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Abstract
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We report on the reaction of Pb(NO3)2 or PbI2 or Pb(SCN)2 with an equimolar amount of the dipyridyl compound N′-(pyridin-2-ylmethylene)isonicotinohydrazide (HL) in methanol or a mixture of methanol and acetonitrile, which yielded novel heteroleptic coordination polymers [Pb(HL)X2]n (X = NO3−, 1; I−, 2) and [PbL(SCN)(H2O)]n (3). Furthermore, it was established that using an equimolar mixture of Pb(ClO4)2 and KSCN as a source of the metal cation also leads to the formation of complex 3. The structures of the resulting complexes 1 and 2 each comprise a neutral ligand HL, while complex 3 contains a monodeprotonated anionic form L of the parent organic ligand. Both forms, HL and L, exhibit an N,N′,O-chelated coordination mode formed by the 2-pyridine and imine nitrogen atoms, and the carbonyl oxygen atom. This resulted in mononuclear doubly charged [Pb(HL)]2+ (in complexes 1 and 2) or monocharged [PbL]+ (in complex 3) cations as the primary building blocks of the resulting coordination polymers. The latter structures are formed primarily due to the additional coordination of the metal cation to the 4-pyridine nitrogen atom arising from the other building block, thus, yielding 1D supramolecular chains of a zig-zag-like structure. These chains are linked into 3D supramolecular aggregates due to the formation of the tetrel bonds with the center-of-gravity of the nitrate O–N–O and the thiocyanate C≡N fragments in the structures of complexes 1 and 3, respectively. In complex 2, one of the iodide anions links two metal cations, thus, exhibiting a μ-bridging coordination mode. This allowed for the production of a 2D double-layered sheet in the structure of complex 2. 3D supramolecular aggregates in the crystal structures of 1–3 are stabilized by π⋯π interactions formed between the pyridine rings, and by the corresponding hydrogen bonds formed due to the NH functionality (in complexes 1 and 2) or due to the water ligand (in complex 3). Complexes 1–3 were established to be emissive in a solution of DMSO due to intraligand transitions. In addition, density functional theory (DFT) calculations were performed to elucidate the nature of key noncovalent interactions, namely hydrogen bonds, tetrel bonds and π-stacking interactions, within the supramolecular assemblies observed in the crystal structures of complexes 1 and 3. Using molecular electrostatic potential (MEP) surfaces, QTAIM and NCIplot analyses, we characterized the role of the Pb2+ center as an electrophilic σ-hole donor. Moreover, combined ELF and RDG calculations were used to distinguish between covalent and ionic Pb⋯I interactions in complex 2.
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