Urea can crystallise with other compounds. These can be called urea adducts or if a solvent is involved, a urea solvate, and the process is called urea extraction crystallization. Urea can also be a neutral ligand if it is coordinated to a central metal atom. Urea can form hydrogen bonds to other oxygen and nitrogen atoms in the substance it crystallises with. This stiffens the solid and raises the melting point.
List nameformularatio urea: solutecrystal systemspace groupunit cellvolumedensitycommentreferencehydrogen peroxide; urea (1:1)HOOH•CH4N2O1:1orthorhombicPncaa 6.732 b 4.8207 c 12.873 Z=4decomposes slowly at room temperatureurea; acetic acid (1:2)2CH3COOH•CH4N2O1:2monoclinicP21/na 7.6549 b 10.1351 c 11.5219 β 99.570° Z=4881.471.358unstable in airoxalic acid; urea (1:1)C2H2O4•CH4N2O1:1monoclinicC2/ca 13.0625 b 6.6437 c 6.8478 β 92.474°oxalic acid; urea (1:2)C2H2O4•2CH4N2O2:1monoclinicP2/ca 5.058 b 12.400 c 6.964 β 98.13°Urea - N,N-dimethylformamide (3:1)3CH4N2O·C3H7NO3:1triclinicPa = 7.525 b = 9.866 c = 10.821 α = 65.61° β = 79.43° γ = 70.76° Z=2689.71.219colourless @150KN,N-dimethylacetamide; urea (1:1)C4H9NO·CH4N2O1:1monoclinicC2/ca = 7.2770 b = 17.5394 c = 7.3789 β = 119.450° Z=4820.111.192colourless @120KUrea malonic acid1:1monoclinicP2/ca 13.091 b 5.455 c 9.933 β 104.39°Maleic acid; urea (1:1)1:1monoclinicCca 5.144 b 9.999 c 14.80 β 95.1°Succinic acid bis urea2:1monoclinicP21/ca 5.637 b 8.243 c 12.262 β 96.75° Z=2565.811.41Maleic acid; urea (2:1)2:1monoclinicP21/na 5.661 b 11.754 c 19.173 β 91.56°Fumaric acid; urea (1:2)1:2monoclinicP21/ca 5.540 b 8.227 c 12.426 β 97.22°D-Tartaric1:1orthorhombicP212121a 17.229 b 9.824 c 5.056 Z=41.63transparent 255 to 1370 nm; SHG 3×KDPDL-Tartaric1:1monoclinicP21a 7.6973 b 23.3310 c 4.8727 β 100.82°NLO; transparent from 240 to 1950 nmGluatric acid bis urea2:1monoclinicC2/ca 11.954 b 10.932 c 9.078 β 97.86° Z=4Itaconic acid1:1monoclinicP21/ca 12.71 b 5.2695 c 13.833β 104.78°2-bromotetradecane ureahexagonalP6122a=8.2582 c=10.9937298K2-bromotetradecane ureaorthorhombicC2221207K2-bromotetradecane ureamonoclinicP21a 16.338(1)Å b 10.9665(8)Å c 16.338(1)Å, α 90° β 60.00°4-amino benzoic acid; urea (2:1)(C7H7NO2)2(CH4N2O)1:2orthorhombicPnaba=7.4159 b=11.870 c=18.750salicylic acid; urea (1:1)1:1monoclinicC2/ca=22.206 b=5.108 c=17.177 β=106.18 Z=8°1.4075-nitro salicylic acid; urea (2:1)(C7H5NO5)2(CH4N2O)1:2triclinicPa=6.9889 b=12.1968 c=12.3622 α=60.923° β=81.169° γ=76.938°3,5-dinitro salicylic acid; urea (1:1)(C7H4N2O7)(CH4N2O)1:1monoclinicP21/ca=4.942 b=22.337 c=10.389 β=100.92°o-phthalic acid; urea (1:1)1:1triclinicPa=7.422 b=7.662 c=10.088 α=85.95° β=82.01° γ=65.14°urea : 1,4-dioxane (1 : 1)C4H8O2•C3H6O1:1monoclinicP2/ca=6.7949 b=4.5234 c=12.2711 β=95.701° Z=2urea : morpholine (1 : 1)C4H8ON•C3H6O1:1orthorhombicPbcma=4.5847 b=19.131 c=8.9047 Z=43,5-dimethylpyridine bis(urea)(CH3)2C5H3N•2C3H6O2:1orthorhombicAbm2a=21.737 b=7.2102 c=15.590 Z=82443.41.236at 173K3,5-dimethylpyridine bis(urea)(CH3)2C5H3N•2C3H6O2:1monoclinicCca=8.5829 b=21.4843 c=7.2050 β=114.405° Z=41209.871.248at 100K2,6-dimethylpyridine urea(CH3)2C5H3N•C3H6O1:1triclinicPa=7.4126 b=7.6720 c=8.1731 α=88.391° β=83.564° γ=80.059° Z=2454.921.221at 100Kurea 2,6-dimethylpyridine(CH3)2C5H3N•C3H6O1:1monoclinicC2/ca=1.426 b=11.1168 c=7.4318 β=101.23° Z=4925.91.200at 200K2,6-dimethylpyridine bis(urea)(CH3)2C5H3N•2C3H6O2:1orthorhombicPnmaa=8.0772 b=7.2986 c=20.4169 Z=41203.641.254at 100K2,6-dimethylpyridine bis(urea)(CH3)2C5H3N•2C3H6O2:1monoclinicC2/ca=11.160 b=11.5421 c=10.910 β=116.12°2-picoline; urea (1:1)C6H7N·CH4N2O1:1orthorhombicPbcaa = 7.471 b = 14.916 c = 15.338 Z = 81709.2Urea pyrazine-2,3-dicarboxylic acid1:1monoclinicP21/na=7.725 b=10.2530 c=12.612 β=97.997°bipyridine ureatriclinicPa=7.200 b=8.297 c=9.835 α=75.849° β=70.744° γ =73.045°bipyridine ureatriclinicPa=7.2862 b=8.3747 c=9.8647 α=76.061° β=72.789° γ=74.121°1,10-Phenanthroline urea1:1monoclinicC2/ca=14.342 b=12.002 c=7.3724 β 116.743°2,9-Dimethyl-1,10-phenanthroline urea1:1orthorhombicCmcma=11.370 b=17.351 c=7.3593sodium chloride; urea; water (1:1:1)NaCl·CH4N2O·H2O1:1triclinica = 6.44 Å, b = 5.245Å, c = 17.312 Å, and a=90°, β=90.15°, γ=90°588.76SHG 1.53 × KDP, birefringence 0.084@1064 nm UV edge 209 nmBis(2-3-isopropyl-7-oxocyclohepta-1,3,5-trien-1-olato)bis[(3-isopropyl-7-oxocyclohepta-1,3,5-trien-1-olato)copper(II)]-urea-acetone (1/6/2)[Cu2(C10H11O2)4]·6CH4N2O·2C3H6O5:1monoclinicP21/ca 17.0125 b 11.0470 c 17.2731 β 110.385° Z=23042.951.371greenaqua(N-salicylidene-rac-alaninato- O,N,O')copper(II)-urea (1/1)1:1triclinicPa = 7.637, b = 8.509 c = 10.716 α = 93.10° β = 97.97° γ= 106.37° Z= 2,658.5blueAqua[9-(1,8-diazafluoren-9-ylidene)amino-1,8-diazafluorenato]hydroxo(urea)zinc(II) urea solvate[Zn(C22H12N5)(OH)(CH4N2O)(H2O)]•C4N2O1:1monoclinicP21/ca 7.8637 b 16.0133 c 17.9513 β 101.358° Z=42216.21.687purplePentakis(carbamide)dioxoneptunium(V) nitrate[NpO2{OC(NH2)2}5](NO3)monoclinicP21a = 11.142, b = 7.6379 c = 11.143 β = 108.9° Z = 2897.1Bis[(isothiocyanato)tetraureadioxoneptunium(V)] urea{NpO2(NCS)[OC(NH2)2]4}2 · OC(NH2)2tetragonalP43212a=7.851 c=56.84 Z=435042.265light green
References