Syntheses, Structures, and Reductive Elimination Studies of Six-Membered Diaryl Platinacycle Complexes

The six-membered platinacycles PtL2(C6H4XC6H4) (X = CH2, O, NMe; L = PEt3, L2 = 1,3-bis(diphenylphosphino)propane (dppp), 4,4′-bis-tert-butyl-2,2′-bipyridine (tBu2bpy)), have been prepared from cis-PtL2Cl2 and the appropriate dilithio reagents. Reductive elimination studies on the platinacycles with L = PEt3 show that the bridging group (X) dramatically influences the reductive elimination rate. Thermodynamic activation parameters were determined for the platinacycles and showed a ΔH⧧ trend X = NMe ≪ O < CH2 with an essentially zero value for ΔS⧧. Rate constants at 95 °C show over a million-fold increase on going from X = CH2 to X = NMe. DFT calculations support direct elimination without phosphine ligand loss and indicate a progressively earlier transition state in the series X = CH2, O, NMe. The earlier transition state and the accelerated rate are associated with the beginning of aromatization in the eliminating organic unit. Computed thermodynamic activation parameters are in good agreement with the experimental results.