Tetrel bonding stabilization of a new coordination polymer constructed from lead(ii) azide and 1-(pyridin-2-yl)ethylidenepicolinohydrazide

Abstract

In this work we report a new Pb^II coordination polymer [Pb₃L₂(N₃)₄]_n, which was obtained from Pb(N₃)₂ and 1-(pyridin-2-yl)ethylidenepicolinohydrazide (HL). The complex exhibits a 1D polymeric chain, constructed from the {[Pb₃(N₃)₄]²⁺}_n backbone, decorated with deprotonated organic L⁻ ligands. The latter ligands are pentadentate and link two metal cations. The Pb^II ions were found to be in a N₅O or N₅O₂ coordination environment, formed by covalent bonds. The azide ligands either bind four metals in a μ⁴-1,1,3,3 coordination mode or are coordinated to metal centers as terminal ligands. All the metal centers exhibit a hemidirected coordination geometry with a pronounced coordination gap, which allows the close approach of an additional nitrogen atom arising from an adjacent terminal azide ligand. A simplified underlying network of [Pb₃L₂(N₃)₄]_n, without considering Pb⋯N tetrel bonds, exhibits the tetranodal 3,4,4,4-connected chain topology, while considering all Pb⋯N tetrel bonds, the binodal 4,5-connected chain topology was shown. The crystal packing of [Pb₃L₂(N₃)₄]_n is reinforced by intermolecular π⋯π stacking interactions, formed by Pb^II-based five-membered chelate metallocycles and pyridine rings. In-depth bonding analysis using ETS-NOCV, IQA and NCI methods demonstrates that although the azide anions experience significant steric crowding due to repulsive azide–azide interactions, they are engaged in strong attractive, Coulomb dominated, dative-covalent Pb–N and tetrel-type Pb⋯N connections, which are further augmented by weak exchange–correlation driven C–H⋯N hydrogen bonds as well as London dispersion forces due to metallocycle units. The diffuse reflectance spectrum of [Pb₃L₂(N₃)₄]_n revealed a broad band with two maxima in the UV region, corresponding to intraligand transitions, and an intense shoulder in the visible region, corresponding to metal-to-ligand charge-transfer. The latter shoulder explains the observed yellow colour of the complex.