Complexes ESR spectra

Reactions of CoCl2(PPh3)2 with TICp subsequently followed by treatment with [M(ArN3Ar)] (M = Cu1, Ag1) afforded CoCp(RN3R)(PPh3). These complexes are representatives of the very small number of paramagnetic monocyclopentadienyl complexes. ESR spectra indicate the presence of low spin Con in an environment of low symmetry (22).,0S... 

The main drawback in the use of thiocarbonyl compounds as spin traps was represented by the fact that in most cases the resulting spin adducts either were as transient as the attacking radicals (aliphatic thioketones and dithioesters) or were characterized by very complex ESR spectra (thiobenzophenone and its derivatives). It was only after the introduction of thiobenzoyltriphenylsilane la that the use of thiocarbonyl compounds in spin trapping experiments acquired some practical value. 

B. Analysis of Experimental ESR Spectra Detailed accounts of interpretation and analysis of complex ESR spectra have been given by many authors (95,101,102) but there are certain aspects which are worth repeating here. We shall be concerned in this section with only isotropic and powder spectra since these are the most likely spectra a photochemist will... 

Often more than one kind of radical may be trapped in UV-irradiated samples at low temperature. Interpretation of the resultant complex ESR spectra requires special care to identify whether some of the ESR lines are part of a hyperfine component or whether they are due to another kind of radical. Such ambiguities may sometimes be resolved in the following ways. 

Figure 1. Comparison of iron and ruthenium oxidation in l,l -bis(diphenylphosphino)ferrocene-ruthenium(II) complexes (ESR spectra [44a] from electrolytically oxidized solutions of precursors [44b, c] in THE at 4 K) Oxidation of [Cp RuH(dppf), dppf= l,l -bis(diphenylphosphino)ferro-cene, and [(Cym)RuCl[dppf)](PF6), Cym = p-cymene, occurs at ruthenium for the former compound but yields a ferrocenium species for the latter.

On coordination to metals, the quinone-CO stretching frequency shifts at least 200-300cm", which infers a formal reduction to catechol with oxidation of the metal. The possibility of an intermediate radical anion coordination mode for o-benzoquinones has been demonstrated by the reversible chemical and electrochemical oxidation of reduced quinone complexes. ESR spectra of [Ir(Cl)(l,2-02QCl4)(C0)(PR3)2] (PR3 = PPhj, PMePh2) indicate that the unpaired electron is localized in the semiquinone ligand. The quinone coordination mode is stron y affected by the oxidizing ability of the quinone and the nature of the complex itself. 

The method to correlate theory with experimental results has turned out to be extremely useful for the analysis of complex ESR spectra. 

Even more complex ESR spectra can occur for nitrogen-containing aromatic radicals of the type in Fig. 3.4. A stickplot analysis is not easily performed. At this level of complexity a combination of ESR and ENDOR measurements, simulation of the observed spectra, and theoretical calculations of hyperfine coupling constants is often applied to obtain a reliable assignment. The procedure to obtain the coupling constants from the ENDOR spectrum of the Wurster blue cation is indicated in Exercise E3.3. ENDOR lines due to can be observed more easily than signals due to " N, a phenomenon that is quite typical in CW-ENDOR studies. The hyperfine couplings due to N nuclei may therefore have to be deduced from simulation of the ESR spectrum. 

Figure 1 shows the complex ESR spectra from isolated cardiac mitochondria. They appear as a superposition of spectra from various paramagnetic components of the mitochondrial ETC. They are mainly iron-sulfur centers, denoted as Nl, N2, N3 - - 4 (located in complex I, NADFi-ubiquinone oxidoreductase), SI (complex II, succinate-ubiquinone oxidoreductase), and the Rieske iron-sulfur protein (complex III, ubihydroquinone-cytochrome C oxidoreductase). The positions of the components... 

ESR spectra, 2, 511 Oxovanadium(V) complexes aqueous solution, 3,1026 Oxyanions coordinated... 

Solid-state, esr spectra of [Cu(Et2dtc)] and [Cu S2P(OPr )2 2] dissolved in coordinating and noncoordinating solvents have been compared with single-crystal and powdered samples diluted with the corresponding complexes of divalent nickel and zinc. With noncoordinating... 

Treatment via chelation has been observed for 2-acetylpyridine thiosemi-carbazone derivatives, which have been found to possess inhibitory activity for the RNA-polymerases of the influenza virus [133]. The iron(III) complexes were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase (no added iron) compared to uncomplexed thiosemi-carbazone [128]. Raina and Srivastava [134] prepared and characterized low spin iron(III) complexes of 2-acetylpyridine thiosemicarbazone, [Fe(8-H)2A] (A = NO3, OH, Cl, N3, NCS or NO2), which were proposed as being seven-coordinate. However, all but the azide complex are 1 1 electrolytes in DMF and their solid ESR spectra are rhombic with the g-values being about 2.20,2.15 and 2.00. Of the six complexes, the azide ion seems to interact ihost strongly with the iron(III) center. 

ESR spectra (Table 1). The JV-cyclohexylthiosemicarbazone, 13, complex formed the expected [Fe(13-H)2] with FeCl as the counterion [141]. However, [Fe(13) (13-H)H20]C104 was isolated from ethanol. Bulkiness of the cyclohexyl group, and the perchlorate ion s greater ability to hydrogen bond are probably both important to the stability of this cation. The iron(III) center is considered six-coordinate with a tridentate 13-H, bidentate 13, and a coordinated water molecule. 

Iron(III) complexes have also been prepared with 2-acetylpyridine N-phenylthiosemicarbazone [142], 14. Three have been formulated as square pyramidal [Fe(14-H)A2] (A = Cl, NCS and NOj) based, in part, on molar conductivities of 37-62 ohm cm mol . Their ESR spectra in frozen DMF are essentially the same as for other Af-substituted species, which likely indicates the presence of [FeL2] ions. More recently [138], [Fe(14-H)2]C104 has been isolated and its solid ESR spectrum is reported to be axial with g > g - Table 1 summarizes the g-values of iron(III) heterocyclic thiosemicarbazone complexes. 

Two copper(II) complexes of 2-acetylpyridine thiosemicarbazone, 8, were included in a study of complexes of 2-formylpyridine thiosemicarbazone [169]. [Cu(8-H)OAc] has a magnetic moment consistent with a monomeric copperfll) center and both it and [Cu(8)Cl2] have d,2-y2 ground state ESR spectra (Table 2). A d-d envelope and a magnetic moment of 1.68 B.M. have led others [178] to propose a distorted tetrahedral environment with metal-metal interaction for the brown complex, [Cu(8)Cl2]. 

Copper(II) complexes have been prepared with the 2-acetylpyridine N-oxide 3-azabicyclo[3.2.2.]nonylthiosemicarbazone, 25, and bonding occurs via the pyridine N-oxide oxygen, azomethine nitrogen and thiol sulfur [128]. Based on electronic and ESR spectra, bonding to copper(II) of uninegative, tridentate 25-H is considerably weaker than the related 2-acetylpyridine thiosemicarbazone, 4-H. The other copper(II) complexes reported to date have been prepared... 

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