Synthesis, crystal structures and physical properties of carbodiimides and related materials
Qiao, Xianji; Dronskowski, Richard (Thesis advisor); Englert, Ullrich (Thesis advisor)
Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2021
This thesis describes the synthesis and structures of several metal cyanamide/carbodiimide compounds and related materials. It then goes on to describe promising electrochemical, photochemical and nonlinear optical properties of this emerging family. As regards to synthesis, a range of different synthetic techniques are employed such as solid-state metathesis (SSM) reactions, aqueous synthetic routes and non-aqueous liquid techniques. SSM reactions are well-suited in the preparation of metal carbodiimide or cyanamide compounds, as is demonstrated in the CdNCN, PbNCN and Li2MgSn2(NCN)6 cases. These reactions involve metal halides and alkali-metal carbodiimide sources (A2(NCN), A = Li, Na) to stimulate an exothermic double exchange of ions under relatively mild conditions. In some cases, however, even the relatively mild conditions typical for SSM reactions are too harsh for the preparation of the target compounds. In the present cases, we prepare the Ag(C2N4H4)NO3·½ H2O, Hg3(NCN)2Cl2, Hg2(C(NH2)3)Cl5 compounds through aqueous synthetic routes. However, in the case of Bi2(NCN)3 and Bi2(NCN)3·NH3, the air/moisture sensitive nature of the reaction products precludes such routes necessitating non-aqueous liquid-state low temperature ammonolysis reactions. Metal carbodiimides or cyanamides often adopt [NaCl] and [NiAs] derived motifs. However, metal elements with larger cation sizes or those with specific coordination preferences yield alternate topologies that are often unique to solid-state metal cyanamide or carbodiimide chemistry. The crystal structure of CdNCN is redetermined to adopt [NaCl]-type motifs with R¯3m symmetry. Bi2(NCN)3 and Bi2(NCN)3·NH3 adopt low-symmetry monoclinic modifications of the [corundum]-like R¯3c M2(NCN)3 architecture (vacancy ordered [NiAs] derived motifs) due to the presence of the aspherical Bi3+ cation. The crystal structure of Li2MgSn2(NCN)6 is also related to those of the binary [NiAs]-type MNCN phases by a lattice expansion in the basal plane with P¯31m symmetry with ⅙ of the octahedral metal sites vacant. However, the PbNCN crystal structure is not derived from a traditional [NaCl] and [NiAs] motif but instead features alternate double layers of NCN2− anions and Pb2+ cations. In addition, an interesting structural nuance of this family is seen in the shape of the NCN2− moiety which assumes either the symmetric carbodiimide –N=C=N– shape with two C=N double bonds (1.22 Å) or lower symmetry cyanamide N≡C–N2– shape with single ( 1.30 Å) and triple bond character ( 1.16 Å). The symmetric carbodiimide –N=C=N– shape was clearly reflected in the IR spectra of the CdNCN, Bi2(NCN)3 and Bi2(NCN)3·NH3 cases due to the relative hard nature of metal cations. By contrast, the cyanamide N≡C–N2– character was confirmed through IR in the PbNCN, Hg3(NCN)2Cl2 cases due to the softer Pb2+ and Hg2+ cations and in the Li2MgSn2(NCN)6 case due to the distinct environments of the terminal nitrogen atoms. In addition, we found several mixed-anion compounds Hg3(NCN)2Cl2, Hg2(C(NH2)3)Cl5 and Ag(C2N4H4)NO3·½ H2O, whose crystal structures exhibit significantly different character with metal carbodiimides or cyanamides, with the later two examples held together through H-bonding interactions. Furthermore, in this work, the electrochemical, photochemical and nonlinear optical properties have been investigated. We find that Ag(C2N4H4)NO3·½H2O adopts a crystal structure with an essentially planar configuration of H4C2N4 molecules and nitrate anions. Additionally, we find a parallel arrangement of the NCN2− group in the crystal structure of Hg3(NCN)2Cl2 as well as opposite polar orientations of C(NH2)3+ group in Hg2(C(NH2)3)Cl5 crystal structure. As such, we investigated the NLO properties of these materials since such structural characters are known to be good hosts for nonlinear optical effects. The nonlinear optical properties were calculated by DFT in Ag(C2N4H4)NO3·½ H2O, Hg3(NCN)2Cl2 and Hg2(C(NH2)3)Cl5. The results show that Ag(C2N4H4)NO3·½ H2O has a relatively large NLO effect with a calculated SHG coefficient d14 of 0.455 pm/V which is about 1.2 times that of KH2PO4 (KDP). DFT calculations indicate that Hg3(NCN)2Cl2 has good NLO potential with a large SHG coefficient (d33 = ‒6.78 pm/V) and birefringence values (0.446@532nm). The second-harmonic generation measurements were performed on powder Hg3(NCN)2Cl2 samples, suggesting the intensity of Hg3(NCN)2Cl2 is two times larger than quartz. The disagreement between observation and theory may be caused by the different grain size. For Hg2(C(NH2)3)Cl5, both theory and experiment show that the SHG coefficient is quite small, possibly induced by the almost antiparallel directions of the molecular dipole moments. In the case of PbNCN, Pb is coordinated by five short N atoms and two longer N atoms. This is very similar to the four-coordinate Pb environment in the PbO litharge structure. We can infer from this the presence of active Pb 6s2 lone pairs, which may possibly imbue PbNCN with hole conduction as in PbO. As such, we explored its photochemical properties. Mott-Schottky experiments indicate that PbNCN is a p-type semiconductor. The photoelectrochemical measurements developed cathodic photocurrent for water reduction. Its stability has been confirmed by PXRD and IR. The LSV measurements were carried out in different scan directions, reconfirming the p-type nature of PbNCN as inferred from MS plot. These findings motivated us to further explore metal cyanamides as new functional materials for solar energy applications.