Tautomeric forms study of 1H-(2′-pyridyl)-3-methyl-5-hydroxypyrazole and 1H-(2′-pyridyl)-3-phenyl-5-hydroxypyrazole. Synthesis, structure, and cytotoxic activity of their complexes with palladium(II) ions

Figures & data

Scheme 1. Synthesis of compounds 1 and 2.

Scheme 2. Tautomeric forms of compounds 1 and 2.

Figure 1.  ¹H NMR spectrum of compound 1.

Figure 2.  ¹H NMR spectrum of compound 2.

Table 1.  Percentage of tautomeric forms in solution calculated from ¹H NMR.

	Compound 1		Compound 2
	Form B/C (%)	Form A (%)	Form B/C (%)	Form A (%)
Chloroform	93.4	6.6	98.8	1.2
DMSO	95.3	4.7	100^a	0^a
Methanol	98.3	1.7	—^b	—^b

^aThe signal of tautomeric form A was not present.

^bThe spectrum was not obtained due to the poor solubility of compound 2 in methanol.

Figure 3.  Calculated structures of the lowest energy conformers of each tautomer of 1 in the gas phase with selected dihedral angles (°).

Figure 4.  Calculated structures of the lowest energy conformers of each tautomer of 2 in the gas phase with selected dihedral angles (°).

Table 2.  Enthalpy and percentage of tautomers calculated at B3LYP/6-31+G(d,p) level of theory.

Compound	Enthalpy of most stable conformer (a.u.)				Percentage of tautomer (%)
	Gas	Chloroform	DMSO	Methanol	Gas	Chloroform	DMSO	Methanol
1A	−587.6918	−587.7057	−587.7120	−587.7119	0.016	0.80	1.13	1.12
1B	−587.7008	−587.7106	−587.7148	−587.7148	99.80	59.79	8.09	8.47
1C	−587.6942	−587.7095	−587.7165	−587.7163	0.184	39.41	90.78	90.41
2A	−779.3863	−779.4021	−779.4095	−779.4096	0.010	0.31	0.56	0.51
2B	−779.3943	−779.4069	−779.4127	−779.4129	99.91	82.38	25.32	24.92
2C	−779.3876	−779.4055	−779.4137	−779.4140	0.080	17.31	74.12	74.57

Table 3.  Gibbs free energy and percentage of tautomers calculated at B3LYP/6-31+G(d,p) level of theory.

Compound	Gibbs free energy of most stable conformer (a.u.)				Percentage of tautomer (%)
	Gas	Chloroform	DMSO	Methanol	Gas	Chloroform	DMSO	Methanol
1A	−587.7435	−587.7549	−587.7613	−587.7611	0.58	0.92	0.53	0.58
1B	−587.7490	−587.7588	−587.7631	−587.7631	98.80	21.43	1.32	1.47
1C	−587.7435	−587.7594	−587.7666	−587.7664	0.62	77.65	98.15	97.95
2A	−779.4446	−779.4601	−779.4690	−779.4673	0.084	1.29	5.58	1.01
2B	−779.4508	−779.4635	−779.4692	−779.4693	99.805	72.42	12.50	10.37
2C	−779.4443	−779.4627	−779.4710	−779.4716	0.111	26.29	81.92	88.62

Scheme 3.  Synthesis of complexes 3 and 4.

Figure 5.  ORTEP view of molecular structure of Pd(II) compound 3 with atom numbering scheme and ellipsoids at 30% probability level.

Figure 6.  ORTEP view of molecular structure of Pd(II) compound 4 with atom numbering scheme and ellipsoids at 30% probability level.

Figure 7.  Comparison of compound 3 structures from X-ray crystallography (gray scale) and DFT calculations (black).

Figure 8.  Comparison of compound 4 structures from X-ray crystallography (gray scale) and DFT calculations (black).

Table 4.  IC₅₀ values of tested compounds.

Compound	IC50^a (μM)
	HL-60	NALM-6	WM-115
1	13.7 ± 3.4	37.9 ± 5.2	68.9 ± 2.4
2	5.6 ± 0.3	22.8 ± 4.6	168 ± 32.7
3	32.4 ± 4.7	58.9 ± 2.7	68.4 ± 10.3
4	6.1 ± 0.1	23.4 ± 4.2	54.4 ± 1.9
Cisplatin	0.8 ± 0.1	0.7 ± 0.3	18.2 ± 4.3
Carboplatin	4.3 ± 1.3	0.7 ± 0.2	422.2 ± 50.2

^a IC₅₀ is the concentration of tested compound that reduces the amount of surviving cells to 50% as compared to control, non-treated cells. Mean values of IC₅₀ ± SD from three independent experiments are presented.