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-C6H4, 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*}][PF6] (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-C5H4); 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)CH2CH2N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)CH2CH2NH2 (2), is reported with its crystal structure showing partial delocalization 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 × 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.