Double Metalation of Acetone by a Nickel-NHC Complex: Trapping of an Oxyallyl Ligand at a Dinickel Center Anna Magdalena Oertel, † Vincent Ritleng, † Asleche Busiah, † Luis F. Veiros, ‡ and Michael J. Chetcuti* ,† † Laboratoire de Chimie Organome ́ tallique Applique ́ e, UMR CNRS 7509, Ecole Europe ́ enne de Chimie, Polyme ̀ res et Mate ́ riaux, Universite ́ de Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France ‡ Centro de Química Estrutural, Complexo I, Instituto Superior Te ́ cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal * S Supporting Information ABSTRACT: We report the first example of a structurally characterized oxyallyl -CH 2 C(O)CH 2 - group, trapped as a bridging ligand that spans two nickel centers. The ligand results from the double metalation of acetone on a nickel- NHC platform. A competing reaction leads to a single metalation of acetone. R ecent research from our group has targeted the chemistry of nickel complexes bearing NHC ligands. 1 We recently reported that nickel-NHC complexes are able to activate one of the C-H bonds of a labile acetonitrile ligand in the presence of a strong base. The reaction results in the formal loss of a hydrogen atom from the ligand, which does a sharp flip to give a neutral cyanomethyl-nickel complex (Scheme 1). 2 Here, we describe the synthesis of the dinickel complex [{Cp- (Mes 2 NHC)Ni} 2 {μ-CH 2 C(O)CH 2 }], in which an oxyallyl group is trapped as a bridging ligand spanning the two metal centers. The complex results from the deprotonation of two C-H bonds from the two different methyl groups of an acetone molecule on a nickel -NHC platform. While trimethylenemethane has been trapped as a ligand in many stable organometallic complexes, 3 the isoelectronic oxyallyl group has never been isolated free or even structurally characterized as a ligand. 4 Treatment of [Ni(Mes 2 NHC)ClCp] with AgBF 4 in acetone affords the red cationic η 1 -acetone complex [Ni(Mes 2 NHC)- {(O)C(CH 3 ) 2 }Cp] + BF 4 - , 1, in high yield (Scheme 2). The 1 H and 13 C{ 1 H} NMR spectra in acetone-d 6 show that the (CH 3 ) 2 C(O) ligand is labile in solution. The acetone ν(CO) stretch is observed at 1656 cm -1 in the IR spectrum. An X-ray diffraction study of the cation of 1, which is presented here (Figure 1), reveals a C2-O-Ni angle of 132°. The rest of the structure is typical of Cp-Ni-NHC fragments in [CpNi- (NHC)(X)] complexes 1,2 and deserves no particular comment. When 1 is treated with a suspension of KOtBu in toluene, two new species are formed. One of these is the brown nickel acetonyl complex [Ni(Mes 2 NHC){CH 2 C(O)CH 3 }Cp], 2 (Scheme 2). In contrast to the heavier group 10 elements, Pd and Pt, where metal-acetonyl or so-called C-bound enolate complexes are prevalent, 5 the latter have rarely been reported for nickel: 6,7 similar reactions with this metal usually generate O-bound enolate nickel complexes. 8 Spectroscopic data for the CH 2 C(O)CH 3 ligand in 2 are consistent with spectral signals observed for other reported acetonyl ligands: the CH 2 protons appear at δ = 1.07 ppm in the 1 H NMR spectrum (in CDCl 3 ), Received: September 1, 2011 Published: November 14, 2011 Scheme 1. Base-Promoted C-H Activation of Acetonitrile 2 Scheme 2. Formation of the Acetone Complex 1 and the Subsequent Metalation of Acetone to Give the Acetonyl Complex 2 and the Oxyallyl Complex 3 Note pubs.acs.org/Organometallics © 2011 American Chemical Society 6495 dx.doi.org/10.1021/om200819c | Organometallics 2011, 30, 6495-6498