Metal–coumarin complexes: synthesis and characterization of 7-isocyanocoumarin ligands and Mo(CO)4(7-isocyanocoumarin)2 complexes. X-ray crystal structure of Mo(CO)4(7-isocyano-4-trifluoromethylcoumarin)2

The preparation and characterization of four different 7-isocyanocoumarin ligands is described. The four ligands are 3,4-dimethyl-7-isocyanocoumarin (Dmic), 7-isocyano-4-methylcoumarin (Mic), 3-chloro-7-isocyano-4-methylcoumarin (Cmic) and 7-isocyano-4-trifluoromethylcoumarin (Tic). Reaction of the four 7-isocyanocoumarin ligands with Mo(CO)4(pip)2 (pip=piperidine) gave the Mo(CO)4L2 complexes. IR and UV–vis spectra of the complexes indicate that the 7-isocyanocoumarin ligands are significantly stronger π-acids than simple aromatic isocyanide ligands. An intense visible absorption band, assigned to a metal-to-ligand charge transfer (MLCT) transition, is present in the UV–vis spectra of the four Mo(CO)4L2 complexes. Excitation into the MLCT band of the Mic, Cmic, and Dmic complexes gave yellow–orange emission at room temperature in methylene chloride solution. Photolysis of Mo(CO)4(Mic)2 with P(Ph)3 in THF solution produced Mo(CO)3(PPh3)(Mic)2.