1206, 1171, 1128, 1075, 1035, 1000, 972, 907, 878, 827, 755, 743, 702, 680, 663, 621, 555, 489, 430, 406.
PtL4b [Pt{N,N'-bis[phenyl(pyridin-2-yl)methylene]1,3-diamino-2,2-dimethylpropane}](PF6)2
UV-Vis (CH3CN) [λmax, nm (ε, M-1cm-1)]: 238 (2.2 × 103), 243 (sh), 285 (11.0 × 103), 309 (9.7 × 103), 354 (4.5 × 103), 383 (5.9 × 103). 1H NMR (400 MHz, CD3CN) [δ, ppm]: 8.77 (d, 2H, J = 4.8, Ha), 8.06 (m, 6H, He,3,4,5), 7.98 (t, 2H, J = 7.6, Hc), 7.62 (d, J = 7.3, 2H, Hd), 7.56 (t, J = 5.8, 2H, Hb), 7.50 (m, 4H, He2,6), 2.03 (s, 4H, Hf), 1.27 (s, 6H, Hh). IR (cm-1): 3283, 3247, 3118, 3067, 2975, 2917, 2850, 1670, 1609, 1534, 1462, 1451, 1426, 1377, 1318, 1293, 1239, 1163, 1099, 1043, 999, 953, 814, 767, 741, 722, 692, 644, 554, 471, 436, 391, 380.
PdL4b [Pd{N,N'-bis[phenyl(pyridin-2-yl)methylene]1,3-diamino-2,2-dimethylpropane}](PF6)2
UV-Vis (CH3CN) [λmax, nm (ε, M-1cm-1)]: 212 (3.7 × 103), 270 (sh), 289 (4.2 × 103). 1H NMR (400 MHz, CD3CN) [δ, ppm]: 8.81 (d, 2H, J = 5.7, Ha), 8.33 (t of d, J = 7.8, 1.3, 2H, Hc), 8.04 (t of d, J = 6.7, 1.5, 2H, Hb), 7.77 (m, 6H, He,3,4,5), 7.53 (d, J = 7.3, 2H, Hd), 7.50 (m, 4H, He2,6), 3.39 (s, 4H, Hf), 1.04 (s, 6H, Hh). 13C NMR (125 MHz, CD3CN) [δ, ppm]:
182.4 (Ci), 158.4 (Cp), 152.4 (Ca), 144.2 (Cc), 133.2 (Ce), 131.1 (Cd), 129.9 (Cb,e3,4,5), 128.7 (Ce2,6), 63.1 (Cf), 39.8 (Cg), 24.1 (Ch). IR (cm-1): 3298, 3211, 3110, 2915, 1664, 1585, 1493, 1475, 1446, 1405, 1377, 1345, 1266, 1218, 1164, 1033, 999, 916, 903, 877, 827, 793, 774, 752, 739, 702, 680, 659, 647, 626, 620, 555, 518, 479, 425.
From here the range was extended by adding groups onto the imine carbon, first a methyl and then a phenyl. The effect that these groups will have on the structure, reactivity and the activity of the ligands was of interest. NMR for the original ligands were all assigned except for L6, for which only trace amounts were obtained. Despite obtaining relatively clean LC-MS data for the methyl-substituted ligands (with the ligand being the prominent peak) the NMR data were not as straightforward. It seems that the methyl-ligands generally behaved differently in solution which caused difficulty for the assignment of the NMR data. In the case of L3m and L5m 13C NMR spectra were not obtained due to poor resolution; however, the assigned 1H NMR data is presented. The phenyl-substituted ligands showed clean LC-MS spectra and NMR data was assigned for all four of the synthesised phenyl-substituted ligands (L1b–L4b), as well as for the cyclised imidazole-containing bidentate ligand L5bh. Only trace amounts of the ‘fully- reacted’ bis(imine) ligand (L5b) were found in the reaction mixture which could not be isolated. The ligand L6b has not been synthesised in this work.
Yields of more than 60% were obtained for all the original six ligands (except for L6 formed in trace amounts only). The methyl-ligands all had high yields, while the phenyl- ligands did not. This may be explained by the mere bulk of the phenyl-ligands and the presence of unreacted starting material in the methyl-ligand samples. It may also be due to the fact that the phenyl-ligand reactions had a greater tendency to form the cyclised hexahydropyrimidine- or imidazole-containing bidentate ligands. This was also seen for L2 and L6 of the original ligands; L2h and L6h. Crystal structures have been obtained for these cyclised bidentate ligands: L2h, L6h, L1bh and L2bh (see Chapter 3), which formed during the syntheses of the full ligands. Once one equivalent of aldehyde was added to the diamine it cyclised before another equivalent of the aldehyde could be added. There seemed little would break the new C–N bond making it a stable (and crystalline) product of some of the reactions. The acyclic single condensation intermediate was not stable and subsequently the equilibrium shifted towards the cyclic structure forming a hexahydropyrimidine or imidazole ring.175
In an endeavour to avoid these 1:1 ratio bidentate ligands and their subsequent cyclisation another method was attempted, particularly for PtL6 and PdL6. The first involved the formation of [M(br)Cl2] (where M = Pt or Pd and br = bridging diamine) and then the subsequent addition of the pyridine rings along with the formation of the imine
bonds. Unfortunately this mostly yielded a complex with one addition of the pyridine-2- carboxaldehyde and was not successful in obtaining any tetradentate metal complexes.
Metallations of the other synthesised ligands were performed using methods altered from literature. The first complexes synthesised had a chloride counter ion from the metal salt.
This produced mainly powder and therefore SbF6
- was introduced in order to form high quality crystals suitable for X-ray diffraction and spectroscopic characterisation. Limited success was observed for SbF6
- and therefore BF4
- and PF6
- were introduced as far more suitable counter ions. This resulted in a total of eight novel metal complexes producing X-ray quality crystals. Despite identical methods with equal conditions being used not all of the thirty attempted metal complexes could be synthesised. However, five other metal complexes, which produced powder, have been spectroscopically characterised where possible. The yields for the metal complexes were not calculated due to the bulk material not being pure.
The metal complexes did not give uncontaminated or even identifiable LC-MS spectra despite many of the samples being crystalline and clean; as seen in the 1H NMR spectra.
The samples were even run in different solvents, but similar results were obtained, ruling out solvent effects. Thus the use of LC-MS has not been implemented to characterise the metal complexes.‡‡ The UV-Vis spectra gave the expected bands for the non planar free ligands. For the metal complexes, intraligand π-π* transition bands were observed as well as the MLCT bands (in the range of 320–380 nm for Pt(II) complexes and around 300 nm for Pd(II)). Extinction coefficients were determined for the novel metal complexes in acetonitrile, where possible. Despite numerous attempts it was observed that Beer’s Law was not obeyed in the region 200–220 nm for PtL2, PdL2 and PdL4m.
There is an obvious change in the shift of the 1H NMR peaks for the metallated derivatives from the spectra of the free ligands. These shifts take place downfield from the respective free ligands ranging from 0.1 to 0.8 ppm for the palladium complexes or 0.4 to 0.9 ppm for the platinum complexes. This is due to the electron donating/accepting ability of the metal centre causing a redistribution of the electron density in the ligand and
‡‡ Some samples were submitted for elemental analysis to the University of KwaZulu Natal in Westville, but no meaningful data has been presented due to inconsistencies in the results obtained, which were attributed to instrument failure. In lieu of this samples have been prepared for submission to Galbraith Laboratories in the USA.
hence changes the resonance values for its protons. The 195Pt NMR spectra were obtained for PtL2 and PtL4, as well as a much weaker signal for PtL2b (Figure 2.4);
however, no data is presented for the other platinum complexes as the peaks could not be observed despite numerous attempts.
Figure 2.4: The 107 MHz 195Pt NMR spectra at 11.7 T recorded at room temperature for PtL2, PtL4 and PtL2b.