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Molecular and crystal structures of UO2(NO3)2(NRP)2 (NRP = N-alkylated 2-pyrrolidone derivative) have been investigated by using single crystal X-ray analysis. All UO2(NO3)2(NRP)2 complexes have typical structural properties of UO2(NO2)2(L)2 (L = unidentate ligand), i.e., hexagonal-bipyramidal geometry, two NRP and two NO3 − located in trans positions in an equatorial plane of the uranyl moiety, UOyl ≃ 1.76 Å, U−ONRP = 2.38−2.41 Å, U−ONO3 = 2.50−2.54 Å, and a bond angle between the U−ONRP bond and the carbonyl group of NRP (ca. 135°). In the crystals of the uranyl nitrate complexes with N-n-propyl-2-pyrrolidone (L3a) and N-iso-propyl-2-pyrrolidone (L3b), polymorphism between monoclinic (293 K) and triclinic forms (173 K) was observed, indicating the presence of significant voids in the crystal lattices of these compounds. From this result, an approach for construction of efficient packing of UO2(NO2)2(NRP)2 was proposed, i.e., the efficient packing would be built up when an alkyl chain of NRP fills the voids in the crystal lattice of UO2(NO3)2(L3a)2 or UO2(NO3)2(L3b)2 without significant deformation of their crystal structures. On this basis, the molecular and crystal structures of UO2(NO3)3(NRP)3 with C4- and C5-alkyl chains were examined. Consequently, it was found that N-iso-butyl-2-pyrrolidone (L4b) completely satisfies that requirement. The high packing efficiency of UO2(NO3)2(L4b)2 was demonstrated quantitatively by introducing “compactness parameter” (C p) defined as a mean volume occupied by one carbon atom of the alkyl chain in the crystal of UO2(NO3)2(NRP)2.

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