Mechanistic Information on the Reductive Elimination from Cationic Trimethylplatinum(IV) Complexes to Form Carbon−Carbon Bonds

• Published in: Inorganic chemistry 2005-10, Vol.44 (22), p.7732-7742
• Main Authors: Procelewska, Joanna, Zahl, Achim, Liehr, Günter, van Eldik, Rudi, Smythe, Nicole A, Williams, B. Scott, Goldberg, Karen I
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic050478+

Cationic complexes of the type fac-[(L2)PtIVMe3(pyr-X)][OTf] (pyr-X = 4-substituted pyridines; L2 = diphosphine, viz., dppe = bis(diphenylphosphino)ethane and dppbz = o-bis(diphenylphosphino)benzene; OTf = trifluoromethanesulfonate) undergo C−C reductive elimination reactions to form [L2PtIIMe(pyr-X)][OTf] and ethane. Detailed studies indicate that these reactions proceed by a two-step pathway, viz., initial reversible dissociation of the pyridine ligand from the cationic complex to generate a five-coordinate PtIV intermediate, followed by irreversible concerted C−C bond formation. The reaction is inhibited by pyridine. The highly positive values for ΔS ⧧ obs = +180 ± 30 J K-1 mol-1, ΔH ⧧ obs = 160 ± 10 kJ mol-1, and ΔV ⧧ obs = +16 ± 1 cm3 mol-1 can be accounted for in terms of significant bond cleavage and/or partial reduction from PtIV to PtII in going from the ground to the transition state. These cationic complexes have provided the first opportunity to carry out detailed studies of C−C reductive elimination from cationic PtIV complexes in a variety of solvents. The absence of a significant solvent effect for this reaction provides strong evidence that the C−C reductive coupling occurs from an unsaturated five-coordinate PtIV intermediate rather than from a six-coordinate PtIV solvento species.