Synthesis and Nonlinear Optical Properties of New Heptapentaenylidene Complexes:  Study on the Second Harmonic Generation Efficiencies of Amino-Substituted Group 6 Cumulenylidenes

Reaction of the ynamine MeC⋮CNEt2 with the pentatetraenylidene complexes [(CO)5MCCCCC(NMe2)2] (M = Cr (3a), W (3b)) at room temperature affords the new heptapentaenylidene complexes [(CO)5MCCCCC(NEt2)C(Me)C(NMe2)2] (M = Cr (5a), W (5b)) in good yield. The spectroscopic data and the X-ray structural analysis of 5a indicate that these amino-substituted metallacumulenes are best described as hybrids of a cumulene and several strongly polarized alkynyl structures. For 5a and the related metallacumulenes [(CO)5MCCC(NMe2)2] (M = Cr (1a), W (1b)), [(CO)5CrCCC(NEt2)C(H)C(NMe2)2] (2a), and 3a,b the molecular first hyperpolarizability β was determined by hyper-Rayleigh scattering experiments. The insertion of a cumulated C2 unit into a metallacumulene quadruples β, while the insertion of a conjugated C2 unit results in only slight enhancement of the second harmonic generation response. Substitution of tungsten for chromium has only a minor effect on β. All observed β values are reasonably high compared with those of other organometallic compounds.