During the reaction of potassium cyamelurate with rare-earth salts, products are formed that can be divided into five groups:
1. Y1, Pr1, Nd1, Sm1, Eu1, Gd1, Tb1, Dy1, Ho1, Er1 obtained at room temperature and Sm2, Eu2, Gd2, Tb2, Dy2 obtained at elevated temperature (Fig. S1 – S3);
2. Tm1, Yb1, Lu1 obtained at room temperature (Fig. S4);
3. Y2, Ho2, Er2, Tm2, Yb2, Lu2 obtained at elevated temperature (Fig. S5);
4. Pr2 and Nd2 obtained at elevated temperature (Fig. S6);
5. La1 and La2 obtained at room and at elevated temperatures respectively (Fig. S7).
Within the group, the resulting products are isostructural; only for the second group of substances the structure has not been established. The main crystallographic characteristics for the solved structures Er1, Gd1, Pr1, Er2, Nd2 and La2 are shown in Table S1. Since the substances in the group Er1, Gd1, Pr1 are isostructural, only Er1 structure is considered in detail below.
The formula of erbium cyamelurate synthesized at room temperature can be represented as [Er(H2O)7C6N7O3], since Er1 contains neutral complexes. The coordination number of metal atom is 9, it is surrounded by 7 water molecules and is coordinated by the cyamelurate anion through the oxygen O2 and nitrogen N2 atoms (Fig. 1).
In the Er1 structure, flat anions are arranged in stacks, which are displaced and rotated relative to each other by an angle of 180° (Fig. 2a). The distances between two neighbor anions in the stack are 3.19 Å and 3.36 Å (Fig. 2b). Adjacent stacks are arranged so that an angle of 34.13° is formed between the anions.
It is interesting to note that cyamelurate anions do not form endless chains typical for known cobalt(II) and cupper(II) cyamelurates , arising due to the polydentity of C6N7O33− anion, and hence the ability to act as a bridging ligand. Hydrogen bonds connect individual [Er(H2O)7C6N7O3] molecules together, and they are formed between all water molecules and oxygen and nitrogen atoms of the anion (except N1, N2, N4 atoms).
The structure of erbium cyamelurate Er2 obtained at elevated temperatures is shown in Figure 3.
Erbium atom is surrounded by 4 water molecules and is bound to two cyamelurate anions through the oxygen atoms O3, O2 and nitrogen atoms N5, N2 (coordination number is 8). The angle between the planar cyamelurate anions bound to the erbium cation is 75.26°. Such self-organization of alternating cations and bridging cyamelurate anions leads to endless undulating chains (Fig. 4a). The distance between planar anions of adjacent chains is 3.19 Å (Fig. 4b). Outer-sphere O8 water molecules are located between the chains. Although this complex is also neutral, but it consists of infinite polymer-like chains, therefore it can be represented by the formula [Er(H2O)4C6N7O3]n·nH2O.
The structure of neodymium cyamelurate N2 obtained by boiling the reaction mixture is shown in the Figure 5.
In Nd2 crystal structure, anions act as bridging ligands coordinating neodymium cation through oxygen O2, O2i, O3ii and nitrogen N2i atoms (i = –x, 1–y, 1–z; ii = x, y, –1+z). Two coordination polyhedra have a common edge O2–O2i (Fig. 6a). Five water molecules complement the coordination sphere of neodymium (coordination number is 9). Nd2 contains endless chains, [Nd(H2O)5C6N7O3]n, forming stacks of parallel cyamelurate anions, the distance between which is equal to 3,14 Å, two neodymium atoms are located between the stacks (Fig. 6b).
The chains are tied together by hydrogen bonds between nitrogen (except N1, N2) and oxygen atoms of anions and all five water molecules.
The structure of lanthanum cyamelurate is shown in Figure 7. The composition of La2 based on structural data corresponds to the formula [La(H2O)6C6N7O3]·H2O.
One lanthanum atom is coordinated by two cyamelurate anions through O1, O1iv and N2, N2iv atoms (where iv = x, –y, 0.5–z). Six water molecules complete the first coordination sphere. The coordination number of central atom is 10. One water molecule is located in the cavities formed between the stacks of anions and is held in the structure via hydrogen bonding O7–H7…O2, O7–H7…N3, O4ii–H4Aii…O7 (Fig. 7).
Neighboring lanthanum polyhedra have one common O1 vertex. Flat ribbons are realized in the structure (Fig. 8a). The ribbons are stacked (Fig. 8b), and the adjacent anions are located one above the other so that the distance between the ribbons displaced relative to each other is roughly equal to 3.3Å or 0.5a, where a – unit cell parameter.
As in the case of lanthanide melonates , some rare-earth cyamelurates have different structures depending on the synthesis temperature. Both thermodynamic and kinetic products were isolated in the case of Nd, Pr,Y, Ho, Er, Tm, Yb, Lu cyamelurates.
Despite the variety of structures, it is possible to distinguish common features among kinetic and thermodynamic products. For example, the former contain single molecules [Ln(H2O)7C6N7O3] (Ln=Y, Pr, Nd, Ho, Er), the latter have endless polymer chains [Ln(H2O)4C6N7O3]n (Ln=Y, Ho, Er, Tm, Yb, Lu), [Ln(H2O)5C6N7O3]n (Ln=Nd, Pr), [La(H2O)6C6N7O3]n formed due to the fact that the cyamelurate ion acts as a bridging ligand and binds neighboring cations. Also, metastable phases of lanthanide cyamelurates (as well as melonates ) contain more water molecules compared to thermodynamically stable ones.
In the case of yttrium and heavy lanthanides Ho and Er, the ion radius most likely affects the decrease in the coordination number from 9 to 8 with an increase in the reaction temperature. Since the central metal atom is surrounded by two large anions, and not one, as in the kinetic product, steric hindrances arise. The water molecule, which could be in the first coordination sphere, is held by hydrogen bonds in the second coordination sphere. On the other hand, the coordination number of neodymium and praseodymium cations is maintained as in the structure obtained at room temperature, although each atom is coordinated by three anions.
In each groups of isostructural complexes, from light to heavy lanthanides, the unit cell parameters decrease (Tables S2–S4), which is consistent with a lanthanide contraction. Also, powders of the first group, obtained at room temperature have unit cell parameters slightly less (by a few hundredths of an angstrom) as those synthesized at elevated temperatures.
Powders of samarium, europium, gadolinium, terbium, and dysprosium cyamelurates obtained both at room and elevated temperature have the same structure, isotypic to Er1. Therefore, this crystalline modification for the listed lanthanides is stable and rapidly forming, and this is the common case when thermodynamic and kinetic products are the same . This also applies to lanthanum cyamelurate, which differs from the other rare-earth cyamelurates presented in this work. On the other hand, it is possible that the boiling point of the reacting mixture is insufficient to obtain the most stable modification, and, as in the case of samarium melonate , hydrothermal conditions are required. However, our attempt to synthesize nickel cyamelurate under hydrothermal conditions led to the hydrolysis of the cyameluric acid residue . For this reason, we abandoned this type of synthesis.
It is possible that nucleation of rare-earth cyamelurates, as well as zinc cyamelurates , takes place in colloidal electric double layer micelles formed in solution. Sequential complexation reactions occur inside such micelles. First, coordination of one anion to the cation aqua-complex lead to the gradual displacement of water molecules, and second, with an increase in temperature, the monomeric complex is transformed into polymeric one due to binding of neighboring monomers via polydentate bridging anions. The second stage also proceeds with the displacement of water molecules from the inner sphere of the complexes. This is clearly seen when comparing the structures of Er1 and Er2, Nd1 (isostructural to Er1) and Nd2.
The π–π stacking of anions in the crystal structures of different metal and ammonium cyamelurates was already mentioned in this article as well as in our previous ones [1, 8, 12]. Coulomb interactions between cations and anions and hydrogen bonds present in all obtained structures, but in Ni(C6N7O3H)·5H2O  Coulomb interactions make a crucial contribution to the crystals potential energy; as a result, this crystal structure is a derivative of the CsCl-type structures (Fig. S14). Other studied structures (Fig. S15-S19) could not be attributed to known structural types, which is inevitable if Coulomb interactions exist not between individual cations and anions, but between ion aggregates collected in stacking columns under the influence of quadrupole  or diradicaloid interactions , as well as hydrophobic-hydrophilic balance. This means that relatively weak stacking effect can determine the type of crystal structure without a determinative contribution to the potential energy of crystals.