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-[PtCln(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-[PtCln{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)2N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 k2 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.