Dendrimers, specifically suited to construct site-isolated groups due to their well-defined hyperbranched structure, have been used as a ligand design element for the construction of nickel catalysts for ethylene oligomerization. The dendritic P,O ligand indeed suppresses the formation of inactive bis(P,O)Ni complexes in toluene, as is evident from NMR studies, and, as a consequence, outperforms the parent ligand in catalysis in this solvent. The dendritic effect observed in methanol is more subtle because both the dendritic ligand 1 and the parent 2 form bis(P,O)nickel complexes in solution according to NMR spectroscopy. Unlike the parent complex 8, the dendritic bis(P,O)Ni complex 7 derived from dendrimer ligand 1 is able to dissociate to a mono-ligated species under catalytic conditions, that is, 40 bar ethylene and 80 degrees C, which can enter the catalytic cycle. Indeed, dendritic ligand 1 gives much more active nickel catalysts for the oligomerization in methanol than does 2.

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N,N ‘ -Diaryl-α-diimine-ligated Pd(II) dimethyl complexes (tBu 2 ArDABMe)PdMe2 and {(CF 3 ) 2 ArDABMe}PdMe2 {tBu 2 ArDABMe:  ArNC(CH3)−C(CH3)NAr, Ar = 3,5-di-tert-butylphenyl; (CF 3 ) 2 ArDABMe:  Ar = 3,5-bis(trifluoromethyl)phenyl} undergo protonolysis with HBF4(aq) in trifluoroethanol (TFE) to form cationic complexes [(α-diimine)Pd(CH3)(H2O)][BF4]. The cations activate benzene C−H bonds at room temperature. Kinetic analyses reveal trends similar to those observed for the analogous platinum complexes:  the C−H activation step is rate-determining (KIE = 4.1 ± 0.5) and is inhibited by H2O. The kinetic data are consistent with a mechanism in which benzene substitution proceeds by a solvent- (TFE-) assisted associative pathway. Following 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 to the dimeric μ-hydroxide complex after the oxidative C−C bond formation. The regioselectivity of arene coupling was investigated with toluene and α,α,α-trifluorotoluene as substrates.

To examine the effects of cyclopentadienyl and olefin substitution on preferred stereochemistry, a series of singly [SiMe 2 ]-bridged ansa-niobocene and -tantalocene olefin hydride complexes has been prepared via reduction and alkylation of the corresponding dichloride complexes. In this manner, [Me 2 Si(η 5 -C 5 H 4 )(η 5 -C 5 H 3 -32 )]Ta(CH 2 dCHR′)H (R′ ) H, C 6 H 5 ) have been prepared and characterized by NMR spectroscopy and, in some cases, X-ray diffraction. The doubly [SiMe 2 ]-bridged ansatantalocene ethylene hydride complex [(1,2-SiMe 2 ) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]-Ta(CH 2 dCH 2 )H has been prepared from thermolysis of the methylidene methyl complex [(1,2-SiMe 2 ) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]Ta(CH 2 )CH 3 . Addition of an excess of propylene or styrene to the tantalocene ethylene hydride results in olefin exchange and formation of the olefin hydride complexes [(1,2-SiMe 2 ) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]Ta(CH 2 dCHR′)H (R′ ) CH 3 , C 6 H 5 ). These compounds 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 metallocene olefin hydride complexes reveals that alkyl substitution on the cyclopentadienyl ligand array may have a significant 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 the following 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 (η 5 -C 5 Me 5 ) 2 Ta(CH 2 CH 2 )H and much slower than a singly bridged complex, [Me 2 Si(η 5 -C 5 -Me 4 ) 2 ]Ta(CH 2 CH 2 )H. 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.