Synthesis, characterization and crystal structures of some tungsten and triosmium carbonyl complexes with monopyridyl and dipyridyl azo ligands

Substitution reactions of [W(CO)₅(THF)] with the azo-functionalized monopyridyl and dipyridyl ligands, namely, 4-phenylazopyridine L₁ and 4,4′-azopyridine L₂, readily afford two tungsten carbonyl complexes [W(CO)₅(NC₅H₄-N=N-C₆H₅)] (1) and [(CO)₅W(μ-NC₅H₄-N=N-C₅H ₄N)W(CO)₅] (2) in good yields. Ligation of L₁ and L₂ with the activated triosmium carbonyl cluster [Os₃(CO)₁₀(NCMe)₂] by ortho-metallation reaction provides [Os₃(μ-H)(CO)₁₀(μ-NC₅H ₃-N=N-C₆H₅)] (3) and the linking cluster [Os₃(μ-H)(CO)₁₀(μ-NC₅H ₃-N=N-C₅H₃N)Os₃(μ-H)(CO) ₁₀] (4) in satisfactory yields. All of the complexes have been fully characterized by IR, ¹H-NMR, UV-vis spectroscopies, fast atom bombardment (FAB) mass spectrometry and electrochemical measurements. The solid-state molecular structures of 1-4 have been ascertained by single-crystal X-ray diffraction methods. The structures of 1 and 3 involve the coordination of L₁ to one {W(CO)₅} and {Os₃(CO)₁₀} moieties, respectively. In the case of 2 and 4, two identical metal cores are linked together by the bridging ligand L₂. Spectroscopic investigations revealed that the metal-to-ligand charge transfer (MLCT) transitions of 1 and 2 demonstrate strong solvent dependency, displaying a large negative solvatochromism in a wide range of organic solvents.