Synthesis and conformational study of a novel macrocyclic chiral(salen) ligand and its uranyl and Mn complexes

Article abstract of DOI:10.3390/molecules15031442

A novel chiral macrocyclic ligand incorporating a chiral salen moiety into a framework containing two biphenyl units was synthesized. Structural properties and conformational aspects of the free ligand and an UO₂ complex were studied by using N

Full text of DOI:10.3390/molecules15031442

Molecules 2010, 15, 1442-1452; doi:10.3390/molecules15031442
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Conformational Study of a Novel Macrocyclic
Chiral(Salen) ligand and its Uranyl and Mn Complexes
Maria E. Amato, Francesco P. Ballistreri, Andrea Pappalardo, Gaetano A. Tomaselli *
and Rosa M. Toscano
Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, Catania 95125, Italy;
E-Mails: eamato@unict.it (M.E.A.); fballistreri@unict.it (F.P.B.); andrea.pappalardo@unict.it (A.P.);
rmtoscano@unict.it (R.M.T.)
* Author to whom correspondence should be addresses; E-Mail: gtomaselli@unict.it.
Received: 21 December 2009; in revised form: 20 January 2010 / Accepted: 2 March 2010 /
Published: 9 March 2010
Abstract: A novel chiral macrocyclic ligand incorporating a chiral salen moiety into a
framework containing two biphenyl units was synthesized. Structural properties and
conformational aspects of the free ligand and an UO2 complex were studied by using NMR
spectroscopy in solution and MM calculations. The Mn(III) complex was tested as catalyst
in enantioselective oxidation of prochiral unfunctionalized olefins to the corresponding
optically active epoxides under very mild conditions.
Keywords: macrocyclic chiral ligand; salen; enantioselectivity; epoxidation
1. Introduction
The successful design, synthesis and use of molecules capable of the selective recognition of other
species is of great interest in catalysis, separations, enzyme functions and other fields involving
molecular recognition [1]. Chiral salen-metal complexes are currently recognized versatile, practical
and efficient catalysts for a large number of asymmetric reactions [2–12]. Moreover, in recent years,
much attention has been devoted to the synthesis of new neutral ditopic salen receptors able to
simultaneously bind chiral ammonium cations and their counteranions [13]. Generally, in addition to
electrostatic interactions, hydrogen bonding and dispersive non bonding π-π interactions cooperate for
binding affinity of intimate contact ion pair in organic solvents.

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In this paper we describe the synthesis of a novel 21-membered macrocyclic ligand incorporating a
chiral salen moiety into a framework containing two biphenyl units. The salen moiety, due to the
presence of two stereogenic carbon atoms in the diimine bridge, generates a chiral pocket which can
coordinate metal cations (via imine nitrogen and oxygen phenolic atoms), such as uranyl or Mn. In
particular the uranyl cation can be employed as a Lewis acidic site able to bind ion pairs [13] whereas
the Mn metal center can act as a catalytic site in the enantioselective epoxidation of olefins [14].
2. Results and Discussion
Synthesis of the macrocyclic diimine ligand 5 is shown in Scheme 1. The bis(hydroxymethyl) dimer
1 was prepared in 40% yield as reported elsewhere [15]. The selective alkylation of both phenolic
hydroxyl groups with (CH3)2CH(CH2)3Br in refluxing dry acetonitrile in the presence of one
equivalent of potassium carbonate as a base, afforded 2, which was purified by column
chromatography (67% yield).
Scheme 1. Synthesis of the macrocyclic salen 5 and its metal complexes.
R-Br
,
O
KOH
K
2 CO3
,
CH
3 CN
HO
OH
40 C
HO
OH
OR
OR
OH
OH
OH
R =
1
2
,
TsOH
OH
Benzene
SnCl4, Cl2CHOCH3
CH2Cl2
OH
OR
OR
OH
O
O
OH
OR
OR
OH
3
4
EtOH
H2N
NH2
Mn(OAc)32H2O
OR
O
OR
O
OR
OR
HO
OH
M
N
N
N
N
5
6
M = UO
2, Mn(III)

Molecules 2010, 15
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Compound 2 was converted into 3 by acid-catalyzed condensation with a large excess of p-t-butyl-
phenol in the presence of TsOH in benzene giving after column chromatography a white solid (76%
yield). The subsequent formylation [14] of 3 afforded dialdehyde 4 (70% yield), which finally was
cyclized with (1R,2R)-1,2-diphenylethylenediamine in refluxing ethanol under high diluting conditions
(40% yield) to yield the macrocycle 5.
The structural characterization of all new compounds 2–4 was achieved by ESI-MS measurements
and 1D and 2D-NMR investigations. The 1H- and 13C-NMR spectra of compounds 2, 3 and 4 in CDCl3
consist of relative simple patterns of resonances showing only one set of signals for each pair of
identical groups. The formation of the macrocyclic ligand 5 was fully supported by the ESI-MS
measurements (m/z = 1,077 [MH]+).
1
The H-NMR spectrum of 5 in CDCl3 exhibits two sharp singlet signals at 8.42 and 4.73 ppm
assigned to the azomethine CH=N protons and the diimine bridge protons, respectively, while the
broad singlet at 13.36 ppm arise from phenolic hydroxylic protons. The AB system centered at 4.24 ppm
(Δδ = 0.05 ppm, J = 16.9 Hz) was assigned to the diasterotopic methylene protons located between the
two p-phenyl-phenoxy groups. The AB system centered at 4.12 ppm (Δδ = 0.22 ppm, J = 15.8 Hz)
integrating for four protons was easily attributed to the remaining two methylene groups. Aromatic
protons resonate in the appropriate low field region and alkyl ether substituents gave one set of the
expected multiplets (1–2 ppm). This simple spectrum, supported by the T-ROESY data (Figure 1a),
indicates that the ligand assumes a C1 averaged symmetric cyclic structure in a non-interconvertible
cone conformation ensured by the presence of sufficiently bulky 4-methylbutyl groups at the lower
rim. In agreement with NMR spectroscopic data, MMFFs force field [16] calculation produced the
computed lowest-energy structure of macrocycle 5 (Figure 1b).
1
Figure 1. (a) H-NMR (500 MHz, CDCl3, 300K) T-ROESY map and (b) computed
lowest-energy structure of macrocycle 5.
(a)
(b)

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Ligand 5 was utilized to prepare both UO2(VI) and Mn(III) complexes 6-UO2 and 6-Mn (Scheme 1).
To a stirred solution of ligand 5 in EtOH, solid (AcO)2UO2•2H2O or Mn(AcO)3•2H2O respectively
was added. The mixtures were allowed to stir overnight at room temperature and were monitored by
TLC. Evaporation of the solvent gave a residue, which was dissolved in CH2Cl2, filtered and
concentrated to produce the uranyl or Mn complex in a nearly quantitative yield. ESI-MS spectra
confirmed the formation of the mono-metallic complexes.
Structural information on the uranyl(VI) complex (6-UO2 ), in solution, were obtained from 1D and
2D-NMR studies. The 1H-NMR spectrum in acetone-d6 of 6-UO2 complex showed an almost unvaried
pattern of signals with respect to the free ligand, even considering the expected broadening and
downfield shift of the resonances arising from the protons close to the coordination site. The ROE
relationships measured by the phase-sensitive T-ROESY spectrum in acetone-d6 are shown in Figure 2a.
The dipolar correlation contacts identified in the 2D map suggested that the symmetrical structure is
maintained in solution upon metal complexation. The strong ROE correlations between aromatic
protons signals and azomethine CH=N protons, diimine bridge protons and t-Bu groups allowed safe
assignment of the partially overlapped signals of the aromatic protons system. Besides the
characteristic bond-through ROE correlations in the aromatic moiety, no additional dipolar contacts
were observed either between aromatics rings or between the aromatic protons and iso-hexyloxy-
substituents. Interestingly, this finding ruled out any complex conformation where the diimine bridge
phenyl rings were arranged in an almost anti-periplanar conformation. In agreement with MM
calculated lowest energy structure, the complex adopts in solution a less hindered “disk-shaped” rather
a “cup-like” shape (Figure 2b).
1
Figure 2. (a) H-NMR (500 MHz, (CD3)2CO, 300K) T-ROESY map and (b) modelled
structure of complex 6-UO2.
(a)
(b)

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1
The broadening of the lines of the H spectrum might envisage the flipping motion of the
salicylaldehyde framework already observed for uranyl salophen and salen complexes [13,17–19]. In
order to enlighten on possible exchanging process occurring in solution, the proton signals of the
uranyl complex were monitored carefully at lower temperature (Figure 3). As the temperature was
lowered all the signals progressively broadened. At 193 K the signals of CH=N and diimine bridge
protons were no longer visible and methylene protons located between the macrocycle aromatic rings
coalesced into an unresolved envelope. Further cooling to 183 K did not produce additional detectable
changes in the spectral profile neither new signals could be revealed. Only trivial modifications were
observed in the range 183–213 K in the aromatic spectra, easily attributed to slower bi-phenyl rotation.
1
Figure 3. Variable temperature H NMR spectrum of 6-UO2 complex (500 MHz,
(CD3)2CO) at relevant temperatures.
T = 293 K
T = 273 K
T = 253 K
T = 233 K
T = 213 K
T = 183 K
7.0
9.0
8.0
6.0
ppm
The 6-Mn complex was characterised by ESI mass spectroscopy (m/z = 1129 [M]+). The catalytic
ability of 6-Mn was tested in the epoxidation reactions of some prochiral alkenes. The reactions were
performed in CH2Cl2/H2O at 25° using NaClO as oxygen donor and 4-phenyl-pyridine N-oxide (4-
PPNO) as coligand. Enantiomeric excess values for the formation of epoxides were determined by
capillary GLC analysis employing chiral columns [14] for 1,2-dihydronaphthalene, and by 1H-NMR in
the presence of Eu(+)(hfc)3 reagent shift for chromene derivatives epoxides.

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The results are reported in Table 1. The reactions were allowed to proceed overnight (24 h)
reaching 80%–90% completion (entries 2–4) and quantitative yield in the corresponding epoxides
(entries 1–5).
Table 1. Enantioselective Epoxidation reactions of Alkenes with NaClO catalyzed by
6-Mn in CH2Cl2/H2O at 25 °C. a
Entry
1
Alkene
Conv. (%) Yield (%)
ee (%)
Conf.c
50d
90b
30b
1R,2S
2
3
4
5
80
90b
50b
63f
50f
52f
1R,2S
3R,4R
3R,4R
3R,4R
O
90e
90e
50e
100e
100e
100e
CH3O
O
O
CN
O
N
2
a In all experiments [Alkene] = 0.14 M, [Catalyst] = 0.007 M, [Coligand] = [4-PPNO] = 0.07 M,
b
[NaClO] = 0.14 M, [Na2HPO4] = 0.05 M at pH = 11.2 as buffer. Determined by GC on a chiral
c
column; ee values are referred to the major cis epoxide (eecis). Determined by measuring the
optical rotation. d No coligand added. eYield by weight of the isolated product. f Determined by 1H-
NMR analysis in the presence of Eu(+)(hfc)3.
The observed effect of the coligand (compare entries 1 and 2) and the absolute configuration of the
cis-epoxides were in agreement with literature data [14,20–22].
Previously reported studies indicated that the dissymmetry of diimine bridge should favour the
approaching of the si enantioface of the alkene towards the Mn-oxo site of chiral salen catalysts [14].
The observed ee values for the alkenes reported in Table 1 are in line with this finding. However the
degree of enantioselectivity displayed is moderate. In order to try to understand the observed
behaviour, molecular mechanics calculations were performed on the complexes. Figure 4 shows the
attack to the catalyst of both re and si face of the dihydronaphthalene. Very small differences were
found for calculated energies (<5 kJ mol−1) suggesting that the attack pathways by the re and si face
are energetically equivalent. Similar results were obtained in all the modelled structures of the
examined alkenes and the catalyst. This behaviour might be ascribed to the presence of the biphenyl
rings, which can undergo atropoisomerization and then can adapt their spatial position to the
approaching guest, favouring therefore π-π interactions with the aromatic framework of the alkene
regardless of the alkene exposed face.

Molecules 2010, 15
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Figure 4. Computer optimized structures concerning the re (a) and si (b) face attack of
dihydronaphthalene to the 6-Mn catalyst.
3. Experimental
3.1. General
Melting points were determined on a Kofler hot stage apparatus and are uncorrected. The NMR
13
experiments were carried out at 27 °C on a 500 MHz spectrometer (1H at 499.88 MHz, C-NMR at
125.7 MHz) equipped with pulse field gradient module (Z axis) and a tunable 5mm inverse detection
probe. The chemical shifts (ppm) were referenced to TMS as internal standard. Gas chromatographic
analyses of the reaction mixtures were carried out on a gas chromatograph equipped with a flame
ionization detector and program capability on a DMePeBETACDX (1,2-dihydronaphthalene)
(25 m × 0.25 mm ID, 0.25 μm film) against an internal standard (n-decane). ESI mass spectra were
obtained by employing an ESI-MS spectrometer equipped with an ion trap analyzer. The absolute
configuration of (1R,2S)-1,2-epoxy-1,2,3,4-tetrahydronaphthalene, (3R,4R)-3,4-epoxy-6-methoxy-2-2-
dimethyl chromene, (3R,4R)-3,4-epoxy-6-cyano-2-2-dimethylchromene and (3R,4R)-3,4-epoxy-6-nitro-2-
2-dimethylchromene were determined by measuring the optical rotation with a polarimeter.
20
Measurements of optical rotation gave [α]D = +17.5 (c = 0.20, CHCl3) for 1,2-epoxy-1,2,3,4-
20
tetrahydronaphthalene, [α]D
= +10.8 (c = 1.1, CHCl3) for 3,4-epoxy-6-methoxy-2-2-
dimethylchromene, [α]D20 = +34.3 (c = 0.94, CHCl3) for 3,4-epoxy-6-cyano-2-2-dimethylchromene,
[α]D20 = +11.1 (c =0.1, CHCl3) for 3,4-epoxy-6-nitro-2-2-dimethylchromene. Absolute configurations
were assigned by comparison of the measured [α]D20 values with those reported in the literature [14,23].
Commercial reagents were used as received without further purification unless otherwise noted.
6-Cyano-2,2-dimethylchromene, 6-nitro-2,2-dimethylchromene and 6-methoxy-2,2-dimethylchromene
were synthesized by the Bergmann and Gericke procedure [24]. Dichloromethane was freshly distilled
from calcium hydride before use.
3.2. General Procedures for the Epoxidation Reactions
To a stirred solution of alkene (0.35 mmol), catalyst (0.0175 mmol) and 4-phenylpyridine-N-oxide
(4-PPNO, 0.175 mmol) in CH2Cl2 (2.5 mL), kept in a round bottom flask and maintained at 25 °C in a

Molecules 2010, 15
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thermostatic bath, buffered bleach (0.35 mmol, buffered to pH = 11.2 with 0.05M Na2HPO4) is added.
The course of the reaction was monitored by GC. After 24 h, the phases were separated, and the
aqueous layer was extracted with CH2Cl2. The combined organic phases were dried over Na2SO4 and
concentrated. The crude product was purified by PLC (SiO2).
3-[3-(Hydroxymethyl)-5-phenylsalicyl]-5-phenyl-2-hydroxybenzyl alcohol (1). Compound 1 was
prepared according to literature procedure [15]: a mixture of p-phenylphenol (30 g, 0.18 mol) and
formaldehyde 37% (150 mL, 1.8 mol) was cooled in an ice bath, treated slowly with KOH (20.4 g,
0.36 mol), and then stirred for 7 days at 40 °C. Crystallization from CH3OH yielded 30 g (40%) of 1 as
a white powder: mp 127–128 °C. 1H-NMR (acetone-d6) δ ppm: 7.55 ArH (d, 6H, J = 7Hz) 7.37 ArH (t,
4H, J = 7.4Hz,) 7.32 ArH (s, 2H), 7.24 ArH (t, 2H, J = 7.4Hz), 4.90 CH2OH (s, 4H), 4.11 CH2 (s, 2H),
3.30 OH (br, 2H). ESI-MS : m/z 435 [M Na]+.
3-[2-Isohexyloxy-3-(hydroxymethyl]-5-phenyl-2-isohexyloxybenzyl alcohol (2). A stirred mixture of 1
(5.43 g, 13 mmol), 1-bromo-4-methylpentane (10 g, 61 mmol) and anhydrous K2CO3 (4.2 g,
30 mmol) in dry MeCN (200 mL) was refluxed for 24 h. After filtration and evaporation of the solvent,
the residue was dissolved in CH2Cl2. The organic layer was washed with water, dried over anhydrous
Na2SO4 and concentrated. Crystallization from CH2Cl2/n-hexane yielded 5.1 g (67%) of 2 as a white
powder: mp 90–91 °C. 1H-NMR (CDCl3) δ ppm: 7.47 ArH (m, 6H), 7.36 ArH (t, 4H, J = 7.5 Hz), 7.28
ArH (m, 4H), 4.81 CH2OH (d, 4H, J = 6 Hz), 4.17 CH2 (s, 2H), 3.86 OCH2 (t, 4H, J = 6.5 Hz), 2.19
CH2OH (t, 2H, J = 6 Hz), 1.82 OCH2CH2 (m, 4H), 1.59 (CH3)2CH (m, 2H), 1.35 OCH2CH2CH2 (m, 4H),
0.92 CH3 (d, 12H, J = 7 Hz). 13C-NMR (CDCl3) δ ppm: 155.93, 153.30, 150.94, 142.79, 140.59,
138.07, 133.51, 128.62, 128.07, 127.76, 127.05, 127.02, 126.97, 126.30, 126.22, 124.20, 75.67, 63.68,
35.01, 33.99, 31.55, 29.70, 27.99, 27.92, 22.54; ESI-MS : m/z 603 [MNa]+. Anal. Calcd for C39H48O4:
C, 80.69; H, 8.28. Found: C, 80.98; H, 8.16.
2-[3-[3-(5-tert-Butylsalicyl)-5-phenyl-2-isohexyloxybenzyl]-5-phenyl-2-isohexyloxybenzyl]-4-tert-
butylphenol (3). A stirred mixture of 2 (5.0 g, 9 mmol), p-tert-butylphenol (13 g, 86 mmol) and p-
toluenesulfonic acid (370 mg, 0.44 mmol) in benzene (30 mL) was refluxed for 24h. After evaporation
of the solvent and remove excess of p-tert-butylphenol with sublimate under vacuum at 115 °C the
residue was dissolved in CH2Cl2. The organic layer was washed with water, dried over anhydrous
Na2SO4, concentrated and purified by column chromatography (SiO2, eluent n-hexane/AcOEt 7:1, v/v)
1
to give (3) (5.5 g, 76%) as a white powder: mp 99–100 °C. H-NMR (CDCl3) δ ppm: 7.40–7.12 ArH
(m, 18H), 6.78 ArH (d, 2H, J = 8.5 Hz) 4.19 CH2 (s, 2H), 3.98 OCH2 (t, 2H, J = 6.5 Hz), 3.95 CH2 (s,
4H), 1.90 OCH2CH2 (m, 4H), 1.56 (CH3)2CH (m, 2H), 1.35 OCH2CH2CH2 (m, 4H), 1.29 CCH3 (s,
18H), 0.92 CH3 (d, 12H, J = 7 Hz). 13C-NMR (CDCl3) δ ppm: 152.95, 152.25, 142.97, 140.50, 138.38,
133.62, 133.39, 128.61, 128.10, 127.77, 127.12, 127.02, 126.39, 125.62, 125.11, 116.06, 114.73,
75.88, 34.9, 33.98, 32.25, 31.62, 31.52, 29.98, 27.88, 22.46; ESI-MS: m/z 867 [MNa]+. Anal. Calcd for
C59H72O4: C, 83.89; H, 8.53. Found: C, 80.87; H, 8.59.
3-[3-[3-[3-(Formyl)-(5-tert-butylsalicyl)]-5-phenyl-2-isohexyloxybenzyl]-5-phenyl-2-isohexyloxy-
benzyl]-5-tert-butyl-salicylaldehyde (4). To a chilled solution of 3 (1.12 g, 1.32 mmol) in dry CH2Cl2
(50 mL) was added Cl2CHOCH3 (2.34 mL, 26.4 mmol) under N2. A 1M solution of SnCl4 in CH2Cl2

Molecules 2010, 15
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(2.64 mL, 2.64 mmol) was then added and after 20 minutes the reaction was stopped with 1M HCl
[14]. The mixture was washed with a saturated aqueous solution of NaHCO3 and then with water, and
dried over anhydrous Na2SO4. The solvent was removed under reduced pressure and the resulting oil
was purified by column chromatography (SiO2, eluent: n-hexane/acetone 6:1) to give 4 as a white
1
solid. (0.83 g, 70%). H-NMR (CDCl3) δ ppm: 11.18 ArOH (s, 2H), 9.88 CHO (s, 2H),
7.50–7.18 ArH (m, 18H), 4.22 CH2 (s, 2H), 4.12 CH2 (s, 4H), 3.84 OCH2 (t, 4H, J = 6.5 Hz ), 1.90
OCH2CH2 (m, 4H), 1.59(CH3)2CH (m, 2H), 1.35 OCH2CH2CH2 (m, 4H), 1.26 CCH3 (s, 18H), 0.91
CH3 (d, 12H, J = 7 Hz). MS-ESI: m/z 923 [MNa]+. Anal. Calcd for C61H72O6: C, 81.33; H, 8.0. Found: C,
81.74; H, 8.41.
Condensation of 3-[3-[3-[3-(formyl)-(5-tert-butylsalicyl)]-5-phenyl-2-isohexyloxybenzyl]-5-phenyl-2-
isohexyloxybenzyl]-5-tert-butyl- salicylaldehyde with (1R,2R)-diphenylethylendiamine (5). Solutions
of 4 (0.27 g 0.3 mmol) in EtOH (27 mL) and (1R,2R)-1,2-diphenylethylendiamine (0.064 g,
0.3 mmol), in EtOH (14 mL), were dropped separately but synchronously from two dropping funnels
into boiling abs EtOH (200 mL) under rapid stirring. The reaction mixture was refluxed for an
additional 24 h, and cooled. After removal of the solvent, the crude product was column
chromatographed (SiO2, eluent n-hexane/acetone 6:1, v/v) to afford 5 (0.13 g, 40%). 1H-NMR (CDCl3)
δ ppm: 13.36 ArOH (s, 2H), 8.42 CH=N (s, 2H), 7.37–7.00 ArH (m, 26H), 4.73 CH-N (s, 2H), 4.26 CH2
(d, 2H, J =15 Hz), 4.23 (s, 2H), 4.04 CH2 (d, 4H J =15 Hz), 3.83 OCH2 (t, 4H, J = 6.5 Hz), 1.79
OCH2CH2 (m, 4H), 1.50(CH3)2CH (m, 2H), 1.30 OCH2CH2CH2 (m, 4H), 1.1 CCH3 (s, 18H), 0.8 CH3
13
(d, 12H, J = 7 Hz). C-NMR (CDCl3) δ ppm: 166.76, 156.722, 155.71, 141.14, 140.94, 139.56,
136.54, 134.30, 133.85, 131.09, 128.84, 128.49, 128.25, 128.00, 127.71, 127.51, 127.39, 126.93,
126.58, 126.44, 117.55, 35.17, 33.80, 31.39, 31.31, 30.94, 29.31, 28.35, 27.96, 22.55; ESI-MS: m/z
1077 [MH]+. Anal. Calcd for C75H84N2O4: C, 83.64; H, 7.81; N, 2.60. Found: C, 83.31; H, 7.86; N, 2.62.
Uranyl complex (6-UO2). (AcO)2UO2•2H2O (10 mg, 0.025 mmol) was added as a solid to a stirred
solution of the ligand 5 ( 20 mg, 0.02 mmol) in MeOH (10 mL). The mixture was allowed to stir
overnight at room temperature and was monitored by TLC (eluent: 20% acetone in n-hexane). The
solvent was removed on a rotary evaporator under vacuum and the residue was dissolved in CH2Cl2,
filtered and concentrated to produce the pertinent uranyl complex in a nearly quantitative yield.
1H-NMR (acetone d6) δ ppm: 9.41 CH=N (br, 2H), 7.79–7.08 ArH (m, 26H), 6.23 CH-N (s, 2H), 4.52
CH2 (s, 2H,), 4.28 CH2 (s, 4H), 3.96 OCH2 (bt, 4H), 1.82 OCH2CH2 (s, 4H), 1.65 (CH3)2CH (m, 2H),
1.35 OCH2CH2CH2 (m, 4H), 1.26 CCH3 (s, 18H), 0.91 CH3 (d, 12H, J = 7 Hz). ESI-MS: m/z 1345
[MH]+. Anal. Calcd for C75H82N2O6U: C, 66.96; H, 6.10; N, 2.08. Found: C, 66.73; H, 6.13; N, 2.10.
[Mn(III)(L)] complex (6-Mn). To a solution of the ligand 5 (65 mg, 0.06 mmol) in CH2Cl2 (8 mL) was
added a solution of Mn(OAc)3•2H2O (16 mg, 0.064 mmol) in EtOH (8 mL). The dark solution was
allowed to stir overnight at room temperature and was monitored by TLC (eluent: 20% acetone in
n-hexane). The solvent was removed under vacuum and the residue was dissolved in CH2Cl2, filtered
and concentrated to produce the Mn(III) catalyst in a nearly quantitative yield. ESI-MS: m/z 1129
[M]+. Anal. Calcd for C77H85MnN2O6: C, 77.78; H, 7.15; N, 2.36. Found: C, 77.68; H, 7.20; N, 2.40.4.

Molecules 2010, 15
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4. Conclusions
We have synthesized a new macrocyclic chiral salen ligand and its UO2 and Mn(III) complexes.
NMR studies of the (salen) UO2 complex are in agreement with MM calculated structures which
indicate that the shallow conformation of the receptor cavity and, probably, the mobility of biphenyl
rings are responsible of the observed moderate enantioselectivity. At any rate, the catalyst synthesized
in this work represents the first example of salen derivatives combined with biphenyl units and the
work is in progress to avoid the atropoisomerization of biphenyl units, in order to control more
efficiently molecular recognition.
Acknowledgements
We thank University of Catania for financial support.
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Sample Availability: Samples of the compounds are available from the authors.
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