Design and Synthesis of new Nanosized C ₃-Symmetrical Tricarboxylic Acids: Key Elongated Ligands for the Preparation of Highly Porous MOFs

Abstract

The synthesis of two new C ₃-symmetrical nanosized tricarboxylic acids bearing triphenylmethane cores and three imines linkages is presented. The new elongated tripods were designed to stabilise highly porous metal-organic frameworks (MOFs) and were synthesized by a threefold Schiff base condensation reaction between the reduced leuco form of pararosaniline hydrochloride salt and an appropriate aldehyde.

Key words

Schiff bases - condensation reaction - tricarboxylic acid ligands - metal-organic frameworks (MOFs)

• References and Notes

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• 7 4,4′,4′′-Triaminotriphenylmethane (5) was prepared according to a literature procedure.⁶ Purification was performed by column chromatography on silica gel (acetone–Et₃N = 9:0.08) to afford product 5 as a greyish-pink solid (52%);⁶ mp 210 °C (dec.). R_f  = 0.65 (acetone–Et₃N = 9:0.08). IR (ATR): ν_max = 3468 (br w), 3427 (br w), 3377 (br w), 3348 (br w), 3211 (br w), 3011 (w), 2357 (w), 2318 (w), 1622 (m), 1510 (s), 1435 (w), 1273 (m), 1176 (m), 1122 (w), 1084 (w), 1016 (w), 827 (m), 779 (m). cm^–1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 6.70 (6 H, d, J = 8.1 Hz, CH arom.), 6.44 (6 H, d, J = 8.1 Hz, CH arom.), 4.97 (1 H, s), 4.83 (6 H, s, 3 × NH₂). ¹³C NMR (125 MHz, DMSO-d ₆): δ = 146.17 (C arom.), 132.78 (C arom.), 129.14 (CH arom.), 113.51 (CH arom.), 53.80 (CH). HRMS (TOF MS AP+): m/z [M + H]⁺calcd for C₁₉H₂₀N₃: 290.1657; found: 290.1652. UV/vis (DMSO): λ_max (ε/dm³ mol^–1 cm^–1): 259 nm (34792), 298 nm (8418).
• 8 Synthesis of 4,4′,4′′-{[Methanetriyltris(benzene-4,1-diyl)-tris(azanylylidene)]tris(methanylylidene)}tribenzoic Acid (4a) A solution of 4-formylbenzoic acid (6, 189 mg, 1.25 mmol) in absolute EtOH (15 mL) was added into a stirred solution of 4,4′,4′′-triaminotriphenylmethane (5, 110 mg, 0.38 mmol) in absolute EtOH (40 mL), in a predried 100 mL single-neck round-bottomed flask equipped with a magnetic stirrer. The resulting mixture was stirred for seven days at r.t., under an atmosphere of dry nitrogen. The reaction mixture was then filtered under suction on a sintered funnel to give a greyish pink solid which was washed with absolute EtOH (60 mL). Purification of the solid was performed by column chromatography on silica gel (CHCl₃–MeOH–Et₃N = 9:1:0.15) to afford product 4a as a grey solid (232 mg, 89%); mp 237 °C (dec.). R_f  = 0.38 (CHCl₃–MeOH–Et₃N = 9:1:0.15). IR (ATR): ν_max = 2991 (br w), 2879 (br w), 2667 (br w), 2540 (br w), 2358 (br), 2328 (br), 1691 (s), 1624 (m), 1570 (w), 1500 (m), 1421 (m), 1286 (s), 1196 (m), 1172 (w), 1112 (w), 1014 (m), 958 (w), 889 (w), 856 (w), 839 (w), 771 (s) cm^–1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 13.08 (3 H, br s, 3 × CO₂H), 8.74 (3 H, s, N=CH), 8.06 (12 H, app. q, J = 7.9 Hz, CH arom.), 7.31 (6 H, d, J = 8.2 Hz, CH arom.), 7.23 (6 H, d, J = 8.2 Hz, CH arom.), 5.76 (1 H, s, CH). ¹³C NMR (125 MHz, DMSO-d ₆): δ = 166.84 (CO₂H), 159.61 (N=CH), 149.20 (C arom.), 142.13 (C arom.), 139.65 (C arom.), 132.87 (C arom.), 129.84 (CH arom.), 129.69 (CH arom.), 128.62 (CH arom.), 121.26 (CH arom.), 54.42 (CH). HRMS (TOF MS AP+): m/z [M + H]⁺ calcd for C₄₃H₃₂N₃O₆: 686.2291; found: 686.2323. UV/vis (DMSO): λ_max (ε/dm³ mol^–1 cm^–1): 260 nm (79873), 344 nm (25047).
• 9 Synthesis of 4′-Formyl-(1,1′-biphenyl)-4-carboxylic Acid (8) NaOH (497 mg, 12.4 mmol) was added into a stirred solution of methyl 4-(4-formylphenyl)benzoate (7, 2.13 g, 8.9 mmol) in MeOH–H₂O (3:1, 500 mL) in a 1000 mL single-neck round-bottomed flask, and the resulting mixture was stirred at an elevated temperature of 55–60 °C for two weeks. During this time the consumption of the starting material was monitored by means of TLC (hexane–EtOAc = 4:1). Upon completion, MeOH was evaporated under reduced pressure, and 1 M NaOH (25 mL) was added. The aqueous solution was washed with EtOAc (3 × 150 mL) and finally made acidic with 3 M HCl. The precipitate was then filtered under suction on a sintered funnel to give an off-white solid which was washed successively with water, CHCl₃, MeCN, and finally with Et₂O. Residual solvent traces were removed under high vacuum to afford product 8 as an off-white solid (1.54 g, 77%). IR (ATR): ν_max = 3072 (br), 3043 (br), 2951 (m), 2372 (w), 1713 (m), 1682 (m), 1603 (s), 1558 (w), 1425 (w), 1404 (w), 1296 (m), 1247 (m), 1219 (m), 1172 (w), 1113 (w), 1007 (m), 827 (s) cm^–1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 13.10 (1 H, br s, CO₂H), 10.08 (1 H, s, CHO), 8.07–8.02 (4 H, m, 4 × CH arom.), 7.98 (2 H, d, J = 8.0 Hz, 2 × CH arom.), 7.90 (2 H, d, J = 8.0 Hz, 2 × CH arom.). ¹³C NMR (125 MHz, DMSO-d ₆): δ = 192.72 (CHO), 166.90 (CO₂H), 144.55 (C arom.), 142.78 (C arom.), 135.54 (C arom.), 130.53 (C arom.), 130.11 (CH arom.), 129.96 (CH arom.), 127.67 (CH arom.), 127.28 (CH arom.). HRMS (TOF MS AP+): m/z [M + H]⁺ calcd for C₁₄H₁₁O₃: 227.0708; found: 227.0713. UV/vis (DMSO): λ_max (ε/dm³ mol^–1 cm^–1): 252 nm (13111), 295 nm (21370).
• 10 Synthesis of 4′,4′′′,4′′′′′-{[Methanetriyltris(benzene-4,1-diyl)tris(azanylylidene)]tris(methanylylidene)}tris(1,1′-bi-phenyl)-4′-carboxylic Acid (4b) A solution of 4′-formyl-(1,1′-biphenyl)-4-carboxylic acid (8, 203 mg, 0.89 mmol) in absolute EtOH (15 mL) was added into a stirred solution of 4,4′,4′′-triaminotriphenylmethane (5, 78.6 mg, 0.27 mmol) in absolute EtOH (35 mL), in a dry 100 mL single-neck round-bottomed flask. The resulting mixture was stirred for 10 d at r.t., under an atmosphere of dry nitrogen. The reaction mixture was then filtered under suction on a sintered funnel to give an off-white solid which was washed successively with EtOH, CHCl₃, MeCN, THF, and finally with Et₂O. Residual solvent traces were removed under high vacuum to afford product 4b as an off-white solid (157 mg, 63%). IR (ATR): ν_max = 2989 (br w), 2871 (br w), 2656 (br w), 2543 (br w), 2362 (br), 1697 (s), 1610 (m), 1571 (w), 1510 (m), 1413 (m), 1364 (w), 1298 (s), 1199 (m), 1168 (w), 1117 (w), 1015 (w), 962 (w), 857 (w), 838 (w), 786 (m) cm^–1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 13.06 (3 H, br s, 3 × CO₂H), 8.72 (3 H, s, N=CH), 8.04 (13 H, br s, 13 × CH arom.), 7.90 (13 H, br s, 13 × CH arom.), 7.31 (5 H, br s, 5 × CH arom.), 7.25 (5 H, br s, 5 × CH arom.), 5.76 (1 H, s, CH). ¹³C NMR (125 MHz, DMSO-d ₆): δ = 167.04 (CO₂H), 159.83, 159.56, 149.53, 143.24, 141.88, 141.58, 135.82, 129.99, 129.82, 129.29, 127.33, 126.92, 121.19, 66.98 (CH). HRMS (MALDI TOF, positive mode, reflectron, HCCA matrix on Prespotted AnchorChip PAC II 384/96): m/z [M + H]⁺ calcd for C₆₁H₄₄N₃O₆: 914.3225; found: 914.3221. UV/vis (DMSO): λ_max (ε/dm³ mol^–1 cm^–1): 258 nm (57011), 303 nm (77785), 351 nm (37277).