Kinetic and Mechanistic Study of the Pt(II) versus Pt(IV) Effect in the Platinum-Mediated Nitrile−Hydroxylamine Coupling

The metal-mediated coupling between coordinated EtCN in the platinum(II) and platinum(IV) complexes cis- and
trans-[PtCl2(EtCN)2], trans-[PtCl4(EtCN)2], a mixture of cis/trans-[PtCl4(EtCN)2] or [Ph3PCH2Ph][PtCln(EtCN)]
(n = 3, 5), and dialkyl- and dibenzylhydroxylamines R2NOH (R = Me, Et, CH2Ph, CH2C6H4Cl-p) proceeds smoothly
in CH2Cl2 at 20−25 °C and the subsequent workup allowed the isolation of new imino species [PtCln{NHC(Et)ONR2}2] (n = 2, R = Me, cis-1 and trans-1; Et, cis-2 and trans-2; CH2Ph, cis-3 and trans-3; CH2C6H4Cl-p, cis-4
and trans-4; n = 4, R = Me, trans-9; Et, trans-10; CH2Ph, trans-11; CH2C6H4Cl-p, trans-12) or [Ph3PCH2Ph][PtCln{NHC(Et)ONR2}] (n = 3, R = Me, 5; Et, 6; CH2Ph, 7; CH2C6H4Cl-p, 8; n = 5, R = Me, 13; Et, 14;
CH2Ph, 15; CH2C6H4Cl-p, 16) in excellent to good (95−80%) isolated yields. The reduction of the Pt(IV) complexes
9−16 with the ylide Ph3PCHCO2Me allows the synthesis of Pt(II) species 1−8. The compounds 1−16 were
characterized by elemental analyses (C, H, N), FAB-MS, IR, 1H, 13C{1H}, and 31P{1H} NMR (the latter for the
anionic type complexes 5−8 and 13−16) and by X-ray crystallography for the Pt(II) (cis-1, cis-2, and trans-4) and
Pt(IV) (15) species. Kinetic studies of addition of R2NOH (R = CH2C6H4Cl-p) to complexes [Ph3PCH2Ph][PtIICl3(EtCN)] and [Ph3PCH2Ph][PtIVCl5(EtCN)] by the 1H NMR technique revealed that both reactions are first
order in (p-ClC6H4CH2)2NOH and Pt(II) or Pt(IV) complex, the second-order rate constant k2 being three orders of
magnitude larger for the Pt(IV) complex. The reactions are intermolecular in nature as proved by the independence
of k2 on the concentrations of added EtC⋮N and Cl. These data and the calculated values of ΔH⧧ and ΔS⧧ are
consistent with the mechanism involving the rate-limiting nucleophilic attack of the oxygen of (p-ClC6H4CH2)2NOH
at the sp-carbon of the C⋮N bond followed by a fast proton migration.