Angular Distortions at Benzylic Carbons Due to Intramolecular Polarization-Induced Metal–Arene Interactions: A Case Study with Open-Shell Chromium(II) NHC Complexes

Abstract

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.

A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer

“…Single crystal X-ray diffraction demonstrated that 2 is a square planar complex (Figure ). The Cr–C bond distances in complex 2 range from 2.076(3) to 2.132(3) Å which is within the expected range (2.08–2.18 Å) given the limited number of Cr­(II) NHC complexes. − The trans C1–Cr–C1′ and C2–Cr–C2′ angles are 170.54(14)° and 177.39(12)°, respectively, and the Cr atom sits on the calculated centroid of the plane of the NHC carbenes.…”

N-Phosphanyl- and N,N′-Diphosphanyl-Substituted N-Heterocyclic Carbene Chromium Complexes: Synthesis, Structures, and Catalytic Ethylene Oligomerization

“…by reacting excess ligand with [CrCl 3 (thf) 3 ] or [Cr­(Me)­Cl 2 (thf) 3 ], led unexpectedly to the reduction of Cr III to Cr II and the isolation of 1 – 4 . Similar redox reactions between a Cr III complex and N , N ′-diisopropylimidazol-2-ylidene have been observed previously, but their mechanisms were not fully elucidated …”

“…The Cr–C benzyl bond distances in 5 and 6 (2.095(5)–2.149(1) Å) fall within the expected range , and are significantly shorter than the Cr–C NHC bonds (2.216(1) Å for 5 and 2.227(4) Å for 6 ). In both 5 and 6 , the three angles subtended at C benzylic show considerable differences: the C32–C31–Cr1 angle (107.20(8)° in 5 , 99.8(3)° in 6 ) is more acute than the other two (C39–C38–Cr1 121.57(8), 124.7(3)° and C25–C24–Cr1 117.07(9), 118.1(3)° in 5 and 6 , respectively).…”

Diamido tantalum(V) complexes derived from a diazamacrocycle

“…The involvement of polarization induced interactions as contributing factor at the angular distortions at the benzylic C has been proposed recently. 16 Tantalum tribenzyl complexes with bulky monodentate aryloxides adopt trigonal bipyramidal geometry with equatorial benzyls, 17 and with bulky chelating bis aryloxides trigonal bipyramidal with two axial and one equatorial benzyls; 18 tridentate amine bis-phenolate ligands support octahedral geometry with meridional benzyls. 14 with Ta(CH2 t Bu3)3Cl2 in toluene giving, following work-up, a brown solid (2).…”