Heteroleptic Copper(I) Complexes Prepared from Mono- and Tetra-phenylbenzene-substituted Phenanthroline Ligands

Article abstract of DOI:10.1002/ejic.202100331

Phenanthroline ligands bearing two biphenyl (L1) or two tetraarylbenzene (L2) substituents have been synthetized and used to prepare bis-phenanthroline copper(I) complexes. Steric constraints are limited in the case of the biphenyl-substituted ligand and

Full text of DOI:10.1002/ejic.202100331

European Journal of Inorganic Chemistry
Accepted Article
Title: Heteroleptic Copper(I) Complexes Prepared from Mono- and
Tetra-phenylbenzene-substituted Phenanthroline Ligands
Authors: Brigino Ralahy, Uwe Hahn, Emeric Wasielewski, and Jean-
Francois Nierengarten
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100331
Link to VoR: https://doi.org/10.1002/ejic.202100331
01/2020

10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
Heteroleptic Copper(I) Complexes Prepared from Mono- and
Tetra-phenylbenzene-substituted Phenanthroline Ligands
Brigino Ralahy,[a] Uwe Hahn,[a] Emeric Wasielewski,[b] and Jean-François Nierengarten*[a]
Dedicated to Prof. Michael J. Chetcuti on the occasion of his 65th birthday
[a]
B. Ralahy, Dr. U. Hahn, Dr. J.-F. Nierengarten
Laboratoire de Chimie des Matériaux Moléculaires
Université de Strasbourg et CNRS (UMR 7402 LIMA), Ecole Européenne de Chimie, Polymères et Matériaux
25 rue Becquerel, 67087 Strasbourg Cedex 2, France
E-mail: nierengarten@unistra.fr – Homepage URL: http://nierengartengroup.com
Dr. E. Wasielewski
[b]
Plateforme RMN Cronenbourg
Université de Strasbourg et CNRS (UMR 7402 LIMA), Ecole Européenne de Chimie, Polymères et Matériaux
25 rue Becquerel, 67087 Strasbourg Cedex 2, France
Supporting information for this article is given via a link at the end of the document.
Abstract: Phenanthroline ligands bearing two biphenyl (L1) or two
tetraarylbenzene (L2) substituents have been synthetized and used
to prepare bis-phenanthroline copper(I) complexes. Steric constraints
are limited in the case of the biphenyl-substituted ligand and
[Cu(L1)2](BF4) has been obtained by reaction of L1 with
[Cu(CH3CN)4](BF4). In contrast, the tetraarylbenzene substituents of
L2 are large enough to totally prevent the formation of the
corresponding homoleptic bis-phenanthroline copper(I) complex.
Both L1 and L2 have been also combined with 2,9-dimethyl-1,10-
phenanthroline (dmp) to prepare the corresponding heteroleptic bis-
phenanthroline copper(I) complexes. All the copper(I) complexes
obtained from L1 and L2 revealed dynamic conformational exchange
between several atropisomers. Detailed NMR studies and Density
Functional Theory (DFT) calculations have been carried out to assess
their conformations in solution.
heteroleptic complex. Another very efficient strategy to control the
coordination of two different ligands around a copper(I) cation has
been developed by Schmittel and co-workers.[7] In this case, one
of the two chelating ligands is decorated with large substituents
preventing the formation of a stable homoleptic copper(I) complex.
This construction principle is shown in Figure 1 for the heteroleptic
complex resulting from the combination of 2,9-dimethyl-1,10-
phenanthroline (dmp) and 2,9-dimesityl-1,10-phenanthroline
(dMesp).
N
N
N
N
N
N
N
N
Cu+
Cu+
+
+
-
-
BF4
BF4
N
N
[Cu(dmp)2](BF4)
[Cu(dMesp)](BF4)
dMesp
Introduction
dMesp
The coordination of diimine aromatic ligands with copper(I) has
been widely used for the construction of fascinating metallo-
supramolecular nanostructures.[1] Classical examples include
helicates,[2] cages[3] and molecular grids.[4] In these particular
cases, the kinetic instability of the copper(I) complexes leading to
ligand dissociation in solution is a clear advantage. The dynamic
character allows actually for correction of possible errors during
the self-assembly process and the equilibrium is therefore totally
driven towards the most stable product.[1] On the other hand, this
kinetic instability means also that the coordination of different
ligands around the same copper(I) cation is particularly difficult to
control.[5] The pioneering work of Sauvage has shown that the
heteroleptic coordination of copper(I) can be favored by
combining a macrocyclic chelating ligand with an acyclic one.[6]
N
N
N
N
2
Cu+
N
N
dmp
-
BF4
[Cu(dmp)(dMesp)](BF4)
Figure 1. Control of the thermodynamic complexation scenario based on steric
constraints developed by Schmittel and co-workers. The formation of
heteroleptic complex [Cu(dmp)(dMesp)]+ is favoured because all the ligands are
used to generate coordinatively saturated copper(I) complexes. Formation of
[Cu(dmp)2]+ is disfavoured because it would generate a frustrated metal-ligand
assembly for ligand dMesp.
The assembly of two macrocyclic ligands around
a
Steric constraints resulting from the bulky mesityl substituents of
dMesp prevent the formation of the homoleptic complex
[Cu(dMesp)2]+. Consequently, formation of the homoleptic
complex from dmp would generate a mixture of free ligand
(dMesp) and partially coordinated [Cu(dMesp)]+. According to the
tetracoordinated copper(I) cation is not possible and formation of
a homoleptic complex from the acyclic ligand would generate a
frustrated metal-ligand assembly in which all the metal-binding
sites of the macrocycle are not utilized. As a result, the dynamic
coordination equilibrium is totally displaced in favor of the
1
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
maximum occupancy principle,[8] such a situation is not favorable
from a thermodynamic point of view. In contrast, formation of the
heteroleptic complex [Cu(dmp)(dMesp)]+ generates only
coordinatively saturated copper(I) cations with all the ligands
incorporated in complexes. In this case, the perfect match
between the number of ligands and the coordination number of
the copper(I) cations fulfils the maximum occupancy principle. As
a result, formation of the heteroleptic complex is by far more
favorable. As part of this research, we became interested in
evaluating the potential of new diaryl-1,10-phenanthroline ligands
for the formation of stable heteroleptic copper(I) complexes. In the
design of these ligands, the two aryl subunits have been only
substituted in one ortho position. As a result, their relative
orientation is either Syn or Anti thus leading to an unprecedented
isomerism in the resulting copper(I) complexes. Interestingly, the
Anti isomers have been exclusively observed in the solid state.
Detailed dynamic NMR studies have been carried out to assess
the conformations present in solution. These experimental results
have been analyzed with the help of Density Functional Theory
(DFT) calculations.
The preparation of ligand L2 relies on the Diels-Alder reaction of
a
terminal alkyne with
a
tetraphenylcyclopentadienone
derivative.[10] Phenanthroline building block 3 was prepared
according to a previously reported procedure.[11] Compound 4
with its two t-butyl solubilizing groups was selected as the
cyclopentadienone reagent.[12] Treatment of 3 with a slight excess
of 4 (2.05 equiv.) in o-xylene under reflux for 12 h afforded L2 in
71% yield.
Ligands L1 and L2 were fully characterized by NMR and IR
spectroscopies as well as by mass spectrometry. In both cases,
there are two possible relative orientations for the o-substituents
of the phenyl moieties attached to the phenanthroline core leading
to a Syn/Anti isomerism (Figure 2). 1H NMR spectra were
recorded at low temperature but under our experimental
conditions rapid exchange between the two conformers remained
faster than the NMR timescale even at 203 K (see the
Supplementary Information). Computational studies were
performed at the B3LYP/6-31G* level in order to further
understand the conformation of L1 and L2. The calculated Syn
anti Anti atropisomers of both L1 and L2 are shown in Figure 2.
Results and Discussion
Preparation of the ligands. The preparation of ligands L1 and
L2 is depicted in Scheme 1. Treatment of 2-bromo-1,1’-biphenyl
(1) with n-butyllithium (nBuLi) followed by reaction of the resulting
[1,1’-biphenyl]-2-yl lithium with 1,10-phenanthroline under the
conditions developed by Sauvage[9] gave 2 in 72% yield. The
second biphenyl unit was then introduced by reaction of [1,1’-
biphenyl]-2-yl lithium with
2 followed by hydrolysis and
rearomatization with MnO2. Ligand L1 was thus obtained in 82%
yield.
Figure 2. Calculated structures of the C2-symmetrical Anti and CS-symmetrical
Syn atropisomers of L1 and L2 (conformations minimized at the B3LYP/6-31G*
level).
N
N
N
N
(i), (ii), (iii)
(iv), (v)
Br
1
2
The energy barrier calculated for the rotation about the
phenanthroline-phenyl C-C bond in L1 and L2 was also evaluated
by DFT calculations. In both cases, the rather low calculated
energy barriers were fully consistent with the observation of an
average NMR spectra even at 203 K (see the Supplementary
L1
O
1
Information). In the particular case of L1, the H NMR spectrum
H
H
recorded at low temperature revealed however the coalescence
of the signals corresponding to the biphenyl units (see the
Supplementary Information). The energy barrier for the rotation
about the C-C bond of the biphenyl moieties of L1 was effectively
estimated significantly higher by DFT calculations (39.5 kJ/mol,
see the Supplementary Information). This value is in perfect
agreement with the experimental and theoretical data reported by
Schlosser and co-workers for the rotational barriers of ortho-
substituted biphenyl derivatives.[13] At room temperature, the
dynamic exchange of the diastereotopic pairs of H-atoms of the
biphenyl unit is faster than the NMR timescale and they appear
as enantiotopic under these conditions. At 203 K, this exchange
is significantly slower explaining the observation of a coalescence.
Under our experimental conditions, it was however not possible
4
N
N
N
N
(vi)
3
L2
Scheme 1. Preparation of ligands L1 and L2. Reagents and conditions: (i) nBuLi,
Et2O, 0°C to rt; (ii) 1,10-phenanthroline, 0°C then H2O; (iii) MnO2, CH2Cl2 (72%);
(iv) [1,1’-biphenyl]-2-yl lithium, Et2O, 0°C to rt then H2O; (v) MnO2, CH2Cl2 (82%);
(vi) o-xylene, D (71%).
2
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
to record NMR spectra at temperature lower than 203K in order
to observe a fully resolved spectrum showing sharp signals for all
the biphenyl signals. A similar behavior was also observed for
ligand L2 due to the restricted rotation about the phenyl-phenyl C-
C bonds of the tetraaryl-substituted phenyl subunits (see the
Supplementary Information). The calculated energy barriers were
fully consistent with the experimental values reported by Gust for
hexaarylbenzene derivatives.[14]
evidenced by the variable-temperature NMR investigations, a
conformational analysis of [Cu(L1)2]+ was carried out by DFT
calculations. As already discussed in the previous section, ligand
L1 may adopt either a C2-symmetrical Anti or a Cs-symmetrical
Syn conformation. Consequently, the combination of two L1
moiety around a copper(I) cation can give rise to seven
atropisomers (3 pairs of enantiomers and an achiral one, see
Figure 4). Conformer A combines two Anti-L1 ligands of opposite
chirality ([Cu(S-Anti-L1)(R-Anti-L1)]+; S4-symmetrical; achiral).
Conformer B and its enantiomer en-B combine one Syn-L1 ligand
with an Anti-L1 ligand ([Cu(Syn)(R-Anti)]+ and [Cu(Syn-L1)(S-
Anti-L1)]+; C1-symmetrical). Conformer C and its enantiomer en-
C combine two Anti-L1 ligands of similar chirality ([Cu(R-Anti-
L1)2]+ and [Cu(S-Anti-L1)2]+); D2-symmetrical). Conformer D and
its enantiomer en-D combine two Syn-L1 ligands ([Cu(Syn-L1)2]+;
two possible relative orientations leading to a pair of enantiomers,
C2-symmetrical). DFT calculations revealed that achiral
conformer A is substantially destabilized by negative steric effects.
This is also the case for the D2-symmetrical conformer (D) but to
a lesser degree. Finally, the most stable conformers, B and C, are
almost isoenergetic and are expected to be the major species in
solution. The Boltzmann distribution calculated from the free
energy values of the different isomers shows that the dynamic
equilibrium is largely displaced in favor of B/en-B (63.6%) and
C/en-C (36.2%). This is fully consistent with the NMR data
recorded at 203 K. Interconversion between the different isomers
occurs by rotation around the phenanthroline-phenyl C-C bonds.
The calculated rotational energy barriers are in good agreement
with a fast-dynamic exchange between the different conformers
at room temperature (Figure 4). These values are also consistent
with the observation of isomers in slow exchange on the NMR
timescale at 203 K.
Homoleptic copper(I) complexes. Steric constraints are
sufficiently important to totally prevent the formation of homoleptic
complex [Cu(L2)2]+ by treatment of L2 with [Cu(CH3CN)4](BF4). In
contrast, a stable homoleptic copper(I) complex was obtained by
reaction of L1 with [Cu(CH3CN)4](BF4) (Scheme 2).
N
(i)
N
N
Cu+
N
N
N
-
BF4
[Cu(L1)2](BF4)
L1
Scheme 2. Preparation of [Cu(L1)2](BF4). Reagents and conditions: (i)
[Cu(CH3CN)4](BF4), CH3CN/CH2Cl2, rt (42%).
Pure [Cu(L1)2](BF4) was isolated in 42% yield by column
chromatography followed by crystallization in Et2O/CH2Cl2. The
MALDI-TOF mass spectrum of [Cu(L1)2](BF4) was in full
agreement with the proposed structure. Effectively, the expected
pseudo-molecular ion peak is observed as the base peak at m/z
- +
= 1031.3 ([M-BF4 ] , calcd for C72H48CuN2: 1031.32). The
absorption spectrum of [Cu(L1)2](BF4) also shows the
characteristic
features
of
bis-phenanthroline
copper(I)
complexes[15] with a strong ligand-centered absorption in the UV
region and a much weaker band in the visible region (lMax = 442
nm, e = 5400 M-1 cm-1). The latest is attributed to the metal-to-
ligand charge transfer band characteristic of bis-phenanthroline
copper(I) complexes and at the origin of their orange-red color.[15]
As shown in Figure 3, the 1H NMR spectrum recorded for
[Cu(L1)2](BF4) at room temperature exhibits the expected
features with the characteristic signals arising from the biphenyl
groups as well as two doublets and a singlet for the two equivalent
coordinating phenanthroline moieties. However, some signals are
broad at room temperature suggesting a dynamic effect. This was
confirmed by a variable-temperature NMR study. By increasing
the temperature, a perfectly reversible narrowing was observed.
At high temperatures, the dynamic exchange between the
possible conformers of [Cu(L1)2]+ is faster than the NMR
timescale and the average 1H NMR spectrum recorded under
these conditions is consistent with an apparent D2d symmetry. In
contrast, by decreasing the temperature, the dynamic exchange
resulting from the rotation around the phenyl-phenanthroline
bonds becomes slower as attested by the dramatic broadening of
all the signals. At 203 K, the 1H NMR spectrum is consistent with
a mixture of two major atropisomers in a 2:1 ratio but it was difficult
to properly assign all the signals. Under these conditions, the
major isomer is C1-symmetrical while the minor has a two-fold
symmetry. In order to fully understand the dynamic exchange
Figure 3. 1H NMR of [Cu(L1)2](BF4) recorded at different temperatures (400
MHz, CD2Cl2). At 203 K, four pairs of non-equivalent H(3-4) could be identified
for the major C1-symmetrical atropisomer (indicated in red) and two for the minor
C2-symmetrical one (indicated in blue) with the help of a 2D COSY NMR
spectrum.
3
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
Figure 4. Free energy profile calculated for [Cu(L1)2]+ at the B3LYP/6-31G* level (relative energies in kJ/mol; for clarity, the C atoms are represented in pale blue
for one L1 ligand and in gray for the second one; the phenyl-phenanthroline C-C bond involved in the isomerization is highlighted in yellow in the transition states).
Boltzmann distribution calculated from the computed G0 values at 298.15 K: A: 0.0005%; B: 31.8%; en-B: 31.8%; C: 0.05%; en-C: 0.05%; D: 18.1%; en-D: 18.1%.
Heteroleptic copper(I) complexes. The preparation of
heteroleptic copper(I) complexes from L1 and L2 is shown in
Scheme 3.
Treatment of L1 with dmp (1 equiv.) and [Cu(CH3CN)4](BF4) in
CH2Cl2/CH3CN gave a mixture of complexes. Analysis of the H
1
NMR spectrum recorded in CD2Cl2 at rt for the crude mixture
revealed the presence of [Cu(L1)(dmp)](BF4), [Cu(L1)2](BF4),
[Cu(dmp)2](BF4) in a 9:1:1 ratio. The heteroleptic complex
[Cu(L1)(dmp)](BF4) was however isolated pure by slow diffusion
of Et2O into a CH2Cl2 solution of the crude mixture. In contrast,
the successive addition of L2 (1 equiv.) and dmp (1 equiv.) to a
solution of [Cu(CH3CN)4](BF4) (1 equiv.) in CH2Cl2/CH3CN
afforded exclusively the heteroleptic complex [Cu(L2)(dmp)](BF4).
In this case, the steric constraints are sufficiently important to
totally prevent the formation of [Cu(L2)2]+ and the coordination
scenario is totally driven towards the heteroleptic complex.
Ph
Ph
Ph
N
N
N
N
N
N
Cu+
Cu+
+
+
N
N
Cu+
N
N
N
N
-
BF4
L1
L2
-
-
BF4
BF4
Ph
Ph
Ph
(i)
[Cu(dmp)2](BF4)
N
N
[Cu(L1)(dmp)](BF4)
[Cu(L1)2](BF4)
Ar
(ii)
Ar
Ph
dmp
Ph
Recrystallization
in
Et2O/CH2Cl2
provided
pure
N
N
[Cu(L2)(dmp)](BF4) in 66% yield.
Cu+
=
Ar
N
For both [Cu(L1)(dmp)](BF4) and [Cu(L2)(dmp)](BF4), crystals
obtained by slow diffusion of Et2O into a CH2Cl2 solution of the
complex were suitable for X-ray crystal structure analysis. The
structures are shown in Figures 5 and 6. Selected bond lengths
and angles are given in Table 1. Two pseudo-C2 symmetrical
[Cu(L1)(dmp)]+ cations are present in the crystal lattice. In both of
them, L1 adopts an Anti-conformation (Figure 5A). As a result, the
cations are chiral in the solid state and compound
[Cu(L1)(dmp)](BF4) crystalized as a racemate. For both cations,
the two enantiomers are present in the crystal lattice. They are
related by the crystallographic inversion centers (P-1 triclinic
space group). The coordination about the copper(I) center in
N
Ph
-
BF4
Ph
Ar
Ar
[Cu(L2)(dmp)](BF4)
Scheme 3. Preparation of heteroleptic complexes from L1 and L2. Reagents
and conditions: (i) [Cu(CH3CN)4](BF4), CH2Cl2/CH3CN (the 1H NMR spectrum of
the crude mixture revealed the presence of [Cu(L1)(dmp)](BF4), [Cu(L1)2](BF4),
[Cu(dmp)2](BF4) in a 9:1:1 ratio; [Cu(L1)(dmp)](BF4) was isolated pure by
crystallization in 62% yield); (ii) [Cu(CH3CN)4](BF4), CH2Cl2/CH3CN (the 1H
NMR spectrum of the crude mixture revealed only the presence of
[Cu(L2)(dmp)](BF4); [Cu(L2)(dmp)](BF4) was isolated pure by crystallization in
66% yield)
4
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
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[Cu(L1)(dmp)]+ is flattened tetrahedral with angles of 100.6 and
103.7° between the mean planes of the two phenanthroline
ligands around Cu(1) and Cu(2), respectively. (Figure 5C). In this
way, inter-ligand p-p interactions between dmp and the two
biphenyl moieties of L1 are maximized. A very small rocking of
the dmp ligand is also observed, thus explaining the slightly longer
C(1)-N(2) and Cu(2)-N(6) bond lengths (Table 1). Detailed
inspection of the crystal lattice of [Cu(L1)(dmp)](BF4) reveals
linear chains alternating cations of opposite chirality in which
intermolecular p-stacking interactions between the biphenyl
subunits of neighboring cations take place (Figure 5E).
Table 1. Bond length (Å) and bond angles (°) within the coordination spheres
for [Cu(L1)(dmp)]+ and [Cu(L2)(dmp)]+.[a]
[Cu(L1)(dmp)]+
[Cu(L2)(dmp)]+
Cu(1)-N(1)
Cu(1)-N(2)
2.031(2)
2.050(2)
2.028(3)
2.025(3)
82.66(8)
120.31(8)
131.82(8)
128.41(8)
116.85(8)
83.14(8)
2.033(2)
2.043(2)
2.025(2)
2.024(2)
82.67(8)
135.24(8)
116.57(9)
117.44(8)
128.30(9)
83.31(9)
2.051(4)
2.027(3)
2.051(3)
2.056(3)
83.0(1)
Figure 5. (A) View of the two [Cu(L1)(dmp)]+ cations present in the crystal lattice
of [Cu(L1)(dmp)](BF4).(Et2O); (C) detailed view of the coordination sphere
around the Cu(I) centers. Back (B) and side (D) views of [Cu(L1)(dmp)]+
highlighting the flattening distortion (angle between the mean planes of L1 and
dmp: q = 100.6°) of the coordination sphere around Cu(1) and the very limited
rocking distortion (ORTEP plots; C: gray for L1 and pale blue for dmp, N: blue,
Cu: orange; the H atoms, the counteranions and the co-crystallized solvent
molecules are omitted for clarity; the thermal ellipsoids are shown at the 50%
probability level). (E) Stacking within the [Cu(L1)(dmp)](BF4).(Et2O) lattice
highlighting the intra- and inter-molecular p-p interactions between neighbouring
phenanthroline and biphenyl subunit (space filling representations; left: C: gray
for L1 and pale blue for dmp, N: blue, Cu: orange; H: white; right: same view
with the cations incorporating Cu(1) in green and Cu(2) in blue).
Cu(1)-N(3)
Cu(1)-N(4)
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(3)
N(2)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
Cu(2)-N(5)
116.2(1)
114.8(1)
137.1(1)
127.4(1)
81.2(1)
2.057(4)
2.040(3)
2.073(3)
2.056(3)
82.3(1)
As shown in Figure 6A, two distinct cations are present in the
crystal lattice of [Cu(L2)(dmp)](BF4). As observed for the
corresponding copper(I) complex obtained from L1, ligand L2
Cu(2)-N(6)
Cu(2)-N(7)
adopts
a pseudo-C2 symmetrical Anti-conformation in the
[Cu(L2)(dmp)]+ cations and compound [Cu(L2)(dmp)](BF4)
crystallizes as a racemate. The coordination geometry of the
copper(I) centers is distorted from a D2d pseudotetrahedral
geometry expected for a d10 ion.[16] Indeed, a significant rocking
distortion is observed (Figure 6C). As a result, the close-to-
tetrahedral geometry is distorted towards a trigonal pyramidal
geometry. Such a distortion is typically associated with an
elongation of the Cu-N bond moving to the axial position.[16] This
is not really the case for both [Cu(L2)(dmp)]+ cations present in
the crystal lattice. On one hand, the copper(I) center is out of the
plane of the L2 ligand by ca. 0.225 Å thus limiting the elongation
of the axial Cu-N bond. On the other hand, all the Cu-N bonds are
slightly longer than those typically observed for phenanthroline
copper(I) complexes.[16] This is ascribed to the steric constraints
imposed by the large tetraarylbenzene substituents of L2. While
an important rocking distortion is observed for the [Cu(L2)(dmp)]+
cations, their two phenanthroline moieties are almost
perpendicular (Figure 6D).
Cu(2)-N(8)
N(5)-Cu(2)-N(6)
N(5)-Cu(2)-N(7)
N(5)-Cu(2)-N(8)
N(6)-Cu(2)-N(7)
N(6)-Cu(1)-N(8)
N(7)-Cu(1)-N(8)
112.5(1)
125.2(1)
135.7(1)
124.7(1)
81.9(1)
[a] See Figures 5 and 6 for the numbering.
5
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
ligand exchange leading to the formation of the homoleptic
complexes.
Figure 7. 1H NMR recorded at 298 K for (A) a freshly prepared CD2Cl2 solution
of [Cu(L1)(dmp)](BF4) (out of equilibrium), (B)
[Cu(L1)(dmp)](BF4) in the presence of CH3CN after the equilibrium was reached
(signals corresponding to [Cu(L1)2](BF4) are indicated with black marks and
those of [Cu(dmp)2](BF4) with blue marks), (C)
[Cu(L1)2](BF4), and (D) a CD2Cl2 solution of [Cu(dmp)2](BF4).
a CD2Cl2 solution of
Figure 6. (A) View of the two [Cu(L2)(dmp)]+ cations present in the crystal lattice
of [Cu(L1)(dmp)](BF4).(Et2O) (view along crystallographic a axis); (B) detailed
view of the coordination sphere around the Cu(I) centers; (C) side and (D) back
views of [Cu(L2)(dmp)]+ highlighting the rocking distortion of the coordination
sphere around Cu(1) and the very limited flattening (angle between the mean
planes of L2 and dmp: q = 87.1°), (ORTEP plots; C: gray for L2 and pale blue
for dmp, N: blue, Cu: orange; the H atoms and the counteranions are omitted
for clarity; the thermal ellipsoids are shown at the 50% probability level).
a CD2Cl2 solution of
In the solid state, only the Anti conformers were observed for
[Cu(L1)(dmp)](BF4) and [Cu(L2)(dmp)](BF4). In order to also
assess the conformations in solution for these compounds,
detailed variable-temperature NMR studies and Density
Functional Theory (DFT) calculations were carried out. For both
heteroleptic complexes, the Syn-Anti isomerization was
conveniently monitored by following the evolution of the signals
arising from the methyl substituents of the dmp ligand in 1H NMR
spectra of [Cu(L1)(dmp)](BF4) and [Cu(L2)(dmp)](BF4) recorded
at different temperatures (Figure 8). At room temperature, the
Syn-Anti isomerization is faster than the NMR timescale as
attested by the observation of a singlet for the methyl groups of
dmp in both cases. In contrast at low temperature, the dynamic
exchange is slower than the NMR timescale and the two
conformers are clearly observed. In both cases, a singlet is
observed for the two equivalent methyl groups of the C2-
symmetrical Anti conformer, while two singlets are seen for the
non-equivalent methyl groups of the Cs-symmetrical Syn
conformer. The proportion of both conformers is temperature
dependent. For [Cu(L1)(dmp)](BF4), the Anti/Syn ratio estimated
from the integration of the CH3 groups in 1H NMR spectra was ca.
6/4 at 243 K and 7/3 at 213 K corresponding to a difference in free
energy of ca. 1 kJ/mol in favor of the Anti-isomer. Similarly, the
Anti/Syn ratio was ca. 94/6 at 243 K and 97/3 at 208 K for
[Cu(L2)(dmp)](BF4). Based on these values, the difference in free
energy between the Anti and the Syn conformers was estimated
Both [Cu(L1)(dmp)](BF4) and [Cu(L2)(dmp)](BF4) are stable in the
solid state. Their stability was also evaluated in solution. While
CH2Cl2 or CH3CN solutions of [Cu(L2)(dmp)](BF4) were stable for
days, this was not the case for [Cu(L1)(dmp)](BF4). The 1H NMR
recorded for a freshly prepared CD2Cl2 solution from recrystallized
[Cu(L1)(dmp)](BF4) revealed the almost exclusive presence of the
heteroleptic complex (Figure 7). A very slow evolution was
however observed and traces of the homoleptic complexes
[Cu(L1)2](BF4) and [Cu(dmp)2](BF4) were detected after a few
hours. In this solvent, the evolution is extremely slow and the
equilibrium is not reached after one week. In contrast,
equilibration was found faster in the presence of CH3CN thus
suggesting that this coordinating solvent is catalyzing the ligand
exchange reaction.[17] At equilibrium, the 1H NMR spectrum
recorded in CD2Cl2 revealed the presence of [Cu(L1)(dmp)](BF4),
[Cu(L1)2](BF4) and [Cu(dmp)2](BF4) in a 9:1:1 ratio (Figure 7). The
heteroleptic/homoleptic ratio is indeed identical to the one
observed in the crude mixture obtained upon mixing L1, dmp and
[Cu(CH3CN)4](BF4). The proportion of the different species results
actually from their relative thermodynamic stability. All these
observations
revealed
that
heteroleptic
complex
[Cu(L1)(dmp)](BF4) may be investigated out of equilibrium in
CH2Cl2 solutions, however one should always consider the slow
6
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
to be ca. 6 kJ/mol. By monitoring the coalescence of the H(5)
signal from the dmp ligand, the activation free energies for the
Syn-Anti isomerization through rotation about the phenanthroline-
phenyl bond was estimated as DG‡ = 48 and 60 kJ/mol for
[Cu(L1)(dmp)](BF4) and [Cu(L2)(dmp)](BF4), respectively.
conformer is clearly destabilized by steric factors and this is in
perfect agreement with our experimental observations. Finally,
the computed free energy barriers for the Syn/Anti isomerizations
are in good agreement with experimental values obtained by the
variable-temperature NMR data for both [Cu(L1)(dmp)]+ and
[Cu(L2)(dmp)]+.
Table 2. Total electronic energies (E) and Gibbs energies at 298.15 K (G0) for
the Anti and Syn conformers of [Cu(L1)(dmp)]+ and [Cu(L2)(dmp)]+ as well as
the transition states (TS) for the Syn-Anti isomerizations computed at the
B3LYP/6-31G* level in the gas phase.[a]
E
Relative E
G0
DG0
(Hartree)
(kJ/mol)
(Hartree)
(kJ/mol)
[Cu(L1)(dmp)]+
Anti
Syn
TS
-3786.326910
-3786.327019
-3786.314321
0.3
0
-3785.726125
-3785.727352
-3785.709887
3.2[b]
0[b]
33.3
45.8
[Cu(L2)(dmp)]+
Anti
Syn
TS
-5801.657948
-5801.652126
-5801.637529
0
-5800.237871
-5800.232074
-5800.216821
0[c]
15.3
53.6
15.2[c]
55.3
[a] The calculated structures are shown in Figure 8. [b] Boltzmann distribution
calculated from the computed G0 values at 298.15 K: Anti: 21.4%; Syn: 78.6%.
[c] Boltzmann distribution calculated from the computed G0 values at 298.15 K:
Anti: 99.8%; Syn: 0.2%.
Figure 8. Top: calculated structures of the Anti and Syn conformers of
[Cu(L1)(dmp)]+ and [Cu(L2)(dmp)]+ as well as the transition states for the Syn-
Anti isomerizations computed at the B3LYP/6-31G* level in the gas phase (for
clarity, the C atoms are represented in pale blue for dmp and in gray for L1 and
L2; the phenyl-phenanthroline C-C bond involved in the isomerization is
DFT studies of the equilibrium between homo- and hetero-
leptic copper(I) complexes. In order to evaluate the steric
constrains of the different ligands discussed in the present paper,
the equilibrium between [Cu(L)(dmp)]+ (L = L1, L2 or dMesp) and
the corresponding homoleptic complexes was analyzed by
theoretical calculations. The computed free energy values and the
corresponding Boltzmann distributions obtained at the B3LYP/6-
31G* level in the gas phase are summarized in Table 3. It can be
noted that the same calculations were also done at the semi-
empirical PM6 level (see the Supplementary Information). The
PM6 level is actually sufficient to rapidly evaluate the possibility to
prepare stable heteroleptic copper(I) complexes from two ligands,
however the calculated DG0 values for the homoleptic/heteroleptic
equilibrium are clearly underestimated. In contrast, the DG0
obtained at the B3LYP/6-31G* level are most likely overestimated
but the comparison between the values obtained for the different
systems provides a good estimation of the relative steric
constraints resulting from the nature of the L ligand. In the case
of L1, the biphenyl group is not sufficiently large to prevent the
formation of the homoleptic complex. Accordingly, a relatively
modest DG0 value was calculated for the homoleptic/heteroleptic
equilibrium (-13.2 kJ/mol). In contrast, the large DG0 values
calculated in the case of L2 and dMesp are in good agreement
with the experimental observations. Effectively, the steric
constraints are sufficiently important in both cases to prevent the
formation of the corresponding homoleptic complexes and their
associated high energy contributes largely to favor the
heteroleptic complex. In this case, the steric constraints are
minimized as the second ligand, namely dmp, is not substituted
with sterically demanding groups. Comparison of L1 and L2
highlighted in yellow in the transition states). Bottom: aliphatic region of the 1
H
NMR spectra recorded at different temperatures for [Cu(L1)(dmp)](BF4) (left)
and [Cu(L2)(dmp)](BF4) (right) showing the resonance of the Me groups of the
dmp ligand (* = trace of [Cu(dmp)2]+).
The Syn-Anti isomerization of cations [Cu(L1)(dmp)]+ and
[Cu(L2)(dmp)]+ was also investigated by DFT calculations at the
B3LYP/6-31G* level in the gas phase. The calculated Syn and
Anti isomers as well as the transition states for the Syn-Anti
isomerization through rotation about the phenanthroline-phenyl
bond are shown for both heteroleptic copper(I) complexes in
Figure 8. The associated total electronic (E) and Gibbs (G0)
energies are given in Table 2. The E values obtained for the Syn
and Anti isomers of [Cu(L1)(dmp)]+ are very similar as observed
experimentally. There is however a difference of 3.2 kJ/mol
between their G0 values with a preference for the Syn isomer. This
discrepancy with the experimental data might be ascribed to the
idealization of low-frequency vibrators as harmonic oscillators.[13]
On the other hand, calculations have been performed in the gas
phase. In solution, the relative energy of the two isomers is also
influenced by their dipole moments. The significant difference (ca.
1.7 D) between the calculated dipole moments of the Syn and Anti
isomers may also influence their behavior in solution. As one may
expect, it appears that the most abundant conformer in CD2Cl2 is
the one having the smallest dipole moment. In the case of
[Cu(L2)(dmp)]+, the relative E and G0 values between the two
conformers revealed a clear preference for the Anti. The Syn
7
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
shows that the three additional aryl substituents on each phenyl
units attached to the phenanthroline core play a key role in the
steric destabilization of the homoleptic complex with an energy
penalty of 144.7 kJ/mol. In this way, their presence contributes to
favor the formation of the heteroleptic complex. However, the
unsubstituted ortho-position led to a reduced steric constraints of
65.1 kJ/mol when compared to dMesp in which the two ortho-
positions are substituted. It is also known that replacing the Me
groups of dMesp by larger i-Pr substituents generates enormous
steric constraints that also destabilize the heteroleptic complex.[18]
As a result, the latest is not stable and decoordination is observed
in solution. The present results show that using mono-substituted
ortho-phenyl groups is an appealing strategy to further increase
the steric demand without destabilizing the heteroleptic complex.
This is extremely important in the perspective of future
developments of strongly luminescent copper(I) complexes as
increased steric constraint prevents severe distortion of the
coordination sphere in the excited state.[15]
Owing to limited steric constraints, the homoleptic/heteroleptic
equilibrium is not totally displaced in favor of [Cu(L1)(dmp)](BF4).
This compound has been however isolated pure in the solid state
by crystallization. When dissolved in a non-coordinating solvent,
the ligand exchange reaction is very slow and [Cu(L1)(dmp)](BF4)
can be investigated out of equilibrium. In contrast, in coordinating
solvents, the ligand exchange reaction is fast and
a
thermodynamic equilibrium is rapidly reached. The proportion of
the different species, namely [Cu(L1)(dmp)](BF4), [Cu(L1)2](BF4)
and [Cu(dmp)2](BF4), results from their relative thermodynamic
stability. In the case of L2, the steric constraints are sufficiently
important to completely drive the coordination scenario towards
the exclusive formation of [Cu(L1)(dmp)](BF4). Interestingly, only
the Anti conformers have been observed in the solid state for both
[Cu(L1)(dmp)](BF4) and [Cu(L2)(dmp)](BF4). In contrast, detailed
NMR studies have shown a dynamic Syn/Anti conformational
equilibrium in solution. DFT calculations have been useful to fully
elucidate the conformational equilibrium observed for the
copper(I) complexes reported in this study. They are also helpful
to assess the steric constraints in such compounds and are thus
essential in the design of stable heteroleptic bis-phenanthroline
copper(I) complexes. The design principle reported herein will be
further investigated, in particular for the development of new
copper(I) photocatalysts. Work in this direction is underway in our
laboratory.
Table 3. Relative Gibbs energies at 298.15 K (G0) for [Cu(dmp)2]+ + [Cu(L)2]+
⇄
2 [Cu(dmp)(L)]+ computed at the B3LYP/6-31G* level in the gas phase. The
Boltzmann distributions have been calculated based on the DG0 values at
298.15 K.
Relative G0
(kJ/mol)
Boltzmann
distribution
[Cu(dmp)2]+ + [Cu(L1)2]+ ⇄ 2 [Cu(dmp)(L1)]+
Experimental Section
[Cu(dmp)2]+ + [Cu(L1)2]+
2 [Cu(dmp)(L1)]+
0
0.5%
General. Reagents (reagent grade) and solvents (analytical grade) were
purchased and used without further purification. Compounds 3,[11] 4,[12]
and [Cu(dmp)2](BF4)[19] were prepared according to literature procedures.
All reactions were performed in standard glassware under an inert argon
atmosphere. Column chromatography: silica gel 60 (70-230 mesh, 0.063-
0.200 mm) was purchased from E. Merck. Thin-layer chromatography
(TLC) was performed on aluminium sheets coated with silica gel 60 F254
purchased from E. Merck, visualization was done by irradiation with UV
light. Melting points (M.p.) were measured on a Gallenkamp melting point
apparatus. IR spectra (cm-1) were measured on a Perkin Elmer Spectrum
2 instrument. UV/Vis spectra have been recorded on a Perkin Elmer
spectrophotometer Lambda 365 (λmax in nm, ε in M-1 cm-1). NMR spectra
were recorded on a Bruker Avance I (300 MHz), Bruker Avance III HD (400
MHz) or a Bruker Avance III HD (500 MHz) with solvent peaks as reference
and at 298 K if not indicated otherwise (br: broad signal). ). MALDI-TOF-
mass spectra (m/z, % relative intensity) were carried out on a Bruker
ULTRAFLEXTM matrix-assisted laser desorption ionization time-of-flight
mass spectrometer. A saturated solution of 1,8,9-anthracenetriol (dithranol,
ALDRICH) in CH2Cl2 was used as matrix. Elemental analyses were
performed on a Flash 2000 apparatus of ThermoFisher Scientific at the
Analytical Service of the University of Strasbourg, France.
-13.2
99.5%
[Cu(dmp)2]+ + [Cu(L2)2]+ ⇄ 2 [Cu(dmp)(L2)]+
[Cu(dmp)2]+ + [Cu(L2)2]+
[2 [Cu(dmp)(L2)]+
0
0
-157.9
100%
[Cu(dmp)2]+ + [Cu(dMesp)2]+ ⇄ 2 [Cu(dmp)(dMesp)]+
[Cu(dMesp)2]+ + [Cu(L)2]+
2 [Cu(dmp)(dMesp)]+
0
0
-223.0
100%
Conclusion
Two phenanthroline ligands substituted in their 2,9-positions with
mono- and tetra-arylated phenyl groups have been prepared. In
the case of the tetraarylphenyl-substituted ligand (L2), steric
effects totally prevent the formation of a bis-phenanthroline
copper(I) compex. In contrast, the steric constraints resulting from
the presence of two biphenyl substituents in L1 are not sufficient
to prevent the formation of a stable homoleptic copper(I) complex.
Owing to the possible relative Syn and Anti orientation of the two
biphenyl moieties in L1, a complicated conformational equilibrium
has been evidenced for the resulting homoleptic copper(I)
complex. Detailed dynamic NMR studies combined with DFT
calculations have shown that the dynamic conformational
equilibrium favors two specific conformers out of the four possible
ones. Both L1 and L2 have been also combined with dmp to
generate the corresponding heteroleptic copper(I) complexes.
Compound 2. n-BuLi (1.6M in hexanes, 2.14 mL, 3.43 mmol) was added
slowly to a solution of 1 (800 mg, 3.43 mmol) in dry diethyl ether (20 mL)
under argon at 0°C. After 10 min, the solution was allowed to warm to rt
and stirring was continued at rt for 2h. This solution was then transferred
slowly over 15 min to a suspension of 1,10-phenanthroline (309 mg, 1.72
mmol) in dry diethyl ether (15 mL) under argon and at 0°C. The resulting
red reaction mixture was allowed to warm to rt and stirring was continued
for 4 h. A saturated aqueous NH4Cl solution (5 mL) was added, the organic
phase separated, and the aqueous phase extracted with CH2Cl2 (2x10 mL).
The combined organic phases were washed with H2O (10 mL) and brine
(10 mL), dried with Na2SO4 and evaporated. The crude was dissolved in
CH2Cl2 (20 mL), MnO2 (3.00 g, 3.44 mmol) was added and the mixture
stirred at rt for 2 h. The mixture was filtered over celite (CH2Cl2) and
evaporated. Column chromatography (SiO2; eluent: CH2Cl2 to
8
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
CH2Cl2/MeOH 50:1) gave 2 (410 mg, 72%). Highly viscous pale-yellow oil.
IR (neat, cm-1): n = 3042, 1487, 851, 746, 701; 1H NMR (500 MHz, CDCl3):
δ = 9.28 (dd, J = 4, 1 Hz, 1H), 8.28 (dd, J = 8, 2 Hz, 1H), 8.13-8.06 (m, 1H),
7.86 (d, J = 8 Hz, 1H), 7.78 (d, J = 9 Hz, 1H), 7.72 (d, J = 9 Hz, 1H), 7.65
(dd, J = 8, 4 Hz, 1H), 7.13 (d, J = 8 Hz, 1H) ppm; 13C NMR (126 MHz,
CDCl3): δ = 160.1, 150.4, 146.3, 146.2, 141.4, 140.6, 140.1, 136.3, 134.8,
132.1, 130.3, 130.1, 129.0, 128.9, 128.3, 128.0, 127.0, 126.9, 126.6, 126.3,
125.7, 122.9 ppm; Elemental analysis calcd. for C24H16N2 x 2/3 CH2Cl2: C
76.16, H 4.49, N 7.20; found: C 75.99, H 4.46, N 7.21; MALDI-TOF MS
(m/z): 333.16 ([M+H]+, calcd for C24H17N2: 333.14).
1/3 CH2Cl2: C 75.69, H 4.28, N 4.88; found: C 75.44, H 4.41, N 4.80;
MALDI-TOF MS (m/z): 1031.31 ([M-BF4 ] , calcd for C72H48CuN2: 1031.32).
- +
[Cu(L1)(dmp)](BF4). A solution of L1 (100 mg, 0.206 mmol) in CH2Cl2 (5
mL) was added to a solution of [Cu(CH3CN)4]BF4 (64.9 mg, 0.206 mmol)
in MeCN (5 mL) under argon. The solution turned yellow. After 10 min, a
solution of dmp (43.0 mg, 0.206 mmol) in CH2Cl2 (5 mL) was added. The
resulting red solution was stirred for
1 h at rt and evaporated.
Crystallization from CH2Cl2/diethyl ether gave [Cu(L1)(dmp)](BF4) (107.9
mg, 62%). M.p.: 148-149°C; Dark red solid. IR (neat, cm-1): n = 3059, 1494,
1477, 1051, 1035, 859, 743, 729, 701; UV/Vis (CH2Cl2): λmax (ε, M-1 cm-1)
= 275 (39900), 295 (sh, 35000), 310 (sh, 28000), 461 (5200);; 1H NMR
(400 MHz, CD2Cl2): δ = 8.33 (d, J = 8 Hz, 4H), 8.30 (d, J = 8 Hz, 4H), 8.08
(s, 2H), 7.85 (s, 2H), 7.57 (d, J = 8 Hz, 2H), 7.49 (d, J = 8 Hz, 2H), 7.38
(dd, J = 8, 1 Hz, 2H), 7.20 (m, 2H), 7.12 (m, 4H), 6.98 (dd, J = 8, 1 Hz, 2H),
6.84 (td, J = 8, 1 Hz, 2H), 6.78 (m, 4H), 6.22 (td, J = 8, 1 Hz, 2H), 2.28 (s,
Ligand L1. n-BuLi (1.6M in hexanes, 2.14 mL, 3.43 mmol) was added
slowly to a solution of 1 (800 mg, 3.43 mmol) in dry diethyl ether (20 mL)
under argon at 0°C. After 10 min, the solution was allowed to warm to rt
and stirring was continued at rt for 2h. This solution was then transferred
slowly over 20 min to a suspension of 2 (570 mg, 1.72 mmol) in dry diethyl
ether (25 mL) under argon and at 0°C. The resulting purple reaction
mixture was allowed to warm to rt and stirring was continued for 17 h. A
saturated aqueous NH4Cl solution (15 mL) was added, the organic phase
separated, and the aqueous phase extracted with CH2Cl2 (10 mL). The
combined organic phases were washed with brine (10 mL), dried with
Na2SO4 and evaporated. The crude was dissolved in CH2Cl2 (20 mL),
MnO2 (3.00 g, 3.44 mmol) was added and the mixture stirred at rt for 2 h.
The mixture was filtered over celite (CH2Cl2) and evaporated. Column
chromatography (SiO2; eluent: cyclohexane / diethylether 4:1 to 3:1) gave
L1 (680 mg, 82%). Colorless solid. M.p.: 109-110°C; IR (neat, cm-1): n =
3055, 1492, 1476, 855, 745, 701; 1H NMR (500 MHz, CD2Cl2): δ = 8.00
(dd, J = 4, 1 Hz, 2H), 7.91 (d, J = 8 Hz, 2H), 7.70 (s, 2H), 7.58 (m, 4H),
7.54 (m, 2H), 7.30 (m, 4H), 7.23 (m, 6H), 7.20 (d, J = 8 Hz, 2H) ppm; 13C
NMR (126 MHz, CD2Cl2): δ = 159.9, 146.8, 142.0, 141.5, 140.6, 135.4,
132.0, 131.1, 130.6, 129.3, 128.6, 128.3, 127.6, 127.3, 126.7, 125.5 ppm;
Elemental analysis calcd. for C36H24N2xC6H14: C 88.38, H 6.71, N 4.91;
found: C 88.33, H 5.81, N 5.30; MALDI-TOF MS (m/z): 485.13 ([M+H]+,
calcd for C36H25N2: 485.20).
- +
6H) ppm; MALDI-TOF MS (m/z): 755.22 ([M-BF4 ] , calcd for C50H36CuN4:
755.22).
[Cu(L2)(dmp)](BF4). A solution of L2 (100 mg, 0.086 mmol) in CH2Cl2 (5
mL) was added to a solution of [Cu(CH3CN)4]BF4 (27.0 mg, 0.086 mmol)
in MeCN (5 mL) under argon. The solution turned yellow. After 10 min, a
solution of dmp (17.9 mg, 0.086 mmol) in CH2Cl2 (5 mL) was added. The
resulting red solution was stirred for 1 h at rt. The resulting red solution
was stirred for 1 h at rt and evaporated. Crystallization from CH2Cl2/diethyl
ether gave [Cu(L2)(dmp)](BF4) (86 mg, 66%). Red crystalline solid. M.p.:
>300°C; IR (neat, cm-1): n = 2960, 1500, 1056, 857, 839, 701; UV/Vis
(CH2Cl2): λmax (ε, M-1 cm-1) = 253 (100500), 320 (sh, 35000), 453 (4800);
1H NMR (500 MHz, CD2Cl2): δ = 8.17 (d, J = 8 Hz, 2H), 8.07 (br s, 2H),
8.01 (s, 2H), 7.75 (br s, 2H), 7.69 (br s, 2H), 7.30 (br s, 2H), 7.25 (br m,
2H), 6.97 (m, 4H), 6.72-88 (br m, 14H), 6.70 (m, 4H), 6.40 (br s, 4H), 6.17
(br d, J = 8 Hz, 4H), 6.02 (br s, 2H), 5.96 (br s, 2H), 5.90 (br s, 2H), 2.20
(s, 6H), 1.11 (s, 18H), 1.08 (s, 18H) ppm; 1H NMR (400 MHz, CD2Cl2, -
40°C): δ = 8.15 (d, J = 8 Hz, 2H), 8.06 (d, J = 8 Hz, 2H), 7.99 (s, 2H), 7.73
(s, 2H), 7.67 (s, 2H), 7.27 (d, J = 8 Hz, 2H), 7.15 (d, J = 8 Hz, 2H), 6.95
(m, 4H), 6.72-6.85 (m, 12H), 6.67 (m, 6H), 6.37 (dd, J = 8, 2 Hz, 2H), 6.34
(dd, J = 8, 2 Hz, 2H), 6.13 (d, J = 8 Hz, 4H), 5.92 (d, J = 8 Hz, 4H), 5.89
(dd, J = 8, 2 Hz, 2H), 5.84 (dd, J = 8, 2 Hz, 2H), 2.11 (s, 6H), 1.06 (s, 18H),
1.03 (s, 18H) ppm; 13C NMR (126 MHz, CD2Cl2): δ = 159.4, 157.1, 149.4,
149.0, 144.4, 143.1, 142.4, 141.7, 139.8, 139.4, 139.3, 139.2, 138.8, 137.2,
136.8, 136.7, 136.2, 132.3, 131.6, 131.1, 130.8, 130.4, 129.5, 128.2, 128.1,
127.5, 127.5, 127.3, 127.0, 126.6, 126.2, 125.7, 124.2, 124.1, 123.6, 34.6,
34.5, 31.5, 31.4, 25.9 ppm; Elemental analysis calcd. for C102H92BCuF4N4
x 1/5 CH2Cl2: C 79.65, H 6.04, N 3.64; found: C 79.74, H 6.13, N 3.71;
Ligand L2. A mixture of 3 (159.2 mg, 0.70 mmol) and 4 (762.2 mg, 1.54
mmol) in o-xylene (10 ml) was heated at 80°C under argon for 30 min. The
temperature was slowly raised to reflux within 1h and the mixture kept at
reflux overnight. The solvent was removed by using a gentle stream of air
while heating at 100°C and the resulting solid was further dried under high
vacuum. Column chromatography (SiO2; eluent: CH2Cl2/MeOH 100:1 to
20:1) gave L2 (582 mg, 71%). Beige solid. M.p.: 207-208°C; IR (neat, cm-
1): n = 2960, 1495, 838, 699; UV/Vis (CH2Cl2): λmax (ε, M-1 cm-1) = 246
(121300), 285 (sh, 58600), 310 (sh, 43300), 355 (sh, 5100); 1H NMR (300
MHz, CD2Cl2): δ = 7.89 (d, J = 8 Hz, 2H), 7.84 (s, 2H), 7.65 (s, 2H), 7.27
(d, J = 8 Hz, 2H), 7.24 (m, 4H), 7.13 (m, 6H), 7.01 (m, 4H), 6.96 (AB system,
4H), 6.90 (AB system, 4H), 6.84-6.88 (m, 2H), 6.78-6.83 (m, 8H), 6.74 (AB
system, 4H), 1.17 (s, 18H, tBu), 1.15 (s, 18H, tBu) ppm; 13C NMR (126
MHz, CD2Cl2): δ = 160.4, 149.0, 148.7, 146.4, 142.7, 142.6, 141.5, 141.4,
141.0, 140.7, 139.9, 137.9, 137.8, 135.2, 132.3, 131.7, 131.7, 131.6, 130.6,
128.0, 127.4, 127.3, 126.7, 126.5, 126.1, 125.1, 124.1, 123.8, 34.6, 34.6,
31.5, 31.5 ppm; Elemental analysis calcd. for C88H80N2•1/3CH2Cl2: C
88.86, H 6.81, N 2.35; found: C 88.56, H 6.75, N 2.35; MALDI-TOF MS
(m/z): 1165.46 ([M+H]+, calcd for C88H81N2: 1165.64).
- +
MALDI-TOF MS (m/z): 1435.62 ([M-BF4 ] , calcd for C102H92CuN4:
1435.66).
X-ray crystal structures. The crystallographic data and the refinement
parameters are reported in the Supporting Information for all the
compounds. The X-ray crystal structures have been deposited at the
Cambridge Structural Database (CCDC deposition numbers: 2079617 for
[Cu(L1)(dmp)](BF4) and 2079620 for [Cu(L2)(dmp)](BF4)). These data are
provided free of charge by the joint Cambridge Crystallographic Data
Centre and Fachinformationszentrum Karlsruhe Access Structures service
(https://www.ccdc.cam.ac.uk/structures).
[Cu(L1)2](BF4). A solution of L1 (150 mg, 0.309 mmol) in CH2Cl2 (15 mL)
was added to a solution of [Cu(CH3CN)4](BF4) (48.6 mg, 0.155 mmol) in
MeCN (15 mL) under argon. The resulting orange solution was stirred for
1 h at rt and evaporated. Column chromatography (SiO2; eluent: CH2Cl2 to
CH2Cl2/MeOH 80:1 to 50:1) followed by crystallization from CH2Cl2/diethyl
ether gave [Cu(L1)2](BF4) (72.8 mg, 42%). Dark orange-red solid. M.p.:
184-186°C; IR (neat, cm-1): n = 3058, 1494, 1477, 1054, 861, 743, 701;
UV/Vis (CH2Cl2): λmax (ε, M-1 cm-1) = 319 (59200), 442 (5400); 1H NMR
(500 MHz, CD2Cl2): δ = 8.15 (d, J = 8 Hz, 4H), 8.01 (s, 4H), 7.50 (br s, 4H),
7.26 (d, J = 8 Hz, 4H), 7.12 (m, 4H), 7.07-6.97 (m, 16H), 6.51 (d, J = 8 Hz,
8H), 6.18 (br s, 4H) ppm; 13C NMR (126 MHz, CD2Cl2): δ = 158.5, 144.4,
141.6, 140.1, 138.1, 136.5, 130.9, 130.1, 129.8, 129.2, 128.9, 128.7,
Acknowledgements
Financial support by the International Center for Frontier
Research in Chemistry and the LabEx “Chimie des Systèmes
Complexes” is gratefully acknowledged. We thank L. Karmazin
and C. Bailly for the X-ray crystal structure resolutions and J.-M.
Strub for the mass spectra.
127.8, 127.6, 126.2 ppm; Elemental analysis calcd. for C72H48BCuF4N4
x
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European Journal of Inorganic Chemistry
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Keywords: Conformation • Copper • Dynamic NMR studies •
Heteroleptic complexes • Phenanthroline
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European Journal of Inorganic Chemistry
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Entry for the Table of Contents
Ar
Ar
Ar
Ar
Ph
Ph
Ph
Ph
N
N
N
N
N
N
N
N
Cu+
Cu+
Ph
-
-
BF4
BF4
Ar
Ph
Ph
Anti
Ar
Ar
Ar
Ph
Syn
Heteroleptic bis-phenanthroline copper(I) complexes have been prepared from phenanthroline ligands substituted in their 2,9-positions
with mono- and tetra-arylated phenyl groups. The resulting copper(I) complexes revealed dynamic conformational exchange between
several atropisomers. Detailed NMR studies and Density Functional Theory (DFT) calculations have been carried out to assess their
conformations in solution.
Institute and/or researcher Twitter usernames: @INC_CNRS - @Unistra - @LIMA_UMR7042 (Institutions) / @Giin0 -
@nierengarten6 (Researchers).
11
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
B. Ralahy obtained his Bachelor (2017) and Master (2019) degrees in biochemistry at the University of Strasbourg,
France. During his Master, he worked on the synthesis of phenanthroline ligands and their copper(I) complexes in
the group of J.-F. Nierengarten. In 2020, he joined the group of L. Peng (CINaM, Aix-Marseille University and
CNRS) to prepare a PhD. He is currently working on supramolecular PAMAM dendrimers for biomedical
applications.
U. Hahn obtained his PhD in 2003 from the University of Bonn (Germany) under the supervision of F. Vögtle
working on dendrimers for diagnostic applications. After postdocs in the groups of L. De Cola and J.-F. Nierengarten,
he moved in 2007 to the Universidad Autónoma de Madrid (Spain), where he worked in the group of T. Torres as
Ramón y Cajal fellow on hydrophilic phthalocyanines for photodynamic therapy. Since 2011 he works as a
researcher for the CNRS in the group of J.-F. Nierengarten (ECPM, University of Strasbourg and CNRS). His
current research activities are mainly dedicated to polyaromatic compounds and nanographenes.
E. Wasielewski studied structural biology and liquid state NMR spectroscopy at the University of Strasbourg, France,
and obtained his PhD in 2004 under the supervision of B. Kieffer. He worked as a post-doctoral fellow with G. Mer
at the Mayo Clinic College of Medicine, Rochester MN, USA (2004-2009), then worked as NMR lab manager in the
CNC institute of the University of Coimbra (2009-2015). He joined the CNRS in 2015 to manage the new NMR
Core Facility of the Cronenbourg Campus, as part of the LIMA (Laboratoire d’Innovation Moléculaire et Applications,
UMR 7042).
12
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10.1002/ejic.202100331
European Journal of Inorganic Chemistry
FULL PAPER
J.-F. Nierengarten studied biochemistry and chemistry at the University of Strasbourg, France, and received his
doctoral degree under the supervision of J.-P. Sauvage and C. Dietrich-Buchecker (1994). He worked as a post-
doctoral fellow with F. Diederich at the ETH-Zürich, Switzerland (1994-1996). He then obtained a CNRS position
in 1996. Currently he is Directeur de Recherche at the CNRS (DR 1) and head of the laboratoire de Chimie des
Matériaux Moléculaires (ECPM, University of Strasbourg and CNRS). His research focuses on the chemistry of
fullerenes for applications in materials science and biology. Other research interests are in the area of dendrimers,
luminescent transition metal complexes, porphyrins, π-conjugated systems and macrocyclic compounds. He is co-
chair of the Editorial Board of Chemistry-a European Journal.
13
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