Studies on Three-Coordinate [Co{N(SiMe 3 ) 2 } 2 L] Complexes, L = N‑Heterocyclic Carbene Alexandre Massard, † Pierre Braunstein,* ,† Andreas A. Danopoulos,* ,†,‡ Sylvie Choua,* ,§ and Pierre Rabu* ,∥ † Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite ́ de Strasbourg, 4 rue Blaise Pascal, F-67081 Strasbourg Cedex, France ‡ Universite ́ de Strasbourg Institute for Advanced Study (USIAS), F-67083 Strasbourg, France § Institut de Chimie, Universite ́ de Strasbourg, 1 rue Blaise Pascal, BP 296 R8, F-67008 Strasbourg Cedex, France ∥ Institut de Physique et Chimie des Mate ́ riaux de Strasbourg (IPCMS) (UMR7504 CNRS - Universite ́ de Strasbourg), 23 rue du Loess, F-67034 Strasbourg, France * S Supporting Information ABSTRACT: A series of three-coordinate N-heterocyclic carbene bis(trimethylsilyl)amide complexes of cobalt, [Co{N- (SiMe 3 ) 2 } 2 (NHC)] (NHC = N,N′-diarylimidazolin-2-ylidene, aryl = 2,6-diisopropylphenyl, SIPr (1), aryl = mesityl, SIMes (2); κ 1 -N,N′-diphosphanylimidazol-2-ylidene, PCP (5); κ 1 -N- phosphanyl-N′-2,6-diisopropylphenylimidazol-2-ylidene, PC (6); cyclic alkyl amino carbene :C(Cy)CH 2 CMe 2 N-2,6- Pr i 2 C 6 H 3 , cAAC Cy (7)) were prepared by the reaction of [Co{N(SiMe 3 ) 2 } 2 ] with the corresponding NHCs. The complexes exhibited interesting transamination reactivity in reactions with bulky 2,6-diisopropylaniline (NH 2 (DiPP)), where adjustment of the stoichiometry and reaction conditions resulted in the substitution of one or two N(SiMe 3 ) 2 ligands by the anilido ligand NH(DiPP), giving [Co{N(SiMe 3 ) 2 }{NH(DiPP)}(NHC)] (NHC = N,N′-bis(2,6-diisopropylphenyl)- imidazol-2-ylidene, IPr (8)) and [Co{NH(DiPP)} 2 (NHC)] (NHC = SIPr (9); IPr (10); PCP (11)). X-ray crystallography revealed trigonal-planar coordination geometry at Co for all of the new complexes and long Co−C NHC bond lengths; in the cases of 5, 8, 9, 10, and 11, intramolecular Co···H and/or H···P interactions may provide further stabilization of specific conformations. Magnetic and electron paramagnetic resonance studies showed that the three-coordinate Co(II) centers in 1−7 behave as S = 3 / 2 spins with strong anisotropy arising from the low symmetry of the coordination site (C 2v ). The anisotropy results in large values of the zero-field splitting parameter D. ■ INTRODUCTION The complexes [M{N(SiMe 3 ) 2 } 2 ] (M = Mn, Co, Fe) 1 constitute versatile, reactive, hydrocarbon-soluble starting materials with a multitude of synthetic applications by virtue of the basicity of the coordinated silylamides 2 and the low- coordination-number metal geometries that they feature. In the gas phase they are monomeric with two-coordinate metals, 3 and in the solid-state they are dimeric, with three-coordinate metals and amido bridges. 4 In solution in noncoordinating solvents, they participate in monomer−dimer equilibria. 4b Besides their synthetic utility and structural versatility, there is growing interest in the study of their magnetic properties. 5,6 The low coordination number of the metals leads to high magnetic moments with orbital contributions or zero-field splitting (ZFS) and anisotropic g tensors; 7 however, only one detailed study for M = Co has recently appeared. 5a Catalytic applications of the silylamide complexes [M{N(SiMe 3 ) 2 } 2 ] (M = Mn, Co, Fe) are still limited but emerging. 8 Monomeric three-coordinate adducts [M{N(SiMe 3 ) 2 } 2 L] in which L is a monodentate neutral donor have been prepared with various classical donors, e.g., for M = Fe, L = THF, 4b P(cyclohexyl) 3 ; 9 for M = Co, L = THF, 5a pyridine, 5a,b PR 3 . 5a,c In general, depending on the electronic and steric properties of L, the adducts feature distortion from the trigonal-planar arrangement, including variation of interligand angles and the M−N(SiMe 3 ) 2 bond distances. In the case where L is an N-heterocyclic carbene (NHC), monomeric three-coordinate adducts [M{N(SiMe 3 ) 2 } 2 (NHC)] (NHC = N,N′-diarylimidazol-2-ylidene, aryl = 2,6-diisopropyl- phenyl (IPr), mesityl (IMes)) have recently been reported for M = Fe(II) 10 and, while this article was in preparation, Co(II). 11 Furthermore, upon heating in toluene, the complexes Special Issue: Mike Lappert Memorial Issue Received: November 26, 2014 Published: February 24, 2015 Article pubs.acs.org/Organometallics © 2015 American Chemical Society 2429 DOI: 10.1021/om501178p Organometallics 2015, 34, 2429−2438