VERIFICACIÓN DE HIPÓTESIS O IDEA A DEFENDER

This chapter covers the coordination chemistry of seven disubstituted benzene ligands with various silver(I) salts. Ligand 1.28 was synthesised from a Wittig reaction with

terephathalic aldehyde. Ligands 2.4 – 2.6 were created through simple Stille coupling reactions and ligands 2.7 – 2.9 were prepared from the allylation of dihydroxybenzenes. Only ligand 2.5 was a previously unreported compound.

Ligands 1.28, 2.6 and 2.9 gave one-dimensional polymers with AgClO4. Both 1.28 and

2.9 gave very similar one-dimensional polymers that continuously folds back upon

themselves to enable π-π stacking interactions between the aromatic ligand cores. Alternatively, ligand 2.6 gave a linear one-dimensional polymer which was enabled, in part, by the increase of flexibility of the functional arm. The other factor influencing the formation of the linear polymer is the solvent toluene molecules in the crystal lattice which reduces the need for the polymer to fold as it provides additional π-π stacking interactions. With a change in counter anion from perchlorate to tetrafluoroborate, ligand

2.9 gave a two-dimensional polymer, with 2.9 acting in a bi- and tri-dentate manner.

Similarly on changing from a perchlorate to trifluoroacetate counter anion, ligand 2.6 gave a three-dimensional interpenetrated network. Ligand 1.28 gives a second polymorph with AgClO4 and not only forms a one-dimensional polymer, but also a [2+2]

macrocycle.

Ligand 2.8 interacts in a tridentate manner with silver(I) which is more analogous to

ortho-divinylbenzene as opposed to the corresponding meta-divinylbenzene. Similar to ortho-divinylbenzene, 2.8 forms a two-dimensional metallopolymer. The two-

dimensional sheets stack in a way which creates one-dimensional hydrophilic pores which are filled with hexafluorosilicate counter anions.

Ligand 2.7, as a result of the additional flexibility in the functional arms, gives rise to helical one-dimensional polymers as opposed to the more rigid 1.26, which gives a two- dimensional network. Remarkably ligand 2.7 has the same solid state conformation with both AgClO4 and AgPF6 and, although in a different conformation, gives a related one-

Silver(I)-olefin species are detectable in solution and the silver(I) to ligand ratio of the solid complex is determinable. The exact nature of the species present in solution is unknown, however, it is clear there is an equilibrium between the coordinated and free ligand. The nature of the equilibrium has shown a dependency on the counter anion present.

Chapter Three

Two Armed Ligands with

Naphthalene Cores

C

3

Two Armed Ligands with Naphthalene Cores

3.1 Introduction

An advantageous feature of metallosupramolecular chemistry is the ability to control the dimensions and geometry of the basic components to give the desired product. The previous chapter explored the effects of using small spacer groups and substitution patterns around a benzene core. Another way to affect the separation of functional groups is to change the core itself. There are many different potential ligand cores from single atoms such as carbon, nitrogen or oxygen to large extended aromatic systems.109-111 Naphthalene is an effective core as it offers ten possible disubstituted isomers compared to the three (ortho, meta, para) isomers of disubstituted benzene.

NH2 NH2 O O O O P(Ph)2 P(Ph)2 O O N N N N N N 3.1 3.2 3.3 3.6 3.7 N N O O O O O O N N O O O O O O 3.4 3.5

Figure 3.1 – A selection of ligands with a variety of functional groups and

Naphthalene has been successfully used as a core in bisubstituted ligands with a variety of functional groups. Shown in Figure 3.1 are some such examples.112-117 Ligand 3.1 is used as a simple bridging co-ligand with 2,2’-dipyridylamine to form a dimeric copper complex where the dimers are linked together into a two-dimensional network via hydrogen bonding.112 It was again used as a bridging co-ligand in a different study, this time with 4,4’-bipyridine, which resulted in a two-dimensional square grid when reacted with Ni(II) or Co(II).118 Molecule 3.2 has been used as a bridging ligand to bridge two Cu(I)(trifac)2 (trifac = 1,1,1-trifluoropentane-2,4-dionato) units113 and ligand 3.3 can

chelate to Au(II) to form dinuclear gold complexes.114,115

Ligand 3.5 has been used, in combination with copper, to form a tetraanionic dinuclear Cu(II) metallacyclophane complex.117 The intermetallic distance in the complex was calculated to be 8.3 Å. Further calculations showed that the estimated intermetallic distance in an analogous complex using ligand 3.4 would be 10.09 Å. This is representative of the ability to control metal-metal separation through ligand substitution patterns around a naphthalene ring.

Ligand 3.6 is chiral and has been used by Bernardinelli et al. to form chirally predetermined helicates. It was combined with Ag(I) to give two intertwined single- stranded helicates that form an infinite, highly symmetric double helix.119

Ligand 3.7 is of particular note as it is one of a series of disubstituted isomers based around a naphthalene core. It has the same 180 ° geometry as an analogous 1,4 disubstituted benzene and both result in a dimetallocyclophane when combined with silver(I) nitrate.116,120 The metal-metal separation is extended from 10.38 Å in the benzene derived dimetallocyclophane to 12.58 Å by the incorporation of a naphthalene core into the ligand.

Also of interest is that the solid state structure of species produced from this ligand series showed a remarkable dependence on the specific substitution patterns in the naphthalene core. As previously mentioned 3.7, when combined with silver(I) nitrate, gives a M2L2

dimetallocycle and the 1,3-isomer will also give rise to a M2L2 dimetallocycle. However

using the same silver salt the, 1,5- and 1,6-isomer give one-dimensional zig-zag metallopolymers of a similar nature, and the 1,7-isomer yielded a one-dimensional single- stranded helicate.116

With naphthalene’s larger aromatic system, there is greater potential for η2 arene-silver(I) coordination to occur as any electron cloud perturbation introduced by the coordination of silver(I) can effectively be spread more over a two ring system than a one ring system.121 This offers the opportunity to evaluate the comparative favourability of forming a silver(I)-alkene, silver(I)-arene or a silver(I)-ether oxygen interaction.

Unlike benzene, naphthalene does not have uniform carbon-carbon bond lengths. Naphthalene has three resonance contributors, as shown in Figure 3.2. Two of the resonance contributors have double bonds localised between C1-C2, C3-C4, C5-C6 and C7-C8 with only one resonance contributor where the double bonds are localised between C1-C8a, C2-C3, C4-C4a, C4a-C5, C6-C7 and C8-C8a. This results in the bonds between carbons with the double bond localised in two contributors having shorter lengths compared to carbons with double bonds localised in only one contributor. The shorter bond lengths are generally 1.37 Å and the longer lengths 1.42 Å.

2 3 4 4a 8a 1 5 6 7 8 2 3 4 4a 8a 1 5 6 7 8 2 3 4 4a 8a 1 5 6 7 8

Figure 3.2 – Three resonance contributors of naphthalene.

Theoretical calculations based on delocalization energies done in the early 1960’s by Fukui et al. predict silver will preferentially coordinate to naphthalene at the C1-C2, C3- C4, C5-C6 and C7-C8 positions.122 The first crystal structure of a naphthalene silver(I) complex was published in 1969 revealing naphthalene bridging four silver atoms via η2 coordination at all four of the predicted positions.121,123 A search of the CCD yielded two silver(I) complexes of disubstituted naphthalenes with η2 silver coordinated at the

predicted positions124 and one monosubstituted naphthalene with η2 coordination at the unexpected position of the C2-C3 bond.125

The ability to incorporate chirality into assemblies is an important aspect of metallosupramolecular chemistry. Chirality can be achieved with either the ligand or metal component or a combination of both. Most commonly it is incorporated through the use of chiral ligands. Chiral ligands can arise from either modified natural products like

3.6 or the resolution of synthetic compounds like 1,1'-binaphthalene.

(R)-(+)- (S)-(-)-

Figure 3.3 – Enantiomers of 1,1'-Binaphthalene.

1,1'-Binaphthalene has axial chirality as a result of restricted rotation around the transannular bond. This unit is used as a ligand core with substitution at the 2,2’-position. Substitution at this position with oxygen gives 1,1'-binaphthyl-2,2'-diol (BINOL), nitrogen gives 2,2'-diamino-1,1'-binaphthyl- (BINAM) and diphenylphosphine gives 2,2'- bis(diphenylphosphino)-1,1'-binaphthyl (BINAP).

OH

OH NH₂

NH₂ P(Ph)₂

P(Ph)₂

These compounds and in particular BINOL and BINAP are among the most widely used ligands for both stoichiometric and catalytic asymmetric reactions in recent times.126,127 BINOL and BINAM are used routinely in metallosupramolecular chemistry.128

BINOL is widely used in metallosupramolecular chemistry as it can be functionalized on the naphthalene ring, the hydroxyl groups or both.128 Some examples of assemblies arising from ligands with a BINOL core are; cyclic tetramers with rhenium,129 molecular triangles with platinum,130 and triple stranded helicates with zinc.131 Unmodified (R)-(+)- 1,1'-binaphthyl-2,2'-diol was used as a ligand, in combination with silver(I), in closley related work by Munakata et al.132

Figure 3.4 – View of the complex of (R)-(+)-1,1'-binaphthyl-2,2'-diol and

silver(I) perchlorate.132

(R)-(+)-1,1'-Binaphthyl-2,2'-diol was combined with silver(I) perchlorate monohydrate in a 1:1 ratio resulting in a three-dimensional porous network. In the resulting complex the free hydroxyl groups are not coordinated to the silver and the silver coordination sphere contains two perchlorate oxygens, one η2 interaction with the chiral ligand and an η2 coordinated benzene acting as an auxiliary ligand. The η2 coordination of (R)-(+)-1,1'- binaphthyl-2,2'-diol to silver occurs at the C7-C8 position, labeled C1-C2 in Figure 3.4. The torsion angle between the two substituted naphthyl rings is 113.7 ° and is larger than in the free ligand which has an angle of 99.2 °.133