Ligand pentadentate

Iron complexes with the pentadentate ligand 3 derived from pyridyl and prolinol building blocks containing a stereogenic center were reported from the group of Klein Gebbink (Scheme 4) [34]. In alkene oxidations with hydrogen peroxide,... 

Recently, Nam, Fukuzumi, and coworkers succeed in an iron-catalyzed oxidation of alkanes using Ce(IV) and water. Here, the generation of the reactive nonheme iron (IV) 0x0 complex is proposed, which subsequently oxidized the respective alkane (Scheme 16) [104]. With the corresponding iron(II) complex of the pentadentate ligand 31, it was possible to achieve oxidation of ethylbenzene to acetophenone (9 TON). 0 labeling studies indicated that water is the oxygen source. 

There have been no reports of cobalt(III) complexes of these potentially pentadentate ligands. 

Diacetylpyridine bis(S-methylisothiosemicarbazone), 46, formed [Co(46-H)l2] H20 from a mixture of cobalt(II) acetate, 46, and hydrogen iodide in heated ethanol [203]. The complex is 7-coordinate with 46 functioning as a NNNNN pentadentate ligand. 

Solution studies of nickel(II) complexes of 2,6-diacetylpyridine bis(thiosemi-carbazone), 28, have been carried out [147]. From a refluxed methanolic solution containing 28 and nickel(II) chloride, [Ni(28)Cl2] was isolated which behaved as a 1 1 electrolyte in methanol [152], Based on spectral information it is proposed that the complex is a distorted octahedron with a SNNNS pentadentate ligand. 

Synthesis of complex 1. The pentadentate salen catalyst 1 was synthesized as described (9). In short, the tosylated 2-[2-(2-methoxyethoxy)-ethoxy]-ethanol 2 (10) was reacted with 2,4-dihydroxybenzaldehyde 3 to yield 4-alkoxy salicylaldehyde 4 after chromatographic purification (eq. 1). Subsequent condensation of 4 with 1,3-diaminopropanol yielded water-soluble salen ligand 5 in sufficient purity and 89% yield (11). The formation of an azomethine bond is indicated by a shift of the NMR signal for the carbonyl carbon from 194.4 ppm in aldehyde 4 to 166.4 ppm for the imino carbon in 5. The pentadentate ligand 5 was then treated with copper(ll) acetate in methanol to obtain the dinuclear copper(ll) complex 1 as a green solid (eq. 2) (11). 

Due to the steric constraints of the ligand, the partially protonated H2cdta binds as a pentadentate ligand in psri(H2cdta)(H20)] where a water molecules completes the octahedral coordination sphere.1191, 192... 

A cyclam derivative with an amine pendent donor attached at the bridgehead carbon atom was presynthesized from diethylaminomalonate and l,9-diamino-3,7-diazanonane in refluxing MeOH, followed by BH3-THF reduction.1507 It acts as a pentadentate ligand at pH 8.5 and forms a six-coordinate complex (592) with a water molecule sitting at the remaining axial site. 

Pentadentate ligands with N4S donors based on methyl 2-aminocyclopent-l-ene-l-dithiocar-boxylate with pendent pyrazolyl groups (113) form complexes with zinc including a structurally characterized trigonal-bipyramidal zinc compound.891... 

The zinc complex of a tripodal N2S20 pentadentate ligand undergoes amide alcoholysis of a coordinated amide group.893 Examples of amide hydrolysis are known for other ligand systems.894... 

Schiff bases provide useful mixed donor sets. The carbonyl function of the most frequently used ligands is derived from either 1,3-dicarbonyl compounds or salicylaldehyde. Favourable combinations involve O-, N- and S-donor atoms. A range of technetium and rhenium complexes exist with bi-, tri-, tetra- and pentadentate ligands. The geometry of these complexes depends on the number and type of coordinating atoms as well as on the chain length between the donor atoms in the SchifF-base ligands. 

Pentadentate ligands have been prepared by attaching two pendant 2-pyri-dylmethyl arms to 1,4,7-triazacyclononane. The systems with R=H were investigated by Spiccia et al. [24] (13). 

In the only instances where spin crossover has been observed for a system involving a pentadentate ligand this has been an N302-donating Schiff base and the sixth coordination site has been occupied by a nitrogen donor, giving rise to an FeN402 coordination core. Most examples involve salten (Fig. 17). 

It is not surprising that the substitution processes on seven-coordinate 3d metal ions follow an interchange, rather than a limiting dissociative mechanism. In the case of seven-coordinate Fe(III) complex we even found an associative interchange mechanism for the substitution of solvent molecules as a result of the high n-acceptor ability of the fully conjugated pentadentate ligand system present in its equatorial plane 44). 

Employing asymmetric pentadentate ligands, such as H3-4-X-bhedsd (H3-4-X-bhedsd = A-(4-X-salicylidene)-A, iV -bis(2-hydroxyethyl)ethylenediamine and X = Q, Br, or OMe) (Figure 18), gave hexanuclear manganese(III) carboxylate complexes with an [Mn6(//3-0)2(//-0Me)6] + core. 

Three /i-alkoxo dinuclear complexes with pentadentate ligands have been reported [Mn2-(2-OH-5-Cl-SALPN)2(MeOH)] MeOH ((157)-MeOH),"" [Mn2(2-OH-SALPN)2(THF)] (158)," and [Mn2(2-0H-SALPN)2(H20)] 3Me0H (159)." It is unexpected that the two metal centers in this complex are not equivalent. The Mn- -Mn separation in (159) is the longest yet reported for a monoalkoxy-bridged species. 

Several bis(/i-alkoxo) complexes have been reported for various pentadentate ligands. Some examples are [Mn2(2-0H-3-N02-SALPN)2] 2DMF (160-DMF),"" [Mn2(2-0H-5N02-... 

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