Compound 3

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InChIKey UNJRGTLHORMBML-UHFFFAOYSA-N

Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

Compound 3 as the NHC precursor was obtained by first synthesizing the corresponding bromide salt according to the literature procedure [Park, H. J. et al. Unsymmetric Ru(II) complexes with N-Heterocyclic carbene and/or terpyridine ligands: Synthesis, characterization, ground- and excited-state electronic structures and their application for DSSC sensitizers. Inorg. Chem. 49, 7340–7352 (2010).] and then carrying out counter-ion metathesis in the following way: the crude bromide salt (2.0 g) was dissolved in 10 ml of water and the hexafluorophosphate salt of 3 was precipitated by adding an excess of solid NH₄PF₆. The precipitate was collected on a glass sinter (porosity #4) and was triturated with a minimum amount of CH₃CN (the product turned out to be little soluble in CH₃CN, which is quite unexpected). Park et al. identified their product as the dibromide salt containing a carboxylic acid group. The proton chemical shifts of 3 were almost the same as those reported by Park et al. However, the elemental analysis measurement, which was not reported by Park et al., identified the product as the mono-hexafluorophosphate salt of 3 with the carboxylic acid group deprotonated. This was also supported by the IR vibration frequency of the carbonyl group of the carboxylate group, matching the one of sodium isonicotinate. This could be due to the deprotonation of the carboxylic acid by the excess of 1-methylimidazole used in the reaction. The product (2.3 g, yield 60 %) was obtained as a beige powder. ¹H NMR (400 MHz, DMSO-d6) δ (ppm) 10.58 (s, 2 H), 8.85(t, J=2 Hz, 2H), 8.51 (s, 2H), 8.02 (t, J=2 Hz, 2H), 4.02 (s, 6H); ¹³C NMR (100.3 MHz, DMSO-d6) δ (ppm) 163.50, 158.61, 145.18, 136.66, 124.72, 119.22, 113.36, 36.45. IR (neat) cm^-1 3198, 3178, 3025, 1641 (C=O), 1614, 1588, 1538, 1433, 1374, 1349, 1323, 1263, 1238, 1198, 1114, 1022, 996, 977, 894, 878, 842, 785, 752, 726, 705, 664, 636, 626, 607, 555, 472. ESI–HRMS: calc. for [3-(PF₆)]⁺ 284.1147, found 284.1145. Elemental analysis: calc. for 3 (C₁₄H₁₄F₆N₅O₂P) C, 39.17; H, 3.29; N, 16.32; found C, 38.86; H, 3.46; N, 16.17.