Crystal structures of rare earth cyamelurates obtained under kinetic and thermodynamic controls

If various compounds exist in the system “metal cation–C 6 N 7 O 3 3− –H 2 O” (where C 6 N 7 O 3 3− is cyamelurate anion), the synthesis temperature can affect the isolation of a particular product. Short reaction times together with low temperatures favor the release of kinetic products, and an increase in temperature, on the contrary, can lead to the formation ofthermodynamic ones. It is found that several structural types exist in the row of rare earth cyamelurates. Room temperature synthesis leads to the formation of [Ln(H 2 O) 7 C 6 N 7 O 3 ] (Ln = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er); an increase in temperature yields thermodynamically more stable [Ln(H 2 O) 4 C 6 N 7 O 3 ] n · n H 2 O (Ln = Y, Ho, Er, Tm, Yb, Lu) and [Ln(H 2 O) 5 C 6 N 7 O 3 ] n (Ln = Pr, Nd). The change in the synthesis temperature did not affect the structures of Sm, Eu, Gd, Tb, and Dy cyamelurates, as well as the structure of lanthanum cyamelurate [La(H 2 O) 6 C 6 N 7 O 3 ]·H 2 O. In [Ln(H 2 O) 4 C 6 N 7 O 3 ] n · n H 2 O and [Ln(H 2 O) 5 C 6 N 7 O 3 ] n synthesized at increased temperature, cyamelurate anion acts as a bridging ligand, leading to polymeric chains. Kinetically trapped Y, Pr, Nd, Ho, and Er cyamelurates, in contrast, consist of individual complex molecules [Ln(H 2 O) 7 C 6 N 7 O 3 ]. Probably, steric difficulties caused a decrease in the coordination number of Y, Ho, and Er from 9 to 8 in the thermodynamic product. The coordination number of Pr and Nd remains equal to 9 in both types of compounds.