II Schiff-base complexes

Considerable work has been expended on elucidating the mechanism of catalytic oxidation of hindered phenols. Reactions of this kind proved to be amenable to kinetic studies and structural work on isolated intermediates. Systems involving cobalt chelate complexes as catalysts have received much attention. In this section we discuss in some detail the most important mechanistic versions proposed thus far. 

Nishinaga et al. [27] have studied the formation of peroxyquinolinatocobalt complexes 17 and have shown that it does not occur via the hydroperoxide route assumed to be operative in the autoxidation of hydrocarbons [28,29]  

This suggestion, having far-reaching mechanistic consequences, is supported by studies on the decomposition of the isolated hydroperoxide 20 in the presence of Co(salpr)  

The products of this reaction are distinctly different from those observed in the catalytic oxidation of 2,6-di-t-butyl-4-alkylphenols la. Nishinaga et al. [27] interpret the formation of peroxyquinolinatocobalt(III) complexes as major products in terms of H-atom abstraction from the phenol by the superoxocobalt(III) species which has been known [30-32] from earlier work. 

Another possibility would in principle be proton transfer from the phenol to coordinated dioxygen in the superoxo complex. Free superoxide is a moderately strong base (pK = 4.7 [153]) and not a very strong 

---

An X-ray photoelectron spectroscopic study (132) of some cobalt(II) Schiff base complexes and their dioxygen adducts finds that the cobalt core electron binding energies in... 

NMR spectra of (p-cymene)ruthenium (II) Schiff base complex, derivative of (S)-(a-methylbenzyl) and 3,5-di-ferf-butylsalicylaldimine, at room temperature in CDCI3 solution evidenced the presence of diastereomers at the ratio of 88 12.93 On the basis of a detailed analysis of 2D NMR spectra (ROESY) measured at 293 and 233 K, the (RRu,Sc) configuration of the major diastereomer in solution was suggested. 

Palladium(II) and nickel(II) Schiff-base complexes such as 67 catalyze hydrogenation of 1-hexene in dimethylformamide at ambient conditions (542). 

No information concerning the distribution of the unpaired electron on the chelate ligands of four-coordinated low-spin Co(II) Schiff base complexes is available from single crystal EPR spectra. In particular, no nitrogen hf interaction, which is sensitive to the ground state configurations, is observed. 

The first single crystal EPR study on a five-coordinated low-spin Co(II) Schiff base complex has been reported by Jorin et al.80. From the magnetic data of Co(salen)py diluted into a single crystal of Zn(salen)py, the orientation of the principal axes of the g and A 30 tensor with respect to the molecular structure could be determined, and the orientations of the in-plane g and hf tensor axes predicted by theory could be verified. 

Fig. 2 a, b. EPR and ENDOR spectrum of the low-spin Co(II) Schiff base complex Co(acacen) diluted into a Ni(acacen) 1/2 H20 single crystal, temperature 8K. a) EPR spectrum the two magnetically nonequivalent sites coincide for this particular orientation (EPR observer is marked by an arrow) b) ENDOR spectrum of H, 13C (enriched) and 14N ligand nuclei vp free proton frequency denote the AmN = 2 nitrogen ENDOR transitions. (From Ref. 12)... 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

Cobalt(II)-Schiff-base complexes with a nitrogenous base are also well known to bind molecular oxygen in an organic solvent at room temperature. 

A more complete article (58b) further details the properties of faujasite EMT-entrapped cobalt(II)-Schiff base complexes as compared to the same complexes entrapped in zeolite Y. The Schiff bases used to... 

A Co(II) Schiff-base complex converts 1- and 2-alkenes into methyl ketones and the corresponding secondary alcohols in the presence of oxygen or H2O2 in primary alcohol solvent.543 A radical oxidation with cobalt hydroperoxide through the formation and subsequent decomposition of alkyl hydroperoxide was suggested.543 An efficient conversion of alkenylarenes to ketones was achieved by the use of molecular oxygen and EtjSiH in the presence of a catalytic amount of Co(II) porphyrin in 2-propanol.544... 

There have been very few reports of the Raman spectra of spin-equilibrium complexes. In one experiment the presence of both high-spin and low-spin isomers of an iron(II) Schiff base complex was observed by the resonance Raman spectra of the imine region (11). The temperature dependence of the spectra was recorded for both solid and solution samples. Recently differences were described in the resonance Raman spectra of four- and six-coordinate nickel(II) porphyrin complexes which undergo coordination-spin equilibria. These studies are extensions of a considerable literature on spin state effects on the Raman spectra of iron porphyrins and hemes. There are apparently no reports of attempts to use time-resolved Raman spectra for dynamics experiments. 

Metal complexes with Schiff base ligands have useful applications in organic optoelectronics due to their outstanding photoluminescent (PL) and electroluminescent (EL) properties, and their ease of synthesis, which readily allows structural modification for optimization of material properties.28 Hamada and co-workers pioneered the use of zinc(II) Schiff base complexes as blue to greenish white emitters for EL devices. We have demonstrated Pt(II) Schiff base triplet emitters as yellow dopants for organic light-emitting devices... 

OLEDs) and achieved white EL with Pt(II) Schiff base complexes in 2004.29 Metal Schiff base complexes are potential candidates for the development of high performance PLEDs. 

The reaction of benzothiazoline 148 with RhCl(PPh3)3 gives rhodium(III) complex 149 as determined by X-ray diffraction <03CL1058>. The nickel(II) Schiff base complex 150 is also synthesized from the corresponding benzothiazoline <03BCJ127>. 

The authors utilized mild reaction conditions and a Co(II) Schiff-base complex to oxidize a number of pyridines to pyridine iV-oxides <03AG(E)1265>. 

Although there has been a great deal of research concerning how plahnum(II) complexes bind to biological molecules and the hkely mechanism of antitumor activity of these platinum-containing species, far less attention has been paid to the properties of other metal complexes in this arena. Recent attention has fallen on cobalt(II)-Schiff base complexes, as several have been discovered to have promise as antiviral agents. A review of recent work has appeared elsewhere [64], so the topic will not be covered here however, in addition to focusing on recent developments, emphasis is placed on the introduction of the new head unit, 3,6-diformylpyridazine (13), into Schiff-base macrocyclic electrochemistry. 

Figure 2 shows the Q band (35 GHz) powder spectrum of Co(acacen) doped into Ni(acacen) as a typical example of the EPR spectra of planar, four-fold coordinated, low-spin Co(II) Schiff base complexes. The structure, due to the three principal values of the g and A tensors is nicely resolved. Both tensors are clearly orthorhombic, with one large and two similar, smaller principal values. Powder spectra yield however no information on the orientation of the tensors with respect to the molecular frame. This information is extremely important in complexes having low molecu-... 

Figure 8.6 Direct anchoring onto AC of an octahedral Ni(II) Schiff-base complex with pendent amine groups coordinated in the axial position. (From ref. 76, with permission from Elsevier.)...

In situ photoacoustic spectroscopy has been used to study the redox process on the surface of an electrode using copper metal in alkaline solution. The values of copper(II) Schiff base complexes " absorbed on optically transparent thin-layer electrodes (OTTLE) have been... 

PAMAM [G4] with Co(II) Schiff base complexes 14,196" 4.5" H20,MeOH = Hydrolysis of phosphate esters Dialysis [134]... 

More recently, Drago and coworkers [23] have shown that the alcohol solvent takes part in the reaction by reducing the Rh(III) to Rh(I). This is followed by reaction of Rh(I) with O2 and a proton to give a Rh(III) hydroperoxide complex which oxidizes the olefin. Similarly, Drago [24] and Nishinaga [25] found that cobalt(II) Schiff base complexes catalyze the co-oxidation of olefins and primary alcohols (reaction 13). 

---