Segmentación de la cartera de crédito

Five-coordinate 16 valence electron complexes are known to be short-lived; some can only be speculated as reaction intermediates. The addition of oxidants that add via a two-step SN2 mechanism allowed for these intermediates to be trapped. This included the use of

auxiliary ligands, bulky ligands (to block coordination sites), and intramolecular interactions (such as agostic interactions). This chapter looked at the addition of the chlorine-based oxidant PhICl2 to C^N^C dicyclometallated PtII complexes with larger and

bulkier ligands, building on the past work in the Rourke group.141,150,158

After the addition of PhICl2 to the various PtII starting materials, dichloro- complexes were

observed in all cases. These are the expected products based on the initial addition of Cl+ to the 16 valence electron PtII _{complex, followed later by combination with Cl}- _{to give}

coordinatively saturated 18 valence electron PtIV _species.

However, the dichloro- complexes were not the only observed compounds. Oxidation of 1- Ph gave a five-coordinate species which preceded the dichloro- complex, where the bulky phenyl groups blocked the open coordination site. Oxidation of 1-Bu, 1-Pr or 1-Tol gave transcyclometallatation products formed by trapping the five-coordinate intermediate with an intramolecular agostic interaction.

The driving force in the formation of the transcyclometallation product is thought to be the release of bond strain in the five-membered ring. For this reason, future work should focus on monocyclometallated PtII starting materials, or less rigid dicyclometallated complexes. More specifically, it might be worth studying the oxidation of a triethylphosphine (or ethyldiphenylphosphine) to see if a four-membered ring can be synthesized.

62

Oxidative Addition with RX Compounds and Subsequent

Reductive Elimination

3.1. Introduction

RX compounds (e.g. MeI) add to square planar PtII complexes via an SN2-type reaction. An

SN2 reaction has been proved by the dependence of the rate on the concentration of both

reagents, and the increase in reaction rate with polar solvents.159 The rate of reaction is also greatly affected by the level of congestion at the metal centre, as the metal needs to be able to attack the R group. This also has implications for the size of the R group, as any congestion at the metal centre will reduce the rate of oxidative addition, and increase the rate of reductive elimination. The rate difference for increasing the size of the R group can be large; Puddephatt showed that some rates of oxidative addition of MeI were up to a hundred times faster than for comparable reactions with EtI.160

SN2 reactions are by definition, a two-step mechanism. In the case of MeI undergoing

oxidative addition to a 16 valence electron square planar complex, the metal first attacks the methyl carbon, which results in the breaking of the C-I bond. The oxidation state of the five-coordinate metal complex increases by two. The iodide later coordinates to form an 18 valence electron octahedral complex.

At low temperatures, Puddephatt et al have observed direct evidence of an intermediate of the oxidative addition reaction (Scheme 3.1).161 After initial reaction of the platinum complex with MeI at -40 °C in a coordinating solvent (acetonitrile), the methyl group has added, but the remaining coordination site has been taken by a solvent molecule. Upon warming to room temperature, the anion displaces the solvent to form the expected product.

Scheme 3.1

Reductive elimination is the reverse of oxidative addition, and therefore shares the same mechanism pathways, but in reverse. Work in the Goldberg group looked at competing reductive elimination reactions at PtIV centres. They started with (P^P)PtIVMe3I, where

63 there is a methyl group trans to the iodide. Heating the complex led to the formation of two PtII species. As the mechanism of an SN2 reductive elimination reaction is expected to go

via the reverse of the expected oxidation reaction, the natural first step is the dissociation of the iodide. The iodide would then abstract the methyl group from the metal, and reduce the oxidation state of the metal by two.162

Dissociation of the iodide reveals a coordinatively unsaturated intermediate. The coordinatively unsaturated complex allowed for the concerted reductive elimination of ethane, leading to the formation of the second PtII _{complex. The S}

N2 reaction was therefore

in competition with the concerted reaction, giving two possible products. Other work in their group demonstrated that methyl groups can be abstracted from five coordinate complexes by other five-coordinate complexes (also an SN2 mechanism).163

Scheme 3.2

Next, they synthesized the tetra-methyl derivative P^P PtIVMe4. With no iodide to

dissociate, an SN2-type mechanism wasn’t possible. With longer heating times and higher

temperatures (50 h, 165 °C in benzene) ethane was still produced in a concerted reductive elimination reaction (Scheme 3.3).164

Scheme 3.3

A concerted reductive elimination mechanism is highly disfavoured at coordinatively saturated complexes because the electrons donated from the coupled group would be put into a highly antibonding orbital, which would destabilise the complex (Figure 1.9). Therefore, to allow a concerted mechanism to take place, a group would first have to leave to reveal a coordinatively unsaturated complex. Because methyl groups are unlikely to dissociate (as the carbanion would be highly unstable), it is likely that it is a phosphorus

64 that dissociates. Platinum-phosphorus bonds can be very strong, due to the phosphorus orbitals allowing efficient forward and back bonding. The strength of these bonds explains the need for the high temperatures and long reaction times, as the reductive elimination reaction is fast (and irreversible). Lower temperatures of reductive elimination could be achieved with monodentate-phosphines. Diphosphines will dissociate less readily than similar monodentate-phosphines because of the chelate effect, increasing the temperature required. In the examples mentioned above, all of the possible coupling groups were methyl groups. The methyl groups could move freely to different places on the metal (in the coordinatively unsaturated intermediate), and so there is no selectivity over which methyl groups would couple.

Cyclometallated ligands restrict the movement of other ligands by taking up two or more adjacent coordination sites. With less free movement in the coordinatively unsaturated intermediate, the products of reductive elimination will depend more on the original positions of the ligands before a group was removed (i.e. the starting material). Removal of a ligand from a six-coordinate 18 valence electron octahedral complex will give a five- coordinate reactive species which can take two different (but quite similar) geometries; square-based pyramidal and trigonal bipyramidal. Favoured and disfavoured coupling pathways are shown in Figure 3.1.165

Figure 3.1 Illustration of favoured and disfavoured coupling geometries at five-coordinate metals.

Le-Le and La-Lb are favoured orientations for reductive coupling reactions because the

65 donated from the coupling groups is donated into a non-bonding orbital. Conversely, La-Le

and Lb-Lb coupling reactions are disfavoured, because (like the coordinatively saturated

complex) the electron density would be donated into a high energy anti-bonding orbital that would destabilise the complex. It has been shown computationally that alkenes may eliminate without the need of a coordinatively unsaturated intermediate, and without breaking the 18 valence electron rule. 166

An example of the dependence of geometry for the selectivity of reductive coupling is shown in (Scheme 3.4). The monocyclometallated C^N complex of PtIV_(PPh

3)Me2I has

fac-PtC3 geometry (aryl and two methyl groups) with the iodide trans to the aryl

group.167,168 Addition of AgBF4 to this complex (to abstract the iodide) gave two isomers

of a single product; the aryl group had coupled with a methyl. There was no evidence of methyl-methyl coupling, showing selectivity of C(sp2)-C(sp3) coupling instead of C(sp3)- C(sp3), forced by the geometry of the intermediate.

Scheme 3.4

The five-coordinate intermediate will have a square-based pyramidal geometry with the aryl group taking the axial position, and the other groups taking basal positions. Axial- basal reductive elimination reactions are favoured, whilst basal-basal are disfavoured. Without any rearrangement of the ligands taking place, the only favoured reaction is the coupling of an aryl and a methyl group, giving the observed PtII products. Axial-basal reductive elimination of aryl-P is favoured geometrically, however the product of the reductive coupling reaction is far less stable, and therefore not seen. Similar results have also been seen for tridentate N^N^C ligands.169

This chapter looks at the reaction of RX compounds with dicyclometallated C^N^C PtII complexes. The PtIV products were then treated with AgBF4 to generate five-coordinate

intermediates. As the C^N^C ligand is forced to take a mer geometry at the metal centre, movement of ligands about a five-coordinate species are even more restricted. The

66 products of reductive elimination have been rationalized, and the five-coordinate intermediates have been trapped.