Modulation of coordination in pincer-type isonicotinohydrazone Schiff base ligands by proton transfer

Mahmoudi, Ghodrat and Khandar, Ali Akbar and Afkhami, Farhad Akbari and Miroslaw, Barbara and Gurbanov, Atash V. and Zubkov, Fedor I. and Kennedy, Alan and Franconetti, Antonio and Frontera, Antonio (2018) Modulation of coordination in pincer-type isonicotinohydrazone Schiff base ligands by proton transfer. CrystEngComm. ISSN 1466-8033

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    Abstract

    We present here two different coordination polyhedra of pincer type N2O hydrazone based ligands supplemented with thiocyanate ions. The compounds namely [Hg(SCN)2(HL1)] (1) and [Hg(SCN)2(HL2)] (2) have a common isonicotinohydrazone fragment and have been prepared by using a coordination driven self-assembly of the Hg(SCN)2 with two different ligands including 2-benzoylpyridine-isonicotinoylhydrazone (HL1), and 2-acetylpyridineisonicotinoylhydrazone (HL2). In compound 1 the ligand coordinated to the mercury center in the keto form (=N–NH=C=O) whereas, in compound 2, the proton at the hydrazine group have been shifted to the uncoordinated pyridine ring and the ligand acted as a zwitterion. The structures provide a complementary system for proton transfer within the ligand molecule involving the keto-enol tautomerization of amide group and 4-pyridyl N protonation. As a result, the relative location of orbitals and ligands in the complexes are different as well as the bonding strength and the coordination polyhedra. We have also studied electrostatically enhanced π···π (either conventional or involving the chelate ring) interactions observed in the solid state of both compounds and analyzed them using DFT calculations, Molecular Electrostatic Potential surface and the Bader’s theory of atoms in molecules.