Download Coordination Chemistry of cis,trans-1,3,5

Coordination Chemistry of cis,trans-1,3,5-Triaminocyclohexane –
From Mononuclear Complexes to Supramolecular Architectures
by
Georg Peter Maria Seeber
A thesis submitted to
The University of Birmingham
for the degree of
DOCTOR OF PHILOSOPHY
School of Chemistry
The University of Birmingham
September 2003
University of Birmingham Research Archive
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1st of 3 files
Sections 3 to 8 (page 28 to end)
are in two additional files
ABSTRACT
The coordination chemistry of the rigid, aliphatic triamino ligand cis,trans-1,3,5triaminocyclohexane (trans-tach) is investigated. With closed shell transition metals, transtach forms 1-D {Ag(OTf), ZnCl2} networks and a 3-D {AgNO3} coordination polymer with
unprecedented topology. Coordination to the open shell transition metals {NiCl2, Ni(NO3)2,
Cu(NO3)2, CuBr2, CuCl2, CuF2, CuSO4} leads to discrete diligand complexes. Protonation of
the copper(II) complexes predominantly forms monoligand species, which can aggregate into
higher nuclearity clusters. Protonated Cu(NO3)2 and CuBr2 complexes (HNO3 and HBr,
respectively) remain mononuclear. CuCl2 and CuBr2 complexes protonated with HCl,
however, form trinuclear species comprising trigonal planar µ3-chloro ligands that aggregate
into 1-D trinuclear copper(II) chains via hydrogen-bonded interactions. Protonated CuF2
forms a tetranuclear cubane type structure that aggregates into a 3-D nanoporous network via
hydrogen-bonded interactions. Protonation of the CuSO4 diligand complex maintains the
diligand coordination, forming a 1-D nanoporous network through hydrogen-bonded
interactions. Coordination to square planar palladium(II) ions forms all possible coordination
motifs (‘Tail’ = monodentate, ‘Head’ = bidentate coordination). ‘Head-to-Head’ coordination
results in diligand complexes, ‘Head-to-Tail’ coordination forms cyclic hexanuclear structures
and ‘Tail-to-Tail’ coordination leads to a trinuclear species. Extension of trans-tach via Schiff
base formation with pyridine-2-carboxaldehyde results in a hexadonor ligand suitable for
metal coordination. Depending on the stoichiometry of the metal, mono- or polynuclear
complexes are formed.
Meinen Eltern
“The essence of chemistry is not only to
discover but to invent and, above all, to create.”
Jean-Marie Lehn
ACKNOWLEDGEMENTS
This project was carried out between October 2000 and September 2003 under the supervision
of Dr. Leroy Cronin at the Universities of Birmingham and Glasgow.
Thanks to
•
my friend and supervisor for this inspiring project and his support over the last three
years.
•
my colleagues in the group for a good time and teaching me how to survive in chaos.
Particularly, I would like to thank Ms. Alex Pickering, not only for counselling me
through the writing up process, but also for her invaluable help in finishing this thesis.
Many thanks to Dr. De-Liang Long for thoroughly correcting each chapter and pointing
out details that had been overlooked. Thanks to Mr. Geoffrey James Tapio Cooper for his
help in labelling pictures when time was melting away too quickly.
•
Dr. De-Liang Long and Dr. Benson M. Kariuki for the X-ray crystallography, even when
the crystals were of bad quality, and for sharing their crystallographic expertise. Dr. Neil
Spencer, Dr. Rycroft and Jim Gall for the NMR spectroscopy. Dr. Rycroft for correcting
my NMR chapter. Peter R. Ashton for the mass spectrometry. Kim Wilson and Lianne
Hill for the elemental analysis.
•
Jim McIver and Alex Burns for their help in setting up the lab in Glasgow as well as
introducing me to the proper ´Glaswegian´ accent (special thanks also to Darren Orr).
•
Hamera Abbas, Laura Carnochan, Narinder Kaur, Jill Meadows and Helen L. Uttley for
contributing to this thesis by their project work.
•
all the people in Birmingham for a fabulous two years: Robyn, Hanna, Rosa, Margarita,
Lulu, Tindi, Katia, Zé; Frenchi, Jodi, Alex, Kevin, Simon, Jon, Pete, Chris and everybody
else who I have forgotten to mention.
Above all, I would like to thank my parents for supporting me over the period of my studies
and making it all possible.
TABLE OF CONTENTS
I
TABLE OF CONTENTS
1
INTRODUCTION ...................................................................................................................1
1.1
Supramolecular Chemistry ........................................................................................1
1.2
Self-assembly ............................................................................................................3
1.2.1
1.2.1.1
‘Symmetry interaction’ model........................................................................4
1.2.1.2
‘Molecular library’ model ..............................................................................5
1.2.2
2-D polygons .....................................................................................................6
1.2.2.1
Dinuclear assemblies ......................................................................................6
1.2.2.2
Trinuclear assemblies .....................................................................................7
1.2.2.3
Tetranuclear assemblies..................................................................................8
1.2.2.4
Higher nuclearity assemblies..........................................................................9
1.2.3
3-D cages and polyhedra .................................................................................11
1.2.3.1
Triangular prism and cylinders.....................................................................11
1.2.3.2
Tetrahedra.....................................................................................................12
1.2.3.3
Octahedra......................................................................................................13
1.2.3.4
Cubes ............................................................................................................14
1.2.3.5
Cuboctahedra ................................................................................................15
1.2.4
Helicates ..........................................................................................................16
1.2.4.1
Single stranded .............................................................................................17
1.2.4.2
Double stranded............................................................................................18
1.2.4.3
Triple stranded..............................................................................................19
1.2.5
Interlocked structures and topology ................................................................19
1.2.5.1
Zero-dimensional..........................................................................................20
1.2.5.2
One-dimensional...........................................................................................22
1.2.5.3
Two-dimensional ..........................................................................................23
1.2.5.4
Three-dimensional ........................................................................................24
1.2.6
2
Design principles ...............................................................................................4
Application and outlook ..................................................................................25
AIMS .................................................................................................................................26
TABLE OF CONTENTS
3
II
RESULTS AND DISCUSSION ...............................................................................................28
3.1
Optimised synthesis of trans-tach ...........................................................................29
3.1.1
Spectroscopy of trans-tach ..............................................................................31
3.2
Crystal structures of trans-tach ● 3 HX (X = Cl, Br, NO3, ClO4)............................33
3.3
trans-Tach as a ligand in coordination chemistry ...................................................36
3.4
Open shell TM coordination chemistry ...................................................................41
3.4.1
Nickel(II) coordination 34 and 35 ...................................................................41
3.4.2
Copper(II) coordination...................................................................................45
3.4.2.1
3.4.2.1.1
Cu(NO3)2 at unadjusted pH, complex 36 ● 0.5 HNO3.....................................................47
3.4.2.1.2
Cu(NO3)2 adjusted to pH 6.5 with nitric acid 37 ............................................................48
3.4.2.2
Copper(II) bromide.......................................................................................49
3.4.2.2.1
CuBr2 at unadjusted pH, complexes syn-38, twist-38 and 39 .........................................49
3.4.2.2.2
CuBr2 adjusted to pH 6.5 with hydrobromic acid 40......................................................51
3.4.2.3
Copper(II) chloride .......................................................................................53
3.4.2.3.1
CuCl2 at unadjusted pH, complex 41 ● 2 HCl.................................................................53
3.4.2.3.2
CuCl2 and CuBr2 adjusted to pH 6.5 with hydrochloric acid 42 and 43.........................54
3.4.2.4
Copper(II) fluoride .......................................................................................60
3.4.2.4.1
CuF2 at unadjusted pH, complex 44 ...............................................................................60
3.4.2.4.2
CuF2 adjusted to pH 7.7 with hydrofluoric acid 45 ........................................................61
3.4.2.5
Copper(II) sulphate.......................................................................................68
3.4.2.5.1
CuSO4 at unadjusted pH, complex 46 ............................................................................68
3.4.2.5.2
CuSO4 adjusted to pH 8.8 with sulphuric acid 46 ● H2SO4.............................................69
3.4.3
3.5
Copper(II) nitrate..........................................................................................47
Palladium(II) coordination ..............................................................................74
3.4.3.1
‘Head’ coordination - monoligand complexes 47 and 48 ............................75
3.4.3.2
‘Head to Head’ coordination – anti-diligand complexes 49 and 50.............77
3.4.3.3
‘Head to Tail’ coordination - hexanuclear ring structures 51 and 52...........80
3.4.3.4
‘Tail to Tail’ coordination - trinuclear complex 53......................................86
3.4.3.5
1
H-NMR data for complexes 47 - 52............................................................89
Closed shell TM coordination chemistry.................................................................94
3.5.1
Ag(I) coordination ...........................................................................................95
3.5.1.1 3-D coordination network with AgNO3 54...................................................95
3.5.1.2 1-D coordination network with AgOTf 55 ..................................................98
3.5.2
Zn(II) coordination ........................................................................................101
3.5.2.1 1-D coordination network with ZnCl2 56 ...................................................102
TABLE OF CONTENTS
3.6
Schiff base derivatives of trans-tach .....................................................................103
3.6.1
cis,trans-1,3,5-tris-Benzaldimino-cyclohexane (tbc, 57)..............................103
3.6.1.1
3.6.2
4
5
III
Spectroscopy of tbc ....................................................................................106
cis,trans-1,3,5-tris-(Pyridine-2-carboxaldimino)-cyclohexane (ttop, 59) .....107
3.6.2.1
Spectroscopy of ttop ...................................................................................109
3.6.2.2
1
3.6.2.3
Complexation of ttop with one equivalent of CuCl2 61 .............................113
3.6.2.4
Complexation of ttop with two equivalents of CuCl2 62............................114
H-NMR-titration of ttop with ZnCl2 .........................................................110
CONCLUSIONS AND FURTHER WORK ..............................................................................116
4.1
Coordination networks ..........................................................................................117
4.2
Supramolecular architectures ................................................................................117
4.3
Building block approach........................................................................................119
4.4
Schiff base derivatives of trans-tach .....................................................................120
4.5
Analogues of trans-tach ........................................................................................121
EXPERIMENTAL ..............................................................................................................122
5.1
Materials ................................................................................................................122
5.2
Instrumentation......................................................................................................122
5.3
Synthesis................................................................................................................123
5.3.1
1,3,5-Cyclohexanetrionetrioxime (chtt, 31) ..................................................123
5.3.2
1,3,5-Triaminocyclohexane (tach, 29 and 30)...............................................124
5.3.2.1
Isolation of cis- and trans- tach isomers from its nickel(II)
complexes 34 and 33 ..................................................................................126
5.3.3
[anti-bis(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)-nickel(II)]
dinitrate (34) ..................................................................................................128
5.3.4
[anti-bis(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)nickel(II)] dichloride (35)..............................................................................129
5.3.5
[anti-bis(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)copper(II)] dinitrate (36)................................................................................129
5.3.6
[tris-Nitrato-(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)copper(II)] (37) ..............................................................................................130
TABLE OF CONTENTS
5.3.7
IV
[syn-bis(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)-copper(II)]
dibromide (syn-38),
[bis(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)-copper(II)]
dibromide (twist-38) and
[bromo-syn-bis(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)copper(II)] bromide (39)................................................................................131
5.3.8
[Dibromo-(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)copper(II)] bromide (40)................................................................................132
5.3.9
[(µ2-Chloro)-di-{syn-bis(cis,trans-1,3,5-triaminocyclohexane-κ2N,N’)-copper(II)}] trichloride dihydrochloride (41 ● 2 HCl) ........................132
5.3.10
[(µ3-Chloro)-tris{dichloro-(cis,trans-1,3,5-triaminocyclohexane-κ2N,N’)-copper(II)}] dichloride (42) ................................................................133
5.3.11
[(µ3-Chloro)-tris{dichloro-(cis,trans-1,3,5-triaminocyclohexane-κ2N,N’)-copper(II)}] dibromide (43)................................................................134
5.3.12
[Fluoro-syn-bis(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)copper(II)] fluoride (44) ................................................................................135
5.3.13
[tetrakis{(µ3-Fluoro)-(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)copper(II)}] octafluoride (45)........................................................................136
5.3.14
[Sulphato-syn-bis(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)-copper(II)] (46).............................................................................................137
5.3.15
[Sulphato-syn-bis(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)copper(II)] sulphate (46 ● H2SO4)..................................................................137
5.3.16
[Dichloro-(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)palladium(II)] chloride (47)...........................................................................138
5.3.17
bis[Dibromo-(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)palladium(II)] tetrabromopalladate (48)........................................................139
5.3.18
[anti-bis(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)palladium(II)] tetranitrate (49) ......................................................................139
5.3.19
[anti-bis(cis,trans-1,3,5-Triaminocyclohexane-κ2-N,N’)palladium(II)] bis-sulphate (50) ....................................................................140
5.3.20
[bis{Dichloro-µ2-(cis,trans-1,3,5-triaminocyclohexane-κ3-N,N’,N’’)palladium(II)}-trans-{dichloro-palladium(II)}] (53) ....................................141
TABLE OF CONTENTS
5.3.21
V
cyclo[hexakis{Chloro-µ2-(cis,trans-1,3,5-triaminocyclohexane-κ3N,N’,N’’)-palladium(II)}] hexachloride (51) ................................................141
5.3.22
cyclo[hexakis{Bromo-µ2-(cis,trans-1,3,5-triaminocyclohexane-κ3N,N’,N’’)-palladium(II)}] hexabromide (52)................................................143
5.3.23
poly{[µ3-(cis,trans-1,3,5-Triaminocyclohexane-κ3-N,N’,N’’)silver(I)] nitrate} (54) ....................................................................................144
5.3.24
catena-poly{[µ3-(cis,trans-1,3,5-Triaminocyclohexane-κ3-N,N’,N’’)silver(I)] trifluoromethanesulphonate} (55) ..................................................145
5.3.25
catena-poly{[Dichloro-bis(µ2-chloro)-µ2-(cis,trans-1,3,5-triaminocyclohexane-κ2-N,N’)-di-zinc(II)] chloride} (56) .........................................146
5.3.26
cis,trans-1,3,5-tris-Benzaldimino-cyclohexane (tbc, 57)..............................146
5.3.27
cis,trans-1,3,5-tris(Pyridine-2-carboxaldimino)-cyclohexane (ttop, 59) ......147
5.3.28
[Chloro-(cis,trans-1,3,5-tris(pyridine-2-carboxaldimino)-cyclohexaneκ4-N1,N2,N3,N4)-copper(II)] chloride (61).....................................................148
5.3.29
[bis{Dichloro-(µ2-chloro)-µ2-(cis,trans-1,3,5-tris(pyridine-2carboxaldimino)-cyclohexane-κ6-N1,N2,N3,N4,N5,N6)di-copper(II)}] dichloride (62) ......................................................................149
6
CRYSTALLOGRAPHY .......................................................................................................150
6.1
X-ray summary table .............................................................................................151
6.2
trans-tach ● 3 HCl (29 ● 3 HCl).............................................................................154
6.3
trans-tach ● 3 HBr (29 ● 3 HBr).............................................................................156
6.4
trans-tach ● 3 HNO3 (29 ● 3 HNO3).......................................................................158
6.5
trans-tach ● 3 HClO3 (29 ● 3 HClO4).....................................................................160
6.6
[Ni(trans-tachH)2)](NO3)4 (34 ● 2 HNO3) ............................................................163
6.7
[Ni(trans-tach)2)]Cl2 (35) .....................................................................................166
6.8
[Cu(trans-tach)2](NO3)2 ● 0.5 HNO3 (36 ● 0.5 HNO3) .........................................169
6.9
[Cu(trans-tachH)(NO3)3] (37) ..............................................................................172
6.10
[Cu(trans-tach)2]Br2 (syn-38) ...............................................................................175
6.11
[Cu(trans-tach)2]Br2 (twist-38).............................................................................177
6.12
[Cu(trans-tach)2Br]Br (39) ...................................................................................180
6.13
[Cu(trans-tachH)Br2]Br (40) ................................................................................182
6.14
[(µ2-Cl){Cu(trans-tach)2}2]Cl3 ● 2 HCl (41 ● 2 HCl) ............................................184
TABLE OF CONTENTS
7
8
VI
6.15
[(µ3-Cl){Cu(trans-tach)Cl2}3]Cl2 (42)..................................................................187
6.16
[(µ3-Cl){Cu(trans-tach)Cl2}3]Br2 (43) .................................................................190
6.17
[Cu(trans-tach)2F]F (44) ......................................................................................193
6.18
[{Cu(trans-tachH)F}4]F4 (45) ..............................................................................196
6.19
[Cu(trans-tach)2(SO4)] (46)..................................................................................199
6.20
[Cu(trans-tachH)2(SO4)]SO4 (46 ● H2SO4)...........................................................202
6.21
[Pd(trans-tachH)Cl2]Cl (47) .................................................................................204
6.22
[Pd(trans-tachH)Br2]2[PdBr4] (48) .......................................................................206
6.23
[Pd(trans-tachH)2)](NO3)4 (49) ............................................................................208
6.24
[Pd(trans-tachH)2](SO4)2 (50) ..............................................................................211
6.25
[Pd{Pd(µ2-trans-tach)Cl2}2Cl2] (53) ....................................................................214
6.26
cyclo[{Pd(µ2-trans-tach)Cl}6]Cl6 (51) .................................................................217
6.27
cyclo[{Pd(µ2-trans-tach)Br}6]Br6 (52) .................................................................220
6.28
poly{[Ag(µ3-trans-tach)](NO3)} (54)...................................................................223
6.29
catena-poly{[Ag(µ3-trans-tach)](CF3SO3)} (55) .................................................226
6.30
catena-poly{[Zn2(µ2-trans-tach)(µ2-Cl)2Cl2]} (56) ..............................................229
6.31
tbc (57)..................................................................................................................232
6.32
[Cu(ttop)Cl]Cl (61)...............................................................................................235
6.33
[{Cu2(µ2-ttop)(µ2-Cl)Cl2}2]Cl2 (62) .....................................................................238
SUPPLEMENTARY DATA .................................................................................................242
7.1
Table of Θ and χ for all complexes.......................................................................242
7.2
Speciation resolution of a triprotic ligand A .........................................................243
7.3
Re-numbered hexanuclear ring complexes 51 and 52...........................................245
7.4
Correlated NMR spectra for complex 51 ..............................................................247
REFERENCES ...................................................................................................................250
ABBREVIATIONS
VII
ABBREVIATIONS
B.p.
Boiling point
chtt
1,3,5-Cyclohexanetrionetrioxime
CI
Chemical ionisation
CN
Coordination number
COSY
Correlated spectroscopy
CSD
Cambridge crystallographic data base
dec.
Decomposition
EI
Electrical ionisation
en
Ethylenediamine
HSQC
MALDI
Heteronuclear single quantum coherence spectroscopy
Matrix assisted laser desorption ionisation
M.p.
Melting point
MS
Mass spectrometry
MS-ES+
NOE
triflate, OTf
Positive ion electrospray mass spectrometry
Nuclear Overhouser Effect
Trifluormethanesulphonate (CF3SO3-) anion
pz
Pyrazine
rt
Room temperature
cis-tach
trans-tach
cis,cis-1,3,5-Triaminocyclohexane
cis,trans-1,3,5-Triaminocyclohexane
tbc
cis,trans-1,3,5-tris-Benzaldimino-cyclohexane
tcm
Tricyanomethanide (CCN3-) anion
TM
Transition metal
TOF
Time of flight
ttop
cis,trans-1,3,5-tris-(Pyridine-2-carboxaldimino)-cyclohexane
1
INTRODUCTION
1
1 INTRODUCTION
1.1 Supramolecular Chemistry
“Molecular chemistry has established its power over the covalent bond. The time has come to
do the same for non-covalent intermolecular forces. Beyond molecular chemistry lies the field
of supramolecular chemistry, whose goal it is to gain control over the intermolecular bond.”1
This consequential development represents the next step in increasing complexity beyond the
molecule towards the supermolecule and organised polymolecular systems held together by
non-covalent interactions. Molecular interactions form the basis of the highly specific
processes that occur in biology such as recognition, reagent transport and regulation.2,3 The
design of biomimetic systems capable of processes of highest efficiency and selectivity
requires correct manipulation of the energetic and stereochemical features of the non-covalent
intermolecular forces within a defined molecular architecture.
The concept of ‘supramolecular’ stems back as early as 1906, when Paul Ehrlich recognised
that molecules “do not act if they do not bind”.4 This introduced the concept of molecules
acting as receptors. Further to this, a notion presented by Emil Fischer that binding must be
selective5 led consequently to the ‘lock and key’ model of steric fit, implying a certain degree
of geometrical complementarity. Thus the foundations of molecular recognition were laid.
Finally, selective binding requires interaction, an affinity between the partners that may be
related to the idea of coordination introduced by Alfred Werner.6 Therefore, supramolecular
chemistry can be thought of as a generalization of coordination chemistry.7 With these three
concepts, receptor, recognition and coordination, the scientific basis for supramolecular
chemistry was established.
The field began developing with the selective binding of alkali metal cations by natural8-14 as
well as by synthetic macrocyclic and macropolycyclic ligands, predominantly crown
ethers15,16 and cryptands.17-20 A novel area of chemical research emerged by the identification
of molecular recognition processes,20 extension to intermolecular interactions and expansion
into other areas - growing into supramolecular chemistry. The term ‘supramolecular
chemistry’ was introduced in 197821 as a development and generalisation of earlier work20:
1
INTRODUCTION
2
“Just as there is a field of molecular chemistry based on the covalent bond, there is a field of
supramolecular chemistry, the chemistry of molecular assemblies and of the intermolecular
bond.” It has been reformulated on various occasions, e.g. “Supramolecular chemistry may be
defined as ‘Chemistry beyond the molecule’, bearing on the organised entities of higher
complexity that result from the association of two or more chemical species held together by
intermolecular forces.”22 A chemical species is thereby defined by its components, by the
nature of the bonds that hold them together and by the resulting geometrical and topological
features. The objects of supramolecular chemistry are individual entities, supermolecules
possessing features as well defined as those of molecules themselves. It can be postulated that
supermolecules are to molecules and the intermolecular bond what molecules are to atoms
and the covalent bond.22,23 The supermolecule therefore represents the next level of
complexity of matter after the fundaments of the elementary particle, the nucleus, the atom
and the molecule.
In modern terminology, the meaning of supramolecular becomes a strategy for controlled
self-assemblies, tightly linked to coordinative interactions between metals and organic
ligands, which also interact through intermolecular forces.24 These intermolecular forces, such
as hydrogen bonding, donor-acceptor, van-der-Waals and π-π stacking interactions25-30 are in
general weaker than covalent bonds.† This means that supramolecular species are
thermodynamically less stable, kinetically more labile and dynamically more flexible than
molecules. The dynamic behaviour of such fast processes is important for the thermodynamic
control of multi-component assemblies.24,32,33
Major goals of supramolecular chemistry are to mimic the structure and function of the
magnificent ensembles found in molecular biology, as well as to generate new materials based
on complex architectures. Approaches to the assembly of large arrays and organisation of
multiple components are divided into principally different concepts: serendipity and rational
design. Not limited by the boundaries of imagination, some of the polymetallic complexes
with the most interesting physical properties have been made, at least initially, by accident.
For example, the initial discovery of ‘single molecular magnets’ was achieved
unintentionally,34 a concept now of immense current interest due to possible applications as
†
Coordination bonds can have a substantial proportion of covalency.31
1
INTRODUCTION
3
information storage devices. Serendipitous examples of such self-organised architectures are
not amenable to initial design, yet allow fascinating insights into the processes that govern
spontaneous supramolecular assembly.35,36 Contrary to this, rational design utilises carefully
chosen metal centres and ligand systems to facilitate the geometrical requirements for a given
architecture. A common feature of such design approaches is the use of rigid and
polyfunctional organic ligands as geometrically predetermined building blocks. Rigidity is an
important factor to ensure the formation of the desired architecture that represents the most
favoured structure as a function of the geometrical restraints. Elegant examples of the rational
design approach can be seen in the architectures based both on the chelating ligands used by
Raymond,37,38 Lehn,39,40 and Saalfrank41,42 that show increased preorganization and high
formation constants,43 and on monodentate ligands,44-48 exemplified by the work of Stang.33,49
In his innovative work on palladium(II) coordination chemistry, Fujita44,50 uses the concept of
‘molecular panelling’ to create polynulcear complexes containing palladium(II) fragments.
1.2 Self-assembly
Coordination driven motifs allow great directionality offered by metal-ligand coordinative
bonding compared to weak electrostatic, π-π stacking or hydrogen-bonded interactions. In
addition they allow great versatility due to the variety of transition-metals and multidentate
ligands available as building blocks. Self-assembled multinuclear coordination structures are
considered supermolecules. Although supermolecules have – in common with covalently
constructed molecules – well defined molecular weight, formula, and connectivity, they also
have other properties that are unique to the assembly, and unattainable with the individual
component molecules.
On such basis, the following common features for metal-mediated supramolecular selfassembly can be assigned: (i) self-assembling units are held together by coordinative
interactions; ii) the assembly of subunits into larger architectures is selective to form the most
stable aggregate; iii) the assembled units display different properties to their individual
subunits and iv) descrete assemblies are generally thermodynamically favored over polymeric
systems.
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INTRODUCTION
4
Several approaches have been developed towards self-assembly of discrete multinuclear
species. The two main rational design approaches are based on either control of the bonding
vector direction between the building blocks (‘symmetry interaction model’) or control of the
overall symmetry of interaction between the molecular components (‘molecular library
model’).33,38
1.2.1 Design principles
1.2.1.1 ‘Symmetry interaction’ model
Figure 1 Geometric relationship between symmetry operations in tetrahedra (LHS) and helicates (RHS).33
The ‘symmetry interaction’ model takes advantage of multibranched chelating ligands, as
exemplified by Lehn, Raymond and Saalfrank, which show increased preorganisation and
stronger binding energies as a result of the chelate effect.43 Geometric relationships between
ligand and metal ion form the conceptual basis to understand the formation of a given
supramolecular architecture. For example, a tetrahedron contains four C3 and six C2 axes
(Figure 1, LHS). The structure may be formed from the combination of four C3-symmetric
tris-bidentate chelated octahedral metal centres spanned by six C2-symmetric bis-bidentate
ligands (M4L6 tetrahedron). This approach places the metal ions at the vertices of the
tetrahedron and the ligands on the edges. However, not any combination of C2-symmetric bisbidentate ligands and octahedrally coordinated metal centres will generate a tetrahedron. The
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INTRODUCTION
5
structure of the ligand is critical in achieving the correct orientation of symmetry elements in
the target structure (e.g. 54.7° between the C2 and C3 axes for a tetrahedron).33 Another
rationally designed example is a M2L3 triple helicate with D3 symmetry, where it is important
that the C3 and C2 symmetry axes are perpendicular to each other (Figure 1, RHS).33
1.2.1.2 ‘Molecular library’ model
Figure 2 Molecular library of 2-D cyclic molecular polygons constructed via combination of ditopic building
units with fixed geometry.33
The ‘molecular library’ model takes advantage of multibranched monodentate ligands and
was first applied by Verkade45 and later elaborated by Fujita44,48 and Stang.46,47 It describes
metal and ligand components as angular and linear pieces to be combined in the formation of
two-dimensional polygons and three-dimensional polyhedra. A matrix of combinations of
these simplified elements provides a formula for the synthesis of an array of self-assembled
architectures (Figure 2). For example, a molecular square can be assembled in several
different ways, such as combining four linear with four 90° angular building blocks or by
combining four 90° angular subunits. Similarly, three dimensional polyhedra can be
constructed where a minimum of one building unit has to be a tritopic subunit.33
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INTRODUCTION
6
1.2.2 2-D polygons
A variety of cyclic arrangements forming two-dimensional polygons are reported in literature.
These examples vary greatly in nuclearity and a small selection of complexes is presented in
this thesis.
1.2.2.1 Dinuclear assemblies
Figure 3 A selection of dinuclear metallamacrocyles.
One of the first cyclic self-assembled host molecules was reported by Maverick51 and
co-workers (Figure 3, Top left). This complex 1 comprises two copper(II) ions and two bis(beta-diketone) ligands functioning as a host molecule for small amino bases such as pyrazine
or pyridine.51,52
By introducing a non-linear spacer between the two pyridine rings of bipyridine, Fujita and
co-workers53 assembled the water-soluble macrocycle 2 upon coordination to [Pd(en)(NO3)2]
(en = ethylenediamine), which represents a 90° corner unit by coordinative blockage of two
cis-sites on the square planar palladium(II) centre with the ancillary ‘en’ ligand (Figure 3,
Bottom). Due to the perfluorinated phenylene subunits, the complex was capable of
recognizing electron-rich compounds, such as naphthalene, in aqueous solution.
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INTRODUCTION
7
Hannon and co-workers employed an interesting strategy to overcome the need of ancillary
ligands commonly found in self-assembly processes with palladium(II) corner units.33,44 They
incorporated a chelating unit and a monodentate coordination site within one ligand system,54
thus forming the 2:2 metal to ligand complex 3 (Figure 3, Top right).
1.2.2.2 Trinuclear assemblies
Figure 4 Equilibrium between molecular squares (4, LHS) and triangles (5, RHS) in solution.33
Although de novo construction of a molecular triangle does not seem to be synthetically
complex, only a surprisingly small number of compounds have been reported. Such examples
include observations by Fujita55,56 when reacting [Pd(en)(NO3)2] with one equivalent of
several bis-heteroaryl ligands in water (Figure 4). Under the reported reaction conditions, two
different self-assembled macrocycles were observed: molecular squares 4a-d and molecular
triangles 5a-d. The products are in equilibrium as confirmed by NMR-spectroscopy, with the
major species in solution being the molecular square 4. Assignments were supported by the
observation that the equilibrium is concentration-dependent: at higher concentrations the ratio
shifted towards the formation of the less strained molecular square 4. This can be rationalized
by thermodynamic considerations: molecular squares are less strained and hence more stable
in terms of enthalpy, while entropically the molecular triangles are favored since they
assemble from a smaller number of components.
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INTRODUCTION
8
Figure 5 Molecular iron(II) triangle 6.57
Similar equilibria between multinuclear species were observed by Ziessel and co-workers57
upon formation of the trinuclear iron(II) complex 6 (Figure 5).
1.2.2.3 Tetranuclear assemblies
Figure 6 Molecular squares 7a and 7b.55
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INTRODUCTION
9
Molecular squares represent one of the most commonly formed supramolecular assemblies.
These macrocyclic species exhibit 90° corners and therefore a wide variety of transition-metal
complexes with two accessible cis-coordination sites are suitable to form molecular squares.
These include metals with square planar, trigonal bipyramidal or octahedral coordination
geometries. In addition to the earlier described equilibria between molecular squares and
triangles, Fujita55,58 also reported the quantitative formation of molecular squares by reaction
of 4,4´-bipyridine and [M(en)(NO3)2] (M = Pd (7a), Pt (7b)) in water (Figure 6). These
squares show the ability for molecular recognition of neutral aromatic guests such as
naphthalene and benzene. A great variety of tetranuclear molecular squares and rectangles is
present in literature, including both homo- and heteronuclear assemblies.59-65
1.2.2.4 Higher nuclearity assemblies
Figure 7 Anion templated formation of cyclic penta- (8, LHS) or hexanuclear (9, RHS) iron(II) clusters.66
Lehn and co-workers66 have reported the self-assembly of the iron-containing molecular
pentagon 8 and hexagon 9. Both structures facilitate octahedral iron(II) centres coordinated by
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INTRODUCTION
10
a tris-bipyridyl ligand forming circular, double-stranded helical structures (Figure 7). These
reactions demonstrate the remarkable influence of anion templating effects on the formation
of polynuclear assemblies. The smaller chloride ions template the formation of the smaller
pentagon 8, whereas the larger hexagon 9 is formed upon templation of the larger sulphate
counterions.
Other molecular hexagons include work reported by Saalfrank (Fe6),67 Bernardinelli (Ag6)68
and Matsumoto (Cu6).69 Higher nuclearity assemblies such as molecular heptagons (Fe7)70 and
octagons (Co8, Pt8)71,72 reported by Köhler, McCleverty and Lin are also present in the
literature. It is interesting to note that in the case of the molecular copper(II) hexagon 11
reported by Matsumoto and co-workers, the cyclic arrangement can be ‘switched’ on and off
by controlling the pH in solution. In the mononuclear complex 10, the ligand coordinates to
one copper(II) ion in a tridentate fashion with a neutral imidazol-unit. The square planar
copper(II) coordination sphere is completed by a water molecule. Deprotonation by an
increase in pH results in condensation of 10 via coordination between the imidazolate
nitrogens and copper(II) ions (Figure 8).
Figure 8 pH controlled reversible ‘switch’ from mononuclear (10, LHS) to hexanuclear (11, RHS) assemblies.69
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INTRODUCTION
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1.2.3 3-D cages and polyhedra
A great number of self-assembled three-dimensional complexes has been reported in this
rapidly developing field of supramolecular chemistry and are subject to a number of review
articles.33,38,44,73-79 Only a select few examples of three-dimensional polyhedra can be
presented in this thesis.
1.2.3.1 Triangular prism and cylinders
Figure 9 Selection of triangular prismatic (12, Left) and cylindrical (13, Centre and 14, Right) complexes.33
Triangular prism represent the simplest three-dimensional architectures assembled from the
fewest number of components. Their design requires only five building blocks; two angular
tritopic subunits and three linear connecting units. Formation of such a cage-like molecule
was reported by Fujita and co-workers,80 whose complex 12 is preferentially formed in the
presence of hydrophobic guest molecules (Figure 9, Left).
Further extension of triangular arrangements leads to the construction of self-assembled
cylinder-type structures, which require design and choice of components that fulfil criteria
such as recognition, orientation and termination. Examples involving two types of ligand and
several copper(I) ions were reported by Lehn and co-workers.81 By choosing linear bis- or
tris-bipyridyl ligands in combination with suitable µ3 bridging ligands, reaction with copper(I)
resulted in the formation of the cylindrical complexes 13 and 14 (Figure 9, Centre and Right).
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INTRODUCTION
12
1.2.3.2 Tetrahedra
Figure 10 Molecular assemblies of M4L6 (15a-d, LHS) and M4L4 (16, RHS) tetrahedra.33
Two common types of tetrahedra are present in the literature: M4L6 and M4L4 clusters. In the
case of M4L6 complexes, six C2 symmetric bis-bidentate ligands form the six C2 symmetry
axes of the tetrahedron coordinating to four octahedral metal centres at the vertices (Figure
10, LHS). Saalfrank and co-workers82,83 were the first to report M4L6 complexes, which since
then have been elaborated on and more generalised by others.84-86 The first examples were
formed from dimalonic ester ligands with a variety of metal ions such as magnesium(II),
manganese(II), cobolt(II) and nickel(II) (15a-d). Further studies led to the design principles
described earlier and resulted in a great variety of tetrahedra with different metal ions and
varying cavity sizes by increasing the length of the organic linker molecules.87-90
Interestingly, it was shown that guest molecules could be incorporated in the rigid cavities of
these complexes,87,90-92 which is of great interest for guest exchange91,92 and application of
these hydrophobic pockets in catalytic processes.93,94
Contrary to µ2 bridged metal centres, four C3 symmetric tris-bidentate ligands coordinate to
four octahedral metal centres at the vertices of M4L4 tetrahedra. Utilizing a tris-catechol
ligand, the titanium(IV) M4L4 tetrahedron 16 is formed,95 where each ligand represents a face
of the tetrahedron (Figure 10, RHS). Further investigations into these types of M4L4
complexes were carried out by Raymond95 and others.96
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INTRODUCTION
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1.2.3.3 Octahedra
Figure 11 Molecular assembly of the M6L4 octahedron 17.50
One of the most studied supramolecular cages is the octahedron 17 reported by Fujita and coworkers50 This cage assembles from four ‘panels’, C3-symmetric tris-monodentate pyridine
donor ligands and six cis-capped palladium(II) ions (Figure 11). The large size of the host
cavity allows encapsulation of multiple guest molecules that show extensive host-guest
interactions. For example, Fujita reported the catalysis of styrene oxidation in aqueous
solution in the presence of 17.93 The nano-cavity thereby acts as a phase transfer catalyst for
styrene and product between organic and aqueous phase. No reaction occured when styrene
was treated with the catalyst in the absence of the nanovessel, indicating the catalytic process
occurring inside the cavity of 17. Furthermore, stereoselective [2+2] photo-addition reactions
are accelerated hundred-fold within the cavity of 17.94
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INTRODUCTION
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1.2.3.4 Cubes
Figure 12 Molecular cubes from a variety of metal ions (dark spheres): Co(III), Cr(III) and Mo(III). The
octahedral metal ions are linear bridged by cyanide ligands and face-capped to prevent oligomerization reactions.
A variety of cubic cages have been reported in literature. Long and co-workers have produced
a number of discrete cubic cages with eight cobolt(III), chromium(III) and molybdenum(III)
ions located at the vertices and cyanide as bridging ligands. Face capping ligands facilitated
the formation of the molecular box rather than a three-dimensional network by prohibiting
further coordination to the metal centres (Figure 12).97-101
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INTRODUCTION
15
1.2.3.5 Cuboctahedra
Figure 13 Formation of molecular cuboctahedra by eight tritopic and twelve ditopic building units.102
Stang and co-workers were the first to report the synthesis of molecular cuboctahedra.102
These types of macromolecules are the first examples of artificial synthetic assemblies
exhibiting O-symmetry. The complexes were formed by two different combinations of eight
C3-symmetric planar tridentate units with twelve C2-symmetric ditopic units to spontaneously
assemble the cuboctahedra (Figure 13). Both complexes were characterised in solution and
their sizes were assessed to contain 5 nm voids, which makes them extremely interesting as
supramolecular host molecules.
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INTRODUCTION
16
1.2.4 Helicates
Figure 14
Absolute configurations at metal centres of bis-bidentate (Top) and tris-bidentate (Bottom)
complexes. Views are along the principal axis.
The term ‘helicate’ was first introduced by Lehn and co-workers in 1987 as a description of a
polymetallic helical double-stranded complex.103a The concept of helicates extends the idea of
classical coordination complexes towards more complex supermolecules in which two or
more metal ions lie on a helical axis.1,103a-c Thus, a helicate is a discrete supramolecular
complex formed by one or more covalent organic strands entwined and coordinated to a series
of metal-ions defining the helical axis.
Helical arrangements implicitly result in chirality of the coordinated metal ions, producing
right- (∆ = P) or left-handed (Λ = M) helicity around the principal axis (Figure 14)103b,104-107
Variations in helical arrangements can be described (i) by the number of coordinated strands,
corresponding to single, double and triple stranded helicates and (ii) by the symmetry of the
coordinating strands. Identical strands correspond to homotopic arrangements, whereas nonsymmetrical strands represent heterotopic helicates, which can form isomers by different
orientations of the coordinating binding units (‘Head to Head’ or ‘Head to Tail’) (Figure 15).
From the myriad of reported helical structures, only a few examples are represented in this
thesis.
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INTRODUCTION
17
Figure 15 Overview of homotopic (LHS) and heterotopic (RHS) helical arrangements.77
1.2.4.1 Single stranded
Figure 16 Representation of the single stranded helicate 18 (RHS) formed from quinquepyridine 19 (LHS) and
ruthenium(II) ions.77
The quinquepyridine ligand 19 can form a heterotopic, dinuclear single stranded helicate with
ruthenium(II) (18).108 The crystal structure shows two different six-coordinate metallic sites,
with one ruthenium(II) coordinated to the terpyridine subunit of 19 and the second
ruthenium(II) bound to the remaining bipyridine subunit (Figure 16). The helical twist of the
strand (74.9°) results from the torsion between the two connected pyridine rings of each
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INTRODUCTION
18
subunit. The remaining vacant coordination sites around ruthenium(II) are occupied by one
chloro and two tridentate terpyridine ligands.
1.2.4.2 Double stranded
Figure 17 Representation of the double stranded helicate 20 (RHS) formed from sexipyridine 21 (LHS) and
copper(I) ions.77
Although it was not possible to crystallise the trinuclear complex 20, unambiguous
characterisation by FAB-MS and 1H-NMR (D2-symmetric in solution) was reported by
Constable and co-workers.109,110 Two tris-bidentate sexipyridine ligands 21 wrap themselves
around three copper(I) centres forming the trinuclear double-stranded helicate 20 (Figure 17).
Other examples of double-stranded helicates are the circular penta- and hexanuclear iron(II)
complexes 8 and 9 described earlier.
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INTRODUCTION
19
1.2.4.3 Triple stranded
Figure 18 Self-recognition of triple stranded helicate formation (RHS) with bis-catechol ligands (LHS) and
iron(III) or gallium(III) ions.33
The spontaneous, highly selective process of helicate formation has been reported to exhibit
self-recognition.111,112 In the case of gallium(III) or iron(III) coordination to bis-bidentate
catechol ligands, Raymond and co-workers have reported the exclusive formation of
complementary homo-triple helicates in high yield without traces of oligomeric or mixed
species (Figure 18). The high degree of conformational rigidity of each donor subunit
combined with differences in the distance between binding sites in each ligand provide a
possible explanation for the self-recognition of the assembled complexes.
1.2.5 Interlocked structures and topology
Multi-dimensional interpenetrating networks and molecular entanglements observed in
catenanes, rotaxanes and molecular knots have long fascinated chemists with beautiful
examples of ingenious design. Framework solids, often accessible by self-assembly under
mild conditions, are particularly interesting due to their chemical and structural diversity and
challenging construction.113 Conceptual approaches to the construction of frameworks are
based on the idea of a net. Nets are abstract mathematical entities consisting of a collection of
points or nodes with clearly defined connectivity or topology. The topology of a net can be
described by the general term (n,p) where n is the number of nodes in the smallest closed
circuit and p is the number of circuits surrounding the node. However, more complete is the
Schläfli notation n p(p-1)/2, where n is the number of nodes in the smallest closed circuits in the
net and p(p-1)/2 represents the combinations of connections to two neighboring nodes. Thus
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INTRODUCTION
20
in Figure 19 (LHS), the number 6 in the symbol 63 indicates that the smallest complete
circuits in the net are hexagons, and the number 3 indicates that each node is surrounded by
three hexagonal circuits. In the case of the net shown on the right hand side, the complete
notation should be 4462. This was, however, simplified by Wells114 to 44 by arbitrarily
excluding circuits involving co-linear connections.
X
X
63
X
44
Figure 19 Network topologies for 63 (LHS) and 44 (RHS) nets.
It is interesting to note that both 63 nets in Figure 19 represent different geometries but
identical topologies. This example illustrates the general point that nets may be geometrically
deformed to any extent provided no connections are broken.
1.2.5.1 Zero-dimensional
Figure 20 Topological representation of rotaxanes (Top) and catenanes (Bottom).113
Unlike helicates, interpenetrating zero-dimensional structures can only be disentangled by
breaking of the composite strands. There are a number of ways in which these strands can be
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INTRODUCTION
21
interlocked. For example, catenanes consist of two or more independent rings locked together
(Figure 20, Bottom), whereas rotaxanes comprise a ring encircling the shank of an
independent dumbbell-like component (Figure 20, Top). Molecular knots show per definition
self-entanglement.115 Synthesis and characterisation of these types of structures are
particularly attractive aesthetically, and because of the challenge presented in designing
interpenetrating molecular architectures. Fascinating examples of ‘interlocked’ cages were
recently reported by Fujita and co-workers, in which two identical three-fold catenated units
interpenetrate to form the overall cage structure. (Figure 21).116
Figure 21 Two examples of three-fold catenated cages: Top: Topological representation; Bottom: Structural
diagram (LHS) and space filling representation (RHS) of ‘interlocked’ cages, M = Pd and Pt.116
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INTRODUCTION
22
1.2.5.2 One-dimensional
Figure 22 Topological representation of 1-D interpenetrating polymers. LHS: ligand 22 facilitates the formation
of ladder-like chains composed of [Cd2(22)(NO3)4], resulting in the 3-D interlocked structure found in 23; RHS:
ligand 24 facilitates the formation of a polyrotaxane type architecture composed of [Ag2(24)](NO3)2, forming the
2-D interlocked framework found in 25.113
One-dimensional interpenetrating polymers are extremely rare, in fact only two examples are
reported in literature. The first is based on the ligand 22 and affords infinite, 1-D ladder-like
chains of the composition [Cd2(22)(NO3)4],117 resulting in a three-dimensional interlocked
structure (23, Figure 22, LHS). The second structure is based on the ligand 24 and forms a
1-D polyrotaxane-type coordination polymer with the composition [Ag2(24)](NO3)2, resulting
in a two-dimensional interlocked framework (25, Figure 22, RHS).118
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INTRODUCTION
23
1.2.5.3 Two-dimensional
Figure 23 Topological representation of 2-D interpenetration polymers. Top: Parallel interpenetration resulting
in the 2-D interlocked structure found in 26, which is composed of [Ag(tcm)]; Bottom: Inclined interpenetration
forming the 3-D interlocked framework found in 27, which is composed of [Cu2(pz)3](SiF6).113
Two major categories of interpenetrating 2-D coordination polymers can be described as
parallel and inclined interpenetration (Figure 23). Parallel interpenetration involves
corrugated sheets arranged with their average planes parallel to each other, so that each is able
to pass through the other an infinite number of times. The resulting structure from this
interpenetration is itself two-dimensional, with the composite sheets stacked on top of one
another. One example of parallel interpenetration is given in the [Ag(tcm)] (tcm =
tricyanomethanide) complex forming a 63 net with three connected nodes provided by
alternating central carbon atoms of the tcm ions and three-coordinate silver(I) centres (26,
Figure 23, Top right).119,120
Inclined interpenetration polymers arrange one sheet in a tilted fashion with respect to
another, resulting in an overall three-dimensional interlocked structure. One example of this
type of interpenetration is given in the complex [Cu2(pz)3](SiF6) (pz = pyrazine). This
complex is based on a 63 net with three connected nodes provided by copper(I) centres linked
to three neighbouring copper(I) centres by pyrazine bridges (27, Figure 23, Bottom right).121
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INTRODUCTION
24
1.2.5.4 Three-dimensional
A variety of interpenetrating 3-D coordination polymers are reported. The topology varies
from uniform three-, four- or six-connected nets to multi-nodal combinations of nets
containing three- and five- or three- and six-connected nodes. Each notation can be further
classified into subgroups with slight topological variations in the resulting network. A
comprehensive review of literature examples has been published by Batten and Robson,113
and an overview of the more theoretical aspects of topological systems is given by Wells.114
Figure 24
Topological representation of 3-D interpenetrating polymers. LHS: A chiral 103-a net; RHS:
Enantiomorphic interpenetration of two 103-a nets with opposing handedness found in 28, which is composed of
[Ag2(2,3-Me2pz)3](SbF6).113
One example of three-connected 3-D nets is represented by 103-a nets (the a indicates the
most symmetrical of a number of possible variations). The network is shown in (Figure 24,
LHS) in its geometrically most symmetrical cubic form. Every node shows a trigonal planar
environment with 120° angles. A characteristic feature of the 103-a net is the presence of
four-fold helices, all of identical handedness, running parallel with the three cubic axes. The
net as a whole is therefore chiral. However, interpenetration can occur with an identical net of
the same handedness122,123 and also with a net of opposing handedness, in which case the
structure represents a three-dimensional racemate. Such enantiomorphic interpenetration is
found in the crystal structure of [Ag2(2,3-Me2pz)3](SbF6) in which the silver(I) atoms provide
the three-connected nodes for the 103-a net (28, Figure 24, RHS).124
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INTRODUCTION
25
1.2.6 Application and outlook
Progress in utilizing cavity-containing supramolecular hosts in catalysis raises expectations to
further increasing the applicability of such systems. By providing a unique molecular
environment isolated from the bulk solvent, a high local concentration of reagents is observed
within the cavities. Thus, reactions of hydrophobic substrates can occur in aqueous media
within the hydrophobic cavities resulting in stereospecific and catalytic reactions otherwise
not observed in bulk solution. Furthermore, fixation of unstable intermediates or
products101,125,126 within these cavities can provide insight into reaction mechanism. Further
application involves deposition of the catalytically active complexes onto surfaces as
demonstrated by Reinhoudt and co-workers.127
Self-assembly has proven to be a powerful tool for the formation of large, multifunctional
architectures. Most metal ions can be incorporated into supramolecular assemblies as
demonstrated by various examples in literature. This rapidly growing field of chemistry
routinely produces new structures that further deepen our understanding of structural design
principles and give insight into the processes that govern supramolecular formation.
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AIMS
26
2 AIMS
Current interests in supramolecular chemistry lie within the synthesis and characterisation of
suitable building blocks to construct supramolecular architectures. Since rigidity of a given
ligand system plays an important role in rational design approaches, the main focus so far has
been on aromatic ligands with in-built rigidity due to aromatic planarity, delocalisation and
conjugation.33,38,44,48 However, there are less examples that utilise aliphatic based ligands to
form supramolecular assemblies.128-130
The aim of this work is to exploit the use of rigid aliphatic ligand systems in supramolecular
chemistry and to demonstrate their usefulness in constructing supramolecular architectures.
The design of such aliphatic ligand systems in this thesis is based on certain requirements:
(i) rigidity must be maintained in order to rationally design supramolecular assemblies,
(ii) multiple donor groups are necessary to link metal centres together via coordinative
interactions, (iii) the binding sites must be non-interacting to ensure the formation of
polynuclear complexes. Differentiating these binding sites might further prove useful in
specifically addressing different parts of the molecule by (iv) dividing the donor groups into
sets of binding sites with varying denticity, (v) incorporation of hard and soft donor groups,
thus varying their coordination preferences, (vi) enabling second order interactions like
hydrogen-bonded interactions.
Consideration of these ligand requirements leads to a set of molecules based upon the rigid
cyclohexane backbone. The cyclic structure provides rigidity and the equatorial and axial
positions on the chair conformation differentiate between multiple binding sites. As a starting
point, the simplest case will be investigated as proof of concept. In this case, three binding
sites in 1,3,5-position on the cyclohexane ring are chosen to be in a cis,trans-configuration.
This places two donor groups in cis position and a third trans, thus generating two sets of
non-interacting binding sites. Furthermore, the donor atoms are chosen to be identical, namely
amino groups. Amino functionalities are excellent donor groups in coordination chemistry,
which also have the ability to forming strong hydrogen-bonded interactions, particularly upon
protonation.
2
AIMS
27
This project investigates the coordination chemistry of cis,trans-1,3,5-triaminocyclohexane
(trans-tach, 29) and its potential as a building block in supramolecular chemistry. The
coordination chemistry131-136 and application137-142 of its geometrical isomer, cis,cis-1,3,5triaminocyclohexane (cis-tach, 30) is well established; cis-tach acts predominantly as a
tridentate chelating ligand, preferentially facial coordinating in octahedral complexes. This
tridentate coordination mode is achieved by ring-flip from the more stable tris-equatorial to
the tris-axial conformation upon complexation. Presumably entropic effects, as well as
ligand-field stabilisation energies, overcome the enthalpy necessary for the transformation of
one conformation into the other. Similarly it is anticipated that trans-tach is also capable of
conformational change from the more stable bisequatorial-monoaxial conformation into the
bisaxial-monoequatorial conformation upon complexation. This is demonstrated by the
related ligands cis-1,3-diaminocyclohexane143-146 and cis-3,5-diaminopiperidine,147,148 where
the cis-axial amino groups can act as a bidentate chelating moiety upon coordination to
various metal ions. In addition to the cis-bidentate moiety upon ring-flip, it is anticipated that
the equatorial amino group provides a monodentate coordination site independent from the
bidentate chelating moiety. Furthermore, ligand derivatisation, i.e. upon Schiff base
formation, are anticipated to be straightforward as demonstrated on the cis,cis-isomer.149-151
These ligand systems are anticipated to exhibit modified coordination properties.131,152