Lead(II) Thiocyanate Complexes with Bibracchial Lariat Ethers: An X-ray and DFT Study

Compounds of formula [Pb(L2)(NCS)2] (1) and [Pb(L4)(SCN)2] (2) (where L2 is the lariat crown ether N,N‘-bis(3-aminobenzyl)-4,13-diaza-18-crown-6 and L4 is the Schiff-base lariat crown ether N,N‘-bis(3-(salicylaldimino)benzyl)-4,13-diaza-18-crown-6) were isolated and structurally characterized by X-ray diffraction analyses. The X-ray crystal
structures of both compounds show the metal ion coordinated to the six donor atoms of the crown moiety, leaving
the corresponding pendant arms uncoordinated. The coordination sphere of lead(II) is completed by two thiocyanate
groups that coordinate either through their nitrogen (1) or sulfur (2) atoms. The organic receptor adopts a syn
conformation in 1, while in 2 it shows an anti conformation. To rationalize these unexpected different conformations
of the L2 and L4 receptors in compounds 1 and 2, as well as the different binding modes found for the thiocyanate
ligands, we have carried out theoretical calculations at the DFT (B3LYP) level. These calculations predict the syn
conformation being the most stable in both 1 and 2 complexes. So, the anti conformation found for 2 in the solid
state is tentatively attributed to the presence of intermolecular π−π interactions between phenol rings, for which
the dihedral angle between the least-squares planes of both rings amounts to 2.6° and the distance between the
center of both rings is 3.766 Å. On the other hand, the analysis of the electronic structure has revealed that the
Pb−ligand bonds present highly ionic character in this family of compounds. They also suggest a greater transfer
of electron density from the NCS- ligands when they coordinate through the less electronegative S atom. The
Pb−SCN covalent bond formation mainly occurs due to an effective overlap of the occupied 3pz orbitals of the S
atoms and the unoccupied 6pz AO of the Pb atom, while the Pb−NCS bonding interaction is primarily due to the
overlap of the 6s and 7s AO of Pb with sp1.10 hybrids of the N donor atoms. Our electronic structure calculations
can rationalize the different coordination of the thiocyanate groups in compounds 1 and 2:  the simultaneous formation
of two Pb−SCN bonds is more favorable for S−Pb−S angles close to 180°, for which the overlap between the
occupied 3pz orbitals of the S atoms and the unoccupied 6pz AO of the Pb atom is maximized.