INICIATIVAS DE FORMACIÓN: PLANES DE FORMACIÓN

The investigation of reactions between the POCOPH ligand 1 and platinum starting materials revealed the propensity of this phosphinite ligand to form a large number of dimeric and oligomeric structures, which possessed both cis and trans coordination geometries. The advantage of investigating this chemistry with platinum complexes was that the magnitude of the platinum-phosphorus coupling revealed information about the nature of the ligand coordinated trans to the phosphorus-donor, and so

Figure 3.8 Stacked plot of31_{P NMR spectra showing the course of the metallation}

provided a crucial tool in distinguishing between cis and trans configurations. With the spectroscopic data for the cis-, trans-, and cis,trans-dimers established using platinum chemistry, this spectroscopic data could be employed to help elucidate the structure of products formed in analogous reactions with palladium.

Reactions between 1 and [PdCl₂(NCMe)₂] were initially performed in benzene-d6,

as this solvent had produced the most interesting results with platinum precur- sors. Analysis of the reaction mixtures by 31_{P NMR spectroscopy indicated the}

initial formation of the symmetric complex 22, which possessed one phosphorus environment in the 31_{P NMR spectrum, at δ}

P = 88.3 ppm. This was likely to be trans-[(POCOPH)PdCl2]2 owing to the phosphorus chemical shift slightly downfield

of the free ligand (mutually trans phosphorus coordination was observed to appear at chemical shifts close to that of the free ligand in the 31_{P NMR spectra of plat-}

inum complexes, whereas phosphorus coordination trans to a chloride produced a significant upfield shift of this phosphorus resonance). However, this complex pos- sessed broad signals in the 1_{H NMR spectrum and was seen to react further to give}

a poorly soluble yellow precipitate, hindering the characterisation of species in these reaction mixtures.

The reactions were repeated in dichloromethane-d2 to increase the solubility of the

products in solution. NMR spectroscopy revealed the immediate formation of the previously observed trans-dimer 22, which was observed to isomerise into the corre- sponding cis,trans-dimer (23) over the course of about an hour at room temperature (Scheme 3.7). This product displayed signals in the31_{P NMR spectrum in a 1:1 ratio}

at 88.7 and 78.5 ppm (for the P-trans-P and P-trans-Cl environments respectively), shifted downfield from similar environments in the platinum analogue 19, as the less electron-rich palladium did not shield the phosphinite ligands to the extent that platinum did. The 31_{P NMR resonance of the phosphorus nuclei attached to}

the cis-coordinated palladium centre also appeared slightly broader than those on the trans-coordinated palladium, an effect also seen in the spectra of the analogous platinum complex 19. The proton NMR spectrum of 23 also resembles that of the platinum cis,trans-dimer 19: the chemical shifts and lineshapes for the resonances of protons H-2, H-4, and H-6 are similar, but the H-5 signal for the palladium com- plex appears approximately 0.5 ppm upfield of that in the platinum complex. It is not immediately clear why this may be; however, it may indicate that these similar species adopt slightly different conformations in solution.

It was found that 23 could be readily obtained from dichloromethane solutions of 1 and [PdCl2(NCMe)2] heated at reflux overnight. The dimeric nature of this cis,trans

O O PR2 PR2 [PdCl2(NCMe)2] CH2Cl2, rt where R = C6F5 Cl Pd Cl P R2 R2 P O O O O R2P R2P Pd Cl Cl 83% O Pd O Pd O O R2P PR2 R2P PR2 Cl Cl Cl Cl ∆ 1 22 23

Scheme 3.7 Formation of cis,trans-[(POCOPH)PdCl2]2 species (23) via the trans-[(POCOPH)PdCl₂]₂ intermediate (22).

1995 amu. This also confirmed that the initial reaction product 22 was likely to be a dimer (rather than a higher oligomer), due to its ready isomerisation to 23 and its solubility in benzene. On continued reflux in dichloromethane, 23 was not seen to react any further; however, upon heating above 100 ◦_{C in toluene quanti-}

tative conversion to the metallated [(POCOP)PdCl] pincer species was observed. During this metallation reaction a transient intermediate was observed by31_{P NMR}

spectroscopy as a single resonance at δP = 45.6 ppm; while this chemical shift was

suitably downfield of the signals corresponding to 23, insufficient data were gath- ered on this species to speculate as to whether this may indicate the presence of a

cis-monomeric complex as a metallation intermediate.

The initial formation of the palladium trans-dimer 22 in dichloromethane, whereby under identical conditions for platinum a cis-dimer is favoured, is likely to be due to electrostatic effects. The greater ionic character of metal-ligand bonding on pal- ladium serves to enhance the electrostatic repulsion between mutually cis chloride ligands, destabilising the cis geometry favoured by antisymbiotic and solvent ef- fects. This also serves to account for the formation of a trans-dimer in the absence of strongly trans directing ligands. The preference of platinum and palladium dimers of the type [(POCOPH)MCl2]2 to adopt a stable cis,trans configuration is interest-

ing, especially considering that the three examples of cis,trans dimers (19, 21, and 23) represent complexes both with and without strongly trans directing ligands, and rearrange to form these asymmetric dimers from the initial formation of both

cis and trans complexes. For coordination complexes of the phosphinite ligand

1, the cis,trans-dimeric structure must offer a balance between antisymbiotic and electrostatic effects that is energetically favourable for all compounds, despite their differences.