Kinetic Formation of Pt-Pt Dimers of Cationic and Neutral Platinum(II) Complexes under Rapid Freeze Conditions in Solution.

We report the formation of M-M dimers (M = Pt or Pd) of cationic [M(dpb)(CHCN)] [dpbH = 1,3-di(2-pyridyl)benzene] and neutral [M(dpb)Cl] complexes resulting from the rapid freezing of solutions. Dimers based on M-M dz overlap were found to preferentially form rather than the thermodynamically favored head-to-tail π-stacking structures typically observed in the crystalline state. Kinetic dimers in glassy frozen solutions generated broad metal-metal-to-ligand charge-transfer emissions within the range of 600-800 nm at 77 K. These emissions were red-shifted relative to monomer emissions. As expected, the degree of aggregation of these complexes was affected by the concentration in each solution. Photoexcitation evidently accelerated Pt-Pt dimerization even at ambient temperature. Electrostatic attraction between [Pt(dpb)Cl] and [Pt(dpb)Cl] ions resulting from disproportionation due to photoinduced electron transfer is thought to have driven excimer formation. [Pt(dpb)(CHCN)]OTf (OTf = trifluoromethanesulfonate ion) and its Pd(II) analogue were determined to have isostructural crystals, but a Pd-Pd stacked polymorph was not observed and the photophysics of the two complexes are evidently different.