Luminescent P-chirogenic copper clusters

Article abstract of DOI:10.1021/ic400498j

P-chirogenic clusters of the cubanes [Cu₄I₄L ₄] (L = chiral phosphine) were prepared from (+)- and (-)-ephedrine with L = (S)- or (R)-(R)(Ph)(i-Pr)P (with R = CH₃ (seven steps) or C₁₇H₃₅ (1

Full text of DOI:10.1021/ic400498j

Article
pubs.acs.org/IC
Luminescent P‑Chirogenic Copper Clusters
Antony Lapprand,† Mathieu Dutartre,‡ Naïma Khiri,‡ Etienne Levert,† Daniel Fortin,† Yoann Rousselin,‡
Armand Soldera,† Sylvain Juge,*,‡ and Pierre D. Harvey*,†,‡
́
†Dep
́
artement de Chimie, Universite
ulaire de l’Universite
* Supporting Information
́
de Sherbrooke, 2500 Boul. Universite,
́
Sherbrooke, Queb
́
ec, Canada J1K 2R1
de Bourgogne, Dijon, France
‡Institut de Chimie Molec
́
́
de Bourgogne (ICMUB, UMR 6302), Universite
́
S
ABSTRACT: P-chirogenic clusters of the cubanes [Cu4I4L4] (L =
chiral phosphine) were prepared from (+)- and (−)-ephedrine with
L = (S)- or (R)-(R)(Ph)(i-Pr)P (with R = CH3 (seven steps) or
C17H35 (10 steps)) with e.e. up to 96%. The X-ray structure of
[Cu4I4((R)-(CH3)(Ph)(i-Pr)P)4] confirmed the cubane structure
with average Cu···Cu and Cu···I distances of 2.954 and 2.696 Å,
respectively. The cubane structure of the corresponding
[Cu4I4((S)-(CH3)(Ph)(i-Pr)P)4] was established by the compar-
ison of the X-ray powder diffraction patterns, and the opposite
optical activity of the (S)- and (R)-ligand-containing clusters was
confirmed by circular dichroism spectroscopy. Small-angle X-ray
scattering patterns of one cluster bearing a C17H35 chain exhibit a
weak signal at 2θ ∼ 2.8° (d ∼ 31.6 Å), indicating some molecular
ordering in the liquid state. The emission spectra exhibit two
emission bands, both associated with triplet excited states. These two bands are assigned as follows: the high energy emission is
due to a halide-to-ligand charge transfer, XLCT, state mixed with LXCT (ligand-to-halide-charge-transfer). The low energy band
is assigned to a cluster-centered excited state. Both emissions are found to be thermochromic with the relative intensity changing
between 77 and 298 K for the clusters in methylcyclohexane solution. Several differences are observed in the photophysical
parameters, emission quantum yields and lifetimes for R = CH3 and C17H35. The measurements of the polarization along the
emission indicate that the emission is depolarized, consistent with an approximate tetrahedral geometry of the chromophores.
Copper(I) halides display a wide diversity of complexes with
phosphines either as mono- or multinuclear forms where the
coordination number ranges from 2 to 4.10 Moreover, the
tetra(copper(I) iodide) clusters, [Cu4I4L4] (L = organic
ligand), are particularly appealing because of their rich
photoluminescence properties, which can be exploited to
access materials exhibiting original optical properties, including
thermochromic luminescence.11 One of the key features is the
presence of two emissions arising from cluster-centered (CC*,
i.e. within the Cu4X4 skeleton) and halide-to-ligand charge
transfer (XLCT) for the low and high energy bands,
respectively. We now report the synthesis, the X-ray structure,
and the photonic properties of the first examples of tetracopper
iodide clusters bearing both enantiomers of P-chirogenic
phosphines, 4 and 6 (Chart 1). These clusters are found to
be strongly luminescent and thermochromic in solution.
INTRODUCTION
■
Copper(I) catalyzed coupling reactions for carbon−carbon and
carbon−heteroatom bond formation have greatly improved
synthesis methodologies, motivated by environmental and
economic reasons.1,2 The asymmetric versions of these
reactions based on chiral ligated copper salts or copper
hydrides have also achieved high enantioselectivities.3 The
copper transmetalation from various organometallic reagents
also contribute to the stereoselective reactions notably in the
case of functionalized substrates.4 Most chiral copper(I)
complexes are prepared from phosphorus ligands bearing the
chirality on the carbon in the backbone.5,6 The interest in
phosphine ligands bearing the chirality on the phosphorus atom
(P-chirogenic) greatly expanded recently, due to the develop-
ment of new stereoselective methods based on the borane
complex chemistry.6−8 Surprisingly, only a few chiral copper(I)
catalyzed reactions are described using a P-chirogenic
phosphorus ligand.7 Yet their use in asymmetric catalysis
presents much interest due to their structural design, which is
relatively accessible.6−8 P-chirogenic ligands are also interesting
due to the possibility that the chirality can be bared by both the
phosphorus and carbon atoms, thereby increasing the number
of stereoisomers.6,9 To the best of our knowledge, no P-
chirogenic copper complexes have been described so far.
EXPERIMENTAL SECTION
■
Materials and General Procedure. All reactions were carried out
under an argon atmosphere in dried glassware with magnetic stirring.
Solvents were dried prior to use. Tetrahydrofuran (THF) and toluene
were distilled from sodium/benzophenone and stored under argon.
Received: February 26, 2013
Published: June 24, 2013
© 2013 American Chemical Society
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(R)-Heptadecylphenyl-i-propylphosphine (6). This compound
was prepared using (−)-ephedrine, and the NMR characterization data
are identical as previously described.9f
Chart 1
(S)-Heptadecylphenyl-i-propylphosphine (6). This enantiomer
was prepared using a similar procedure as for (R)-6 but using
(+)-ephedrine. The NMR and mass spectra are identical to the (R)-6
described.9f
Preparation of the Copper Complexes 7 and 8. Tetrameric-
[iodo(methylphenyl-i-propylphosphine)copper(I)]-(RP)-7. In a 50 mL
two-necked flask equipped with a magnetic stirrer and an argon inlet,
0.17 g of (R)-phosphine 4 (1.0 mmol) and 0.19 g of CuI (1.0 mmol)
were dissolved in 5 mL of toluene. The reaction mixture was heated at
60 °C overnight (a white solid appears). The solid is filtered on
Millipore and washed with toluene to remove the excess copper. The
solvent is evaporated, and the residue is triturated in methanol to
afford 0.27 g of the desired complex as a white solid. Yield: 75%. White
solid. [α]D25 = −49.5 (c 1.0, CHCl3). Mp = 143 °C. 1H NMR (CDCl3,
500 MHz): δ 7.75 (m, 2H, CHarom), 7.31 (m, 3H, CHarom), 2.14 (m,
1H, CH, i-Pr), 1.52 (d, 3H, J = 5.2 Hz, CH3), 1.17 (dd, 3H, J = 15.6,
7.0 Hz, CH3, i-Pr), 1.08 (dd, 3H, J = 16.0, 7.0 Hz, CH3, i-Pr). 13C
NMR (CDCl3, 126 MHz): δ 133.3 (s, Carom), 133.1 (d, J = 12.9 Hz,
Carom), 128.1 (d, J = 8.9 Hz, Carom), 128.0 (d, J = 8.9 Hz, Carom), 28.4
(d, J = 18.1 Hz, CH3), 18.5 (d, J = 5.4 Hz, CH, i-Pr), 7.6 (d, J = 17 Hz,
CH3, i-Pr), 7.5 (d, J = 16.3 Hz, CH3, i-Pr). 31P NMR (CDCl3, 202
Dichloromethane (CH2Cl2) was distilled from CaH2. Hexane and
propan-2-ol for HPLC were of chromatographic grade and used
without further purification. Isopropyllithium (0.7 M in pentane),
methyllithium (1.6 M in Et2O), n-butyllithium (2.5 M in hexane), 1-
bromo-hexadecane (C16H33Br), 1,4-diazabicyclo[2.2.2]octane
(DABCO), BH3·SMe2, and K2[PtCl4] 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
on a rotary evaporator. (2R,4S,5R)-(+)-3,4-Dimethyl-2,5-diphenyl-
1,3,2-oxazaphospholidine-2-borane (1) and its enantiomer (2S, 4R,
5S)-(−)-1 were prepared from appropriate (−)- or (+)-ephedrine, as
previously described.12 (Rp)-(+)-N-Methyl-N-[(1S,2R)(1-hydroxy-2-
methyl-1-phenyl-2-propyl)]amino-i-propyl phenyl phosphine borane
(2) and (S)-(+)-methylphenyl-i-propylphosphine borane (3) were
prepared from (−)-ephedrine according to the published procedure.12
The toluene HCl solution was obtained by bubbling HCl gas, and the
resulting solution was titrated before use. Reactions were monitored
by thin-layer chromatography (TLC) using 0.25-mm E. Merck
precoated silica gel plates. Visualization was accomplished with UV
light and/or appropriate staining reagents. Flash chromatography was
performed with the indicated solvents using silica gel 60 (particle size
35−70 μm; Acros) or aluminum oxide 90 standardized (Merck).
(R)-Methylphenyl-i-propylphosphine (4; Prepared Using
(−)-Ephedrine).12a In a 50 mL two-necked flask equipped with a
magnetic stirrer and an argon inlet, 0.18 g of (R)-(−)-methylphenyl-i-
propylphosphine borane (3; 1.0 mmol) and 0.56 g of DABCO (5
mmol) were dissolved in 10 mL of toluene. The reaction mixture was
heated at 50 °C for 12 h. After cooling, the crude product was rapidly
transferred via canula into a column previously evacuated and filled
with argon containing neutral alumina. The reaction solution was
filtered using degassed 9:1 toluene/ethyl acetate as the eluent. After
removal of the solvent under a vacuum, 0.11 g of the free phosphine
MHz): δ −24.0. HRMS calcd for C40H60Cu4I4P4: [M − I]+
=
1296.79581. Found: 1296.80912. Anal. calcd for C40H60Cu4I4P4
(1426.61): C, 33.68; H, 4.24. Found: C, 33.67; H, 4.08. White
crystals suitable for X-ray diffraction were obtained by slow diffusion of
methanol in dichloromethane.
The enantiomeric complex (Sp)-7 was prepared using a similar
procedure as for (Rp)-7, but starting from the phosphine (S)-4. The
1H NMR spectrum is identical: 31P NMR (CDCl3, 202 MHz): δ
−24.8.
Tetrameric[iodoheptadecylphenyl-i-propylphosphine)copper(I)]-
(RP)-8. In a 50 mL two-necked flask equipped with a magnetic stirrer
and an argon inlet, 0.39 g of phosphine (R)-6 (1 mmol) and 0.2 g of
CuI (1 mmol) were dissolved in 5 mL of toluene. The reaction
mixture was heated at 60 °C overnight (a white solid appears). The
solid is filtered on Millipore and washed with toluene to remove the
excess copper. The solvent is evaporated, and the residue was purified
by column chromatography on silica gel using a mixture of 95:5
hexane/ethyl acetate as the eluent to obtain 0.36 g of a colorless oil.
25
1
Yield: 62%. Colorless oil. [α]D = −12.3 (c 0.8, CHCl3). H NMR
(CDCl3, 500 MHz): δ 7.75 (m, 2H, CHarom), 7.31 (m, 3H, CHarom),
2.14 (m, 1H, CH, i-Pr), 2.03−1.61 (m, 4H, CH2), 1.25−1.14 (br.s,
31H, CH3 and C17H35), 0.98 (dd, 3H, J = 15.6, 7.0 Hz, CH3, i-Pr), 0.81
(t, 3H, J = 6.7 Hz, CH3, C17H35). 13C NMR (CDCl3, 126 MHz): δ
133.9 (d, J = 12.9 Hz, Carom), 131.9 (d, J = 25.3 Hz, CHarom), 129.8
(br.s, CHarom), 128.2 (d, J = 9.0 Hz, CHarom), 32.0 (s, CH2), 31.7 (d, J
= 13.1 Hz, CH2), 29.8 (s, CH2), 29.7 (s, CH2), 29.6 (s, CH2), 29.3 (d, J
= 32.7 Hz, CH2), 28.1 (d, J = 18.7 Hz, CH), 24.8 (d, J = 3.7 Hz, CH2),
24.1 (d, J = 16.8 Hz, CH), 22.7 (s, CH2), 18.8 (d, J = 6.9 Hz, CH3),
18.7 (d, J = 4.9 Hz, CH3), 14.1 (s, CH3). 31P NMR (CDCl3, 202
1
was obtained as a colorless oil. Yield: 65%. Colorless oil. H NMR
(CDCl3, 600 MHz): δ 7.52−7.46 (m, 2H, CHarom), 7.37−7.31 (m, 3H,
CHarom), 1.78 (m, 1H, CH), 1.30 (d, 3H, J = 3.0 Hz, PCH3), 1.06 (dd,
3H, J = 13.6, 7.0 Hz, CH3, i-Pr), 0.93 (dd, 3H, J = 14.9, 7.0 Hz, CH3, i-
Pr). 31P NMR (CDCl3, 121 MHz): δ −19.2.
The enantiomeric purity of (R)-4 (>94% e.e.) was checked by
comparison with a racemic sample, by 31P NMR in the presence of
(+)-di-μ-chlorobis{2[1-(dimethylamino)ethyl]phenyl-C,N}-
dipalladium. 31P NMR (CDCl3, 121 MHz): δ +30.2 [(R)-enantiomer].
(S)-Methylphenyl-i-propylphosphine (4). This enantiomer was
prepared using a similar procedure as for (R)-4, but using
(+)-ephedrine.12a The enantiomeric excess of (S)-4 was checked by
comparison with a racemic sample, by 31P NMR in the presence of
(+)-di-μ-chlorobis{2[1-(dimethylamino)ethyl]phenyl-C,N}-
dipalladium. 31P NMR (CDCl3, 121 MHz): δ +28.3 [(S)-
enantiomer)].
(R)-(−)-Heptadecylphenyl-i-propylphosphine Borane (5).
This compound was prepared using (−)-ephedrine, and the NMR
characterization data are identical to those previously described.9f
HRMS calcd for C26H50BNaP: [M + Na]+ = 427.36401. Found:
427.36763. Anal. calcd for C26H50BP (404.47): C, 77.21; H, 12.46.
Found: C, 77.00; H, 12.60.
MHz): δ −12.7. HRMS calcd for C104H188Cu4I4P4: [M − I]+
=
2193.79741. Found: 2193.79031. Anal. calcd for C104H188Cu4I4P4
(2324.33): C, 53.74; H, 8.15. Found: C, 53.44; H, 8.41.
The enantiomeric complex (Sp)-8 was prepared using a similar
procedure as for (Rp)-8, but starting from the phosphine (S)-6. The
1H NMR spectrum is identical. 31P NMR (CDCl3, 202 MHz): δ
−12.0.
Instruments. The NMR spectra (1H, 13C, and 31P) were recorded
on Bruker 500 Avance DRX, 300 Avance, or 600 Avance II MHz or
Bruker Avance 400 MHz FT-NMR spectrometers at room temper-
1
ature using tetramethylsilane as an internal standard for H and 13C
nuclei or 85% H3PO4 as an external standard for the 31P nucleus. Data
are reported as s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet, br.s = broad singlet, integration, coupling constant(s) in Hz.
Melting points were measured on a Kofler bench melting point
apparatus and are uncorrected. Optical rotation values were
determined at 25 °C on a Perkin-Elmer 341 polarimeter, using a 10
(S)-Heptadecylphenyl-i-propylphosphine Borane (5). This
enantiomer was prepared using a similar procedure to that for (R)-
5, but using (+)-ephedrine. The NMR and mass spectra are identical
to the (R)-5 described.9f
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a
Scheme 1. Synthesis of (R)-(−)-3 and (R)-6
a
Note that the syntheses of the (S)-analogues are the same.
cm quartz vessel. Mass spectral analyses were performed on a Bruker
Daltonics microTOF-Q apparatus or on a Bruker AutoflexSpeed
MALDI-TOF. The circular dichroism spectra were measured in
dichloromethane on a JACSO J-810 spectropolarimeter. UV−visible
spectra were obtained on an HP-8453 diode array spectrophotometer
or on a Varian Cary 300 spectrophotometer. Emission and excitation
spectra were measured on a Fluorolog 2 from SPEX or a LS100 from
Photon Technology International (PTI). The emission lifetimes were
measured on either an LS100 for phosphorescence or an N2 laser
system for fluorescence both from PTI. The N2 source exhibited a
fwhm ∼ 1400 ps, and the fluorescence lifetimes were obtained from
deconvolution or distribution lifetimes analysis. The uncertainties were
∼50−100 ps. The phosphorescence lifetimes were measured using a 1
μs tungsten-flash lamp (fwhm ∼ 1 μs) and were also obtained from
deconvolution.
Procedures. The quantum yield measurements using 9,10-
diphenylanthracene as a standard were performed in methylcyclohex-
ane at 298 K.13 Three different measurements (i.e., different solutions)
were prepared for each photophysical datum (quantum yields and
lifetimes). The solutions for the samples and the reference were
prepared under inert atmosphere in a glovebox. Solutions prepared for
both the reference and the samples were adjusted to obtain
absorbencies around 0.05 where three different solutions for each
measurement were used. Each absorbance value was measured six
times for better accuracy in the measurements of the quantum yields
and was adjusted to be the same as much as possible for the standard
and the sample for a measurement.
X-Ray Equipment and Refinement. Diffraction data were
collected on a Nonius Kappa Apex II diffractometer equipped with a
nitrogen jet stream low-temperature system (Oxford Cryosystems).
The X-ray source was graphite monochromated Mo Kα radiation (=
0.71073 Å) from a sealed tube. The lattice parameters were obtained
by least-squares fit to the optimized setting angles of the entire set of
collected reflections. No significant intensity decay or temperature
drift was observed during the data collections. Data were reduced by
using HKL Denzo and Scalepack14 software without applying
absorption corrections, the missing absorption corrections were
partially compensated by the data scaling procedure in the data
reduction. The structure was solved by direct methods using the
SIR9215 program. Refinements were carried out by full-matrix least-
squares on F2, using the SHELXL-9716 program on the complete set of
reflections. Anisotropic thermal parameters were used for non-
hydrogen atoms. All H atoms, on a carbon atom or oxygen atom,
were placed at calculated positions using a riding model with C−H =
0.95 Å (aromatic), 0.99 Å (methylene), and 0.98 Å (methyl) with
Uiso(H) = 1.2Ueq(CH) or Uiso(H) = 1.5Ueq(CH3). Absolute
configuration was determined reliably from anomalous scattering,
using the Flack method.17 A table and data are available in the
Supporting Information.
approximately 0.3 × 0.3 × 0.3 mm3, placed on a needle and mounted
on a Bruker APEX DUO X-ray diffractometer. Six correlated runs with
a phi scan of 360° and exposure times of 360 s were collected with the
Cu microfocus anode (1.54184 Å) and the CCD APEX II detector at a
150 mm distance. These runs, from −12 to −72 2-theta and 6 to 36
omega, were then treated and integrated with the XRW2 Eval Bruker
software to produce the X-ray powder diffraction patterns from 2.5 to
82° 2-theta. The pattern was treated with Diffrac.Eva version 2.0 from
Bruker.
Small-Angle X-Ray Scattering. The SAXS patterns were
collected with a Bruker AXS Nanostar system equipped with a
microfocus copper anode at 45 kV/0.65 mA, MONTAL OPTICS, and
a VANTEC 2000 2D detector at a 67.75 mm distance from the
samples calibrated with a silver behenate standard. The solutions were
prepared saturated in distilled THF and placed in quartz cells for
measurements. The blanks were measured first and subtracted from
the measured data. The diffracted intensities were then integrated from
0.15 to 5.00° 2-theta and treated with the Primus GNOM 3.0 program
from ATSAS 2.3 software, to determine the particle sizes by pair
distance distribution.18
Computer Modeling. These computations based on molecular
mechanics (MMX) were performed using the commercially available
package PCModel v.7.0 from Serena Software. No bond or angle was
fixed.
RESULTS AND DISCUSSION
Ligand Synthesis. The stereoselective synthesis of the (R)-
and the (S)-phosphine ligands 4 and 6 was performed in several
■
Scheme 2
steps using (−)- and (+)-ephedrine,9f respectively, where the
precursors 1−3 are already known (Scheme 1).12a (R)-(−)- or
(S)-(−)-methylphenyl-i-propylphosphine borane (3) reacts
with n-BuLi to form the α-carbanion LiCH2P(BH3)(Ph)(i-
Pr), which reacts with the hexadecyl bromide C16H33Br to
afford the heptadecyl derivative 5 in 56% yield. Deprotection of
3 and 5 with DABCO leads to the corresponding free
phosphines 4 and 6 in good to excellent yields (98%) and
with e.e. up to 96%.
Synthesis and Characterization of the Copper
Complexes. The complexes were prepared from both
enantiomers of phosphines 4 and 6 by reacting them directly
with CuI in toluene at 60 °C (Scheme 2). For the sake of
X-Ray Powder Diffraction Patterns. The powder was mixed
with Paratone Oil to obtain a paste like sample. This sample was cut to
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Table 1. Selected Bond Lengths (Å) for Compound (Rp)-7
Cu(1)−Cu(2)
Cu(1)−Cu(3)
Cu(2)−Cu(3)
Cu(2)−Cu(4)
Cu(3)−Cu(4)
Cu(1)−Cu(4)
2.9573(13)
2.8797(13)
2.8808(14)
2.9522(13)
2.9791(13)
3.0736(13)
Cu(1)−I(1)
Cu(1)−I(2)
Cu(1)−I(4)
Cu(2)−I(1)
Cu(2)−I(2)
Cu(2)−I(3)
Cu(3)−I(2)
Cu(3)−I(3)
Cu(3)−I(4)
Cu(4)−I(1)
Cu(4)−I(3)
Cu(4)−I(4)
2.7052(10)
2.6324(10)
2.7439(11)
2.6085(10)
2.7976(11)
2.7014(10)
2.6547(11)
2.6923(10)
2.7445(11)
2.7253(11)
2.7103(10)
2.6375(11)
Figure 1. Top: ORTEP19 view of compound (Rp)-7. Hydrogen atoms
have been omitted for clarity. Bottom: Highlighting of triakis
tetrahedron in compound (Rp)-7. Nonmetallic atoms have been
omitted for clarity. The thermal ellipsoids at the 50% probability level
for both representations.
Figure 3. CD spectra of (Rp)-7, (Sp)-7, (Rp)-8, and (Sp)-8 in CH2Cl2
at 298 K. The θ [deg cm2 dmol−1] data are provided in Table S5 for
convenience.
independently of the absolute configuration of the phosphorus
centers in the complexes.
Clusters (Sp)-8 or (Rp)-8 are oily materials at room
temperature. So no crystal was obtained for these species.
Conversely, colorless prismatic chiral crystals were obtained by
slow evaporation of (Rp)-7 in a dichloromethane/methanol
mixture. Single crystal X-ray analysis showed that this
compound contains a triakis tetrahedron of copper atoms,
with facially capping iodides (Figure 1), and chirality is
apparent from the presence of a pseudo-C3 axis formed by three
of the phosphine ligands (Figure 2).
Bond lengths of the “Cu4I4” skeleton are presented in Table
1 and are found to be normal in comparison with distances
reported for other cubane complexes containing the Cu4I4P4
frame. More specifically, the Cu···Cu and Cu···I distances range
from 2.8797(13) to 3.0736(13) Å, averaging 2.954 Å, and from
2.6085(10) to 2.7976(11) Å, averaging 2.696 Å, respectively.
Although these average distances fall substantially short
comparatively to those observed for Cu4I4(t-Bu3P)4 (Cu···Cu:
3.560 and Cu···I: 2.731 Å),20 Cu4I4(P(C7H8)3)4 (Cu···Cu:
3.247 and Cu···I: 2.713 Å),21 and Cu4I4(P(NH2)3)4 (Cu···Cu:
3.386 and Cu···I: 2.726 Å),22 these distances compare more
Figure 2. Space filling model of (Rp)-7 showing the pseudo (or local)
C3 axis (I = purple; Cu = dark orange; P = light orange). Note that the
fourth phosphine ligand behind the cluster is not considered.
simplicity, the nomenclature of the copper complexes 7 (or 8)
prepared from the phosphine (S)- or (R)-4 (or 6) are called
(Sp)-7 or (Rp)-7 (or (Sp)-8 or (Rp)-8), respectively,
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Figure 4. Absorption (black), excitation (blue), and emission (red) spectra of (Rp)-7 (top) and (Sp)-7 (bottom) at 298 K (left) and 77 K (right) in
the solid state. The uncertainty on the maxima is 5 nm.
Figure 5. Absorption (black), excitation (blue), and emission (red) spectra of (Rp)-7 (top) and (Sp)-7 (bottom) at 298 K (left) and 77 K (right) in
methylcyclohexane. The uncertainty on the maxima is 5 nm.
favorably to those reported for Cu4I4(P(C6H5)2CH3)4
(Cu···Cu: 2.930 and Cu···I: 2.705 Å),23 Cu4I4(P(C6H5)3)4
(Cu···Cu: 2.968 and Cu···I: 2.693 Å),24 Cu4I4(P(C6H5CH3)3)4
(Cu···Cu: 3.023 and Cu···I: 2.680 Å),25 Cu4I4(P(C6H5)2Fc)4
(Fc = ferrocenyl, Cu···Cu: 3.071, and Cu···I: 2.692 Å),26
Cu4I4(P(C6H5)2C3H7)4 (Cu···Cu: 3.143 and Cu···I: 2.690 Å),27
and Cu4I4(P(C6H11)3)4 (Cu···Cu: 3.142 and Cu···I: 2.692 Å).10i
Three cubanes exhibiting the smallest Cu···Cu and Cu···I
distances are those for the recently reported structures of
Cu4I4(P(C6H5)3)4 (Cu···Cu: 2.873 and Cu···I: 2.681 Å)11e and
Cu4I4(P(C6H5)3(pyrimidine-2,4(1H,3H)-dione-P))4 (Cu···Cu:
2.885 and Cu···I: 2.677 Å).10 X-ray powder diffraction
spectroscopy was used to confirm that both (Rp)-7 and (Sp)-
7 isomers are the enantiomers of each other. Indeed, the
calculated pattern for (Rp)-7 from the X-ray structure was
almost identical to that measured for (Sp)-7 (Figure S17).
Because of the presence of long chains on (Sp)-8 and (Rp)-8
and their very viscous behavior, one of them, (Sp)-8, was further
investigated under a polarized microscope searching for a
mesophase. No birefringence activity was observed with
polarized light under a microscope all the way down to −10
°C, even after several hours. Small-angle X-ray scattering
patterns were also measured (Figure S18). A weak and broad
scattering was observed at ∼2.8° (2θ) in silicone matrices,
indicating the presence of organization. Assuming that most of
the scattered X-rays arise from the heavy elements (i.e., Cu4I4),
the intercluster distance, d, is found to be 31.6 Å. Assuming full
side-by-side C17H35···C17H35 contacts, computer modeling
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Figure 6. Absorption (black), excitation (blue), and emission (red) spectra of (Rp)-8 (top) and (Sp)-8 (bottom) at 298 (left) and 77 K (right) in
methylcyclohexane. The uncertainty on the maxima is 5 nm.
the (R)- and (S)-enantiomers are essentially identical as
expected, and the band maxima in the absorption, excitation
(when appropriate), emission spectra, and Stokes shifts
(Δ(cm−1)) are placed in Table 2.
Table 2. Spectroscopic Data for the Chiral Clusters in the
Solid State or in Methylcyclohexane
298 K
77 K
λabs
λemi
Δ
λ
λemi
Δ
a
abs b
The absorption spectra exhibit features at wavelengths
shorter than 400 nm, consistent with the off white coloration
of the solid (7) and liquid (8), and resemble those reported by
Perruchas and co-workers.11c The emission spectra, for most
cases, consist of the characteristic double emission, i.e. two
bands, one in the range of ∼400−500 and the other in the
∼500−700 nm window known for Cu4I4P4 cubanes11 and other
Cu4I4L4-containing materials (L = pyridine28 or mono- and
dithioethers,29 for examples). These two emissions are
characterized by Stokes shifts of 4000−9000 and of 12 800−
17 300 cm−1 for the high- and low-energy emissions,
respectively. The large Stokes shifts and the microsecond
time scales of their lifetimes (placed below) indicate
phosphorescence. The relative intensity of the two bands
changes drastically for these species in solutions going from 298
down to 77 K, which is reminiscent of the known
thermochromic behavior of several Cu4I4P4 cubanes.11b−f The
assignment of the two excited states involved in these two
emissions was previously investigated by different research
groups. Indeed, Ford and collaborators addressed this problem
in the 1990s using ab initio calculations,30 and more recently
using density functional theory (DFT).31 The main conclusion
is that these two excited states are either cluster centered (CC*,
i.e within the Cu4X4 skeleton) or halide-to-ligand charge
transfer (XLCT). The recent calculations reported by Vega and
Saillard also agreed with these conclusions.10e More recently,
Perruchas and co-workers also used DFT to address the nature
of low and high energy emission bands and came to a similar
conclusion with a slight modification. The low energy emission
(T1→S0) is assigned to a combination of a halide-to-copper
charge transfer transition (XMCT) and a copper-centered
transition (3d→4s, 4p, i.e., CC*), which is essentially
independent of the nature of the ligand. The high energy
a
(nm) (nm) (cm−1
)
(nm)
(nm) (cm−1
)
(Rp)-7 solid state
(Sp)-7 solid state
(Rp)-7 solution
336
336
298
590
588
372
616
370
615
369
594
357
592
12800
315
315
314
580
14500
c
12800
6700
580
14500
8600
430
573
432
572
410
554
410
550
17300
6500
14400
8600
(Sp)-7 solution
(Rp)-8 solution
(Sp)-8 solution
298
315
307
299
17300
4000
14500
8200
b
321
14300
5300
14500
9000
b
300
16400
15300
a
Δ(cm−1) = (107/λabs (nm)) − (107/λemi (nm)); i.e. Stokes shift in
b
cm−1, a linear scale of energy. The position of the maximum is
c
extracted from the excitation spectra. A weak signal is also noted at
∼425 nm.
indicates that the average (Sp)-8 cluster/(Sp)-8 cluster distance
is ∼29.2 Å (Figure S19). In conclusion, the clusters 8 in the
pure state exhibit some structural organization that greatly
depends on the presence of alkyl chains. Because of the
tetrahedral geometry of the cluster, the formation of a gel is not
excluded.
Finally, the circular dichroism (CD) spectra of (Rp)-7, (Sp)-7,
(Rp)-8, and (Sp)-8 in CD2Cl2 exhibit the expected mirror image
relationship of each other (Figure 3), confirming their relative
chirality. Their polarized activities occur in the 220−350 nm
range in line with the position of their absorption spectra below
(Figures 4−6).
Electronic and Luminescence Spectra. These absorption
and emission spectra of the strongly emissive (Rp)-7, (Sp)-7,
(Rp)-8, and (Sp)-8 clusters in the solid state and in solution
(methylcyclohexane as noncoordinating solvent) are shown in
Figures 4−6, and a picture of a glowing solid sample is placed
in the Supporting Information (Figure S20). The spectra for
3
emission (T2→S0) corresponds to a XLCT/3MLCT mixed
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Figure 7. Top and side views of a computer model (vide infra) of (Sp)-8 stressing on the possible C17H35 chain autoassembly.
One very interesting observation is the striking difference in
relative intensity ratio of the high energy band/low energy band
between (Rp)-7 and (Sp)-7 vs (Rp)-8 and (Sp)-8 (Figures 5 and
6). The clusters exhibit essentially opposite trends in these
relative emission intensities at both temperatures. The role of
the long chains in the temperature dependence of the relative
intensity of these two emissions remains unknown, although
computer modeling suggests that the C17H35 chains can
autoassemble by pairs (Figure 7). Finally, weak and structured
emissions are depicted in the vicinity of 350 nm. This feature is
believed to be associated with an intraligand (C6H5P) ππ*
emission, likely fluorescence due to the spectral proximity to
the absorption. This assignment is supported by the detection
of a nanosecond lifetime decay at this wavelength (see below).
The quantum yield data, Φ, at 298 K are placed in Table 3.
The emission lifetimes, τe, for these two emissions range
from 5 to 200 μs (Table 4) and compare favorably to that
reported for other Cu4I4L4-containing materials (for example L
= pyridine28 or mono- and dithioether),29,32 including those
which contain phosphine ligands (details are provided in Table
S6 for convenience, SI). As previously stated above, the
microsecond time scale also indicates that these emissions arise
from triplet manifolds so that these are better referred to as
phosphorescence (i.e., their lifetimes are noted as τP). Several
other features are noted. First, the τP values are expectedly
Table 3. Fluorescence and Phosphorescence Quantum
Yields Measured Methylcyclohexane at 298 K
a
compound
ΦF (× 10−2
)
ΦP (× 10−2
)
(Rp)-7
(Sp)-7
(Rp)-8
(Sp)-8
28.6
28.6
8.9
8.7
8.7
6.3
6.3
8.9
a
The uncertainties are 10%.
transition.11c On the basis of the similitude in Cu···Cu (2.954)
and Cu···I (2.696 Å) distances for (Rp)-7 with that of
Cu4I4(P(C6H5)3)4 used by Perruchas and co-workers
(2.9012(6) and 2.6838(5) Å),11c the nature of these emissions
are reasonably assumed to be the same: 3XLCT/3MLCT (T2→
3
S0) and CC (T1→S0).
Interestingly, the low energy emission is profoundly more
intense for (Rp)-7 and (Sp)-7 in the solid state at both 298 and
77 K (Figure 4). However, the relative intensity ratio of the
high energy band/low energy band becomes larger when (Rp)-
7 and (Sp)-7 are in solution (2-methylcyclohexane; Figure 5),
again at both temperatures, in comparison with the solid state
spectra. This behavior strongly suggests that the rigidity of the
medium plays a role in this property, where the solid state is
more rigid than both fluid and frozen solutions.
a
Table 4. Fluorescence and Phosphorescence Lifetimes As Solid or in Methylcyclohexane (MCH)
298 K
77 K
compd.
τF (ns)
τP (μs)
τF (ns)
τP (μs)
(Rp)-7 solid state
(Sp)-7 solid state
(Rp)-7 in MCH
5.2 0.1 (590)
5.2 0.1 (590)
9.2 0.1 (375)
13.0 0.1 (620)
9.2 0.1 (375)
12.7 0.1 (620)
13.7 0.1 (360)
12.8 0.1 (595)
13.4 0.01 (360)
12.9 0.1 (595)
13.1 0.1 (585)
13.1 0.1 (575)
119.0 0.4 (430)
107.0 0.1 (585)
119.0 0.5 (430)
106.8 0.2 (585)
193.0 0.1 (415)
39.8 0.3 (560)
193.0 0.8 (415)
39.4 0.1 (560)
4.5 0.2 (350)
4.6 0.4 (350)
4.7 0.4 (350)
4.9 0.6 (350)
15.5 0.6 (350)
15.3 0.2 (350)
21.9 0.2 (350)
21.5 0.4 (350)
(Sp)-7 in MCH
(Rp)-8 in MCH
(Sp)-8 in MCH
a
The values in parentheses are the emission maxima also where the measurements were performed.
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Figure 8. Polarization ratios of the excitation (left; λem = 420 nm) and emission (right; λexc = 330 nm) spectra of (Rp)-8 and (Sp)-8 in
methylcyclohexane at 77 K. The N values approaching and below 300 and exceeding 350 nm are uncertain due to weak instrument response and a
weak signal, respectively. The uncertainties on the measurements are 0.2.
3CC emissive state is Φe = 3T2 and Φs = Rx,y,z (R is for rotation
identical (within the uncertainties) for both enantiomers.
Second, τP increases upon cooling the temperature for all cases,
which is consistent with the increase in medium rigidity. Third,
the τP values increase upon going from the solid state to the
or spin; i.e. t1) whereas the diamagnetic ground state is Φe′=
1A1 but also Φs′ = a1. Assuming that the cluster sits on the
lowest energy vibrational level v = 0, then Φv and Φv′ are totally
solution, suggesting that intermolecular proximity induces more
nonradiative excited state deactivation processes. This could
also explain the very low intensity or absence of the high energy
band. Fourth, the weak structured emission appearing in the
vicinity of 350 nm exhibits decays on the nanosecond time scale
(4−5 ns, 298 K; 15−22 ns, 77 K), which is fully consistent with
the intraligand (C6H5P-) ππ* fluorescence assignment. Finally,
the τP values of the high and low energy emissions are similar
(in solution) for all cases except that for (Rp)-8 and (Sp)-8 at 77
K, for which these lifetimes are significantly shorter for the low
energy band in comparison with the high one, which is
consistent with the much weaker intensity.
The polarization ratios, N, of both the emission and
excitation spectra were measured using the photoselection
method.33 The N values are obtained from N = (I∥ × I⊥)∥/(I∥ ×
I⊥)⊥, where I∥ and I⊥ are the emission intensity when the
polarizers are placed parallel and perpendicular to each other
with the excitation polarizer placed either parallel or
perpendicular to the floor for (I∥ × I⊥)∥ and (I∥ × I⊥)⊥,
respectively. With molecules rigidly held in frozen solutions, the
relative orientations of the transition moments are randomly
distributed, and the N values may have maximum and
minimum theoretical values of 3.0 and 0.5, respectively. The
former and latter values indicate that the emitted light is
respectively polarized parallel and perpendicular with respect to
the transition moment of the absorption. For the case where N
= 1, the emitted light is polarized along the three Cartesian
axes.
symmetric (i.e., a1). With μe = Tx,y,z (T is for transition moment
or translation; here t2), the product of the seven terms is, in the
same order as above, P = T2 × t1 × a1 × t2 × A1 × a1 × a1. The
product T2 × t1(Rx,y,z) gives a1 + e + t1 + t2, and then the
product (a1 + e + t1 + t2) × t2 includes the operation t2 × t2 =
(t2)2, which gives an answer that contains the totally symmetric
term a1. Note that the product of any term by a1 gives this same
term. The conclusion of this analysis is that this spin forbidden
radiative process 3T2 → 1A1 occurs with a low probability with a
transition moment polarized along the x, y, and z axes (i.e., t2).
Consequently, N is bound by symmetry to be 1 for the pure
electronic transitions. This theoretical prediction is indeed
experimentally verified for the (Rp)-8 and (Sp)-8. Noteworthy is
that both 3XLCT/LXCT (T2→S0) and 3CC (T1→S0)
emissions exhibit N = 1 (Figure 8).
Final Comments. Copper complexes of the general formula
[Cu4I4L4] (L = P-chirogenic ligand) have been synthesized
from the very convenient (−)- and (+)-ephedrine methodology
providing species with e.e. up to 96%. The clusters were fully
characterized, including X-ray structure, X-ray diffraction
patterns, and CD spectra. For further applications, it is also
reasonable to suspect that the chiral induction for reactions
utilizing the (R)-6 and (S)-6 ligands (with C17H35) will be
different with respect to those using (R)-4 and (S)-4 (with
CH3) based on a simple steric argument. Moreover, the
electronic spectral signatures and photophysical properties
arising from the Cu4I4P4 core clearly demonstrate the close
similarity with those for the few achiral analogues reported so
far. One particularity is the presence of the long C17H35 chains
that influences the photophysical parameters (i.e., decrease of
ΦP and τP upon cooling) with respect to those for CH3.
Autoassembling of the C17H35 long chains (i.e., pairing) may
most likely occur at lower temperatures. This would be in line
with the SAXS findings. If verified, then the design of nematic
liquid crystals may also be possible by finding the appropriate
substituents to be placed on the phosphorus atom, but with the
particularity that they contain a thermochromic luminescent
core. Moreover, preliminary results on the catalytic addition of
In the case of emissions arising from triplet states, the
probability for the radiative transitions, P, is given by P = ∫
ΦeΦsΦv|μe|Φe′Φs′Φv′ dτ where Φe and Φe′, Φs and Φs′, and Φv
and Φv′ are respectively the electronic, spin, and vibrational
wave functions of the starting and arriving states, and μe is the
transition moment operator. This expression can be treated
using the group theory where the product of all these terms
must give an answer that contains the totally symmetric term.
Assuming that an approximate Td symmetry can be applied
(i.e., T1→S0; a1 → t2),11c then the symmetry of the low energy
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Asymmetric Catalysis; Springer: New York, 2004. (c) Shimidzu, H.;
Nagasaki, I.; Saito, T. Tetrahedron 2005, 61, 5405−5432. (d) Arrayas,
R. G.; Adrio, J.; Carretero, J. C. Angew. Chem., Int. Ed. 2006, 45, 7674−
7715. (e) Hargaden, G. C.; Guiry, P. J. Chem. Rev. 2009, 109, 2505−
2550.
phenylmagnesium bromide on cyclohex-2-enone give the
racemic adduct phenyl-3-cyclohexanone in 81% conversion
after one hour using both (Rp)-7 and (Rp)-8 as a catalyst (SI).
Despite these modest preliminary results, further works are in
progress to improve the enantioselectivity of this reaction using
functional P-chirogenic phosphines.
(6) (a) Borner, A. Phosphorous Ligands in Asymmetric Catalysis;
Wiley-VCH: Weinheim, Germany, 2008; Vol. 3. (b) Phosphorus (III)
Ligands in Homogeneous Catalysis: Design and Synthesis; Kamer, P. C. J.,
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic structure of (Rp)-7 in CIF format. H,
■
S
1
13C, and 31P NMR spectra of (R)- and (S)-4, (R)-5, (R)-6, and
cluster 8 prepared from (R)- and (S)-6. Photography of (Rp)-7
under natural light (left) and UV lamp 254 nm (right). CD data
for chiral compounds 7 and 8. X-ray powder spectra for (Sp)-7
and calculated for (Rp)-7. Log plot of the SAXS traces of cluster
(Sp)-8 in silicone compared to silicone. Molecular modeling of
two (Sp)-8 clusters interacting via C17H35···C17H35 contacts.
General procedure for the conjugate addition of phenyl-
magnesium bromide with cyclohex-2-enone 9, in presence of
copper(I) complexes (Rp)-7 or (Rp)-8. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Synthesis 1998, 1391−1415. (d) Crepy, K. V. L.; Imamoto, T. Top.
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AUTHOR INFORMATION
Corresponding Author
■
́
270. (c) Moulin, D.; Darcel, C.; Juge, S. Tetrahedron: Asymmetry 1999,
10, 4729−4743. (d) Darcel, C.; Moulin, D.; Henry, J. C.; Lagrelette,
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2078−2090. (e) Khiri, N.; Bertrand, E.; Ondel-Eymin, M.-J.;
Rousselin, Y.; Bayardon, J.; Harvey, P. D.; Juge, S. Organometallics
2010, 29, 3622−3631. (f) Lapprand, A.; Khiri, N.; Fortin, D.; Juge, S.;
Pierre, D.; Harvey, P. D. Inorg. Chem. 2013, 52, 2361−2371.
*Tel.: 1-819-821-7092 (P.D.H.), +33(0)3 80 39 61 13 (S.J.). E-
mail: Pierre.Harvey@USherbrooke (P.D.H.), Sylvain.Juge@u-
bourgogne.fr (S.J.).
́
́
́
Notes
The authors declare no competing financial interest.
(10) (a) Churchill, M. R.; Kalra, K. L. Inorg. Chem. 1974, 13, 1899−
1904. (b) Attar, S.; Bowmaker, G. A.; Alcock, N. W.; Frye, J. S.;
Bearden, W. H.; Nelson, J. H. Inorg. Chem. 1991, 30, 4743−4753.
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2003, 33, 341−347. (d) Li, Y.; Yung, K.-F.; Chan, H.-S.; Wong, W.-T.
Inorg. Chem. Commun. 2003, 6, 1451−1453. (e) Vega, A.; Saillard, J. Y.
Inorg. Chem. 2004, 43, 4012−4018. (f) Seddigi, Z. S.; Hossain, G. M.
G.; Banu, A. Acta Crystallogr. 2007, E63, m756−m758. (g) Altaf, M.;
Stoeckli-Evans, H. Inorg. Chim. Acta 2010, 363, 2567−2573. (h) Deng,
S.; Schwarzmaier, C.; Zabel, M.; Nixon, J. F.; Bodensteiner, M.;
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Whitwood, A. C.; Lynam, J. M. Inorg. Chim. Acta 2012, 380, 252−260.
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(b) Perruchas, S.; Le Goff, X. F.; Maron, S.; Maurin, I.; Guillen, F.;
Garcia, A.; Gacoin, T.; Boilot, J. P. J. Am. Chem. Soc. 2010, 132,
10967−10969. (c) Perruchas, S.; Tard, C.; Le Goff, X. F.; Fargues, A.;
Garcia, A.; Khalal, S.; Saillard, J. Y.; Gacoin, T.; Boilot, J. P. Inorg.
Chem. 2011, 50, 10682−10692. (d) Maini, L.; Braga, D.; Mazzeo, P.
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Chem. 2011, 21, 19106−19113. (f) Perruchas, S.; Desboeufs, N.;
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ACKNOWLEDGMENTS
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC), le Fonds
■
Queb
(FQRNT), the Centre d′Etudes des Mater
de Sherbrooke (CEMOPUS), and
́
ec
́
ois de la Recherche sur la Nature et les Technologies
́
iaux Optiques et
Photoniques de l′Universite
́
l′Agence Nationale de la Recherche (France) for the grant
MetChirPhos and the Award of a Research chair of Excellence to
P.D.H. We thank Dr. Pierre Lavigne (Faculte
́ ́
de Medecine of
the Universite de Sherbrooke) for letting us use his circular
́
dichroism spectrometer, and Dr. J. Bayardon for helpful
discussion and his help in the preparation of this manuscript.
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