The Palladium-Catalyzed Carbonylation of Nitrobenzene into Phenyl Isocyanate:  The Structural Characterization of a Metallacylic Intermediate

The synthesis and characterization of a family of metallacyclic complexes of palladium, that reported where N-N is a diimine ligand and Ar‘ is a substituted aryl. The complexes were isolated from solutions during the catalytic carbonylation of nitroaromatics using palladium complexes and were proposed to be intermediates in the catalytic process. The X-ray structure determination = 4,4‘-bis- tert -butyl-2,2‘-bipyridyl) was carried out, the complex crystallizing in the triclinic space group P 1̄ (C 30 H 37 N 3 O 3 Pd) 2 ·C 3 H 6 O, Z = 2, a = 15.626(4) Å, b = 18.269(6) Å, c = 12.067(3) Å, α = 107.34(2)°, β = 111.53(2)°, γ = 78.81(2)°. The crystal structure contains two molecules in the asymmetric unit which are assembled in a quasi-dimeric form via stacking interactions. The previously assigned metallacyclic structure for these complexes was confirmed. A mechanistic pathway for the carbonylation process implicating a series of metallacycle intermediates is proposed without intervention of a metal−imido intermediate. Thermal decomposition studies of the metallacycle and some trapping experiments are also presented.