Anion-Induced Assembly of Five-Coordinated Mercury(II) Complexes and Density Functional Theory Calculations to Study Bond Dissociation Energies of Long Hg−N Bonds

A series of new five-coordinated mercury(ΙΙ) coordination complexes (Hg(TpzT)(SCN)2·H2O (1), Hg(TpzT)I2·H2O (2), Hg(TpzT)Br2·H2O (3), 2Hg(TpzT)Cl2·HgCl2·2H2O (4)) have been synthesized by self-assembling the flexible ligand 2,4,6-tri(pyrazole-1-yl)-1,3,5-triazine (TpzT) with HgX2 (X = SCN, I, Br, Cl). Various weak interactions including hydrogen bonds (O−H···X, C−H···X), π−π interactions, and S···S contacts play significant roles in the final topological structures of the four compounds. Unexpectedly, 5 and 6 were obtained accidentally by self-assembling TpzT with MX2 (Zn(NO3)2·6H2O or CdI2) in methanol and were assessed by X-ray crystallography, which indicated that there are nucleophilic substitution reactions. Surprisingly, all the Hg−N bonds of approximately 2.70 Å in length formed by the tridentate ligand and mercuric salt are rather unusual. So density functional theory (DFT) calculations (Amsterdam density functional, ADF) were employed to study the bond dissociation energies (BDE) of Hg−N bonds in 1−4 to assess the nature of the bonds. The calculation reveals the strong coordination nature of Hg−N bonds in 1−4 compared to that of the same coordination mode of compound A ((4′-(4-[4-(imidazole)phenylethylene]phenyl)-2,2′:6′,2′′-terpyridine)HgBr2·CHCl3) with the normal range Hg−N bond lengths. And a similar trend is that the larger the anion and BDE become, the steadier the coordination complexes are.