Effect of Axial Coordination

A notable effect ol rra/i.v comdination has tilso been observed for the aniitnic pttlymeriziition of methacryloni-trile (22) initiated from a living polymer ctf methyl methacrylate (21, R = Me) with an aluminum enolate reactive end (32 ), in which the chain grttwth is promoted by the trails coordination of pyridine, to afford a narrow MWD block copolymer (42)- Ott the other hand, in the absence of axially ccatrdinating pyridine under otherwise identical conditions as described, no block copolymerization of 22 from 32 takes place. 

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The resonance Raman spectra of Co(TMPyP(4)), with cobalt in the oxidation states +2 or +3, where TMPyP(4) = tetrakis(7V-methyl-4-pyridinyl)po-rphyrin, were obtained. Oxidation state marker bands were identified, which suggested that the effects of axial coordination by solvent molecules are greater for the Co(III) than for the Co(II) species.244... 

ChUsmnoff J, Sansinena J-M. Effects of axial coordination of the metal center on the activity of iron tetr henylporphyrin as a nonprecious catalyst for oxygen reduction. J Phys Chem C 2014 118 19139-49. 

In view of the coordination pattern in protein-bound methylcobalamin (see below), the thermodynamic studies on the effect of the coordination of the dimethylbenzimidazole base to the a-side of the cobalt center on the homolytic and heterolytic (Co/3-C)-bond dissociation energy in (4) (thermodynamic effect of the trans hgand or trans influence ) were extended to corresponding investigations with CojS-methyl-imidazolylcobamides, such as (9), where imidazole replaces the dimethylbenzimidazole these studies showed this change of the nature of the axial base to have httle effect on the two relevant bond-dissociation energies of the corresponding methylcobamide. ... 

Kinetic measurements on the concentration of complex B revealed that it docs not react with E -p-mcthylstyrcne directly, acting only as a reservoir of the active 0x0 chromium(V) species A [102]. It was found that coordination of DMF reduced the reactivity of complex A significantly (with respect to that in CHtCN) this could be seen from x a values of complexes A in DMF (ti/2(An, DMF) greater than 1 d ii 2(Aij, DMF)= 6 h). The increased enantiosclectivity (+11% ee upon addition of DMF [35]. stoichiometric procedure, catalyst non-deuterated counterpart of 13) could result from reduced reactivity, thus representing the effect of axial ligation. 

The reverse process, the nucleophile-induced dealkylations of methyl-Co(III)-corrins, has been less studied due to the impediment of the intramolecular coordination of the nucleotide base [134,137]. Indeed, thiolates demethylate methylcobinamide (45) to cob(l)inamide (43) approximately 1000 times faster than MeCbl (3) to B s (40") [137], reflecting the strong stabilizing effect of the coordinated nucleotide in 3 [86,134]. This is of relevance for enzymatic methyl group transfer reactions involving protein bound Co(l)- and methyl-Co(lll)-corrins, where considerable axial base effects are expected [125,138]. 

The selectivity of 2,6-di-t-butylphenol (2,6-DTBP) oxidation catalyzed by Co(salen) derivatives depends on the nature of axially coordinated (5th) ligand as well as on the solvent used. The relevant data of Kothari and Tazuma [23] are listed in Table I. These authors have found that DMF is an effective and selective solvent for benzoquinone (BQ) formation. With Co(salen)py as catalyst, up to 100 % selectivities to BQ can be reached relative to diphenoquinone (DPQ) formation. Mn(salen) in DMF, however, steers the reaction with 99.9 % selectivity to DPQ. The proper choice of reaction conditions permits even monoalkylphenols (2-t-butylphenol and 2-methylphenol) and phenol itself to be oxidized, mostly to the corresponding BQ. Phenol oxidation occurs at 6 bar 0 and ca. 70 °C in DMF. A recent report describes a... 

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