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Comparative spectroscopic techniques have been used to study the properties of ligands L¹ -L⁹.

6.2.1. FTIR spectra

Infrared (ATR-FTIR-IR) spectra were recorded using a smart diamond ATR attachment on a Thermo-Nicolet FT-IR Spectrophotometer (AVATAR 320) over the range 4000 to 400 cm^-1. The FTIR spectral data of ligands L¹-L⁹ are visually presented as an overlay of the IR spectra of L¹-L⁹, as shown in Figure 6.1. These significant peaks for ligands L¹-L⁹ are tabulated in Table 6.3.

Table 6.3: IR frequencies (cm^-1) of the ligands (L¹-L⁹)

Ligand ν(C-H)triaz ν(C=N)py ν(C=C)Ar ν(C=C)triaz ν(N-N)triaz ν(N=N)triaz ν(C-N) ν(C=O)

L¹ 3125 1605 1605, 1573 1557 1177,1035 1520 1229 1682

L² 3118 1597 1592, 1566 1549 1144,1036 1517 1233

dL³ 3142 1604 1592,1574 1549 1154,1022 1515 1258

L⁴ 3141 1603 1588, 1573 1553 1151,1036 1517 1240 1687

L⁵ 3152 1589 1572, 1549 1515 1149,1035 1516 1235

L⁶ 3132 1602 1589, 1570 1550 1146,1037 1501 1236

eL⁷ 3120 1601 1572, 1549 1511 1147,1032 1511 1235

L⁸ 3116 1599 1591, 1567 1544 1147,1035 1502 1237

L⁸ 3122 1615 1593, 1570 1547 1105,1027 1528 1337

dν(C-OMe): 1257 cm^-1, ῡ =1022cm^-1 ; ^e ν(C≡N): 2230 cm^-1

106 Figure 6.1: ATR-FT-IR absorption spectral data of the ligands (L¹- L⁹).

107 Table 6.3 shows the characteristic bands common to all bond types present in click ligands L¹-L⁹. The bands in the region ῡ = 3116-3151cm^-1 were assigned to the stretching of the ν(C-H) bond of the triazole ring [38-48] and that at ῡ = 1615-1598 cm^-1 to the stretching of the ν(C=N) bond of free pyridine that shifts on coordination to the metal to higher wavenumbers, indicating a new compound was obtained. The two bands at region ῡ = 1605-1549 cm^-1 were, attributed to the two characteristic stretching peaks of the ν(C=C)Ar bond which occur as a pair of peaks in substituted benzenes. The bands observed at region 1557-1511 cm^-1 were attributed to stretching of the ν(C=C)triaz bond of the triazole ring [49, 50] and the ῡ = 1520-1500cm^-1 to the ν(N=N) stretching vibration [51, 52]. The band at ῡ = 1258-1220cm^-1 is attributed to the ν(C-N) stretching bond [53], and the bands around ῡ = 1145cm^-1 and ῡ = 1036cm

-1were assigned to the ν(N-N) bond of the triazole ring [54]. The pair of bands in the range at ῡ = 962-977cm^-1 were assigned to ν(N–N=N) stretching vibration [55, 56] of the triazole ring. In addition, substituent specific bands were identified for ligands, L¹ and L⁴ at 1682 and 1687 cm^-1 respectively, for the ν(C=O) stretching of the COOH group and ῡ =2230 cm^-1for the ν(C≡N) stretching of the nitrile group of L⁷. Two bands at ῡ = 1257 cm^-1 andῡ =1022cm^-1 were assigned for the ν(C-O) stretching of the methoxy group of L³ [53].

6.2.2 Hammett Plot Using UV-Vis Data

A plot of the λ maxvs the σp substituent constant for all ligands gave a concave upward deviation in the Hammett plot with a negative slope for the electron donating groups and a positive slope for electron withdrawing groups. This is a change in mechanism in which electron donating groups decrease the positive charge at the triazole ring.

However, electron withdrawing groups enhance the positive charge (see Figure 6.2) at this centre. There were three substituents (H, CF3 and COOH) which are outliers on the plot.

108 Figure 6.2: Hammett plot of correlations of the λ max of L²-L⁹ ligands vs. σp

values for the Hammett substituent parameters electron donor and withdrawing para substituents (R = H, Me, OMe, Cl, F, CN, COOH, and CN).

Figure 6.3: Hammett plot of absorbance maximum of L²-L⁹ ligands vs. σp values for the electron donor and withdrawing para substituents (R = H, Me, OMe, Cl, F, CN, COOH, and CN).

In the series L²-L⁹, the maximum absorption of both bands measured in DMSO (1x10

-6M), gives a linear correlation the between the Hammett σρ constant for the substituent

109 on the p-position of the phenyl group (Figure 6.3). The two substituents which are outliers (COOH and CN) are not included in the plot. This suggests that compounds with these substituents act as stronger acids than one would have predicted from their values. When electron withdrawing due to mesomeric effects can be extended to the reaction centre via “through conjugation”, the result is an even more stabilized species. These types of correlation between the Hammett σρ constant in relation to the substituent of the p-position and the relevant UV absorption frequencies were reported previously in literature [57-58].

Table 6.4: The absorption maxima λ max vs the Hammett parameter of the σρ

substituent correlation in DMSO (C= 1 X10^-6M).

Compound Substituent spectra of previously reported triazoles [38-47], and supported by 2-dimensional spectra (¹H–¹HCOSY and ¹³C–¹H HMQC) to confirm the proposed assignments. The chemical shift (δ) of the ¹H- (C-H) and ¹³C-NMR (=CH) of the triazole moiety have been tabulated for ligands L¹-L⁹ in Table 6.5. This is also shown in Figure 6.4, which indicates that the proton from the triazole ring position shifts according to the substituent in the para-position of the phenyl ring,

110 Figure 6.4: The overlayed spectra of the (C-H) triazole from the ¹H NMR for ligands L¹-L⁹ in CDCl2 solutions.

111 The effect of the electron withdrawing substituents on the phenyl ring manifested itself in shifts of the C-H-triazole peak of the ¹H-1,2,3-triazole system, which was observed in the range between 8.54-9.46 ppm in the ¹H-NMR and 119.69-120.48 ppm in the ¹³C-NMR spectra of ligands L²-L⁹. These values are tabulated in Table 6.5 with literature values reported for triazole ligands with a NO2 and NH2 substituent included [45]. To ascertain whether there was a relationship between the shifts in the C-H-triazole¹H-NMR peak and the substituents present on the phenyl ring, a Hammett type plot was undertaken on the data. Figure 6.5 shows the plot of the shift of δ(C-H) triazole in ppm for ligands L²-L⁹ and literature reported ligands (NH2 andNO2) against the Hammett σ substituent constant [59]. A linear relationship with positive slope (0.18) was observed between δ(C-H) and the σp substituent constant for the ligands, with R²=0.97. L⁴ (COOH) and the NO2 substituent were outliers and were excluded from the linear regression fit. The plot Figure 6.5 suggests that COOH and NO2 groups act as much stronger electron withdrawing groups than expected via the inductive effect based on their σp constant. They therefore cause greater deshielding of the (C-H) of the triazole ring and higher acidity.

Table 6.5: The ¹H- and ¹³C-NMR spectra of the C-H-triazole ligands (L¹- L⁹).

112 Figure 6.5: A plot of δ(C-H) ppm of triazole against the Hammett's para

substituent constant [σρ].

Deviation from the plot by electron withdrawing groups such as the NO2 and COOH, can occur due to mesomeric effects, if these effects can be extended to the reaction centre via “through conjugation” to result in a more stabilised species.

Scheme 6.8: Resonance structures proposed for 2-pyridyl-1,2,3-triazole ligands