The first unpaired electron placed inside a C3-symmetry P-chirogenic cluster

Article abstract of DOI:10.1039/c0dt00542h

The Pd₃(dppm*)₃(CO)ⁿ⁺ enantiomers (n = 2 (2), 1 (3)) were prepared either from (R,R)- or (S,S)-P-chirogenic bis(phenyl-m-xylylphosphino)methane (dppm*; 1) and Pd(OAc)₂ in the presence of CF₃CO₂H, CO and water (n = 2), and then by reductive electrolysis (n = 1). The stable enantiomeric [Pd₃((S,S)- dppm*)₃(CO)]⁺ (3), is the first C ₃-symmetry radical-cation M-M bonded cluster, therefore the odd electron is delocalized onto the Pd₃ frame within this symmetry. The novel chiral species have been characterized by circular dichroism (CD) of both enantiomers of the Pd₃(dppm*)₃(CO)²⁺ clusters (2) and by EPR spectroscopy for the Pd₃((S,S)-dppm*) ₃(CO)⁺ paramagnetic compounds (3, g = 2.041). Evidence for reduced symmetry with respect to the achiral cluster was also obvious from the hyperfine splittings of the EPR signal which display three different hyperfine coupling values: 3 ×A(³¹P) = 83.9 × 10^-4 cm^-1, 3 ×A(³¹P) = 69.7 × 10^-4 cm^-1, 3 ×A(¹⁰⁵Pd) = 12.5 × 10^-4 cm^-1. In the absence of an X-ray structure for the paramagnetic clusters, DFT computations were performed to address the geometry. The optimized geometry of the Pd₃((S,S)-dppm*)₃(CO)⁺ radicals (3) exhibits three phosphorus atoms placed well above the Pd ₃ plane, while the three others are located below the trimetallic frame within C₃-symmetry due to intramolecular steric hindrance. This makes them chemically different with respect to the carbonyl group and explains the experimental EPR spectrum well. Consequently this C₃-symmetry deformation also induces a change in the shape of the SOMO (semi-occupied molecular orbital) towards this same symmetry compared to the corresponding achiral C_3v species. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00542h

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The first unpaired electron placed inside a C3-symmetry P-chirogenic cluster†
Christine Salomon,a,b Sophie Dal Molin,b Daniel Fortin,a Yves Mugnier,b Rene´ T. Boere´,c Sylvain Juge´*b and
Pierre D. Harvey*a
Received 25th May 2010, Accepted 4th June 2010
DOI: 10.1039/c0dt00542h
The Pd3(dppm*)3(CO)n+ enantiomers (n = 2 (2), 1 (3)) were prepared either from (R,R)- or
(S,S)-P-chirogenic bis(phenyl-m-xylylphosphino)methane (dppm*; 1) and Pd(OAc)2 in the presence of
CF3CO2H, CO and water (n = 2), and then by reductive electrolysis (n = 1). The stable enantiomeric
∑
[Pd3((S,S)-dppm*)3(CO)]+ (3), is the first C3-symmetry radical-cation M-M bonded cluster, therefore
the odd electron is delocalized onto the Pd3 frame within this symmetry. The novel chiral species have
been characterized by circular dichroism (CD) of both enantiomers of the Pd3(dppm*)3(CO)2+ clusters
(2) and by EPR spectroscopy for the Pd3((S,S)-dppm*)3(CO)+∑ paramagnetic compounds (3, g = 2.041).
Evidence for reduced symmetry with respect to the achiral cluster was also obvious from the hyperfine
splittings of the EPR signal which display three different hyperfine coupling values: 3 ¥ A(31P) = 83.9 ¥
10-4 cm-1, 3 ¥ A(31P) = 69.7 ¥ 10-4 cm-1, 3 ¥ A(105Pd) = 12.5 ¥ 10-4 cm-1. In the absence of an X-ray
structure for the paramagnetic clusters, DFT computations were performed to address the geometry.
∑
The optimized geometry of the Pd3((S,S)-dppm*)3(CO)+ radicals (3) exhibits three phosphorus atoms
placed well above the Pd3 plane, while the three others are located below the trimetallic frame within
C3-symmetry due to intramolecular steric hindrance. This makes them chemically different with respect
to the carbonyl group and explains the experimental EPR spectrum well. Consequently this
C3-symmetry deformation also induces a change in the shape of the SOMO (semi-occupied molecular
orbital) towards this same symmetry compared to the corresponding achiral C3v species.
the formation of such adducts leads to the loss of C3v symmetry
Introduction
as indicated for the diamagnetic dications by three pairs of
chemically inequivalent 31P nuclei (AA’BB’XX’ system) and in
the radical cations by six different phosphorus hyperfine coupling
constants.9,10
Chiral paramagnetic transition metal complexes are numerous
and have attracted attention for their potential applications
as metalloenzyme models, single-molecule magnets, catalysts
and nanoelectronic components.1–6 On the other hand, chiral
paramagnetic binuclear complexes2 and both M-M bonded
and non-M-M-bonded metal clusters3,4 are scarce and have
been generated by redox5 or electrochemical methods.6 We re-
cently reported thermal and electrochemical preparation of the
first confidently characterized paramagnetic palladium cluster,
[Pd3(dppm)3(CO)]+ (3’, Pd3 ; dppm = (Ph2P)2CH2; Scheme 1).7,8
This species exhibits an unpaired electron delocalized over the
three Pd atoms due to Pd–Pd bonding, hence retaining full C3v
symmetry. Because of its unsaturated face, this cluster undergoes
dismutation to [Pd3(dppm)3(CO)]◦ (4¢) and [Pd3(dppm)3(CO)]2+
(2¢) within an hour in solution. Its stability was improved by
∑
∑
+
Scheme 1
≡
reversibly protecting it with a capping RC CR ligand. However,
The first C3-symmetry P-chirogenic [Pd3(dppm*)3(CO)]2+ clus-
ters in which the modified dppm* ligands bear o-anisyl and phenyl
substituents at each P-centre were just reported by us.11 This
arrangement did not affect the cavity size above the Pd3 center as
the orientation of the OMe groups minimized intramolecular steric
hindrance and thus do not provide stability to the corresponding
radical cation. Conversely, changing all four phenyl groups of
dppm by m-xylyl groups would, on the basis of computer
aDe´partement de Chimie, Universite´ de Sherbrooke, Sherbrooke, J1K 2R1,
Que´bec, Canada. E-mail: Pierre.Harvey@USherbrooke.ca; Fax: 1-819-821-
8017; Tel: 1-819-821-7092
bInstitut de Chimie Mole´culaire de l’Universite´ de Bourgogne (ICMUB)
UMR CNRS 5260, Universite´ de Bourgogne, 9 avenue Alain Savary, BP
47870, 21078 Dijon Cedex, France. E-mail: Sylvain.Juge@u-bourgogne.fr;
Fax: (+33) (0)3 80 39 61 17; Tel: (+33) (0)3 80 39 61 13
cDepartment of Chemistry and Biochemistry, University of Lethbridge,
Lethbridge, T1K 3M4, Alberta, Canada. E-mail: boere@uleth.ca
† Contribution from the Universite´ de Sherbrooke, Sherbrooke, PQ,
Canada, the Institut de Chimie Mole´culaire de l’Universite´ de Bourgogne,
Universite´ de Bourgogne, France, and the University of Lethbridge,
Lethbridge, Alberta, Canada.
∑
+
modeling, completely obscure the open Pd3 face and, while
improving its lifetime, would also prevent any substrate from
∑
+
interacting with the very reactive Pd3 center. Hence, we elected
to substitute two of the four phenyl groups per ligand by m-xylyls
10068 | Dalton Trans., 2010, 39, 10068–10075
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Scheme 2
in a C2-chiral fashion, so that the cavity would still be symmetric
and hopefully be stabilized while remaining accessible.
We now wish to report the synthesis and characterization
of Pd3(dppm*)3(CO)n+ clusters (dppm* = (R,R)- and (S,S)-(m-
XylPhP)2CH2; n = 2 (2), 1 (3)). The paramagnetic cluster (2) is the
first example where the odd electron is retained in a C3-symmetry
Scheme 3
3
Table 1 CD data for 2 in THF at 298 K. dppm* = (R,R)- and (S,S)-(m-
XylPhP)2CH2 (1)
cluster environment. The recently reported [Cu3(m -OH)2]4+ clus-
ter placed inside a tritopic nonaazapyridinophane macrocycle,3
exhibits a general molecular structure of C3-symmetry with three
Cu(II) d9 centers lacking Cu–Cu bonds, so that the odd electrons
are not delocalized over the whole structure. Consequently, those
unpaired electrons are not placed in a truly C3-symmetry environ-
ment. In addition, the electrochemically generated paramagnetic
(R,R)
l max (nm)
289
4.35
4860
452
1.1
1240
529
-0.8
-900
q (mdeg)
q (deg cm2 dmol-1)
(S,S)
l max (nm)
q (mdeg)
291
-3.72
-4430
448
-0.7
-840
526
1.13
1340
∑
q (deg cm2 dmol-1)
2
species [(m3-S)Co3(CO)7(m-1,3-h -NHC(Me)S] - does not belong
to the C3-symmetry point group.6
to be associated with an electronic transition occurring within
the Pd3P6 core (ds→ds* type).18 Knowing that the chiral ligands
absorb only below 300 nm, the CD spectra indicate that the P-
chirality is readily transposed to the Pd3 core.
Results and discussion
The stereoselective synthesis of the (R,R)- and the (S,S)-ligands 1
was performed in several steps using the ephedrine methodology
and (+)- or (-)-ephedine respectively.12,13 The key step of the
synthesis is the methano bridge formation by reaction of the
carbanion derived the methyl phosphine borane 6, with the
chloro phosphine borane 5 (Scheme 2, only the (R,R)-version is
shown).12,13
2 exhibits a chemically irreversible 2-electron reduction wave at
~ -0.65V vs. SCE (wave A3; Fig. 3; [Pd3(dppm*)3(CO)(X)]+ + 2e-
→ [Pd3(dppm*)3(CO)]0 + X-). During the anodic scan, the cyclic
voltammogram (CV) exhibits two oxidation waves, A’2 (-0.56)
and A’1 (-0.24 V vs. SCE). Such behavior has been previously
observed by us for the achiral adducts of [Pd3(dppm)3(CO)(X)]+
Thus, the reaction of the (S)-chlorophenyl-m-xylylphosphine
borane 5 with the MeLi reagent affords the corresponding (R)-
methylphosphine borane 6 with inversion of configuration at the P-
centre. After deprotonation of the methylphosphine borane 6 with
n-BuLi, reaction with the (S)-chloro phosphine borane 5 afforded
the protected diphosphine diborane 7 in good yields (80%). The
enantiomeric purity was checked by chiral HPLC using racemic
samples of 7 and the meso-isomer. The desired free ligand 1 was
obtained cleanly after decomplexation of the diborane complex 7
with DABCO (Scheme 2).
(X = halide or CF3CO2 ),19 and a square scheme description
-
(Scheme 4) for the observed CV traces is provided in Fig. 3 (A’2;
∑
+
[Pd3(dppm)3(CO)]0 (Pd3 ) → [Pd3(dppm)3(CO)]+ (Pd3 ) + 1e-; A’1;
0
∑
[Pd3(dppm)3(CO)]+ (Pd3 ) → [Pd3(dppm)3(CO)]2+ (Pd32+) + 1e-).
+
The 2-electron reduction potential is associated with cluster 2,
-
but the CF3CO2 is located inside the cavity. Upon reduction, the
guest anion escapes the cavity but does not re-enter inside it fast
enough during the anodic sweep. So the species being oxidized
∑
+
0
are the corresponding neutral (Pd3 ) and radical Pd3 clusters.
Of importance to the EPR study described below, the electro-
2+
The chiral Pd3 clusters 2 were prepared using a methodology
∑
+
chemical findings indicate that the Pd3 species exhibit empty
cavities.
similar to that outlined by Lloyd and Puddephatt,14–16 by reacting
the (R,R)- or (S,S)-(m-XylPhP)2CH2 ligands 1 with Pd(OAc)2 in
the presence of CF3CO2H, CO and water (Scheme 3, only the
(R,R)-version is shown). The relative size of the cavity above the
2+
unsaturated Pd3 frame was addressed by space-filling computer
modelling for various dppm* conformations with respect to the
Pd3(CO) framework in comparison with the achiral version (2¢)17
(Fig. 1). The conclusion is that the cavity size is reduced slightly but
can accommodate the CF3CO2- ion inside it. This is corroborated
by electrochemical findings discussed below.
Evidence for chirality at the Pd3 center is provided by circular
dichroism spectroscopy (CD; Fig. 2, Table 1) where positive and
negative signals are observed in the 500-nm region; one known
Scheme 4 Various processes taking place during the CV scans.
Dalton Trans., 2010, 39, 10068–10075 | 10069
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Fig. 1 Comparison of the cavity size above the Pd3 plane for 4 possible conformations of the dppm ligands in [Pd3(dppm)3(CO)]2+ (2¢; first row) and
dppm* ligands in [Pd3(dppm*)3(CO)]2+ (2 ; second row). Up and down refer to the orientation of the PCH2P chain with respect to the Pd3 plane. The
down, down, down conformation has never been observed.
(3) (via the known dismutation reaction [Pd3(dppm*)3(CO)]0 +
∑
[Pd3(dppm*)3(CO)]2+ → 2 [Pd3(dppm*)3(CO)]+ ) as unambigu-
ously identified by the presence of the characteristic oxidation wave
at -0.21 V (A’1) and a reduction peak at -0.58 V.7,8 This radical
cation (3) is stable for several hours and has been characterized by
EPR spectroscopy (Fig. 4).
The experimental EPR spectrum shown in Fig. 4A is strongly
reminiscent of the symmetric septet previously reported for the
∑
[Pd3(dppm)3(CO)]+ cluster (3¢) (the EPR parameters of which
are: g = 2.065, A(31P) = 75.8 ¥ 10-4 cm-1, A(105Pd) = 7.6 ¥
10-4 cm-1)7 except that the five inner lines appear to be doubled.
The origin of this effect can be explained by two slightly offset
quartets, i.e. hyperfine splitting from two sets of three equivalent
31P nuclei, as indicated by the successive “stick diagrams” shown
in Fig. 4C. The observed spectrum covers a large field range (ca.
60 mT), and is strongly line-broadened at high field, leading to
Fig. 2 CD spectra of 2 in THF at 298 K. dppm* = (R,R)- (black) and
significant distortions from the ideal line intensities (Fig. 4A).
(S,S)-(m-XylPhP)2CH2, 1 (grey).
This is attributed to tumbling insufficiently fast to average
out the anisotropy of the hyperfine coupling tensor, an effect
commonly seen for large radicals covering wide field/frequency
ranges. Numerical fitting of the experimental spectrum (Fig. 4B),
after correction for phase and baseline drift, using WinSim 0.98
software20 converged to: 3 ¥ A(31P) = 83.9 ¥ 10-4 cm-1, 3 ¥ A(31P) =
69.7 ¥ 10-4 cm-1, 3 ¥ A(105Pd) = 12.5 ¥ 10-4 cm-1, LW = 5.5 ¥
10-4 cm-1. A very good fit was obtained using purely Lorentzian
lines and a line-width parameter of 7.9 ¥ 10-4 cm-1. The isotropic
g value is 2.041. We think it is significant that the average A(31P)
value of 76.8 ¥ 10-4 cm-1 in the chiral [Pd3(dppm*)3(CO)]+∑ cluster
(3) is very close to the 75.8 ¥ 10-4 cm-1 observed in the achiral
Fig. 3 CV of 2 in THF/Bu4NPF6. Starting potential = 0 V, scan rate =
∑
0.1 V s-1.
[Pd3(dppm)3(CO)]+ (3¢).7 That three hyperfine coupling constants
are significantly larger, and three smaller by a similar amount
from the values in the achiral dppm cluster (3¢), suggests that the
m-xylyl groups induce a systematic change in conformation such
The bulk electrolysis of 2 at -1 V vs. SCE halted after transfer
∑
of one equivalent of electrons leads to [Pd3(dppm*)3(CO)]+
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Fig. 5 [Pd3((S,S)-dppm*)3(CO)]+∑ radical cation (3); Top: side view of the
optimized geometry. Bottom: representation of the SOMO.
∑
Fig. 4 Isotropic CW-EPR spectrum of the putative [Pd3(dppm*)3(CO)]+
radical cation (3) (dppm* = (S,S)-(m-XylPhP)2CH2) as measured at 298 K
on extracts from the bulk electrolysis solution in THF. A) Experimental
spectrum, with (i) low-field branch displaying 3 ¥ intensity of (ii) the
high-field branch. B) Line-fitted simulation calibrated to the average
intensity. C) Interpretation of 31P hfs: (i) free electron; (ii) 1 : 3 : 3 : 1 quartet
from three equivalent P nuclei; (iii) each quartet in (ii) split further into
1 : 3 : 3 : 1 quartets by another three equivalent P nuclei (stick diagram in
absorption mode).
likely caused by the steric interactions among the xylyl groups
since we do not find such distortions when similar calculations are
performed on the achiral dppm cluster (3¢).
Conclusions
that three 31P nuclei gain in spin density, while the other three lose
spin density compared to the achiral structure. We have previously
observed in our studies of alkyne adducts of the achiral cluster (3¢)
that the sum of all six A(P) values remains remarkably stable even
when the individual hyperfine coupling values differ greatly; spin
density in these paramagnetic clusters remains strongly localized
on the Pd3P6 core of the molecule.9,10
We have prepared and characterized the first C3-symmetry para-
magnetic M-M-bonded clusters. This was achieved with the aid
of new P-chirogenic diphosphines belonging to the dppm family.
The C3-symmetry was also confirmed by analyses of the EPR
spectra wherein two sets of three chemically equivalent P atoms
are detected. This conclusion is easily corroborated from DFT
geometry optimizations where three P-atoms are found above
the Pd3 face, whereas the three others are below the face, by
reference to the apical carbonyl. Consequently, the SOMO of the
enantiomeric cluster has evident C3-symmetry, such that the odd
electrons are delocalised over Pd3 metallic frames and placed in
C3-symmetry environment. This is unprecedented.
DFT computations were performed in order to examine
∑
the SOMO of the [Pd3((S,S)-dppm*)3(CO)]+ radical cation (3)
(Fig. 5). First, its geometry was optimized, then the SOMO
computed. This MO (Fig. 5, bottom) is composed of the Pd
dx2-y2 orbitals along the coordinating lone pairs of the P atoms,
but importantly the shape of this MO exhibits distinctive C3-
symmetry. This C3-induced distortion (compared to the expected
C3v point group of the achiral cluster (3¢)) occurs because three of
the phosphorus atoms are placed above the Pd3 plane and three
others are located below (Fig. 5, top). This distortion is most
At the same time, we increased the stability of these highly re-
∑
active species relative to the achiral analogue [Pd3(dppm)3(CO)]+
9,10
≡
without using a capping RC CR ligand.
Consequently, the
cluster cation radical lasts a long time while retaining reactivity
and the C3-symmetry cavity is available for substrate interactions.
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(Rp)-(-)- or (S)-(+)-methylphenyl-m-xylylphosphine borane 6
Experimental
The (S)-chlorophosphine 5 was obtained according to a gen-
eral procedure outlined in ref. 13, and proceeds as fol-
low. In a 50 mL two-necked flask equipped with a mag-
netic stirrer, an argon inlet and a rubber septum was in-
troduced 2 mmol of (Rp)-(-)-N-methyl-[(1R,2S)(2-hydroxy-1-
phenyl)ethyl]-aminophenyl-m-xylyl phosphine borane. A solution
of HCl in toluene (0.38 M, 11.0 mL, 4.2 mmol, 2.1 equiv.) was
next added under stirring at room temperature. After 1 h, the
precipitate of ephedrine hydrochloride was filtered off with a
Millipore 4 mm filter, and the excess HCl was removed by several
vaccum/argon cycles. The toluene solution of chlorophosphine
borane 5 obtained was used without further purification. Then,
this solution was cooled at -78 ◦C and MeLi (0.87M, 5.7 mL,
5 mmol, 2.5 equiv.) was added. The reaction mixture was allowed
to warm to RT during 1 h, and hydrolysed with water. The organic
phase was removed and the aqueous layer was extracted with
CH2Cl2, and the combined extracts were dried over MgSO4, then
concentrated. The residue was purified by chromatography on
a short column of silica gel with toluene/petroleum ether: 7/3
as eluent, to give the methylphenyl-m-xylylphosphine borane 6.
It was recrystallized in a mixture of isopropyl alcohol/hexane,
affording enantiomerically pure phosphine borane as white needle
crystals. The enantiomeric purity was checked by HPLC (Chiralcel
OJ, Hexane/iPrOH 90 : 10, 1 mL min-1, l = 254 nm, tR (R) =
11.1 min, tR (S) = 14.8 min). Yield = 99%; colorless oil; [a]2D5 = -9.9
(c = 0.6, CHCl3) for e.e. > 99%; Rf 0.70 (toluene); IR (KBr, n/cm-1
3074-2852 (C–H), 2380 (B–H), 1604, 1548, 1459, 1271, 1134, 825,
735; 1H NMR (CDCl3) d (ppm) 0.56–1.48 (q, 3H, 3JBH = 96, BH3),
1.86 (d, 3H, 2JPH = 10.2, P–CH3), 2.35 (s, 3H,CH3), 7.14 (br, 1H,
H arom), 7.29 (d, 2H, J = 11.1, H arom.), 7.42–7.54 (m, 3H, H
arom), 7.65–7.81 (m, 2H, H arom.); 13C NMR (CDCl3) d (ppm)
General
All reactions were carried out under an Ar atmosphere in dried
glassware. Solvents were dried and freshly distilled under an Ar
atmosphere over sodium/benzophenone for THF, diethylether,
toluene and benzene and over CaH2 for CH2Cl2. Hexane and
isopropanol for HPLC were of chromatography grade and used
without further purification. Methyllithium (1.6M in Et2O), n-
butyllithium (1.6M in hexanes), 5-bromo-m-xylene, BH3.SMe2,
and 1,4-diazabicyclo[2.2.2]octane (DABCO) were purchased from
Aldrich, Acros or Alfa Aesar, and used as received. (+)- and (-)-
ephedrine were purchased from Aldrich and dried by azeotropic
shift of toluene using a rotary evaporator. The toluenic HCl
solution (0.2–0.4 M) was obtained by bubbling HCl gas in toluene
and titrated by acidimetry before use. The (2S, 4R, 5S)-(-)-
and its enantiomer (2R, 4S, 5R)-(+)-3,4-dimethyl-2,5-diphenyl-
1,3,2-oxazaphospholidine-2-borane, were prepared from the ap-
propriate (+)- or (-)-ephedrine, as previously described.13 The
(Rp)-(-)- and (Sp)-(+)-N-methyl-N-[(1-hydroxy-1-phenyl-prop-2-
yl]aminophenyl-m-xylylphosphine borane were prepared accord-
ing to the general procedure,13 from (-)- or (+)-3,4-dimethyl-2,5-
diphenyl-1,3,2-oxazaphospholidine-2-borane, respectively, and m-
xylyl lithium reagent. Chiral HPLC analysis were performed on
SHIMADZU 10-series apparatus, using chiral columns (Chiralcel
OD–H, Chiralpack OJ), and with hexane/propan-2-ol mixtures
as the mobile phase (Flow rate 1 mL min-1; UV detection
l = 254 nm). Thin layer chromatography (TLC) was performed
on 0.25 mm E. Merck precoated silica gel plates and exposed
by UV, potassium permanganate or iodine treatment. Flash
chromatography was performed with the indicated solvents using
silica gel 60 A, (35–70 mm; Acros) or aluminium oxide 90
standardized (Merck). All NMR spectra data were recorded on
BRUKER 300 AVANCE, 500 AVANCE DRX and 600 AVANCE
II spectrometers at ambient temperature. Data are reported as s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet, brs =
broad singlet, brd = broad doublet, dhept = doublet of heptuplet,
coupling constant(s) in Hertz. Melting points were measured on
a Kofler bench melting point apparatus and are uncorrected.
Optical rotations values were recorded at 20 ◦C on a Perkin-
Elmer 341 polarimeter, using a 10 cm quartz vessel. Infrared
spectra were recorded on a Bruker Vector 22 apparatus. Mass
and HRMS spectra were recorded on Mass, Bruker ESI micro
TOF-Q apparatus, at the Universite´ de Bourgogne (Dijon). The
major peak m/z was mentioned with the intensity as a percentage
of the base peak in brackets. Elemental analyses were measured
with a precision superior to 0.3% at the Microanalysis Laboratory
of the Universite´ de Bourgogne (EA 1108 CHNS–O FISONS
Instrument).
1
11.9 (d, JPC = 40.2, P–CH3), 21.3 (CH3), 128.8 (d, JPC = 10.5,
C arom.), 129.3 (d, JPC = 9.7, C arom.), 130.4 (d, JPC = 22.5,
Cq arom.), 131.0 (d, JPC = 2.6, C arom.), 131.7 (d, JPC = 9.7,
C arom.), 133 (d, JPC = 2.6, C arom.), 138.5 (d, JPC = 10.5, Cq
arom.); 31P NMR (CDCl3) d (ppm) +9.4 (q, 1JPB = 68.9); MS (EI)
m/z (relative intensity) 265 (M+ + Na, 100), 281 (18); HRESI-
MS (CH2Cl2) calcd for C15H20BNaP [M+Na+]: 265.1288; found:
265.1278; Anal. Calcd for C15H20BP (242.109): C 74.41, H 8.33;
found: C 74.04, H 8.34.
The (S)-(+)-methylphenyl-m-xylylphosphine borane 6 , which
was prepared from the (+)-3,4-dimethyl-2,5-diphenyl-1,3,2-
oxazaphospholidine-2-borane derived from (-)-ephedrine, ex-
hibits satisfactory analytical data in agreement with the (R)-(-)-
enantiomer.
Preparation of the (R,R)-(+)- or (S,S)-(-)-bis(phenyl-m-
xylylphosphino borane)methane 7 (dppm*)
Synthesis of the (R,R)- and (S,S)-(m-XylPhP)2CH2 ligands 1
(dppm*)
A 100 mL two-necked flask equipped with a magnetic stirrer,
an argon inlet and a rubber septum was charged with 1.08 g
of the (R)-(-)-phosphine borane 6 (4.40 mmol, 3 equiv.) in
10 mL of THF, and cooled to 0 ◦C. 2.8 mL of n-BuLi
(1.6 M in hexane, 4.40 mmol, 3 equiv.) was added dropwise.
The reaction was maintained at this temperature for 30 min,
then the cooling bath was removed and the reaction stirred
at room temperature for 90 min. After cooling at -78 ◦C, a
The (R,R)-ligand were prepared via its diborane complex, by reac-
tion of the a-carbanion derived from the (R)-(-)-methylphenyl-
m-xylylphosphine borane
6 with the (S)-chlorophenyl-m-
xylylphosphine borane 5 , both prepared according to modified
methodology starting from (+)-ephedrine,13 The (S,S)-ligand 1
was prepared by a similar method starting from (-)-ephedrine.
10072 | Dalton Trans., 2010, 39, 10068–10075
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freshly prepared toluene solution of the chlorophosphine bo-
rane 5 (1.47 mmol, 1 equiv.) from (Rp)-(-)-N-methyl-[(1R,2S)(2-
hydroxy-1-phenyl)ethyl]-aminophenyl-m-xylyl phosphine borane,
was then added dropwise under stirring to the anion solution. The
mixture was slowly warmed room temperature overnight. After
hydrolysis, the aqueous layer was extracted with 3 ¥ 30 mL of
CH2Cl2, and the combined extracts were dried over MgSO4, then
concentrated. The residue was purified by chromatography on
a short column of silica gel with toluene as eluent, to give the
diphosphine as diborane complex. It was recrystallized in CH2Cl2,
by slow diffusion of heptane, affording diastereomerically and
enantiomerically pure white needle crystals. The diastereomeric
and enantiomeric purity of the diphosphine diborane 7 was
controlled by HPLC analysis on a Chiralcel OD–H Daicel column,
eluent: hexane/i-PrOH 98 : 2, 0.25 mL min-1, l = 254 nm; (R,R),
tR = 35 min; (S,S)-enantiomer tR = 37 min; (R,S), tR = 26 min.
Yield = 70%; mp = 154 ◦C; Rf 0.1 (toluene: petroleum
ether/1 : 1); [a]2D0= + 26 (c 0.5, CHCl3) for 99% e.e.; IR (solid)
The (S,S)-(+)-1 which was obtained from the (S,S)-(-)-
bis(phenyl-m-xylylphosphino borane)methane 7 , exhibits satis-
factory analytical data in agreement with the (R,R)-(-)-
enantiomer.
Synthesis of [Pd3(dppm*)3(CO)](CF3CO2)2 2 (dppm* = (R,R)- or
(S,S)-(m-XylPhP)2CH2 1)
A 100 mL two-necked flask equipped with a magnetic stirrer, an
argon inlet and a rubber septum was charged with (R,R)- or (S,S)-
(m-XylPhP)2CH2 1 (51.5 mg, 0.116 mmol) and Pd(OAc)2 (25.4 mg,
0.113 mmol) in acetone (7 mL), trifluoroacetic acid (1.5 mL),
and H2O (1 mL). The mixture was transferred into an autoclave
and was allowed to react under CO pressure (700 psi) for 16 h.
The solvent was removed under vacuum after filtration, and the
resultant dark red oil was extracted with toluene several times.
After filtration, the solvent of the red solution was removed under
◦
1
(cm-1) 3057–2860, 2384, 1437, 1136, 1060. H NMR (CDCl3) d
vacuum. Yield 40% (53 mg), mp= 100 C (decomp.). IR (solid)
1
(ppm) 0.75–1.15 (6H, m, JBH = 116, BH3), 2.17 (6H, s, CH3),
(n/cm-1: 3065–2867, 1814, 1687, 1436, 1196, 1140, 743. 1H NMR
(CDCl3) d :7.79–7.27 (m, 6H, Ar), 7.80 (br, 16H, Ar), 7.18–6.92
(m, 18H, Ar), 6.72 (br, 4H, Ar), 6.66 (br, 2H, Ar), 6.56 (br, 2H, Ar),
4.71 (br, 2H, CH2), 4.11 (br, 2H, CH2), 2.96 (br, 2H, CH2), 2.35
(br, 6H, CH3), 2.28–2.17 (m, 9H, CH3), 2.03 (br, 6H, CH3), 1.98
(br, 6H, CH3), 1.25 (br, 6H, CH3), 0.88 (br, 3H, CH3). 13C NMR
(CDCl3) d: 137.9 (CO), 132.9 (Carom), 132,0 (Carom), 130.8 (Carom),
129.6 (Carom), 129.0 (Carom), 128.2 (Carom), 127.8 (Carom), 125.3 (Carom),
29.7 (CH2), 21.4 (CH3), 20.9 (CH3). 31P NMR (CDCl3) d: -9.1 (s).
ESI m/z (relative intensity): 1703.22 ([C88H90P6Pd3 + Cl]+, 100).
HRESI-MS (CH2Cl2) calcd for [C88H90Cl1O1P6Pd3]: 1701.2205;
found: 1701.2224, calcd for [C90H90F3O3P6Pd3]: 1779.2367; found:
1779.2424. Anal. calcd for C92H90F6O5P6Pd3: C 58.35, H 4.76; O
4.23 found: C 58.02, H 5.04, O 3.98.
2
3.14 (2H, t, JPH = 10.8, CH2), 6.83 (2H, s, Ar), 7.11 (6H, d,
3JHH = 12, Ar), 7.39–7.23 (6H, m, Ar), 7.61–7.48 (4H, m, Ar);
13C NMR (CDCl3) d (ppm): 21.2 (CH3), 23.6 (t, JPC = 26, CH2),
127.0 (d, JPC = 61, Cq), 128.6 (d, JPC = 10, Carom), 129.5 (s, Cq),
130.3 (d, JPC = 10, Carom), 131.2 (d, JPC = 2, Carom), 132.4 (d, JPC
=
10, Carom), 133.2 (d, JPC = 2, Carom), 138.1 (d, JPC = 11, Cq); 31P
NMR (CDCl3) d (ppm): +14,4 (m). HRESI-MS (CH2Cl2) calcd for
C29H36B2NaP2 [M+Na+]: 491.2371; found: 491.2362. Anal. calcd
(%) for C29H36B2P2: C 74.36, H 7.69; found: C 74.11, H 8.15.
The (S,S)-(-)-7 which was prepared from the (S)-(+)-
methylphenyl-m-xylylphosphine borane 6 derived from (-)-
ephedrine, exhibits satisfactory analytical data in agreement
with the (R,R)-(+)-enantiomer. The meso-isomer (R,S)-1 was
prepared by reaction of the a-carbanion derived from the
(R)-(-)-methylphenyl-m-xylylphosphine borane 6 with the (R)-
chlorophenyl-m-xylylphosphine borane 5 obtained using the (-)-
ephedrine pathway.
1
Electrochemical experiments
All manipulations were performed using Schlenk techniques in an
atmosphere of dry oxygen-free argon. The supporting electrolyte,
nBu4NPF6, was degassed under vacuum before use and then
dissolved to a concentration of 0.2 mol L-1. For cyclic voltam-
metry experiments, the concentration of the analyte was almost
10-3 mol L-1. Voltammetric analyses were carried out in a standard
three electrode cell using an EG&G Princeton Applied Research
(PAR) Model 263A potentiostat, interfaced to a computer running
Electrochemistry Power Suite software. The reference electrode
was a saturated calomel electrode (SCE) separated from the
solution by a sintered glass disk. The auxiliary electrode was a
platinum wire. For all voltammetric measurements, the working
electrode was a vitreous carbon disk (nominal diameter = 3 mm).
Under these conditions, when operating in THF, the formal
potential for the ferrocene (+/0) couple was found to be +0.56 V
versus the SCE. Controlled potential electrolysis was performed
withanAmel552potentiostat coupledwithanAmel721 electronic
integrator. Bulk electrolyses were performed in a cell with three
compartments separated with glass frits of medium porosity.
Carbon gauze was used as the working electrode, a platinum plate
as the counter-electrode and a saturated calomel electrode as the
reference electrode.
(R,R)-(-)- or (S,S)-(+)bis(phenyl-m-xylylphosphino)methane 1
The (R,R)-diphosphine diborane 7 (0.11 mmol) was placed in
a three-necked flask fitted with a reflux condenser, a magnetic
stirrer, and an argon inlet. A solution of DABCO (0.44 mmol) in
toluene (4 mL) was added, and the flask was purged three times
with argon. The mixture was heated to 50 ◦C for 12h and the
crude product was rapidly purified on a short column of neutral
alumina (15 cm height, 2 cm diameter) using a degassed toluene–
AcOEt (9 : 1) mixture as eluent. After removal of the solvent, the
free ligand 1 was obtained quantitatively and used without further
purification. Rf 0.87 (toluene; CCM neutral alumina); [a]2D0= - 3.0
1
(c 0.5, CHCl3); H NMR (CDCl3) d (ppm): 2.21 (12H, s, CH3),
2.70 (2H, t, J = 1.0, CH2), 6.84 (2H, s, Ar), 6.96–6.94 (4H, t, J =
3.3, Ar), 7.22–7.18 (6H, m, Ar), 7.39–7.32 (4H, m, Ar). 13C NMR
(CDCl3) d (ppm): 21.3 (s, CH3), 27.8 (t, JPC = 2.3, CH2), 77.2
(Carom), 128.2 (t, JPC = 3.3, Carom), 128.5 (s, Carom), 130.6 (t, JPC
=
10.0, Carom), 132.8 (t, JPC = 10.0, Carom), 137.7 (t, JPC = 3.8, Cq),
138.2 (t, JPC = 2.7, Cq), 139.2 (t, JPC = 3.9, Cq); 31P NMR (CDCl3)
d (ppm): -22.5 (s). HRESI-MS (CH2Cl2) calcd for C29H30NaP2
[M+Na+]: 463.1715; found: 463.1691.
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Computations. DFT
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computer, supported by the Re´seau Que´be´cois de Calcul de Haute
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basis set; Palladium was described by SBKJC VDZ29,35–39 and
Stuttgart-Koeln MCDHF RSC ECP.40–43
Circular dichroism spectra
Circular dichroism measurements were performed on a Jasco J-810
spectropolarimeter equipped with a Jasco Peltier-type thermostat.
The instrument was calibrated with an aqueous solution of (+)-
10-camphorsulfonic acid at 290.5 nm. Samples were loaded into
quartz cells with a path length of 0.1 cm. Far-UV CD spectra
were recorded at the desired temperature from 190 to 700 nm by
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simulation was obtained using Lorentzian lines with WinSim 0.98
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31P nuclei; additional weak features result from satellites from
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Acknowledgements
PDH thanks the Natural Sciences and Engineering Research
Council of Canada (NSERC), Fonds Que´be´cois de la Recherche
sur laNatureet la Technologie, and Centre d’Etudes des Mate´riaux
Optiques et Photoniques de l’Universite´ de Sherbrooke for funding
and Dr P. Lavigne from the Faculte´ de Me´decine of the Universite´
de Sherbrooke for use of his circular dichroism spectrometer.
SJ acknowledges Pr. C. Darcel for fruitful discussions on ligand
synthesis and Ms. M. J. Penouilh (Centre de Spectroscopie, Dijon),
for NMR analysis and mass spectroscopy. RTB thanks NSERC
for ongoing support.
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