Heating at temperatures above 300 °C was carried out in a custom-made resistive furnace, using a Eurotherm 2408 temperature controller. Where applicable, percentage molar yields are calculated assuming the ideal formula of C6N9H3 for melon. Crystalline melem (1 )39 (link), crystalline melon (10 )4 (link), amorphous melem65 (link), amorphous melon18 (link), tri(diethylamino)heptazine (3 )66 , potassium cyamelurate (6 )67 (link), cyameluric acid (7 )68 , tri(p-tolyl)heptazine (8 )69 (link), tri(p-benzoic acid)heptazine (9 )69 (link) and potassium melonate38 (5 ) were synthesized following literature methods and their characterizations are consistent with those previously reported.
Heptazine triphthalimide (4 ) was prepared following the literature method65 (link), except that unreacted melem was not removed by Soxhlet extraction with nitromethane because of safety concerns.
Cyameluric trichloride was prepared following an adapted procedure70 . Potassium cyamelurate (24.4 g) was refluxed in a mixture of PCl5 (6.81 g) and POCl3 (10 ml) until gas evolution has ceased. Unreacted PCl5 and POCl3 were boiled or sublimed off by heating under vacuum. The yellow product was not purified from the side product KCl as it is easier to remove in the subsequent syntheses. Yield 34.1 g (molar yield not provided as residual KCl was not removed). Characterization with FTIR is consistent with literature70 .
Tri(ethylamino)heptazine (2 ) was synthesized by mixing under argon a solution of cyameluric trichloride (1.386 g) in tetrahydrofuran (THF, 20 ml, anhydrous) and ethylamine in THF (8.5 ml, 2 M L −1), and then refluxed for 2 h. The solvent and unreacted ethylamine were evaporated off and the resulting solid was re-dispersed in water, refluxed for 1 h, isolated by filtration, and then washed repeatedly with water and dried. The product was further purified by recrystallization from hot glacial acetic acid. Yield: 933 mg (62%). 1H NMR (DMSO): δ=2.50 (CH2), 1.07 ppm (CH3). FTIR: 3,222, 3,080, 3,029, 2,971, 2,933, 1,641, 1,571, 1,494, 1,433, 1,398, 1,373, 1,346, 1,308, 1,286, 1,178, 1,144, 1,100, 1,069 and 797 cm−1.
Transition and lanthanum metal complexes of melonate were prepared by mixing stoichiometric amounts of potassium melonate and the metal salt, both as aqueous solutions (20 mM). The metal salts employed are: (5a ) AgNO3, (5b ) CeCl3·6H2O, (5c ) Co(NO3)2·6H2O, (5d ) Cr(NO3)3·9H2O, (5e ) Cu(AcO)2·H2O, (5f ) Fe(NO3)2·nH2O, (5g ) La(NO3)3·6H2O, (5h ) Mn(AcO)2·4H2O, (5i ) Nd2(SO4)3, (5j ) Ni(NO3)2·6H2O, (5k ) (NH3)4Pt(NO3)2, (5l ) Tb(NO3)3 and (5m ) Zn(AcO)2·2H2O. The complex precipitated immediately upon mixing the metal salt and the ligand. The complex was isolated by filtration, washed with copious amounts of water, and then dried at 60 °C in a vacuum oven. Product yields were above 90% to quantitative. Characterizations are shown in Supplementary Figs 6–8 .
Amorphous melon with the cyanamide functionalization was prepared following the original synthesis of potassium melonate38 , except that the water-insoluble solid was collected. In detail, melon (800 mg) was thoroughly ground with KSCN (1.6 g, dried at 140 °C in vacuum) and loaded in an alumina boat. In a tube furnace, this mixture was heated under argon to 400 °C at 30 °C min−1 ramp for 1 h, and then to 500 °C at 30 °C min−1 ramp for 30 min. The resulting yellow mass was suspended in water and the insoluble product was isolated by centrifugation, washed with copious amount of water and dried at 60 °C in a vacuum oven. Yield from 800 mg melon is 350–450 mg (35–45% assuming the formula C7N10H1.4K0.6, see elemental analyses inSupplementary Table 4 ).
As post-synthetic annealing may lead to a significant improvement in the photocatalytic activity of melon, we prepared another control sample to verify that the large outperformance of KSCN-treated melon is not attributed to this heating step. Melon in a ceramic boat was heated under argon to 400 °C at 30 °C min−1 ramp for 1 h, and then 500 °C at 30 °C min−1 ramp for 30 min. This sample is denoted as ‘amorphous melon (extra heating step)' inFig. 1a .
Heptazine triphthalimide (
Cyameluric trichloride was prepared following an adapted procedure70 . Potassium cyamelurate (24.4 g) was refluxed in a mixture of PCl5 (6.81 g) and POCl3 (10 ml) until gas evolution has ceased. Unreacted PCl5 and POCl3 were boiled or sublimed off by heating under vacuum. The yellow product was not purified from the side product KCl as it is easier to remove in the subsequent syntheses. Yield 34.1 g (molar yield not provided as residual KCl was not removed). Characterization with FTIR is consistent with literature70 .
Tri(ethylamino)heptazine (
Transition and lanthanum metal complexes of melonate were prepared by mixing stoichiometric amounts of potassium melonate and the metal salt, both as aqueous solutions (20 mM). The metal salts employed are: (
Amorphous melon with the cyanamide functionalization was prepared following the original synthesis of potassium melonate38 , except that the water-insoluble solid was collected. In detail, melon (800 mg) was thoroughly ground with KSCN (1.6 g, dried at 140 °C in vacuum) and loaded in an alumina boat. In a tube furnace, this mixture was heated under argon to 400 °C at 30 °C min−1 ramp for 1 h, and then to 500 °C at 30 °C min−1 ramp for 30 min. The resulting yellow mass was suspended in water and the insoluble product was isolated by centrifugation, washed with copious amount of water and dried at 60 °C in a vacuum oven. Yield from 800 mg melon is 350–450 mg (35–45% assuming the formula C7N10H1.4K0.6, see elemental analyses in
As post-synthetic annealing may lead to a significant improvement in the photocatalytic activity of melon, we prepared another control sample to verify that the large outperformance of KSCN-treated melon is not attributed to this heating step. Melon in a ceramic boat was heated under argon to 400 °C at 30 °C min−1 ramp for 1 h, and then 500 °C at 30 °C min−1 ramp for 30 min. This sample is denoted as ‘amorphous melon (extra heating step)' in
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