IVONNE ADRIANA PALTA

In an attempt to prepare crystals of complex 2 for single crystal X-ray diffraction,

complex 2 was dissolved in the minimum volume of dichloromethane and pentane was

allowed to migrate into the vial slowly over time. After 3 days, the originally red

solution became a yellow colour and small yellow crystals had begun to form on the sides of the vial. To investigate what was occurring, a sample of complex 2 was again

dissolved in dichloromethane in a round-bottomed flask and allowed to stir overnight. After removing the solvent in vacuo and washing the yellow product with pentane, 1H

NMR spectra were acquired of the product. The 1_{H NMR spectrum of the yellow solid in} acetone-d6 gave evidence for a mixture of products as could be concluded from the presence of six distinct methylplatinum resonances, figure 2.21. Integration of these resonances indicated that there were three independent species present in the solution. Each of the three species present possessed two distinct methylplatinum resonances of equivalent integration, which is consistent with the asymmetry of the

dimethylplatinum(II) precursor complex 2. 2J(PtH) coupling constants of the

methylplatinum signals ranged from 70 – 74 Hz which is diagnostic of these resonances arising due to different platinum(IV) species. The coupling constant values being

comparable to other platinum(IV) species provided evidence for the oxidative addition of a dichloromethane solvent molecule at the platinum(II) complex 2. The three different

products observed in the 1_{H NMR spectrum, (figure 2.21), would therefore arise due to} the presence of three distinct isomers. The major product appeared to be that of the trans

oxidative addition of dichloromethane, 12a, which would be followed by trans-cis

isomerization to yield the two possible cis isomers, 12b/12c, as depicted in scheme 2.12.

The major isomer, 12a, was readily identified by the presence of two methylplatinum

resonances of equivalent integration values at 1.26 ppm (MeA) and 1.41 ppm (MeB), with coupling constant values of 70 Hz which is diagnostic of a methyl group being trans to

Figure 2.21. 1H NMR spectrum of the methyl and methylene region for complexes 12a, 12b, and 12c in acetone-d6. The aromatic region is omitted for clarity. CH2r is used to indicate the resonances arising from the protons of cyclooctyl ring in complex 12a.

The remaining four methylplatinum resonances could be attributed to their specific cis isomers through the use of 1H-1H gCOSY and NOESY NMR experiments.

These experiments allowed for the determination of whether the methyl group of 12c or

the chloromethyl group of 12b were adjacent to the ortho proton of the coordinated

pyridyl ring of the ligand. Utilizing these spectra, the ultimate assignment of all isomers of 12 was possible and the values are summarized in table 2.9.

Scheme 2.12. Oxidative addition of CH2Cl2 to complex 2 and the subsequent isomerization.

Table 2.9: Selected NMR data [δ ppm, _J in Hz] for the dichloromethane adducts (see scheme 2.12 for labelling system).

Complex δ(Mea₎ 2_{J(PtH) δ(Me}b₎ 2_{J(PtH) δ(Me}c₎ 2_{J(PtH) δ(CH}

2Cl) 2J(PtH) 12a 1.26 70 1.41 70 3.53 48 3.54 58 12b 1.44 70 0.61 74 4.72 78 4.89 86 12c 1.55 70 0.66 74 4.26 44 4.74 92

The reaction was further investigated through the analysis of the slow reaction of complex 2 with solvent CD2Cl2, figure 2.22. This reaction was observed to produce a mixture of 12a:12b:12c in the ratio 3:2:1 after two hours reaction but 5:1:1 after 48

hours, as determined through monitoring the reaction by 1_{H NMR. In this case, the} oxidative addition is slow and was only about 20% complete after two hours but complete after 24 hours. From this analysis it was concluded that the rate of

isomerization is competitive with the rate of oxidative addition. It was also observed that complex 12b is formed in a higher proportion early in the reaction and is therefore

1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 Chemical Shift (ppm) 2hrs 19hrs 48hrs Complex2 Complex12a Complex12b Complex12c

*

*

_*

*

*

*

*

Figure 2.22. Time series 1H NMR spectrum of the oxidative addition of methylene

Figure 2.23 shows the structure of complex 12a. The chlorine atom of the

chloromethyl group lies above the nitrogen atom of the coordinated pyridyl group, with torsion angle N4-Pt1-C21-Cl1 = 5° and bond angle Pt1-C21-Cl1 = 116.2(4)°. Again it can be observed that the presence of the cyclooctene ring imposes steric hindrance in overall conformation of complex 12a, resulting in the pyridyl and pyridazine rings being

skewed by 17° to reduce the steric congestion. The vacant pyridyl group also exhibits skewing relative to the pyridazine ring by 49° to presumably reduce the steric hindrance.

Figure 2.23. A view of the structure of complex 12a, (hydrogen atoms excluded for

Table 2.10: Bond lengths [Å] and angles [°] for [PtClMe2(CH2Cl)(6-dppd)],

12a

The formation of 12 provided a potential opportunity to access a diplatinum

complex of 6-dppd. It was opined that the presence of the chloromethyl ligand attached

to the platinum center could allow for an additional oxidative addition reaction across the CH2-Cl bond which may then allow for the second platinum center to coordinate in the vacant coordination site as depicted in scheme 2.13. To that end, an acetone solution of

12 was added to an acetone solution of an equivalent of [Pt2Me4(µ-SMe2)2] and stirred for 4 hours. After working up the reaction mixture, no evidence for coordination of a second platinum center was observed. To ensure that the reactivity of the chloromethyl group was not the issue, the correspond bromomethyl and iodomethyl complexes were generated and tested through their in situ generation by stirring 2 in an acetone solution

of dibromomethane or diiodomethane and then adding an equivalent of [Pt2Me4(µ- SMe2)2]. Again the incorporation of a second platinum center was not observed.

Pt(1)-N(1) 2.136(5) Pt(1)-N(4) 2.133(6) Pt(1)-C(22) 2.046(7) Pt(1)-C(23) 2.052(8) Pt(1)-Cl(2) 2.4371(18) Pt(1)-C(21) 2.027(8) N(1)-Pt(1)-N(4) 75.0(2) C(22)-Pt(1)-C(23) 85.6(2) N(4)-Pt(1)-C(22) 99.5(3) N(1)-Pt(1)-C(23) 99.9(3) N(4)-Pt(1)-C(23) 174.4(3) N(1)-Pt(1)-C(22) 174.5(3) C(21)-Pt(1)-C(23) 85.4(3) C(21)-Pt(1)-C(22) 89.5(3) N(1)-Pt(1)-C(21) 91.3(3) N(4)-Pt(1)-C(21) 97.0(3) N(4)-Pt(1)-Cl(2) 86.33(15) C(22)-Pt(1)-Cl(2) 93.1(2) N(1)-Pt(1)-Cl(2) 86.60(15) C(23)-Pt(1)-Cl(2) 91.0(2) C(21)-Pt(1)-Cl(2) 175.4(2) Pt(1)-C(21)-Cl(1) 116.2(4) N(1)-C(15)-C(16)-N(4) 10.0(8) N(2)-C(1)-C(2)-N(3) 132.2(7)

Scheme 2.13. The attempted synthesis of a diplatinated complex of 6-dppd through the

use of a bridging methylene approach.