2.2.1. Synthesis of (R)-phosphonomethylhomoproline (pmhpH3)
The mixture solution of (R)-pipecolic acid (50 mmol, 6.49 g), diethyl phosphite (75 mmol, 6.34 g) and hydrochloric acid (50 mL 6.0 M) in 100 mL three-necked flask was refluxed at 75°C in oil bath. After 1 hour, 2.32 g (75 mmol) of paraformaldehyde are added portion wise and the mixture was refluxed for an additional 12 hours. Then, the reaction solution was completely evaporated to dryness with a rotary evaporator, to which cold ethanol (100 mL) was added with stirring. A large number of white crystals were obtained. Finally, the product was filtrated, washed with ethanol and dried to give a white powder about 8.50 g. Yield: ca. 80% (based on (R)-pipecolic acid). Anal. calcd for C7H14NO5P (%): C, 37.87; H, 6.30; N, 6.39. Found: C, 37.67; H, 6.32; N, 6.27. IR (KBr, cm− 1):3408(s), 3062(m), 2950(w), 2873(w), 2785 − 2098(br), 1722(s), 1456(m), 1365(m), 1317(s), 1232(m), 1170(s), 1122(m), 1043(s), 932(m), 795(w), 757(m), 675(w), 580(w), 507(w), 444(w).
2.2.2 Synthesis of R/S-[Gd6(pmhpH)8(NO3)2(H2O)8]·19H2O
Syntheses of R-1. Gd(NO3)3·6H2O (0.1 mmol, 0.0458 g) and R-pmhpH3 (0.3 mmol, 0.0675 g) were dissloved in 10 mL deionized water, then adjusted to pH = 3.0 with 0.5 M NaOH solution. After evaporation at room temperature for two weeks, colorless bulk crystals were obtained. Yield: 43% (based on Gd). Elemental analysis calcd for C56H112N10O54P8Tb6(%): C, 18.84; H, 4.84; N, 3.89. Found: C, 18.91; H, 4.99; N, 3.94. IR (KBr, cm− 1): 3408(br), 3012(w), 2952(w), 2866(w), 2746(w), 2689(w), 2640(w), 2544(w), 1643(s), 1396(s), 1361(w), 1336(w), 1294(w), 1242(w), 1203(s), 1169(s), 1036(s), 1001(m), 972(m), 934(m), 870(w), 812(w), 775(m), 690(w), 592(m), 559(m), 524(w), 505(m), 476(w), 432(m).
Syntheses of S-1. S-1 was prepared using a similar procedure to R-1 except that R-pmhpH3 was replaced by S-pmhpH3. Yield: 51%. Elemental analysis calcd for C56H112N10O54P8Tb6(%): C, 18.84; H, 4.84; N, 3.89. Found: C, 18.88; H, 4.90; N, 3.91. IR (KBr, cm− 1): 3408(br), 3012(w), 2952(w), 2866(w), 2746(w), 2689(w), 2640(w), 2544(w), 1643(s), 1396(s), 1361(w), 1336(w), 1294(w), 1242(w), 1203(s), 1169(s), 1036(s), 1001(m), 972(m), 934(m), 870(w), 812(w), 775(m), 690(w), 592(m), 559(m), 524(w), 505(m), 476(w), 432(m).