Rb₃[C₆N₇(NCN)₃]·3H₂O and Cs₃[C₆N₇(NCN)₃]·3H₂O – Synthesis, Crystal Structure and Thermal Behavior of Two Novel Alkali Melonates

Abstract

Rubidium melonate trihydrate Rb3[C6N7(NCN)3]·3H2O and cesium melonate trihydrate Cs3[C6N7(NCN)3]·3H2O were obtained by ion exchange reactions in aqueous solution. The structure of the cesium salt was determined by single‐crystal X‐ray diffraction (Pna21, a = 687.34(14), b = 2196.5(4), c = 1232.6(3) pm, V = 1860.9(7)·106 pm3, Z = 4, T = 200 K). The crystal structure of the isotypic rubidium salt was refined from X‐ray powder diffraction data by the Rietveld method (Pna21, a = 674.48(2), b = 2146.4(1), c = 1207.5(1) pm, V = 1753.4(6)·106 pm3, Z = 4, T = 298 K). In the crystal structure planar melonate ions and alkali cations as well as water molecules are arranged in alternating layers. The melonate entities are interconnected by a dense hydrogen bonding network. Rb3[C6N7(NCN)3]·3H2O and Cs3[C6N7(NCN)3]·3H2O were investigated by FT‐IR spectroscopy, TG and DTA measurements.

Silver Melonates and Coordination Modes of the Multidentate [C6N7(NCN)3]3– Anion

“…However, various attempts to solve the structure were unsuccessful, which is related to the flexible coordination modes of the melonate anion as discussed below. Different metal-melonate interactions have been found in the structures of its complexes with K + , [22] Li + , [27] Cs + (Rb + ), [26] Ca 2+ [28] and Cu 2+ . [29] Fortunately, we were able to obtain single crystals of 2c, which shows a new type of coordination.…”

“…Furthermore, it has recently been proposed to abandon the HSAB concept generally due to weak correlation with experimental facts. [37] The protonated melonate derivative described by Schnick et al [26] does not fit into the scheme of orbital-and chargecontrolled reactivity as one would expect to coordinate the hard electrophile at the NCN group. Instead, it was found at the s-heptazine ring in position (a).…”

“…The bond lengths and angles are in accordance with those reported for other melonates and heptazine compounds, and the inner C-N bonds (C2-N7, C4-N7, C6-N7) are significantly longer than the outer ones. [6,7,11,13,16,18,19,22,[26][27][28][29] Figure 1. Asymmetric unit of 2c with numbering scheme.…”

“…[29] The lithium ion coordinates to (c) and (d). [27] Larger cations such as potassium, [22] rubidium and cesium [26] form irregular distorted polyhedra with nitrogen atoms from the melonate anion and further ligands. Multiple coordination sites of the melonate anion are involved in these cases ( Table 3).…”

“…[24] In 2003, the first structural determination of a melonate [potassium melonate (1)] was reported, which confirmed the arrangement of alternating sp 2 -hybridised carbon and nitrogen atoms in the heptazine core and that the -NCN substituents were cyanamide groups. [22] Synthesis and structural determination of further alkali, [25][26][27] alkaline earth [28] and a Cu II melonate [29] followed to give further insights into the molecular structure and thermal behaviour of melonates as promising precursors for graphitic carbon nitrides. [26][27][28][29][30] The crystal structures of the melonates revealed diverse modes of coordination, at which the outer nitrogen atoms of the heptazine nucleus and/or the two nitrogen atoms of the cyanamide sidearms are involved in coordination.…”