Mechanistic studies on mutual isomerization of propargyl- and allenylplatinum(II) complexes

When heated in benzene, phenyl-substituted propargylplatinum(II) complexes, trans-Pt(CH2C drop CPh) (X) (PPh3)(2) (2) (X=Cl, Br, I), isomerized gradually to the more stable allenyl isomers, trans-Pt(CPh=C=CH2) (X) (PPh3)(2) (3), to give rise to an equilibrium mixture of 2/3 (5/95), from which 3 (X = Cl) was isolated as an isomerically pure sample. The allenyl complex 3 also gave rise to the same equilibrium mixture of 2/3 when heated under the same conditions. The rate of propargyl-to allenylplatinum(II) isomerization was examined as the function of the ligand X in 2. The isomerization rate was first-order in the concentration of the propargyl complex, not affected by adding PPh3, and increased in the order X = Cl < Br < I. The organo(propargyl)platinum(II) complex, cis-Pt(CH2C drop CPh) (C drop CPh) (PPh3)(2), isomerized even faster to the corresponding allenyl isomer than 2. These kinetic aspects led us to suggest that the spontaneous propargyl-to allenylplatinum isomerization proceeds via intramolecular rearrangement of an 18-electron eta(3)-propargyl/allenyl intermediate, Pt(eta(3)-CH2C drop CPh) (X) (PPh3)(2). The isomerization from 2 to 3 was accelerated considerably by addition of Pt(PPh3)(2)L-n (L=PPh3, n=2; L=C2H4, n=1), but not at all by addition of PtCl2(PPh3)(2). (C) 1997 Elsevier Science S.A.