Nucleophilic N-heterocyclic carbenes (NHCs) have attracted considerable interest as ligands in organometallic chemistry and catalysis. [1] This attention is largely because of the unique combination of strong s-donor, poor p-acceptor, and steric properties. [2] Typical transition-metal NHC complexes show "normal" binding at C2 because of the acidity of the C2 proton in the precursor azolium salt. [3] "Abnormal" metalation at C5 of the NHC ring was first reported in elegant studies by Crabtree and co-workers for pyridyl-and picolyl-NHC dihydride complexes of Ir iii ; [4] blocking C2 with a substituent, such as Me or Ph groups, will also encourage binding at C5. [5] Nolan and co-workers have recently reported the structure and interesting catalytic reactivity of [PdCl 2 -(NHC) 2 ] complexes containing NHC ligands bound the "wrong way" that are derived from imidazolium salts. [6] In both these studies, the carbene bound at C4/5 was generated through deprotonation by the addition of a base or use of basic ligands on the metal.
In previous studies on metal-carbene complexes, we identified a facile reductive-elimination process, in which the carbene is lost as an imidazolium salt. [7] Conversely, it was found that imidazolium salts (ionic liquids) add oxidatively to low-valent metal centers through C2 activation. [7][8][9] The combination of these two redox steps lead to a new atomefficient catalytic reaction for the substitution of azolium salts at C2. [10] The nature of this reaction and its wider implications is of major interest; for example, ionic liquids may add oxidatively to metal complexes dissolved within them, which generates catalytically active (hydrido)metal(carbene) complexes, and ionic liquids may also be incorporated into the reaction products, thus both problems and opportunities are presented. [11] Herein, we report a novel extension to our investigations on this redox behavior. Platinum hydride complexes have been synthesized that bear NHC ligands in an abnormal binding mode through the oxidative addition of imidazolium salts with the C2 position blocked by a methyl group.
Complexes 2, 4, and 4' were prepared in one-pot reactions by mixing [Pt(norbornene) 3 ], [12] 1,3-bis(2,4,6trimethylphenyl)imidazolin-2-ylidene (IMes), and either 1,2,3,4-tetramethylimidazolium iodide (1), in the case of 2, or 1,2-dimethyl-3-propylimidazolium bromide (3), in the case of 4 and 4', in a molar ratio of 1:1:1.8 (Scheme 1). The reactions yielded the desired products after washing the crude residues with n-hexane and extraction with THF. It is proposed that the reactions proceed through the initial Scheme 1. Oxidative addition at C4 and C5 in imidazolium cations to a Pt 0 center. nbe = norbornene.