Competitive Aryl-Iodide vs Aryl-Aryl Reductive Elimination Reactions in Pt(IV) Complexes: Experimental and Theoretical Studies

The platinum(IV) complex trans-(dmpe)Pt^IV(Ar)₂I ₂ (2, dmpe = 1,2-dimethylphosphinoethane, Ar = 4-FC₆H ₄) rapidly reacts, upon moderate heating in solution under ambient light, via two distinct pathways: isomerization to the corresponding cis-isomer (3) and Ar-I reductive elimination to give (dmpe)-Pt^II(Ar)I (4). Complex 3 undergoes, upon prolonged heating at high temperatures, an exclusive Ar-Ar reductive elimination reaction to give (dmpe)Pt^III₂. Experimental and DFT studies showed that the 2-to-3 isomerization proceeds via three pathways: photochemical or thermal phosphine chelate opening and a mechanism involving cleavage of the Pt-I bond. The isomerization reaction is significantly slowed down but not stopped in the absence of light or in the presence of an excess of tetra-n-butylammonium iodide. On the other hand, the Ar-I reductive elimination from 2 proceeds via the Pt^δ+-I ^δ_ ion pairlike transition state. Use of the rigid dmpe analogue 1,2-dimethylphosphinobenzene (dmpbz) as the ligand shuts down the chelate ring-opening isomerization pathway and enables faster Ar-I reductive elimination thus making the latter reaction the major reaction route for the dmpbz supported trans-diiodo Pt(IV) complex 8.