Results and Discussion
5.1. Synthesis and Characterisation of the Complexes
The platinum(II) complex, [Pt(terpy)Cl]Cl·2H2O (where terpy = 2,2´:6´,2˝-terpyridine) (Pt1) was synthesized according to literature procedure.[1] The compound was characterized by using Infrared (IR) and elemental analysis. The characteristic data obtained for the platinum(II) complex is in good agreement with the structural formula and the literature data available.[2]
The novel platinum(II) complex, [Pt{4-(o-tolyl)-6-(3˝-isoquinoyl)-2,2´-bipyridine}Cl]SbF6
(Pt4) was synthesised and characterised (Chapter 4.2). All the characteristic data obtained are in good agreement with the structural formulae of the intermediates and the final complex. The compound was synthesised by using a slightly modified method of the Kröhnke synthesis.[3] This was achieved by replacing the 2-acetylpyridine in the first stage of Kröhnke synthesis with 3-acetylisoquinoline.
Since 3-acetylisoquinoline was not commercially available, the first challenge in this synthesis was to prepare the isoquinoline precursor. The chosen method for the synthesis of this precursor was a method used by Sauvage et al.[4] which made use of 3-acetylisoquinoline as an intermediate to synthesise cyclo-and non-cyclometalated ruthenium(II) complexes.
This part of the synthesis was carried out under inert conditions.
The first step of this reaction involves a Claisen condensation of the methyl-3-isoquinole carboxylate with sodium ethoxide to produce a β-ketoester. The product was precipitated with sodium hydroxide, followed by acid work-up and subsequent rearrangement.
Subsequent removal of carbon dioxide favoured the formation of the desired enol product in a moderately good yield (66%). The characterisations were done using Nuclear Magnetic Resonance (NMR) 1H NMR, 13C NMR and IR. A strong peak, at 1689 cm-1, for the υ(C=O) bond was observed from the IR spectrum of this precursor. The 1H NMR and 13C NMR spectra were recorded in deuterated chloroform and the data are in good agreement with the structural formula of the molecule.
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In following with Kröhnke methodology, the 3-acetylisoquinoline was reacted with ortho- tolualdehyde by Aldol condensation to give the desired 1-(3-isoquinoyl)-3-(o-tolyl)-prop-2- en-1-one (Chapter 4.2.1.2). The IR spectrum of the enone exhibits a prominent peak at 1664 cm-1, indicating the presence of the ketone carbonyl (C=O) bond and also, distinctive peaks in the range 1621-1400 cm-1 corresponding to the aromatic alkene (C=C) and (C=N) were also observed. Successful synthesis of the enone was confirmed by the 1HNMR data which showed a characteristic vicinal coupling constant, JHH 13.49 Hz, for the trans alkene.[5]
The ligand was synthesised in a comparably low yield (34%) and was characterised using
1H NMR, 13C NMR, IR, Liquid Chromatography-Mass Spectroscopy (LC-MS), elemental analysis and X-ray crystallography. The 1H NMR and 13C NMR spectra were recorded in deuterated chloroform. To aid the spectral assignments for the ligand, absolute value COrrelation Spectroscopy (COSY), Heteronuclear Single Quantum Correlation (HSQC) and Heteronuclear Multiple-Bond Correlation (HMBC) NMR experiments were used. The spectral data obtained is in good agreement with the structural formula of the ligand. Furthermore, the X-ray crystal structure obtained for the ligand indicates that the synthesis of the ligand was successful.
The synthesis of Pt4 was successful as the characteristic data are in good agreement with the structural formula. From the IR spectrum, a small shift to lower wavenumber (ca. < 15 cm-1) of υ(C=C) (aromatic) and υ(C=N) bands for the metal complex compared to that of the free ligand indicates the coordination via the nitrogen atoms of the ligand. The 1H NMR and
13C NMR for Pt4 was recorded in deuterated DMSO. The spectral assignments for the metal complex were made with the aid of COSY, HSQC, Nuclear Overhauser Enhancement Spectroscopy (NOESY) or Nuclear Overhauser Effect (NOE) 15N HMBC.
NOE irradiation at CH3 produced responses at 7.6 ppm and 8.66 ppm which are due to H3/H5 and H3”’. In the NOESY spectrum H3 (δH 8.7 ppm) showed a spatial correlation with H4”
(δH 9.06 ppm, singlet) and a doublet due to H3’ and H6”’ (δH8.48 ppm and 7.9 ppm). The NOE effect of H4” gave responses at 8.66 ppm due to H3/H5 and 7.80 ppm due to H5’’. The NOE effect of H1” gave a response at 8.17 ppm due to H8”. Since selective COSY at 8.5 ppm shows no response at 7.9 ppm, the 1H NMR signal at 7.9 ppm is not part of the same spin system as bulk protons at 8.5 ppm. Multiplicity at 8.5 suggests two overlapping doublets corresponding to H3’/ H6’ and H5’.
In the 15N HMBC spectrum, the nitrogen atoms N1, N2 and N3 (See Figure 5.1) were correlated with the protons around the nitrogen centre. The signals due to H3 and H5 were confirmed from 15N HMBC which showed a signal at 208 ppm (N2). Also from the 15N HMBC
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spectrum the signal at 193 ppm (N1) is connected to the signals at 9.1 ppm and 9.13 ppm due to H1” and H4” respectively. Furthermore, by the signal at 200 ppm (N3) in the 15N HMBC spectrum confirms the protons, H3’, H5’ and H6’ which corresponds to 8.47 ppm and 7.6 ppm.
The signals due to H7” and H6” are confirmed by 2D-COSY experiments and the signal due to H4’ is assigned from 2D COSY and 15N HMBC. Elimination of the processed protons leave the signals due to H4”’, 5”’, 6”’. The coordination of platinum was confirmed by the presence of a
195Pt peak at -2661.76 ppm which is in agreement with the chemical shift observed for NMR
195Pt.[6]
The four platinum(II) complexes and the nucleophiles studied in this investigation are shown in Figure 5.1. The crystal structure for Pt3 has been published before.[7, 8] To ensure that the anticipated substitution reactions have taken place, crystals were grown for the substituted complex of Pt3 with 1-methylimidazole (MIm) as the nucleophile. The procedure followed for crystal growth of the substituted complex was a slight modification of the method employed by Pitteri and Bortoluzzi[9]. X-ray crystal structure and the data are given in Section 4.3.2.
N N N
Pt Cl
CH3
N N N
Pt Cl
CH3
Pt1 Pt2
N N
N Pt Cl
N N Pt N
Cl
Pt3 Pt4
5 N H
4 N 3
5 N H
4 2 N
5 N 4
2 N CH3
5 N 4
2 N
CH3 CH3 5
N H
N N 3
Pz Tz Im MIm DMIm
1 2 1
3 2 1
3 3 4
1 1
1
1 1
1
2 2
2 2
3
3 3
3
Figure 5.1 Structures of platinum(II) complexes and the nucleophiles used in kinetic investigations. Anions are omitted for simplicity.
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