Intramolecular C-H activation directed self-assembly of an organoplatinum(II) molecular square

A Pt(II) complex that contains a N,N-chelate ligand with the formula of Pt(NPA)(CH₃)₂ (1), NPA = N-(2′-pyridyl)-7-azaindole, has been found to undergo a rapid chelate ring opening and "roll-over" process that leads to intramolecular C-H activation and the subsequent formation of a rare organoplatinum Pt₄ macrocycle, Pt₄(CH₃)₄(N,C,N-NPA)₄ (2), without extra linkers. Complex 2 has been fully characterized by NMR, elemental, and X-ray diffraction analyses. Kinetic study using NMR spectroscopy has established that, in the absence of strong coordinating solvent molecules, the rate-determining step for the conversion process of 1 to 2 is the C-H cleavage step, and 2 is formed exclusively. The presence of strong coordinating solvent molecules, such as CH₃CN, terminates the reaction at the Pt(CH₃)(N,C-NPA)(L) formation step where L = the solvent molecule. The facile roll-over intramolecular C-H activation and the facile and selective formation of the cyclic Pt₄ complex by compound 1 are attributed to the poor stability of the N,N-chelate mode and the geometry of NPA.