Περίληψη:
{The present work reports a new family of tripodal Co-II complexes
bearing trianionic triphenylamido-amine ligands with a variety of
pendant arms (aryl, acyl, alkyl). These complexes have been synthesized
by the reaction of anhydrous CoCl2 with the deprotonated ligands and
exhibit stoichiometric and structural variation. The solid-state
structures of these compounds reveal that in all cases the four
nitrogen-atom residues of the ligands are coordinated to the metal
center in a distorted trigonal-pyramidal geometry. In two cases, the
presence of acetonitrile adds a fifth moiety to the coordination sphere.
Among the aryl-armed Co-II reagents, {[}K(L-3)Co-II-NCMe](n) (1),
{[}K(THF)(6)]{[}(L-5)Co-II]center dot 1.5THF (2) and
{[}K(NCMe)(3)(L-13)Co-II-NCMe] (6), the two five-coordinate structures
(1, 6) exhibit distorted trigonal bipyramidal geometries, with that of 6
being the least distorted. The four-coordinate species 2 is the only
compound with a distinct anionic {[}(L-5)Co-II](-) component, since the
K+ ion is solely coordinated by solvent molecules. The acyl-armed Coll
compounds {[}K(THF)(2)(L-8)Co-II](n) (3) and
\{{[}K-2(DMA)(3)(L-10)(2)Co-2(II)]center dot 0.5Et(2)O\}(n) (5) are
strictly four-coordinate species, with the carbonyl moieties oriented
exo with respect to the cavity of the vacant coordination site. Finally,
compound {[}K-2(L-9)(2)Co-2(II)](n) (4) is the only example bearing an
alkyl-armed ligand, and exhibits a geometry featuring a repeating
-{[}Co(1)-K(1)-Co(2)-K(2)](n)- sequence. All compounds have been
characterized by spectroscopic and electrochemical techniques. Compounds
1, 2, 4, and 6 show reversible or semi-reversible Co-II/Co-III redox
couples, whereas the electron-deficient complexes 3 and 5 exhibit
irreversible anodic waves. The catalytic reactivity of these complexes
towards controlled radical polymerization (CRP) of styrene (St) and
methyl methacrylate (MMA) has been studied and preliminary results are
presented. Compounds 4 and 5 seem to be the most reactive with both
monomers, giving high yields of polymers (60-86\%), 3 efficiently
induces styrene polymerization (90\%), whereas 2 and 6 provide lower
yields (12-16\% for PS and 30-45\% for PMMA). The steric factor seems to
play an essential role, since Co-II compounds that feature a less
hindered fifth coordination site show the highest reactivity and better
control over polymerization. The polymers obtained are predominately
syndiotactic, consistent with radical polymerization, with two
exceptions (1, 2) that exhibit unusually high ratios of isotactic
triads. Interestingly, in many cases, the polymers obtained feature
bimodal distributions, while the molecular weight distributions are not
very broad (1.40-2.00), and this strongly indicates that two parallel
mechanisms may be in operation. (C) 2012 Elsevier Ltd. All rights
reserved.}
