Ancillary ligand effects in niobocene olefin hydride complexes and hydrocarbon oxidation by palladium(II) complexes

To examine the effects of cyclopentadienyl and olefin substitution on preferred stereochemistry, the preparation of a series of singly [SiMe ₂]-bridged ansa-niobocene olefin hydride complexes is described. These complexes serve as stable transition state analogues for the much more kinetically labile group 4 metallocenium cationic intermediates in metallocene-catalyzed olefin polymerization. Characterization of the thermodynamically preferred isomers of niobocene olefin hydride complexes reveals that placement of a single alkyl substituent on the cyclopentadienyl ligand array may have a moderate effect on the stereochemistry of olefin coordination.; Using dynamic NMR methods the rates of hydrogen exchange following intramolecular ethylene insertion into the metal-hydride bond have been measured for singly and doubly bridged group 5 ansa-metallocene complexes. The singly bridged ansa-niobocenes exchange up to 3 orders of magnitude faster than unbridged complexes. However, the doubly bridged ansa-tantalocene complex exchanges at a rate comparable to that previously reported for the unlinked and much slower than a singly bridged complex. These “ansa-effects” were investigated by DFT calculations on model complexes. The computed exchange pathway showed the presence of an agostic ethyl intermediate. The calculated barriers for hydrogen exchange of model unbridged, singly bridged, and doubly bridged niobocenes correlate with the experimental results.; N,N′-Diaryl-α-diimine-ligated Pd(II) dimethyl complexes undergo protonolysis with HBF₄ (aq) in trifluoroethanol (TFE) to form the cationic complexes [(α-diimine)Pd(CH ₃)(H₂O)][BF₄]. The cations activate benzene C-H bonds at room temperature. Kinetic analyses reveal trends similar to those observed for the analogous Pt complexes: the C-H activation step is rate determining and is inhibited by H₂O, which is consistent with a mechanism in which benzene substitution proceeds by a solvent- (TFE-) assisted associative pathway. After benzene C-H activation under 1 atm O₂, the products of the reaction are biphenyl and a dimeric μ-hydroxide complex, [(α-diimine)Pd(OH)] ₂[BF₄]₂. The Pd(0) formed in the reaction is reoxidized by O₂ after the oxidative C-C bond formation. Toluene and α,α,α-trifluorotoluene were investigated as substrates to examine the regioselectivity of arene coupling.