196213-73-7Relevant academic research and scientific papers
Mechanistic insight into the protonolysis of the Pt-C bond as a model for C-H bond activation by platinum(II) complexes
Romeo, Raffaello,D'Amico, Giuseppina
, p. 3435 - 3446 (2008/10/09)
The kinetic and NMR features of the protonolysis reactions on platinum(II) alkyl complexes of the types cis-[PtMe2L2], [PtMe 2(L-L)], cis-[PtMeClL2], and [PtMeCl(L-L)] (L = PEt 3, P(Pri)3, PCy3, P(4-MePh) 3, L-L = dppm, dppe, dppp, dppb) in methanol suggest a rate-determining proton attack at the Pt - C bond. In contrast, a multistep oxidative-addition - reductive-elimination mechanism characterizes the methane loss on protonation of the corresponding trans-[PtMeClL2] species. Tools that were particularly diagnostic in suggesting different reaction pathways for the two systems were (i) the different results of kinetic deuterium isotope experiments, (ii) the detection or absence of Pt(IV) hydrido alkyl intermediate species by low-temperature 1H NMR experiments, and (iii) the detection or absence of isotope scrambling and incorporation of deuterium into Pt - CH3, combined with the loss of a range of CH nDn-4 isotopomers. For all systems, the rates of protonolysis are retarded by ligand steric congestion, accelerated by ligand electron donation, and almost unaffected by the chain length along the series of chelate complexes. A straight line correlates the rates of protonolysis of cis-dialkyl and cis-monoalkyl complexes, the difference in reactivity between the two systems being almost 5 orders of magnitude (slope of the line = 6 × 104). Factors controlling the dichotomy of behavior between complexes of different geometry have been taken into consideration. Application of the principle of microscopic reversibility suggests the reason why platinum complexes with nitrogen donor ligands appear to be far more efficient than platinum phosphane complexes in activating the C-H bond.
Structure - Reactivity correlations for the dissociative uncatalyzed isomerization of monoalkylbis(phosphine)platinum(II) solvento complexes
Romeo, Raffaello,Alibrandi, Giuseppe
, p. 4822 - 4830 (2008/10/09)
Complexes of the type a's-[PtL2Me2] (1-14) (L = an extended series of phosphines of widely different steric and electronic properties) were synthesized, characterized, and used as precursors for the formation of cis-monoalkylplatinum(II) solvente species in methanol. The cleavage of the first platinum-alkyl bond by protonolysis is a fast process, but the subsequent cis to trans isomerization of the cationic solvento species [PtL2- (Me)(MeOH)]+ is relatively slow and it can be monitored using 31P NMR or conventional spectrophotometry. A large collection of 1H and 31P NMR data for cis-[PtL2Me2], cis-[PtL2Me(MeOH)]+, and trans-[PtL2Me(MeOH)]+ complexes showed interesting dependencies upon the size, the α-donor capacity, and the mutual position of the phosphines in the coordination sphere of the metal. The rate constants for isomerization of cis-[PtL2Me(MeOH)]+ were resolved quantitatively into steric and electronic contributions of the phosphine ligands, by means of correlations with parameters which reflect their σ-donor ability (Χ values) and steric requirements (Tolman's cone angles, θ). The electronic and steric profiles obtained for these reactions are discussed within the framework of a mechanism which involves dissociative loss of the solvent molecule and interconversion of two geometrically distinct 3-coordinate T-shaped 14-electron intermediates. The factors controlling the stability of these coordinatively unsaturated species are discussed. The electronic and steric influences of phosphines as spectator ligands in a dissociative process are compared with those shown by these ligands when used as nucleophiles in associative substitution processes. The activation parameters ΔH? and ΔS? were measured using both conventional isothermal and non-isothermal spectrophotometric kinetics.
