Synthesis, spectral, structural, second-order nonlinear optical properties and theoretical studies on new organometallic donor-acceptor substituted nickel(ii) and copper(ii) unsymmetrical schiff-base complexes

The synthesis, spectroscopic and structural characterization, linear and nonlinear optical properties, as well as the electrochemical behavior of a series of robust neutral binuclear M[Fc-C(O)CH=C(CH₃)N-X-N=CH-(2-O,5- R-C₆H₃)] (M = Ni (4), Cu (5), X = o-C₆H ₄, R = H; M = Ni (9), X = CH₂CH₂, R = OH), and their corresponding ionic trinuclear [M{Fc-C(O)CH=C(CH₃)N-X-N=CH- (η⁶-2-O,5-R-C₆H₃)RuCp*}][PF ₆] (6, 7, 10), M[ONNO]-type unsymmetrical Salophen and salen complexes featuring ferrocenyl (Fc) donor and the mixed sandwich acceptor [Cp*Ru(η⁶-salicylidene)]⁺ as a push-pull moiety are reported in this paper (Fc = CpFe(η⁵-C₅H ₄); Cp = η⁵-C₅H₅; Cp* = η⁵-C₅Me₅). The single-crystal X-ray structure of the bimetallic iron-nickel derivative 4 indicates a bowed structure of the unsymmetrical Schiff base skeleton. The Ni(II) ion is tetracoordinated in a square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The new metalloligand [Fc-C(O)CH=C(CH₃)N-(H)CH ₂CH₂N=CH-(2,5-(OH)₂C₆H₃)] (8) obtained from the Schiff base condensation of 2,5-dihydroxobenzaldehyde with the half-unit precursor, Fc-C(O)CH=C(CH₃)N(H)CH ₂CH₂NH₂ (2), is reported with its crystal structure showing partial derealization of the heteroconjugated [O-C-C-C-N] frameworks with a dihedral angle between the respective planes of 60.76°. Second order nonlinear optical (NLO) measurements were achieved using the Harmonic Light Scattering technique to probe the role of the M[ONNO] chromophores and of the π-complexation of the salicylidene ring in the nonlinearity. All the complexes exhibit a second-order nonlinear response increasing with the nuclearity, the hyperpolarizability (β) value of the trinuclear complex 10 being 1.5 time larger than that of the metalloligand 8 (β = 155 x 10^-30 esu). A rationalization of the structural, electronic, and redox properties of the title compounds is provided, based on a theoretical investigation at the density functional theory (DFT) level. Their UV-visible spectra has been assigned with the help of time-dependent (TD) DFT calculations. They are dominated by LMCT, MLCT, and π - π* transitions.