The chromium(III) complexes of the type L1CrXY (L1 = 1-(2,6-diisopropylphenyl)-3-[β-(4,7-dimethylindenyl)ethyl]-imidazol-2-ylidene, X, Y = Cl, 2a-Cl; X = Cl, Y = Me, 2a-Me; X = Cl, Y = Ph, 2a-Ph; X, Y = benzyl, 2a-Bz) and L2CrCl2 (L2 = 1-(2,6-diisopropylphenyl)-3-[γ-(4,7-dimethylindenyl)propyl]-imidazol-2-ylidene, 2b-Cl) have been prepared and characterized by analytical and diffraction methods. The salts [L1CrCl(THF)]+[BArF 4]− and [L2CrCl(THF)]+[BArF 4]− (ArF = 3,5-bis(trifluoromethyl)phenyl) were also characterized, having been obtained by abstraction of chloride, or indirectly a methyl, from 2a-Cl or 2a-Me, and 2b-Cl, by NaBArF 4, respectively. Reaction of 2a-Bz with 1 equiv of [H(Et2O)2][BArF 4] in THF gave the thermally unstable salt [L1Cr(Bz)(THF)]+[BArF 4], [2a-Bz] + [BAr F 4 ], which was also characterized crystallographically. The chromium(II) analogue L1CrCl, 3a, was prepared from CrCl2 and L1K, but on prolonged standing in THF, it converted to the dimeric complex 4a. The complexes 2a-Cl, 2b-Cl, 2b-Me, and 2a-Bz in the presence of an Al activator were tested as ethylene polymerization and oligomerization catalysts, in general, showing activity toward polymer formation.

A new family of N,N'-bis-(2,6-diisopropylphenyl)-(2,6-diisopropylphenyl-imidazolium)-acetamidines have been developed as NHC proligands and ligands that are functionalised with neutral amidine and anionic amidinato moieties. On coordination they adopt diverse binding modes, depending on the nature of the metal and the reaction conditions. In the Ag, K and Cr complexes reported in this paper, monodentate κ(1)(NHC), bimetallic bridging-(κ(1)-NHC-κ(1)-amidinato) and bidentate (κ(1)-NHC-κ(1)-amidinato) binding modes were observed, respectively. Two of the novel Cr complexes, in the presence of activators, were tested as catalysts for the polymerisation of ethylene showing low activity.

The synthesis and full characterization of the unprecedented open-shell Cr(II) benzyl organometallic complexes [Cr(NHC)2(benzyl)2] (2) and [Cr(NHC*)(benzyl)2] (3) (NHC = N,N′-diisopropylimidazol-2-ylidene; NHC* = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) from [Cr(benzyl)3(THF)3] and [CrCl2(THF)2]/[Mg(benzyl)2], respectively, uncovered unusually acute angles (93° in 2 and 76° in 3) at the sp3 benzylic C of the coordinated benzyl ligands. Detailed theoretical analyses (DFT and CASPT2) of the four- and three-coordinate Cr(II) species were performed to elucidate the physical origin of the benzyl bending and led to the recognition of a noncovalent, intramolecular polarization-induced metal–arene (PIMA) interaction as being responsible for it. The energetic contribution from a single PIMA interaction is estimated to be ca. 50 kJ/mol. A comparison with the origin of the angular distortions in the d0 [Zr(benzyl)4] complex will also be presented. Sharing the common origin of an induced-dipole charge density scheme with intermolecular anion−π interactions, the intramolecular PIMA interaction concept involving a transition metal unpaired d electron and arene−π orbitals can be viewed as an extension of intermolecular anion−π interactions and leads to remarkable quantitative prediction of the observed structural distortions. PIMA interactions may manifest themselves in diverse structural and dynamic phenomena and have broad implications in chemical sciences.