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Transmetalation of Aryl Units from Gold(I) to Cobalt(III):  A Clean Route to the Synthesis of Anion Cobaltoreceptors

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posted on 28.03.2005, 00:00 by Mike Robitzer, Imenne Bouamaïed, Claude Sirlin, Preston A. Chase, Gerard van Koten, Michel Pfeffer
New organocobalt complexes were synthesized through the transmetalation reaction of lithium derivatives of several N,C,N pincer ligands, Li(2,6-(NR2R1CH2)2C6H2R‘), with [(η5-Cp)CoI2]2. Changing one or both methyl groups on the nitrogen atoms for ethyl, tert-butyl, or a (CH2)4 group led to rather unstable aquo or HX derivatives of the form [(η5-Cp)Co{2,6-(NR1R2CH2)2C6H3}HY]PF6 (where R1, R2 = Me, Et, t-Bu, (CH2)4 and Y = OH, Cl, I), which were characterized by 1H NMR spectroscopy and a crystal structure analysis for the compound in which R1 = R2 = Et. Related organocobalt derivatives could, however, be prepared more readily by using a transauration reaction. Thus, the transmetalation reaction between Ph3PAu(2,6-(Me2NCH2)2C6H3) and [(η5-Cp)CoI2]2 afforded the known [(η5-Cp)Co{2,6-(Me2NCH2)2C6H3}H2O]PF6 in good yield. Two organocobalt complexes containing pincer ligands substituted at the para position by either an electron-withdrawing group, −COMe (5a), or an electron-donating group, −OMe (5b), were thus prepared by the transauration with gold(I) derivatives: Ph3PAu(2,6-(Me2NCH2)2(4-COMeC6H2) and Ph3PAu(2,6-(Me2NCH2)2(4-MeOC6H2) and [(η5-Cp)CoI2]2. The thermodynamic study of these latter compounds indicated that they display high selectivity for acetate versus chloride anion in water. We found for 5b that the acetate versus chloride selectivity reaches a factor of 40. Such results are without precedent in the literature, if one does not take into account the selectivities found for oxyanions in nonprotic solvents such as acetonitrile, dimethyl sulfoxide, and chloroform.