Paramagnetic complexes reactivity

Thus, for paramagnetic complexes the reactivity patterns promoted either by the metal center or by the ligand (equivalent to an inner- vs. an outer-sphere pathway), are essentially triggered by the spin density distribution. 

The mono- and dinuclear paramagnetic complexes are of course extremely reactive. We have shown that the reactions of Cr(CO)J and Cr2(CO)10I with various mono- andbidentate ligands lead to the formation of the respective substitution products of Cr(CO)6 with simultaneous oxidation to elemental I2 (80) ... 

Early transition metal allyl complexes have an enormous practical importance as either catalytic precursors or stoichiometric reagents in organic synthesis [103-108]. In the majority of the Group 4 complexes containing the allyl moiety, the metals exhibit the higher oxidation state (+4). Very few of these compounds are available in the literature with a +3 oxidation state, presumably because of their paramagnetic nature (reactivity) and difficulty in their handling. 

The relatively weak Ar-Br and especially Ar-I bonds would readily dissociate, giving rise to the Ni(I) paramagnetic complex and free aryl radical. This decomposition path is normally disfavored for aryl chlorides with considerably stronger Ar-Cl bonds. As a result, no Ni(I) species formed in the reactions of all chlo-roarenes studied, the only exception being [p-Me3NC6H4Cl]+. The p value of 5.4 obtained by Tsou and Kochi [33] is close to that (8.8 see above) previously reported by Foa and Cassar [32], suggesting that SET (Scheme 1) may play a certain role in some of the reactions of triphenylphosphine nickel(O) complexes with chloroarenes. It is still unclear if every reaction between any chloroarene and Ni(0) always involves the SET step. However, the excellent selectivity of the o-aryl Ni(II) complex formation from ArCl and highly reactive Ni(0) makes chloroarenes especially attractive substrates for various arylation reactions catalyzed by Ni complexes. 

The complexes [Rh°(TMP)], [Rh°(TTEPP)], [Rh (TTiPP)], [Ir°(TTEPP)], and [Ir°(TTiPP)] are sufficiently bulky to completely prevent dimerization (TMP = tetramesitylporphyrinato TTEPP = tetra(2,4,6-triethylphenyl)porphyr-inato TTiPP = tetra(2,4,6-triisopropyl- phenyl)porphyrinato). The EPR spectroscopy, NMR paramagnetic shifts and line-broadening studies have proven useful to smdy the structme and reactivity of these paramagnetic complexes (and their adducts with ethene and CO, see below) (133-135). The EPR parameters of the rhodium complexes [Rh (TMP)J and [Rh TTiPP)] are... 

Bis(l,5-cyclooctadiene)Fe(0), is prepared using Fe atoms impinging into a diene--methycyclohexane solution. This paramagnetic complex is stable to — 30°C in hexane and 0°C in 1,5-cyclooctadiene solution and is extremely reactive with added ligands. 

The sohd-state NMR analysis of paramagnetic powders features in particular strategic applications for the understanding of solid-state reactions, and can strongly contribute to the development of nano-scale and surface chemistry. The ability to evaluate hyperfine shifts in paramagnetic complexes can help explain how the high reactivity of some systems and the lack of expected reactivity of others can be tuned by the choice of the solid support. 

A number of recent papers have also focussed on the identification of paramagnetic transient reactive intermediates in bio-inspired non-heme iron catalysed oxidations. Makhlynets and Rybak-Akimova investigated the mechanism of the substrate oxidation, using H2O2 as oxidant, in an iron aminopyridine complex for aromatic hydro qrlation (Scheme 1). EPR with stopped-flow kinetic measurements, was used to identify the formation of the key Fe" (OOH) intermediate, with rhombic g... 

Most biochemically relevant high-spin systems have such short 7j-relaxation times that their EPR is broadened beyond detection at ambient temperatures. An exception is the class of S = 5/2 Mn" systems with D hx. Also, S = 7/2 Gd"1-based MRI shift reagents exhibit readily detectable room-temperature EPR spectra. Otherwise, aqueous-solution transition ion bioEPR is limited to complexes of S = 1/2 metals, in particular Cu", and to a lesser extent VIV02+, NiIn, Ni1, Mov, and Wv. Cupric is the stable oxidation state of biological copper under aerobic conditions, however, the other metals are stable as Vv, Ni", MoVI, and WVI, and, therefore, the other oxidation states associated with S = 1/2 paramagnetism may exhibit oxidative or reductive reactivity and may thus require specific experimental precautions such as strict anaerobicity over the course of the EPR experiment. 

The initial product of the inhibition step is not known in this case and may be a molecular complex.8 The direct reaction of the ethane with the peroxy radical is an example of a covalent compound giving a reaction resembling that of a related free radical. The molecular weight determination by Gomberg was therefore a necessary part of the proof that he was dealing with radicals and not merely an unusually reactive hydrocarbon. The presence of free radicals has since been confirmed by measurements of the paramagnetic susceptibility and the paramagnetic resonance absorption.9-10 The latter evidence also rules out an alter-... 

Sodium nitroprusside is the only clinically used metal complex of NO, so that its reactions provide an indication of the types of reactivity that metallonitrosyl complexes might be expected to have in physiological environments (see Fig. 1). The in vivo activation of nitroprusside depends on its reduction to [Fe(CN)5NO], which then releases cyanide to give [Fe(CN)4NO] which in turn releases NO and additional CN to yield Fejl,) and [Fe(CN)g] [75]. [Fe(CN)5(NO)] is paramagnetic (g, = 1.9993, g, = 1.9282, g = 2.008,... 

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

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