex
: 1H NMR (600.13 MHz, C
Cl , -10°C); δ = 7.81 (m, 4H, ortho-H of
PPh of chelating P), 7.52 (t, 2H, para-H), 7.48 (t, 2H, para-H), 7.27-7.22
2
Cl
2
/MeOH (99 :1 v/v) as eluent.
= 0.3; yield : 0.102
Cl /MeOH
f
2
2
(
f
2
2
[26]
3
D
2 2
4
Fourier difference map. The structure was refined with SHELXL97 by
2
the full-matrix least-square techniques (use of F ; x, y, z, ij for C, Cl, O,
(
(
m, 8H, meta-H), 6.99 (m, 4H, ortho-H of PPh of monodentate PO), 4.62
P, and Rh atoms; x, y, z in riding mode for H atoms); 573 variables and
q, 2H, 3J = 7 Hz, CH
of chelating PO), 4.42 (d, 2H, 2J = 12 Hz,
5826 observations with I > 2.0 σ(I); calcd. w = 1/[σ (Fo ) + (0.1220P) +
2
2
2
2
CH
3
of chelating PO), 3.91 (d, 2H, 2J = 11 Hz, PCH
of monodentate
12.0163P] where P = (Fo + 2Fc )/3, with the resulting R = 0.0733, Rw =
0.2083, and S
= 1.176, Δρ < 1.413 e Å–3. The atoms C30, O3, C32 ,
2
2
PCH
2
2
3
of monodentate PO), 1.38 (t, 3H, 3J
PO), 3.43 (q, 2H, J = 7 Hz, CH
=
chelating PO) ppm. C{ H} NMR (125.77 MHz, C
1
C=O of chelating PO), 133.68 (d, JPC = 8 Hz, ortho-C of PPh of chelating
PO), 132.76 (d, 2JPC = 8 Hz, ortho-C of PPh of monodentate PO), 131.11
and 130.85 (2s, para-C), 127.41 (d, meta-C of PPh, 3JPC = 11 Hz), 126.81
CH
2 3
W
7 Hz, CH
2
CH
3
of monodentate P), 0.67 (t, 3H, 3J = 7 Hz, CH
2
CH
, 25°C);
3
of
=
C46, O5 et O6 are disordered over two positions. CCDC-1505779
contains the supplementary crystallographic data for this report. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif.
13
1
2
D
2
Cl
4
78.96 (d, 2JPC = 7 Hz, C=O of monodentate PO), 165.61 (d, JPC = 9 Hz,
2
2
Computational details. All calculations were performed with the
(
(
d, meta-C of PPh, 3JPC = 11 Hz), 125.75 (d, ipso-C, JPC = 58 Hz), 124.94
d, ipso-C, JPC = 52 Hz), 65.82 (s, CH CH of chelating PO), 60.11 (s,
CH of monodentate PO), 42.15 (d, PCH
8 Hz), 36.23 (d, PCH of chelating PO, JPC = 35 Hz), 13.19 (s, CH
monodentate PO), 12.62 (s, CH
242.94 MHz, CDCl , -60°C); = 34.17 (P ) and 25.18 (P ) (ABX system,
ARh = 126 Hz, J B
Rh = 111 Hz) ppm. Anal. Calcd (%) for
Rh•0.25 C Cl (Mr = 753.82 + 41.47): C, 49.05; H, 4.37;
Found: C, 49.08; H, 4.37. Complex 4: P{ H} NMR (121.51 MHz, CDCl
[27]
Gaussian 09 program (version D.01), at DFT level of theory, using the
2
3
hybrid B3LYP functional.[28] Dispersion corrections were included using
Grimme’s corrections.[29] The atoms were described using the 6-31+G**
basis set[30] except for rhodium for which an SDD basis set[31] and
CH
3
2
3
2
of monodentate PO, JPC
=
2
3
of
31
1
3
of chelating PO) ppm. P{ H} NMR
pseudopotential were employed. Solvent effects (for CH
2
Cl
using
2
or, in one
the
A
B
(
2
3
mentioned
case,
CH
3
CN)
were
included
J
AB = 24 Hz, J
34Cl
P
P
[32]
polarizable continuum model.
All geometries were fully optimised and
C
32
H
3
O
4
P
2
H
2 2
4
the nature of the encountered stationary points was checked by
frequency calculations. Geometry minima were characterised by
31
1
3
):
14.55 (dt, 2P, trans positioned P's, 2JPP(cis) = 24 Hz, JPRh = 118 Hz),
.93 (dd, 1P, P trans to Cl, 2JPP = 24 Hz, JPRh = 83 Hz). Owing to its facile
a
δ
3
complete set of real frequencies and the transition state by one and only
one imaginary frequency. All the energies discussed are free enthalpies
extracted from the frequency calculation. The reference energy is that of
the most stable conformer found for 3. All the cartesian coordinates are
provided in the SI.
conversion to 3 and phosphine 2 a sample of pure 4 could not be
obtained.
X-ray Crystal Structure Determination for 3. Single crystals of 3
suitable for X-ray diffraction were obtained by slow diffusion of hexane
into a dichloromethane solution of the complex. Crystal data: Mr = 753.79
g mol–1, monoclinic, space group P21/c, a = 9.6654(7) Å, b = 21.4635(15)
Acknowledgements
3
Å, c = 16.9809(11) Å, = 108.9589(11)°, V = 3331.7(4) Å , Z = 4, Dx =
1
.503 mg m–3, μ = 0.884mm–1, F(000) = 1536, T = 173(2) K. The sample
DM is indebted to Prof P. Braunstein (University of Strasbourg)
and Prof J. Harrowfield (ISIS Strasbourg) for many fruitful
discussions. The authors further thank Mr B. Vincent (University
of Strasbourg) for his help with the NMR experiments. We also
thank the HPC pole (University of Strasbourg) for computing
facilities.
(0.300 × 0.200 × 0.150 mm) was studied on a Kappa APEX II
diffractometer (graphite monochromated Mo-Kα radiation, λ = 0.71073 Å).
The data collection (2θmax = 54.8°, omega scan frames by using 0.7°
omega rotation and 30 s per frame, range hkl: h –13,13 k –29,29 l –
2
3,23) gave 27815 reflections. The structure was solved with SIR-97,[25]
which revealed the non-hydrogen atoms of the molecule. After
anisotropic refinement, all of the hydrogen atoms were found with a
Fourier difference map. The structure was refined with SHELXL97[26] by
For internal use, please do not delete. Submitted_Manuscript
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European Journal of Inorganic Chemistry
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FULL PAPER
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10.1002/ejic.201900571
FULL PAPER
FULL PAPER
Phosphine Ligands
Nicolas Renard, Eric Brenner,
Dominique Matt*, Christophe
Gourlaouen*
Page No. – Page No.
Fight for a binding site: Phosphine hemilability in [RhCl
3
(Ph
2
PCH
2
CO
2
Et-
Adaptive behaviour of a ditopic
phosphine ligand
2

2 2 2
PO)(Ph PCH CO Et-P)] occurs "spontaneously" and is best interpreted in terms
of a dissociative, intramolecular mechanism. Steric frictions play an important role
in making the ligand interchange process spectroscopically detectable.
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