Synthesis, crystal structures, and nonlinear optical (NLO) properties of new Schiff-base nickel(II) complexes.: Toward a new type of molecular switch?

An H2L Schiff-base ligand that was obtained from the monocondensation of diaminomaleonitrile and 4-(diethylamino)salicylaldehyde is reported together with four related nickel(II) complexes formulated as [Ni(L)(L')] (L' = MePhCHNH2, (PrNH2)-Pr-i, Py, and PPh3). Crystal structures have been solved for H2L, [Ni(L)(MePhCHNH2)1, and [Ni(L)((PrNH2)-Pr-i)]. Surprisingly, the complexation process leads to the formation of a rather unusual nickel amido (-NH-Ni-II) bond by deprotonation of the primary amine of H2L. A reduction of the quadratic hyperpolarizability (beta) from 38 x 10(-30) to 17.5 x 10-30 cm(5) esu(-1) is evidenced on H2L upon metal complexation by the electric-field-induced second-harmonic (EFISH) technique. Qualitative ZINDO/SCI quantum chemical calculations indicate that, in [Ni(L)(MePhCHNH2)], the beta orientation strongly depends on the laser wavelength. In particular, a P rotation strictly equal to 900 could be obtained with 1.022 mu m incident light on passing from [Ni(L)(MePhCHNH2] to a hypothetical [Ni(HL)(MePhCHNH2](+) protonated complex, thus raising the possibility for a new type of molecular switch.