Metallo-macrocycles with labile coordination sites through self assembly: Binuclear phenyl bridged bisbipyridine manganese complexes

Article abstract of DOI:10.1039/c1dt11589h

Self assembly of phenyl bridged bisbipyridines with manganese perchlorate gave structurally different metallo-macrocycles having cis-labile coordination sites which can catalyse olefin epoxidation with peracetic acid in good yield.

Full text of DOI:10.1039/c1dt11589h

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Self-Assembly in Inorganic Chemistry
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COMMUNICATION
Metallo-macrocycles with labile coordination sites through self assembly:
Binuclear phenyl bridged bisbipyridine manganese complexes†
Kiu-Chor Sham, Guoli Zheng, Yinyan Li, Shek-Man Yiu and Hoi-Lun Kwong*
Received 23rd August 2011, Accepted 26th September 2011
DOI: 10.1039/c1dt11589h
Self assembly of phenyl bridged bisbipyridines with man-
ganese perchlorate gave structurally different metallo-
macrocycles having cis-labile coordination sites which can
catalyse olefin epoxidation with peracetic acid in good yield.
The chemistry of metallo-macrocycles has been of great interest.
Various interesting architectures, for examples triangles, squares,
rectangles, polygons, circle, etc., have been synthesised1–6 and
the applications of them in molecular recognition, molecular
machinery and catalysis have been investigated.7–14 Metallo-
macrocycles can be constructed by metal complex formation using
cyclic ligands15,16 or self assembly of suitable metal ions or metal
Scheme 1 Reaction conditions: (i) BuLi, THF, -50 ◦C, 30 min; ZnCl2,
complexes with ditopic ligands.17–19 Attempts have been made
to functionalise the macrocycles by incorporating coordinatively
unsaturated metal complexes into the macrocycles20–24 and it
will be of interest if metallo-macrocycles inherently having labile
coordination sites can be synthesised.
0
◦C,
2 h; 2,5-dibromopyridine, Pd(PPh3)4, 10 h (ii) Pd(PPh3)4,
1,3-phenyldiboronic acid, CH3OH, H2O, toluene, NaCO3, 70 ◦C; (iii)
AcOH, NH4OAc, 100 ◦C(IV) Pd(PPh3)4, 1,3-phenyldiboronic acid,
CH3OH, H2O, toluene, NaCO3, 70 ◦C.
Crystallisation by diffusion of diisopropyl ether into an ace-
tonitrile solution of 1 and diffusion of diethyl ether into a
mixture of methanol and dichloromethane solution of 2 both
gave single crystals that were suitable for X-ray crystal structural
analysis.§ Complex 1, of formula [Mn2(L1)2(H2O)4](ClO4)4, crys-
tallised in the monoclinic space group C2/c as a racemic mixture.
Fig. 1 shows the ORTEP plot of one of its enantiomers, the
P-1 form. The most significant feature of the structure is that
it is a helical metallo-macrocycle having two labile manganese
centres. Both centres are coordinated to two bipyridine units,
one from each L1, with the remaining sites completed by water
Bipyridine, an important binding motif in supramolecular
chemistry, has been used in different supramolecular architectures
including metallo-macrocycles25–27 and phenyl-bridged oligopy-
ridines are known to form supramolecular architectures with
metal ions.28,29 Herein, we report the synthesis of two manganese
macrocycles with labile coordination sites and their application in
catalytic epoxidation.
As shown in Scheme 1, phenyl-bridged bisbipyridine ligands
L1 and L2 were synthesised through Pd catalysed Suzuki cou-
pling of 1,3-phenyldiboronic acid with an achiral and a chiral
bromo-bipyridine respectively.30 Both ligands have a phenyl linker
connecting two bipyridine units at the 5¢-position. Stirring of L
with one equivalent of manganese perchlorate salt in acetonitrile
at room temperature gave a clear bright yellow solution. Air stable
yellow complexes 1 and 2, from L1 and L2 respectively, were ob-
tained by filtration of the precipitates after addition of diethyl ether
and they were purified and then characterised by CHN elemental
analysis‡ and ESI-MS. In the mass spectra, peaks corresponded to
singly charged species [Mn2(L)2(ClO4)3]+ were observed for both 1
and 2 suggesting the formation of bimetallic species.
Department of Biology and Chemistry, City University of Hong Kong, Tat
Chee Avenue, Kowloon, Hong Kong SAR. E-mail: bhhoik@cityu.edu.hk;
Fax: 852 2788 4706; Tel: 852 2788 7304
† Electronic supplementary information (ESI) available: Fig. 1S, and detail
experimental procedures. CCDC reference numbers 829526 and 829527.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1dt11589h
Fig. 1 The ORTEP plot of cation of P-1 showing partial atom labeling.
The thermal ellipsoids are drawn at the 30% probability level. Hydrogen
atoms, solvent molecules and counterions are omitted for clarity.
12060 | Dalton Trans., 2011, 40, 12060–12062
This journal is
The Royal Society of Chemistry 2011
©

molecules. One of the water molecules is trans to the terminal
˚
pyridine with Mn–O distances 2.170(4) and 2.175(4) A, and the
other one is trans to the pyridine in the middle with Mn–O
˚
distances 2.140(4) and 2.167(4) A. The manganese centres have
distorted octahedral geometry with angles O1–Mn1–O2 and O3–
Mn2–O4 equal to 83.1(2)◦ and 84.1(2)◦ respectively. The Mn–
˚
N distances are in the range 2.232(5)–2.286(5) A and twisting
between the rings of L1 allows L1 to act as a ditopic ligand. The
bipyridine units are slightly non-planar [dihedral angle between
the two pyridines: 5.9(8)–13.5(8)◦] and the major twisting of L1
comes from the twisting between the bridging phenyl ring and
bipyridine units [dihedral angle between the phenyl ring and the
pyridines: 31.4(9)–46.9(8)◦]. Complex 1 has a boat shape structure.
The two manganese centres are located on the same side of a
pocket with a metal–metal distance (Mn1–Mn2) equal to 9.714(9)
˚
A. Interestingly, a perchlorate anion is captured inside the pocket
Fig. 3 The ORTEP plots of cation of M-2 showing partial atom labeling.
by hydrogen bonding between the coordinated water molecules
and the perchlorate anion (Fig. 2). The shortest distance between
the oxygen of the perchlorate anion and hydrogen of coordinating
The thermal ellipsoids are drawn at the 30% probability level.
˚
7.857(2) A. The dihedral angles between pyridines are 6.0(1) and
water molecule is 1.926(6) A.
10.1(1)◦ and the dihedral angles between the bridging phenyl ring
and bipyridine units are 29.2(1) and 29.6(1)◦. In general, twisting
of L2 in 2 is less when compared with L1 in 1.
˚
With labile coordination sites, both 1 and 2 were explored as
catalysts for the epoxidation of olefin.31–33 As depicted in Table 1,
both metallo-macrocycles can catalyse epoxidation of olefin with
peracetic acid (PAA) as oxidant. 1 and 2 are very different catalysts
in terms of reactivity. With 1 mol% 1 as catalyst and 4 eq of PAA,
epoxidation of styrene gave 100% conversion in 15 s (entry 1). The
selectivity of reaction can be improved by adding Na2HPO4 as
an additive which minimises a background epoxide ring opening
reaction. 1 also gives excellent performance with electron deficient
substrates like 4-chlorostyrene and 4-fluorostyrene giving 100%
conversion and more than 90% selectivity (Entries 4 and 5). It
is less selective towards an aliphatic substrate like cyclooctene
(Entry 9). Comparing with our previously reported manganese
double stranded helicates which also have cis-labile coordination
sites, 1 is a more active catalyst.34 We believe a Lewis acid activated
peracid species is responsible for the oxygen atom transfer.35 With
its labile sites cached, 2 needs a much longer reaction time (6 h)
to give full conversion of substrate (Entry 10). Unlike a related
mononuclear complex which gives up to 46% ee in epoxidation of
styrene,36 no enantioselectivity has been observed with pseudo-
racemic 2. At this point we do not know if the difference in
reactivity between 1 and 2 is the result of steric effect or the
different configuration of the manganese. But the relatively rigid
and well defined coordination of these supramolecular systems will
allow us to study the effect of structure and metal configuration on
the reactivity in manganese catalysed epoxidations in the future.
In summary, we have developed the synthesis of two new
coordinatively unsaturated manganese macrocycles 1 and 2 using a
self assembly approach. Although the ligand backbone structures
are very similar, 1 and 2 are structurally quite different, one with
mer-configurations and the other having fac-configurations. Their
potential application in epoxidation of olefins has been explored
and both are found to be active catalysts but with different
reactivities. Further experiments are underway to understand the
effect of structure and metal configuration on reactivity and to
explore the potential of these macrocycles as bimetallic catalysts
for other organic transformations.
Fig. 2 Capped stick drawing of the cation of P-1 with one of the
perchlorate anions. The dotted lines show hydrogen bonding between the
hydrogen of water molecules and oxygen of perchlorate.
Complex 2 crystallised in the orthorhombic space group
C222(1). The crystal lattice contains two different helical
molecules in a 1 : 1 ratio, M-2 of formula [Mn2(L2)2(H2O)2
(ClO4)2](ClO4)2 with both metal centres having K configuration
and P-2 of formula [Mn2(L2)2(CH3OH)2 (ClO4)2](ClO4)2 with
both metal centres having D configuration. Both P-2 and M-2
are metallo-macrocycles with labile coordination sites but their
structures are significantly different from the boat shaped 1. They
can be described as elliptical structures with manganese centres
located at the two focuses of the ellipse. The labile coordination
sites of the manganese centres, cached inside the cavity, are trans to
terminal pyridines and point towards each other. Fig. 3 shows the
ORTEP plots of M-2 as an example. It has a two fold symmetry
with its C2 axis perpendicular to the metal axis. The ◦Mn–Mn
˚
distance is 7.884(3) A. The angle O1–Mn1–O2 is 95.55(6) and the
˚
Mn–N distances are in the range 2.260(2)–2.287(2) A. The dihedral
angles between pyridines are 9.1(1) and 12.7(1)◦ and the dihedral
angles between the bridging phenyl ring and bipyridine units are
27.7(1) and 29.0(1)◦. In this structure, the labile sites of each metal
are occupied by one water molecule and one perchlorate. But in
the structure of P-2 (Fig. 1S†), the vacant sites are occupied either
by two methanol molecules or two perchlorate anions. The C2
axis of P-2 lies along the metal axis and the Mn–Mn distance is
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 12060–12062 | 12061
©

Table 1 Catalytic epoxidation by manganese complexesa
§ Crystal data for 1: C59H55.5Cl4Mn2N11.5O20.5, M = 1505.33, monoclinic,
˚
space group P2/c, a = 31.0270(7), b = 14.2965(2), c = 34.680(1) A, V =
3
-3
˚
˚
13864.4(6) A , Z = 8, Dc = 1.442 mg m , l(Cu-Ka) = 1.54184 A, F000
=
Catalyst, PAA, Na HPO , r.t.
2
4
alkene ⎯⎯⎯⎯⎯⎯⎯⎯→ epoxide
6176, T = 113 K, 45889 reflections measured, 12297 unique, Rint = 0.0410,
R = 0.0797 (I > 2s(I)) and 0.0966 (for all data), wR2 = 0.2443 (I > 2s(I))
and 0.2661 (for all data), CCDC 829527. 2: C165.8H168.6Cl21Mn4N16O36, M =
3925.58, orthorhombic, space group P2221, a = 24.4376(3), b = 29.2923(3),
CH CN
3
Entry Catalyst Substrate
Conversionb (%) Yieldb,c (%) Time (s)
3
-3
1d
2
1
100
100
67
94
15
15
˚
˚
c = 25.2535(3) A, V = 18077.3(4) A , Z = 4, Dc = 1.442 mg m , l(Cu-Ka)
˚
= 1.54184 A, F000 = 8082, T = 173 K, 61660 reflections measured, 12940
unique, Rint = 0.0472, R = 0.0944 (I > 2s(I)) and 0.1141 (for all data), wR2
= 0.2577 (I > 2s(I)) and 0.2717 (for all data), Flack parameter: 0.005(9),
CCDC 829526.
3
4
5
6
7
100
67
90
96
86
87
15
15
15
15
15
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Acknowledgements
The work described in this paper was supported by a grant from
the University Grants Committee of the Hong Kong Special
Administrative Region, China (Project No. [AoE/P-03/08]) and
the City University of Hong Kong SRG grant (CityU 7002607).
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Notes and references
‡ [Mn2(L1)2(H2O)4](ClO4)4: ESI-MS: m/z 986 [Mn(C26H18N4)2](ClO4)+,
1181, [Mn2(C26H18N4)2](ClO4)3
Mn2(C26H18N4)2(H2O)4(ClO4)4·3H2O: C, 43.84; H, 3.72; N, 7.86; Found:
C, 43.89, H, 3.63, N, 7.88%. [Mn2(L2)2](H2O)(MeOH)(ClO4)4: ESI-
MS: 1558.4 [Mn2(C40H38N4)2](ClO4)3+; CHN elemental analysis: calc.
for Mn2(C46H42N4)2(H2O)(MeOH)(ClO4)4·2H2O·0.5CH2Cl2: C, 54.82; H,
4.91; N, 6.27. Found: C, 54.77, H, 4.91, N, 6.35%.
+
;
CHN elemental analysis: calc. for
12062 | Dalton Trans., 2011, 40, 12060–12062
This journal is
The Royal Society of Chemistry 2011
©