Chemistry of Tetraosmium Carbonyl Clusters with Phenylazopyridine Ligands: Synthesis, Structure and Electrochemistry

Two new tetraosmium carbonyl clusters Os₄(-H)₄(CO)₁₁{ ¹-NC₅H₄(N=N)C₆H₅}] (1) and Os₄(-H)₄(CO)₁₀{ ²-NC₅H₄(N=N)C₆H₅}] (2) were synthesized from the reaction of Os₄(-H)₄(CO)₁₂] with two equivalent of 2-phenylazopyridine ligand in dichloromethane at ambient temperature using trimethylamine-N-oxide as the decarbonylation reagent. Subsequent chromatographic purification led to the isolation of 1and 2as stable orange and blue solids in respectively 25 and 13% yields. Complex 1converted to 2in a 25% yield in refluxing chloroform. Treating a solution of Os₄(-H)₄(CO)₁₂] in dichloromethane with two equivalent of 3-phenylazopyridine led to the formation of another two new clusters Os₄(-H)₄(CO)₁₁{ ¹-NC₅H₄(N=N)C₆H₅}] (3) and Os₄(-H)₄(CO)₁₀(NMe₃){ ¹-NC₅H₄(N=N)C₆H₅}] (4), which could be isolated as yellow solids in 34% and 21% yields respectively. Thermolysis of 3or 4in refluxing n-hexane gives Os₄(-H)₃(CO)₁₀{- ³-NC₅H₃(N=N)C₆H₅}] (5) in 24 and 33% yields respectively. Their electronic absorption properties and electrochemical behavior are also reported.