312925-30-7Relevant academic research and scientific papers
Adding a new dimension to the investigation of platinum-mediated arene C-H activation reactions using 20 NMR exchange spectroscopy. Dynamics of Pt(II) phenyl/benzene site exchange
Wik, Bror Johan,Lersch, Martin,Krivokapic, Alexander,Tilset, Mats
, p. 2682 - 2696 (2007/10/03)
Protonation of (N-N)PtPh2 (1; N-N = diimine ArN=CMe-CMe=NAr with Ar = 2,6-Me2C6H3 (a), 2,4,6-Me 3C6H2 (b), 4-Br-2,6-Me2C 6H2 (c), 3,5-Me2C6H3 (d), and 4-CF3C6H4 (e)) in the presence of MeCN at ambient temperature generates (N-N)Pt(Ph)(NCMe)+ (2). At -78 °C, protonation of 1a yielded (N-N)PtPh2(H)(NCMe)+ (3a), which produced benzene and 2a at ca. -40 °C. Protonation of 1a-e in CD 2-Cl2/Et2O-d10 furnished (N-N)Pt(C6H5)(η2-C6H 6)+ (4a-e). The π-benzene complexes 4a-c, sterically protected at Pt, eliminate benzene at ca. 0 °C. The sterically less protected 4d-e lose benzene already at -30 °C. SST and 2D EXSY NMR demonstrate that phenyl and π-benzene ligand protons undergo exchange with concomitant symmetrization of the diimine ligand, most likely via oxidative insertion of Pt into a C-H bond of coordinated benzene. The kinetics of the exchange processes for 4a-c were probed by quantitative EXSY spectroscopy, resulting in ΔH? of 70-72 kJ mol-1 and ΔS? of 37-48 J K-1 mol-1. A large, strongly temperature-dependent H/D kinetic isotope effect (9.7 at -34 °C; 6.9 at -19 °C) was measured for the dynamic behavior of 4a versus 4a-d 10, consistent with the proposed π-benzene C-H bond cleavage. The fact that the π-benzene complex 4a is thermally more robust in the absence of MeCN than is the Pt(IV) hydridodiphenyl complex 3a in the presence of MeCN agrees with the notion that arene elimination from Pt(IV) hydridoaryl complexes occurs via Pt(II) π-arene intermediates that eliminate the hydrocarbon associatively, in this case, promoted by MeCN. Compounds 1a, 1b, 1d, 2a, and 2b have been crystallographically characterized.
Improved one-pot synthesis of mixed methyl - aryl platinum(II) diimine complexes
Lersch, Martin,Dalhus, Bjorn,Bercaw, John E.,Labinger, Jay,Tilset, Mats
, p. 1055 - 1058 (2008/10/09)
A general one-pot synthetic route for mixed methyl-aryl Pt(II) diimine complexes is described. Performing the alkylation in neat Me2S instead of ether or THF greatly reduces the amount of comproportionation products otherwise formed, diminishes separation problems, and improves yields. Treatment of the intermediate methyl - aryl complexes (Me2S) 2Pt(Me)(Ar) with diimines (N-N) furnishes the methyl - aryl Pt(II) diimine complexes (N-N)Pt(Me)(Ar) in 76-84% yields. The Pt methylphenyl complex [p-Tol-N=C(Me)C(Me)=N-p-Tol]Pt(Me)(Ph) has been characterized by X-ray diffraction.
Mechanistic investigation of benzene C-H activation at a cationic platinum(II) center: Direct observation of a platinum(II) benzene adduct
Johansson,Tilset,Labinger,Bercaw
, p. 10846 - 10855 (2007/10/03)
The platinum(II) methyl cation [(N-N)Pt(CH3)(solv)]+BF4- (N-N = ArN=C(Me)C(Me)=NAr, Ar = 2,6-(CH3)2C6H3, solv = H2O (2a) or TFE = CF3CH2OH (2b)) is prepared by treatment of (N-N)Pt(CH3)2 with 1 equiv of aqueous HBF4 in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H2O solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C6H5)(solv)]+BF4- (3). Investigations of the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene coordination, reversible oxidative addition of benzene C-H bonds, reversible formation of a methane C,H-σ complex, and final dissociation of methane. Under conditions where [(N-N)Pt(CH3)(H2O)]+BF4- (2a) is the major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic rate law (inverse first order in [H2O] and first order in [C6H6] to 3.8 M) and the small kinetic deuterium isotope effect for C6H6 vs C6D6 (k(H)/k(D) = 1.06 ± 0.05 at 25 °C). When deuterated benzenes C6D6 and 1,3,5C6H3D3 are used, almost full statistical scrambling of deuterium from one benzene into methane is achieved, indicating that the energetic barriers for dissociating benzene and methane are considerably higher than interconversions of intermediate hydrocarbon complexes and [(N-N)Pt(C6H5)(CH3)H]+. Protonation of (N-N)Pt(CH3)(C6H5) with HBF4 in TFE, which provides an independent route into the manifold of postulated intermediates, gives a mixture of 3 + CH4 (82%) and 2 + C6H6 (18%). Protonation of (N-N)Pt(CH3)(C6H5) with triflic acid in methylene chloride/diethyl ether mixtures at -69 °C allows direct low-temperature NMR observation of a fluxional π benzene complex, [(N-N)Pt(CH3)(C,C-η2-C6H6)]+.
