Nickel(II)-Based Building Blocks with Schiff Base Derivatives: Experimental Insights and DFT Calculations

We have recently reported a series of neutral square planar tridentate Schiff base (L) complexes of the general formula [(L)M(py)], showing relatively high first-order hyperpolarizabilities and NLO redox switching behavior. In the present study, new members of this family of compounds have been prepared with the objective to investigate their potential as building blocks in the on-demand construction of D-pi-A push-pull systems. Namely, ternary nickel(II) building blocks of general formula [(L-A/D)Ni(4-pyX)] (4-7), where L-A/D stands for an electron accepting or donating dianionic O,N,O-tridentate Schiff base ligand resulting from the monocondensation of 2-aminophenol or its 4-substituted nitro derivative and beta-diketones R-C(=O)CH2C(=O)CH3 (R = methyl, anisyl, ferrocenyl), and 4-pyX is 4-iodopyridine or 4-ethynylpyridine, were synthesized and isolated in 60-78% yields. Unexpectedly, the Sonogashira cross-coupling reaction between the 4-iodopyridine derivative 6 and 4-ethynylpyridine led to the formation of the bis(4-pyridyl) acetylene bridged centrosymmetric dimer [{(L-D)Ni}(2)(mu(2)-py-C equivalent to C-py)] (8). Complexes 4-8 were characterized by elemental analysis, FT-IR and NMR spectroscopy, single crystal X-ray diffraction and computational methods. In each compound, the four-coordinate Ni(II) metal ion adopts a square planar geometry with two nitrogen and two oxygen atoms as donors occupying trans positions. In 8, the Ni horizontal ellipsis Ni separation is of 13.62(14) angstrom. Experimental results were proved and explained theoretically exploiting Density Functional Theory calculations.

Automated summary

A series of new neutral square planar tridentate Schiff base complexes were synthesized and investigated as potential building blocks in D-pi-A push-pull systems. The compounds showed high first-order hyperpolarizabilities and NLO redox switching behavior, and unexpected bis(4-pyridyl) acetylene bridged centrosymmetric dimer was formed through Sonogashira cross-coupling reaction. The experimental results were supported and explained theoretically through Density Functional Theory calculations.