Synthesis and Characterisation

Seventeen iridium(III) half-sandwich complexes bearing functionally diverse 2-PhPy ligands (Chart 3.1) were synthesised and characterised by 1H NMR spectroscopy, ESI-MS, CHN analysis and RP-HPLC. Functionalised 2-PhPy ligands bearing either electron-withdrawing or electron-donating substituents were synthesised using Suzuki-Miyaura palladium cross-coupling chemistry following an adapted literature protocol.35 They were purified using flash silica gel chromatography with mixtures

of chloroform/ethyl acetate yielding 2-(Rˈ-phenyl)-R-pyridine ligands L1-17. The synthesis of the corresponding complexes [(η5-Cp*)Ir(2-(Rˈ-phenyl)-R-pyridine)Cl]

(1-17) was carried out using an adapted literature procedure,27involving stirring of 2

mol equiv of 2-(Rˈ-phenyl)-R-pyridine ligand in DCM with 4 mol equiv of sodium

acetate followed by addition of 1 mol equiv of [η5-Cp*)IrCl2]2 at ambient temperature for 18 h. After filtering the reaction mixture through celite, the mixture was concentrated to dryness then recrystallised in chloroform/hexane at 278 K. The synthetic routes for the complexes are shown in Scheme 3.1. The introduction of electron-withdrawing nitro (-NO2) and aldehydic (-CHO) groups and the electron- donating hydroxymethyl group (-CH2OH) in complexes 5-10 are the first to be reported in complexes of the type [(η5-Cp*)Ir(2-PhPy)X]0/+. Complex [(η5-Cp*)Ir(2-

(4ˈ-methoxyphenyl)pyridine)Cl] 17has been previously reported36but investigations into its biological activity have not been performed.

Scheme 3.1. Synthetic route for 2-(Rˈ-phenyl)-R-pyridine ligands L1-17 (A) and [(η5-Cp*)Ir(2-(Rˈ-phenyl)-R-pyridine)Cl]complexes1-17(B).

The X-ray crystal structures of complexes [(η5-Cp*)Ir(2-(2ˈ-

fluorophenyl)pyridine)Cl] 1, [(η5-Cp*)Ir(2-(4ˈ-fluorophenyl)pyridine)Cl] 2and [(η5-

Cp*)Ir(2-(4ˈ-trifluoromethylphenyl)pyridine)Cl] 16 were determined and are shown in Figure 3.1, with X-ray crystallographic data listed in Table 3.1 and selected bond lengths and angles in Table 3.2. The complexes exhibit the expected pseudo- octahedral half-sandwich structure, with the Cp* ring occupying three coordination sites. The 2-PhPy chelating ligand occupies two sites and the monodentate chlorido ligand occupies the sixth coordination position. The Ir-centroid distances range from 1.828-1.837 Å, Ir-C distances from 2.0346-2.0482 Å, Ir-N distances from 2.0797- 2.093 Å and Ir-Cl distances from 2.4026-2.4159 Å. The bond angle between the chelating ligand and iridium centre (C-Ir-N) is the smallest in all cases, ranging from 77.84-78.13°.

Complex 1 was modelled as disordered over two positions related by swapping the position of the pyridine and the 2-fluorophenyl rings. The disorder was simply modelled by interchanging the carbon (C12) and the associated carbon and fluorine atoms on that ring with the nitrogen (N1) bound to the metal. This was refined to an occupancy of 80:20. Therefore, in any particular position the site is occupied 80% by one enantiomer and 20% by the other enantiomer. As there is an inversion centre in the space group (P2(1)/n), there is another position occupied by 80% of the time by the other enantiomer and 20% of the time by the other. This results in an overall 50:50 racemate.

Figure 3.1.X-ray crystal structures of [(η5-Cp*)Ir(2-(2ˈ-fluorophenyl)pyridine)Cl]1, [(η5-Cp*)Ir(2-(4ˈ-fluorophenyl)pyridine)Cl] 2 and [(η5-Cp*)Ir(2-(4ˈ-

trifluoromethylphenyl)pyridine)Cl] 16. Hydrogen atoms have been omitted for clarity and thermal ellipsoids are shown at 50% probability. Disorder found in complex1is omitted for clarity.

Table 3.1.Summary of the X-ray crystallographic data for complexes [(η5-Cp*)Ir(2-

(2ˈ-fluorophenyl)pyridine)Cl] 1, [(η5-Cp*)Ir(2-(4ˈ-fluorophenyl)pyridine)Cl] 2 and [(η5-Cp*)Ir(2-(4ˈ-trifluoromethylphenyl)pyridine)Cl]16

Table 3.2. Selected bond lengths (Å) and bond angles (deg) for complexes [(η5-

Cp*)Ir(2-(2ˈ-fluorophenyl)pyridine)Cl] 1, [(η5-Cp*)Ir(2-(4ˈ-fluorophenyl)-

pyridine)Cl]2and [(η5-Cp*)Ir(2-(4ˈ-trifluoromethylphenyl)pyridine)Cl]16

Bond/angle 1 2 16 Ir-C (Cp* ring) 2.163(2) 2.160(2) 2.157(4) 2.1675(19) 2.1609(19) 2.169(5) 2.177(2) 2.172(2) 2.170(4) 2.239(2) 2.2522(19) 2.265(5) 2.247(2) 2.272(2) 2.284(4) Ir-C (centroid) 1.828 1.833 1.837 Ir-C 2.0482(18) 2.0346(19) 2.045(5) Ir-N 2.0797(17) 2.0931(16) 2.093(4) Ir-Cl 2.4056(5) 2.4026(5) 2.4159(11) C-Ir-N 77.84(7) 78.13(7) 77.95(18) C-Ir-Cl 85.96(5) 85.68(5) 88.40(13) N-Ir-Cl 87.60(5) 87.30(5) 85.94(11) 1 2 16

Formula C21H22ClFIrN C21H22ClFIrN C22H22ClF3IrN

MW 535.05 535.05 585.06

Crystal Colour Yellow Yellow Yellow

Cryst size (mm) 0.30 x 0.20 x 0.12 0.40 x 0.18 x 0.10 0.20 × 0.20 × 0.08

λ (Å) 0.71073 0.71073 1.54184

Temp (K) 150 150 100

Cryst system Monoclinic Monoclinic Monoclinic Space group P2(1)/n P2(1)/n P2(1)/c a(Å) 8.37197(9) 8.27453(11) 15.4953(4) b(Å) 15.71819(18) 15.70747(16) 7.76677(19) c(Å) 13.73371(17) 13.83772(14) 16.4333(3) α(°) 90 90 90 β(°) 92.8873(11) 93.1162(11) 92.690(2) γ(°) 90 90 90 Vol (Å3_{) 1804.95(4) 1795.86(3) 1975.54(8)} Z 4 4 4 R(Fo2_{) 0.0172 0.0199 0.0363} Rw(Fo2_{) 0.0381 0.0490 0.0997} GOF 1.092 1.095 1.050

Weak off-set intermolecular π-π stacking between the 2-PhPy ligands is observed in

the X-ray crystal structures of complexes1and2with a centroid-centroid distance of 4.140 Å and 4.059 Å, respectively (Figure 3.2).

Figure 3.2. Diagram showing the π-π stacking interaction in the X-ray crystal

structures of complexes [(η5-Cp*)Ir(2-(2ˈ-fluorophenyl)pyridine)Cl] 1 (A) and [(η5-

Cp*)Ir(2-(4ˈ-fluorophenyl)pyridine)Cl] 2 (B) where the centroid – centroid distance between pyridyl-phenyl rings of independent molecules is 4.140 and 4.059 Å for 1

and2, respectively. Thermal ellipsoids drawn at 50% with hydrogen atoms removed for clarity.

The electrostatic potential surfaces (EPS) of complexes [(η5-Cp*)Ir(2-(4ˈ-

fluorophenyl)pyridine)Cl] 2, [(η5-Cp*)Ir(2-phenyl-5-fluoropyridine)Cl] 4, [(η5-

Cp*)Ir(2-(4ˈ-nitrophenyl)pyridine)Cl] 7, [(η5-Cp*)Ir(2-phenyl-4-nitropyridine)Cl] 8, [(η5-Cp*)Ir(2-(4ˈ-hydroxyphenyl)pyridine)Cl] 11, [(η5-Cp*)Ir(2-phenyl-5- hydroxypyridine)Cl] 12 and [(η5-Cp*)Ir(2-(2ˈ-methylphenyl)pyridine)Cl] 13 were calculated using DFT methods at the PBE0/Lanl2DZ/6-31+G** level,38-40 based on the crystal structure of complex 2, with functional group modifications being made with GaussView 3.0.41 for the rest of the complexes. The resulting EPS of each complex is shown in Figure 3.3. The phenyl ring in the chelating ligand exhibits a more negative electrostatic potential than the pyridyl ring due to the deprotonated

carbon bonded to the Ir centre. There are no major differences in the charge distribution at the iridium centre, Cp* ring or chlorido ligands among the complexes, indicating that the substituent on the chelating ligand causes only a localised effect. Complexes 7 and8 contain the electron-withdrawing nitro group on the phenyl and pyridyl ring, respectively, causing a more positive surface. Complex 13 bears a methyl group on the phenyl ring which pushes electron density into the ring, causing a more negative EPS. The substituents themselves cause the outer EPS of each complex to alter significantly.

The calculated bond lengths at the metal centre are shown in Table 3.3, which shows minimal difference among the complexes, with the exception of13which exhibits a shorter Ir-N bond. Generally, the calculated bond lengths of complex 2 are in good agreement with those obtained by X-ray diffraction.

Figure 3.3. Electrostatic potential surfaces for complexes 2, 4, 7, 8 and 11-13

(calculated at the PBE0/Lanl2DZ/6-31+G** level) where the EPS are shown both in space (with positive and negative regions in blue and red, respectively) and mapped on electron density (isovalue 0.04) of the molecules. The electrostatic potential is represented with a colour scale ranging from red (-0.040 au) to blue (+0.250 au).

Table 3.3.Calculated bond lengths for complexes 2,4,7,8and 11-13(calculated at the PBE0/Lanl2DZ/6-31+G** level).

Calculated bond lengths (Å)

Complex Ir-Cp* (centroid) Ir-C Ir-N Ir-Cl

2 1.862 2.008 2.078 2.410 4 1.862 2.011 2.076 2.411 7 1.861 2.006 2.078 2.412 8 1.864 2.009 2.069 2.408 11 1.862 2.009 2.078 2.411 12 1.862 2.012 2.077 2.412 13 1.868 2.004 2.058 2.414

3.3.2 Separation of Enantiomers

The complexes studied in this chapter exhibit chirality at the metal centre, resulting in the formation of a racemic solution of R and S enantiomers. The enantiomeric separation of complexes [(η5-Cp*)Ir(2-(2ˈ-fluorophenyl)pyridine)Cl] 1, [(η5-

Cp*)Ir(2-(4ˈ-fluorophenyl)pyridine)Cl] 2, [(η5-Cp*)Ir(2-phenyl-5-pyridine- carboxaldehyde)Cl] 6 and [(η5-Cp*)Ir(2-(4ˈ-trifluorophenyl)pyridine)Cl] 16 was attempted using chiral HPLC, where the stationary phase (CHIRALPAK® IA) is a

chiral sugar (amylose tris(3,5-dimethylphenylcarbamate)) immobilized on 3 μm

silica-gel. The resulting HPLC traces from the injection of the racemic solutions of each complex (EtOH, 1 mg mL-1) are shown in Figure 3.4. Two peaks are present in each case, indicating the presence of two enantiomers in each solution. Integration of each peak shows that a racemic mixture is present for1,6and16, while2appears to have a slight excess of one enantiomer. Complexes1,2and6show good separations of enantiomers while the enantiomers of16exhibit similar retention times (6.94 and 7.50 min).

Figure 3.4.Chiral HPLC traces of complexes1 (A), 2 (B), 6 (C) and16 (D) (1 mg mL-1, λ = 260 nm)

The stability of the separated enantiomers of complex 6 was assessed by collecting the two eluting peaks. The fractions were stored at 273 K for 3 days before being concentrated to dryness under vacuum with no heating, followed by re-dissolution and injection into the column. The resulting HPLC traces are shown in Figure 3.5, which show that each enantiomer epimerises to a racemic mixture of R and S under the conditions they were stored under.

Figure 3.5 Chiral HPLC traces of complex 6 after re-injection of collected peaks. The appearance of two peaks in each fraction indicates epimerisation to a racemic mixture has occurred.

3.3.3 Hydrolysis Studies

The hydrolysis of complexes 1-17 (ca 500 µM) was studied by 1H NMR spectroscopy in 26.7% MeOD-d4/ 73.3% D2O at 310 K. Methanol was required to ensure sufficient solubility of the complexes in aqueous solution. Hydrolysis equilibrium was reached before the time taken to acquire the first spectra (15 min), consistent with previous reports of Ir(III) C,N-chelated half-sandwich complexes.27 Hydrolysis was confirmed by the sequential addition of NaCl, showing an increase in the peaks associated with the Ir-Cl species and a decrease in the peaks associated with the Ir-OD2/OD species, as exemplified for complex11in Figure 3.6 a. Between 66-80% was found to be in the hydrolysed form, based on peak integrations (Figure 3.6 b). With 4 mM NaCl present, almost all of the Ir-OD2/OD species had been converted to the Ir-Cl complexes. Above this concentration of NaCl, precipitation of the complexes was observed. The hydrolysed complexes exhibit a down-field shift of the aromatic protons of around 0.15 ppm compared to the parent chlorido complexes. Determination of the pKa values of the complexes was attempted, but solubility issues prevented reliable analysis by1H NMR spectroscopy.

Figure 3.6.(A) 1H NMR (600 MHz) spectra of complex11(c.a. 500 µM) in 26.7% MeOD-d4/73.3% D2O (v/v) at 310 K. Hydrolysis equilibrium was reached within the time taken to acquire the first spectrum. Sequential addition of NaCl resulted in conversion from the 11-OD2/OD (★) complex to the 11-Cl (▲) complex. (B)

Percentage of hydrolysis (Ir-OD2/OD) for complexes1-17at equilibrium (15 min).