Synthesis and crystal structure of sodium copper tetrametaphosphimate heptahydrate Na2Cu(PO2NH)4·7H2O and sodium potassium copper tetrametaphosphimate heptahydrate KxNa2−xCu(PO2NH)4·7H2O

Na2Cu(PO2NH)4·7H2O and KxNa2−xCu(PO2NH)4·7H2O (x ≈ 0.5) were synthesized by gel crystallization in sodium silicate gels. The crystal structures were solved by single-crystal X-ray methods and found to be isotypic (Pnma, Z = 4; Na2Cu(PO2NH)4·7H2O: a = 627.5(2) pm, b = 1456.0(3) pm, c = 1900.5(4) pm, R1 = 0.0352; K0.47Na1.53Cu(PO2NH)4·7H2O: a = 632.2(2) pm, b = 1460.0(3) pm, c = 1936.4(4) pm, R1 = 0.0345). The P4N4 rings of the tetrametaphosphimate anion exhibit a distorted chair-2 conformation with admixtures of saddle and crown conformation. The M+ ions are six- and sevenfold coordinated by oxygen atoms, the Cu2+ ions are fivefold coordinated, respectively. The MO7 and the CuO5 units form pairs of face-sharing polyhedra, which are connected by common corners forming chains and are further interconnected by tetrametaphosphimate anions, forming a three-dimensional network structure with channels along [100] and [010]. The MO6 units form chains of face-sharing polyhedra, which are situated in the channels along [100]. Extended hydrogen bonding reinforces the three-dimensional framework structure of the compounds. 23Na-MAS NMR experiments were conducted to verify the K/Na distribution on the M sites derived from the X-ray crystal structure refinement.

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