Tetrel Bonding and Other Non-Covalent Interactions Assisted Supramolecular Aggregation in a New Pb(II) Complex of an Isonicotinohydrazide

Abstract

A new supramolecular Pb(II) complex [PbL(NO₂)]_n was synthesized from Pb(NO₃)₂, N'-(1-(pyridin-2-yl)ethylidene)isonicotinohydrazide (HL) and NaNO₂. [PbL(NO₂)]_n is constructed from discrete [PbL(NO₂)] units with an almost ideal N₂O₃ square pyramidal coordination environment around Pb(II). The ligand L^- is coordinated through the 2-pyridyl N-atom, one aza N-atom, and the carbonyl O-atom. The nitrite ligand binds in a κ²-O,O coordination mode through both O-atoms. The Pb(II) center exhibits a hemidirected coordination geometry with a pronounced coordination gap, which allows a close approach of two additional N-atoms arising from the N=C(O) N-atom of an adjacent molecule and from the 4-pyridyl N-atom from the another adjacent molecule, yielding a N₄O₃ coordination, constructed from two Pb-N and three Pb-O covalent bonds, and two Pb⋯N tetrel bonds. Dimeric units in the structure of [PbL(NO₂)]_n are formed by the Pb⋯N=C(O) tetrel bonds and intermolecular electrostatically enforced π⁺⋯π^- stacking interactions between the 2- and 4-pyridyl rings and further stabilized by C-H⋯π intermolecular interactions, formed by one of the methyl H-atoms and the 4-pyridyl ring. These dimers are embedded in a 2D network representing a simplified uninodal 3-connected fes (Shubnikov plane net) topology defined by the point symbol (4∙8²). The Hirshfeld surface analysis of [PbL(NO₂)] revealed that the intermolecular H⋯X (X = H, C, N, O) contacts occupy an overwhelming majority of the molecular surface of the [PbL(NO₂)] coordination unit. Furthermore, the structure is characterized by intermolecular C⋯C and C⋯N interactions, corresponding to the intermolecular π⋯π stacking interactions. Notably, intermolecular Pb⋯N and, most interestingly, Pb⋯H interactions are remarkable contributors to the molecular surface of [PbL(NO₂)]. While the former contacts are due to the Pb⋯N tetrel bonds, the latter contacts are mainly due to the interaction with the methyl H-atoms in the π⋯π stacked [PbL(NO₂)] molecules. Molecular electrostatic potential (MEP) surface calculations showed marked electrostatic contributions to both the Pb⋯N tetrel bonds and the dimer forming π⁺⋯π^- stacking interactions. Quantum theory of atoms in molecules (QTAIM) analyses underlined the tetrel bonding character of the Pb⋯N interactions. The manifold non-covalent interactions found in this supramolecular assembly are the result of the proper combination of the polyfunctional multidentate pyridine-hydrazide ligand and the small nitrito auxiliary ligand.

Keywords: DFT calculations; Hirshfeld surface analysis; crystal structure; isonicotinohydrazide; lead(II); non-covalent interaction; tetrel bond.

Chart 1

Simplified diagram for holodirected and hemidirected coordination spheres around Pb(II).

Scheme 1

Syntheses of [PbL(NO₂)]_n (this work) and [PbL’(NO₂)]_n [37] (tetrel bonds shown as dashed lines).

Figure 1

Molecular structure of the [PbL(NO₂)] unit in the structure of [PbL(NO₂)]_n (ellipsoids are drawn at 50% probability level). Color code: H = black, C = gold, N = blue, O = red, and Pb = magenta.

Figure 2

View on a 1D supramolecular chain in the structure of [PbL(NO₂)]_n, formed through Pb⋯N=C(O) tetrel bonds, π⋯π stacking, and C–H⋯π interactions (ellipsoids are drawn at 50% probability level). Color code: H = black, C = gold, N = blue, O = red, and Pb = magenta; cyan dashed line = Pb⋯N tetrel bond, yellow dashed line = π⋯π stacking interaction, and gray dashed line = C–H⋯π interaction.

Figure 3

(a) View on the 2D supramolecular layer in the structure of [PbL(NO₂)]_n, formed through Pb⋯N=C(O) and Pb⋯N_4-Py tetrel bonds (H-atoms were omitted for clarity). Color code: C = gold, N = blue, O = red, and Pb = magenta; cyan dashed line = Pb⋯N tetrel bond. (b) Simplified underlying network of [PbL(NO₂)]_n, considering all Pb⋯N tetrel bonds, with the uninodal 3-connected fes (Shubnikov plane net) topology defined by the point symbol of (4∙8²). Color code: Pb = magenta, and L = blue.

Figure 4

Two-dimensional and decomposed 2D fingerprint plots of observed contacts for [PbL(NO₂)]_n. The d_i value is the distance from the surface to the nearest atom interior to the surface, and d_e is the distance from the surface to the nearest atom exterior to the surface. Color code: blue = a fraction of the surface points of the corresponding contact.

Figure 5

Two views of the molecular electrostatic potential (MEP) surface of [PbL(NO₂)] (isosurface 0.001 a.u.). The blue color is used for positive MEP values, and the red color for negative values, as indicated in the legend.

Figure 6

Quantum theory of atoms in molecules (QTAIM) analysis of (a) the antiparallel stacking dimer in [PbL(NO₂)]_n as a combination of the Pb⋯N=C(O) tetrel bonds and π⁺⋯π⁻ interactions and (b) of two basic coordination units in [PbL(NO₂)]_n forming a Pb⋯N_4-Py tetrel bond and C–H⋯O bridging interaction. Bond, ring, and cage critical points are represented by red, yellow, and blue spheres, respectively. Bond paths connecting bond critical points are represented by dashed lines. ΔE = accumulated dimerization energies. ρ(r) = densities at the bond critical points.