| Summary: | The oxidative addition products trans-[Pd(NHC)2(Ar)Cl] (NHC = cyclo-C{N`BuCH}2; Ar = Me-4-C6H4, MeO-4-C6H4, CO2Me-4-C6H4) have been isolated in good yields from the reactions of ArCI with the amination precatalyst [Pd(NHC)2] and structurally characterized. The former undergo reversible dissociation of one NHC ligand at elevated temperatures, and a value of 25.57 kcal mol-1 has been determined for the Pd- NHC bond dissociation enthalpy in the case where Ar = Me-4-C6H4. Detailed kinetic studies (via extensive NMR studies) have established that the oxidative addition reactions proceed by a dissociative mechanism. Rate data for the oxidative addition of Me-4-C6H4Cl to [Pd(NHC)2] compared to that obtained for the [Pd(NHC)2}-catalysed coupling of morpholine with 4-chlorotoluene are consistent with a rate-determining oxidative addition in the catalytic amination reaction. The relative rates of oxidative addition of the three aryl chlorides to [Pd(NHC)21(CO2Me-4-C6H4C1> Me-4-C6H4C1> MeO-4-C6H4C1) reflect the electronic nature of the substituents and also parallel observed trends in the coupling efficiency for these aryl halides in aminations. Transmetallations tudies resulted in the isolation and structural characteristaiono f palladacycle [Pd(cyclo-C{N`BuCH}2)(μ-O(2-CH2C6H4))]2 from the reaction of [Pd(NHC)2] with 2-chlorobenzylalcohol. Attempts to isolate and fully characterise [Pd(cyclo-C{N'BuCH}2)(μ-O(2-CH2CH2C6H4))]2 and the amine analogue [Pd(cyclo- C {N`BuCH}2)(μ-NH(2-CH2C6H4))]2 were unsuccessful. Attempts to prepare [Ni(cyclo-C{NtBuCH}Z)] by a conventional route led to the formation of several unexpected Ni-NHC complexes. The reaction between [Ni(1,5- COD)2] and 1,3-bis-tert-butylimidazol-2-ylidene in the presence of silicone grease affords the siloxane bridged dimer [Ni(cyclo-C{N`BuCH}2)(O{Me2SiOSiMe2}-it- 0)]2. Using greaseless apparatus, the same reaction yields the Ni-Ni dimer [Ni(cyclo-C{N`BuCH}2)( -(cyclo-C{N`BuCH} {NCH}))]2, via two structurally characterised intermediates V
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