Structural and mechanistic look at the orthoplatination of aryl oximes by dichlorobis(sulfoxide or sulfide)platinum(II) complexes

Structural and mechanistic aspects of orthoplatination of acetophenone and benzaldehyde oximes by the platinum-(II) sulfoxide and sulfide complexes [PtCl2L2] (2, L = SOMe2 (a), rac-SOMePh (b), R-SOMe(C6H4Me-4) (c), and SMe2 (d)) to afford the corresponding platinacycles cis-(C,S)-[PtII(C6H3-2-CR ′=NOH-5-R)Cl(L)] (3, R, R′ = H, Me) have been investigated. The reaction of acetophenone oxime with sulfoxide complex 2a in methanol solvent occurs noticeably faster than with sulfide complex 2d due to the fact that the sulfoxide is a much better platinum(II) leaving ligand than the sulfide. Evidence is presented that the orthoplatination is a multistep process. The formation of unreactive dichlorobis(N-oxime)platinum(II) cations accounts for the rate retardation by excess acetophenone oxime and suggests the importance of pseudocoordinatively unsaturated species for the C-H bond activation by PtII. A comparative X-ray structural study of dimethyl sulfoxide platinacycle 3b (R = R′ = Me) and its sulfide analogue 3e (R = H, R′ = Me), as well as of SOMePh complex 3c (R = H, R′ = Me), indicated that they are structurally similar and a sulfur ligand is coordinated in the cis position with respect to the σ-bound phenyl carbon. The differences concern the Pt-S bond distance, which is notably longer in the sulfide complex 3e (2.2677(11) Å) as compared to that in sulfoxide complexes 3b (2.201(2)-2.215(2) Å) and 3c (2.2196(12) Å). Whereas the metal plane is practically a plane of symmetry in 3b due to the H-bonding between the sulfoxide oxygen and the proton at carbon ortho to the Pt-C bond, an S-bonded methyl of SOMePh and SMe2 is basically in the platinum(II) plane in complexes 3c and 3e, respectively. There are intra- and intermolecular hydrogen bond networks in complex 3b. An interesting structural feature of complex 3c is that the two independent molecules in the asymmetric unit of the crystal reveal an extremely short Pt-Pt contact of 3.337 Å.