Synthesis, structure, and olefin polymerization with nickel( II) N-heterocyclic carbene enolates{ Benjamin E. Ketz, Xavier G. Ottenwaelder and Robert M. Waymouth* Received (in Berkeley, CA, USA) 4th August 2005, Accepted 26th September 2005 First published as an Advance Article on the web 20th October 2005 DOI: 10.1039/b511202h Two novel N-heterocylic carbene enolate nickel complexes have been prepared and shown to be active for ethylene and propylene polymerization to yield linear polymers. Neutral Ni(II) complexes supported by chelating anionic LX ligands are an important class of olefin oligomerization and polymerization catalysts. 1–9 The pioneering studies by Keim 5,10 formed the basis of the Shell Higher Olefin Process (SHOP) and stimulated the development of a variety of Ni complexes for ethylene oligomerization, polymerization, and copolymeriza- tion. 7,8,10,11 Brookhart’s contributions with cationic Ni and Pd complexes sparked renewed interest in late transition metal catalysts for ethylene polymerization. 2,12,13 The selectivity of neutral Ni catalysts for ethylene polymeriza- tion versus oligomerization depends on several interrelated factors including both the steric 1,3 and electronic 14,15 properties of the ligands, as well as the nature of the neutral L ligand. 9 Keim’s studies implicating that dialkylphosphinoenolate Ni complexes of electron-donating alkyl phosphines lead to higher M n polymers 14,15 stimulated us to investigate anionic enolate ligands based on N-heterocyclic carbenes (NHCs). 16 NHCs are versatile ligands that share many of the coordination properties of phosphines, but are more potent s-donors. 17–20 To date, there are no known nickel complexes with monoanionic chelating NHC ligands, and neutral Ni catalysts based on NHCs have not been used for polymeriza- tion. However, two reports describe the structures of anionic NHC complexes of Pd, 16,21 and cationic Ni NHC complexes have been investigated for ethylene polymerization. 22,23 Herein, we describe the synthesis, structure, and polymerization behavior of two nickel carbene enolate complexes. Initial attempts to prepare the nickel carbene complexes from singly or doubly deprotonated imidazolium salts 1a,b and Ni halides were unsuccessful. Treatment of 1a,b with (Ph 3 P) 2 Ni(Ph)Cl in the presence of 2 equiv. of NaN(TMS) 2 yielded the Ph 3 P-ligated Ni carbene phenyl complex, but removal of the excess Ph 3 P proved difficult. After screening a number of reactions, we developed an expedient synthesis of the pyridine adduct, which we anticipated would be more labile than the phosphine adduct toward substitution by ethylene. 9 Complexes 2a,b were prepared in high yield in a one pot procedure (eqn 1). The imidazolium salt, 1 equiv. of bis(1,5-cyclooctadiene)nickel (Ni(COD) 2 ), and 2 equiv. NaN(TMS) 2 were frozen in pyridine. As this mixture thawed, an excess of chlorobenzene was added and the mixture stirred for 1 h at 25 uC. Removal of the solvent in vacuo and recrystallization by slow diffusion of pentane into a CH 2 Cl 2 solution of 2a layered with Et 2 O provided crystals suitable for X-ray diffraction.{ ð1Þ The structure of 2a (Fig. 1) reveals a square-planar coordination for Ni. The Ni–C1 bond length of 1.848 A ˚ is shorter than that observed by Sigman in the complex (IPr)Ni(Cl)(allyl) (Ni– NHC 5 1.903 A ˚ , IPr 5 N,N-bis(2,6-diisopropylphenyl)imidazol- 2-ylidine), 24 and that of other known Ni–NHC complexes. 25–28 The Ni–O bond of 1.891 A ˚ is slightly shorter than that of an analogous diphenylphosphinoenolate complex (Ni–O 5 1.914 A ˚ ). 29 The bond angles and lengths within the carbene heterocycle are comparable to those of the analogous Pd carbene enolate. 16 The mesityl ring is nearly perpendicular to the NHC, with a C2–N2– C12–C13 dihedral angle of 99.6u for 2a. The conformation of the chelate ring is slightly different between this structure and the analogous Pd complex, presumably due to the different covalent Department of Chemistry, Stanford University, Stanford, California, USA. E-mail: waymouth@stanford.edu; Fax: +01 650 736 2262; Tel: +01 650 723 4515 { Electronic supplementary information (ESI) available: experimental details, representative NMR spectra, and crystal structure data. See DOI: 10.1039/b511202h Fig. 1 ORTEP diagram of 2a, selected bond lengths (A ˚ ) and angles (deg): Ni–C1, 1.848(3); Ni–C21, 1.891(2); Ni–N3, 1.934(2); Ni–O, 1.891(2); C1–N1, 1.375(4); C1–N2, 1.373(4); N1–C4, 1.406(4); C4–C5, 1.353(5); C5– O, 1.315(4); C1–Ni–C21, 93.17(12); C21–Ni–N3, 88.91(10); N3–Ni–O, 85.43(10); O–Ni–C1, 92.56(12); N1–C1–N2, 103.1(2); N1–C4–C5, 122.7(3); C4–C5–O, 122.7(3). COMMUNICATION www.rsc.org/chemcomm | ChemComm This journal is ß The Royal Society of Chemistry 2005 Chem. Commun., 2005, 5693–5695 | 5693