Published: May 19, 2011 r2011 American Chemical Society 3400 dx.doi.org/10.1021/om200246k | Organometallics 2011, 30, 3400–3411 ARTICLE pubs.acs.org/Organometallics Intramolecular Nitrile CH Bond Activation in Nickel NHC Complexes: A Route to New Nickelacycles Anna Magdalena Oertel, † Julien Freudenreich, † Jordan Gein, † Vincent Ritleng,* ,† Luis F. Veiros, ‡ and Michael J. Chetcuti* ,† † Laboratoire de Chimie Organom etallique Appliqu ee, UMR CNRS 7509, Ecole Europ eenne de Chimie, Polym eres et Mat  eriaux, Universit  e de Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France ‡ Centro de Química Estrutural, Complexo I, Instituto Superior T ecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b S Supporting Information ’ INTRODUCTION Since the first isolation of a stable imidazol-2-ylidene as a free ligand, 1 N-heterocyclic carbenes (NHCs) have become an im- portant class of ligands in organometallic chemistry. 2 The easy preparation and handling of their precursors and their high modularity, as well as their strong σ-donor properties, 3 which allow them to form strong NHCmetal bonds that prevent ligand dissociation, 4 have made them popular as supporting ligands in transition-metal catalysis. 2,5 Nickel NHC systems offer significant potential advantages, including a lower cost and a reduced tendency to deposit metallic nanoparticles, 6 as compared to many noble metal NHC systems, but they have been less studied than palladium, ruthenium, or even rhodium NHC systems. Nevertheless, nickel NHC catalysts have now found applications in a vast number of organic transformations, including notably CC cross-coupling re- actions, 710 the amination of arylamines, 11 and [2 þ 2 þ 2] cyclo- additions. 12 Moreover, nickel NHC complexes are capable of activating small molecules such as O 2 13 and CO 2 , 14 as well as the fairly unreactive CS and CC bonds in sulfoxides 15 and organ- onitriles, 16 respectively. We recently described the base-promoted CH activation of a labile acetonitrile ligand on a nickel NHC center. This reac- tion results in the acetonitrile ligand formally losing a proton and doing a sharp flip to give a nickel cyanomethyl complex Received: March 29, 2011 ABSTRACT: Cyclopentadienyl N-heterocyclic carbene (NHC) nickel complexes of general formula [Ni(NHC)XCp] (NHC = 1-(propylnitrile)-3-methylimidazol-2-ylidene, 1-(2,4,6- trimethylphenyl)-3-(butylnitrile)imidazol-2-ylidene, 1-(2,4,6- trimethylphenyl)-3-(pentylnitrile)imidazol-2-ylidene, 1-(2,4,6- trimethylphenyl)-3-(hexylnitrile)imidazol-2-ylidene; X = Cl, Br, I; Cp = η 5 -C 5 H 5 ), which bear an alkylnitrile side chain attached to one of the nitrogen atoms of the NHC ring, were prepared by the direct reaction of nickelocene with the corre- sponding imidazolium salts (NHC 3 HX). The new complexes [Ni{Me-NHC-(CH 2 ) 2 CN}ICp] (1a), [Ni{Mes-NHC-(CH 2 ) 3 - CN}ClCp] (1b), [Ni{Mes-NHC-(CH 2 ) 4 CN}ICp] (1c), and [Ni{Mes-NHC-(CH 2 ) 5 CN}BrCp] (1d) were obtained in good yields and were fully characterized by standard spectroscopic techniques and elemental analyses and, in the cases of 1a,b, by single- crystal X-ray crystallography. Structural studies established their two-legged piano-stool geometry. The cationic derivatives [Ni{Mes-NHC-(CH 2 ) n CN}(NCMe)Cp] þ (2bd; n =35) were prepared from the reaction of their neutral homologues 1bd with KPF 6 in acetonitrile at room temperature. Upon treatment of the neutral complexes 1 or of the cationic compounds 2 with KO- t-Bu, a CH bond R to the nitrile group in each molecule underwent a base-promoted CH activation and the new nickelacyclic complexes [Ni{Me-NHC-CH 2 CH(CN)}Cp] (3a) and [Ni{Mes-NHC-(CH 2 ) n CH(CN)}Cp] (3bd; n =24) were obtained. All of these metallacycles contain NiC σ-bonds, and their synthesis generated a new asymmetric carbon center. The cyclic complexes 3 were fully characterized by 1 H and 13 C NMR spectroscopy, IR spectroscopy, and elemental analyses. The structures of complexes 3b and 3d, which contain six- and eight-membered metallacyclic rings, respectively, were determined by single-crystal X-ray diffraction studies. DFT studies, carried out to probe the mechanism of these cyclonickelation reactions, indicated that the mechanism of formation of these nickelacycles was similar to that observed for the formation of cyanomethyl ligands from coordinated acetonitrile. Nevertheless, the base deprotonation of an R-CH ligand in a side arm, while thermodynamically comparable to that of a simultaneously coordinated acetonitrile ligand, is kinetically favored, and this leads to the formation of nickelacycles rather than cyanomethyl complexes in the case of the base-assisted activation of the cationic complexes 2.