Silver Cyanoguanidine Nitrate Hydrate: Ag(C2N4H4)NO312 H2O, a Cyanoguanidine Compound Coordinating by an Inner Nitrogen Atom

Silver(I) cyanoguanidine nitrate hydrate, Ag(C2N4H4)NO3·1⁄2H2O, was synthesized as the first cyanoguanidine solid-state complex in which monovalent Ag is coordinated through inner nitrogen N atoms. Its structure was characterized by single-crystal X-ray diffraction, crystallizing in the acentric orthorhombic space group P21212 with a = 10.670(3) Å, b = 18.236(5) Å, and c = 3.5078(9) Å. The differing chemical bondings of Ag(C2N4H4)NO3·1⁄2H2O and Ag(C2N4H4)3NO3 were compared on the basis of first-principle calculations.


Introduction
Cyanoguanidine, also called dicyandiamide, is not only the dimer of molecular cyanamide (or carbodiimide) but also a commercially important compound that has found medicinal and industrial applications [1][2][3][4] and has long been known to crystallographers. The molecule has also been widely used as a precursor for synthesizing organonitrogen compounds [5]. It exists in two tautomeric forms, differing in the protonation and bonding of its nitrogen atoms ( Figure 1). The structural conformations of both forms are planar with one tautomer displaying two terminal amine groups ( Figure 1a) and the other expressing only one terminal amine group but two imine groups in the middle of the molecule (Figure 1b). The coordination chemistry of cyanoguanidine has earned significant attention owing to these differing functional groups [6][7][8][9][10][11], which readily coordinate late transition metals, forming complexes with monovalent [6] and divalent copper [7], silver [8,11], and divalent zinc [9,10], all of which have been structurally characterized. As alluded to in Figure 1, the inner N atoms as well as the terminal N atoms are able, at least in principle, to coordinate to metal atoms due to the existence of lone pairs at those N atoms. However, to the best of our knowledge, all reported metal cyanoguanidine compounds involve a coordination only with the terminal cyano N atoms of the cyanoguanidine molecules. In an attempt to grow high-quality Ag(C 2 N 4 H 4 ) 3 NO 3 single crystals by slow diffusion, we unexpectedly made the discovery of the title compound Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O. Here, we report the synthesis and crystal structure of this silver(I) cyanoguanidine nitrate hydrate, the first example of a metal cyanoguanidine compound coordinating with inner nitrogen atoms.

Chemical Bonding Analysis
As the discovery of Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O was a consequence of preparing yet another silver cyanoguanidine nitrate, Ag(C 2 N 4 H 4 ) 3 NO 3 [8], questions of chemical equilibrium as regards the energetic competition between the two phases pop up, so their structural and bonding character was analyzed in more detail. As shown in Figure 2b, the Ag atom in Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O is surrounded by two amine and one imine N atoms of the H 4 C 2 N 4 molecule and also two O atoms of the nitrate anion, while the Ag atom in Ag(C 2 N 4 H 4 ) 3 NO 3 is surrounded by five amine N atoms. Additionally, a comparison of the bond lengths ( Figure 4) indicates that the Ag-N bonds are shorter in Ag(C 2 N 4 H 4 ) 3 NO 3 than the Ag-N and Ag-O bonds in Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O.  In order to quantitatively probe the relative stability of these phases, crystal orbital Hamilton population (COHP) analyses were performed to study the bonding mechanisms, as shown in Figure 5. Note that the integrated COHP (ICOHP) up to the Fermi level measures covalency, with more negative ICOHP values corresponding to stronger bonding interactions. Figure 5 reveals that the enhanced stability of Ag(C 2 N 4 H 4 ) 3 NO 3 , compared with Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O, appears to arise from the increased strength of the two short Ag-N bonds in Ag(C 2 N 4 H 4 ) 3 NO 3 (ICOHP = −1.58 eV for Ag-N1 and −1.31 eV for Ag-N5), which are significantly stronger than the two short Ag-N bonds in Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O (−1.08 eV for Ag-N2 and −1.14 eV for Ag-N3). Furthermore, we note that the inner Ag-N2 mode in Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O is weaker than the terminal Ag-N3 interaction despite being slightly shorter. It seems, therefore, that the inner N coordination mode is less strongly bonding than equivalent terminal N bonds, thus explaining the preferential formation of Ag(C 2 N 4 H 4 ) 3 NO 3 over Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O and the absence of other metal cyanoguanidines that coordinate with inner N atoms.

Synthesis
Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O was prepared by mixing aqueous solutions of AgNO 3 (339.7 mg, 2 mmol in 20 mL; Alfa, Kandel, Rheinland-Pfalz, Germany) and H 4 C 2 N 4 (504.0 mg, 2 mmol in 20 mL; Alfa, Kandel, Rheinland-Pfalz, Germany). This reaction yielded a large amount of white precipitate that was isolated by filtration and washed with water before being dried under vacuum. The product was then recrystallized by dissolving 0.5 g in 20 mL water and stirring for 1 h. Colorless, transparent needle-like crystals were obtained after water evaporation. It should be noted that attempts to grow high-quality single crystals by evaporation were hindered by the preferential formation of yet another silver cyanoguanidine compound, Ag(C 2 N 4 H 4 ) 3 NO 3 [8], whose crystal structure was clarified many years ago. In order to help understand this preferential formation, electronic structure calculations were performed as detailed below.

Chemical Bonding Analysis
The chemical bonding analysis was performed using the LOBSTER package (Version 3.2.0, Richard Dronskowski, Aachen, Germany) [16][17][18][19], starting from a static self-consistency calculation using the entire k-mesh with the symmetry switched off. For bonding analysis, the "pbevaspfit2015" basis set including 1s orbitals for hydrogen, 2s and 2p orbitals for nitrogen, carbon, and oxygen, and the 5s, 4p, and 4d orbitals for silver was employed.

Conclusions
The first silver(I) cyanoguanidine nitrate in which silver is coordinated by inner imine nitrogen atoms was prepared by mixing aqueous solutions of AgNO 3 and H 4 C 2 N 4 . LOBSTER calculations were used to compare the chemical bonding interactions of Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O and Ag(C 2 N 4 H 4 ) 3 NO 3 such as to explain the limited stability of Ag(C 2 N 4 H 4 )NO 3 · 1 ⁄2H 2 O. ICOHP analysis reveals that the inner N interaction in Ag(C 2 N 4 H 4 ) 3 NO 3 · 1 ⁄2H 2 O is somewhat weaker than equivalent terminal N bonding modes, thereby explaining the propensity for terminal N bonding in metal cyanoguanidines.