Phenolate Based Ligands

Phenolate based molecules as templates for dinucleating ligands have been around for more than 40 years [1—3]. The term di- or binucleating ligands was first used by Robson in 1970 who reported a Schiff-ha.se copper complex with a phenol core [3]. While the numbers of applications are vast, the purpose of the phenolic 

Daumann, Spectroscopic and Mechanistic Studies of Dinuclear Metallohydrolases and Their Biomimetic Complexes, Springer Theses, DOI 10.1007/978-3-319-06629-5 2, Springer International Publishing Switzerland 2014 

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Oxygen Donor Ligands 6.63.2.1 Phenol-based ligands... 

A similar mechanistic pathway to that shown in the lower portion of Fig. 58 is proposed for tris(4-nitrophenyl) phosophate hydrolysis promoted by binuclear zinc complexes of phenol-based ligands, albeit no kinetic or mechanistic studies were reported for these reactions.243... 

The presence of a second Co(III) center was suggested to be responsible for a 26-fold increase in the rate of hydrolysis compared to that observed for the corresponding mononuclear complex [27]. It was proposed that the coordinated PNPP was cleaved by an intramolecular attack of a terminal bound hydroxide. Since Sargeson s early work other examples of functional or spectroscopic model complexes for several classes of phosphoesterases enzymes have been reported [28-36], however, only a few use cobalt. The Co(III)Co(Ill) complex with a phenolate based ligand (Fig. 6.4b) showed activity in phosphomonoester cleavage and a bridging coordination of the substrate was confirmed with P NMR [37]. 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Mixed N,S,0 donor sets are useful to compare the coordination ability and the structural consequences of various donor sets and to induce a variety of structural properties. The solid-state structures of [Ni(pyzs)]BF4, [Ni(pyrs)]BF4 and [Ni(pyzo)]BF4 containing dissymetric tetradentate ligands with N2S2 (543) and N2SO (544) donor sets are square planar.1397 The phenolate-based complexes are essentially planar in nitromethane, but distorted octahedral in methanol, while thiophenolate-containing complexes remain square planar in both solvents. 

Attempts have been made to exploit the intrinsic C2 symmetry of the phenolate-based dinickel core in enantioselective catalytic reactions. Therefore, enantiomerically pure C2-symmetric ligands such as (736a) and the corresponding dinickel systems (736b) have been prepared ( Equation (27)),1890 and (736b) was tested in the epoxidation of unfunctionalized alkenes with sodium hypochlorite as the oxidant. The catalytic reaction was found to be highly pH dependent with an optimum at a pH of 9. While the complex is catalytically active, significant enantioselectivity was not achieved. 

The first of two conversions catalyzed by the metalloenzyme tyrosinase is the selective ortho-hydroxylation of a phenol moiety, yielding catechol. Few synthetic systems exist that are capable of selectively performing the same monooxygenase reaction [172-178]. The group of Casella has studied a series of dicopper complexes based on 2,6-disubstituted benzene hgands providing two N3 compartments [179-181] (26a,b) as well as the asymmetrically substituted tripodal amine-based ligand (27) [182]. 

With the benzene-derived Ugands 26a,b the Cu-catalyzed orfho-hydroxyl-ation of methyl-4-hydroxybenzoate was only highly selective at low temperature, as at room temperature bound catecholate was shown to undergo a formal Michael addition with unreacted phenolate to yield methyl 2-[4-(carbomethoxy)phenoxy]-3,4-dihydroxybenzoate [181,270]. With the amine-based ligand 27, no such reactivity was observed and the catechol product methyl 3,4-dihydroxybenzoate was formed selectively. Interestingly, this latter ligand system also displayed some selective reactivity with the neutral 4-hydroxybenzoic acid substrate. 

Li, X., M.S. Lah, and V.L. Pecoraro. 1988. Vanadium complexes of the tridentate Schiff base ligand A,-salicydene-/V -(2-hydroxyethyl)ethylenediamine Acid-base and redox conversion between vanadium(IV) and vanadium(V) imino phenolates. Inorg. Chem. 27 4657 -664. 

In 1985, the hypothesis about the formation of a dicopper-catecholate intermediate at the first stage of the catalytic reaction was further supported by Karlin and co-workers [30], who crystallized an adduct between tetrachlorocatechol (TCC) and a dicopper(II) complex with a phenol-based dinucleating ligand (Figure 5.5). This... 

A broad application of phenolic Sehiff base ligands lies in the synthesis of heterobimetallic complexes. Extended functionalization of the L2 - ligand affords the so-called compartmental ligands which ensure rich coordination chemistry by accomodation of several metals [89, 146]. Some representative ligands are depicted in Fig. 25. 

A series of phenol-based compartmental ligands of types A-C was studied thoroughly as frameworks for dinickel(II) complexes by the groups of Okawa,... 

In fine with the above hypothesis, Momenteau and coworkers showed that with a proximal imidazole ligand, Fe and Mn hanging-base porphyrins, in the presence of H2O2, will carry out both epoxidation and dismutation (of H2O2) reactions, while with a phenolate proximal ligand, dismutation of H2O2 is favored by the Mn porphyrin, and is the exclusive reaction of the Fe porphyrin. Furthermore, Bruice and coworkers have shown that addition of small concentrations of NH3 or CF3CH2NH2 enhances... 

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