The combination of Pd(2)dba(3) x CHCl(3) and hexacationic triarylphosphine-based Dendriphos ligands (1-3) leads to a highly active catalytic system in the Suzuki-Miyaura cross-coupling reaction. Under relatively mild reaction conditions, nonactivated aryl bromides and activated aryl chlorides can be coupled at a low Pd loading (0.1 mol %). The observed activity of this catalytic system, in particular in coupling reactions of aryl chlorides, is dramatically higher than that of conventional Pd catalysts employing triarylphosphine ligands. Through control and poisoning experiments, it is concluded that a homogeneous Pd(0)-Dendriphos complex is the active species in this catalytic system. Despite their triarylphosphine-based structure, Dendriphos ligands behave as very bulky phosphine ligands and lead to a preferential formation of coordinatively unsaturated and catalytically active Pd(0) species, which explains the observed high catalytic activity for these systems. The presence of six permanent cationic charges in the backbone of this class of ligands is proposed to result in a significant interligand Coulombic repulsion and plays a crucial role in their bulky behavior. In the coupling reactions of activated aryl chlorides, a positive dendritic kinetic effect was observed among the different Dendriphos generations, indicating an increased ability of the higher ligand generations to stabilize the active species due to steric effects. For aryl bromides, no dendritic effect was observed due to a shift in the rate-determining step in the catalytic cycle, from oxidative addition for aryl chlorides to transmetalation for aryl bromides.

The hydrogenation of phenylacetone to cyclohexylacetone, in which the aromatic ring is selectively reduced in preference to the carbonyl group, has been achieved with chemoselectivities exceeding 90%. The catalyst (precatalyst) used to achieve this transformation comprises PVP-stabilized Rh nanoparticles dispersed in water with some phosphine ligand additives. Phosphine ligands with different steric and electronic properties and polarities were investigated for this purpose, and several clear trends were observed, showing the potential of welldefined phosphine ligands as modifiers in nanocatalysis.

This work reviews recent developments in the field of organic transformations on sigma-aryl organometallic complexes. The general notion that M--C sigma bonds are kinetically labile, highly reactive, and incompatible with typical reaction conditions met in organic synthesis has limited the use of these synthetic strategies thus far. However, organic transformations on metal-bound sigma-aryl fragments are being used more and more by chemists in both industry and academia. In this Review, emphasis is put on the synthetic methods applied in this field up to now. The simplicity and generally good yields of these methods are very attractive for the construction of functionalized organometallic building blocks that are potentially useful as photochemical molecular devices, biosensors and -conjugates, or molecular switches. Thus, this Review has been tailored for a broader audience with the aim of encouraging the application of these strategies.

2,6-Bis(oxazolinyl)phenylnickel bromide complexes [NiBr(R,R′-Phebox)] (2) were synthesized via two synthetic routes (A and B). In route A, selective bis-ortho lithiation of [R,R′-PheboxBr], followed by a transmetalation reaction with [NiBr 2 (PEt 3 ) 2 ], yielded not only complexes 2 with an η 3 -mer-N,C,N′-bonded Phebox ligand but also [NiBr(R,R′-Phebox)(PEt 3 ) 2 ], 7, where the nickel center is η 1 -C bonded to the intra-annular C ipso of the Phebox ligand. Coordination of two PEt 3 ligands completes the square-planar coordination sphere of the Ni center in 7. When R ) t-Bu, R′ ) H, only complex 7c was formed. Alternatively, when route B (oxidative addition with [Ni(cod) 2 ], cod ) cyclooctadiene) was followed, selective formation of complexes 2 was observed. X-ray crystal structures were obtained for [NiBr-(i-Pr,H-Phebox)] (2b) and [NiBr(bis(ketimine)phenyl)] (3). The Ni centers have square-planar geometries with a planar, η 3 -mer-N,C,N′ coordination of the terdentate ligand systems. Complexes 2 were found to be inactive as catalysts in the atom-transfer radical polymerization (ATRP) reaction of methyl methacrylate (MMA) and in the atom-transfer radical addition (ATRA, Kharasch addition) reaction of CCl 4 to MMA. This is ascribed to the relatively high oxidation potential of Ni II -Phebox complexes, which excludes the (reversible) formation of a d 7 -Ni III -Phebox complex, a crucial condition for subsequent reactions. Cyclovoltammetry (CV) experiments ((n-Bu) 4 NBr as supporting electrolyte) showed no electrochemical waves between -1.00 and +1.50 V (Ag/AgCl reference electrode, (n-Bu) 4 NBr as supporting electrode). Theoretical calculations showed that the energy (E ox ) needed for the oxidation reaction occurring during the CV experiments is considerably higher for [NiBr(Me,Me-Phebox)] (1.87 eV) and [NiBr(bis(ketimine)-phenyl)] (1.90 eV) than for [NiBr(NCN)] (1) (1.45 eV).