The complex Co(C-N-C)Br 2 (C-N-C ) 2,6bis(arylimidazol-2-ylidene)pyridine, aryl ) 2,6-Pr i 2 C 6 H 3 ), prepared by aminolysis of Co[N(SiMe 3 ) 2 ] 2 with the corresponding imidazolium salt (CH-N-CH)Br 2 , was methylated to the square-planar Co(C-N-C)(CH 3 ) and oxidized with BrN(SiMe 3 ) 2 to give Co(C-N-C)Br 3 . The Co(I), Co(II), and Co(III) complexes show no ethylene polymerization activity.

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The “pincer”‐type carbene complexes Rh(C–N–C)Cl and Rh(C–N–N)Cl [C–N–C = 3,3′‐diaryl‐1,1′‐pyridine‐2,6‐diylbis(imidazol‐2‐ylidene), C–N–N = 1‐aryl‐3‐(2,2′‐bipyridin‐6‐yl)imidazol‐2‐ylidene] have been prepared and characterised by spectroscopic and diffraction methods. They bind CO to give cationic complexes but are inert to H2. Both oxidatively add CH2Cl2 to give octahedral RhIII complexes. Theoretical calculations provide insight into the bonding of the Rh(C–N–C)Cl and Rh(C–N–N)Cl complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Replacement of Ishii×s N-hydroxyphthalimide (NHPI) with the novel carbon radical chain promoter N-hydroxysaccharin (NHS) affords, in combination with metal salts, notably Co, or other additives, selective catalytic autoxidation of hydrocarbons, alcohols and alkylbenzenes under mild conditions (25 ± 100 8C, O 2 1 atm). The effects of solvent, temperature and the nature of the additives were investigated to give an optimised oxidation protocol for the various systems. The NHS/Co combination was more reactive than NHPI/Co in the autoxidation of cycloalkanes. In contrast, the opposite order of reactivity was observed in the autoxidation of ethylbenzene and alcohols. It is suggested, on the basis of bond dissociation energy (BDE) considerations, that this is a result of a change in the rate-limiting step with the more reactive ethylbenzene and alcohol substrates. In the autoxidation of the model cycloalkane, cyclododecane, the best results (90% selectivity to a 4 : 1 mixture of alcohol and ketone at 24% conversion) were obtained with NHS/Co(acac) 3 in PhCF 3 at 80 8C. Competition experiments revealed that, in contrast to what is commonly believed, formation of the dicarboxylic acid by ring opening is not a result of further oxidation of the ketone product. It is suggested that ring opened products are a result of˚-scission of the cycloalkoxy radical formed via (metal-catalysed) decomposition of the hydroperoxide. This is suppressed in the presence of NHS (or NHPI) which efficiently scavenge the alkoxy radicals.