Structural and chiroptical properties of acyclic and macrocyclic 1,5-bis(oxazoline) ligands and their copper(I) and silver(I) complexes

Article abstract of DOI:10.1002/1099-0682(200207)2002:7<1738::AID-EJIC1738>3.0.CO;2-O

C2-symmetric acyclic Ligands 1a-1e, macrocyclic ligands 2a-2d and 3, and their copper(I) and silver(I) complexes were studied by UV, NMR and CD spectroscopy. The stability constants of the metal-1a and -1b complexes with 1:1 and 1:2 stoichiometries in acetonitrile were determined by spectrometric titrations. Evidence of tetrahedral coordination for complex [Ag(1a)₂]+ was obtained from the complexation induced shifts (CIS) and NOEs, and corroborated by molecular modeling. Ligands with phenyl rings on the stereogenic centers (1a,c and 2) exhibit a strong Cotton effect (CE) in the region of the 1B_b band, which is absent in the ligands with benzyl groups on the stereogenic center (1b, 3). This effect is ascribed to the exciton coupling (EC) of two aromatic chromophores on the stereogenic center. For ligands la and 2a the EC changes the sign on complexation to Cu⁺, revealing opposite helicity of the free and complexed ligand. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0682(200207)2002:7<1738::AID-EJIC1738>3.0.CO;2-O

FULL PAPER
Structural and Chiroptical Properties of Acyclic and Macrocyclic
1,5-Bis(oxazoline) Ligands and Their Copper(I) and Silver(I) Complexes
[a]
[a]
[a]
[b]
[c]
´
ˇ ´
Miklos Hollosi, Zsuzsa Majer, Elemer Vass, Zlata Raza, Vladislav Tomisic,
[b]
[b]
[b]
[b]
ˇ
ˇ
´
´
Tomislav Portada, Ivo Piantanida, Mladen Zinic, and Vitomir Sunjic*
Keywords: Circular dichroism / Conformation analysis / Copper / Silver / Bis(oxazoline) ligands / Macrocyclic ligands
C2-symmetric acyclic ligands 1a−1e, macrocyclic ligands
ters (1a,c and 2) exhibit a strong Cotton effect (CE) in the
2a−2d and 3, and their copper(I) and silver(I) complexes were region of the 1Bb band, which is absent in the ligands with
studied by UV, NMR and CD spectroscopy. The stability con-
benzyl groups on the stereogenic center (1b, 3). This effect
stants of the metal−1a and −1b complexes with 1:1 and 1:2
is ascribed to the exciton coupling (EC) of two aromatic chro-
stoichiometries in acetonitrile were determined by spectro- mophores on the stereogenic center. For ligands 1a and 2a
metric titrations. Evidence of tetrahedral coordination for
complex [Ag(1a)2]+ was obtained from the complexation in-
duced shifts (CIS) and NOEs, and corroborated by molecular
modeling. Ligands with phenyl rings on the stereogenic cen-
the EC changes the sign on complexation to Cu+, revealing
opposite helicity of the free and complexed ligand.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Introduction
These results are in line with our previous observation,[32,33]
that enantioselectivity enhancement can be achieved by re-
stricting conformational mobility of the ligand by repulsive
or attractive πϪπ interactions.[34Ϫ36]
Chiral bis(oxazoline)Ϫmetal complexes have received a
great deal of attention through their use in various asym-
metric catalytic processes.[1Ϫ4] Among them the complexes
with 1,5-bis(oxazoline) ligands having one carbon spacer
between the oxazoline rings and able to form six-membered
metal chelates, are the most frequently utilized as chiral
catalytic complexes in various enantioselective transforma-
tions.
Structural studies of the ligandϪmetal complexes often
provide valuable information regarding transition state as-
sembly in the catalytic transformation. Already, the solid-
state structures of various bis(oxazoline)Ϫmetal complexes
have been elucidated on the basis of X-ray diffraction
analysis,[5,9,19,26,27,37Ϫ44] but only a few were determined in
solution based on NMR[26,38,43Ϫ45] and CD[44,45] spectro-
scopic investigations. To the best of our knowledge, the
spectroscopic study of 1,5-bis(oxazoline)Ϫcopper(I) and
Ϫsilver(I) complexes in solution, aimed at elucidating their
structure and stability, has not been performed to date.
Herewith we report the results of the combined achiral (UV,
fluorimetric, ES-MS, NMR) and chiroptical (CD) spectro-
scopic studies of the acyclic (1) and macrocyclic ligands (2,
3) (Figure 1) as well as their Cuϩ and Agϩ complexes, cor-
roborated by molecular modeling of some selected com-
plexes. The study was expected to give insight into the topo-
logy of the free ligands and complexes, identify the stoichi-
ometry, stability and structure of the complexes, and con-
sequently to shed more light on the origin of
stereoselectivity exhibited by this type of bisoxazoline li-
gands.
The most explored catalytic complexes are those of
copper(I)[5Ϫ14] (enantioselective cyclopropanation of
alkenes,[5Ϫ11] aziridation of olefins[12] and imines,[13] allylic
oxidation
(DielsϪAlder reaction,[15Ϫ19] Mukaiyama aldol reaction,[20]
MukaiyamaϪMichael addition
reaction,[21,22]
of
cycloalkenes[14]),
copper(II)[15Ϫ24]
FriedelϪCrafts reaction,[23] Canizzaro reaction[24]),
palladium(II)[25Ϫ27] (allylic substitution reaction), and
zinc(II)[28Ϫ30] (allylation reaction). We have recently re-
ported on very high cumulative enantio- and diastereoselec-
tivity in cyclopropanation of styrene catalyzed by copper(I)
complexes of macrocyclic ligands 2aϪ2c (Figure 1).[31]
[a]
´
Eötvös Lorand University, Department of Organic Chemistry
P. O. Box 32, 1518 Budapest, Hungary
[b]
Department of Organic Chemistry and Biochemistry, ‘‘Rudjer
Boskovic’’ Institute,
ˇ
´
P. O. Box 180, 10002 Zagreb, Croatia
Fax: (internat.) ϩ 385-1/468-0195
E-mail: sunjic@rudjer.irb.hr
After this study was completed, a paper of Walsh et al.[46]
appeared reporting on the study of the conformation of
flexible macrocyclic catalysts in solution. The results sup-
port that bis(sulfonamido) ligands, derived from (R,R)-1,2-
[c]
Laboratory of Physical Chemistry, Faculty of Science,
University of Zagreb,
P. O. Box 163, 10001 Zagreb, Croatia
1738
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0707Ϫ1738 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2002, 1738Ϫ1748

Acyclic and Macrocyclic 1,5-Bis(oxazoline) Ligands and Their Copper(I) and Silver(I) Complexes
FULL PAPER
gram.[47] The addition of Cuϩ into the MeCN solutions of
both 1a and 1b caused a hyperchromic effect in the corres-
ponding UV spectra (Figure 2). Among several chemically
reasonable speciations used in the fitting of the spectropho-
tometric data, two (set by assuming the presence of [CuL]ϩ
and [CuL2]ϩ or [CuL2]ϩ and [Cu2L2]2ϩ; L standing for
either 1a or 1b) gave satisfactory results. However, the elec-
trospray mass spectrometry (ES-MS) experiments indicated
formation of [CuL]ϩ and [CuL2]ϩ complexes and no evid-
ence of the presence of [Cu2L2]2ϩ species was observed. The
mass spectra of MeCN solutions of Cuϩ/1a with various
c(Cuϩ)/c(1a) ratios showed peaks at m/z
ϭ
678
{[Cu(1a)(MeCN)]nnϩ} and 1212 {[Cu(1a)2]ϩ}, while in the
nϩ
case of Cuϩ/1b peaks were observed at 665 {[Cu(1b)]n
}
and 1268 {[Cu(1b)2]ϩ, a very weak signal, presumably due
to the fragmentation in the mass spectrometer}. The iso-
topic abundances of the peaks at m/z ϭ 678 and 665 showed
peak separation of 1 Da, supporting the presence of singly
charged species, i.e. [CuL]ϩ. The stability constants of the
Cuϩ complexes calculated from the spectrophotometric ti-
tration data are collected in Table 1. Both, 1a and 1b form
quite stable 1:1 (KCuL ഠ 105 Ϫ1) and 1:2 (KCuL2 ഠ 104

Ϫ1) complexes with Cuϩ in MeCN.
Figure 2. Spectrophotometric titration of 1a (c ϭ 5·10Ϫ5 mol·dmϪ3
)
with [(CH3CN)4Cu]PF6 in MeCN; l ϭ 1 cm; ϕ ϭ 25 °C; c(Cuϩ) ϭ
0 (bottom curve) to 1.7·10Ϫ4 mol·dmϪ3 (top curve); the spectra are
corrected for dilution
Table 1. Stability constants of Cuϩ and Agϩ complexes with 1a
and 1b in MeCN determined by UV, 1H NMR, and fluorimetric
titrations; numbers in parentheses are estimated errors of the last
digit
Figure 1. Formulae 1Ϫ3
diaminocyclohexane, are bound to the catalytic metal ion
in a C2-symmetric conformation in the transition state of
the asymmetric addition of Et2Zn to benzaldehyde.
Species
log K[a]
UV
1H NMR
Fluorimetry
Cu(1a)
Cu(1a)2
Cu(1b)
Cu(1b)2
Ag(1a)
Ag(1a)2
Ag(1b)
Ag(1b)2
5.23(5)
4.2(2)
5.07(3)
4.2(1)
Results and Discussion
Spectrometric Studies of Copper(I) and Silver(I) Complexes
of 1a and 1b
3.49(7)
3.27(8)
3.92(7)
3.1(2)
3.45(6)
3.63(4)
The copper(I) and silver(I) complexing properties of li-
gands 1a and 1b have been investigated by spectrometric
(UV, NMR, and fluorimetric) titrations. The obtained spec-
K ϭ [MLn]/([MLnϪ1][L]); M standing for Cuϩ or Agϩ and L for
[a]
trometric data were processed using the SPECFIT pro- 1a or 1b; n ϭ 1, 2.
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748
1739

ˇ
´
V. S unjic et al.
FULL PAPER
The UV spectral changes recorded during the titrations
of 1a and 1b with Agϩ in MeCN had qualitatively the same
trend as in the case of Cuϩ, but were insufficient to enable
accurate analysis. Therefore, the formation of Agϩ com-
1
plexes was studied by means of H NMR and fluorimetric
titrations. The 1H NMR spectra of 1a and 1b in [D3]MeCN
are concentration-independent in the range of 2·10Ϫ3 to
2·10Ϫ2 mol·dmϪ3. The addition of AgBF4 to 1a up to the
ratio c(Agϩ)/c(1a) ϭ 0.5 induced shielding of all protons
except that of C(5)ϪHtrans (Figures 3 and 4, a). Further ad-
dition [c(Agϩ)/c(1a) ϭ 0.5Ϫ2.0] resulted in a deshielding
effect. The shielding/deshielding effect changed at the ratio
c(Agϩ)/c(1a) ϭ 0.5 indicating the initial formation of
[Ag(1a)2]ϩ species. Accordingly, the titration data were best
fitted with a model including [Ag(1a)]ϩ and [Ag(1a)2]ϩ
complexes. The addition of Agϩ to the solution of 1b
caused deshielding of all proton signals except for those of
the methyl groups, which were shielded. However, the data
were again best fitted with the model including two com-
plex species, [Ag(1b)]ϩ and [Ag(1b)2]ϩ. Comparison of the
determined stability constants of Agϩ complexes with those
of Cuϩ complexes (Table 1) shows that the former are more
than one order of magnitude less stable for both ligands 1a
and 1b.
Figure 4. Complexation-induced shifts for 1a؊ and 1bϪAgϩ com-
plexes; a): (ϩ) CH3; (᭜) C(2Ј)ϪH; (ϫ) C(4Ј)ϪOCH2; (᭿)
C(3Ј)ϪH; (∆) C(4)ϪH; (*) C(5)ϪHtrans; (•) C(5)ϪHcis; b): (Ο)
C(4)ϪCH2
rather good agreement. The stoichiometries of the Agϩ
complexes were additionally confirmed by electrospray
mass spectrometry which showed peaks at m/z ϭ 683
{[Ag(1a)]nnϩ}, 1258 {[Ag(1a)2]ϩ} and 711 {[Ag(1b)]nnϩ}.
The exception was the absence of the signals of the
[Ag(1b)2]ϩ species that could be attributed to the frag-
mentation in the mass spectrometer and to the equilibrium
in solution (see next paragraph and Figure 6). The isotopic
abundances of the peaks at m/z ϭ 683 and 711 showed peak
separation of 1 Da, corresponding to the singly charged
species [Ag(1a)]ϩ and [Ag(1b)]ϩ, respectively.
Figure 3. Experimental (dots) and calculated (line) chemical shifts
of C(2Ј)ϪH of 1a induced by addition of AgBF4 (solvent:
[D3]MeCN)
The stabilities of the 1aϪ and 1bϪAgϩ complexes were
also determined by means of fluorimetry. In MeCN ligands
1a and 1b exhibited a concentration-independent fluores-
cence up to the concentration of 2·10Ϫ5 mol·dmϪ3. The ad-
dition of Agϩ quenched the fluorescence of 1a and 1b for
ca. 90%. Processing of the titration data gave good fits when
1
Complexation-Induced Shifts (CIS) and NOEs in the H
NMR Spectra of the Ag؊1a,b Complexes
Structure of Complex [Ag(1a)2]؉
Addition of Agϩ to 1a produced significant shifts of all
only the [AgL]ϩ complex formation was taken into account the resonances of 1a, without line broadening. This indic-
for both compounds. The absence of [AgL2]ϩ could be ex- ates fast equilibrium kinetics on the NMR time scale. The
plained assuming that fluorimetric properties of this species observed complexation induced shifts of 1a, expressed as
are similar to those of the free ligand causing insignificant ∆δ ϭ δHilig Ϫ δHicompl (ppm), were plotted against the
spectral changes. On the other hand, the [AgL]ϩ complexes c(Agϩ)/c(1a) ratio (Figure 4, a). Agϩ was added until a ra-
exhibit a much weaker emission than free 1a and 1b. Con- tio of 0.5 was reached for the induced upfield shifts of all
sequently, titration data were collected mostly under condi- 1a resonances except that of C(5)ϪHtrans; further additions
tions preferable for 1:1 stoichiometry and only the forma- of the cation produced downfield shifts of all resonances. A
tion of [Ag(1a)]ϩ and [Ag(1b)]ϩ was observed. As can be clear maximum at a c(Agϩ)/c(1a) value of 0.5 suggests ini-
seen in Table 1, the stability constants of these complexes tial formation of the AgϩϪ1a complex in a 1:2 stoichi-
1
determined by fluorimetric and H NMR titrations are in ometry which at higher ratios transforms to the 1:1 com-
1740
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748

Acyclic and Macrocyclic 1,5-Bis(oxazoline) Ligands and Their Copper(I) and Silver(I) Complexes
FULL PAPER
plex. The methyl groups at the bis(oxazoline) bridge as well
as the aromatic C(2Ј)ϪH and C(3Ј)ϪH groups experience
the strongest shielding effects in the [Ag(1a)2]ϩ complex,
which strongly suggests formation of the tetrahedral Agϩ
complex with the cation bound to 4 nitrogen atoms of the
two orthogonally oriented bis(oxazoline) units of the 1a
molecules. The molecular models (see next paragraph and
Figure 7, a) shows that in such a complex each bis(oxazo-
line) unit is located between two phenyl groups of the se-
cond ligand molecule. The C(4)-phenyl groups of two li-
gands end up close to one another, which results in the shi-
elding of C(2Ј)ϪH and C(3Ј)ϪH. The methyl groups and
the C(4)ϪH and C(5)ϪHcis atoms of each bis(oxazoline)
unit are also shielded by C(4)-phenyl groups of the second
ligand. Additional support for the occurrence of such a 1:2
Figure 5. NOE interactions observed in the NOESY spectrum of
complex comes from the analysis of the NOESY spectrum
at a c(Agϩ)/c(1a) ratio of 0.5. The observed NOE interac-
tions are shown in Figure 5. The molecular model of 1a
shows that the majority of the observed NOE crosspeaks
could be explained by the favorable distances between re-
spective hydrogen atoms in a single 1a molecule. However,
a clear crosspeak corresponding to NOE interactions be-
tween C(4Ј)ϪOCH2 and methyl protons of the bis(oxazo-
line) bridge is observed. The closest possible distance be-
[Ag(1a)2]ϩ
ally more stable. This is reflected in the distribution dia-
grams of AgϩϪ1b and AgϩϪ1a systems shown in Fig-
ure 6, a, b. At a c(Agϩ)/c(1b) ratio of 0.5 in the first system
the concentration of the 1:1 complex is close to twice that
of the 1:2 complex (Figure 6, a). For AgϩϪ1a at this con-
centration ratio, a 1:2 complex is present in approximately
four times higher concentration than the 1:1 complex (Fig-
ure 6, b). Consequently, the presence of an AgϩϪ1a 1:2
complex can be detected through pronounced shielding ef-
fects. However, this is not the case for an AgϩϪ1b system
where the complexation induced shifts are dominated by
the prevailing 1:1 complex at low and high c(Agϩ)/c(1b)
ratios. Nevertheless, the observation of shielding effects of
methyl groups on the bridge in 1b indicates similar struc-
tures of 1:2 AgϩϪ1a and AgϩϪ1b complexes.
˚
tween these protons in the model of 1a is ca. 8 A, well
˚
exceeding the limiting value of 4Ϫ5 A, necessary for obser-
vation of NOE interactions. The additional proof for the
structure of the [Ag(1a)2]ϩ complex is also provided by the
observed trend of the complexation induced shifts in the
c(Agϩ)/c(1a) range of 0.5 and 2.0 (Figure 4, a), where all
ligand resonances underwent downfield shifts. This trend
could be explained by the disappearance of the [Ag(1a)2]ϩ
complex and predominant formation of the [Ag(1a)]ϩ com-
plex. The ligand protons close to the metal center [C(4)ϪH,
C(5)ϪH and C(2Ј)ϪH] of the 1:1 complex should be more
deshielded by Agϩ coordination through bond effects than
those in the 1:2 complex, where these effects are distributed
between two bis(oxazoline) units. Indeed, deshielding ef-
fects were observed for the C(4)ϪH and C(5)ϪH protons
Molecular Modeling
The [Ag(1a)2]ϩ complex was modeled[48] using 3Ϫ5 A
˚
on going from a 1:2 to a 1:1 complex. However, the strong- distance constrains between Me and OCH2Ph protons in
est deshielding experienced was of the methyl groups on the accordance with observed NOE interactions. The fully min-
bridge, being four bonds away of the Agϩ coordination site imized final structure gives NϪAgϩ distances of 1.700,
˚
(Figure 4, a). Therefore, the latter effect must come from 1.704, 1.704, and 1.710 A, Me to C(4)ϪPh centroid dis-
˚
the shielding of the methyl groups in the 1:2 complex by tances of 3.12 A and MeϪOCH2Ph CϪC distances of 3.89
˚
the π-system of the C(4)-phenyl groups of the second mole- A, in agreement with observed strong Me shielding and
1
cule of the ligand 1a.
NOEs in the H NMR spectra. The planes containing two
In contrast to 1a, the addition of Agϩ to a CD3CN solu- bis(oxazoline) units form an angle of 71°, showing pseudo-
tion of 1b produced deshielding of all proton resonances tetrahedral coordination (Figure 7, a). The conformational
but only slight shielding of the methyl protons; no clear space of the [Ag(1a)]ϩ complex was then searched using the
indication of a 1:2 complex could be observed (Figure 4, b). systematic search procedure at 30° increments for rotations
However, processing of the NMR titration data gave the of 8 rotatable bonds of PhOϪCH2Ph fragments. Five differ-
best fit for the formation of AgϩϪ1b complexes of 1:2 and ent low-energy conformations with energies within 5 kcal/
1:1 stoichiometries (see previous paragraph). Comparison mol were selected and each was fully minimized (Fig-
of the stability constants determined for AgϩϪ1a and Ϫ1b ure 7, b). The calculated structure reveals high conforma-
complexes (Table 1) shows that the 1:1 complex of 1b is tional mobility of C(4Ј)ϪOCH2Ph fragments and the ab-
around seven times more stable than the 1:2 complex, sence of possible intramolecular aromatic πϪπ interactions
whereas the equivalent complexes of 1a are of approxim- between terminal phenyl groups, is in complete accordance
ately the same stability, with the 1:1 complex being margin- with all experimental results.
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748
1741

ˇ
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V. S unjic et al.
FULL PAPER
Figure 6. Distribution diagrams of the Agϩ/1a and Agϩ/1b systems
in MeCN; c denotes total concentration, while [i] stands for equilib-
rium concentration of i ϭ L (1a or 1b), [ML]ϩ and [ML2]ϩ spe-
cies, respectively
Chiroptical (CD) Study of the Acyclic and Macrocyclic
Ligands and Their Copper(I) and Silver(I) Complexes
UV and CD Spectra of Acyclic Bis(oxazolines) 1
The short-wavelength band (λmax ϭ 194 nm) can be as-
signed to the πCϭNϪπ*CϭN transition in C2-symmetric 1e,
whereas in 1d it (λmax ϭ 190 nm) comprises superimposed
1La bands of the phenyl chromophore. The weak long-wave-
length band at λmax ϭ 264 nm in the UV spectrum of 1e is
Figure 7. a) Structure of [Ag(1a)2]ϩ generated by molecular
modeling; hydrogen atoms are omitted for the sake of clarity; b)
five superpositioned low-energy conformations of [Ag(1a)]ϩ gener-
assigned to the conjugated πϪπ* transition, based on the ated by the systematic search of the conformational space
ab initio 6-31G* calculations and analysis of photoelec-
tronic spectra;[49] 1a and 1b show similar UV spectra. How-

Ϫ1·cmϪ1), only ca 50% less intense then the positive 205-
ever, the spectrum of 1d is strikingly different showing much
less intense bands at 264, 258, and 252 nm corresponding
to phenyl substituents. The much more intense band ob-
served at 275 nm for 1a (ε ϭ 4700 Ϫ1·cmϪ1) as compared
to the band at 264 nm for 1d (ε ϭ 360 Ϫ1·cmϪ1), belongs
to the phenoxy group on the stereogenic center.
Absolute configuration of acyclic bis(oxazolines) are
(R,R) for 1a,c,d and (S,S) for 1b,e. In the CD spectra of 1a
and 1c the intense positive band at 205 nm and 204 nm,
respectively, is accompanied by a negative band near
192 nm as a sign of exciton interaction between the 1Bb
transitions. Ligand 1c with an O(CH2)4O bridge between
the para-phenylene and phenyl rings shows some blue-shift
nm band of 1a and 1c (Table 2). One can argue that this
shift in position and intensity is because of the absence of
electron-donating para-alkoxy groups in 1d. In the CD
spectra of chiral bis(oxazolines) 1b and 1e, having no aro-
matic substituent at the stereogenic center the bands at 204
and 190 nm, respectively, belong to the π-π* transition of
the CϭN chromophore (Table 2). The possibility of a weak
exciton interaction between the 1Bb transitions of the
phenyl chromophores cannot be excluded either.
CD Spectra of Macrocyclic Bis(oxazolines) 2 and 3
As expected, in the CD spectrum of (R,R)-2a with oppos-
and decreased intensity, presumably due to the increased itely configured chiral centers, as compared to 1b and 3, the
conformational mobility of the side chains. The third ligand 1La band (ca. 230 nm) is positive. However, instead of one,
with a phenyl group attached to a stereogenic center (1d) there are two extremely strong bands of opposite sign in the
shows a ca. 10 times decreased negative band at 190 nm (∆ε 1Bb region of the spectrum. In the UV spectrum a strong
Ͻ Ϫ5 Ϫ1·cmϪ1), and a positive band at 198 nm (∆ε ϭ 46.2 absorption was measured at 195 nm that supports the above
1742
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748

Acyclic and Macrocyclic 1,5-Bis(oxazoline) Ligands and Their Copper(I) and Silver(I) Complexes
FULL PAPER
Table 2. CD and UV spectroscopic data for acyclic bis(oxazolines) 1aϪ1e and their metal complexes in MeCN
Ligand (ϩ Metal)
Configuration CD (1st line): λ/nm (∆ε/Ϫ1 cmϪ1
)
UV (2nd line): λ/nm (ε ϫ10Ϫ4/Ϫ1 cmϪ1
)
1Bb
1La, πϪπ*(CϭN)
1Lb, πϪπ* or charge transfer
1a
(R,R)
(S,S)
(R,R)
192 (Ϫ73.6), 205 (64.22)
190 (12.1), 196 sh
192 (Ϫ24.0), 205 (15.0)
196 (20.5), 204.5 (Ϫ13.0) 219.5(7.80), 235 (Ϫ34.0)
204 (Ϫ16.8)
190.5 (12.7), 195.5 sh
196.5 (1.02)
198 (Ϫ7.23)
190.5 (Ϫ58.2), 204 (64.7) 229 (37.6)
195(11.7) 224.5 (2.75)
196 (7.50), 202.5 (Ϫ6.98) 213sh 221(10.8) 235(Ϫ23.2) 285 (0.095)
229.5(41.2)
227 (2.81)
226 (12.0), 237 (Ϫ2.0)
277(Ϫ0.79)
275(0.47)
1a ϩ Agϩ, rAg,1a ϭ 1[a]
1a ϩ Cuϩ, rCu,1a ϭ 1
1b
284.5 (0.20)
282 (0.31)
275.5 (0.51)
ഠ 229sh
225.5(2.7)
206 (Ϫ4.05), 237 (Ϫ1.78)
207 (Ϫ4.23), 229.5 (5.83)
1b ϩ Agϩ, rAg,1b ϭ 1[a]
1b ϩ Cuϩ, rCu,1b ϭ 1
1c
281 (Ϫ0.69)
277 (Ϫ0.94)
1c ϩ Cuϩ, rCu,1c ϭ 1
1d
(R,R)
(S,S)
Ͻ 190 (Ͻ Ϫ5), 198 (46.2) 214 (31.9)
256 (0.50)
Ͻ 190 (Ͼ 12)
190 (Ϫ10.0)
194 (0.88)
207 (3.17)
229 sh (Ϫ3.13)
222 sh (0.09)
252 (0.043), 258 (0.048), 264 (0.036)
[b]
1e
264 (very weak)
[a]
[b]
Continuous spectral changes up to rAg,L ഠ 5. No band could be measured even with a 1-cm cell.
assignment. Clearly, the strong bands at 204 and 194 nm rocycles of type 2 have a positive CD (mostly two positive
are due to the para-substituted phenyl chromophores at- shoulders) between 225 and 205 nm. The lack of an exciton
1
tached to the stereogenic centers (Table 3). Notably, a coup- couplet in the La region can be attributed to the nearly
let in the 1Bb band spectral region was observed for 1a with parallel orientation of the electric transition vector
para-alkoxyphenyl substituents but not for 1b with para- (Table 3). A positive band appears in the spectrum of 2b at
alkoxylbenzyl groups.
220 nm. The complexity of the spectrum clearly indicates
Regardless of the length of the alkyl chain (n ϭ 2Ϫ5), that the CD contribution of the ‘‘lower half’’ of the molec-
the macrocyclic ligands 2aϪd with para-alkoxy-substituted ule cannot be completely neglected. Because of the in-
phenyl groups attached to the stereogenic centers show CD creased flexibility, this part of the molecule likely occurs as
spectra marked by an almost symmetric exciton couplet in a mixture of conformers but the dominant one(s) are ex-
the 1Bb region. The most intense bands turn up in the spec- pected to have the same sign of helical twist of the para-
trum of (R)-2b (n ϭ 3) [204 nm (90.5 Ϫ1·cmϪ1) and alkoxyphenyl rings attached to the C(Et)2 bridge.
193 nm (Ϫ73.0 Ϫ1·cmϪ1), respectively]. The positive sign
The CD spectrum of (S,S)-3 is dominated by two nega-
1
of the couplet is compatible with the orientation of the Bb tive bands at 203.5 and 228 nm which can be assigned to
electric transition moment vectors, as discussed in the final the 1Bb and 1La band, respectively, of the para-alkoxybenzyl
1
section. On the basis of the increased intensity of the La chromophores attached to stereogenic centers. A weak pos-
1
1
bands, a weaker exciton interaction between the La trans- itive and some weaker negative bands are seen in the Lb
itions cannot be excluded either. However, instead of the region of the spectrum (Table 3). In the spectrum of the
expected negative band near 220 nm, the spectra of all mac- related acyclic analog (S,S)-1b (R ϭ BzlϪOϪBzl) the cor-
Table 3. CD and UV spectroscopic data for macrocyclic bis(oxazolines) 2aϪ2d and 3 and their metal complexes in MeCN
Ligand (ϩ metal)
CD (1st line): λ/nm (∆ε/Ϫ1 cmϪ1
)
UV (2nd line): λ/nm (ε ϫ10Ϫ4/Ϫ1 cmϪ1
)
1Bb
1La, πϪπ*(CϭN)
1Lb
(R,R)-2a
(R,R)-2b
(R,R)-2c
(R,R)-2d
(S,S)-3
194 (Ϫ71.0), 204 (83.9)
195 (12.6)
193 (Ϫ73.00), 204 (90.5)
195 (12.9)
191.5 (Ϫ65.5), 204 (75.2)
196 (13.5)
191.5 (Ϫ66.6), 203.5 (81.2) ഠ 211 sh, ഠ 222 sh 232 (37.0) 276 (Ϫ1.07)
196 (13.8)
203.5 (Ϫ19.8)
194.5 (15.3)
ഠ 211 sh, ഠ 218 sh, 233 (37.0) 283 (Ϫ0.61)
226 (3.18)
276 (0.61)
220 (25.8), 232 (37.8)
226 (3.27)
ഠ 212 sh, 231 (38.2)
227 (3.40)
284.5 (Ϫ0.62)
276 (0.73)
277 (Ϫ0.83), 290Ϫ325 (weak positive bands)
276 (0.77)
227 (3.69)
228 (Ϫ6.40)
226 (3.66)
229 (Ϫ6.50)
ഠ 222 sh
276.5 (0.83)
277 (Ϫ0.072), 282.5 (0.07), 299.5 (Ϫ0.05)
ϩ Agϩ, rAg,3 ϭ 1[a] 203.5 (Ϫ14.9)
273.5 sh (Ϫ0.29) weak positive bands above 300 nm
261 (0.68), 287 sh, 311.5 (Ϫ0.145)
ϩ Cuϩ, rCu,3 ϭ 1
209 (Ϫ6.80)
[a]
Continuous spectral changes up to rAg,3 ഠ 4.
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748
1743

ˇ
´
V. S unjic et al.
FULL PAPER
responding bands turn up at 204 nm (Ϫ16.8 Ϫ1·cmϪ1),
Titration of 1b with Agϩ resulted in gradual spectral
229 nm (sh) and 282 nm. The lack of exciton splitting in changes up to rAg,1b ϭ 5 (Figure 10). The formation of a
the far-UV spectral region of 1b and 3 is attributed to the 1:1 complex of exceeding stability could not be detected
difference in geometry and the greater conformational free- (Table 2). No definite spectral changes were seen between
dom of the para-alkoxybenzyl chromophores.
r
Ag,1b ϭ 5 and 10. Addition of Cuϩ caused marked spectral
1
changes in the La band region. Above rCu,1b ϭ 1 the main
feature of the spectra is a positive band at ca. 230 nm. The
formation of no dominant 1:1 complex could be detected
(Figure 11).
CD Spectra of Copper(I) and Silver(I) Complexes
In order to study their complexation by CD, acyclic li-
gands 1a؊c as well as the corresponding macrocyclic anal-
ogs 2a؊d and 3 were titrated with Agϩ and Cuϩ ions.
In the spectrum of the acyclic ligand 1a the exciton coup-
1
let of the Bb transition was destroyed by Cuϩ complexing.
A strong negative and a weaker positive band replaced the
positive 1La band at 229.5 nm (Figure 8). The spectrum
showed only small changes between rCu,1a ϭ 1 and 2
[rM,L ϭ c(M)/c(L)] that suggests formation of a 1:1 complex
of significant stability, in accordance with the results of the
UV titration. Titration of 1a with Agϩ ions gave rise to
a gradual decrease of the band intensities of the couplet.
1
Significant spectral changes were observed also in the La
band region: a relatively strong negative band appeared at
rAg,1a Ն 1, while position and intensity of the band at
205 nm in the spectrum of 1a varied gradually up to
rAg,1a ϭ 5 (Figure 9). This spectral behavior can be ex-
plained by the formation of a 1:1 complex of lower stability
compared to [Cu(1a)]ϩ. Ligand 1c showed a similar spectral
response to Agϩ as 1a. Cuϩ broke the positive couplet at
rCu,1c Ն 0.75 and the titration data indicated formation of
a stable 1:1 Cuϩ complex.
Figure 10. CD spectra of free (S,S)-1b (—), and on addition of
Agϩ; 1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), 1:2 (·····), 1:5 (— - - —), and
1:10 (Ϫ · · Ϫ · ·Ϫ)
Figure 11. CD spectra of free (S,S)-1b (—), and on addition of
Cuϩ; 1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), 1:2 (·····), and 1:5 (— - - —)
Figure 8. CD spectra of free (R,R)-1a (—), and on addition of Cuϩ;
1:0.5 (Ϫ Ϫ Ϫ), 1:0.75 (— · —), 1:1 (·····) and 1:2 (— - - —)
Macrocyclic ligands 2a؊d showed a similar behavior
against Agϩ. The couplet was completely destroyed at an
rAg,2 Ն 2 ratio in the case of n ϭ 2 and 5 (Figure 12); Cuϩ
proved to be a more efficient breaker of the couplet. The
spectra reflected no exciton interaction at rCu,2 Ն 1 for li-
1
gands 2, n ϭ 2 and 4. At rCu,2 Ͼ 1 the La band appeared
1
as a pair of negative and positive bands. The Bb couplet,
characteristic for the spectra of ligands 2a؊d, was replaced
by a shoulder at ca. 210 nm and a pair of weaker blue-
shifted oppositely signed bands (Figure 13) at rCu,2 Ͼ 1.
Increasing the amount of Cuϩ gave rise to further spectral
shifts, both below and above 250 nm. This spectral behavior
can be interpreted as the consequence of the formation of
a 1:1 complex of lower stability. However, the occurrence of
complexes of other stoichiometry cannot be excluded either.
Figure 9. CD spectra of free (R,R)-1a (—), and on addition of
Agϩ; 1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), 1:2 (·····), and 1:5 (— - - —)
1744
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748

Acyclic and Macrocyclic 1,5-Bis(oxazoline) Ligands and Their Copper(I) and Silver(I) Complexes
FULL PAPER
Figure 14. CD spectra of free (S,S)-3 (—), and on addition of Agϩ;
1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), 1:2 (·····), and 1:7.8 (— - - —)
Figure 12. CD spectra of free (R,R)-2a (—), and on addition of
Agϩ; 1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), 1:2 (·····), and 1:5 (— - - —)
Figure 15. CD spectra of free (S,S)-3 (—), and on addition of Cuϩ;
1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), and 1:2 (·····)
Figure 13. CD spectra of free (R,R)-2a (—), and on addition of
Cuϩ; 1:0.5 (Ϫ Ϫ Ϫ), 1:1 (— · —), 1:2 (·····), and 1:5 (— - - —)
Conformations of Free Ligand 1a and Its Copper(I) and
Silver(I) Complexes in Solution
There are few reports on comparative spectroscopic study
of (poly)nitrogen ligands and their metal complexes. In an
early study Bosnich has observed the split of πϪπ* trans-
Addition of Agϩ to the macrocyclic ligand 3 resulted in itions and the nearly equal positive and negative CD bands
an almost continuous spectral change (Figure 14). The po- (couplet) for Zn2ϩ complexes with 1,4-N,N-bidentate li-
1
sition and intensity of the La band at 228 nm were less gands derived from salicylaldehyde (salene), comprising
influenced than that of the more intense 1Bb band at strong conjugated chromophores, and the presence of ex-
203.5 nm. Interestingly, the weak negative band at 299.5 nm citon interaction was demonstrated;[50,51] transoid con-
of 3 with vibrational fine structure changed only to a small formation of the free ligand was observed in solution of
extent, while the weak positive band at 282.5 nm of 3 was 6,6Ј-disubstituted 2,2-bipyridine,[52,53] and its rotation into
blue-shifted, changed sign and appeared with increased in- a cisoid conformer on complexation with Co2ϩ, affording a
tensity upon addition of Agϩ (Table 3). Accordingly, the complex of C2 symmetry, is documented by X-ray analysis
latter band can be assigned to the nϪπ* transition of the of the complex.[54] Williams et al. have recently reported on
CϭN groups, the nitrogen atoms of which coordinate to a the enantiopure helicates, complexes of chiral 2,6-bis(ox-
central cation. Addition of Cuϩ to 3 changed the spectrum azolyl)pyridines as tridentate nitrogen ligands and an Agϩ
continuously up to a 2:1 metal/ligand molar ratio (rCu,3
ϭ
ion.[44,45] They had observed large changes in the CD spec-
2) (Figure 15). Complexing of Cuϩ by 3 gave rise to an trum of the ligand upon complex formation. The free ligand
increase in the intensity of both long-wavelength bands. The shows a weak negative band in the UV region of the spec-
negative band showed a red-shift while the positive one a trum (∆ε296 ϭ Ϫ0.4 Ϫ1·cmϪ1), whereas the complex shows
more definite (Ͼ 20 nm) blue-shift (Table 3). The weak a strong positive band (∆ε301 ϭ 10.15 Ϫ1·cmϪ1, for one
negative band at ca. 310 nm of the 1:1 Cuϩ complex likely metal center), corresponding to a new shoulder in the UV/
belongs to the complexed Cuϩ ion. In the far-UV region, Vis spectrum (ε ϭ 1547 Ϫ1·cmϪ1). This CD activity was
both bands showed definite intensity and wavelength shifts attributed to a fixed helical conformation of the ligand in
up to rCu,3 ϭ 2.
the complex, whereas free ligands may adopt different con-
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748
1745

ˇ
´
V. S unjic et al.
FULL PAPER
formations that cancel out each other in the CD spectrum. by the NMR titration, but only partial inversion of the sign
Similar phenomena have been observed for several of the two bands of the couplet is observed. This reveals
ligands[55Ϫ57] and metal complexes,[44][58][59] and explained different influence of Cuϩ and Agϩ on conformational
by the exciton theory.
change of 1a upon complexation.
For the free bis(oxazoline)-type ligands, the conforma-
Generally, diminished intensity of the short-wavelength
tion in solution is not known. We have therefore performed band on complexation of 1a and 1b is presumably raised by
MM calculations[48] for an analog of 1a (for better visual- depleting π- and n-electron density in the CϭN group on
ization methyl substituents are attached instead of benzyl), coordination to the metal ion. Since any stronger exciton
and the most stable conformation found was the helically coupling between two CϭN chromophores, which are non-
twisted transoid one. In this conformation two phenyl planar in the free ligand, seems to be absent, coplanariz-
groups form a positive (P) twist, and therefore a positive ation of the 4π system on complexation should result in an
1
CE for the Bb band is expected, (Scheme 1, Figure 16). In enhanced intramolecular charge transfer, and consequently
the 1:1 complex CuϩϪ1a two halves of bis(oxazoline) are could influence only the long-wavelength transition band at
fixed in the nearly coplanar cis position with two large ca 280 nm.
groups on the stereogenic centers forming a ‘‘picket-fence’’
topology, important for high enantioselectivity in catalytic
reactions.[60]
Conclusion
Combined achiral and chiral spectroscopic study of 1,5-
dinitrogen ligands with C2-symmetic 1a and 1b and their
copper(I) and silver(I) complexes has shown that they form
stable 1:2 metal/ligand complexes, which transform on ul-
terior addition of metal ions to 1:1 complexes. Modeling
experiments indicate pseudotetrahedral arrangement of co-
ordinating N atoms around the central metal ion in the
[Ag(1a)2]ϩ complex. The stability constants (Table 1) were
determined for CuϩϪ and AgϩϪ1a and Ϫ1b complexes of
1:2 and 1:1 stoichiometries. At low cation/ligand molar ra-
tios, 1:2 complexes are formed with Cuϩ. The stabilities of
1:1 complexes exceed those of 1:2 complexes for one order
of magnitude. The catalytic 1:1 Cuϩ complexs of 1a and
1b used in stereoselective cyclopropanations have stability
1
constants of ca. 105 Ϫ1 in acetonitrile. A H NMR study
of the 1aϪAgϩ complexation reveals the formation of an
Scheme 1. Schematic presentation of interacting chromophores in
1a and 1aϪmetal complex, and position of their 1Bb transition vec-
[Ag(1a)2]ϩ complex with a stability constant of ca. 103 Ϫ1
.
tors in the free ligand and in the complex
The observed complexation induced shifts and NOEs to-
gether with molecular modeling suggest formation of a
pseudotetrahedral complex. The comparative CD study of
free acyclic (1a؊e) and macrocyclic (2a؊d, 3) ligands and
their Cuϩ and Agϩ complexes provided insight into con-
formational changes occurring upon formation of the com-
plexes. For free ligands 1a, 1c and 2a؊d strong exciton
1
coupling in the Bb region was observed and explained by
a positive helicity of the alkoxyphenyl substituents on the
stereogenic centers. Addition of a Cuϩ ion destroys the
couplet and leads to opposite CE. The latter is the con-
sequence of the ligand conformational change from transoid
in the free ligand to cisoid in the Cuϩ complex and forma-
tion of a negative helicity between two alkoxyphenyl sub-
stituents. Such spectral properties are not observed for 1b
and 3 possessing more flexible benzyl substituents at the
stereogenic centers.
Figure 16. Ball-and-stick model of the non-hydrogen atoms of an
analog of 1a based on MM2 calculations
It is interesting that no such straight-forward change of
the CD of 1a is observed on titration with Agϩ ions (Fig-
ure 9). The inflection point at ca. 195 nm is maintained,
Experimental Section
General: Compounds 1d and 1e are commercially available. Pre-
revealing formation of an [Ag(1a)2]ϩ complex, as confirmed paration of the compounds 2a؊d is reported in ref.[31] and the pre-
1746
Eur. J. Inorg. Chem. 2002, 1738Ϫ1748

Acyclic and Macrocyclic 1,5-Bis(oxazoline) Ligands and Their Copper(I) and Silver(I) Complexes
FULL PAPER
[1]
paration of the compounds 1a؊c and 3 will be published elsewhere.
Reagents were purchased from Aldrich or Fluka and were used
without further purification. All solvents were purified and dried
according to standard procedures.
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[2]
[3]
[4]
Spectrometric Titrations: UV titrations were performed at (25Ϯ0.1)
°C by means of a Varian Cary 5 spectrophotometer equipped with
a thermostating device. The spectral changes of 1a and 1b solutions
(c ϭ 5·10Ϫ5 mol·dmϪ3) in MeCN were recorded upon stepwise ad-
dition of copper(I) ions directly into the measuring quartz cell
(1 cm). Copper(I) ions were added as [(CH3CN)4Cu]PF6 solution
(c ϭ 2.6·10Ϫ4 mol dmϪ3) in MeCN. Spectra were sampled at 1-nm
[5]
[6]
[7]
[8]
[9]
intervals. Fluorimetric titrations (λexc
ϭ
277 nm, λem
ϭ
285Ϫ330 nm, filter 290 nm) were carried out using
ϭ
a
PerkinϪElmer LS-50 spectrofluorimeter at ambient temperature.
The MeCN solutions of 1a and 1b (c ϭ1.32·10Ϫ5 mol·dmϪ3) were
placed in the measuring quartz cell (1 cm) and titrated with an
MeCN solution of AgBF4 (c ϭ 0.0116 mol·dmϪ3 and 0.116
mol·dmϪ3). 1H NMR titrations of 1a and 1b were performed at
ambient temperature in CD3CN (data taken as ∆δ/ppm according
to the signal of solvent used as internal standard) with Varian Ge-
mini 300 MHz [c(1a) ϭ 0.02 mol·dmϪ3, V0 ϭ 0.5 mL, c(AgBF4) ϭ
0.1 mol·dmϪ3] and a Bruker 500 MHz instruments [c(1b) ϭ 0.001
mol·dmϪ3, V0 ϭ 0.5 mL, c(AgBF4)ϭ 0.005 mol·dmϪ3]. Aliquots of
the metal ion solution were added into the solution of the ligand
in an NMR probe with Hamilton syringes. The obtained spectro-
metric data were processed using the SPECFIT program.[47] CD
titrations: UV/Vis and CD spectra were recorded in acetonitrile
with a JobinϪYvon VI dichrograph calibrated with epiandros-
terone. The ligands were titrated with AgBF4 and
[(CH3CN)4Cu]PF6, respectively, in both the long-wavelength
(Ͼ 240 nm) and short-wavelength UV regions. Measurements were
performed at room temperature in 0.02- and 0.1-cm cells. The spec-
tral changes of ligand solutions (c ϭ 4.5Ϫ6.0·10Ϫ4 mol·dmϪ3) in
MeCN were recorded upon addition of silver(I) (c ϭ 2.0·10Ϫ2
mol·dmϪ3) and copper(I) (c ϭ 8.0· 10Ϫ3 mol·dmϪ3) solutions in
MeCN, respectively.
[10]
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[14]
[15]
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[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
Molecular Modeling: The metal-free bis(oxazoline) ligand was
docked on the complex Agϩ/1d and the low-energy structure ob-
tained modified by adding two additional NϪheavy atom bonds to
obtain the tetracoordinated complex. The metalϪN bonds of 2.18
˚
A were used as distance constraints and the complex was fully min-
imized. In the next step, a benzyloxy group was attached in the
para position to the stereogenic center of each phenyl ring to gener-
ate the 1:2 Agϩ/1a complex. The molecular modeling of the 1:2
Agϩ/1a complex and the search of the conformational space of the
1:1 Agϩ/1a complex was performed using the SYBYL molecular
modeling software, version 6.3 of TRIPOS Inc. 4,4Ј-Diphenylbis-
(oxazoline) complex 1dϪAgϩ was constructed using the Builder
module. Instead of the Agϩ ion, a heavy atom parameter available
in the SYBYL package was used together with NϪheavy atom dis-
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
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ϩ
˚
ˇ
ˇ
ˇ
tance constraints of 2.18 A corresponding to an NϪAg bond
length found in the X-ray structure of some tetracoordinated
AgϩϪbis(oxazoline) helicates.[44] The GasteigerϪHückel charges
were used with ϩ1 formal charge of the heavy atom and the com-
plex was minimized.
´
´
T. Portada, M. Roje, Z. Raza, V. Caplar, M. Zinic, V. Sunjic,
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ˇ
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´
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Acknowledgments
[35]
[36]
ˇ
´
´
ˇ
The financial support from the Croatian Ministry of Science and
Technology (Program 009807) and the Hungarian Scientific Re-
search Fund (OTKA Nr.T034866) is gratefully acknowledged.
´
Zs. Majer, M. Hollosi, S. I. Kirin, V. Sunjic, Chirality 1996,
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