Conclusions

As a result of this study several novel polynuclear complexes incorporating paramagnetic transition metal complexes and H2L3, H2L4 or H3L6 have been

synthesised from the reaction of the chelating ligands H2L3 or H3L6 with Mn(II), Ni(II)

or Cu(II) salts rather than using the common methodology consisting of building around a preformed metal-carboxylate cluster. The undecanuclear manganese cluster 12 includes a unique MnII4MnIII7 core. Moreover, the most noticeable feature of the reaction leading to 12 is the in situ formation and trapping within the Mn11 product of the pentadentate ligand (L4)2- from the reation of two H2L3 molecules. This ligand has

only been identified previously in a Mn8 and Mn9 cluster.93 However, isolation and structural characterisation of the mononuclear manganese complex 13 including H2L4

has allowed the first mononuclear complex of the pentadentate ligand to be identified. Another noticeable feature of complex 13 is that it exists as two enantiomers, as two non-superimposable forms of the complex are obtained by application of the inversion centre inherent to the space group. Only one reported example of a seven-coordinate manganese centre,221 also crystallising in the P-1 space group, presents the same remarkable feature as complex 13, however the authors in the paper did not highlight this characteristic. We are currently trying to reproduce crystals of complexes 12, in order to study the magnetic properties of 12.

The tetranuclear mixed-valence manganese cluster 14 is a new member of only a handful of examples of polynuclear clusters including H2L3 reported by

Christou et al., [Mn4(O2CMe)2(HL3)6]2+,40,41 [Mn9(O2CEt)12(L3)(HL3)2(L4)2],93 [Mn11O2- (OH)2(nmpd)(HL3)5(L3)5Cl6],94 and [Mn25O18(OH)2(N3)12(L3)6(HL3)6]2+,59 all of which are SMMs.Future work about cluster 14 includes the study of its magnetic properties, to see if like the similar [Mn4(O2CMe)2(HL3)6]2+ cation,40,41 it behaves as a SMM. H2L3

also formed the tetranuclear nickel complex 15 incorporating an open cubane core that has never been reported. However, attempts to reproduce the same Ni4 cluster incorporating HL5 were not successful, only mononuclear complex 16 was isolated. H2L3 has thus proven to be a versatile (N,O,O) chelating and bridging ligand in the

manganese- and nickel-cluster chemistry, being either singly (HL3)- or doubly (L3)2- deprotonated and behaving either as a tridentate ligand or as a bidentate chelate with the protonated oxygen not bound. Beside the discrete polynuclear complexes incorporating H2L3 and H2L4, a novel Cu(II) coordination polymer, 17, incorporating H3L6 has been isolated, resulting from the bridging of the consecutive Cu centres via

two µ2-oxygen atoms from either the (HL6)2- ligand or acetate molecules. Such a

Cu-coordination polymer is unprecedented. These 1D chains then assemble into 2D sheets via hydrogen bonding interactions. Preliminary magnetic studies on compounds

15 and 17 were performed. However, there were not described in this thesis as they were not fitted due to time constraint.

The work presented in this chapter represents how serendipitous discoveries change the direction of scientific research. Initially, work was to focus on the synthesis and structural characterisation of dinuclear complexes of H2L3 or H3L6. In summary,

this study shows that H2L3 is a versatile ligand, which can adopt a wide range of

coordination modes, and is capable of forming a wide variety of polynuclear complexes, and that H3L6 can behave as a bridging ligand. Indeed, the formation of

complexes 12, 13, 14, 15 and 17 was totally unpredictable, and their mechanism of formation unclear and difficult to ascertain.

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Chapter 4

d-Block Metal Complexes Incorporating Tripodal

Schiff Base Ligands.

4.1 Introduction.

Chapter 2 described the synthesis of cis,cis- and cis,trans-1,3,5-tris- (salicylideneamino)cyclohexane (H3L2a and H3L2b) as well as the structure of various

polynuclear d-block metal complexes including the two isomers of this ligand. The current chapter presents the synthesis and complexation of several tripodal Schiff base ligands similar to H3L2a and H3L2b. These three ligands are shown in figure 4.1: H3L7 and L8 both including a tren backbone, to which are respectively attached three

phenol groups and three pyridine groups through an imine function, whereas H3L2 and L9 both include an L1 backbone, to which are respectively attached three phenol groups and three pyridine groups through an imine function. As was already mentioned in Chapter 2, L1 exists as two isomers, cis,cisand cis,trans, which give two isomers for each tripodal ligands: H3L2a and H3L2b, L9a and L9b.

O H N N N N O H OH N N N N N N N N N N N N N N N N N N N H₃L7 L8

L9a (cis,cis-isomer) L9b (cis,trans-isomer)

138

These ligands contain either three acidic phenolic sites or three pyridyl nitrogen atoms as well as three imino nitrogen centres for interaction with metal ions; each arm including two potential centres for coordination, they can act as chelating ligands. As the tren backbone also includes a central nitrogen atom, H3L7 and L8 can potentially

behave as heptadentate ligands, whereas H3L2 and L9 can only behave as

hexadentate ligands.

H3L7 has been extensively used in the preparation of mononuclear complexes

of formula [(L7)MIII], MIII being either any lanthanide of the series except Pm249-254 or transition metals (Co,255 Ru,256 Mn,257-259 or Fe260). Tetranuclear manganese(II) clusters incorporating two molecules of (L7)3- have also been reported.83,84,257 Extensive work has also been carried out with L8 to form mononuclear complexes [(L8)M]2+(X)2 (M(II) = Mn, Co, Fe, Cu, Ni, Zn, Tc; X = ClO4-,PF6-, BF4-)261-269 or [(L8)Cr]3+(ClO4)3.270 Mononuclear complexes incorporating L9a of formula [(L9a)M]2+(ClO4-)2 where M(II) = Co271, Zn271,272 or Ni273 have been reported. However, no complexes incorporating L9b have been reported to date.

As mentioned in Chapter 1, previous work within the Kruger group involved the

C3-symmetric tris-bidentate ligand, L, featuring a triphenylamine core appended by pyridylimine coordination sites. This was reacted with Ag(I) to form the trinuclear double helicate complex [Ag3L2]3+: the Trinity helix (see figure 1.24 in Chapter 1).161,162 Initially, the aim of the synthesis and complexation of the ligands used in the present body of work was to investigate the influence of the flexibility of the ligand on the formation of such assemblies, the tren or L1 backbone incorporated in these ligands being much more flexible than the planar tertiary amine featured in L. However, the work presented in the following chapter represents how serendipitous discoveries change the direction of scientific research. This study developped into the synthesis and structural characterisation of various trinuclear and mononuclear d-block metal complexes, which are reported herein.

In this chapter it will be interesting to compare the complexes incorporating these four ligands and to see what differences are observed on the complexation when the phenol groups in the ligand are replaced by pyridine groups. Moreover, the tren backbone being much more flexible than the rigid L1 backbone, their influence on the structure of the corresponding metal complexes will be discussed.