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⋮CNEt<sub>2</sub> with the pentatetraenylidene complexes [(CO)<sub>5</sub>MCCCCC(NMe<sub>2</sub>)<sub>2</sub>] (M = Cr (<b>3a</b>), W (<b>3b</b>)) at room temperature affords the new heptapentaenylidene complexes [(CO)<sub>5</sub>MCCCCC(NEt<sub>2</sub>)C(Me)C(NMe<sub>2</sub>)<sub>2</sub>] (M = Cr (<b>5a</b>), W (<b>5b</b>)) in good yield. The spectroscopic data and the X-ray structural analysis of <b>5a</b> indicate that these amino-substituted metallacumulenes are best described as hybrids of a cumulene and several strongly polarized alkynyl structures. For <b>5a</b> and the related metallacumulenes [(CO)<sub>5</sub>MCCC(NMe<sub>2</sub>)<sub>2</sub>] (M = Cr (<b>1a</b>), W (<b>1b</b>)), [(CO)<sub>5</sub>CrCCC(NEt<sub>2</sub>)C(H)C(NMe<sub>2</sub>)<sub>2</sub>] (<b>2a</b>), and <b>3a</b>,<b>b</b> the molecular first hyperpolarizability β was determined by hyper-Rayleigh scattering experiments. The insertion of a <i>cumulated</i> C<sub>2</sub> unit into a metallacumulene quadruples β, while the insertion of a <i>conjugated</i> C<sub>2</sub> 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.