Metal-center steric effects

The preceding discussion clearly demonstrates the important role that metal-centered steric effects can exert on enolate diastereoface selection in the aldol process. A recent publication from this laboratory provides an additional example of the importance of this... 

The role of the other ligands ("ancillary") is to avoid the precipitation of the metal and to insure a correct stereoelectronic balance (electronic density on the metal center, steric effect, trans effect) allowing all the individual reactions along the catalytic cycle to proceed at a good rate and with a good selectivity. 

In contrast to the case of Cp2ZrX2/MAO giving atactic poly(alkene)s, Cp MCl2/MAO, M = Zr (139) and Hf (140), are the catalyst precursors of the syndiotactic polymerization of 1-butene and propylene [176]. Triad distribution indicated that this is chain-end controlled syndiospecific polymerization. The syndiospecificity is attributed to the increase of steric encumbrance around the metal center. Thus, Cp HfX2 is the most effective syndiospecific catalyst component in this system. 

N-heterocycles are a class of neutral ligands with strong coordination affinity to many metal ions. Since a number of neutral N-donors ligands are available, a wide range of oxo-centered triruthenium complexes with various N-heterocyclic ligands have been prepared through axial ligand substitution. By judicious selection of the N-heterocyclic type and modification of the substituents with different electronic and steric effects, the electronic, redox, and spectroscopic properties in these oxo-centered triruthenium derivatives are controllable. 

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