Single chain magnets

Another class of compounds that are of interest to magneto-chemists are single chain magnets (SCM).93 These are 1D polymeric materials which display slow relaxation of magnetisation and can show some hysteresis behaviour at low temperature.93,131 In 2001, Sessoli and co-workers reported the first example of SCM behaviour within a 1D polymer chain, [Co(hfac)2(NITPhOMe)] (hfac =

hexafluoroacetylacetonate, NITPhOMe = 4′-methoxy-phenyl-4,4,5,5-tetramethyl-

imidazoline-1-oxyl-3-oxide), that consist of alternating CoII centres and organic radical moieties.132 This compound reveals a magnetic hysteresis below 4 K and the

24 1D chains behave as superparamagnetic nanowires.132 Since the publication of this seminal work, numerous synthetic approaches have been developed for the generation of SCMs.131,133,134 To achieve SCM behaviour a number of criteria must be met.131 Firstly, The 1D chains must possess a strong uniaxial anisotropy (D) together with a net spin magnitude (S > 0) along the chain. This means it is preferable that the spin centres of the chains are ferro- or ferrimagnetically coupled.131 However, spin canted antiferromagnetic interactions may also be employed in the formation of SCMs.131 The generation of SCM systems also requires that the inter-chain interactions must be much smaller than the intra-chain ones.131 This requirement can be overcome by the use of extended organic linkers that act as spacers between the 1D chains.135,136

Liu and co-workers have utilised a series of azido bridges to link CoII _{ions into}

homonuclear chains, [Co(2-2’-bithazoline)(N3)2]n (Figure 1.16a).137 Within these 1D

polymer chains, bithazoline ligands are also employed as peripheral spacers which are required to reduce the inter-chain magnetic interactions.137 _{This compound shows}

intriguing magnetic behaviour with a large hysteresis cycle at temperatures below 1.85 K (Figure 1.16b).137

Although the development of these SCM materials has progressed rapidly there remains one major flaw with regard to the potential application of these compounds for device purposes: As with SMMs, SCMs only display bistability at

Figure 1.16: a) The coordination polymer [Co(2-2’-bithazoline)(N3)2]n and b) its temperature

dependent magnetic susceptibility displaying single chain magnet behaviour.137

25 very low tempertures.93,131 However, it is thought that the coupling of the 1D magnetic properties of SCMs with other physical phenomenon such as spin crossover or photo-magnetism could in future produce desirable multifunctional devices.113,138

1.7 Schiff base ligands in supramolecular chemistry

Since they were first discovered by Hugo Schiff in 1864,139 Schiff base ligands have become one of the most widely used ligand type in coordination chemistry.140,141 The predictable binding modes of these imine type ligands can be exploited in transition metal directed self-assembly to produce a variety of metallo-supramolecular architectures such as macrocycles,142,143 grid structures,144 metallohelicates145,146 and coordination cages.147.

Schiff base ligands are easily prepared via a nucleophilic addition reaction

between a primary amine and a carbonyl compound. The reaction are usually carried out in alcoholic solutions.140 The mechanism of the imine formation (Figure 1.17) involves a nucleophilic attack on the carbonyl functionality by the lone pair electrons of the amine, yielding a tetrahedral intermediate.149 Proton transfer from the nitrogen atom to the oxygen atom then occurs, resulting in the formation of a neutral carbinolamine species. The hydroxyl group is then protonated by an acid catalyst

Figure 1.17: The reaction mechanism for the formation of Schiff bases (R and R’ represent various

26 followed by the expulsion of water to give an iminium ion. Subsequent loss of a proton from the nitrogen atom gives the neutral Schiff base product.148,149

Owing to their ease of formation, a large variety of Schiff base ligands, ranging from monodentate species150 to large macrocyclic moieties,142 have been described in the literature.140,141 One of the most prominent examples are the so called salen-type ligands.151 These tetradentate ligands exhibit a {N2O2} binding mode and

result from a condensation reaction between a salicylaldehyde derivative and a diamine.151 Salen derivatives have been used in the formation of polynuclear transition metal complexes that display appealing catalytic and magnetic properties.104,152, For example, Jacobsen and co-workers have developed a chiral salen-type ligand, (R,R)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2- cyclohexanediamine, which was used to form a mononuclear MnII _{complex that}

displays activity as an epoxidation catalyst.153

Salen-type ligands have also been utilised for the synthesis of coordination compounds that display fascinating magnetic properties.154,105 It was demonstrated that the [MnIII(salen)]+ complex exhibits axial elongated Jan-Teller distortion which

Figure 1.18: a) Schematic representation of the 1D polymer chain in (NEt4)[Mn2(salmen)2

(MeOH)2Fe(CN)6].156 b) Field dependence of the magnetisation of (NEt4)[Mn2(salmen)2

(MeOH)2Fe(CN)6] below 1.1 K.157 b)

27 results in a large uniaxial anisotropy (D).155 For this reason, [MnIII(salen)]+ and derivative complexes have recently been attracting much attention as a source of uniaxial magnetic anisotropy for the design of molecular superparamagnets such as SMMs or SCMs.154 Miyasaka and co-workers have developed the compound (NEt4)[Mn2(salmen)2(MeOH)2Fe(CN)6] (salmen = rac-N,N‘-(1-

methylethylene)bis(salicylideneiminate)) (Figure 1.18a) which consists of a network of discrete dinuclear [Mn2(salmen)2(MeOH)2Fe(CN)6] complexes linked by H-

bonding.156 It was shown that this compound behaves as a SMM at temperatures below 1.1 K, with a S = 9/2 ground state (Figure 1.18b).157

Macrocylic Schiff base complexes have also attracted much attention as they can be used as model systems for complicated matalloproteins, such as haemoglobin.141,158 _{Many macrocyclic complexes have been prepared using template}

synthesis.140,141 This synthetic approach involves the condensation of a dicarbonyl precursor with an appropriate polyamine in the presence of a metal ion.141,159 _This

template effect has been known since 1964 when Curry and Busch reported the FeII- templated condensation of 2,6-diacetylpyridine with triethylenetetramine to give a FeIII pentaazadiimino macrocycle. Since then researchers have developed synthetic approaches for the generation of complex macrocycles that can be applied as biological models.160 For instance, McKee and co-workers have developed a series of dinuclear CuII macrocycles derived from the condensation of 2,6-diacetylpyridine and 1,n-diamino-n'-hydroxyalkanes (where n,n' = 3,2; 4,2; and 5,3).161 These dinuclear

CuII macrocyclic compounds are used as model systems for haemocyanin which is the dioxygen-carrying metalloprotein found in arthropods and molluscs.161

Schiff bases have also shown the ability to self-assemble into molecular helicates in the presence of transition metal ions.73,145,146 A series of ligands which consist of two imine binding groups linked by a diaminophenylmethane core have been extensively exploited for this purpose (Figure 1.19).75,76 As discussed in section 1.4, Hannon and co-workers have used these diimine ligands to develop a series of dinuclear helicates that were shown to bind and coil with DNA.76,77,78 They have also reported the RuII helicate, [Ru2L3]4+ (where L = N,N’-bis-(2-pryidine-2-yl-

methylene)-4,4’-diaminodiphenylmethane), which displays some cytotoxic activity against the human breast cancer cells HBL-100 and T47D.77

28

Figure 1.19: Examples of flexible Schiff base ligands that have been used in the self-assembly of

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