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

In this work we report a new Pb-II coordination polymer [Pb3L2(N-3)(4)](n), which was obtained from Pb(N-3)(2) and 1-(pyridin-2-yl)ethylidenepicolinohydrazide (HL). The complex exhibits a 1D polymeric chain, constructed from the {[Pb-3(N-3)(4)](2+)}(n) backbone, decorated with deprotonated organic L- ligands. The latter ligands are pentadentate and link two metal cations. The PbII 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 mu(4)-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(N-3)(4)](n), without considering Pb center dot center dot center dot 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(N-3)(4)](n) is reinforced by intermolecular stacking interactions, formed by Pb-II-based five-membered chelate metallocycles and pyridine rings. Indepth 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 center dot center dot center dot N hydrogen bonds as well as London dispersion forces due to metallocycle units. The diffuse reflectance spectrum of [Pb3L2(N-3)(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.