Kinetic and Thermodynamic Aspects of the Regioselective Addition of Bifunctional Hydroxylaminooxime-type HO-Nucleophiles to Pt-Complexed Nitriles

The coupling between coordinated propiononitriles in trans-[PtCl n (EtCN)2] (n = 2, 4) and the 1,2-hydroxylaminooximes HON(H)CMe2C(R)NOH (R = Ph 1, Me 2) proceeds smoothly in CHCl3 at ca. 40−45 °C and gives trans-[PtCl n {NHC(Et)ON(H)CMe2C(R)NOH}2] (n = 2, R = Ph 5, Me 6; n = 4, R = Ph 7, Me 8) in 80−85% isolated yields. The reaction is highly regioselective, and both spectroscopic (IR; FAB+-MS; 1D 1H, 13C{1H}, and 195Pt NMR; and 2D 1H,13C HMQC, 1H,13C HMBC, and 1H,15N HMQC NMR) and X-ray data for 6 − 8 suggest that the addition proceeds exclusively via the hydroxylamine moiety of the 1,2-hydroxylaminooxime species; the existence of an oxime group remote from the nucleophile was also confirmed. Heating of 6 in air leads to its conversion to the unusual nitrosoalkane complex [PtCl2{HONC(Me)C(Me)2 NO}] (9), whereas in the case of 5, only the metal-free salt [H3NC(Me)2C(Ph)NOH]2(NO3)Cl·H2O (10) was isolated. To compare the kinetic aspects and trends in the addition of both types of nucleophiles (oximes and hydroxylamines; for the latter, see our recent work: Inorg. Chem. 2005, 44, 2944) to coordinated nitriles, a kinetic study of the addition of HONC(CH2Ph)2 to [Ph3PCH2Ph][PtCl5(EtCN)] (11) to give [Ph3PCH2Ph][PtCl5{NHC(Et)ONC(CH2Ph)2}] (12) was performed. The calculated rate constant k 2 of 3.9 × 10-6 M-1 s-1 at −20 °C for the addition of the oxime indicates that the hydroxylamine is, by a factor 1.7 × 104, more reactive toward the addition to nitriles than the oxime. Results of the synthetic, kinetic, and theoretical (at the B3LYP level of theory) studies have demonstrated that the high regioselectivity of the reactions of the 1,2-hydroxylaminooximes with ligated nitriles is both kinetically and thermodynamically controlled.